Keywords: synthesis, CdSe/CdS dot-in-rods, ligand-induced chirality, circularly polarized luminescence, photodeposition, hybrid semiconductor-metal nanoparticles

a b c Bare CdSe/CdS DRs 2.8 nm two end 1.9 nm one end d e f CdS rods Multi-sites Au (big) Big-tip Au (6 nm) +Multi-sites Multi-sites Au (small) Single-tipped Au (6.2 nm)

i

References page x

Comme mentionné dans le titre, l'objectif de cette thèse est la synthèse de tiges quantiques semi-conductrices qui peuvent être utilisées pour une étude approfondie et apporter de nouvelles connaissances sur les propriétés de chiralité induite et les processus de photodéposition lors de l'utilisation de tels substrats. Dans cette thèse, nous nous concentrerons sur les nanocristaux semi-conducteurs (NC) traditionnels à base de cadmium II -VI, en particulier les bâtonnets quantiques, comme les CdSe / CdS dot-in-rods (DRs).

Ces dernières années, de grands progrès ont été réalisés dans la conception de la croissance contrôlée et de façon sélective de petits cristaux métalliques sur les pointes des nanobâtonnets à base de cadmium, où le métal est en contact cristallin direct avec la partie semi-conductrice. 1,2 Les stratégies de synthèse, la taille, l'emplacement, la composition et la forme des différents composants hybrides de nanoparticules semiconductrices métalliques (HNP) peuvent être contrôlées, de sorte que leurs propriétés synergiques uniques puissent être étudiées plus avant et utilisées dans diverses applications. [3][4][5] Une application importante implique l'utilisation de ces nanoparticules (NP) comme photocatalyseurs. Il existe plusieurs façons d'améliorer les propriétés catalytiques, par exemple, en facilitant la séparation efficace et durable des porteurs de charge et en minimisant rétro-réaction des intermédiaires, ainsi les structures en bâtonnets quantiques ensemencés, tels que les bâtonnets CdSe / CdS (DRs), [6][7][8][9][10][11] se sont révélées très actives pour la production d'hydrogène (Figure 1 a et b). 8 De même, il a été montré que la réduction des colorants organiques (MV 2+ ) était 1,5 fois plus efficace dans une structure CdSe / CdS que dans le cas d'un nanorod CdS pur. L'activité accrue a été attribuée à une localisation plus forte du trou photo-induit au niveau du domaine riche en trous, ce qui a effectivement empêché sa recombinaison avec un électron. 12 D'autre part, Amirav et ses collègues ont découvert qu'un catalyseur base sur un nanorod et une pointe métallique se révèle être le plus actif pour la production d'hydrogène (27 fois plus élevé que les pointes multiples) (Figure 1 c). 6,7 En outre, Zamkov et ses collègues ont montré que le noyau CdSe exposé améliore encore les taux associés de génération de H2 (Figure 1 d). 13 CdS non ensemencées à pointe de platine (jaune), et de cinq échantillons différents de tiges à pointe de platine, avec des diamètres de germes de 3,1 (rouge) ou 2,3 nm (vert). Sous chaque barre se trouve la longueur moyenne correspondante de l'échantillon. 8 b) Activité photocatalytique de divers nanocristaux semi-conducteurs présentant différents degrés de séparation de charge photo-induite.

L'axe vertical montre le rendement quantique de photogénération radicalaire de MV +• utilisant le tris(bipyridine)ruthenium(II) (Ru(bipy)3 2+ ) en fonction de différents types de semiconducteurs. 14

c)

Efficacité quantique photocatalytique pour la demi-réaction de réduction d'hydrogène obtenue avec des photocatalyseurs nanorodc CdSe / CdS décorés avec un site de réduction Pt simple, double ou multiple 6 d) Enlèvement de la couche externe de CdS dans une structure CdSe / CdS DR résultats dans la morphologie où les composants du donneur et de l'accepteur sont en contact direct avec l'environnement externe, améliorant les taux associés de génération d'H2. 13 D'autre part, la préparation contrôlée des DRs CdSe / CdS peut également être cruciale en termes de chiralité. Kuang et ses collègues 15 ont montré que des DR longs avec des rapports d'aspect (AR) élevés peuvent améliorer le facteur g dans la plage de 445 à 461 nm (de 5,9 × 10 -4 à 1,2 × 10 -3 ). Gun'ko et ses collègues 16 ont montré que la réponse chirale des points quantiques CdSe / CdS (QDs) était fortement améliorée à mesure que l'épaisseur de la coquille CdS diminuait, certainement en raison de la a b d c

iii References page x distance décroissante entre le trou présent dans le noyau CdSe et les ligands chiraux coordonnés au niveau de la surface du QD. Outre le processus de photodéposition, la maîtrise de la préparation des NCs de semi-conducteur peut simultanément conduire à d'excellentes performances en chiralité et en catalyse. Cependant, à notre connaissance, malgré les nombreux travaux entrepris sur ce sujet ; il y a encore un manque de recherche systématique sur la préparation de ces bâtonnets quantiques, notamment en termes de contrôle de haute précision de la synthèse des DRs en terme d'uniformité de couche en deça de la couche, qui sont essentiels pour la chiralité et la catalyse. Un objectif clé de cette thèse est le développement de méthodes expérimentales pour contrôler sélectivement la forme et la taille des nanocristaux (NCs) de semi-conducteurs et ensuite donner un guide pour une large gamme de matériaux.

Les études expérimentales actuelles sur les nanocristaux colloïdaux stabilisés par ligands et leurs propriétés proviennent des travaux pionniers du groupe de Brus et Henglein, 17,18 qui a commencé à étudier l'effet de confinement quantique des nanocristaux colloïdaux sphériques de CdS il y a plus de deux décennies. Ces échantillons de NCs semi-conducteurs ont été synthétisés à température ambiante.

Bien que les propriétés optiques aient présenté des effets de confinement quantique, leurs distributions granulométriques étaient larges et les particules ne présentaient qu'une faible luminescence, ce qui était dû à des défauts cristallins et une mauvaise passivation de surface. Par la suite, Bawendi et ses collègues 19 ont développé une méthode de synthèse à haute température pour décomposer les précurseurs organométalliques dans un solvant de coordination et produire des nanocristaux semiconducteurs sphériques à facettes avec des propriétés optiques grandement améliorées. Cette méthode ascendante a également été étendue avec succès au contrôle de forme des nano-objets, réalisant ainsi différentes morphologies. En ajustant les paramètres de réaction, Alivisatos, Peng et ses collègues ont présenté pour la première fois la synthèse de nanorods en 2000. 20 Cette méthode de croissance contrôlée par un surfactant a été réalisée en ajoutant des co-ligands qui se lient préférentiellement à certains plans cristallins dans le mélange réactionnel, de sorte qu'il est devenu beaucoup plus facile pour le cristal de se développer sur des plans iv References page x avec une passivation plus faible. Un contrôle précis de la forme et de la taille a été réalisé pour différents types de semi-conducteurs. Des structures dendritiques plus complexes ont également été réalisées grâce à une méthode de croissance cristalline secondaire sur les bras aux extrémités d'une tige ou d'un tétrapode. 21 Bien que ces dernières années, des progrès significatifs aient été réalisés dans la compréhension de la façon de contrôler la morphologie des nanocristaux, des recherches plus systématiques sur le contrôle de haute précision de la synthèse des NCs semiconducteurs sont encore nécessaires. Ainsi, il est constamment obligatoire et pertinent de trouver une approche synthétique efficace pour réaliser de tels NCs semiconducteurs avec des propriétés accordables. Dans ce travail, nous reviendrons une fois de plus sur le contrôle précis de la synthèse des bâtonnets quantiques, et les utiliserons à des fins de chiralité et de dépôt métallique induit par laser. ll existe deux types de chiralité dans le cas des NCs semi-conducteurs, le premier est la «chiralité intrinsèque», qui est obtenue par la chiralité de la nanoparticule ellemême, telle que la chiralité induite par la dislocation, [22][23][24] auto-assemblage, [25][26][27] et modèle supramoléculaire de nanohélices. 28,29 Cependant, ses applications potentielles sont limitées en raison du processus de préparation complexe et du manque de contrôle. L'autre type est la «Transmission de Chiralité», et utilise des molécules chirales, elle transfère sa chiralité aux nano-objets. Différents niveaux de structures chirales peuvent être construits; cela signifie que les informations chirales sont transmissibles. L'un des travaux expérimentaux pionniers de Gun'ko et de ses collègues a démontré que les couches superficielles des NCs semi-conducteurs CdS pouvaient acquérir la propriété chirale des ligands énantiopurs de pénicillamine attachés à la surface des points quantiques. 30 Un certain nombre d'articles théoriques et expérimentaux ont suivi, qui ont amélioré cette technique de fabrication, et ont théoriquement abordé les origines les plus courantes de la chiralité des nanocristaux.

Grâce à l'universalité de la transmissibilité chirale, l'induction de la chiralité et / ou de l'activité optique dans les nanocristaux semi-conducteurs inorganiques est un moyen prometteur d'introduire une nouvelle dimension de complexité intrigante et utile à leurs propriétés, qui peut être utilisée dans la synthèse stéréosélective, 24,26,31 la reconnaissance chirale, 32,33 les dispositifs d'affichage, 34,35 la biodétection, 36,37 la v References page x catalyse asymétrique, 38 et ainsi de suite. [39][40][41] Cependant, le signal de luminescence à polarisation circulaire (CPL) est encore rare. Jusqu'à présent, il est toujours difficile d'observer un signal CPL pour les NCs de semi-conducteurs recouverts de ligands chiraux. Bien que une luminescence à polarisation circulaire induite par ligands ait été observée sur simple transfert de phase de QDs 0D CdSe du toluène en phase aqueuse contenant de la L-ou D-cystéine chirale par Balaz et ses collègues, 42 à ce jour, très peu de cas de NCs de semi-conducteurs actifs enCPL ont été reportés.

Les nanoparticules hybrides (HNP) ont suscité un grand intérêt dans une variété de domaines de recherche et d'applications, notamment l'optique, l'électronique et la catalyse. Par rapport à la croissance thermique de particules d'or multi-sites, la croissance induite par la lumière plus efficace en énergie conduit à un dépôt d'or hautement sélectif dans les HNPs. Outre le contrôle de la taille et de la forme des CNs de semi-conducteurs, l'évolution du contrôle des sites de dépôt joue également un rôle clé dans la propriété catalytique des HNPs. Par la maîtrise des méthodes thermiques et de photo-réduction, Banin et ses collaborateurs 9 ont réussi à fabriquer des bâtonnets décorés Au CdS et CdSe / CdS, avec soit plusieurs points Au sur la surface, soit un grand dot d'Au à la pointe. Cependant, les HNPs à une seule pointe sans minuscules NPs Au entourant la surface du bâtonnet sont toujours un défi. Comme le montre la Figure 1, non seulement la forme des NCs semi-conducteurs mais aussi le nombre de NP métalliques peuvent être un paramètre crucial et la controverse n'est pas encore terminée dans la littérature. 6,7,[43][44][45][46][47] Donc, il y a une urgence nécessité d'une méthode de préparation, qui augmenterait le ratio de HNP à une seule pointe de manière contrôlée. Les grandes lignes de la thèse sont les suivantes: Dans la partie 1, nous décrirons la recherche bibliographique sur la synthèse de contrôle, les propriétés de chiralité et la photodéposition sur les NCs de semiconducteurs II -VI à base de cadmium. Nous sommes partis de la dépendance de forme passée et présente des CNs de semi-conducteurs II -VI à base de cadmium, et avons souligné les lacunes des méthodes actuelles de contrôle de forme. Elle est suivie par l'étude de la transmission de la chiralité dans les NCs de semi-conducteurs, dans laquelle nous résumons les facteurs g actuels d'anisotropie CD et CPL glum des NCs de semi-conducteurs II-VI à base de cadmium chiraux induits par de petits ligands chiraux vi References page x et avons fait un guide pour une compréhension simple de la progression actuelle des ligands induit la chiralité. Dans la dernière partie de la partie 1, nous avons fait un survol de la littérature sur les études récentes des HNPs Au-CdSe / CdS. Nous avons principalement résumé les dépôts métalliques actuels sur les CNs desemi-conducteurs à base de cadmium, en particulier pour une analyse approfondie de l'or le plus étudié, afin de faire un guide pour une compréhension directe des progrès actuels dans ce domaine. Dans la deuxième partie, nous avons introduit la préparation contrôlée de nanocristaux de semi-conducteurs de morphologies différentes. En détail, cette partie est divisée en trois chapitres. Tout d'abord, le chapitre 1 est consacré à l'étude bibliographique des points quantiques noyau-coquille (QD). Ici, nous rapportons une nouvelle méthode pour synthétiser des points quantiques à structure à coque épaisse (TSQD) avec une émission verte saturée où la tri-n-octylphosphine (TOP) a été utilisée à la fois comme ligand et comme solvant pour extraire les ions redondants de la surface des QDs et éliminer les imperfections du réseau avant tout revêtement de surface inorganique couche par couche. Les TSQD tels que préparés démontrent des propriétés luminescentes améliorées, y compris un QY élevé atteignant jusqu'à 75%, une largeur totale à mi-hauteur (FWHM) restant proche de 26 nm et des propriétés d'émission précises réglables (532 nm), qui peuvent être utilisées pour effectuer 91% de la Recommandation BT de l'Union internationale des télécommunications (UIT). 2020 (Rec.2020) pour les écrans haute définition et gamme de couleurs. Dans le chapitre 2, nous avons étudié pour la première fois l'influence de la température sur la croissance de la coquille des bâtonnets quantiques point-dans-bâtons (DR), et observé que la température est le secret pour la préparation de bâtonnets quantiques à coquille ultra-mince. Un processus en deux étapes est proposé pour expliquer le processus de croissance de la coquille des nanocristaux CdSe / CdS dot-in-rods (DRs).

Le procédé combine une croissance de coquille isotrope rapide et finement réglée dans la première étape et ensuite une croissance de longueur anisotrope le long de l'axe [001] dans la deuxième étape. Cela permet un contrôle systématique de l'épaisseur de la coque avec des rapports d'aspect variables tout en maintenant un degré élevé de mono-dispersité au niveau du lot. Nous avons observé une amélioration significative de la chiralité induite par le ligand après la croissance de la vii References page x coquille ultra-mince CdS sur le noyau CdSe, la valeur du facteur g du dichroïsme circulaire (CD) pourrait atteindre 10 -3 juste par une modification de petite molécule (cystéine), qui était le plus grand signal chiral induit par un seul ligand chiral à ce jour.

La préparation de nanocristaux semi-conducteurs de morphologies différentes a ensuite été présentée au chapitre 3. Nous rapportons une méthode non conventionnelle en une seule tape pour générer des NCs en forme de têtard CdSe / CdS en utilisant des noyaux wurtzite de CdSe en présence de ligands mixtes (cadmium carboxylate et cadmium phosphonate), sans aucun processus de croissance secondaire. Le rapport entre les ligands et la concentration des coeurs a un impact significatif sur la morphologie de la croissance de CdS. Sur la base de cette méthode, les morphologies des NCs de CdSe / CdS varient des têtards aux nanofleurs, des dotsin -rods et des tétrapodes ont également pu être obtenus. Ce mécanisme spécial de croissance cristalline peut être appliqué à d'autres méthodes basées sur la croissance sur germes pour produire des nanostructures anisotropes.

Dans la troisième partie, nous nous sommes principalement concentrés sur l'analyse de la chiralité des NCs de semi-conducteurs avec ces différentes morphologies. Dans le chapitre 4, nous avons d'abord étudié le mécanisme de transmission chirale induite par le ligand grâce à la gravure top-down du domaine sélectif. Nous avons montré qu'une gravure chimique descendante (top-down) de nanoparticules colloïdales peut faciliter un contrôle précis des nanocristaux de même concentration, ce qui ne peut être obtenu que par une technologie d'injection à chaud ascendante. Cette méthode peut être prouvée en synthétisant un DR CdSe / CdS à deux composants, qui permet d'obtenir des nanocristaux avec des coques différentes à la même composition et concentration par gravure. Ceci est hautement souhaitable pour l'étude du mécanisme de conduction chirale induite par un ligand, car il élimine efficacement l'influence de la concentration de nanoparticules et de ligands en même temps, et il n'est pas uniquement basé sur le facteur g, qui est lié à l'absorbance. Les On peut se référer aux Annexes pour une meilleure compréhension des informations détaillées concernant les techniques de caractérisation (Annexe 1) et la prédiction théorique de la taille du métal (Annexe 2).

References page 11

As mentioned in the title, the objective of this thesis is the synthesis of semiconductor quantum rods which can be used to in depth study and bring some new insights on the induced chirality properties and photodeposition processes when using such substrates. In this thesis, we will focus on the traditional cadmium-based II-VI semiconductor nanocrystals (NCs), especially the quantum rods, such as CdSe/CdS dot-in-rods (DRs).

In the last two decades, great advances have been made toward designing the controlled growth of small metal crystals selectively on the tips of cadmium-based rodshaped nanocrystals, where the metal is in direct crystalline contact with the semiconductor part. 1,2 By using advanced synthesis strategies, the size, location, composition and shape of different hybrid semiconductor-metal nanoparticles (HNPs) components can be controlled, so that their unique synergistic properties can be further studied and utilized in various applications. [3][4][5] A significant and challenging application involves the use of these nanoparticles (NPs) as photocatalysts. There are several ways to enhance the catalytic properties, for instance, due to the facilitated efficient long-lasting charge carriers separation and minimized back-reaction of the intermediates, the seeded quantum rod structures, such as CdSe/CdS dot-in-rods (DRs), [6][7][8][9][10][11] were found to be highly active for the hydrogen production (Figure 1 a andb). 8 Likewise, it was shown that the reduction of organic dyes (MV 2+ ) was 1.5 times more efficient in a CdSe/CdS structure than in the case of a pure CdS nanorod. The enhanced activity was attributed to a stronger localization of the photoinduced hole at the hole-rich domain, which effectively prevented its recombination with an electron. 12 On the other hand, Amirav and coworkers found that a single-tip metal catalyst decorated in the semiconductor rods demonstrate to be the most active for hydrogen production (27 times higher than multiple tips) (Figure 1 c). 6,7 Furthermore, Zamkov and coworkers show that the exposed CdSe core will further improve the associated rates of H2 generation (Figure 1 d). 13 In order to achieve the HNPs with excellent catalytic properties, we need to perform first the controllable preparation of the required CdSe/CdS DRs. CdSe/CdS core/shell QDs of similar lowest exciton energy (CS-SE) as DR (dot-in-a-rod), CdSe/CdS core/shell QDs of similar volume (CS-SV) as DR, and CdSe/CdS dot-in-a-rod (DR) structures. 14

References page 11

c)

Photocatalytic quantum efficiency for the hydrogen reduction half-reaction obtained with CdSe/CdS nanorod photocatalysts decorated with a single, double, or multiple Pt reduction sites. 6 d) Removal of the outer CdS layer in a CdSe/CdS DR structure results in the morphology where both donor and acceptor components are in direct contact with the external environment, improving the associated rates of H2 generation. 13 On the other hand, the controllable preparation of CdSe/CdS DRs can also be crucial in terms of chirality. Kuang and coworkers 15 showed that long DRs with high aspect ratios (AR) can enhance the g-factor in the range of 445-461 nm (from 5.9×10 -4 to 1.2×10 -3 ). Gun'ko and coworkers 16 showed that the chiral response of the CdSe/CdS quantum dots (QDs) strongly enhanced as the CdS shell thickness decreased, certainly because of the decreasing distance between the hole present in the CdSe core and the

a b d c

References page 11 coordinated chiral ligands at the surface of the QD. Beside to the photodeposition process, mastering the controllable preparation of semiconductor NCs can simultaneously lead to excellent performance in chirality and catalysis. However, to the best of our knowledge, despite the many works undertaken on this topic; there is still a lack of systematic research on the preparation of these quantum rods, especially in terms of a high-precision control of the synthesis of DRs with a uniform thin-shell with less than one layer, which are essential for chirality and catalysis. One key goal of this thesis is the development of experimental methods to selectively control the shape and size of semiconductor NCs and then give guide for a wide range of materials.

Current experimental studies of ligand-stabilized colloidal nanocrystals and their properties originated from the pioneering work by Brus and Henglein's group, 17,18 who began to study the quantum confinement effect of spherical colloidal CdS nanocrystals more than two decades ago. These semiconductor NCs samples were synthesized at room temperature. Although the optical properties exhibited quantum confinement effects, their particle size distributions were broad and the particles showed only weak luminescence, which was due to the result of crystal defects and a poor surface passivation. After that, Bawendi and co-workers 19 later developed a high-temperature synthesis method to decompose organometallic precursors in a coordinating solvent and produce spherically faceted semiconductor nanocrystals with greatly improved optical properties. This bottom-up method has also been successfully extended to the shape control of nano-objects, thus realizing different morphologies. By adjusting the reaction parameters, Alivisatos, Peng and coworkers first presented the synthesis of nanorods in 2000. 20 This surfactant-controlled growth method was carried out by adding co-ligands which preferentially bind to certain crystal planes in the reaction mixture, so that it became much easier for the crystal to grow on planes with lower passivation. A precise control of the shape and size has been achieved for different types of semiconductors. More complex dendritic structures have also been achieved through a secondary crystal growth method on the arms at the ends of a rod or tetrapod. 21 Although in recent years, significant progress has been made in the understanding of how to control the morphology of nanocrystals, a more systematic research on the high-precision control of the synthesis of semiconductor NCs is still References page 11 needed. Thus, it is constantly mandatory and relevant to find out an effective synthetic approach to achieve semiconductor NCs with tunable properties. In this work, we will come back once more on the precise control of the quantum rods synthesis, and use them for chirality and metal laser-induced deposition purposes.

There are two types of chirality in the case of semiconductor NCs, the first one is the "Intrinsic Chirality", which is obtained through the chirality of the nanoparticle itself, such as dislocation-induced chirality, [22][23][24] self-assembly, [25][26][27] and supramolecular templating nanohelices. 28,29 However, its potential applications are limited due to the complex preparation process and uncontrollability. The other type is the "Chirality Transmission", which uses chiral molecules, and transfers their chirality to the nanoobjects, different levels of chiral structures can be built; this means that chiral information is transmissible. One of the pioneering experimental works by Gun'ko and co-workers demonstrated that surface layers of CdS semiconductor NCs could acquire the chiral property from enantiopure penicillamine ligands attached to the quantum dot surface. 30 A number of theoretical and experimental papers followed, which improved this fabrication technique, and theoretically addressed the most common origins of nanocrystal chirality. Thanks to the universality of chiral transmissibility, induction of chirality and/or optical activity in inorganic semiconductor nanocrystals is a promising way to introduce a new dimension of intriguing and useful complexity to their properties, which can be used in stereoselective synthesis, 24,26,31 chiral recognition, 32,33 display devices, 34,35 biosensing, 36,37 asymmetric catalysis, 38 and so forth. [39][40][41] However, the circularly polarized luminescence (CPL) signal is still scarce. Up to now, it is still a great challenge to observe a CPL signal for semiconductor NCs capped with chiral ligands. Although ligands induced circularly polarized luminescence has been observed upon a simple phase transfer of 0D CdSe QDs from toluene into an aqueous phase containing chiral L-or D-cysteine by Balaz and coworkers, 42 to date, very few cases of CPL-active semiconductor NCs have been reported.

Hybrid nanoparticles (HNPs) have gained great interest in a variety of research fields and applications, including optics, electronics and catalysis. Compared with the thermal induced growth of multi-sites gold particles, the more energy efficient lightinduced growth leads to a highly selective deposition of gold in HNPs. In addition to References page 11 the size and shape control of semiconductor NCs, the evolution of the deposition sites control also plays a key role in the catalytic property of HNPs. By the control of thermal and photo-reduction methods, Banin and coworkers 9 succeeded in fabricating the Au decorated CdS and CdSe/CdS rods, with either multi Au dots on the body or a large Au at the tip. However, the single-tipped HNPs without tiny Au NPs surrounding the surface of rod is still a challenge. As shown in Figure 1, not only the shape of semiconductor NCs but also the number of metal NPs can be a crucial parameter and the controversy is not over yet in the literature. 6,7,[43][44][45][46][47] So that, there is an urgent need for a preparation method, which would increase the ratio of single-tipped HNPs in a controlled way. The outline of the thesis is the following:

In Part 1, we will describe the literature research on the control synthesis, the chirality properties of and the photodeposition on cadmium-based II-VI semiconductor NCs. We started from the past and present shape dependence of cadmium-based II-VI semiconductor NCs, and pointed out the deficiencies of current shape control methods. It was followed by the study of chirality transmission in semiconductor NCs, in which we summarized the current CD anisotropy g-factors and CPL glum of chiral cadmium-based II-VI semiconductor NCs induced by small chiral ligands and made a guide for straightforward understanding of the current progress in ligands induced chirality. In the last part of the Part 1, we made a literature overview on the recent studies of Au-CdSe/CdS HNPs. We mainly summarized the current metal deposition on the cadmium-based semiconductor NCs, especially for in-depth analysis of the most studied gold, so as to make a guide for straightforward understanding of the current progress in this field.

In the second Part, we introduced the controlled preparation of semiconductor nanocrystals with different morphologies. In detail, this part is divided into three Chapters. First, Chapter 1 is devoted to the bibliographic study of the core-shell quantum dots (QDs). Herein, we report a novel method to synthesize thick-shell structure quantum dots (TSQDs) with saturated green-emitting where tri-noctylphosphine (TOP) was used as both a ligand and a solvent to extract the redundant ions from the QDs surface and remove the lattice imperfections before any surface inorganic layer-by-layer coating. The as-prepared TSQDs demonstrate enhanced References page 11 luminescent properties including high QY reaching up to 75%, full width at half maximum (FWHM) remaining close to 26 nm and tunable precise emission properties (532 nm), which can be utilized to perform 91% of the International Telecommunication Union (ITU) Recommendation BT. 2020 (Rec. 2020) for high definition and color gamut displays. In Chapter 2, we investigated for the first time the influence of the temperature on the shell growth of dot-in-rod quantum rods (DRs), and observed that the temperature is the secret for the preparation of ultra-thin shell quantum rods. A two-step process is proposed to explain the shell growth process of CdSe/CdS dot-in-rods (DRs) nanocrystals. The process combines a fast finely tuned isotropic shell growth in the first stage and then an anisotropic length growth along the [001] axis in the second stage. This allows for a systematic control of the shell thickness with variable aspect ratios while maintaining a high degree of monodispersity at the batch level. We observed a significant enhancement in the ligandinduced chirality after the ultra-thin CdS shell growth onto the CdSe core, the circular dichroism (CD) g-factor value could reach 10 -3 just by a small molecule (cysteine) modification, which was the largest induced chiral signal by a single chiral ligand to date. The preparation of semiconductor nanocrystals with different morphologies were then presented in Chapter 3. We report an unconventional one-pot method to grow CdSe/CdS Tadpole shape NCs using wurtzite CdSe cores in the presence of mixed ligands (cadmium carboxylate and cadmium phosphonate), without any secondary growth process. The ratio between the ligands and the cores concentration have a significant impact on the morphology of the CdS growth. Based on this method, the CdSe/CdS NCs morphologies varied from tadpoles to nanoflowers, dot-in-rods and tetrapods could also be obtained. This special crystal growth mechanism can be applied to other seed-mediated methods to produce anisotropic nanostructures.

In the third Part, we mainly focused on the chirality analysis of semiconductor NCs with these different morphologies. In Chapter 4, we first studied the mechanism of ligand-induced chiral transmission through the top-down selective domain etching.

We showed that a top-down chemical etching of colloidal nanoparticles can facilitate a precise control of nanocrystals of the same concentration, which cannot be obtained only by bottom-up hot injection technology. This method can be proved by References page 11 synthesizing a two-component CdSe/CdS DRs, which can achieve nanocrystals with different shells at the same composition and concentration by etching. This is highly desirable for the study of ligand-induced chiral conduction mechanism, as it effectively eliminates the influence of nanoparticles and ligands concentration at the same time, and it is not just based on g-factor, which is related to absorbance. The results showed that when comparing the chirality signals of an individual nanoparticle, the shell layer had a negative correlation on the first exciton peak chirality, but positive correlation on the shell absorption chirality. At the same time, the integrity of the core is crucial for the DRs to maintain high CD and CPL signals. The optically active CdSe/CdS nanoplates (NPLs) with induced CD and CPL are shown in Chapter 5. Here, a one-pot approach for CdSe/CdS core-shell NPLs was presented with varying the islands-like CdS shell upon a 4MLs CdSe NPL core. We describe the successful preparation of Land D-cysteine-capped CdSe/CdS NPLs with tunable circular dichroism (CD) and CPL behaviors and a maximum anisotropic factor (glum) of 5.71 × 10 - 4 . It has been found that the induced chiroptical response showed a direct relationship with the formation of islands-like CdS shell in the first stage and second stages and a clear evolution of the signal, whereas, in the third stage with the full cover shell formation, the CD and CPL signals of the samples were inversely proportional to the CdS shell thickness. The islands-like CdS shell gave birth to the CPL signal, while the formation of full shell decreased the induced chirality. Furthermore, in Chapter 6, we studied the ligandinduced chirality of semiconductor nanocrystals with different morphologies. Herein, we present CdSe/CdS NCs with morphologies varying from nanoflowers to tadpoles and DRs with induced chirality derived from the surface chiral ligands. The observed CD and CPL activities are closely associated to the geometrical characters of the nanostructures such as the shell thickness and the aspect ratio of the CdSe/CdS NCs.

For instance, the CD response is mainly attributed to the CdS shell and exhibits a maximum value for a ~45 nm tail length while the CPL activity is only related to the CdSe core and exhibits an optimum value for NCs with thin CdS shells and relatively high photoluminescence quantum yield (QY). Further theoretical model demonstrated the aspect-ratio-dependent g-factor and QY variation in such asymmetric nanostructures.

References page 11

Finally, in the last Part, the photodeposition of Gold (Au) nanodots on DRs semiconductors induced by laser was studied, with two Chapters. In Chapter 7, we described the setup configuration and laser control for the photodeposition process.

We describe three different setups built and used to perform the photochemical deposition of the metallic dots onto the CdSe@CdS DRs. A description of the specific parameters and configuration of the setups is fully detailed. And in Chapter 8, we explore the laser photodeposition growth mechanism of the gold nanocrystals onto the preformed CdSe@CdS DRs to form the hybrid metal nanocrystal/semiconductor nanorod colloids by adding methanol as hole scavenger. By manipulating the growth conditions, it is possible to obtain nanostructures with a single Au nanocrystal mainly at one tip of the nanorods. We have systematically studied the effects of the experimental parameters on photodeposition sites, such as the presence of a hole scavenger, the irradiation intensity, the deposition time, the Au/DRs ratio and so on.

Our results compare quite well with a model for the growth of single Au nanocrystal.

We then expect that the demonstrated strategy can raise a common methodology concerning the synthesis of single-tip HNPs nanocrystals, leading to advanced nanoparticles architectures for applications in areas such as photocatalysis, photovoltaics, and light detection. This thesis ends up with Conclusions and Perspectives with a reflection about the as-obtained results and some possible improvements for further research.

One may refer to the Appendices for a better understanding of the detailed information concerning the characterization techniques (Appendix 1) and the theoretical prediction of the metal size (Appendix 2) Part 1 The ability to control and manipulate the physical and chemical properties of materials as we desire is one of the challenging issues in chemistry and materials science. Modern chemists have been exploring ways to obtain such a control at the nanometer scale. Since physical properties of nanocrystals (NCs) are strongly influenced by their size, early investigations focused on the nanoscale size effect. [1][2][3][4][5][6][7][8][9][10] For example, it has been revealed both theoretically and experimentally that the electronic band of a crystal is gradually quantized as the crystal-size is reduced, resulting in an increase in the band-gap energy. The size of the NCs also significantly influences the electron-transport process. Recently, more and more works focused on the shape of NCs which also plays a crucial role on the determination of their properties. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] The shape of nanocrystals can be simply divided into zero-dimensional (0D), 1D, 2D by their dimensionality (Figure 1). In this thesis, I mainly focused on semiconductor NCs (SNCs).

State of the art

Table of Contents

Figure 1 Basic motifs of inorganic nanocrystals: 0D spheres, cubes, and polyhedrons; 1D rods and wires; 2D discs, prisms, and plates. 13 SNCs with tailored geometries exhibit unique shape-dependent phenomena and their subsequent use as building blocks for the fabrication of nanodevices is of significant interest. Thanks to their tunable emission property, they have been broadly applied in light emitting diodes (LEDs) displays, [25][26][27][28][29] solar cells [30][31][32] and biomedical applications, 18,[33][34][35][36][37][38] etc. Herein, we review the birth and growth of quantum dots (QDs)

and the recent developments in the shape control of II-VI semiconductor NCs with a focus on traditional cadmium-based NCs obtained by non-hydrolytic hightemperature thermal reaction methods, 4 which have been used for the synthesis of semiconductor and metal oxide nanocrystals with various geometrical motifs. The currently proposed shape-guiding mechanisms are presented and the guidance for the controllable shape of SNCs are also discussed.

1.1.1 The birth stage of semiconductor NCs.

0D are the most basic and symmetric motifs among shaped nanocrystals, they were the first to be successfully prepared and studied. Since their discovery in the 1980s, these remarkable nanoparticles (NPs) have held out tantalizing prospects for all kinds of new technologies, ranging from paint-on lighting materials and solar cells to quantum computer chips, biological markers, and even lasers and communications technologies. quantum size effect (Figure 2 a). 3 At this stage, quantum dot nanocrystals were mainly prepared by co-precipitation methods, and the size tunability and monodispersity of these nanocrystals were also limited.

In 1993, Bawendi and co-workers later developed a ground-breaking nonhydrolytic method to prepare CdSe nanocrystals by injecting a solution containing dimethyl cadmium (Cd(CH3)2) and trioctyl phosphine (TOP) selenide (TOP-Se) into hot trioctyl phosphine oxide (TOPO). 4 The obtained nanocrystals were highly monodisperse and their size could vary from 1.2 to 11.5nm (Figure 2 b). Optical spectra clearly demonstrated size-dependent quantum confinement effects indicating high monodispersity and crystallinity. This non-hydrolytic chemical route has been the most widely adopted method to synthesize high-quality inorganic NCs. [5][6][7][39][40][41][42] 1.1.2 The great development period of semiconductor NCs.

However, these preparations of SNCs are based on organometallic precursors, which are extremely toxic and reactive reagents. High level of purity and safety requirements for laboratory conditions are necessary, making the preparation of such QDs difficult in a common laboratory. In 2001-2004, a "greener" synthetic route in non-aqueous solution was proposed by Peng and co-workers, 8,9,43 by using CdO or Cd-fatty-acid precursors rather than highly reactive alkyl cadmium precursors (Cd(CH3)2). The environmentally more benign synthesis of Cd-based chalcogenide (CdE, E=S, Se, Te) NCs was then more accessible (Figure 3a). Based on this method, more straightforward works were performed, such as the research on the relationship between the NCs diameter and the absorption peak position, 9 which gives a simple guide on the QDs synthesis beyond the TEM test, and promoted the rapid development of SNCs.

Another breakthrough was performed in 2002, when Weller and co-workers 44 reported a new approach to synthesize photostable thiol-capped CdTe nanocrystals (Figure 4). CdTe NCs promoted by reflux. 44 Post-preparative size-selective precipitation and selective photochemical etching have also been developed as methods providing an increase of the photoluminescence quantum efficiency of the nanocrystals with values reaching up to 40 %. This method made the synthesis of SNCs possible in most laboratories.

1.1.3 The bottleneck period of semiconductor NCs.

Based on these two revolutionary experiments, SNCs research carried out explosive.

The semiconductor QDs have been broadly applied in various fields, such as LEDs, 25,[45][46][47][48][49] solar cells, 30,50,51 biomedical applications, [34][35][36] and so on. In these applications, it

References page 70 is a prerequisite to improve their chemical stability, photoluminescence (PL) and quantum yield (QY) without changing their characters of facile tunable emission and narrow full width at half maximum (FWHM). To reach these targets, it becomes necessary to:

i. Improve the fluorescence properties by rubbing out the residual surface defects or obtaining the exciton emission by passivation of the surface exhibiting defects.

ii. Protect the photoactive core with respect to the surrounding environment and in particular from photo-oxidation.

iii. Obtain structures with a wider fluorescence emission range as compared to those of the materials taken separately.

To address these issues, chemists synthetized core/shell QDs, which are usually synthesized by the epitaxial growth of inorganic shells on the cores with a consequent removal of the surface defect states. 52 When the core/shell structure is such that the valence and conduction bands of the core and shell materials are positioned as in Figure 5, the core/shell QDs are Type I because the sulfide designed for the shell construction has a band gap energy higher than that of the core. CdSe/ZnS 39 or CdSe/CdS 53 are typical examples of this kind of QDs where the conduction band of the shell has a higher energy than that of the core, whereas the valence band, on the contrary, has a lower energy (Figure 5). QY of the obtained core/shell QDs is routinely in the 10-40% range, 39, 52, 54-57 although much higher QY of 50-80% could be achieved, 39,54 but a sharp decrease in QY is observed as soon as the shell grows by more than 1.5 layers. 39,54,55 Therefore, the synthesis of multilayered core/shell QDs with extremely improved QY still remains a great challenge. This dilemma was solved in 2012-2014. One way to decrease the source of defects was to decrease the lattice mismatch between the core and the shell, for instance, Peng and co-workers improved the Successive Ion Layer Adsorption and Reaction (SILAR) method and used small lattice mismatch core/shell materials to form a thickshell of CdS on CdSe/CdS core/shell quantum rods that maintained a high quantum yield (more than 80% QY after 5 monolayers) (Figure 6a). 58 Hao et al. using the tri-noctylphosphine (TOP) modified SILAR method also got thick shell "core/shell QDs" by introducing a buffer layer, which could maintain 95% QY after 6 monolayers (Figure 6b). 59,60 Another way consists in forming alloyed QDs, Jang and co-workers synthesized a green alloy CdSe//ZnS QD type with a nearly 100% quantum yield (Figure 6c). 26 Li and co-workers also synthesized alloyed QDs with different colors in a one- 64 However, there are still some challenges for QDs beyond the QY, such as the photoluminescence (PL) blinking, photo-bleaching and their intrinsic solubility, which were also recently solved by Peng and co-workers. Quantum dots tend to blink on and off because either one or both of the generated photo-excited carriers are susceptible to escape to the nanocrystal surface. This blinking is not random (it obeys a "power law"), but it also is not predictable. Thus, individual particles might go dark only for nanoseconds or remain dark for minutes at a time or some intervals in between. 65 As shown in Figure 8 a, even with a coating of 7 monolayers (MLs) ZnS shell, the 2.1 nm single CdSe NC blinking still remains via a non-radiative Auger recombination or is intercepted before thermalization. 66 Recently, with non-blinking quantum dots and antibleaching coating, most part of the visible window was covered by using a new synthetic strategy, i.e., confining the excited-state wave functions of the core/shell QDs within the core and its inner shells (≤∼5 monolayers) (Figure 8 b). [67][68][69] The solubility was solved by bleaching quantum dots with uniform-alloy shell. 67 Figure 9. Improvement of the QDs solubility by changing the entropic power of the ligands. 71 These key works have greatly increased the application range of SNCs and allowed their explosive development in color filters, [72][73][74][75][76][77] LEDs, 27,61,72,[78][79][START_REF] Hao | A Facile Route to Synthesize CdSe/ZnS Thick-Shell Quantum Dots with Precisely Controlled Green Emission Properties: Towards QDs Based LED Applications[END_REF][START_REF] Fang | Atomic Layer Deposition Assisted Encapsulation of Quantum Dot Luminescent Microspheres toward Display Applications[END_REF] photocatalysis, [START_REF] Costi | Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells[END_REF][START_REF] Aronovitch | Stability of Seeded Rod Photocatalysts: Atomic Scale View[END_REF] and solar cells. 31,[START_REF] Li | Doctor-Blade Deposition of Quantum Dots onto Standard Window Glass for Low-Loss Large-Area Luminescent Solar Concentrators[END_REF][START_REF] Bradshaw | Nanocrystals for Luminescent Solar Concentrators[END_REF][START_REF] Coropceanu | Core/Shell Quantum Dot Based Luminescent Solar Concentrators with Reduced Reabsorption and Enhanced Efficiency[END_REF] However, there are still limitations to the precise regulation of the wavelength, especially for green quantum dots, which may be due to the ability to control the small size of the involved CdSe cores which exhibit a very large surfaceto-volume ratio (S/V). [START_REF] Fitzmorris | Optical Properties and Exciton Dynamics of Alloyed Core/Shell/Shell Cd1-xZnxSe/ZnSe/ZnS Quantum Dots[END_REF][START_REF] Agrawal | Recent Developments for the Synthesis of Air Stable Quantum Dots[END_REF] Thus, it is still mandatory and relevant to find out an effective synthetic approach to achieve thick shell QDs with a pure green emission, a high PL, a good QY, a narrow FWHM and tunable precise emission properties, which will be done in Chapter 1. [START_REF] Peng | Shape Control of CdSe Nanocrystals[END_REF][START_REF] Peng | Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth[END_REF] properties which arise from their anisotropic shape, and elongated structures which exhibit an attractive set of emergent properties, such as a large degree of fluorescence anisotropy 12,18,20,[START_REF] Carbone | Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach[END_REF][START_REF] Zhang | Ligand Shell Engineering to Achieve Optimal Photoalignment of Semiconductor Quantum Rods for Liquid Crystal Displays[END_REF] and an enhanced potential of electron-hole separation. [START_REF] Ben-Shahar | Hybrid Semiconductor-Metal Nanorods as Photocatalysts[END_REF][START_REF] Wu | Hole Removal Rate Limits Photodriven H2 Generation Efficiency in CdS-Pt and CdSe/CdS-Pt Semiconductor Nanorod-Metal Tip Heterostructures[END_REF][START_REF] O'connor | The Effect of the Charge-Separating Interface on Exciton Dynamics in Photocatalytic Colloidal Heteronanocrystals[END_REF][START_REF] Ma | Chiral Semiconductor Nanorod Heterostructures with High Photocatalysis Activity[END_REF] Since CdSe nanorods were first reported in 2000, [START_REF] Peng | Shape Control of CdSe Nanocrystals[END_REF] a non-hydrolytic, hightemperature-injection method has been widely used for the syntheses of high-quality 1D nanocrystals. In the same way as for 0D semiconductor NCs, the CdSe nanorods were also prepared by a thermal reaction between highly reactive Cd(CH3)2 and TOP-Se in a hot surfactant mixture of TOPO and hexyl phosphonic acid (HPA). 11 The formation of extremely high aspect ratio CdSe nanorods (30:1), as well as arrow-, teardrop-, tetrapod-, and branched tetrapod-shaped CdSe NCs, has been achieved by growth of the nanoparticles in a mixture of hexyl phosphonic acid (HPA) and trioctyl phosphine oxide (TOPO) (Figure 10). The most influential factors in the shape control are the ratio of surfactants, the injection volume, and the time-dependent monomer concentration. Peng and co-workers then proposed another synthetic method utilizing less-reactive cadmium precursors. 8,[START_REF] Peng | Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth[END_REF] By using CdO or Cd-fatty-acid precursors rather than highly reactive alkyl cadmium ones, they obtained Cd-based chalcogenides (CdE, E=S, Se, Te) nanorods more benign and environmentally friendly.

However, this method is still poor terms of size tunability and uniformity of nanorods. Although the rapid initial progress in the development of CdSe/CdS nanorods resulted in the achievement of various aspect ratios (ARs) and colors, 15,20,[START_REF] Carbone | Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach[END_REF][START_REF] Zhang | Ligand Shell Engineering to Achieve Optimal Photoalignment of Semiconductor Quantum Rods for Liquid Crystal Displays[END_REF][106][107][108] further improvement of the morphology control at the nanoscale, especially of the ultra-thin shell, proved elusive using conventional fast-injection-based preparations.

This was mainly due to the ultra-high temperature reaction conditions, which generally exceeded 350 o C, [START_REF] Carbone | Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach[END_REF][108][109][110][111][112][113] making it more difficult to exercise a precise control on the process. However, the controllable preparation of the shell thickness, especially thin one, has a positive significance on the improvement of both the catalytic performance 103 and the chirality. 114,115 However, only few papers mentioned the control of the DRs shell thickness. Bawendi and co-workers 109 used a second overcoating process to focus on a thicker shell of DRs, which enhanced the quantum yield (QY) to near 100 %. She et al. 16 studied the quantum yields (QYs) and photophysical processes by systematically tuning the shell and core sizes, which resulted in a thin shell layer, but the shell layer increased with the increase in length.

Kim et al. 20 proposed what they called "a dual-diameter structure semiconductor nanorods" with a thin diameter of CdS tail while still with a thick-diameter shell onto CdSe core. Khon et al. 103 obtained DRs with a thin shell by etching a 0.7 nm CdS layer from CdSe/CdS nanorods, which enhanced the catalytic activity by over 3 times as compared to the original CdSe/CdS nanorods. Nevertheless, there is still some room for a systematic research on the preparation of these quantum rods, especially to improve the high-precision control of the synthesis of DRs with uniform thin-shell of less than one layer. We will present a solution method in Chapter 2.

2D nanocrystals

Unlike 1D rods, the formation of plate-shaped nanocrystals is less common in colloidal systems. In a kinetically driven growth regime, the 1D nanorod growth is typically promoted with a faster growth along a specific direction (e.g. z axis; Figure 13 a). On the other hand, when the growth along an axis is blocked and the preferential growth then takes place in the perpendicular plane (e.g. (x,y) one), the formation of disc-shaped nanocrystals is facilitated (Figure 13b). 13 References page 70 119, 122, 123 and core/shell (vertically grown shell) (Figure 14 (a, b, c)) 116,119,120,124 architectures have been synthesized and studied intensively.

Advanced Shapes of Nanocrystals

Inorganic nanocrystals with unique shapes can be used as key components for electronic devices. As described above, NCs shape controls have been widely studied for 0D spheres, 1D rods, and 2D plates. These basic building blocks have unique geometries and properties and can be used further as components in more complex nanostructures. There is another very much studied nanostructure: the tetrapod. The first tetrapod-shaped CdSe was obtained by Alivisatos and co-workers, 11 who opened the door to this special morphology. In 2003, they reported the growth of CdSe and CdTe tetrapods with controlled both lengths and diameters (Figure 15 a, c). 11,125 They found that the ratio of Cd/Te could be used to control the arm's length and the alkyl phosphonic acid/Cd ratio to determine their diameter. These CdTe tetrapods exhibit shape-dependent optical properties. 11 Even more complex dendritic structures can be achieved through a secondary crystal growth method on the arms at the ends of rods or tetrapods. As shown in tetrapods and structural isomers in branched tetrapods (the bottom 2). 23 Even for the most studied CdSe/CdS NCs systems, the perfection of their shape structure has not reached yet a satisfactory level, especially for the anisotropic one.

A common strategy to break the symmetric growth of a shell is to use additional ligands such as phosphonic acids 14,133 and halide ions 17 which selectively bind onto specific crystal facets and control the surface energy and growth rate of these facets.

Peng and coworkers claimed that, even if the side facet surface of the wurtzite rods and tetrapods were typically nonpolar ones, they could be stabilized by cadmium phosphonate complexes. [START_REF] Peng | Shape Control of CdSe Nanocrystals[END_REF][START_REF] Peng | Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth[END_REF] The difficulty in controlling the structural perfection of the different shapes is the alternation of isotropy and anisotropy, which in turn results

a b

Second injection

Second injection d c

References page 70 in specific morphologies. Although a secondary shell growth process is a solution, however, it always makes the preparation process complicated and increases the cost due to repeated purification and re-growth procedures. 11,21,23 The step-to-come required for the success of this field would be the development of more versatile and reliable but simple synthetic schemes, and a better understanding of the guiding principles of nanocrystal growth. In Chapter 3, we will propose a two-type ligandmediated core/shell growth method in detail, which is based on a one-step reaction involving a controlled ratio of cadmium phosphonate and cadmium carboxylate mixed ligands, approach which is different from the most commonly used single-type based ligands, 13,14,52,129,134 which were up to now used to control the anisotropic shapes.

Semiconductor nanoparticles chirality transmission

Chirality is one of the most remarkable geometrical properties with a variety of manifestations in the material world. Chiral molecules are molecules with two mirror images, which are not superimposable in three dimensions. They attracted researchers' interest since the first chiral molecule reported by Louis Pasteur in 1848, through his seminal work on left-and right-handed crystals of sodium ammonium tartrate. 135 Potentially, any nanocrystal (NC) can be chiral since they frequently have low symmetry due to the presence of chiral defects in the bulk and at the surface. 136 However, since their chirality is random, the NCs at the macroscopic scale usually do not show optical activity (circular dichroism, CD) in solution, thereby producing an equal rate of the possible enantiomers. Nevertheless, chiral inorganic nanocrystals can be readily designed and manufactured by various methods. The past decade has seen a tremendous growth in the number of studies focused on chiral inorganic NCs of various shapes and compositions. 130,[136][137][138] Chiral SNCs are especially attractive for these applications owing to their tunable photoluminescence signatures, resistance to photobleaching, and adjustable shape. 14,23,67,68 The main advantage of such NCs over small chiral molecules is their much higher dissymmetry in the interaction with circularly polarized light thanks to their higher number of atoms. There are two types of chirality for SNCs, the first one is Intrinsic Chirality, which is obtained through the chirality of the nanoparticle itself, such as dislocation-induced chirality, 139-141 self-assembly, [142][143][144] and supramolecular templating nanohelices. 145,146 However, its potential applications are limited due to the complex preparation process and lack of control. Another way is the Chirality Transmission, which based on chiral molecules absorbed at the surface of the SNCs, can induce different levels of chiral structures, meaning that chiral information is transmissible. One of the pioneering experimental works by Gun'ko and co-workers demonstrated that surface layers of CdS SNCs could acquire the chiral structure from enantiopure penicillamine ligands attached to the quantum dot surface. 147 This work was then followed by a number of theoretical and experimental papers, which improved this fabrication technique, and theoretically addressed the most common origins of nanocrystal chirality. Thanks to the universality of the chirality transmissibility, the induction of chirality and/or optical activity in inorganic semiconductor nanocrystals is a promising way to introduce a new dimension of intriguing complexity to their existing properties, which can be used in stereo-selective synthesis, 141, 142, 148 chiral recognition, 149,150 display devices, 151, 152 bio-sensing, 153, 154 asymmetric catalysis, 155 and so forth. 130,131,138 Among these applications, we are interested in manipulating light propagation in the visible range with a focus on chirality transmission from chiral molecules to supramolecular structures at nanometric scales.

In this part, we mainly focused on small ligands induced chirality in the case of cadmium-based II-VI SCNs, an area relatively new and experiencing an active growth.

Circular dichroism (CD) of cadmium-based II-VI semiconductor NCs

Circular dichroism, or simply CD, is the difference between the absorption of left and right circularly polarized lights, which is a manifestation of diastereomers discrimination. 156 In 2007, Gun'ko and co-workers found chirality in semiconductor NPs for the first time. 147 After that, a number of theoretical and experimental papers was proposed recently. As the ligand-induced chirality is the main focus in this part, only a brief discussion of Intrinsic Chirality will be presented at the end of this paragraph. with a g-factor of 8 × 10 -4 several times higher than the values previously observed for spherical dots. 157,164,166 The authors attributed this to the higher surface to volume ratio and the zinc blende (ZB) structure of the NPLs. In addition, by increasing the thicknesses (3, 4 to 5 monolayers (MLs)) of the chiral ZB CdSe NPLs, the authors showed that the CD peaks continuously red-shifted, and explained this with the quantum confinement effect. (1.4 nm) could reach value as high as 5.3 × 10 -3 , while for the same thickness of 4.5

MLs ZB-CdSe NPLs (1.4 nm) the g-factor was only 6.7× 10 -4 . This eight-fold improvement is obviously due to the wurtzite structure; to the best of our knowledge, this result is the current maximum value of CD g-factor for ligand-induced chirality on Cadmium-based II-VI semiconductor nanocrystals.

Vasiliev and co-workers 159 focused on the improvement for ZB NPLs, showing that the rolled-up thin (2.5 MLs) CdSe NPLs could be used to obtain a high dissymmetry gfactor (up to 3× 10 -3 ), providing another way to improve the chirality of semiconductor NCs.

Effects of shell-thickness

The shell-thickness also has a great effect on the chirality signals. In 2016, Markovich and coworkers first studied the variation of the absorption and CD of Cys-CdSe/CdS core/shell QDs with different shell thicknesses, 150 (Figure 19 a). In the limit of a very thick shell, the hole is confined to the core, since the valence band of CdSe is higher than that of CdS (Figure 5). The strong CD signal observed for the CdSe cores is completely suppressed in this sample, as expected for an effect mediated by the interaction of the hole with the surface. However, the behavior for samples in between the bare cores and the thickest shell is more complex and non-monotonic.

The authors explained that this was probably due to two synergy factors: The presence of the CdS shell serves as an energy barrier for the hole then decreasing the chirality on the one hand, even if the addition of shell layers reduces the confinement energy of the hole, shifting the VB edge higher and increasing the chirality on the other hand.

A different variation was obtained by Gun'ko and coworkers in 2017; 115 who showed that the resulting chiral response of the sample was strongly reduced as the CdS shell thickness increased, result they imparted to the increased distance between the hole present in the CdSe core and the coordinated chiral ligands on the CdS surface of the QD (Figure 19 b). In 2018, Yang and co-workers also studied the shell effect on the CdSe/CdS NPLs, 161 showing that the chirality could be maintained in core-crown CdSe/CdS NPLs, while it was decreased sharply in the core/shell type NPLs (Figure 19 c). The effects of the shell is then complex and still unclear, even in the same CdSe/CdS core/shell QDs system, two kinds of g-factor variation rules seem to appear depending on the research groups (first increase and then decrease, 150 or gradual decrease 115 ).

Therefore, it is very important to go on studying the mechanism of the chirality generation on individual nanoparticles, discover the nature of this chirality, this will be further studied in Chapter 4. ). The spectral analysis shows that the CD shift is due to the exciton splitting caused by the ligand-nanocrystal interaction. Subsequent inspections of a series of chiral carboxylic acid ligands showed that through relatively small changes in the structure of the ligands, the g factor could vary by 30 times. However, the current mechanism of action is still unclear. Very recently, Gun'ko and co-workers presented a model to explain the Cys ligands concentration effect on the chirality, 167 they found that the QD CD signal intensity increased with the Cys concentration at the beginning, then reached a maximum and decreased at higher Cys concentration (Figure 20). This will give a guide for the development of potential applications of optically active NCs. bidentate, S --CO 2-bidentate, and tridentate (bottom). 167

Intrinsic chirality of individual nanoparticles

Apart from this ligand-induced chirality, we also consider here briefly the intrinsic chirality in semiconductor NCs. This type of chirality is due to the presence of chiral defects such as screw dislocations and discussed in terms of structure-property relationship.

References page 70 Inorganic nanowires (NWs) represent a special case in chiral nanostructures, because their chirality can be expressed as a unique spiral geometric structure, which cannot be developed in NCs. Many synthetic processes for helical or twisted semiconductor materials were carried out. For instance, the assembled templates of polymers for synthesis of helical nanostructures with either right-or left-handed helices can be used for the growth of CdS helices (Figure 23). 145,146 The CdS helices had a pitch of 40-60 nm, but this process is not enantioselective. In order to achieve CPL-activity, the use of luminescent organic or inorganic materials is generally required, which limits the research in ligand-induced CPL. Although semiconductor NCs prepared with chiral stabilizers, such as CdS or CdTe, showed significant CD, their circularly polarized luminescence (CPL) was inactive. Figure 24 a shows the preparation of a typical procedure to get CPL from unpolarized light via a combination of a linear polarizer and a quarter-wave retarder, which leads to an energy loss after the plates, and in the case of devices application, to more complex and bulky device architectures. 151 Therefore, there is a need to develop efficient devices that can emit CPL directly, which require the synthesis of circularly polarized luminescence NCs. When the material is excited, the left-handed and right-handed CdS QDs and provide a CPL signal (Figure 26 a). 173 The authors suggested that due to the presence of the chiral template, the entire (core and surface) CdS QDs had a chiral structure, which contributed to the circular polarized emission.

Since the ligand-induced chirality is the most effective at the surface of NCs, their self-assembly can significantly amplify the overall circular polarization of CPL-active materials due to the precise control of their arrangement. For instance, Liu and coworkers reported a full-and white color circularly polarized luminescence of QDs via their supramolecular self-assembly, 152 the core/shell QDs were attached on the surface of a chiral gelator with a high glum about 10 -3 (Figure 26 b). However, up to now, there are still no CPL reports about the NPLs, even if the PL QY can be improved by the surface modification of the NPLs either core/crown (laterally grown shell) 119,122,123 or core/shell (vertically grown shell) 119,120,124 . The PL-QY can be significantly enhanced in the core/crown NPLs, because of the passivation of trap sites originating from the sidewall of core-only NPLs. 119 However, since the CdS-crown layers are grown only in the lateral direction, they still suffer from serious stability issues. The PL-QY will then be sharply quenched after the chiral ligand exchange, 175 which is due to a lack of proper passivation on their large lateral surfaces.

In (ZB) and wurtzite (WZ)), the ligands types (mercapto and thiol-free carboxylic acids), the ligands concentration, the NCs shell-thickness and aspect ratio (AR).

i. The morphology of bare NCs is the most significant parameter for CD anisotropy g-factors, the chirality increasing with the anisotropy (2D>1D>0D), for instance, the maximum gCD of Cys-NPLs can reach 5.7×10 -3 , 163 while it is 4.83×10 -4 for Cys-nanorods 162 and -2.1×10 -4 for Cys-QDs.

References page 70

ii. Differences in crystal structure can lead to a difference of up to 8-9 fold in chirality (5.7×10 -3 for wurtzite NPLs vs. 6.7×10 -4 for zinc blende NPLs). 163 iii. The type of anion chalcogenide in the QDs has only a trivial effect on the CD anisotropy g-factors, the CD signals of CdTe, CdSe and CdS dots are relatively close. 157,166,171 iv. Chiral ligands on the opposite have a huge impact on the chiral signals, for instance, the g-factors of thiol-free carboxylic acids (L-tartaric acid) 165 can reach -7.0×10 -4 compared with -2.1×10 -4 for Cys-QDs 157 and 0.5×10 -4 for N-Ac-Cys -QDs, 166 which may be due to the difference in the number of stereocenters. 165 The concentration of the ligands does affect their binding modes to the quantum dots, which further affects the CD anisotropy. 167 v. For the core/shell spherical NCs, the shell plays an important role on the chiral responses. Gun'ko et al. 115 Kuang et al. [START_REF] Ma | Chiral Semiconductor Nanorod Heterostructures with High Photocatalysis Activity[END_REF] also showed the impact of the DRs aspect ratio (AR) on the CD anisotropy, enhancing the g-factor in the range of 445-461 nm (from 5.9×10 -4 to 1.2×10 -3 ). Abbreviations: Cys: cysteine; L-HomoCys: L-homocysteine; N-Ac-L-Cys: N-acetyl-L-cysteine; L-MA: Lmalic acid; L-TA: L-tartaric acids; L-CPA: (S)-(-)-2-chloropropionic acid; AR: aspect ratio; T: shell thickness; D-his: D-histidine.

Based on the above-mentioned literature, we found that the shapes of NCs, their aspect ratio (AR) and ligands play a key role on chirality signals, and then the core/shell DRs were born for CPL (their anisotropy improves chirality and their shell protection improves QY). Therefore, although the underlying mechanism is still unclear, in Part 3:

i. we will focus on the study of CdSe/CdS NCs with different morphologies so as to tentatively unveil the changing laws of CD and CPL, and study the chirality transmission mechanism;

ii. Try to form thinner shell DRs for a higher chirality, and use the etching method to further explore the chirality variation as the shell is changing;

iii. try to get CPL signals from 2D NPLs. opening the path for their function as photocatalysts in redox reactions, including hydrogen generation by water splitting, [START_REF] Ben-Shahar | Hybrid Semiconductor-Metal Nanorods as Photocatalysts[END_REF]103,104,132,179,180 generation of radicals and photodegradation of organic contaminants. 181 Many methods have been used to synthesize semiconductor-metal HNPs, including chemical reduction, [START_REF] Ma | Chiral Semiconductor Nanorod Heterostructures with High Photocatalysis Activity[END_REF]103,[182][183][184] physical vapor deposition, 185, 186 impregnation-deposition 187, 188 photodeposition, 179, 182, 189-192 and so on, among which the photodeposition method has been widely used for its simplicity, low energy cost and contactless approach.

Applications of photodeposition and comparison to other methods

The illumination of the mixed solution of the semiconductor nanoparticles and the metal salt results in the deposition of the metal or metal oxide on the surface of the semiconductor, and this phenomenon is commonly referred to as photodeposition.

Photodeposition attracted researchers' interest since the first photodeposition experiments reported by Vondjidis and coworkers in 1965, 193 who succeeded in the formation of metallic silver on titanium dioxide (TiO2) after illuminating the TiO2 solution containing silver nitrate (AgNO3). There are two types of photodeposition:

One is oxidative photodeposition, and the other is reductive photodeposition resulting from the generation of an electron-hole pair on the semiconductor, which is trapped before recombination. Due to the excellent catalytic property of metal-semiconductor HNPs, [START_REF] Ben-Shahar | Hybrid Semiconductor-Metal Nanorods as Photocatalysts[END_REF]132,180,[194][195][196] we will focus on the reductive photodeposition, the metal NPs being able to improve the performance of semiconductors in response to light Beyond photodeposition, several methods can also be used to load metal NPs on the semiconductor surface, such as hydrothermal reactions, 198 thermal evaporation, 199 chemical reduction, [START_REF] Ma | Chiral Semiconductor Nanorod Heterostructures with High Photocatalysis Activity[END_REF]103,[182][183][184] physical vapor deposition, 185,186 impregnation-deposition, 187, 188 electrodeposition, 200 and sputtering. 201 The photodeposition method is one of the easiest and most attractive methods to synthesize noble metal/cadmium-based semiconductor HNPs by using the band gap of the semiconductor. Compared with most other methods that require high temperature, additional redox agents, electric potential or multi-step processing, the photo-induced reduction method only needs to irradiate the mixture solution or slurry with a light source. In addition, the size of the noble NPs can be controlled by adjusting the concentration of the metal precursor, the intensity and the duration time of the irradiation. The advantage of the photodeposition may also include that the deposition sites can be varied by changing the intensity of irradiation, which is beneficial to the realization of single-point deposition at high intensity.

Photodeposition of nanoparticles on the surface of cadmium-based II-VI semiconductor nanocrystals

In this part, we shall review the gold NP loading on the surface of cadmium-based II-VI semiconductor NCs using a photodeposition method. The reader can refer to Wenderich's review for photodeposition of other metals on QDs. 202 The most common methods to be discussed, are the metal growth at room temperature or at higher temperatures (thermal growth) and the light-induced photodeposition. This allows metals to grow preferentially on the rods instead of forming undesirable free-standing metal NPs, thus resulting in the formation of two-component crystalline phases on the same HNP. Applying one or a sequential combination of these strategies can well control the structural characteristics of the HNP according to its morphology and composition, resulting in different chemical and physical properties. This section focuses on the synthesis and structural characterization of semiconductor-metal quasi-1D hybrid NR structures, with particular emphasis on single-tip HNPs that are more susceptible to improve their catalytic performance. Based on their previous work, Banin and coworkers further studied the effect of the temperature and the surface ligands on the site selective deposition. 111 As show on Figure 29, the multi-sites Au deposition was found to be substantially suppressed by a suitable temperature control and effective choice of surface ligands, which directly led to the selective growth of a single metal NP on the top of the CdSe/CdS DR.

Chemical conditions

Moreover, beyond the thermodynamic factors that affect the noble NPs deposition on the semiconductor surface, chemical conditions can also control the site deposition process. For instance, in 2006, Banin reported on the site selectivity by adjusting the atmosphere (Figure 30). 205 The inert atmosphere leads to the growth of a singletipped Au-CdS HNPs, while, in the presence of air and humidity, an etching of the NR provides additional nucleation sites for the metal, resulting in multiple non-epitaxial gold domains. The dumbbell-like structures gradually appears (< 90 min), followed by multi-sites HNPs for longer reaction times. And the same ripening process takes place but for even longer reaction times, in which small Au areas dissolve back into the solution and redeposit on the tip, thus forming a single large Au tip (Figure 28 b).

However, even if the ripening process can form single-tipped HNPs, due to the uncontrollable nature of the overall process, multi-sites are always observed. gold grows only at one tip. In the presence of oxygen, the Cd-terminated end is etched, providing a second high-energy site for the gold growth, followed by the side growth. TEM images of the gold growth under argon (b) and under air (c) after 3 h. Without oxygen, gold growth is slowed and occurs only at one end, producing small (typically ≤ 2 nm) nanocrystals. With oxygen, growth occurs at both tips, followed by growth onto defect sites. 205 In the organic phase, defects could be effectively restricted by long-chain organic ligands, which gave the possibility for single-tipped deposition. However, after the phase transfer, the protection of the NRs by the ligands used in aqueous medium revealed to be much less efficient so that the multi-sites formed in an uncontrollable way 182 then reducing the catalytic performances of the nano-objects. 192 Therefore, there is an urgent need for a preparation method to increase the ratio of single-tipped HNPs.

Gold precursors

For a decoration in organic solvents, gold precursors (such as AuCl3, HAuCl4 or primary amines (such as dodecyl amine (DDA) or octadecylamine (ODA)). However, their reactivity has been claimed as rather low and favoring the Ostwald ripening as opposed to a direct single-tipped deposition. In 2010, Ryan and coworkers presented a new preparation method using the highly reactive monovalent gold precursor (AuCl) 184 instead of trivalent gold precursors (AuCl3 and HAuCl4) allowing an instantaneous gold tip formation onto CdS nanorods. The single-tipped HNPs can be maintained at room temperature only adding the shorter dodecyl amine ligand (DDA), which was known to always form multi-sites with trivalent gold precursors. At the same time, it could achieve large gold NDs size preparation (up to a few tens of nanometers). However, this method also had drawbacks, which made its controllability even worse.

Shapes of the semiconductor NCs

Recently, Pyun and coworker obtained by selective deposition, CdSe@CdS tetrapods that carry a single dipolar Au@Co NP tip. 189,206 The UV-irradiation was the key factor for the single Au NP tip deposition, which further enabled the selective deposition of a dipolar Co shell onto the tetrapod tip. This system can be used in a further step to form complex architectures and compositions by controlled selfassembly. Additionally, cone-shaped CdSe/CdS tetrapods show another example of morphological dependence. Compared with ordinary rod like-arms, the selectivity of single Au tip deposition is improved thanks to their conical arm structure. 207 Since the control of the metal deposition sites on the semiconductor surface is important with respect to the charge transfer and hence to the photocatalytic properties of the HNPs, 104,192 it seemed necessary for us to undertake a systematic study on it using a laser approach.

Morphology: Size and Shape Control

Morphological changes in shape and size of the metal co-catalyst has also been reported to play a key role in controlling and modifying the photocatalytic performance. As shown in Figure 31 a, a set of nickel-tipped CdSe@CdS DRs with various sizes of Ni NPs were studied, and an optimal size of 5.2 nm was found for improved performances in catalysis. 208 The nature of the connection between semiconductor and metal nanoparticles is sensitive to the size of the metal component, which is reflected in the activity to hydrogen production, emission quantum yield and charge separation. It was also confirmed by Banin and coworker, [START_REF] Ben-Shahar | Hybrid Semiconductor-Metal Nanorods as Photocatalysts[END_REF]179,209 who showed that the Ausemiconductor HNPs with a large Au tip benefited to electron transfer to the metal domain.

HNPs with different sizes of deposited metal co-catalysts can be obtained by a variety of synthetic methods, sizes from small-cluster-size to several and a few tens of nanometers can be reached. 179,184,210 For instance, Banin and coworkers present an example of successful metal domain size control for Au-CdS HNPs (Figure 32) . 179 A dark reaction was used to obtain site-selective growth of small Au-tipped CdS HNPs.

Then, these tiny Au domains were used as seeds for the light-induced step, which was achieved at low temperature (2-4 o C) in an inert environment. Furthermore, Banin also showed by a statistical analysis that during prolonged reaction, whether Au NPs were deposited on the NRs tip or not, the length and diameter of the latter did undergo a size reduction. 211,[START_REF] Costi | Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells[END_REF] This was probably due to the dissolution of the rods in the presence of the bromide ion of tertiary ammonium salt (DDAB, TOAB) or the fact that the chalcogenide anions from the rods can reduce Au 3+ inducing the rods etching. For instance, the average length is shortened from 38 nm for the rods in water to 22 nm after Au deposition in gold-tipped CdSe nanodumbbells. [START_REF] Costi | Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells[END_REF] Highly active AuCl would have been a solution, which achieves large-size Au ends in a short time (< 2min), but the controllability is poor. 184 Adding a hole scavenger is another way to increase the Au deposition rate. As far as silver is concerned the deposition seems difficult, it has been convincingly demonstrated by Alivisatos and co-workers, 220,221 that a cation exchange between cadmium and silver ions takes place and is the main mechanism rather than the silver cation photoreduction to elementary silver which explains why no Ag NPs formed, but CdS-Ag2S superlattices along the originally CdS NRs. 221 Then the formation of small, randomly distributed islands of Ag2S takes place, resulting in a type-I HNPs with NIR emission tunable between 1000 and 1400. To the best of our knowledge, there is still no report about single-tip Ag deposition for cadmium-based II-VI semiconductor nanocrystals. The photoluminescence is rapidly and almost completely quenched during the multi-sites Au NPs deposition processes, as shown in Figure 33 a and b, Banin and coworkers observed a significant quenching upon the growth of gold domains onto the tips and surface of CdS 205 and CdSe/ZnS 211 nanorods. However, in single-ended or double-ended samples, the fluorescence quenching could be effectively slowed down (Figure 33 c). 223 In the single-tipped HNPs samples, the semiconductor NRs still showed photoluminescence emission, even if a 8 nm Au NP was deposited. 184 These quenching processes may be the result of electron [START_REF] Costi | Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells[END_REF] or energy 224 transfer to the metal. The absorption spectra, on the contrary, show an increased absorbance in the UVvisible part, and a clear plasmon peak visible in the case of bigger colloidal gold NDs. 111,191,205 Even if the diameter of the Au NDs reaches a size of 4.5 nm in multi-sites deposition, 205,211 (as shown in Figure 33 d and e), there is still no obvious plasmon peak, probably because of the interactions between the multiple gold dots. Banin and coworkers studied the absorption spectra of different types of deposition (Figure 33 f), and confirmed that, compared with the single-tipped HNPs, the multi-sites deposition prevented the plasmon peak of gold. 111 Through the analysis of the fluorescence and absorption spectra, we can preliminarily infer the deposition type of these obtained HNPs. For instance, if the emission is strongly quenched without gold plasmon peak, it means that multi-sites deposition HNPs formed. On contrary, if a clear plasmon peak becomes visible while maintaining a fluorescence emission, this may be the sign of the single-tipped deposition. This can quickly predict the experimental results without TEM testing, and improve the experimental efficiency. SNCs with the aid of a 473 nm laser (Figure 34 b). 111 Although the light intensity can be a critical parameter in the photodeposition process, the light source power cannot increase unlimitedly, especially when using a lamp. Indeed, some additional thermal effects may be induced near the exposure area, which will interfere with the formation of the deposits. For instance, the incident light may be scattered because of the bubbles formed near the irradiation area or the excessive heating may complicate the deposition process. 111 However, to the best of our knowledge, apart from the research mentioned above, there are almost no reports on the use of a laser as a light source for the photodeposition processes in cadmium-based SNCs.

While a deeper mechanistic insight into the formation of these heterostructures has yet to be gained, these results highlight that both the seed structure and the irradiation conditions are delicate factors on which the topology of such metalsemiconductor HNPs critically depend. In order to improve this area, more work needs to be done.

Summary and outlook

Semiconductor-metal hybrid NPs have been proven to be effective photocatalysts, and their unique synergistic properties are characterized by their ability to finely-tune their optical, chemical and physical properties, thereby meeting the wide and diverse requirements of photocatalytic reactions. Hence, in this part, we mainly summarize the current metal deposition on the cadmium-based SNCs, especially for in-depth analysis of the most studied gold, to make a guide for straightforward understanding of the current progress in this field. As shown in mainly reduces the deposition on the NRs wall surfaces, thereby improving the singletip ratio. Laser or lamp-assisted photodeposition did also improve the frequency of the single-tip deposition. Even if Ryan and coworkers reported that the more active AuCl could achieve single-tip deposition more easily in short time (< 120 s), 184 its uncontrollable double-edged sword "high activity" also generated a multi-site deposition of tiny Au NDs on the semiconductor NRs surface.

Based on the above-mentioned literature, we found that most of the data only reported results of single-tip deposition with a high probability, and there was still no literature reporting on how to accurately control the deposition ratios of different sites, that is, how to achieve true single-tip deposition in a statistical sense.

Furthermore, although Banin's group has done some work related to laser or lamp induced photodeposition, [START_REF] Costi | Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells[END_REF]111,179,209 the study of the effects of the laser power, especially the influence of the incident intensity directly on the sample for deposition, is still to be performed. Additionally, the hole scavenger plays a vital role in accelerating the photodeposition process on semiconductors (such as TiO2), and can be used to increase the light deposition rate of metals on semiconductor surfaces. 202, 228-231 However, it is rarely mentioned in the deposition on cadmium-based semiconductor NCs, [START_REF] Aronovitch | Stability of Seeded Rod Photocatalysts: Atomic Scale View[END_REF]191,225 probably because even without hole scavenger, chalcogenide anions from the rods may reduce the Au 3+ accompanied by etching of the rods, [START_REF] Costi | Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells[END_REF]206 or dodecylamine (DDA) & didodecyldimethyl ammonium bromide (DDAB) as both the reducing agent and catalyst help to reduce the Au 3+ . [START_REF] Aronovitch | Stability of Seeded Rod Photocatalysts: Atomic Scale View[END_REF]206,223 Until now, no systematic research has been performed on the parameters affecting the deposition of single-tip, especially affecting the ratio evolution of single-tip, double-tip and multiple sites HNPs, even if the sites and size control have a great impact on catalysis 104 and even chirality [START_REF] Ma | Chiral Semiconductor Nanorod Heterostructures with High Photocatalysis Activity[END_REF] . Therefore, in Part 4, we tried to fill this gap and discovered the optimal laser-induced deposition parameters through a systematic study of the evolution of the ratio of single-tip, double-tip, and multiple sites HNPs, which will provide guidance for future research. To do this, we chose methanol as hole scavenger, coupled with the low activity gold precursor, KAuCl4, 223,232 which can improve the operability and repeatability of experimental results.

Introduction

Semiconductor quantum dots (SQDs), thanks to their tunable emission property, have been broadly applied in light emitting diodes (LEDs) displays, 1-5 solar cells [6][7][8] and biomedical applications, [9][10][11] etc. In these applications, it is a prerequisite to improve their chemical stability, photoluminescence (PL) and quantum yield (QY) without changing their characters of facile tunable emission and narrow full width at half maximum (FWHM). To reach these targets, thick-shell QDs (TSQDs) are necessary and are usually synthesized by the epitaxial growth of inorganic shells on the cores with a consequent removal of the surface defect states. 12 In this way therefore, QDs with a pure color emission and a FWHM narrower than 30 nm can be obtained, accordingly achieving QD-based display devices with a promising color gamut. 13 Based on previous literature data, CdSe QDs with thick CdS shells exhibit reliable photoluminescence (PL) properties for saturated red emitting from 630 nm to 650 nm, with QY up to 97%, and FWHM of approximately 30 nm. 1,8,[14][15][16] However, one color remains challenging to obtain: the green one. An efficient saturated green-emitter ranging from 520 nm to 540 nm for CdSe based TSQDs is yet a challenge due to the two following main factors:

(i)
the large surface-to-volume ratio (S/V), which is proportional to 3/r and increases as the size decreases. The QDs then get more and more sensitive to their environment, and their PL intensity is easier to quench caused by more surface trap states; 17 (ii) not only is it difficult to fabricate such small CdSe semiconductor nanocrystals (SNCs) with a high QY and a uniform size distribution, but also it is hard to achieve a high chemical stability by their coating with an inorganic semiconductor shell with a widerband-gap. 18,19 The conventional routes to synthesize green-emitting QDs are relying on expensive reagents, such as 1-tetradecylphosphonic acid (TDPA), which forms strong binding link with cadmium ions in the complexes resulting in a slow formation of the QDs, and a subsequent small size of the resulting nanoobjects with a homogeneous size dispersion. 3,20 Up to now, none of these QDs were used to form thick-shell green-emitting QDs. Other attempts were based on alloyed QDs, such as ZnCdSe, 21,22 ZnCdSSe, 18,23,24 which provided large-sized QDs with green emission, however a systematic control over the emission wavelength of these QDs was extremely difficult not to say impossible. Thus, it is still mandatory and relevant to find out an effective synthetic approach to achieve TSQDs with a pure green emission, a high PL, a good QY, a narrow FWHM and tunable precise emission properties.

Tri-n-octylphosphine assisted successive ionic layer adsorption and reaction (TOP-assisted SILAR) method is known to be an efficient route to form shells of precise thickness on QDs core and that successfully synthesized thick-shell QDs are able to provide an accurate controllable emission over a wide color range, with a high QY and a narrow FWHM (Scheme 1 a). 25,26 The core/shell QDs in the 570-615 nm range were synthetized this way. The QY of three-color core-shell QDs improves with the increased ZnS shell thickness respectively. The highest QY could reach up to 98%, 95%, and 69%, respectively, after coating with a 3monolayered ZnS shell. In particular, the QY of the orange emission core-shell QDs could reach a value of 95% after coating with a 3-monolayered ZnS shell and was maintained even after coating with a 6-monolayered ZnS shell. This is the highest observed QY for core-shell QDs coated with a thick shell of more than 3 layers, breaking through the bottleneck of low QY at high coverage of the shell. 26 The effects of the Se/Cd ratio showed that the TOP-assisted SILAR method works well in the Se-rich systems, but in the Cd-rich systems, the red shift caused by the shell growth could not be prevented. For instance, a red-shift of 31 nm was observed for the Cd-rich CdSe core, much larger than the 4 nm found for the Se-rich CdSe core. 25 Although this method can realize a controllable preparation of core-shell quantum dots, especially for cores with a complete reaction of Se excess (DCdSe > 3.2 nm and is particularly suitable for the preparation of yellow and red CdSe-based core-shell quantum dots, it is not suitable for the pure green ones (DCdSe < 2.5 nm), due to the incomplete reaction process, the residual precursor having a great influence on the shell layer growth.

Herein, we improve this TOP-SILAR method with an additional TOP-assisted extraction process (Scheme 1 b) to fabricate CdSe/ZnS TSQDs, pure enough to be susceptible to extend this wavelength range down to the green emission (532 nm) with the same accuracy. This TOPassisted extraction method took advantage of the TOP ligand excellent dissolving capability 14,27 to clean-up the QD core surface before proceeding to the layer-by-layer coating. The resulting as-prepared TSQDs exhibited a high quantum yield (QY) close to 75% and a narrow 1.2 SILAR and TOP-SILAR syntheses of QDs

Principle of the synthesis

The Successive Ionic Layer Adsorption And Reaction (SILAR) is the most widely used technique to synthesize core-shell QDs. 28 We introduced tri-n-octylphosphine (TOP) in this shell growth process, which is named as TOP-SILAR method. TOP served as both a surfactant and a solvent to synthesize the nanoparticles, playing a crucial role in not only dissolving and activating the precursors but also in controlling the size, morphology and stability of the nanoparticles, breaking through the bottleneck of the known low QY at high coverage of the shell. 26 The TOP-assisted extraction method is based on the TOP-SILAR method with some modification. The QDs cores are first purified by the TOP-assisted extraction process, which consists in adding TOP to the hexane/methanol extraction solvent. This mixture is more effective to purify the solution resulting from the incomplete reaction from the precursors producing a solution of pure cores for the TOP-SILAR process to be undertaken and lead to the further growth of the core-shell QDs.

The small size CdSe cores are prepared by a swift cooling process in order to generate cores as small as possible (see 1.5 experimental part for details p 108). The slow reaction rate of CdSe generation and the swift quenching of the temperature provide the required tiny dots, however the reaction is far from being complete. This is why this first step has to be followed by this so-called TOP-assisted extraction process. This washing step revealed to be an efficient one to eliminate the excess of Cd 2+ , all the side-products present on the CdSe core or in the solution, and which might affect the subsequent TOP-SILAR technique. The subsequent in-situ ZnS growth using TOP as an activator led to the CdSe/ZnS core/shell TSQDs. Since the TOP molecules are present all along the shell growth process, they can, not only efficiently dissolve the isolated ZnS NCs which may form and but also remove any surface defects formed during the shell growth process, so as to achieve nanodots with a narrow emission spectral width.

Calculations for the injections

The amount of CdSe seeds was determined with absorption spectroscopy using the empirical formulas by Peng et al., 20 which correlate the position of the first electronic transition to the diameter of the CdSe particles (equation ( 1)) and the molar extinction coefficient of the particles to their diameter (equation ( 2)). D = 1.6122 × 10 -9 • λ 4 -2.6575 × 10 -6 • λ 3 + 1.6242 × 10 -3 • λ 2 -0.4277 • λ + 41.57

(1) 𝜀 = 5857 • (𝐷) 2.65 (2) Therefore, the Beer-Lambert law can be used to easily determine the concentration of CdSe core through UV/Vis spectroscopy (equation ( 3)):

A = ε • c • l (3)
where A, ε, c, and l are the absorbance, the molar absorptivity (L mol -1 cm -1 ), the NC concentration (mol L -1 ), and the path length of the cuvette in which the sample is contained (cm), respectively. With the knowledge of the CdSe core diameter and the molar quantity, the amount of precursor for the growth of ZnS shell with a desired thickness of x monolayers on the surface of core, can be calculated by using the bulk crystal parameters of ZnS shell, as follows:

Chapter 1

Controlled preparation of core-shell quantum dots 93

References page 113

𝑉 𝑍𝑛𝑆 (𝑀𝐿 𝑥 ) = 4/3 • 𝜋 • (( 𝐷 𝐶𝑑𝑆𝑒 2 + 𝑥 • 𝑑) 3 -( 𝐷 𝐶𝑑𝑆𝑒 2 ) 3 ) (4)
N 𝑍𝑛𝑆 (𝑀𝐿 𝑥 ) = 𝜌 𝑍𝑛𝑆 • 𝑉 𝑍𝑛𝑆 (𝑀𝐿 𝑥 ) • 10 -27 /𝑚 𝑍𝑛𝑆 (5)

n 𝑍𝑛𝑆 = 𝑛 𝐶𝑑𝑆𝑒 * N 𝑍𝑛𝑆 (𝑀𝐿 𝑥 ) (6)
where VZnS(MLx) is the volume of the shell comprising x monolayers per core-shell NC (nm 3 ), DCdSe is the diameter of the NCs core (nm), d is the thickness of one shell monolayer (nm), NZnS(MLx) is the number of ZnS monomer units per core/shell NC contained in x shell monolayers (dimensionless), ρZnS is the density of the bulk shell material (kg•m -3 ), mZnS is the mass of a shell monomer unit (kg) (mZnS=MZnS/NA), nZnS is the molar quantity of precursor ZnS needed for the growth of x monolayers (mmol), and nCdSe is the molar quantity of NCs core used for the synthesis of the core-shell QDs (mmol), which can be obtained by the Beer-Lambert law. The SILAR technique is based on the alternating injections of the Zn and S precursors into the solution containing the CdSe NCs for the growth of CdSe/ZnS core/shell

NCs. The amounts of zinc or sulfur precursors required for each layer are determined by the number of the surface atoms of a given size of a core/shell NC (equations ( 4), ( 5), and ( 6)). We estimated the number of monolayers assuming each layer of ZnS had a thickness of 0.35 nm, so one additional layer growth would increase the diameter of a NC by 0.7 nm. 29 For example, in a typical experiment with 1 × 10 -5 mmol of 2.5 nm core, 2.27 × 10 -3 mmol of Zn and S precursors are needed for the first layer of the shell growth, and an additional 3.53 × 10 -3 mmol of Zn and S precursors complete the growth of the second layer. nm for ZB CdSe, JCPDS No. 00-019-0191). [30][31][32][33] The PL emission of pure green CdSe core can be maintained at 529 nm with a narrow FWHM (25 nm). Furthermore, the ZnS shell thickness can be fairly estimated by the subtraction of the core size from that of the prepared core/shell particles. As an example, the average diameter of CdSe/ZnS(7) TSQDs with 7 layers, as measured by (TEM), is 7.2 nm (± 0.9 nm) (Figure 2 (d2))

leading to an average shell thickness of 4.7 nm (± 0.9 nm). This value effectively corresponds to 7 monolayers (MLs) of shell (1 ML of ZnS inducing a 0.7 nm estimated increase in diameter/size as based on previous data). 2,28,34 A closer analysis of the high-resolution TEM (HRTEM) of the CdSe/ZnS(7) core/shell TSQDs show images of representative single NCs. All the particles show high crystallinity with continuous lattice fringes, and between the core and the shell, no evidence of interface can be detected. This means that the shell growth process occurs in the case of coherent epitaxial region, and excludes any independent homogeneous nucleation of the shell precursors. 34 As shown in the HRTEM image of CdSe/ZnS( 7) TSQDs (inset in Figure 3), the distance of interplanar spacing is now 0.317 nm, which is in good agreement with a cubic ZnS in the lattice spacing of the ( 111) planes (0.312 nm). 30,35

XRD

In parallel, X-ray diffraction (XRD) measurements were employed to examine the crystal After the growth of the ZnS thick-shells, those of the TSQDs slightly red-shift to 519 nm and 532 nm, respectively. In addition, the FWHMs of the PL peaks remain the same (25 nm and 26 nm), and the QY value consequently increases from an initial 21% to a final 75%. The PL emission spectra of CdS@ZnS core/shell QDs exhibit a slight red-shift with the shell growth as compared to that of the CdSe QD cores, and the FWHM of the obtained core/shell QDs remains very narrow (Figure 6).

Based on the emitting wavelength and FWHM obtained from their PL spectrum, these To probe the mechanism of synthesis of these green-emitting TSQDs, the effect of the chemical nature of the core precursor was first investigated. For this, we chose two commercially available precursors: an expensive one (TDPA), which is also a strong ligand for the cadmium cation, and a cheap one (stearic acid (SA)) (Figure 7), which is also a much weaker ligand for Cd 2+ . Cd(TDPA)2, and Cd(SA)2 were respectively generated as precursors of the green-emitting QD cores. The later ones were then used in the typical TOP-SILAR method synthesis of CdSe/ZnS core/shell QDs. TOP-assisted extraction purification process we observed that for TDPA (Figure 8, solid line), a large amount of metal ion precursors and non-nanocrystalline side products still co-existed in the reaction system due to the low reaction rate of the precursor. 42,43 The main results are:

(i) very small cores (λ < 500 nm); (ii) is the presence of residues on the core surface which induce an uncontrollable red-shift of more than 40 nm of the CdSe/ZnS(3) core/shell QDs PL peak after the typical TOP-SILAR shell growth as illustrated in Figure 8. In the case of the SA ligand, the fabrication of the CdSe cores is easier and faster, the QD cores are then bigger and we assume that there's no excess of remaining Cd 2+ in solution (Figure 8, dashed line) 44 , since there is only a small red-shift of 6 nm after the shell growth.

However, it is still difficult to prepare a fully reacted CdSe QDs with an emission wavelength below 550 nm.

Based on these data that SA is more efficient and cheaper, from now on we shall work with Cd(SA)2 as a precursor and detail the positive influence of the TOP extraction method as illustrated in the next paragraph.

Chapter 1

Controlled preparation of core-shell quantum dots 100

References page 113

Improvement thanks to the TOP-assisted extraction

The green-emitting CdSe cores are generated using the cheap reagent (SA) and a rapid cooling process (emission at 525 nm). Ren et al. studied the influence of the ratio of alkane amino groups on the PL emission wavelength, 25 and showed there was a good linear relationship between the emission wavelength of the CdSe cores and the ODA:Cd ratio where ODA is octadecyl amine, it varied from 620 nm to 570 nm by changing the ratio of ODA:Cd (from 80 to 20). Due to the high reaction temperature, it takes 1-5 minutes to cool down to room temperature after removing the heat source, and the core continues to grow uncontrollably during this process. Based on these results, we further reduced the ratio of ODA:Cd to reduce the core diameter, and at the same time, achieved a rapid cooling process by injecting cooled deoxygenated octadecene (ODE), the reaction was then quickly stopped in 10-20 s (temperature below 373 K). These cores are then submitted (Figure 9 Comparing these two conditions, it appears that:

(i) this purification step is crucial to efficiently remove the excess amount of Cd 2+ ions and non-nanocrystalline side products whether they are on the surface of CdSe cores (as shown in Scheme 1) or in solution.

(ii) the CdSe/ZnS core/shell QDs can effectively be obtained by this layer-by-layer technique without modification of the emission maximum. 25 To explain the significant red shift of the emission peak (Figure 9 dashed line and Figure 10), when no TOP extraction process is applied before the typical TOP-SILAR method, it is assumed that the excess of Cd 2+ is probably forming an external corona around the shell which will react first. This Cd 2+ is still available due to the incomplete reaction and if this part of the precursor is effectively removed, a CdS buffer layer or a Cd-doped shell layer is formed during the shell growth process. Compared with the ZnS shell, the CdS shell cannot provide potential barriers that are large enough to prevent the leakage of the exciton into the shell, 45 causing the significant red-shift of the absorption spectra. Since it is very hard to analyze quantitatively the excess amount of Cd 2+ ions and of the side products, it is also hard to evaluate the thickness of the shell, the emission properties of the core-shell QDs often encounter problems with reproducibility, justifying once more a real need for a purification process leading to reliable QDs. Starting with the same batch of CdSe cores, we repeated the shell growth process based on the TOP extraction method several times to test its efficiency, and the results are gathered in Table 2. The emission peak, the FWHM and the QY show a high level of reproducibility, which is controlled with a respective variation range ± 1.5 nm, ± 1.2 nm, and ± 2%. These results definitely show that the traditional purification processes (hexane/methanol, toluene/methanol) do not provide a high enough level of performance in terms of cleaning ability and they were unable to prevent secondary reactions of the core precursors in the incomplete reaction core samples. 42 Moreover, there is a real need of the effective elimination of redundant Cd 2+ since it reduces the lattice mismatch between the core and shell materials, which is greatly beneficial to improve the QY of the final TSQDs. Since these islands like redundant Cd 2+ precursors may be embedded in the ZnS shell during the encapsulation process, it will aggravate the lattice mismatch and affect the luminescence performance of the thick-shell quantum dots. The extraction process is, thus, proved to be a key factor in controlling the emitting wavelength of the TSQDs especially in the green region (< 550 nm) (corresponding to the smallest CdSe cores).

Generalization to improved synthesis of red-emitting core/shells QDs

This TOP extraction process is of course general and can also provide red core/shell QDs with a good emission, as shown in Figure 12, in which the emission maximum moved by only 2 nm from 632 nm to 634 nm after the whole process, and the FWHM of the PL peaks could be maintained (26 nm and 28 nm). Additionally, if these red-emitting TSQDs are combined with the green ones, they form a hybrid spectrum, which can be used to achieve around 91% of the Rec. 2020 high color gamut display devices (Figure 14), this is of great implications for the development of advanced display devices.

1.3.4

The thermal stability of these green-emitting QDs

The photoluminescence (PL) of QDs is known to decrease gradually with increasing the temperature, and to be difficult to recover after a high temperature heating process. 46,47 In order to simulate the effect of temperature on the thermal stability of these quantum dots, we placed the green-emitting QDs under different temperature conditions to study the stability and performance of their quantum yield with time (Figure 15). The is effectively temperature dependent but it can stand temperatures as high as 150°C

while keeping QY of 90 % for 4h. A systematic PL QY decrease is observed with time at 200°C. 4h lead to a decrease of the QY down to 35%.

Set-up of LED chips

Based on previous considerations that the blue LED chips can generate a lot of heat, which can damage the QDs placed in the same layer, we need to find an effective method to control this phenomenon. [48][49][50] At present, there are two main packaging methods suitable for QD-LEDs based on blue LED chip: the on-chip type and remote type. For a typical on-chip type QD-LED, a blue InGaN LED chip with peak wavelength of 455 nm and yellow-greenish YAG:Ce phosphor were used. 51 As shown in Figure 16 a, for the preparation of QDs-WLEDs, 70 μL of QDs-chloroform solution and 0.12 g of phosphor were mixed with 1 g of silicone. Then the mixture was heated at 60 °C to remove the chloroform completely. Finally, the mixed gel was coated onto the surface of LED chip, followed by a curing process in a vacuum oven at 150 °C for 1 h. In this way, the QDs are uniformly distributed in the silica gel medium and directly in contact with the chip. The heat power of QDs-phosphor layer is close to the LED chip at low current (Figure 16 c). On contrary, for a typical remote type LED (Figure 16 b), a polymer (such as PMMA, PS) was chosen as the polymer matrix of the QDs film. 48 The QDs were dispersed into the polymer matrix by in-situ polymerization or directly mixed with the polymer in a toluene solution. Finally, the resulting film was cut into squares to fit LED module. The heat power of QD-polymer layer is far away from the LED chip, which means that the remote type can efficiently reduce the impact of the chip heat generation on the QDs.s 48,51 The red dash lines are a guide to the eye.

In this work we chose the remote type QD-LED, which is based on the separation of the QDs layer from the blue LED excitation light source. Then temperature in the QDs layer is much lower due to the cooling effect of the space between the LED excitation light source and the QD layer, 48,51 as shown in Figure 17 a. The detailed fabrication method for the green-QD-LEDs is described in the experimental part (1.5.6 and 1.5.7). First, an independent green-emitting- 1.5 Methods

Chemicals

Cadmium oxide (CdO) (99.99%), zinc oxide (99.99%, powder), stearic acid (SA, 95%), selenium powder (Se) (99.5% powder), oleic acid (OA, 90%), and sulfur (S) (99.5%, powder)

were obtained from Sigma, USA. 1-octadecene (ODE, 90%), tri-n-octylphosphine (TOP, 97%), octadecyl amine (ODA, 90 %), and trioctyl phosphine oxide (TOPO, 90%), and other organic The synthesis is based on the procedure from the literature except for the rapid cooling process after the Se-TOP injection. 26,44,52 Typically, 0.4 mmol of stearic acid and 0.2 mmol of CdO are mixed in a 25-mL three-neck flask, which is heated to 220°C so as to obtain a colorless clear solution under argon atmosphere. The mixture is then allowed to cool to room temperature, ODA (2 mmol, ODA:Cd=10:1) and ODE (8 mL) are added into the flask, and the mixture is reheated to 270°C under argon atmosphere. As soon as the heating device is removed, 2 mL of a 1M Se-TOP (2 mmol) and 4 mL of ODE are injected sequentially over a period of 5-10 s (rapid cooling process). Finally, the reaction mixture is cooled to less than 60 o C, and a typical hexane/methanol extraction procedure is used to purify the NCs. 28,44 The obtained green-emitting CdSe cores (529 nm, FWHM 25 nm) are dispersed in hexane.

TOP assisted extraction process

4 mL of ODE was added to the CdSe-hexane solution, the mixture was pumped at room temperature to remove the hexane and then further pumped at 100 °C to remove any residual air from the system. 25 After the argon atmosphere was switched to the system, 0.5 mL of TOP was injected as an activator, and the mixture solution was further increased to 160°C for 10 min. 52 Then the solution was allowed to cool to less than 60 o C, and the typical hexane/methanol (volume ratio: 1:2) extraction procedure was used for the further purification. The TOP assisted extraction process was repeated 3 times, then the purified NCs dispersed in the hexane/ODE mixture phase were used for shell growth process, and the adsorbed Cd 2+ maybe remained in the alcohol phase and could be removed.

SILAR method

High quality green-emitting CdSe/ZnS core/shell QDs can be synthesized by the modified tri-n-octylphosphine-assisted successive ionic layer adsorption and reaction (TOP-SILAR) described in our previous papers. 25,26,53 Using the TOP assisted extraction process purified CdSe cores were generated.

For a typical reaction, the QDs dissolved in 2.5 mL of hexanes are mixed with 0.8 g of ODA and 4.0 mL of ODE in a 25-mL three-neck flask. The flask was pumped down at room temperature with a mechanical pump for 30 min to remove the hexanes and at 100 o C for another 10 min to remove any residual air from the system. Subsequently, the system was switched to argon atmosphere and heated to 140 o C for the injections.

For in-situ growth of the buffer layer, 0. were added consecutively via syringe to the reaction flask at the interval of 10 min for the growth of second ZnS monolayer. Cycling of injections and growths continued for the increased monolayers of ZnS shell, the amounts of subsequent injection solutions were calculated using the method described in our previous work. 26 The reaction was terminated by allowing the reaction mixture to cool. The final product was diluted by hexanes followed by a methanol extraction. The extraction procedure was repeated for three times, and the top hexane layer was stored. If necessary, heating or further addition of ODE can be used for the better extraction. The NCs were further purified by precipitating with acetone or methanol.

Highly pure NCs were obtained by repeating the above purification procedure for several times. The obtained green-emitting TSQDs (7-monolayered ZnS shell) had an emission peak at 532 nm, and 26 nm of FWHM.

Synthesis of red emitting core/shell QDs by TOP-SILAR method

The red CdSe/ZnS core-shell QDs was made by the same process as the green-emitting TSQDs using the 632 nm CdSe core (FWHM 26 nm).

As for the red emission CdSe QDs, 0.8 mmol of stearic acid and 20 mmol of ODA was used, and the reaction was maintained at 250 °C for 5 min. The CdSe was purified by TOP assisted extraction process (2.5.2.2), and the shell was then grown as in 2.5.2.4. The obtained red core/shell QDs had an emission peak at 634 nm (FWHM 28 nm).

1.5.3 Synthesis of the CdSe@CdS@ZnS core/shell QDs

The CdSe//CdS/ZnS core-shell QDs were synthesized based on typical TOP-SILAR method using the green-emitting CdSe core (2.5.2.1) directly without TOP assisted extraction process. 25 The same process as 2.5.2.4 was used and QDs with an emission peak at 573 nm were obtained. The green-emitting CdSe/ZnS using Cd(TDPA)2 as core precursors was synthesized as in the literature. 2,11,20 Typically, CdO and TDPA were mixed in ODE. The mixture was degassed, using the obtained CdSe core. 25 The same process as 2.5.2.4 was used and the emission peak with 530 nm was obtained.

1.5.5 Synthesis of CdSe/ZnS core/shell QDs by typical TOP-SILAR method using the CdSe without rapid cooling process 1.5.5.1 Synthesis of CdSe core by stearic acid without rapid cooling process

The highly fluorescent CdSe NCs were prepared by the procedure in literatures as the QDs core. 44 The synthesis process was the same as Cd 2+ adsorbed CdSe core (1.2.1) except for without rapid cooling procedure. Typically, a mixture of 0.2 mmol of CdO and 0.4 mmol of stearic acid in a 25-mL three-neck flask was heated to about 220 °C under argon atmosphere to obtain a colorless clear solution. After cooling to room temperature, 2 mmol of ODA (ODA:Cd=10:1) and 8 mL of ODE were added into the flask, and reheated to 270 °C under argon atmosphere. 2mmol of Se in 2 mL of TOP was injected, and the reaction was maintained at 250 °C for 5 min. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to purify the NCs from side products and unreacted precursors.

The obtained CdSe core (558 nm) was dispersed in hexane.

Synthesis of CdSe/ZnS core/shell QDs

The core/shell QDs synthesized by typical tri-n-octylphosphine-assisted successive ionic layer adsorption and reaction (TOP-SILAR) were described in our precious research. 25 The same process as in 3.2.2.4 was used and QDs with an emission peak at 564 nm was obtained.

Fabrication of QDs-PS composite plate

A physical mixing method was used to form a polymer plate with a green-emitting TSQDs as in the references. 4,54 Typically, polystyrene (PS) polymer particles (1 g) was dissolved in chloroform (3 mL), and then mixed with the prepared QDs solution (1 mL). The QDs-PS composite plate was obtained after natural drying in the mold for more than 20 h under an argon atmosphere, and a further compression molding with a vulcanizing machine (4 MPa) at 100 °C for about 10 min.

Chapter 1

Controlled preparation of core-shell quantum dots 112

References page 113

Assembling of TSQDs-LED

The blue In-GaN/GaN LEDs with the peak emission at 455 nm were used. The QDs-PS composite plate was first mixed with silica gels (silicone 6550 gel A/gel B = 1:1) and then pasted onto the SMD-type LED. The LED chip was allowed to cure at 100°C (30 min) so as to obtain the TSQDs based LED. 4 The EL spectra were tested in an integrated sphere and a spectrograph system.

Thermal stability test of QDs

The QDs-toluene solution was dropped on a quartz plate to form a uniform film. The sample was packaged by another quartz plate using the UV curing adhesive. The QDs thermal stability sample was tested under heating over different times, and then the sample was cooled down at room temperature for the determination of quantum yield.

Conclusion

In conclusion, we report here an improved facile synthetic method to obtain CdSe/ZnS thick-shell QDs with reproducible saturated green and high luminance emission. The cores are generated through a rapid cooling process using cost-effective reagents (stearic acid and CdO).

The shell is build-up on a Lego-type basis associating a TOP extraction method to clean-up the QD surface, and a TOP-SILAR method. QDs with a high QY of 75%, a narrow FWHM of 26 nm and a controllable emission can be obtained and their green emission (532 nm) can be maintained even after their coating with a thick ZnS shell (7-monolayer). This inexpensive and high-output method can also be used to obtain multi-color core/shell QDs with a high precision. In addition, the experiment results obtained in this research are beneficial to understand the mechanism of TOP-assisted method, and provide:

(i) a real new access to QDs with properties exhibiting a high level of reproducibility in terms of size, purity, wavelength and QY.

(ii) a promising low-cost way to mass fabrication of LED as pure-colored backlight in displays.

Chapter 2

Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 121 References page 141

Introduction

The versatility of inorganic nanocrystals (NCs) is greatly expanded by the possibility of modifying their electronic properties by changing not only their size, but also their dimensionality. In particular, elongated structures such as nanorods exhibit an attractive set of emergent properties that set them apart from spherical quantum dots, such as a large degree of fluorescence anisotropy [1][2][3][4][5] and enhanced potential of electron-hole separation. [6][7][8][9] Among visible-light-emitting materials, CdSe/CdS dot-in-rods (DRs) nanocrystals (structures consisting of a spherical CdSe core surrounded by an elongated CdS shell) have emerged as robust material systems combining a high degree of monodispersity with bright and spectrally narrow photoluminescence. [10][11][12] However, while a rapid initial progress in the development of these CdSe/CdS nanorods (NRs) resulted in the achievement of various aspect ratios (ARs) in different colors, [2][3][4][13][14][15][16] a further improvement of the morphology control at the nanoscale, especially for the ultra-thin shell, proved elusive using conventional fast-injection-based preparations.

Even if the controllable preparation of the shell thickness, especially a thin one, has a positive significance on the improvement of the catalytic performance 17 and the chirality, 18,19 as far as we know, up to now, no one has studied how to directly implement the uniform thin shell quantum rods with various ARs. Only few papers mentioned the control of the DRs shell thickness. Bawendi's group 11 used a second over-coating process and focused on DRs with thicker shells, which enhanced the quantum yield (QY) to near 100 %. She et al. 20 studied the quantum yields (QYs) and photophysical processes by systematically tuning the shell and core sizes, which resulted in a thin shell layer, but the shell layer increased as the length increased.

Kim et al. 3 proposed a dual-diameter structure semiconductor nanorods with a narrow diameter of CdS shell and a controlled core position, while still with a thick-shell onto the CdSe core. Khon et al. 17 (2) For various ARs, the growth is controlled by the second stage, which can be achieved by easily changing the core concentration and the growth time.

In particular, core-shell quantum rods with ultra-thin shell layers and different ARs were successfully prepared. Under optimal conditions, thin shell quantum rods (< 0.35 nm) can be obtained with aspect ratio (AR) from 4.8 to 20.2, and the CD g-factor value enhanced from 0.23×10 -3 to 1.30×10 -3 . Moreover, we envision that the ultra-thin shell adjustment can be expanded to other semiconductor nanoparticles, and could be beneficial to their chiral signal and catalytic properties with clear applicative importance.

Results and discussion

Comparison of the DRs made by different groups

We gathered in Table 1 and Figure 1, the results obtained by various groups. It seems clear that the shell thickness control is rather poor with different research groups having different results. Moreover, even in the same group, at different temperatures, with different researchers, and in different seasons, the results are at variance.

Then, this work aims to find out a growth mechanism for these quantum rods and explore how to improve the reproducibility of their preparation, especially in terms of the control of the shell thickness.

Control synthesis of different wurtzite-type CdSe cores

In order to systematically study the growth law of the DRs, we tried to prepare cores with different wavelengths. As shown in Figure 2, the wavelength of the CdSe cores can be stepchanged just by varying the ratio of Se and Cd precursors; it can be changed from 547 nm to 614 nm just by varying the ratio from 1:3 to 3:1 (CdSe-1, CdSe-2 and CdSe-3). In order to precisely control the emission wavelength, we also studied the effect of the reaction time and the ligand, as shown in Table 2 and

DRs with different shell thicknesses obtained at different temperatures

The shell thickness control of the CdSe/CdS DRs is poor at the high injection temperature (> 623 K), and the results obtained at different times and by different operators also vary widely (Figure 1 and Table 1). In order to find out the reasons why, we investigated for the first time the effect of the temperature on the growth of these DRs.

We manage to have a temperature control directly inside the solution to have a better follow-up of the reaction. Typically, 3.2 nm WZ-CdSe cores 26 (UV 557nm, Figure 4) were chosen as the starting cores sample, to study the growth process over a wide range of temperatures from 503 to 653 K. Figure 5, Figure 6 and Table 3 illustrate and detail the variation of the CdS shell thickness, the overall length of the DRs as well as that of the aspect ratio, and the increase of the overall volume of the nanoobjects. The injections are performed at room temperature in the solution at the desired one. The shell thickness can be controlled with the temperature at which the injection is proceeded, this one should be less than 593 K. As shown in Table 3, on the basis of the similarity of the mean diameter of CdSe/CdS DRs-4 (∼3.9 nm) and the size of the CdSe cores is likely to be one monolayer (0.35 nm). 17 The shell thickness can then be maintained to less than one monolayer as long as the injection temperature is below this temperature limit.

When the temperature reaches values above 623 K, the shell thickness increases significantly and exponentially. Additionally, we also can observe from Figure 6, that the DRs length and therefore their volume seem to show a good exponential correlation with the temperature. These results show that the temperature is the key factor to control the shell thickness in the synthesis of the CdSe/CdS DRs, and then the key factor for thin shell DRs (less than one monolayer). This is a new contribution to the synthesis of this type of DRs, which up to now, have been difficult to obtain because most of the works were based on higher temperature reaction procedures (above 623 K). 2,11,15,[21][22][23][24] The dot-in-rod (DR) shape of our samples were confirmed by a TEM EDX mapping as shown Furthermore, even if the thin shell can be maintained when the injection temperature is less than 593 K (Table 3), from the length standard deviation ratios study of the DRs under different injection temperatures, we can see that the standard deviation gradually increases as the temperature gets lower than 563 K (Table 4).

From the above data analysis, we can see that 593 K can maintain a thin shell with more uniform size distribution, and at the same time achieves an efficient growth of the quantum rods (advantages to lower temperature: faster growth ratio and larger adjustable range), so that from now on, we will choose 593 K for the further studies.

Chapter 2

Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 129 References page 141

Variation of the DRs size with the reaction time

In order to study the shell growth mechanism of CdSe/CdS DRs, we studied the effect of reaction time when decreasing the core concentration so as to achieve a larger range of length variations (Figure 8, Figure 9 and Table 5).

Under a reaction temperature of 593 K, the shell-thickness is maintained less than 1 monolayer in less than 8 min (Table 5), which gives us the chance to perfectly control the morphology of the DRs (Figure 8). As shown in Figure 9a, the length of DRs will gradually grow as the reaction time increasing less than 8 min, and the volume can also well fit with the reaction time. While longer time will form thicker shell DRs with shorter length, which may due to the Ostwald ripening process, [31][32][33][34][35][36] this may also the reason why most previous work are based on 6-10 min. 2,11,15,[21][22][23][24] In this work, we chose 8 min for the further studies. and HPA ligands, which are known to favor this crystalline form of CdS. 37 After the injection of the shell precursors, the reaction temperature of the solution (593 K) decreases to around 560 K and then recovers to the injection temperature after a certain time (40 s). This step is the one which determines the thickness of the overall shell. One monolayer CdS shell can be maintained till 593 K. A higher temperature gives rise to a thicker shell and a shorter DR since for the same duration time it seems that the total volume and then the precursor consumption follow the general trend depicted in Figure 6 as opposed to the length. (2) Stage 2: Due to the prevention of sides growth by the phosphonate ligands (ODPA and HPA), which adsorbed on the (100), (110) planes, the nanocrystals tend to grow along the [001] axis. 38 Additionally, due to the high energy of the (001) plane, the CdS shell is easy to continue to grow along the [001] axis, which also prefers to form a rod-like structure. 38 Under these two combined actions, a controllable morphology of quantum rod is finally obtained.

2.2.5.1

The effects of the heating mantle power.

The time to recover the reaction temperature after the precursor injection may also be a In the control experiment, 593 K was chosen as the reference reaction temperature, due to the lower heating rate. after the precursor injection, more time (from 40 s to 140 s) will be needed to come back to the injection temperature (593 K), and this will extend the duration of the first stage accordingly, and a thicker shell CdSe/CdS DRs should form.

We tested the growth of DRs under the same conditions with different heating power, as shown in Table 6, a thicker shell DRs-15 with 0.8 nm (90 W) was obtained as compared to DR-11 with 0.2 nm shell (180 W) (Table 5). We can infer that the increase in the shell thickness is due to the prolonged growth time in this very first stage, let's say from 1 min to 4 min. In short, high-power heating yields thin-shell DRs, while low-power heating yields thick-shell DRs As shown in Table 1 and Table 7, we can see that when the laboratory ambient temperature is not controlled by air conditioning, the period, season can also become a key parameter in the preparation CdSe/CdS DRs, the shell thickness of nanorods obtained under the same conditions can be as different as 0.35 nm in summer and 0.6 nm for winter the cooler season. This also confirm this two stages reaction process with a first stage taking place during the time when the reaction solution come back to its set temperature. In winter, due to the lower ambient temperature (291 K), the time in the first stage is longer, in the same way as for the low heating mantle power. The main result from all these data is: the shorter the time in the first stage, the easier it is to obtain a thin-shell DR; while, the longer time in the first stage, the thicker the shell forms. Furthermore, this growth mechanism can also be used to explain the shell-thickness increase when increasing the reaction temperature. Although the period of stage 1 is almost the same, reactivity of the shell precursors is getting higher at higher temperature allowing a further increase of the thickness of the isotropic CdS shell. Additionally, the higher injection temperature, the more difficult it is to go back to the set temperature before injection using the same heating mantle, and the longer the time for the first stage, so the thicker shell layer.

So that, as the injection temperature increases (from 503 to 653 K), the diameter of the DRs gradually increases. Therefore, the thickness of this CdS shell in the first stage is the key factor for the control of the diameter of DRs. In order to obtain the best results, we choose to work using the higher power heating mantle (180 W), at the higher ambient temperature (303 K), and at a relatively low injection temperature (593K) with 8 min shell growth process. Under these conditions, thin shell CdSe/CdS DRs were achieved, and their reproducibility was considerably improved. 26 DRs with different aspect ratios are mostly obtained by varying the core diameter, 2,20,22,39 reaction time, 3, 15 ligands 7 or amounts of precursors. 2,[40][41][42] In this work, the ultra-thin shell CdSe/CdS DRs with various aspect ratios are obtained simply by just varying the cores concentration (Table 8, Figure 10 and Figure 11). 7,42 This further confirmed that the best conditions we prepared are beneficial for the morphology control. Under these optimal conditions, different concentrations of cores are added to obtain DRs with different aspect ratios. The length of the DRs increases as the core concentration decreases (Figure 10 f), while their shell-thickness can be maintained less than 1 monolayer (Table 8) (the average thickness of one monolayer of CdS was taken as 0.35 nm 43 ).

As shown in Figure 11, the length and volume of the DRs are well fitting with the core concentration, which induces a variation of the AR over a large range from 4.8 to more than 20.

For instance, we are able to achieve the DRs with AR = 20.2 by using 2.8 nmol/mL CdSe core, AR = 10.8 by using 11.2 nmol/mL CdSe core, and AR = 4.8 by using 33.6 nmol/mL CdSe core. can also be obtained by using different sizes of CdSe cores (Table 9). In order to keep the DRs with same aspect ratio, the smaller core required higher concentration. Therefore, under this optimal condition (high power heating mantle (180 W), in high ambient temperature (303 K), and at a relative low reaction temperature (593K) before injection with a total 8 min shell growth process), we can get the same shell thickness (<1 layer of CdS) and DRs with different aspect ratios just by simply controlling the cores concentration.

Preparation of chiral DRs

Chirality in QDs can be obtained by several methods, 41,[44][45][46] ligand-induced chirality is the most common one and is a widely used way, in which a strongly induced chirality in aqueous phase can be obtained just by replacing the original achiral ligands with a chiral one on the semiconductor nanoparticles. 41,[47][48][49][50] Gun'ko and co-workers were the first ones to report on the observation of a circular dichroism (CD) response induced in the excitonic transitions of CdS quantum dots (QDs) capped with chiral molecules. 51 However, the CD g-factor they found is still low (mostly 10 -4 or even 10 -5 ), 41,[52][53][54][55] and the induced chirality signals of core-shells further decrease as the shell thickness increases. 19 Earlier, we found that the CdSe/CdS DRs had a great potential for large ligand-induced chirality. 18,41 Hence, in this work, we wanted to study the ligand-induced chiral signal in these ultra-thin shell quantum rods (Figure 12). In order to prepare these chiral DRs, the colloidal oil phase DRs was transferred to the aqueous phase by using cysteine as in our previous methods (see also Part 3 chapters 4 and 6 of this thesis) (Figure 12). 18,41 For instance, the cysteine was dissolved in DI water (5 mL, To study the effects of the shell-thickness on the CD g-factor (see Chapter 4 for more details on g-factor) signal for DRs, we compared the shell-thickness from 0.35 to 2.5 nm (DR-4, DR-5, DR-6 and DR-24). The thicker shell was obtained at a slow injection of Cd and S precursors at high temperature to decrease the defect band and improve the QRs optical properties as reported by Bawendi's group. 11 The shell thickness increased to 2.5 nm by one slow-growth step (Figure 14), and can be further increased by one more slow-growth step as long as a purification step is performed in between both steps. UV-vis. and CD studies were performed on these samples. As shown in Figure 15, the CD gfactor signal decreases by more than 5 times as the shell thickness increases, as also observed in Purcell-Milton's paper. 19 The results inspire us an in-depth study of the DRs with ultra-thin shells. As illustrated in Figure 16 a-e, we run the CD spectra of the different DRs for both the Land D-modified surfaces. The CD signal intensities are symmetrical and increase with the aspect ratio (Table 8). As a consequence, the CD g-factor sharply increases from 0.23×10 -3 when AR = 4.8 to 1.3×10 -3 for AR = 10.8, and is then maintained at more than ×10 -3 as the AR goes on increasing. As far as we know, this is the first report on CdSe/CdS DRs with a more than 10 -3 induced chirality using a single small molecule. Although Kuang's group 7 reported a CD g-factor with 1.2×10 -3 it was using a mixture of D-cysteine and D-histidine as ligands, and the single ligands induced CD g-factor of 0.52×10 -3 , which was probably due to the thicker CdS shell of their DRs which reduced the chirality.

Methods

Chemicals

Tri-n-octylphosphine oxide (TOPO, 99%), tri-n-octylphosphine (TOP, 97%), tributyl phosphine (TBP, 97%), n-octadecylphosphonic acid (ODPA, 97%), n-tetradecyl phosphonic acid (TDPA, 97%) and n-hexyl phosphonic acid (HPA, 97%) were purchased from Strem Chemicals. Cadmium oxide (CdO, 99.99%), sulfur (S, 99.98%) and selenium (Se, 99.99%) were purchased from Sigma-Aldrich. Oleic acid (OA, AR) were purchased from Aladdin. All chemicals were used as received without further purification. Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 139

References page 141

Synthesis of the CdSe Core

The synthetic procedure was based on the procedure reported in the literature. Typically, TOPO (1.5 g), ODPA (0.140 g) and CdO (0.030 g) were mixed in a 50 mL flask, heated to 150 °C and alternately exposed to vacuum and argon at least 5 times until CdO was a brown solid and the rest of the reagents were a colorless liquid. Then, to dissolve the CdO, the solution was heated to above 300 °C under argon until it became optically clear and colorless, which indicated that the reaction between CdO and ODPA was complete. Then, the temperature was increased to 370 °C, and 1.5 mL of TOP was injected into the flask, which caused the temperature to naturally decrease to 300 °C. Then, a Se:TOP solution (0.4 mL, 1 mol/L) was injected at 380 °C and reacted for several minutes. The size of the CdSe core could be adjusted by altering the temperature, reaction time and type of phosphonic acid (ODPA or TDPA).

Purification the CdSe-TOP Core

The reaction mixture was cooled to 70~80 °C, and 4 mL of ethanol and a certain amount of TBP were added into the above solution, which was centrifuged at 10000 rpm for 3 minutes.

Then, the precipitate was dissolved in a small amount of toluene while the supernatant was discarded. After centrifugation again, the precipitate was dissolved in TOP (CdSe-TOP). Due to the TBP purified process, the CdSe core can be better purified, which can remove the adsorbed ions on the surface and the by-products in solution. This is essential for uniform morphology control and high quantum yield.

Synthesis of the CdSe/CdS DRs

In a typical synthesis of CdSe/CdS nanorods via seeded growth, CdO (30 mg) was mixed in a 50 mL flask together with TOPO (1 g), ODPA (100 mg) and HPA (30 mg). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 300 °C to make the solution become completely transparent liquid without any solids. Then, the temperature was increased to 320 °C and a mixed solution of S:TOP (0.5 mL, 2.5 mol/L) and the above CdSe-TBP solution (100 µL) were injected into the flask, which caused the temperature to naturally decrease to 300 °C. After injection, the temperature dropped to 270~300 °C and then recovered to the pre-injection temperature within two minutes. The CdSe/CdS DRs were allowed to grow for approximately 8 minutes after the injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors. The shell thickness can be controlled by changing the injection temperature, heating mantle or the ambient temperature. And, the CdSe/CdS DRs with different aspect ratio were synthesized by adjusting the concentration of the core.

Conclusion

In conclusion, we have revisited a simple known method and shown that by controlling the temperature in a more accurate way it becomes easy to grow an ultra-thin CdS shell on the CdSe cores, yielding highly monodisperse nanorods with highly tunable AR. Notably, precise synthetic conditions were developed to prepare monodisperse ultra-thin shell DRs ranging from 18.3 to 74.6 nm in length and form the basis of the first example of a fully mapping of the effects of the temperature. This study shows that the synthesis process of DRs is in fact divided into two stages, and that the temperature has the greatest influence during the first stage, because it is at this stage that it affects the thickness of the primary shell. During the second stage, the thin shell is maintained while length growth proceeds at temperatures which can be lower than those generally adopted (<593 K).

The key factors controlling the morphology of the quantum rods are as follows: (1) The temperature mainly controls the diameter, such as the reaction temperature before injection, the heating mantle power, and the ambient temperature; (2) The cores concentration, the size of the cores can be the easiest way to control the length of the DRs; (3) As the reaction time increases, the Ostwald ripening process will gradually take place, and then affect the shell-thickness and length of the DRs.

These nanorods exhibit up to now the highest ligands induced chirality, with a CD g-factor value of 1.3×10 -3 just by introducing a single type small molecule, giving the structures great potentials for further chirality applications. Most of all, this study provides a platform for the exploration of diverse dimensions of nanostructures with controllable anisotropy. We believe that these new materials may be good candidates for a variety of chiral optical and catalytic applications.

Introduction

The synthesis of size-and shape-controlled core-shell colloidal semiconductor nanocrystals in their quantum confinement size regime has achieved a great success in the past couple decades. [1][2][3][4][5][6][7] Extensive studies revealed that semiconductor nanocrystals have generated great interest in catalysis 8,9 , optoelectronics 10,11 , chirality 12,13 , and self-assembly. The preferred nanocrystals should be not only highly monodispersed in size but also exhibit identical shape.

Unfortunately, even for the most studied CdSe/CdS nanocrystal (NC) systems, perfection of the shape structure of the nanocrystals has not reached a satisfactory level yet, especially from the anisotropy point of view. The results described below suggest that, instead of considering the core crystal structure, it is also necessary to take into account the interface and the effects of the surface ligands as well as the core concentration. In the specific case of the Tadpole shape of CdSe/CdS NCs, we will reveal that the surface effects dominate and control the differences in shapes during both formation and growth of the CdS shell.

Very recently, the seed-mediated core/shell growth method has been attempted to produce anisotropic NCs, which can balance the nucleation and growth processes, providing a better control in the mono-dispersity and even in the morphological uniformity as compared to the one-pot synthesis. 1,6,[14][15][16] The anisotropic NCs exhibit optoelectronic properties different from those of the spherical quantum dots (QDs) due to a dimension-dependent quantum confinement. 2,[17][18][19][20][21] In particular, anisotropic core@shell NCs have both efficient charge separation and carrier transport, which make them promising in a wide range of optoelectronic applications, for instance, photovoltaics, 22 electron transistor, 23 optical strain gauge, 24 chirality, 12,13,25 and photocatalysis. 8,9 The anisotropic morphology of the shells mainly comes from a different growth rate in the different spatial directions; this growth is itself due to a difference in the surface energy between the various crystal facets of the cores. 6 A common strategy to break the symmetry growth of the shell is to use additional ligands such as phosphonic acids 2,18 and halide ions 26 which selectively bind onto specific crystal facets of the core and then control their surface energy and their growth rate. The side facets of the wurtzite rods and tetrapods are typically nonpolar ones, which are claimed to be stabilized by cadmium phosphonate complexes. 27,28 Generally speaking, the preparation of polar NCs in specific shapes often requires an alternation of isotropic and anisotropic growth process. This

References page 172 means that a secondary shell growth process is always needed 6,15,16 complicating the preparation process and increasing the cost due to repeated purifications and re-growth processes.

In this paper, we propose a two-type ligand-mediated core@shell growth method, which based on a controlled combination of mixed phosphonic acid and carboxylic acid complexes acting as ligands, different from the most commonly used single type ligand syntheses is capable of controlling the NCs morphologies and sizes. [1][2][3][4]24 No additional purification and injection processes are required, and the anisotropic growth is achieved in a one-pot process.

This method is based on the assumption that the control of the shape of the CdSe/CdS NC systems is regulated by the bond interaction strength of these ligands with the exposed surface ions of the NCs. A systematic study of different capping ligands and of their different ratios has then been carried out to control and obtain the formation of specific shapes other than the known rods or tetrapods.

Results and discussion

By judiciously designing the reaction system, it is possible to separate the initial nucleation stage from the following growth process, and somewhat demonstrate different trends of the ligand effects. Like in other Chapters wurtzite (W)-CdSe cores of 2.8 nm diameter were used as starting materials for all the experiments and reacted at 320 °C in the presence of various ratios of n-octadecyl phosphonic acid (ODPA) and oleic acid (OA). Other parameters such as the core concentration, an incremental or one-shot injection were carried out.

The effects of octadecyl phosphonic acid (ODPA) as a source of symmetry breaking

Since ODPA is known as a key factor for the anisotropic growth of W-CdSe/W-CdS NCs, 15,17,21,24,27,29 we undertook a systematic study of its influence as a perturbing ligand of OA when varying its ratio up to 20 %. The main features are illustrated in Figure 1, which significantly illustrates the role played by the phosphonate. In the absence of such a ligand and a large excess of OA, a nanoflower type morphology is obtained which can be considered as rather symmetrical (Figure 1a) as compared to those obtained (Figure 1b-d) as soon as ODPA is added.

151 References page 172 The difference in growth rate between the different crystallographic directions was then accentuated. 1,15 Then, because of the stronger bonding between the cadmium cation and the phosphonate anion, 30,31 the presence of ODPA in the cadmium precursor solution changes the composition of the ligands around the CdSe core, this in turn modifies the shape of the growing NC. Therefore, for a given amount of OA when no additional ODPA is introduced the resulting nanoobjects exhibit a rather symmetrical growth with resulting nanoflower morphology (Figure 1a, Table 1), probably because the ODPA quantity released in solution from the core surface is too low to induce any symmetry breaking during the growth process.

However, by adding and adjusting the amount of ODPA in the injection solution and therefore adjusting the amounts of ligands (ODPA/(OA+ODPA) = 8.6, 12.4, 21.2%), other morphologies were obtained that we shall name Tadpoles from now on. Tadpoles with three to one tail were obtained at the nanometric scale depending on the ligand ratios (Figure 1bd, Table 1). The head of the Tadpoles, their aspect ratio (AR) and the number of tails systematically decrease when the ODPA ratio increases confirming, here in a nice way, the strong implication of ODPA not only in the symmetry breaking but also in the morphology Tadpoles. They exhibit a head bigger than the core precursor prolonged by one long tail, as also illustrated on the large area image (Figure 3). This morphology is very different from the already reported dot-in-rods (DRs) or tetrapod structures obtained from the classical seeded growth procedures. 2,17,24,26,27 As shown in the HR-TEM image of a single Tadpole (Figure 2b), the lattice fringes of the tail have an interplanar spacing of 0.341 nm, corresponding to the (002) planes of CdS in the wurtzite phase. The lattice fringes of the head in Figure 2c show a clear hexagonal packing pattern with the dspacing of 0.216 nm corresponding to the (110) crystal planes of the W-CdS. This indicates that the Tadpole did grow along the c-axis of the W phase. The CdS shell crystallizes in the wurtzite type for the whole core/shell NCs, in good agreement with the literature, which mentions that CdS prefers to grow in the wurtzite phase from W-CdSe seeds. 2 These data are confirmed by the XRD pattern in Figure 2d which also shows a wurtzite crystal structure for the CdSe/CdS Tadpoles. The intense and sharp peak of the (002) crystal planes suggests that References page 172 a large (002) domain of the wurtzite phase is present in the Tadpole, 26,27,32 which is consistent with the HRTEM data in Figure 2b,c. The optical properties of the Tadpoles were characterized with UV-vis. absorption and photoluminescence (PL) spectroscopy (Figure 2e). The absorption spectrum of the Tadpole displays a classical broad band below 500 nm, due to the absorption of the CdS. The PL spectrum exhibits a peak at 631 nm. A more specific study (Figure 4, Figure 5 and Table 2) illustrates the change in the absorption and PL spectra of the nanocrystals and the size and shape of the NCs during the reaction. In Figure 4a, the first excitonic absorption peak red-shifts with time (from 0 to 8 min), as typically observed during the CdS shell growth. 3,33 As the CdS continues to deposit on the nanocrystal (Figure 5), the intensity of the broad absorption band below 500 nm gradually increases (Figure 4a). 34 Additionally, the PL maximum wavelength red-shifts from 554 to 631 nm with a first decrease and then increase of the full width at the half maximum (FWHM) (Figure 4b). This FWHM decrease is attributed to the reduction of the particle size distribution during the epitaxial growth; the subsequent band broadening likely results from the directional growth of the objects and the inhomogeneity of the final Tadpole structures.

These data were also confirmed by the TEM analysis of the samples which was performed at different reaction times on at least 100 nanoobjects (Figure 5) to investigate the growth mechanism, especially the tail growth process from the W-CdSe seeds. These results show that the Tadpole growth process seems to be divided into mainly two stages (Scheme 1). Stage I is the formation of a wurtzite-type CdS shell around the core (Figure 5 b-c); 1, 2 it is easy to form at high reaction temperature (320 °C) since at this temperature CdS does adopt this wurtzite crystal phase whether the seeds are W-CdSe or ZB-CdSe seeds. 1,2 This initial spherical shape of W-CdSe/CdS (< 2 min) is due to the high reactivity of cadmium dicarboxylate, 35 which first grows on the CdSe surface and forms the isotropic layer leading to an isotropic growth. It This hypothesis is illustrated with literature data, which shows that cadmium diphosphonate fosters growth of anisotropic rods and tetrapods, 27,28,36 as opposed to cadmium carboxylates which favor the growth of isotropic spherical QDs. 3,33,35,37 This difference between carboxylate and phosphonate ligands in inducing such different shapes is directly linked to their respective coordination abilities. Indeed, a phosphonate group can coordinate with up to three or more cationic centers, instead of one (or two) for the carboxylate group, the anion is also bigger and then less polarized than the carboxylate. 35,38,39 This is why, employing a mixture of both ligands to complex the Cd cation in the precursor solution, yielded the Tadpole shape wurtzite CdSe/CdS NCs with very high structure purity (~100% in Figure 3). Cadmium dicarboxylate, the most reactive, dominated the structure control for the spherical tadpole head formation in stage I, and the "selective adhesion" 1,15 of the cadmium diphosphonate to the (100) and (110) surfaces of the growing crystallites orientates the unidirectional growth, along the [001] direction to gradually form the long tail.

The series of ligands ratios of these ligands, effectively showed that introducing ODPA not only increased the tail length but decreased the head size, blocking the access of OA to the surface even if it is still in excess.

The effects of oleic acid (OA)

In order to study the impact of OA on the Tadpole growth and size, we also varied its concentration at a fixed content of ODPA (285 mg, 8.5 10 -4 mol) as shown in Figure 6 a-e and Table 3. When compared to a situation when no OA is used and nanorods are formed (Figure 6 e), the addition of OA not only induces the asymmetric growth of the structure (Figure 6 ad) but also influences the length itself of the formed nanostructure. Indeed, the size of Tadpole head tends to reach rapidly a limit (11 nm) with increasing the OA ratio while both its length and its diameter go on increasing. Assuming no CdSe cores are dissolved, we explain this result with an increase of the reaction rate dependent the acid concentration. At a constant ODPA concentration, it looks like increasing the ratio of OA in the system does change the composition of the cadmium complexes ratio, which in turn also seems at This is once again consistent (1) with the growth of the first step, and the high activity of cadmium carboxylate complexes. (2) the "selective adhesion" of ODPA, and the subsequent growth of shells along the [001] direction to form the long tail shape.

However, when the ratio of OA becomes too high (OA 1.5 mL), the homogeneity of the Tadpoles gets worse, and some tetrapods appear, reducing the morphological uniformity.

Additionally, the anisotropic growth of the CdS tail will decrease with decreasing the ratio of OA. So, we chose the 1 mL OA sample as the best one to further study the effects of ODPA.

The results in this subsection combined with the results about the evolution with OA (Figure 6) reveal that the ligand effects on the tail growth stage can be separated from that in the head stage.

In fact, playing with the difference of (i) reactivity between (Cd(OA)2) and (Cd(ODPA)2) on the one hand and (ii) the respective binding strengths of the CdSe-bond ligands on the other hand it becomes possible to finely tune and target the desired CdSe/CdS morphology.

3.2.4

The effects of CdSe core concentration on Tadpole shape We also performed experiments to test the possibility to grow very high AR Tadpoles. The experimental results revealed that both the length and the selectivity in Tadpole shape NCs also depended on the cores concentration. To investigate the role of the cores concentration on the NCs shape evolution, a set of shell growth experiments using W-CdSe seeds were carried out varying the concentrations of the CdSe cores. Under the ligands ratio used in the reference conditions (RC), as shown in Figure 7, not only the Tadpole shape is maintained, but the Tadpole tail and aspect ratio significantly increase while decreasing the core concentration reaching an overall length of 181 nm (Figure 8 and Table 4). On the contrary, the diameter of the tail systematically measured at the beginning position of tail unchanged (mainly 5.5-6 nm). This may be due to the unchanged ratio between the two available ligands ODPA and OA, which adsorbed on the tail surface at this time of the reaction and affects the growth of the tail side in the same way. Our results suggested a formation mechanism where the two-step growth processes are independent; the head grows first due to the introduction of OA. And only then, under the influence of ODPA, the tail grows along the [001] direction in a way independent of the head growth process.

However, when the cores concentration becomes too low (5 nmol), the structural purity of the final NCs grown on the W-CdSe seeds becomes gradually poorer (Figure 7(a-b), and some tetrapods appear. Indeed, when the cores concentration is too low, the relative concentrations of Cd and S increase, so that the CdS growth around each core can become isotropic and a tetrapod structure can form as already mentioned in the literature. 2,15,21,24,27,29 This is confirmed in the case of the DRs synthesis process which using the typical Cadmium phosphonate for 1D shape control, as shown in Figure 9, in the typical mixture ligands (ODPA & HPA). The tetrapods shape begins to be observed at low cores concentrations (5 nmol), while DRs shape is obtained at a high concentration (50 nmol). The combined results from Figure 10 and Figure 7b reveal that the use of a mixture of both cadmium diphosphonate and cadmium dicarboxylate complexes in identical reaction conditions, promotes the growth of a Tadpole shape no matter the crystal structure of the initial cores. 35 This is consistent with the fact that the core composition at this temperature is not a key parameter, and with our hypothesis that the morphologies of the Tadpole CdSe/CdS NCs during both the shell formation and the growth steps are determined mostly by the interface and the chemical nature of the ligands present at this interface. This further confirms the two-step mechanism already mentioned with first a Tadpole head growth followed by the tail one. In this approach, we have underlined that playing with mixed ligands, controls the growth rate difference between the different crystallographic directions in the first stage.

Independently of the crystal structure of the seeds, the addition of cadmium dicarboxylate complexes promote the growth of NCs heads and provides Tadpoles morphologies hard to obtain with a single ligand. In the case of the ZB-CdSe core (3.95 nm), the head final size is larger, meaning that the CdS shell is even getting thicker in the first stage, may be because this structure is exposing faces with a packing density at the surface of the initially formed nanocrystal different from that of W-CdSe. Another suggestion was made that at the used temperatures the rate of the ligands attachment to and detachment from the surface of the ZB NCs is faster than that of W NCs. 7 162

References page 172 Therefore, this synthesis is not only ligand ratio dependent but also temperature dependent which provides a wider set of opportunities to control the size and shape of these Tadpoles and then their subsequent properties.

Role of different reaction temperatures

3.2.5.3

One-shot injection of the shell precursors.

We eventually investigated the impact of the injection mode on the resulting nanoobjects comparing the three incremental injections used above to a single shot keeping all the other parameters identical as for the synthesis of Tadpoles in Figure 1d and Figure 3. The resulting Tadpoles have basically the same size, but the one-shot injection of the shell precursors is less selective and gives rise to a large number of tetrapod structures (Figure 12). Then we also confirm that in the presence of a high concentration of the precursors, the uniformity of the objects morphology is destroyed. 40,41 A trioctylphosphine selenide (Se-TOP) stock solution of concentration 1.0 M selenium in TOP was prepared by addition of selenium powder (1184.4 mg, 15 mmol) to a 20 mL vial with a stir bar. The vial was purged with Ar and then, 15 mL of argon-purified TOP (97%) was added to the vial via syringe, and the resulting mixture was sonicated, stirred, and vortex mixed until a homogeneous mixture was obtained.

The synthetic procedure was based on the procedure reported in the literature. 2,41,42 Typically, TOPO (1.5 g, 3.88 mmol), ODPA (0.140 g, 0.419 mmol) and CdO (0.030 g, 0.234 nmol)

were mixed in a 50 mL flask, heated to 150 °C and alternately exposed to vacuum and argon at least 5 times until CdO was a brown solid and the rest of the reagents was a colorless liquid.

Then, to dissolve the CdO, the solution was heated above 300 °C under argon until it became optically clear and colorless, which indicated that the reaction between CdO and ODPA was complete. Then, the temperature was increased to 370 °C, and 1.5 mL of TOP was injected into the flask, which caused the temperature to naturally decrease to 300 °C. Then, a Se:TOP solution (0.4 mL, 1 mol/L,) was injected at 380 °C and reacted for several minutes. The size of 165 References page 172 the W-CdSe core could be adjusted by altering the temperature, reaction time and type of phosphonic acid (ODPA or TDPA).

Synthesis of the Zinc blende (ZB) CdSe core

Preparation of Cadmium Myristate. The synthetic procedure was based on the procedure reported in the literature. 2,41 Typically, CdO (115.5 mg, 0.9 mmol), myristic acid (435 mg, 1.9 mmol), and 1-ODE (7. The synthesis of ZB-CdSe was based on the procedure reported in the literature. 2,41 Typically, 0.34 g (0.6 mmol) of cadmium myristate was dissolved in 37 mL of 1-octadecene and the yellow oil was less than 0.5 mL.

Purification of the CdSe-TOP Core

The W-or ZB-CdSe cores solution was maintained at 70~80 °C, and 4 mL of ethanol and a certain amount of TBP were added into the above solution, which was centrifuged at 10,000 rpm for 3 minutes. Then, the precipitate was dissolved in a small amount of toluene while the supernatant was discarded. After a second centrifugation, the precipitate was dissolved in TOP (CdSe-TOP). Due to the TBP purification process, the CdSe core can be better purified, which removes the adsorbed ions on the surface and the by-products in solution. This is essential for a uniform control of the morphology and a high quantum yield.

Calculations for the diameter and concentration of CdSe

The amount of CdSe seeds was determined with absorption spectroscopy using the empirical formulas by Peng et al. 34 A trioctylphosphine sulfur (S-TOP) stock solution of concentration 2.5 M sulfur in TOP was prepared by addition of sulfur powder (1200 mg, 37.5 mmol) to a 20 mL vials with a stir bar.

The vial was filled with Ar and then, 15 mL of Ar-purged TOP (97%) was added to the vial via a syringe, and heated at 120 o C, the resulting mixture was sonicated, stirred, and vortex mixed until it became homogeneous. The 0.5 M S-TOP used for the NC growth was diluted by adding TOP.

Synthesis of CdSe/CdS Tadpoles

In a typical synthesis of CdSe/CdS Tadpoles via the seeded growth procedure, the seeding solution consisted in CdO (115.8 mg, 0.9 mmol) which was mixed in a 50 mL flask together in TOPO (3 g, 7.76 mmol), with ODPA (285 mg, 0.852 mmol) and OA (908 mg; 1.02 mL; 3.21

References page 172 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated at 320 °C to make the solution become completely transparent and dissolve all the solids. Due to the mixture of ODPA and OA, the reaction rate was very slow, and needed more than 2 h. This was very different from pure ODPA or OA systems, for instance, only 30 min were required in the case of the pure ODPA and the transparent solution was light yellow, not colorless. 1.5 mL of TOP was added, and then, the temperature was maintained at 320 °C, and 50 nmol of the previously obtained W-CdSe-TOP solution was added. The S-TOP (0.5 M, 3 mL; 1.5 x 10 -3 mol) was divided into four incremental injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were then allowed to grow at 320 °C for approximately 8 minutes after the CdSe core injection. At various time intervals, an aliquot amount was removed from the reaction mixture with a needle-tip and diluted in chloroform. Finally, the reaction mixture was cooled down to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors. For the control experiments, the S-TOP was injected in a single step, which is shown in Figure 12.

3.3.3.3

The influence of OA

The Tadpole morphology was also achieved by changing the amounts of OA in a solution containing 50 nmol of W-CdSe core. For instance, the OA amount was varied from 0.33 mL, 0.5 mL, 1.0 mL to 1.5 mL (see Table 3).

3.3.3.4

The influence of W-CdSe core concentration

Tadpole shape CdSe/CdS NCs with different tail lengths were synthesized by adding different amounts of W-CdSe core, varying from 5 nmol to 100 nmol (see Table 4).

Synthesis of different shapes CdSe/CdS NCs based on W-CdSe

Nanoflowers, three-tail Tadpoles, two-tail Tadpoles, DRs and Tetrapods were also synthesized by the same process as that used for the Tadpole shape NCs just playing on the ligand types and mixtures and/or core concentration. For instance, by changing the ligands, nanoflower shape can be obtained by ODPA (0 mg, 0 mmol) and OA (1.02 mL, 3.21 mmol), three tails Tadpole can be obtained by ODPA (100 mg, 0.299 mmol) and OA (1.02 mL, 3.21

References page 172 mmol), two tails Tadpole can be obtained by ODPA (150 mg, 0.448 mmol) and OA (1.02 mL, 3.21 mmol). And different aspect ratio (AR) of DRs can also be obtained by changing the ratio of ligands, for example ODPA (600 mg, 1.79 mmol) and HPA (0 mg, 0 mmol), ODPA (285 mg, 0.852 mmol) and HPA (0 mg, 0 mmol), or ODPA (600 mg, 1.79 mmol) and HPA (162 mg, 0.975 mmol). The Tetrapod shape can be synthesized by the same process as DRs just decrease the amount of CdSe core, from 50 nmol to 5 nmol.

Synthesis of CdSe/CdS Nanoflowers

In a typical synthesis of CdSe/CdS Nanoflowers via seeded growth, CdO (115.8 mg, 0.9 mmol) was mixed in a 50 mL flask together with TOPO (3 g, 7.76 mmol), ODPA (0 mg, 0 mmol) and OA (1.02mL, 3.21 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 320 °C to make the solution become completely transparent. 1.5 mL of TOP were added, the temperature maintained at 320 °C and 50 nmol of W-CdSe-TOP solution was added. The S-TOP (0.5 M, 3 mL, 1.5×10 -3 mol) was divided into four injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were allowed to grow at 320 °C for approximately 8 minutes after the CdSe core injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors.

Synthesis of CdSe/CdS three tail Tadpoles

In a typical synthesis of CdSe/CdS three tail Tadpoles via seeded growth, CdO (115.8 mg, 0.9 mmol) was mixed in a 50 mL flask together with TOPO (3 g, 7.76 mmol), ODPA (100 mg, 0.299 mmol) and OA (1.02mL, 3.21 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 320 °C to make the solution become completely transparent liquid without any solids. 1.5 mL of TOP was added, and then, the temperature was maintained at 320 °C and 50 nmol of W-CdSe-TOP solution was added. The S-TOP (0.5 M, 3 mL) was divided into four incremental injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were allowed to grow at 320 °C for approximately 8 minutes after the CdSe core injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors.

References page 172

Synthesis of CdSe/CdS two tail Tadpoles

In a typical synthesis of CdSe/CdS two tail Tadpoles via seeded growth, CdO (115.8 mg, 0.9 mmol) was mixed in a 50 mL flask together with TOPO (3 g, 7.76 mmol), ODPA (150 mg, 0.449 mmol) and OA (1.02mL, 3.21 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 320 °C to make the solution become completely transparent liquid without any solids. 1.5 ml of TOP was added, and then, the temperature was maintained at 320 °C and 50 nmol of W-CdSe-TOP solution was added.

The S-TOP (0.5 M, 3 mL, 1.5 × 10 -3 mol) was divided into four injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were allowed to grow at 320 °C for approximately 8 minutes after the CdSe core injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors. of short DRs (8.7±0.7 nm). ODPA (285 mg, 0.852 mmol) and HPA (0 mg, 0 mmol) can be used for the synthesis of medium DRs (14.0±2.4 nm), while ODPA (600 mg, 1.79 mmol) and HPA (162 mg, 0.975 mmol) can be used for the synthesis of long DRs (58.5±7.3 nm).

Synthesis of CdSe/CdS Tetrapods

In a typical via seeded growth, CdO (115.8 mg, 0.9 mmol) was mixed in a 50 mL flask together with TOPO (3 g), ODPA (600 mg, 1.79 mmol) and HPA (162 mg, 0.975 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 320 °C to make the solution become completely transparent liquid without any solids. 1.5 mL of TOP were added, and then, the temperature was maintained at 320 °C and 5 nmol of W-CdSe-TOP solution was added. The S-TOP (0.5 M, 3 mL, 1.5×10 -3 mol) was divided into four injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were allowed to grow at 320 °C for approximately 8 minutes after the CdSe core injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors. References page 172

Conclusion

By adjusting the ratio between two ligands usually used separately in the synthesis of CeSe/CdS NCs, OA and ODPA, we performed in a one-pot synthesis, nanostructures with different controlled morphologies such as Tadpoles, nanoflowers, dot-rods and tetrapods.

Unlike previous works, 2,24 this study shows that the one-pot configuration synthesis of anisotropic Tadpole shape CdSe/CdS nanocrystals in the commonly known size range, i.e., 22 nm to 180 nm, is possible provided the proper choice of ligands is performed and this whatever the initial crystal structure (W-or ZB-) of the CdSe seeds. The experimental results

showed that the ratio of the mixed ligands and the CdSe cores concentrations had more influence on the anisotropic control of the CdS shell growth than the core crystal structure.

In summary, W-CdSe/CdS core/shell Tadpole were synthesized using a mixed ligands method to control the anisotropic growth of CdS on the CdSe core in one-pot. Specifically, Wurtzite CdSe was used as seeds and anisotropic growth of CdS shell on the seeds was achieved by only adjusting the ligands ratio and the concentration of the cores. Our results suggested that more than the interior core crystal structure, the surface ligands were found to be key factor in determining the shape of CdSe/CdS NCs, i.e., Tadpoles versus nanoflowers, dot-rods and tetrapods. Overall, the results above showed that these mixed ligands promoted the formation of variable shapes structure in both nucleation and growth. The addition of cadmium oleate can improve the growth of the early isotropy, and cadmium phosphonate determines the structure of anisotropy. Conversely, the core concentration, core crystal structure, and temperature did not show decisive effects. This controllable polytypism of shape structures provides us new insights in symmetry breaking and anisotropic morphology formation in nanocrystals colloidal synthesis. This kinetically controlled strategy can be extended potentially to other nanocrystal synthesis, providing an alternative morphology control protocol. We anticipate various changes of anisotropic shapes, which may be useful for the design of chirality probes, anisotropic catalysts and complex optoelectronic transistors with semiconductor nanocrystals.

Introduction

In the second Part of this manuscript we have designed and/or improved synthetic procedures to better control semiconducting quantum dots (SQDs) in terms of size, shape, and morphologies. In this part we shall focus on how to improve their chiral properties. Indeed, as already mentioned in Part 1, chiral SQDs are an emerging field in nanomaterials, showing potential applications in asymmetric catalysis, 1 bio-sensing, 2, 3 chiral recognition, 4,5 and as a circularly polarized light sources, 6,7 attributed to the coupling of chirality with their inherent photoluminescence (PL) and optical activity. Chirality in QDs can be obtained by several methods, [8][9][10][11] however the ligand-induced chirality is the most common one and is widely used.

It is based on the principle that a strongly induced chirality in the aqueous phase can simply be obtained by replacing on the semiconductor nanoparticles the original achiral ligands of the organic phase by a chiral ligand soluble in water (cysteine as an example). 10,[12][13][14][15][16] Moreover, these studies have shown a complex evolution of the circular dichroism (CD) spectra with the size and the composition of the QD 12,17 The CD was found to be dependent upon the surface/volume ratio, 18,19 the shell thickness, 5,17 or the absorption ratio of shell vs. core (ARSC). 10 Two methodological research approaches related to chirality have been used to study the induced chiral mechanism. One is to study the mechanism of chiral conduction based on varying the ligands. [20][21][22][23][24] It studies the different ligand binding forces and the distances and the chiral center and the different binding sites, to rationalize the behavior and setup a the mechanism of induced chirality by different chiral molecules. The other way mostly relies on the bottomup colloidal process which involves the comparison of series of nanoparticles preparations to study the mechanism of action, such as different sizes, 5,12 different shapes, 10, 25-28 different shell-thicknesses, 5,17,29 and self-assemblies. 11,30,31 However, the precise control of the different morphologies especially anisotropic NC dimensions, such as DRs width and length, under kinetic growth conditions remains quite challenging because the reaction does not proceed toward a desired single (thermodynamically stable) endpoint. 32 Therefore, as the kinetic growth proceeds, the structural uncontrollable aspects increase: the size distribution of the anisotropic NCs becomes broader, resulting in a poor control over their optical properties, such as emission wavelengths and bandwidths. Furthermore, the relative extinction coefficient per mole of References page 217 particles strongly depends on the volume of the NCs, whether it is a single NC or a core-shell NC such as CdSe/CdS. 33,34 So that, the classical g-factor is not enough to characterize the chiral signal evolution of a single nanoparticle, because it just eliminates the effect of absorbance from the CD signals.

As the molar absorption coefficient of NPs increases, the number of particles decreases at the same absorbance, that is, the number of nanoparticles constituting the g-factor becomes less. Therefore, in a system with a large change in the molar absorption coefficient (such as different shell volume of the CdSe/CdS NCs), it is not possible to simply compare the g-factor change to show the chiral properties of the individual nanoparticles, in which maybe show the real chiral evolution for different morphologies. For instance, in the same CdSe/CdS core-shell

QDs system, two kinds of g-factor evolution rules appear in the CdSe/CdS core-shell QDs of different research groups (first increased (1 to 3 MLs) and then decreased, 5 or gradually decreased 17 ). Therefore, it is very important to study the mechanism of chirality generation of individual nanoparticles.

Meanwhile, the control of different morphologies at the same concentration is difficult to achieve only by bottom-up colloidal hot injection technology. This is because the reaction yield and purification efficiency are difficult to control and cannot be simply considered to give the same concentration as that of the core. On the other hand, for the calculation of the concentration of the composite semiconductor NCs, combining the comprehensive use of various characterization methods are required, such as TGA, XPS, TEM, and so on. This strategy, however, is hampered by the fact that the calculation of the molar concentration of core-shell nanocrystals requires a large amount of nanocrystals (usually tens of milligrams for TGA), which is difficult to achieve due to the small amounts of products and their high cost. In particular, for the preparation of anisotropic nanocrystals, such as CdSe/CdS DRs (having a higher chiral signal relative to other morphologies), 10,28,35 which requires expensive reagents such as TDPA, HPA, TOP, etc., it makes the concentration calculation even more impossible to achieve. Therefore, it is very important to find a low-cost and controllable method to prepare samples with the same concentration.

The top-down etching of colloidally grown nanocrystals provides a versatile tool to shape the structure of a series of comparative experiments starting with the same mother solution. The inorganic core can be oxidized, etched, and even completely dissolved, which normally defeats the function of the original nanocrystal/ligands complexes. 36 Several groups have reported the References page 217 modification of structural and optical properties of the as-grown NCs through top-down etching processes. Peng and coworkers have successfully used this method to study the evolution of NPs size and absorption of the first exciton peak. Their results greatly simplified the study of QDs, and the particle size could be calculated by a simple UV-vis test instead of TEM, which is of great significance. 37 On the other hand, due to the chemical or photooxidative dissolution of surface atoms, the top-down etching technology has been successfully used in semiconductors to tune the size of binary nanocrystals and study the interface of dots or peanut shape coreshell QDs. For instance, etching-mediated shape control has been reported for several semiconductor nanostructures, CdX, [37][38][39][40] PbS, 41 InP 42 binary nanocrystals or CdSe/CdS, 43 CdSe/CdTe, 44,45 CdSe/ZnS/CdSe 46 core-shell NCs.

Herein, based on apparent potential of the top-down etching-based methods in nanoparticle synthesis, we propose a technique that uses the same kind of cores and obtains different coreshell DRs structures through top-down etching techniques to study the mechanism of ligandinduced chiral transfer. The method is exemplified by performing isotropic etching of the colloidal nanostructures consisting of two spatially asymmetric material domains. Additionally, this etching method relies on the irreversible etching of the chalcogenides in a controlled way by a benzoyl peroxide solution. [44][45][46] The etching process can be monitored in situ using a UV-vis.

spectrometer. As shown by our results, the top-down etching process of CdSe/CdS DRs can be divided into two stages: First, a slow isotropic etching process takes place at the CdS shell level, because of the complete protection of the CdSe core. We show that the chemical etching of the CdSe/CdS dot-in-rod (DRs) structures promotes a partial dissolution of the CdS shell from surface of the CdSe dots, which induced an increased chiral signal for a single particle. However, in the second stage, after the further etching of the CdS shell, the CdSe core is gradually exposed, and a faster etching process begins. This is due to the different oxidation activities of the two chalcogenides. 45 By controlling the top-down etching and ligand exchange processes, we can achieve the comparison of solutions of nanoparticles with the same concentration, that is, the chiral signal of a single particle can be compared. In particular, due to our special etching process, we can further analyze the reason why the DRs structure has a high chiral signal. As shown in the references, the chirality of CdSe/CdS DRs 10,35 is significantly higher than that of CdSe or CdS QDs. 5,20 In the second stage, since the shell cannot completely protect the CdSe core, the defects of the core increase, which may affect the conduction of electrons and holes between the core and the shell. This may be the cause of the sharp decrease of the chiral signal References page 217 during the CdSe core etching. The novelty of this top-down etching is that it can precisely tune the quantum-confinement dimension or quantum size of NCs based on solutions with the same concentration. In this sense, etching seems to be the most natural and potentially the easiest way to study the evolution process of the chiral signal of a single particle. These findings suggest that the greatest capacity of the etching approach is anticipated from the application of this technique to multi-domain nanocrystals, where it can result in novel chiral signal that are unattainable by conventional colloidal methods.

Discussion

Exploring the mechanism of chiral action by top-down etching technology

To evaluate the ligand-induced chirality of NPS, the CD anisotropy factor (also known as Kuhn's dissymmetry ratio, (g) is used for comparison and to explore the mechanism in most of literature papers. 10,12,14,17,[25][26][27]47 The optical activity of chiral systems is often given through the dimensionless Kuhn factor (gfactor): 48

g = Δε/ε ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (1)
Where ε is the molar extinction coefficients (L/(mol•cm)). The circular dichroism (CD) is usually measured as the differential absorbance of left (AL) and right circularly polarized (AR) light, and so can be expressed as:

ΔA = 𝐴 𝐿 -𝐴 𝑅 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (2)
Since induced electronic CD signal presents the power on light polarization of a certain medium in UV range and it is based on the adsorption by the substance, and the absorption can be easily understood as Beer Lambert's law:

References page 217 A = ε • c • l •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (3)
Using Beer Lambert 's law to induce the concentration and pathlength of the incident light,

we can obtain the molar circular dichroism (Δε):

Δε = 𝜀 𝐿 -𝜀 𝑅 = ΔA/(c • l) ••••••••••••••••••••••••••••••••••••••••••••••••• (4)
Where εL and εR are the molar extinction coefficients for left circularly polarized light LCP and right circularly polarized light R CP light respectively. c is the molar concentration (mol/L) and l is the path length (cm).

The degree of ellipticity (θ) is defined as the tangent of the ratio of the minor to major elliptical axis, and θ with the unit of in mdeg is the ordinate of the CD spectrum. Cheng et al. 49 present the detailed calculation process between θ and ΔA via mathematic tangent relation of ellipticity, and is illustrated as below:

𝑡𝑎𝑛𝜃 = 𝐸 𝑅 -𝐸 𝐿 𝐸 𝑅 +𝐸 𝐿 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (5) θ(radians) = (𝐼 𝑅 1/2 -𝐼 𝐿 1/2 ) 𝐼 𝑅 1/2 +𝐼 𝐿 1/2 = 𝑒 ∆𝐴 𝑙𝑛10 2 -1 𝑒 ∆𝐴 𝑙𝑛10 2 +1 •••••••••••••••••••••••••••••••••••••••••• (6) θ(degrees) = ∆A( 𝑙𝑛10 4 )( 180 𝜋 ) ••••••••••••••••••••••••••••••••••••••••••••••• (7)
Which means:

ΔA = θ(degrees)/32.98 ••••••••••••••••••••••••••••••••••••••••••••••••••(8)
Where θ with the unit of in mdeg is the ordinate of the CD spectrum, the CD anisotropy factor is defined as:

g = Δε/ε = (A 𝐿 -A 𝑅 )/A = θ/(32980 × A) •••••••••••••••••••••••••••••••••• (9)
Chapter 4

Study of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 186 References page 217

where A represents the conventional absorbance of nonpolarized light, θ in mdeg is the ordinate of the CD spectrum, and AL and AR are the absorptions of the left and right circularly polarized light, respectively. 12 The CD anisotropy factor g is independent of the concentration and of the path length if the CD and absorbance spectra are taken on the same sample.

However, the absorbance at different wavelengths varies greatly, especially for the core-shell nanocrystals (NCs) with large "effective Stokes shift" (a large wavelength gap between excitation and emission spectra). [50][51][52] For instance, in the CdSe/CdS dot-in-rods (DRs) samples, the ratio of the absorption ratio of the shell to core (ARSC) changes by more than 12 times in different comparison samples, and in many cases the ARSC is higher than 10. 10 Due to their great change of molar absorptivity in the CdS shell absorbance, based on Beer Lambert's law, we can get that the number of nanoparticles does change dramatically for a same value of the absorbance. In this case, if only the g-factor is used to study the evolution process of the chiral signal of a single nanoparticle, the result will be complicated, 5,17 and the true change law may be hidden. Furthermore, due to the influence of the measurement range and sensitivity of the CD spectra, it is better to use different concentrations of nanoparticles to achieve the same OD to analyze the chiral signals in different band area. 53

Characterization of the single DRs

In order to explore the ligand-induced chirality variation law, we just use the top-down etching method to provide a series of NCs solutions, all with the same concentration, so that the chirality study of individual nanoparticles becomes possible just by the ordinate of CD spectrum (𝜃), which excludes the influence of changes in concentration and absorbance on the exploration of the chiral mechanism. It should be pointed out that the anisotropic g-factor at the first excitonic transition shows a 10-fold enhancement as the shape changes from spherical QDs to one-dimensional quantum rods (QRs), 27 and these excellent high chiral signals are displayed in both the core (500-700 nm) and the shell (< 500 nm) of the CdSe/CdS dot-in-rod (DRs). 10,35 We therefore choose the CdSe/CdS DRs system for the top-down etching study.

The synthesis conditions are the same as shown in Chapter 2, different shell-thicknesses can be obtained by varying the injection temperature, for instance the thin shell is obtained at 593 K. As shown in Figure 1 and Table 1, in order to prove the universality of the etching experiment, a Diameter of the CdSe core determined from the absorption spectrum by Peng's equation (nm). 37 DRs with different shell thicknesses were also obtained by varying the temperature as shown in Figure 2, Figure 3 and Table 2. The shell thickness is three monolayers (DR-1), two monolayers (DR-2) and one monolayer (DR-3), respectively. The average thickness of one monolayer of CdS was taken as 0.35nm, so one additional layer growth would increase the diameter of a nanocrystal by 0.7 nm. 54 As shown in Figure 3, after the first exciton peak is normalized, it can be seen that the absorption below 500 nm increases sharply as the shell increases, and the PL emission is simultaneously red-shifted with the absorption peak. This is due to the CdS shell which cannot provide potential barriers that are large enough to prevent the leakage of the exciton into the shell. 55 This causes a gradually red-shift of the absorption band with the shell-thickness increase. Surprisingly, the FWHM of CdSe/CdS DRs can be maintained at 23 nm (Table 2) which is narrower than that of the CdSe core.

188 References page 217 a Diameter of the CdSe core determined from the absorption spectrum by Peng's equation (nm). 37 These CdSe/CdS DRs were transferred to a water solution containing the L-or D-cysteine solution (0.2 mol/L, 2 mL), and the reaction mixture was deoxygenated and stirred at room temperature under argon in the absence of light for 24 h to obtain the chiral DRs.

Chiral evolution of a single DR with time

In particular, to study the process of the chiral evolution of the single DRs, benzoyl peroxide References page 217 was used to etch in a controlled way the CdSe/CdS DRs in the presence of a large excess of benzyl amine in solution. 43 Ideally, the indication of such a controlled top-down etching process is that the overall size distribution of the DRs should not change significantly, although the diameter of the DRs decreases with time. Experimentally, the surface ligands of the nanocrystals were exchanged from ODPA/TOPO to benzyl amine. The rigid and short tail of benzyl amine, in comparison to the long and flexible hydrocarbon chain of ODPA/TOPO, made the etching process efficient. The top-down etching process can be monitored in situ by UV-vis absorption. 46 The etching process is described in paragraph 4.3.4. The CD spectra of the DR-1 samples exciton region is presented in Figure 4, which shows that the CD peaks correspond to the excitonic bands of DRs (Figure 5d). In addition, the CD spectra split at the absorption excitonic maxima, crossing zero and changing sign, behavior characteristic of the "Cotton effect" in chirooptical spectra. 56 This has been assigned to the splitting of the excitonic hole level (valence band) due to coupling interactions with the HOMO of the cysteine ligand producing two sub-bands, which are preferentially excited by alternating circular polarizations, though at present the precise mechanism of this coupling is not clear. 5 following the same trend as the g-factor (Figure 5, Figure 6 and Table 3). This is consistent with the literature, 17 Indeed, it can be seen from Table 4, Figure 7, Figure 8 and Figure 9 that the g-factor first increases and then decreases, while, the CD signal of solutions of same concentration of NPs gradually weakens (Figure 7 and Figure 9). This may be due to the fact that the volume of the CdS shell decreases as the shell is etched, the available surface for the ligand binding is reduced accordingly, thereby reducing the chiral signal provided by the shell. Additionally, the decrease of the shell volume induces a large change in the absorption (Figure 8 and Figure 9). Although the CD of a single nanoparticle decreases, the g-factor increases at the beginning of etching process, which may due to the absorption variation. As a result, this may be one of the reasons for the complicated laws behaviors observed in chiral changes. Here, we demonstrate that the top-down chemical etching of the semiconductor nanocrystals represents a powerful tool to reduce the difficulty of analyzing comparative experiments, which gives rise to the improvement of the operability of the experiment.

Shell thickness obtained using the temperature as a key parameter

To validate these etching procedures, we also prepared thinner shell quantum rods. DRs with different shell thicknesses were also obtained by varying the temperature as shown in Figure 2, Figure 3 and Table 2. The shell thickness is three monolayers (DR-1), two monolayer (DR-2) and one monolayer (DR-3), respectively. The average thickness of one monolayer of CdS was taken as 0.35nm, so one additional layer growth would increase the diameter of a nanocrystal by 0.7 nm. 54 These CdSe/CdS DRs were also transferred in a water solution containing the L-or D-cysteine solution (2 mL), and the reaction mixture was deoxygenated and stirred at room temperature under argon in the absence of light for 24 h to obtain the chiral DRs.

Chiral evolution of the shell-thickness DRs obtained using the temperature as a key parameter

The chiral signal comparison of these different DRs is shown in Figure 10, Figure 11, Figure 12, Figure 13 and Table 5, Table 6, they are consistent with the etching results (Figure 5 and Figure 6). The chiral signal of CdSe cores increases when the shell-thickness decreases (Figure 10, Figure 11 and Table 5), which is the same behavior as reported in the literature, 17 indicating that the smaller the distance between the ligand and the core, the stronger the chiral signal. However, due to the poor controllability of the one-pot bottom-up method, it is difficult to prepare a single variable sample (the shell-thickness and length of DRs both changed). As shown in Figure 12, Figure 13 and Table 6, different from what is observed in the etching process which can present the true evolution, the chiral signal of the CdS shell (below 500 nm) test in bulk solution is also complex, this further obstructed the study of the chiral change law of individual particles.

This motivated us to try to study the comparison of chiral signals of individual particles with a single variable and explore the evolution rules of the chirality. Based on the above research results about the increase of chirality with the decrease of shell-thickness, we decided to study thinner shells and try to obtain core-shell DRs with exposed cores in order to further increase the chiral signals as much as possible.

Chiral evolution of the etched thin-shell DRs (DR-3)

In order to further improve the chiral signal, we try to etch the thinnest shells (the one monolayer (1ML) one). It is expected that through further etching, the CdS protection might be removed and the CdSe core can be completely exposed and directly in contact with the chiral ligands, which minimizes the distance between the ligands and the CdSe core, thereby further improving the chiral conduction effect and enhancing the chiral signal. 17,25 Surprisingly, as shown in Figure 14, Figure 15, Figure 16 and Table 7, the experimental results are opposite to the expectations. Our results show that as the etching proceeds from 1 to 4 cycles, the CD signal at the first exciton absorption peak of individual nanoparticles does not increase as expected but decreases (Figure 14), while the corresponding UV-vis absorption peak undergoes a blue shift (Figure 15 f). This is probably due to the influence of the core etching, which we will discuss in the following part. Additionally, the corresponding g-factor still shows a different evolution process as compared to the CD signal, since it increases first and then decreases (Figure 15 and From all these results we may also draw the conclusion that 1monolayer is the best compromise to get the highest values of both g and CD.

Study of the change of behavior of the chiral signal of the first exciton absorption peak

In order to explore the law of this abnormal change in chirality, we conducted an in-depth study on the one monolayer shell DRs sample (DR-3) discovered the reasons for this decreased CD in the dot-in-rod structure. The etched nanocrystals at this stage were isolated and checked by TEM and XRD pattern (Figure 17). First, it can be seen that as the etching process continues, obvious defects appear at the position of the CdSe core (the core was eaten, as shown by the red outlines in Figure 17b-d The crystalline structure of the synthesized CdSe/CdS DRs (DR-3-1) and the etched samples (DR-3-2 and DR-3-4) was then characterized by X-ray diffraction (XRD) measurements. As shown in Figure 17 f (black), the XRD spectra and corresponding diffraction peaks of DR-3-1 match well with w-CdS (JCPDF No. 41-1049). 57 While as the etching progresses (Figure 17 f (red and blue)), the crystal shape of the nanocrystal becomes worse and the signal gradually decreases. This may be due to the fact that Se is more easily oxidized 45 and the CdS shell cannot fully protect the CdSe anymore as its thickness is less than one layer; as a consequence this preferential etching of the CdSe core leads to an anisotropic etching process. On the contrary, in the thickshell samples (DR-1), the isotropic etching of the CdS shell is observed. This result is consistent with the literature where Brock's and Peng's groups have reported that peroxides can etch CdSe much more efficiently than CdS. 46,58 Only a well-protected CdSe core nanocrystal can be used for the isotropic etching of shell, any pinholes in the CdS shell allows an etching of the inner CdSe core. 44,46 This is the main reason for the reduction of CD signals of the thin-shell CdSe/CdS DRs after etching (Figure 15 and Figure 16). This core etching affects transfer of electron holes between the core and the shell, thereby reducing the ligand-induced chiral transfer in this domain of wavelength (< 500 nm). This can be confirmed by the TEM images (Figure 17), which show that in the case of the thin-shell DRs-3, the etching causes the CdSe core to be preferentially attacked (Figure 17b-d), which eventually leads to the nanorod break

Comparison of the chiral signals of the two types of DRs

This result can also be verified by monitoring the changes in the UV-vis absorption spectra.

Due to the different etching speeds of the CdS shell and the CdSe core, samples with different shell thicknesses (DR-1, DR-3) show different trends under the same etching process. In the thin-shell DRs-3, the anisotropic etching process induces a significant blue shift of the emission wavelength, and at the same time the CdSe first exciton peak sharply decreases or even disappears (Figure 15f). While, in the thick-shell DRs (DR-1, Figure 8d), thanks to the better protection of the CdSe core, the absorption spectrum gradually shifts to higher energy (blue spectrum) under the isotropic etching and maintains the contour and the peak width of the UVvis spectrum of the original DRs. Additionally, due to the etching of CdSe core, more defects on the core surface affect the electron-hole transport between the core and the shell, which may prevent the ligand-induced chiral conduction and greatly reduce the CD signal (Figure 14, Figure 16).

Another item to note is the behavior of the g-factor: it first increases and then decreases, with the CdSe core etching, which decreases the absorption of the first exciton absorption peak at a given concentration (Figure 15f) and the CD signal. Then, in such a situation, even if the two CD and UV-vis signals decrease, the calculated g-factor at the first exciton absorption peak can increase anyway in a given range (Figure 15) because the two curves CD and UV-vis follow different slopes (Figure 16).

This result can be very confusing when considering only one aspect of the growth process. If a study is based only on the g-factor, it may lead to a different and probably wrong approach of the mechanism of action.

205 References page 217 Moreover, after the most severe etching (Figure 23 e), the DRs show a signal of the same order (10 -5 ) as the CdS quantum dots (as in Figure 17e). 20 At this point, the CdSe core is completely etched, and the chiral transmission between CdS and the ligands is the only process which can occur. Through the study of the etching experiment, we can see that the CdSe core is also a key factor in the process of generating high chiral signals in DRs. This may also be the reason why when compared to CdSe or CdS NCs (dots or even rods) 5,20 , the CdSe/CdS DRs exhibit much higher CD signals 10,35 , especially in the range below 500 nm for the CdS shell area. While, when directly using the g-factor for comparison, the results can be complicated and misinterpreted (Figure 8, Figure 9 and Figure 22, Figure 23). We can conclude that, comparative experiments in between series, cannot be confidently performed since the molar absorption coefficient of a single nanoparticle often changes greatly especially in the area below 500 nm, and this affects the number of nanoparticles per mole for a given value of the absorbance. In this case, the g-factor exhibits chiral signals with different number of nanoparticles per mole, which does not provide the real chiral signal of a single nanoparticle.

The same trouble will present in the etching process to study chirality by g-factor, which makes the g-factor seriously affected by the change of the molar absorption coefficient of a single nanoparticle and then hides the true chiral change law.

Role of the core size

In order to find the right way to further improve the chiral signal of thin shell DRs, we also studied the DRs with cores of different sizes (DR-4 and DR-5 in Table 2), the thin shells (< 1 ML)

show that as the core increases, the chiral signal decreases (Figure 24), especially in the CdS shell range (< 500nm). This may be because DRs with a large size core are of type I structure 63 which implies that the confinement of the electron is in the shell rather than in the core, while, DRs with a small CdSe core are closer to a quasi-type II, 64 which makes the electrons easily delocalized, thereby enhancing the transmission of chiral signals.

4.2.12 CPL study of the thin-shell DRs.

The development of circularly polarized luminescent (CPL) materials has drawn extensive attention due to the numerous potential applications in optical data storage, displays, backlights in 3D displays, and so on.

In developing CPL materials, the key issue is to obtain a high luminescence dissymmetry factor (glum), which is used to quantify the level of CPL. CPL spectroscopy is based on the differential spontaneous emission of left and right circularly polarized radiation by luminescent systems. the differential emission intensity (ΔI) of the left-and right-handed circularly polarized light is defined as follows:

∆𝐼 = 𝐼 𝐿 -𝐼 𝑅 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (10)
where IL and IR are the intensity of the left-and right-handed circularly polarized emissions, respectively. However, it is difficult to evaluate absolute I and ΔI values. Therefore, the degree of dissymmetry of CPL is usually quantified by the luminescence dissymmetry factor (glum), which represents the ratio of the intensity difference divided by the average total luminous intensity (with the similar mathematical expression as CD g-factor)

𝑔 𝑙𝑢𝑚 = 2 × ∆𝐼/𝐼 = 2 × (𝐼 𝐿 -𝐼 𝑅 )/(𝐼 𝐿 + 𝐼 𝑅 ) •••••••••••••••••••••••••••••••• (11)
The maximum value of |glum| is 2, which means completely left or right circularly polarized light. 65 Thus, pursuing large glum value is one of the most urgent issues in CPL research.

Experimentally, the CPL was measured using a JASCO CPL-300 spectrometer, , the value of glum is defined as glum = 2 ×[(ellipticity)/(32980/ln10)]/(total fluorescence intensity at the CPL extremum). 30 Here, we also studied the change in the CPL signal with etching, as illustrated on Figure and Figure 26, which show that as the etching progresses for the thin shell DR-3, the CPL signal gradually decreases. It is probably linked to a reduced chiral fluorescence signal. This further confirms that the generation of the CPL signal is caused by the core and should be correlated to chapter 2, which shows that the CPL signal gradually decreases with the shell thickness increase. 28 In order to obtain a high CPL signal, we then need to reduce the core defects while trying to obtain a thin anisotropic shell. During the second stage, the etching rate becomes significantly faster as indicated by the quick changes of the first exciton absorption peaks in the absorption spectra (Figure 18 d) and the TEM images of the etched DRs which exhibit worse size distributions (Figure 17 c-e). Indeed, when BPO reaches the more reactive CdSe surface, the etching rate speeds up as observed experimentally (Figure 15 f, Figure 18 d), 46,62 sulfides being much more difficult to oxidize than selenides, leading to a rapid oxidation and even dissolution of the CdSe cores and the generation of a population of smaller CdS quantum rods. This explanation is consistent with the disappearance of the first exciton absorption peak in the absorption spectra, which originates from CdSe core.

In fact the overall mechanism is still not fully understood even if some insights began to appear recently in the literature, 14,67 the thiol group induces the orbital hybridization of the highest occupied molecular orbital (HOMOs) of cysteine and the valence band of the DRs, leading to the decrease of the energy band gap and a concomitant red shift of CD and fluorescence spectra as compared to the bare DRs.

What is also certain is that the presence of CdSe as a core and its interaction with the CdS shell in a rod rather than simply a rod-like CdS structure, thereby greatly improves the chiral signal. Basically, the core is coated by a shell of another semiconductor with a larger band gap, a conduction band higher in energy and a valence band lower in energy than that of the core.

This structure has the effect of confining the excited state to the core of the nanoparticle and when the shell is corroded enough to observe the core etching, more defects form and the nanorods eventually break, which cause a sharp decrease of the chiral signal. In other words, it can indirectly explain that the coexistence of the core and a one-monolayer shell is crucial to a high chirality signal.

The synthetic procedure was based on the procedure reported in the literature. 61,64,69,70 Typically, TOPO (1.5 g), ODPA (0.140 g) and CdO (0.030 g) were mixed in a 50 mL flask, heated to 150 °C and alternately exposed to vacuum and argon at least 5 times until CdO was a brown solid and the rest of the reagents were a colorless liquid. Then, to dissolve the CdO, the solution was heated to above 300 °C under argon until it became optically clear and colorless, which indicated that the reaction between CdO and ODPA was complete. The temperature was further increased to 370 °C, and 1.5 mL of TOP was injected into the flask, which caused the temperature to naturally decrease to 300 °C. Then, a Se:TOP solution (0.4 mL, 1 mol/L) was injected at 380 °C and reacted for several minutes. The size of the CdSe core could be adjusted by altering the temperature, reaction time and type of phosphonic acid (ODPA or TDPA).

Purification of the CdSe-TOP Core

The CdSe core solution was maintained to 70~80 °C, and 4 mL of ethanol and a certain amount of TBP were added into the above solution, which was centrifuged at 10000 rpm for 3 minutes. Then, the precipitate was dissolved in a small amount of toluene while the supernatant was discarded. After centrifugation again, the precipitate was dissolved in TOP (CdSe-TOP). Due to the TBP purified process, the CdSe core can be better purified, which can remove the adsorbed ions on the surface and the by-products in solution. This is essential for uniform morphology control and high quantum yield.

Calculations for the diameter and concentration of CdSe

The amount of CdSe seeds was determined with absorption spectroscopy using the empirical formulas of Peng et al. 37 which correlates the position of the first electronic transition with the diameter of the CdSe particles (equation 1) and the molar extinction coefficient of the particles and their diameter (equation 2). As shown in equation 1 and 2 in Chapter 1.

In a typical synthesis of CdSe/CdS dot-in-rods (DRs) via seeded growth, CdO (180 mg) was mixed in a 100 mL flask together with TOPO (6 g), ODPA (600 mg) and HPA (180 mg). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 300 °C to make the solution become completely transparent liquid without any solids. 3.0 mL of TOP was added, and then, the temperature was increased to 320 °C and a mixed solution of S:TOP (3.0 mL, 2.5 mol/L) and the above CdSe-TOP solution (600 µL) were injected into the flask, which caused the temperature to naturally dropped to 270~300 °C and then recovered to the pre-injection temperature within two minutes. The CdSe/CdS DRs were allowed to grow for approximately 8 minutes after the injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors.

4.3.3.3

The CdSe/CdS DRs with different shell thickness synthesized under different injection temperature

The CdSe/CdS DRs with different shell thickness can be synthesized under different injection temperature. For instance, using the CdSe with PL emission at 578 nm as core, as changing the injection temperature from 320 to 380 °C , the shell thickness changed from 0. For example, using the CdSe with PL emission at 601 nm as core, as changing the CdSe core from 112 nmol to 224 nmol, the AR of DRs will decrease from 10.2 to 5.7.

Controlled etching

Controlled etching of nanocrystals with benzoyl peroxide (BPO) was performed following the technique described in the reference. 43,44,46 4.3.4.1 Prepare of the BPO precursor solution 0.2 g BPO was dissolved in 4 mL toluene/methanol (1:1) mixture solution.

Ligand exchange by benzyl amine

Prior to etching of CdSe/CdS DRs with BPO, their surface ligands were exchanged with a shorter benzyl amine by mixing respectively 2 mL of purified CdSe/CdS DRs in chloroform with 5 mL of benzyl amine, and the resulting suspension was sonicated for 20 min until the solution turned clear.

Etching process with BPO

For the further etching process, 36 mL of a toluene/methanol (3:1) mixture was added to the DRs mixture solution, and then 0.3 mL of BPO solution was further injected into the nanocrystals suspension and allowed to react for 30 min, upon which the absorbance was measured. This step was repeated several times for the following study about the mechanism of chiral conduction of Cys-DRs. Etched NCs were precipitated with ethanol and re-dispersed in chloroform.

Conclusion

In conclusion, we proposed a method to verify the evolution of ligand-induced chiral conduction mechanism using a top-down etching method, which can be applied to any type of complex nanocrystals in principle, especially core-shell structure materials. This technique complements the traditional approach to chiral conduction mechanism by offering a series of homologous nanocrystals that cannot be easily obtained using the bottom-up colloidal synthesis methods. In the present study, a facile top-down etching procedure was employed to fabricate CdSe/CdS DRs where both the CdSe core and CdS shell components were etched in a controlled step by step process. Once the outer layer CdS was removed by etching, the core was gradually exposed on the rods side and preferentially etched at this location in the second stage, because of the much higher high reactivity of selenium relative to sulfur. This top-down approach gives rise to a series of solutions of identical concentrations of DRs with various shell thicknesses. It provides an opportunity that the bottom-up approach is at this point not able to do, since a reaction per number of layers and temperature has to be launched independently.

Then, this etching approach allows the comparison of solutions of NPs at exactly the same concentration. As long as the core is not etched, the comparison of such solutions of NPs shows that the chiral signal of the CdSe core gradually decreases with the shell-thickness increase, while the chiral signal of the shell layer increases. Through a series of comparative studies of chiral signals induced by solutions of NPs at the same concentration, the results show that the chiral signal comparison of a single nanoparticle can be better used to study the chiral change rules compared to the g-factor in bulk system.

The CD and CPL activities could be further modulated via top-down chemical etching, and the origin of high induced chirality for DRs was elucidated to mainly involve the dot-in-rod structure, especially the interaction between the core and the shell. To our knowledge, the present work is the first demonstration of the mechanism of ligand-induced chiral transmission through a top-down etching process. We expect this etching-after-growth approach will find versatile applications in the future to design widespread applications in different chiral mechanism research systems with the highest precision.

Introduction

Induction of chirality and/or optical activity in inorganic semiconductor nanocrystals is a promising way to introduce a new dimension of intriguing and useful complexity to their properties. It can be used in various areas such as stereoselective synthesis, 1-3 chiral recognition, 4, 5 display devices, 6,7 biosensing, 8,9 asymmetric catalysis, 10 and so forth. [11][12][13] Gun'ko and co-workers were the first ones to report on the observation of a CD response induced in the excitonic transitions of CdS quantum dots (QDs) capped with chiral molecules. 14 Chirality in quantum dots finds its origin in a range of mechanisms including a presence of a chiral shell layer, [14][15][16][17] chiral dislocations and defects, 1,3,18 chiral ligand-induction, [19][20][21] and through chiral assemblies. 2,[22][23][24] Among these mechanisms, a ligand exchange with chiral molecules produces tremendous developments in obtaining QDs with a uniform size distribution and providing diverse choices in designing the surface chemistry of the QDs to impart chemical properties. However, circularly polarized luminescence (CPL), a counterpart of CD in terms of emission, has seldom been reported in semiconductor NCs because of the low accessibility of the CPL instruments, the insufficient theoretical background on this topic, and the complex experimental control that is required in terms of synthesis. Up to now, only spherical and rod-shaped semiconductor NCs samples with chiral-ligand-induced CPL have been reported 20,[25][26][27] , in which L-and D-cysteine induced the CPL signals on achiral NCs, and other morphologies are scarce. Therefore, an active dissymmetry factor in CPL with fine tunability still has a great research significance in chiral-ligand-induced quantum NCs, and more importantly, the question arises as to whether we can identify key factors to regulate these optical phenomena through feasible parameters.

In semiconductors, shape is an important parameter due to its huge effects not only on the electronic spectrum of the charge carriers but also on the optical properties of the nanocrystals. 28 For example, the density of electron and hole states changes from discrete levels for zero-dimensional quantum dots (QDs), to a saw-like quasi-continuum in onedimensional (1D) nanorods, and finally to the step-like quasi-continuum in two-dimensional (2D) nanoplatelets (NPLs). Due to the excellent properties of NPLs, such as a very short fluorescence lifetime, 29 wide tunability of narrow absorption and photoluminescence lines, 30- compared with spherical QDs and nanorods, NPLs have a much stronger quantum confinement in the transverse direction, and a larger surface to volume ratio, which are critical parameters for the induction of chirality. However, similar to other classes of colloidal semiconductor nanocrystals, [34][35][36][37] core-only NPLs suffer from a poor colloidal stability owing to their large surface area that results in a low photoluminescence quantum yield (PL-QY), 31,38 however a necessary condition for CPL signals. Up to now, to enhance PL-QY and colloidal stability of NPLs, core/crown (laterally grown shell) 31,39,40 and core/shell (vertically grown shell) 31,32,41 architectures have been synthesized and intensively studied. The PL-QY can be significantly increased in the core-crown NPLs, thanks toto the passivation of the trap sites originating from the sidewall of the core-only NPLs. 31 However, since the CdS-crown layer is grown only in the lateral direction, they still suffer from serious stability issues. The PL-QY can be greatly quenched in the core/crown NPLs after a common chiral ligands exchange process, 42 because of lack of proper passivation of their larger lateral surfaces.

Thanks to the coating with a flat CdS shell, the optical properties of the CdSe/CdS core-shell NPLs have been shown to be enhanced with respect to the core-only NPLs, 31,32,41 This shell build-up is carried out by a colloidal atomic-layer-deposition (c-ALD) technique. This layer-bylayer method has the ability to improve the NPLs stability in the aqueous phase compared to core-only NPLs, while the intense CD signal almost completely disappears after coating with a very thin flat CdS shell 21 and no CPL is reported. Although the layer-by-layer processes are very useful when a fine control of the shell composition is needed, when thick shells are needed, the application of this protocol is too time-consuming. In particular, the nanocrystals need to be carefully washed between each layer to avoid secondary nucleation. Recently, Dubertret and coworkers 30,32 presented a simple continuous shell growth in a one-pot approach performed at room temperature, in which the exciton was also confined within the core of the heterostructure and away from the lateral surface traps and possible dangling bonds.

Additionally, with the formation of polytypism shell layers, a significant amount of spectral red-shifts is also observed in the absorption and emission spectra depending on the thickness and composition of the shell layers. 32 Moreover, the island-like shell coincides with a chiral anisotropy, and a high level of PL QY (up to 80%) can be achieved, with then a high potential to get CPL signals. 30 Here, we report the comprehensive investigation of the combined effects of a chiral ligand upon the induced CD and CPL, by varying the thickness of the islands-like shell, leading to an optimal NPL nanostructure with both a strong excitonic emission and a well-expressed chiral ligand-induced optical activity. Surprisingly, when compared to flat shell NPLs obtained by c-ALD, the islands-like shell modified NPLs prepared by the one-pot synthesis method at room temperature give rise to both a CPL signal and also exhibit significantly enhanced CD signal, this may be due to both the chiral anisotropy of the islands-like shape & the small distance between ligand and the still accessible core based on this shape. We found that the process of chiral evolution can be divided into three stages: First, with the formation of discrete islands-like CdS shells covering the periphery of the CdSe-core NPLs, the PL QY being enhanced as the trap sites are gradually passivated. Additionally, the presence of these discrete islandlike shells can enhance the geometry anisotropy and also provide the possibility for the chiral ligands to come into close contact with CdSe-core NPL or even directly, 19 which may be the key factor for the excellent anisotropic properties. And the interfacial strain energy can be reduced as the CdS shell is tuned from flat to islands-like, 43 generally, crystal defects induced by epitaxial growth are decreased. The strain energy induced by the lattice mismatch between the two materials accumulates and above a critical thickness, three-dimensional island growth is favored to relieve the misfit strain, leading to a better stability. As a result, in this stage, the CPL signal is born along with the enhanced CD signal. In the second stage, the density of islands-like shells increases, coupling with the enhanced CPL and CD signals. Whereas, in the third stage, further growth of the CdS shell finally leads to a gradual emission intensity quenching accompanied by a gradually decreased induced chirality. In this process, although the polytypism islands growth is beneficial to release the misfit strain energy and improve the anisotropic properties, the actual thickness of the CdS shell still increases during the islands growth, producing a decrease in the chiral conduction. This result is in excellent agreement with previous results mentioning that the CD signals decrease as the shell thickness increases. 19,21 Indeed, as the CdS shell thickness increases, the distance of the hole present in the CdSe core from the coordinated chiral ligands on the surface of the QD increases.

Additionally, the anisotropy will reduce even more due to the full cover shell. Therefore, we have shown the critical importance of anisotropic morphology control during the shell growth to push the gain performance from CPLs. These findings can provide an important guideline for the induced CPL in colloidal semiconductor nanocrystals. Beside its fundamental interest, we believe that our study could help identify potential applications for core/shell NPLs.

Results and discussion

Synthesis of CdSe and CdSe/CdS NPLs.

As reported in the literature, when the lateral size of a NPL (length x width) is less than 100 nm 2 , the CD signal increases dramatically as the size decreases, however, the g-factor or optical activity only hardly change for values > 100 nm 2 , 21 While in the one-pot shell growth process, as shown in Table 1 (at least100 NPs were used for the calculation), the lateral size will significantly increase with the shell growth. In order to reduce the impact of the size changes on the chiral signal, we chose S0 (Lateral size ~ 117 nm 2 and the emission at 515 nm) as the CdSe core NPLs. We have developed the ligand exchange protocols, as well as the core shell syntheses, on four monolayers (4 ML) CdSe NPLs core (S0) with lateral dimensions ∼ 5.3nm × 21.9nm and 1.4 nm thickness (Figure 1), and these NPLs have a zinc blende (ZB) structure, and their two largest facets end with a cadmium rich plane passivated by carboxylate ligands. 42,44 They were obtained as indicated in 5.3.3 and purified from NPs (Figure 1c-d).

Islands formation Full-covered

The They are 512 nm emitting CdSe NPLs (as shown in Figure 2, see also Table 2)). The full widths at half maximum (FWHM) are 14 and 9 nm for light and heavy hole bands, respectively (Figure 2). The extremely narrow peaks are attributed to the strong quantum confinement effect in these ultrathin 2D nanostructures. The observed absorption peaks indicate that the NPLs thickness herein is around four monolayers (4 ML) according to previous studies. 21,31,32,42 The L/D-Cysteine (L/D-Cys) were chosen as the chiral induction ligands, the absorption spectrum of the D-Cys NPLs is shown in Figure 3 and Table 3. After the ligands exchange with Cys, a significant red-shift can be seen in the absorption spectrum of the Cys stabilized CdSe NPLs. This strong shift is due to the exciton extension into the two additional monolayers of the thiolate functions once the ligand exchange is complete. 32 Additionally, the first two excitonic transitions, involving light hole (e-lh) and heavy hole (ehh), are not as sharp as in the case of the CdSe core-only NPLs. This broadening of the excitonic transitions is mentioned as related to the ligand density variations between NPLs. 32 However, even if the CdSe NPLs can reach a high QY (30%), after the ligands exchange, their PL emission is almost quenched (less than 5%), and makes it impossible to obtain CPL signals. For the purpose of activating the CD and CPL responses, a one-pot approach was used to form the core-shell structure, as indicated in the Methods section 5.3.4. We synthesized them using the same process described in the literature with little modifications so as to further study the formation process of the islands-like shell. 30,32 When thioacetamide (TAA) is introduced in the NPL solution, the PL emission of the CdSe NPLs was quenched and the solution changed its color from yellow to orange in 10 min. With the addition of the Cd precursor, the islands-shape NPLs formed and the NPLs luminescence gradually increased, while the QY could be maintained at more than 40%. After the ligand exchange, the NPLs were still fluorescent, but their quantum yield had dropped by a quarter to 30%. The CD and CPL responses of the islands-like CdS shell modified CdSe/CdS NPLs (S3) are presented in Figure 4.

They show an excellent anisotropy, with a maximum of CD g-factor maintained at 5.43 × 10 -4 ;

to the best of our knowledge, such a large g-factor for CdSe/CdS ZB-NPLs induced by ligands exchange has not been reported so far.

Additionally, the CPL signal was initiated along with the enhanced CD signal, which has a maximum anisotropic factor (glum) of 5.29 × 10 -4 (Figure 4 d, D-S3). This is the first report about CPL signals in ligand-induced chiral NPLs. In addition, our results also clearly demonstrate that the purification process is also crucial during the growth of the shell. As shown in references, 30,32 although the one-pot preparation easily produces CdS self-nucleation by-products, thanks to the ease of purification, pure nanosheets can also be obtained through a simple process. As shown in the TEM images in are needed so as to get the pure core-shell NPLs. 31,32 Each time before adding the Cd or S precursors, the mixture solution needs to be purified to remove the excess of each precursor and prevent the secondary nucleation. When thick shells are needed, the application of such a layer-by layer protocol becomes too time-consuming. Additionally, these washing steps also break the remaining S or Cd bonds on the NPLs surface and produces reactive sites. It's more difficult to get pure NPLs by c-ALD method, which affects the CD signals.

Effect of reaction time

As shown in Figure 7a, three hours after the addition of the cadmium precursor, the PL emission of the islands-like NPLs in the organic phase gradually red-shifted from 579 nm to 638 nm, and the FWHM increased from 12 nm to 23 nm. The trend of the emission wavelength reaches a plateau the reaction time (Figure 7b). This red-shift may due to the relaxing vertical confinement of the electrons and holes and the change in the dielectric medium of the NPLs. The FWHM displays a gradual narrowing as the shell thickness increases, indicating that the size distribution of the islands-like NPLs improves during the shelling procedure. Although a significant amount of broadening of the emission is observed in the islands-like shell CdSe/CdS NPLs as compared to the CdSe core NPLs, it is still narrower than that of other classes of colloidal semiconductor nanocrystals including QDs and NRs. 34,37 This is completely different from the step-like quasi-continuum NPLs obtained by c-ALD. For instance, although the c-ALD method offers great advantages including the formation of a smooth CdS shell layer with atomically precise shell thickness control, the emission jumped from 586 nm, 630 nm and 645 nm without continuous evolution, consistent with the growth of one, two and three CdS monolayers. 31 Compared with the steplike shell growth process, a finer tuning can be achieved in the islands-like shell process, which gives rise to the precise regulation of the nanoparticle morphology and the chiral signal. It was also found that the PL QY of these NPLs varies from 5 to 40 % across the samples S1-S5 (Table 2), with the PL QY initially increasing up to S4 and then showing a minor reduction when the shell growth proceeded. The further minor decrease in QY may be due to the crystal defects into the shell during the shell growth or the effects of the disordered structure present in the islands-like shell increased. 45,46 The islands-like shell shape after the CdS growth is visible in Figure 8 where most of the core-shell NPLs appear flat on the TEM grid and only a few of them are still able to stand on their edge. And from the HRTEM (Figure 9) we can further confirm the islands shape CdS shell growth around the CdSe NPLs. The reaction time is started after the source of cadmium is added. In fact, the strain energy induced by the lattice mismatch between the CdSe core and CdS shell accumulates and above a critical thickness, a three-dimensional island growth is favored in order to relieve the misfit strain, leading to the island shape shell formation. 43 Despite the lattice mismatch between CdS and CdSe which is relatively small (5-6%) (even if it depends on authors), 47 and the large specific surface area of the NPLs which even causes the curl, 29 it is more likely to observe lattice mismatch due to the generation of defects during the shell growth process. Additionally, due to the islands-like shell formation, even if the NPL thickness gradually increased with the reaction time, from 1.4 nm for the core to 6.0 nm after 3 h, their absorption and emission spectra are similar to that of the core-shell NPLs by c-ALD. 31,32 The thicker CdS shell is also illustrated by a strong absorption increase below 510 nm (Figure 10), value close to the CdS bandgap. The HRTEM images of the NPLs after shell growth (Figure 9 (b-f)) reveal that the CdS shell is not homogeneous, this anisotropic structure may be the key factor to get an excellent chiral signal in a one-pot process compared with the c-ALD method. Furthermore, for the core/islands-shell NPLs, the emission FWHM can be kept narrow while the QY steadily increased up to maximal values of ~40% (at reaction times between 60 and 120 min). However, although the islands growth is beneficial to release the misfit strain energy, the inter-island spacing CdS shell thickness also eventually increased during the islands growth, producing interfacial defects, which led to the gradual quenching of the emission intensity. The samples were measured at similar optical densities (at the first band edge peak position, ~ 1.0) to make sure the CD strength does not depend on the relative absorption. It can be observed that CD peaks correspond to the excitonic bands of the QDs, following the same red shift with the increase of the shell thickness. In addition, unlike the literature reports, 21 the "Cotton effect" gradually appear as the islands-like shell growth, though at present the precise mechanism of this coupling is unknown. The dependence of the g-factor value on the reaction time is illustrated in Figure 12 (see also Table 4). From these data, the chiral response intensity is quite complicated, the g-factor of the islands-like NPLs increases with the shell growth, goes through a maximum value before decreasing again (Figure 13).

) time (min)

Figure 13. The maximum g-factor variation between 450 nm to 650 nm as the islands-like shell grows.

We then propose a three-step growth mechanism for the islands-like shell NPLs growth process. In the first stage, a shell made of discrete CdS islands begins to form around the CdSe core NPLs (0-30 min). With time, a much higher density of islands-like CdS shell is reached around the CdSe core NPLs (30-90min). In the final stage, the surface of the nanosheets is completely covered with an additional layer forming in between the pre-existing islands, and the CdS shell further goes on growing in this existing CdS surface. This can also be verified by TEM as shown in Table 1, the physical properties were each measured for more than 100 NCs. Indeed, since the surface of the CdSe core NPLs still has blank sites, the CdS shell is still mainly grown on the CdSe surface, which does not increase the lateral size. Finally, as the growth proceeds for more than 120 min (S4, S5), then the surface of the CdSe core NPLs is overall covered, the islands-like CdS shell can only grow on the surface of exit CdS shell, which leads to the significantly increase of the lateral size again(❸).

As shown in

In the first and second stages, due to the formation of a small number of island-like structures, there are still many exposed parts on the surface of the CdSe core NPLs, and the ligand can directly contact the core or be in close contact with it. The high CD signals can then be maintained. At the same time, due to the islands-like structure, the additional anisotropy of the NPLs may further improve the chiral signals. However, in the third stage, due to the complete encapsulation of the CdSe core NPLs, after more shell precursors have reacted, the resulting chiral response of the sample is strongly reduced because it then depends on the increased distance between the hole present in the CdSe NPLs core and the coordinated Cyschiral ligands on the surface of the NPLs, so that the CD signals of the NPLs is greatly decreased accordingly. 21 This results was unexpected because the initial anticipation was that the strong CD signal observed in CdSe core NPLs would almost completely disappear in the core-shell NPLs. 21 Instead, it was determined that the presence of a islands-like shell did increase the measured CD response. We also studied the CD response evolution for CdS shells (between 350 nm to 500 nm), as shown in Table 5, Figure 15, Figure 16 and Figure 17, this is mainly induced by the anisotropy of islands-like CdS shell. In the first stage, the anisotropy increases with the islands-like shell formation. While in the second stage, it decreases due to the more density of CdS shell. Finally, in the third stage, the growth of the CdS shell layer further reduces the anisotropy. The activation of the chiral emission from these chiral islands-like shell NPLs can therefore be rationally envisioned after the successful observation of this enhanced induced chirality in the CD measurements. The corresponding CPL spectra of these samples are displayed in Figure 18 for the opposite enantiomers. As expected, opposite line shapes are observed for the CPL spectra within the PL-active region The PL and PL QY are known to be highly important optical properties of the NPLs, especially in relation to the CPL activity. Similar to other classes of colloidal semiconductor nanocrystals, 37 core-only NPLs have suffered from a poor colloidal stability owing to their large surface area that result s in lower photoluminescence quantum yield (PL-QY). 31 The stability issues and low PL-QYs prevent CPL signals from being generated. A common consequence of ligand exchanges is a strong reduction in NCs photoluminescence, due to the large specific surface area, it's especially serious for NPLs. In the core-crown system, 21,31 due to the formation of surface trap states in the large bare lateral area of CdSe exposed on both sides, after the ligand exchange, the fluorescence is almost completely quenched (<5%), unable to generate CPL signal. In the c-ALD method, although the high QY can be maintained, due to the thick-shell of CdS overall coating, up to now, there's no CPL signal reported.

In the one-pot reaction process, as shown in Table 3, the PL QY is maintained to more than 25% during the ligands exchange from the organic phase to the aqueous phase due to the islands-like shell effect which can reduce the stress caused by lattice dislocation. 43 The three stages growth process is also confirmed by the CPL tests. As shown in Figure 18, due to the partial exposure of CdSe in the initial stage, after the ligand exchange, more defects are generated, the fluorescence quenching is severe, and no CPL signal can be obtained. In the second stage, since more and more CdSe surface is protected, a high quantum yield can be maintained after the ligand exchange, which lays the foundation for CPL generation. At the same time, due to the islands-like structure, the distance between the ligand and the NPL core is small, which is beneficial to inducing the generation of emission chirality.

While in the final stage, the island shape shell will gradually form a complete overcoating shell, this increases the lattice misalignment resulting the PL QY decrease. Additionally, the same as in the reference, 31 with increasing the CdS-shell thickness, a decrease of the PL-QY is observed for the further growth of islands-like shell. This decrease trend of PL-QY with the shell thickness increase can possibly be attributed to our shell deposition method, which is performed at room temperature under ambient atmosphere, resulting in the formation of defect sites. Coupling with a decrease of the anisotropy, the CPL signals will die.

Additionally, although the exact mechanism for the CPL induction still needs to be determined, the chiral interactions between the cysteine molecules and the NPLs are asserted to be a key parameter in producing CPL, similar to their role in producing the CD response.

Synthesis of the CdSe Core NPLs Preparation of Cadmium Myristate precursor

Cadmium myristate was prepared according to the recipe given in the literature. 31 For a typical synthesis, 1.23 g of Cd(NO3)2•4H2O was dissolved in 40 mL of methanol and 3.13 g of sodium myristate was dissolved in 200 mL of methanol. After complete dissolution of Cd(NO3)2•4H2O and sodium myristate powders, solutions were mixed and stirred vigorously around 1 h. Then, the cadmium myristate (Cd(myristate)2) powders were precipitated by centrifugation and re-dispersed in methanol. The washing step with methanol was performed at least three times to remove any unreacted and/or excess precursors. After successive washing steps, the precipitated part was completely dried under vacuum overnight.

Synthesis of the CdSe Core NPLs

CdSe nanoplatelets (NPLs) having four monolayers (4 MLs) were prepared according to the procedure described in the literature with little modifications. 31 For a typical synthesis, 0.17 g (0.

The purification process for CdSe NPLs

The resulting 4 ML CdSe NPLs are separated from other reaction products with successive purification steps. First, the resulting mixture is centrifuged at 14,500 rpm for 10 min, and the supernatant is removed from the centrifuge tube. The precipitate is dried under nitrogen, redispersed in hexane, and centrifuged again at 4,500 rpm for 5 min. In the second step, the supernatant is separated into another centrifuge tube, and 30 % ethanol is added into the supernatant solution. In the last step, after the turbid solution is centrifuged at 4,500 rpm for 5 min, the precipitate is dissolved in hexane and filtered with a 0.20 μm filter.

Synthesis of the CdSe/CdS core-shell NPLs

Preparation of the Cd(OA)2 precursor solution Cd(OA)2 were prepared by the same method in the literature. 47 The cadmium injection solution (0.2 M) was prepared by dissolving CdO (4 mmol) in oleic acid (16 mmol) and ODE (15 mL) at 250 °C under argon atmosphere. After the clear solution obtained, the Cd injection solution was allowed to cool to about 60 °C . For each injection, the Cd(OA)2 solution was diluted with same volume of octylamine and was taken with a syringe using a standard airfree procedure.

Continuous CdS shell growth using TAA and Cd(OA)2

CdSe/CdS core-shell NPLs were prepared according to the procedure described in the literature with little modifications. 32 For a typical synthesis, 400 μL of the as-synthetized NPLs in hexane was diluted in 4 mL chloroform. 20 mg of TAA and 200 μL of octylamine were added The reaction rate can be controlled by changing the concentration of TAA, octylamine and Cd(OA)2.

The purification process for CdSe/CdS NPLs

In the CdS shell growth process on the initial CdSe NPLs, there are an important secondary nucleation of CdS NCs occurred but the growth of the CdS shell was still effective. After synthesis, the CdSe/CdS core/shell NPLs can be purified from the secondary nucleation by precipitation with a few drops of ethanol and suspended in CHCl3.

Ligand exchange of the NPLs with Chiral Cysteine Molecules

The purification process in oil phase is essential for reliable chiral analysis. The spherical CdSe NCs sub-product in the core preparation process and the self-nucleation of CdS NCs in the shell preparation process will significantly affect the reproducibility of chiral test. The Land D-cysteine ligand exchange was carried out using a previously reported method. 20,21,42 Preparation of the L-/D-Cys solution D-or L-cysteine hydrochloride monohydrate (0.2 M) was dissolved in DI water, and then the pH of solution was adjusted to 12 by TMAH to form the cysteine solution.

Ligand exchange of the CdSe NPLs Ligand exchange of the CdSe/CdS NPLs 2 mL CHCl3 solution containing CdSe/CdS NPLs was mixed with 2 ml of cysteine solution.

The mixture was stirred at room temperature for more than 48 h (although the NPs can transfer to water phase in several hours, enough time incubation is crucial). The reaction mixture was left to stand for 1 h to allow the phases to separate. The top aqueous layer was got with a syringe, and the Cys-CdSe/CdS NPLs were purified by precipitation with ethanol/DI water (4:1, two times). The purified Cys-NPLs were re-dispersed in DI H2O and stored at room temperature in the dark.

The CdSe/CdS NPLs can be dispersed in CHCl3, but difficult to dispersed in hexane or toluene.

Conclusion

To the best of our knowledge, we have reported here one of the first chiral ligand induced CPL signal on a 2D colloidal semiconductor CdSe/CdS NPLs, with a maximum anisotropic factor (glum) of 5.82 × 10 -4 . Their CD and CPL activities were found to be sensitive not only to their surrounding chiral environment but also to their intrinsic geometrical properties. Because of the formation of a islands-like CdS shell using a continuous shell growth in a one-pot approach, chiral CdSe/CdS NPLs present both a strong CD response and a CPL one for the first time, which is very different from the normal flat core-shell NPLs. 21 By controlling the shell growth rate, we showed that three different shell growth stages can be observed, from the formation of discrete island-shaped shell structures, the increase of density of these islands-like structures, to the completely covered shell formation.

(1) First, the formation of discrete islands-like CdS shells covers the periphery of the CdSecores only NPLs. This structure can also effectively improve the stability of NPLs even after the ligands exchange process. In addition, due to the islands-like shell, CPL signals are generated coupling with the enhanced CD signals.

(2) In the second stage, the density of islands-like shell increases, and the trap sites are strongly passivated in the CdSe/CdS NPLs. Thus, these NPLs exhibit systematically higher PL- QY and better stability when compared to the core-crown or plan NPLs, which is the key factor for ligand induced CPL. In addition to the enhanced PL-QY, the anisotropy of NPLs will increase, which also enhanced the ligand induced CD compared to the CdSe-core only NPLs.

(3) In the final stage, the fully coated shell may form as the shell grows, which increases the distance between the ligand and the CdSe core, and the induced chiroptical response was inversely proportional to the CdS shell thickness. This trend of decreasing chiral signal relative to shell thickness matches previously published results concerning CdSe/CdS QDs. 19 The islands-like CdS shell gave birth to the CPL signal, while the formation of full cover shell decreased the induced chirality.

Moreover, we envision that the shell adjustment can be expanded to other semiconductor nanoparticles, where the anisotropic islands-like shell can be used to enhance the ligand induced effects which are beneficial to the chiral signal with clear applicative importance as well. These findings can also provide an important guideline for 2D semiconductor quantum wells with induction of chiroptical activity, which can be used for better understanding of the origin of ligand-induced chiroptical effects in semiconductor nanoparticles and may potentially aid in technological development of chiral synthesis and recognition, optical devices, and multifunctional material design.

Chapter 6

Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies

Introduction

This Chapter was published this year in ASC NANO. 1 Since the first synthesis of core-shell CdSe/CdS quantum dots (QDs) with featured photoluminescence properties, great efforts have been made to explore a diversity of shape-controlled CdSe/CdS QDs using different synthetic approaches: cadmium sources, capping ligands, solvents etc. Indeed, a variety of morphologies such as spheres, 2 rods, 3 and tetrapods 4 were fabricated so far with a fine systematic control over the synthesis. However, the scientific demands to extend fundamentally the scope of shape-controlled synthesis and design materials with unique physicochemical properties are overwhelmingly growing with respect to the great potential of these quantum materials in the area of nanoscience, bio-nanotechnology and optoelectronics. [5][6][7][8][9][10][11][12] On the other hand, ligandinduced chirality in nanostructures such as metal nanoparticles and clusters, [13][14][15][16][17] QDs, [18][19][20][21] carbon-based materials [22][23][24] and semiconductors, [25][26][27][28][29][30] has revolutionized the motif of basic lightmatter interactions into a chiroptic realm where more sophisticated phenomena such as circularly polarized luminescence (CPL) and circular dichroism (CD) are dominant. [31][32][33] With post-synthetic ligand exchanges, the optical chirality can be transferred to the NPs via ligand-metal interactions. This chirality is very sensitive to the size, the morphology and the surface chemistry of the NPs, [34][35][36][37][38][39][40] indicating that the NPs ligand-induced chiroptical responses can be involved from the bench-top synthesis stage providing critical information about the shape evolutions as well as the ligand-particle interactions during the NPs formation. 41,42 In the previous chapter, we presented a facile synthesis of CdSe/CdS NPs with various anisotropic shapes from nanoflowers to tadpoles with one to three tails, and dot-in-rods (DRs) exhibiting tunable absorption and luminescence properties as illustrated in Scheme 1. We now wish to investigate the induced chiroptical properties of these nanostructures with CD and CPL spectrometers to shed light upon the geometry effects of the ligand-induced chirality of these CdSe/CdS based NPs. Furthermore, in-situ CD/CPL observations during the formation of the chiral tadpoles are applied to unveil the growth mechanism as well as the tail length effect on the chiroptical responses of the CdSe/CdS tadpoles. With these investigations, we hope with these asymmetric morphologies, to bring in brand-new insights unveiling the relationship between ligand-induced chirality at a molecular level and geometrical effects of the quantum objects at the nanoscale.

References page 294 of octadecyl-or hexyl-phosphonic acid ligands (n-octadecylphosphonic acid (ODPA) or nhexylphosphonic acid (HPA)) (Figure 1 a-c), spherical nanoparticles are more likely to form, which results in nanoflowers, tadpole-III, and tadpole-II shapes. On the contrary, with increasing "selective adhesion" type ligands (ODPA or HPA), tadpole-I and DRs shapes can be formed because (100) and (110) surfaces are more favorable to the adhesion leading to the growth of shells along the [001] direction to form the long tail or rod-like shapes.

The crystalline structure of synthesized CdSe/CdS core/shell Tadpole-I was then characterized by X-ray diffraction (XRD) measurements. As shown in Figure 2 The cysteine ligand exchange reaction was then carried out using a previously reported method 21 with some modifications. Cysteine hydrochloride monohydrate (702.52 mg) was exhibit only a slight blue-shift of 1-3 nm, as was shown in Figure 3 h (Table 1 andTable 2), which indicates that this ligand exchange process does not induce any aggregation or destruction of the NCs. After the CdS shell growth; the absorption peak of the CdSe cores is red-shifted from 539 nm to 605, 606, 605, 606, 568 and 572 nm for the different morphologies respectively (Table 1), while the PL quantum yield (QY) increases from 1.5 % to 52 % simply because the CdS shell repairs the defects on the surface of CdSe cores (Table 1). As shown in Table 1 and Table 2, the shell thickness of DR-1 is only 0.2 nm, which produces a small red-shift (25 nm). In contrast, the shell thickness of Tadpole-I is 4.3 nm, which produces a large red-shift (77 nm). The DR-2, Tadpole-II, and Tadpole-III shapes are all in agreement with the change rule of the PL red-shift caused by an increase in the shell thickness, but the case of nanoflowers is not completely consistent, this is probably due to the gaps between the petals.

The QY of the different NCs gradually decreases as the number of shell layers increases (Figure 4 a) due to the defects and lattice mismatch of the shell, which is consistent with the literature as well. 45,46 Moreover, as shown in Figure 4 b and Table 3, the FWHM value increases with the increase of the standard deviation (SD) ratio, which indicates that a uniform size distribution is more likely to achieve a narrower FWHM. Since the first observation of the CD responses of CdS QDs capped with chiral molecules, 47 studies of chiral CdSe/CdS NCs with ligand-induced chirality have rapidly increased over the past decade, with a focus on the origin of the chirogenesis, CPL and biological applications. However, as shown in Part 1, Table 1, the current CD and CPL studies on chiral cadmium-based NCs induced by small chiral ligands are still incomplete, with accessibility issues to CPL instruments and still to come theoretical/experimental supports to understand the chirality induction mechanism.

In order to better understand the significance of this work, and make a guide for a straightforward understanding of the current progress in this field, we compare our results

with other related works in this table. Here, we mainly summarize the current CD anisotropy (i) The morphology of the bare NCs is the most significant parameter impacting the CD anisotropy g-factors; the chirality increases with the anisotropy (2D>1D>0D), for instance, maximum g of the Cys-NPLs can reach 5.7×10 -3 , 48 while it is only 4.83×10 -4 for Cys-nanorods 18 and -2.1×10 -4 for Cys-QDs. 21 (ii) Different crystal types can lead up to 8-9 fold difference in chirality (5.7×10 -3 for wurtzite NPLs vs. 6.7×10 -4 for zinc blende NPLs). 48 (iii) The nature of the anion chalcogenide in the quantum dots has only a trivial effect on the CD anisotropy g-factors, the CD signals of CdTe, CdSe and CdS dots are relatively close. 21,40,49 (iv) Chiral ligands also have a great impact on the chiral signals; for instance, the g-factors of thiol-free carboxylic acids (L-tartaric acid) 50 can reach -7.0×10 -4 compared with -2.1×10 -4 for the Cys-QDs 21 and 0.5×10 -4 for the N-Ac-Cys -QDs, 40 which may be due to the difference in the number of stereo-centers. 50 The concentration of ligands affects the binding modes of the ligands on the quantum dots, which further affect the CD anisotropy. 51 (v) For the core/shell spherical NCs, the shell plays an important role on the chiral responses.

Gun'ko et al. 52 showed the impact of the shell thickness on the CD and photoluminescence (PL) performances of CdSe/CdS QDs. Although the PL measurements showed a direct relationship with the shell thickness, the induced CD was inversely proportional to the thickness, and decreased more than 5 times only by changing the CdS shell from 0.45 nm to 1.2 nm.

(vi) Due to the great influence of the shell layer on the chirality of spherical NCs, it becomes difficult for the spherical NCs to achieve both high CD anisotropy and CPL signals. Then, anisotropic NCs (1D, 2D) may be a solution.

As shown in a previous work, 20 even if the CdS shell thickness reaches 1.89 nm, due to their anisotropic shape, the DRs still show obvious CD anisotropy g-factors (0.46 x 10 -4 ) in the first absorption peak range (550 -650 nm), which are much larger than the spherical NCs (0.08 x 10 - 4 for 1.2 nm CdS shell). 52 Moreover, active CPL signals can also be obtained with this type of DR structure. Additionally, Kuang et al. 53 showed the impact of the aspect ratio (AR) of DRs on the CD anisotropy, with an enhancement of the g-factor in the range of 445-461 nm from 5.9 x 10 - 4 to 12 x 10 -4 ).

Analysis of the different structures 6.3.2.1 CD results of different structures

Based on the above-mentioned literature, we found that (i) the shapes of NCs, their AR and the ligands chosen play a key role on the chirality signals, and (ii) the core-shell DRs were born for CPL (anisotropy improves chirality and shell protection improves QY). Therefore, although the underlying mechanism is still unclear, in this chapter, we focus on the study of CdSe/CdS NCs with different morphologies to attempt to decipher the change trends in the CD and CPL.

After the proper ligand exchange, the electronic CD measurements were performed on the cysteine-functionalized CdSe/CdS NCs. In the vicinity of the exciton absorption of each nanostructure, mirrored CD line shapes are presented when L-and D-cysteine are separately To quantitatively compare the CD responses of these nanostructures, their anisotropic gfactors 54 are calculated and exhibited in Figure 6, where the highest value of the g-factor, defined as |gCD --gCD + |/2, 20 is used as an indicator for the magnitude of the induced chirality (Table 2 andTable 4). Note that all the nanostructures share the same batch of CdSe cores with the same ligand exchange treatment. We then assume that the differences in the CD spectra were mainly related to the geometry-induced by the variations in the nanostructures.

For a more in-depth analysis, the CD and UV spectra were divided into two regions:

(i) the 400-500 nm region where the CdS absorption dominates;

(ii) the 500-700 nm region where CdSe core absorption dominates.

With this consideration, the nanoflower-(Figure 5 2) increases as the morphology evolves from nanoflowers to tadpoles and then DRs, i.e. with increasing the asymmetric morphologies.

This means that a high AR provides a large induced CD response (Figure 7), since the chirality is strongly related to the degree of symmetry breaking (σ, i, Cn or Sn). Similar to Tang 18

and

Kuang's 53 works on their chiral quantum rods (QRs), the induced chirality can be recognized as the array of Cd-S bonds in the direction of the long axis of the asymmetric NCs. The total effective number of cysteine molecules increases to some extent as the AR (or the NC length) increases, resulting in enhanced chiral coupling, which gives rise to a stronger CD intensity. Nonetheless, this tendency is not applicable to the CdSe core absorption in DRs (Figure 8).

While the absorption of the CdS shell is below 500 nm, the CD activity in the 500-700 nm region is mainly due to the CdSe core. Increasing the length of the chiral NCs (Figure 8) results in a decrease of this CD intensity because of the CdS shell thickness, which prevents the chiral interactions between the CdSe core and the chiral ligands.

Purcell-Milton et al. 52 claimed in their paper that two competing effects occurred when increasing the shell thickness:

(i) increasing the distance between the ligands and the holes localized in the CdSe cores would decrease their coupling effects, and accordingly, the CdS shell would act as an energy barrier for the holes;

(ii) subsequent addition of CdS layers reduces the hole energy and shifts the hole level closer to the energetic resonance of the HOMO of the ligand which increases the CD response.

Our observations are in good agreement with the first effect, even though the mechanism is still unclear.

CPL results of different structures

The corresponding CPL activities of these samples are shown in Figure 9 and The CPL behavior of these samples also exhibits an inverse dependency on the CdS shell thickness, and is proportional to the photoluminescence QY, as summarized in Table 2 and We see here a two-step process. First, at high reaction temperature (320 °C), a wurtzitephase spherical CdS shell (from 0 to 2 min) is formed 4,5 due to the high reactivity of cadmium carboxylates. 56 As a result, the CD measurements exhibit relatively weak activities just after the ligand exchange. Afterwards, with further tail growth (> 2 min), the CD intensity changes from almost inactive to active as confirmed by the corresponding CD and UV spectra (Figure 13) and anisotropic g-factors (Figure 14), which is attributed to the CdS tail formation. Indeed, since the shell becomes thicker by a factor of 4 (from 1.0 nm to 4.3 nm), the interactions between the ligands and the core are becoming weaker and weaker as well as the chirality transfer between the two (Table 6 and Table 7). 52 The UV-vis absorption of the CdSe core (500-700 nm) gets weaker to nearly null, that of the CdS shell becomes stronger with a shoulder appearing at approximately 470 nm while the PL peaks red-shift from 554 to 631 nm (Figure 15, Table 8). 45,57 Due to the strong complexing ability of the phosphonate group, which coordinates to three or more cation centers, instead of one (or two) for the carboxylate group, 50,56 the alkylphosphonic acid ligands (ODPA) selectively bind to the (100) and ( 110) surfaces. This facilitates the growth along the [001] direction to gradually favor long tail formation, as already explained before. This anisotropic shape is responsible of the variation in the chiroptical signals in tadpoles during the growth, which is in agreement with previous statements that thin shells facilitate core-ligand interactions and that asymmetric morphologies benefit the induced CD responses in the CdS shell absorption region. We also investigated the CPL behavior of these NCs (Figure 21, Figure 22 and Table 10), as shown in Figure 23, Tadpole-I-2 to Tadpole-I-4 showed a nearly inactive CPL response. This may be due to a quenching effect during the ligand exchange with a low QY (< 10%, References page 294 best way to obtain multiple CPL and CD line shapes with high intensity is to achieve chiral tadpoles such as Tadpole-I. These Tadpoles have a thin shell, a reasonable QY for the photoluminescent core (Tadpole-I-6, glum = 5.71×10 -4 ) and a moderate tail length/AR for the CdS tail (Tadpole-I-1, gCD = 9.79×10 -5 ), as indicated in Table 10. However, the best parameters to obtain both CD and CPL activities are in fact not necessarily the same and not necessarily centered on the same Tadpole.

The studies when performed on the L-Cys-CdSe/CdS Tadpole-I provided the same trend and the glum values are given in Table 12). These observations could provide critical information for the rational design of CdSe/CdS Tadpole-I structures with tunable chiroptical activities, which should also be applicable for other related asymmetric multidimensional morphologies, such as DRs and tetrapods, since hole level-chiral ligand interactions are always the key issue in chiral quantum semiconductor heterostructures. 59 Moreover, these observations should also be applicable for other related asymmetric morphologies, as one should always consider both chirality at the molecular level and the nanometric scale at the same time.

The g-factor

To better understand the chiral origin and simulate the chiroptical behavior of our tadpole system, a brief theoretical modeling strategy is presented as follows. The g-factor was examined first in a very complicated manner, by solving a Schrodinger equation (1) with a chiral geometry. The model was then found too redundant because it actually represents a much simpler penetrating nature of the chirality from the surface. The chirality of rod-like tadpoles can come from two different mechanisms (Figure 24). The first one has been detailed in Li's paper, where a Schrodinger equation was solved 61 for the excitations in a screwed quantum rod. The chirality in that case comes from the screw dislocation, i.e. the symmetry breaking in the geometry of that quantum rod. The symmetry breaking mechanism was endorsed via the Laplacian operator, in which the 𝜙 and 𝑧 coordinate of the cylindrical coordinate system was correlated via the screw Burgers vector 𝛽. The Schrodinger equation was: Such a case must be solved mathematically via Schrodinger equation since the wave function would also be spiral. A spiral wave equation would then couple with the circularly polarized light operator, which is also spiral. In that way, the problem cannot be simplified due to the coupling of a spiral operator and a spiral wave function.

- ℏ 2 2𝑚 [
The second mechanism is the one presented here. The chirality comes from the excitation of electrons from the tadpoles to the chiral organic ligands (cysteine) on the surface. The chirality therefore is from the geometry of the molecules rather than the tadpoles. Of course, one can solve another Schrodinger equation for such a system, where no spiral geometry is added, i.e. the terms:

2𝛽 𝑟 2 𝜕 𝜕𝜑 𝜕 𝜕𝑧 - 2𝛽 2 𝑎 2 𝑟 2 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (2)
in the above Schrodinger equation should be removed. Such an equation can also be solved, which was done as mentioned, to yield a Bessel function with no spiral property. In such a case, the expectation value calculation for the circularly polarized light is actually only a function of the penetration depth of the light since the wave function is not chiral. Thus, this system cannot be described via the Schrodinger equation ( 1) method since it cannot describe the surface electron excitation, which has nothing to do with the geometric confinement of excitons or a chiral potential that will yield a chiral wave function. This is because the Schrodinger equation describes the geometry of the system via Laplacian or the potential, while in our case there is no such potential since the chirality comes from the chirality of the surface molecules.

Because of the above reason, we sought for a solution that can explain the surface effect of this penetration of chirality from the ligands. Since the interactions between excitons is electromagnetic, and the effect is concerned about surface penetration, London equation was adopted.

The magnetic field is believed to come from the directed movement of electrons in the ligand when excited. The field then couples with the excited moving electrons in the QDs, and therefore affects their excitations since the excitations of the ligands are chiral. Thus, the penetration of the magnetic field into the QD surface can describe the injection of chirality into the QDs. For the semiconductor case, the term 𝑖𝜔𝜏 in the conductivity 𝜎 is much smaller than 1, i.e., 𝑖𝜔𝜏 ≪ 1, and can be ignored. Thus,

2 𝐵 •••••••••••••••••••••••••••••••••••••• (3) 𝐵(𝑟) = 𝐵 0 𝑒𝑥𝑝 (- 𝑅-𝑟 𝜆 𝑠 ) •••••••••••••••••••••••••••••••••••••••••••••••••••••• (4)
where 𝜆 𝑠 = √ 𝑚 𝜇𝑛𝑒 2 is the penetration depth, 𝑟 is the distance from the center of the QD, 𝑅 is the radius, 𝜇 is the permeability (NA -2 ), 𝑛 is the density of the electrons, and 𝑚 is the effective mass of electrons. Since this magnetic field penetration decays exponentially, CD injection must also be performed. Given the speed of light 𝑐, the intensity of this injected electromagnetic wave injected by the chiral ligand is calculated as:

𝐼 = 𝐵 2 𝑐 2𝜇 = 𝐵 0 2 𝑐 2𝜇 𝑒𝑥𝑝 (-2 𝑅-𝑟 𝜆 𝑠 ) ••••••••••••••••••••••••••••••••••••••••••••••• (5)
As for the spherical case, the calculation of the chiral dichroism considers both the decay of the light intensity along the incident direction and its interaction with the electron flux above.

Thus,

𝐶𝐷 𝑠𝑝ℎ𝑒𝑟𝑒 (𝜔, 𝑅) ∝ ∫ ∫ 2𝜋𝑟𝑒 -𝑥 𝜆 𝑒 -2 𝑅-𝑟 𝜆 𝑠 𝑑𝑥𝑑𝑟 2√𝑅 2 -𝑟 2 0 𝑅 0 ••••••••••••••••••••••••••••• (6)
where 𝜆 is the wavelength of the incident light, 𝑥 is the penetration direction of the circularly polarized light, 𝑟 is the radius of the circle at which the light is incident, 2𝜋𝑟 is the multiplicity of that circle, and the range 2√𝑅 2 -𝑟 2 denotes the depth of 𝑥 incident on 𝑟. Similarly, the CD should be proportional to this integral.

In computing such a model, an important factor is the size dependence (length, shellthickness) of the imaginary part of the dielectric constant 𝜖 2 , which relates 𝜆 (when 𝜔𝜏 ≪ 1) to the penetration depth by 𝑛 𝑟𝑒𝑎𝑙 ≈ 𝑛 𝑖𝑚𝑎𝑔 = √𝜖 2 2 ⁄ and 𝜆 = (𝑛 𝑖𝑚𝑎𝑔 𝜔 𝑐 ⁄ ) -1 . The size dependence was taken to be roughly 𝜖 2 = 𝜖 2 0 𝑒𝑥𝑝(𝑟/𝑟 𝑐 ) 1+𝑒𝑥𝑝(𝑟/𝑟 𝑐 )

, where 𝑟 𝑐 is a numerical critical size that governs the approximating property of 𝜖 2 . This size dependence of 𝜖 2 is valid since the 𝜖 2 will converge to the bulk value as the size grows to weaken the quantum confinement, which agrees with this. In this way, the calculated results of cysteine-capped CdSe/CdS nanostructures For the tadpole, similarly, since there is no difference in 𝑛 along the 𝑧 direction, it is simpler than the spherical case:

𝐶𝐷 𝑡𝑎𝑑𝑝𝑜𝑙𝑒 (𝜔, 𝑙) ∝ ∫ 𝑒 -𝑧 𝜆 𝑑𝑧 𝑙 0 ••••••••••••••••••••••••••••••••••••••••••••••• (7)
where 𝜆 ≈ 𝑐 𝑙𝑖𝑔ℎ𝑡

2𝜅𝜔

, 𝜅 is the imaginary part of the complex refractive index, 𝑅 is the radius of the tadpole head, and 𝑙 is the length of the tadpole tail. A further assumption made is that 𝜅 ≈ √ 𝜖 2 and that 𝜖 2 increases first with the length of the tadpole tail and then converges to the bulk value of the material. As shown in Figure 25 b, the g-factor also fit well with our experiments results.

The quantum yield

In calculating the quantum yield of the rod-like tadpoles, we apply the electronic transition theory of excitons of Förster, the FRET (Förster resonance energy transfer) (Figure 26). Förster talked about the long-range excitation transfer in detail in one of his publications. 62 It was said that the transition was allowed even for weak interactions between distant atoms or molecules.

Due to the electronic nature of the interaction, his analysis on molecules applies also on the atomic cases, for example the long-range excitation in tadpoles, for they are in nature electric dipole-dipole interaction. During the absorption process, the sensitizer is excited to a high vibrational level of its first electronic excitation state. From there it is converted to lower vibrational levels of the same electronic state obtaining a thermal equilibrium with the surrounding medium. Suppose that the energy difference for one of these possible deactivating processes in the sensitizer corresponds exactly to that for a possible absorption transition in a nearby acceptor, then with sufficient energetic coupling between these molecules, both processes may occur simultaneously, resulting in a transfer of excitation from the sensitizer to the acceptor. Naturally, the interaction energy is of a dipole-dipole nature, depending on an inverse proportionality to the third power of the molecular distance. The probability of energy transfer is then proportional to the square of this interaction energy and decreases, therefore, with the sixth power of the distance. 63 Chapter 6:

Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 288

References page 294

According to basic theories, 64 the interaction between dipoles is inverse proportional to the third power of distance. And according to Förster, 63 the mean radiation duration can be expressed as:

𝑡 0 ∝ ℏ 𝑈 ∝ ℏ𝑅 3 𝑀 2 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (8)
where 𝑈 is the energy of the dipole interaction and 𝑀 is the intensity of the dipole moments, and 𝑅 is the distance between the interacting dipoles. Another factor concerning the probability of transition is the comparison between the dipole-dipole interaction and the oscillator between them, which makes the total probability

𝑤 ∝ 𝑈 ℏ 1 Ω 1 𝑡 0 ∝ 𝑈 2 ∝ 𝑅 -6 ••••••••••••••••••••••••••••••••••••••••••••••••••• (9)
Thus,

𝑛 𝑆 * →𝐴 * = 9𝑙𝑛10𝜅 2 128𝜋 6 𝑛 4 𝑁 𝐴 𝜏 𝑠 0 𝑅 6 ∫ 𝑓 𝑠 (𝜈)𝜖 𝐴 (𝜈)𝑑𝜈 𝜈 4 ∞ 0 ••••••••••••••••••••••••••••••••••••• (10)
where 𝜈 is the wavenumber, 𝜖 is the decadic extinction coefficient, 𝑓 the spectral distribution of fluorescence, 𝑁 the Avogadro's number and 𝜏 𝑠 0 the intrinsic or radiative lifetime of the excited sensitizer, 𝑛 the refractive index of the surroundings, 𝑅 the mutual distance and 𝜅 the orientation factor. More specifically,

𝜅 = 𝑐𝑜𝑠𝜙 𝑆𝐴 -3𝑐𝑜𝑠𝜙 𝑆 • 𝑐𝑜𝑠𝜙 𝐴 ••••••••••••••••••••••••••••••••••••••••••• (11)
where 𝜙 𝑆𝐴 is the angle between the transition moment vectors and 𝜙 𝑆 , 𝜙 𝐴 are the angles between the respective transition moment and the direction 𝑆 → 𝐴. The average value for a random directional distribution 65 is 𝜅 2 = 2/3.

Rewrite the equation 10 into where 𝜏 𝑆 is the actual mean lifetime of the excited sensitizer, which is connected to the quantum yield 𝜙 𝐷 by:

𝑛 𝑆 * →𝐴 * = 1 𝜏 𝑆 ( 𝑅 0 𝑅 ) 6 ••••••••••••••••••••••••••••••••••••••••••••••••••••• (12)
𝜏 𝑆 = 𝜙 𝐷 𝜏 𝑆 0 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (13)
Then 𝑅 0 is the critical transfer distance for which the excitation transfer and spontaneous deactivation are of equal probability, and it can be expressed by: 66

𝑅 0 6 = 9𝑙𝑛10 128𝜋 5 𝑁 𝐴 𝜅 2 𝜙 𝐷 𝑛 4 𝐽 = ( 78×10 -3 𝜋 ) 6 𝜅 2 𝜙 𝐷 𝐽 𝑛 4 ••••••••••••••••••••••••••••••••••• (14)
where the overlapping integral was written as 𝐽, and the 𝑅 is written as 𝑅 𝐷𝐴 , 67 denoting the distance between the donor and acceptor.

Till now, the model has involved the quantum yield but not the diffusion length of the excitons, which can be introduced via the diffusion coefficient through the Smoluchowski-Einstein mechanism of random walk by 68

𝐷 =

(𝐿 𝐷 𝑆 )

2 𝜏 𝑆 = 𝑅 𝐷𝐴 2 6𝜏 𝐻 𝑆 ••••••••••••••••••••••••••••••••••••••••••••••••••••••• (15)
Thus, the Förster transfer rate and the diffusion length can be related by

𝑛 𝑆 * →𝐴 * = 1 𝜏 𝐻 𝑆 = 6(𝐿 𝐷 𝑆 ) 2 𝜏 𝑆 𝑅 𝐷𝐴 ••••••••••••••••••••••••••••••••••••••••••••••••••• (16) Since 𝑛 𝑆 * →𝐴 * = 1 𝜏 𝑆 ( 78×10 -3 𝜋 ) 6 𝜅 2 𝜙 𝐷 𝐽 𝑛 4 𝑅 𝐷𝐴 6 •••••••••••••••••••••••••••••••••••••••••••• (17)
we now have As expected, the calculation demonstrates the AR effect on the g-factor and QY in tadpoles (Figure 25 b), however, it should be noted that when Tadpole-I grows even longer, the interparticle interactions should be taken into account as well.

𝜙 𝐷 = 6𝜋 2 𝑅 𝐷𝐴 4 (𝐿 𝐷 𝑆 ) 2 48×10 -6 𝑛 4 𝜅 2 𝐽 ••••••••••••••••••••••••••••••••••••••••••••••••••• (18)
6.5 Methods

Chemicals

Tri-n-octylphosphine oxide (TOPO, 99%), tri-n-octylphosphine (TOP, 97%), tributyl phosphine (TBP, 99%), n-octadecyl phosphonic acid (ODPA, 97%) and n-hexyl phosphonic acid (HPA, 97%) were purchased from Strem Chemicals. Cadmium oxide (CdO, 99.99%), sulfur (S, 99.98%) and selenium (Se, 99.99%) were purchased from Sigma-Aldrich. Oleic acid (OA, AR), Lcysteine hydrochloride monohydrate (L-Cys, 99%), D-cysteine hydrochloride monohydrate (D-Cys, 98%) and tetramethyl ammonium hydroxide solution (TMAH, AR, 25 wt. % in H2O) were purchased from Aladdin Industrial Co., Ltd. (Shanghai, China). Deionized (DI) water was purchased from Hangzhou Wahaha Group Co., Ltd, China. All chemicals were used as received without further purification.

Synthesis of the CdSe core

The synthesis procedure was based on the procedure reported in the literature. 3,69 Typically, TOPO (1.5 g), ODPA (0.140 g) and CdO (0.030 g) were mixed in a 50 mL flask, heated to 150 °C and alternately exposed to vacuum conditions and an argon atmosphere at least 5 times until CdO was a brown solid and the remaining reagents were a colorless liquid. Then, to dissolve the CdO, the solution was heated to above 320 °C under argon until it became optically clear and colorless, which indicated that the reaction between CdO and ODPA was complete. Then, the temperature was increased to 350 °C, and 1.5 mL of TOP was injected into the flask, which caused the temperature to naturally decrease to 300 °C. Then, a Se:TOP solution (0.4 mL, 1 mol/L) was injected into the flask at 380 °C and was allowed to react for several seconds. The size of the CdSe core could be adjusted by altering the temperature and reaction time.

Purification and acidification of the CdSe-TOP core

The reaction mixture was cooled to 70~80 °C, and a certain amount of TBP and 4 mL of ethanol were added into the above solution for the purification process; the mixture was centrifuged at 10000 rpm for 3 min. Then, the precipitate was dissolved in a small amount of toluene, and the supernatant was discarded. After centrifugation was performed again, the precipitate was redispersed in TOP (CdSe-TOP). The cysteine ligand exchange reaction was carried out using a previously reported method 21 with some modifications. Cysteine hydrochloride monohydrate (702.52 mg) was dissolved in DI water (40 mL, [Cys] = 0.1 M). The pH of the resulting solution was adjusted to 12.0 with TMAH solution (AR, 25 wt. % in H2O). A solution of CdSe/CdS NCs in n-hexane (5 mL, 1× 10 -6 M) was added to the cysteine solution (5 mL), and the reaction mixture was deoxygenated and stirred at room temperature under argon in the absence of light for 72 h. The reaction mixture was left to stand for 1 h to allow the phases to separate. The bottom aqueous layer was taken with a syringe, and the Cys-CdSe/CdS NCs were purified 5 times with DI H2O by using ultra centrifugal filter units (Millipore, 10,000 d, 15 mL). The purified Cys-NCs were diluted and dispersed in DI water, and stored at room temperature in the dark.

Conclusion

In conclusion, cysteine-capped chiral asymmetric CdSe/CdS nanostructures with tunable morphologies were synthesized using the shape-directing ligands. The measurements of induced chiroptical activities of these nanostructures showed that:

(i) the induced CD intensity of the CdSe core existed only in the short DR sample, which is due to the barrier effect of the CdS shell (> 1.0 nm);

(ii) the symmetry breaking effect also accounted for the CD responses, and asymmetric samples with larger ARs or longer lengths would be beneficial for the CD activities;

(iii) the step-by-step growth mechanism of Tadpole-I was investigated, confirming that increasing the tail length enhanced the CD intensity; however, further growth of the tail (iv) CPL measurements were applied in parallel, showing that the chiral-ligandphotoluminescent-core interactions were the key indicator for CPL performance. In general, asymmetric samples with thin CdS shells and a high PL QY were the most efficient in terms of CPL activities. However, a sample with the highest CD performance does not necessarily also exhibit an efficient CPL response.

These findings prove that induced CD and CPL can serve as indicators of the extent to which a chiral ligand can affect the hole wave function through an energy barrier such as a CdS shell with different morphologies. 59 We expect these observations to provide experimental support to manipulate the chirality of CdSe/CdS-based nanostructures with potential uses in stereoselective catalysis, chiroptical sensing and chirality-based devices for both optics and biological applications.

Reference page 316

In this chapter, we will describe three different setups built and used to perform the photochemical deposition of the metallic dots onto the CdSe/CdS dot-in-rods (DRs)

semiconductor nanoparticles. 1) The first one uses a linear polarized continuous blue laser with a waist large enough to correctly illuminate a 1 cm wide quartz cuvette, later called "cuvette setup using blue laser", 2) the second one uses the same blue laser set up with an additional λ/4 plate to get circularly polarized light, either left or right, called later on "cuvette setup using blue CPL laser" and 3) finally we will use a linearly polarized continuous UV laser with a waist large enough to correctly illuminate a 1 cm wide quartz cuvette, later called "cuvette setup using UV laser". The setups either use different laser sources (blue or UV laser)

or variable blue L-/D-circularly polarized light (CPL). The results obtained from cuvette setups using either blue/UV laser or CPL laser will be presented and discussed.

Cuvette setup using large blue laser exposure

The setup includes a blue laser as the light source. The exposure area was designed to be around 1 cm in diameter to fill a 1 cm wide cuvette. The setup also enables magnetic stirring inside the vessel, making sure that photochemical reactions are performed in homogeneous conditions.

Schematic picture of the blue cuvette setup

A schematic drawing and a photograph of the cuvette setup using the blue laser are shown in Figure 1. After coming out from the laser head, the beam is reflected vertically by a first mirror and then reflected by another mirror at the same height as the reaction cuvette. Two

Lenses then enlarge the laser beam and then a third mirror collimates the laser light onto the cuvette with a laser spot of 1 cm in diameter. The cuvette is placed on a magnetic stirring machine to continuously homogenize the particle solution and prevent sedimentation; the stirring speed was set at 300 rpm unless otherwise stated. All experiments were performed under room temperature. Here we use a MLL-N-473nm-300mW-19012905 (PO#G1900567) blue laser from the CNI company. The wavelength of the used laser is 472.65 nm, and the beam power range varies from 0 to 389 mW when varying the current, with a maximum current achievable of 5.0 A.

Figure 2 shows the linear relationship between exit power and current in the only accessible current control mode. 𝑃 𝐻𝑒𝑎𝑑 = 120.9 × 𝐼 -238.0

•••••••••••••••••••••••••••••••••••••••••••••••••••• (1)

Beam power transmission by the setup vs current

For the reaction vessels, we mainly use Quartz cuvette (Hellma, 110-10-40) cuvette with volume 10×10×40 mm 3 , but always filled with 1 cm 3 solution corresponding to the area exposed to the blue radiation. Before arriving at the cuvette, the laser is reflected by mirrors, and passes through the lenses, so that energy losses are inevitable. A clear linear relationship between the incident to the cuvette and the driving current was measured (Figure 3). A two-parameter linear fit gives

𝑃 𝐼𝑛𝑐 = 99.3 × 𝐼 -199.0 •••••••••••••••••••••••••••••••••••••••••••••••••••••• (2)
Normally for the two sets of calibration (laser power at the exit of the laser head and incident to the cuvette), the current value at laser emission (P = 0 mW) should be the same.

However, since we perform linear fits with two unknown values, then the mean square fitting may lead to slightly different values. We got from the measurements at the exit of the laser laser: I (P=0 mW)= 1.9686 A and from the measurements just before the cuvette: I (P=0 mW)= 2.0040 A. As these values should be theoretically the same, we propose to take the mean value (I = 1.9863 A) and fit again these data by forcing this value for P=0 mw, so that the fits are of the form P=a(1.9863-I). As shown in Figure 4, the two new fits are still very good. So now we can use them in a coherent way and get a relationship between them:

𝑃 𝐼𝑛𝑐 = 0.8067𝑃 𝐻𝑒𝑎𝑑 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (3)
The transmission measurements in quartz cuvettes was also checked based on CdSe/CdS dot-in-rods 1 (DR-1, OD473 nm=0.11) (Figure 5). As shown in Figure 5, after the irradiation of the DR-1 (OD473 nm=0.11), the transmitted power shows a good linear relationship with the Incident Power.

𝑃 𝑇𝑟𝑎𝑛𝑠 = 0.7414 × 𝑃 𝐼𝑛𝑐 = 0.598𝑃 𝐻𝑒𝑎𝑑 •••••••••••••••••••••••••••••••••••••••••• (4)
The power in the middle of the cuvette can be written as:

Reference page 316

𝑃 1/2 = 𝑃 𝐼𝑛𝑐 √ 𝑃 𝑇𝑟𝑎𝑛𝑠 𝑃 𝐼𝑛𝑐 = √0.7414𝑃 𝐼𝑛𝑐 = 0.861𝑃 𝐼𝑛𝑐 •••••••••••••••••••••••••••••••••• (5) Then 𝑃 1/2 = 0.695𝑃 𝐻𝑒𝑎𝑑 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (6)
Where PHead is the power at the exit of the laser, PInc is the power incident to the cuvette, PTrans is the power transmitted by the cuvette including a DR-1 solution (OD473 nm=0.11), and P1/2 is the power in the middle of the cuvette, i.e. the power really involved in the photodeposition process.

In this way, the applied power inside the cuvette P1/2 can be adjusted just by changing the current of the blue laser.

Intensity

Although the type and power of laser and lamps used are often mentioned in experimental procedures of photodeposition, [1][2][3][4][5][6][7][8] the photon flux (in mW/cm 2 ) arriving at the semiconductor particle solution is not. This should be measured and provided, since the photon flux does primarily affect the deposition rate and the resulting size of the deposited metallic nanoparticles. Reporting of the photon flux is also essential to allow comparison of studies aimed at determining the effects of the different involved parameters on photodeposition rates.

For most of the papers which mentioned the intensity, the values vary from 1 to 3 mW/cm 2 , 9-11 and the exposure duration for the photodeposition process, usually using a UV lamp, takes hours, which may be a drawback for materials synthesis. Although large Au NPs can be deposited on one end, multiple tiny Au NPs will still form on the surface of the semiconductor rods. 10,11 In the other hand, high-energy LEDs and lasers, [1][2][3][4] which are advantageous for short-time and single-tip deposition, usually do not provide this photon flux or beam intensity information.

In order to study the law of pure single-tip deposition preventing the presence of additional tiny metal NPs, it is necessary to conduct a systematic study in light flux, especially at higher laser intensities, which is benefit to single-tip photodeposition.

Intensity in the blue CPL cuvette vs Intensity on panel

The addition of a λ/4 plate leads to extra energy losses. A clear linear relationship between the incident power and the current was obtained (Figure 7 for +45°-CPL and Figure 8 for -45°-CPL). A two-parameter linear fit gives:

𝑃 𝐼𝑛𝑐+45 = 91.2 × 𝐼 -176.8 ••••••••••••••••••••••••••••••••••••••••••••••••••• (7)
Using the same method as shown in 7.1.2, i.e. by forcing the current at laser emission at i=1.9863 A, we can get the relationship between them:

𝑃 𝐼𝑛𝑐+45 = -93.61 × (1.9863 -𝐼) = 0.7670𝑃 𝐻𝑒𝑎𝑑 •••••••••••••••••••••••••••••••• (8)
After irradiation on the DR-1 (OD473 nm=0.11), the power inside the cuvette (P1/2) is:

𝑃 1/2 = 0.861𝑃 𝐼𝑛𝑐+45 = 0.660P 𝐻𝑒𝑎𝑑 ••••••••••••••••••••••••••••••••••••••••••••• (9)
Reference page 316

The same calibration procedure is used for the angle -45° of the /4 waveplate. A two-parameter linear fit gives:

𝑃 𝐼𝑛𝑐-45 = 94.4 × 𝐼 -189.1 •••••••••••••••••••••••••••••••••••••••••••••••• (10)
And using the same method as shown in 7.1.2, we finally can get the relationship between them:

𝑃 𝐼𝑛𝑐-45 = -93.55 × (1.9863 -𝐼) = 0.7665𝑃 𝐻𝑒𝑎𝑑 ••••••••••••••••••••••••••••• (11)
This proportionality value is, as expected, very close to that obtained with a +45° rotation.

After irradiation on the DR-1 (OD473 nm=0.11), the power inside the cuvette (P1/2) is then:

𝑃 1/2 = 0.861𝑃 𝐼𝑛𝑐-45 = 0.660P 𝐻𝑒𝑎𝑑 •••••••••••••••••••••••••••••••••••••••••• (12)
Since our reaction system will be exposed to several fixed power with 1 cm beam diameter, the irradiation intensity in the middle of the cuvette is shown in Table 2. In this way, the applied power inside the cuvette can be adjusted by changing again the current of the blue laser. After coming out from the laser head, the beam is reflected by a first mirror and then elevated by two other mirrors to reach the same height as that of the reaction cuvette. The initial beam being much smaller than that of the blue laser, we used a microscope objective and a lens to enlarge and collimate the laser light onto the UV cuvette with a laser spot size of 1 cm in diameter. The cuvette is still placed on a magnetic stirring machine to continuously homogenize the particle solution and prevent sedimentation; the stirring speed is set again at 300 rpm unless otherwise stated. All experiments were performed under room temperature. Here we use a continuous Argon ion UV laser (Innova 306C, Coherent). The UV laser apparatus is shown in Figure 10. The wavelength of the UV laser is 363.8 nm, and the nominal power given by the apparatus ranges from 5 to 200 mW, with a maximum current achievable of 50 A. This laser has two driving modes, current and light, so that one can drive laser emission directly in power

Intensity calibration in the cuvette vs driving emitted power

Before arriving at the cuvette, the UV laser is reflected by mirrors, and passes through the lens and the objective, so that energy losses are inevitable. A clear linear relationship between the incident power and the panel power was obtained (Figure 11).

𝑃 𝐼𝑛𝑐 = 0.787𝑃 𝑃𝑎𝑛𝑒𝑙 •••••••••••••••••••••••••••••••••••••••••••••••••••••••• (13)
The transmission measurements in UV laser of quartz cuvettes was also checked based on CdSe/CdS dot-in-rods 1 (DR-1, OD473 nm=0.11). especially for the exploration of growth mechanism.

Most of the research concerning single-tip HNPs was based on the low temperature, 24,26 long alkyl amine ligands, 24 anaerobic conditions 23 or/and high reactivity gold precursor (AuCl3, 24 AuCl 12 ), which intend mainly to reduce defect deposition, thereby improving the ratio of single-tip growth. Moreover, the presence of a hole scavenger has been shown to plays a vital role in the photodeposition process of semiconductors (such as TiO2), [27][28][29][30] however, it has rarely been mentioned as such in the photodeposition on cadmium chalcogenide semiconductor. 19,31,32 This may be due to the fact that even without dedicated hole scavenger, the chalcogenide anions from the rods may reduce the Au 3+ accompanied by etching of the rods, 33,34 or dodecylamine (DDA) and di-dodecyl dimethyl ammonium bromide (DDAB) as the reducing agent and the catalyst help to reduce the Au 3+ . 32,34,35 Until now, there has still been no systematic research on the parameters affecting the single-tip photodeposition, especially concerning the ratio evolution of single, double and multiple sites HNPs nor on the modeling of the gold nanodot (ND) growth.

Hence, in this work, we try to fill in this gap and optimize the laser-induced deposition parameters through the systematic study of the evolution of the ratio between single, double, and multiple sites, which will provide guidance for future research. By using methanol (MeOH)

as hole scavenger, we show that it plays a major role for single-tip deposition, and provides milder reagents to be selected to improve the operability and reproducibility of experimental results. We also use the stable KAuCl4 as a gold precursor, no additional ligands injection, and deposition at room temperature, which can effectively improve operability and reproducibility.

These results are at variance with reported data in the absence of MeOH, 20, 24-26, 31, 33, 35-37 where the deposition results varied widely, and where the true effect of the parameters (such as light intensity, Au/DRs ratio) were not identified either. This is probably why the current reported results up to now vary so widely from one experiment to another (see Table 2 in Part 1).

Additionally, we address the effect of a tiny hole scavenger on the photodeposition process,

Discussion

A growth prediction model for Au photodeposition on CdSe/CdS DRs

In order to better understand the significance of our work, using a focused laser instead of a lamp, and make a guide for straightforward understanding of the current progress in this field, we compared our results with other related work (Table 2 of Part 1). We mainly summarized the current metal deposition on cadmium chalcogenide semiconductors (Cadmium-based II-VI semiconductor NCs), especially for in-depth analysis of the most studied gold deposition, which provides nanoobjects with a great potential for high performance catalysis. As shown in this table, we examined different effects: the reaction temperature, the ligands, the light induced deposition, the reaction time, the type of semiconductors, and the type of gold precursors. The results comparisons are based on the deposition sites, such as single-tip, double-tip, multi-sites, and so on. The most effective methods now tend to lower the deposition temperature, 20,24,26,31,36,37 shorten reaction time, 12,20 and use longer alkyl amine ligands. 20,24,36 Laser or lamp-assisted photodeposition did also help in getting single-tip deposition more selectively. Although O'Sullivan et al.

reported that the more active AuCl could achieve single-tip deposition more easily than AuCl3 and in shorter time (< 120 s), 12 it appears that its uncontrollable double-edged sword-"high reactivity", leads to simultaneous multi-site deposition of tiny Au NPs deposition on the semiconductor DRs surface. Based on this literature, it appears that most of the papers only report the results of single-tip deposition with a high probability, and there is still no literature reporting on how to control accurately the deposition ratio of different sites, that is, how to Chapter 8

Hole scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 324 References page 364 achieve true single-tip deposition in a statistical sense. Additionally, although Banin's group has done some work related to the laser or lamp induced photodeposition, 20,24,33,36 the study of effects of the laser Intensity, especially the influence of the flux of photons directly on the sample for deposition, has still never been investigated. Therefore, in this work, we focus on the study of the ratio variation of single-tip HNPs. To do this, we chose the lower activity gold precursor, KAuCl4, 35,38 which can improve the operability and repeatability of the experimental results.

To perform such an investigation, we first summarize briefly the main steps of a general model used to analyze the metallic growth on the semiconductor nanoparticles photo-excited in their bandgap. 39 Upon light absorption within the band gap of the used DRs, electron-hole pairs are produced in their volume and at their surface at a rate g, which is proportional to the optical absorption  of the solution at the used wavelength, and to the light intensity. The electrons, of concentration [e], either recombine with the holes with a rate kr, are trapped on defects (we neglect this trapping effect due to the large amount of electrons and holes produced using a focused laser), or reduce the Au 3+ ions adsorbed at the surface at an effective rate ke. In an analogous way, holes, of concentration [h], may recombine with electrons, be trapped on defects (effect that is still neglected due to the large amount of electrons and holes produced) or oxidize the hole scavenger HS (MeOH in experiments) adsorbed at the surface of the DRs at a rate kh. These different stages are summarized by eq. 1:

{ [DR + ℎ𝜈 𝑔𝑘 𝑟 ⇇ DR (𝑒 -, ℎ + )] bulk , [HS + ℎ + 𝑘 ℎ → HS + ] interface , [Au 3+ + 3𝑒 -𝑘 𝑒 → Au 0 ] interface , (eq. 1)
We have to make some assumptions to solve these equations. First, since the diffusion time of the carriers inside the DRs is typically in the picosecond range, we assume that the electrons and holes are rapidly distributed homogeneously inside and over the DRs. We can as well consider that the concentrations of the adsorbed

{ 𝑑[𝑒] 𝑑𝑡 = {𝑔 -𝑘 𝑟 [𝑒][ℎ]} bulk -{𝑘′ 𝑒 [𝑒]} ads , 𝑑[ℎ] 𝑑𝑡 = {𝑔 -𝑘 𝑟 [𝑒][ℎ]} bulk -{𝑘′ ℎ [ℎ]} ads , 𝑑[HS + ] 𝑑𝑡 = {𝑘′ ℎ [ℎ]} ads , 𝑑[Au 0 ] 𝑑𝑡 = {𝑘′ 𝑒 [𝑒]} ads .
(eq. 3a)

(eq. 3b)

(eq. 3c)

(eq. 3d)

Note that the diffusion of Au 0 atoms at the surface of the DRs has been neglected in eq. 3d since a calculation shows that it is negligible compared to the reaction term {𝑘′ ℎ [ℎ]} ads . Since we expect to investigate growth mechanisms at long exposure time, we assume that the number of electron-hole pairs is preserved during the entire laser-deposition process (

𝑑[𝑒] 𝑑𝑡 = 𝑑[ℎ] 𝑑𝑡
). Using the above assumptions, the electron concentration is given by:

𝑑[𝑒] 𝑑𝑡 = 𝑔 -𝑘′ 𝑒 [𝑒] ( 𝑘 𝑟 𝑘′ ℎ [𝑒] + 1) (eq. 4)
The solution of eq. 4 shows that two main behaviors can be devised depending on the value of the term ( The calculation of the growth law of the gold nanodot is performed using an approach analogous to that used for dew growth on partially wetting solid substrates. 40 By equating the variation of the volume of the growing metallic dot and the flux of produced [Au 0 ] at its perimeter to fulfil mass conservation, we finally find for the late stage growth:

𝑅 3 (𝑡) -𝑅 𝐶 3 = 3 𝜅 𝑁 𝐴 𝑣 Au 𝑁 [Au 0 ](𝑡) (eq. 5)
where NA is the Avogadro's number, νau is the volume of a gold atom, N is the number of DRs per liter and  is a dimensionless factor related to the contact angle of the gold nanodots photodeposited onto the DR surface. 𝑅 𝐶 = 𝑅(𝑡 = 0) is a critical radius and [Au 0 ](𝑡 = 0) = [𝑒](𝑡 = 0) = 0, t = 0 s is the time at which the laser exposure starts. Then, the late stage growth of a gold nanodot induced by a focused laser (so-called high field regime above) is described by: 𝐷 3 (𝑡) -𝐷 𝐶 3 = ( where we used the diameter D = 2R in agreement with measurements. This modeling predicts that the diameter D of the gold nanodot should (i) behave in time as t 1/3 , (ii) vary in beam intensity as I 1/6 , and (iii) as [Au 3+ ]ads 1/6 and [HS]ads 1/6 in concentrations. These growth predictions will be confronted to experiments in the following paragraphs.

Choice of the DRs for the laser deposition

The DRs with different shapes were made by the method we described in Chapter 2 by varying the synthesis parameters. CdSe cores were used as starting materials for all the experiments and other parameters such as injection temperature, reaction time, and core concentration were used for the different DRs. Briefly, DR-1 were prepared using 3.9 nm CdSe cores and a growth at 350 o C, DR-2 to DR-5 were made using 3.2 nm CdSe cores and a growth at different injection temperatures (from 320 to 380 o C), DR-6 to DR-9 were synthesized using

References page 364

We also studied the case when no DRs samples were added. Under laser excitation even for exposure times as long as 32 min coupled with doubled gold precursors, there are only very few small size Au NDs not even detectable by UV-vis. spectroscopy, which means that DDA modified KAuCl4 in toluene is stable enough for our experiments and will not affect the analysis of the experimental results.

To summarize it appears that in our experimental conditions, the results are not exactly as in the literature. Indeed, Banin and coworkers showed that low temperature and long-chain organic ligands were the key factors for HNPs with a single gold tip, 24 while, the tiny Au NDs were still difficult to prevent. In our case, MeOH and the laser are the main factors for single gold tip. However, it has to be mentioned that in very few cases, the hole scavenger is mentioned as such and identified as a short chain alcohol; 19,41,42 most of the time it is never mentioned and TEM images show a QDs and DRs corrosion taking place at the same time as the gold deposition. Carbone, et al. 19 presented that the alcohol mainly effected the uniformity and size of Au NDs, while in our results, the deposition yield also changed greatly.

In this work, in the absence of identified hole scavenger, the ratio of single-tip is limited to 42.1%; when coupling with the absence of laser, it goes down to 11.7%. The multi-site samples ratio increases to 75.1%; and if both laser and MeOH are absent, the non-Au deposition ratio doed maintain at a high level (62.34%).

Role of the different parameters

The influence of the different parameters will be divided into two major aspects: the influence on the distribution of the HNPs in between mono-bi-& multi-sites DRs, and the nucleation and growth process of the metallic dots.

Hole scavenger effect on Au photodeposition

According to the previous paragraph, the hole scavenging seems to play a vital role in the photodeposition process. If efficient enough, it consumes the holes generated on the CdSe/CdS DRs by the blue laser excitation fitting the section of the cuvette. As the reaction takes place in an oil phase (toluene), the hole scavenger has to be miscible/dissolvable in toluene solution. In our study, MeOH is chosen to play this role, since its scavenging efficiency in the presence of TiO2 under UV activation is widely demonstrated, 39,43 motivating us to We also studied the influence of the concentration of the hole scavenger on the degree of etching of the DRs (Figure 6) and showed its positive role with regard to the integrity of the DRs. It is further confirmed that the hole scavenger can reduce the etching of the chalcogenide anions on the surface of DRs, thereby reducing the number of defect sites for deposition and increasing the single-tip deposition ratio. The size of the Au NDs increased by 2 nm (from C1 to S1) just after the addition of MeOH, which means the hole scavenger does also increase the deposition rate; 19 we shall come back to this later on. Note that the ratio of raw DRs is maintained almost the same. It then appears that the hole scavenger, scarcely mentioned in photodeposition processes on cadmium chalcogenide in the literature, is the most reliable choice for a single-tip deposition with a high yield in CdSe/CdS DRs and may also be the reason why the results reported in the literature have been so vastly different.

Laser intensity effect on Au photodeposition

Furthermore, in literature data, the information about the photon intensity applied to the mixture is often missing, while it is an essential parameter, as demonstrated in this work. To study the effect of illumination intensity on the ratio of single-tip photodeposition process; the gold photodeposition experiments are performed with the cuvette setup using a blue laser.

In these experiments, the MeOH concentration as a hole scavenger is fixed at 2.5 M, the 4, Figure 8). The ratio of singletip deposition increases as the laser intensity increases (Figure 9), and changes sharply from Indeed, the photodeposition using lamps (or LEDs), of typically a ten of mW/cm 2 , is known to lead to the deposition of multidots; this behavior is retrieved here at low laser intensity. 39 Conversely, we find single dot deposition at high intensity. By plotting the corresponding slopes in Figure 9, we find a crossover between the low and the high field regimes raised by our modeling at 24 mW (exit laser) leading to an intensity inside the sample of 16.68 mW (PInc 24.65 mW/cm 2 ) (see Chapter 7 for the calculation). This value is in the right order of magnitude of the largest intensities used with lamps (or LEDs). Figure 9 thus offers an illustration of the transition from the low to the high field regimes in photodeposition. As a final remark, these results are consistent with previous report on TiO2 from the group, 39 which showed that high laser intensity can effectively reduce multi-site deposition, which is

Time effect on Au photodeposition

As shown in the different references, the reaction time also plays an important role in controlling the different sites depositions. However there are several different results: 1)

Pavlopoulos et al. 31 showed that multiple sites were obtained at short times (< 30 s), and ripening of small Au NPs occurred toward a single one over time (> 2.5 min); 2) O'Sullivan et al. 12 showed that it was easy to achieve single-tip HNPs in a short time (< 2 min); 3) Banin's group 20,24,36 found that single-tip HNPs could be obtained under much longer deposition times (> 30 min) at low temperature (< 277 K). These different results make the underlying mechanism even more unclear.

In order to try to clarify this effect of time in the case of the laser photodeposition we compared the deposition sites evolution with and without a hole scavenger (Figure 11 and Table 5, Table 6). As shown in Figure 11, in the presence of the hole scavenger, the ratio of single-tip deposition HNPs can be maintained at about 80% with time, and multi-site deposition can be maintained below 1%. Conversely, if there is no added hole scavenger, as the time increases, the ratio of single-tip deposition significantly decreases (from 44.9% to 25.4%), while at the same time the multi-site deposition increases sharply from 1.6% to 50.3%. Consequently, the presence of hole scavenger is absolutely required to obtain single-tip HNPs. Note that, in the absence of the hole scavenger, the disparity is also illustrated in our experimental results, which become more complicated to interpret. A single-tip HNP is mainly obtained at short time, and as the time evolves, it becomes progressively multi-sites deposition probably because of the increasing attack and corrosion of the DR itself.

We propose the following point of view: the hole scavenger (here MeOH) greatly slows down the double & multi-sites deposition, and especially prevents the multi-sites HNPs at the end of the procedure, thereby restoring the real impact of the parameters.

In order to fully study the effect of irradiation time on the laser deposition, especially the selection of the conditions to selectively achieve single-tip HNPs, we studied the evolution of photodeposition in the presence of a hole scavenger over a period of time as long as 64 min.

As shown in Figure 12 and Figure 13, the ratio of single-tip HNPs can reach up to 87% after 8 min, and then decrease a little for a longer irradiation time. This could result from the relationship between DRs etching process and irradiation time (Figure 14), as on the one hand for the reaction time less than 8 min, there is little change in the length of DRs and the etching ratio can be maintained to less than 2% and on the other hand, for increasing time (> 8min), the DRs are etched to shorter length and the etching ratio changes by more than 6.5%. Therefore, even in the presence of a hole scavenger, when the irradiation time is too long, the etching of the chalcogenide anions on the surface of DRs does take place and the ideal timing for these conditions of power and concentrations seems to be around 8 min value which will be confirmed later on with the model. The diameter of the single-tip Au NP gradually increases with time (from 4.5 to 6.9 nm), and then seems to decrease for longer irradiation time (Figure 13 a). This later phenomenon may be due to the large lattice dislocation between CdS and Au (> 30%), 44 Au NPs may fall off from the nanorods under long-term laser irradiation, 45 thereby reducing the single-tip ratio of Au NDs on the surface of the CdSe/CdS DRs and generating free Au NPs. This is observed on the TEM image (Figure 15), where few free Au NPs are retrieved on the TEM grid. We also noted that they increased in number after 32 min of laser exposure; this was also comforted by the fact that the contact angle of the gold dot onto the DRs is close to 180°, see 7 shows the TEM images, the diameter variation of Au NDs in and the respective distributions of the resulting Au-CdSe/CdS HNPs obtained at Au 3+ /DRs ratios varying from 1,000 to 128,000. As the concentration of the Au 3+ precursor increases from 2.5 × 10 -6 M to 400 × 10 -6 M, the single-tip yield of the Au-CdSe/CdS HNPs greatly increases from 2.1% to 87%, and then even up to 94%. As shown in Figure 16, Figure 17 and Figure 18, we can infer that:

i. the diameter increases with the Au/DRs ratio (from 2.3 nm to 10.2 nm, and a high ratio of single-tip HNPs can be obtained from 6 nm to 10.2 nm),

ii. the ratio of single-tip HNPs increases with the Au/DRs ratio, and then maintains at high level (~ 90%),

iii. the non-Au deposition ratio greatly decreases accordingly with an Au/DRs ratio increase, iv. the double & multi-sites can be maintained constant at a low level. Curves are guides for the eyes.

The relationship between the etching degree of the DRs and the concentration of Au precursor in Figure 19, shows the DRs etching ratio can be maintained at less than 2% as long as the concentration of the Au 3+ precursor is smaller than 40 × 10 -6 M, it then gradually increases. This means that even if the hole scavenger can significantly decrease the multi-site deposition process, a large amount of Au precursor injected in a single time can in parallel on the DRs volume even if in this case it is rather small (< 10%). Curves are guides for the eyes. We also varied the ratio of Au/DRs by changing the concentration of CdSe/CdS DRs, and we got the same trend in terms of gold NDs size (Table 8, Figure 21). Indeed, according to our model, an increase of the DR concentration increases the absorption of the solution and thus

Does the gold nucleation and growth model work?

By testing all these parameters, we gathered some experimental data which besides the variation of the ratio between the mono-bi-and multi-sites NHPs, seemed to reveal that the diameter of the grown gold NDs also depends on the MeOH & gold concentrations, the exposure time, and the laser intensity. We therefore try to interpret our data according to the model previously described and successfully used for the growth of metal NDs on TiO2 semiconductor, 39 the goal being to rise some new insights in the generation of NHPs and to see if it is possible to generalize our theoretical description to the case of DRs.

Growth law and Assumptions

Growth law with [MeOH] Based on some assumptions 39 the growth of a gold nanodot for a pseudo-first order reaction onto a DR is expected to behave as (eq. 7):

𝐷 3 [HS] bulk ) , (eq. 8)

In eq. 7, let remind that g corresponds to the number of electron-hole pairs produced by the laser excitation, and is proportional to I, where  is the absorption of the solution and I the beam intensity inside the solution. This representation is presented in linear (Figure 24) and in log-log (Figure 25) coordinates for the whole data set, that is variations in time, intensity, concentrations in MeOH and gold ions, when keeping [DR]= 2.5 × 10 -9 M. Two main behaviors emerge from this data set:

i. Whatever the variable (intensity, gold, MeOH and time (provided t < 8 min), the results merge on a single curve in a highly coherent way with a 1/3 behavior (a free exponent power law gives 0.28 instead of 0.33). This rationalization shows how experiments performed in so different conditions can be interpreted according to a general scheme and then may offer a predictive route to synthetize HNPs with a targeted size and a targeted morphology.

ii. If we look closer at the data in exposure time (cyan dots), they follow the predicted behavior of eq. 9 up to 8 min of exposure, but then seem to undergo a saturation process (already suggested by the variation in power when looking in detail (Figure 26). This seems to suggest that a significant consumption of gold ions took place in these specific conditions leading to a completion of the growth. Using an archetypal behavior in D = a[1 -e - t/t* ] 1/3 to mimic the dynamics towards completion (refer to dashed line on Figure 27), we can see that for the given experimental conditions a completion time is found around 500 s. To at completion, which seems to be verified in Figure 28 (correlation 0.9). In conclusion, it seems that all the data can be quite reasonably interpreted using our (3) to go to completion. A calculation in Appendix 2, nevertheless shows that completion is reached when half of the initial gold ion concentration is consumed.

What about the role of pre-existing ligands on the DRs?

In order to further verify the role of the pre-existing ligands at the surface of the DRs, noctadecyl phosphonic acid (ODPA) and n-hexyl phosphonic acid (HPA), we let the samples in solution for 14 days before performing any photodeposition to let a balance settle between the surface and the solution and decrease the number of surface ligands of the CdSe/CdS DRs.

We then study the influence of the laser intensity, the time and the concentration of gold precursor. = 0 M for 8 min and various intensitys. As shown in Table 9 and Figure 29, this reaction is different from the evolution process with a hole scavenger, the ratio of multi-sites deposition does not decrease but on the opposite increased greatly as the laser intensity increased, as for the ratio of double sites, while the ratio of single-tip deposition increased first (PHead< 50 mW) and then decreased. This behavior is much less coherent than in the presence of the HS. This is very different from the deposition process with a hole scavenger seen above and the work done based on TiO2 (the multi-sites decreased as the intensity increased), 39 and this is certainly due to the lack of protection initially provided by the ligands and the propensity to higher defects rate. Although the high intensity is supposed to increase the ratio of single-tip deposition, the reactivity of the gold precursor also increases with the illumination intensity, which increases the double-sites & multi-sites deposition, so that the ratio of single-tip HNPs is first increased but then decreases.

Time effect on Au photodeposition

The effect of time on photodeposition on DR-1 is described in Table 10, Figure 30 and 3), and goes on to decrease by 17 % at 64 min. This is consistent with reports from the literature, 33,34 which already mentioned this drawback: in the absence of hole scavengers, the quantum rods are gradually etched as the deposition proceeds. On the contrary, the etching can be effectively reduced by adding hole scavenger. As reported in the literature, 19 no significant etching is observed when the starting sample is dispersed in a mixture of EtOH:CHCl3=10:90 (v:v) ratio, which is similar to our results at short irradiation time (Figure 14). We also studied the influence of the concentration of Au precursor on the etching degree of the DRs with more defects DRs (Figure 34), even with only 10 × 10 -6 M Au precursor, the quantum rod etching did exceed 3%. This is different from the case with the hole scavenger presented in Figure 19, where the same degree of etching required 80 × 10 -6 M Au 3+ precursor. Comparing the different samples, we can conclude that the prevention of deposition on defects is a key point for the process to improve the single-tip Au-DRs synthesis, especially to get very high yields of single-tip HNPs.

There are two main ways: 1) directly decrease the defects on the surface of the DRs, taking We then focused on the Au photodeposition experiments to study the effect of the DRs morphology on the photodeposition process; DR-2 to DR-5 from Table 1 were used for this study. We use the optimal conditions to try to deposit on different quantum rod surfaces. In these experiments, MeOH (2.5 M) was used as a hole scavenger, CdSe/CdS as substrate (optical density (OD) of 0.11 at wavelength 473 nm), the power (200 mW at the laser exit).

The effect of the DRs shell thickness

As shown in Table 12, Figure 35 and Figure 36, when the shell-thickness is less than 10 monolayers (C21 & C22) on a 3.2 nm core, the double-& multi-sites can be maintained at a low ratio, which is beneficial to improve the ratio of single-tip HNPs. However, they increase significantly, for a shell thickness larger than 10 monolayers (C23 & C24). This is something we still do not understand and which is still under investigation.

Synthesis of spherical CdSe seeds with different diameters

The synthetic procedure was based on the procedure reported in the literature. 46,47 Typically, TOPO (1.5 g), ODPA (0.140 g) and CdO (0.030 g) were mixed in a 50 mL flask, heated to 150 °C and alternately exposed to vacuum and argon at least 5 times until CdO was a brown solid and the rest of the reagents were a colorless liquid. Then, to dissolve the CdO, the solution was heated to above 320 °C under argon until it became optically clear and colorless, which indicated that the reaction between CdO and ODPA was complete. Then, the temperature was increased to 350 °C, and 1.5 mL of TOP was injected into the flask, which caused the temperature to naturally decrease to 300 °C. Then, a Se:TOP solution (0.4 mL, 1 mol/L) was injected at 380 °C and reacted for several seconds. The size of the CdSe core could be adjusted by altering the temperature and reaction time.

The reaction mixture was cooled to 70~80 °C, and 4 mL of ethanol and a certain amount of TBP were added into the above solution, which was centrifuged at 10,000 rpm for 3 minutes.

Then, the precipitate was dissolved in a small amount of toluene while the supernatant was discarded. After centrifugation again, the precipitate was dissolved in TOP (CdSe-TOP).

Synthesis of CdSe/CdS dot-in-rod (DRs)

In a typical synthesis of CdSe/CdS DRs via seeded growth, CdO (30 mg) was mixed in a 50 mL flask together with TOPO (1 g), ODPA (100 mg) and HPA (30 mg). After alternately exposing

General procedure for phase transfer of Au precursor in toluene

For the preparation of the Au precursor, 10 mM KAuCl4 (0.1 mmol, 37.8 mg) was dissolved in water (10 mL), yielding a homogenous clear solution. This will subsequently be referred to as the Au solution and diluted for phase transfer. The transfer of KAuCl4 from water to toluene was performed following a procedure of Yang et. al. 35,38 All the solutions before mixing were degassed by bubbling argon for 10 min. In a centrifuge tube, Au (2 mM, 1 mL) solution was mixed with DDA (1 mL, comprised of 280 mg of DDA dissolved in 10 mL ethanol) stock solution.

After mixing well, let stand for more than three hours. 1 mL of degassed toluene was then added to the mixture solution, the light yellow gold toluene solution is on the upper layer. A pipette was used carefully to remove it and bubble argon for 10 min, at last, additionally deoxygenated toluene was added to fix it to 1mL (2 mM Au in toluene). Different concentration of Au was made by using different concentration of KAuCl4 in water with the same process.

Experimental methods in cuvette setup

The DRs samples was diluted in toluene and purged with Ar for at least 10 min, to remove the dissolved oxygen in the solution before transfer to a cuvette. The laser exposure area is designed to be circular with a 1 cm diameter in order to fill a 1 cm wide square cuvette, so the volume of the solution is always kept to 1 cm 3 to be sure that the solution is exposed to the laser light. The cuvette is then capped under Ar, and the solution is continuously stirred at a speed of 300 rpm by a magnetic stirrer. The intensity and illumination time of the experiment can be easily adjusted. All the experiments are performed at room temperature.

The reaction solution is prepared in the dark by mixing as-already made DRs solution and freshly made Au precursor. The order of adding samples to the cuvette is as follows: Toluene, DRs in toluene (density), Au in toluene, hole scavenger (MeOH).

In a typical process: 780 μl toluene, 100 μl DRs in toluene, 20 μl Au in toluene (2 mmol/L), and 100 μl MeOH was mixed under stirring, and then 205.42 mW/cm 2 blue laser was exposed for 8 min. Then the reaction was stopped by adding 3 times acetone and store in dark in fridge before purification. Purified by centrifugation at 4000g, and then redispersed in chloroform solution.

Control experiments

In order to test the role of each component in the reaction mixture, we carried out several control experiments.

The TEM grid preparation

The TEM grid preparation is performed according to the following procedures: The solution is sonicated for 10 min to disperse the nanoparticles, and then one drop (~3-5 μL) is deposited on a TEM grid. A paper tissue can help absorb the liquid from the carbon film side from the back side of the grid, which allows only the liquid to pass through, and leaves the nanoparticles on the top side. Two or more CHCl3 drops are also deposited onside, to wash away the remaining unreacted metal salts if any on DRs.

Conclusion

We have revisited the synthesis of single-tip Au/CdSe/CdS HNPs and rationalized the various results from the literature. We have performed this single-tip deposition process, by a photochemical deposition on DRs of a DDA modified KAuCl4 precursor in toluene using a blue laser (473 nm within the bandgap of the DRs) in a cuvette setup. We have shown that the optimal yield of single-tip HNPs can reach 94% and is achieved only by introducing an external small hole scavenger such as MeOH in the reaction mixture.

Beside this high ratio of single-tip HNPs, we can also just by one patch injection have access to various gold NDs diameters, between 6 nm to 10 nm with a perfect control. Additionally, we showed the key role of hole scavenger for laser-induced deposition, and that without hole scavengers, the deposition results will vary widely, because of the DRs corrosion so that the true effect of the parameters (such as intensity, Au/DRs ratio) cannot be identified. Based on our experimental results, we propose experimental parameters on how to increase the deposition ratio of single tips.

i. Adding a sufficient amount of the hole scavenger (such as: CMeOH>0.625 M);

ii. Using relative high intensity of laser (> 100 mW/cm 2 ) to be in the high field regime;

iii. Adding an appropriate amount of gold precursor (the ratio of Au/DRs from16,000 to 63,200) to avoid both the starvation process but also the beginning of the DR corrosion; iv. Using moderate deposition time, less than 32min, 8 min seem ideal timing;

v. The morphology of the nanocrystal also affects the deposition, thin shell and long length are more adapted for single-tip photodeposition process.

to study the mechanism of ligand-induced chiral transmission (Chapter 4). The CD and CPL activities were further modulated via this method, which can achieve nanocrystals with different shells in solutions with the same composition and concentration by chemical etching. This top-down approach gives rise to a series of solutions of identical DRs concentration but various shell thicknesses. It provides an opportunity to study the optical properties of a single NC, that the bottom-up approach is, at this point, not The UV laser was also used for the photodeposition. As shown in Figure 2, without the hole scavenger (MeOH), the ratio of single-tipped Au-CdSe/CdS HNPs is also lower (~55 %)

which is almost the same as Blue laser. While the etching process of DRs is more serious than Blue laser. More works are still needed to be done to explore the photodeposition conditions. Since the cysteine (Cys) modified DRs exhibit excellent CD chiral properties, this

gives the chance to study the effect of a CPL laser on the photodeposition process.

Interestingly, the chiral signal can be increased by 2-5 times after Au deposition, which can even higher than 10 -3 for the CPL glum. Due to the short chain of Cysteine, multisites Au-DRs HNPs formed frequently and as shown in

  Afin d'obtenir les HNPs avec d'excellentes propriétés catalytiques, nous devons d'abord effectuer la préparation contrôllée des DRs CdSe / CdS requis. ii References page x

Figure 1 a

 1 Figure 1 a) Efficacité quantique relative pour la production d'hydrogène, obtenue à partir de tiges de

  photodéposition, comme la présence d'un capteur de trous, l'intensité de l'irradiation, le temps de dépôt, le rapport Au / DRs, etc. Nos résultats se comparent assez bien avec un modèle de croissance d'un seul nanocristal Au. Nous nous attendons ensuite à ce que la stratégie démontrée puisse susciter une méthodologie commune concernant la synthèse de nanocristaux HNPs à pointe unique, conduisant à des architectures de nanoparticules avancées pour des applications dans des domaines tels que la photocatalyse, le photovoltaïque et la détection de la lumière.Cette thèse se termine par des Conclusions et des Perspectives avec une réflexion sur les résultats obtenus et quelques améliorations possibles pour de futures recherches.

Figure 1 a

 1 Figure 1 a) Relative quantum efficiency for hydrogen production, obtained from platinum-tipped unseeded CdS rods (yellow), and five different samples of platinum-tipped seeded rods, with seed diameters of 3.1 (red) or 2.3 nm (green). Underneath each bar is the corresponding average sample length. 8 b) Photocatalytic activity of various semiconductor nanocrystals featuring different degrees of photo-induced charge separation. The vertical axis shows the quantum yield of MV +• radical photogeneration using tris(bipyridine)ruthenium(II) (Ru(bipy)3 2+ ), CdSe spherical NCs, CdS nanorods,

Part 1

 1 State of the art .......................................................................... 1.1 Cadmium-based II-VI semiconductor nanocrystals: the past, present and shape dependent .......................................................................................... 1.1.1 The birth stage of semiconductor NCs. ............................................ 1.1.2 The great development period of semiconductor NCs. ..................... 1.1.3 The bottleneck period of semiconductor NCs. .................................. 1.1.4 The development of shape control ................................................... 27 1.2 Semiconductor nanoparticles chirality transmission ......................... 1.2.1 Circular dichroism (CD) of Cadmium-based II-VI semiconductor NCs . ......................................................................................................... 1.2.2 Induced circularly polarized luminescence properties of Cadmiumbased II-VI semiconductor nanocrystals .......................................................... 1.2.3 Summary and outlook ...................................................................... 1.3 Laser induced deposition of Au-CdSe/CdS hybrid nanoparticles ...... 1.3.1 Applications of photodeposition and comparison to other methods .. 1.3.2 Photodeposition of nanoparticles on the surface of cadmium-based II-VI semiconductor nanocrystals ......................................................................... 1.3.3 Study of laser-induced photodeposition ............................................ 1.3.4 Summary and outlook ...................................................................... 1.4 Conclusion ....................................................................................... 1.5 Bibliography. .................................................................................... past, present and shape dependent

Figure 2 .

 2 Figure 2. a) Lowest allowed electronic transition energy vs. size, 3 b) Room temperature optical absorption spectra of CdSe nanocrystallites in hexane and ranging in size from ~ 12 to 115 A.4

Figure 3 .

 3 Figure 3. a) Temporal evolution of the size and size distribution of CdTe, CdSe, and CdS NCs, 8 b) Sizing curves for CdTe, CdSe, and CdS NCs.9

Figure 4 .

 4 Figure 4. Schematic presentation of the synthesis of thiol-capped CdTe QDs. First stage: formation of CdTe precursors by introducing H2Te gas. Second stage: formation and growth of

Figure 5 .

 5 Figure 5. Schematic representation of the band structure of a CdSe/CdS QD.

Figure 6 a

 6 Figure 6 a) UV-vis and PL spectra (left panel), PL QY (top right), and FWHM (bottom right) of the PL peaks for CdSe/CdS core/shell nanocrystals, 58 b) QY variation of core/shell CdSe-ZnS QDs with the shell thickness, 59 c) Schematic structures, UV-vis and PL spectra of alloy CdSe//ZnS.26

  [61][62][63][64] Due to their low cost and easy large-scale preparation, these new well-established protocols can provide up to 15 g of high quality alloyed Zn1-xCdxSe core/shell nanocrystals in a single-batch reaction (Figure7).64

Figure 7 (

 7 Figure 7 (a-c): TEM images of plain Zn1-xCdxSe cores; (a1, b1): (a) and (b), respectively, plus 3 monolayers of ZnSe, 9 monolayers of ZnSexS1-x and 3 monolayers of ZnS. (c1): (c) plus 3 monolayers of CdS, 9 monolayers of Zn1-xCdxS and 3 monolayers of ZnS; (a2, b2 and c2): HRTEM images of a1, b1 and c1; (a3, b3 and c3): purified powder samples from a1, b1 and c1.64

Figure 8 a

 8 Figure 8 a) Fluorescence intensity versus time traces of a "bare", TOPO/TOPSe-passivated nanocrystal compared with that of a ZnS-overcoated one with a shell thickness of 7 monolayers at the same excitation intensity (I≈0.70 kW/cm 2 ) and a sampling interval of 20 ms, 65 b) non blinking and anti-

Figure 10 . 1 State of the Art 28 References page 70 AnFigure 11

 101287011 Figure 10. (a) Arrow-shaped (b) pencil-shaped (c) arrow-shaped (d) pine tree-shaped (e) pine treeshaped (f) rod-shaped nanocrystal. 11

Figure 12

 12 Figure 12 The applications for DRs. a) a single CdSe/CdS NR recorded simultaneously in two perpendicular polarization directions, 12 b) CdSe-Dot/CdS-Rod NC with Induced Chirality and Circularly Polarized Luminescence, 100 c) Photocatalytic quantum efficiency for the hydrogen reduction half reaction obtainedwith CdSe@CdS NR photocatalysts decorated with a single, double, or multiple Pt reduction sites.104

Figure 13 . 13 Figure 14 .

 131314 Figure 13. 1D rod versus 2D plate growth. a) Preferential growth along one direction (e.g. z axis) of a seed results in rod-shaped nanocrystals, b) growth in two directions (x,y plane) leads to the formation of disc-shaped nanocrystals.13

Figure 15 b 1 State of the Art 32 References page 70 developments

 1513270 Figure 15 b and d, the nucleation of CdTe on either end of CdSe nanorods to form polar tetrapods with different lengths of arms; similarly core tetrapod(CdSe)-branched tetrapod(CdTe) dendritic structures are also obtained through a second injection of Cd(CH3)2 and TOP-Te into a solution containing CdSe tetrapods.23 Multiply branched heterostructures can further be obtained through a careful control of the crystallinephase. The resulting NCs have morphologies which even include thorny balls, tree-like structures, and cobweb-like structures.126 These nanocrystal hybrid materials coupled with other inorganic materials or organic polymers will be potentially useful in photovoltaics, 127 electron transistor, 128 optical strain gauge, 129 chirality, 100, 130, 131 and photocatalysis.104, 132 Unfortunately, although there have been many new interesting

Figure 15 .

 15 Figure 15. a) Proposed model of a CdTe tetrapod. The exploded view of one arm illustrates the identical nature of the (111) zinc blende (ZB) and (0001-) wurtzite (WZ) facets of the nucleus and the arms, respectively (Cd atoms are yellow, Te ones are blue). Phosphonic acid molecules selectively bind to the lateral facets of the arms, as suggested in the figure (for clarity, only two facets are shown covered). 125 b) Growth mechanism of the second injection process. 23 c) a typical tetrapod-shaped CdSe nanocrystal. 11 d) A closer look at nanocrystal heterostructures. Isolated particles allow examination of structural aspects such as 'back branching' in branched rods (the top 2), extended

  The ligand-induced chirality was first demonstrated by Gun'ko and co-workers in the absorption region of CdS semiconductor NCs stabilized with chiral penicillamine molecules during the aqueous syntheses of NPs.147 They found that the chiral activity was, most likely, caused by the chiral penicillamine-induced distortions of the NCs surface atoms. This discovery opened the door to the induced chirality. In the recent decade, many efforts were made to study this induced chirality in SNCs depending on a number of factors, including the size of the NCs, 157-159 their shape, 100, 160-162 their crystal structure, 163 their shell thickness when any,115, 150 and the types 164-166 or concentration 167 of ligands.Size of NCs.In 2013, Balaz and co-workers157, 168 studied the induced CD of cysteine (Cys)-capped CdSe NCs with different diameters from 2.5 to 5.2 nm. They then proposed a model in which the chirality of these NCs could result from the hybridization of the ligand highest occupied molecular orbital (HOMO) of the ligand with the NCs valence band states (Figure 16 a). L-or D-cysteine-stabilized QDs in the water phase exhibited a sizedependent CD and circularly polarized luminescence (CPL) anisotropy. Unsurprisingly, L-and D-cysteine capped CdSe QDs showed CD and CPL signals with opposite mirror images (Figure 16). The CD distribution and CD anisotropy varied with the size of the CdSe NCs to reach the highest anisotropy value for the 4.4 nm CdSe NCs.

Figure 16 a

 16 Figure 16 a) Induced CD spectra of L-and D-cysteine capped CdSe QDs and optimized geometries of (left) L-Cys-(CdSe)13 and (right) D-Cys-(CdSe)13 nanoclusters, b) CPL (upper curves) and total luminescence (lower curve) spectra of aqueous solution of L-Cys-CdSe (red dots, Ø = 2.9 nm) and D-Cys-CdSe (blue dots, Ø = 2.9 nm) QDs.157

Figure 17 ab

 17 Figure 17 a) TEM images of CdSe nanorods with various aspect ratios (AR). Diameter is about 3 nm., b)CD spectra of the CdSe nanorods in the viable range and g factor with different AR nanorods.162

Figure 18 .

 18 Figure 18. a) CD spectra of l-cysteine-stabilized CdSe NPLs with different lateral sizes, 161 b) Diagrams of crystal surfaces and structures of WZ and ZB CdSe NPLs and the corresponding CD spectral. 163

Figure 19 .

 19 Figure 19.a) The variation of the absorption and CD with the growing shell for Cys-CdSe/CdS core/shell QDs by Markovich and coworkers, 150 b) CD and g-factor graphs of D-cysteine (blue) and L-cysteine (green) functionalized CdSe/CdS QDs with varying CdS shell thickness, marked S1-S5 by Gun'ko and coworkers, 115 c) CD spectra of 5 MLs CdSe NPLs and CdSe-CdS core-crown NPLs (left) and CD spectra of 4 MLs CdSe NPLs and CdSe/CdS core/shell NPLs (right). 161

Figure 20 The

 20 Figure 20 The CD signal variation during the change of the Cysteine concentration (top); possible binding modes of the Cys ligand on the QD surface (from left to right): S -monodentate, S --NH2

Figure 21 a

 21 Figure 21 a) Chiral memory of thiol-capped CdTe nanocrystals. QDs stabilized with chiral ligands retained their optical activity even after exchange with an achiral thiol, 169 b) Atomistic models for CdSe/ZnS QDs with (-) Burgers vector (left) and (+) Burgers vector (right) screw dislocations, c) Spectra of induced (left) and intrinsic (right) CD in Cys-CdSe/ZnS QDs.Absorption spectra are shown as dotted lines.141

Figure 22 .b

 22 Figure 22. a) Chirality of single CdTe nanoribbons. CD (solid line) and g-factor (dotted line) spectra for solutions of nanoribbons obtained after 50 h of illumination with circularly polarized light(left), surface rendering of the 3D TEM tomographic reconstruction of a LH and a RH nanoribbon (right). 142 b) Origin of enantio-purity in NPs assembly into helices. CdTe NPs pack into helices with chiral bias exerted by the chiral ligands at the interface. 144

Figure 23 .1. 2 . 2

 2322 Figure 23. a) Schematic representation of templating pathways. Nucleation and growth on one side of the twisted ribbons (blue) leads to single helices of CdS (yellow), while nucleation and growth on both sides of the ribbon leads to double helices. b, c) TEM images of a mature CdS single helix (b) and CdS double helices (c).145

  CPL emission intensities produced by the chiral luminescent sample are different and mirror images (Figure16 b). The CPL spectrum reflects the difference in emission intensity; therefore, it is always accompanied by the emission spectrum (Figure24b).172 It should be noted that the CD signal of the chiral luminescence system could not ensure the observation of a CPL signal. By contrast, CPL active materials generally have a Cotton effect on CD.

Figure 24 .b

 24 Figure 24. a) The relationship between unpolarized light, and linearly and circularly polarized light. b) Schematic illustration of circularly polarized luminescence (CPL).172

Figure 25 .b

 25 Figure 25. a) CD and CPL signals of Cys-CdSe/CdS DRs, b) CPL spectra and corresponding anisotropic glum factors of the L-and D-Cys-CdSe/CdS DRs with different ARSC values in aqueous solution. 100

Figure 26 ab

 26 Figure 26 a) CdS QDs with CPL prepared by a hollow protein chiral template, 173 b) CPL spectra of green-color QDs doped co-gel, assembly and disassembly regulated CPL (top), and different spacer lengths of capping reagents (C3 and C12) could also determine the feasibility of the chirality transfer from the chiral nanotubes to QDs (bottom). 152

Figure 27 .

 27 Figure 27. Schematic illustration of the self-assembled CNC-QR composite solid film. The QRs bind onto fibrillary CNCs by electrostatic interactions in an aqueous suspension. When above a critical concentration, the CNCs are self-assembled to form a lyotropic cholesteric (LC) phase with a lefthanded helical structure. The helical ordering is preserved upon drying to form a solid chiral film. The composite chiral film enables RCP emission due to the forbidden propagation of LCP light. 174

  The site selective deposition of noble NPs can be determined through various synthetic approaches, such as the surface coating, the deposition temperature, the deposition time, the activity of precursors, the concentrations and so on. In 2005, Banin and coworkers reported the single-tipped CdS-Au and CdSe-Au nanorods,203 which were achieved assuming an Ostwald ripening mechanism following multiple-site depositions on defect sites on the surface (Figure28 a). Manna and coworkers also got single-tipped CdSe-Au HNPs from multi-sites by a thermal annealing in 2010.204

Figure 28 .

 28 Figure 28. Sketch of pathways leading to topology evolution in HNCs based on decorated nanorods: (a) conversion of dumbbells to matchsticks by intra-particle metal ripening, 203 (b) conversion of multiple-to single-domain functionalized nanorods by thermal coalescence.204, 205

Figure 29

 29 Figure 29 Single-tip deposition control: Au growth on seeded CdSe/CdS nanorods (56 × 5.3 nm) after 30 min of 473 nm laser irradiation with different temperatures and ligands: (a) 293 K and DDA, (b) 273 K and DDA, (c) 293 K and ODA, and (d) 273K and TOA. The scale bar is 20 nm in all cases. 111

Figure 30 a

 30 Figure 30 a) Scheme of the growth process of Au nanocrystals onto CdS nanorods. Without oxygen,

  KAuCl4) are usually associated to tetraalkyl ammonium bromides (such as dodecyl dimethyl ammonium bromide (DDAB) or tetraoctadecyl imine bromide (TOAB)) and

Figure 31

 31 Figure 31 Photocatalytic quantum efficiency for the water reduction half reaction obtained withCdSe@CdS nanorod photocatalysts decorated with different-sized Ni tips.208

Figure 32

 32 Figure 32 Growth of size-controlled semiconductor-metal nanohybrids. (a) A scheme of the two-step metal growth deposition. (b) UV-vis absorbance spectra of CdS-Au hybrid nanoparticles showing the development of the plasmonic feature at 540 nm as the Au tip size increases. TEM images of CdS-Au hybrid NPs with 1.5 ± 0.2nm Au tip size after a 1 h dark synthesis (c) and light-induced synthesis for 30 min with various CdS:Au molar ratios leading to Au tip size of 3.0 ± 0.5nm (d) 4.8 ± 0.7nm (e), and 6.2 ± 0.8nm (f). (g) size distribution histogram of the Au metal tip diameters. 179

  Beyond TEM, the noble metal deposition can also be verified by optical characterization. Hybrid nanostructures have unique characteristics, which come from those of the individual materials involved in the system and those coming from the synergy brought about by the metal-semiconductor contact. Due to the proximity of the semiconductor and the metal NDs, different effects on the optical performance may be expected. Starting from the fluorescence, which is the central characteristic of the colloidal semiconductor QDs. The proximity of the metal induces a strong coupling and a quenching of the QDs luminescence via a non-radiative pathway leading to electron transfer to the Au, 211, 214 and the opposite the enhancement effect for gold when used as a shell around CdTe NW.222 The fluorescence quenching may be caused by the transfer of energy from excitons in the semiconductor to the metal, explaining most of the HNPs obtained by photodeposition. On the contrary, Kotov and coworkers observed an enhanced quantum yield accompanied by a decreased fluorescence lifetime which was attributed to the interaction between the collective surface plasmons of the metal nanoparticles and the excitons in the semiconductor nanowires.222

Figure 33 .

 33 Figure 33. The typical photoluminescence quenching and absorption spectra enhancement. a) Both the band edge and trap-state emission of CdS are quenched upon the growth of multi-sites Au NPs, 205 b) Similar quenching effects are observed upon the growth of different sizes (2-4.5 nm) of gold tips onto the CdSe/ZnS core/shell, 211 c) The normalized PL spectra of rods with 1.9 nm diameter Au at one end (red), 2.8 nm diameter Au at both ends (blue), and 'treated' bare CdSe/CdS DRs (black), 223 d) and e) the corresponding absorption spectra of a) and b), f) Normalized absorption spectra of CdS NRs (black line) and the hybrid nanorods samples (colored lines). 111

1. 3 . 3

 33 Study of laser-induced photodepositionWhen looking at the experimental processes related to photodeposition, most researchers used UV light, Xeon lamps or blue LEDs as light sources to load metal/metal oxide on the semiconductor surface. A limited number of papers used another light source, such as blue laser (Figure34).

Figure 34

 34 Figure 34 Sketch of light-assisted pathways leading to semiconductor-metal HNPs with varying topologies: a) laser photocatalytic reduction of Pt on CdS and CdSe@CdS NRs, b) transition of multiple Au-decorated CdS and eccentric CdSe@CdS NR heterostructures to corresponding quasi matchsticklike HNPs that carry one larger Au head and residual Au patches along the longitudinal sidewalls. 111

1. 5 . 4

 54 Synthesis of green-emitting CdSe/ZnS core/shell QDs using Cd(TDPA)2 as core precursor ............................................................................................................. 110 1.5.5 Synthesis of CdSe/ZnS core/shell QDs by typical TOP-SILAR method using the CdSe without rapid cooling process ............................................................................ 111 1.5.6 Fabrication of QDs-PS composite plate ......................................................... 111 1.5.7 Assembling of TSQDs-LED ........................................................................... 112 1.5.8 Thermal stability test of QDs ......................................................................... 112 1.6 Conclusion ........................................................................................................ 112 1.7 Bibliography ...................................................................................................... 113 Chapter 1 Controlled preparation of core-shell quantum dots 89 References page 113

FWHM of 26

 26 Scheme 1. a) The schematic growth of multilayered core/shell QDs by traditional TOP-SILAR method, 26 b) the growth scheme of modified TOP-SILAR method coupled with TOP extraction process eliminating Cd 2+ adsorbed CdSe core.

Figure

  Figure 1a and UV-vis and PL spectroscopy on Figure 1b. We can see from the inset in Figure 1a, a 0.348 nm interplanar spacing value derived for the CdSe QDs core, which is good agreement with the crystal lattice plane (111) of bulk cubic zinc blende (ZB) CdSe (d111=0.351

Figure 1 . 2 .

 12 Figure 1. TEM and HRTEM (inset) images of a) bare green-emitting CdSe cores (2.5 nm); b) UV-vis and PL of the same bare green-emitting CdSe cores (2.5 nm).

Figure 2 .

 2 Figure 2. TEM of CdSe/ZnS TSQDs with the increase of the shell thickness: a) 1-layer ZnS shell; b) 3-layer of ZnS shell; c) 5-layer of ZnS shell; d) 7-layer of ZnS shell. (a1-d1): Corresponding HRTEM of images a-d. (a2-d2): Corresponding histograms of the diameter distributions of images a-d.

Figure 3 .

 3 Figure 3. TEM and HRTEM (inset) images of green-emitting CdSe/ZnS(7) thick-shell QDs (7.2 ± 0.9 nm), lattice fringes with interlayer distances of 0.348 nm, and 0.317 nm are displayed. The crystallinity is confirmed by the distinct lattice fringes.

  structure of the TSQDs. The XRD pattern of the CdSe cores shows peaks at 25.3, 42.1, and 49.7°, which correspond to the (111), (220), and (311) lattice planes (Figure4), respectively. These results are completely in conformity with the standard JCPDS No. 00-019-0191.18 There are peak shifts towards the high angles and peak shrinkage when the ZnS shells grow on the zinc blende CdSe cores. It is also clearly shown that as the thickness of the shell increases, the XRD peaks becomes narrower, indicating that the QDs are getting bigger, an indication of the successful epitaxial growth of the ZnS shell.26, 36, 37

Figure 4 .

 4 Figure 4. Powder XRD patterns of the green-emitting CdSe cores and the CdSe/ZnS(7) TSQDs as shown in Figure 3. The XRD peaks position of the zinc blende (ZB) bulk CdSe (JCPDS No. No. 00-019-0191) and bulk ZnS (JCPDS No. 80-0020) are respectively indicated by vertical solid lines below and above. Dash lines are a guide to the eye.

Figure 5 .

 5 Figure 5. a) PL spectra and absorption spectra of the green-emitting CdSe cores and the green-emitting CdSe/ZnS(7) TSQDs; b) The corresponding CIE coordinates of green-emitting CdSe/ZnS(7) core/shell TSQDs synthesized by modified TOP-SILAR method using TOP extraction process. (Insets: as prepared TSQDs and the PL spectrum).

Figure 6 .

 6 Figure 6. Normalized fluorescence spectra of CdSe/ZnS QDs with the increase of the shell thickness: CdSe cores (black), 1 layer (red), 3 layers (blue), 5 layers (magenta), and 7 layers of ZnS shell (orange). Their corresponding full-width at half-maximum (FWHM) is 25 nm, 28 nm, 28 nm, 26 and 26 nm, respectively.

41 1. 3

 413 TSQDs exhibit the following color coordinates (0.194, 0.763) (Figure 5 b), which almost correspond to the ideal saturated green point (0.170, 0.797) in ITU-R Recommendation BT.2020 (Rec. 2020) posted in 2012 for the new generation of high definition displays. 38-Further insights on the role of various parameters 1.3.1 Effect of ligands: Stearic acid (SA) or 1-tetradecylphosphonic acid (TDPA)?

Figure 7 . 1 Controlled

 71 Figure 7. Chemical structure of 1-tetradecylphosphonic acid (TDPA) and stearic acid (SA).

Figure 8 .

 8 Figure 8. Normalized PL spectra of CdSe/ZnS(3) core/shell QDs synthesized using different precursors: Cd(TDPA)2 (solid line, inadequate reaction) and cadmium stearate (Cd(SA)2) (dashed line, adequate reaction) as CdSe precursors.

Figure 9 ,

 9 , red line) or not (dashed blue line) to a TOP-assisted extraction prior to the TOP-SILAR method process. The resulting CdSe/ZnS core/shells exhibit a green emission which successfully overlays that of the core when the TOP extraction is used i.e. < 550nm.

Figure 9 .

 9 Figure 9. Normalized PL spectra of QDs in different shell formation synthetic processes, CdSe/ZnS(3) core/shell QDs synthesized by typical TOP-SILAR method based on the rapid cooling process core (black solid line), with TOP-assisted extraction purification process (red solid line), without TOP assisted extraction purification process (blue dashed line).

Figure 10 .

 10 Figure 10. The schematic growth of the typical TOP-SILAR method using Cd 2+ adsorbed CdSe core.

Figure 11 Figure 11 .

 1111 Figure 11. Fluorescence spectra of core/shell QDs synthesized by TOP-SILAR method. Using the rapid cooling CdSe cores (The same as shown in Figure 9 black line), a 3-monolayered ZnS shell was grown after purification processes performed different times. TOP assisted extraction purify process performed 3 times (red); 2 times (orange); 1 time (black); and typical hexane/methanol extraction procedure (blue).

Figure 12 .

 12 Figure 12. Fluorescence spectra and absorption spectra of the red CdSe cores (FWHM 26 nm) and red CdSe/ZnS(3) core/shell QDs (FWHM 28 nm) synthesized by TOP-SILAR method coupled with the TOP assisted extraction purification process.

Figure 13 .

 13 Figure 13. High resolution TEM images of a) bare red CdSe cores; b) corresponding red CdSe/ZnS(3) core-shell QDs, lattice fringes with interlayer distances of 0.349 nm, and 0.329 nm are displayed. 18 c) The diameter of the bare core CdSe NCs is 5.6 nm, and d) 7.7 nm for the core-shell QDs. The stripes are all in the same direction, indicating that the NC is a single crystal; c and d the respective histograms of their diameter size distributions.

Figure 14 .

 14 Figure 14. The corresponding CIE coordinates of green-emitting and red-emitting QDs synthesized by modified TOP-SILAR method using TOP extraction process. The TSQDs can be utilized to realize about 91% Rec. 2020 high color gamut display devices.

Figure 15 .

 15 Figure 15. Temperature-dependent photoluminescence quantum yield (PL QY) of green-emitting QDs.

Figure 16 .

 16 Figure 16. The schematic diagram of two types of QDs-LED chips: a) on-chip type WLEDs, b) remote type WLED; and measured heat generation of each layer in LED chip: c) on-chip type WLED under 60 mA, 200 mA and 300 mA, d) remote type WLED under 20 mA and 60 mA.48, 51 The red dash lines are a guide to the eye.

  composite plate is prepared using a green-emitting CdSe/ZnS(7) TSQDs with a QY of 75%. The polystyrene (PS) polymer particles once dissolved in chloroform were mixed with the green-emitting QDs, and then a subsequent QD-PS composite plate was obtained after natural curing under an inert atmosphere. A photograph of the flexible QD-PS composite plate highly fluorescent under a 365nm UV irradiation is shown in Figure17b. The QD-based remote-type LED is simply prepared by cutting the plate into a suitable size and combined it with a silicone resin (silicone 6550 gel A/gel B = 1:1) on an In-GaN/GaN blue LED chip (455 nm).As shown in Figure17 c, both the blue LED and the QD-LED spectra were operated at a current of 20 mA at 2.7 V. The emission of the TSQDs layer is slightly shifted to 533 nm and the FWHM remains as narrow as 25 nm, indicating the achievement of a green LED. The photo of a green-LED operated at a driving current of 20 mA is shown in the insert of Figure17 c. This example demonstrates that our TSQDs can be easily applied in QD-LEDs.

Figure 17 .

 17 Figure 17. (a) Scheme of a remote packaging LED with green-emitting TSQDs; (b) The photograph of a freestanding green-emitting QDs-PS composite plate under a 365 nm UV illumination; (c) the EL spectra of a blue LED and a green QD-LED operated at a current of 20 mA at 2.7 V, the inset is the photograph of a greenemitting LED under daylight (top) and operating at 20 mA (bottom).

  from J & K Chemical Reagent Company, China. All the chemicals were used as received without further purification. 1.5.2 Synthesis of green-emitting thick-shell core/shell QDs 1.5.2.1 Synthesis of green-emitting CdSe cores by stearic acid through rapid cooling process

  green-emitting thick-shell core/shell QDs by TOP-

  4 mL of TOP solution was injected as an activator and the reaction mixture was further maintained at 200 o C for 30 min. Then, calculated amount of Zn solution (0.1 M) was injected and maintained at 200 o C for 20 min. After that, without further purification, the reaction temperature was decreased from 200 o C to 180 o C. Then the same amount of S precursor (0.1 M) solution was added into the reaction flask via syringe and the temperature was increased to 220 o C immediately, for the growth of ZnS outer-layers. After 60 min, the first ZnS monolayer was grown in-situ, and then the temperature was decreased to140 o C. After 0.4 mL of fresh TOP solution was injected, the temperature was increased immediately to 180 o C. The Zn and Se precursor solutions (calculated for the second layer)

1. 5 . 4

 54 Synthesis of green-emitting CdSe/ZnS core/shell QDs using Cd(TDPA)2 as core precursor 1.5.4.1 Synthesis of CdSe based on Cd(TDPA)2

1 Controlled

 1 heated to 280 °C under argon atmosphere to obtain a colorless clear solution. The CdSe core QDs were prepared by quick injection of 1M of Se-TOP precursor into the Cd-containing reaction mixture at 300 °C. After 1 min, the reaction mixture was cooled to 50-60 °C. The obtained QDs with an emission peak at 486 nm, and FWHM 25 nm, and a typical hexane/methanol extraction procedure was used to purify.1.5.4.2Synthesis of green-emitting CdSe/ZnS core/shell QDsThe green-emitting core/shell QDs was synthesized based on typical TOP-SILAR method Chapter

  obtained thin shell DRs by etching a 0.7 nm CdS layer from the CdSe/CdS nanorods, which enhanced their catalytic activity by over 3 times as compared to the original CdSe/CdS nanorods. Nevertheless, there is a lack of a systematic research on the preparation of these quantum rods, especially for the high-precision control of the synthesis of DRs with uniform thin-shell with less than one layer.Herein, we focused on the temperature control during the synthesis process and studied Chapter 2 Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 122 References page 141 the growth mechanism of the quantum rods by systematically controlling the impact of the hot injection temperature, the heating mantle power, and ambient temperature. Based on these results we propose a two-step growth mechanism. (1) By controlling the first stage, wurtzite CdS spherical shells with different thicknesses are performed, which can be achieved by controlling the temperature, such as hot injection temperature, power of the heating mantle, and ambient temperature.

a

  Diameter of the CdSe core determined from the absorption spectrum by Peng's equation (nm); 26 *The ambient temperature was 303 K (in summer); **The ambient temperature was 291 K (in winter); The reaction time was 8min, except for the time marked.

Figure 1 .

 1 Figure 1. Range of shell thicknesses for a given temperature and a reaction time of 8 min based on data from Table 1.

Figure 2 .

 2 Figure 2. Control synthesized of CdSe core with different wavelengths by varying the ratio of precursor from 1:3 to 3:1. a) The UV absorption spectra of different CdSe cores, b) the PL emission spectra of the same CdSe cores, the insert shows the images of toluene solutions under normal and UV lights.

Figure 3 .

 3 The wavelength of the CdSe core can be continuous changed by varying the reaction time from 0 s to 60 s, and the ligands also contribute to the wavelength control, longer (ODPA, C18) can be used to control smaller CdSe cores, while shorter n-tetradecylphosphonic acid (TDPA, C14) provides larger CdSe cores. These data for core wavelength controllable synthesis can further provide guidance for the Chapter 2 Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 125 References page 141 controllable preparation of DRs.

Figure 3 .

 3 Figure 3. Controlled synthesis of CdSe cores with different wavelengths by varying the nature of the capping ligands and the reaction time. a) Using ODPA as ligands reaction for different times (0s, 5s, 15s, and 60s), b) using TDPA as ligands reaction for different times (0s, 10s, 30s, and 60s). The insert images show the corresponding CdSe cores in toluene under UV lights.

Figure 4 .

 4 Figure 4. a) TEM of CdSe core (3.2 nm); the insert image shows the corresponding CdSe core in toluene under UV light; and b) HRTEM images of the same CdSe cores (3.2 nm).

Figure 5 .

 5 Figure 5. TEM images of the DRs synthesized at different temperatures using the same CdSe cores. a) 503 K, b) 533 K, c) 563 K, d) 593 K, e) 623 K, f) 653 K.

( 3 .

 3 2 nm), we conclude that the layer of the CdS material covering the surface of the CdSe dot Chapter 2 Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 127 References page 141

Figure 6 .

 6 Figure 6. Variation of the length and the volume of the DRs with the temperature.

128 References page 141 in Figure 7 ; 30 Figure 7 .

 1417307 Figure 7. TEM EDX mapping of CdSe/CdS DRs. a) STEM images of CdSe/CdS DRs, the insert is HAADF-STEM images, b-d) the representative EDS mapping images for Cd, S and Se, e) combination of Cd, S, and Se maps, f) line-scan distribution profile.

Figure 8 .Figure 9 .

 89 Figure 8. TEM images of DRs synthesized under different reaction times starting with the same CdSe cores solution. a) 0.5 min, b) 1 min, c) 2 min, d) 4 min, e) 8 min, f) 16 min.

Scheme 1 .

 1 Scheme 1.The growth mechanism for dot-in-rod (DR) nanorods.

( 1 )

 1 Stage 1: First, a CdS shell with a given and controlled thickness is generated, which depends on the reaction temperature. This CdS shell presents the wurtzite structure for 2 reasons: (i) epitaxy on the crystal structure of the CdSe crystal core (ii) the choice of the ODPA powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 131 References page 141

  key factor. So as to test this aspect, we designed control experiments, with a different heating mantle. All the heating mantles are made by Glas-Col but with different setups. (1) Typically, the experiments were performed by using a 180 W Series O spherical mantle (high capacity) covering both the top and the bottom of the flask to improve safety and efficiency; (2) we chose the 50% (90 W) input power controlled by the Glas-Col controller of lower power for control experiments, which will decrease the heating rate (from ~12 K/min to ~5 K/min for TOPO solution).

Chapter 2 Temperature

 2 , a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 132 References page 141

Chapter 2 Temperature

 2 , a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 133 References page 141

2. 2 . 6 Figure 10 .Figure 11 .

 261011 Figure 10. TEM images of DRs synthesized under different core concentration by the same CdSe core. a) 33.6 nmol/mL, b) 16.8 nmol/mL, c) 11.2 nmol/mL, d) 5.6 nmol/mL, e) 2.8 nmol/mL, f) the length evolution of DRs under different core concentrations.

  References page 141Using this method, thin shell multi-color DRs (green DR-21, orange DR-22, and red DR-23)

Chapter 2 Temperature

 2 , a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 136 References page 141

Figure 12 .

 12 Figure 12. The ligands exchanged process of DRs from oil phase to aqueous phase.

[

  Cys] = 0.1 M), and the pH of the resulting solution was adjusted to 12.0 with tetramethyl ammonium hydroxide pentahydrate (TMAH). A solution of CdSe/CdS DRs in n-hexane (5 mL, 1× 10 -6 M) was added to the cysteine solution, and the reaction mixture was deoxygenated and stirred at room temperature under nitrogen in the absence of light for 24 h. The reaction mixture was left to stand for 1 h to allow the phases to separate. The bottom aqueous layer was taken out with a syringe, and the DRs water phase was purified by precipitation with ethanol/DI water/chloroform (4:1:1, 2 times). The purified Cys-DRs were dissolved in DI H2O and stored at room temperature in the dark. As shown in Figure13, we compared their luminescence before and after the ligands exchange, from left to right are the CdSe QDs, the DRs with different aspect ratios, respectively. The luminescence of the DRs is well preserved, but that of the pure CdSe cores is almost completely quenched.

Figure 13 .

 13 Figure 13. Images comparing the luminescence before and after the ligands exchange. (a, b) The oil phase QDs and DRs in Hexane; (c, d) the aqueous phase QDs and DRs. (a) and (c) is under natural light, and (b) and (d) is under the UV light (365 nm). From left to right: the CdSe QDs, DRs with different aspect ratios.

Figure 14 .

 14 Figure 14. The TEM image of DR-24 with a second over-coating process, the shell thickness is 2.5 nm.

Figure 15 .

 15 Figure 15. DRs CD g-factor variation with the shell-thickness.

Figure 16 .

 16 Figure 16. The variation of CD chirality of the DRs with different aspect ratios. (a-e) The CD signal of DR-17, DR-4, DR-18, DR-19 and DR-20; (f) The resulting CD g-factor variation of the same DRs.

Chapter 2 Temperature

 2 , a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods 140 References page 141

Figure 1 .

 1 Figure 1. TEM images of W-CdSe/W-CdS NCs showing the evolution of the different shapes when changing the molar ratio ODPA/(ODPA+OA), for a constant amount of OA of 1 mL (3.18 mmol). (a) ODPA, 0%; (b) ODPA, 100 mg, 8.5 %; (c) ODPA, 150 mg, 12.2 %; (d) ODPA, 285 mg, 21.0 %.

Chapter 3 :

 3 Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure 152 References page 172

  anisotropy. It also confirmed the selective binding to the {100} and {110} surfaces of the growing crystallites orientating the growth pattern along the [001] direction to eventually form a long tail. At low concentrations or in the absence of ODPA, due to the weak "selective adhesion" process, the faceted nanocrystals are more likely to form flowers, Tadpoles with three and two-tail. When increasing the ratio of "selective adhesion" ligand (ODPA), the Tadpole shape with one tail prevail. In fact, in between (Cd(OA)2) and (Cd(ODPA)2) cadmium dicarboxylate is the most reactive one and will first participate to generate a symmetrical shell around the CdSe core.

Figure 2

 2 Figure 2 (a) Representative TEM image of CdSe/CdS Tadpoles; (b, c) HR-TEM image of tail and head of Tadpoles nanocrystals; (d) XRD pattern of CdSe/CdS Tadpoles; (e) UV-vis. and photoluminescence (PL) spectra of CdSe/CdS Tadpoles.

Figure 2 a

 2 Figure 2 a shows a representative high magnification TEM image of these CdSe/CdS

Figure 3

 3 Figure 3 TEM image of CdSe/CdS Tadpoles over a large area as shown in Figure 2.

Figure 4

 4 Figure 4 (a) UV-vis. spectral variation of the CdSe/CdS Tadpoles during their synthesis from CdSe seeds. (b) and their PL spectra.

Figure 5 .

 5 Figure 5. TEM images evolution during the synthesis of CdSe/CdS Tadpoles from W-CdSe. a) CdSe core (S0), b) 0.5 min (S2), c) 1min (S3), d) 2min (S4), e) 4min (S5), f) 8min (S1). 1 mL OA and 21.2% ODPA.

Scheme 1

 1 Scheme 1 Two-stage growth process of Tadpole morphology. In the first stage, the isotropic CdS shell growth; then in stage 2, the Tadpole shape gradually forms by shell growth along the c direction.

Chapter 3 :

 3 Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure 157 References page 172

Figure 6 .

 6 Figure 6. TEM evolution of the different shapes as changing the ratio of carboxylic acid ligand, ODPA being constant at 285 mg, 8.5 10 -4 mol. a) OA v (mL), molar % (OA/(OA+ODPA)), 0 mL (0); b) OA, 0.33 mL (55.2); c) OA, 0.5 mL (65.1); d) OA, 1 mL (79.2); (e) OA, 1.5 mL (85); (f) the relationship between OA and length.

Chapter 3 :

 3 Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure 158 References page 172 some point to modify this composition at the available surfaces of the NCs head and increase its size.

Figure 7 .Chapter 3 :

 73 Figure 7. Effect of the cores concentration on the morphology. (a-d) The ligands are OA (1 mL, 3.21 mmol) and ODPA (285 mg, 8.5 10 -4 mol), the cores concentrations changed from 5 nmol to 15 nmol, 50 nmol (RC), and 100 nmol, respectively.

Figure 8

 8 Figure8 Variation of the Tadpoles length with the concentration in seeds. The line is a guide for the eyes.

Chapter 3 :

 3 Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure 160 References page 172

Figure 9 .Figure 10 .

 910 Figure 9. Effect of the core concentration on the morphology. The ligands are ODPA (600 mg), HPA (162 mg), the core concentrations are 5 nmol and 50 nmol for a) and b), respectively

Figure 11 .

 11 Figure 11. TEM of Tadpole-shaped NCs made at different temperatures. a) 260 o C, c) 290 o C; b) and d) are the corresponding HRTEM images.

  At 260 o C, only deformed tetrahedral nanoparticles, with an aspect ratio of 1.5 are observed; which means a very short tail are grown at this temperature. Additionally, the lattice fringes still maintain at 0.351 nm, which corresponds to the (002) planes of W-CdSe phase, this also confirms that only a thin CdS shell forms. While, at 290 o C, the Tadpole shape is effectively formed with a 19 nm tail. As shown in the HR-TEM image of a single Tadpole (Figure11b and Chapter 3: Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure 163 References page 172 d), the lattice fringes of the long tail have an interplanar spacing of 0.341 nm, corresponding to the (002) planes of the W-CdS phase.

Figure 12

 12 Figure 12 TEM of the Tadpole-shaped NCs made by One-shot injection of the shell precursors.

  5 mL) was mixed in a 200 mL flask. After alternately exposing the flask to vacuum and argon at least 5 times at 100 °C, the resulting solution was heated to 250 °C to obtain a completely transparent liquid without any solids. Over course of 10 minutes, the solution changed from deep red and opaque (characteristic of CdO) to colorless and transparent, indicating complexation of cadmium and formation of the active Cd-myristate species. Upon the observed color change, 1-ODE (48.0 mL) was injected and the solution was cooled to room temperature.

  degassed upon heating at 100 o C under vacuum for 30 min. The solution was cooled to room temperature, and 240 mg (0.304 mmol) of 200 mesh Se powder (99.99%) were added to the reaction mixture, followed by degassing at 70 °C under vacuum for 30 min. The reaction mixture was heated to 240 °C under Ar atmosphere. A degassed solution of 0.1 mL of oleic acid (OA) (89.0 mg, 0.315 mmol) and 1 mL of oleylamine (OAM) (828 mg, 3.10 mmol) in 4 mL of 1-octadecene (1-ODE) was added dropwise to the reaction mixture in 4 min after approaching 240 °C to stabilize the nanoparticle growth. The growth of nanocrystals can be visually monitored by the change of the solution color from yellow to deep red and by taking aliquots for UV-vis and PL measurements. After the ODE/OAM/OA injection, the reaction was quenched by rapid injection of 1-ODE (40.0 mL) at the desired time point. The ZB-CdSe cores were purified by adding the mixture of isopropanol (IPA) and methanol (MeOH), 15 mL of IPA and 15 mL of MeOH were added to the 15 mL of supernatant in each tube (for a typical synthesis), resulting in a cloudy white emulsion. Centrifugation at 10,000 rpm for 20 minutes yielded a viscous yellow oil (~3 mL) and a clear/colorless supernatant. The multiple yellow oil droplets suspended in the supernatant were allowed to settle for 5 minutes prior to a careful removal of the supernatant via a syringe. The purification process was repeated 3 times until 166 References page 172

  which correlate the position of the first electronic transition with the diameter of the CdSe particles (Chapter 1, equation 1) and the molar extinction coefficient of the particles and their diameter (Chapter 1, equation 2).3.3.3 Synthesis of Tadpole shape CdSe/CdS NCs based on W-CdSe 3.3.3.1 Preparation of 2.5 M S-TOP Stock Solution

3. 3 . 4 . 4

 344 Synthesis of CdSe/CdS DRsIn a typical synthesis of CdSe/CdS DRs via seeded growth, CdO (115.8 mg, 0.9 mmol) was mixed in a 50 mL flask together with TOPO (3 g, 7.76 mmol), ODPA (600 mg, 1.79 mmol) and HPA (162 mg, 0.975 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 320 °C to make the solution become completely transparent liquid without any solids. 1.5 mL of TOP was added, and then, the temperature was maintained at 320 °C and 50 nmol of W-CdSe-TOP solution was added. The S-TOP (0.5 M, 3 mL, 1.5×10 -3 mol) was divided into four injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were allowed to grow at 320 °C for approximately 8 minutes after the CdSe core injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors.As shown in Chapter 2, different lengths of DRs can be adjusted by different core concentrations. In this work, we show another way to get different aspect ratios (ARs), here, different lengths of DRs can be obtained by changing the amounts of ligands. For only ODPA, the AR of DRs increases as the amount of ligands decreases. For a given amount of ODPA when increasing the amount of HPA also allows to greatly increase the length of the DRs. As an example, ODPA (600 mg, 1.79 mmol) and HPA (0 mg, 0 mmol) can be used for the synthesis References page 172

3. 3 . 5

 35 Synthesis of Tadpole shape CdSe/CdS NCs based on ZB-CdSeIn a typical synthesis of CdSe/CdS Tadpoles NCs based on ZB-CdSe (3.95 nm) via seeded growth, CdO (115.8 mg, 0.9 mmol) was mixed in a 50 mL flask together with TOPO (3 g, 7.76 mmol), ODPA (285 mg, 0.852 mmol) and OA (1.02 mL, 908 mg, 3.21 mmol). After alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 320 °C to make the solution become completely transparent. TOP (1.5 mL) was added, and then, the temperature was maintained at 320 °C and 5 nmol of ZB-CdSe-TOP solution was added. The S-TOP (0.5 M, 3 mL, 1.5 × 10 -3 mol) was divided into four injections at 2 min intervals (0, 2, 4, 6 min). The CdSe/CdS NCs were allowed to grow at 320 °C for approximately 8 minutes after the CdSe cores injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors.

Chapter 4 Study

 4 of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 187 References page 217we first present three CdSe cores with different emissions (yellow, orange and red). Their FWHM can be maintained at around 30 nm, indicating that they are good in size uniformity.

Figure 1 .

 1 Figure 1. The UV and PL of different CdSe core for the control synthesis of DRs.

Figure 2 .

 2 Figure 2. TEM images of DRs with different shell thicknesses: (a) DR-1, three monolayers; (b) DR-2, two monolayers; (c) DR-3, one monolayer.

Figure 3 .Figure 2 :

 32 Figure 3. The UV-vis and PL of the different DRs synthesized under different reaction temperatures as shown inFigure 2: DR-1, 653 K; DR-2, 623 K; DR-3, 593 K.

Figure 4 .

 4 Figure 4. The CD spectrum evolution of DR-1 at the first exciton absorption peak during the etching process. (a) The CD spectrum of benzyl amine modified DRs-1 after Cys exchange; (b) The CD spectrum of DRs-1 etching one cycle after Cys exchange; (c) The CD spectrum of DRs-1 two etching cycles after Cys exchange.

  mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 190 References page 217

Figure 5 .

 5 Figure 5. The corresponding g-factors variations and UV-vis spectra of DR-1 at the first exciton absorption peak as shown in Figure 4. a) The CD spectrum of Benzyl amine modified DRs-1-1 after Cys exchange; b) The CD spectrum of DRs-1: after one etching cycle and Cys exchange; c) The CD spectrum of DRs-1: after two etching cycles and Cys exchange; d) The corresponding UV-vis absorption spectra of different etching processes.

Figure 6 .

 6 Figure 6. The CD, UV-vis and g-factor variations of DR-1 at the first exciton absorption peak during the etching process. The UV-vis value was maximum at the first exciton absorption peak.

Figure 4 ,

 4 Figure 4, shows that as the etching proceeds the shell layer decreases, the DRs CD signal increases at the first exciton absorption peak (~ 600 nm, response of the signal of core),

Figure 4

 4 Figure4should directly reflect the chiral properties per mole of nanoparticles. Although we do not know the specific number of nanoparticles, the concentration of nanoparticles can be controlled by UV-Vis absorption monitoring. This explain the small change in the absorption intensity of the CdSe core of the single DRs during the etching process (Figure5d), so in this case, the g-factor in the bulk solution can represent the same evolution of the chiral signal as in a single particle However, when we focus in the range of 400-500 nm corresponding to the CD signals of CdS shell, the chiral signals of the shell are more intense, the results can become complicated.

Figure 7 .Figure 4 and

 74 Figure 7. The CD spectrum variations of DR-1 (< 500 nm) during the etching process. a) The CD spectrum of benzyl amine modified DRs-1 after Cys exchange; b) The CD spectrum of DRs-1 one etching cycle after Cys exchange; c) The CD spectrum of DRs-1 two etching cycles after Cys exchange. The concentration is 5 times lower than in Figure 4 and Figure 5.

  mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 192 References page 217

Figure 8 .Figure 4 and

 84 Figure 8. The corresponding UV-vis and g-factor variations of DR-1 (< 500 nm) as shown in Figure 7. (a) The CD spectrum of the benzyl amine modified DRs-1 after Cys exchange, (b) The CD spectrum of DRs-1 after one etching cycle and Cys exchange, (c) The CD spectrum of DRs-1 after two etching cycles and Cys exchange, (d) The corresponding UV-VIS absorption of the different etching processes. The concentration is 5 times lower than in Figure 4 and Figure 5.

Figure 9 .

 9 Figure 9. The CD, UV-vis. and g-factor evolution of DR-1 (< 500 nm) during the etching process. The UV-vis. value is the wavelength of the maximum value of the CD signal.

Chapter 4 Study

 4 of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 194 References page 217

Figure 10 .

 10 Figure 10. The corresponding CD, UV-vis and g-factor variations of these DRs with different shell thicknesses at the first exciton absorption peak. a) Three-monolayer DR-1 after Cys exchange, b) two-monolayer DR-2 after Cys exchange, c) one-monolayer DR-3 after Cys exchange.

Figure 11 .

 11 Figure 11. The CD, UV-vis and g-factor variations of DRs with different shell thicknesses at the first exciton absorption peak. The UV-vis value is the maximum value of at the first exciton absorption peak.

  mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 195 References page 217

Figure 12 .

 12 Figure 12. The corresponding CD, UV-vis and g-factor variations of DRs with different shell thicknesses (< 500 nm). a) Three-monolayer DR-1 after Cys exchange. b) two-monolayer DR-2 after Cys exchange, c) One-monolayer DR-3 after Cys exchange. The concentration is 5 times lower than in Figure 10.

Figure 13 .

 13 Figure 13. The CD, UV-vis and g-factor evolution of DRs with different shell thicknesses (< 500 nm). The UV-visvalue is wavelength of the maximum value of CD signal.

  mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 196 References page 217

Figure 16 ) 4 217 Figure 14 .

 16421714 Figure16), this further indicates that it may also not be possible to simply use the g-factor to accurately represent the change of behavior of the single nanoparticle chiral signal.

Figure 15 .Figure 16 .

 1516 Figure 15. The corresponding UV-vis and g-factor variations of DR-3 at the first exciton absorption peak as shown in Figure 14. (a) The CD spectrum of benzyl amine modified DRs after Cys exchange, (b)-(e)The CD spectrum of DRs one cycle to four etching cycles after Cys exchange, (f) The corresponding UV-vis absorption of different etching processes.

  ), and broken nanorods appear as the etching further proceeds (Figure17 d-e, orange ellipses). 199 References page 217

Figure 17 .

 17 Figure 17. TEM images and XRD pattern of DR-3 after etching process. a) TEM of benzyl amine modified DRs; b)e) TEM of DRs after one cycle to four etching cycles. The red outline represents the etched part, the blue represents the CdSe core position, and the orange underlines a quantum rod which is etched and broken. f) The corresponding XRD of DR-3 after etching processes (a, b and d).

  mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 200 References page 217down (Figure17 d-e). This means that thin-shell CdSe/CdS DRs cannot be used in top-down chemical etching process for a further improvement of chirality as proposed, because they are probably already in the best configuration with a good compromise between high chiral signals and appropriate CPL properties.Additionally, based on the etched position, we can infer the position of the CdSe core. As shown in Figure17 b-d(the blue circle), most of the DRs cores are located at 1/4 to 1/2 of one of the tips, which is consistent with the literature.[59][60][61] This top-down chemical etching technology can also be used to characterize the position of the core in DRs, in a much cheaper way than the expensive HAADF-STEM one generally reported in the literature for such determination.59, 61

Figure 18 Figure 18 .

 1818 Figure18 plots the dependence of the absorbance on the etching time for DR-1 and DR-3 at two wavelengths (450 nm and the first exciton absorption peak). In the case of the thin shell DR-3, the change in the slope of these dependences clearly indicates the transition between two different etching stages. The initial slow change in absorbance is associated with the removal of CdS shell, while the second one relevant of faster changes in the absorbance is due to a quicker dissolution of the CdSe core, because Se is more easily oxidized.45 While for the thick shell DR-1, the etching time linearly varies with time for DR-1; no obvious stages can be seen, indicating that it is mainly the etching process of CdS shell.

Figure 22 .

 22 Figure 22. The CD, UV-vis and g-factor variations of DR-3 during the etching process (< 500 nm). The UV-vis value is the maximum value of the CD signal. The concentration is 5 times lower than that in Figure 16.

Figure 23 .

 23 Figure 23. The corresponding UV-vis and g-factor variations of DR-3 as shown in Figure 21 (< 500 nm). a) The CD spectrum of benzyl amine modified DRs after Cys exchange, b-e) the CD spectrum of DRs after one to four etching cycles and Cys exchange, f) the corresponding UV-vis absorption of different etching process. The concentration is 5 times lower than Figure 15.

Chapter 4 Study

 4 of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 206 References page 217

Figure 24 .

 24 Figure 24. The g-factor variation of DRs with different core sizes. a) DR-3, b) DR-4, c) DR-5, d) the g-factor value variation for the position of core and shell (The peaks are marked by black or red arrow, respectively).

Chapter 4 Study

 4 of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 208 References page 217

Figure 25 .

 25 Figure 25. The CPL variation of DR-3 during the etching process. a1) The CPL of benzyl amine modified DRs after Cys exchange; b1-e1) The CPL of DRs after one to four etching cycles and Cys exchange. a2-e2) The corresponding DC in volts, which stands for fluorescence intensity.

Figure 26 .

 26 Figure 26. The corresponding glum variation of DR-3 as shown in Figure 5. (a) The glum of benzyl amine modified DRs after Cys exchange; (b)-(e) the glum of DRs after one to four etching cycles and Cys exchange; (f) the glum variation of DR-3 during the etching process.

4. 2 .Scheme 1 .

 21 Scheme 1. The schematic diagram of the DRs etching process.

Chapter 4 Study

 4 of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching 216 References page 217

5. 2 . 2 5 Optically

 225 The CD and CPL signals of islands-like CdS shell modified NPLs & chiral mechanism study ........................................................................................................ 5.3 Methods ............................................................................................................... 5.3.1 Chemicals .................................................................................................. 5.3.2 Synthesis of the CdSe Core NPLs ............................................................. 5.3.3 Synthesis of the CdSe/CdS core-shell NPLs ............................................. 5.3.4 Ligand exchange of the NPLs with Chiral Cysteine Molecules ................... 5.4 Conclusion ........................................................................................................... 5.5 Bibliography ......................................................................................................... Chapter Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 225 References page 251

Chapter 5 Optically

 5 Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 226 References page 251

Chapter 5 Optically

 5 Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 227 References page 251

Chapter 5 Optically

 5 Scheme 1. Growth mechanism of islands-like shell CdSe/CdS NPLs. The one-pot growth process can be divided into three stages: In the first stage, the islands-like shell formed on the surface of CdSe NPLs. And then, in the second stage, the all covered shell will form. Last, the thick shell will further growth.

Figure 1 .

 1 Figure 1. (a, b) The TEM images of purified CdSe NPLs; (c, d) The TEM images of supernatant of the CdSe NPLs after centrifugation.

Figure 2 . 5 Optically

 25 Figure 2. UV and PL spectra of CdSe NPLs core.

Figure 3 .

 3 Figure 3. The absorption spectrum of OA caped and L-cysteine stabilized CdSe NPLs.

5. 2 . 2

 22 The CD and CPL signals of islands-like CdS shell modified NPLs & chiral mechanism study CD and CPL signals of islands-like CdS shell modified NPLs

Chapter 5 Optically

 5 Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 232 References page 251

Figure 4 .

 4 Figure 4. The CD and CPL signals of islands-like CdS shell modified NPLs. (a, b) The UV absorption and CD g-factor of islands-like CdS shell modified NPLs, (c, d) The PL emission and CPL glum of islands-like CdS shell modified NPLs, (e) The schematic diagram of chiral signals of NPLs, (f) The TEM image of islands-like CdS shell modified NPLs. The CD and CPL originates from interactions with the chiral cysteine molecules as depicted in (e).

Figure 5 .

 5 Figure 5. The CD signals of CdSe NPLs with (a) & without (b) purification. The g-factor of CdSe NPLs is: 2.1 × 10 -4 and 6.9 × 10 -5 for D-Cys CdSe NPLs with and without purification.

Figure 6 a

 6 Figure 6 a-d, the islands-like shell NPLs were purified and isolated from the secondary nucleation of CdS nanocrystals by precipitation with a few drops of ethanol. Although the selfnucleation has little effect on the fluorescence spectrum (CdS has no visible light emission) (Figure 6 f), it has a great effect on the absorption spectrum (Figure 6 e).By pre-removing CdS NPs resulting from the secondary nucleation, which is easily separated from the NPLs because of their difference in size, the CD signals of the nanosheets is more truly reflected. As comparison, during the c-ALD method, more purification processes

Figure 6 .

 6 Figure 6. (a, b) TEM and HRTEM images of the purified islands-like CdSe/CdS NPLs; (c, d) TEM and HRTEM images of the supernatant of the islands-like CdSe/CdS NPLs after centrifugation. (e, f) The comparison of PL and UV-vis spectra before and after purification.

Chapter 5 Optically 251 Figure 7 .

 52517 Figure 7. The PL evolution during the islands-shell growth process. (a) The normalized PL intensity of CdSe/CdS NPLs. (b) Trend of the emission wavelength with the reaction time. Red curve is a guide line for the eyes.

Figure 8 .

 8 Figure 8. The growth evolution of the islands-like CdS shell modified NPLs. (a) S0-CdSe NPLs, (b) S1-10min, (c) S2-30min, (d) S3-60min, (e) S4-120min, (f) S5-180min.

Figure 9 . 5 Optically

 95 Figure 9. The HRTEM images corresponding to Figure 8.

Figure 10 .

 10 Figure 10. Comparison of UV-vis absorption before and after the shell growth.

Chapter 5 Optically

 5 Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 238 References page 251 CD and CPL signal changes with the growth of the shell To develop a better understanding of the reasons of these excellent chiral signals, such as the birth of the CPL signals and the enhancement of the CD signals, we studied their variation with the growth of the islands-like shell. The CD and CPL signals changed as the CdS shell grew from controlled islands-like shell to real core-shell NPLs. This ligand induced-chirality of NPLs was obtained by a ligand exchange from the organic phase to the aqueous one thanks to Cysteine, and a storage in an excess of aqueous cysteine solution overnight prior to measurements. The optical properties of the samples are examined using absorption dichroism, circularly polarized luminescence. The absorption and induced CD response of a series of CdSe NPLs core and islands-like CdSe/CdS core/shell NPLs as a function of the growing shell between 450 nm to 650 nm are presented in Figure 11 and Figure 12.

Figure 11 .Figure 12 .

 1112 Figure 11. The g-factor value between 450 nm to 650 nm of a series of CdSe NPLs core(a) and island-likeCdSe/CdS core/shell NPLs (b-f). The islands-like shell is gradually formed as the shell growth.

Figure 14 a 5 Optically

 145 , the lateral size increases quickly in less than 30 min (S0-S2) (❶), which corresponds to the formation of the discrete islands-like CdS shell. Due to the nonuniform size of the islands-like shell, the lateral size increases significantly. While it remains Chapter Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 241 References page 251almost constant in the second stage from 30 min to 90min (S2-SX)(❷), this may be due to the coverage of the CdSe core NPLs between of the already existing islands-like CdS shell.

Figure 14 .

 14 Figure 14. a) The evolution of lateral size with the shell growth. b) The evolution of NPL thickness as the shell growth.

Figure 15 .Figure 16 .

 1516 Figure 15. The g-factor value between 350 nm to 500 nm of a series of CdSe NPLs cores (a) and island-likeCdSe/CdS core/shell NPLs (b-f). The islands-like shell was gradually formed with the shell growth.

Figure 17 .

 17 Figure17. The g-factor variation between 350 nm to 500 nm (corresponding to the CdS shell) with the islandslike shell growth.

Figure 18 .

 18 Figure 18. The glum value for the CPL of a series of CdSe NPLs cores and islands-like CdSe/CdS core/shell NPLs.

5 Optically

 5 3 mmol) of Cd(myristate)2, 0.012 g (0.15 mmol) of Se and 15 mL of ODE were introduced into a three-neck flask. The solution was degassed at 70 °C for an hour and 100 °C for 10 min Chapter Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 247 References page 251 under vacuum. After degassing step, the solution was heated to 240 °C under argon atmosphere. 0.055 g (0.2 mmol, in 2 ml degassed ODE) of Cd(Ac)2•2H2O was swiftly added to the reaction solution. After 10 min growth at 240 °C, 1 mL of OA was injected further protect the NPLs, and the temperature of the solution was decreased to room temperature. By changing the cadmium precursor injection temperature and the growth time, different shape and size of CdSe core NPLs can be obtained.

Chapter 5 Optically

 5 Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 248 References page 251 in the flask and the mixture was sonicated until the complete dissolution of the TAA. 200 μL of a 0.1 M Cd(OA)2 solution in ODE and octylamine mixture solution was then added to the reaction mixture. The reaction was allowed to proceed for different time at room temperature (from 10 min to 3 h). At various time intervals, aliquots with a needle-tip amount of the reaction mixture were removed and diluted by chloroform for further purification.

2 5 Optically

 5 mL hexane solution containing CdSe NPLs was mixed with 2 ml of cysteine solution. The mixture was stirred at room temperature for more than 48 h (although the NPs can transfer to water phase in several hours, enough time incubation is crucial). The reaction mixture was left to stand for 1 h to allow the phases to separate. The bottom aqueous layer was got with a syringe, and the Cys-CdSe NPLs were purified by precipitation with ethanol/DI water (4:1, Chapter Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 249 References page 251 two times). The purified Cys-NPLs were re-dispersed in DI H2O and stored at room temperature in the dark.

Chapter 5 Optically

 5 Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence 250 References page 251

6. 2

 2 Choice of the with various shapes ........................................................................ 6.2.1 Synthesis and properties of the anisotropic CdSe/CdS NPs in the organic solvent .................................................................................................................. 6.2.2 Ligand exchange on the CdSe/CdS NCs with chiral cysteine molecules and transfer to water. ......................................................................................................... 6.3 Study on the ligand-induced chirality of CdSe/CdS NPs ....................................... 6.3.1 State of the art ........................................................................................... 6.3.2 Analysis of the different structures ............................................................. 6.3.3 In-situ observations of the formation of chiral Tadpole-I ............................. 6.4 Simulation of the AR-dependent g-factor and the QY variation ............................. 6.4.1 The g-factor ............................................................................................... 6.4.2 The quantum yield ..................................................................................... 6.5 Methods ............................................................................................................... 6.5.1 Chemicals .................................................................................................. 6.5.2 Synthesis of the CdSe core ....................................................................... 6.5.3 Purification and acidification of the CdSe-TOP core................................... 6.5.4 Synthesis of CdSe/CdS tadpole NCs ......................................................... 6.5.5 Synthesis of CdSe/CdS NCs with different shapes .................................... 6.5.6 Ligand exchange of the CdSe/CdS NCs with chiral cysteine molecules..... 6.6 Conclusion ........................................................................................................... 6.7 Bibliography .........................................................................................................

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 259 References page 294

  , the XRD pattern and corresponding diffraction peaks of cores and the tadpoles match well with both W-CdSe (JCPDF No. 02-0330) and W-CdS (JCPDF No. 41-1049), respectively. The results demonstrate that as the shells grow thicker, the crystalline structures changed from pure W-CdSe crystals to pure W-CdS, and the diffraction peaks, such as (100), (101) and (110), show a slight shift towards larger 2θ directions due to the smaller lattice plane spacing of CdS shells.

Figure 2 X

 2 Figure 2 X-ray diffraction (XRD) patterns of powdered samples of CdSe cores (upper line) and CdSe/CdS Tadpoles (lower line). The XRD PDF standard card of CdSe (upper bars, JCPDF of No. 02-0330) and CdS (lower bars, JCPDF of No. 41-1049) are also exhibited under XRD results, respectively, as the references.

6. 2 . 2

 22 Ligand exchange on the CdSe/CdS NCs with chiral cysteine molecules and transfer to water.

Figure 3 .

 3 Figure 3. UV-vis absorptions and PL spectra before ligand exchange excited by a 450 nm monochromatic light of (a) CdSe cores with a Stokes shift of 15 nm, (b) CdSe/CdS Nanoflower with a quasi-Stokes shift of 162 nm, (c) CdSe/CdS Tadpole-III with a quasi-Stokes shift of 172 nm, (d) CdSe/CdS Tadpole-II with a quasi-Stokes shift of 167 nm, (e) CdSe/CdS Tadpole-I with a quasi-Stokes shift of 160 nm, (f) CdSe/CdS DR-1 with a Stokes shift of 12 nm, and (g) CdSe/CdS DR-2 with a quasi-Stokes shift of 121 nm. (h) The corresponding UV-vis absorption and PL spectra of the different shapes illustrated for the D-Cys-CdSe/CdS NPs.

Figure 4 .

 4 Figure 4. The relationship between PL spectra and shapes of different NCs. (a) The relationship between the quantum yield (QY) and shell-thickness of the different NCs, (b) The relationship between the size uniformity of the different NCs and the FWHM of PL spectra. (SD: Standard Deviation). All the data including shell-thickness, QY, FWHM and SD are imported from Tables 1, 2 and 3 for the different shaped NCs.

  g-factors and CPL glum of chiral cadmium-based nanocrystals induced by small chiral ligands for bare cadmium based QDs, nanorods, nanoplatelets (NPLs), and CdSe/CdS core-shell NCs (QDs and DRs). The comparison is based on the types of NCs (CdTe, CdSe and CdS), the shapes of NCs (0D, 1D and 2D), the crystal types (zinc blende (ZB) and wurtzite (W)), ligand types (mercapto-and thiol-free carboxylic acids), ligand concentration, shell-thickness of NCs and aspect ratio (AR) of NCs.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 267 References page 294

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 268 References page 294 used as surface ligands (Figure5and Figure6) where from now on, both D and L cysteine modified samples will be shown and compared on the Figures (L: black curves and D: red ones)).

  a), tadpole-(Figure 5 (b-d)) and long DR-(Figure 5 f) shaped samples show only a CdS CD activity because they have relatively thick shells (> 1 nm) with a large volume of CdS compared to the small CdSe core. While the short DRs (Figure 5 e) with thin shell still show a CD response corresponding to the CdSe cores, which agrees with previous work on chiral CdSe/CdS DRs. 20 It is particularly worth mentioning that Tadpole-I is able to maintain a high CD activity, as shown in Figure 5 d, which may be due to its special anisotropic morphology, even if the shell thickness is large (>10 monolayers).

Figure 5 . 6 :

 56 Figure 5. CD spectra of the L-/D-CdSe/CdS NCs with different shapes. (a) Nanoflowers, (b) Tadpole-III, (c) Tadpole-II, (d) Tadpole-I, (e) short dot/rods, and (f) long dot/rods.

Figure 6 . 1 a

 61 Figure 6. Corresponding anisotropic gCD factors and UV/vis absorption spectra of Figure 5. (a) Nanoflower, (b) Tadpole-III, (c) Tadpole-II, (d) Tadpole-I, (e) short dot/rods (DRs), and (f) long DRs.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 270 References page 294 Moreover, considering the CD responses of the CdS shell, the anisotropic gabs-factor maximum (~ 470 nm, Figure 6, Table

Figure 7 Figure 8 . 2 . 6 :

 7826 Figure 7 The dependence of gCD factors of D-Cys-CdSe/CdS NPs on the ratio of the total length to the largest diameter of the nanostructures. (F: Nanoflowers, T-III: Tadpole-III, T-II: Tadpole-II, T-I: Tadpole-I, DR-1: DR-1, DR-2: DR-2)

Figure 9 .

 9 Figure 9. Corresponding glum factors of different shaped CdSe/CdS NCs in Figure 8: (a) Nanoflowers, (b) Tadpole-III, (c) Tadpole-II, (d) Tadpole-I, (e) short DRs, and (f) long DRs.

6 :

 6 Figure 10 with the glum-factor, 55 defined as glum = 2(IL -IR )/(IL + IR ), where IL and IR are the emission intensities Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 272 References page 294 of the left-and right-handed polarized lights, respectively. They increase in the order of nanoflowers (Figure 10 a), tadpoles (Figure 10 (b-d)) and DRs (Figure 10 (e-f)).

Figure 10 .

 10 Figure 10. Mirror-image CPL spectra of the CdSe/CdS NCs with different shapes: (a) Nanoflowers, (b) Tadpole-III, (c) Tadpole-II, (d) Tadpole-I, (e) short dot/rods (DR-1), and (f) long dot/rods (DR-2).

Figure 11 .

 11 Figure 11. The same trend can be observed for the L-Cys-CdSe/CdS NCs (Table5). As stated

Figure 11 .

 11 Figure 11. The dependence of the glum factor values of D-Cys-CdSe/CdS NPs (a) and the photoluminescence QY (b) on the CdS shell thickness. (F: Nanoflowers, T-III: Tadpole-III, T-II: Tadpole-II, T-I: Tadpole-I, DR-1: DR-1, DR-2: DR-2).

6. 3 . 3

 33 In-situ observations of the formation of chiral Tadpole-I The induced chiral responses of these asymmetric nanostructures are chosen in the case of chiral Tadpole-I as a typical sample for the in-situ observation of their growth during the tail formation. As illustrated by the TEM images in Figure 12, the chiral Tadpole-I step-by-step siteselective growth of the CdS tail from a spherical CdSe/CdS QDs formation is confirmed.

Figure 12 .

 12 Figure 12. TEM and HRTEM images of the tadpole-I-shaped CdSe/CdS NPs growth evolution process. (a) Tadpole-I-2, 30 s, (b) Tadpole-I-3, 1 min, (c) Tadpole-I-4, 2 min, (d) Tadpole-I-5, 4 min, and (e) Tadpole-I-1, 8 min.

Figure 13 .

 13 Figure 13. CD spectra and UV/vis absorption spectra of the tadpole-I shape CdSe/CdS NPs growth evolution process. (a) Tadpole-I-2, 30 s, (b) Tadpole-I-3, 1 min, (c) Tadpole-I-4, 2 min, (d) Tadpole-I-5, 4 min, and (e) Tadpole-I-1, 8 min. (a-1 to e-1) CD spectra and (a-2 to e-2) UV/vis absorption spectra of tadpole shape NPs at different reaction times.

Figure 14 .

 14 Figure 14. Corresponding anisotropic gCD values of Figure 6. (a) Tadpole-I-2, 30 s, (b) Tadpole-I-3, 1 min, (c) Tadpole-I-4, 2 min, (d) Tadpole-I-5, 4 min, and (e) Tadpole-I-1, 8 min.

Chapter 6 :Figure 15 .

 615 Figure 15. The UV-vis absorption (a) and PL (b) spectra excited by a 450 nm monochromatic light of Tadpole-I CdSe/CdS NCs at different reaction times before ligand exchange. Tadpole-I-2, 30 s; Tadpole-I-3, 1 min; Tadpole-I-4, 2 min; Tadpole-I-5, 4 min; Tadpole-I-1, 8 min.

Tadpolea

  Diameter of the CdSe core determined from the absorption spectrum by Peng's equation (nm).43 b The head diameter of tadpoles. c,d PL emission peak position of the CdSe/CdS NCs and their full-width half-maximum (FWHM) before ligand exchange. e PL QY of the DRs before ligand exchange. f The absorption peak of CdSe cores. g The wavelengths of these CdS shell absorption features (λCdS).

Figure 16 .

 16 Figure 16. TEM and HRTEM images of Tadpole-I-shaped CdSe/CdS NPs with different lengths. (a) Tadpole-I-6, 22.5 nm, (b) Tadpole-I-7, 30.9 nm, (c) Tadpole-I-1, 43 nm, (d) Tadpole-I-8, 109.5 nm and (e) Tadpole-I-9, 180.8 nm.

Figure 20 .

 20 Figure 20. The dependence of gCD factors of D-Cys-CdSe/CdS Tadpoles-I on the ratio of the total length to the largest diameter of the nanostructures. (T-1: Tadpole I-1, T-2: Tadpole I-2, T-3: Tadpole I-3, T-6: Tadpole I-6, T-7: Tadpole I-7, T-8: Tadpole I-8, T-9: Tadpole I-9).

Figure 21 .

 21 Figure 21. Mirror-image CPL spectra of tadpole-shaped CdSe/CdS NCs with different lengths (a1-e1). (a) Tadpole-I-6, 22.5 nm, (b) Tadpole-I-7, 30.9 nm, (c) Tadpole-I-1, 43 nm, (d) Tadpole-I-8, 109.5 nm and (e) Tadpole-I-9, 180.8 nm. (f) The corresponding DC spectra of D-Cys-CdSe/CdS Tadpole-I NCs(a2-e2).

Figure 23

 23 c), even though they do have a thin CdS shell (< 2 nm, Figure 23 b), the CPL signal is difficult to observe. Beyond the small samples (Tadpole-I-2 to Tadpole-I-4), we can observe that the overall glum decreased straightforwardly with the aspect ratio increase (Figure 23 a), and the highest

6. 4

 4 Simulation of the AR-dependent g-factor and the QY variation This modeling part was performed, in collaboration with Theoreticians Prof. Xi ZHU's group from Shenzhen Institute of Artificial Intelligence and Robotics for Society (AIRS), The Chinese University of Hong Kong, Shenzhen. The model consists in two parts: the g-factor and the quantum yield of the quantum dots and their size dependence. The following explanations will detail both models.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 283 References page 294

Figure 24 .

 24 Figure 24. Sketches of circular dichroism from different mechanisms and methods of description. The ligands on the right part are cysteine, which attaches on the surface as shown. 60 𝑀 in the wave function 𝜓 denotes the Whittaker M function, 𝜆 𝑠 on the right denotes the penetration depth of the magnetic field.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 286 References page 294 with different shell thicknesses are illustrated in Figure25 a, in which the g-factor show a good correlation with our experimental results and match with previously published data reported by Purcell-Milton et al.52

Figure 25 . 6 :

 256 Figure 25. (a) Theoretical calculations of the spherical CdSe/CdS fitted with the experimental data. All the CdSe cores have a diameter of 2.8 nm, and we assume that one layer of CdS has a thickness of approximately 0.35 nm. 57 (b) Theoretical results fitted with the data of the tadpole morphology. The model aligns well with the experimental data, and the CD signal (g-factor) of tadpoles yielded a different size dependence from that of the spheres, whereas both the g-factor and QY have the same size dependence behavior.

Figure 26 .

 26 Figure 26. Scheme of the FRET (Förster resonance energy transfer) mechanism in tadpole morphology. Becausethe head is larger in size, the quantum confinement on the head is less severe, resulting in a smaller bandgap, while the excitation on the tail is higher. This difference in the excitation energy yields the FRET phenomenon, and thus the diffusion length is considered to be proportional to the length of tadpoles.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 289 References page 294

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 290References page 294According to the work by Singh,67 both the QYs of the singlet and the triplet 𝜙 𝐷 are proportional to the exciton diffusion length 𝐿 𝐷 , i.e. 𝜙 𝐷 ∝ 𝐿 𝐷2 . Since the exciton diffusion length is proportional to the size of the QDs, it can be assumed to be a second-order relationship with both the number of shells and the aspect ratio, given that the core and the head of the tadpole barely change in size. With the tadpoles growing larger, the diffusion length, which refers to the standard deviation of the displacement of the exciton would grow proportionally. Thus, the diffusion length 𝐿 𝐷 𝑆 is also proportional to the size of the tadpoles, which makes 𝜙 𝐷 also proportional to the size. As shown in Figure 25 a and b (blue line and triangles), the corresponding calculated QY curves fitted with our experimental data on different shellthickness QDs and a series of Tadpole-I structures with different AR values.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 291 References page 294

6. 5 . 4

 54 Synthesis of CdSe/CdS tadpole NCsIn a typical synthesis of CdSe/CdS tadpole NCs via seeded growth, CdO (115.8 mg) was mixed in a 50 mL flask together with TOPO (3 g), ODPA (285 mg) and OA (1.02 ml). After alternately exposing the flask to vacuum conditions and an argon atmosphere at least 5 times at 150 °C, the resulting solution was heated to 320 °C to cause the solution to become a completely transparent liquid without any solids. Then, 1.5 mL of TOP was added; the temperature recovered to 320 °C and 50 nmol of CdSe was added. The sulfur-tri-n-octylphosphine (S-TOP, 0.5 M, 3 mL) was divided into four injections at 2-min intervals (0 min, 2 min, 4 min, and 6 min).The CdSe/CdS NCs were allowed to grow for approximately 8 min after the injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was performed to separate the NCs from the side products and unreacted precursors. Tadpoles NCs with different tail lengths were used by adding different amounts of CdSe core, ranging from 5 nmol to 200 nmol.

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 292 References page 294 6.5.5 Synthesis of CdSe/CdS NCs with different shapes Different shapes of NCs can be synthesized just by changing the ligands. By changing the ratio of ODPA and OA, different shapes can be obtained, such as nanoflower, Tadpole-III, Tadpole-II, Tadpole-I, and DRs. For different lengths of DRs, different ratios of HPA were added instead of OA. 6.5.6 Ligand exchange of the CdSe/CdS NCs with chiral cysteine molecules

Chapter 6 :

 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies 293 References page 294 length (> λ/10) attenuated the chiroptical responses because inter-particle interactions affected the overall chirality;

  Photo-deposition of Gold

Figure 1 a

 1 Figure 1 a) Schematic drawing, and b) photograph of cuvette setup using blue laser.

Figure 2 .

 2 Figure 2. The PI curve for exit power of Blue laser.

Figure 3

 3 Figure 3 Power Incident to the cuvette as a function of the driving current.

Figure 4 .

 4 Figure 4. Fits of the variations of the power at the laser head and of the Incident to the cuvette as a function of the driving current with same onset of the laser emission.

Figure 5 .

 5 Figure 5. Transmitted power as a function of the incident power for a 1 cm quartz cuvette filled with a CdSe/CdS DRs solution.

7. 2 . 1 6 .

 216 Schematic picture of the Blue CPL cuvette setup A schematic drawing and a photograph of the setup, using the same blue laser and including a /4 wave plate are shown in Figure This is the same setup as before where a λ/4 plate is added at the right angle between the thrid mirror and the cuvet used to get a L-/D-polarized beam. The cuvette is still stirred to continuously homogenize the particle solution and prevent sedimentation; the stirring speed is set again at 300 rpm unless otherwise stated. All experiments were performed under room temperature.

Figure 6 a

 6 Figure 6 a) Schematic drawing, and b) photograph of the CPL setup, note the presence of a /4 wave plate.

Figure 8

 8 Figure 8 Incident Power as a function of current for Blue -45°-CPL laser.

7. 3 . 1

 31 Figure 9.

Figure 9

 9 Figure 9 (a) Schematic drawing, and (b)photograph of the cuvette setup when using a UV laser.

Figure 10

 10 Figure 10 UV laser apparatus a) photo of the laser, b) schematic functional parts

Figure 11

 11 Figure 11 Power incident to the cuvette as a function of panel power.

using

  CdSe/CdS DRs as a model system. By a combination of light intensity, deposition time, Au/DRs ratio, and different morphologies, we observe and explain the dependence model of the evolution of gold deposition sites. 1) The concentration of hole scavenger plays a crucial role to decrease the double & multi-sites deposition, and with a little contribution of nondeposition. 2) The laser beam intensity plays a role to decrease the multi-sites deposition, and with little contribution for double & non-deposition. 3) The Au/DRs ratio plays a role in Chapter 8 Hole scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 323 References page 364 decreasing the ratio of non-deposition, with little contribution of double & multi-sites deposition. 4) The reaction time plays little contribution for the single-tip deposition at the beginning, however, with long-term deposition, the double & multi-sites deposition seems to increase gradually. We finally provide a model which gathers the whole set of experiments on a master curve showing how the gold ND growth depends collectively on the hole scavenger and gold precursor concentrations (chemical parameters) as well as time and light intensity (physical parameters).

3. 2 nm

 2 CdSe cores with different concentrations. As an example, in Figure 1, we present the TEM and spectroscopy data of DR-1; due to the large volume of CdS shell, its UV-vis absorption spectrum shows an excellent property for the absorption of a Blue laser, which gives the Chapter 8 Hole scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 327 References page 364 chance for a blue laser photodeposition. The physical properties of the DRs with different shapes we used for deposition are given in Table1. Unless otherwise noted, the DR-1 nanoobjects were used as substrates.

Figure 1 .

 1 Figure 1. TEM and spectroscopy of DR-1. a-b) TEM images of DR-1, c) The UV absorption spectral of DR-1, d) The PL emission spectral of DR-1.

First, in order 2 and 2 ,

 22 photodeposition, we conducted some comparative experiments, as shown in Figure2and Table2, where the various parameters were tested separately. 1) Laser and MeOH on their own can both generate the gold deposition process, however, the yield and the selectivity are rather low. 2) In the absence of MeOH (C1), even if the main goal is the synthesis of single-tip nanoheterodimers (NHDs), it is difficult to achieve a high ratio of single-tip HNPs due to the uncontrollable tiny Au NDs. 3) As a comparison, Banin and coworkers made a 1.5-1.8 nm Au tip in the dark at room temperature in 1 hour. While in our work, the 2.34 nm Au NDs can deposited on the DRs surface with a ratio of 94% within 8 min (C2). This means that without laser, MeOH as a hole scavenger can effectively increase the reaction rate in the dark and produce deposition at defects, multi-sites (75%) deposition being then dominant. In the absence of either of them (C3), and during the average reaction time used for the laser deposition time, the reactivity is very low and the bare DRs remain predominant.

Figure 2 .

 2 Figure 2. Control experiment results: a) C1: no MeOH, power: 200 mW (exit laser head), KAuCl4 (4×10 -5 M), deposition time (8min); b) C2: no laser, MeOH (2.5 M), KAuCl4 (4×10 -5 M), deposition time (8min); c) C3: no laser, no MeOH, KAuCl4 (4×10 -5 M), deposition time (8 min).

Figure 3 .

 3 Figure 3. The effects of MeOH on the photodeposition process. a) 0 M, b) 0.313 M, c) 0.625 M, d) 1.25 M, e) 2.5 M. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min, power: 200 mW (exit laser head).

Figure 4 .

 4 Figure 4. The variation of the ratio of single-tip HNPs with the MeOH concentration as shown in Figure 3. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min, power: 200 mW (exit laser head).

Figure 5 .

 5 Figure 5. The effects of MeOH on the photodeposition process. a) The ratio variation of mono-, double-sites, & multi-sites HNPs and pure CdSe/CdS DRs without Au deposition, b) the ratio variation of mono-tip compared to the sum of double-sites & multi-sites HNPs. [DR-1] = 2.5 × 10 -9 M; [A 3+ ] = 4 × 10 -5 M, time: 8 min, power: 200 mW (exit laser head).

Figure 6 .

 6 Figure 6. The relationship of etching degree of DR-1 with the concentration of the hole scavenger. a) The variation of DRs length with the hole scavenger (MeOH) concentration, b) The decrease of DRs etching rate as the concentration of hole scavenger (MeOH) increases. Red lines are guides for the eyes. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min, power: 200 mW (exit laser head).

7 .

 7 CdSe/CdS DR-1 as the substrate at [DR-1] = 2.5 × 10 -9 M, and the gold salt at [Au 3+ ] = 4 × 10 -5 M. The representative TEM images of the Au-CdSe/CdS HNPs obtained under beam intensities of 20, 50, 100, 200, 300 mW at the head of the laser are shown in Figure They are to be compared to those with the no laser deposition and in the dark where the single-tip DRs are only 11.7% yield and have a gold NDs size of only 2.6 nm (Table

Figure 7 .

 7 Figure 7. The effects of the laser intensity on the photodeposition process. a) 20 mW, b) 50 mW, c) 100 mW, d) 200 mW, e) 300 mW at the laser head. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min.

Figure 8 .Figure 9 .

 89 Figure 8. Results of the reaction mixture when let in the dark. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min. b) is a zoom of part of a).

Chapter 8 Hole

 8 scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 335 References page 364 beneficial to the realization of single-point deposition.

Figure 10 .

 10 Figure 10. The effects of laser intensity on the photodeposition process. a) The ratio variation of mono-, double-sites, and multi-sites HNPs and pure CdSe/CdS DRs without Au deposition, b) the ratio variation of mono-tip compared to the sum of double-sites and multi-sites HNPs. [DR-1] = 2.5 × 10 -9 M; [Au 3+ ] = 4×10 -5 M, time: 8 min, [MeOH] = 2.5 M.

Figure 11 .

 11 Figure 11. The effects of irradiation time on the photodeposition process with and without hole scavenger. a) the variation of the ratio of multi-sites deposition [MeOH] = 0 M; b) the variation of the ratio single-tip deposition & of multi-sites deposition [MeOH] = 2.5 M; c) d) and e) comparison by types of deposition.at 0 and 2.5M. Curves are guides for the eyes. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, 200 mW (exit laser head).

Figure 12 . 8 364 Figure 13 .

 12836413 Figure 12. The effect of irradiation time on the photodeposition process. a) 1 min, b) 2 min, c) 4 min, d) 8 min, e) 16 min, f) 32 min, g) 64 min. with [MeOH] = 2.5 M, [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M power: 200mW (exit laser head).

Figure 14 .

 14 Figure 14. The relationship between the etching degree of the DRs with the irradiation time. a) The variation of the length of DRs with the irradiation time, b) The etching ratio of DRs as irradiation time changes. with [MeOH] = 2.5 M, [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, power: 200 mW (exit laser head).. Curve are guides for the eyes.

Figure 15 c.

 15 Additionally, the double & multi-sites also increased a little more than at 32 min as also observed after long reaction time in the literature.7, 19,[23][24][25]

Figure 15 .

 15 Figure 15. TEM images of long time irradiations: a) 32min; b) 64min; c) the contact angle of the gold dot onto the DRs. [MeOH] = 2.5 M, [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, power: 200 mW (exit laser head).

8. 2 . 4 . 4

 244 Gold concentration: High yield of single-tip Au-CdSe/CdS HNPseffect of Au/DRs ratio Based on the above analysis, we can conclude that the hole scavenger and laser intensity are essential to achieve a high ratio of single-tipped deposition. We then fixed these two parameters and studied the influence of the gold precursor concentration on the single-tip distribution. The photodeposition experiments using the DDA modified KAuCl4 in toluene were

Figure 17 and

 17 Figure 17 and Table7shows the TEM images, the diameter variation of Au NDs in and the

Figure 16 . 8

 168 Figure 16. The effects of Au 3+ /DRs ratio on the photodeposition process with hole scavenger. a) 1,000, b) 2,000, c) 4,000, d) 8,000, e) 16,000, f) 32,000, g) 64,000 h) 128,000. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, time: 8 min, power: 200 mW (exit laser head).

Figure 17 .

 17 Figure 17. The diameter variation of Au NPs in HNPs. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, time: 8 min, power: 200 mW (exit laser head).

Chapter 8 HoleFigure 18 .

 818 Figure 18. The effects of Au/DRs ratio on the photodeposition process by changing the concentration of the Au 3+ precursor: variation of single-tip HNPs, the double-sites + multi-sites HNPs, and of the pure CdSe/CdS DRs without Au deposition. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, time: 8 min, power: 200 mW (exit laser head).

Figure 19 .

 19 Figure 19. The relationship between the etching degree of the DRs and the concentration of Au 3+ precursor in solution. a) The variation length of DRs as the irradiation time changes, b) The etching ratio of DRs as irradiation time changes. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, time: 8 min, power: 200 mW (exit laser head).

Figure 20 .

 20 Figure 20. TEM images of high Au/DRs ratio: a) 64,000, the ratio of free Au NPs is 3.9%; b) 128,000, the ratio of free Au NPs is 23.5%.

Chapter 8 HoleFigure 21 .

 821 Figure 21. The effects of Au/DRs ratio on the photodeposition process by changing the concentration of CdSe/CdS DRs. The ratio evolution of single-tip HNPs, the ratio evolution of double-sites + multi-sites HNPs, the ratio evolution of pure CdSe/CdS DRs without Au deposition. [MeOH] = 2.5 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min, power: 200 mW (exit laser head).

1 ) 2 )

 12 Since by definition 𝛼 = 𝜎 𝑎𝑏𝑠 . ( 𝑁 𝑉 ) where N/V is the number of DRs per unit volume and 𝜎 𝑎𝑏𝑠 their absorption cross-section, we can deduce that 𝛼 ∝ [DR]. Moreover, the expression of the beam intensity is 𝐼 ≈ 𝐼 0 = 2𝑃 𝜋𝜔 0 2 , where P is the mean beam power in the middle of the cuvette (𝑃 = 0.695 ⋅ 𝑃 ℎ𝑒𝑎𝑑 from the calibrations) and 𝜔 0 = 0.5 cm to cover the cuvette. As 0 is kept constant, we finally find that 𝑔 ∝ 𝑃[DR]. The adsorbed hole scavenger concentration [HS] ads may be obtained by a Langmuir-Hinshelwood-type model for adsorption. Since we got difficulties to get values of KHS and (CT)HS from the literature, we first try to fit our data (from Figure 4) according to the low [HS]bulk limit of the Langmuir-Hinshelwood model. A fit of D according to the expected power law behavior D = a[HS]bulk 1/6 all other variables being constant for all the tested HS concentrations is illustrated in Figure 22, and shows, with a correlation better than 0.84, that our growth data as a function of [HS]bulk are quite reasonably well retrieved.

Figure 22 .] 3 ) 23 ,

 22323 Figure 22. Variation of the gold diameter with the bulk MeOH concentration. [DR-1] = 2.5 10 -9 M; [Au 3+ ] = 4 × 10 - 5 M, time: 8 min power: 200 mW (exit laser head).

Figure 23 .

 23 Figure 23. Variation of the gold diameter with the bulk Au 3+ concentration, [DR-1] = 2.5 × 10 -9 M, [MeOH] = 2.5 M, time: 8 min power: 200 mW (exit laser head).

Chapter 8 HoleFigure 24 .Figure 25 .

 82425 Figure 24. Variation of the gold diameter with Sqrt([MeOH]bulk.[Au 3+ ]bulk.[DR].P).t

Figure 26 .

 26 Figure 26. Variation of the gold NDs diameter with the exposure time. [DR] = 2.5 × 10 -9 M, [MeOH] = 2.5 M and [Au 3+ ] = 4 × 10 -5 M, power: 200 mW (exit laser head).

Figure 27 . 8

 278 Figure 27. Variation of the gold NDs diameter with the exposure time. [DR] = 2.5 × 10 -9 M, [MeOH] = 2.5 M and [Au 3+ ] = 4 × 10 -5 M, power: 200 mW (exit laser head).

Figure 28 .

 28 Figure 28. Variation of the gold NDs diameter with the DRs concentration in log-log representation also showing a power law fit with an exponent -1/3. A pure power law fit gives an exponent of 0.31.

  model (before completion) assuming pseudo first order reactions. This description allows understanding how the different parameters involved in the photodeposition of Au 3+ on DR can assemble to propose a general point of view to control growth via different means. Moreover, insights raised by the investigation of the transition to completion suggest that from a practical point of view a way to get a given single gold ND size is (1) to add enough MeOH to ensure one gold dot per DR, (2) to choose the right [Au 3+ ]/[DR],

Chapter 8 Hole

 8 scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 350 References page 364

Scheme 1 .

 1 Scheme 1. Schematic diagram of the change of the ligand on the surface of CdSe/CdS DRs.

Figure 29 .

 29 Figure 29. The effects of the laser power on the photodeposition process without hole scavenger. a) With the ratio variation of single-tip HNPs, the ratio variation of double-sites + multi-sites HNPs, and of pure CdSe/CdS DRs without Au deposition. b) with multidots HNPs on its own. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, [MeOH] = 0 M, time: 8 min. Curves are guides for the eyes.

Figure 31 paragraph 8 . 2 . 4 . 1 ,

 318241 Figure31 for times as long as 64 min. As compared to the evolution process with a hole

Figure 30 . 364 Figure 31 .

 3036431 Figure 30. The effect of the irradiation time on the photodeposition process on unprotected DRs (irradiation power: 100 mW). a) 4 min, b) 8 min, c) 16 min, d) 32 min, e) 64 min. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, [MeOH] = 0 M

Figure 31 b

 31 Figure31 b, also shows that the length of the DRs strongly changes even after only 4 min of irradiation (9.6%) (remember this % is reached only after an irradiation of 64 min in the presence of HS ( § 8.2.4.3), and goes on to decrease by 17 % at 64 min. This is consistent with

Figure 32 and Figure 33 ,

 3233 Au 3+ precursor effect on Au photo-deposition DR-1 were also chosen for these experiments, [DR-1] = 2.5 × 10 -9 M, [MeOH] = 0 M time is 8 min and the power is 100 mW at the laser exit. As shown in Table11, the ratios of single-tip, double-and multi-sites follow the same trend as the laser intensity evolution is observed and is very different from what is obtained with a hole scavenger, where the single-tip ratio is significantly improved and the low double-& multi-sites are maintained relatively.

Figure 32 . 8 364 Figure 33 .

 32836433 Figure 32. Au 3+ /DRs ratio impact on the photodeposition process by changing the concentration of the Au precursor based on photodeposition on DRs with more defects. [DR-1] = 2.5 × 10 -9 M, [MeOH] = 0 M, time: 8 min, power: 100 mW at the laser exit. a) 1,000, b) 2,000, c) 4,000, d) 8,000, e) 16,000.

Figure 34 .

 34 Figure 34. The relationship between the etching degree of the unprotected DRs and the concentration of Au 3+ precursor based on the photodeposition on the unprotected DRs. a) The variation of the length of DRs as the irradiation time changes, b) The etching ratio of the DRs as irradiation time changes. [DR-1] = 2.5 × 10 -9 M, [MeOH] = 0 M, time: 8 min and the power: 100 mW at the laser exit. Lines are guides for the eyes.

7

 7 CdSe/CdS NPs morphology effect on Au photodeposition

Chapter 8 Hole

 8 scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 361 References page 364 the flask to vacuum and argon at least 5 times at 150 °C for more than 1 h, the resulting solution was heated to 300 °C to make the solution become completely transparent liquid without any solids. Then, the temperature was increased to 350 °C and a mixed solution of S:TOP (0.5 mL, 2.5 mol/L) and the above CdSe-TOP solution (100 µL) were injected into the flask, which caused the temperature to naturally decrease to ~310 °C , and then recovered to the pre-injection temperature within two minutes. The CdSe/CdS DRs were allowed to grow for approximately 8 minutes after the injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the nanocrystals from the side products and unreacted precursors. CdSe/CdS DRs with different shell thicknesses was made by different reaction temperatures and reaction times (for instance, 320 °C for thin shell (DR-2) and 380 °C for the thick shell (DR-5)), and the different aspect ratio samples (from DR-6 to DR-9) were synthesized by adjusting the concentration of the core and the proportion of the shell precursor.

Chapter 8 Hole

 8 scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 362 References page 364

  i. Without MeOH; ii. Without laser expose; iii. Without MeOH and laser expose; iv. Without CdSe/CdS semiconductor DRs; v. Using more defects CdSe/CdS semiconductor DRs without MeOH; vi. Using different shell thickness of CdSe/CdS semiconductor DRs; vii. Using different length of CdSe/CdS semiconductor DRs.

Chapter 8 Hole

 8 scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 363 References page 364

Furthermore, in Chapter 6 , 1 .

 61 photoluminescence quantum yield (QY).In Part 3, I focused on the photodeposition of Gold (Au) nanodots on the DRs semiconductors induced by laser. First, I described the setup configuration and the laser control for the photodeposition process (Chapter 7). Three different setups were built and used to perform the photochemical deposition. In Chapter 8, I revisited the synthesis of single-tip Au/CdSe/CdS HNPs by a photochemical deposition on DRs of a DDA modified KAuCl4 precursor in toluene using a blue laser (473 nm within the bandgap of the DRs) in a cuvette setup. The yield of single-tip HNPs has been maximized and the optimal systematically reproducible one is 94% and was achieved simply by introducing an external small hole scavenger such as MeOH in the reaction mixture. Beside this high ratio of single-tip HNPs, we can also just by one patch

Figure 1 .

 1 Figure 1. TEM images of single-tipped Au-CdSe/CdS HNPs on different morphologies made in cuvette setup by blue laser. a) Tadpoles, b) tetrapods, c) QDs. With [MeOH] = 2.5 M, [Au 3+ ] = 4 × 10 -5 M power: 200 mW (exit laser head), time: 8 min, CdSe/CdS as substrate (optical density (OD) of 0.11 at wavelength 473 nm).

Figure 2 .

 2 Figure 2. TEM images of Au-CdSe/CdS HNPs made in UV laser cuvette set up. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, [MeOH] = 0 M, power: 57 mW (Ppanel), time: 8 min.

Figure 3 ,

 3 the DRs have a tendency to self-assemble which maybe the reason why the chirality increases. Moreover, the CPL glum is different based on L-or D-Cys-DRs. However, the PL emission is easily quenched on multi-sites Au-DRs HNPs (Au need less than 2 nm), which limits the generation of CPL signals. More research based on the CD chirality still needs to be done. As a comparison, we studied the photodeposition by CPL laser without chiral ligands in oil phase, as shown in Figure 4, there's no obvious difference (5.4 ± 1.1 nm and 5.6 ± 1.4 nm). Which means that without the CD chiral properties, the CPL laser does not contribute to difference. Anyway, more studies remain to be performed on this topic.

Figure 3 .

 3 Figure 3. TEM images of multi-site Au-DRs HNPs made in cuvette set up by exposed CPL blue laser. The Cys-DRs was used as the substrate. [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 8 × 10 -5 M, power: 200 mW (exit laser head), reaction time, 8min.

Part 1 State of the Art 27

References page 70

state of the art of the current progress in ligand-induced chirality in Table 1 where, we mainly summarize the current CD anisotropy g-factors and CPL glum for chiral cadmium-based II-VI semiconductor NCs induced by small chiral ligands.

	Part 1	State of the Art
	1.2.3 Summary and outlook
	Gun'ko and co-workers were the first to report on the observation of a CD response
	induced in the excitonic transitions of CdS quantum dots (QDs) capped with chiral
	molecules. 147 Chirality in QDs may be due to a range of mechanisms including a chiral
	shell layer, 147, 169, 176, 177 chiral dislocations and defects, 139, 141, 148 ligand induction, 100,
	115, 161 and chiral assemblies. 142, 143, 146, 171 Among these mechanisms, the ligand
	exchange with chiral molecules produced tremendous advances to obtain QDs with
	divers choices to design their surface chemistry and impart their chemical and optical
	properties. However, CPL, a counterpart of CD in terms of emission, has been seldom
	reported in semiconductor NPs because of the low accessibility of the CPL instruments,
	the theoretical background on this topic and the complex experimental control that is
	required over the synthesis. Up to now, only the spherical and rod-shaped
	semiconductor NPs samples with chiral-ligand-induced CPL have been reported 100, 101,
	157, 174 , in which L-and D-cysteine induced CPL signals on achiral NCs. This may be due
	to the increased defects on the bare NCs after the ligand exchange that result in a
	lower photoluminescence quantum yield (PL-QY), which is a necessary condition for
	CPL signals. Although a shell layer can efficiently improve the PL-QY of NCs, the
	increase in this shell thickness in spherical NCs does significantly reduce the chiral
	conduction, which limits the development of CPL.
	the case of CdSe/CdS core/shell NPLs made by colloidal atomic-layer-deposition We gathered the As shown in Table 1, we compared the bare cadmium based QDs, NRs, NPLs, and
	(c-ALD) technique, a flat CdS shell coating could be obtained, and the optical CdSe/CdS core/shell NCs (QDs and DRs). The comparison is based on the type of NCs
	properties of these NPLs were enhanced with respect to the core-only NPLs. Recently (CdTe, CdSe and CdS), the shapes of NCs (0D, 1D and 2D), the crystal types (zinc blende
	Gaoling et al. used this layer-by-layer method to improve the chemical stability of the
	NPLs in aqueous phase, while the intense CD signal almost completely disappeared
	after coating with a very thin flat CdS shell and no CPL was reported. 161 Here, we will
	try to find a solution for the CPL in NPLs in chapter 5. In which, we chose a one-pot
	approach for CdSe/CdS core/shell NPLs first reported by Dubertret and coworkers. 120
	48
	References page 70

Part 1 State of the Art 50 References page 70Table 1 .

 1 CD anisotropy g-factors and CPL glum for chiral cadmium-based NPs induced by smallmolecule chiral ligands

	Part 1						State of the Art
	CdSe/3C dS		576	4.5; T=0.85		0.12/584
	NPs Type CdSe/4C dS CdSe/5C dS CdSe CdSe/Cd S CdS CdTe CdSe/Cd S	Shape Dots Dots Dots dots	λabs, nm a 581 584 596.2 543.0 584 410.1 550	Ø b 4.9; T=1.05 5.2; T=1.2 4.4 2.9 5.2 3.7 3.2 3.58; AR=7.2	Ligands L-Cys 0.15 mg/ml Cys 0.42 mg/ml Cys L-Cys 1.60 mg/ml Cys L-Cys D-Cys	gCD(×10 -4 ) c / λCD (nm) d 0.10/588 0.08/590 -2.1 /587.8; 0.35/605 +1.9/569.0 -0.8/532.2; 0.55/605 +0.7/548.8 0.8/379.5 0.22/605 0.9/520 5.2/445-461	glum (×10 -4 ) / λCPL (nm) e / 30 / / / /	ref 7
	CdSe CdS CdSe CdSe/Cd S CdSe a Band-edge absorption maximum (in nm); b Diameter (in nm) of NCs; c CD anisotropy g-factors; Dots 595.0 4.4 L-HomoCys 1.3 /550.4 / 595.2 4.4 N-Ac-L-Cys 0.5 /584.6 546.4 2.9 N-Ac-L-Cys 0.4 /584.6 Dots 407.5 3.7 N-Ac-L-Cys DRs 3.58; 12/445-461 AR=7.2 / D-cys, D-his / 3.66; 8.4/445-461 AR=3.6 0.4/401.0 / 426.2 4.3 N-Ac-L-Cys 0.4/415.5 Dots / 4.2 N-Ac-L-Cys* 3.45; 5.9/445-461 AR=3.1 -1.7/572; +2.0/538 N-acetyl-L-aspartic acid* -3.2/560; +6.0/528 L-TA* +3.8/557; -7.0/528 L-MA* -2.2/557; +3.6/528 L-lactic acid* -1.2/572; +1.0/538 L-CPA* DRs 615 5.24; 2.03/630.0; -4.46/664 AR=3.3; 1.14/598.8 .3 T=0.77 D-Cys 618 7.11; 0.46/626; -1.77/671 AR=3.0; 0.22/601.8 .8 T=1.89 -0.25/572; +0.26/538 Dots / 3.0 L-MA +0.44/539; -0.48/517 / CdSe Dots / 3.0 L-Cys +1.24/518; -0.89/536 d Wavelengths of the most intense positive or negative CD bands (nm); / CdSe Dots / 3.0 L-TA +0.56/515; -0.43/541 e CPL anisotropic glum at the most intense CPL wavelength (nm); / 516 2.96; f Different monolayer of zinc blende nanoplatelets (ZB-NPLs); +0.39/528; -0.12/509 g 3.5 monolayers of wurtzite nanoplatelets (3.5 ML WZ-NPLs); AR=1 551 2.98; AR=1.7 h The lateral size was chosen as the comparison for the size of nanoplatelets. +3.38/549; -3.22/529 * Using DMF as solvent; ** Using hexane or NMF as solution;	0
	CdSe	NRs	553 571	2.96; AR=1.9 2.97; AR=2.7	D-Cys	+4.25/555; -3.95/533 +4.83/571; -3.30/553	/
			576	3.03; AR=4.2		+4.29/577; -2.33/557
			578	3.02; AR=7.3		+3.51/582; -1.40/552
	CdSe	NRs	/	20.4; AR=1.8	D-Cys	0.79/539-558	/
	CdSe	4.5 ML NPL f 3.5 ML WZ NPL g	462 456	/ /	L-Cys	-6.7/491 +53/456	/ /
	CdSe	5 ML NPL f	554.8	21.7 h	Cys	8.0 /558	/
	CdSe	2.5 ML NPL f	396	400 h 150 h 80 h	N-Ac-Cys**	-29/438; +25/452 -11/433; +11/445 -3.6/431; +4.0/445	/
	CdSe/1C dS CdSe/2C dS	Dots	560 572	3.7; T=0.45 4.1; T=0.65	D-Cys	0.42 /571 0.20/578	/
					51
	References page 70

Part 1 State of the Art 52

	1.3 Laser induced deposition of Au-CdSe/CdS hybrid
	nanoparticles
	Over the last decade, colloidal hybrid semiconductor-metal nanoparticles (HNPs)
	have been the object of a great interest due to the synergistic properties produced by
	different combinations of conductive and semiconductor structures within a single
	nano-object. These HNPs show both metal and semiconductor properties with
	possible applications in various fields, such as light-induced charge separation, 178
	References page 70

by Chen et al. in an in situ photodeposition of cobalt on CdS nanorod 219 By contrast to the gold deposition, 110 a site selectivity was not observed.

	and increase the deposition rate. However, when Singh et al. used a one to one ratio
	of water and isopropanol for his photodeposition, he only obtained multi-sites
	NHPs. 212, 213 There is then still some room to work on this Hole Scavenger problematic.
	1.3.2.3	Deposition with different metal NPs
	In 2008, the decoration of CdSe, 214 CdS, 215 and CdSe/CdS 216 nanorods (NRs) with
	platinum (Pt) has been obtained by the groups of Banin, Mokari, and Alivisatos,
	respectively. The pH-controlled reduction of Pt precursor in aqueous solution, 214 the
	reduction of a platinum precursor in an organic solvent, 215 and the light-induced
	photodeposition of Pt from an organic solution on the semiconductor surface in the
	presence of triethy lamine (TEA) as a hole scavenger 216 have been employed. However,
	as compared with the deposition of Au, it happened that the formation of multi-sites
	was the easiest to do, this may be due to the surface defects or sites of incomplete
	passivation having a greater impact on the deposition than in the case of Au. It was
	found very difficult to obtain colloidal semiconductor nanostructures with Pt
	nanoparticles bigger than 5-6 nm. Moreover, the average size of photodeposited Pt
	NPs varied only slightly upon experimental conditions, remaining in the range of 1.5
	to 2.7 nm for CdSe/CdS NRs, 216 or 4.3 to 5.7 nm for CdS NRs 215 .
	Cobalt NPs on the tips of CdSe 217 and CdSe/CdS 218 NRs also have been reported in
	2009, and recently,
		Carbone
	et al. 191 found a method by adding 10% of ethanol (EtOH) in chloroform (CHCl3) under
	UV light, to create large gold domains (up to 15 nm) exclusively on one side of CdS or
	CdSe/CdS quantum rods. The advantage of this method is that the EtOH acts as an
	electron donor, reacting with the photogenerated holes in the semiconductor, thus
	allowing the photogenerated electron on the rod to reduce the gold cation. So that,
	this method allowed the synthesis of large Au domains on CdSe@CdS or CdS NR tips
	References page 70

Table 2

 2

	, we divided the effects based

Part 1 State of the Art 65 References page 70

Table 2 .

 2 Recent works about the Au deposition on cadmium chalcogenide semiconductors.

			Deposition conditions			DRs information		Au concentration	Results (single, double, multi-site)	ref
	T (K)	Ligands	Light	Time	Type of semiconductor	Length	Diamet er	Core diamete r	Shell-thickne ss
	293K 273K	DDA, DDAB			CdSe/CdS, 1.7 ml,						Multi-sites Single-tip
	293K 273K 293K 273K 273K 313K 313K 273K	ODA, DDAB TOA, DDAB No amine ligands DDA, DDAB	473 nm 40 mW, laser Dark 473 nm Dark	30min 1h	OD=1.2 (450-470 nm); ε=1.5*10 -7 L/(mol cm) CdS	56 nm 120 nm 58nm	5.3 nm 6 nm 5.1nm	2.9 nm 3.4 nm CdS	1.2 nm 1.3 nm 0.9nm	AuCl3; Au/QR= 65000	Single-tip Multi-sites Multi-sites, aggregation Single-tip (have tiny Au) Multi-sites (all small) Multi-sites (one big at tip) Multi-sites	111
	273K		473 nm								Single tip
	303K	ODA, DDAB	dark 473 nm laser	1h 1h	CdSe/CdS	37 nm	4.9 nm	3.4 nm	0.7 nm		Multi-sites CdSe location
										HAuCl4, Au injection
										over 25 min, slowly	Double-tip
	RT									added (drop-wise)
	323K	DDA, DDAB, + EtOH (EtOH: CHCl3=10:90)	/ 366 nm, UV-	60min	CdSe/CdS 10 -9 mol OD=0.48 (452 nm)	/	/	/	/	HAuCl4, Au injection over 2-3 min HAuCl4, Au injection over 25 min, slowly added (drop-wise)	Multi-sites, CdSe seed position	191
	RT		lamp, 2-3 mW/cm 2 , FWHM of 18	60 min	CdSe/CdS	28 nm	3.8 nm	/	/	HAuCl4, Au injection over 25 min, slowly added (drop-wise)	Single large Au NPs at one end
			nm.

Part 2 Controlled preparation of semiconductor nanocrystals with different morphologies Chapter 1 Controlled preparation of core-shell quantum dots

Table of Contents

 of Chapter 1 Controlled preparation of core-shell quantum dots ............................ 85 1.1 Introduction .........................................................................................................89 1.2 SILAR and TOP-SILAR syntheses of QDs ..........................................................91 The thermal stability of these green-emitting QDs ......................................... 105 1.4 Set-up of LED chips .......................................................................................... 105 1.5 Methods ............................................................................................................ 107 1.5.1 Chemicals ..................................................................................................... 107 1.5.2 Synthesis of green-emitting thick-shell core/shell QDs .................................. 108 1.5.3 Synthesis of the CdSe@CdS@ZnS core/shell QDs ...................................... 110

	1.2.1 Principle of the synthesis .................................................................................91
	1.2.2 Calculations for the Injections ..........................................................................92
	1.2.3 Characterizations of the CdSe cores and CdSe/ZnS core/shell TSQDs ..........93
	1.3 Further insights on the role of various parameters ..............................................98
	1.3.1 Effect of ligands: Stearic acid (SA) or 1-tetradecylphosphonic acid (TDPA)? ..98
	1.3.2 Improvement thanks to the TOP-assisted extraction ..................................... 100
	1.3.3 Generalization to improved synthesis of red-emitting core/shells QDs .......... 103
	1.3.4

Table 1

 1 The total molar quantity of precursor ZnS needed for the growth of x monolayers

	Monolayer	1	3	5	7
	nZnS (mmol)	2.27 × 10 -3	1.09 × 10 -2	2.66 × 10 -2	5.18 × 10 -2

1.2.3 Characterizations of the CdSe cores and CdSe/ZnS core/shell TSQDs 1.2.3.1 Characterizations of the CdSe cores The cores are characterized by transmission electron microscopy (TEM) as illustrated on Chapter 1 Controlled preparation of core-shell quantum dots 94 References page 113

Table 2 .

 2 Reproducibility of shell growth process for the TOP-assisted extraction method.

	Sample No.	Core λem. (nm)	Core-shell λem. (nm)	FWHM (nm)	QY (%)
	1	529	532	26	75
	2	529	533	26	70
	3	529	535	28	73
	4	529	531	25	72
	5	529	533	25	70
	Average	-	533 ± 1.5	26 ± 1.2	72 ± 2

Table 1 :

 1 list of various conditions used in the literature to make DRs

	No.	UV Abs (nm)	Core diameter (nm)	Injection at temperature (K)/time	DR length (nm)	DR diameter (nm)	DR AR	Shell-(nm) thickness	ref
	1		3.2	623	53±4	3.9±0.2	13.6	0.35
	2	/	3.3	623	35±2	4.2±0.4	8.3	0.50	2
	3		3.3	653	19±1	5.0±0.5	3.8	0.90
	4	585	4.0 a	623/10min	44±4	5.5±0.2	8.0	0.75	11
	5		3.9 a	638	28±4	8±1	3.5	2.05
	6	/	3.4 a	623	44±8	4.9±0.7	9.0	0.75	21
	7		3.1 a	623	108±25	4.6±0.5	23.5	0.75
	8 9	/	3.4 a 2.9 a	653 633	37±3 56±3	4.9±0.6 5.3±0.6	7.6 10.6	0.75 1.20	22
				623/20min	22	7	3.1	2.15
		536	2.7	623 653/20min	22 35	4 7	5.5 5.0	0.65 2.15	15
				623/40min	58	7	8.3	2.15
		/	4.2	623	42	7.5	5.6	1.65
		/	3.5	623/6min	24.0±2	5.0±0.5	4.8	0.75	23
		557	3.2 a	623	36.5	7.3	5.0	2.05	24
		/	4.2	593	49±2.5	5.8±0.5	8.4	0.8	25
				653*	30.4±2.5	5.8±0.8	5.2	1.3
		557	3.2 a	623* 593*	32.5±2.0 24.3±1.6	4.9±0.5 3.8±0.4	6.6 6.4	0.85 0.35	This work
				593**	24.5±2.

Table 2 .

 2 Controlled synthesis of different CdSe cores by changing the reaction time and the ligands.

	Name	Ligands	Temperature (K)	Time (s)	PL (nm)	FWHM (nm)
	CdSe-4	ODPA	653	0	532	26
	CdSe-5	ODPA	653	5	567	31
	CdSe-6	ODPA	653	15	573	30
	CdSe-7	ODPA	653	60	595	30
	CdSe-8	TDPA	643	0	563	27
	CdSe-9	TDPA	643	10	586	26
	CdSe-10	TDPA	643	30	596	30
	CdSe-11	TDPA	643	60	617	33

Table 3 .

 3 The physical properties of CdSe/CdS DRs under different injection temperatures on the same CdSe cores solution (D = 3.2 nm). The core concentration was 16.8 nmol/mL.

	Name	Injection at temperature (K)	Length (nm)	Diameter (nm)	Aspect ratio (AR)	Shell-thickness (nm)	Volume (nm 3 )
	DR-1	503	6.2±0.7	3.6± 0.4	1.7	0.2	63.1
	DR-2	533	8.3±0.8	3.7±0.5	2.2	0.25	89.2
	DR-3	563	12.0±1.0	3.8±0.4	3.2	0.3	136.1
	DR-4	593	24.3±1.6	3.8±0.4	6.4	0.35	275.6
	DR-5	623	32.5±2.0	4.9±0.5	6.6	0.85	612.9
	DR-6	653	30.4±2.5	5.8±0.8	5.2	1.3	803.2

Table 4

 4 Length

	Name	503 K	533 K	563 K	593 K	623 K	653 K
	Length (nm)	6.2	8.3	12.0	24.3	32.5	30.4
	Standard deviation (nm)	0.7	0.8	1.0	1.6	2.0	2.5
	Standard deviation (%)	11.3	9.6%	8.3%	6.6%	6.2%	8.2%

standard deviation ratio study of DRs under different injection temperatures in the same CdSe cores (D=3.2 nm)

Table 5 .

 5 The physical properties of CdSe/CdS DRs under different reaction times starting with the same CdSe

	core (D = 3.2 nm) at a concentration of 12 nmol/mL
	Name	Reaction time (min)	Length (nm)	Diameter (nm)	Aspect ratio (AR)	Shell-thickness (nm)	Volume (nm 3 )
	DR-7	0.5	5.5±0.9	3.4±0.5	1.6	0.1	49.9
	DR-8	1.0	8.0±1.0	3.6±0.5	2.2	0.2	81.4
	DR-9	2.0	15.7±3.0	3.6±0.4	4.4	0.2	159.8
	DR-10	4.0	21.0±2.7	3.8±0.4	5.5	0.3	238.2
	DR-11	8.0	38.7±3.5	3.6±0.4	10.8	0.2	393.9
	DR-12	16.0	35.8±2.4	5.4± 0.8	6.6	1.1	819.9
	2.2.5 Proposition of growth mechanism

Table 6 .

 6 The physical properties of CdSe/CdS DRs under different reaction times under the same conditions as

Table 5

 5 except that the heating rate uses the lower power. Core concentration of 12 nmol/mL

Name Reaction time (min) Length (nm) Diameter (nm) Aspect ratio (AR) Shell-thickness (nm) Volume (nm 3 )

	DR-13	2.0	14.9±3.7	4.3±0.6	3.5	0.55	216.4
	DR-14	4.0	17.5±2.8	4.7±1.0	3.7	0.75	303.6
	DR-15	8.0	26.2±2.2	4.8±0.5	5.5	0.80	474.1
		2.2.5.2	The effects of ambient temperature.

Table 7 .

 7 The

	Name	Ambient temperature (K)	Length (nm) Diameter (nm)	Aspect ratio (AR)	Shell-thickness (nm)	Volume (nm 3 )
	DR-4	303	24.3±1.6	3.8±0.4	6.4	0.35	275.6
	DR-16	291	24.5±2.0	4.4±0.5	5.6	0.6	372.5

physical properties of CdSe/CdS DRs under different ambient temperature based on the same CdSe core (D = 3.2 nm). The core concentration was 16.8 nmol/mL, the injection temperature was 593 K.

Table 8 .

 8 The physical properties of CdSe/CdS DRs under different core concentrations based on the same CdSe cores (D = 3.2 nm).

	Name	[Core]* (nmol/mL)	Length (nm)	Diameter (nm)	AR	Shell-thickness (nm) Volume (nm 3 )
	DR-17	33.6	18.3±2.8	3.8±0.6	4.8	0.3	207.5
	DR-4	16.8	24.3±1.6	3.8±0.4	6.4	0.3	275.6
	DR-18	11.2	38.7±3.5	3.6±0.4	10.8	0.2	393.9
	DR-19	5.6	52.7±3.7	3.6± 0.4	14.6	0.2	536.4
	DR-20	2.8	74.6±7.1	3.7± 0.5	20.2	0.3	802.1

*The core concentration was calculated from the absorption spectrum by

Peng's equation (nm)

Table 9 .

 9 The Physical properties of thin shell CdSe/CdS DRs using different sizes of CdSe cores.

	Name	Diameter of the CdSe core (nm)	Length (nm)	Diameter (nm)	Aspect ratio (AR)	Shell-thickness (nm)	Volume (nm 3 )
	DR-21	2.1	14.0±3.1	2.9±0.4	4.8	0.4	92.5
	DR-17	3.2	18.3±2.8	3.8±0.6	4.8	0.3	207.5
	DR-22	3.9	23.9±2.5	4.2±0.4	5.7	0.15	331.1
	DR-23	4.7	26.3±2.5	5.1±0.5	5.2	0.2	537.3

Table of Contents

 of Chapter 3 Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure . 3.1 Introduction ..........................................................................................................149 3.2 Results and discussion .........................................................................................150 3.2.1 The effects of octadecyl phosphonic acid (ODPA) as a source of symmetry breaking .................................................................................................................. 3.2.2 Characterization of Tadpole shape NCs based on W-CdSe ....................... 3.2.3 The effects of oleic acid (OA) .....................................................................

3.2.4 The effects of CdSe core concentration on Tadpole shape ........................ 3.2.5 Controlled experiments to verify further the mechanism............................. 3.3 Methods ...............................................................................................................164 3.3.1 Chemicals .................................................................................................. 3.3.2 Synthesis of the CdSe Core ....................................................................... 3.3.3 Synthesis of Tadpole shape CdSe/CdS NCs based on W-CdSe ............... 3.3.4 Synthesis of different shapes CdSe/CdS NCs based on W-CdSe .............. 3.3.5 Synthesis of Tadpole shape CdSe/CdS NCs based on ZB-CdSe .............. 3.4 Conclusion ...........................................................................................................171 3.5 Bibliography .........................................................................................................172 Chapter 3: Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface Ligands versus Core Crystal Structure 149 References page 172

Table 1

 1 Physical Different These experiments confirm the suggestions made by Alivisatos, A. P. et al. and Cheon, J. et al. who claimed that the surface energy of the nanocrystals was modulated by introducing surfactants that were susceptible to adsorb onto the different surfaces of the growing crystallites.

	Sample	ODPA (mg)	ODPA/ (OA+ODPA)	Length (nm)	Total NCs Head diameter (nm)	Aspect ratio (AR)
	C1	0	0 %	23.5±1.8	20.1±2.4	1.2±0.2
	C2	100	8.5 %	34.4±3.5	19.0±2.6	1.8
	C3	150	12.2 %	32.5±5.4	17.0±2.6	1.9
	S1	285	21.0 %	43.0±4.4	11.3±1.0	3.8

characterizations of CdSe/CdS NCs as shown in Figure 1. shapes: C1: nanoflower; C2: Three-tail Tadpole; C3: Two-tail Tadpole. S1: One-tail Tadpole.

Table 2

 2 Physical characterizations of W-CdSe/W-CdS Tadpoles at different reaction times as inFigure 5.

	Sample	Time (min)	Length (nm)	Total Tadpoles Head diameter (nm)	Aspect ratio (AR)	Tail Length (nm)
	S0	0	2.8 a	\	\	\
	S2	0.5	4.7±0.9	\	\	spherical
	S3	1.0	4.8±0.8	\	\	spherical
	S4	2.0	8.1±1.0	5.3±0.6	1.5	triangle
	S5	4.0	24.3±3.1	10.0±0.8	2.4	16.1±3.2
	S1	8.0	43.0±4.4	11.3±1.0	3.8	33.2±3.1

a Diameter of the CdSe cores determined from the absorption spectrum by Peng's equation (nm).

Table 3

 3 Physical

	Sample OA (mL)	% mol (OA/ (OA+ODPA)	Length (	Total Tadpoles	Tail

characterizations of W-CdSe/W-CdS Tadpoles as shown in Figure 6.

nm) Head diameter (nm) Aspect ratio (AR) Length (nm)

	S6	1.5	84.7	47.0±6.2	11.3±1.2	4.2	37.4±8.1
	S1	1.0	79.0	43.0±4.4	11.3±1.0	3.8	33.2±3.1
	S7	0.5	65.0	30.9±4.0	10.8±1.5	2.9	20.9±5.4
	S8	0.33	55.0	22.5±4.2	9.4±1.3	2.4	14.9±3.6
	S9 a	0	0	14.0±2.4	3.6± 0.4	3.9	/

a DRs

Table 4 .

 4 Physical characterizations of CdSe/CdS NCs as shown in Figure 7.

	Sample	CdSe amount (nmol) a	Length (nm)	Total Tadpoles Head diameter (nm)	Aspect ratio (AR)	Length (nm)	Tail	Width (nm)
	S10	5	180.8±38.5	21.1±6.3	8.6	168.6±37.4		6.0±1.2
	S11	15	109.5±12.1	14.9±2.9	7.3	96.7±17.6		5.4±0.7
	S1	50	43.0±4.4	11.3±1.0	3.8	33.2±3.1		5.5±0.8
	S12	100	39.2±3.0	11.4 ±0.9	3.4	27.6±2.9		5.7±0.7
	a The ligands are OA 1 mL (3.21 mmol) and ODPA 285 mg ( 8.5 10 -4 mol)

Table 5

 5 Physical characterizations of CdSe/CdS Tadpole based on ZB-CdSe core as shown in Figure 10.

	Sample	core	Length (nm)	Total Tadpole Head diameter (nm)	Aspect ratio (AR)	Tail Length (nm)
	S13	ZB CdSe, 3.95 nm	98.5±17.6	17.7±4.1	5.6	88.1±15.1
	S10	W-CdSe, 2.84 nm	180.8±38.5	21.1±6.3	8.6	168.6±37.4

Table 6

 6 Physical properties of CdSe/CdS NCs as shown in Figure 11.We also checked the temperature since it is always a crucial parameter in such a synthesis, we tested two lower reaction ones, 260 o C and 290 o C (Figure11) using 50 nmol of W-CdSe cores with ODPA (285 mg, 0.85 mmol, 21.2%) and OA (1.02 mL, 908 mg, 3.21 mmol) (Table6).

	Sample	T (°C)	Length (nm)	Total Tadpoles Head diameter (nm)	Aspect ratio (AR)	Length (nm)	Tail Width (nm)	Volume (nm 3 )
	S14	260	7.0±0.8	4.6±0.5	1.5	Triangle2.4	/	/
	S15	290	25.9±4.6	10.5±1.3	2.5	18.7±4.2	4.6±0.6	103.5±32.0
	S1	320	43.0±4.4	11.3±1.0	3.8	33.2±3.1	5.5±0.8	266.4±88.2

Table 7 .

 7 Physical properties of CdSe/CdS NCs as shown in Figure 12. Chapter 3: Shape Control of CdSe/CdS Nanocrystals during the Shell Growth and Formation: Dominating Effects of Surface

			Total Tadpoles			Tail
	Sample	Length	Head diameter	Aspect ratio	Length	Width	Volume
		(nm)	(nm)	(AR)	(nm)	(nm)	(nm 3 )
	S1	43.0±4.4	11.3±1.0	3.8	33.2±3.1	5.5±0.8	266.4±88.2
	S16	41.2±5.2	11.0±2.1	3.7	32.6±5.1	4.7±0.6	194.5±59.9

Different injection times: S1: divided to 4 times, continuous injection; S21: Total injected in one time.

were purchased from Aladdin Industrial Co., Ltd. (Shanghai, China). 1-octadecene (ODE, 90%) and all other organic solvents were obtained from J & K Chemical Reagent Company, China.

All chemicals were used as received without further purification.

3.3.2 Synthesis of the CdSe

Core

3.3.2.1 Synthesis of the Wurtzite (W) CdSe Core Preparation of 1.0 M Se-TOP Stock Solution.

Table 1 .

 1 The Physical properties of different CdSe core.

	Name	Abs (nm)	Diameter (nm) a	PL (nm)	FWHM (nm)
	CdSe-1	557	3.2	578	33
	CdSe-2	583	3.9	601	30
	CdSe-3	603	4.7	619	32

Table 2 .

 2 The physical characterizations of the different CdSe/CdS DRs.

Name Core Diameter (nm) a Diameter (nm) Length (nm) PL (nm) FWHM (nm) QY (%) Abs (nm)

	DR-1	3.2	3.9±0.4	24.3±1.6	597	23	41	591
	DR-2	3.2	4.9±0.5	32.5±2.0	604	23	40	597
	DR-3	3.2	5.3±0.6	25.5±1.8	612	23	61	605
	DR-4	3.9	4.2±0.4	23.9±2.5	620	23	55.5	613
	DR-5	4.7	4.9±0.6	17.5±1.7	631	24	41	623

Table 3 .

 3 The CD and g-factor of L-Cys-DR-1 at the first exciton absorption peak during the etching process.

	Name	UV-vis (nm)	CD + /λCD a	CD -/λCD b	|CD + |/2 c	-CD -	gCD+/λCD d	gCD-/λCD e |gCD+ -gCD-|/2 f
	DR-1-1	602.5	1.787/590.6	-1.295/608.2 1.541	0.787/588.2	-0.572/611.6	0.680
	DR-1-2	601.4	1.930/586.0	-1.827/608.2 1.879	1.002/585.2	-0.847/611.8	0.925
	DR-1-3	600.2	1.944/585.6	-1.895/608.0 1.920	1.085/582.8	-1.032/614.0	1.059

a, b CD anisotropy at the most intense positive and negative CD bands (mdeg/nm), c Magnitude of the CD signal, defined as |CD + -CD -|/2, d, e CD anisotropy gCD-factors at the most intense positive and negative CD bands

(10 - 4

/nm), f Magnitude of the gCD factor, defined as |gCD+ -gCD-|/2.

Table 4 .

 4 The

	Name	UV-vis (nm) (core)	CD + /λCD a	CD -/λCD b	|CD + -CD -|/2 c	gCD+/λCD d	gCD-/λCD e	|gCD+ -gCD-|/2 f
	DR-1-1 602.5	9.596/468.7 -19.296/482.3 14.446	1.204/469.0 -4.231/485.2	2.718
	DR-1-2 601.4	7.888/468.9 -14.205/482.6 11.047	1.367/468.9 -4.555/485.8	2.961
	DR-1-3 600.2	4.282/468.1 -8.143/482.5	6.213	0.996/468.6 -3.566/485.8	2.281

CD and g-factor of L-Cys-DR-1 (< 500 nm) during the etching process. a, bCD anisotropy at the most intense positive and negative CD bands (mdeg/nm), cMagnitude of the CD signal, defined as |CD+ -CD-|/2, d, eCD anisotropy gCD-factors at the most intense positive and negative CD bands

(10- 4/nm)

, fMagnitude of the gCD factor, defined as |gCD+ -gCD-|/2.

Table 5 .

 5 The

	Name	UV-

CD and g-factor of L-Cys-DRs with different shell-thicknesses at the first exciton absorption peak.

vis (nm) CD + /λCD a CD -/λCD b |CD + -CD -|/2 c gCD+/λCD d gCD-/λCD e |gCD+-gCD-|/2 f

	DR-1	602.4	2.539/589.2 -2.691/609.4	2.615	1.748/588.0 -1.717/613.0	1.733
	DR-2	595.8	3.907/581.2 -4.002/600.8	3.955	1.972/578.2 -1.629/605.1	1.801
	DR-3	587.8

3.410/574.2 -5.252/593.6 4.331 1.650/572.3 -2.324/596.9 1.987 a, b CD anisotropy at the most intense positive and negative CD bands (mdeg/nm), c Magnitude of the CD signal, defined as |CD + -CD -|/2, d, e CD anisotropy gCD-factors at the most intense positive and negative CD bands (10 -4 /nm), f Magnitude of the gCD factor(|gCD+ -gCD-|/2). The DRs were proceeded without benzyl amine ligand exchange.

Table 6 .

 6 The

	Name UV-vis (nm)	CD + /λCD a	CD -/λCD b	|CD + -CD -|/2 c	gCD+/λCD d	gCD-/λCD e	|gCD+ -gCD-|/2 f
	DR-1	601.9	13.772/468.4 -27.786/482.3	20.779	1.733/468.7 -6.092/485.0	3.913
	DR-2	595.7	35.315/462.8 -45.223/477.0	40.269	4.317/462.7 -9.776/480.6	7.047
	DR-3	587.8	23.027/454.3 -24.913/469.6	23.970	4.905/454.1 -8.793/472.6	6.849

CD and g-factor of L-Cys-DRs with different shell-thickness (<500 nm).

a, b CD anisotropy at the most intense positive and negative CD bands (mdeg/nm), c Magnitude of the CD signal, defined as |CD + -CD -|/2, d, e CD anisotropy gCD-factors at the most intense positive and negative CD bands

(10 -4

/nm), f Magnitude of the gCD factor, defined as |gCD+ -gCD-|/2. The DRs were proceeded without benzylamine ligand exchange.

Table 7 .

 7 The

CD and g-factor of L-Cys-DR-3 at the first exciton absorption peak during the etching process.

Name UV-vis (

nm) CD + /λCD a CD -/λCD b |CD + -CD -|/2 c gCD+/λCD d gCD-/λCD e |gCD+ -gCD-|/2 f

	DR-3-1	586.8	2.251/572.0 -2.170/593.6	2.211	1.161/569.2 -1.055/599.4	1.108
	DR-3-2	580.2	1.991/565.0 -2.093/589.6	1.853	1.135/563.4 -1.276/595.0	1.206
	DR-3-3	575.0	1.402/558.0 -1.412/586.6	1.407	0.735/559.4 -0.915/591.6	0.825
	DR-3-4	568.8	1.354/555.0 -0.185/582.2	0.770	0.566/554.8 -0.099/583.0	0.333
	DR-3-5	/	/	/	/	/	/	/

a, b CD anisotropy at the most intense positive and negative CD bands (mdeg/nm), c Magnitude of the CD signal, defined as |CD + -CD -|/2, d, e CD anisotropy gCD-factors at the most intense positive and negative CD bands

(10 -4

/nm), f Magnitude of the gCD factor, defined as |gCD+ -gCD-|/2.

  35 nm to 1.05 nm. CdSe/CdS DRs with different AR were synthesized by adding different amount of CdSe core.

	Chapter 4	Study of the mechanism of Ligand-induced chiral transmission through the top-down selective domain etching
		4.3.3.4	The CdSe/CdS DRs with different aspect ratio (AR) synthesized by
		various core concentrations
			214
	References page 217

Chapter 5 Optically Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized LuminescenceTable of Contents

 of Chapter 5 Optically Active CdSe/CdS NPLs with Induced Chirality and Circularly Polarized Luminescence ........................................................................................ 5.1 Introduction .......................................................................................................... 5.2 Results and discussion ......................................................................................... 5.2.1 Synthesis of CdSe and CdSe/CdS NPLs. ..................................................

Table 1

 1 Size distributions of the NPLs in organic phase

	Sample NPL type	Reaction time (min)	Width (nm)	Length (nm)	Lateral size (nm 2 )	Thickness (nm)
	S0	CdSe	0	5.3±0.8	21.9±2.6	116.6±22.3	1.4±0.3
	S1	CdSe/CdS	10	7.1±1.0	24.1±2.7	170.7±22.1	1.8±0.4
	S2	CdSe/CdS	30	9.9±1.1	26.0±2.3	257.2±35.2	2.8±0.6
	S3	CdSe/CdS	60	10.4±1.2	26.4±2.4	274.7±31.3	3.2±0.8
	SX	CdSe/CdS	90	10.9±1.3	26.0±2.5	284.9±43.3	3.9±0.6
	S4	CdSe/CdS	120	12.7±1.4	27.8±2.5	352.3±44.1	4.3±0.8
	S5	CdSe/CdS	180	16.5±1.9	30.0±2.6	493.5±67.7	6.0±0.9

Table 2 .

 2 The Spectral properties of the NPLs in organic phase before ligand exchange. PL emission peak position of the NPLs and their FWHM before ligand exchange. c PL QY of the NPLs before ligand exchange.d The absorption peak of NPLs. e,f Excitonic transitions involving light hole (e-lh) and heavy hole (e-hh).

	Sample	NPL type	Reaction time (min)	PL emission PL (nm) a FWHM (nm) b	QY (%) c	Abs (nm) d λe-lh e	λe-hh f
	S0	CdSe	0	515	12	30	484	513
	S1	CdSe/CdS	10	579	27	5	535	569
	S2	CdSe/CdS	30	604	27	27	556	594
	S3	CdSe/CdS	60	620	24	38	569	611
	S4	CdSe/CdS	120	632	23	40	575	623
	S5	CdSe/CdS	180	638	23	35	577	629
	a,b

Table 3 .

 3 The Spectral properties of the D-Cys-NPLs in aqueous phase after ligand exchange.

	Sample	NPL type	Reaction time (min)	PL emission PL (nm) a QY (%) b	λe-lh d	Abs (nm) c λe-hh e
	S0	CdSe	0	528	2	500	526
	S1	CdSe/CdS	10	584	2	541	574
	S2	CdSe/CdS	30	608	13	560	598
	S3	CdSe/CdS	60	624	20	573	615
	S4	CdSe/CdS	120	634	25	578	625
	S5	CdSe/CdS	180	639	22	580	630

a,b PL emission peak position of the NPLs and their FWHM before ligand exchange. c PL QY of the NPLs before ligand exchange.

d

The absorption peak of NPLs. e,f Excitonic transitions involving light hole (e-lh) and heavy hole (e-hh).

Table 4 .

 4 CD

	Sample	λmax a	gCD+/λCD b	gCD-/λCD c	glum/λCPL (nm) d
	S0		2.10×10 -4 /535.2	-0.73×10 -4 /492.6	/
	S1		2.11×10 -4 /567.9	0.01×10 -4 /504.0	/
	S2		2.28×10 -4 /572.1	1.02×10 -4 /586.7	2.52×10 -4 /600.6
	S3		5.44×10 -4 /596.6	-0.24×10 -4 /552.0	5.29×10 -4 /620.3
	S4		1.66×10 -4 /613.2	0.09×10 -4 /569.2	5.82×10 -4 /630.4
	S5		1.06×10 -4 /617.8	-0.15×10 -4 /563.2	3.77×10 -4 /637.1

anisotropy factors of D-Cys-NPLs between 450 to 650 nm and CPL anisotropy factors of D-Cys-NPLs in water.

a Excitonic transitions involving heavy hole (λe-hh). b, c CD anisotropy gCD-factors at the most intense positive and negative CD bands (nm). d CPL anisotropic glum at the most intense CPL wavelength (nm).

Table 5 .

 5 CD anisotropy factors of D-Cys-NPLs between 350 to 500 nm in water.

	Sample	λmax a	gCD+/λCD b	gCD-/λCD c
	S0	526	-0.47×10 -4 /400.2	-1.57×10 -4 /429.6
	S1	574	1.59×10 -4 /385.1	-1.54×10 -4 /450.7
	S2	598	0.95×10 -4 /447.1	-1.98×10 -4 /483.2
	S3	615	1.01×10 -4 /438.6	-0.88×10 -4 /497.7
	S4	625	0.98×10 -4 /474.9	-0.29×10 -4 /503.9
	S5	630	0.84×10 -4 /479.3	0.10×10 -4 /506.2

a Excitonic transitions involving heavy hole (λe-hh). b, c CD anisotropy gCD-factors at the most intense positive and negative CD bands (nm).

Table of Contents

 of Chapter 6 Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies ............................................................................................ 6.1 Introduction ..........................................................................................................

Table 2 .

 2 CD and CPL anisotropy factors of D-Cys-CdSe/CdS NPs in water.

	Name	Shell-

thickness (nm) a Ratio c PL (nm) d |gCD+ -gCD-|/2 e QY (%) f glum/λCPL (nm) g

  The CdS shell thickness around the CdSe core.b The shell thickness of the Tadpoles head. c Ratio of the total length to the largest diameter of the nanostructures. d PL emission peak position of Cys-CdSe/CdS NPs in water phase. Magnitude of the gCD-factor, defined as |gCD+ -gCD-|/2. f PL QY of the NPs after ligand exchange. g CPL anisotropic glum at the most intense CPL wavelength (nm).

	Chapter 6:		Study of ligand-induced chirality of semiconductor nanocrystals with different morphologies
	Nanoflower	8.7	1.2	624	0.68×10 -5	2	/
	Tadpole-III	8.1 b	1.8	632	1.79×10 -5	8	/
	Tadpole-II	7.1 b	1.9	633	4.41×10 -5	12	1.71×10 -4 /636.9
	Tadpole-I-1	4.3 b	3.8	629	9.79×10 -5	37	5.12×10 -4 /632.9
	DR-1	0.2	2.7	577	9.36×10 -5	44	8.46×10 -4 /584.9
	DR-2	1.0	12.4	583	39.18×10 -5	39	7.45×10 -4 /585.8
				264
	References page 294

a e

Table 3 .

 3 Diameter standard deviation ratio study of NCs with different shapes

	Name	Nanoflower	Tadpole-III	Tadpole-II	Tadpole-I	DR-1	DR-2
	Diameter (nm)	20.1	19.0	17.0	11.3	3.2	4.7
	Standard deviation (nm)	2.4	2.6	2.6	1.0	0.3	0.5
	Standard deviation (%)	11.94	13.68	15.29	8.85	9.38	10.64

Table 5 )

 5 . As stated before, we assert that the number of chiral interactions between the CdSe cores and the chiral surface ligands determines the CPL intensity that can be achieved, indicating that a thin CdS shell is favorable to obtain strong CPL signals. It is however worth noting that some related articles have demonstrated that for rod-like morphologies, proper AR or length would achieve larger CPL signals, although this tendency is not obviously observed in this work.18

Table 5 .

 5 CPL anisotropy factors of L-Cys-CdSe/CdS NCs in water.

	Morphologies	glum/λCPL (nm) a
	Nanoflower	/
	Tadpole III	/
	Tadpole II	-1.43×10 -4 /636.6
	Tadpole I-1	-4.26×10 -4 /621.5
	DR-1	-7.55×10 -4 /577.0
	DR-2	-5.31×10 -4 /581.1

a CPL anisotropic glum at the most intense CPL wavelength (nm).

Table 6 .

 6 CD Anisotropy Factors of D-Cys-CdSe/CdS Tadpole-I at Different Reaction Times. Shell thickness of the tadpole heads. b Ratio of the total length to the largest diameter of the nanostructures. c PL emission peak position of Cys-CdSe/CdS NCs in the water phase. d, e CD anisotropy gCD-factors at the most intense positive and negative CD bands (nm). f Magnitude of the gCD factor, defined as |gCD+ -gCD-|/2. g PL QY of the NCs after the ligand exchange.

	Name	Shell thickness (nm) a	Ratio b	PL (nm) c	gCD+/λCD d	gCD-/λCD e	|gCD+-gCD|/2 f	QY(%) g
	Tadpole-I-2	0.9	1.0	572	8.95×10 -6 /490.4	-6.06×10 -6 /454.4	0.75×10 -5	2
	Tadpole-I-3	1.0	1.0	581	1.78×10 -5 /480.9	6.02×10 -6 /459.7	0.59×10 -5	6
	Tadpole-I-4	1.3	1.5	596	3.62×10 -5 /491.6	1.67×10 -5 /464.0	0.98×10 -5	8
	Tadpole-I-5	3.6	2.4	614	9.75×10 -5 /483.1	-1.38×10 -5 /453.0	5.57×10 -5	17
	Tadpole-I-1	4.3	3.8	629	14.43×10 -5 /486.1	-5.15×10 -5 /460.1	9.79×10 -5	37

a

Table 7 .

 7 CD anisotropy factors of D-Cys-CdSe/CdS Tadpole-I for different reaction times.

	Name	gCD+/λCD a	gCD-/λCD b
	Tadpole I-2	8.95×10 -6 /490.4	-6.06×10 -6 /454.4
	Tadpole I-3	1.78×10 -5 /480.9	6.02×10 -6 /459.7
	Tadpole I-4	3.62×10 -5 /491.6	1.67×10 -5 /464.0
	Tadpole I-5	9.75×10 -5 /483.1	-1.38×10 -5 /453.0
	Tadpole I-1	14.43×10 -5 /486.1	-5.15×10 -5 /460.1
	a, b CD anisotropy gCD-factors at the most intense positive and negative CD bands (nm).

Table 8 .

 8 Physical properties of tadpoles at different reaction times in the organic phase.

	Name	Diameter (nm) Length (nm)	PL (nm) c FWHM (nm) d	QY (%) e Abs (nm) f λCdS g
	Core	2.8 a	/	554	32	1.5	539	/
	Tadpole I-2	4.6±0.9 b	/	573	25	5	559	467
	Tadpole I-3	4.8±0.8 b	/	583	25	8	572	473
	Tadpole I-4	5.3±0.6 b	8.1±1.0	598	27	12	589	487

Table 10 .

 10 CD and CPL Anisotropy Factors of D-Cys-CdSe/CdS Tadpole-I with Different Lengths. The shell thickness of the tadpole heads. b Ratio of the total length to the largest diameter of the nanostructures. c PL emission peak position of Cys-CdSe/CdS NCs in the water phase. d Magnitude of the gCD factor, defined as |gCD+ -gCD-|/2. e PL QY of the NCs after ligand exchange. f CPL anisotropic glum at the most intense CPL wavelength (nm).

	Name	Shell-thickness (nm) a	Ratio b	PL (nm) c	|gCD+ -gCD-|/2 d	QY(%) e	glum/λCPL (nm) f
	Tadpole-I-6	3.3	2.4	613	5.60×10 -5	20	5.71×10 -4 /613.7
	Tadpole-I-7	4.2	2.6	624	7.71×10 -5	28	5.32×10 -4 /625.4
	Tadpole-I-1	4.3	3.8	629	9.79×10 -5	37	5.12×10 -4 /632.9
	Tadpole-I-8	6.1	7.3	639	5.41×10 -5	13	2.38×10 -4 /638.7
	Tadpole-I-9	9.2	8.6	640	4.03×10 -5	11	1.02×10 -4 /647.2

a

Table 11 .

 11 CD anisotropy factors of D-Cys-CdSe/CdS Tadpole-I with different lengths.It is, however, worth noting that a related article reported that for rod-like QDs, a proper length with twice the exciton Bohr radius would achieve the strongest CD signals, although this tendency is not obviously observed in this work.18

	Morphologies	gCD+/λCD a	gCD-/λCD b
	Tadpole I-6	9.70×10 -5 /473.6	-1.49×10 -5 /447.5
	Tadpole I-7	8.84×10 -5 /475.9	-6.58×10 -5 /453.4
	Tadpole I-1	14.43×10 -5 /486.1	-5.15×10 -5 /460.1
	Tadpole I-8	5.74×10 -5 /486.5	-5.08×10 -5 /460.0
	Tadpole I-9	2.28×10 -5 /473.3	-5.77×10 -5 /453.8

a, b CD anisotropy gCD-factors at the most intense positive and negative CD bands (nm).

Table 12 .

 12 CPL anisotropy factors of L-Cys-CdSe/CdS Tadpole-I with different lengths.

	Morphologies	glum/λCPL (nm) a
	Tadpole I-6	-4.83×10 -4 /620.0
	Tadpole I-7	-4.58×10 -4 /617.1
	Tadpole I-1	-4.26×10 -4 /621.5
	Tadpole I-8	-1.84×10 -4 /635.7
	Tadpole I-9	-0.88×10 -4 /639.3

a CPL anisotropic glum at the most intense CPL wavelength (nm).

Table of Contents

 of Chapter 7 Setup Configuration and Laser Control ............................... 7.1 Cuvette setup using large blue laser exposure ..................................................... 7.1.1 Schematic picture of the blue cuvette setup ............................................... 7.1.2 Beam power transmission by the setup vs current ..................................... 7.1.3 Intensity ..................................................................................................... 7.2 Cuvette setup using large Blue CPL laser exposure ............................................. 7.2.1 Schematic picture of the Blue CPL cuvette setup ...................................... 7.2.2 Intensity in the blue CPL cuvette vs Intensity on panel .............................. 7.3 Cuvette setup using large UV laser exposure ....................................................... 7.3.1 Schematic picture of the UV cuvette setup ................................................ 7.3.2 Intensity calibration in the cuvette vs driving emitted power ....................... 7.4 Bibliography .........................................................................................................

Table 1

 1 The relationship between the laser power at different places in the setup and the driving current.

	Current A	PHead mW	Pinc mW	PTrans mW	P1/2 mW	Irradiation intensity mW/cm 2
	/	0	0	0	0	0
	2.05	20	16.13	11.96	13.90	20.54
	2.39	50	40.34	29.90	34.75	51.36
	2.92	100	80.67	59.80	69.50	102.71
	3.61	200	160.34	119.60	139.00	205.42
	4.48	300	242.01	179.40	208.50	308.14
	7.2 Cuvette setup using large Blue CPL laser exposure
	In this section, we build the Blue CPL cuvette setup based on 7.1. A further λ/4 plate was
	used at ±45° to produce a L-/D-polarized beam.

Table 2 .

 2 The relationship between Blue CPL laser intensity and current. Cuvette setup using large UV laser exposureIn this section, we discuss another experimental setup arrangement used during the present work. While still keeping the cuvette as reaction vessel, we use a UV laser instead of the blue one. Both setups are complementary. For the same deposition in the solution under UV laser exposure, less light energy will be needed because the optical absorption in UV is larger than in the visible range.

	Current A	PHead mW	Pinc+45 mW	Irradiation intensity	Pinc-45 mW	Irradiation intensity (-
				(+45) mW/cm 2		45) mW/cm 2
	/	0	0	0	0	0
	2.05	20	15.34	19.53	15.33	19.52
	2.39	50	38.35	48.83	38.33	48.80
	2.92	100	76.70	97.66	76.65	97.59
	3.61	200	153.40	195.31	153.30	195.19
	4.48	300	230.10	292.97	229.95	292.78

  KAu and KHS are respectively the Au 3+ and HS adsorption equilibrium constants, and CT is the highest concentration of active sites for Au3+ and HS adsorption at the surface of the DRs.Finally, since [Au 3+ ]bulk and [HS]bulk are initially in excess in the DRs solution, we suppose that [Au 3+ ]ads and [HS]ads do not vary and are equal to their equilibrium values given by eq. 2 all along the laser-deposition process (this hypothesis may be invalidated at very long exposure time in our experiments in cuvettes). Then, we can decrease the order of the interface reactions by setting 𝑘′ 𝑒 = 𝑘 𝑒 [Au 3+ ] ads and

	Chapter 8		Hole scavenger, the key factor for single-tip Au-CdSe/CdS HNPs
	{	[Au 3+ ] ads = (𝐶 𝑇 ) Au [HS] ads = (𝐶 𝑇 ) HS	𝐾 Au [Au 3+ ] bulk (1+𝐾 Au [Au 3+ ] bulk ) (1+𝐾 HS [HS] bulk ) . 𝐾 HS [HS] bulk	,		(eq. 2)
				hh = kk '	  ads HS	and solve the following set of
	equations:
				[Au 3+ ]ads and [HS]ads, involved in the reaction
	with photo-produced electrons and holes, can be deduced from their bulk values [Au 3+ ]bulk and
	[HS]bulk using a Langmuir-Hinshelwood type equation (eq. 2):
				325
	References page 364

Table 1 .

 1 Physical properties of the different DRs

	Name Core Diameter (nm) Diameter (nm)	Length (nm)	(nm) Shell-thickness	Aspect ratio (AR)
	DR-1	3.9	7.6±0.8	48.0±2.1	1.84	5.3
	DR-2	3.2	4.9±0.5	26.2±2.2	0.9	5.3
	DR-3	3.2	8.8±2.2	24.1±3.8	2.8	2.7
	DR-4	3.2	12.2±1.5	23.5±4.1	4.5	1.9
	DR-5	3.2	14.9±2.3	20.1±4.5	5.9	1.3
	DR-6	3.2	5.6±0.7	62.4±6.4	1.2	11.1
	DR-7	3.2	4.3±0.7	21.4±2.6	0.6	5
	DR-8	3.2	3.5±0.4	12.1±2.0	0.15	3.5
	DR-9	3.2	3.6±0.5	8.2±1.4	0.2	2.3
		8.2.3 Control experiments
	To proceed to the single-tip deposition process, photodeposition experiments using a DDA
	modified KAuCl4 precursor in toluene were performed using a blue laser (473 nm within the
	bandgap of the DRs) and a cuvette setup, with MeOH (varying concentrations) as hole

scavenger, with CdSe/CdS DRs (varying concentrations) as the substrate, at varying laser irradiation intensities and time exposures.

Table 2 .

 2 Control experiments for gold deposition on DR-1

	Samples	Conditions	Au Size	Single-tip (%)	Double-sites	Multi-sites	Non-Au (%)
			(nm)		(%)	(%)
	C1 a	No MeOH	3.84±1.03	42.10%	18.90%	27.80%	11.20%
	C2 b	No laser	2.34±0.62	11.65%	7.63%	75.10%	5.62%
	C3 c	No laser, no	1.83±0.94	13.85%	7.36%	16.45%	62.34%
		MeOH

a no MeOH, 200 mW (exit laser head), KAuCl4 (4×10 -5 M), deposition time (8 min); b no laser, MeOH (2.5 M), KAuCl4 (4×10 -5 M), deposition time (8 min); c no laser, no MeOH, KAuCl4 (4×10 -5 M), deposition time (8 min).

Table 3 .

 3 The effects of MeOH on the distribution of the gold NDs on the DRs after photodeposition*

	Samples	MeOH (M)	Size of Au (nm)	Single-tip (%)	Double (%)	Multi-sites (%)	Non-Au (%)
	C1	0.00	3.84±1.03	42.10%	18.90%	27.80%	11.20%
	S1	0.313	5.80±1.23	68.50%	5.56%	17.04%	8.89%
	S2	0.625	5.92±1.08	81.30%	3.20%	4.52%	10.97%
	S3	1.25	6.01±1.40	82.90%	3.00%	1.30%	12.70%
	S4	2.50	6.46±1.22	83.00%	1.70%	0.00%	15.30%

*The reaction conditions: [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min., power: 200 mW (exit laser head).

Table 4 .

 4 The

	Samples	Power (mW)*	Size of Au	Single-tip(%) Double (%)	Multi-(%)	Non-Au (%)
			(nm)
	C2	0 (no laser)	2.34±0.62	11.65%	7.63%	75.10%	5.62%
	S5	20	5.06±1.09	67.42%	0.37%	14.98%	17.23%
	S6	50	5.29±1.09	69.91%	1.43%	11.17%	17.48%
	S7	100	5.55±1.08	80.51%	1.03%	6.15%	12.31%
	S8	200	5.79±1.04	82.33%	2.47%	0.71%	14.49%
	S9	300	6.16±1.00	84.23%	1.92%	1.92%	11.92%

effects of laser intensity for photodeposition with [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, time: 8 min. *The laser power is given at the exit of the laser head. 334 References page 364

Table 5 .

 5 The

		effect of time for photodeposition with [MeOH] = 2.5 M, [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M 200
	mW (exit laser head).
	Samples	Time
		(min)

Size of Au (nm) Single-tip (%)

					Double	Multi-(%)	Non-Au
					(%)		(%)
	S10	1	4.50±1.14	72.20%	1.54%	0.00%	26.25%
	S11	2	4.63±1.07	72.15%	1.77%	1.27%	24.80%
	S12	4	5.46±1.11	78.96%	0.00%	0.82%	20.22%
	S13	8	5.98±1.38	87.00%	2.17%	0.72%	10.11%
	S14	16	6.14±1.23	75.44%	2.05%	0.29%	22.22%
	S15	32	6.91±1.37	83.38%	5.26%	1.94%	9.42%
	S16	64	5.93±1.08*	69.63%	6.54%	6.07%	17.76%

Table 6 .

 6 The

	effect of time for photodeposition with [MeOH] = 0 M, [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M power:
	200 mW (exit laser head).
	Samples	Time (min)	Size of Au (nm)	Single-tip (%)	Double (%)	Multi-(%)	Non-Au (%)
	C4	4	3.3±1.11	44.90%	5.80%	1.60%	47.80%
	C1	8	3.84±1.03	42.10%	18.90%	27.80%	11.20%
	C5	16	5.4±1.48	25.40%	17.40%	50.30%	7.00%

Table 7 .

 7 The effect of the Au/DRs ratio (different Au concentrations) for photodeposition. [MeOH] = 2.5 M, [DR-1] = 2.5 × 10 -9 M, time: 8 min, power: 200 mW (exit laser head).

	TEM	Au/DR	Au	Size of Au	Single-	Double	Multi-	Non-
		ratio	(10 -6 M)	(nm)	tip (%)	(%)	(%)	Au(%)
	S17	1,000	2.5	2.26*	2.21%	0.74%	5.88%	91.18%
	S18	2,000	5	3.96±1.10	30.06%	0.43%	2.13%	67.38%
	S19	4,000	10	4.25±1.11	30.79%	1.27%	11.70%	56.23%
	S20	8,000	20	4.87±1.09	43.26%	1.25%	3.76%	51.72%
	S13	16,000	40	5.98±1.38	87.00%	2.17%	0.72%	10.11%
	S21	32,000	80	6.97±1.43	94.09%	0.54%	2.15%	3.23%
	S22	64,000	160	8.22±1.21	92.35%	3.57%	1.02%	3.06%
	S23	128,000	320	10.18±2.37	87.83%	3.70%	2.12%	6.35%
	*There are very few Au NPs

Table 8 .

 8 The effect of Au/DRs ratio (different DRs concentrations) for photodeposition with MeOH (2.5 M)

	TEM Au/DR ratio DRs (10 -9 M) Size of Au (nm) Single-tip (%) Double (%) Multi-(%) Non-Au (%)
	S24	1975	20.00	4.42±1.30*	17.59%	0.50%	2.51%	79.40%
	S25	3950	10.00	3.96±1.14	37.10%	1.61%	4.30%	56.99%
	S26	7900	5.00	5.06±1.29	43.80%	0.00%	1.16%	55.04%
	S27	15800	2.50	5.98±1.29	71.43%	2.01%	1.25%	25.31%
	S28	31600	1.25	7.79±1.14	77.18%	1.94%	0.49%	20.39%

  (𝑡) -𝐷 𝐶 3 ∝ √ 𝑘 ℎ [HS] ads 𝑘 𝑒 [Au 3+ ] ads 𝑔 Hole scavenger, the key factor for single-tip Au-CdSe/CdS HNPs 345 References page 364where DC is a critical diameter which can be neglected according to previous fits. Moreover, one has (eq. 8):

	Chapter 8
	{	[Au 3+ ] ads = (𝐶 𝑇 ) Au [HS] ads = (𝐶 𝑇 ) HS	𝐾 Au [Au 3+ ] bulk (1+𝐾 Au [Au 3+ ] bulk ) 𝐾 HS [HS] bulk (1+𝐾 HS	,
			𝑘 𝑟	⋅ 𝑡	(eq. 7)

Table 9 .

 9 The effects of the laser intensity on the photodeposition on unprotected DRs. [DR-1] = 2.5 × 10 -9 M, [Au 3+ ]

	C3	No laser, no MeOH	1.83±0.94	13.85%	7.36%	16.45%	62.34%
	C6	20	3.6± 1.28	31.40%	15.40%	41.10%	12%
	C7	50	5.0±1.19	36.20%	22.60%	28.00%	13.20%
	C8	100	5.0±0.77	31.20%	32.40%	32.40%	4.10%
	C9	200	5.1±0.85	5.20%	20.40%	73.70%	0.70%
	C10	300	5.3±0.89	1.20%	7.60%	91.20%	0%

= 4 × 10 -5 M, [MeOH] = 0 M, time: 8 min, power: 100 mW. Samples Power (mW)* Size of Au (nm) Single-tip (%) Double (%) Multi-(%) Non-Au (%) *Measured at the exit of the laser head

Table 10 .

 10 The effect of time for photodeposition on unprotected DR-1 with [DR-1] = 2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, [MeOH] = 0 M, power: 100 mW.

	C11	4	2.8± 0.90	53.40%	15%	6.10%	25.60%
	C12	8	4.6±0.84	29.90%	22.00%	44.10%	4.00%
	C13	16	5.5±0.97	21.60%	28.80%	45.10%	4.50%
	C14	32	5.3±0.80	13.00%	23.60%	62.20%	1.20%
	C15	64	5.6±0.90	5.10%	29.00%	63.50%	2.40%

Samples

Time (min) Size of Au (nm) Single-tip (%) Double (%) Multi-(%) Non-Au (%)

Table 11 .

 11 The effect of the Au/DRs ratio (different Au 3+ concentrations) for photodeposition on DRs with more defects. [DR-1] = 2.5 × 10 -9 M, [MeOH] = 0 M, time 8 min and power 100 mW at the laser exit.

	Samples Au/DR ratio	Au 3+ (10 -6 M)	Size of Au (nm)	Single-tip (%)	Double (%)	Multi-(%) Non-Au (%)
	C16	988	2.5	2.0±1.01	10.40%	3.00%	1.40%	85.30%
	C17	1975	5	4.3± 0.76	17.00%	6.60%	4.70%	71.60%
	C18	3950	10	5.2± 0.92	25.10%	5.90%	4.00%	65.00%
	C19	7900	20	4.0± 0.70	29.30%	40.50%	25.10%	5.10%
	C20	15800	40	5.5±0.97	21.60%	28.80%	45.10%	4.50%

[225][226][227] shorten the time,179, 184 and use long trialkyl amine ligands,111, 179, 209 which

Figure 19. The CD spectrum variations of DR-4 at the first exciton absorption peak during the etching process. a) The CD spectrum of benzyl amine modified DRs after the Cys exchange, b-c) the CD spectrum of DRs after one and two etching cycles and Cys exchange, d) The CD signal evolution of DR-4 at the first exciton absorption peak during the etching process.

Figure 4. TEM images of Au-CdSe/CdS HNPs made in cuvette set up by exposed CPL blue laser. a) +45-CPL, b) -45-CPL. [MeOH] = 2.5 M, [DR-1] =2.5 × 10 -9 M, [Au 3+ ] = 4 × 10 -5 M, power: 200 mW (exit laser head), reaction time, 8min.

Conclusions and Perspectives

Chapter 2

Temperature, a powerful tool to fill the gap in the controllable synthesis of ultra-thin shell quantum rods Although it is impossible to further enhance the chiral signal of thin-shell DRs (less than 1 monolayer) by etching, we can still analyze the reason for this DR high chiral signal through the etching process. In order to explore the mechanism of ligand-induced chiral action, in addition to studying the change of the chiral signal at the first exciton peak of the CdSe core, we also studied the chiral signal of the CdS shell. It was set up in the literature that in DR nanoparticles, the chiral signal of the CdS shell is often found to be much larger than that of the core. 10,28,35 H. Kuang et al. showed for example that based on the two amino acid post-synthetic ligand exchange strategy, longer CdSe/CdS semiconductor nanorods could induce strong chirality properties with large g-factors even higher than 10 -3 (1.2×10 -3 /461 nm), 35 which had great potential for chiral applications of the CdS shell.

Considering the results obtained in paragraph 4. The research on the chiral signal of the CdS shell (< 500 nm) revealed that as the etching progressed, the CD signal of the individual nanoparticles weakened regardless of the nature of the shell, thick or thin (compare Figure 9, Figure 22).

In the thin shell samples (DR-3 )(Figure 22), the CD signal decreased during the etching process, for probably two reasons: (i) the surface decrease of CdS shell, thereby decreasing the amount of chiral ligand adsorption and reducing the chiral signal; on the other hand, (ii) the etching of CdSe core in a second time. This can be confirmed by the plots the dependence of the absorbance on the etching time for DR-3 in Figure 18 b and d, which show that there are two stages: slow decrease at the first stage and then an abrupt turning point 45,62 takes place at the second stage. The CdSe core etching will further affect the electron and hole transport between the core and the shell, thereby preventing the CD signal of the shell.

References page 294

Scheme 1

The mechanism of ligands induced shape variation. When changing the composition and the ratio of the ligands, various anisotropic shapes from nanoflowers, tadpoles with one to three tails, and dot-in-rods can be obtained. for each nanostructure are measured for more than 100 NPs and given in Table 1. The wurtzite (W)-CdSe was used here as the core material, since CdS is known to preferentially grow in the wurtzite phase from W-CdSe seeds, this can be confirmed in Figure 1 (a-2, d-2 and e-2). 4 Alivisatos 44 and Cheon 5 also showed that the surface energy of the nanoparticles could be modulated by introducing surfactants that adsorb onto the different surfaces of the growing crystallites. As the surfactants stabilize a given surface by "selective adhesion", the growth rate difference between the different crystallographic directions is then accentuated. As already mentioned in a previous chapter at low concentration or in the absence References page 294 However, further growth of the tail does not necessarily result in an enhanced CD activity.

Figure 16, Figure 17 and Figure 18 demonstrate that when the length of the tail grows even longer (from 22.5 to 180.8 nm), the recorded CD responses become weaker with a gradual redshift in the PL and UV spectra (Figure 19, Table 9). When nanostructures are much smaller than the incident light wavelength (typically λ/10), the weak spatial dispersion theory can be applied. Under these circumstances, the local effects dominate, and non-local effects can introduce chirality coefficients, which can be recognized as a manifestation of first-order spatial dispersion, and the homogenization of the microscopic Maxwell equations can be applied. 58 When the length of the chiral Tadpole-I reaches a value that is one tenth of the incident wavelength (for example, approximately 40-50 nm), a further increase in the length does not necessarily further enhance the chirality, indicating that Because CPL measures the emission property of the CdSe/CdS nanostructures, the greater the increase in the tail length, the more difficult it becomes to induce ligand-based chirality, even if a longer tail may introduce some degree of geometrical anisotropy for CD increments, as previously stated. Interestingly, although the geometrical relationships are still complex, the As shown in Figure 12, after irradiation on the DR-1 (OD473 nm=0.11), the transmitted power shows a good linear relationship with the Incident Power.

The power inside the cuvette can be written as:

Then

Where Ppanel is the power of UV laser indicated on the panel, Pinc is the power incident to the UV cuvette, PTrans is the power transmitted after irradiation on the DR-1 (OD473 nm=0.11), and P1/2 is the power in the middle of the cuvette.

In this way, the applied power inside the UV cuvette is perfectly known and can be adjusted by changing the panel power. We explore the laser photodeposition growth mechanism of gold nanocrystals onto preformed CdSe/CdS dot-in-rods (DRs) to form hybrid metal nanocrystal/semiconductor nanorod colloids by adding methanol (MeOH) as hole scavenger. By manipulating the growth conditions, it is possible to obtain nanostructures exhibiting the growth of single Au nanocrystal mainly at one tip of the nanorods. We have systematically studied the effects of experimental parameters on the photodeposition sites, such as the presence of a hole scavenger, the irradiation intensity, the deposition time, the Au/DRs ratio and so on. Our results compare quite well with a model for the growth of a single Au nanocrystal. We then expect that the demonstrated strategy can raise a common methodology in the synthesis of single-tip semiconductor-metal hybrid nanoparticles (HNPs) nanocrystals, leading to advanced nanoparticles architectures for applications in areas of photocatalysis, photovoltaics, and light detection.

Introduction

The synergistic optical and chemical properties of semiconductor-metal hybrid nanoparticles (HNPs) lead to light-induced charge separation, 1 opening the path for their function as photo-catalysts in redox reactions, including hydrogen generation by water splitting, 2-7 generation of radicals and photo-degradation of organic contaminants. 8 Many methods have been used to synthesize semiconductor-metal HNPs, including chemical reduction, 3,[9][10][11][12] physical vapor deposition, 13,14 impregnation-deposition 15,16 and photodeposition, 10,[17][18][19][20][21] and so forth, among which the photodeposition method has been widely used for its simplicity, low energy cost and contactless approach.

In recent years, great advances have been made toward designing the controlled growth of small Au crystals selectively on the tips of CdSe and CdS rod-shaped nanocrystals, where the metal is in direct crystalline contact with the semiconductor part. 22,23 Due to their high catalytic performance, seeded rod metal tip samples, such as CdSe/CdS dot-in-rods (DRs), 4, 11, 19, 21, 24, 25 have been widely studied; they can facilitate an efficient long-lasting charge carrier separation and minimize back reaction of intermediates. 11 Furthermore, Amirav's group found that a single-tip metal catalyst decorated semiconductor rod revealed to be the most active for the hydrogen production (27 times higher than the multiple dots covered DRs). 4,21 However, up to now, the study of single-tip HNPs photodeposition method is still limited As shown in Table 13, Figure 37, Figure 38 and Figure 39, we chose the DRs with thin shell (less than 10 monolayers) to study the effect of the DRs length. Ratios of single tips higher than 50% were obtained whatever the Rods length. The ratio of double-& multi-sites depositions still increased when the length was very short (C27 & C28, less than 12.1 nm).

While for the longer samples, the double-& multi-sites could be maintained at relative low level. This aspect is still under scrutiny.

Table 13. The effect of DRs length on Au size and tips distribution Samples Length (nm) Size of Au (nm) Single-tip(%) Double (%) Multi-(%) Non-Au (%) C25 62.4 Lines are guides for the eyes.

Although the exact mechanism is still unclear, this can still lead to the deposition process.

A high ratio of single-tip HNPs can be obtained by selecting nanocrystals with appropriate morphology.

Length (nm)

Single ratio Double+Multi ratio Non ratio

Conclusions and

Perspectives

In this conclusion, I would like to highlight the following issues:

The goal of my thesis was:

(1) to prepare different morphologies of traditional cadmium-based II-VI semiconductor nanocrystals (NCs),

(2) to study the mechanism involved in the ligand-induced chirality especially that found the circularly polarized luminescence (CPL) signals of these NCs, and

(3) to control the photochemistry process in terms of the distribution and growth law of metal nanoparticles (NPs) during their photodeposition on CdSe/CdS dot-inrods (DRs).

In this context, I have accomplished the controlled preparation of semiconductor nanocrystals with different morphologies (Part 2). By using the tri-n-octylphosphine (TOP)-assisted extraction method and coupling with the TOP-SILAR method, I managed to prepared thick-shell structure quantum dots with a saturated green-emitting (Chapter 1). These QDs exhibit a high QY of 75%, a narrow FWHM of 26 nm and a controllable green emission (532 nm) which can be maintained constant even after a thick ZnS shell (7-monolayer) coating. In Chapter 2, I investigated, for the first time, the influence of the temperature on the shell growth of dot-in-rod quantum rods (DRs), and by using this method of temperature control (< 593 K), I obtained highly monodisperse nanorods with an ultra-thin CdS shell coated on the CdSe cores, with highly tunable aspect ratios (ARs). The preparation of the semiconductor nanocrystals with different morphologies presented in Chapter 3) was adjusted thanks to the judicious ratio between two ligands usually used separately in the synthesis of CdSe/CdS NCs, (OA and ODPA). Nanostructures with controlled morphologies as different as Tadpoles, nanoflowers, dot-rods and tetrapods can be obtained in a onepot synthesis without any secondary growth process. Our results suggested that more than the bulk core crystal structure, the surface ligands, their chemical nature and their respective ratios were the key factors to determine the final shape of CdSe/CdS NCs. Thermogravimetric analysis was carried out under nitrogen atmosphere using a DSC/TGA (SDT) Q600 instrument and software from TA Instruments.

Chirality analysis of the semiconductor

The UV-vis absorption spectra were recorded with a SHIMADZU UV-3600 Uv-Vis-NIR spectrophotometer and a TU-1901 double-beam UV/vis spectrophotometer (Beijing Purkine General Instrument Co. Ltd., China) using 10 mm cuvettes. The PL spectra were recorded on a fluoroSENS spectrophotometer (Gilden Photonics). The 378 absolute PLQYs of the QDs solutions were measured using an Ocean Optics FOIS-1 integrating sphere coupled with a QE65 Pro spectrometer. 1,2 The electroluminescence (EL) spectra, Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the as-fabricated TSQDs-LED were measured using an ATA-500 Spectral Radiation Analyzer (EVERFINE Corporation) with an integrating sphere at room temperature.

CD measurements:

The measurements were conducted on a JASCO J-1500 CD spectrometer at a scan rate of 20 nm/min. All CD experiments were carried out in Milli-Q water with a quartz cuvette (0.1 cm path length, Hellma). In designing chiral nanoparticles for application, the anisotropy or g-factor is a key parameter to determine the optical activity due to its readily availability by comparing CD spectrum with absorption spectrum. The equation is as follows:

where ɵ is the vertical coordinate of CD spectrum, and A is the value in the absorption spectrum at the same concentration used to measure the CD spectrum.

CPL measurements:

The CPL measurements were performed on a JASCO CPL-300 spectrometer in Milli-Q water with a quartz cuvette (0.1 cm path length, Hellma) with an excitation wavelength of 400 nm.

All optical measurements were performed at room temperature under ambient conditions.

Appendix 2

Prediction of the size of the gold deposited on CdSe/CdS dot-in-rods nanoparticles

381

In this part, we introduce a general approach to estimate theoretically the expected size of the gold nanoparticles deposited during the photodeposition process using KAuCl4 as a gold precursor.

A2.1 Assumptions

For the sake of simplicity, we first assume a volume of a reactant mixture solution of 1 mL. Then, for the photodeposition of gold at the surface of CdSe/CdS dot-in-rods (DRs), we neglect the contact angle between the gold and DRs tip and assume a spherical shape; this approximation seems quite justified when looking at the morphology of the photoproduced nanoheterodimers, as illustrated on Figure 1.

Figure 1. Nano heterodimer based on DR1

Then, the volume of the metal dot is

Prediction of the size of the gold deposited on CdSe/CdS dot-in-rods nanoparticles

382

A2.2 Determination of the number of CdSe/CdS DRs particles in solution

All the calculations are based on CdSe/CdS DR-1 support, with an average diameter (D) of (7.6 ± 0.8) nm and length (l) of (48.0 ± 2.1) nm; the diameter of CdSe core is 3.9 nm (Figure 2). The volume of a single CdSe/CdS DR-1 is:

And dV=D(ldD/2+D/4.dl)=553.7 nm 3 The volume of a single CdSe core is:

The density of CdSe/CdS DR-1 is then:

The molar mass for CdSe/CdS DR-1 is then:

Due to the presence of ligands on the surface of the DRs, the weight of the pure quantum rod cannot be obtained directly, and the proportion of ligands needs to be deducted from TGA analysis.

Where A: absorbance, b: cell path, c: concentration used for the UV-Vis. analysis (Figure 4).

For the Blue laser photodeposition process, the mother solution was diluted 400 times, to get a concentration of DR of 2.5 nmol/L, OD473 nm=0.11.

A2.3 Example of Au-CdSe/CdS HNPs

In order to get an upper limit of the synthesis of Au-CdSe/CdS hybrid nanoparticles (HNPs), let us assume that the metal precursor is fully photo-reduced to metallic gold, that is, the consumption rate is ε = 100%.

For a solution with volume Vsol =1 mL and a concentration cAu3+=40 μM (the ratio of Au/DRs=16000), the total mass of reduced gold is:

Then, according to the TEM results, we obtained a heterodimer yield Y = 83-94% of Au-CdSe/CdS (by counting more than 200 NPs), which means a ratio between Au-CdSe/CdS HNPs and the total CdSe/CdS DRs of nearly 100%. Let us assume that the yield of Au-CdSe/CdS HNPs is 100%. Then, the total number of Au NPs after completion of the photodeposition process is:

𝑁 𝐴𝑢 = 𝑌𝑁 𝐷𝑅-1 = 100% × 2.5 × 10 -12 × 6.02 × 10 23 = 1.5 × 10 The mass of a single Au NP is then

and the volume of a single Au NP is

As assumed before, this value is equal to the volume of the metal sphere, that is:

Then, in the case of a gold consumption rate ε = 100% and a heterodimer yield Y = 100%, a reasonable estimation of the largest expected diameter of Au NPs (Au size) photodeposited onto each CdSe/CdS DR should be in average DAus = 8.0 nm.

Generally speaking, the experiments show that the gold dot diameter after completion is around D = 6.5 nm (issued from a fit of the data) for a mean nanoheterodimer yield of 90 % (from 83 to 94%). Based on this diameter, we can also try to estimate the mass of gold effectively photodeposited as heterodimer.

The number of photodeposited gold dots is 𝑁 𝐴𝑢 = 𝑌𝑁 𝐷𝑅-1 = 90% × 2.5 × 10 -12 × 6.02 × 10 23 = 1.35 × 10 Their average experimental volume is:

And the total mass of gold effectively photodeposited experimentally as nanoheterodimers becomes:

Which means that in reality, experiments allow to photodeposit half of the gold present in the solution as nanoheterodimers.

To be slightly more precise, we can consider the experiment HJ11-C1-HJ3-P107 performed at a laser exposure of 32 minutes, i.e. near expected completion. Data are the following: D = 6.91 ± 1.37 nm, yield in heterodimers: 83%, two tiny dot deposition:

5.26%, multi tiny dot deposition 1.94 % and no observable deposition 9.42%. Then,

The number of photodeposited gold dots corresponding to nanoheterodimers is:

𝑁 𝐴𝑢 = 𝑌𝑁 𝐷𝑅-1 = 83% × 2.5 × 10 -12 × 6.02 × 10 23 = 1.25 × 10 Their average experimental volume is

And the total mass of gold effectively photodeposited experimentally on the form of nanoheterodimers becomes:

Then, experimentally, the mass of gold effectively photodeposited as nanoheterodimers represents for this long exposure time experiment 4.16/7.88 = 53%

of the Au 3+ initially present in the solution.

Part of this 'missing mass' is photodeposited in the form of second dot of the twotip of photodeposited samples (Some of them contain large Au NPs) and the multi tiny dots (typical size: <2 nm) and another part is expected to remain in solution in agreement with the thermochemical equilibrium conditions. Indeed, the disproportionation mechanism of Au(II) in the Au(III) reduction process supplies continuously the reformation of half of the Au(III). And if we add the fact that some Au(III) is initially lost during the different stages of the preparation of the samples to be photoexposed, then it may be concluded that 50% of gold readily photodeposited on the CdSe/CdS DRs to form 90% nanoheterodimers remains a quite good yield.

Abstract :

Controlled morphologies of traditional cadmium-based II-VI semiconductor nanocrystals (NCs) are presented. Different morphologies can be achieved by using the tri-n-octylphosphine (TOP) extraction and purification process, such as nanodots, nanoflowers, tadpoles, dot-in-rods and tetrapods. CdSe/CdS dot-in-rods (DRs) were specifically chosen for the further study on chirality and photodeposition due to its potential ligand-induced chirality and catalytic performances. The mechanism of ligand-induced chiral transmission was studied by the top-down selective domain etching process. The results showed that when comparing the chirality signals of an individual nanoparticle, the shell layer had a negative correlation with the first exciton peak chirality, but positive correlation with the shell absorption chirality. We present the induced chirality circularly polarized luminescence (CPL) signals in CdSe/CdS nanoplates (NPLs) synthesized by a onepot approach for the first time. The ligand induced chirality of semiconductor nanocrystals with different morphologies are further studied, and the observed circular dichroism (CD) and CPL activities are closely associated to the geometrical characters of the nanostructures such as the shell thickness and the aspect ratio of the CdSe/CdS Tadpoles. Finally, the laser-induced photodeposition growth mechanism of gold nanocrystals onto preformed CdSe/CdS dot-in-rods (DRs) is presented. The hybrid NPs (HNPs) Au-CdSe/CdS are achieved by using a blue-laser light. The effects of the hole scavenger for the synthesis of single-tipped HNPs are studied deeply for the first time. Additionally, other parameters are also studied, such as the irradiation intensity, the deposition time, the Au/DRs ratio and so on. Our results compare quite well with a model developed for the growth of single Au nanocrystal.