S H Yu

Y Lee

S K Jang

J Kang

J Jeon

C Lee

J Y Lee

H Kim

E Hwang

S Lee

Acs Nano

F H L Koppens

T Mueller

P Avouris

M. S A C Ferrari

M Vitiello

Nat Polini

Nanotechnol

E Lee

Y Hwang

J S Choi

S Kim

Lee

T Xiang

J Liu

J Y Xu

Z Tan

B Hu

Y Lei

J Zheng

A H C Wu

P Wang

J Wang

W Wang

X Hu

N Zhou

H Guo

S Huang

H Sun

T Shen

M Lin

L Tang

A Liao

Jiang

X Li

Y Wang

S Chen

F Zhu

G Gong

C Wu

L Meng

L Liu

T Wang

S Lin

X Shen

J Hu

X Sun

J Meng

Nanotechnology Chu

] R C G Naber

K Asadi

P W M Blom

D M De Leeuw

B De Boer

H Qiu

Y Zhao

Z Liu

M Herder

S Hecht

P Samorì

] H Qiu

Y Yao

M.-A Stoeckel

M Gobbi

T Leydecker

M Eredia

S Bonacchi

R Verucchi

M Timpel

M V Nardi

E Orgiu

En conclusion, nous avons exploité l'approche molé culaire sensible aux stimuli pour fabriquer des FET multifonctionnels dont le courant de sortie peut ê tre contrôlé de maniè re ré versible non seulement par son champ é lectrique interne, mais é galement par divers stimuli externes, comme la lumiè re, la chaleur et le champ é lectrique. En particulier, des FET à ré ponse binaire ont é té fabriqué s en inté grant des 2DS avec des molé cules photochromiques DAE. Des FET à ré ponse ternaire ont ensuite é té ré alisé s en ajoutant un troisiè me composant, le polymè re ferroé lectrique P(VDF-TrFE). En tirant pleinement parti de la bistabilité des é lé ments sensibles aux stimuli, les dispositifs ont dé montré leur application en tant Ré sumé VII que mé moires non volatiles à haute densité . Enfin, en remplaç ant DAE par SP, des FET à ré ponse quaternaire aux stimuli ont é té fabriqué s. Notre architecture de dispositif est gé né ralement applicable à d'autres 2DS et molé cules ré actives aux stimuli, ouvrant de nouvelles opportunité s pour l'é lectronique multi-stimuli-sensible de nouvelle gé né ration et favorisant le dé veloppement de technologies «More than Moore» en enrichissant la diversification.

Déclaration sur l'honneur

Declaration of Honour J'affirme être informé que le plagiat est une faute grave susceptible de mener à des sanctions administratives et disciplinaires pouvant aller jusqu'au renvoi de l'Université de Strasbourg et passible de poursuites devant les tribunaux de la République Française.

Je suis conscient(e) que l'absence de citation claire et transparente d'une source empruntée à un tiers (texte, idée, raisonnement ou autre création) est constitutive de plagiat.

Au vu de ce qui précède, j'atteste sur l'honneur que le travail décrit dans mon manuscrit de thèse est un travail original et que je n'ai pas eu recours au plagiat ou à toute autre forme de fraude.

I affirm that I am aware that plagiarism is a serious misconduct that may lead to administrative and disciplinary sanctions up to dismissal from the University of Strasbourg and liable to prosecution in the courts of the French Republic.

I am aware that the absence of a clear and transparent citation of a source borrowed from a third party (text, idea, reasoning or other creation) is constitutive of plagiarism.

In view of the foregoing, I hereby certify that the work described in my thesis manuscript is original work and that I have not resorted to plagiarism or any other form of fraud.

Ré sumé

Le travail mené dans le cadre de cette thè se s'est axé sur la fabrication de dispositifs multifonctionnels basé s sur des semi-conducteurs bidimensionnels (2DS) dé coré s par de molé cules. Les 2DS ont attiré une attention considé rable tant dans la science fondamentale que dans la technologie, du fait de leurs proprié té s exceptionnelles. Dans l'è re post-silicium, leurs é paisseurs à l'é chelle atomique ont validé ces maté riaux comme les candidats prometteurs dans l'industrie é lectronique avancé e, et ils ont é té utilisé s dans un large é ventail d'applications comme les photodé tecteurs, [1] les mé moires non volatiles à haute densité [2] et les capteurs biocompatibles. [3] Bien que des avancé es impressionnantes aient é té ré alisé es, des dé fis se posent encore en matiè re de commercialisation. L'un des vé ritables verrous est le contrôle du type de porteur de charge et du niveau de dopage dans les 2DS, qui est la condition essentielle pour la production de dispositifs et de circuits à logique complé mentaire. [4] Notre projet de recherche consiste à fournir une solution prometteuse et polyvalente pour ré soudre ce dé fi, c'est-à -dire inté grer des molé cules sensibles aux stimuli dans des dispositifs à base de 2DS, non seulement pour ré aliser le contrôle du transport des porteurs de charge, mais é galement pour ajouter de nouvelles fonctions au circuits numé riques, enrichissant la diversification des technologies «More than Moore». [5] Les molé cules sensibles aux stimuli sont particuliè rement inté ressantes en raison de leur capacité à ê tre interconverties entre deux ou plusieurs é tats, ayant des proprié té s nettement distinctes, lorsqu'elles sont soumises à des stimuli externes tels que des champs é lectromagné tiques, é lectriques ou magné tiques ou des changements dans l'environnement (pH, tempé rature, etc). [6] En incorporant de telles molé cules dans des transistors à effet de champ (FET) classiques, une nouvelle capacité de ré ponse aux stimuli peut ê tre confé ré e au dispositif, permettant ainsi de contrôler leurs proprié té s é lectriques par diverses entré es externes. De plus, ayant un rapport surface: volume é levé , les 2DSs en monocouches et en quelques couches ont une surface extrê mement sensible au moindre changement environnemental avoisinant, ce qui ouvre la possibilité de modifier leurs proprié té s intrinsè ques. [7] Des é tudes ré centes innovantes sur l'utilisation de molé cules ré pondants aux stimuli pour dé corer la surface des 2DSs ont montré que cette straté gie est prometteuse pour moduler la densité des porteurs de charge de maniè re non destructive, en dé plaç ant l'interrupteur déclenché par les stimuli au niveau moléculaire par la modification des proprié té s des 2DS sans introduire de dé faut. [8] Parmi les stimuli externes possibles, la lumiè re et le champ é lectrique sont apparus comme des stimuli externes attrayants, tous deux non invasifs, contrôlables à distance et numé riquement avec une ré solution spatiale et temporelle é levé e. En tant que stimulus doux et bien appliqué , la tempé rature repré sente é galement l'une des voies les plus largement exploré es dans la conception de plates-formes sensibles aux stimuli.

Ré sumé II Les molé cules photochromiques sont des systè mes optiquement ré actifs, capables de subir une photoisomé risation ré versible lorsqu'ils sont irradié s avec de la lumiè re à des longueurs d'onde spé cifiques. [9] Les molé cules photochromiques les plus populaires rapporté es à ce jour sont les azobenzè nes, les spiropyranes (SPs) et les diarylé thè nes (DAEs). Les deux premiè res molé cules mentionné es ont d'abord é té combiné es avec des 2DSs pour conduire à un dopage ré versible optiquement contrôlé par la variation de leurs moments dipolaires molé culaires entre leurs deux photoisomè res. [10] Par contre, notre recherche utilise le mé canisme de transfert de charge pour ré aliser des FET à base de 2DS optiquement sensible en les combinant avec des molé cules DAE approprié es.

Cette molé cule possè de des avantages significatifs pour les applications é lectroniques numé riques car elle combine une photoisomé risation efficace à l'é tat solide avec une ré sistance à la fatigue é levé e, une bistabilité thermique et des niveaux é nergé tiques phototunables strictement dé finis. [11] D'autre part, des maté riaux ferroé lectriques ont é galement é té utilisé s pour contrôler pré cisé ment le niveau de dopage des 2DS. [12] Selon la direction du champ é lectrique externe, leur polarisation bistable intrinsè que peut ê tre ré orienté e, conduisant à une accumulation ou un é puisement des porteurs de charge dans le maté riau semi-conducteur interfacé . [13] Fait inté ressant, ces transistors à effet de champ ferroé lectriques (FeFET) tirent parti des é tats de polarisation restants binaires du ferroé lectrique pour stocker des informations et ont donc é té utilisé s avec succè s dans des mé moires non volatiles. [14] Dans le premier projet, des FETs optiquement commutables ont é té ré alisé s en dé corant la surface des 2DSs, du WSe2 de type n et du phosphore noir (BP) de type p, avec des molé cules DAE conç ues de maniè re à moduler leur transport d'é lectrons et de trous, respectivement. [15] Les molé cules DAE subissent une isomé risation photochimique entre leur isomè re en cycle ouvert et en cycle fermé par la lumiè re UV et visible, accompagné e d'un changement de leur orbitale molé culaire occupé e la plus é levé e (HOMO) et de leur orbitale molé culaire inoccupé e la plus basse (LUMO), offrant la possibilité d'affecter diffé remment le processus de transport de charge lorsqu'ils sont combiné s avec des 2DSs.

Par consé quent, la né cessité de ré aliser une telle photomodulation repose sur l'adaptation des niveaux Aprè s avoir dé montré l'applicabilité gé né rale et l'efficacité de cette approche 2DS/DAE pour la modulation de transport d'é lectrons et de trous en fonction de la nature de la cible 2DS, dans le deuxiè me projet, nous abordons la commutation ambipolaire complè te du transport de charge dans le 2DS. [16] Inspiré par le phé nomè ne selon lequel le porteur principal de charge dans WSe2 é volue avec son é paisseur, allant du type p, ambipolaire au type n, nous visons des multicouche de WSe2 à quelques couches avec un comportement ambipolaire plus é quilibré , avec ses courants d'électrons et de trous à la mê me é chelle (10 -6 A). Nous proposons ici un FET multiniveaux optiquement commutable en interfaç ant WSe2 avec un mé lange DAE à deux constituants composé s de deux molé cules avec des Ré sumé IV niveaux d'é nergie spé cifiques, qui sont conç ues pour ê tre capables de pié ger les é lectrons ou les trous de WSe2 par des stimuli lumineux à distance (Figure 2). Il est inté ressant de noter que la modulation du courant de sortie pour les trous atteint 97%, tandis que pour les é lectrons, la modulation atteint 52%. V molé cules rapporté s, une seule face du 2DS est interfacé e aux molé cules, n'exploitant ainsi pas une asymé trie potentielle dé terminé e par les deux surfaces qui pourraient ê tre rendues iné gales par exposition à deux environnements diffé rents. Un tel scé nario peut ê tre ré alisé par la fonctionnalisation indé pendante de chaque surface avec une molé cule fonctionnelle diffé rente, [17] offrant des perspectives inté ressantes vers la ré alisation de FET multi-stimuli-ré actifs. Pour ré soudre ce problè me, dans le troisiè me projet, nous rapportons une approche sans pré cé dent basé e sur la dé coration de feuillets WSe2 à quelques couches avec deux (macro)molé cules diffé rentes ré actives aux stimuli gé né rant ainsi un maté riau Janus 2D. En particulier, sur la surface infé rieure reposait une couche de DAE_1 sensible à la lumiè re et la surface supé rieure é tait revê tue d'une couche de copolymè re poly(fluorure de vinylidè netrifluoroé thylè ne) (P(VDF-TrFE)) ré actif à l'é lectricité (Figure 3a). De cette maniè re, un FET sensible à plusieurs stimuli a é té fabriqué dans lequel le courant de sortie peut ê tre modulé de maniè re ré versible et pré cise au moyen soit d'une irradiation lumineuse soit d'un champ é lectrique. Par rapport aux FET conventionnels mono-stimuli sensibles, cette nouvelle architecture de dispositif ternaire combine à la fois une complexité fonctionnelle amé lioré e avec une simplicité de fabrication. Nous fournissons des preuves sans é quivoque de la compatibilité de la ré ponse du dispositif aux stimuli optiques et é lectriques, offrant ainsi une prise en main orthogonale sur le fonctionnement de l'appareil, ce qui est é galement favorable pour les applications de mé moire. En tirant parti de l'effet synergique entre les deux commutateurs (macro)molé culaires, le dispositif a atteint une modulation amé lioré e du courant de sortie, avec un rendement pouvant atteindre 87%. De plus, comme chaque stimulus manifeste un grand nombre de sorties diffé rentes sans interfé rences mutuelles (9 niveaux induits par l'é lectricité et 84 niveaux induits par la lumiè re), le dispositif produit un nombre total de 756 é tats distincts, surpassant le transistor mono-stimuli sensibles, en fournissant ainsi un pas en avant dé cisif pour ré pondre à la demande de mé moires de stockage à haute densité . La fiabilité de chaque é tat est en outre reflé té e par les tests d'endurance et de ré tention pour 5 é tats arbitraires, pré sentant une endurance notable pendant plus de 10 cycles et une excellente ré tention des donné es dé passant 1000 heures. De plus, comparé e à la synthè se complexe de molé cules uniques incorporant diffé rents groupes fonctionnels pour ré aliser les é lé ments multi-adressables, cette structure ternaire modifié e Janus repose sur un processus de fabrication simple et elle semble ê tre une straté gie plus universelle qui peut ê tre é tendue à d'autres 2DS et molé cules fonctionnelles, offrant une plate-forme polyvalente pour la construction de dispositifs multi-stimuli-sensibles.

Dans le projet ci-dessus, la couche de DAE physiosorbé e offre une voie prometteuse vers la fonctionnalisation non destructive du maté riau du canal. Cependant, l'interaction faible de van der Waals entre les molé cules et le substrat Si/SiO2 permet d'enlever facilement les molé cules au cours d'une simple procé dure de rinç age. En consé quence, le processus de photolithographie conventionnel n'est pas faisable dans une telle architecture de dispositif, et le choix du second é lé ment sensible à des stimuli est é galement limité . Pour surmonter ce problè me, dans le quatriè me projet, nous avons Ré sumé VI synthé tisé un dé rivé de SP à terminaison silane, qui peut densé ment s'assembler sur un substrat Si/SiO2 grâ ce à une silylation covalente forte. Une telle chimisorption non volatile de celui-ci facilite la fabrication du dispositif et offre une meilleure compatibilité lors de l'inté gration d'un troisiè me é lé ment dans le systè me. De plus, la ré action photochromique du SP à la mé rocyanine (MC) peut ê tre dé clenché e par un é clairage à la lumiè re UV, et l'isomé risation inverse de MC à SP peut ê tre accomplie par illumination de la lumiè re visible ainsi que par chauffage, ce qui ajoute une nouvelle capacité de ré ponse thermique aux dispositifs FET correspondants par rapport au systè me composant DAE. Suivant la structure modifié e Janus similaire, la molé cule photochromique SP à terminaison silane et le polymè re ferroé lectrique P(VDF-TrFE) ont é té incorporé s dans des dispositifs FET à base de WSe2 (Figure 3b).

Avec cette combinaison, nous avons obtenu un FET sensible aux stimuli quaternaires capable de percevoir les stimuli comme la chaleur, la lumiè re et de champ é lectrique, et de moduler le courant de sortie en consé quence. Une telle architecture permet un contrôle indé pendant du courant de sortie sous de multiples stimuli distincts. Profitant de l'effet synergique entre les stimuli, avec une efficacité de modulation de courant d'é lectrons significative de 84% de SP, et de 99% de P (VDF-TrFE), les é lectrons dans le canal peuvent ê tre totalement é puisé s en combinant les deux commutateurs et le dispositif peut ê tre modulé du type n au type p. De plus, en regroupant collectivement plusieurs stimuli en tant qu'entré es, la dé pendance excessive à un seul stimulus a é té ré duite, ce qui augmente en outre la sensibilité et la conformité de la plate-forme de ré ponse aux stimuli molé culaires.

Symbols and abbreviations

Chapter 1 Introduction

Moore's law, stating that the number of transistors per silicon chip doubles every two years, predicted by Gordon Moore in 1965, has been the golden rule for electronic industry over 50 years. The main driver of Moore's law has been the miniaturization of integrated circuit. However, such projection cannot be sustained indefinitely, and transistors would eventually reach the limits of miniaturization at atomic levels. [1] In view of this, the International Technology Roadmap for Semiconductors defined three distinct paths to keep the semiconductor industry continually growing: More Moore, focusing on the continued scale down of CMOS technology; More than Moore, calling for a disruptive change in the architecture and integration strategies; and Beyond Moore, exploiting innovative technologies as complimentary to CMOS and to overcome its limit. [2] Silicon has historically been used in most electronic devices because of its wide availability and ideal semiconductor properties. However, achieving its physical dimensions near the deeper nanoscale regime of 10 nm or beyond is challenging for existing technologies. At the nanoscale, the silicon circuits tend to possess high leakage current with increased static power consumption that generating more heat in a unit area. As silicon is reaching its limit, it is necessary to find a replacement in electronic circuits.

Graphene and other two-dimensional semiconductors (2DSs) have emerged as plausible candidates, due to their breakthrough in scaling limits and preserved device performance. [3] These 2DSs combine several unique physico-chemical properties including atomic thickness, sizable bandgap, and high charge carrier mobility, opening up a variety of opportunities for the construction of high-performance devices with ultrathin body.

Albeit being outstanding, the tuning of the properties of these 2DSs in a controllable manner has been a challenge, which is of paramount importance in terms of production of practical complementary-logic devices and circuits. The combination of 2DSs with molecular system represents a promising approach to not only control the charge carrier doping, but also confer new properties to the former components.

Thus, the aim of this thesis work is to contribute to the reversible control over the charge carrier transport via integrating of molecular system into the 2DS-based electronic devices. In particular, stimuli-responsive molecules including photochromic molecules diarylethene (DAE), spiropyrans (SPs) and ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) have been employed to confer their intrinsic capacities to the interfacial 2DS and fabricate multifunctional devices. Such molecular tailored strategy promotes the development of "More than Moore" technologies in terms of enriching diversification in electronic systems.

The thesis is divided into 8 chapters in the following order:

Chapter 1 places the work carried out in thesis in scientific context.

Chapter 2 provides a brief theoretical background of this work, including principles of semiconductor physics and charge transport mechanism in electronic devices, doping engineering of 2DSs, in particular focus on the molecular chemistry approaches that have been developed so far to control the chargecarrier doping along with other properties. In solid-state physics, the band theory is a useful way to distinct between the conductors, insulators and semiconductors (Figure 2-1). [1] Electrons in an atom are present in different energy levels. The valence band (VB) involves energy levels that consist of valence electrons present in the outermost shell of an atomic structure, and the conduction band (CB) involves energy levels that consist of free electrons that are responsible for conduction. Since the CB possesses higher energy level, they are generally empty.

The CB in semiconductors can accept the electrons from the VB on receiving sufficient external energy, and the conducting electrons in the CB results in the flow of current. The energy difference between the highest occupied state of the VB and the lowest unoccupied state of the CB is known as the band gap, an indicator of the electrical conductivity. The greater the band gap, the higher is the energy needed to excite valence electrons to the CB. The insulators are characterized by a large band gap, so a prohibitively large energy is required to promote electrons into the CB.

Conductors exhibit an overlap between the CB and VB, hence electrons can jump freely between the two bands, meaning the material is profoundly conductive. Semiconductors are characterized by a small band gap allowing a meaningful fraction of the valence electrons to get promoted into the CB when a certain amount of energy is provided to the system. Their conductivities are therefore in-between those of conductors and insulators, which is the main reason for their application in electrical circuits.

Types of semiconductors

Semiconductors can be classified as intrinsic semiconductors and extrinsic semiconductors. An intrinsic semiconductor is chemically pure and is made up of only a single type of atoms, such as silicon (Si), germanium (Ge), tin (Sn) in column IV, and selenium (Se), tellurium (Te) in column VI of the periodic table. There are also numerous compound semiconductors, which are composed of two or more elements, such as gallium arsenide (GaAs), a combination of gallium (Ga) in column III and arsenic (As) in column V. [2] At sufficiently high temperatures, some of the valence electrons in the semiconductor can break their bonds in the lattice structure and become available as conduction electrons, resulting in the current flow in the semiconductor. (a) Bond structure, reproduced from Ref. [3] and (b) its energy-band diagram of intrinsic semiconductor.

Fermi level is an important concept used to describe the thermodynamical equilibrium of charges in the semiconductor. It is defined as the energy level where the probability of occupation by an electron is 0.5. In the intrinsic semiconductor, the number of electrons in the CB is equal to the number of holes in VB. The probability of occupation of energy levels in CB and VB is therefore equal. Hence, the Fermi level lies in the middle of forbidden band (Figure 2-2).

Apart from heating, the conductivity of semiconductors can be greatly improved by introducing different types of atoms called impurities. These impurities modify the electrical properties of the semiconductor, making it more suitable for electronic applications. Impurities that are added to control charge carrier concentrations are called dopants. The process of adding impurity atoms is called doping, and the doped semiconductor is called extrinsic semiconductor. The extrinsic semiconductor can further be classified into n-type semiconductor and p-type semiconductor, as shown in Figure 2-3 and 2-4. [3] Figure 2-3. (a) Bond structure, reproduced from Ref. [3] and (b) its energy-band diagram of n-type semiconductor.

The intrinsic semiconductor silicon is pure and contains negligible impurities, with each silicon atom sharing its four valence electrons with the four neighboring atoms. When a silicon atom is replaced by a dopant that has the same electronic structure, the bonding behavior is substantially unchanged, resulting in only minor changes in bonding. While when the dopant has more or fewer electrons than the atom it replaces, it creates an extra electron or hole. This extra electron or hole can escape from the dopant and move freely in the semiconductor, causing a change in the conductivity. For example, when a silicon atom is replaced by a substitutional phosphorous (P) atom which has five valence electrons, four electrons out of five of phosphorous bond with the four electrons of silicon, leaving the fifth negative-charged electron free and donated to the CB. The phosphorous atom is thereof called donor.

The energy level of the donor lies slightly lower than that of the CB. Hence, electrons can move into the CB with minimal energy. Since the number of free electrons is increased by the additional dopants, they exceed the number of holes and become the majority carriers, eventually leading to a n-type semiconductor.

In the energy band structure, the probability of occupation of energy levels by the electrons in the CB is greater than the probability of occupation of energy levels by the holes in the VB. The Fermi level is therefore shifted up near the CB.

Conversely, when a boron atom which has three valence electrons replaces a silicon atom, its three valence electrons bond with three electrons of the semiconductor, leaving a hole between the fourth atom of the silicon atom and the boron atom. The hole accepts an electron from the neighborhood, creating a conductive hole. The boron atom is thereof referred to as an acceptor. The energy level of the acceptor is just above the VB. Electrons from the VB can thus move to the acceptor level with minimal energy. This process provides excess holes to the intrinsic semiconductor, making them the majority carriers, creating a p-type semiconductor.

In the p-type semiconductor, the probability of occupation of energy levels by the holes in the VB is greater than the probability of occupation of energy levels by the electrons in the CB. The Fermi level is therefore shifted down near the VB. (a) Bond structure, reproduced from Ref. [3] and (b) its energy-band diagram of p-type semiconductor.

Semiconductor applications

Application of semiconductors are ubiquitous in our daily life, including mobile devices, data centers, industrial equipment, and the Internet of Things. Without semiconductors, our life would be much differentthey are as critical to modern life as air or water.

The semiconductor industry started from 1947, when the first point-contact transistor based on germanium (Ge) was invented by Bardeen, Brattain, and Shockley at Bell Laboratories in the US. They were later awarded the Nobel Prize in Physics in 1956 for their contributions to the semiconductor research. Since then, the semiconductor industry grew rapidly.

Until the early 1950s, germanium was the main material used in semiconductor industry and was considered the most effective semiconductor material due to its high mobility. However, it turns out that such material was found not suitable for many applications, mainly due to two reasons: one is devices made of germanium have worse leakage currents in the OFF state, the other is they are restricted to 0 to 70 °C operation, which limits their use in rugged applications. [4] Since the middle 1960s, silicon has replaced germanium and has become the dominant semiconductor material for the device manufacturing since it is more abundant and less expensive. Moreover, devices made of silicon are found to have much lower leakage current compared with those made of germanium due to the larger bandgap of silicon, and they are able to operate at a larger temperature range, from -55 to 125°C . In addition, silicon dioxide (SiO2), a high-quality insulator, can be easily employed in the Si-based devices. [5] Therefore, silicon technology has become widespread since then and silicon devices has accounted for more than 95% of all semiconductor products nowadays. [6] Owing to their reliability, compactness, power efficiency and low cost, semiconductors have been employed in the manufacture of various electronic devices, including diodes, transistors and integrated circuits. [7] These devices are manufactured for both discrete devices and integrated circuits, which consist of from a few to billions of devices manufactured on a single semiconductor substrate.

Diode

It is one of the simplest semiconductor devices, which is typically made up of a single p-n junction. [8] It consists of a single piece of silicon, half of which is doped with impurities of n-type, and the other half doped with impurities of p-type. Both regions are coated with metal contacts so that wires can be attached. To obtain a maximum current in the low resistance, a positive voltage must be applied to the anode with respect to the cathode. The main advantage of the device is to allow the electric current to pass in one direction and prevent it in other direction, the diode is therefore a unidirectional device.

Diodes have many applications in electric and digital circuits, such as half-wave rectifier, full-wave rectifier, photodiode, and light-emitting diode (LED). [9] Figure 2-5. Semiconductor diode characteristic. Reproduced from Ref. [10] The device performance of a diode is usually characterized by a current-voltage curve (Figure 2345).

Even without application of an external bias, the electrons in the n-type region can move freely to the p-type region and recombine with the holes due to the concentration differences, creating uncovered negative ions in the p-type region. In the same way, the holes diffuse into the n-type region and create uncovered positive ions. Such uncovered immobile ions form a layer that is set up across the junction line, forming a region in the middle of the diode. The region is called depletion region since all the charge carriers get combined here. The electric field across the region, which is known as the junction potential, will further prevent the migration of charge carriers from one side to the other (Figure 23456). Conversely, in the reversed bias condition, a negative bias is applied on the p-type region and a positive bias is applied on the n-type region. In such condition, the holes in the p-type region would be repelled away from the junction region, leaving more uncovered negative ions at the junction. Similarly, the electrons would also be repelled and leave uncovered positive ions. Therefore, the depletion region becomes wider, no majority carriers cross the junction. In this way, flow of current is blocked (Figure 2345678).

Transistor

It is a three-terminal device based on doped semiconducting material which is used in most electronic circuits. It can perform two basic functions: amplification and switching. Amplification consists of magnifying an input signal into a greater output signal, whereas switching means to control the input current through a pair of terminals while applying a voltage or current at another pair of terminals. [11] There are a variety of transistors available in the market, including bipolar junction transistors (BJTs)

and field-effect transistors (FETs). A BJT is a type of transistor that contains both electrons and holes as charge carriers and join three sections of semiconductor material with different doping concentrations.

It is built up of three terminals, named as the emitter, the collector and the base. A small current injected at the base terminal will pass to the emitter and be modified or switched into a much larger current flowing between the collector and emitter terminals. [12] While different from the BJTs, the FETs The introduction of FET devices will be further discussed in Chapter 4.4.5.

Tuning properties of two-dimensional materials

Over the past decades, the evolution of silicon technology is based on Moore's law which predicts that the number of transistors in integrated circuits would double every two years. This is achieved by steadily shrinking the size of transistors. However, this reduction in size is reaching a nanometer-scale limit in which parasitic effects such as the quantum tunnel effect have to be taken into account. To solve this problem, several solutions such as searching for a replacement material with comparable or even much higher performances than silicon have been considered.

The emergence of graphene in 2004 and later related two-dimensional semiconductors (2DSs) offer prospects of unprecedented advances in device performance at the atomic limit. Their amazing potential has proven to overcome the limitations of silicon technology, where the combination of 2DSs with silicon chips promises to surpass the current technological limitations. They have been appealing components for numerous applications including flexible/wearable electronics, sensing technologies and energy storage/conversion.

Although 2DSs have been widely used in various fields due to their extraordinary physical properties, we are still facing numerous challenges in broadening their applications. In the first market products based on 2DSs, there are two major technological challenges that required to be tackled, one is the synthesis of large-area and high-quality films of monolayer materials with low densities of defects and impurities, [13] and the other is the development of strategies to control the charge-carrier type and the doping level in 2DMs. [14] Whereas a lot of efforts has been devoted on the former, [15] relatively less work has been focused on the latter, which is nevertheless a critical hurdle for the realization of 2DSs in electronic applications. In this section, we will introduce some classic doping techniques that have been developed.

Conventional doping techniques

Doping refers to the process of adding impurities to a semiconductor to modify its intrinsic optical or electrical properties. The most common doping methods include thermal diffusion and ion implantation. [16] Thermal diffusion occurs when impurity atoms or molecules migrate from an area of high concentration to an area of low concentration. Diffusion usually occurs in two steps: predeposition and drive-in.

During the predeposition process, the impurity dopants, mainly in three forms, gases, liquids and solids, are introduced to the wafer substrates in a furnace at the temperature around 1000-1250 º C. The wafers are then put into a second furnace at a higher temperature about 1300 º C to drive-in the dopant. The introduced dopants in this process diffuse deeper into the wafer to create the required concentration gradient. Note that the drive-in process usually occurs in an oxidizing atmosphere so that a protective layer of SiO2 is grown over the diffused layer. The Figure 2-10 below shows the setup for the thermal diffusion. When comparing these two techniques, thermal diffusion usually takes place at a higher temperature, it is an isotropic process and involves lateral diffusion. Conversely, ion implantation is a fundamental process used to make microchips which requires lower temperature. It is an anisotropic process and can form very shallow junctions. However, they are both challenging to be implemented in combination with 2DMs, since damages or defects may be induced during such processes, which can cause detrimental effects on their structural and electrical characteristics, resulting in degraded device performances.

Doping techniques for 2DMs 2.3.2.1. Alloying

Alloying of semiconductors tunes their intrinsic optical and electrical properties by altering the energy band structure. 2D alloy is single atomic layer of alloy that is incommensurate with underlying substrate. [17] The monolayer alloys can be fabricated by mechanically exfoliation from bulk alloys, physical vapor deposition from the powders of the end materials at a low temperature, chemical vapor deposition from sulfurization and selenization of transition metal oxides, and chalcogen exchange by hetero-chalogen atoms. [18] form Ref. [19] 2.3.2.2.

Transition metal substitution/incorporation

Atomic doping is a technique that can alter the structure and properties of the target material depending on the dopant atoms. [20] The intrinsic atoms in the lattice can be directly substituted if the dopant atoms are well matched in terms of size, valence, and coordination. For layered materials, dopant atoms may also intercalate between layers. MoS2, (c) Re-MoS2 for several temperatures. Reproduced from Ref. [20] 2.3.2.3. Plasma-assisted doping the control and H2-plasma-treated WSe2 FETs. Reproduced from Ref. [21] The plasma-assisted doping method is a mature technique which uses various gas molecules including N2, O2 and H2 to generate ions with high energy. With these ions bombarded on the surface of 2DM, vacancies are prone to created. [22] Figure 2-14 shows the investigation of mild H2 plasma treatment on the defect engineering in the WSe2 lattice. [21] The n-type WSe2 field-effect transistors (FETs) with plasma-treated contact regions that exhibit contact resistance significantly lower than that of devices made without plasma treatment of the contacts. Moreover, such improved contacts result in nearly ideal subthreshold swing value at room temperature, with the value of 66 mV/dec.

Molecular chemistry approach

As the conventional doping techniques introduced above like ion implantation and diffusion may unavoidably damage the 2D crystals and result in degraded device performances. In recent years, the use of molecular systems interfacing 2DSs has emerged as a promising versatile, not only for controlling the charge carrier doping, but also imparting them novel functionalities. 2DSs are extremely sensitive to surrounding variables due to their large surface to volume ratio. Their intrinsic properties can thereof be modified by any surface modification. Taking advantage of this character, the molecules on the surface of the 2DSs can modulate their charge-carrier density via electrical dipoles or charge-transfer process through both covalent and non-covalent interactions. Especially, molecules with functional groups possessing optical and electrical properties can activate specific interactions with 2DSs. Since an unlimited variety of molecules can be synthesized with programmed properties, such hybrid structure based on 2DS/molecule system holds great potential as the device architecture for on-demand applications.

Covalent interaction between molecules and 2DSs

Covalent functionalization of 2DSs with molecules mainly including two strategies, defect engineering and phase conversion.

The crystals of 2DSs are enriched by a variety of intrinsic defects, such as vacancies, adatoms, grain boundaries, and substitutional impurities (Figure 2-15a), which strongly influence on their optical and electronic properties, rendering it crucial to develop novel strategies of defect engineering for the realization of high-performance devices. [23] In defect engineering approach, the chemical reactivity of the 2DS's surface is first activated by the generated defects, enabling the chemical bonds be formed between the defective sites and ad hoc molecules by S vacancy self-healing or low-temperature thiol chemistry treatment. To date, various techniques such as electron irradiation, thermal annealing, plasma treatment and physical bombardments with charged particles, have been employed to engineer chalcogen vacancies in MoS2 nanosheets, or selenium vacancies in WSe2, Molecules carrying thiol group like (3-mercaptopropyl)trimethoxysilane are often used to functionalize the defective MoS2 nanosheets, which results in a significant recovery of the electrical characteristics of the related FET devices. [24] Molecules carrying selenol group are expected to repair the defective WSe2. In Figure 2-15c, the effect of the S vacancy self-healing on the electrical properties of a MoS2 transistor was investigated. A decrease of current can be observed in the output characteristic and the threshold voltage dramatically shifted toward zero after the S vacancy self-healing, indicating that the electrons or sulfur vacancies in the as-grown MoS2 was removed and the healing reduced the interface trap states. [25] Figure 2-15. (a) Two-dimensional materials with diverse types of defects. Reproduced from Ref. [23] (b) The HAADF images of MoS2 surface before (left) and after (right) poly(4-styrenesulfonate)-induced S vacancy selfhealing. (c) Output and transfer characteristics of a monolayer MoS2 FET before (left) and after (right) poly(4styrenesulfonate)-induced S vacancy self-healing. Reproduced from Ref. [25] When it comes to phase conversion approach, it relies on the reversible switch of the 2DS between the functionable metallic polytype 1T/1T' phase and the inert semiconducting polytype 1H/2H phase via chemical and thermal stimuli, like alkali metal intercalation, strain, electron beam and laser beam irradiation. For example, the exposure of substrate-supported CVD MoS2 nanosheets to n-butyllithium can result in the phase change from 1H to 1T/1T'. The 1T/1T' phase can then be easily functionalized by methyl iodide and iodoacetamide electrophiles via covalently bonding between the functional groups and sulfur atoms. The functionalized MoS2 nanosheets feature a different optical property compared with the pristine material without functionalization. Therefore, by choosing ad hoc electrophile molecules, the optical and electrical properties of the target 2DSs are expected to be modified for specific applications. Moreover, mixing the two phases in a monolayer 2DS could also be interesting, which may principally modulate its property. Figure 2-16b illustrates the Ar-plasma treatment used to activate the 2H to 1T phase transition for MoS2. [26] The kinetic energy of Ar ions was tuned well to wrench the S-Mo bond to induce the lateral sliding of top S-layer. By performing a selective-area phase transition in 2H MoS2, we can clearly see the PL quenching in the 1T phase. MoS2. (d) Performance of the FET device with a heterophase structure in MoS2. Reproduced from Ref. [26] 2.3.3.2.

Non-covalent interaction between molecules and 2DSs

Although being less robust compared with the above-mentioned covalent approach, the non-covalent approach turns out to be noninvasive. Without altering the crystal structure and formation of new chemical bonds, the approach can largely preserve the intrinsic superior properties of the 2DSs. Van der Waals interaction and π-π stacking are the most common interactions between the molecules and the target 2DS. The molecules usually comprise an anchoring group possessing a high affinity to promote the physisorption with the basal plane of the 2DS, and a functional group to impart a novel function to the system. 2-17a). Such effect is evident from the evolution of the photoluminescence (PL) spectra. In the F4-TCNQ-doped MoS2, the weight ratio of the negative trion X -is reduced while the contribution of the neutral exciton X becomes dominant due to the electron depletion. When the F4-TCNQ-doped WS2 is integrated as the channel material in the back-gated FET, as shown in Figure 2-17c, with increasing concentration F4-TCNQ, the threshold voltage is found to shift towards more positive gate voltage values indicating an enhanced p-type doping, in good agreement with the optical modulation. Different from F4-TCNQ, nicotinamide adenine dinucleotide (NADH) possesses a redox potential higher than the CBM of monolayer MoS2, which can therefore induce n-type doping. After physisorption of NADH molecules, the corresponding PL spectra of monolayer MoS2 show opposite behavior and is characterized by a higher weight ratio of trion X -due to electron accumulation. This example demonstrates that the molecular physisorption approach can induce charge-carrier doping and effectively modulate the optical as well as electrical properties of the target 2DS. Reproduced from Ref. [28] (b)

PL spectra of MoS2 before and after being doped with F4-TCNQ. (c) Transfer evolution of WS2 FET as-prepared and after various F4-TCNQ doping steps. Reproduced from Ref. [29] (d) PL spectra of MoS2 before and after being doped with NADH.

Dipolar interaction is another mechanism to tune the doping level in 2DSs. A molecule with an intrinsic dipole moment can generate an electric field at the interfaced 2DS. When a large quantity of the same molecules possessing the same dipole orientation lies on the 2DS surface, each single electric field sums up and results in a significant electric field. The target 2DS thus experiences an additional non-zero gate voltage, leading a shift of its Fermi level. In this way, the molecular layer leads to a doping effect depending on the dipole direction.

The molecule induced dipolar interaction can be realized by interfacing with either the bottom or the top surface of the 2DS. By using functional self-assembled monolayers (SAMs) to decorate the substrate surface, the bottom surface of the later transferred 2DS can thus be modified. The molecules possessing head groups like thiols, silane, phosphonates have been widely studied to form SAM on the Au or Si/SiO2 substrate. For example, by inserting a 3-aminopropyltriethoxysilane (APTES) passivation layer at the MoS2/SiO2 interface, the threshold voltage hysteresis of MoS2 FET is significantly reduced. [30] The top surface of the 2DS can also be decorated with molecules via physisorption.

Interestingly, the molecule with same functional groups is investigated to modify the top surface of MoS2 and is found to induce n-type doping. The results are in agreement with those obtained at the bottom surface modified 2DS FET. [31] with and without APTES beneath. Reproduced from Ref. [30] (c) Schematic illustration of APTES/MoS2/SiO2 structure. (d) Transfer curves of the MoS2 FETs with and without APTES on top. Reproduced from Ref. [31] As mentioned above, molecules with functional groups interfacing with 2DS may enable technologically relevant applications. For example, device based on organic dye molecules/2DS can work as an ultrasensitive photodetector, due to the enhancement of its electrical response to light. [32] As in Figure 2-19a, rhodamine 6G (R-6G) treated MoS2 photodetectors showed an enhanced performance with a broad spectral photoresponse and a high photoresponsivity compared with the properties of the pristine MoS2 photodetectors, due to the charge transfer of the photoexcited electrons from the R-6G molecules to the MoS2 layer. Among all the functional molecules, one interesting group is the stimuliresponsive molecules. They possess two different states with each state exhibiting well-defined characteristics; these two states can reversibly switch between each other in response to the according stimuli, like light irradiation, electric field, magnetic field, change in pH, etc. With different states inducing different doping levels in the target 2DS, devices based on such hybrid structure are thereof endowed with the novel stimuli-responsive function. For example, switchable 2D FET with a chemically programmed electrochemical signal as a remote control can be fabricated by functionalize the MoS2 channel with a suitably designed ferrocene-substituted alkanethiol (Fc-SH) molecule. [33] The molecule not only improves the FET performance by healing the sulfur vacancies with thiol functional groups, but also show reversible electrochemical switching on the surface of MoS2. As illustrated in Figure 2-19d, after the adsorption of Fc-SH molecules, a small droplet of ionic liquid was applied onto the devices. A Pt wire, serving as the top gate electrode was immersed in the ionic liquid. two redox peaks can be attributed to the chemically reversible oxidation-reduction between Fc-SH and Fc + -SH.

The two states can induce different doping effects, which effectively tune the charge transport in the underlying MoS2 layer. Our research work is based on the combination of stimuli-responsive photochromic and ferroelectric molecules with 2DS and towards the fabrication of multi-stimuliresponsive FETs, which will be discussed in the following chapter. (c) Photocurrent at Vg = 0 V. Reproduced from Ref. [32] Ref. [33]

Chapter 3 Materials and molecules

This chapter is focused on the description of the graphene and related two-dimensional semiconductors (2DSs) as well as molecules used within this thesis work. Different types of stimuli-responsive molecules such as light-responsive photochromic molecules and electric-responsive ferroelectric polymers, were employed by interfacing them with 2DMs and modulate their intrinsic properties.

Two-dimensional materials

Low-dimensional nanomaterials can be broadly classified into 3 types according to the total number of their nanoscopic dimensions: 0DM (zero-dimensional material): If all three dimensions of a material are nano-sized, it would be called 0DM. A typical example of 0DM is the nanoparticle.

1DM (one-dimensional material): If two dimensions of a material are nano-sized, with the other dimension much larger, it would be 1DM, such as nanotube or nanowire.

2DM (two-dimensional material): If only one dimension of a material is nano-sized, it would be a 2DM.

It is often possible to reduce the thickness of a 2DM down to a single atom.

If none of the dimensions of a material nano-sized, then it is not a nanomaterial. Instead, it should be referred to as a bulk material. Examples for 0D, 1D, 2D, and 3D carbon nanostructures. Reproduced from Ref. [1] Following the Moore's law, there have been much effort devoted to scaling down the dimensions of FETs. However, FETs cannot be scaled down infinitely and the scaling limitation of silicon FETs was predicted to be 5 nm, under which severe short channel effects occur. [2] As Moore's law is reaching its physical limit, 2DMs have attracted considerable attention due to their their excellent gate electrostatics and the reduced short channel effects. Especially, their inherent advantage of being capable of scaling down into atomic thickness has opened the perspective for the extension of the Moore's law into 'More than Moore'. [3] 3.1.1. Graphene

Graphene, the ancestor of the 2DMs, is an atom-thick honeycomb sheet of carbon atoms. It was first isolated from graphite by Ander Geim and Konstantin Novoselov via scotch tape method in 2004. Since then, graphene has emerged as one of the most promising nanomaterials due to its unique combination of properties. [4] For this historic contribution to graphene science, the two scientists were awarded the Nobel Prize in physics 2010. Graphene is the 2D building block for other sp2 carbon allotropes, including 0D fullerenes, 1D nanotubes, and 3D graphite. [5] In each carbon atom, there is a total number of 6 electrons: 2 in the inner shell and 4 in the outer shell.

The outer shell electrons are available for chemical bonding. While in graphene, only 3 out of the 4 electrons in the atom are covalently bonded with sp2 hybridizations, leaving 1 electron freely available for electronic conduction. These hybrid orbitals form strong in-plane σ bonds, with the interatomic distance of 0.14 nm, offering graphene with outstanding mechanical properties. While on the other hand, the left unhybridized p orbital on each carbon atom overlaps with the unhybridized orbitals from the adjacent atoms to form out of plane weak π bonds, which is responsible for graphene's excellent electronic properties. [6] Due to such atomic arrangement, the electronic dispersion of graphene presents cones in the Brillouin zone, where its valence and conduction band meet at a single precise point, called Dirac point (Figure 3-2). [7] Because of this unique electronic structure, many fascinating properties of pristine graphene were discovered in the past few years. When it comes to the optical property, graphene has a high optical transmittance with an optical absorption of only 2.3% over a broad wavelength range, making it almost transparent thus suitable for application as transparent electrodes that are necessary components in modern devices such as touch screens and solar cells . [8] For electrical property, it has an extremely high charge carrier mobility for both electrons and holes (up to 2×10 6 cm 2 /Vs at room temperature), being much higher than the silicon based transistor (1400 cm 2 /Vs). [7,9] In addition to that, graphene also possesses high thermal conductivity (K exceeding ~3000 W mK -1 near room temperature), [10] the highest intrinsic strength known to exist (130 GPa), being 200 times stronger than steel, [11] the highest theoretical specific surface area (2630 m 2 /g), which means a single gram of graphene can cover an entire football pitch, [12] and high Young modulus of 1 TPa [11] . These unique extraordinary properties make graphene indispensable component in applications like flexible electronics, optoelectronics, solar cells, batteries, etc. [13] from Ref. [7] The market of graphene applications is essentially driven by progress in the production of graphene. Graphite is a layered material and can be considered as a stack of individual graphene layers held together via Van der Waals interactions. In the scotch tape method, adhesive tape is used to detach the top few layers of graphite from a highly oriented pyrolytic graphite (HOPG). By repeated peeling the multilayer graphene is cleaved into various flakes of few-layer graphene. Then the tape is pressed against a substrate resulting in some materials transferring to it (Figure 3-3). [14] Since the Van der Waals force between the graphite interlayer is not as strong as the force between the graphite and the substrate, graphene flakes are possible to be obtained during such peeling process. Finally we can seek for the monolayer graphene under an optical microscope. The different colors refer to different thicknesses of flakes, from monolayer (the pale purple), few layer (the intermediate purple), and bulk (the white).

This method was demonstrated for the first time in 2004 and has been then applied to other materials. [15] The process of this method is relatively simple, and the obtained graphene is of highest quality and cleanliness with almost no defects. However, the size of the flake is limited by the crystal grains in the starting HOPG, which is generally below the order of millimeters. This drawback limits its large-scale applications, instead, the material prepared by this method is usually used for fundamental research and device fabrication. We have been employed this method during the thesis to obtain high-quality 2DMs.

b. Liquid phase exfoliation

Liquid-phase exfoliation of graphene via sonication was initiated in 2008, and it became an industrialscale method to produce high-quality graphene in the form of liquid suspensions, inks, or dispersions. [16] It has also been extended to produce other 2DMs. [17] This method comprises a number of different techniques: sonication, shear mixing, microfluidization, electrochemical exfoliation, etc. The most common procedure is to disperse graphite in an organic solvent, which has a similar surface energy to graphite in order to reduce the energy that needed to overcome in order to detach a graphene layer from the crystal. The solution is then sonicated and centrifuged in order to disperse the thicker flakes. By optimize the solvent and the sonication time, concentration of the graphene dispersion can be improved. [18] Although the size of obtained flake is usually limited by the intense exfoliation process therefore not suitable for electronic devices fabrication, the liquid exfoliation method is relatively fast, low-cost, scalable, the quality of the obtained material is high, and the operating conditions are mild. The material produced by liquid phase exfoliation can be deposited on the desired substrate via drop casting, spray coating, ink-jet printing, etc. Moreover, it allows additional applications such as composites and hybrids by simple mixing of dispersion of different kinds of materials. It is reported that solution-based graphene has been integrated in high-frequency flexible electronic with a current gain cut-off frequency of 2.2 GHz. [19] Figure 34. Schematic illustration of liquid phase exfoliation process. Reproduced from Ref. [20] c. Chemical vapor deposition (CVD) CVD has been emerged as an important method for the production of graphene since it was first reported in 2008 and 2009, using Ni and Cu substrates, which was followed by a variety of research activities to use different transition metal substrates. [21] CVD is the process to grow graphene film on the surface of a metal substrate (which are also used as the catalysts) by exposing it to a gaseous hydrocarbon precursor such as CH4. The reaction usually takes place in a chamber under the atmosphere of H2 and Ar. Parameters like temperature, gas composition and deposition time should be controlled during the reaction. The CH4 molecules first dehydrogenate on the metal surface, resulting in CHx species. The atomic C species then diffuse and attach to the graphene domain edge, incorporated into the graphene lattice. The growth of the graphene nuclei continues until consuming all the surrounding adsorbed C species and coalesces to form a continuous film. [22] For practical uses, the graphene film formed by CVD technique usually requires transferring to another target substrate by using wet or dry transfer technique, which remains the main drawback of CVD method since the transfer process may not preserve the quality of the as-grown graphene. Another drawback of the method is that it is relatively expensive owing to the large energy consumption. [23] While despite of these drawbacks, it is a robust and promising technique to produce continuous graphene films with uniform properties over centimeter-scale areas.

d. Epitaxial growth

Graphene growth by decomposition of silicon carbide (SiC) crystal is another technique that holds great potential. [24] In this technique, graphene is prepared by simply sublime the SiC in a furnace filled with Ar or in ultra-high vacuum. At high temperature more than 1400 °C , the sublimation of Si is much faster than C owing to its higher vapor pressure, therefore the remaining C can form a graphene film on the target substrate surface. [25] The quality of the graphene films derived from SiC are highly dependent on the operating temperature, heating rate and pressure. Generally, the graphene obtained contains a mixture of multilayer graphene with a high surface roughness. The main advantage of this technique is that the resulting graphene film can be directly obtained on a commercially available substrate, avoiding the transfer process.

Molybdenum disulfide MoS2

MoS2 is the most well-known among all the TMDs and shares various similarities with graphite. In the single layer of MoS2, each Mo atom is sandwiched between two S atoms, forming a S-Mo-S monolayer stacking, whereas the adjacent layers are interacted by relatively weak van der Waals forces. [27] Monolayer MoS2, with a thickness of ~0.65 nm, was first isolated in 2005 by Novoselov and co-workers using scotch-tape method. [28] It is stable in ambient conditions, and usually consists of a mixture of two major polytopes with similar structure but different stack orders and metal atom coordination, namely 2H and 1T. 2H is an intrinsic semiconductor, where each Mo atom is covalently bonded to 6 S atoms in trigonal prismatic coordination with 3 layers per repeat unit. 1T is metallic and metastable, with Mo and S atoms arranged in octahedral coordination and 1 layer per repeat unit (Figure 34567). [29] trigonal prismatic; (c) 1T octahedral. Reproduced from Ref. [30] particular, monolayer MoS2 possesses a high absorption of of 5%-10% in the visible range, [32] making it a promising material for nanoelectronic and optoelectronic applications such as phototransistors and solar cells. [33] Figure 345678. Band structure of MoS2 with different thicknesses. Reproduced from Ref. [34] Despite the common features, the different nature of MoS2 has enable the material being used into wider has a direct bandgap of 1.9 eV, and the bandgap varies with the number of layers (Figure 345678). [34] Such change in the band structure is due to quantum confinement and the resulting change in hybridization between p orbitals on S atoms and d orbitals on Mo atoms. The presence of this bandgap makes it interesting for applications in nanoelectronics where it allows the fabrication of transistors with low power dissipation. The mobility of a monolayer device is in the range 0.1-10 cm 2 V -1 s -1 , which can be implemented by dielectric screening. The first implementation of a top-gate FET based on monolayer MoS2 was reported by Kis and co-workers in 2011. [35] The top-gating with high k dielectric (HfO2) geometry largely improves the device mobility due to the suppression of Coulomb scattering. The device showed excellent a mobility of > 200 cm 2 V -1 s -1 , ON/OFF current ratio of ∼10 8 , and subthreshold swing of 74 mV per decade. vapor. Reproduced from Ref. [30] Reliable production of MoS2 is essential for translating their intrinsic properties into various applications. Therefore, substantial efforts have been devoted to preparing thin-layer MoS2 (Figure 3- 9), including top-down methods like scotch tape based mechanical cleavage as we introduced in the previous section, the same technique that were developed for graphene, liquid phase exfoliation via ultrasonication by intercalation of ionic species like N-butyllithium between the MoS2 atomic planes to increase the distance between adjacent layer and favor the separation, [36] and bottom up synthesis like CVD method (Figure 3-9 c-e) by vaporizing and depositing S and MoO3 powder onto the substrate; [37] depositing a thin layer of Mo onto a wafer heated with solid S; dipping substrates in a solution of (NH4)2MoS4 and heating in the S vapor atmosphere, and hydrothermal synthesis to grow single crystals from an aqueous solution in an autoclave via the reaction of molybdic acid with thiourea. [38] 3.1.2.2.

Tungsten diselenide WSe2

Another exemplary case of TMD is WSe2, which holds great potential for complementary digital logic applications since n-type, p-type, and ambipolar transport can all be achieved by thickness modulation and electrode engineering in the corresponding transistor. [39] WSe2 is thermally stable in 2H phase(Figure 3-10a), with a hexagonal crystalline structure similar to MoS2, where Se atoms locate in the lattice position of a hexagonal structure. [40] The metallic octahedral coordination 1T phase is less stable and are commonly obtained via distortion of the 2H phase as by electrical gating, [41] electron transfer during chemical treatment (i.e. lithiation during exfoliation or organolithium treatment) [42] or applied mechanical strain. [43] Band alignments of some potential contact metals and Al2O3 with respect to that of WSe2. Reproduced from Ref. [44] (d) WSe2 FETs with hole transport behavior. Produced from Ref. [45] (e) WSe2 FETs with electron transport behavior.

Bulk WSe2 is an indirect bandgap (~1.2 eV) semiconductor, whereas monolayer WSe2 exhibits a direct bandgap of ~1.65 eV, [46] offer opportunities for its potential applications in optoelectronics. [47] While in comparison with widely studied MoS2, studies focusing on WSe2 are still very limited. The main reason is that WSe2 tends to form a substantial Schottky barrier with most metals that commonly used for making electrical contacts. [48] The high contact resistance between the metal electrode and WSe2 thereby induce poor injection, significantly influence device performance. Therefore, selecting proper metal contacts to form low contact resistance is necessary for WSe2. Metals with high work have been selected

for hole injection, and monolayer WSe2 is the first TMD material in which p-type behavior is observed by using high work function metal Pd as the contact, with the hole mobility of ~250 cm 2 /V• s. [45] Metal with low work function have been selected for electron injection, and monolayer WSe2 FET with In contact can attain an electron mobility of 142 cm 2 /V• s. [44] 3.1.

Black phosphorous BP

As the most stable allotrope of phosphorus, BP is a newly discovered 2DM that has been the focus for recent research for various applications. It is a layered material similar to graphene, whose individual atomic layers are stacked together by weak van der Waals interactions, with the distance between two adjacent layers of about 5 Å. Such layered structure allows the obtaining of monolayer or few-layer thick BP from the bulk crystal by mechanical or liquid exfoliation. In a single layer, each phosphorous atom is covalently bonded with three neighboring atoms forming a puckered 2D honeycomb structure due to sp3 hybridization with zigzag configuration (Figure 3-11). [49] Different from graphene, the three bonds take up all three valence electrons of phosphorus, constructing a quadrangular pyramid structure. Three-dimensional representation of BP structure. Reproduced from Ref. [49] BP has attracted tremendous attention since it bridges the energy gap between graphene and TMDs.

Monolayer BP is a semiconductor with a direct bandgap of 2 eV. For few-layer BP, each added layer reduces the bandgap due to interlayer interactions, and eventually reaches 0.3 eV for bulk. [50] Figure 3-12 shows the band structures for monolayer to 5 layers. [51] Such thickness-dependent direct bandgap feature leads to remarkable applications in optoelectronics covering from visible to mid-infrared range,

where TMD are limited because of their relatively large bandgap. [52] Figure 3-12. Band structures for BP from for monolayer to 5 layers obtained from HSE06 hybrid functional calculations. Reproduced from Ref. [51] BP possesses many fascinating physical properties. FETs based on bulk BP shows the mobilities on the order of 10,000 cm 2 V -1 s -1. While the exfoliated BP flakes have presents a clear ambipolar transport behavior with a high carrier mobility over 1,000 cm 2 V -1 s -1 , together with a high ON/OFF ratio of 10 5 . [53] Such distinguished virtue are hard to achieve simultaneously by either graphene or monolayer transition metal dichalcogenides. Moreover, It has strong photon absorption and in-plane anisotropic properties, which is beneficial for applications in high performance optoelectronics, such as broadband photodetectors. [52a] Albeit exhibiting outstanding properties, BP shows environmental instability in ambient conditions especially in its single and few-layer forms. The flake surface roughens rapidly within few hours in air, resulting in deteriorated electrical transport properties. By employing ab initio electronic structure calculations and molecular dynamics simulations, the understanding on the ambient degradation of fewlayer BP are illustrated by a 3-step picture: generation of superoxide under light, dissociation of the superoxide, and eventual breakdown under the action of water (Figure 3-13. [54] Figure 3-13. The light-induced ambient degradation process of BP. Reproduced from Ref. [54] a. Generation of O 2-through a charge transfer reaction under ambient light on the surface of BP. 𝑃 𝑥 𝑂 𝑦 + 𝐻 2 𝑂 → 𝑃 𝑥 + + 𝑂 𝑦 -Various protection strategies have been proposed to protect BP against moist air. As physical protection, capping layers have been implemented to isolate BP from moist air, [55] such as Al2O3 layers, [56] PMMA layer, and other 2D materials like graphene and h-BN. Chemical routes rely on the surface treatment of BP with chemical species like metoxybenzene, nitrobenzene, [57] and poly(phenylenevinylene), [58] in an effort to transform it from environmentally active to being inert.

Stimuli-responsive molecules

Photochromic molecules

Photochromic molecules are compounds that can undergo a reversible photo-induced isomerization between two or more states upon absorption of photons with well-defined energy. [59] The two isomers feature different physical properties, which means during such reversible photoisomerization, their physical properties such as absorption spectra, fluorescence emission, conjugation, electron conductivity, electrochemical properties, magnetic properties, coordination properties, dipole moment, refractive index, dielectric constant and geometrical structure, can be modulated by light. [60] In view of such unique characteristics, scientists have used this kind of molecules as active components in optoelectronic devices that can be addressed by light. Possible applications include processing of information in the molecular-scale, biological engineering, and high-density data storage. Currently, the most popular photochromic molecules are azobenzenes, spiropyrans (SPs), diarylethenes (DAEs), and dihydroazulenes (DHAs).

Azobenzenes

The photochromic properties of azobenzenes were first investigated by Hartley and co-workers in 1937. [61] Azobenzene is a diazene (HN=NH) derivative where both hydrogens are replaced by phenyl groups. The N=N double bond have two geometric isomers and the azobenzene molecules can thereby exist in either the cis or trans configurations. The trans isomer is s 10-12 kcal mol -1 more stable than the cis, and therefore is predominant in the dark. [62] The trans isomer can switch to the cis by irradiation with UV light at 340 nm, and the cis isomer can reverse to trans by vis light at 450 nm or spontaneously in the dark owing to the thermodynamic stability of the latter (Figure 3-14a).

Apart from thermal stability, the two isomers possess many other distinct properties. In the unsubstituted azobenzene, the trans isomer is planar with a quasi-zero dipole moment, while the cis isomer has a bent conformation and a dipole moment of 3 Debye. [63] Moreover, for the UV-vis absorption spectra, at 440 nm, the trans isomer shows a weak n-π* peak while cis isomer has a much stronger peak. The trans isomer has another strong π-π* peak near 320 nm, while the cis has peaks at shorter wavelength between 280 nm and 250 nm (Figure 3-14b). [64] Figure 3-14. (a) Photoisomerization of azobenzene between trans and cis isomer. (b) A typical example of UV/vis absorption spectra of trans and cis isomers of azobenzene. Reproduced from Ref. [64] The azobenzene molecules has been applied to electronic devices to impart their photo-responsivity capacity to the initially non light-switchable devices. Our group exploited a thiol-terminated biphenyl azobenzene to form SAM on the electrode of an OFET to act as active layer interface (Figure 3-15ac). [65] The P3HT based device presented reversible photoresponsitivity, with output current increased when the device was exposed to the UV light and restored to initial state after relaxation in the dark.

The modulation behavior was attributed to three possible factors, the decreased carrier injection barrier by cis-isomer, and the change of interface film morphology and also the variation of the work function of the contacts with the electrodes. The large conformational change of azobenzene molecules during photoisomerization also enable its use for multifunctional applications. [66] As shown in Figure 3-15d-f, with graphene working as the bottom and top electrode, and azobenzene molecules grafting onto the bottom electrode surface. The corresponding devices showed light-controlled electrical transport. After UV light irradiation, the output current increased by one order of magnitude and later decreased back to the previous value upon exposure to visible light. Such modulation behavior was explained by the variation of the molecular length by switching, thus altering the vertical distance of the bottom and top electrodes. Azobenzene molecules have also been employed to decorate the 2DM surface to impart a hybrid with light responsive capacity. [67] The azobenzene/MoS2 hybrid system possesses light-tunable PL spectra behavior, whose PL peak blue shifts and the intensity increases after UV irradiation, and convert back to the initial state after irradiation with visible light. The corresponding FET device also showed light-controlled charge transport (Figure 3-15g-i). These phenomena were induced by isomerization of azobenzene molecules between different states exhibiting diverse dipole moment. The carrier concentration and even carrier polarity of the interfaced 2DM can therefore be directly modulated via dipolar interactions. The photoinduced current switching for over 50 cycles. Reproduced form Ref. [66] (g) Schematic illustration of the FET with azobenzene on the MoS2 surface. (h) PL spectra and (i) Transfer evolution of the FET for one switching cycle of trans/cis azobenzene. Reproduced from Ref. [67] 3.2.1.

Spiropyrans

During the past few years, SPs has emerged as the molecule-of-choice for the construction of novel dynamic materials, due to their capacity of being able to undergo isomerization in response to a variety of orthogonal stimuli, which ranges from light, temperature, solvents, acids and bases, metal ions, redox potential, to mechanical stress. [68] The structure of SP molecule is depicted in Figure 3 ). [68] Therefore, the MC form may exist in both cis and trans isomers, while only the trans form is observed at room temperature due to the low thermal stability of the cis form. The MC isomer can return to SP spontaneously in the dark or can be accelerated by vis light illumination and heating. [69] and protonated spiropyran (SPH + ). Reproduced from Ref. [68] (b) UV/vis absorption spectra, (c) Emission spectra of SP and MC isomers. Reproduced from Ref. [70] The structure difference between SP and MC isomers induces a markedly difference in their optical and electrical properties, which allows their widespread utility. First of all, density functional theory calculations have shown that the dipole moment of SP normally lies in the range of 4-6 Debye, while the value for MC is largely increased to 14-18 Debye due to the formation of charge-separated species. [71] Secondly, the SP isomer is optically colorless in the vis range and shows two localized bands at the UV range (Figure 3-16b). One band located at ∼272-296 nm is attributed to the π-π* electronic transition in the indoline moiety, and the other band at ∼323-351 nm originates from the chromene moiety. While the MC isomer appears deep blue and has strong absorption band at ~550-600 nm.

Thirdly, the two isomers shows different emission behavior (Figure 3-16c): the SP isomer has no emission, while MC has an intense emission band centered at ~650 nm. [70] Fourthly, the structure difference between the two isomers makes the SP occupy less volume than MC, which may give a mechanical response when they are functionalized on the planar surface, beneficial for biophysical systems. [72] Based on these characteristics, various investigations on the study of SP-based dynamic materials have been reported, such as SP-functional polymers, photo-control inorganic nanoparticles and so on.

Optically controllable transistors based on SP molecules have also been proposed. Guo et co-workers reported the fabrication of OFET by blending P3HT with SP to form a hybrid active layer, whose output current increased when UV light turned on and restored under the irradiation of visible light (Figure 3-17a-c). [73] Such modulation behavior may be explained by two possible mechanisms. One is that the charge-separated MC form possess larger dipole moment, which can form an electrostatic environment that act as a local negative gate-voltage. Another possible reason is that the HOMO level of MC is lower than that of SP. As a result, the increased energy gap between the HOMO levels of P3HT and MC enhance the driving force for hole transport. Later on, a similar work was proposed by replacing P3HT

with poly(triarylamine) (PTAA), while obtaining the opposite switching behavior: the output current reduced by UV irradiation and reversed under vis irradiation (Figure 3-17d-f). [74] The possible mechanism for this phenomenon is that the ionic polarized SP-open works as a carrier scattering site and trap the carriers in blend film thus leading to the decrease of current. When combining SP with 2DMs, optically switchable FETs can be realized. Figure 3-17g-i presents the switching behavior of FET based on graphene and MoS2 with a spin-coated SP layer on top, both showing significant n-type doping effect induced by MC molecules after UV irradiation. [75] Such doping effect was induced by the presence of positive vertical dipoles in MC molecules. With positively charged side is lifted up in each single molecule, the MC assembly resulted in a significant electrical dipole oriented perpendicular to the 2D surface. Reproduced from Ref. [74] (g) Schematic illustrations of FET based on 2DM with SP. (h) Transfer evolution by alternating UV and vis irradiation for graphene/SP FET (left) and MoS2/SP FET (right). (i) Representative snapshot of the calculated dynamic evolution of the SP and MC assembly. Reproduced from Ref. [75] 3.2.1.

Diarylethenes

DAEs, being another typical example of photochromic molecules, was pioneered by Irie and co-workers in 1980s. [76] Since then, they have received growing attention since they are particularly suitable as switching units and fulfil the requirements to be applied in various practical applications due to their excellent properties: high fatigue resistance, high quantum yield, short response time, thermal bistabiliy and photo-tunable energetic levels. the population ratio of the antiparallel conformation, which is usually 50%. [59] As increasing the antiparallel ratio can result in an increase in quantum yield, therefore various strategies have been proposed, for example, introduction of bulky isopropyl groups at 2 and 2' positions of bisbenzothienylethene to increase the relative stability of the antiparallel conformation (Figure 3-18b), [77] or, bridge the 2 and 4 positions of thiophene rings by a considerably long -CH2SCH2linkage to fix the antiparallel conformation (Figure 3-17c). Reproduced from Ref. [78] b. Thermal stability Thermal stability of the ring-closed isomers is one of the most promising advantages for DAE molecules serving as active elements in memory applications, which is controlled by the enthalpy difference between the ring-open and ring-closed isomers in the ground state. [60a] It was demonstrated that such stability is dependent on the aryl groups: (a) aromatic stabilization energy: molecules with thiophene, benzothiophene, or furan heterocycles serving as aryl groups are much stale than those with phenyl, pyrrolyl, or indolyl rings; (b) electron-withdrawing substituents: introduction of electron-withdrawing substituents can weaken the central carbon-carbon bonds in the closed-ring isomers thus decrease its stability; (c) steric hindrance of the substituents at the reactive carbons: bulky substituents can lead to thermal cycloreversion reactions of the closed-ring isomer. [79] c. Fatigue resistance Fatigue resistance is another necessary property when applying the molecular switches as functional units in various applications. It has been observed that distinct DAE derivatives have different photochemical side reactions, e.g., oxidation or elimination reactions of the ring-closed isomer. In particular, the formation of an annulated byproduct was considered as the major source of fatigue over the broad range of DAE structures. [80] A typical example of the absorption spectra of DAE switches during the course of UV irradiation is shown in Figure 3 during repetitive UV/vis switching cycles with a high fatigue resistance. Reproduced from Ref. [81a] d. Modulating the charge transport in semiconductors via energy-level phototuning One of the most important and special features of DAE molecules is their phototunable energy levels, which means that when irradiated with UV/vis light, the molecules can switch between their ring-open and ring-closed isomers, accompanied with a switch of their corresponding HOMO/LUMO energy level.

This phenomenon may be explained as the number of double bonds in the open form is larger than that in the closed form, thus the energy of HOMO in the open form is expected to be lower than that of the closed form. In the ring-open form, the conjugation is localized on the heteroaromatic rings. While in the ring-closed form the molecule belongs to a conjugated system, whose HOMO and LUMO orbitals are essentially delocalized π orbitals extending over the whole backbone of the molecule, therefore the HOMO-LUMO gap is significantly smaller after the switching. [83] Such characteristic allows the molecules to induce different trapping behavior when they are combined with semiconducting materials with suitable energy levels. In particular, for p-type semiconductor, if the HOMO level of the DAE molecule in one form is higher than its VB and the other form is lower, the molecule will act as a trap for holes in one form and not the other. Conversely, for n-type semiconductor, tunable LUMO levels can make a molecule switch from scattering center to charge trap for electrons. This effect can be exploited to modulate the charge carrier transport of a semiconductor upon irradiation at the defined wavelengths. switching of an OTFT under several UV/vis irradiation cycles. Reproduced from Ref. [84] (c) Schematic illustration of the optically switchable OSOLET based on light-emitting polymers with DAE. (d) and (e) Reversible modulation of OSOLET (d) current, (e) luminance during UV/vis irradiation cycles. Reproduced from Ref. [85] (f) Switching behavior of DAE-graphene micro-supercapacitor by alternating UV/vis irradiation. Reproduced from

Ref. [86] Such mechanism was first realized by using blends of DAEs with a polymeric semiconductor P3HT acting the electroactive material to fabricate optically switchable organic thin-film transistors (OTFTs). [84] The dual functionality in these OTFTs was achieved by engineering the energy levels of the blend through the insertion of the DAE phototunable energy levels in the polymer matrix. As shown in Figure 3-20a and b, a dynamic control of the output current of the device could be achieved by UV/vis irradiation in a reversible manner without significant fatigue for several irradiation cycles. To go a step further, optically switchable organic light-emitting transistors (OSOLETs) can be fabricated by integrating DAEs into the light-emitting semiconducting layer of organic light-emitting diode (Figure 3-20c-e). Both output current and electroluminescence can be simultaneously modulated by irradiating the devices at distinct wavelengths. Another example was the fabrication of photoswitchable microsupercapacitor based on a DAE-graphene composite film (Figure 3-20f and g). [86] The light-responsive CV curves showed that the total capacitance increased under UV irradiation, recovered to original value under white light irradiation, and could be modulated for several cycles alternating light irradiation.

Dihydroazulenes

DHA molecules were investigated for the first time by Daub and co-workers in the 1980s. [87] They can undergo a 10-electron retro-electro-cyclization to vinylheptafulvene (VHF) isomer by UV irradiation.

The initial VHF isomer in its s-cis-conformation is metastable and will convert into its more stable strans-conformer. The VFH can in turn, back to DHA by a thermally induced cyclization. [88] The ringopening reaction from DHA to VHF occurs with a high quantum yield and is accompanied with significant changes in the physical properties, such as dipole-moment, emission and absorption spectra.

The DHA exhibits a characteristic absorption band around 350 nm while the VHF has a more intense absorption band around 470 nm. Furthermore, the DHA is usually fluorescent while VHF is generally non-fluorescent because of a radiation-less deactivation of the excited state. Ref. [89] (b) UV/vis absorption spectra of DHA and VHF isomers. Reproduced from Ref. [90] 3.

Ferroelectric materials

Ferroelectric materials were first discovered in 1920 in the form of bulk single crystals of Rochell salt. [91] They belong to the broad family of dielectrics that have spontaneous stable polarization states switchable by application of a sufficient external electric field.

The term "ferroelectricity" stems from the "ferromagnetism", which is used to describe magnetic materials when measuring the magnetization of a ferromagnet as a function of magnetic field. Similarly, ferroelectricity is the characteristic of ferroelectric materials when measuring the electric polarization as a function of electric field.

When most materials are polarized with an electric field E, the polarization P induced is almost exactly proportional to the E, resulting in a linear function. While for ferroelectric material, its polarization does not vary with the electric field in a linear way. It has a remnant polarization Pr even when the applied electric field is zero. Such ferroelectric polarization originates from the alignment of dipoles inside the material and is bistable, i.e., when the direction of the applied electric field is reversed, orientation of the dipole moment will also inverse, giving rise to a ferroelectric hysteresis loop. ). [92] The α phase possesses a trans-gauche-trans-gauche (TGTG; T-trans, G-gauche)) conformation, in which the F and H substituents are at 180 ° and at ± 60 ° alternatively. The absence of steric strains between the F molecules makes it the lowest potential energy state among the four polymorphs. The dipole moment in this state is arranged antiparallelly within a single unit cell so that each dipole is neutralized by its neighbor that is oriented in the opposite direction. Therefore, α phase the most stable while non ferroelectric. The β phase has an all-trans planar zigzag conformation, with all the F and H molecules aligned in the same direction perpendicular to the carbon backbone. Therefore, the β phase owns the largest spontaneous polarization among all the phases. [93] Since the direction of the polarization can be switched according to the direction of the external electric field, the β phase is responsible for the ferroelectricity of the PVDF. The γ phase is in the intermediate T3GT3G' conformation, with weaker polarization compared with the β phase. [94] The δ phase is the parallel version of the α phase, which can be obtained by poling α phase under a high electric field. [95] The γ phase and δ phase are also polar, while their spontaneous polarization is only half that of the β phase. Chain conformations of the α, β, γ and δ phases of PVDF. Produced from Ref. [96] In order to exploit the ferroelectricity of the PVDF, the polar β phase is the most desirable, while thin PVDF films processed directly from the melt or from the solution is mostly the non-polar α-phase due to its thermodynamic stability. [97] Therefore, a lot of efforts have been devoted to explore effective ways for inducing piezoelectric β phases. One of the most common methods is to mechanically stretch the αphase films at a suitable temperature. [98] By stretching the PVDF film, the polymer chains can be aligned into an all-trans planar zigzag conformation, the crystals with β phase is therefore induced. [START_REF] Sencadas | α-to β Transformation on PVDF Films Obtained by Uniaxial Stretch[END_REF] Another method is by crystallization from polar solvents like N,N-dimethylformamide (DMF) or N,N-dimethyl acetamide (DMAc).

In 1968 Lando and Doll suggested that the introduction of a small amount of trifluoroethylene (TrFE)

into PVDF to form a copolymer poly(vinylidene fluoride trifluoroethylene (P(VDF-TrFE)) can induce direct crystallization of the material into the polar β phase by melting or solvent casting. [100] The structure of the copolymer P(VDF-TrFE) with the monomer unit of (CH2CF2)n-(CHFCF2)m is illustrated in Figure 3-23b. Because of the similar size of the hydrogen and fluorine atoms, VDF and TrFE units are randomly distributed along the main carbon chain, allowing for the formation of the all-trans conformation in analogous with the PVDF in β phase. [101] Moreover, the interchain distance in the β phase is slightly larger than that of pure PVDF, therefore it is easier for the dipole rotation around the carbon backbone for the copolymer. Reproduced from Ref. [102] One important parameter for PVDF-based materials is the Curie temperature, TC, that marks the transition of the material from the ferroelectric phase to the paraelectric phase and vice versa. Above TC the material will be paraelectric and lose their permanent ferroelectricity, i.e., the dipoles are randomly oriented. Note that the Curie temperature is absent in pure PVDF because it melts before the phase transition.

Another classification of P(VDF-TrFE) copolymers was introduced to describe the crystal phase, the low-temperature (LT) phase with all-trans conformation, similar to PVDF β-phase, the hightemperature (HT) phase with TGTG conformation similar to PVDF α-phase, and the cooled (CL) phase similar to PVDF γ-phase. [103] The material can undergo phase transition between each other if it is annealed, stretched or poled. The LT phase transforms into the HT phase via the CL phase during heating above the Curie temperature, and the HT phase then transforms into the CL phase during the cooling process. The CL phase can be converted into the more regular LT phase by stretching or by applying an electric field at room temperature. [104] Therefore it is proposed that when the copolymer P(VDF-TrFE) is annealed at a temperature between the TC and TM (the melting temperature), there occurs an enhanced β-phase fraction and further crystallization of the material. [105] Consequently, their ferroelectric response can be enhanced after annealing. Figure 3-25 shows the comparison of the hysteresis loop of a 56/44 mol% PVDF-TRFE copolymer film as prepared by spin coating from dimethylformamide and after annealing, which clearly demonstrates that the annealing induces higher crystallinity and thus larger remnant polarization of the sample. [102] Compared with pure PVDF, the preferential formation of β-phase of the copolymer P(VDF-TrFE) give them a broader variety of applications in electroactive applications. Note that the copolymer can afford a crystallinity reaching 90% while the value for PVDF is around 50%. Therefore, larger polarization is expected in the copolymer P(VDF-TrFE). [106] Due to its intrinsic ferroelectric and piezoelectric characteristics, robust chemical resistance and high thermal stability, P(VDF-TrFE) is appealing for applications like memories, wearable electronics, and pressure sensors. One of the simplest types of ferroelectric memories is the thin-film metal-ferroelectricmetal capacitor, in which information is stored by aligning the direction of the internal polarization either in the upward or downward direction and can be retrieved by applying a bias voltage to obtain a higher or a lower charge displacement. Figure 3-26a illustrates the external poling of a P(VDF-TrFE) based capacitor. [107] The vertical poling was achieved by the application of an external electric field on a top electrode on top of a spin-coated P(VDF-TrFE) film, while lateral poling was achieved by depositing Al strips on top. Figure 3-25c shows the C-V curves from three capacitors with the same thickness of the P(VDF-TrFE) film but different poling conditions: vertical-poled, lateral-poled, and unpoled. The curve exhibits a hysteresis with butterfly loops due to polarization reversal, indicating the change of capacitance from an accumulation state to an inversion state. The vertical-poled device possesses the highest value of capacitance, followed by unpoled and lateral-poled films, demonstrating that the polarization orientation in the ferroelectric layer directly impacts capacitor performance.

Apart from capacitors, metal-insulator-semiconductor diodes is another type of two-terminal device, whose capacitance can be changed via accumulating, depleting, and inverting charges at the interface of semiconductor and the dielectric by sweeping the bias voltage. Figure 3-26d displays the representative C-V curves of pentacene diodes with different dielectric materials: nonferroelectric PMMA and P(VDF-TrFE). [108] No hysteresis is observed when PMMA is used as the dielectric layer, for vertical-poled, lateral-poled, and unpoled P(VDF-TrFE) films. Reproduced from Ref. [107] (d) Schematic of metal-insulator-pentacene diodes. e, f) C-V curves from pentacene diodes with PMMA (e) and P(VDF-TrFE) (f) dielectric layers. Reproduced from Ref. [108] Great interest has been given to organic ferroelectric field-effect transistors (FeFETs) because of their nondestructive readout, rewritability and low power consumption. In a conventional FET, the conducting channel is in contact with the gate dielectric layer, so that the surface characteristics of the gate dielectric crucially influence its charge carrier transport. When the gate dielectric is ferroelectric, the polarization field can either accumulate or deplete charge carriers in the conducting channel, resulting in resistive nonvolatile memory properties for information storage and charge sensing. In 2012, Cheolmin Park et al. reported a flexible nonvolatile FeFET device based on P3HT (Figure 3-26a). [109] The transfer characteristic of the device shows current hysteresis arising from the ferroelectric polarization of the PVDF-TrFE layer (Figure 3-26b). To study the memory retention, the writing voltage was applied to the gate, the the source-train current was recorded as a function of time. Finally, the device shows highly reliable data retention up to approximately 10 5 s (Figure 3-25c). Moreover, multiple data write/erase endurance measurements were launched, and consistent rewritable cycles were observed over 100 cycles, as shown in Figure 3-26d. endurance cycles in FeFET. Reproduced from Ref. [109] The integration of 2DMs with ferroelectric materials has also been investigated. Figure 3-27a shows a typical example of FeFET using P(VDF-TrFE) as the top gate and MoS2 as the channel material. Its transfer curve in top-gate configuration shows a large memory window which confirms the ferroelectric polarization switching process. [110] While the transfer curve in back-gate configuration with SiO2 serving as the dielectric only shows a small hysteresis which is due to the surface adsorbates like water and oxygen molecules. The stable remnant polarization of P(VDF-TrFE) can provide an ultrahigh local electrostatic field in the MoS2 channel. Based on the polarization direction of P(VDF-TrFE): without polarization (Pfresh), upward polarization (Pup), and downward polarization (Pdown), the device can in turn achieve three different current levels. The Pup state corresponds to a depletion of the charge carriers in the channel, which shows the lowest current value compared with the other two states. On the countary, the Pdown state corresponds to the accumulation of carriers in the MoS2 channel, thereof possesses the highest current value. The photoresponse at 635 nm of the device is then compared for these three states.

As shown in Figure 3-27d, the photoresponse is very small in both Pfresh and Pdown state. While in the Pup state, a significant photoresponse can be observed, which can be explained as the photogenerated current dominants the channel current, resulting in a high-efficient photocurrent extraction, and ensuring their application as photodetectors with high detectivity. Apart from photodetectors, FeFET based on 2DMs can also be applied in memory applications. The memory switching current dynamics and the retention properties of a typical MoS2 FeFET is displayed in Figure 3-27c and d. [111] When periodic positive or negative top-gate voltage bias is applied to the device, the P(VDF-TrFE) layer is efficiently switched, and very clear current dynamic responses with nearly constant ratios of more than 10 4 is observed. The retention properties of the two states are measured by recording the current after the top-gate bias. In Figure 3-27d, the two distinctive states can maintain for 10 min without serious degradation. Reproduced from Ref. [110] (e) Current dynamics of the MoS2 FeFET under periodic top-gate bias. (f) Retention properties of the MoS2 FeFET at Pup and Pdown states. Reproduced from Ref. [111] Chapter 4 Experimental techniques Fraunhofer Institute. These n doped Si were covered by a thermally grown SiO2 as gate dielectric. Note that the substrates were treated with hexamethyldisilazane and covered with a layer of photoresist to provide protection during the wafer dicing process. The as-received substrates were first rinsed with acetone to roughly remove the photoresist, followed by acetone and isopropanol (IPA) bath respectively for 20 min in an ultrasonic cleaner. Finally, they were dried with a flow of nitrogen. The HDMS can be removed by UV-ozone before using the substrate when it is essential.

The basic principle for removal of surface contaminants by UV-ozone cleaning involves 3 steps. [1] a. The oxygen in the atmosphere absorbs the UV radiation at 184.9 nm, creating free oxygen radicals and ozone.

2O 2 → O * + O 3
b. The generated ozone then absorbs the UV radiation at 253.7 nm, forming free oxygen radicals.

Simultaneously, the contaminant molecules on the surface also absorb the UV radiation and are dissociated into excited molecules or free radicals.

O 3 → O * + O 2
Contaminant molecules → Excited molecules + Free radicals c. The dissociated molecules react with free oxygen radicals, forming gaseous molecules like H2O and CO2 and removing from the surface.

Polyethylene terephtalate substrates

Polyethylene terephtalate (PET) substrates used for flexible applications were purchased from Thorlabs.

They were coated with a layer of indium tin oxide (ITO) serving as gate material. They follow the same cleaning procedure with the rigid Si/SiO2 substrate. The monolayer or few-layer thick 2D flakes were obtained by mechanical exfoliation via the conventional scotch tape method as described in the previous chapter. [2] A small piece of bulk crystal of the target 2D material was first transferred onto a clean glass wafer, and then an adhesive tape was carefully placed to detach the top multilayers. By repeated peeling the multilayers on the tape, the material would be cleaved into various few-layer flakes and fully cover the tape. Finally, the tape was pasted to a pre-cleaned Si/SiO2 substrate and slowly detached, leaving the substrate covered with exfoliated materials. The high-quality monolayer or few-layer 2D flakes can be identified in assistance of an optical microscope. Figure 4-1e shows an example of a monolater graphene flake. For fabrication of field-effect transistor (FET) devices based on a 2D flake, the electrical contacts were patterned in assistance of laser writer photolithography, a writing technique used for high resolution lithography. The sample substrate was first baked on a hotplate at 120 °C for 1 min to remove moisture.

Mechanical exfoliation

Laser writer photolithography

Then a uniform thin layer of positive photoresist (AZ1505) was coated on the substrate (3000 rpm, 45s), followed by baking the substrate on a hotplate at 120 °C for 1 min to drive off excess photoresist solvent.

After baking, the CleWin5 software was used to design the shape of future electrodes on the chosen flakes. The photoresist film was irradiated very precisely in the pattern we designed by an intense laser at 405 nm within a Microtech laser writer. Note that the power of the laser beam is optimized to order to write channel length with a high resolution around 1.5 𝜇m. If the power is not strong enough, some photoresist residue will be left on the patterned aera. While if the the power is too intense, the patterned aera will be over exposed, shortening the channel length. In the patterned area, the photoresist was isomerized, and its solubility was thus changed, enabling being etched by a special solution called 'developer' (726 MIF). The sample was finally immersed in the developer solution and agitated gently for 15 s and rinsed with distilled water.

Metal evaporation

After photolithography, metal was deposited on the substrate to construct electrodes. We placed desired metal in a crucible and fixed the substrate on a sample-holder. The chamber was then sealed and pumped the atmosphere first with a rotary pump, followed by a turbo pump in order to reach an ultra-high vacuum. The metal evaporation usually starts at 10 -7 bar to avoid contaminant absorbed on the substrate.

When the vacuum was reached, an electric current was slowly applied through the crucible to heat the metal by Joule effect. When the temperature was high enough to melt the metal, the metal started to vaporize. The shutter switched from close to open so that the metal vapor would condensate on the substrate. In this way, a layer of desired metal was deposited on the whole surface of the substrate, and its thickness is accurately monitored through a resonance. Note that the evaporation rate was kept lower than 0.05 nm/s to ensure the homogeneity and intensity of the metal film. Once the metal film deposited on the substrate reached the desired thickness, the shutter would be closed. Then the current slowly deceased to 0 and the substrate could be taken out from the chamber after 20 min when it was totally cooling down. The metal film now covered the whole substrate and the area where there should be no contacts could be removed by washing the remaining photoresist. For this purpose, the lift-off method was applied in which the sample was first immersed in an acetone bath at 50 °C for one hour, followed by a gently bubbling of acetone to remove the photoresist. Finally, the substrate with desired pattern is fabricated.

Molecular deposition by spin-coating

Spin-coating is a widely used technique to deposit a uniform thin film with nanoscale thickness onto a substrate. The desired molecules were first dissolved in a volatile solvent. Then the substrate was fixed on a rotary plate, the solution dissolving desired molecules (~ 100 𝜇𝐿 ) was dispensed on the substrate to cover the whole surface. Parameters like speed, time, and acceleration were defined which can influence the thickness of the coated film. Note that the film thickness is also related to the concentration and viscosity of the solution. The solvent was evaporated during the spin process, resulting in a relatively planar molecular film.

Polymer-assisted CVD 2D material transfer process

For UV/vis absorption measurement, the CVD 2D material on Si/SiO2 substrate was transferred onto a quartz substrate by using the PMMA-assisted transfer technique. The detailed steps were similar as the FET device transfer process.

Synthesis of the DAE derivatives used in the thesis

Three DAE derivatives have been employed in the thesis. The synthesis was carried out at Humboldt-Universitä t zu Berlin. The reaction sketch and the synthesis procedure are presented as follows:

DAE_1: 1,2-Bis(2-(3,5-bis(trifluoromethyl)phenyl)-5-methylthiazol-4-yl)perfluorocyclopent-1-ene 1 eq. of bromothiazole, 6 eq of triethylamine and 0.05 eq. of Pd(PPh3)2Cl2 were dissolved in 15 mL of dry toluene and the mixture was degassed with argon. After the addition of 3 eq. of pinacolborane, the mixture was refluxed for 4 h at 120 °C. After cooling to 80 °C, an aqueous solution of Na2CO3 was slowly added. Then, 0.33 eq. of 1,2-dichlorohexafluorocylopent-1-ene and 0.02 eq. of Pd(PPh3)4 were added and the mixture, and stirred at 100 °C for 18 h. After cooling down to room temperature the mixture was extracted with ethyl acetate and the combined organic layers were washed with brine and dried over MgSO4.

DAE_2: 1,2-Bis(5-(4-N,N-dimethylaminophenyl)-2-methylthien-3-yl)cyclopent-1-ene 1 eq. of 1,2-bis(5-chloro-2-methylthiophen-3-yl)cyclopent-1-ene was dissolved in 15 mL of anhydrous THF, then 3 eq. n-BuLi was added dropwise at room temperature. Then, 3 eq. tri(n-butyl) borate was added in one portion and the resulting mixture was stirred at room temperature for 1h. In the meantime, 5 eq. 4-N,N-dimethylaminobromobenzene was dissolved in 15 mL of toluene, adding 0.1 eq. Pd(PPh3)4.

An aqueous solution of 13 eq. Na2CO3 and the previously prepared borylated bisthienylcyclopentene solution were added. The mixture was stirred at 80 °C for 16 h. After cooling to room temperature, 100 combined organic phases were washed with brine, dried over MgSO4 and evaporated.

DAE_3: 1,2-Bis(5-(4-methoxyphenyl)-2methylthien-3-yl)cyclopent-1-ene Similar with DAE_2, 1 eq. of 1,2-bis(5-chloro-2-methylthiophen-3-yl)cyclopent-1-ene was dissolved in hexane, then 3 eq. n-BuLi was added dropwise at room temperature. Then, 3 eq. tri(n-butyl) borate was added in one portion and the resulting mixture was stirred at room temperature for 1h. In the meantime, 5 eq. 4-bromo-anisole was dissolved in 15 mL of toluene, adding 0.1 eq. Pd(PPh3)4. An aqueous solution of 13 eq. Na2CO3 and the previously prepared solution were added. The mixture was stirred at 80 °C for 16 h.

Characterization techniques

Surface characterization: Atomic Force Microscopy

The Atomic Force Microscopy (AFM) is the most versatile technology to study the surface morphology of the sample at the nanometer scale by measuring the force between the probe and the sample surface.

It was first invented by Gerd Binning et al. in 1986 based on the concept of scanning tunneling microscope (STM). [3] While the STM only allows to investigate conductive samples, the AFM has extended the application to imaging almost any type of samples, including polymers, ceramics, glass and composites. A typical AFM consists of an oscillating cantilever with a sharp silicon nitride tip to scan the sample surface, a laser, a 4-quadrant photodiode and a scanner (Figure 4567). The technique is based on the attractive and repulsive forces reflected by the cantilever. As the tip approaches the sample surface, the attractive force between the tip and the surface causes the cantilever to deflect towards the surface.

While if the distance gets even closer by about 0.1 nm, the increasing repulsive force will cause the cantilever to deflect away from the surface. The deflection is then measured by the laser spot reflected from the top surface of the cantilever into an array of the photodiode. Regarding the cantilever as a spring, the strength of the interaction between the tip and sample surface can thus be characterized with Hooke's Law:

𝐹 = -𝑘 • 𝑥
Where 𝐹 is the force, 𝑘 is the spring constant, 𝑥 is the cantilever deflection.

The AFM can be operated in two modes: contact mode and tapping mode. In the contact mode, the tip is in a soft and constant contact with the sample surface. It can provide an accurate image with high resolution but the sample may be damaged since its surface is scratched by the tip. In the tapping mode, the distance between the tip and the sample is kept intermediate in the tens-of-nanometer scale so that the cantilever is kept oscillating without touching the surface. This mode is less destructive to the surface and the tip compared with the former while the resolution is decreased. Note that in order to increase the quality of the results obtained, each spot is being recorded twice by trace and retrace.

The morphology of the sample surface induces changes in the amplitude of the cantilever oscillations, which is detected by the laser beam. Different interactions between the tip and the sample surface may generate different intensities of the reflected light. Such difference can be recorded by the photodiode.

With a voltage applied to the piezoelectric scanner which moves in the Z direction at each point, the signal is sent to the controller feedback loop which keeps the distance between the cantilever and the sample maintaining a constant. Therefore, an accurate topological illustration of the sample surface is generated. When the electrons undergo internal energy transitions before relaxing to its ground state by emitting photons, some of the absorbed energy is dissipated so that the emitted light photons are of lower energy than those absorbed, the process is called fluorescence. This is a prompt process that occurs very shortly.

Raman and photoluminescence spectroscopy

Phosphorescence is along-lived photoluminescence, in which the absorbed energy undergoes intersystem crossing into a state with a different spin multiplicity before relaxing to the ground state.

Therefore, phosphorescence is even rarer than fluorescence, since a molecule in the triplet state has a good chance of undergoing intersystem crossing to ground state before phosphorescence can occur. Raman spectroscopy is a widely used spectroscopic technique to characterize a material, which is based on the inelastic scattering of light. [5] It The intensity of the reference beam, which should have suffered no light absorption, is defined as I0.

The intensity of the sample beam is defined as I. The absorption of the sample may be presented as transmittance 𝑇 = 𝐼/𝐼 0 or absorbance 𝐴 = -𝑙𝑜𝑔𝑇 = -log ( 𝐼 𝐼 0

).

With the known absorbance, the concentration of the sample can be determined by using Beer-Lambert Law.

𝐴 = 𝜀𝑙𝑐

Where A is the measured absorbance, 𝜀 is the absorption coefficient, 𝑙 is the path length, 𝑐 is the concentration.

Water contact angle

The water contact angle has been an important parameter to determine the wetting ability of a sample surface. The shape that a water drop takes on a surface depends on the surface tension of the fluid and the nature of the surface. At the boundary between droplets and the gaseous environment, the surface tension causes a curved contour. At the edge of the drop, where the contour merges into the bearing surface, the contact angle between the interface liquid/solid and the tangent to the interface liquid/ gaseous forms.

If the liquid runs evenly on the solid surface, complete wetting is present with a contact angle of 0°.

An angle between 0° and 90° corresponds to a wettable hydrophilic surface.

An angle between 90° and 180° indicates a not wettable hydrophobic surface.

If the angle clearly approaches the value of 180°, the surface is ultra-hydrophobic and is completely liquid-repellent, which is defined as lotus effect.

During the measurement, the sample is placed on a mobile sample stage. A syringe filled will water is fixed above the sample and an optical camera is aligned and focused on the sample. After a 2 μL drop of water is deposited on the sample, the camera takes a picture and the instrument software will measure the corresponding contact angle.

Energy level determination

The CB/VB energy levels for 2DMs were taken from reported work, which were obtain by calculation using the plane wave code CASTEP (WSe2)/VASP (BP) with the framework of DFT. Screen-exchange hybrid functional are utilized.

The energy levels for DAE molecules were determined by cyclic voltammetry (CV), in reference to the ferrocene redox couple (Fc/Fc + ), derived from the reduction/oxidation peak potentials. The measurement was performed using a PG310 USB (HEKA Elektronik) potentiostat. A three-electrode configuration contained in a non-divided cell consisting of a platinum disc (d = 1 mm) as working electrode, a platinum plate as counter-electrode, and a saturated calomel electrode with an agar-agarplug in a Luggin capillary with a diaphragm as reference electrode was used. Field-effect transistors

The field-effect transistor (FET) represents the building block for modern electronics. The concept of it was first patented by Lilienfeld in 1925 and by Heil [6] in 1934. [7] The first working FET was fabricated by Bardeen and Brattain in 1947, followed by Shockley's bipolar junction transistor in 1948. In 1956 they were conferred with the Nobel prize and the FET was defined as "the most important invention of the 20 th century". top/bottom gate with top/bottom contact. Since each geometry has its own merits and drawbacks, the choice of the geometry is decided by practical fabrication conditions. When the source and drain electrodes are deposited on top of the semiconductor, while the gate at the bottom, it forms top contact, bottom gate geometry. In this geometry, the contact area between the source electrode and the semiconductor is relatively large, and their interface is intimate, which allows a facilitated injection.

While if the source and drain electrodes are directly deposited on the dielectric, with the semiconductor on top, it forms the second geometry: bottom contact, bottom gate. In this geometry, the substrate gate/dielectric with pre-patterned electrodes can be purchased in advance since the deposition of the semiconductor is the last step. This geometry can therefore largely facilitate the device fabrication process. The top contact, top gate geometry is used when a high 𝑘 dielectric compared with SiO2 is employed, which allows to improve the device mobility. The drawback is that it requires the greatest number of production steps.

Extraction of electrical parameters

The electrical properties of the semiconductor are usually determined by performing the current versus voltage measurement from the FET device, which allows to trace the transfer curve and the output curve. The key parameters to define the intrinsic electrical properties of the semiconductor and the FET device performance can be extracted from these two curves. In particular, the charge carrier mobility (𝜇), the threshold voltage (Vth) and the Ion/Ioff ratio.

The charge carrier mobility defines how quickly the carrier can move through the semiconductor when an electric field E is applied. [8] 𝑉 𝑑 = 𝜇 • 𝐸

Where 𝑉 𝑑 is the carrier drift velocity caused by the electric field E.

The charge Q induced at the gate voltage Vg that higher than Vth at the certain position x can be expressed as:

𝑄 = 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ -𝑉 𝑥 )
The drain current Ids can therefore be expressed as:

𝐼 𝑑𝑠 = 𝑊 • 𝑄 • 𝜇 • 𝐸 = 𝑊 • 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ -𝑉 𝑥 ) • 𝜇 • 𝐸
Where W is the width of the channel.

Known that 𝐸 = 𝑑𝑉 𝑑𝑥

,

𝐼 𝑑𝑠 = 𝑊 • 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ -𝑉 𝑥 ) • 𝜇 • 𝑑𝑉 𝑑𝑥
As we can see, the transfer curve has two different regimes: linear regime (when Ids changes linearly with Vds) and saturation regime (when Ids reaches saturation).

Ids in the linear regime can therefore be defined as:

𝐼 𝑑𝑠,𝑙𝑖𝑛 = 𝑊 𝐿 • 𝐶 𝑖 [(𝑉 𝑔 -𝑉 𝑡ℎ ) • 𝑉 𝑑𝑠 - 1 2 𝑉 𝑑𝑠 2 )] • 𝜇 𝑙𝑖𝑛 Since 𝑉 𝑑𝑠 ≪ 𝑉 𝑔 -𝑉 𝑡ℎ , 𝐼 𝑑𝑠,𝑙𝑖𝑛 = 𝑊 𝐿 • 𝐶 𝑖 • (𝑉 𝑔 -𝑉 𝑡ℎ ) • 𝑉 𝑑𝑠 • 𝜇 𝑙𝑖𝑛
The mobility in the linear regime can therefore be extracted as:

𝜇 𝑙𝑖𝑛 = 𝜕𝐼 𝑑𝑠,𝑙𝑖𝑛 𝜕𝑉 𝑔 𝐿 𝑊 • 1 𝐶 𝑖 • 1 𝑉 𝑑𝑠 While in the saturation regime, 𝐼 𝑑𝑠,𝑠𝑎𝑡 = 1 2 • 𝑊 𝐿 • 𝐶 𝑖 • (𝑉 𝑔 -𝑉 𝑡ℎ ) 2 • 𝜇 𝑠𝑎𝑡 Therefore, 𝜇 𝑠𝑎𝑡 = 2 • ( 𝜕 √ 𝐼 𝑑𝑠,𝑙𝑖𝑛 𝜕𝑉 𝑔 ) 2 𝐿 𝑊 • 1 𝐶 𝑖
While when using the above expression to determine the charge carrier mobility in a FET device, the value might be underestimated since they consider the total resistance Rtotal including contact resistance Rcontact instead of only channel resistance Rchannel.

The contact resistance is an indicator of how easily the charge carrier can be injected from the electrode to the semiconductor. Actually, when the charges are injected, the flow will first be reduced by the metal resistance Rm. Then it is injected from the metal to the channel. Due to the energy level mismatch between the HOMO/LUMO of the semiconductor and the work function of the metal electrode, there is an injection barrier which will further reduce the flow, leading to the contact resistance. Later, the flow in the semiconductor will be reduced again by the channel resistance Rsemi, which is dependent on the channel length. To be collected on the other electrode, the flow will experience for the second time the injection barrier and the metal resistance. Note that Rm is small and can usually be neglected.

There are several techniques to measure contact resistance, one of the most prevalent the transmission line method (TLM). According to TLM, the total resistance can be plotted as a function of the channel length L: [9] 𝑅 𝑡𝑜𝑡𝑎𝑙 = 2𝑅 𝑚 + 2𝑅 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 + 𝑅 𝑠𝑒𝑚𝑖 ≈ 2𝑅 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 + 𝑅 𝑠ℎ𝑒𝑒𝑡 𝑊 • 𝐿 = 2𝑅 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 + 1 𝜇 0 𝐶 𝑖 (𝑉 𝑔 -𝑉 𝑡ℎ )𝑊

• 𝐿

Where μ0 is the intrinsic mobility excluding contact effect. Gated four-probe measurement is another alternative way to address the contact resistance. [10] As shown in Figure 4-17, the device consists of four probes arranged linearly in a straight line at equal distance from each other. During the measurement, a constant current is passed through the four probes and the potential drop across the middle two probes is measured. In the linear regime, the charge-carrier density in the channel is almost uniform, giving a linear potential profile in the channel from source to drain.

Since the potentials V1 and V2 at the positions L1 and L2 are known from the measurement, they can be used to linearly extrapolate the channel potential to the source and drain contacts (Figure 4 The threshold voltage Vth is another important parameter that defined as the minimum gate voltage Vg that is required to create a conducting path between the source and drain electrodes. The value can be obtained by using a linear extrapolation in the transfer curve. [11] The Ion/Ioff ratio is the ratio value of the maximum current Ids to the minimum current Ids. It defines the device capacity to switch from ON state to OFF state. A high Ion/Ioff is usually preferred since a high ON current represents high performance of the device, and a low OFF current represents less leakage power.

The subthreshold swing is defined as the change in gate voltage which must be applied in order to create a one decade change in the output current. It describes an exponential behavior of the current as a function of voltage. It can be calculated using the formula:

𝑆𝑆 = 𝐼𝑛10 𝑘𝑇 𝑞 (1 + 𝐶 𝑑 𝐶 𝑜𝑥 )
Where 𝑘𝑇 𝑞 is the thermal voltage, Cd is the depletion capacitance,

Cox is dielectric capacitance.

The minimum subthreshold swing of a conventional device can be found by letting Cd → 0, yielding 𝑆𝑆 = 𝐼𝑛10 𝑘𝑇 𝑞 . Therefore, at room temperature, SS has a thermionic limit of 60 mV/dec.

In 2DM based FETs, the charge carrier transport and scattering are confined to the plane of the material.

The charge carrier mobility is affected by several scattering mechanisms: a) Acoustic and optical phonon scattering. The mobility is increasingly affected by phonon scattering with increasing temperature. At low temperatures (T < 100 K), the acoustic component dominates, but at higher temperatures the optical component dominates.

b)

Coulomb scattering at charged impurities. Coulomb scattering is caused by random charged impurities located within the material. The improvement of room-temperature mobility of MoS2 transistors from ~ 0.5-3 cm 2 V -1 s -1 to ~ 200 cm 2 V -1 s -1 can be realized by dielectric screening of Coulomb scattering on charged impurities by using gate dielectric materials with a high dielectric constant. The carrier concentration and bandgap can be tuned by adding ionic impurities, but the mobility is also decreased through scattering, so the choice of doping level in a particular device can strongly influence its performance. By considering the mean free path of carriers, the impurity concentration required to make impurity scattering dominant over phonon scattering is calculated to be ~5 × 10 11 cm -2 , which corresponds to heavy doping.

c)

Surface interface phonon scattering and roughness scattering. Such effect can be very important in extremely thin 2DMs.

The degree to which these scattering mechanisms affect the charge carrier mobility is influenced by layer thickness, carrier density, temperature, effective carrier mass, electronic band structure and phonon band structure.

Introduction

For application of 2DSs in opto-electronics, more and more efforts have been devoted to the investigation and device optimization based on 2DSs such as transition metal dichalcogenides (TMDs) and more recently also to BP since they combine efficient charge transport characteristics, with a finite band-gap, to enable controlled optimal transistors operation. [1] Unfortunately, the 2DS's structures define their properties, which are difficult to be tuned or modified via the conventional doping strategies. [2] In this regard, it is highly appealing to manipulate the electronic properties of 2DSs, by also conferring them new functions, towards emergence of novel applications in opto-electronics and related technologies. In view of the highest sensitivity of 2D materials to subtle changes in the local environment, [3] their functionalization with photosensitive molecules represents a powerful strategy for modifying their intrinsic properties when exposed to electromagnetic fields. [4] The modification of the properties of 2D materials when interacting with molecules can result in doping effects due to either charge transfer processes or interfacial dipoles. [5] Recently, in order to go beyond the mere doping effect induced modification, much attention has been given to the combination of 2DMs with a molecular building blocks which could impart reversible and phototunable characteristics. [4] Among various approaches, the use of photochromic molecules as optically responsive components makes it possible to form stable hybrid structures with molecular-driven photoresponsivity. [6] Switchable electronic properties can be realized with such hybrid systems, thus enabling them to serve as active materials for transistors/memories that can be switched on and off via light irradiation at specific wavelengths.

Moreover, the use of light as external stimuli offers a fast, non-invasive and easily addressable way to trigger molecular switches and modulate the property of the hybrid materials.

The most popular photochromic molecules reported so far are azobenzenes, [7] spiropyrans [8] and DAEs. [9] During the last five years the former two molecules have been combined with 2DMs to fabricate multifunctional FETs. [10] Due to a variation of the molecular dipole moment between their two photoisomers local electrostatics are modified, leading to light-controlled reversible doping. [10a, 10c] Instead, the use of DAEs in combination with 2DMs is still rather unexplored. DAEs possess significant advantages for applications in (digital) electronics as they combine efficient photoisomerization in the solid state with high fatigue resistance, thermal bi-stability and strictly defined phototunable energetic levels. [11] Upon irradiation with UV or vis light, DAEs can be toggled between their cross-conjugated ring-open (DAE_o) and -conjugated ring-closed (DAE_c) isomers, accompanied by a reversible change of their corresponding HOMO and LUMO energy levels. Thereby, the charge transfers between the 2DS and DAE component and thus the density of charge carriers in the 2DM can be modified. This makes DAEs suitable candidates to reversibly adjust the electrical performance of FETs based on 2DMs.

In this chapter, we present the use of charge transfer as a physical process to realize light responsive FETs based on few-layer WSe2 and few-layer BP by combining them with suitable DAE molecules.

We demonstrate that a reversible control on the electron and hole transport of the FET device could be achieved upon illumination at specific wavelengths, without exhibiting significant fatigue over at least 16 irradiation cycles. The field-effect mobility photo-modulation is around 61% for electrons in WSe2, and 42% for holes in BP, thus outperforming the one observed for 2DMs coupled with spiropyran and azobenzene previously. In addition, the devices exhibit an excellent data retention capacity as evidenced by negligible variation of output current upon storage for one week in the dark. Furthermore, five distinguishable output current levels with a high accuracy readout are attained, making our system highly promising for its potential applications in multilevel memories when fast light pulses with low areal power density are employed.

Experimental methods

Match of energy levels between DAE and 2DM

Three DAE derivatives [11][12] displaying different highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels have been designed and synthesized in view of the frontier orbitals of WSe2 [13] and BP [14] . The requirement to achieve photo-modulation relies on the matching of the energy levels between the host 2DS and DAE molecules, to enable the appearance of trapping states. Likewise when blended with organic semiconductors, [15] the two isomers, DAE_o and DAE_c, feature different energy levels and can be expected to affect differently the charge transport process through the 2DSs. A schematic illustration of the energy level alignment between WSe2/DAE_1 is depicted in Figure 5-1.

For the intrinsically n-doped WSe2, the energy of the conduction band minimum (CBM) is located between the LUMO levels of DAE_1o and DAE_1c. In other words, the LUMO of DAE_1o features a higher energy value compared to the CBM of WSe2, thereby impeding the electron transfer from WSe2 to DAE_1o. Conversely, the DAE_1c isomer behaves as trapping site for WSe2 electrons, resulting in a decrease of electron density in the FET device.

Similarly, for the BP FET with p-type dominant transport, the VBM of BP lies energetically between the HOMO levels of DAE_2o and DAE_2c. The driving force for hole trapping therefore exists only in the closed form of DAE_2, leading to a reduced hole density. To further reveal the role of energy levels of DAEs with respect to the tuning of the charge transport in 2DM, we extended our study to BP/DAE_3.

With the HOMO of DAE_3c being isoenergetic to the VBM of BP, the photo-triggered hole trapping is nullified.

Device preparation

High-quality flakes of 2D target materials (few layer WSe2 or BP) are obtained by typical micromechanical exfoliation from bulk crystals (from HQ Graphene) with scotch tape and pressed onto highly n doped SiO2 (90 nm)/Si substrates. Electrodes (Au 50 nm for WSe2, Cr/Au 5 nm/45 nm for BP) are defined by laser writer, followed by thermally metal evaporation and lift-off process. The devices are annealed overnight (WSe2 at 110 °C and BP at 200 °C) in ultra-high vacuum to remove the photoresist residues. The photoactive film is obtained by spin-coating DAE molecules from chloroform solutions (0.02 mg/mL) onto the substrate. (Details on the syntheses, photochemistry in solution and energy level determination of three DAE molecules have been previously reported. [12,16] A mild baking at 55 °C for 1 hour is later performed to ensure the evaporation of chloroform. Since BP undergo degradation upon exposure to ambient air, the exfoliation and lift-off process are all performed in glovebox filled with N2.

Sample preparation for UV-vis absorption spectroscopy

Monolayer chemical vapor deposition (CVD) WSe2, DAE film, and the WSe2/DAE hybrid film is prepared on quartz substrate for UV-vis absorption spectroscopy.

CVD grown WSe2 is purchased from SixCarbon. The poly(methyl methacrylate) (PMMA) solution is purchased from micro resist technology GmbH (MicroChem 950 PMMA C2). The as purchased CVD grown WSe2 (on SiO2/Si substrate) is transferred onto a pre-cleaned quartz substrate (size: 1 × 1 cm 2 ) by using the PMMA-assisted wet transfer method, as introduced in chapter 4.3.

The DAE film is obtained by spin-coating (1000 rpm, 60 s) chloroform solutions (0.02 mg/mL) onto the quartz substrate or on WSe2. A post baking at 55 °C for 1 hour after spin-coating is performed to ensure the complete chloroform evaporation. Note that the DAE film is prepared by using the same procedure used for fabricating FETs.

Characterization methods

UV/vis absorption spectra in film are recorded with a JASCO V-650 spectrophotometer. UV light irradiation is carried out by using an analysis lamp (312 nm, 6 W, Herolab), and vis light irradiation is performed using a white lamp (Dolan-Jenner MI-150 Illuminator) coupled with a long pass filter (> 520 nm).

Atomic force microscopy (AFM) images are recorded using a Nanoscope (Veeco Multimode V) in tapping mode under ambient conditions.

Raman spectra are recorded by using a Renishaw inVia spectrometer equipped with 532 nm laser.

The isomerization induced charge carrier density in the linear region of the transfer curve can be estimated by the formula below: [17]

∆𝑛 = 𝐶 𝑖 ∆𝑉 𝑡ℎ 𝑒

Where Ci is the gate capacitance per unit area;

Vg is the gate voltage, Vth is the threshold voltage.

The charge carrier mobility can be extracted in the linear regime from the transfer curve using the following equation: [18] 𝜇 = 𝑑𝐼 𝑑𝑠 𝑑𝑉 𝑔

• 𝐿 𝑊 • 1 𝐶 𝑖 • 1 𝑉 𝑑𝑠
Where L and W are the channel length and width of the FET respectively; Vds and Ids is the source-drain voltage and current. To evaluate the tendency of DAE to undergo efficient photochemical isomerization when deposited on WSe2, UV-vis absorption spectra of bare WSe2, DAE film and WSe2/DAE in the dark and after UV/vis irradiation have been recorded in Figure 5-5. The spectrum of monolayer WSe2 possesses 4 predominant bands at ~ 420, 520, 600, and 740 nm in the range of 200-850 nm, in agreement with previous reports. [19] The absorption spectrum is unchanged upon exposure to UV or vis light. The spin-coated DAE films on WSe2 displayed a spectrum which resembles the sum of WSe2 and DAE, as it combines all their representative bands. Upon UV irradiation, the photochemical isomerization from DAE_1o to DAE_1c occurs, with a spectroscopic behavior being similar to the neat DAE_1 film. The band at 312 nm from DAE_1o clearly diminishes, while the band at 530 nm which originates from DAE_1c appears and superimposes with another band from WSe2 at 520 nm, turning into a broader band. The initial spectrum can be completely re-established after sequential vis light irradiation. These observations provide clear evidence that the photoisomerization of DAE_1 is not perturbed by the underlying WSe2 surface.

Results and discussion

Characterization of 2DSs

The 2D flakes after exfoliation were first identified by optical microscope and Raman spectra and their thicknesses are further characterized by AFM. Note that both of our chosen 2DSs show thickness-dependent physical properties and transport characteristics, especially for band structure evolution and carrier mobility. Though the electron trapping by DAE is favored by WSe2 with higher CBM level thus thinner thickness, a high electron mobility is essential for achieving multilevel storage with a quantity of distinguishable current levels.

Given that the highest electron mobility value for exfoliated WSe2 is achieved for 12 atomic layers, [20] in order to find a good compromise between the ratio of electron transport modulation and the high device mobility, we have targeted WSe2 flakes with an average thickness of 3.3 nm (corresponding to 6 atomic layers). Moreover, when compared to monolayer thick films, the few-layer flakes are found to have a better stability and tendency to form lower Schottky barriers with metal contacts. [21] Similarly, for BP the hole trapping is favored with lower VBM level, thus thinner flakes and 12 nm thickness exfoliated BP FET exhibits the highest hole mobility. [22] For this reason we have focused on flakes with a thickness of ca. 4.6 nm (corresponding to 8 atomic layers). Back gate sweeping (from 30 V to -30 V) is also implemented (Figure 5-8b). The obtained transfer curves present a similar evolution as forward gate sweeping, indicating that the transport modulation of WSe2 applies to both forward and backward gate sweeping in spite of the hysteresis. In order to rule out the possibility that the contribution to the modulation coming from WSe2, it is mandatory to perform a blank test in which the electrical characteristics of the device based on bare WSe2 without DAEs under the same irradiation conditions are measured. As displayed in Figure 56789, the device exhibits no sign of photoresponsive behavior. In view of the thermal stability of both photo-isomers, the devices incorporating DAEs are expected to exhibit good retention characteristics. Figure 5-14 displays the Ids evolution upon different storage times.

The standard derivation for these two levels is calculated respectively and divided by the current difference between DAE_1o and DAE_1c (Ids_1o -Ids_1c, Table 5-2). The negligible variation of both current levels after one week, as quantified by a standard deviation below 1.5% of the total current difference, provides unambiguous evidence for the excellent retention capacity of our devices, which renders them interesting elements for potential application as non-volatile memories. [23] Note that here the modulation of Ids is up to 84.6%, and the intrinsic value of electron mobility (1.7 cm 2 /V• s) is 10 times lower than the above device (16.6 cm 2 /V• s). This is because the thickness of WSe2 is thinner, which favors the electron transfer to DAE_1c and lowers the mobility. The result is consistent with our preliminary prediction.

The device is kept under dark except UV/vis irradiation to trigger the DAE photoswitching process.

The higher current level belongs to DAE_1o and the lower belongs to DAE_1c.

Ids difference = Ids_1o -Ids_1c

To further elucidate the effect of DAE photoswitching, the dynamic variation of Ids with time is measured under alternating dark and illumination conditions, revealing similar trends as in previous experimental observations (Figure 5-14a). Owing to the bias stress effect when applying a constant positive gate voltage, the Ids decays with time. [24] To address separately the contribution of DAE photoswitching, the bias-induced decay is fitted and subtracted from the curve (Detailed fitting curve and fitting parameters is provided in Figure 5-14c and Table 5-3). photocurrent, which can be attributed to the conventional band-to-band transition that generates electron-hole pairs. [25] The Ids then decreases gradually with time, as a result of the isomerization to DAE_1c, a process accompanied with electron donation from WSe2 to DAE_1c. When UV irradiation is terminated, the decrease in photocurrent leads a rapid drop of Ids owing to the exciton relaxation. [25] Similarly, for the case of vis illumination, the Ids showed a sudden jump which is the photoresponse, The curve is fitted by using the formula: y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0 For the sake of comparison, the Ids-time measurement is also performed for the device based on bare WSe2. As illustrated in Figure 5-15, the device shows only photoresponse, with Ids ramps upon illumination and suddenly resumes to origin value under dark condition. The blank test unequivocally proves that WSe2 is not responsible for the electron transport modulation under illumination. As shown in Figure 5-17, the modulation of Ids at Vg = -30 V is around 37.4 % and is stable over the measured 4 illumination cycles.

To support above-mentioned interpretation, we have extended our study to BP/DAE_3. Clearly, the Ids decreases after DAE_3 deposition, while no further variation is observed as a result of either UV or vis light irradiation. Taking into account the energy levels depicted in Figure 5-2, the result is not surprising because of the absence of driving force for hole transfer from BP to DAE_3c. This phenomenon provides clear evidence that suitably designed DAEs possessing ad hoc energy level alignment with the semiconducting 2DMs is required to achieve efficient optical modulation of the charge transport in the hybrid devices.

Conclusion

In summary, we have fabricated two-dimensional semiconductor-based light-responsive FETs via energy level phototuning. The device's electrical performance can be reversibly modulated by light illumination with well-defined wavelengths. In particular, the physisorbed photochromic DAEs act as optical switching elements, thereby allowing for the fine-tuning and remote control over the charge transport of the host 2DM. To prove the general applicability of our approach we have challenged it by realizing n-and p-type dominated ambipolar FETs containing a few-layer WSe2 and BP, respectively.

In each pair of 2DM/DAE, the modulation is achieved when the CBM or VBM of 2DM is energetically sandwiched between the LUMO or HOMO gap of the two photo-isomers of DAEs, respectively. Our approach is thus proved feasible for both electron and hole transport modulation depending on the nature of target 2DM without sacrificing their intrinsic performance, confirming its general applicability and effectiveness. Our strategy can be extended to modulate the electrical characteristics of devices integrating other 2DMs by simply choosing suitable DAEs, thereby opening the door for multiresponsive 2DM/DAEs based FETs that can be switched ON and OFF upon irradiation with light.

Noteworthy, the FET device based on such 2DM/DAE system displayed excellent data retention capacity and a high accuracy readout with several distinguishable current levels, opening perspectives towards their use as multilevel non-volatile memory. Furthermore, such controlled optical switching of the charge injection and transport in 2DM-based systems may pave the way to a new era of electronics, which incorporate novel functions in a single device based on high-performing 2D materials.

Chapter 6 Simultaneous optical tuning of hole and electron transport in ambipolar WSe2 interfaced with a bicomponent photochromic layer

The interfacing of two-dimensional semiconductors (2DSs) with photochromic molecules provides an efficient solution to reversibly modulate their outstanding electronic properties and offers a versatile platform for the development of multifunctional field-effect transistors (FETs). Herein optically switchable multilevel high-mobility FETs based on few-layer ambipolar WSe2 are realized by applying on its surface a suitably designed bicomponent diarylethene (DAE) blend, in which both, hole and electron transport can be simultaneously modulated for over 20 cycles. The high output current modulation efficiency (97% for holes and 52% for electrons) ensures 128 distinct current levels, corresponding to a data storage capacity of 7 bit. The device is also implemented on a flexible and transparent polyethylene terephthalate substrate, rendering the 2DSs/DAEs hybrid structures promising candidates for flexible multilevel nonvolatile memories.

Introduction

Atomically thin 2DMs combining exceptional electronic, optical, and mechanical properties, as well as chemical stability, are promising candidates as key components for various applications spanning from (opto)electronics, [1] photonics, [2] sensing, [3] to ultra-flexible and wearable electronics, [4] in the postsilicon era. In the past decade the scientific community has witnessed the booming of fundamental studies as well as technological advancements in the field of 2DSs. Alongside exploring their intrinsic properties, various physical and chemical modification strategies have been adopted in order to tune the 2DS's functions and device performances. [5] Among them, achieving a tunable control over the electronic properties of 2DSs without introducing any defects still remains a major challenge, [6] which is at the same time highly appealing for the development of practical multilevel stimuli-responsive electronics. [7] The interfacing with molecular systems possessing tunable structures and functionalities constitutes an efficient and versatile approach to provide different charge carrier doping levels and thus to reversibly modulate the opto-electronic properties of 2DSs. [8] For example, it has been demonstrated that photochromic molecules, capable of undergoing reversible isomerization between two different states induced by light stimuli, [9] can serve as optically-responsive components when employed to functionalize unipolar 2DSs. [10] Importantly, such hybrid structures take full advantage of the ultra-high sensitivity of 2DSs to any subtle environmental change, [8b] which will emerge as an additional molecular-driven photoresponsivity. This enables the 2DSs/photochromic molecules hybrid structures to serve as active components in superior multifunctional transistors/memories that can optically be addressed and operated due to efficient and highly sensitive ON/OFF switching upon irradiation with light at defined wavelengths. [11] In the previous chapter we have demonstrated the unipolar modulation of either electron transport in few-layer WSe2 or hole transport in few-layer black phosphorous-based optically switchable FETs by interfacing suitable photochromic molecules as optical switching elements. [12] However, the simultaneous photo-modulation of both electron and hole transport in the target 2D materials has not been achieved before. It would be of utmost importance for a practical multilevel transistor as well as for the development of responsive organic complementary circuits (CMOS like) and photo-tunable inverters, and as a consequence the applicability of such 2DSs/photochromic molecules hybrid systems could largely be extended.

In last chapter, we focused on WSe2 flakes possessing an n-type dominant ambipolar behavior, with its electron current (10 -5 A) three orders of magnitude larger than the hole current (10 -8 A). Therefore, the modulation of hole current by operation of the photochromic molecule was not visually distinguishable.

Inspired by the phenomenon that the main type of charge carrier in WSe2 changes with its thickness, ranging from p-type, ambipolar to n-type, [13] in this work we tackle full ambipolar switching of charge transport in 2DM/photochromic molecule blends. For this purpose, we target few-layer WSe2 flakes with a more balanced ambipolar behavior, with its electron and hole current on the same scale (10 -6 A).

We propose an optically switchable multilevel FET by interfacing WSe2 with a bicomponent DAE blend consisting of two DAE molecules with specific energy levels, which are engineered in order to be capable of trapping either the electrons or the holes of WSe2. The device is demonstrated to be able to modulate both electron and hole transport simultaneously by remote light stimuli. The photomodulation is demonstrated to be efficient and reversible with negligible switching fatigue after over 20 illumination cycles. Noteworthy, the output current modulation for holes is as high as 97%, while for electrons the modulation amounts to 52%. Moreover, such current modulation ratio can be readily tuned by controlling the light dose, which means by adjusting the irradiation duration the FET device can reach a multitude of current states. In particular, the balanced ambipolar characteristic of WSe2 enables our device to attain 64 distinct current levels for both holes and electrons, as evidenced through a precise control and fine-tuning over the output current thereby highlighting its potential to be further developed into multilevel memories with data storage capacity of 7 bit. [14] Additionally, by taking advantage of the thermal bistability of DAE molecules at both photo-isomeric states, [15] the device is shown to possess a prolonged retention time exceeding 20 days in the dark. This result outperforms the current state-of-the-art memories based on other photochromic molecules, such as spiropyrans and azobenzenes, [16] and is highly promising for applications as non-volatile memories. [17] Remarkably, taking full advantage of the superior mechanical properties of ultra-thin 2D WSe2, our device is also able to be operated on a flexible and transparent polyethylene terephthalate (PET) substrate, without losing its optical switching capability, which is attractive for future generation of multifunctional wearable electronics. When compared with memories based on organic semiconductors blended with DAE molecules, [18] our memories based on 2DSs show similar features while hold higher mobilities and miniaturizing size at the atomic level thickness.

Experimental methods

Match of energy level between ambipolar WSe2 and DAEs.

In order to realize two charge carrier modulation, here we employed a bicomponent DAE blend, which consists of two different molecules namely DAE_1 [19] and DAE_3, [20] both of which feature tunable lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) energy levels above or below conduction band (CB) and valence band (VB), respectively, of WSe2. [21] Therefore, we are able to utilize the optically responsive nature of the photochromic molecules to modulate the charge transport of both electrons and holes in the WSe2 channel.

WSe2 FET on PET substrate

The as-fabricated WSe2 FET on Si/SiO2 substrate is transferred to PET/ITO/CL-PVP substrate by using the PMMA-assisted transfer process, as introduced the chapter 4.3. CVD grown WSe2 was purchased from SixCarbon. The as purchased CVD grown WSe2 (on Si/SiO2 substrate) was transferred onto a pre-cleaned quartz substrate (size: 1 × 1 cm 2 ) by using the poly(methyl methacrylate (PMMA) assistant wet transfer method, as introduced in chapter 4.3.

The DAE_1 film, DAE_3 film, DAE blend film was obtained by respectively spin-coating (1000 rpm, 60 s) chloroform solutions (0.02 mg/mL) onto the quartz substrate. A post baking at 55 °C for 1 hour after spin-coating was performed to ensure the complete chloroform evaporation. Note that the DAE film was prepared by using the same procedure used for fabricating FETs.

Chapter 6 Modulating the charge transport in ambipolar WSe2 123 6.2.4. Characterization methods UV/vis absorption spectra of the DAE blend film on quartz substrate is recorded with a JASCO V-650 spectrophotometer. The molecular switching is triggered either by an analysis lamp at 312 nm (1.2 mWcm -2 , from Herolab) for 40 s or a white lamp (from Dolan-Jenner MI-150 Illuminator) coupled with a long pass filter (> 520 nm) for 1 min.

Raman spectra were recorded by a Renishaw inVia spectrometer equipped with 532 nm laser.

Atomic force microscopy (AFM) imaging was carried out by a Nanoscope (Veeco Multimode V) in tapping mode under ambient conditions.

All electrical measurements were carried out by using a probe station connected to a Keithley 2636 in a glovebox filled with nitrogen. Device irradiation is realized by using a monochromator set-up, with either UV light (at λ = 312 nm, 0.15 mWcm -2 ) or vis light (λ = 530 nm, 11.8 mWcm -2 ). The PSS is reached under UV irradiation for 150 s and recovery within 20 min under vis light.

The charge carrier mobility is extracted in the linear regime from the transfer curve by the equation: [22] μ =

dI ds dV g × L W × 1 V ds C i
Where L is the channel length, W is the channel width, and Ci is the gate capacitance per unit area.

The molecular isomerization-induced is estimated by the equation: [23] △

n = C i △V th e
Where △Vth is the difference of threshold voltage before and after molecular isomerization, e is the elementary charge.

Results and discussion

Optical characterization

The syntheses, photochemistry both in solution and in films and energy level determination of the two

The absorption spectra of DAE_1 and DAE_3 show same trend after UV/vis irradiation (Figure 6-5).

The initial spectrum displays a characteristic band at 312 nm, which originates from DAE_o. After UV irradiation, this band diminishes while another band centered at 530 nm appears which can be attributed to DAE_c. The initial spectrum can be recovered after subsequent vis illumination, confirming the switching ability of the DAE compounds in the solid state. Importantly, both two DAEs show similar molar absorptivities, speed of photoisomerization (quantum yields), and almost quantitative formation of the ring closed isomer under UV irradiation, [20] allowing for their fully simultaneous operation in the device. To assess the photo-switching ability of DAE molecules after physisorption on WSe2, UV-vis absorption spectra of the WSe2/DAE blend hybrid film was initially recorded on quartz substrates. The evolution of the absorption spectra of DAE blend was similar to single molecules as DAE_1 and DAE_3

and shows same trend after UV/vis irradiation (Figure 6-6a).

The spectrum of WSe2 possesses 4 prominent bands at ~ 420, 520, 600, and 740 nm, in accordance with previous reports [24] . It remains unchanged when subjected to either UV or vis light illumination.

For the WSe2/DAE blend hybrid, it combines all the representative peaks of both DAE and WSe2.

following UV irradiation, the intensity of the peak around 312 nm corresponding to open isomers clearly decreases, while another peak around 530 nm originating from the closed isomers appears. The original absorption features are recovered after vis irradiation. Therefore, the DAE molecules in the solid-state blended film are still able to undergo reversible photo-isomerization when physiosorbed onto the surface of WSe2.

WSe2 characterization

The WSe2 flakes after exfoliation were first identified by optical microscope and Raman spectra, and their thicknesses were further determined by AFM imaging.

In order to obtain a multilevel device with a quantity of distinguishable current levels, a high mobility for both electron and hole is required, therefore we target on WSe2 flakes with an average thickness of 4.3 nm (corresponding to 7 atomic layers). Figure 6-7a shows a typical optical microscopy image of the flake, and Figure 6-7b displays its Raman spectrum at point A. The Raman spectrum possesses three active modes, E 1 2g, A1g and B 1 2g, suggesting a good material quality.

AFM topographical imaging of WSe2 flake enabled to visualize the flakes and quantify their thickness as ~4.3 nm. The surface roughness of WSe2 flake was enhanced after molecular deposition with the average thickness increasing to ~ 6.9 nm, thereby indicating a thickness of the DAE blend film of ~2.6 nm. WSe2 device without DAE molecules features a typical ambipolar behavior with pristine hole mobility of 71.5 cm 2 V -1 s -1 and electron mobility of 21.6 cm 2 V -1 s -1 . Once the DAE blend is physisorbed on the WSe2 surface, the threshold voltages for both holes and electrons move towards negative directions, whereas the hole mobility decreases to 49.2 cm 2 V -1 s -1 and the electron mobility increases to 25.1 cm 2 V -1 s -1 . These phenomena indicate that the DAE blends, with both photochromes in their open state, induce an n-type doping effect on WSe2. As a result of UV irradiation, a decrease of mobility can be observed for both holes (μ ℎ + = 2.1 cm 2 V -1 s -1 ) and electrons (μ 𝑒 -= 16.9 cm 2 V -1 s -1 ), accompanied with a downshift of threshold voltage for holes △ V 𝑡ℎ-ℎ += -4.8 V and a upshift for electrons △ V 𝑡ℎ-𝑒 -= 19.1 V. Therefore, the closed-form of DAE in the blend simultaneously deplete electrons and holes from WSe2 with △ n ℎ + = 3.8 × 10 11 cm -2 and △ n 𝑒 -= 1.5 × 10 12 cm -2 , indicating a n-type doping effect on hole transport while p-type doping effect on electron transport. The modulation process was proven to be reversible by successive vis light irradiation. In this process, switching the DAE molecules back to the ring-open form fully restores the initial electrical performance. An additional backward gate sweeping measurement is illustrated in Figure 6-8c, which shows the same trend as the forward sweeping, proving that the hysteresis has no influence on the carrier transport modulation.

To eliminate the contribution from WSe2, transfer curves of the reference device with pristine WSe2 were also measured, which shows no variation under dark and after UV/vis illumination. In order to prove the reproducibility of such current modulation, we investigated 8 devices with each based on WSe2 flakes with similar thicknesses. The devices show stable current modulation efficiencies under the same experimental conditions, with hole current modulation efficiencies vary from 95% to 98% and electron current modulation efficiencies vary from 48% to 53%. Such minor difference is attributed to the interface quality between WSe2 and DAE blends. Due to the full recovery of all the electrical characteristics of the FET device after subsequent vis light irradiation, more illumination cycles can be launched to test the stability of such photo-modulation of the current. In Figure 6-9a we plot the normalized drain current Ids for both holes and electrons taken from the transfer curves obtained during the 10 measured cycles (I0 is the current value measured in step 0, which corresponds to the current initially after molecular deposition. Figure 6-9b plots the detailed transfer curves. The hole current is collected at Vg = -90 V and the electron current is collected at Vg = 90 V. The current modulation is as high as 97% for holes and 52% for electrons, showing a more efficient trapping for holes than for electrons. Note that different mix weight ratios of the two DAE molecules are tested (from 1:10 to 10:1), but the current modulation efficiency remains unchanged.

Therefore we attribute the observed difference of modulation efficiency to the fact that the energy difference between the VBM of WSe2 and the HOMO of DAE_3c (0.4 eV) is larger than that between the CBM of WSe2 and the LUMO of DAE_1c (0.2 eV), thus triggering the a more efficient trapping of holes when compared to electrons. This result is also in accordance with our previous established models when DAE molecules are blended with different organic semiconductors. [25] The absence of degradation after 10 illumination cycles shows that our devices possess reasonable stability and robustness. Note that the curves are corrected for gate bias stress effect to single out the sole contribution of molecular switching (original curves and gate bias curves are provided in Figure 6-12). [26] By comparing the curves obtained, we find that the electron and hole current show the same modulation tendency under light irradiation. When the light is switched ON and OFF, the current displays a sudden increase or drop behavior, which is described as the photocurrent triggered by the generation and relaxation of electron-hole pairs. [27] During the illumination of UV light, the conversion of open to closed isomers is a time-resolved process, which leads to a gradual depletion of electrons and holes from the WSe2 channel, resulting in a gradual decrease of the current. Conversely, under subsequent vis light illumination the current increases gradually owing to the relaxation of molecules from closed to open isomers. At the end of each cycle, the current recovers to the original value confirming that all the charge carriers are transferred back to WSe2. In Figure 6-11a and 6-11c, we observe that after over 20 illumination cycles the modulation shows no noticeable fatigue, indicating the excellent endurability of our device. The control experiment on pristine WSe2 device is also performed under the same UV/vis illumination conditions. As displayed in Figure 6-13, the device exhibits only photoresponse upon light exposure whereas no significant modulation is observed. Moreover, the photocurrent in pristine WSe2 is one order of magnitude smaller than that decorated by the DAEs. Therefore, the photo-modulation of charge transport in WSe2 can be ascribed to the molecular photoswitching behavior. The unique thermal bistability of the DAE molecules can be expected to endow to the devices a good retention capacity. By keeping the device in the dark, we independently measure its hole and electron current after different storage times. The results are plotted in Figure 6-18. They reveal a highly reliable retention characteristic with the current maintains stable for both isomer states over 20 days. For each current level, the standard deviation is below 1.5% of the total current difference (Table 6-3), demonstrating a remarkable retention capacity, which is essential for a commercial non-volatile memory.

Table 6-3. Calculated standard deviation for Ids_1o and Ids_1c in the Figure 6-18.

Adaptability on flexible substrate

In order to demonstrate the general adaptability of our multilevel device into future flexible/wearable memories, the as-fabricated WSe2 FET on Si/SiO2 substrate is transferred onto a flexible PET substrate with ITO serving as back-gate electrode and 80 nm CL-PVP as dielectric material.

The device exhibits a hole mobility of 35.5 cm 2 V -1 s -1 and electron mobility of 43.3 cm 2 V -1 s -1 , which is comparable to values obtained on rigid Si/SiO2 substrates, suggesting that the transfer process does not significantly diminish the device's electrical performance. The transfer characteristics evolution after molecular deposition and light irradiation is displayed in Figure 6-19a. The device retains its photomodulation ability with a hole current modulation of 96% and electron current modulation of 49%, being slightly lower than the obtained values on Si/SiO2 substrate. We attribute this to the difference of interface quality between the substrates. The dynamic electron current variation at 90 V gate voltage during 6 illumination cycles is also investigated, revealing similar trend as on Si/SiO2 substrate: UV light yields a sizable drop of the current while vis light resumes the current to original value.

Furthermore, a series of bending tests are performed to investigate the influence of bending cycles on the current modulation efficiency. Each cycle consists of 5 s of bending at a bending radius of 8 mm, followed by 20 s of resting time. After specific bending cycles, the device is exposed to one illumination cycle with alternate UV/vis irradiation and the corresponding transfer curves are collected. As displayed in Figure 6-19c, the effect of bending cycles on the current modulation efficiency is found to be negligible. Over 100 bending cycles are monitored, both hole and electron current can virtually preserve their modulation efficiency, suggesting a good mechanical stability of our device to work on the flexible substrate.

Conclusions

In conclusion, we have successfully fabricated WSe2-based optically switchable FET by interfacing such 2DM with a blend of two photochromic DAE molecules acting as light-sensitive components.

Upon light illumination, the device can modulate hole and electron transport simultaneously for at least suitably designed DAE molecules with phototunable frontier molecular orbital energy levels can systematically tune the charge transport of the target 2DM. We clearly demonstrate the feasibility and generality of our approach to optically control the transport of electrons, or holes, or both charge carriers, depending on the nature of the target 2DM, thus it offers intriguing perspectives for optically switchable ambipolar FETs based on photochromic molecules/2DMs hybrid systems. Noteworthy, our device exhibits a combination of 128 distinct current levels with a high accuracy readout and over 20 days of retention time, which is of paramount importance for future applications as multilevel non-volatile memories. Our devices are also compatible to work on flexible PET substrates as their light responsive properties are successfully preserved, and thus offer a versatile platform for applications in flexible and transparent electronics.

Introduction

The emerging "More than Moore" technologies, which are characterized by integrating novel analog functionalities within semiconductor-based devices, bring vast diversification rather than pure miniaturization to CMOS digital circuitry. [1] The integration of molecular components into digital circuits has been demonstrated to be a powerful strategy to endow functional diversification, taking advantage of the almost infinite variety of molecules that can be synthesized. [2] Towards this end, stimuli-responsive molecules are particularly interesting because of their capacity to be interconverted between two or more states, having markedly distinct properties when subjected to external stimuli such as electromagnetic, electric or magnetic fields or changes in the environment (pH, temperature, etc.). [3] decisive step forward to address the demand for high density storage memories. [12a, 19] The reliability of each state is further reflected by the endurance and retention tests for 5 arbitrary states, exhibiting a notable endurance for over 10 cycles and an excellent data retention exceeding 1000 hours. Moreover, compared with cumbersome synthesis of single molecules incorporating different functional groups to realize the multi-addressable elements, [20] our Janus modified ternary structure relies on a simple fabrication process and appears to be a universal strategy that can be extended to other 2D materials and functional molecules, offering a versatile platform for the construction of multi-stimuli-responsive devices. change in their corresponding energy levels. [21] By taking advantage of this unique characteristic, a DAE derivative has been designed and synthesized to possess well-defined lowest unoccupied molecular orbital (LUMO) energy levels in the two states (Figure 7-1) with respect to the conduction band of WSe2 thereby exhibiting two different and reversible regimes of charge transfer upon light irradiation.

Experimental methods

In particular, the LUMO level of DAE_O lies higher than the conduction band minimum of WSe2 which blocks electron transfer from the latter. On the contrary, the LUMO level of DAE_1C lies comfortably below the conduction band minimum of WSe2 and thus significant electron trapping is expected. [22] In The bottom electrode (60 nm Au) was first evaporated onto a Si/SiO2 substrate through a shadow-mask.

The P(VDF-TrFE) (molar ratio of 70%/30%, from Solvay) layer was created by spin-coating a 8 wt% solution in methylethylketone on another Si/SiO2 substrate and cured at 140 °C for 2 h to increase its crystallinity. [23] The SiO2 layer is then etched in a NaOH bath to release the layer from the substrate.

After washing with distilled water, the ferroelectric layer was transferred to the pre-electrodeevaporated substrate (or to the substrate with pre-fabricated DAE/WSe2 FET, Figure 7-2). In the final step top gate electrode (60 nm Au) was deposited on top of the ferroelectric layer. Note that the P(VDF-TrFE) layer preparation and transfer method remain the same for both capacitors and FET devices. requires curing at 140 °C for 2 hours to increase its crystallinity, which may deteriorate the switching capacity of the DAE molecules. [24] Finally, a semi-transparent top gate electrode (20 nm Au) was evaporated on top of the P(VDF-TrFE) layer.

Characterization methods

Raman spectra were recorded by a Renishaw inVia spectrometer equipped with 532 nm laser.

Atomic force microscopy (AFM) imaging was carried out by a Nanoscope (Veeco Multimode V) in tapping mode under ambient conditions.

All electrical measurements were carried out by using a probe station connected to a Keithley 2636 in a glovebox filled with nitrogen. Device irradiation is realized by using a monochromator set-up, with either UV light (at λ = 312 nm, 0.15 mWcm -2 ) or vis light (λ = 530 nm, 11.8 mWcm -2 ). The PSS is reached under UV irradiation for 150 s and recovery within 20 min under vis light.

The charge carrier mobility is calculated in the linear regime from the transfer curve by using the expression: [25] μ =

dI ds dV g × L W × 1 V ds × 𝑑 𝜀 0 𝜀 𝑟 (1)

Optical switching capacity of the DAE/WSe2 FET

The photomodulation effect induced by DAE molecules was first examined in a DAE_1/WSe2 FET. As displayed in Figure 7-8, the initial device reveals a typical n-type dominant behavior with electron mobility of 13.8 cm 2 V -1 s -1 . Upon subsequent exposure to UV or vis light, the electron current could be reversibly modulated with an efficiency of 74%. Such value is even higher than the one obtained for the inverse heterostructure, i.e. WSe2 coated with a top DAE layer. [12b] The improved efficiency is attributed to a better interface contact between the DAE and WSe2 layers. To cast light onto the ferroelectric properties of P(VDF-TrFE) polymer, it was integrated as the dielectric material into a capacitor. Figure 7-9 shows the typical hysteresis loops for a P(VDF-TrFE) capacitor with the thickness of 500 nm. The coercive electric field appears at ≈ 0.9 MVcm -1 (corresponding to a coercive voltage of 38 V) and the remnant polarization value is ≈ 7.3 µCcm -2 , indicating a good quality of the P(VDF-TrFE) film. [28] The measurement cycle was repeated 10 times to gain insight into the ferroelectric stability and no fatigue was observed. In top gate configuration

Regarding the DAE/WSe2/FeFET device, its transfer characteristics were first investigated in back gate configuration, which showed a typical n-type dominant behavior with an electron mobility of 18.2 cm 2 V -1 s -1 (Figure 7-11, black curve). The enhanced mobility compared with DAE/WSe2 device by adding P(VDF-TrFE) as top gate is correlated to its high dielectric constant (r = 10), which suppresses the Coulomb scattering in the WSe2 channel. [29] Next, the electrical performance of the same device was characterized in top gate configuration with P(VDF-TrFE) serving as the dielectric. As the top gate voltage (Vtg) sweeps between -60 V and +60 V ( cycles, proving the retained dielectric and ferroelectric properties of the P(VDF-TrFE). The electron mobility derived from the top gate configuration is 32.3 cm 2 V -1 s -1 . Due to the better dielectric shielding effect of P(VDF-TrFE) compared to SiO2, [30] the value is much higher than that obtained from the back gate configuration. FET, which slightly decreases from 74% to 59%, suggesting that the wet transfer process does not significantly diminish the light responsive property of DAE molecules. In an effort to demonstrate that the device can respond synergistically to light irradiation and an electric field, the two stimuli were incorporated to switch the device. In Aiming at verifying the compatibility between the two stimuli, we further recorded the transfer evolution by reversing the stimuli sequences. Figure 7-13 shows that the 4 distinct states generated above can be freely addressed independently of the stimuli order, i.e., S0-0 Pup/DAE_C can be acquired from S1-1 Pdown/DAE_O by 2 routes: UV irradiation + negative bias and negative bias + UV irradiation.

This phenomenon demonstrated the absence of mutual interference between the light irradiation and electric field, providing evidence for the full compatibility of the electric-responsive P(VDF-TrFE) polymer with the photo-responsive DAE molecules. Figure 7-13 plots the normalized current modulation efficiency during the 4 switching cycles. In view of the synergic effect between these two molecular building blocks, the total electron current modulation efficiency reaches up to 87%. The 4 states were found to be stable during the measured cycles with different order of applied stimuli, confirming a good stability and reversibility of both electrical and optical modulation. The switching behavior was furthermore elucidated by tracking the real time current dynamics under repeated voltage and illumination operation. In Figure 7-14a, the Ids current at Vbg = 60 V in back gate configuration was collected under cyclic ±60 V pulses applied to the top gate (each cycle contains 2 s +60 V, red boxed region; 28 s 0 V; 2 s -60 V, violet boxed region; 28 s 0 V). The measurement started from the current state with the ferroelectric polymer in the upward direction. The device was first triggered by a +60 V pulse, leading to a current spike. When the bias was reset to 0 V, the current ramps promptly to a higher level and stays constant at that value, indicating the switch of the ferroelectric polymer into downward direction. Conversely, in response to a symmetric negative pulse -60 V, the current declines back to the lower level and holds stable at that value. Under such successive cycles of ±60 V pulses, the device switched steadily between the generated two states. The current behavior at Vbg = 0 V modulated by periodic UV/vis illumination is shown in Figure 7-14c (each cycle contains 5 s UV, orange boxed region; 15 s dark; 15 s vis, green boxed region; 15 s dark). When the illumination was switched on, there was a surge of the current, which is attributed to the photogenerated electronhole pairs. As the DAE isomerization progressed, the current presented a gradual decrease during UV illumination, due to the continuous trapping of electrons by photogenerated DAE_C. After the UV illumination was terminated, the current exhibited a lower value, and such reduced current could recover progressively by subsequent vis illumination. At the end of the illumination cycle, the current level resumed to the pre-irradiation value, in good accordance with the results obtained from the transfer measurements. In a control experiment, WSe2/FeFET without DAE molecules was tested under the same illumination conditions, exhibiting only photoresponse yet no modulation behavior (Figure 7-15). Moreover, for both electrical and optical switching, the absence of degradation over 20 consecutive cycles confirms the durability and robustness of the device, which are crucial prerequisites for memory applications. Since both electrical and optical modulation display a high efficiency, the suitability of our FET device as a multibit memory can be envisaged. Upon adjusting the population ratio between Pup and Pdown, which can be controlled by the applied top gate voltage, intermediate ferroelectric current levels can be obtained between these two saturated states during the P(VDF-TrFE) polarization process. [31] Similarly, by tuning the population ratio between DAE_O and DAE_C, which can be controlled by the light exposure, multiple photogenerated current levels can be achieved accordingly. [12a] In order to fulfill the demand of high density storage in practical memory applications, the maximum number of levels that can be achieved by each stimulus have been investigated. fully polarized states (S1-1, S1-0.35, S0-0.35, S0-0) were steadily maintained for over 1000 h with negligible degradation, whereas S0.5-0.35 shows a weak deterioration. This phenomenon can be explained as the partial polarization states of the ferroelectric are not thermodynamically stable and suffer from depolarization. [32] In general, the device shows reliable non-volatile characteristics. The observed robust cyclic endurance and excellent retention time is promising for application of our DAE/WSe2/FeFET as a high density non-volatile memory.

Conclusion

In summary, by unprecedentedly interfacing 2D semiconductors with both light and ferroelectricresponsive components, we have demonstrated the operation of a multi-stimuli-responsive WSe2 FET device. By harnessing reversible isomerization of DAE molecules and the polarization of P(VDF-TrFE) polymer, the device could respond orthogonally to light irradiation and an electric field. We observed a synergetic effect between the two stimuli, with total absence of mutual interference, yielding to an overall modulation of the output current up to 87%. Such current modulation was found being robust for over 20 cycles without perceptible attenuation. Significantly, we could achieve 9 distinguishable ferroelectric states and 84 photogenerated states, yielding to as many as 756 current levels storage in a single device. In addition, by taking full advantage of the bistability of both DAE molecules and P(VDF-TrFE) polymer, the devices demonstrated an excellent performance in cyclic endurance (10 cycles) and data retention (over 1000 h), paving the way for their application as high-density non-volatile memories.

This novel ternary Janus device architecture is generally applicable to other 2D materials and stimuliresponsive molecules, thereby opening new opportunities for next-generation multi-stimuli-responsive electronics and promoting the development of "More than Moore" technologies by enriching diversification.

Chapter 8: Quaternary-Responsive Field-Effect Transistors Based on Asymmetrically Functionalized Janus Few-Layer WSe2

The projected saturation of Moore's law and the expiration of Dennard scaling has led to the demand for integrating novel analog functionalities within semiconductor-based devices. Herein, photochromic silane-terminated spiropyran (SP) derivative has been designed and synthesized, which can transform to the merocyanine (MC) isomer in response to UV illumination and reverse back by either heat or vis illumination. The SP molecule was incorporated at the dielectric-semiconductor interface in a WSe2based field-effect-transistor (FET), coated with a ferroelectric poly(vinylidene fluoridetrifluoroethylene) layer. A quaternary-responsive FET device has thereof been unprecedently fabricated, whose output current can be remotely and reversibly controlled by heat, light as well as electric field.

Thanks to the efficient tuning of each of the stimuli-responsive element and their synergic effect between each other, the device can be modulated from n-type to p-type with electrons been fully exhausted. The rational mechanism for the observed switching behaviour from the SP molecules has also been investigated, with the ionic polarized MC isomer acting as a carrier scattering site to effectively suppress electron transport in the interfaced WSe2.

Introduction

Taking advantage of their extraordinary physical properties and compatibility with silicon-based technology, two-dimensional materials (2DMs), including graphene and transition metal dichalcogenides (TMDs), have been the target of extensive fundamental studies and technological advances over past decade. [1] Within the current More-than-Moore era, the realization of multifunctional manipulations within individual 2DM-based devices is being explored. Owing to their atomic thickness and reduced screening effect, the intrinsic properties of 2DMs are extremely sensitive to the external environment and can thereof be modulated by surface modification. [2] Various functional molecules have been incorporated into transistor operations, yet the approach is still in the embryonic phase. [3] In particular, during the last five years, considerable workload has been dedicated to the incorporation of the stimuli-responsive molecules, which are able to exploit precise nanoscale actuations in response to chemical or physical inputs, as fundamental building blocks in the conventional electronic devices. [4] When tailored with such dynamic molecular entities, the external stimuli may act as an additional processing unit to operate the target devices, endowing them with new properties for specific applications as memories, [5] UV sensors, [6] synapse simulation, [7] and so on.

Light and electric field have been emerged as attractive external stimuli, both of which are noninvasive, remotely and digitally controllable with high spatial and temporal resolution. [8] As a mild and conveniently applied stimuli, temperature also represents one of the most widely explored inputs in the design of stimuli-responsive platforms.

Triggered by light, photochromic reactions have been recently demonstrated to be a simple-buteffective strategy to integrate optical switching into the manipulation of the carrier transport in 2DMbased FETs. [9] The development of such stimulus-responsive FETs has then been boosted by synergistically cooperating between electronic physics and multiple organic chemistry reactions. We previously achieved ternary-stimuli-responsive FETs by integrating two different stimuli-responsive elements, photochromic molecule diarylethene (DAE) and ferroelectric poly(vinylidene fluoridetrifluoroethylene) (P(VDF-TrFE) into a single WSe2-based FET, whose output current can be reversibly controlled in accordance with the photoisomerization or ferropolarization. The physisorbed DAE layer provides a promising way towards non-destructive functionalization of the channel material, however, the weak van der Waals interaction between the molecules and the Si/SiO2 substrate results in easy remove of the molecules during a simple rinsing procedure. As a consequence, conventional photolithography cannot be used in such device architecture, and the choice of the second stimuliresponsive element is also limited. To overcome this issue, here we synthesized a silane-terminated spiropyran (SP) derivative, another representative photochromic molecule, which can densely pack on Si/SiO2 substrate through a strong covalent silylation. [10] Such nonvolatile chemisorption thereof facilitates the device fabrication and provides a better compatibility when integrating a third element into the system. Moreover, the photochromic reaction from SP to merocyanine (MC) can be triggered by UV light illumination, and the reverse isomerization from MC to SP can be accomplished by vis light illumination as well as heating, adding a further thermal-responsive nature to the corresponding FET device when compared with DAE system. [11] Herein, silane-terminated SP photochromic molecule and ferroelectric polymer P(VDF-TrFE) have been incorporated into WSe2-based FET devices. With this combination, we obtained quaternarystimuli-responsive FET capable of perceiving heat, light and electric field stimuli inputs and modulate output current accordingly. Following the similar Janus modified structure presented in chapter 7, the bottom surface of the WSe2 flake is attached with a SP film, and the top surface is decorated with the P(VDF-TrFE) layer. Such hierarchical architecture enables an independent control of the output current under distinct multiple stimuli. Taking advantage of the synergic effect between the stimuli, with a significant electron current modulation efficiency of 84% from SP, and 99% from P(VDF-TrFE), the electrons in the channel can be totally exhausted when combining the two switches and the device thereof can be modulated from n-type to p-type. Moreover, by collectively pool multiple stimuli as inputs, the overreliance in a single stimulus has been reduced, which moreover increases the sensitivity and compliance of the molecular-tailored stimuli-responsive platform.

Experimental method

SP film functionalization

The formation of SP film via silylation of hydroxyl functions on the SiO2 surface with silylating agents is sketched in Figure 8-1. The protocols can be described as: cleaning and activation of the Si/SiO2 substrate to maximize the number of reactive hydroxyl groups on the surface by exposing them to UV/O3 for 20 min. Then, these freshly UV/O3 treated substrates were immediately immersed in a SP solution with a concentration of about 1% (dissolved in toluene) in a sealed reactor. The solution was heated at 60 °C for one hour and then left at room temperature for overnight. After taking out from the SP solution, the substrates were thoroughly rinsed with toluene, and sonicated 5 s in hexane to remove any physically adsorbed reagents.

Finally, the substrates were baked at 60 °C for one hour. The mechanically exfoliated few-layer WSe2 flakes from bulk crystal (from HQ Graphene) were transferred onto the pre-SP functionalized SiO2 substrates. The source and drain electrodes were patterned by using conventional photolithography method, followed by metal evaporation and liftoff process. With the underlying physisorbed SP layer guaranteeing the light responsive capacity, the top surface of WSe2 is ready for being interfaced with the ferroelectric P(VDF-TrFE) layer to impart the ferroelectric responsive nature. The P(VDF-TrFE) layer was spin-coated on top of the SP/WSe2 device, followed by curing at 140 °C for 2 hours to increase its crystallinity. Finally, a semi-transparent top gate electrode (20 nm Au) was evaporated on top of the P(VDF-TrFE) layer.

Characterization methods

UV-vis absorption spectra and isomerization kinetics of the SP film on quartz substrate were recorded at room temperature with a Jasco V650 spectrophotometer. The molecular switching triggered by light irradiation was performed with optical fiber-coupled LEDs (from ThorLabs), with wavelength of 365 nm (1.2 mW/cm 2 ) or λ = 530 nm (4 mW/cm 2 ), by heating was performed with a hair drier. For photoisomerization kinetics, irradiation was performed in situ within the spectrophotometer chamber, by shining a beam of UV light on the front face of the quartz substrate. UV irradiation was performed for fixed intervals, and Abs550nm was recorded immediately after switching off UV light.

Water contact angle was measured by a drop shape analyzer instrument Kruss DSA 100.

were acquired with 0.5 mm steps in 15 x 15 mm 2 areas. Each spectrum was taken at 0.1 s of integration time and 1 accumulation.

Atomic force microscopy (AFM) imaging was carried out by a Nanoscope (Veeco Multimode V) in tapping mode under ambient conditions.

All electrical measurements were carried out by using a probe station connected to a Keithley 2636 in a glovebox filled with nitrogen. Device irradiation is realized by using a monochromator set-up, with either UV light (at λ = 312 nm, 0.15 mWcm -2 ) or vis light (λ = 530 nm, 11.8 mWcm -2 ). The PSS is reached under UV irradiation for 150 s and recovery within 20 min under vis light.

The charge carrier mobility is calculated in the linear regime from the transfer curve by using the expression: [12] μ =

dI ds dV g × L W × 1 V ds × d ε 0 ε r (1)
Where L is the channel length, W is the channel width, and ε r is the dielectric constant, ε r = 3.9 for SiO2 and ε r = 10 for P(VDF-TrFE).

Results and discussion

Characterization of SP film

The reversible transformation between SP and MC is schematically illustrated in Figure 8-4a. The successful functionalization of SP film on SiO2 surface was confirmed through UV-vis absorption spectra, water contact angle measurements, atomic force microscopy (AFM) and PL mapping. UV-vis absorption spectra The typical UV-vis absorption spectra of SP film on quartz substrate were initially recorded during the course of UV irradiation until reaching the photostationary state (PSS) (Figure 8-4). The typical band of MC chromophores centered at 550 nm appeared after a few seconds of UV irradiation. Therefore, the kinetics of the photoisomerization between SP and MC can be followed by monitoring the evolution of the absorbance at 550 nm (Abs550nm). Abs550nm kept rising by UV irradiation until all SP converted to MC after 180 s, which can be fitted by a simple exponential curve following the equation (inset in Figure 8-4c and Figure 8-5a): [13] Abs550nm (t) = Abssat -B × exp (-t/τhν), where τhv represents the time constant. After reaching PSS, MC can be restored to SP in three routes by showing a decrease of Abs550nm in accordance with a first-order decay: spontaneously in the thermal condition, vis light irradiation and heating. 8-6. The initial clean SiO2 surface displayed hydrophilic performance, and its wettability decreased after SP functionalization. The MC, being more polar than SP, [11] is characterized by an increased hydrophilic nature when explored by contact angle measurements. More precisely, the The morphology of the SP film was investigated by means of AFM (Figure 8-7). AFM imaging performed on SP-functionalized SiO2 surface revealed a rather flat morphology, with the root-meansquare roughness value in an area of 1 μm x 1 μm amounting to ca. 0.17 nm. As evidenced by the topographical profile, the height of the film was 1.3 ± 0.5 nm, which correlates well with the length of a single SP molecule, which is ca. of 1.5 nanometer. The appearance of relatively larger aggregates is likely due to the self-polymerization, i.e., the silane-based SP molecules covalently bonded with neighboring molecules. [14] 8.3.1.4.

PL mapping

To unveil the interaction between SP and WSe2, 2D mapping of PL intensity over chosen WSe2 flakes with different thicknesses was carried out. Before focusing the attention to the PL of WSe2, the PL evolution of the SP/MC molecules were first investigated by integrating the wavelength from 560 nm to 650 nm, the range in which MC is photoluminescent while SP is not. As summarized in Figure 8-8, the as-prepared SP/WSe2 was found to be scarcely photoluminescent. After UV illumination, the PL of the region outside the flake was strongly enhanced, indicative of an effective SP to MC isomerization.

Interestingly, the region inside the flake showed a weaker PL, which can be ascribed to the quenching effect by WSe2. Note that the applied green laser power and the obtained peak intensity were compromised, and a relatively weak laser power was utilized during the measurement in order to minimize the interaction between the laser and the MC molecules. In order to trigger the molecule back to SP form, a stronger laser power was utilized to scan the same region. As clearly indicated in Figure 8-8a, the PL after laser scanning turned back to the original state, indicative of the recovery of SP.

Figure8-8b depicts the detailed PL spectra evolution in the points labeled as A and B. To investigate if the SP photoswitch is accompanied with a modulation of the optical properties of WSe2, wavelength from 750 nm to 900 nm the range in which WSe2 is photoluminescent were integrated. Spatial variations in the intensity across the flake were observed due to the variation of the flake thickness. The representative spectra taken from the bilayer (highlighted in C) and few-layer (highlighted in D) regions, are presented in Figure 8-8d. Point C represents a bilayer region, and its PL spectra shows a broad band corresponding to emission from both the indirect and direct bandgaps. For the few-layer region, the emission intensity decreased strikingly compared with thinner regions, and accordingly the PL spectra contains two separate peaks, originated from the indirect and direct bandgaps emission, respectively. [15] Note that we have chosen flakes with different thicknesses for PL mapping, and similar evolution trend was obtained: An overall quenching of the PL intensity for WSe2 was observed when molecules were in MC form compared with SP, providing evidence for the occurrence of electronic interaction between the two components. Such MC-induced PL quenching might be explained by the charge accumulation (either holes or electrons) induced Auger-like nonradiative decay. [6a] The reduced PL intensity could be completely reversed by further laser irradiation. Monolayer WSe2 possesses direct band gap thus strong emission can be observed in its PL spectra, with a prominent peak A centered at ~760 nm. As the number of layers increases, peak I emission spectra appears, originating from indirect band gap emission, which is virtually absent in the monolayer. The emerging peak I would compete with peak A deriving from direct bandgap emission, leading to a decay of the PL quantum efficiency. It should also be noted that peak I reveals a gradual redshift and a decay of intensity with the number of layers increasing. [15] Here by comparing the PL mapping of SP/WSe2 flakes with thickness ranging from monolayer, bilayer to few-layer, it is obvious that they all showing the same tendency after UV and laser irradiation: the molecule luminescence increases after UV irradiation, and recovers after laser irradiation. The WSe2 luminescence decreases after UV irradiation, also recovers after laser irradiation. To cast light onto the electronic properties of the SP/WSe2, they have been integrated as the channel material into the FET device. The original device revealed typical n-type behavior with electron mobility of 12.4 cm 2 V -1 s -1 (Figure 8-11). Obviously, the electron current was altered upon UV light irradiation with a modulation efficiency of 82% and the mobility decreased to 1.1 cm 2 V -1 s -1 , attributable to the SP conversion to MC. After heating, the current reversed almost to the initial value, which coincides well with the reversible change in the absorption spectra and PL mapping. The optical/heating switching capacity of SP/WSe2/FeFET was investigated in a back-gate configuration. With the dielectric constant (εr = 10) being higher than SiO2 (εr = 3.9), the deposition of P(VDF-TrFE) is believed to suppress the Coulomb scattering in WSe2. [1b] As a consequence, an improved electron mobility (μe-= 18.2 cm 2 V -1 s -1 ) was observed in SP/WSe2/FeFET compared with SP/WSe2 FET. In response to UV irradiation, the electron current decreased notably with a modulation efficiency of 84%, accompanied with the mobility decreased to 2.6 cm 2 V -1 s -1 . Figure 8-13b depicts the reversible variation of the current during three cycles when different inputs were utilized to switch back the molecule to SP form: heating, vis irradiation and thermally at room temperature. By heating at 140 °C for 6 min and vis irradiation at 530 nm for 30 min, the effects of the previous UV irradiation were completely removed, and electrical performances resumed to original states. While by leaving device that treated with UV irradiation for overnight at room temperature, 63% of the current was restored. The kinetics in electrical measurement matched well with the UV-vis absorption spectra results.

. The device exhibited more balanced ambipolar behavior with hole current comparable to the electron current (μh+ = 0.118 cm 2 V -1 s -1 , μe-= 0.024 cm 2 V -1 s -1 ). On the contrary, a +80 V pulse can give rise to the reversed arrangement of the dipoles in the downward direction. In this case, with positively charged C-H bond facing the channel, the current state resumed to the original state. Such reversible tuning could be repeated for at least 3 cycles without attenuation upon the successive +80 V and -80 V bias operation. In the following, to prove the compatibility between these two switching elements, we combined the light and bias stimuli in the same switching cycle. As shown in Figure 8-15a, after first stimuli of UV irradiation, the current decreased in the same ratio as the results above. More strikingly, followed by a negative bias, the electron current further decreased accompanied with an increase of hole current. The To better illustrate the current modulation by different stimuli, the top-gate bias and light irradiation were supplied separately as input signals, and the resulting drain current was collected as the output signal (Figure 8-16). Following the current, when a negative bias was applied, the current value greatly reduced, and stayed constant at that lower current state. Then in response to the positive bias, a current After proving the photoswitching phenomenon by SP, we then turned our attention to analyzing the switching mechanism. The isomerization between SP and MC isomer is accompanied by a variation of the dipole moment, which may alter the local electrostatic environment. Previous work has demonstrated that SP and MC isomer would assemble differently on the substrate. SP was found to be conformationally flexible, while MC can orderly assembled with the positively charged side lifted up, resulting in a n-type doping effect to the interfaced 2D material. [9a] However, this is opposite to our above electrical results, thus being excluded. Another possibility is to apply the electronic band structure to analyze the electron transfer between the SP/WSe2 system. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of SP and MC were estimated on the basis of their cyclic voltammograms. The band alignment diagram is illustrated in Figure 8-18, with conduction band (CB) and valence band (VB) values of WSe2 calculated in the screened exchange functional. [16] Our device based on few-layer WSe2 displayed electron-dominant transport behavior, so the CB of WSe2 and LUMO of SP/MC were taken into account. Since the CB of WSe2 is lower than the LUMO of both SP and MC isomer, electrons in WSe2 can transfer to neither of these two isomers, hence the possibility of the MC molecules acting as the electron trap site can be excluded. On the other hand, when it comes to mono/bilayer flake which possesses hole-transport behavior, the VB of WSe2 and HOMO of SP/MC should be taken into consideration. In this case, the HOMO of MC is slightly higher than the VB of WSe2, therefore MC isomer could act as a shallow trap for holes, which might result in a drop of hole current in the corresponding device. Electrical characterization on monolayer WSe2 was presented in Figure 8-19, which shows a reversible hole current modulation. Finally, we apply the ionic impurity scattering mechanism as previously reported. [17] It is generally agreed that charge carrier mobility/current of a semiconducting material is limited by the interfaced charged impurities centers, i.e., the lower current state is indicative of a higher density of scatters. [18] In our study, SP is neutral and non-polarized, while MC isomer is ionic polarized because of its opposing separated charges: the N + group and the O -group. Both of these cationic and anionic parts may serve as the scattering center, inducing an attractive or repulsive force with the electrons in the channel. Due to their Coulombic interactions between each other, the trajectory of the electrons will be disturbed and follows a gradual deviation, reducing of current. The scattering manner is similar to that of Coulombic scattering by ionic impurities in semiconducting materials. [19] HOMO (eV) LUMO (eV) To confirm such hypothesis, we performed a comprehensive analysis of the hysteresis by varying gate sweep rate and sweep range in bare WSe2 FET, and SP/WSe2 FET when molecules in SP and MC form.

Hysteresis in the transfer characteristics of TMD-based FET devices has been attributed to several mechanisms, such as adsorption of O2 and H2O molecules, [20] gate-oxide [21] and oxide-TMD interface traps, [22] gate voltage stress [23] and intrinsic defects in the material. [24] It is worth noting that the only altered variable was the SP layer, as the experimental conditions and measurement environment have been kept identical. A full gate sweep consists of both forward and backward sweep, i.e., from negative gate voltage Vbg to positive gate voltage Vbg then back to negative Vbg. Similar trend can be found from the measured devices: a hysteresis with clockwise loop direction with its magnitude been deterministically tuned via both gate voltage sweep range and sweep rate. Figure 8-20d illustrates the comparison of the hysteresis for various sweep rates. In WSe2 FET, the hysteresis was relatively small, which exhibited significant dependence on the sweep rate and increased from 3 V at 3.2 V/s to 22 V when the sweep rate decreased to 0.7 V/s. This is in accordance with the previous reports since the faster sweep rate inhibits electron from trapping during the measurement. [25] Larger hysteresis is observed in SP/WSe2 and MC/WSe2, while the variation in magnitude with the sweep rate is marginally. Therefore, the main contribution for such hysteresis can be attributed to the SP/MC layer. Ionic impurity scattering is considered the main interaction between ionic adsorbates and the charge carriers in the interfaced TMD materials, which may alter the charge distribution at the interface between SiO2 and TMD material due to the field induced polarization effect, thus have been proposed one of the major sources of the hysteresis. [26] When comparing the hysteresis in SP/WSe2 and MC/WSe2, a more pronounced magnitude is clearly observed in the latter. Such phenomenon is coincident with the assumption that the ionic-polarized MC isomer serve as the scattering center. Besides sweep rate, hysteresis variations have also been investigated with different sweep ranges (Figure 8-21). For WSe2, the hysteresis is negligible in the sweep range of 30 V, whose magnitude turns larger as the range becomes wider. It indicates that hysteresis originates from charge trapping/detrapping process at higher gate bias. [24] When it comes to SP/WSe2, the hysteresis variation turns more prominent. These variations are plotted in Figure 8-21d, where it can be clearly seen that when molecules were in MC form, the sweep range dependent hysteresis variation enhanced as compared to SP/WSe2, which confirms that MC isomer acts as ionic scattering center that play a main role in the device performance.

Conclusion

In conclusion, we have designed and synthesized a photochromic silane-terminated SP derivative, offering a robust molecular platform for fabrication of multifunctional FET devices. A quaternaryresponsive WSe2 FET has later been built up by combining SP and ferroelectric P(VDF-TrFE). The output current could be kindly and independently modulated by external stimuli including heating, light irradiation and electric field. Significantly, the synergic effect between the different stimuli enables a significant modulation of the charge carrier transport in the channel, which switches from n-type to ptype with electron been fully exhausted. The light induced modulation behavior from MC has been explained as the ionic polarized MC isomer plays the role of scattering center. It is anticipated that our design strategy of Janus modified structure can be extended to other TMDs, providing a general guideline to fabricate multifunctional FET devices via double-sided interfacial engineering. blend consisting of two molecules with specific energy levels, which are engineered to be capable of trapping either the electrons or the holes of WSe2 by remote light stimuli. UV-vis absorption spectra of the DAE blend/WSe2 film were initially recorded on quartz substrates. Note that both DAEs show similar molar absorptivities, speed of photoisomerization (quantum yields), and almost quantitative formation of the ring closed isomer under UV irradiation, allowing for their fully simultaneous operation in the device. For the electrical characterization on FET device, the output current modulation for holes is as high as 97%, while for electrons the modulation amounts to 52%. By taking advantage of the high fatigue resistance and the thermal bi-stability from DAE molecules, the devices are further employed to memory applications. The photomodulation was demonstrated to be efficient and reversible with negligible switching fatigue after over 20 illumination cycles. Moreover, the modulation ratio can be readily tuned by controlling the light dose, which means by adjusting the irradiation duration the FET device can reach a multitude of current states. In particular, the balanced ambipolar characteristic of WSe2 enables our device to attain 64 distinct current levels for both holes and electrons, highlighting its potential to be further developed into multilevel memories with data storage capacity of 7 bit. Additionally, the device is shown to possess a prolonged retention time exceeding 20 days in the dark. Remarkably, by taking full advantage of the superior mechanical properties of ultrathin 2D WSe2, our device is also able to be operated on a flexible and transparent poly(ethylene terephthalate) substrate with indium tin oxide serving as back-gate electrode and cross-linked poly(4-vinylphenol) as dielectric material, without losing its optical switching capability. Moreover, over 100 bending cycles are monitored on the PET device, both hole and electron currents can virtually preserve their modulation efficiency, suggesting a good mechanical stability of our device on the flexible substrate. Therefore, the new device architecture presents paramount importance for multilevel nonvolatile wearable memory applications.

In the above two chapters, optically switchable FETs have been demonstrated, by directly spin-coating the DAE molecules on top of the upper channel surface, leaving the bottom surface of the channel material undecorated. In the third experimental chapter, in order to best exploit both two surfaces of the 2D flake, we designed an unprecedented approach based on the decoration of WSe2 flakes with two chemisorption thereof facilitates the device fabrication and provides a better compatibility when integration a third element into the system. Moreover, the photochromic isomerization from SP to merocyanine (MC) can be triggered by UV light illumination, and the reverse switch from MC to SP can be accomplished by vis light illumination as well as heating, adding a further thermal-responsive nature to the corresponding FET device compared with DAE system. The functionalization of SP film on SiO2 surface has been confirmed through UV-vis absorption spectra, water contact angle measurements, atomic force microscopy and PL mapping. In particular, the kinetics of the photoisomerization between SP and MC has been followed by monitoring the evolution of the absorbance at 550 nm. For the electrical characterization, following the similar Janus modified structure, silane-terminated SP photochromic molecule and ferroelectric polymer P(VDF-TrFE) have been incorporated into WSe2-based FET devices. With this combination, we obtained quaternary-stimuliresponsive FET capable of perceiving heat, light and electric field stimuli inputs and modulate output current accordingly. Such hierarchical architecture enables an independent control of the output current under distinct multiple stimuli. Taking advantage of the synergic effect between the stimuli, with a significant electron current modulation efficiency of 84% from SP, and 99% from P(VDF-TrFE), the electrons in the channel can be totally exhausted when combining the two switches and the device thereof can be modulated from n-type to p-type. The analysis of the switching mechanism of SP molecules revealed a key role played by the ionic impurity scattering mechanism. SP is neutral and nonpolarized, while MC isomer is ionic polarized because of its opposing separated charges: the N + group and the O -group. Both of these cationic and anionic parts may serve as the scattering center, inducing an attractive or repulsive force with the electrons in the channel. It is anticipated that our design strategy of Janus modified structure can be extended to other TMDs, providing a general guideline to fabricate multifunctional FET devices via double-sided interfacial engineering. By collectively pooling multiple stimuli as inputs, the overreliance in a single stimulus has been reduced, which moreover increases the sensitivity and compliance of the molecular-tailored stimuli-responsive platform.

In conclusion, during the four experimental chapters, we have exploited the stimuli-responsive molecular approach to fabricate multifunctional FETs whose output current can be remotely and

d'é nergie

  entre les 2DSs hôtes et les molé cules DAE, pour former l'é tat de pié geage. Une illustration sché matique de l'alignement du niveau d'é nergie entre 2DS/DAE est repré senté e sur la Figure 1. Pour WSe2 intrinsè quement dopé de type n avec des é lectrons agissant comme les porteurs de charge principaux, l'é nergie de sa bande de conduction minimale (CBM) doit se situer entre les niveaux LUMO de DAE_1O et DAE_1C. C'est-à -dire que la LUMO de DAE_1O pré sente une valeur d'é nergie plus é levé e que le CBM de WSe2, empê chant ainsi le transfert d'é lectrons de WSe2 à DAE_1O. D'autre part, DAE_1C avec un niveau d'é nergie infé rieur se comporte comme un site de pié geage pour les é lectrons de WSe2, ce qui entraî ne une diminution de la densité é lectronique. Dans le dispositif FET correspondant, la conversion de DAE_1o en DAE_1c par irradiation UV induit un dopage de type p avec une modulation de courant de 61%. De mê me, pour BP avec un transport dominant de type p, son maximum de bande de valance (VBM) doit se trouver é nergiquement entre les niveaux HOMO de Ré sumé III DAE_2O et DAE_2C. La force motrice pour le pié geage des trous n'existe donc que dans DAE_2O, conduisant à une densité de trous ré duite. Dans le dispositif FET, l'irradiation UV ré vè le une diminution du courant de sortie d'une modulation de 42%. Pour ré vé ler davantage le rôle des niveaux d'é nergie des DAE par rapport à la modification du transport de charge dans 2DS, nous avons é galement é tendu notre é tude à BP/DAE_3. La HOMO de DAE_3C é tant isoé nergé tique par rapport au VBM de BP, le pié geage de trou photodé clenché est annulé dans cette paire, donc aucun comportement de modulation n'est observé dans le dispositif FET par irradiation UV/vis. Il convient de noter que les dispositifs FET basé s sur un tel systè me 2DS/DAE affichent une excellente capacité de ré tention des donné es et une lecture de haute pré cision avec plusieurs niveaux de courant distincts, ouvrant des perspectives vers son utilisation comme mé moire non volatile à plusieurs niveaux.

Figure 1 .

 1 Figure 1. Molé cules, niveaux d'é nergie et structure de l'appareil. (a, b) Diagramme du niveau d'é nergie du transport d'é lectrons entre WSe2/DAE_1 (a) et du transport de trous entre BP/DAE_2 et BP/DAE_3 (b). Les structures chimiques des molé cules DAE sont é galement illustré es. Les valeurs de niveau d'é nergie pour les DAE sont dé terminé es par voltamé trie cyclique; pour WSe2 et BP, ils sont calculé s dans la fonction d'é change filtré e. (c) Repré sentation sché matique du FET basé sur 2DS/DAE.

  Une telle photomodulation s'est avé ré e efficace et ré versible avec une fatigue de commutation né gligeable aprè s plus de 20 cycles d'é clairage. De plus, le rapport de modulation peut ê tre facilement ré glé en contrôlant la dose de lumiè re, ce qui signifie qu'en ajustant la duré e d'irradiation, le dispositif FET peut atteindre une multitude d'é tats courants. En particulier, la caracté ristique ambipolaire é quilibré e de WSe2 permet à notre appareil d'atteindre 64 niveaux de courant distincts pour les trous et les é lectrons, soulignant son potentiel à ê tre davantage dé veloppé en mé moires à plusieurs niveaux avec une capacité de stockage de donné es de 7 bits. De plus, en tirant parti de la bistabilité thermique des molé cules de DAE dans les deux é tats photoisomè res, il est dé montré que le dispositif possè de un temps de ré tention prolongé dé passant 20 jours dans l'obscurité . Il est à noter que, tirant pleinement parti des exellentes proprié té s mé caniques du 2D WSe2 ultramince, notre dispositif est é galement capable de fonctionner sur un substrat en poly(té ré phtalate d'é thylè ne) (PET) flexible et transparent, sans perdre sa capacité de commutation optique, ce qui est attrayant pour la nouvelle gé né ration d'é lectronique portable multifonctionnelle.

Figure 2 .

 2 Figure 2. Diagramme de niveau d'é nergie, structure molé culaire et du dispositif. (a) Structures chimiques de DAE_1 et DAE_3, en tant qu'isomè res ouverts et fermé s. Diagramme de niveau d'é nergie du transport d'é lectrons entre WSe2/DAE_1 et transport de trous entre WSe2/DAE_3. (b) Illustration sché matique de l'architecture du dispositif FET basé e sur WSe2/DAE sur substrat Si/SiO2.

Figure 3 .Chapter 9 :

 39 Figure 3. (a) Sché ma de principe du dispositif DAE_1/WSe2/FeFET avec dé coration double face: la face

Chapter 3 Chapter 2 background 2 . 1 .

 3221 introduces the materials employed in the work, 2DSs and stimuli-responsive materials, including their properties and characteristics. Chapter 4 presents the experimental techniques used to characterize materials and electronic devices, as well as the device fabrication procedures. Chapter 5 focuses on the fabrication of 2DS-based light-responsive field-effect transistors (FETs) via energy-level phototuning. In particular, the physisorbed photochromic DAEs with well-defined energy levels act as optical switching elements, thereby allowing for the fine-tuning and remote control over the charge transport of the host 2DM, either in terms of electrons in n-type dominated few-layer WSe2 or holes in p-type dominated few-layer BP. Chapter 6 is centered on the fabrication of ambipolar WSe2-based optically switchable FET by interfacing the channel flake with a blend of two photochromic DAE molecules acting as light-sensitive components. The device capacity to work as memory is evaluated by the basic parameters like data Theoretical Basic concept of semiconductor physics 2.1.1. Band theory

Figure 2 - 1 .

 21 Figure 2-1. Band theory representation of different materials.

Figure 2 - 2 .

 22 Figure 2-2. (a) Bond structure, reproduced from Ref.[3] and (b) its energy-band diagram of intrinsic semiconductor.

Figure 2 - 4 .

 24 Figure 2-4. (a) Bond structure, reproduced from Ref.[3] and (b) its energy-band diagram of p-type semiconductor.

Figure 2 - 6 .

 26 Figure 2-6. Zero biased PN junction diode.

Figure 2 - 7 .

 27 Figure 2-7. PN junction diode with forward bias.

Figure 2 - 8 .

 28 Figure 2-8. PN junction diode with reversed bias.

  controls the output current by the input voltage.As the diagram given below, the BJTs come into two types: the NPN type and the PNP type, based on the doping types of the three main terminal regions. In an NPN transistor, a lightly p-type doped base is sandwiched between a heavily n-type doped emitter and another n-type doped collector, where the electrons are the dominant charge carriers. The emitter possesses a high concentration of electrons, so it emits electrons into the base, these electrons are then collected by the collector and sent to the following part of a circuit. Whereas for a PNP transistor, a lightly n-type doped base is sandwiched between a heavily p-type doped emitter and another p-type doped collector. It shares the analog behavior with the NPN transistor but with holes as the dominant charge carriers.

Figure 2 - 9 .

 29 Figure 2-9. Layout of BJT transistors. Left: PNP type; Right: NPN type.

Figure 2 - 10 .

 210 Figure 2-10. Thermal diffusion setup.

Figure 2 - 11 .

 211 Figure 2-11. Ion implantation setup.

Figure 2 -

 2 12 illustrates the synthesis of large-area 2D semiconductor alloys comprising MoS2(1-x)Se2x monolayer by direct vaporization of MoSe2 and MoS2 powders. As from the electrical characterization, a much higher mobility was obtained for the MoS2(1-x)Se2x monolayer FET where x = 0.30, in comparison to that of the pure MoS2 monolayer FET where x = 0.

Figure 2 - 12 .

 212 Figure 2-12. (a) Illustration of three-zone furnace for the growth of the MoS2(1-x)Se2x monolayer. (b) HAADF-STEM image of MoS1.6Se0.4 monolayer in false color. Mo sites: yellow; S2 sites: light blue; SSe sites: yellow, and Se2 sites: red. (c) and (d) Transfer curves of MoS2(1-x)Se2x monolayer. (c) for x = 0; (d) for x = 0.30. Reproduced

Figure 2 -

 2 13 shows the STEM image of the doped MoS2 monolayers with rhenium (Re) impurities during the chemical vapor deposited growing process. By comparing the output characteristics of MoS2 and Re-MoS2 based FET devices, the Re doping is found to significantly improve the contacts between the metal and MoS2, enabling ohmic behavior to be maintained over a wide range of temperatures.

Figure 2 - 13 .

 213 Figure 2-13. (a) Aberration-corrected Z-contrast STEM image from monolayer Re-doped MoS2. The brown, white, and dim purple dots represent Mo, Re, and S atoms respectively. (b) and (c) Output curves of (b) pure

Figure 2 - 14 .

 214 Figure 2-14. (a) Schematic of an WSe2 flake as exfoliated (left) and after the H2 plasma treatment. (b) Transfer characteristics of the control (no plasma treatment) and H2-plasma-treated WSe2 FETs. (c) Subthreshold swing of

Figure 2 -

 2 12d shows the typical transfer characteristics of the three types devices. Large gate modulation of the 2H-channel can be seen in type I, whereas the 1T channel has much weaker gate dependence, which is expectable since the pure 1T-phase is metallic. With 1T phase on the electrode areas, the contact resistance could substantially decrease, and devices' electrical performances in turn improved.

Figure 2 - 16 .

 216 Figure 2-16. (a) Phase conversion of MoS2 from 1H to 1T/1T'. Reproduced from Ref. [27] (b) Schematic representation of the Ar plasma-treated process. (c) PL intensity mapping on a selective-area plasma treated 2H

Figure 2 - 17 .

 217 Figure 2-17. (a) Band energy alignment between MoS2 and n-and p-type dopants. Reproduced from Ref.[28] (b)

Figure 2 - 18 .

 218 Figure 2-18. (a) Schematic illustration of MoS2/APTES/SiO2 structure. (b) Transfer curves of the MoS2 FETs

Figure 2 -

 2 Figure 2-19. (a) Schematic illustration of MoS2 photodetectors treated with R-6G. (b) Photoinduced transfer characteristics of the pristine and R6G-sensitized MoS2 photodetectors under different illumination wavelengths.

  (d) Schematic illustration of monolayer MoS2 FET functionalized with Fc-SH molecules. (e) The cyclic voltammogram of Fc-SH at the MoS2-IL interface. (f) Transfer characteristics of monolayer MoS2 FET with different Fc-SH electrochemical states. Reproduced from

Figure 3 - 1 .

 31 Figure 3-1.Examples for 0D, 1D, 2D, and 3D carbon nanostructures. Reproduced from Ref.[1]

Figure 3 - 2 .

 32 Figure 3-2. (a) Lattice arrangement of graphene, where 𝑎1 and a2 are the lattice unit vectors, 𝛿1, 𝛿2 and 𝛿3 are the nearest neighbor vectors. (b) Electronic dispersion relation of graphene lattice in first Brillouin zone. Reproduced

Figure 3 - 3 .

 33 Figure 3-3. (a) Illustrative procedure of the scotch tape exfoliation. Reproduced from Ref. [14] (b) An optical microscopy image of exfoliated graphene flake with different thicknesses.

Figure 3 - 5 .

 35 Figure 3-5. Sketch of CVD technique to grow graphene.

Figure 3 - 6 .

 36 Figure 3-6. Schematic illustration of epitaxial graphene growth of on silicon carbide. Reproduced from Ref.[26]

Figure 3 - 7 .

 37 Figure 3-7. (a) 3D representation of MoS2 structure; (b) and (c) Two polytopes of single layer MoS2: (b) 2H

  photoluminescence. While a strong PL emerges when it is thinned to monolayer, indicating an indirect

Figure 3 - 9 .

 39 Figure 3-9. Synthesis of thin layer MoS2 by different techniques. (a) Stable suspensions obtained by liquid-phase exfoliation (top), and thin films derived from vacuum filtration (bottom). (b) Suspension obtained by lithiumintercalated exfoliation in water. (c) CVD growth from solid S and MoO3 precursors; (d) CVD growth from Mo exposed to S vapor on SiO2. (e) CVD growth from a dip-coated precursor and growth in the presence of Ar + S

Figure 3 - 10 :

 310 Figure 3-10: (a) side and top view of the 2H and IT structure of WSe2. (b) band structure of monolayer WSe2. (c)

Figure 3 -

 3 Figure 3-11.Three-dimensional representation of BP structure. Reproduced from Ref.[49]

𝑂 2 +

 2 ℎ𝑣 → 𝑂 2-+ ℎ + b. Dissociation of O 2-at the BP surface and formation of dangling P-O bonds. 𝑂 2-+ 𝑃 + ℎ + → 𝑃 𝑥 𝑂 𝑦 c. Removal of O and P from the surface by water molecules and dissolution of the top layer.

Figure 3 - 15 .

 315 Figure 3-15. (a) Schematic illustration of the OFET with azobenzene-functionalized electrodes. (b) Transfer evolution of the OFET for one switching cycle of trans/cis azobenzene. (c) Variation of output current for consecutive switching cycles of azobenzene. Reproduced from Ref. [65] (d) Schematic illustration of graphene-aryl azobenzene-graphene device. (e) Current density evolution of the devices during one switching cycle of trans/cis azobenzene. (f) The photoinduced current switching for over 50 cycles. Reproduced form Ref.[66] (g) Schematic

  -16a, which comprises an indoline and a chromene moiety bound through a spiro junction and oriented perpendicular with respect to one another. UV irradiation of the neutral closed-ring SP molecules gives rise to the zwitterionic open-ring isomer merocyanine (MC). The transformation begins with the cleavage of the C-O bond in the pyran ring, resulting in cis-MC. Then the rotation of the central C=C double bond in cis-MC ultimately yields thermally stable trans-MC (Figure 3-16a

Figure 3 -

 3 Figure 3-16. (a) Photoisomerization of SP between the four states: SP, MC, protonated merocyanine (MCH + ),

Figure 3 - 17 .

 317 Figure 3-17. (a) Schematic illustration of the optically switchable OTFT based on P3HT and SP. (b) Dynamic current behavior under UV/vis irradiation cycles. (c) Energy level diagram and working model of P3HT-SP hybrid systems(d). Reproduced from Ref.[73] (d) Schematic illustrations of the transistor involving PTAA and SP. (e) Transfer evolution by alternating UV and vis irradiation. (f) Drain current modulation during 2 switching cycles.

  a. Quantum yield The DAE molecules can undergo reversible photocyclization between their ring-open isomer and ringclosed isomer under UV/vis light. The ring-open isomers exist two conformations, which can interconvert between each other at room temperature. One conformation has two aromatic rings in parallel orientation and the other in antiparallel orientation (Figure 3-17a). Since the photocyclization can only occur from the antiparallel conformation, the quantum yield of the cyclization is dependent on

Figure 3 -

 3 Figure 3-18. (a) Photoisomerization of DAE between ring-open isomer with antiparallel conformation and ringclosed isomers. Reproduced from Ref.[78]

  -18. The ring-open isomer possesses a strong absorbance in the UV range. Upon UV irradiation, a new band centered at 522 nm appears that is characteristic for the ring-closed isomer. The band intensity keeps increasing until reaching the photostationary state (PSS). When the sample is further subjected to UV illumination, a slow decrease of the vis absorbance can be observed, accompanied by a small hypochromic shift of the band, indicating the formation of the annulated byproduct. The formation process is initiated by photochemical C-S bond cleavage. Radical migration and bicyclohexane formation lead to the condensed-ring byproduct. The byproduct formation process is not reversible, thus leading to a significant loss of photochromic material after repeating such illumination cycle for several times (Figure3-19c). To improve the fatigue resistance and suppress the byproduct formation, strategies have been proposed like introduction of strong acceptor groups (CF3 molecule with SF5 group, which shows a high fatigue resistance. Another strategy is to introduce acetyl groups at 6-and 6′-positions, which are considered to suppress the photochemical C-S bond cleavage and the radical migration.[82]

Figure 3 -

 3 Figure 3-19. (a) Absorption spectra of DAE molecules during the course of UV irradiation, from blue to red: initial to PSS; from red to green: further irradiation after reaching PSS. (b) Evolution of absorbance at the vis range with UV irradiation time. (c) Evolution of absorbance at the vis range during repetitive UV/vis switching cycles with a low fatigue resistance. (d) Evolution of absorbance at vis range of DAE molecule with SF5 group

Figure 3 -

 3 Figure 3-20. (a) Schematic illustration of the optically switchable OTFT based on P3HT and DAE. (b) Dynamic

Figure 3 -

 3 Figure 3-21. (a) Photoisomerization between ring-closed DHA and ring-open VHF forms. Reproduced from

Figure 3 -

 3 Figure 3-22. A typical example of ferroelectric hysteresis loop, right images are schematized ferroelectric material with defined orientation of the ferroelectric domains at different polarization states.

Figure 3 -

 3 Figure 3-23. (a) Chemical structure of PVDF and its monomer. (b) Chemical structure of P(VDF-TrFE) and the monomer of TrFE.

Figure 3 -

 3 Figure 3-24.Chain conformations of the α, β, γ and δ phases of PVDF. Produced from Ref.[96]

Figure 3 - 25 .

 325 Figure 3-25. Hysteresis loops of a 56/44 mol% PVDF-TRFE copolymer film as prepared and after annealing.

  while a clockwise direction of hysteresis in the P(VDF-TrFE) based diode, indicative of the ferroelectric polarization.

Figure 3 -

 3 Figure 3-26: a, b) Schematic of vertical (a) and lateral (b) poling of P(VDF-TrFE), respectively. (c) C-V curves

Figure 3 - 26 :

 326 Figure 3-26: (a) Schematic illustration of the FeFET device with a P3HT channel and P(VDF-TrFE) insulator. (b) Transfer curve of the device with hysteresis characteristic. (c) retention characteristics and (d) multiple write/erase

Figure 3 -

 3 Figure 3-27. (a) Schematic illustration of the MoS2 FeFET. (b) The transfer characteristic of MoS2 FeFET in topgate configuration with P(VDF-TrFE) serving as electrode. The transfer characteristic of the same device in backgate configuration with SiO2 serving as dielectric is shown in inset. (c) Output characteristic of the device with three states of ferroelectric layer: Pfresh, Pup, and Pdown. (d) Photoresponse of the device with three states of ferroelectric layer. Reproduced from Ref.[110] (e) Current dynamics of the MoS2 FeFET under periodic top-gate

Figure 4 - 1 .

 41 Figure 4-1. (a) to (d) Conventional scotch tape exfoliation process. (e) Optical microscopy image of a monolayer graphene flake.

Figure 4 - 2 .

 42 Figure 4-2. The fabrication process of a FET device by photolithography: (a) The target 2D flake is positioned on the substrate. (b) The substrate is covered with a player of photoresist. (c) The photoresist film is irradiated with a laser writer in the pattern we designed. (d) the substrate is immersed in a develop solution to remove the irradiated photoresist. (e) The needed metal is evaporated on the substrate. (f) Metal in unwanted area is removed by washing away the rest photoresist.

Figure 4 - 3 .

 43 Figure 4-3. Sketch of a metal evaporation setup.

Figure 4 -

 4 Figure 4-4 shows two representative images of a FET device based on a few-layer WSe2 flake. The

Figure4- 4 .

 4 Figure4-4. Optical microscopy images of a FET device based on a few-layer WSe2 flake.

Figure 4 - 5 . 4 . 3 .

 4543 Figure 4-5. Sketch of the spin-coating process.

Figure 4 - 6 .

 46 Figure 4-6. Transfer process of a FET device from Si/SiO2 substrate to PET/ITO/CL-PVP substrate. (a) FET device is first fabricated on a Si/SiO2 substrate. (b) The device is coated with a layer of poly(methyl methacrylate) (PMMA). (c) The PMMA-coated device is floated in a NaOH bath to etch the SiO2 layer. (d) and (e) The PMMA supported FET layer is transferred to a deionized water bath and later transferred to a PET/ITO/CL-PVP substrate. (f) The PMMA layer is finally removed by soaking the sample in hot acetone.

Figure 4 - 7 .

 47 Figure 4-7. Sketch of an Atomic Force Microscopy setup.

Figure 4 - 8 .

 48 Figure 4-8. (a) Light interaction with the sample molecules, reproduced from Ref. [4] (b) Representative energy levels for Rayleigh, Stokes, Anti-stokes scattering and photoluminescence.

Figure 4 - 9 .

 49 Figure 4-9. Sketch of an Atomic Force Microscopy setup.

  gives the vibrational spectrum of the sample material, of which the shift in wavelength provides the chemical and structural information. It can be used to observe rotational, vibrational and other modes of low frequency within a system. PL can provide information about the composition and solid-state structure of the material. The high spectral resolution of a Raman spectrometer can be useful in performing PL spectroscopy of solid-state materials. A schematic layout of the Raman microscope is shown in Figure 4-9. The sample substrate is placed on the microscope stage. The excitation light from a laser (532 nm) is reflected by the Notch filter and focused down onto the sample through the objective lens of the microscope. The resulting Raman scattering from the sample is collected by the objective lens and transmitted back to the Notch filter. Note that the filter attenuates the incident excitation light and only passes the Raman scattering. The remaining Raman scattering then enters the monochromator which uses a diffraction grafting to disperse the beam into a different band of wavelengths. Finally, a CCD camera is used to detect the resulting spectrum. 4.5.3. UV-vis absorption spectroscopy UV-vis absorption spectroscopy is a quantitative technique that can measure how much a molecule is capable to absorb light, by comparing the light intensity that passes through the sample molecule with respect to the reference. A UV-vis spectrophotometer usually consists of a light source, a diffraction grating to separate the different wavelengths of light, a holder for the sample/reference and a detector (Figure 4-11). The light source is often a Halogen filament (300-2500 nm for visible range) and a deuterium arc lamp (190-400 nm for UV range). A beam of light from the source is separated by a diffraction grating. The monochromatic beam in turn is split into two equal intensity beams by the chopper. The sample beam passes through the cuvette containing the sample solution. The reference beam passes through the cuvette containing only the solvent. The intensities of these light beams are then measured by the detectors and compared.

Figure 4 - 10 .

 410 Figure 4-10. Sketch of a UV-vis spectrophotometer.

Figure 4 - 11 .

 411 Figure 4-11. (a) Sketch of water-contact angle setup. (b) Example of contact angle.

Figure 4 - 12 .

 412 Figure 4-12. Cyclic voltammetry of (a) DAE_1, (b) DAE_2 and (c) DAE_3 in acetonitrile/0.1 M Bu4NPF6.

Figure 4 - 13 .

 413 Figure 4-13. Sketch of a field-effect transistor.

Figure 4 - 14 .

 414 Figure 4-14. Different geometries of FET devices: (a) top contact, bottom gate; (b) bottom contact, bottom gate; (c) top contact, top gate; (d) bottom contact, top gate.

  The transfer curve is obtained by recording the source-drain current Ids, when sweeping the gate bias Vg and applying a constant source-drain bias Vds. The output curve is obtained by recording Ids at a constant Vg when sweeping the Vds. In most cases, multiple curves for different Vg are shown in output measurement.

Figure 4 - 15 .

 415 Figure 4-15. A typical example of the (a) Transfer; (b) Output curve for a n-type semiconductor.

Figure 4 - 16 .

 416 Figure 4-16. (a) Schematic illustration of TLM structure. (b) Example of total resistance versus channel length plot.

  -15b). The contact resistances can thus be calculated from the potential drops at the source (∆VS) and at the drain (∆VD) using 𝑅 𝑆 = ∆ 𝑉 𝑠 𝐼 𝐷 and 𝑅 𝐷 = ∆ 𝑉 𝐷 𝐼 𝐷, while µ0 can be obtained from the potential drop across the channel ∆Vch.

Figure 4 - 17 .

 417 Figure 4-17. (a) Schematic of the measurement configuration for four-probe field-effect mobility. (b) Potential profile in the channel of device in the linear regime.

  characterization was performed using a Keithley 2636A Dual-channel System Source Meter, with a Cascade Microtech M150 probe-station in a glove box filled with nitrogen. The sample was placed on a metallic holder which can be biased with a gate voltage Vg. Two micromanipulators allow to move two tungsten probes that carry the electric current connecting to the source and drain electrodes respectively, localized with the help of the optical microscope. The source electrode is connected to ground and the drain would be imposed with a voltage Vds. A monochromator setup can be used to illuminate the sample.

Figure 4 - 18 .Chapter 5

 4185 Figure 4-18. Sketch of a probe-station setup for electrical characterization.

Figure 5 - 1 .

 51 Figure 5-1. Energy level diagram of electron transport between WSe2/DAE_1.

Figure 5 - 2 .

 52 Figure 5-2. Energy level diagram of hole transport between BP/DAE_2 and BP/DAE_3.

Figure 5 -

 5 Figure 5-3a shows an example of optical microscopy image of the WSe2 FET device and 5-3b shows

Figure 5 - 3 .

 53 Figure 5-3. (a) An optical microscopy image of the WSe2 FET device; (b) Schematic of the 2DM/DAE device structure.

5. 3 . 1 .

 31 Optical characterization UV/vis absorption spectra of the three DAE molecules in film on quartz substrate were first characterized during the course of UV irradiation until reaching the photostationary state (PSS). In general, the PSS is reached under UV irradiation for 2 min and recovery within 5 min under vis light. As displayed in Figure 5-4, the three molecules show similar band evolution during UV irradiation. The initial spectrum for open isomer has a typical band at 312 nm. Following UV irradiation, the intensity of the peak around 312 nm clearly decreases, while another peak around 530 nm, originating from the closed isomers appears. The peak intensity gradually increases until reaching PSS state. The initial spectrum can be reobtained after vis illumination.

Figure 5 - 4 .

 54 Figure 5-4. UV/vis absorption spectra of (a) DAE_1 (b) DAE_2 and (c) DAE_3 in film on quartz substrate during the course of UV irradiation until reaching the PSS.

Figure 5 - 5 .

 55 Figure 5-5. (a) to (c): UV/vis absorption spectra of (a) WSe2, (b) DAE film and (c) WSe2/DAE in dark and after UV/vis irradiation; (d) to (e): same dataset with (a) to (c) while plotted in another combination. UV/vis absorption spectra of WSe2, DAE film and WSe2/DAE (d) in the dark, (e) after UV irradiation, (f) after vis irradiation.

Figure 5 - 6 .

 56 Figure 5-6. (a) Optical microscopy image, (b) Raman of few-layer WSe2, (c) and (d) AFM image of the same flake (c) before (d) after DAE deposition.

Figure 5 -

 5 Figure 5-6a shows a typical optical microscopy image of few layer WSe2 flake. The Raman spectra at point A is shown in Figure 5-6b, which has three active modes, E 1 2g, A1g and B 1 2g. It suggests the few-

Figure 5 -

 5 Figure 5-7a shows an optical microscopy image of few layer BP flake. Its Raman spectra at point B shows three prominent peaks around ∼360, 440 and 470 cm -1 , namely A 1 g, B2g and A 2 g, respectively,

Figure 5 - 7 .

 57 Figure 5-7. (a) Optical microscopy image, (b) Raman and (c) AFM image of few-layer BP.

Figure 5 - 8 .

 58 Figure 5-8. (a) Transfer evolution of bare WSe2, WSe2/DAE_1 as prepared and after UV/vis irradiation. (b) Transfer characteristics of WSe2/DAE_1 via backward gate sweeping (Vg from 30 V to -30 V).

Figure 5 - 9 .

 59 Figure 5-9. blank test on bare WSe2 as prepared and after UV/vis irradiation.

Figure 5 -

 5 Figure 5-10 displays the output characteristics of WSe2/DAE_1o and WSe2/DAE_1c respectively. The

Figure 5 - 10 .

 510 Figure 5-10. Output evolution of WSe2/DAE_o and WSe2/DAE_c for Vg from -30 V to 30 V in steps of 10 V.

Figure 5 - 11 .

 511 Figure 5-11. (a) WSe2/DAE_1o-WSe2/DAE_1c switching cycles for 10 times; (b)Ids modulation over 10 illumination cycles with alternative UV (violet shaded areas) and vis (grey shaded areas) light. All current values are normalized to the initial value obtained from the as prepared WSe2/DAE_1o. The connecting lines are used as guides to the eye.

Figure 5 - 13 :

 513 Figure 5-13: Ids evolution with storage time in the dark.

Figure 5 -

 5 14b displays an example of one switching cycle (each cycle contains 15 s UV ON + 45 s light OFF + 120 s vis ON + 60 s light OFF). The violet boxed regions indicate the period when the UV light is on and the grey boxed regions indicate the period when the vis light is on. The initial current increase upon UV light is due to

  photoswitching. The absence of fatigue for at least 16 static illumination cycles demonstrates that the

Figure 5 - 14 .

 514 Figure 5-14. Ids-time measurement under alternative dark and illumination conditions at Vg = 30 V and Vds = 2 V (a) over 16 illumination cycles; (b) an example of one illumination cycle. Each cycle contains 15 s UV (violet shaded areas) and 2 min vis (grey shaded areas) light. The curve is corrected for bias stress effect; (c) Original curve without fitting; (d) fitting curve to correct the bias stress.

Figure 5 - 15 .

 515 Figure 5-15. Control experiment on WSe2. Ids-time measurement under alternative dark and illumination conditions at Vg = 30 V and Vds = 2 V: (a) over 11 illumination cycles, and (b) example of 1 illumination cycle.

Figure 5 - 16 .

 516 Figure 5-16. Electrical characterizations of BP/DAE_2 devices. (a) Transfer evolution of bare BP, BP/DAE_2 as prepared and after UV/vis irradiation. Inset: the same curves represented on a logarithmic scale; (b) Transfer characteristics via backward gate sweeping (Vg from 30 V to -30 V); (c) Transfer evolution of bare BP as prepared and after UV/vis irradiation; (d) Output evolution of BP/DAE_2o and BP/DAE_3c for Vg from -30 V to 30 V in steps of 10 V.

Figure 5 - 17 .

 517 Figure 5-17. (a) Transfer evolution of BP/DAE_2 under 4 UV/vis irradiation cycles; (b)Normalized Ids modulation over 4 illumination cycles with UV (violet shaded areas) and vis (grey shaded areas) light at Vg = -30 V and Vds = 2V.

Figure 5 - 18 .

 518 Figure 5-18. Transfer evolution of bare BP, BP/DAE_3 as prepared and after UV/vis irradiation at Vds = 2 V.

Figure 6 - 1

 61 illustrates the detailed energy-level diagram of DAE_1, DAE_3, and WSe2. The yellow bands represent the CB/VB of few-layer WSe2. The electron transport modulation is realized by DAE_1, since the conduction band minimum (CBM) of WSe2 (-3.5 eV) lies energetically between the LUMO levels of the open isomer (DAE_1o, -2.7 eV) and the closed isomer (DAE_1c, -3.7 eV). The lower LUMO of DAE_1c behaves as electron-accepting level (trapping the transferred electrons from WSe2, thus reducing the current to gain modulation), while higher LUMO energy of DAE_1o impedes the electron transfer from WSe2 (no modulation). Similarly, the valence band maximum (VBM) of WSe2 (-5.1 eV) is located between the HOMO of DAE_3o (-5.4 eV) and DAE_3c (-4.7 eV), enabling the modulation of hole transport. DAE_3c behaves as a trapping site (modulation) while DAE_3o only represents a scattering center for the holes (no modulation) travelling in WSe2. In general, upon UV irradiation at 312 nm, both DAE molecules are converted to their closed photo-isomeric states, accompanied with a simultaneous transfer of electrons and holes from WSe2 channel to DAEs, inducing a decrease of charge carrier density in the FET device as well as the current. Conversely, upon visible (vis) light irradiation at 530 nm the DAE molecules switch back to their open isomer states, resulting in full recovery of the charge carrier density within WSe2 also the current. Note that in order to largely preserve the intrinsic properties of WSe2 and to minimize the perturbation of DAE molecules, the DAE blends are simply spin-coated onto the substrate and form a non-covalent bonding with WSe2 flakes.

Figure 6 - 1 .

 61 Figure 6-1. Energy level diagram of electron transport between WSe2/DAE_1 and hole transport between WSe2/DAE_3.

Figure 6 - 2 .

 62 Figure 6-2. An example of optical microscopy image of the WSe2 FET device

Figure 6 -

 6 photoresist residues.Figure 6-2 shows an example of optical microscopy image of the WSe2 FET device

Figure 6 - 3 .

 63 Figure 6-3. Schematic illustration of the FET device architechture based on WSe2/DAE blend on Si/SiO2 substrate.

Figure 6 - 4 .

 64 Figure 6-4. (a) Photograph of a typical WSe2/DAE blend FET device on flexible and transparent PET substrate. (b) Schematic (top) and corresponding optical microscopy images (bottom) of FET device.

Figure 6 - 5 .

 65 Figure 6-5. UV-vis absorption spectra for (a) DAE_1, (b)DAE_3 film during the course of UV irradiation.

Figure 6 - 6 .

 66 Figure 6-6. UV-vis absorption spectra for (a) DAE blend, (b) WSe2, (c)WSe2/DAE blend in dark and after UV/vis irradiation on quartz substrate.

Figure 6 - 7 .Figures 6 -

 676 Figure 6-7. (a) Optical microscopy image and (b) Raman spectra of few-layer WSe2; (c) and (d) Topographical AFM image of the same flake (c) before (d) after DAE blend deposition.

Figure 6 - 8 .

 68 Figure 6-8. Electrical characterization of WSe2/DAE blend FET device. (a) and (b) Transfer evolution of pristine WSe2, WSe2/DAE blend as prepared and after UV/vis irradiation in (a) linear and (b) logarithmic scale; (c) Transfer revolution of WSe2/DAE blend via backward gate sweeping (Vg from 90 V to -90 V); (d) Control experiment on bare WSe2 FET device.

Figure 6 - 9 .

 69 Figure 6-9. (a) WSe2/DAE_o -WSe2/DAE_c switching cycles for 10 times; (b) Drain current Ids modulation as both open and closed isomers over 10 illumination cycles with alternative UV (red shaded areas) and vis (blue shaded areas) light. All current values are normalized to the initial value obtained from the as prepared WSe2/DAE blend. The connecting lines are used as guides to the eye.

Figure 6 -

 6 Figure 6-10 displays the output measurement of the FET based on DAE blend decorated WSe2. The

Figure 6 - 10 .Figure 6 -

 6106 Figure 6-10. Output evolution of WSe2/DAE blend after UV (dashed lines) and vis (solid lines) illumination for Vg from -90 V to 90 V in steps of 30 V.

Figure 6 - 11 .

 611 Figure 6-11. Dynamic Ids-time measurement (corrected for bias stress effect) under alternative dark and UV/vis illumination conditions at Vg = -90 V and Vds = 2 V for WSe2/DAE blend duing (a) 20 cycles; (b) 1 singe cycle; (c) and (d) Same measurement at Vg = 90 V and Vds = 2 V.

Figure 6 - 12 .

 612 Figure 6-12. Dynamic Ids-time measurement under alternative dark and illumination conditions without correcting the bias stress effect (black) and the corresponding gate bias curve (red) V for WSe2/DAE blend for (a) holes at Vg = -90 V and Vds = 2 V, (b) electrons at Vg = 90 V and Vds = 2 V.

Figure 6 - 13 .

 613 Figure 6-13. Ids-time measurement under alternative dark and UV/vis illumination conditions at (b) Vg = -90 V and (c) Vg = 90 V, Vds = 2 V for WSe2 FET device.

Figure 6 - 14 .

 614 Figure 6-14. Transfer evolution of (a) and (b) WSe2/DAE_1, (c) and (d) WSe2/DAE_3 after UV/vis irradiation in (a) and (c) linear, (b) and (d) logarithmic scale.

Figure 6 - 15 .

 615 Figure 6-15. Multilevel characterization of WSe 2 /DAE blend FET device. (a) 5 distinct current levels over 5 illumination cycles for holes recorded at V g = -90 V. The levels are obtained by illuminating the device for different fixed times; (b) Same measurement recorded at V g = 90 V for electrons.

Figure 6 -

 6 15b. The drain current level for both holes and electrons are proportional to the population ratio between DAE_o and DAE_c molecules, which can be controlled by the light illumination dose. In Figure 6-15a and 6-15b we portray the multilevel current for holes and electrons respectively, which are obtained by illuminating the device at different fixed times at a well-defined areal power density. We start from Level 1, at which the current state is the result of both two DAE molecules in open forms. Later, Level 2 to Level 5 are reached after UV irradiation for 10 s, 20 s, 40 s, and 80 s, and Level 5 is the current state when all the molecules are in closed forms. By subsequent vis illumination, the current value can be restored to Level 1. Such illumination cycle is repeated for 5 times and the values obtained for each level are quite stable. Their standard deviations are calculated to be below 1.5% of the total current difference (Ids_o -Ids_c), thus demonstrating that the device can attain multiple current levels precisely by remote light control (detailed values for each level are provided in Table 6-1 and 6-2). To further investigate the number of levels our device can reach, we perform the Ids-time test by exposing the device to a periodic UV light irradiation at intervals of 20 s (60 s -540 s: 0.5 s per 20 s; 540 s -860 s, 1 s per 20 s; 860 s -1020 s: 2 s per 20 s, 1020 s -1180 s: 4 s per 20 s; 1180 s -1340 s: 8 s per 20 s).

Figure 6 -

 6 Figure 6-16a and 6-17a display the results: the current follows a progressive decrease after each light

Figure 6 - 16 .

 616 Figure 6-16. Dynamic hole current I ds -time curves under periodic UV irradiation per 20 s recorded at V g = -90 V and Vds = 2 V, (a) 64 levels are achieved in total (corrected for bias stress); (b) Original curve without correcting the bias stress effect (black) and the corresponding fitting curve (red); (c-g) an enlargement of 5 levels.

Figure 6 - 17 .

 617 Figure 6-17. Dynamic hole current Ids-time curves under periodic UV irradiation per 20 s recorded at Vg = 90 V and Vds = 2 V, (a) 64 levels are achieved in total (corrected for bias stress); (b) Original curve without correcting the bias stress effect (black) and the corresponding fitting curve (red); (c-g) an enlargement of 5 levels.

Figure 6 - 18 .

 618 Figure 6-18. Ids evolution with different storage time in the dark (a) for holes at Vg = -90V and Vds = 2 V, (b) for electrons at Vg = 90V and Vds = 2 V.

Figure 6 - 19 .

 619 Figure 6-19. Electrical characterization of WSe2/DAE blend FET device on flexible substrate. (a) Transfer evolution of pristine WSe2, WSe2/DAE blend FET as prepared and after UV/vis irradiation; (b) Dynamic Ids-time measurement under alternative dark and UV/vis illumination conditions during 6 cycles at Vg = 90 V, Vds = 2 V for WSe2/DAE blend FET; (c) Drain current modulation efficiency as a function of bending cycles.

Chapter 7 :

 7 photo-generated states ensure 756 current levels in a single device. The over 10 cycles of cyclic

7. 2 . 1 .

 21 Match of energy level between WSe2 and DAEs.

Figure 7 - 1 .

 71 Figure 7-1. Energy level diagram of electron transport between WSe2/DAE_1.

Figure 7 - 2 .

 72 Figure 7-2. Transfer process of the P(VDF-TrFE) layer from Si/SiO2 substrate.

Figure 7 - 3 .

 73 Figure 7-3. Optical microscopy images captured at four stages during the fabrication process. (a) Target WSe2 flake with the underneath DAE_1 layer. (b) DAE_1/WSe2 FET with evaporated source-drain electrodes. (c) DAE_1/WSe2/FeFET with transferred P(VDF-TrFE) film. (d) DAE_1/WSe2/FeFET with evaporated top gate electrode.

Figure 7 -

 7 Figure 7-3 shows the optical microscopy images captured at four stages during the fabrication process:

Figure 7 - 4 .

 74 Figure 7-4. Schematic diagram of the DAE_1/WSe2/FeFET device with double-sided decoration: the bottom surface with DAE film and the top surface with P(VDF-TrFE) layer.

Figure 7 - 6 .

 76 Figure 7-6. Raman spectra of few-layer WSe2 flake.

Figure 7 - 7 .

 77 Figure 7-7. The thickness measurement of the P(VDF-TrFE) film by a profilometer.

Figure 7 - 8 .

 78 Figure 7-8. Optical switching capacity of DAE/WSe2 FET by UV/vis illumination.

Figure 7 - 9 .

 79 Figure 7-9. (a) The ferroelectric hysteresis loop and (b) corresponding current of a 500 nm P(VDF-TrFE) capacitor during 10 cycles.

7. 3 . 4 .

 34 Electrical characterization of DAE/WSe2/FeFET 7.3.4.1.

Figure 7 -

 7 10a), the transfer curve shows a large memory hysteresis window (≈ 45 V). The generation of such memory window is entirely related to the polarization switching process of the P(VDF-TrFE). The state of the P(VDF-TrFE) can be programmed by the top gate bias: The positive voltage exceeding its coercive voltage can polarize the ferroelectric into the downward direction Pdown. With C-H bond facing the WSe2 channel, electrons in the WSe2 will be engaged to compensate for the polarization charges. Conversely, the application of a negative voltage exceeding the coercive voltage will induce a polarization of the P(VDF-TrFE) into the upward direction Pup. In this case, with C-F bond facing the channel, the compensation charge converts from electrons to holes, leading to a depletion of electrons in the channel. Moreover, the corresponding gate leakage current Itg exhibits two apparent peaks near the coercive voltage, which further proves the polarization behavior of the P(VDF-TrFE) (Figure7-10b). The polarization remains stable during the measured 10

Figure 7 - 10 .

 710 Figure 7-10. Transfer curve of a DAE/WSe2/FeFET in top gate configuration in logarithm scale during 10 cycles (blue curve) and corresponding leakage current (red curve).

Figure 7 -Figure 7 -

 77 Figure 7-11a shows a representative transfer evolution cycle. In the Pdown state, an increase of the

Figure 7 - 11 .

 711 Figure 7-11. Electrical and optical switching capacity of DAE_1/WSe2/FeFET. (a) and (b) Transfer evolution by ±60 V top gate bias in linear (a) and logarithm (b) scales. (c) and (d) Transfer evolution by UV/vis irradiation in linear (c) and logarithm (d) scales.

Figure 7 -

 7 12a, the starting point was the state possessing the highest current level: S1-1 Pdown/DAE_O. The device was first exposed to UV light to switch the DAE molecules from their ring-open to their ring-closed form. With some electrons trapped by the ring-closed isomer, a decrease of the current was observed, and the device reached a second state S1-0: Pdown/DAE_C. Next, the polarization direction of the polymer was switched from downward to upward by applying a negative top gate bias (-60 V). This process further depleted the electrons in the channel and the device thereof reached its lowest current level, corresponding to a third state S0-0: Pup/ DAE_C. Subsequently, vis light was used to switch the DAE molecules back to their ring-open form. In the channel, the trapped electrons were released, and the current was partially recovered, reaching a fourth state S0-1: Pup/DAE_O. Ultimately, by applying a positive gate bias (+60 V), the P(VDF-TrFE) was switched to downward direction, electrons would again be accumulated, reversible current modulation behavior provides unambiguous evidence that the optical switching can be incorporated in an orthogonal manner with the electrical switching.

Figure 7 - 12 .

 712 Figure 7-12. Transfer evolution by alternative UV/vis irradiation and ±60 V bias under different stimuli order.

Figure 7 - 13 .

 713 Figure 7-13. Current modulation efficiency at Vbg = 60 V during the 4 switching cycles with different stimuli orders.

Figure 7 - 14 .

 714 Figure 7-14. (a) and (b) Dynamic Ids current switching behavior induced by applying alternating Vtg pulses of ±60 V at Vbg = 60 V and Vds = 2 V during (a) 20 cycles and (b) one single cycle. (c) and (d) Dynamic Ids current switching behavior induced by applying alternating UV and vis illumination at Vbg = 0V and Vds = 4 V during (c) 20 cycles and (d) one single cycle.

Figure 7 - 15 .

 715 Figure 7-15. Blank experiment on WSe2/FeFET without DAE molecules. a) Transfer evolution after UV/vis illumination. b) Dynamic current switching behavior under UV/vis illumination during 4 cycles.

Figure 7 - 16 .

 716 Figure 7-16. Multilevel storage between S1-1 and S0-1. a, c) Transfer evolution when partially polarizing the P(VDF-TrFE) from upward to downward direction (a), and downward to upward direction (c) by sweeping top gate at various ranges, with DAE in ring-closed form; b, d) Ids current at Vg = 60 V in (a) and (c) as a function of programmed sweep.

Figure 7 - 1 .

 71 Figure 7-16 depicts the transfer evolution from S1-1 to S0-1 programmed by various top gate sweeps

Figure 7 - 17 .

 717 Figure 7-17. Multilevel storage between S0-0 and S1-0. a, c) Transfer evolution when partially polarizing the P(VDF-TrFE) from upward to downward direction (a), and downward to upward direction (c) by sweeping top gate at various ranges, with DAE in ring-closed form; b, d) Ids current at Vg = 60 V in (a) and (c) as a function of programmed sweep.

Figure 7 - 19 .

 719 Figure 7-19. Dynamic Ids current evolution behavior under exposure of short UV pulses at Vbg = 0 V and Vds = 4 V: (a) totally obtained 84 levels and (b-d) enlarged 5 levels in region Ⅰ-Ⅳ.

Figure 7 - 20 .

 720 Figure 7-20. (a) Multilevel current over five cycles by different stimuli orders. (b) Enlarged levels in region A.

Figure 7 - 21 .

 721 Figure 7-21. Memory characteristics of DAE_1/WSe2/FeFET. a) Endurance performance of 5 chosen states during 10 voltage/illumination cycles. b) Time dependent retention characteristics of the 5 states.

Figure 8 - 1 .

 81 Figure 8-1. (a) chemical structure of SP derivative, (b) SP film on Si/SiO2 substrate, (c) protocol of SP film formation of Si/SiO2 substrate.

Figure 8 - 2 .

 82 Figure 8-2. Optical microscopy images captured at four stages during the fabrication process. (a) Target WSe2 flake with the underneath SP film, (b) SP/WSe2 FET, (c) SP/WSe2/FeFET with P(VDF-TrFE) film, (d) SP/WSe2/FeFET with evaporated 20 nm semi-transparent top gate Au electrode.

Figure 8 -

 8 Figure 8-2 shows the optical microscopy images recorded at four stages during the fabrication process:

Figure 8 - 3 .

 83 Figure 8-3. Schematic diagram of the SP/WSe2/FeFET device with double-sided decoration: the bottom surface with SP film and the top surface with P(VDF-TrFE) layer.

Figure 8 - 4 .

 84 Figure 8-4. Optical characterization of the SP photoswitching. (a) Chemical structures and photoisomerization reaction between SP and MC isomer. (b) Absorption spectra of SP film on quartz substrate during the course of UV irradiation. (c) to (f) Kinetics of the SP film, as monitored by plotting the absorbance at 550 nm under UV irradiation and afterwards (c) in the dark, (d) under vis irradiation, (e) under heat, (f) in the combination of dark, vis irradiation and heat treatment.

Figure 8 -

 8 4c show that ca. 37.5% of the molecules in the MC form were transformed to SP after 11 hours in the thermal condition. The switching process could be accelerated by vis light irradiation, which lasted ca. 45 min to convert all the MC into SP. Heating turned out being the most efficient stimuli as evidenced by a kinetic which required only 4 min. When the 3 routes were combined in the same measurement, the different kinetics can be clearly singled out.

Figure 8 - 5 .

 85 Figure 8-5. Fitting of the time evolution of absorbance at 550 nm with the equation Abs550nm (t) = Abssat -B × exp (-t/τhν). Red dots are experimental data and blue lines are fitted curves. The values extracted from the fit for Abssat, B and τhν are listed in Table 8-1.

Figure 8 - 6 .

 86 Figure 8-6. Water contact angle of (a) untreated Si/SiO2 substrate, (b) SP treated Si/SiO2 substrate, (c) UV treated SP/Si/SiO2 substrate, (d) Heat treated SP/Si/SiO2 substrate. The measurement was done by Krüss DSA 100 upon deposition of a 2 μL drop of Milli-Q water on the substrate surface.

Figure 8 - 7 .

 87 Figure 8-7. Topographical AFM image of SP treated Si/SiO2 substrate on an area of (a) 5 x 5 µm 2 , (b) 1 x 1 µm 2 . The panel below shows a representative height profile. The measurement was carried out by a Nanoscope (Veeco Multimode V) in tapping mode under ambient conditions.

Figure 8 - 8 .

 88 Figure 8-8. (a) Optical microscopy image of a SP/WSe2 flake (left) and its PL intensity mapping (right) integrated from 560 nm to 650 nm as prepared, after UV irradiation, and after laser irradiation. (b) PL spectra evolution in the points labeled as A and B. (c) same flake with PL intensity mapping integrated from 750 nm to 900 nm. (d) PL spectra evolution in the points labeled as C (bilayer) and D (few-layer).

Figure 8 -

 8 Figure 8-9 and 8-10 shows the PL evolution of SP/WSe2 flakes with monolayer and few-layer thickness.

Figure 8 - 9 .

 89 Figure 8-9. (a) Optical microscopy image of a SP/WSe2 flake (left) and its PL intensity mapping (right) integrated from 560 nm to 650 nm as prepared, after UV irradiation, and after laser irradiation, (b) PL spectra evolution in the points labeled as A and B, (c) same flake with PL intensity mapping integrated from 750 nm to 900 nm, (d) PL spectra evolution in the points labeled as C (monolayer) and D (few-layer).

Figure 8 - 10 .

 810 Figure 8-10. (a) Optical microscopy image of a SP/WSe2 flake (left) and its PL intensity mapping (right) integrated from 560 nm to 650 nm as prepared, after UV irradiation, and after laser irradiation, (b) PL spectra evolution in the points labeled as A and B, (c) same flake with PL intensity mapping integrated from 750 nm to 900 nm, (d) PL spectra evolution in the points labeled as C (few-layer).

Figure 8 - 11 .

 811 Figure 8-11. AFM image of a SP/WSe2 flake, (b) transfer evolution of SP/WSe2 FET as prepared and after UV, heat treatment.

8. 3

 3 .2.2. SP/WSe2/FeFET a. SP/WSe2/FeFET in top-gate configuration To add a further functionality for the as-fabricated optically switchable SP/WSe2 FET, the upper surface of WSe2 was then decorated with a ferroelectric polymer P(VDF-TrFE), followed by the deposition of a semi-transparent top-gate electrode (Au, 20 nm), to control the polarization direction of the P(VDF-TrFE) film. The transistor properties in top-gate configuration with P(VDF-TrFE) serving as dielectric material was first characterized (Figure 8-12a), which reveals obvious electron-transport-dominated ambipolar behavior. The corresponding leakage current is depicted in Figure 8-12b, with two apparent current peaks occur near the coercive field of P(VDF-TrFE), verifying that P(VDF-TrFE) has good dielectric and ferroelectric characteristics.

Figure 8 - 12 .

 812 Figure 8-12. Electrical characterization of SP/WSe2/FeFET in top gate configuration with P(VDF-TrFE) serving as dielectric (a) transfer curve, (b) corresponding leakage current.

Figure 8 - 13 .

 813 Figure 8-13. (a) Optical/thermal switching capacity of SP/WSe2/FeFET by UV, and heat/vis/dark treatment. (b) Current modulation efficiency at Vbg = 90 V during the 3 switching cycles.

Figure 8 - 14 .

 814 Figure 8-14. (a) Electrical switching capacity of SP/WSe2/FeFET by ±80 V top gate bias, the inset shows the zoomed curve when P(VDF-TrFE) in upward direction. (b) Current modulation efficiency at Vbg = 90 V during the 3 electrical switching cycles.

  electrons in the channel were totally exhausted and the final device turned into p-type. Successive vis irradiation and positive bias led to the restoration of the original performance. Then the stimuli order was switched to first polarize the P(VDF-TrFE) into the upward direction, then switch the SP molecules into MC. The current modulation efficiency at Vbg = 90 V was plotted in Figure8-15b. As expected, similar results were obtained, confirming that the two switching elements were able to work independently without any interference.

Figure 8 -

 8 Figure 8-15. (a) Orthogonal switching of SP/WSe2/FeFET by UV/heat/vis/±80 V top gate bias, the inset shows the zoomed curve of MC/Pdown, and MC/Pup state. (b) Current modulation efficiency at Vbg = 90 V during the 2 switching cycles.

  photoisomerization of SP, lowering the current. Such current modulation cycle can be operated for at

Figure 8 - 16 .

 816 Figure 8-16. Dynamic current tracking under alternative -80V, +80V and UV illumination conditions at Vbg = 20 V, Vds = 4 V of SP/WSe2/FeFET during 5 cycles.

Figure 8 - 17 .

 817 Figure 8-17. Cyclic voltammetry curves of (a) SP, (b) MC and (c) ferrocene (FC), (d) UV-vis absorption spectra of SP/MC in toluene.

Figure 8 - 18 .

 818 Figure 8-18. Energy level diagram of electron transport between SP/MC and WSe2.

Figure 8 - 19 .

 819 Figure 8-19. Optical switching capacity of a p-type SP/WSe2 FET device.

Figure 8 - 20 .

 820 Figure 8-20. Hysteresis characterization of (a) bare WSe2, (b) SP/WSe2 and (c) MC/WSe2 FET under different scan rates, (d) Hysteresis comparison between the three conditions.

Figure 8 - 21 .

 821 Figure 8-21. Hysteresis characterization of (a) bare WSe2, (b) SP/WSe2 and (c) MC/WSe2 FET under different scan ranges, (d) Hysteresis comparison between the three conditions.

First

  and foremost, I want to thank my supervisor Prof. Paolo SAMORì , who accepted me as a master student as later a PhD student in his research team. I feel great fortune to work under his guidance and have received relentless assistance from him throughout these years. I am grateful for the patience, trust, and freedom he gave me in the scientific areas. Moreover, I highly appreciate his suggestion and guidance for my further work. I am also thankful to the jury members M. David BELJONNE, M. Dominique VUILLAUME, and M. BIANCO Alberto, for accepting to be referees and dedicating time to this thesis. I will always be grateful to Dr. Marco Gobbi, my supervisor during the master internship, who guided into the world of 2D materials. He imparted me important theoretical basis and experimental techniques, which has benefited me throughout the PhD research. And Dr. Simone Bertolazzi, my co-supervisor during the first year of the PhD. With an extreme patience and a religious altitude towards the science, he had a positive impact on my research career. Many special thanks to my colleagues in the Nanochemistry Group, from whom I received strong support. All of them will remain in my memory, for their skills in the lab and sincere friendship. I am deeply grateful to Dr. Zhaoyang Liu, Dr. Rafael Furlan de Oliveira, Stefano Ippolito and Dr. Yuda Zhao, whom I can always go to whenever have doubt or question. Their valuable scientific suggestions had avoided many detours on my research road. My collaborators from Berlin also played an important role, who synthesized molecules with interesting features. My work would not have been the same without their support. It has been an enormous privilege to receive funding from Chinese Scholarship Council. Last but not the least, I thank people in my family for their endless love, tenderness, and support, especially my parents. I owe them everything. Also, I thank my bestie Lu and her little baby Simon, who brought me great joy and energy in life outside the work. Haixin

Table 5 - 1 .

 51 Calculated standard deviation for each current level presented in Figure5-13.

		Level 0 Level 1 Level 2 Level 3 Level 4
	Ids cycle 1 (μA)	10.22	6.01	4.89	4.37	3.96
	Ids cycle 2 (μA)	10.13	6.03	4.90	4.36	3.96
	Ids cycle 3 (μA)	10.08	6.13	4.88	4.37	3.99
	Ids cycle 4 (μA)	10.15	6.02	4.97	4.43	3.94
	Ids cycle 5 (μA)	10.08				3.92
	Ids average (μA)	0.05	0.05	0.03	0.03	0.02
	Standard deviation (μA)	10.13	6.05	4.91	4.38	3.96
	Standard deviation/ Ids gap	0.008	0.008	0.005	0.004	0.004

Table 5 - 2 .

 52 Calculated standard deviation for Ids_1o and Ids_1c in the Figure 5-13.

		Average Ids (µA)	Standard deviation (μA)	Standard deviation/ Ids difference
	Ids_1o	0.90	0.004	0.005
	Ids_1c	0.11	0.011	0.013

Table 5 - 3 .

 53 Parameters for the bias stress fitting curve.

		Value	Standard deviation
	y0	7.34E-08	3.27E-10
	A1	7.83E-08	5.66E-10
	t1	29.74977	0.5013
	A2	2.84E-07	3.15E-10
	t2	974.05481	3.50119

Table 6 -

 6

1. Calculated standard deviation for each current level presented in Figure 6-15a.

Table 6 - 2 .

 62 Calculated standard deviation for each current level presented in

  other words, in view of such reciprocal energy levels, electron transfer from WSe2 to DAE_1C is foreseeable by UV irradiation, and trapped electrons can get restored in WSe2 by subsequent vis

	irradiation.
	7.2.2. Device fabrication
	7.2.2.1.	P(VDF-TrFE) capacitor

Table 7 -

 7 drain current at Vbg = 60 V for these 5 states during 10 cycles. Interestingly, we observed only a minor fluctuation for each state and no deterioration, indicating that each programmed current level is highly reproducible. The retention property for each state was evaluated by independently monitoring the current as a function of storage time. It is worth noting that the device is kept without perturbation of electric field and illumination. As illustrated in Figure7-21b, P(VDF-TrFE)

	Level	Ids (µA)	Cycle1 Cycle2 Cycle3 Cycle4 Cycle5	Standard deviation
	S1-1	6.77	6.78	6.77	6.77	6.71	0.03
	S1-0.55	5.20	5.27	5.27	5.21		0.04
	S1-0.35	4.24	4.24	4.25			0.01
	S1-0.15	3.49	3.49				0.00
	S1-0	2.90
	S0-1					2.20
	S0-0.55				2.02	1.98	0.03
	S0-0.35			1.70	1.70	1.75	0.03
	S0-0.15		1.40	1.44	1.39	1.14	0.03
	S0-0	1.11	1.16	1.21	1.17	1.13	0.04

1. Calculated standard deviation for each current level presented in Figure 7-19 in the main text.

Table 8

 8

-1.

Table 8 - 1 .

 81 Parameters extracted from the fitting curves of the photoisomerization kinetics.

	8.3.1.2.	Water contact angle
	Water contact angle measurements of pristine, SP-treated, and after UV/heat treatment SiO2 surfaces
	are displayed in Figures

  SP to MC isomerization exhibit a drop of the contact angle by 15.2°. The value could be recovered by further heating treatment, confirming the reversible nature of the switching from MC to SP

		Abs sat	B	τ hv
	UV	0.00811	0.00395	1.0273
	dark	0.00627	-0.00159	248.89
	vis	0.00497	-0.00549	1253.97
	heat	0.00491	-0.03539	85.29

infé rieure avec film DAE_1 et la face supé rieure avec couche P(VDF-TrFE). (b) Sché ma de principe du dispositif SP/WSe2/FeFET avec dé coration double face: la face infé rieure avec film SP et la face supé rieure avec couche P(VDF-TrFE).

cycles with a good endurance. The electrical characterizations provide unambiguous evidence that

Acknowledgements ...........................................................................................................................

Symbols and abbreviations

XIV

Acknowledgements

Acknowledgements

The moment has finally arrived, the end of the thesis, the end of my life in France, for nearly 8 years. I would like to express my great gratitude to people I have met during this challenging and exciting PhD experience.

over 4 cycles. Level 0 is the current state when DAEs are all in their open form, corresponding to the current value obtained at Vg = 30 V in the transfer measurement of DAE_1o. Level 1 is reached upon 2 s of UV irradiation, and Levels 2, 3 and 4 are obtained by further UV irradiation for 4 s, 8 s and 16 s, respectively. Note that the entire UV irradiation time (30 s) is the same as we used in the former illumination cycles and Level 4 converges to the exact value of Ids when DAEs are all in closed form.

We performed 4 switching cycles; the standard deviation of each level is calculated and then divided by the total current difference between DAE_1o and DAE_1c (Table 5-1). The value obtained is below 1% for all these 5 levels, indicating that the device can attain a multitude of current levels with a high accuracy readout. By incorporating such molecules in a conventional FET, novel stimuli-responsive capacity can be imparted to the device thereby enabling to manipulate their electrical output by means of various external inputs. Such approach is instrumental for various applications in advanced electronic devices such as memories, [4] sensors [5] and smart logic gates. [6] Recent efforts have been dedicated to the fabrication of stimuli-responsive devices based on 2D TMDs, which are atomically thin semiconductors possessing extraordinary electronic properties compared to their organic counterparts. [7] Their unique physico-chemical characteristics have endorsed TMDs as promising channel material in low-power and high-performance digital electronic switches. [8] Moreover, due to their high surface-tovolume ratio, monolayer and few-layer TMDs have an exquisite surface sensitivity to any subtle surrounding environmental change, which opens up the opportunity to modify their intrinsic properties. [9] Recent studies involving stimuli-responsive molecules to decorate the surface of TMDs demonstrated this approach as a promising strategy to non-destructively modulate the charge carrier density by translating the stimuli-triggered changes at the molecular level to the modification of TMD's properties without introducing any defect. [10] Among the possible external stimuli, light and electrical inputs are particularly interesting since they are efficient, non-invasive, and spatiotemporally precise. Photochromic molecules are optically responsive systems able to undergo reversible photoisomerization when irradiated with light at specific wavelengths. [11] Hitherto they have been utilized as optical switching elements to translate photonic information into electrical output in TMD-based FETs. [12] We recently reported a proof of this concept by exploiting tailor-designed photochromic DAE molecules to reversibly tune the charge carrier transport in few-layer WSe2 and demonstrated a potential application of the corresponding FET as optically switchable multilevel non-volatile memory. [12a, 12b] On the other hand, ferroelectric materials have been employed to precisely control the doping level of 2D materials. [13] According to the direction of the external electric field, their intrinsic bistable polarization can be reoriented, leading to an accumulation or depletion of the charge carriers in the interfaced semiconducting material. [14] Interestingly, these ferroelectric field-effect transistors (FeFETs) take advantage of the binary remnant polarization states of the ferroelectric to store information and therefore have successfully been utilized in practical non-volatile memories. [15] To date, multifarious molecularly functionalized 2D-mono-stimuli-responsive FETs have been reported, in which direct deposition of functional molecules on top of the upper channel surface served as effective and easily addressable approach to fabricate the device. [16] The inversed device structure could also be obtained by first depositing the molecules on a suitable substrate, followed by transfer of the 2D materials. [17] In all the previous studies only one face of the 2D materials was functionalized by molecules and thus a potential asymmetry arising from unequal exposure of the two surfaces to two different local environments was not exploited. Such a scenario can be achieved by the independent functionalization of each surface with a different functional molecule, [18] providing intriguing perspectives for the realization of multi-stimuli-responsive FETs.

Along these lines, here we report an unprecedented approach based on the decoration of few-layer WSe2 flakes with two different stimuli-responsive (macro)molecules thereby generating a Janus 2D material.

In particular, the bottom surface was sitting on a light-responsive DAE layer and the top surface was coated with an electrically responsive copolymer layer of P(VDF-TrFE). In this way, a multi-stimuliresponsive FET has been fabricated in which the output current could be reversibly and precisely modulated by means of either light irradiation or an electric field. Compared with conventional monostimuli-responsive FETs, our novel ternary device architecture combines enhanced functional complexity with fabrication simplicity. We provide unambiguous evidence for the compatibility of the device response to both optical and electrical stimuli, thereby offering an orthogonal handle for operating the device, which is beneficial for memory applications as well. By taking advantage of the synergic effect between the two (macro)molecular switches, the device has attained an enhanced modulation of the output current with an efficiency reaching up to 87%. Furthermore, with each stimulus manifesting a large number of different outputs without mutual interferences (9 electricinduced levels and 84 light-induced levels), the device yields a total number of 756 distinct states, outperforming the current state-of-the-art mono-stimuli-responsive transistors thereby providing a SiO2 and r = 10 for P(VDF-TrFE).

Results and discussion

Characterization of DAE molecules, WSe2 flake and P(VDF-TrFE) film

The surface morphology of the DAE-coated Si substrate was studied by AFM, which reveals a rather flat film without huge aggregates, only some small dots scattered onto the surface. The room-mean squared roughness Rq as determined on a 5 x 5 µm 2 area is of 0.39 nm. The value is enhanced compared with bare Si substrate, which is around 0.2 nm. (Figure 7-5a). [26] The thickness of the target WSe2 flake was also determined by AFM, which was found to be 5. suggesting a good quality of the material. [27] When it comes to optical switching, starting from S1-1, the device was exposed to a periodic short time UV pulses. The corresponding dynamic current behavior was displayed in Figure 7 ferroelectric states can be generated at each fixed DAE state. Therefore, with 84 photogenerated levels, the device can ideally yield a maximum number of 756 current levels, corresponding to a memory with a data storage capacity exceeding 9 bit.

Chapter 9: Conclusion and perspectives

Due to their atomic thickness, large surface-to-volume ratio, free of dangling bonds, and excellent propensity to act as platform for molecular adsorption, 2D semiconductors (2DSs) are undoubtedly an ideal building block for the fabrication of high-performance stimuli-responsive systems and devices. In this thesis, we have investigated the fabrication of multifunctional field-effect transistors (FETs) by interfacing stimuli-responsive molecules with 2DSs.

In the first experimental chapter, optically switchable In the second experimental chapter, based on the previous demonstrated applicability and effectiveness of 2DS/DAE approach for charge carrier modulation on unipolar 2DS, we tackle full ambipolar 2DS to realize a simultaneous remote control of two charge carriers. Here we target on few-layer WSe2 flakes with a more balanced ambipolar behavior, with its electron and hole current on the same scale (10 -6 A).

We propose an optically switchable multilevel FET by interfacing WSe2 with a bicomponent DAE different stimuli-responsive (macro)molecules thereby generating a Janus 2DS structure. In particular, the light-responsive DAE_1 layer was shifted on the bottom surface of WSe2 to preserve the lightresponsive capacity, and the top surface was coated with an electric-responsive copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) layer. With such architecture a multistimuli-responsive FET has been fabricated in which the output current could be reversibly and precisely modulated by means of either light irradiation or electric field. Compared with conventional mono-stimuli-responsive FETs, this novel ternary device architecture combines enhanced functional complexity with fabrication simplicity, offering an orthogonal handle over the device operation. Thanks to the synergic effect between the two (macro)molecular switches, the device has attained an enhanced modulation of the output current, with an efficiency reaching up to 87%. When integrating to memory application, with 9 electric-induced levels and 84 light-induced levels, the device yields a total number of 756 distinct states, outperforming the current state-to-art mono 2D stimuli-responsive transistors thereby providing a decisive step forward to address the demand for high density storage memories.

The reliability of each state is further reflected by the endurance and retention tests for 5 arbitrary states, exhibiting a notable endurance for over 10 cycles and an excellent data retention exceeding 1000 hours.

Moreover, compared with cumbersome synthesis of single molecules incorporating different functional groups to realize the multi-addressable elements, this Janus modified ternary structure relies on a simple fabrication process and it appears to be a more universal strategy that can be extended to other 2DSs and functional molecules, offering a versatile platform for the construction of multi-stimuli-responsive devices.

In the above chapter, the physiosorbed DAE layer provides a promising way towards non-destructive functionalization of the channel material. However, the weak van der Waals interaction between the molecules and the Si/SiO2 substrate can result in easy removal of the molecules during a simple rinsing procedure. As a consequence, conventional photolithography cannot be exploit with such device architecture, and the choice of the second stimuli-responsive element is also limited. To overcome this issue, in the final experimental chapter, we designed and synthesized a silane-terminated SP derivative, which can densely pack on Si/SiO2 substrate through a strong covalent silylation. Such nonvolatile reversibly controlled not only by its internal electric field, but also by various external stimuli including light, heat and electric field. The projected saturation of Moore's law and the expiration of Dennard scaling has highlighted the demand for integrating novel analog functionalities within semiconductorbased devices. Stimuli-responsive molecules, which are characterized by spatial, temporal and dosage control, can be responsive to many other stimuli, like pH, redox, enzyme, magnetic field, etc, and are available commercially or through chemical synthesis. Therefore, such combination of 2DS with the virtually infinite number of stimuli-responsive molecular systems, offers countless opportunities to engineer the hybrid with characteristics on demand for device applications. In such hybrids, new functionalities can be introduced in a controlled manner, thereby opening further possibilities for developing multiresponsive devices. With more stimuli being applied into single transistor, the device turns increasingly intelligent and controllable.

In the future, the control over the nanoscale molecular ordering might be further investigated. By mastering molecular self-assembly it would be possible to interpret specific functional groups at predetermined spatial locations with an atomic presision, enabling a direct correlation with the modulation in the electrical characteristics of the host 2DS. Moreover, our molecular chemistry approach can be extended to van der Waals heterostructures that are composed of superimposed different 2DSs with "corrected" and optimized properties. Molecules can be incorporated into these layered structures to generate novel hybrid systems in which various molecular capabilities are conferred to different 2DSs.

Furthermore, to promote the industrialization, instead of mechanical exfoliation, CVD-synthesised 2DS

with controlled thickness or solution-processed high-quality 2DS might be integrated into the device to ease of fabrication and allow many integrated devices over large areas. Towards the improvement of the quality of our lives, the development of the defined hybrid structure shall be focused on wearable, portable, and foldable multiresponsive electronic devices, opening a new technological era. All in all, the full exploitation of the infinite options offered by molecular science in terms of functional diversities shall yield to the emergence of new, disruptive and exotic technologies based on 2DS or related heterostructures.

Statement of work

All experiments and data analysis reported in this thesis have been carried by myself, except for the following:

Chapter 4: The synthesis of DAE molecules was carried out by Dr. Martin Herder (Berlin).

Chapter 5: The AFM study was performed by Dr. Yuda Zhao (Hangzhou).

Chapter 6: The AFM study was performed by Stefano Ippolito.

Chapter 7: The AFM study was performed by Stefano Ippolito.

Chapter 8: The synthesis of SP molecules was carried out by Dr. Agostino Garanti (Bristol). The AFM study was performed by Stefano Ippolito. The CV measurement was carried out by Dr. Zhaoyang Liu.

Prof. Paolo Samorì helped on designing various experiments and data analysis through very helpful discussion.

List of publications

List of communications

Optical tuning of hole and electron transport in ambipolar WSe2 interfaced with a bicomponent photochromic layer: from high mobility transistors to flexible multilevel memories.

Haixin Qiu, Zhaoyang Liu, Yifan Yao, Martin Herder, Stefan Hecht, and Paolo Samorì .