Dans un premier temps, nous avons synthétisé des feuillets 2D du graphène, MoS2, GaS, GaSe et InSe en utilisant deux techniques : la méthode d'exfoliation mécanique et la méthode de collage anodique. Nous avons modifié la méthode d'exfoliation mécanique en utilisant un substrat chaud (100 °C -125 °C) pour augmenter la force d'adhérence entre les précurseurs et le substrat. Pour notre procédé de collage anodique, nous avons obtenu des paramètres optimaux pour chaque matériau par la préparation d'un très grand nombre d'échantillons. Des matériaux 2D de grande surface et de haute qualité ont été sélectionnés pour la fabrication de dispositifs. Dans un deuxième temps, ces matériaux 2D ont été caractérisés en utilisant plusieurs outils comme la microscopie optique, la microscopie à force atomique (AFM), la microscopie Raman et de Photo-Luminescence (PL) et la microscopie électronique à transmission (TEM). Le contraste optique, étudié expérimentalement et par simulation, peut être facilement utilisé pour identifier le nombre de couches. Ceci est un large gain de temps par rapport à la caractérisation AFM. À partir des spectres Raman, des propriétés de vibration des matériaux 2D ont été obtenues en fonction du nombre de couches. À partir des spectres PL, les changements de largeurs de bande en fonction du nombre de couches ont été observés dans ces matériaux. Par exemple, dans MoS2, la bande interdite présente une transition indirectedirecte lorsque l'épaisseur de MoS2 diminue de 2 à 1 feuillet. Toutefois, pour InSe, la bande interdite a une transition inversée, présentant une bande interdite directe pour le matériaux massif et indirecte pour des épaisseurs inférieures à 7 couches. La microscopie TEM a été utilisée pour examiner la structure du réseau de GaS, GaSe et InSe et en accord avec les analyses Raman de confirmer le polytype ε de notre précurseur de InSe. Dans un troisième temps, des photo-détecteurs basés sur quelques feuillets de MoS2, GaSe et InSe ont été fabriqués et caractérisés avec une longueur d'onde d'excitation lumineuse de 532 nm. Pour le photo-détecteur basé sur quelques feuillets de MoS2, une photosensibilité de 316 A/W et un rendement quantique externe (EQE) de 6,96×10 4 % ont été obtenus dans notre dispositif. Ceux-ci sont beaucoup plus élevés que les performances publiées pour les photodétecteurs basés sur MoS2 multicouche. Nous avons fabriqué des photo-détecteurs basés sur quelques feuillets de GaSe sur substrats de verre ainsi que sur substrats SiO2/Si, soit avec un masque physique soit par lithographie électronique, et mesuré la distribution de photo-courant. Pour les photo-détecteurs basés sur quelques feuillets de InSe, nous avons fabriqué les dispositifs avec des contacts en carbone amorphe ou or et étudié le mécanisme de fonctionnement et la distribution de photo-courant. Enfin, des hétéro-structures à base de graphène ont été fabriqués afin d'améliorer les performances par rapport aux photo-détecteurs basés sur les semi-conducteurs 2D seuls. Ceci donne l'avantage supplémentaire de protéger les semi-conducteurs 2D ultra-mince des dégradations à l'air ambiant. Pour la première fois, nous avons fabriqué un photo-détecteur à partir d'une hétéro-structure graphène/InSe qui a montré des performances largement améliorées par rapport au dispositif sans le graphène. La photosensibilité et le EQE sont augmentés par quatre ordres de grandeur et la couche ultra-mince de InSe n'est plus dégradée par les conditions ambiantes. De plus, des photo-détecteurs basés sur les hétéro-structures verticales graphène/InSe/graphène et graphène/InSe/Au ont été fabriquées par transfert par voie humide et transfert par voie sèche respectivement, et leurs performances analysées en l'absence de tension de grille et de tension de polarisation. Ces structures sont plutôt «fragiles» mais les deux montrent un effet photovoltaïque qui peut être facilement compris par l'alignement des bandes électroniques aux interfaces. Des hétérostructures InSe/MoS2 qui combinent des TMDCs avec des matériaux III-VI ont été fabriqués par transfert à sec pour la première fois. Les propriétés optiques de ces hétérostructures ont été analysés par simulation et mesurées expérimentalement par l'intensité des modes Raman. Un comportement de type photodiode a été observé alors que les porteurs majoritaires dans InSe et MoS2 sont des électrons. Ce dispositif a ensuité été amélioré par l'utilisation du graphène comme une couche de transport et un nouveau photo-détecteur avec une photo-réponse ultra rapide, une distribution uniforme de courant photoélectrique et un fort effet photovoltaïque a finalement été obtenu. Le mécanisme correspondant a été expliqué par un modèle de dispositif simple et d'alignement des bandes.

Résumé

Le graphène a été étudié d'un point de vue théorique dès 1947 par P. R. Wallace et fabriqué expérimentalement en 2004 par K. Novoselov, A. Geim et leurs collaborateurs. C'est une monocouche de graphite dont les atomes de carbones forment un réseaux hexagonal en nid d'abeille. Le graphène possède de nombreuses propriétés électroniques, optiques, mécaniques et thermiques exceptionnelles, ce qui en fait un matériau phare de la famille des matériaux bidimensionnels (2D). Par exemple, la mobilité des porteurs dans le graphène peut atteindre 10 5 cm 2 /Vs à température ambiante, et 10 6 cm 2 /Vs à basse température. De plus, le module d'Young du graphène a été mesuré jusqu'à près de ~ 1 TPa, ce qui fait du graphène le matériau le plus dur sur terre.

Cependant, le graphène présente aussi des inconvénients notamment une bande interdite électronique nulle ainsi qu'une faible absorption de la lumière, ce qui limite ses applications dans l'électronique et l'optoélectronique. Les semi-conducteurs 2D similaires au graphène sont donc recherchés pour améliorer les performances des dispositifs. Les premiers feuillets 2D du semi-conducteurs MoS2 étudiée par K. F. Mak et al. en 2010 ont une bande interdite a couplage direct pour le mono-feuillet et indirecte du bi-feuillet au matériaux massif. Ce dichalcogénures de métaux de transition (TMDCs), a un fort potentiel pour être intégré dans des circuits électroniques, des photo-détecteurs et des diodes électroluminescentes (DEL). La découverte des propriétés du mono-feuillet de MoS2 à ouvert la voie à l'étude d'autres matériaux tels que les TMDCs WSe2, MoSe2 et WS2. Dans toute cette famille de matériaux, pour le mono-feuillet le minimum de la bande de conduction (CBM) et le maximum de la bande de valence (VBM) coïncident au point K, ce qui en fait des semi-conducteurs à bande directe ; alors que pour les multi-feuillets, le VBM est au point Γ ce qui en fait des semiconducteurs à bande indirecte.

Dans les trois dernières années, des semi-conducteurs 2D ont été exfoliés à partir des matériaux massifs GaS, GaSe, InSe, GaTe, SnS2 et phosphore noir, et les dispositifs optoélectroniques fabriqués à partir de ces feuillets ont montrés d'excellentes performances.

Contrairement au cas des TMDCs, ces semi-conducteurs 2D ont la caractéristique commune d'avoir une réponse Raman et de photoluminescence (PL) très faibles pour les mono-feuillets par rapport aux matériaux massifs. La largeur de la bande interdite dans les feuillets de InSe varie avec l'épaisseur de manière opposée au cas des TMDCs et change de directe pour le massif à indirecte pour les épaisseurs inférieures à 7 feuillets. Ces matériaux peuvent donc être utilisés avec quelques feuillets d'épaisseurs, mais un problème critique doit être affronté : ces couches ultra-minces peuvent se dégrader à l'air, contrairement au graphène et aux TMDCs 2D qui sont relativement stables.

Que pouvons nous obtenir en combinant le graphène et les semi-conducteurs 2D ou si nous empilant les semi-conducteurs 2D ensemble? Ce nouveau champ de recherche amène l'espoir que cette innovation pourra révolutionner l'électronique traditionnelle, et certaines prouesses technologiques ont déjà été réalisées dans ce sens. Par exemple, la photosensibilité d'un photo-détecteur basé sur une hétéro-structure graphène/MoS2 est améliorée par neuf ordres de grandeur par rapport au graphène et quatre ordre de grandeur par rapport à MoS2. De plus, les jonctions p-n ultra-minces ont récemment attiré beaucoup d'attention et des dispositifs tels que des diodes, des LED et des photodiodes ont été fabriqués. Un paramètre important dans les diodes à semi-conducteur est l'épaisseur de la jonction, située généralement entre 10 nm et 1 μm. Toutefois, dans les jonctions p-n ultra-minces de semi-conducteurs 2D, toute l'épaisseur de hétéro-jonction peut être dans l'échelle nanométrique, de sorte que l'effet tunnel quantique des électrons devient de plus en plus important par rapport à la dérive et à la diffusion des porteurs de charge dans les diodes à semi-conducteurs conventionnels. D'autres comportements uniques attendent sûrement d'être explorés dans les hétéro-structures ultraminces de semi-conducteurs 2D.

Dans cette thèse, nous avons étudié de nouveaux matériaux 2D au-delà du graphène. Ceuxci comprennent MoS2, GaS, GaSe et InSe. Le travail a commencé avec la préparation de ces échantillons 2D en utilisant conjointement la méthode traditionnelle d'exfoliation et notre méthode originale de collage anodique. Il a continué avec la caractérisation approfondie des matériaux de quelques feuillets d'épaisseurs, notamment par le contraste optique, les propriétés vibrationnelles et la structure du réseau atomique. Les matériaux 2D ont ensuite été utilisés pour fabriquer des dispositifs. Premièrement des photo-détecteurs homo-structuraux basés sur quelques feuillets de MoS2, GaSe ou InSe ont été fabriqués et analysés. Enfin, diverses hétéro-structures tels que : graphène/InSe, graphène/InSe/graphène, graphène/InSe/Au, InSe/MoS2 avec et sans couche de transport de graphène ont été fabriqués et leurs propriétés de photo-détection caractérisées par rapport aux dispositifs homostructuraux. La comparaison a montré une amélioration des performances et l'apparition de nouvelles propriétés telles que l'effet photovoltaïque.

General Introduction Scientific Context

Graphene was theoretically studied by P. R. Wallace as early as in 1947 [1] and experimentally prepared by K. Novoselov and A. Geim et al. in 2004 [2] . It is a monolayer of graphite with carbon atoms arranged in hexagonal honeycomb lattice. Numerous outstanding electronic, optical, mechanical and thermal properties [3][4][5][6] are found in graphene which makes it a representative material in the two dimensional (2D) material family. For example, the carrier mobility of graphene can be as high as 10 5 cm 2 /Vs at ambient temperature and 10 6 cm 2 /Vs at low temperature [7][8] . In addition, the Young's modulus of graphene has been found to be as high as ~ 1 TPa which makes graphene the highest strength material on the earth [6] .

However graphene also has drawbacks such as zero band gap and weak light absorption which limits its applications in electronics and optoelectronics. Graphene-like 2D semiconductors are thus expected to improve device performances. The first monolayer 2D semiconductor molybdenum disulfide (MoS2) investigated by K. F. Mak et al. in 2010 has a direct band gap in the monolayer and indirect band gap from bilayer to bulk [9] . MoS2 a transition-metal dichalcogenides (TMDCs), has the potential to be incorporated into digital circuits, photo-detectors and light-emitting diodes (LED) [10][11][12] . The discovery of the properties of single layer MoS2 open a door to the study of other TMDCs materials such as WSe2, MoSe2 and WS2. In the single layer of these materials, the conduction band minimum (CBM) and valence band maximum (VBM) coincide at the K point, making them direct band gap semiconductors. However in the multilayers, the VBM is at the Γ point which is above K point thus resulting in an indirect band gap [13][14] .

Are there any other 2D semiconductors which can expand the 2D family with potential in device application? In the last three years 2D semiconductors from bulk GaS, GaSe, InSe, GaTe, SnS2 and black phosphorus have been successfully exfoliated and the corresponding photoelectric devices fabricated with excellent performances reported [15][16][17][18][19][20][21] . In these 2D

semiconductors, the common feature as opposed to 2D TMDCs is that the Raman and photoluminescence (PL) spectra in the single layer are very weak compared to the bulk. The band gap has an inverse variation with respect to 2D TMDCs changing from direct in the bulk to indirect in the monolayer. These materials can thus be used in few layer form but a critical problem has to be confronted: ultrathin layers may be degraded in the air as opposed to graphene and 2D TMDCs which are relatively stable.

What will happen if we combine graphene with 2D semiconductors or stack 2D

semiconductors together? This is a new area and the expectation is that this innovation may bring a revolution in traditional electronics and some amazing achievements have been realized. For example, in a photo-detector based on graphene/MoS2 heterostructure, the photoresponsivity is improved by nine orders of magnitude with respect to graphene and four order of magnitude with respect to MoS2 [22] . In addition, recently, the ultrathin p-n heterojunction has attracted a lot of attention and devices such as rectifying diodes, photodiodes and LED have been fabricated [23][24][25] . A key parameter in semiconductor diodes is the thickness of the junction normally 10 nm to 1 μm. However in p-n ultrathin 2D semiconductors, the whole heterojunction thickness can be in the nm scale, so that quantum tunneling of electrons becomes increasingly important with respect to drift and diffusion of charge carriers in conventional semiconductor diodes [26] . More unique behavior is surely waiting to be explored in ultrathin 2D semiconductor heterostructures.

Objectives of this thesis

III-VI layered semiconductors GaS, GaSe and InSe have been the object of many theoretical and experimental investigations in the last few decades because of their photoelectric properties, and the corresponding photovoltaic, switching and memory devices have been fabricated [27][28] . In these materials, the layers are bonded with weak van der Waals force, which makes it possible to exfoliate 2D materials. Before we started work on this thesis, there were no experimental reports on 2D III-VI materials. This made it all the more meaningful and urgent to explore this untouched area according to the following sequence:

preparation, characterization and device application (photo-detector in our thesis) of few-layer GaS, GaSe and InSe.

As we said before, graphene is not the best material for photoelectric device because of the semimetal behavior. However, in the photo-detection area based on 2D semiconductors, the carrier mobility being relatively low (compared to graphene for example), efficiency can suffer due to fast recombination of photo-excited electron-hole pairs, leaving room for further improvement. With this in mind, devices can be fabricated with heterostructures of graphene stacked with 2D semiconductors to combine the conversion efficiency of the semiconductor with the charge transport characteristics of graphene. In this thesis, graphene/InSe and graphene/InSe/graphene heterostructures are fabricated and the performances of the devices are analyzed.

In addition to the graphene/2D semiconductor based heterostructures, 2D

semiconductor/2D semiconductor heterostructures are also potentially interesting. This research area is mainly focused on the p-n heterojunction of TMDC/TMDC semiconductors [23][24][25] . In this thesis, we extend this to our 2D III-VI materials, such as a InSe/MoS2 heterostructure, which opens a window in the study of interaction between III-VI materials to TMDCs.

Organization of the manuscript

The manuscript is organized in five chapters:

In chapter 1, the basic electronic structure and properties of graphene, MoS2, GaS, GaSe and InSe are introduced briefly. In addition, the state of art in 2D GaS, GaSe and InSe is reviewed.

In chapter 2, the experimental setups used in this thesis are discussed in detail. The experiment techniques include: Anodic boning and improved mechanical exfoliation of 2D samples; optical microscope, atomic force microscopy (AFM), Raman spectroscopy and transmission electron microscopy (TEM) for sample characterization; wet and "real" dry transfer process for device fabrication.

In chapter 3, 2D samples such as graphene, MoS2, GaS, GaSe and InSe are fabricated and characterized by the tools we discussed in chapter 2. It is worth noting that we solve an urgent problem concerning the degradation of ultrathin III-VI materials in ambient conditions by sealing with grapheme which gives a solution for the general problem of 2D material degradation.

In chapter 4, photo-detectors based on few-layer MoS2, GaSe and InSe are fabricated, performances analyzed, and their functioning explained by band alignment arguments. In our few-layer MoS2 photo-detector, we obtained the highest photoresponsivity compared with other few-layer MoS2 photo-detectors reported earlier.

In chapter 5, various heterostructures are fabricated and the corresponding device performances are characterized. For the first time a graphene/InSe heterostructure is fabricated and the photoresponsivity of the device increases several orders of magnitude compared with few-layer InSe. In addition, both graphene/InSe/graphene and Au/InSe/graphene heterostructures show a photovoltaic effect which is in contrast to the device based on 2D homojunctions. Moreover, the light out-coupling (total Raman signal measured from the heterostructure) in InSe/MoS2 is studied by both simulation and Raman characterization, and the performances of photo-detectors based on InSe/MoS2 and graphene/InSe/MoS2 heterostructures are analyzed. We obtain photodiode-like behavior and ultra-fast photo-response time.

Chapter 1 Introduction to 2D materials -graphene, MoS 2 , GaS, GaSe and InSe

In this Chapter, we will briefly introduce the electronic structures and fundamental properties of 2D materials such as graphene, MoS2, GaS, GaSe and InSe which we will fabricate, characterize and measure in this thesis.

Graphene

Graphene is the thinnest material in nature so far experimentally isolated and characterized in 2004 by A. Geim and K. Novoselov who won the Nobel Prize in 2010 [1] .

Graphene is a semi-metal with a peculiar electronic structure and exhibits outstanding electronic, optical, mechanical and thermal properties. In this section, we will introduce the electronic structure and properties of graphene from the photovoltaic and photoelectric device application point of view.

Electronic structure of graphene

Graphene is composed by carbon atoms arranged in a honeycomb structure. The sp 2 hybridization between one s orbital and two p orbitals results in a trigonal structure with a formation of σ bond between the nearest-neighbor carbon atoms. The structure of graphene can be seen in Figure 1.1 (a) with a triangular Bravais lattice with atoms A and B basis [2][3] .

The distance between the two neighbor atoms is a = 0.142 nm. The lattice vectors can be written as:

𝒂 𝟏 = 𝑎 2 (3, √3), 𝒂 𝟐 = 𝑎 2 (3, -√3) (1.1)
The three vectors δ1, δ2 and δ3 which connect a site on the A sublattice with B sublattice are: where t is the nearest-neighbor (A-B) hopping energy between different sublattices and t' is the second nearest-neighbor (A-A or B-B) hopping energy in the same sublattice. The energy spectrum of graphene with t= 2.7 eV and t'= -0.2 t is depicted in the right part of Figure 1.1 (c). However, when the band structure is close to the Dirac point, the energy dispersion can be written as:

𝜹 𝟏 =
𝐸 ± (𝜹𝒌) ≈ ±ħ𝜈 𝐹 |𝜹𝒌| + 𝑂[(𝜹𝒌/𝑲) 2 ] (1.7)
where ħ is the reduced Planck constant, νF is the Fermi velocity (≈ 1×10 6 m/s) and δk can be defined as k=K+ δk. It consists of two bands labeled by the index ± in equation 1. 5. The sign -corresponds to the valence band (π) and the sign + corresponds to the conduction band (π*).

The Fermi level is situated at the point (Dirac point K and K') where the π band touches the π* band.

Properties of graphene

In the following, the properties of graphene such as carrier mobility, electrostatic doping, transparency and chemical stability will be briefly introduced and the corresponding device applications mentioned.

High carrier mobility:

The relationship between carrier mobility and carrier density in graphene is 𝜎 = 𝑛𝑒𝜇, where σ is the conductivity, n is the carrier density and μ is the carrier mobility. Theoretically, μ can reach 2×10 6 cm 2 V -1 s -1 [4] , however this value decreases considerably with the presence of a substrate or charge impurities. Experimentally, mobilities of 10000-15000 cm 2 V -1 s -1 are measured in exfoliated graphene on SiO2/Si substrate [5] . A higher mobility of 6×10 4 cm 2 V -1 s -1 is found in the graphene on hexagonal boron nitride (h-BN) substrate [6] . All of these show a great potential in the high frequency device application.

In our photodetectors based on graphene/InSe and graphene/InSe/MoS2 heterostructure, the photoexcited electrons can flow toward the electrodes before recombination owing to the high mobility of graphene.

Electrostatic doping: For undoped graphene, the Fermi level is located at the Dirac point.

At this point, the resistance of graphene is very high because the number of carries is very low, as confirmed experimentally. The Fermi level needs to be moved through doping to change carrier density and resistance. In a graphene based field effect transistor (FET) device, the Fermi level can be moved easily by electrostatic doping using gate voltage. For example, in a graphene FET deposited on SiO2/Si substrate, SiO2 is a dielectric which allows a gate voltage application. A FET device and corresponding electrostatic doping results are shown in Figure Transparency: Graphene as a transparent conductor might have huge implications for the photovoltaic and photoelectric device in the future. As we known, indium tin oxide (ITO) and fluorine tin oxide (FTO) have been widely used as window electrodes in these devices.

However the element indium (In) is rare and transparency of ITO in the near-infrared region is limited. Graphene can be seen as an ideal transparent conductor in these devices. For example, transparent and conductive graphene electrodes are fabricated in dye sensitized solar cells (DSSC) with a high conductivity of 550 S/cm and a transparency of more than 70% over 1000-3000 nm [8] . Large area chemical vapor deposited (CVD) graphene was also prepared with very low resistance (~ 125 Ω/□) and very high optical transmittance (97.4%) and demonstrated in touch screen panel device [9] . In our photo-detectors, the transparency of the top layer is critical and we used graphene as the top layer in the heterostructure.

Chemical stability: Very recently, monolayer graphene has been used as a protective coating material to prevent oxidation of different underlying substrates [10] . This results from the fact that the genuine sp2 hexagonal distribution of carbon atoms provides an impermeable physical barrier that avoids substrate-environment interaction [11] . The applications of graphene as an anti-corrosion layer have been proposed for Cu, NiTi and other metals [10,12] .

As we know, some 2D materials are not stable in air, which will limit their applications. In our work we used graphene as an anti-corrosion layer which effectively protected few-layer InSe as well as serving for the transport of charge.

MoS2

MoS2, as a TMDC material, has been investigated for its potential in the field of energy conversion and storage, hydrogen evolution reaction and switching and optoelectronic devices [13][14][15][16] . Particularly, the monolayer of MoS2 has shown extraordinary properties in photoelectric and photovoltaic devices in recent years due to the direct band gap [17][18] . In this section, the electronic structure of MoS2 according to the number of layers is introduced and the properties of monolayer and multilayer MoS2 are also mentioned.

Electronic structure of MoS2

MoS2 is formed by S-Mo-S layers stacking together with weak noncovalent interactions.

Each layer consists of two close-packed sheets of S atoms and a sheet of Mo atoms. By the different stacking sequences, different polytypes of MoS2 are obtained such as 2H-MoS2 (two layers per repeat distance, space group of P63/mmc, 𝐷 6ℎ 4 ) and 3R-MoS2 (three layers per repeat distance, space group of R3m, 𝐶 3𝑣 5 ). For single layer MoS2 of course there is no polytype. In our experiment, we use 2H-MoS2 precursor and the corresponding structure is shown in Figure 1.3 (a-b). For the unit cell of 2H-MoS2, the lattice constant a is 3.16 Å and c is 12.9 Å [19] . The thickness of monolayer is 0.645 nm. The top view of 2H-MoS2 structure in The electronic structure of MoS2 evolves with the number of layers, an aspect which is critical for applications. From both theoretical calculations and photoluminescence (PL) as well as optical absorption experimental results [20][21] , monolayer MoS2 has direct band gap around 1.8 eV however the bilayer and above have indirect band gap. This behavior arises from the d-orbital related interactions in MoS2. The theoretical result from A. Splendiani et al.

is shown in Figure 1.3 (c-f) [20] . From this results, the direct excitonic transition energy barely changes at the K point of Brillouin zone with layer thickness, however the indirect band gap increases remarkably when the number of layers decreases and particularly, the indirect transition energy becomes higher than direct transition energy in monolayer MoS2 resulting in a direct band gap. In our PL spectra of MoS2, the results are in accord with this change.

Properties and applications of MoS2

Monolayer and few-layer MoS2 have interesting mechanical, optical absorption and thermal conductivity properties compatible with many applications such as flexible electronics, photo-detectors, superconductors, spintronic and valleytronics [17,[21][22][23] . In this thesis, we investigate the vibrational properties of few-layer MoS2 and fabricate a photodetector based on three-layer MoS2, with better performance compared to other reports on multilayer MoS2.

Mechanical properties: Monolayer MoS2 is supposed to have high mechanical strength and high Young's modulus as opposed to the bulk [24] . In addition, few-layer MoS2 shows high flexible properties in a transistor device, which has no degradation of electronic properties when MoS2 is significantly bent to a curvature radius of 0.75 mm [21] . From the first principle calculation, monolayer has piezoelectric properties which can be used in sensitive mechanical transducers [25] .

Optical absorption:

In photovoltaic device applications monolayer MoS2 can absorb up to 5-10% incident sunlight, which is one order of magnitude higher than the most commonly used solar absorbers such as GaAs and Si [26] . Based on this property, monolayer MoS2 heterojunction solar cells were fabricated with a power conversion efficiency of 5.23% [18] . In photoelectric devices, monolayer MoS2 also shows amazing performances. For example, an ultrasensitive photo-detector based on monolayer MoS2 was fabricated by O. Lopez-Sanchez et al. and the photoresponsivity of the device is up to 880 A/W [17] which is several orders of magnitude more than in graphene.

III-VI materials: GaS, GaSe and InSe

III-VI materials such as GaS, GaSe and InSe have been investigated in recent decades because of their interesting optical, photoelectric and other physical properties [27][28][29][30] . In these materials again each layer is bound by weak van der Waals force, thus they can be exfoliated to 2D materials. In III-VI 2D materials, just a few recent reports focus on preparation and photo-detection [31][32][33] and much remains to be done. In this thesis, we fabricated III-VI 2D materials and homostructural and heterostructural photo-detectors based on these materials for the first time. In this section, we will discuss the crystal structures, the band gaps and the potential and problems for applications of 2D III-VI materials.

Crystal structures of GaS, GaSe and InSe

Single layer GaS, GaSe and InSe have the same crystal structure which consists of four close-packed monoatomic sheets in the sequence of X -M -M -X where X = S or Se and M = In or Ga. However, different polytypes can be formed by stacking the single layers with particular sequences. The most commonly described polytypes are β, γ and ε with the corresponding unit cells shown in Figure 1.4 (a-c) respectively. For β structure, the unit cell is of hexagonal structure and with two-antiparallel layers obtained by a rotation of a layer by 180º and translation onto another. It belongs to the non-symmorphic 𝐷 6ℎ 4 space group and each unit cell contains eight atoms. This polytype has been experimentally observed in GaS, GaSe and InSe [34][35][36] . For ε structure, the unit cell is also of hexagonal structure but with two-parallel layers obtained by a translation of one layer onto another. It belongs to the symmorphic 𝐷 3ℎ 1 space group this time and each unit cell also contains eight atoms. This polytype has been experimentally observed in GaSe and InSe [27] . However for γ structure, the unit cell is of rhombohedral structure and with three-parallel layers obtained by two translations. It belongs to the symmorphic 𝐶 3𝑣 5 space group and each unit cell contains four atoms. It is worth noting that the structure in Figure 1.4 (c) is a non-primitive unit cell, which is transformed into a hexagonal system just for comparison of β and γ polytypes, and this nonprimitive unit cell contains twelve atoms. This polytype has also been experimentally observed in GaSe and InSe [35] . In this thesis, we distinguish ε polytype of our few-layer InSe by both Raman spectroscopy and TEM. In addition, the lattice constants of GaS, GaSe and InSe with the common polytypes are given in Table 1.1. From the c values in this table, we can confirm the thickness of monolayer GaS, GaSe and InSe of 0.77 nm, 0.80 nm and 0.85 nm respectively. Table 1. 1 The lattice constants a and c of GaS, GaSe and InSe with β, ε and γ respectively.

Polytype

Band gaps of GaS, GaSe and InSe

The band gaps of GaS, GaSe and InSe are well known in the bulk materials from both experiment and theory [38][39][40][41] . From experiments, the band gaps of bulk materials are: GaS, direct band gap of 3.04 eV; GaSe, direct band gap of 2.02 eV; InSe, direct band of 1.25 eV.

However, when the samples are exfoliated into 2D materials, the calculation results are not in accord with the experiment results until now [42] . Therefore, much work needs to be done in order to understand the band gap shift or transition of the materials from bulk to monolayer.

Experimentally, the band gaps have a shift when the thickness decreases from bulk to monolayer from PL characterization. For example, a blue shift of 20 meV and a significant intensity decrease are observed from bulk to bilayer GaSe, which is possibly due to the modification of band gap structure caused by the thickness decrease [43] . In addition, in fewlayer InSe, the band gap has a significant blue shift (up to 200 meV) from 1.25 eV in the thick layers (> 15 nm) to 1.47 eV in the seven layers, and no peaks are detected in the thinner layers, which indicates a direct-indirect band transition when the number of layers decrease from bulk to monolayer [44] .

Potential and problems for applications of ultrathin III-VI materials

Since the successful exfoliation of III-VI 2D materials such as GaS, GaSe and InSe a lot of studies have focused on photo-detection. For example, high performance and bendable fewlayer InSe photo-detector was fabricated with a broad spectral response (450-785 nm), high photoresponsivity (12.3 A/W) and fast response time (~50 ms) [33] . However, these materials are not like other 2D materials such as graphene and TMDCs. They are often unstable in ambient conditions and the monolayer is not the best choice in the photovoltaic or photoelectric device application due to its indirect band gap.

Stability: Ultrathin layers of III-VI materials are not stable and easily degrade in ambient condition as reported by A. K. Geim and I. V. Grigorieva in 2013 [45] . This degradation will critically affect the electronic transport properties in the device based on these ultrathin materials. For example 2D-GaSe can be degraded within few seconds, as seen by the increase in roughness from AFM and from electrical transport [46] . We effectively show from Raman characterization that ultrathin InSe is not stable in the air and give a solution to solve this critical problem.

Few-layer or monolayer: Are monolayer III-VI materials as useful in photovoltaic and photoelectric device applications as graphene and TMDCs such as MoS2? Up to now, there is no experimental evidence that monolayer GaS, GaSe or InSe has direct band gap. The PL signal is impossible to detect in the monolayer which is in contrast to few-layer samples. Fewlayer samples may have a direct band gap (at least for InSe, the direct band gap appears above seven layers) and also they are much easier to prepare and show greater stability in air.

Therefore, few-layer samples are the best choice in photovoltaic and photoelectric device application.

Chapter 2 Experimental Techniques

In this Chapter, we will describe all the experimental techniques used in this thesis. These include: precursors and sample preparation; characterization techniques including Raman spectroscopy and atomic force microscopy; nanolayer manipulation and transfer; device fabrication; photoelectrical measurements.

Precursors and sample preparation technique

All precursors we used to prepare 2D materials in this thesis are cut from high quality single crystals some coming from the mineralogy collection of the IMPMC/UPMC (Institut de minéralogie, de physique des matériaux et de cosmochimie/ Université Pierre et Marie Curie) and others synthesized earlier by A. Chevy et al. [1][2][3] . The substrates carrying the samples are borosilicate glass (Schott D263T 0.5 mm thick) and SiO2 (285 nm thick)/Si (pdoped by boron, 0.5 mm thick) respectively.

The optical images of bulk precursors, substrates and the corresponding Raman spectra (532 nm laser excitation) are shown in Figure 2.1. The G and 2D modes in the Raman shift of graphite assures of the high precursor quality used to prepare monolayer graphene for devices.

From the large size of single crystals of bulk semiconductors such as MoS2, GaS, GaSe and InSe in the optical images, very flat and clean surface are observed in the bulk materials which is very critical to prepare large areas of monolayer or few-layer samples. Moreover, all

Raman spectra indicate high quality bulk precursors without any impurity modes. In addition we also characterize the two kind of substrates by their Raman spectra. In glass this diffuse background can be deducted from measurements which show low sample intensity. In SiO2/Si substrate the peak from silicon at around 521 cm -1 is sharp and easily identified. Before sample preparation, obligatory substrate cleaning is normally divided into two steps: a 5 minutes ultrasound clean in acetone and a following 5 min ultrasound clean in ethanol, the substrates are finally dried using high pressure nitrogen gaz. Physical and chemical methods to fabricate few layers of materials exist such as mechanical exfoliation and chemical vapor deposition. In particular, physical methods have the advantage of easy and fast preparation, low cost and chemical stability. For our sample preparation, we used two kind of physical methods to prepare 2D samples: mechanical exfoliation method and anodic bonding method. In the following, we will describe the principle and the fabrication steps based on these methods. The detailed parameters used during the preparation processes are given in Chapter 3.

Mechanical exfoliation:

In recent years the most widely used method to prepare 2D materials has been the mechanical exfoliation method. High quality and surface clean samples are made on arbitrary substrates by using this method at room temperature. However, it is a random technique, whose yield can be affected by various conditions. In our sample fabrication, we improve this technique by adding a hot plate, which increases the adhesion possibility of ultrathin layers. The schematic of our method is shown in Anodic bonding method: Conventionally, Anodic Bonding refers to a simple and reliable bonding method used in microelectronics industry [4][5][6] . Its main advantage is the fact that it allows to join two macroscopic clean and optically polished surfaces, one of which is borosilicate glass (Pyrex for example) without the need for intermediate glue. Typically it allows to bond a Silicon substrate to glass or two Silicon substrates together by using an intermediate Pyrex layer.

We expand this method to the preparation of 2D materials on glass substrate (borosilicate Schott D263T in this thesis) [7][8][9] . This is a reliable method with large throughput and sample size as compared to mechanical exfoliation and also permits sample transfer to other substrates. Principle of this technique is illustrated by Figure 2.3. At high temperature, the glass substrate can be seen as a negatively charged fixed matrix (mainly SiO2 units) with mobile positively charged ionic species, in our case Na + ions. The mobility of sodium ions is high and the presence of an electric field across the glass substrate makes them migrate towards the cathode at its back. They leave behind a negative space charge in the region of the interface arising from the static oxygen ions. Thus a high internal electric field is created with positive charges in the precursor which pulls the latter and the substrate into intimate contact.

This results in a strong electrostatic bonding of the precursors with glass ultimately.

Characterization technique for 2D materials

In this thesis, we use four kind of techniques to characterize our 2D samples: Optical microscopy, atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). From the optical microscope and AFM, one can determine the surface quality and number of layers in 2D samples. From

Raman and PL spectroscopy, the vibration modes of the sample, band gap, number of layers (for graphene and MoS2) and the doping level (for graphene) can be estimated quickly. In addition from the TEM, the electronic structure and eventually the polytype of the sample can be obtained. In this section, the principles of the four techniques and the experimental parameters during our characterization are described. Further characterization results of our 2D samples will be shown in Chapter 3.

Optical microscope

The optical images of graphene, MoS2, GaS, GaSe and InSe are recorded by optical microscopy (Leica DM2500) and a CCD camera. All the images are observed in the brightfield imaging mode and 5X, 10X, 50X, 100X objectives. In addition, the optical contrasts in optical images are analyzed to determine the number of layers. The optical contrast is different according to different materials and substrate. The optical contrast can be examined by the software Image J experimentally, and can be also theoretically simulated using the Fresnel law which is discussed in the Appendix.

Atomic force microscopy (AFM)

AFM was invented by G. Binning, C. F. Quate and Ch. Gerber in 1986 based on the scanning tunneling microscope (STM). This microscopy can be applied on not only electrically conductive samples but semiconductors and insulators [10] . The schematics of AFM principle and the tapping mode (also called AC mode) of AFM are shown in Figure 2.5.

Here is the basic principle of AFM: The AFM consists of a cantilever with a probing tip used to scan the sample surface. The cantilever will be deflected in response to the force between tip and sample when the tip is brought into proximity of a sample surface according to Hooke's law. Typical forces range from 10 -11 to 10 -6 N between probing tip and samples. By scanning the sample relative to the probing tip and digitizing the deflection of the cantilever or the z-movement of the piezo as a function of the lateral position x and y, non-destructive AFM images can be taken [12] . There are three kinds of imaging modes in AFM operation according to the working distance of probe-sample and the movement type of the tip: contact mode (also called static mode), non-contact mode and tapping mode (also called AC mode or intermittent contact). Normally, for the contact mode, the tip and sample are very close, the overall force is repulsive, and the tip is controlled by static deflection. In the non-contact mode, the tip and sample are relatively far, the overall force is attractive, and the tip is controlled by resonance frequency. In the intermediate tapping mode, in which the working distance is between contact mode and non-contact mode, the tip is controlled by oscillation amplitude.

For the AFM technique that is applied on 2D materials in our experiment, the advantages and disadvantages of these three modes can be compared. In the contact mode there are certain advantages: high scan speed; 'atomic resolution' is possible and it is easier to scan rough samples with extreme changes in vertical topography. However, the disadvantage is that it can distort the surface of 2D sample because of the lateral forces which should be avoided. The non-contact mode involves low force exerted on the sample surface which means no damage caused. However it has lower lateral resolution limited by tip-sample separation. Another disadvantage here is that the experiments are usually performed in ultrahigh vacuum in order to obtain high resolution images which is not convenient for our setup Therefore, the best choice for our samples is the tapping mode, with high lateral resolution, no damage to samples and ambient conditions though with slower scan speed compared to contact mode.

In our experiments, AFM scanning was performed on Scanning Probe Microscope SmartSPM-1000 instrument (AIST-NT) in ambient condition. We used tapping mode (AC mode) for all the 2D sample and high resolution images are obtained which are shown in Chapter 3. distance. Reprinted from Ref [10] and [11].

Raman scattering and Raman microscopy

The Raman effect was discovered in 1928 by C. V. Raman and K. S. Krishnan and C. V.

Raman was awarded the Nobel Prize in 1930 [13] . It originates from the interaction of an electromagnetic wave with the vibrations of a material (molecular vibrations or photons in solids). As a result, the scattered radiation can be divided in three categories depending on the frequency of the out-coming photons (ν0) which is shown in The principle of Raman scattering: The classical description of Raman scattering (due to Brillouin) is that an incident electromagnetic (EM) wave induces a dipole moment during the light-material interaction. The strength of the induced dipole moment P, is given by:

P = [α]E (2.1)
where E is the strength of electric field of the incident EM wave, and [α] is the polarizability of the materials, which quantifies the deformation of the electron clouds induced by the presence of the EM wave and is a second-rank tensor. Both classical and quantum mechanical treatments of Raman scattering are based on this equation.

Classically, the emission of a scattered radiation is explained by the periodic variations of P. To simplify calculations it is assumed that E and P are parallel, [α] is no longer a tensor in this case. Consider the incident optical electric field to be governed and the displacement (dQ) of the atoms due to the vibration can be expressed by the following equations:

E = E0cos(2πν0t) (2.2) dQ = Q0cos(2πνvibt) (2.3)
where ν0 is the frequency of the incident EM, νvib is the frequency of the vibrational mode, Q0

is the maximum displacement about the equilibrium position. Typically, the displacement dQ is very small, and the polarizability α can be approximated as a Taylor expansion:

α= α 0 + ( ∂α ∂Q ) 0 dQ (2.4)
where α0 is the polarizability of the molecular mode at equilibrium position. Therefore, the dipole moment P can be written as:

P= αE= α 0 E 0 cos(2πν 0 t) + ( ∂α ∂Q ) 0 Q 0 E 0 cos(2πν 0 t)cos(2πν vib t) (2.5)
Using a trigonometric identity, the above relation can be rewritten as:

P= αE= α 0 E 0 cos(2πν 0 t) + defined by the resolution of the grating, for example, for a high resolution grating, the Raman spectrum has more points in a given range of wavelength. However higher resolution means the spectral window will be smaller and less photons will be detected by each pixel of the CCD, result in a long acquisition time.

Electron diffraction in crystals and TEM

Electron diffraction: As is well known diffraction occurs when a wave encounters an obstacle or a slit and the corresponding patterns are formed because of constructive and destructive interference between various diffracted waves. In crystals, the diffraction phenomenon can be expressed by the Bragg's law:

2dsinθ = nλ (2.9)
where d is the inter-planar spacing of atomic planes, θ is the scattering angle, n is an integer and λ is the wavelength of the electrons. Therefore, from this equation, the intensity of bright spots observed in experimental diffraction patterns comes from the constructive interference of the diffracted beams which depends on n, d, λ and the angle θ of crystal orientation with respect to the incident electron beam.

Transfer of 2D samples

After the preparation of 2D materials on a certain substrate the following step is often the transfer process. During this process, the 2D samples can be transferred not only on any arbitrary substrate, but also on other 2D materials to form heterostructures. However, the samples must be kept clean in the transfer process otherwise the quality will be affected.

There are two kind of transfer methods, wet transfer and dry transfer, and each method includes several processes. In this section, we will briefly discuss each method and then the detailed transfer process we used in our experiment.

Wet transfer method

Wet transfer method is a water containing process in which the 2D samples are immersed into deionized water and removed from the original substrate to the target substrate. This fast and precise method is widely used in nanostructure materials transfer. The schematic of the general process for micrometer size of samples is shown in Figure 2.10. The sample 1 on a substrate is firstly dip-coated by a layer of polymer which is the widely used poly(methyl methacrylate) PMMA or cellulose acetate butyrate (CAB). Secondly, the substrate with the coated sample is dipped into water (for CAB) or sodium hydroxide solution (1 mol/L, for PMMA) to release the polymer and sample. It is also called wedging transfer because the physical driving force yielding the lift-off is the capillary force exerted by water that invades the hydrophilic (clean glass and SiO2)/hydrophobic (PMMA and CAB) interface, resulting in the dynamic separation of both surfaces [16] . After the releasing process, the target substrate with sample 2 can be precisely aligned in contact with the sample 1 and polymer. Finally, the polymer can be dissolved by using acetone (for PMMA) or ethyl acetate (for CAB) thus forming the heterostructure of sample 1/sample 2. The key point of the wedging transfer is that the polymer must be hydrophilic and the substrate must be hydrophobic. In addition, there are other processes which also belong to large area wet transfer such as bubbling transfer and etching transfer of CVD graphene [17][18] , which we will not discuss here.

Dry transfer method

Some drawbacks of wet transfer can be improved by the dry transfer method. Firstly, the surface of sample is possibly contaminated because of trace impurities in water which remain in the interface of heterostructures. Secondly some 2D samples are very sensitive to water which should be avoided during the transfer process. In the dry transfer method, the samples are not directly in contact with water and the method can be divided into two processes which are dual polymer based dry transfer and pick up transfer technique.

The brief processes of the two kind of transfer processes are shown in Figure 2.11. In the dual polymer transfer process [19] , sample 1 is firstly prepared on dual polymer by mechanical exfoliated method. The dual polymer consists of a water-soluble layer and a layer of PMMA on top. By dissolving the water-soluble layer in the water, the PMMA/sample 1 can be released, which can be further aligned onto the target substrate with sample 2 and the heterostructure can be fabricated by dissolving the top polymer PMMA finally. However, this process also has its drawbacks. The process is time-consuming and it needs to be replicated for multi-layer stacking. Polymer residuals possibly remain in the interfaces. Recently, an improvement process of dual polymer transfer is the pick up technique found by L. Wang et al. in 2013 [20] . The technique is based on the fact that the van der Waals force between the two samples is stronger than between the sample and the rough substrate. In the pick up transfer process, the sample 1 is firstly mechanical exfoliated (or by the same pick up process) on poly-propylene carbonate (PPC)/polydimethylsiloxane (PDMS)/glass substrate and the sample 2 is also prepared on a substrate such as SiO2. After a precisely alignment, sample 2 is picked up by sample 1 when the two samples touch each other and then loosen. Finally, the sample 1/sample 2 heterostructure can be released on a target substrate by removing PDMS and dissolving PPC. However, the residual of polymer in the surface of heterostructure cannot be neglected and the size of sample 2 should be smaller than sample 1, therefore the etching of samples are further needed to form contacts in device applications.

Here we introduce a new simple dry transfer process which can be used in multiple mechanical exfoliation process. It is a 'real' dry process which avoids polymer contamination in the interfaces and surface though the flakes are stacking randomly. We will discuss the process in the following section.

Our wedging transfer process

We improved the wedging transfer method (for CAB) in the 2D materials by adapting the conditions and parameters to our use. In our process, the CAB polymer was spin coated on a substrate and treated on a hot plate, which are in contrast with the conventional dip coating and dried naturally without heating. The experimental comparison of the conventional transfer process and our improved process is shown in Figure 2.12. From the conventional transfer process, our few-layer GaS has cracks coming from polymer lift off. The cracks are also confirmed by AFM characterization in Figure 2.12 (d). In our improved wedging transfer method, CAB solution (35 mg/mL in ethyl acetate) was spin coated on few-layer GaS on glass at 1000 rpm for 10 seconds and then 3000 rpm for 60 seconds, which makes the CAB polymer very thin and uniform. The CAB coated glass substrate was then baked on a hot plate at 100°C for 2 minutes to dry CAB quickly. The CAB supported few-layer GaS was then transferred onto SiO2/Si substrate after dipping into de-ionized water. From the AFM scanning in Figure 2.12 (h), we can see the surface of transferred few-layer GaS is clean and uniform without any cracks. We used this process in all our graphene/few-layer InSe and graphene/few-layer InSe/graphene heterostructure preparations.

Radom dry transfer process -A 'real' dry transfer

Both the dual polymer dry transfer process and pick up technique have the shortcoming that the surface of the sample might have polymer residuals. Additionally the processes are extremely time-consuming which is not convenient for the fabrication of van der Waals heterostructures. In our heterostructure fabrication process, we experimented direct random mechanical exfoliation of sample 2 onto sample 1/substrate. The schematics of this technique are shown in Figure 2.13. Firstly, the sample 1 is mechanically exfoliated on a substrate (glass or SiO2/Si) on a hot plate. Secondly sample 2 is also mechanically exfoliated on sample 1/substrate on the hot plate. This process is very fast and ultraclean as regards the surface and interface during the process. An example of InSe/MoS2 heterostructure prepared on SiO2/Si substrate is shown in Figure 2.13 (b-c). However, it is worth noting that it is a random technique, which is limited to the fabrication of multi-layer/multi-layer heterostructure, and the thicknesses of the both samples cannot be precisely controlled. In our case, it is very suitable for few-layer InSe/few-layer MoS2 fabrication in our experiment.

Fabrication of 2D devices

We finally fabricate devices based on 2D materials and heterostructures. During the device fabrication, physical stencil mask and lithography (including both optical lithography and electron beam lithography) techniques are used in the fabrication of the contacts. In addition, metal evaporation, resist lift off and vacuum annealing steps are essential for a final device. In this section, we will discuss the techniques of device fabrication using of physical mask and optical lithography respectively. The detailed parameters of metal evaporation, lift off and vacuum annealing are also mentioned. Finally an example of heterostructure fabrication process is shown in this section.

Physical mask

For fabrication of a device using a physical stencil mask, in principle is a very simple, fast and clean method. It was widely used in device applications such as solar cells. However, for the physical mask in the 2D material devices, it is difficult because of the small size of the samples. This makes mask-sample alignment difficult and the layout of the mask cannot change arbitrarily according to the sample form. Despite this it is still the cleanest method in the fabrication of homostructural 2D materials since it avoids polymer residual in the lithography process and the performance of the resulting device is supposed to be the best.

The schematics of device fabrication with physical mask is shown in

Lithography

As we said before, physical mask has some drawbacks in the 2D materials application. In this case, lithography can be seen as an effective tool to design the device contacts. Optical lithography and electron beam lithography differ technically but are very universal in nanoscale device applications. The advantage of electron beam lithography compared to optical lithography in device application is that precision can reach nm scale and the mask can be designed every time according to the requirement of samples. We did optical lithography and electron beam lithography in the Süss Micro Tec MJB4 mask aligner and Raith e-line system respectively in the clean room of École normal supérieure-Paris (ENS), which is shown in Figure 2.15 (a-b). The schematic of optical lithography and electron beam lithography is depicted in Figure 2.15 (c). Firstly, the resist is spin coated onto substrate (for optical lithography) or sample (for electron beam lithography) and dried in a hot plate.

Secondly, the UV light (for optical lithography) or electron beam exposes the patterns of the mask changing the resist properties. After the exposure the pattern can be developed in a solvent. Thirdly, the metals (Cr/Au in our case) are deposited on the patterned sample, and finally lift off is performed in order to remove the metals/resist layer creating the device. In our devices, the substrate (glass or SiO2/Si) is pre-patterned by optical lithography and evaporated Cr/Au metals. There are two purposes here: The large area contacts (Figure 2.15 (c)) are formed using optical lithography because of the speed of the process. Secondly these structures serve as reference coordinates on the substrate for the precise electron beam lithography step. Samples are then mechanically exfoliated or transferred onto the prepatterned substrate on which masks for electron beam lithography can be designed using Klayout software according to the coordinates of the samples. Electron beam lithography is then performed and the contacts for samples fabricated followed by the metal evaporation. As an example, the results of optical lithography and electron beam lithography are shown in Optical lithography:

1. The substrate is cleaned with acetone and ethanol (or isopropanol) in ultrasound machine for 5 minutes respectively.

The photoresist (AZ5214E

) is spin coated onto substrate for 30 seconds at the speed of 4000 rpm/s.

3. The photoresist/substrate is baked on a hot plate for 2 minutes at 120 C, and the thickness of photoresist is around 1.4 μm.

4. The photoresist/substrate is aligned and exposed by UV light in the Süss Micro Tec MJB4 mask aligner using hard contact mode for 10 seconds.

5. The photoresist is developed in AZ726 developer for 30 seconds and then flushed by de-ionized water.

Electron beam lithography:

1. The resist of PMMA is spin coated in our sample/substrate for 30 seconds at the speed of 4000 rpm/s.

2. The PMMA/sample/substrate is baked on a hot plate for 15 minutes at 155 C, and the thickness of PMMA is around 550 nm.

3. If the sample is on a glass substrate, a layer of Aluminum (Al, 20 nm) is evaporated on top of the PMMA layer to evacuate the electrons from the lithography process. for 30 seconds to dissolve the Al layer, and then in the methyl isobutyl ketone solvent (IPA:MIBK is 1:3) for 70 seconds and finally isopropanol (IPA) for 30 seconds. For sample on SiO2/Si substrates, the KOH development is not used.

Metal evaporation, lift off and annealing

Metal evaporation: After the physical mask alignment or the lithography and development process, the metals are evaporated on substrates and devices. The metals we used are chrome, gold and carbon. For normal contacts, we evaporate 5 nm Cr followed by 70 nm Au followed. For one device in few-layer InSe, we evaporate carbon with thickness of ~ 30 nm as contacts.

Lift off (only for the device made by lithography):

After the lithography and metal evaporation, the resist with metal on top is lifted off the substrate. The step is proceeded in hot acetone (temperature is around 50-60 C), the resist is dissolved and the metals on top of resist can be lifted off within 2 hours, which is much shorter than the universal lift off process with the time of about one night.

Annealing (only for the device made by lithography):

This process is very important and essential in the device fabrication. In this process, the residual of resist or impurities might be removed in vacuum or in flowing inert gas atmosphere. In our experiment, we performed the annealing process in high vacuum at around 2 × 10 -7 mTorr at the temperature of 250 C for 6 hours, the heating ramp speed is 50 C per hour and cooling down to room temperature naturally. In addition, to obtain a good contact between metal and sample, we always input a certain current through the device to remove the impurities in the interface of metal/sample before measurement, which is critical for the device fabricated by wet transfer method.

An example of device fabrication process

Here we show an example of graphene/InSe/graphene device fabrication process using optical images shown in Figure 2.16 with three wedging transfer steps followed by electron beam lithography. The steps are the following: firstly, monolayer graphene is prepared on glass by anodic bonding method and then is wedging transferred onto patterned SiO2/Si substrate as bottom graphene in the device. Secondly, few-layer InSe is prepared on glass by anodic bonding method and then wet-transferred on top of the bottom graphene by precise alignment using micro-manipulators. The optical images of InSe/graphene coved with CAB polymer and after dissolving CAB are shown in Figure 2.16 (d-e). Thirdly, the top graphene is prepared on glass and then transferred on the top of InSe/graphene by precise position controlling, and the graphene/InSe/graphene heterostructure is shown in Figure 2.16 (g). It is worth noting that the sample is annealed after every wedging transfer process. For the contacts fabrication, electron beam lithography is performed and the optical image of sample after pattern development is shown in Figure 2.16 (h). Finally, the graphene/InSe/graphene device is formed by evaporating Cr/Au the followed lift off process.

Measurement setup

After device fabrication, the performance of the device is measured using the setups for I-V characterization, photo-response and photocurrent mapping discussed below.

I-V characterization:

The schematic of setup for I-V characterization is shown in the left side of Figure 2.17

Chapter 3 Fabrication and Characterization of Two Dimensional Materials

This chapter is partly based on our publication: Z. Chen, K. Gacem, M. Boukhicha, J.

Biscaras and A. Shukla, "Anodic bonded 2D semiconductors: from synthesis to device fabrication", Nanotechnology, 24 (41), 415708, 2013

Preparation of two dimensional materials

Since graphene was first made by the classical mechanical exfoliation method in 2004 [1] , many other methods have been developed to prepare 2D materials in recent years. For example, the common fabrication methods include: mechanical exfoliation [2][3][4][5] , anodic bonding [6][7][8][9] , photo-exfoliation [10][11][12] , chemical vapor deposition (CVD) [13][14][15] , molecular beam epitaxy (MBE) [16][17][18] , liquid phase exfoliation [19][20][21] and growth on SiC [22][23][24] . However, these methods are mostly adapted for the fabrication of graphene though few methods can also be used for the fabrication of MoS2, GaSe and other 2D materials. A brief description of each method follows:

Mechanical exfoliation: Also called 'scotch tape' method, this is the earliest and most common method to prepare 2D materials on the 'lab' scale. This method is applicable for the fabrication of all 2D materials and has been used occasionally in this thesis, in particular for the fabrication of 2D materials directly on SiO2/Si substrates. We describe it in more detail in section 3.1.2.

Anodic bonding:

A variation of the mechanical exfoliation method for fabricating 2D materials on glass substrates, it gives potentially larger 2D samples and higher throughput.

Moreover, this method has a unique advantage of doping during the preparation process and has been the principal fabrication method in this thesis in particular for graphene. We will discuss it in detail in section 3.1.3.

Photo-exfoliation:

As a kind of novel laser ablation, it can be used to prepare few layer layers because of ablation threshold saturation due to specific heat of the dominant acoustic mode. In addition, as of now the photo-exfoliation method is mature essentially for monolayer or few layer graphene (N<7) preparation.

Chemical vapor deposition (CVD):

For the preparation of 2D materials, the CVD method has the unique advantage of making large area samples which is very attractive for commercial applications. The growth of monolayer or multi-layer graphene on metal substrates such as Cu, Co, Pt, Ir, Ru, and Ni has been shown using the CVD method. For example, high quality graphene films (by wet-chemical doping, the sheet resistance is ~125 Ω□ -1 with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect) with size of 30-inch were produced successfully on flexible copper substrate and transferred via a roll to roll process [25] . In addition graphene grown directly on semiconductors or insulators such as sapphire [26] , Si3N4/Si [27] , MgO [28] and hBN [29] has been shown because of the requirement in electronic applications of graphene on dielectric surfaces. Moreover, CVD method can be used for other 2D materials. For example, monolayer of TMDCs such as MoS2, MoSe2, WS2 and WSe2 can all produced on SiO2/Si substrates directly using CVD method [30- 33] . In addition, ultrathin GaSe and few layer SnS2 are also synthesized by CVD method [34][35] .

Molecular beam epitaxy (MBE):

This is a well-known and widely used technique to produce high quality epitaxial layers of metals, insulators and superconductors of research and industrial quality. It is very interesting that the in-situ growth of heterostructures can be realized by directly growing graphene onto various dielectric and metallic substrate such as h-BN using this method. In addition, another advantage of MBE is that it is compatible with insitu vacuum characterization, such as scanning tunneling microscopy, x-ray photoemission spectroscopy and reflection high-energy electron diffraction.

Liquid phase exfoliation: High quality graphene sheets can be produced in the liquid phase by sonicating tetrabutylammonium hydroxide and oleum-intercalated graphite in N, Ndimethylformamide [36] . In addition, graphene can be also prepared by supercritical fluidic exfoliation in organic solvents and in ethanol solutions containing the sodium salt of 1pyrenesulfonic acid [37] . Using the liquid phase exfoliation method, graphene can be fabricated as transparent conducting films to substitute indium-tin oxide (ITO), and the solution processed transparent graphene electrodes have been applied in liquid crystal devices and flexible electrodes [38][39] .

Growth on SiC: One advantage of using this method is that one does not need to transfer graphene onto another substrate for the device application because of the semi-insulating nature of SiC. However, the growth rate of graphene on SiC depends on the specific polar SiC crystal face. The drawback of this technology for large area production is the cost of the SiC wafer, as well as the hard control on the layer thickness homogeneity. In addition, this kind of method is only used for graphene preparation but not other 2D materials.

In our 2D sample preparation, we used two methods: mechanical exfoliation and anodic bonding. The samples are of high quality and very clean using both methods, which is very critical for the high performance of the devices.

Mechanical exfoliation parameters

Monolayer graphene was firstly made by mechanical exfoliation method in 2004, and the largest cleaved graphene is of the order of millimeters up to now [40] . In addition, various other 2D materials such as TMDCs can also prepared by this method. Using this method, graphene and other 2D materials can be prepared on desirable substrates. Moreover, this technique gives the clean and highest quality samples, which is the best choice to investigate both new physics and new device concepts. In the traditional mechanical exfoliation experiment 2D precursors (flakes cleaved from bulk material) were firstly cleaved by special adhesive tape, then the adhesive tape with the 2D precursors stuck on it was mechanically pressed on the desired substrates, which leaves some ultra-thin layers of 2D materials. We have introduced the parameter of temperature control and further improved this method. We tested several kinds of 2D materials using our method and got some novel findings. For example, monolayer and few-layer graphene, MoS2, InSe, GaSe and GaS were prepared on both SiO2/Si and glass substrates by this technique.

During our experiments we find the key parameter that impacts the yield is the temperature during the cleavage using adhesive tape. Here we give two examples of temperature control for 2D materials in large scale of optical images. Few-layer InSe was prepared by mechanical exfoliation on SiO2/Si substrate on a hot plate and the optical images for samples obtained with different substrate temperatures were compared in Figure 3.1 (a-f). At room temperature, some small flakes were left on the substrate which indicates poor sticking between the precursor and the substrate. In this case, there are some few-layer InSe samples but the sizes are less than 10m. When the temperature is increased to between 50 ºC and 75 ºC, the sizes of flakes increases, which means sticking is improved by the enhancement of temperature. When the temperature is between 100 ºC to 125 ºC the flake size on the substrate increases further with larger fewlayer InSe flakes. This is the optimum temperature for few-layer InSe since for temperatures up to 150 ºC the flakes on the substrate become very thick and the adhesive tape leaves residues.

Few-layer MoS2 fabrication under similar conditions is shown in Figure 3.2 (a-e), and the results are identical with an optimal substrate temperature of between 100 ºC and 125 ºC by the mechanical exfoliation method. In For all materials shown here, no bonding occurs at sample temperatures lower that 130°C. This is expected since ionic mobility in the glass substrate is a prerequisite and temperature is needed to dissociate metallic oxides in the glass. The higher the temperature, the greater the number of free ions and below a certain threshold the space charge creation leading to bonding is not efficient. At sample temperatures higher than 200°C, generally bad quality samples are obtained. Apart from the thermal stability of the precursor material, which must be guaranteed, higher temperatures result in a bigger space charge and enhanced electrostatic interaction between the substrate and the material. This leads to sample charging and bonding of thick flakes, which results in an ineffective peel-off step and/or destruction of thin flakes.

Anodic bonding parameters

The space charge of course depends on the total charge transferred and thus both on the current and the bonding time. In our experiment the bonding time was typically between 10 min to 15 min.

Characterization of two dimensional materials

After the 2D materials preparation, the next step is the characterization of these 2D materials. Micro-scale characterization techniques are needed because of the micrometer size of 2D materials. For example, the common methods to characterize 2D materials include: optical microscopy, TEM, AFM, scanning tunneling microscopy (STM), micro-Raman scattering spectroscopy (micro-Raman), micro-photoluminescence (micro-PL) and X-ray photoelectron spectroscopy (XPS). From this characterization, one can determine the number of layers, the structure of 2D materials, the quality of 2D materials and even the electronic properties of 2D materials. A very brief description of each method follows:

Optical microscopy: this is the most common and essential tool in the characterization of 2D materials. From the optical image, one can determine the size of 2D materials, and furthermore the number of layers may also be determined through optical contrast.

TEM: Transmission Electron Microscopy is a special technique in which the electron beam is transmitted through an ultra-thin 2D material prepared on TEM grid, and the electron beam interacts with the 2D material as it passes through. The direct space, small angle scattered electrons give the imaging information while the reciprocal space wide angle scattered electrons give the structural information. One can thus determine the structure (from the diffraction pattern) and the polytype of the 2D materials.

AFM: Atomic Force Microscopy is the most precise way to determine number of layers in 2D materials. In addition, the quality of the surface can also be obtained at the same time.

However, the disadvantage of AFM is it is time consuming and the limited scanning size.

STM: Scanning Tunneling Microscopy is a technique for very high resolution surface

imaging. The resolution of the image is on the order of nanometer or even less. However, the 2D materials must be prepared on conducting substrate which makes STM not very convenient for the characterization of 2D materials.

Micro-Raman: this is the most common and popular technique to characterize 2D materials and eventually their quality. The Raman shift, linewidths and intensities can depend on number of layers, sample quality and other electronic properties. Particularly, one can determine the number of layers in graphene and MoS2 from the Raman shift. In addition, in monolayer graphene, the defect and doping information can be also obtained.

Micro-PL:

Similarly to micro-Raman, micro Photo-Luminescence can be sensitive to the thickness of 2D materials and especially to their electronic properties. One can determine the band gap of 2D materials such as TMDCs and its eventual variation with sample thickness.

XPS: X-ray Photoemission Spectroscopy is a surface-sensitive quantitative spectroscopic

technique which can determine the elemental composition, empirical formula, chemical state and electronic state of the elements in 2D materials.

In this work, we use several methods to characterize our 2D materials, and the tools include: AFM, optical contrast, micro-Raman spectroscopy, micro-PL and TEM.

AFM characterization of 2D GaS, GaSe, InSe and Mica

AFM in the AC mode (also called tapping mode) is the most common tool to characterize the thickness of 2D materials on glass or silicon wafer substrates. The corresponding number of layers can be precisely calculated which is an essential requirement to study 2D materials.

In addition, the surface information can also be acquired during AFM scanning. In our experiment, AC mode AFM is used to check the surface quality and measure the substrate to sample step height.

Examples of optical images, the corresponding AFM images and height profiles extracted from AFM data of few-layer GaS, GaSe, InSe and mica are shown in Figure 3.7 (a-l). For GaS the triangular sample of GaS was firstly mechanical exfoliated onto SiO2/Si substrate (Figure 3.7 (a)). The corresponding AFM of GaS was then performed in Figure 3.7 (b). From the two figures, smooth and clean surface of GaS is observed which indicates the good quality of the few-layer GaS we prepared. From Figure 3.7 (c), the thickness of GaS is obtained using the height profile which is extracted from the A line in Figure 3.7 (b), and the height is 3 nm, confirming the presence of 4 layers in our sample. The same characterization of few-layer GaSe was also performed in Figure (d-f), and the result shows the high quality of 2 layers GaSe we obtained using mechanical exfoliation method. We also show an example of characterization of 2D samples made by anodic bonding method. For example, in Figure 3.7 (g-i), monolayer and 3-layer InSe were prepared on glass substrate and the quality of InSe was analyzed by AFM. From both optical images and AFM characterization, one can also observed the clean and smooth surface in our sample, which indicates the anodic bonding is an excellent method to prepared 2D materials. In addition, large scale and high quality multilayer mica was also prepared on glass substrate by anodic bonding method shown in

Optical contrast of 2D Graphene, MoS2, GaS, GaSe and InSe

Optical contrast is an important tool to characterize the number of layers in 2D materials.

The principle of this method is to use Fresnel theory to calculate the reflection and finally the optical contrast of 2D materials prepared on a certain substrates. After that, the experimental data can be compared by analyzing the optical image of the 2D materials taken from optical microscopy. It was first applied to graphene characterization in 2007 by Z. H. Ni, P. Blake and I. Jung [41][42][43] . For example, the visibility of graphene prepared on SiO2/Si wafer was investigated by using different optical filters, and the effect of thickness of SiO2 dielectric was also studied. The results of graphene optical contrast taken from the reference are shown in Figure 3.9. The optical contrast, if quantified, can be used as a standard method to detect layer numbers as already shown for graphene, MoS2 and NbSe2. In our experiment, we measured the optical contrast of 2D materials on glass substrate.

For the first time, we calculated the expected optical contrast of few layer GaS, GaSe and InSe for borosilicate glass substrate and in normal incidence, and then compared with the experiment value from optical images of the samples. Figure 3.9 depicts the optical reflection and transmission for few-layer samples with thickness d and complex refractive index n1 deposited on the substrate of glass, where Re(n1) is the optical refractive index and -Im(n1) is the absorption coefficient. In order to calculate the optical contrast from the measured optical images, we generalize the model for broadband illumination and take the color camera response function 𝑆(𝜆) into consideration. The total optical contrast under illumination is calculated (from a wavelength of 400 nm to of 700 nm) by the formula:

𝑂𝑝𝑡𝑖𝑐𝑎𝑙 𝐶𝑜𝑛𝑡𝑟𝑎𝑠𝑡 = ∫ 𝑆(𝜆)𝐶(𝜆)𝑑𝜆 750 390 ∫ 𝑆(𝜆)𝑑𝜆 750 390 (3.1)
where 𝐶(𝜆) is the wavelength dependent optical contrast and depicted as:

𝐶(𝜆) = 𝑅 𝑔𝑙𝑎𝑠𝑠 -𝑅 𝑠𝑎𝑚𝑝𝑙𝑒 𝑅 𝑔𝑙𝑎𝑠𝑠 +𝑅 𝑠𝑎𝑚𝑝𝑙𝑒 (3.2)
where R is the intensity of reflected light. The detailed calculation processes are shown in Appendices.

For graphene optical contrast shown in the left of Figure 3.10, we used three different reflective indices that mentioned in the literatures and obtained the optical contrast as a function of layer numbers. For monolayer graphene, the optical contrast is around 5% and increases almost linearly when the layer numbers increase. For the optical contrast of MoS2

shown in the right of Figure 3.10, the monolayer has 6.43% optical contrast, and increases linearly under 5 layers but the value starts to saturate when the layer numbers are more than 7. The corresponding calculation results for few layer GaS, GaSe and InSe are shown in

Raman characterization of 2D Graphene, MoS2, GaS, GaSe and InSe

Raman spectroscopy is a very important tool for characterizing 2D materials and compare them to bulk materials. Raman characterization is an ideal tool for graphene because it is not only fast and non-destructive, but also contains high resolution, gives structural and electronic information which can be applied at both laboratory and commercial production scales. In addition, from the Raman shift, layer thickness is easily determined for graphene and MoS2.

In our work, we use Raman spectroscopy to characterize monolayer and few-layer graphene, MoS2, GaS, GaSe and InSe respectively.

Graphene: From the Raman shift of graphene, it is well known that the information of doping, strain and stress, disorder as well as magnetic field and interlayer coupling can be characterized [44][45][46][START_REF] Das | Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor[END_REF][START_REF] Zhao | Intercalation of few-layer graphite flakes with FeCl3: Raman determination of Fermi level, layer by layer decoupling, and stability[END_REF][START_REF] Mohr | Splitting of the Raman 2D band of graphene subjected to strain[END_REF][START_REF] Yoon | Strain-dependent splitting of double resonance Raman scattering band in graphene[END_REF][START_REF] Mohiuddin | Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Gruneisen parameters, and sample orientation[END_REF][START_REF] Ni | Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening[END_REF][START_REF] Venezuela | Theory of double-resonant Raman spectra in graphene: Intensity and line shape of defect-induced and two-phonon bands[END_REF][START_REF] Lucchese | Quantifying ion-induced defects and Raman relaxation length in graphene[END_REF][START_REF] Bonini | Phonon anharmonicities in graphite and graphene[END_REF][START_REF] Basko | Interplay of Coulomb and electron-phonon interactions in graphene[END_REF][START_REF] Basko | Calculation of the Raman G peak intensity in monolayer graphene: role of Ward identities[END_REF][START_REF] Basko | Theory of resonant multiphonon Raman scattering in graphene[END_REF] . The G and 2D vibration modes, can be used to estimate the quality and the doping level of graphene. In the following we give some typical results obtained for graphene mechanically exfoliated on SiO2/Si substrates, made by anodic bonding on glass and transferred to SiO2/Si substrates and finally mechanically exfoliated onto glass. We prepared graphene samples by mechanical exfoliation firstly, and the optical image of the graphene is shown in Figure 3.12 (a). Then we performed the corresponding area Raman mapping under the green laser 532nm, and the I(G)/I(2D) ratio mapping of graphene was shown in Figure 3.12 (b). From the Raman mapping, one can see the monolayer has the value around 0.5, which is largely different from multilayer which is larger than 2. Using this method, we can identify monolayer graphene from multilayer, which is a fast method compared to AFM. In addition, we characterized Raman shift of few-layer graphene according to the number of layers shown in Figure 3.12 (c). From the monolayer graphene prepared on SiO2/Si, there is no defect peak (D) peak around 1350 cm -1 , which implies the high quality of graphene we obtained. The 2D peak in monolayer graphene is single and sharp, which splits in several components in bilayer and above. These results are in accordance with the extensive results on exfoliated graphene published before. However, in our sample, the positions of G mode and 2D mode in monolayer graphene are at 1590.9 cm -1 2672.3 cm -1 respectively, which is blue shifted with respect to undoped pristine graphene. This indicates an interaction with the SiO2/Si substrate resulting in slight doping of graphene. We then prepared graphene with the anodic bonding method and 'wedging transferred' it to SiO2/Si substrate afterwards. This graphene is studied and compared with 'pristine' graphene which we also prepared on the glass substrate by mechanical exfoliation. For the 'pristine' graphene made by mechanical exfoliation, the G mode is at 1581.5 cm -1 and 2D mode is at 2670.8 cm -1 , which indicates that the graphene mechanically exfoliated on glass is not doped [START_REF] Berciaud | Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers[END_REF] . In the graphene prepared by anodic bonding method on glass no defect peaks are observed and the two modes have a clear blue shift, at 1598.8 cm -1 and 2678.1 cm -1

indicating the hole doping [7] because of the principle of anodic bonding method. For the transferred anodic bonded graphene, the Raman peaks are at 1587.5 cm -1 and 2685.1 cm -1 for G and 2D modes respectively, which now shows hole doping compared with pristine graphene. The hole doping can result from the wedging transfer or from water vapor [START_REF] Shim | Water-gated charge doping of graphene induced by mica substrates[END_REF][START_REF] Zhang | Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures[END_REF] .

Therefore, we can prepare high quality pristine or doped monolayer graphene by mechanical MoS2: From the Raman shift of MoS2, one can also precisely analyze the number of layers in few-layer MoS2 [START_REF] Lee | Anomalous Lattice Vibrations of Single-and Few-Layer MoS2[END_REF][START_REF] Li | From Bulk to Monolayer MoS2 : Evolution of Raman Scattering[END_REF] . We prepared few-layer MoS2 on SiO2/Si using mechanical exfoliation method and characterized the vibrational modes in Figure 3 respectively. However, this relative difference between the two modes is detectable in MoS2

under 7 layers and it saturates when the layer number increases [START_REF] Mak | Atomically Thin MoS2: A New Direct-Gap Semiconductor[END_REF] .

GaS:

The Raman shift of GaS is largely different from graphene and TMDCs such as MoS2 that we discussed above. We prepared few-layer GaS by mechanical exfoliation and the corresponding Raman mapping is performed in Figure 3.14 (a-b). From the Raman mapping of the A1g 1 vibration mode, we can see some differences between different thickness layers, (less intensity in thinner layers) similar to MoS2 Raman mapping. In addition we mechanically exfoliated few-layer GaS on glass substrate and its Raman shift is shown in Figure 3.14 (c-d). However, there is no signal in monolayer GaS and the signals are very weak in ultra-thin GaS unlike graphene or MoS2. We characterized the Raman shift of A1g 1 vibration mode of GaS, which has a large blue shift in bilayer to four layers but the shift is not obvious when the layer numbers increase beyond four layers. At the same time, the full width at half maximum (FWHM) also varies from around 4 cm -1 to 22 cm -1 . Raman analysis is thus useful only for distinguishing bilayer and 3-layer GaS and not an efficient way to distinguish number of layers. The optical contrast method is more useful and easier in this case. GaSe: The Raman shift in few-layer GaSe is very similar to GaS that we just discussed.

The optical image and corresponding Raman mapping of A1g 1 mode in GaSe are shown in As the number of layers increase, the peak frequency blue shifts, probably corresponding to a slight increase in inter-layer interaction while the FWHM decreases towards the bulk value as the influence of strain and interaction with the substrate decreases.

The same trend has been observed earlier in our GaS and GaSe samples. This peak can also be used to characterize sample thickness and cross-check with the optical characterization.

Like GaS and GaSe, Raman signals are extremely weak from monolayer and bilayer InSe, which means optical contrast is needed for a precise identification. Comparing the InSe data in Figure 3.16 (c) with published results for the bulk [START_REF] Lee | Anomalous Lattice Vibrations of Single-and Few-Layer MoS2[END_REF][START_REF] Li | From Bulk to Monolayer MoS2 : Evolution of Raman Scattering[END_REF][START_REF] Mak | Atomically Thin MoS2: A New Direct-Gap Semiconductor[END_REF][START_REF] Carlone | Second order Raman spectrum and phase transition in InSe[END_REF][START_REF] Kumazaki | Far-Infrared Reflection and Raman Scattering Spectra in γ-InSe[END_REF] , we identify the peaks at 114.3 cm -1 , 176.1 cm -1 and 226.3 cm -1 with A1g 1 , E2g 1 and A1g 2 modes of the β or γ polytype. In few layer InSe however we often find a weak peak at around 200 cm -1 (3-layer and 5-layer samples in Figure 3.16 (c)) which corresponds to the ε polytype [START_REF] Jandl | Raman spectrum of crystalline InSe[END_REF] . This peak is systematically absent in the bulk samples indicating perhaps that the occurrence of the ε polytype provides an easier preferential cleaving plane for the sample. The polytype of our InSe sample is further analyzed and confirmed combining with TEM characterization shown in part of 3.2.5.

In summary for the Raman analysis of 2D graphene, MoS2, GaS, GaSe and InSe: Raman characterization is a useful way to check the quality of 2D materials and the number of layers by Raman shift. However, for III-VI materials such as GaS, GaSe and InSe, the Raman signal in monolayer and bilayer is very weak which is in contrast to monolayer graphene and MoS2.

Optical contrast is essential for a precise identification. MoS2: PL characterization of few-layer MoS2 is shown in Figure 3.17. From the PL mapping of the intensity of the 1.8eV peak which corresponds to the direct band-gap in the monolayer in Figure 3.17 (b) it can be seen that the intensity of the peak decreases dramatically from monolayer to few layers. The red portions in the map corresponding to the highest intensity of the PL correspond clearly to the monolayer in the optical image in Figure 3.17 GaSe: The variation of PL intensity peaks with thickness is completely different in GaSe as shown in Figure 3.18. In multi-layer GaSe (~ 30 layers), the PL peak at 1.97 eV is very sharp and intense. However, when the thickness decreases to 9-layer, the intensity has significantly dropped and the position has blue-shifted to 2.02 eV. The blue shift continues when the number of layers decrease to 5. It should be mentioned here that it is very difficult to detect PL signal of GaSe below 5 layers. Our measurements indicate that their might exist an indirect to direct band gap transition by increasing number of layers a fact that has been estimated from first principle calculation [START_REF] Ma | Tunable electronic and dielectric behavior of GaS and GaSe monolayers[END_REF] . shows the optical image and the corresponding AFM of few-layer InSe chacterizing the number of layers. From the PL spectra in Figure 3.19 (c), we can see that PL peak blue-shifts and the intensity decreases rapidly when the number of layers decreases from 17-layer to 8layer, with a corresponding band gap shift from 1.26eV to 1.35eV. The trend is similar to GaSe discussed above. Moreover, we cannot detect PL when the thickness is under 8-layer, which corresponds to the band gap transition from indirect to direct when the thickness increases to 8 layers and above. Similar results are also reported by G. W. Mudd et al. [START_REF] Mudd | Tuning the Bandgap of Exfoliated InSe Nanosheets by Quantum Confinement[END_REF] by experiment and theoretical modeling.

Photoluminescence of 2D MoS2, GaSe and InSe

TEM characterization of 2D GaS, GaSe and InSe

TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen and interacts as it passes through the sample. For 2D materials, which are ultra-thin and with micro-meter lateral size, TEM is a good choice to characterize lattice structure. However, the obstacle for its use is sample preparation because it implies transfer of 2D materials to a TEM grid which is complicated in some cases. In our TEM experiment, we characterized few-layer GaS, GaSe and InSe by wedging transferred samples onto TEM grids.

GaS:

The optical image of few-layer GaS transferred on to a TEM grid, the low magnification TEM image, the corresponding high resolution TEM (HR-TEM) and TEM selected area electron diffraction pattern (TEM SAED pattern) are shown in Figure 3.21. In the optical image, one can observe a piece of few-layer GaS (roughly 10 layers checked by optical contrast before the transfer) transferred on a TEM grid which has holes with 50μm × 50μm size. Figure 3.21 (b) gives a typical low magnification TEM image of few-layer GaS.

The corresponding HR-TEM is characterized and the lattice constant for GaS is 0.31 nm from To determine the polytype of our few-layer InSe, a simultaneous TEM and Raman analysis is shown in Figure 3.24. In conclusion, we can exclude the γ polytype, but SAED cannot distinguish between β and ε polytypes. Raman spectroscopy can discriminate between these through a mode around 200 cm -1 which is present in the ε polytype but absent in the β polytype. To exclude the possibility of mode activation by on or off-resonance conditions both 532 nm and 638 nm wavelength excitation were used on exfoliated twelve-layer samples as shown in Figure 3.24 (e). We observe the 202 cm -1 mode in each case with intensity stronger than the E2g 1 mode at 178 cm -1 . This conclusively indicates that our few-layer samples are of the ε polytype. The structure of the ε polytype in few-layer InSe is shown in

Stability of few-layer InSe in ambient condition

There are many 2D materials which can be successfully exfoliated down to the monolayer in ambient conditions and related applications have also been reported in recent years.

However, there a critical problem that is often neglected is that of the stability of ultra-thin layers in the air. As the representative 2D materials, Graphene, BN and MoS2 are very stable when the monolayer is exposed to the air. Unfortunately, some 2D materials such as monolayer black phosphorous and GaSe are very sensitive to water or oxygen in the air.

Therefore, it is necessary to study the stability of our 2D materials in the ambient conditions.

The whole 2D materials family (until the year of 2013) and the proposed stability are classified by A. K. Geim et al. [START_REF] Geim | Van der Waals heterostructures[END_REF] and the table is shown below. In the table, there are several groups of 2D materials, which are marked by different colors according to the sensitivity of the materials exposing to the air. From the table, the layered materials such as GaSe and InSe are not stable in the ambient conditions. However, the stability is not characterized quantitatively and raises the question of a solution to overcome the degradation.

Table 3.1 Monolayers proved to be stable under ambient conditions (room temperature in air) are shaded blue; those probably stable in air are shaded green; and those unstable in air but that perhaps stable in inert atmosphere are shaded pink. Grey shading indicates 3D compounds that have been successfully exfoliated down to monolayers. Reprinted from Ref [70].

In this section we study the stability of ultra-thin InSe by Raman spectroscopy, and give a reliable solution to solve the degradation of ultra-thin InSe in the ambient condition.

We

Introduction: photo-detectors based on 2D materials

A photo-detector is a device which can detect optical signals through electronic processes, and therefore is potentially useful in the areas of imaging, communication devices, sensors for safety monitoring, display technology and fundamental science applications [1][2] . The birth of photo-detectors can be traced back to 1873 when W. Smith discovered photoconductivity in selenium. A photo-detector based on 2D materials was first fabricated in 2009 in the shape of an ultrafast graphene photo-detector which worked without external bias voltage and showed stable photo-response for optical modulation up to 40GHz [3] .

As the prototype 2D material, graphene has high carrier mobility, a broad absorption spectrum and a fast response time which are vital ingredients for high performance photodetectors. However, the weak absorbance of graphene is a strong limitation and the highest photoresponsivity based on an exfoliated graphene photo-detector is 6.1 mA/W to date [4] .

Recently, the successful fabrication of few-layer 2D transition metal dichalcogenide semiconductors (TMDCs) with much larger optical absorption has raised this limit by several orders of magnitude and thus largely improved photo-detector performance [5][6] . For example, the photoresponsivity of a photo-detector based on exfoliated monolayer MoS2 is found to be 880 A/W at 561 nm [7] . The electronic band gap of TMDCs can also be tuned in some cases between indirect and direct as a function of the number of layers, a fact that could be crucial for designing efficient photo-detectors.

In this chapter, we focus on homogenous photo-detectors based on a given 2D material in contrast to heterogeneous photo-detectors with a p-n junction or a heterostructure prepared by

Physical mechanisms in photo-detection

For photo-detection, as we defined above, the key physical process is the conversion of absorbed photos into an electrical signal. However, there are several possible mechanisms identified in literature. These are: photoelectric effect, photo-thermoelectric effect, bolometric effect and photo-gating effect. The mechanisms of bolometric effect and photo-gating effect are found in the graphene photo-detector and we will not discuss them below.

Photoelectric effect: This quantum effect is the excitation by a photon of an electron-hole pair which contributes to the photocurrent. A related effect is the photovoltaic effect which is the generation of voltage when a device is exposed to light [8] . This is often encountered in a 2D homogeneous photodetector because in the region of the interface between metallic contacts and the 2D material a junction forms due to band alignment. Thus the built-in electric field can separate electron-hole pairs without external bias voltage. A convenient example is the photo-detector based on monolayer graphene published by E. J. H. Lee et al. [9] which is shown in where ϕ 𝑀 and ϕ 𝐺 are the work function of metal and graphene; ∆ 𝐸 𝐹 is the Fermi level shift of graphene caused by metal doping; 𝑉 𝑔𝑠 is the gate bias applied; 𝑉 𝑔𝑠 𝐷𝑖𝑟𝑎𝑐 is the gate bias under which graphene is in its charge-neutral point; α is the carrier concentration induced by back gate voltage which is 7.2 × 10 10 cm -2 V -1 for 300 nm SiO2; and ħv 𝐹 is 5.52 eVÅ. However the effective area of this device is very small since it is limited to the metal contact interface which causes a low responsivity of the photovoltaic device. To use the photoelectric effect and extract charge carriers throughout the device an external bias voltage needs to be applied, which improves the performance of the device. By applying the external bias voltage, the effective area can expand to the whole device [10] , and the photocurrent can be described as:

𝐼 𝑝ℎ = 𝐴𝑉𝑒𝜇∆𝑛 (4.2)
where 𝐴 is the cross section of the active layer; 𝑉 is the applied bias voltage; 𝑒 is the unit electron charge; 𝜇 is the charge carrier mobility; ∆𝑛 is the photo induced carrier density. The key parameters are then the charge carrier mobility and the photo induced carrier density.

Graphene has very high carrier mobility but very short carrier lifetime and low absorption of photo induced carriers, which caused low responsivity and quantum efficiency [3] . Photo-thermoelectric (PTE) effect: Thermal detectors detect light by sensing the temperature rise when light energy is absorbed. They are relevant for long wavelengths, in particular the infra-red or far-infrared. PTE effect is the thermoelectric effect caused by light illumination. The mechanism is widely studied in graphene and MoS2 photo-detectors [11][12][13] .

We may cite the PTE effect of monolayer MoS2 investigated by M. Buscema et al. [11] shown in

∆ V PTE = (S MoS 2 -S TiAu )∆T (4.3)
where S is the Seebeck coefficient described by the Mott relation [14][15] in (4.4) and ∆T is the temperature difference between the locally heated gold electrode and MoS2 distant from the junction.

S = π 2 k B 2 T 3e dln(σ(E)) dE │ E=E F (4.4)
where k B is the Boltzmann constant, T is the temperature,e is the unit electron charge, σ(E) is the conductivity as a function of energy, and the derivative is evaluated at the Fermi energy EF. In their device, a large range Seebeck coefficient between -4×10 2 and -1×10 5 µVK -1 was observed, which paves a way to new applications of monolayer MoS2 such as on-chip thermospower generation and waste thermal energy harvesting.

Terminology and key figures of merit in photo-detector based on 2D materials

The key figures of merit used to characterize the performance of photo-detectors are:

photo-gain (Gph), photo-response time, on-off ratio, external quantum efficiency (EQE) and internal quantum efficiency (IQE), photoresponsivity (Rλ) and detectivity (D).

Photo-gain: Gph is defined as the number of detected charge carriers per single incident photons:

𝐺 𝑝ℎ = 𝜏 𝑡𝑟 𝜏 𝑡𝑟𝑎𝑛𝑠𝑖𝑡 = 𝜏 𝑡𝑟 𝜇𝑉 𝑏𝑎𝑖𝑠 𝐿 2 (4.5)
where 𝜏 𝑡𝑟 is the life time of the charge carriers, 𝜏 𝑡𝑟𝑎𝑛𝑠𝑖𝑡 is the drift transit time, μ is the mobility of the charges and L is the source-drain separation distance.

Photo-response time:

It is the time for the current change until saturation when the light is turned on from the dark status. In different 2D materials and related devices this time can varies from µs to minutes, which will limit the application of the photo-detector.

On-off ratio: different from the device on-off ratio in the FET, the on-off ratio in photodetector is the ratio of the value of the current with light on and the dark current. where e is the unit electron charge, ℎ𝜈 is the energy of a single photon, 𝑃 𝑖𝑛 is the power of incident light.

It is worth noting here that EQE can be larger than 1 in a photo-detector because the photogain can be larger than 1. For example [16] : Suppose that photo-excited electrons travel faster than holes, the electron will complete its trip sooner than the hole under the bias voltage.

However the requirement of current continuity forces the external circuit to provide another electron immediately. This new electron might also complete its trip before the hole. This process continues until the electron recombines with the hole. In this case, a single photon can therefore result in an electron passing through the external circle many times, and the photocurrent can be amplified by the single photo-excited electron-hole pair. where 𝐴 𝑎𝑏𝑠 is the absorbed fraction.

Internal quantum efficiency:

Photoresponsivity: Rλ is defined as the photocurrent Iph generated per unit incident power 𝑃 on the effective area of the photo-detector (S):

𝑅 𝜆 = 𝐼 𝑝ℎ 𝑃 𝑖𝑛 = 𝐼 𝑝ℎ 𝑃×𝑆 (4.8)
Detectivity: D represents the ability of the detector to distinguish signal from noise:

𝐷 = 𝑅 𝜆 √𝐴 √2𝑒𝐼 𝑑𝑎𝑟𝑘 (4.9)
where 𝐼 𝑑𝑎𝑟𝑘 is the dark current.

In the following section we report our work on photo-detectors based on few-layer MoS2, GaSe and InSe. The performance of these photo-detectors in light of the figures of merit described above is measured and analyzed. We made the choice of fabricating few-layer devices in all the materials. In section 4.2 we show that in MoS2, despite the indirect gap a device made of three layers has many advantages. However in GaSe and InSe, the direct gap is obtained only in multi-layer samples whereas the monolayer is indirect. In section 4.3 we analyze the performance of two 6-layer GaSe photodetectors. Finally in section 4.4 we analyze the performance of three InSe photodetectors (8 layers, 11 layers and 20 layers). The aim is not only to understand the mechanisms governing the workings of these ultra-thin devices, but also to lay a foundation for the hybrid devices which are expected to work in vertical geometry and which are the object of the next chapter.

Photo-detector based on few-layer MoS2

For optoelectronic applications MoS2 is a promising alternative material to graphene because it has a band gap. There are two kind of photo-detectors based on MoS2, based on direct band gap monolayer MoS2 or based on indirect band gap few-layer MoS2. For monolayer MoS2, the first photo-detector was fabricated by Z. Yin et al. in 2011 [17] , with a photoresponsivity of 7.5 mA/W, which is comparable to graphene based photo-detectors. By far, the highest measured photoresponsivity of devices based on monolayer MoS2 is 880 A/W in ambient condition which is several orders of magnitude higher [7] . A CVD monolayer MoS2 based photo-detector on the other hand showed photoresponsivity and photogain up to 2200 A/W and 5000, respectively, though in high vacuum condition [5] . Photo-detectors based on few-layer MoS2 have a broad photo-response range compared to monolayer MoS2 though the band gap is indirect. For example a multi-layer MoS2 photo-detector with wide spectral response (< ∼900 nm) and high photoresponsivity (> 100 mA/W) was fabricated by W. Choi et al. in 2012 [18] . A bilayer MoS2 photo-detector showed a fast photo-response time (∼70 μs and ∼110 μs corresponding to the rise and decay). In addition, this device had high thermal stability and a high photoresponsivity of 570 mA/W under 10 V bias [19] . In another study topgated photo-detectors based on single-, double-, and triple-layer MoS2 were compared and the results indicate that the device with triple-layer exhibited improved photo-detection capabilities for red light, while the others with single-and double-layers were more useful for higher energy green light detection [20] .

In this section we report on a few-layer MoS2 photo-detector with very high photoresponsivity when compared to the values published before. We characterize the EQE, on-off ratio and photo-response time and compare them to published values. In addition we analyze the photo-detection mechanism in detail.

I-V characterization of a few-layer MoS2 photo-detector

In the fabrication of our device, few-layer MoS2 was prepared using mechanical exfoliation method on a 285 nm SiO2/Si substrate which serves as the back gate. Electrical contacts were evaporated (Cr (5 nm) followed by Au (120 nm)) after electron beam lithography. The optical image of our device is shown in The device can be seen as a field effect transistor with an n-type channel and a threshold of Vt = -10V under dark conditions, in which the on/off ratio > 10 5 . The on-off ratio logically decreases when gate voltage increases or when illumination decreases. Though higher on-off ratios have been published (~10 8 in [21][22] ) accurate determination also depends on the minimum current measurement which is of the order of 10 -12 A in our case. From the Ids-Vg curve in the dark condition, we can estimate the field effect mobility (μ) in our device, which is described in the following the equation:

µ = 1 𝐶 𝑖 𝐿 𝑊 1 𝑉 𝑑𝑠 𝑑𝐼 𝑑𝑠 𝑑𝑉 𝑔𝑠 (4.10)
𝐶 𝑖 = (𝜀 0 𝜀 𝑟 ) 𝑑 ⁄ is the SiO2 layer (d = 285 nm) capacitance per unit area between the conducting channel and the back gate, where 𝜀 𝑟 = 3.9 and 𝜀 0 = 8.854 × 10 -14 Fcm -1 are respectively the relative and absolute dielectric constants. Based on these values we obtained a field effect mobility of 15.86 cm 2 V -1 s -1 . This value is much higher than the values published in monolayer MoS2 photo-detectors [7,17] . This can be understood by sensitivity of the monolayer to interaction or defects induced by the substrate [25] . This difficulty is overcome in a few-layer device. The performance of our 3-layer MoS2 photo-detector was characterized using the key figures of merit, such as photoresponsivity, EQE, on-off ratio and detectivity, shown in Figure 4.5 (b-d). For the photoresponsivity and EQE, the highest values at 316 A/W and 6.96×10 4 % respectively were obtained under the illumination power of 12.6 W/m 2 and Vgs = 20V and both decrease when the gate voltage decreases or the illuminated power increases. These values are several order of magnitude higher than the multi-layer MoS2 photo-detector published before [18][19] but lower than those found in monolayer MoS2 [7] . Finally for the detectivity, the highest value in our device is 4.77×10 11 Jones (cm Hz 1/2 /W), which is the maximum in the literature for photo-detectors based on monolayer or and few-layer MoS2.

Photo-response of few-layer MoS2 photo-detector

The time resolved photo-response of photocurrent for our 3-layer MoS2 photo-detector was analyzed and the results are shown in Figure 4.6. The response is divided into a fast and a slow regimes both for the excitation and the decay. The photocurrent rises rapidly within 0.39 s under the irradiation of 532 nm depicted in the red square of the figure, and then increases by a slower process till saturation after ∼50 s. For the decay process, the fast decay lasts 0.85 s after the irradiation is turned off, and the subsequent slow decay lasts ∼240 s before attaining the dark current. The photo-response is thus a factor to be improved in our device. In particular the low bias in our device with respect to values used in literature may be invoked, the electric field responsible for charge transport being low due to the relatively big sourcedrain distance in our device [7,23] . Alternatively parameters such as surface hydrophobicity or the fact that the two regimes could be related to two modes of excitation, photoelectric for the rapid part and thermal for the slow response could also play a part. Further studies and device optimization are needed to improve the photo-response of our few-layer MoS2 device. In Table 4.1 we compare the performance of our device to photo-detectors based on monolayer MoS2 and few-layer MoS2 published previously. Comparisons with published results are delicate because they depend on parameters such as Vds and Vgs as well as excitation wavelength and intensity which differ from experiment to experiment, but in general our device shows an excellent performance with high mobility and photoresponsivity and detectivity higher than published values. We have mentioned the possible role of the weak electric field for the unsatisfactory photo-response and it can be seen in this comparison that it is indeed much inferior to published work. It can be hoped that our device can be further improved in all respects by increasing the electric field during our measurement, probably using a more compact geometry and smaller source-drain distance.

Device Electric field [V/μm] Rλ

[A/W] μ [cm 2 V -1 s -

Mechanism in few-layer MoS2 photo-detector

The underlying mechanism of few-layer MoS2 photo-detectors concerning the band alignment and various working conditions are analyzed in Figure 4.7. The work functions of Au and MoS2 are different (5.4 eV and 4.6 eV respectively [24][25] ) and when the two materials are in contact the Fermi levels must coincide at the interface. In the equilibrium state, the electronic bands of MoS2 bend and small Schottky barriers are formed in the interfaces as a consequence. The band diagram of the device is depicted in Figure 4.7 (c). In this case there is no current under dark conditions without bias and without back gate voltage. Three situations are discussed, either under focused laser illumination for photocurrent mapping with micron beam or under large spot uniform irradiation (the whole device was covered by the light). Secondly for positive Vds (the negative Vds is the opposite case) and Vgs < Vt shown in

Photo-detector based on few-layer GaSe

We first cite a few recent devices reported in literature based on few layer GaSe. Using exfoliated GaSe P. Hu et al. [26] measured a high photoresponsivity (2.8 W/A) and fast response (0.02 s). A photo-detector based on CVD GaSe was fabricated and a responsivity of 17 mA/W for 405 nm light was measured [27] . More recently a flexible device based on van der Waals epitaxy GaSe was made on a mica substrate with a photoresponsivity of around 30 mA/W [28] . All of these applications indicate that few-layer GaSe is a promising material for the novel photo-detectors. In this part, we prepared few-layer GaSe on glass using anodic bonding and made the device through a simple physical mask. In addition, few-layer GaSe thus fabricated was also transferred onto a SiO2/Si substrate and a device made using e-beam lithography. The performance of our devices was characterized using the critical parameters such as photoresponsivity, photo-response time and EQE.

The insets of with sharp rise and decay which compares favorably to values published earlier [26] . In addition, we compare photocurrent distribution in three different areas in the photo-detector based on SiO2/Si, as shown in the inset of Figure 4.8 (a). The result indicates that the photocurrent is higher when the illumination is closer to the source of our device similar to devices in CdS nanowires [29] . We can infer that the anodic bonding process in this case induces an intrinsic doping [30] in the GaSe layers corresponding to a back gate voltage of about -40 V. We can also infer that the transfer process does not change the properties of the material. In addition, we find photoresponsivity (λ = 532 nm) = 1.1×10 -2 A/W, which is lower than published values. For the EQE, we find 2.6% under full laser illumination for our GaSe photo-detector. The limited values of photocurrent, as well as photoresponsivity, could be improved several orders by fabricating vertically stacked heterostructures as shown in other materials [31][32] .

Photo-detector based on few-layer InSe

Finally we discuss our few layer InSe based devices. Such a photo-detector can have a broad spectral response because the direct band gap is at around 1.3 eV when the thickness is more than 7 layers and thus the device can be performing also in the infra-red. For example, few-layer InSe photo-detectors with a high photoresponsivity (34.7 mA/W) and fast photoresponse time (488 µs) were reported by S. Lei et al. in 2014 [33] . Later, a flexible photodetector based on few-layer InSe was made by S. R. Tamalampudi et al. [34] with a photoresponsivity reaching 12.3 A/W and broadband photo-detection from the visible to nearinfrared region (450 nm -785 nm). As we know, the contacts of the devices play a key role in the performance of FETs and photo-detectors. For example, various metals for contacts on few-layer InSe FETs were examined by W. Feng et al.. They found that InSe FETs with In contacts achieved a high electron mobility of 162 cm 2 V -1 s -1 and on/off ratio of 10 8 , a significant increase with respect to Al, Ti, Cr contacts [35] . In addition a few-layer InSe photodetector was reported to have an ultra-fast photo response time (100 µs) with graphene contacts and the highest photoresponsivity (700 A/W) with Ti/Au contacts [36] .

In this section we first report on a photo-detector based on 8-layer InSe with carbon contacts (~ 20nm). We then present two devices with Cr (5nm)/ Au (80nm) contacts using 11layer and 20-layer InSe.

Photo-detector based on few-layer InSe with carbon contacts

8-layer InSe was prepared on glass using anodic bonding and transferred to a SiO2/Si substrate followed by electron beam lithography and electrode evaporation of carbon wire to fabricate a 2D photo-detector device. The effective area of this device is 3.17 µm 2 . The photoresponse time, on-off ratio, responsivity as well as EQE were analyzed with 532nm laser illumination at a power of 12.6×10 3 W/m 2 as shown in Figure 4.9. The device is characterized by fast response (less than 150 ms) and a reasonable on-off ratio (10 3 ). The highest values for photoresponsivity and EQE (2 A/W and 460% respectively) are reached with a 50V back gate voltage. The field effect mobility is 4.86×10 -3 cm 2 V -1 s -1 from the transfer characteristics that is three order of magnitude lower than the mobility in the MoS2 device of section 4.

Photo-detector based on few-layer InSe with Cr/Au contacts

For the first such detector we used mechanically exfoliated 11-layer InSe on SiO2/Si.

During the photocurrent measurement, the whole device is illuminated by the laser at the wavelength of 532 nm. From the optical image and AFM in

Photocurrent distribution in multilayer InSe photo-detector

To investigate the photocurrent distribution and the regime in which the drift current dominates in the photo-detector, photocurrent mapping was performed in multilayer InSe with a focused laser beam of 20 µW under Vds = 1 V and Vgs = 0 V. From the optical image and the corresponding photocurrent shown in Figure 4.12 (a-b), we can clearly see that the photocurrent is mainly collected close to the source contact because the photocurrent decreases greatly when the laser is focused in the area closing to the drain contact. The underlying mechanism can be explained using the band diagram shown in Figure 4.12 (c-d).

The band bending under the equivalent condition is the same to the band diagram shown in few-layer MoS2 because of the different work functions between Au and InSe (Au is at 5.4 eV and InSe is at 5.2 eV respectively [24,37] ). The band bending at the source contact is increased locally under a positive bias voltage, which results in an increase in the magnitude of the photocurrent. However, the band bending is reduced locally at the drain contact which suppresses the photocurrent in this case. Similar photocurrent distributions are also found in some literatures [38][39] .

Summary

In summary of this Chapter, we analyzed the performance of photo-detectors based on 2D materials such as few-layer MoS2, GaSe and InSe respectively. For the photo-detector based on three-layer MoS2, we find the highest detectivity (> 10 11 ), photoresponsivity (316 A/W) and EQE (6.96×10 4 %) in our device compare to the values in multilayer MoS2 published before. Moreover, these values can be further improved by increasing Vds and Vgs which will be comparable or exceed with the performance in monolayer MoS2 device. The mechanism of the device is finally explained by simultaneously using photocurrent mapping and band alignment diagram. For photo-detector based on six-layer GaSe, a photoresponsivity of 1.1×10 -2 A/W and fast photo-response (less than 400 ms) are observed which can be further improved by using thicker layers because of a direct band gap. For the photo-detector based on few-layer InSe, we firstly used carbon as the contact material however the Schottky contact we obtained. The Cr/Au contacts are then designed in both 11-layer and 20-layer InSe, in which the photoresponsivity and EQE in the two devices are 0.025 A/W, 1.13% and 0.101 A/W, 23.5% without back gate respectively. A Vgs of 60 V is finally applied in the devices and the performance is hundred times increased. However, as we showed in Chapter 3, the III-VI materials such as InSe are not stable exposing in the air which is an obstacle to the device application but can be overcomed by coving it with graphene. In addition, Carrier mobility in these semiconductors being relatively low compared with graphene, efficiency can suffer due to fast recombination of photo-excited electron-hole pairs, leaving room for further improvement. In the next Chapter, hybrid and vertical heterostructures are fabricated in order to both protect materials in the air and enhance the performance of the photo-detector.

Chapter 5 Photo-detector based on van der Waals heterostructure

This chapter is partly based on our publication: Z. Chen, J. Biscaras and A. Shukla, "A High performance graphene/few-layer InSe photo-detector", Nanoscale, 7, 5981-5986, 2015

Introduction of vertical van der Waals heterostructure

Van der Waals heterostructures made by stacking various 2D materials on each other are attractive because of the possibility of combining specific properties of different materials in a device which maintains a 2D character [1][2][3][4][5] . The principle is easy: it is an artificial structure where one manages the order of atomic layers by sequential transfer. The schematics of van der Waals heterostructures are shown in Figure 5.1. In the heterostructures, strong covalent bonds provide in-plane stability of 2D materials, and van der Waals like forces are sufficient to keep the stack together [1] . To date a lot of van der Waals heterostructures have been investigated both through theoretical simulation and experiment, and some of them show great potential for applications in electronics and optoelectronics. In this section, we will briefly introduce the progress of graphene based heterostructures, TMDCs based heterostructures and other 2D heterostructures from both the theoretical and experimental points of view.

Graphene based heterostructures

Graphene based heterostructures seek to benefit from the outstanding properties of graphene. In the following, graphene/h-BN stacks, graphene/TMDCs stacks and graphene/other 2D materials stacks are discussed.

Graphene/h-BN heterostructure: Theoretically it is found that a band gap of 53 meV can be opened in graphene by stacking monolayer graphene on top of lattice matched h-BN. This is because of the creation of inequivalent carbon sites due to the underlying BN and could lead to some specific electronic applications [3] . Experimentally a very high mobility up to 6×10 4 cm 2 V -1 S -1 was found in graphene/h-BN heterostructure which indicates that h-BN is a perfect substrate for graphene electronics [4] . In addition, atomically thin h-BN can act as barrier layer between two graphene layers in FET devices. For example, a switching ratio of ≈50 was achieved in graphene/h-BN/graphene FET and such kind of device has potential for high frequency operation and large scale integration [5] .

Graphene/TMDCs heterostructures: Graphene and TMDCs have distinct properties, but can be integrated into hybrid structures to achieve novel functionalities. By combining the high mobility of carriers in graphene and band gap in TMDCs, various devices based on graphene/TMDCs heterostructures have been fabricated and further investigated [6][7][8][9][10][11] . For example, the doping mechanisms in the graphene/MoS2 interface were investigated by a combination of first principles calculations and photovoltaic measurements, which proved the applicability of the heterostructure for photo-detection [6] . In addition, L. Britnell et al.

investigated the photovoltaic characteristics of a device with graphene/WS2/graphene (Figure 5.2 (a-c)) where strong light-matter interactions lead to large photon absorption and photocurrent production [7] . Ideally, there is no preferred direction for electron/hole transfer because of the symmetric structure of the device. However, in the presence of an electric field due to the applied gate voltage or by doping the top graphene layer with water vapor, the photo excited electron-hole pairs generated from WS2 are separated and transferred in opposite directions easily because of the nm scale transfer length. Further density functional theory (DFT) calculations indicated the localized character of the electronic wave functions in WS2 associated with van Hove singularities in the density of states. Furthermore, K. Roy et al.

demonstrate that graphene/MoS2 heterostructure (Figure 5.2 (d-f)) displays remarkable dual optoelectronic functionality, including highly sensitive photo-detection (nearly 1×10 10 A/W at 130 K and 5×10 8 A/W at room temperature) and gate-tunable persistent photoconductivity [8] .

The hybrids could also function as a rewritable optoelectronic switch or memory, where the persistent state shows almost no relaxation or decay within experimental timescales, indicating near-perfect charge retention. Graphene/other 2D material heterostructures: Graphene based heterostructures are not limited to h-BN and TMDCs. Theoretically, a large number of graphene based vertical heterostructures such as graphene/silicene, graphene/phosphorene and graphene/g-ZnO have been simulated recently, and the results provide a promising experimental roadmap for electronic, electrochemical, photovoltaic photoresponsive and memory devices [12] . For example, the structural and electronic properties of single-layer and bilayer phosphorene with graphene were studied by means of DFT calculations [13] . The results show that the properties of graphene and phosphorene are preserved on contact, and it is possible to tune the position of the band structure of phosphorene relative to that of graphene through the application of an external electric field perpendicular to the system. However experimentally, there are very few reports focusing on other 2D materials combined with graphene. For the first time, we built a graphene/InSe heterostructure and characterized the performance of the resulting device [14] . These results are shown in section 5.2. Contemporarily, sandwiched vertical and planar graphene/InSe/graphene heterostructures were fabricated by G. W. Mudd et al.

showing high photoresponsivity and broad spectral response [15] .

TMDCs based heterostructures

2D layers with different lattice constants can be stacked together in TMDC/TMDC structures with weak interlayer bonding as traditional heterostructures grown epitaxially.

Though we have not experimented with these heterostructures we give a brief overview because based on this structure, novel ultra-thin p-n vertical diodes and LEDs with van der Waals TMDCs heterointerfaces were studied and underlying microscopic processes identified are shown to be different from the traditional p-n junction devices [16][17][18][19] . TMDCs based heterostructures such as MoS2/WS2 heterostructures and MoS2/WSe2 heterostructures reported in literature are discussed below.

MoS2/WS2 heterostructure: The MoS2/WS2 hetero-bilayers system has a direct band gap and the lowest energy electron and highest energy hole states in the optically active K point are localized on different monolayers. In this sense, the Mo-W bilayer forms a type II heterostructure, with future promise for optospintronics in MoS2/WS2 heterostructure [20] .

Experimentally, high quality vertical and lateral MoS2/WS2 heterostructure with clean and atomically sharp interfaces is fabricated directly by CVD method which paves a way for the creation of unprecedented 2D materials with exciting properties [21] . In addition, ultrafast charge transfer in photo excited MoS2/WS2 heterostructure is experimentally observed using both photoluminescence mapping and femtosecond pump-probe spectroscopy by X. Hong et al. [22] . The results show that the hole transfer from MoS2 layer to WS2 layer takes place within 50 fs after optical excitation, which is a remarkable rate for van der Waals coupled 2D layers. Furthermore, vertical and planar multilayer MoS2/WS2 heterostructure is made and the optoelectronic properties are studied [19] . The vertical structure has a rectifying and bipolar behavior which can act as photovoltaic cell and self-driven photodetector with photoswitching ratio exceeding 10 3 . The planar structure has much higher FET on/off ratio, electron mobility and photo-responsivity compared to isolated multilayer MoS2 or WS2.

MoS2/WSe2 heterostructure: Strong interlayer coupling in MoS2/WSe2 hetero-bilayers is observed from a large Stokes-like shift of ∼100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission which is shown in Figure 5.3 (a-c) [23] . The coupling at the interface can be readily tuned by inserting dielectric layers into the van der Waals gap, consisting of h-BN. In addition, atomically thin MoS2/WSe2 p-n junction is fabricated and the optoelectronic properties are characterized by C. Lee et al. shown in Figure 5.3 (e-f) [17] . From their findings, there are gate tunable diode like current rectification and photovoltaic response across the p-n interfaces, and the collection of photo-excited carries is enhanced by adding top and bottom graphene layers. Reprinted from Ref [23] and [17].

Other 2D materials based heterostructures

Other 2D materials based heterostructures such as GaSe/Si, GaTe/Si, phosphorus/MoS2

and GaSe/InSe have been reported and all of them have diode like behavior. For example, the wafer-scale arrayed p-n junctions based on GaSe/Si and GaTe/Si are fabricated by X. Yuan et al. [24][25] , and both photodiodes show steady rectifying characteristics, high EQE (23.6% and 62% respectively) and super-fast photo-response times (60 μs and 22 μs respectively). In addition, p-type black phosphorus/n-type monolayer MoS2 heterojunction is formed and this ultra-thin p-n diode shows a maximum photo-detection responsivity of 418 mA/W at the wavelength of 633 nm and photovoltaic energy conversion with an EQE of 0.3% [26] .

123 Furthermore, heterojunction diode formed by combining layers of p-type GaSe and n-type

InSe emits photons at lower energies, which is attributed to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface [27] .

However, the thickness of this device is more than 1 μm.

We have chosen to fabricate a graphene/InSe photo-detector, and the performance of this device is characterized in section 5.2. In section 5.3 graphene/InSe/graphene and graphene/InSe/Au sandwich heterostructures are also fabricated and a photovoltaic effect is found. Furthermore, we have also combined 2D III-VI materials such as InSe with TMDCs such as few-layer MoS2 and eventually added graphene as a transport layer. An interesting feature of heterostructures in general and the InSe/MoS2 heterostructure in particular are the interference enhancement of the Raman scattering signal in the normal geometry and with a SiO2/Si substrate. This is simulated and compared to experimental Raman spectra in such samples in section 5.4. Finally in section 5.5 an InSe/MoS2 heterostructure device is investigated.

Photo-detector based on graphene/InSe heterostructure

There are two advantages of this heterostructure: First, ultra-thin InSe is protected by the hermetic, covering graphene which makes the device stable in air (shown in Chapter 3).

Second, the performance of the graphene/InSe photo-detector is greatly enhanced compared with a few-layer InSe photo-detector.

I-V characterization and mechanism of Graphene/InSe heterostructure

We fabricate a graphene/InSe heterostructure photo-detector and characterize it with 532 nm light illumination as schematized in We use 20 layers as a compromise between the need to use ultra-thin material for the vertical geometry and the need to increase absorption. In In Figure 5.4 (e-g) we show schematic band diagrams of the graphene/InSe interface for various regimes of the device as defined by the gate voltage. The work functions of graphene and InSe are different (4.56 eV and 5.2 eV respectively) [28][29] and when the two materials are in contact the Fermi levels must coincide at the interface. The electronic bands of InSe bend as a consequence. Considering Figure 5.4 (d) again, we notice firstly that the CNP for the graphene layer in the dark device is at 20 V which indicates hole doping, probably introduced by the transfer process and confirmed through the measurements of Raman shifts. As noted above, the CNP shifts under illumination to 6 V which means that graphene is now less hole doped. Electrons generated by illumination in InSe flow into graphene, nearly compensating the initial hole doping.

Therein lies the mechanism for generating ample photocurrent as the difference between these two shifted transfer curves for the top graphene layer. This is also the reason why the photocurrent changes sign [9]

Performance comparisons of photo-detectors based on few-layer InSe and graphene/InSe heterostructure

The variation of the photocurrent with illumination power is extracted from Figure 5.4 (c) and shown in Figure 5. 5 (a). This variation can be used to parameterize our device. The photocurrent can generally be expressed as [30][31] :

I ph =WC i µ∆V gs V ds /L (5.1)
where Ci is the total capacitance of the gate dielectric and few-layer InSe per unit area, μ is the carrier mobility of graphene, ΔVg is shift of the voltage at the Dirac point of graphene, W and L are the width and the length of the channel, respectively. In addition, ΔVg has a nonlinear power law dependence on the illumination power which can be fitted phenomenologically:

∆V gs = 𝜶P β (5.2)
where P is the illumination power density per unit surface, and α and β are parameters. Using relations (5.1) and (5.2) we can extract the dependence of the photocurrent on two critical parameters: the source-drain voltage and the illumination power density:

I ph =αWC i µV ds P β /L =γV ds P β (5.3)
where γ is a dimensional parameter characterizing the device. Relation (5.3) shows the dependence of Iph on both Vds and P and is used to parameterize the dataset of Under the same illumination condition (532 nm wavelength light at a power density of 12.6 W/m 2 ) we obtain for the graphene/InSe heterostructure R=0.94×10 3 A/W and EQE =2.18×10 5 % with Vds=-50 mV and Vgs=0 V. This is four orders of magnitude higher than the values of R=0.101 A/W and EQE=23.5 % in the few-layer InSe device with Vds=5 V and Vgs=0 V. In the case of the heterostructure, the enhanced EQE represents the gain in efficiency but not the number of photo-excited pairs per absorbed photon anymore since the photocurrent is generated by a different mechanism. As expected, efficiency decreases as incident power increases. This variation follows a power law by taking equation (5.3) into account:

R=I ph /(P×S) =γV ds P β /(P×S) =γV ds P β-1 /S (5.4)

and we find the exponents β≈0.39 and β≈0.78 for the graphene/InSe heterostructure and fewlayer InSe respectively. It is worthwhile noting that the values of R and EQE depend critically on the conditions of measurement as seen in equation (5.4). R increases linearly with Vds but in particular can be increased greatly at very low power as seen by the inverse dependence in relation (5.4). For example, if we refer to Figure 5.5 (a), dIph/dP is nearly 50 times higher at an illumination of 12.6 W/m 2 compared to an illumination of 6.3×10 3 W/m 2 . Clearly the heterostructure is a spectacular improvement with respect to the homostructural few-layer InSe device which has similar absorption and photoelectron generation but with R and EQE four orders of magnitude lower.

Comparison of photo-detectors based on few-layer InSe and graphene/InSe heterostructure

A rough estimation of this increase in efficiency can be made by comparing active volumes between the heterostructure and the few-layer InSe device shown in Figure 5.6. In InSe single crystal, the values of mean carrier mean free path are 1.75 nm and 5.08 nm in the directions parallel and perpendicular to the c-axis respectively [32] . This means that in our fewlayer InSe device (shown in Therefore, adding a graphene monolayer provides very efficient transport of the photoelectrons as compared to InSe where recombination is high except in areas within nanometric distances from the source and drain contacts. Since the InSe layer is only a few nanometers thick, photoelectrons from the whole volume can be potentially extracted vertically into the graphene layer. Graphene is not an efficient photo-detector, with high dark current and low absorption and photoresponsivity. However when InSe is used for generating photoelectrons we have seen that the generated charge is enough to produce significant changes in the Fermi level of the adjacent graphene monolayer. Finally we have also shown that the graphene layer spectacularly enhances the performance not only through its electronic properties but also as a protective packaging for the InSe layers conferring ambient condition compatibility to the device. A possible improvement to this device is to use a dry transfer process to achieve better interface conditions between graphene and the semiconductor layer.

Graphene/InSe/graphene and Graphene/InSe/Au heterostructures with photovoltaic effect

As discussed above, photo-detectors based on few-layer InSe have low photo-responsivity and EQE compared to graphene/InSe heterostructure in which the photo-excited carries can be extracted vertically into the graphene layer before recombination and then transported through the graphene layer. However, the shortcoming of the graphene/InSe device is that the dark current is very high because of the graphene layer which connects the source and drain.

Indeed the dark current is of the order of a few tens of A. The photocurrent which is the difference between the light and dark currents, is relatively high, of the order of a A, but the high dark current results in a very low on-off ratio and restricts the application of photodetectors based on this structure. An improvement on this structure is to fabricate graphene/InSe/graphene or graphene/InSe/Au sandwiched heterostructures, in which the photo-excited carries generated from InSe can also be effectively transported by graphene or Au but the dark current is very low because the source and drain are each connected to different conducting layers, separated by the InSe semi-conducting layer. This results in a high on-off ratio and easily detectable photovoltaic effect in these devices.

In this section, we fabricated two types of sandwich heterostructures:

graphene/InSe/graphene and graphene/InSe/Au. The performance of the devices based on these heterostructures are characterized and the corresponding mechanisms are explained.

Photo-detector based on graphene/InSe/Graphene heterostructure

A Graphene/InSe/graphene heterostructure is fabricated on SiO2/Si substrate by precise wedging transfer (described in Chapter 2). The schematic of the device based on this heterostructure is shown in electron-hole pairs in InSe will be recombined without any potential between the two pristine graphene. However in our case, there is a built-in potential difference between the two graphene layers which have the different Fermi levels resulting from the different p-doping during the device fabrication. Indeed graphene is most often p-doped due to ambient contamination, usually from water vapor or from the wedging transfer process or charging due to the substrate oxide (see W. J. Yu et al. [11] ). This 'uncontrolled' p-doping is generally different in the two graphene layers. In our device for example the bottom layer is in contact with the substrate and has been transferred twice using the wedging technique. The different Fermi levels in the top and bottom graphene result in a built-in potential and the photo-excited electrons can drift into the bottom graphene thus generating photocurrent without an applied bias voltage. Of course the doping level of bottom graphene can also be changed in a controlled manner by electrostatic doping using the back gate.

Photo-detector based on graphene/InSe/Au heterostructure

Though our graphene/InSe/graphene heterostructure has a photovoltaic effect and high performance of photo-detection without Vds and Vgs, the doping levels in graphene are only partially controlled and depend on the particular fabrication process which might cause unstable performance of the device. To try and improve on this shortcoming we fabricate graphene/InSe/Au sandwich device by random dry transfer on glass. The schematics of this heterostructure is shown in Figure 5.9 (a). Firstly, the bottom monolayer graphene is prepared on glass using the anodic bonding method which generates large area or many pieces of small high quality areas. This makes the random dry transfer possible. Secondly, few-layer InSe is mechanically exfoliated onto graphene on glass using the optimum parameters detailed in Chapter 3. Lastly, the top Au layer contact and other electrodes are evaporated after electron beam lithography. Since the Au contact is opaque, during our optoelectronic measurement the light is shining through the glass onto the heterostructure. The front view of the device is shown in Figure 5.9 (b) and the bottom graphene is highlighted using red dashed line. The effective area in our device is around 1.378×10 3 μm 2 .

The corresponding photo-response (Vds=0 V) and the I-V characterization are performed without Vg as shown in Figure 5.9 (c-d). From the photo-response of the device under the 532 nm laser spot focused on the center of our device, the complete rise and decay times are within 20 ms and 10 ms respectively which indicates a very fast photo-response device comparable to the graphene/InSe/graphene heterostructure. Similarly, the dark current is very low and the on-off ratio of our device is higher than 10 3 . However the photocurrent is negative which is opposed to photo-detector based on graphene/InSe/graphene. In the following we will explain this observation using band diagrams.

In addition, from I-V characterization under illumination of the whole device, there is a negative photocurrent around -0.6 nA (Isc), and Voc is 0.18 V, which means a photovoltaic effect also exists in this device. Moreover, the absolute values of photoresponsivity and EQE are calculated at 0.035 A/W and 8.16% respectively which are much lower than our graphene/few-layer/graphene device and could be further improved by adjusting Fermi level of graphene during anodic bonding process.

Friability of vertical heterostructure during photoelectric measurement

These vertical van der Waals heterostructures are very 'fragile' during measurement because of the atomic scale transport path and very high electric field between the different layers compared to the traditional lateral 2D devices. They often break down during measurement and the device failure is hard to distinguish from simple optical observation.

Here we show examples of two devices which broke down during measurement from the optical images and the corresponding electrical characteristics.

From the optical image of graphene/InSe/graphene and the photo-response of the device before breakdown shown in We also examined the performance of graphene/InSe/Au heterostructure after breakdown as shown in Figure 5.11 (d-f). From the optical image we do not observe differences in the device before and after breakdown. However, from photocurrent mapping, the photocurrent is negative along the whole device except several areas with high positive photocurrent which are highlighted by black circles, which means the electrons changed flow direction there compared to the surrounding area. It is worth noting that the photocurrent is uniformly distributed in the whole device before breakdown. Thus the current inhomogeneity is caused by the breakdown of the device. In particular the bottom graphene may touch the Au layer due to damage of the InSe layer which changes the built-in potential. In addition, from the I-V characterization in Figure 5.11 (c), the curve is linear which is similar to graphene and very different from the I-V curve of Figure 5.9 (d). This behavior is also a sign that graphene touched Au after breakdown.

The light out-coupling in InSe/MoS2 heterostructure

Light-matter interactions in 2D materials and optoelectronic devices based on these materials are at the heart of a sustained research effort. For example, incorporating plasmonics and microcavities with 2D materials has been shown to improve coupling with light and achieve higher light absorption [33][34] . In addition, recently, it was demonstrated that the substrates play an important role in the coupling of light with the materials or heterostructures deposited on them. The Raman intensity of 2D materials (such as graphene, MoS2, WSe2) deposited on substrates can thus be enhanced or attenuated with varying thickness of SiO2/Si substrates [35][36][37] . The interaction of light with van der Waals heterostructure is more complicated and can be seen in the enhancement or attenuation of Raman modes in the heterostructure as a function of the wavelength of the excitation [19,26] . In this section, we study the interface light out-coupling in InSe/MoS2 heterostructure by the variation of Raman intensity with the thickness of InSe and MoS2. Experimental measurements from Raman spectroscopy are compared to results from simulation using a simple model for normal incidence of light on a heterostructure. Raman spectra with 638 nm illumination:

Experimental Raman shift of

For MoS2 Raman modes no conclusions can be derived because we approach resonance conditions at 638 nm and comparisons become meaningless.

For InSe Raman modes, by changing the wavelength of excitation the earlier trends are inversed. For example the A 1 1g Raman mode of InSe from area C is highly attenuated compared to the Raman intensity in area A shown in Figure 5.11 (g). This is also verified by the Raman map in Figure 5.12 (e). The peaks at 175 cm -1 in Figure 5.11 (g) in areas B and C are due to a resonant Raman MoS2 mode.

Correspondingly for the Si Raman mode, intensity is enhanced by 2 times in region A compared to region E which means the overlying InSe layer amplifies Raman intensity by trapping light. The corresponding intensity Raman map for the Si mode is shown in Figure 5.12 (f) (compare with Figure 5.12 (d), 532 nm).

Simulation of light out-coupling in InSe/MoS2 heterostructure

These apparently contradictory results with enhancement or attenuation of the same modes according to the wavelength used can be explained by introducing a multiple reflection, normal incidence model, as shown in Figure A3 of Appendics A1.2.

A simple physical picture of the phenomenon is the following: The factor multiplying the intensity of the Raman mode depends on two quantities: The total incident light at the depth x in a given layer of the heterostructure. This in turn will depend on the incoming intensity of the incident light and the reflection/transmission at the different interfaces; The total 'emission' from the layer at depth x. This will depend on the initial field (calculated in 1) times the coefficient coming from the reflection/transmission at the different interfaces of the inelastically scattered light. The factor multiplying the Raman intensity will thus be the square modulus of these two quantities, integrated over the depth x to account for the whole layer thickness.

Considering the multiple interfaces in this heterostructure, incident light will encounter the boundaries air/InSe, InSe/MoS2, MoS2/SiO2 and SiO2/Si, and undergo multiple reflections.

The four media including air, InSe, MoS2, SiO2 and Si are labeled 0, 1, 2, 3 and 4 respectively.

As argued above, taking into account both the incident light at a given depth x in the layer and light emission from the same depth x, the Raman intensity out-coupled from InSe layer can be defined as:

𝐼 = ∫ |𝐹 𝑖𝑛 (𝑥)𝐹 𝑒𝑚 (𝑥)| 2 𝑑𝑥 𝑑 0 (5.5)
where d is the thickness of InSe layer, and 𝐹 𝑖𝑛 (𝑥) and 𝐹 𝑒𝑚 (𝑥) are the amplitude of the incident light and the amplitude of the emitting light at depth x in the layer after taking into account interference effects due to reflection and transmission at the various interfaces. The detailed calculation can be seen in the Appendix. As an example, the calculated result for changes in Raman intensity from InSe due to this interference effect corresponding to the regions A, B and C are shown in Figure 5.13 (a-b). We also simulate the same as functions of InSe thickness and MoS2 thickness under different incident light (Figure 5.13 (c-d), respectively).

In Figure 5.13 (a) we show our simulations for enhancement of InSe Raman intensity in three different MoS2 thicknesses as a function of the InSe thickness under incident wavelength of 532 nm. From this result, we find a large enhancement in 28.1 nm InSe/33.9 nm MoS2 heterostructure (corresponding to area C), and much smaller intensity for 28.1 nm InSe (area A) followed by 28.1 nm InSe/20.2 nm MoS2 heterostructure (area B). This is in complete agreement with the experimental result of Figure 5.11 (d) and Figure 5.12 (b). For an incident wavelength of 638 nm this sequence is changed which is shown in Figure 5.13 (b). Now the Raman intensity is highest from 28.1 nm InSe (area A) and much lower from 28.1 nm InSe/33.9 nm MoS2 heterostructure (area C)and finally lowest from 28.1 nm InSe/20.2 nm MoS2 heterostructure (area B). This is again in complete agreement with the experimental result of Figure 5.11 (g) and Figure 5.12 (e), where care must be taken to disregard the Raman lines at about 170 cm -1 which are due to resonant scattering from MoS2. Thus a simple normal incidence interference model successfully explains the Raman intensity variation in different areas of our heterostructure. In areas A, B and C the thickness of InSe is uniformly 28.1 nm.

In our simulation we have investigated the influence of InSe layer thickness from 0 to 120 nm and MoS2 layer thickness from 0 to 100 nm under different wavelengths of incident light as shown in Figure 5. 13 (c-d). The white dashed lines in the maps indicate the results for 0 nm MoS2, 20.2 nm MoS2 and 33.9 nm MoS2 that we discussed above. From these three maps, we can clearly see the variation of the measured Raman intensity as function of both thicknesses of InSe and MoS2 due to interference effects. In a device based on photocurrent measurement an efficient coupling of the incident light to the device is essential and for example this is a constant source of improvement in photovoltaic devices. Reflection at the device surface needs to be minimized and light absorption inside the device needs to be maximized. Using this kind of calculations one can design a heterostructure by choosing the optimum thickness, substrate and stacking to maximize device performance.

Photo-detector based on InSe/MoS2 heterostructures

A key parameter in semiconductor diodes is the thickness of the junction, which is typically 10 nm to 1 μm depending on the diode type. For van der Waals heterostructures, the thickness is on the lower end of this scale which is ideal for achieving a high efficiency photoelectrical device. Moreover van der Waals p-n junction have attracted recent attention because of photovoltaic effect and diode like behavior which is of enormous interest in applications such as photodiodes, light emitting diodes (LED) and solar cells [16][17]38] . For example, ultra-thin TMDC/TMDC p-n diodes, p-black phosphorus/n-MoS2 and p-GaTe/n-Si heterojunctions have been fabricated and show outstanding optoelectronic properties [16,[25][26] .

However, few devices have been reported where the junction is between materials with the same kind of majority charge carriers like for example the n-WS2/n-MoS2 heterostructure. The vertical device built around such a structure has novel rectifying properties and bipolar behavior and the planar structure shows good performance for photo-detection [19] . In this section, for the first time, we combine the 2D III-VI material InSe with TMDC material MoS2.

We achieve efficient photodiode like behavior and show that both photo-detection and photovoltaic effect can be obtained based on these structures. Photo-detectors based on

Photo-detector based on InSe/MoS2 vertical heterostructure

The optical image of InSe/MoS2 photo-detector is shown in Figure 5.15 (a-b), the thickness of InSe is around 50 nm so as to maximize light absorption. The MoS2 layer is only 6 nm so as to maximize electrostatic doping of it by the back gate.

We firstly characterized the device by photocurrent mapping without Vds as shown in We can further calculate two important parameters such as fill factor (FF) and power conversion efficiency (𝜂 𝑃𝑉 ) for the photovoltaic application. The FF is defined as the radio of the maximum power (Pmax) from the photovoltaic device to the product of Voc and Isc, and the formula is FF=Pmax/(Voc×Isc). The 𝜂 𝑃𝑉 is the percentage of the incident light energy that is converted into electrical energy, 𝜂 𝑃𝑉 = P max /P in , where Pin is the incident light power. In our device, FF and 𝜂 𝑃𝑉 are calculated at 0.21 and 0.11% respectively under an illumination power of 3150 W/m 2 . Moreover, as a photo-detector, the photoresponsivity and EQE are calculated as a function of illumination power shown in Figure 5.15 (f) and the maximum values are 0.17 A/W and 39.6% which are comparable to published values for other van der Waals p-n heterostructures [26,38] . Two questions can be asked regarding this device:

(1) Why is there a negative photocurrent generated in our device? (we use a diode symbol here because of the rectifying behavior). There are two key factors to remember here. Firstly the sheet resistance in InSe (black symbols) is several order of magnitude more than in MoS2 (grey symbols). Secondly the top Au contact does not cover the whole InSe surface. When light is focused close to the top Au contact, hole transport through InSe to the Au contact is easy (high resistance but short distance) while the electron transport through MoS2 is also easy (long distance but low resistance). Thus the total resistance for current through the device can be schematized by R1+ RD+ R5 +R6 +R7+ R8. On the contrary, when light is focused away from the Au contact, total resistance for the current through the device is much higher due to the difficult hole transport through InSe (high resistance and long distance) schematized by R1+ R2+ R3 +R4 +RD+ R8. Therefore, the photocurrent is much stronger close to the top Au contact than away from it.

In the next section we address this problem so as to improve the device.

Photo-detector based on InSe/MoS2 heterostructure with graphene transport layer

To further improve the distribution of the photocurrent in the whole device, a photodetector based on InSe/MoS2 vertical van der Waals heterostructure is fabricated with graphene as transport layer on the top. There are three advantages of using graphene as top transport layer: First, graphene has very low sheet resistance. Second, graphene has high transparency. Third, graphene can effectively protect InSe in the air as we have discussed in Chapter 3.

We now briefly describe the fabrication of the device. Few-layer MoS2 (about 6 nm thick) was mechanically exfoliated onto SiO2/Si substrate firstly, and then few-layer InSe (about 40 nm thick in our case) was random dry transferred onto MoS2. This step was followed by wedging transfer of graphene onto the InSe/MoS2 heterostructure. The optical images of InSe/MoS2 heterostructure with graphene transport layer are shown in Figure 5.17 (a-b).

We now characterize our device. The photocurrent map at zero bias and under 532 nm focused laser illumination is shown in Figure 5.17 (c). The photocurrent is around -25 nA in the whole device except the area close to the Au contact where it is higher. This is in contrast to the InSe/MoS2 heterostructure without the graphene transport layer where the photocurrent decreases fast out of the contact.

Ids-Vds characterization was performed under uniform illumination of the whole device and varying illumination power as shown in Figure 5.17 (d-f). In the closeup in panel (e), the dark response shows diode-like behavior. However when we shine light on the device, the photocurrent decreases fast when the voltage decreases from 0 V to negative voltage and starts to saturate at a certain point. Moreover, this device also has a photovoltaic effect and Isc and Voc are both higher than the values in InSe/MoS2 heterostructure without the graphene transport layer. For example, the Isc and Voc in this device are -4.8 nA and 220 mV under illumination power of 1260 W/m 2 which are much higher than the values in InSe/MoS2 heterostructure without graphene transport layer as shown before. The photo-response of photo-detector based on InSe/MoS2 heterostructure with graphene transport layer is further characterized in Figure 5.18. The photocurrent can be modulated by applying Vgs which adjusts the Fermi level in MoS2. Moreover, the device shows ultra-fast photo-response both current rise and decay being completely within 1 ms. This photoresponse time is three orders of magnitude less than that in homostructural InSe, GaSe and MoS2 photo-detectors. The mechanism of our device is analyzed using band diagram alignment and a simple model shown in Figure 5.20. The band diagram is the same as for the InSe/MoS2 heterostructure without graphene transport layer. It is worth noting that the Vg can change the Fermi level in MoS2, which will increase or decrease the Shottky barrier and thus effectively influence the photocurrent. As we discussed before, the sheet resistance of InSe plays a key role in the photocurrent distribution. However in this device model, the sheet resistance of InSe is inconsequential because of the graphene sheet in parallel with the InSe layer. In addition, the resistance at the graphene/InSe interface (Ro) can be considered ohmic or nearly so [15] . Because the sheet resistance of graphene is less than that in MoS2, the photocurrent is still not completely uniform in the whole device, with a somewhat higher current in the region close to the Au contact.

Summary

In this chapter we report firstly on the fabrication of a graphene/InSe heterostructure photo-detector by transferring anodic bonded graphene on top of mechanically exfoliated fewlayer InSe. This hybrid device shows a four order of magnitude enhancement in photoresponsivity and EQE with respect to a simple few-layer InSe device under the working conditions of low Vds and high incident power. By adjusting the gate voltage the device can perform either with holes or with electrons with a photocurrent of the order of a microampere. This kind of performance, earlier seen in TMDC based heterostructures is possible thanks to the combination of absorption and photoelectron generation characteristics of InSe and efficient charge transport in graphene.

In addition photo-detectors based on graphene/InSe/graphene and graphene/InSe/Au heterostructures are also fabricated. Thanks to the built-in potential these sandwich devices have a photovoltaic effect, fast photo-response time and high on-off ratio compared to graphene/InSe photo-detector. Furthermore we also investigate the interference effects of substrate and InSe/MoS2 heterostructures on the detected Raman intensity from a given layer in the heterostructure.

Simulation of the interference effects from the transmitted and reflected rays at the different interfaces with a simple normal incidence model is in accordance with experimental results.

The correct description of the observed Raman enhancement or attenuation means that such optoelectrical devices can be tuned to higher performance according to substrate characteristics and layer thickness.

Finally, Photo-detectors based on InSe/MoS2 heterostructure with and without graphene as transport layer are characterized. The photovoltaic effect and diode-like behavior are observed in this structure, and the photo-response time is within 1 ms which is nearly three orders of magnitude less than the photo-detectors based a single 2D material.

General Conclusion and Perspectives

General Conclusion

In this thesis, we investigated novel 2D materials beyond graphene. These include MoS2, GaS, GaSe and InSe. The work started with the preparation of these 2D samples using the universal exfoliation method and our unique anodic bonding method simultaneously. It continued with the extensive characterization of the few-layer materials including optical contrast, vibrational properties and lattice structures. The 2D materials were then used to make devices. First homostructural photo-detectors based on few-layer MoS2, GaSe or InSe were fabricated and analyzed respectively. Finally, various heterostructures such as graphene/InSe, graphene/InSe/graphene, graphene/InSe/Au, InSe/MoS2 with and without graphene transport layer were fabricated and their photo-detection properties characterized and compared to the homostructural devices. The comparison showed much improved performance and the appearance of new properties such as the photovoltaic effect. Firstly, we synthesized 2D materials of graphene, MoS2, GaS, GaSe and InSe using two techniques, the mechanical exfoliation method and the anodic bonding method. We modified the mechanical exfoliation method by using a hot substrate (100 ºC -125 ºC) to increase the adhesion force between precursors and the substrate. For our anodic bonding method, we obtained optimum parameters for each material by preparing a very large number of samples.

Large area and high quality 2D materials were selected for device fabrication.

Secondly, these 2D materials were characterized using several tools such as optical microscopy, AFM, micro-Raman, micro-PL and TEM. The optical contrast, studied by both simulation and experiment, can be easily used to identify the number of layers. This is largely time-saving compared to AFM characterization. From micro-Raman, the vibrational properties of 2D materials were obtained as a function of the number of layers. From micro-PL, the shifts of band gaps as function of the number of layers were observed in these materials. For example, in MoS2, the band gap exhibits an indirect-direct transition when the thickness of MoS2 decreases from bulk to monolayer. However for InSe, the band gap has an opposite transition, exhibiting a direct-indirect transition when the thickness of InSe decreases from bulk to 7 layers and below. TEM was used to examine lattice structures of GaS, GaSe and InSe and confirm the ε polytype of our InSe precursor by combining with Raman analyses.

Thirdly, photo-detectors based on few-layer MoS2, few-layer GaSe and few-layer InSe were fabricated and characterized with 532 nm wavelength light. For the photo-detector based on few-layer MoS2, photoresponsivity of 316 A/W and EQE of 6.96×10 4 % were obtained in our device. These are much higher than the published performance of a multilayer MoS2 photo-detector. For the photo-detectors based on few-layer GaSe, we fabricated these on both glass and SiO2/Si substrates with physical mask and electron beam lithography and tested the photocurrent distribution. For the photo-detectors based on few-layer InSe, we made the devices with carbon and Au contacts, and studied the mechanism of functioning and the photocurrent distribution.

Lastly, graphene based heterostructures were made in order to improve the performance with respect to the photo-detectors based on the 2D semiconductors. This had the additional advantage of protecting ultrathin 2D semiconductors from degradation in ambient condition.

For the first time, we fabricated a graphene/InSe heterostructure photo-detector which showed a largely improved performance compared to the device without graphene. The photoresponsivity and EQE are enhanced by four orders of magnitude and the ultra-thin degradation prone InSe is very stable in this structure. In addition photo-detectors based on graphene/InSe/graphene and graphene/InSe/Au vertical heterostructures were made by wet transfer and dry transfer respectively, and their performances were analyzed in the absence of gate and bias voltages. These structures are rather 'fragile' but both show a photovoltaic effect which can be easily understood by the alignment of electronic bands at the interfaces.

InSe/MoS2 heterostructures which combine TMDCs with III-VI materials for the first time were fabricated by dry transfer. The light out-coupling properties of this heterostructure were analyzed by simulation and measured experimentally through the intensity of Raman modes. Photodiode like behavior was observed though the majority carriers in both InSe and MoS2 are electrons. This device was further improved by using graphene as a transport layer and a novel photo-detector with ultrafast photo-response time, uniform photocurrent distribution and high photovoltaic effect was finally achieved. The corresponding mechanism was explained by a simple device model and band alignment. Each layer have a complex index of refraction: n i = n i -ik i where n i and k i are the real and imaginary part of the complex refraction index of the i (the media number: 1, 2, 3 or 4) layer. The Fresnel reflection (r ij ) and transmission (t ij ) coefficients for a light travelling from medium i to medium j and impinging on the 𝑖𝑗 interface are described as follows:

r ij = n i -n j n i +n j (A6) t ij = 2n i n i +n j (A7)
From the description, one can get the relations: r ij = -r ji and t ij t ji -r ij r ji = 1.

The phase will changed during the light traveling and the phase factors can be defined as:

β i = 2πn i d i λ
with n ≥ 1. For example, at a point x in the depth of InSe layer, the phase difference can be defined as β x = 2πn 1 x λ . Mots Clés: Graphène, InSe, MoS2, dispositifs ultra-minces, hétéro-structures, photodétecteur

A2-List of Abbreviations

𝑎 2 ( 1 , 2 ( 1 , 3 ) 4 )

 212134 √3), 𝜹 𝟐 = 𝑎 -√3), 𝜹 𝟑 = -𝑎(1, 0) (1.2)In addition, the reciprocal lattice vectors in Figure1.1 (b) can be given as:In the first Brillouin zone of graphene, the Dirac points K and K' are at the corners of the Brillouin zone:The tight-binding Hamiltonian can be used in the calculation of energy bands in graphene, and the energy band can be derived as:𝐸 ± (𝒌) = ±𝑡√3 + 𝑓(𝒌) -𝑡′𝑓(𝒌)(1.5)𝑓(𝒌) = 2 cos(√3𝑘 𝑦 𝑎) + 4cos (

Figure 1 . 1

 11 Figure 1.1 (a) The honeycomb lattice. The vectors δ1, δ2 and δ3 connect carbon atoms, separated by a distance a = 0.142 nm. The vectors a1 and a2 are basis vectors. (b) The first Brillouin zone with the center Γ and two inequivalent corners K and K'. b1 and b2 are the unit vectors in the reciprocal lattice. (c) Electronic dispersion in the honeycomb lattice Right: zoom in of the energy bands close to one of the Dirac points. Reprinted from Ref [3].

1. 2

 2 (a). A back gate voltage applied to Si induces a charge in graphene moving the Fermi level into the conduction band or valence band. The relationship between the shift of Fermi level 𝜀 𝐹 and the induced charge carrier concentration n is: 𝜀 𝐹 = √𝑛𝜋ħv 𝐹 . The change of resistivity as function of back gate voltage is shown in Figure 1.2 (b). For pristine graphene, the Dirac point lies at Vg = 0 V, and the charge carriers are holes when Vg < 0 V and electrons when Vg > 0 V. The measured Dirac point for a graphene sample in a given environment may be at a non-zero voltage due to parasitic doping. For example in a graphene/InSe heterostructure described later in this work, the Dirac point is at Vg = 20 V due to the residual doping during fabrication process.

Figure 1 . 2

 12 Figure 1.2 Electrostatic doping of graphene by back gate voltage (a) and the resistivity of pristine graphene as function of back gate voltage (b). When Vg < 0 V, graphene is p doped; When Vg > 0 V, graphene is n doped. Reprinted from Ref [7].

Figure 1 .

 1 Figure 1.3 (b) clearly shows the hexagonal structure.

Figure 1 . 3

 13 Figure 1.3 The crystal structure and band structure of MoS2. (a) The unit cell of 2H-MoS2 (b) Hexagonal structure of 2H-MoS2 from top of view (c-f) Band structure of bulk MoS2, quadrilayer MoS2, bilayer MoS2 and monolayer MoS2.The Figure (c-f) is reprinted from Ref [20].

Figure 1 . 4

 14 Figure 1.4 Unit cells of three kinds of Polytypes in III-VI materials such as GaS, GaSe and InSe. (a) β polytype (b) ε polytype (c) γ polytype.

Figure 2 . 1

 21 Figure 2.1 Optical images and the corresponding Raman spectra of bulk precursors and substrates.(a) and (d) are from bulk precursor of graphene (b) and (e) are from bulk precursor of MoS2 (c) and (f)are from bulk precursor of GaS (g) and (j) are from bulk precursor of GaSe (h) and (k) are from bulk precursor of InSe (i) is the borosilicate glass substrate (l) is the SiO2/Si substrate.

Figure 2 .

 2 2 (a). Firstly, the substrates are heated at around 100 C on a hot plate. Secondly, the precursors are exfoliated several times by adhesive tape shown in Figure 2.2 (b). Thirdly, the adhesive tape containing precursors is pressed on the substrates and then peeled off. Finally monolayer or few-layer samples are identified by optical microscopy as shown in Figure 2.2 (c).

Figure 2 . 2

 22 Figure 2.2 Mechanical exfoliation method. (a) Schematic of improved mechanical exfoliation method in our experiment (b) Graphene precursors exfoliated by an (blue) adhesive tape (l) An example of graphene prepared on SiO2/Si substrate.

Figure 2 . 3 A

 23 Figure 2.3 A schematic of Anodic Bonding principle.

Figure 2 . 4

 24 Figure 2.4 Anodic bonding technique. Left: the apparatus of our anodic bonding setup. Right: the control panel of our anodic bonding setup.

Figure 2 . 5

 25 Figure 2.5 (a) Description of the principle operation of an AFM. The tip follows contour B, in order to maintain constant force between tip and sample. (b) Setup of a dynamic force microscope operated in the tapping mode. A laser beam is deflected by the back side of the cantilever, and its deflection is detected by a split photo-diode. The cantilever vibration is caused by an external frequency generator driving an excitation piezo. A lock-in amplifier is used to compare the cantilever driving with its oscillation. The amplitude signal is held constant by a feedback loop controlling the cantilever sample

Figure 2 . 6 .

 26 Radiation with the same frequency ν0 to the incident electromagnetic wave is called Rayleigh scattering (elastic scattering). Scattered radiation with a different frequency (ν0±νvib), comes from the process called Brillouin or Raman scattering (inelastic scattering), depending on whether the frequency shift νvib is in the range of acoustic (GHz) or optical (THz) vibrations. The intensity of Brillouin or Raman scattered light is 10 3 to 10 6 times less than the intensity of Rayleigh scattering which means that a laser as light source is indicated to observe the Brillouin or Raman effect. In this part, we will explain the basic principle of Raman effect first and introduce Raman spectroscopy as well as the particular aspects important in 2D material study.

Figure 2 . 6

 26 Figure 2.6 Schematic illustration of Rayleigh scattering as well as Stokes and anti-Stokes Raman scattering. The laser excitation frequency (nL) is represented by the upward arrows and is much higher in energy than the molecular vibrations. The frequency of the scattered photon (downward arrows) is unchanged in Rayleigh scattering but is of either lower or higher frequency in Raman scattering. The dashed lines indicate the "virtual state". Reprinted from Ref [14].

Figure 2 . 7

 27 Figure 2.7 Optical image and the corresponding schematic of our Raman spectroscopy.

Figure 2 . 8

 28 Figure 2.8 Optical images of few-layer InSe before (a) and after (b) focused laser exposing with the powers of 12 μW, 120 μW and 1.2 mW respectively.

Figure 2 . 9

 29 Figure 2.9 The two basic operations of the TEM imaging system involve (A) projecting the diffraction pattern on the viewing screen and (B) projecting the image onto the screen. In each case the intermediate lens selects either the back focal plane or the image plane of the objective lens as its object. Reprinted from Ref [15].

Figure 2 . 10

 210 Figure 2.10 The wedging transfer process for sample 1/sample 2 heterostructure.

Figure 2 . 11

 211 Figure 2.11 Dry transfer methods. (a) Dual polymer based dry transfer (b) Pick up transfer technique.

Figure 2 . 12

 212 Figure 2.12 Comparison of wedging transfer method for few-layer GaS. (a-d) The conventional wedging transfer process from optical images of few-layer GaS prepared on glass, GaS with polymer on SiO2/Si substrate, GaS on SiO2/Si substrate and the corresponding AFM image (e-h) Our improved wedging transfer process which is the same sequence of optical images and AFM. The white rectangles in (c) and (g) represent the corresponding AFM areas.

Figure 2 . 13

 213 Figure 2.13 Schematic of multiple mechanical exfoliation technique in (a) and InSe/MoS2 heterostructure fabrication process by multiple mechanical exfoliation technique in (b-c). The MoS2 samples is shown in (b) and is highlighted by red dash line in (c).

Figure 2 .

 2 14 (a). The 2D material is firstly prepared on SiO2/Si substrate, and physical mask is then precisely aligned onto the sample and tightly stuck on the substrate. The contacts are made by evaporating metals and the device is finally fabricated by removing the physical mask. An example of our few-layer GaS device is shown in Figure2.14 (b-c). The size of our sample is larger than 50 μm × 50 μm, the minimum size allowed by the mask.

Figure 2 . 14

 214 Figure 2.14 Device fabrication using physical mask. (a) Schematic of device fabrication using physical mask (b) Our physical mask with six contacts (c) Few-layer GaS device fabricated using our physical mask.

Figure 2 . 15

 215 Figure 2.15 Optical lithography and electron beam lithography techniques. (a) Süss Micro Tec MJB4 mask aligner for optical lithography (b) Raith e-line system for electron beam lithography (c) Schematic of lithography process (d) Samples on the pattern made by optical lithography (e) Final device after electron beam lithography. The contacts prepared by electron beam lithography are highlighted by red circles and one device is zoomed in.

Figure 2 .

 2 Figure 2.15 (d-e). The detailed optical lithography and electron beam lithography steps and parameters we used in our experiments are depicted in the following.

4 .

 4 PMMA/sample/SiO2/Si or Al/PMMA/sample/glass is exposed by the electron beam in the Raith e-line system. Aperture: 120 μm for the wide parts of the contacts (far away from the sample) and 10 μm for the narrow parts (very close to the sample) contacts; Extra high tension (EHT): 20 kV; Dose: 350-420 μC/cm 2 . 5. Al/PMMA/sample/glass is firstly developed in potassium hydroxide solution (KOH)

Figure 2 . 16

 216 Figure 2.16 Fabrication process of Graphene/InSe/Graphene device (a) bottom graphene prepared on glass by anodic bonding method (b) graphene transferred on a patterned SiO2/Si substrate (c) InSe prepared on glass (d) InSe/graphene coved by CAB polymer (e) InSe/graphene after dissolving CAB polymer (f) top graphene prepared on glass (g) graphene/InSe/ graphene heterostructure (h) after ebeam lithography and development (i) final device. The top and bottom graphene on glass are highlighted using white dash line. Graphene/InSe/graphene sandwiched structure in the final device is highlighted using yellow dash line.

  Photo-response: For the photo-response measurement, we used a current-voltage amplifier (Stanford research systems-Model SR570) and an oscilloscope (Agilent technology DSO-X 2024A) in the output of the loop to record the current response as functions of milliseconds or microseconds.

Figure 2 .

 2 Figure 2.17 I-V measurement setup and one of photocurrent mapping setup (a) I-V measurement setup under light illumination (b) photocurrent mapping setup based on WITec system.

Figure 3 . 1

 31 Figure 3.1 Comparison of optical images of few-layer InSe in large scale with different temperature prepared on SiO2/Si substrate. (a) Room temperature (b) 50 ºC (c) 75 ºC (d) 100 ºC (e) 125 ºC and (f) 150 ºC.

  Figure 3.3 (a-d), few layer InSe and monolayer MoS2 are shown on SiO2/Si and glass substrates respectively as obtained by mechanical exfoliation with optimum conditions, showing the large size of samples we obtained.

Figure 3 . 2

 32 Figure 3.2 Comparison of optical images of few-layer MoS2 in large scale with different temperature prepared on SiO2/Si substrate. (a) Room temperature (b) 50 ºC (c) 75 ºC (d) 100 ºC (e) 125 ºC (f) 150 ºC.

Figure 3 . 3

 33 Figure 3.3 Large size of few-layer InSe and monolayer MoS2 prepared on SiO2/Si and glass substrates. (a) Few-layer InSe on SiO2/Si (b) 4-layer InSe on glass (c) Monolayer MoS2 on SiO2/Si (d) Monolayer MoS2 on glass.

Figure 3 . 4 Figure 3 . 5 .

 3435 Figure 3.4 Optical images of monolayer graphene prepared on borosilicate glass substrate. (a) Large size of graphene and (b) a higher magnification image of a smaller sample showing surface quality.

Figure 3 . 5

 35 Figure 3.5 Optical micrographs showing examples of 2D flakes obtained by anodic bonding from some materials: single layer and bilayer GaS, GaSe with 1, 2 and 3 layers, single layer and bilayer InSe, single layer and bilayer MoS2, single layer and bilayer CuGeO3, few-layer Mica.

Figure 3 . 6

 36 Figure 3.6 Optimum parameters during anodic bonding for GaS, GaSe, InSe and MoS2.

Figure 3 .

 3 Figure 3.7 (j-l).

Figure 3 . 7

 37 Figure 3.7 The optical images, the corresponding AFM and height profile of few-layer GaS, GaSe, InSe and mica. (a) optical image of 3 nm thickness GaS on SiO2/Si substrate (b) AFM of GaS (c) the corresponding height profile of GaS (d) optical image of 1.6 nm thickness GaSe on SiO2/Si substrate (e) AFM of GaSe (f) the corresponding height profile of GaSe (g) optical image of 0.8 nm and 2.5 nm thickness InSe on glass substrate (h) AFM of InSe (i) the corresponding height profile of InSe (j) optical image of 30 nm and 80 nm thickness mica on glass substrate (k) AFM of mica (l) the corresponding height profile of mica.

Figure 3 . 8

 38 Figure 3.8 Left: Graphene crystallites on 300 nm SiO2 imaged with white light (a), green light and another graphene sample on 200 nm SiO2 imaged with white light (c) Right: Color plot of the contrast as a function of wavelength and SiO2 thickness. The color scale on the right shows the expected contrast. Reprinted from Ref [42].

Figure 3 . 9

 39 Figure 3.9 Schematic of optical reflection and transmission for 2D layered materials prepared on glass substrate.

Figure 3 . 10

 310 Figure 3.10 Optical contrast of graphene and MoS2 calculated by different refractive index on glass substrates.

Figure 3 .

 3 Figure 3.11 (c), (f) and (i) respectively. Moreover, the experimental optical contrast is partly shown in Figure 3.11 (a-b) for few layer GaS, Figure 3.11 (d-e) for few layer GaSe and Figure3.11 (g-h) for few layer InSe. The layer numbers have been determined using AFM measurements in order to calibrate the relationship between layer numbers and optical contrast values. The higher the optical contrast the easier the optical detection. For monolayer GaSe, the optical contrast value is 3.64%, which is higher than 1.28% for monolayer GaS and 2.66% for monolayer InSe. The optical contrast is dependent on the nature of materials and substrates, such as reflective index and thickness. These values are, for example, inferior to those reported for monolayer graphene prepared on 285 nm SiO2 on Si and monolayer MoS2 prepared on 270 nm SiO2 on Si because in these cases the SiO2 layer induces an interference enhancement of the contrast. In addition, these values are also less than the values we obtained for monolayer graphene and MoS2 prepared on glass. In other words, it is less visible than graphene and MoS2 from the optical images. The experimental results are compared with the calculated values in Figure3.11 (c), (f) and (i) showing that with this technique, the thickness of GaS, GaSe or InSe few-layer samples can be determined rapidly by comparing the experimental contrast value with the calculated value.

Figure 3 . 11

 311 Figure 3.11 Optical images and the corresponding optical contrasts of few layer GaS, GaSe and InSe prepared on borosilicate glass. Layer numbers are quantified using AFM in advance. Shown are the optical image, the optical contrast profile along the marked lines in the optical image and the variation of the measured and calculated optical contrast as a function of the number of layers, (a)-(c) for GaS, (d)-(f) for GaSe and (g)-(i) for InSe, respectively.

Figure 3 . 12

 312 Figure 3.12 Raman characterization of monolayer and few-layer graphene (a) Optical image of graphene prepared on SiO2/Si by mechanical exfoliation method (b) I(G)/I(2D) mapping corresponded to the area highlighted by the yellow square in (a) (c) Raman spectra of few-layer graphene prepared by mechanical exfoliation method (d) Comparison of Raman shift of pristine graphene prepared on glass, anodic bonded graphene and graphene wedging transferred on SiO2/Si. The green lines indicate the positions of G and 2D vibration modes in pristine graphene.

  exfoliation and anodic bonding methods and the doping level can be precisely checked by Raman shift of G and 2D modes. By using the pristine or doped graphene, different optoelectronic devices can be made by using the different Fermi leveled of graphene as shown in Chapter 5.

Figure 3 . 13

 313 Figure 3.13 Raman characterization of monolayer and few-layer MoS2 prepared on SiO2/Si using mechanical exfoliation method (a) Optical image of MoS2. (b) I(E2g 1 ) mapping corresponded to the area highlighted by the yellow square in (a) (c) Raman spectra of few-layer MoS2 (d) Raman shift of MoS2 with E2g 1 and A1g modes respectively.

. 13 .

 13 From the Raman mapping of E2g 1 mode of MoS2 shown in Figure3.13 (b), we can distinguish the number of layers roughly from intensity since in the absence of a double resonance as in graphene, the thinner layers have weaker intensity compare to thicker layers. However, this rule is not absolute because of the different light absorption with different thickness and the Raman signal might be weaker in very thick layers (as shown in the top part of Figure3.13 (b), a fact we will discuss in Chapter 5). There is another way to precisely distinguish the layer numbers of ultra-thin MoS2 which is from the relative difference in the positions of the E2g 1 and A1g modes shown in Figure3.13 (c-d). In monolayer MoS2 these peaks are at 383.67 cm -1 and 402.10 cm -1 while in bilayer MoS2 they are located at 382.99 cm -1 and 404.55 cm-1

Figure 3 . 14

 314 Figure 3.14 Raman characterization of monolayer and few-layer GaS prepared on SiO2/Si using mechanical exfoliation method (a) Optical image of GaS (b) I(A1g 1 ) mapping corresponded to the area highlighted by the yellow square in (a) (c) Raman spectra of few-layer GaS (d) Raman shift of GaS with A1g 1 mode.

Figure 3 . 15

 315 Figure 3.15 Raman characterization of monolayer and few-layer GaSe prepared on SiO2/Si using mechanical exfoliation method (a) Optical image of GaSe (b) I(A1g 1 ) mapping corresponded to the area highlighted by the yellow square in (a) (c) Raman spectra of few-layer GaSe (d) Raman shift of GaSe with A1g 1 mode.

Figure 3 .

 3 Figure 3.15 (a-b). From the Raman mapping, it is very hard to distinguish monolayer and bilayer from substrate and the intensity increases according to number of few layers. The Raman spectra are then extracted from the Raman mapping data which are shown in Figure 3.15 (c-d). From the spectra, Raman modes of A1g 1 in monolayer and bilayer GaSe are

Figure 3 . 16 Figure 3 . 16 .

 316316 Figure 3.16 Raman characterization of monolayer and few-layer InSe prepared on SiO2/Si using mechanical exfoliation method (a) Optical image of InSe (b) I(A1g 1 ) mapping corresponded to the area highlighted by the yellow square in (a) (c) Raman spectra of few-layer InSe (d) Raman shift of InSe with A1g 1 mode.

  Photoluminescence (PL) is a fast, contactless and nondestructive technique to characterize the electronic structure of materials. For 2D semiconductors, it is an effective way to analyze the band gap transition depending on the number of layers. For example in MoS2 (a representative TMDC material) the band gap transits from direct in monolayer to indirect in bilayer and above which can be characterized easily by PL. In this work we used PL spectroscopy to characterize few-layer MoS2, GaSe and InSe and the results are discussed below.

  (a). In addition the green portions which correspond to the next lower intensity level map with the bilayer in the optical image. The lowest intensity (blue) corresponds to multilayer MoS2 in the top part of the map, showing the significant difference from monolayer to multilayer discussed before. Individual PL spectra from characteristic points in monolayer, bilayer, 5-layer, 7-layer and multilayer (~ 20-layer) MoS2 under the same measurement conditions are are shown in Figure3.17 (c). The band gap can be followed by analyzing the lowest energy peak in these PL spectra, labeled using arrows and corresponding to the band gaps in different number of layers. For monolayer MoS2, the single peak located at 1.84 eV has very high intensity which means a direct band gap. However, in bilayer MoS2, another peak appears at 1.52 eV which is very weak and almost negligible compared to the peak at 1.84 eV. This result indicates that in bilayer MoS2 the fundamental band gap is already indirect. Similar peaks are found in 5-layer, 7-layer and multilayer MoS2, located at 1.37 eV, 1.34 eV and 1.33 eV respectively. In summary this corresponds to the direct-indirect band gap transition from monolayer to bilayer and above.

Figure 3 . 17

 317 Figure 3.17 Photoluminescence of few-layer MoS2 prepared on SiO2/Si (a) Optical image of MoS2 (b) Intensity at 1.84 eV of MoS2 PL mapping corresponded to the area highlighted by the yellow square in (a) (c) PL spectra of few-layer MoS2.

Figure 3 . 18

 318 Figure 3.18 Photoluminescence of few-layer GaSe prepared on SiO2/Si (a) Optical image of GaSe. (b) PL spectra of few-layer GaSe.

Figure 3 . 19

 319 Figure 3.19 Photoluminescence of few-layer InSe prepared on SiO2/Si (a) Optical image of InSe (b) Corresponding AFM of InSe (c) PL spectra of few-layer InSe.

Figure 3 .

 3 Figure 3.20 (a) Optical image of few-layer InSe. Areas A, B, C, D and E are ~35 layers, 18 layers, 14 layers, 10 layers and 7 layers respectively and the corresponding PL mapping based on sum of PL spectra is shown in (b-d) (b) PL mapping based on sum of the PL spectra from 1.18eV to 1.26eV (c) PL mapping based on sum of the PL spectra from 1.13eV to 1.36eV (d) PL mapping based on sum of the PL spectra from 1.30eV to 1.41eV. The scale bars are 10µm.

Figure 3 .

 3 Figure 3.21 (c). In addition, the SAED pattern indicates that few-layer GaS is formed by single crystals. Moreover, the hexagonal structure of GaS is confirmed and the orientation of the sheets is along < 100 > zone axis from the SAED pattern in Figure 3.21 (d).

Figure 3 .

 3 Figure 3.21 (a) Optical image of few-layer GaS transferred on a holey carbon grid (b) TEM image of few-layer GaS at low magnification (c) The corresponding HR-TEM of GaS with the lattice spacing of 0.31 nm (d) TEM SAED pattern of few-layer GaS with the beam parallel to the c axis.

Figure 3 .

 3 22 (b) shows a typical low magnification TEM image of few-layer GaS. In addition, the corresponding HR-TEM is characterized and the lattice constant for GaSe is 0.32 nm from Figure3.22 (c). From the SAED pattern of few-layer GaSe in Figure3.22 (d), the hexagonal structure are confirmed and the orientation of the sheets is along < 100 > zone axis, which indicates the few-layer GaS is formed by single crystals.

Figure 3 .

 3 Figure 3.22 (a) Optical image of few-layer GaSe transferred on a holey carbon grid (b) TEM image of few-layer GaS at low magnification (c) The corresponding HR-TEM of GaSe with the lattice spacing of 0.32 nm (d) TEM SAED pattern of few-layer GaSe with the beam parallel to the c axis.

Figure 3 .

 3 Figure 3.23 (a) Electron diffraction in ε polytype crystal structure of InSe (b) Electron diffraction in β polytype crystal structure of InSe (c) Electron diffraction in γ polytype crystal structure of InSe.

Figure 3 .

 3 24 (a) shows the TEM image of InSe at low magnification. Figure 3.24 (b) and (c) show the high resolution (HR) TEM of 10-layer InSe and the corresponding selected area electron diffraction (SAED) pattern, prepared by mechanical exfoliation and transferred to a TEM grid. The sample is oriented with the <001> zone axis parallel to the electron beam. The spatial frequency measured for the first reflections is 0.294 Å -1 , and is therefore compatible with the position of the 100 diffraction spot of both β and ε polytypes which are shown in Figure 3.23 (a-b). HR-TEM also allows a direct measurement of the d-spacing of 0.34 nm which is coherent with the {100} plane family of β and ε InSe. The experimental results are not compatible with the electron diffraction pattern in γ polytype.

Figure 3 .

 3 Figure 3.24 (d), in which the thickness of the monolayer is 0.85 nm.

Figure 3 .

 3 Figure 3.24 (a) TEM image of few-layer InSe at low magnification (b) The corresponding HR-TEM of InSe on a holey carbon grid with the lattice spacing of 0.34 nm (c) TEM SAED pattern of 10-layer InSe with the beam parallel to the c axis (d) ε polytype crystal structure of InSe with the c-axis vertical. Orange and purple spheres represent Se and In atoms respectively (e) Raman spectra of 12-layer InSe on SiO2/Si substrate.

  analyzed the stability of a 4-layer InSe sample as a function of exposure time in air as shown in Figure3.25. After sample preparation by mechanical exfoliation, the Raman modes at 114.8 cm -1 , 178.1 cm -1 , 202.1 cm -1 and 227.3 cm -1 are clearly seen. However, the intensity of these modes decreases and fast merges into a rising background ranging from 150 cm -1 to 250 cm -1 when the sample is left in air over twelve days. All the Raman modes disappear after sixteen days which indicates that 4-layer InSe is degraded completely in ambient conditions after this time. This practical problem of degradation can restrict the application of ultra-thin InSe. Following earlier work of graphene use for countering corrosion, we transfer graphene on top of 5-layer InSe as shown in Figure3.25 (c). In Figure3.25 (d) Raman spectra are shown simultaneously for bare 5-layer InSe and graphene covered 5-layer InSe. The bare 5layer InSe is degraded completely within twenty days. However, for the 5-layer InSe covered by graphene, the Raman signal remains unchanged. We conclude that a graphene covering layer is also an effective packaging material to protect ultra-thin InSe from degradation in ambient conditions. This solution is exploited in Chapter 5 in the fabrication of a vertical heterostructure device with InSe and graphene.

Figure 3 .Chapter 4 Photo-detectors based on few-layer MoS 2 ,

 342 Figure 3.25 (a) Optical image of ultra-thin InSe prepared on SiO2/Si using mechanical exfoliation (b) The corresponding instability of 4-layer InSe in ambient conditions characterized by Raman spectroscopy. (n D: after n days) (c) Optical image of 5-layer InSe partly encapsulated by the top monolayer graphene, the graphene is demarcated by an orange dashed line (d) Raman spectroscopy comparison of stability of 5-layer InSe in ambient conditions with and without graphene protection. (n D: after n days)

stacking various 2D materials dealt in Chapter 5 .

 5 The schematics of a photo-detector based on 2D materials are shown in Figure 4.1. The principle of the photo-detector is simple: it is a field effect transistor (FET) based on monolayer or few-layer 2D materials. The voltage between the source and back gate changes the Fermi level of the 2D material by electrostatic doping. The voltage between the source and drain determines the device current. Light shining on such a device creates electron-hole pairs by photoelectric effect and they are collected by various mechanisms (discussed in section 4.1.1) by the source and drain, constituting the device current and finally a detectable photocurrent. There are three steps in the photo-detection process: light harvesting, electron-hole separation and charge transport to source and drain. The detailed physical mechanism and the key figures of merit to characterize photo-detector performance are discussed in the part below.

Figure 4 . 1

 41 Figure 4.1 Schematic program of photo-detector based on 2D materials fabricated on SiO2/Si substrate. (a) Side view (b) Front view.

Figure 4 . 2 .

 42 They performed photocurrent mapping of the whole device and found more efficient carrier separation in the graphene-Au interfaces compared to the region far away from source and drain (Figure 4.2 (b)), which means the built-in electric field plays a role without external bias voltage. The built-in voltage in the graphene-Au interface can be describe as: ∆ 𝑉 = ϕ 𝑀 -ϕ 𝐺 -∆ 𝐸 𝐹 + sgn(𝑉 𝑔𝑠 -𝑉 𝑔𝑠 𝐷𝑖𝑟𝑎𝑐 )ħv 𝐹 √𝜋𝛼 √𝑉 𝑔𝑠 -𝑉 𝑔𝑠 𝐷𝑖𝑟𝑎𝑐 (4.1)

Figure 4 . 2

 42 Figure 4.2 Photocurrent response of a graphene device. (a) AFM image of a graphene monolayer contacted by four gold electrodes (b) Photocurrent image taken at Vds = Vgs = 0 V (c) Ids-Vds characteristic recorded in the dark and with the laser positioned as marked in (b). Reprinted from Ref [9].

Figure 4 . 3 .

 43 They used a 750 nm light source with energy lower than the band gap of monolayer MoS2, and found a photo induced current close to the contacts shown in Figure 4.3 (a), which cannot be explained by the photoelectric effect. It is caused by the local heating of the junction between gold and MoS2 caused by the light absorption resulting in a voltage difference (∆ V PTE ) and a corresponding PTE current generated at the edge of the metal contacts. ∆ V PTE can be described as:

Figure 4 . 3

 43 Figure 4.3 Photo-thermoelectric effect in monolayer MoS2. (a) Photo-voltage map of a single layer MoS2 FET using an excitation wavelength of 750 nm (b) Photocurrent profile across the linecut in panel a (c) Schematic of the photo response mechanism in a device dominated by photothermoelectric effect. Reprinted from Ref [11].

  photons (photo flux ϕin):

  Slightly different from EQE, IQE is defined as the ratio of the number of electron-hole pairs collected to produce the photocurrent Iph at a given energy Eph by the number of incident photons absorbed by photo-detector:

Figure 4 .

 4 Figure 3.14.

Figure 4 . 4

 44 Figure 4.4 Photo-detector based on 3-layer MoS2. (a) Optical image of the device (b) Raman mapping of I(E 1 2g) corresponded to the area highlighted by the yellow square in (a) (c) Ids-Vds sweep from dark to different illumination conditions (d) Ids-Vds sweep with different Vgs in dark condition.

Figure 4 . 5

 45 Figure 4.5 (a) Ids-Vgs sweep of 3-layer MoS2 photo-detector from dark to different illumination conditions, Vt is indicated at the back gate voltage of 10 V (b) Photoresponsivity and EQE as the function of illumination power with different Vgs (c) On-off ratio as the function of illumination power with different Vg (d) Detectivity as the function of illumination power with different Vg.

Figure 4 . 6

 46 Figure 4.6 Time resolved response time of photocurrent in 3-layer MoS2 photo-detector. Left: rise. Right: decay.

Firstly, without

 without Vds and Vgs, the electrons generated from few-layer MoS2 should flow from Au to MoS2 when the laser is focused on the metal/semiconductor contact interfaces according to our mechanism (Figure4.7 (c)). Photocurrent mapping verifies this hypothesis as shown in Figure4.7 (a-b). Without Vds and Vgs photocurrent is generated in the interfaces between Au and MoS2 under focused laser illumination. The photocurrent is around 0.6 nA for illumination at the source interface and -0.6 nA for illumination at the drain interface, respectively. In addition, there is no photocurrent for illumination in the area far away from the interfaces. It should be noted here that the total photocurrent is 0 nA when large spot uniform irradiation covers the whole device because of the neutralization of the current generated from the two interfaces.

Figure 4 . 7

 47 Figure 4.7 Mechanism of photo-detector based on few-layer MoS2. (a) Optical image of the device (b) Photocurrent mapping under Vds = 0 V and Vgs = 0 V (c) The corresponding band diagram of the device with the laser illumination under Vds = 0 V and Vgs = 0 V (d) Band diagram of the device with light under Vds > 0 V and Vgs < Vt (e) Band diagram of the device with light under Vds > 0 V and Vgs > Vt.

Figure 4 .

 4 Figure 4.7 (d), the device is in the off-state, in which the Schottky barriers gets higher and less photo generated electrons flow from Au to MoS2, with less photocurrent in accordance with

Figure 4 .

 4 Figure 4.5 (a).

Figure 4 .

 4 8 (a) and (d) show the six-layer GaSe devices prepared respectively on SiO2/Si and as fabricated on glass.

Figure 4 .

 4 8 (a) and (b) show the photo response of the device on SiO2/Si under 532 nm wavelength laser illumination. It can be seen that there is negligible dark current and a response time of less than 400 ms (Vds = 5 V and Vgs = 0 V)

Figure 4 . 8

 48 Figure 4.8 Six layer GaSe photo-detector performances on SiO2/Si (a), (b), (c) and glass (d), respectively. (a) Photo-response of three different areas under Vds = 5 V. Inset: Optical image of GaSe photo-detector prepared on SiO2/Si and three illuminated position A, B and C (b) Photo-response time

Figure 4 .

 4 Figure 4.8 (c) shows the Vgs dependence of the Ids-Vds curves of the same device, while in Figure 5(d) the I-V curve for the gateless device on glass is shown. We can infer that the

  2. In addition, the Ids-Vds curves are not symmetric which can imply the existence of large Schottky barriers. Clearly we need to change contact material to form more ohmic contacts. In contacts being technologically impossible we opted for the standard Cr/Au for our next devices.

Figure 4 . 9

 49 Figure 4.9 Eight layered InSe photo-detector performances on SiO2/Si substrate (a) Optical image the photo-detector with carbon contacts (b) Photo-response time during photocurrent rise and decay with the light switching on and off, and a bias voltage of 10V without back gate voltage (c) Ids-Vds characterization of InSe under dark and light conditions without back gate voltage (d) Ids-Vgs characterization of InSe with back gate sweep from -40 V to 50 V and Vds at 10 V.

Figure 4 .

 4 photocurrent is of order of magnitude ~ nA. The rise time is within 180 ms which indicates the fast response time in the device compared to our few-layer GaSe device. In addition, the photocurrent is measured under different illumination powers as a function of Vds and under the illumination power of 12.6 W/m 2 as a function of Vgs respectively shown in Figure4.10 (e-f). Our first conclusion is that ohmic contacts are obtained as seen by the symmetric Ids-Vds curves. The photocurrent increases as a function of Vgs similar to our MoS2 device. In this device, the effective area is 35.6 µm 2 which will be used to calculate photoresponsivity and EQE below.

Figure 4 .

 4 Figure 4.10 (a) Optical image of 11-layer InSe photo-detector prepared on SiO2/Si substrate (b) The corresponding AFM image of the device (c) Photo-response time under Vds = 10 V and Vgs = 0 V in different light illumination conditions (d) Photo-response time: rise (e) Power dependent photocurrent as a function of Vds (f) Photocurrent as a function of Vgs.

Figure 4 .

 4 Figure 4.11 (a) Power dependent photocurrent as a function of Vds at Vgs = 0 V. Insert: optical image of 20-layer InSe (b) Photocurrent as a function of Vgs at Vds = 5 V.

Figure 4 .

 4 Figure 4.12 (a) Optical image of multilayer InSe photo-detector (b) Photocurrent mapping at Vds = 1 V and Vgs = 0 V under the 532 nm laser illumination (c) Band diagram of the laser focusing on the area of InSe closing to the source contact (d) Band diagram of the laser focusing on the area of InSe closing to the drain contact.

Figure 5 . 1

 51 Figure 5.1 Schematics of van der Waals heterostructures. Left: different kind of atomic layers are stacked together. Right: h-BN/graphene/h-BN/TMDC/h-BN/graphene/h-BN heterostructure operating as an optoelectronic device. Reprinted from Ref [1] and [2].

Figure 5 . 2

 52 Figure 5.2 High performance photovoltaic and memory devices. (a) Schematic illustration of graphene/WS2/graphene heterostructure. (b) The corresponding photovoltaic characterization of the device. (c) Band diagram of the device with a built-in electric field to separate the generated electronhole pairs. (d) Schematic of a device with graphene/MoS2 heterostructure. (e) Evolution of the switching with different light intensities at Vgs = -40 V. (f) Charge exchange process of the device with graphene and MoS2 for Vgs ≪ VT. Reprinted from Ref[7] and Ref[8].

Figure 5 . 3 (

 53 Figure 5.3 (a) Optical microscope image of a WSe2/MoS2 hetero-bilayer (b) PL and absorbance spectra of single-layer WSe2, MoS2, and the corresponding hetero-bilayer, (c) PL spectra from WSe2/MoS2 heterostructure with n layers of h-BN (n = 0, 1, and 3). (d) Schematics of MoS2/WSe2 junction sandwiched between top and bottom graphene electrodes. (e) Optical image and photocurrent map (for Vds = 0 V) of the device. (f) Current-voltage curves of the device in (e). Inset: Schematic of the band structure. The bottom graphene is slightly p-doped from the SiO2 substrate.

Figure 5 .

 5 4 (a). The optical image of the device based on a graphene/20-layer InSe heterostructure on a SiO2/Si substrate is shown in Figure 5.4 (b).

Figure 5 .

 5 4 (c) we show the photocurrent as a function of source-drain voltage (Vds) by varying the illumination power and with gate voltage Vgs=0 V. As explained below, we are thus in the regime of negative photocurrent (purple region of Figure 5.4 (d)).

Figure 5 .

 5 Photo-carrier generation in graphene is inefficient and the current in an isolated graphene layer changes little between the dark and illuminated conditions, contrary to classical semiconductor based photo-detectors. However for the graphene layer placed on top of InSe, the charge neutral point (CNP) undergoes considerable shift with photo-generation of electron-hole pairs in InSe. The vertical geometry and the interface electronic structure make for efficient transfer of electrons to the graphene layer while holes remain trapped in the InSe layer.

Figure 5 . 4 .

 54 Figure 5.4. (a) Schematic diagram of our photo-detector based on graphene/InSe heterostructure. (b) Optical image of the device. Graphene is demarcated by the blue line. (c) Photocurrent characterization as a function of Vds for different illumination intensities and without gate voltage. (d) Ids of the device in dark and illuminated conditions as a function of Vgs. The Vds is 100 mV and the illumination power is 12.6 W/m 2 . (e-g) Schematic band structure diagram of graphene/InSe heterostructure in different Vgs regions.

  photocurrent changes sign[9] , a phenomenon not observed in InSe or other homostructural photo-detectors. The Fermi levels in graphene are shown inFigures 5.4 (e-g) by a grey line (dark device) and a red line (illuminated device), the illuminated Fermi level always being at higher energy as a result of the electrons flowing in from the InSe. In the central green shaded region of Figure5.4 (d) the Fermi level of graphene for the dark device varies between the neutral point and slight hole doping while the Fermi level of graphene for the illuminated device varies between neutrality and slight electron doping. The particular case of zero photocurrent is illustrated in Figure5.4 (f) and around this region the device is evidently not in a favorable regime. In the purple shaded region of Figure5.4 (d) the illuminated sourcedrain current is always less than the dark current giving a negative photocurrent. In effect, graphene is hole doped in the dark and becomes less hole doped when illuminated with the inflow of photoelectrons from InSe (Figure5.4 (e)). In the pink shaded region ofFigure 5.4 (d) on the contrary, the illuminated source-drain current is always more than the dark current giving a positive photocurrent because graphene is electron doped in the dark and becomes more electron doped when illuminated again due to an inflow of photoelectrons from InSe (Figure5.4 (g)).

3 W/m 2 (

 32 Figure 5.5 (a) giving β≈0.39.What is the gain in quantum efficiency in going from few-layer InSe to the graphene/InSe heterostructure? In Figure5.5 (b) we compare the two structures (the same thickness of fewlayer InSe device was also fabricated, seen in Chapter 4) by computing the photo-responsivity (R) and EQE as a function of illuminated power ranging between 12.6 W/m 2 to 6.3×10 for reference, polychromatic solar radiation at the earth's surface is roughly 10 3 W/m 2 ).

Figure 5 . 5 .

 55 Figure 5.5. (a) Photocurrent as a function of illumination power for different Vds in the graphene/20layer InSe heterostructure. Symbols: experimental data, continuous lines: parameterized fit of relation (3). (b) R and EQE as a function of illuminated power for graphene/20-layer InSe heterostructure and 20-layer InSe homostructure. Symbols: experimental data, continuous lines: parameterized fit of relation (5.4).

Figure 4 .

 4 photoexcited electrons can flow into the drain contact before recombination (defined by the mean free paths and the drain/InSe interface) is 5.3×10 -5 µm 3 . However in the graphene/InSe heterostructure (shown in Figure5.4 (b)), the effective junction area is the whole graphene covered surface of the device. The effective junction volume is 5.4×10 -2 µm 3 which is more than 10 3 times larger than in few-layer InSe which gives an estimation of the increase in efficiency expected in this device.

Figure 5 . 6

 56 Figure 5.6 (a) Schematics of photo-detector based on few-layer InSe. (b) Schematics of photodetector based on graphene/InSe heterostructure. The highlighted pink region demarcates the effective volume in the devices from which charge carriers can be extracted. The full red circles represent photo-excited electrons.

Figure 5 .

 5 photo-response time of more than 100 ms. Moreover, from the photocurrent of ~ 0.7 nA without any Vds and Vgs, we can calculate the photoresponsivity and EQE of the device which are 0.139 A/W and 32.42% respectively. In addition, from I-V characterization with a laser spot focused on the sandwich part of the device, there is a large photocurrent ~40 nA called the short circuit current (Isc, with Vds=0 V), and an open circuit voltage (Voc, with Ids=0 A) of -0.21 V is obtained, which means there is a photovoltaic effect in our device. As explained above, the dark current is very low and the on-off ratio of our device is higher than 10 3 .

Figure 5 . 7

 57 Figure 5.7 (a) Schematics of graphene/InSe/graphene photo-detector based on SiO2/Si substrate. (b) The corresponding optical image of the device. The sandwiched part is labeled using yellow dash line. (c) Photo-response time with 532 nm laser illuminating the whole device without bias voltage. (d) Ids-Vds characterization under the 532 nm laser point focusing on the sandwiched part with Vgs = 0 V.

Figure 5 . 8

 58 Figure 5.8 Schematic band structure diagram of graphene/InSe/graphene heterostructure. Left: Ideal case, with pristine top graphene and bottom graphene under equivalent condition. Right: Our case, the bottom graphene is highly p-doped and the top graphene is less p-doped under equivalent condition.

Figure 5 . 9

 59 Figure 5.9 (a) Schematics of graphene/InSe/Au photo-detector based on glass substrate from the back side view. (b) The corresponding optical image of the device from the top view. The bottom graphene is labeled using red dash line and Au is labeled using black solid line. (c) Photo-response time with 532 nm laser point focusing on the sandwiched part without bias voltage. (d) I-V characterization with 532 nm laser illuminated the whole device.

Figure 5 . 10

 510 Figure 5.10 Schematic band structure diagram of graphene/InSe/Au heterostructure. Left: The heterostructure under light illumination without Vds. Right: The heterostructure under light illumination with Vds >0.18 V.

Figure 5 .

 5 10 (a) and (c), a clean surface and a photocurrent of 2 pA are observed which means the photo-detector works normally. Barely visible holes in the optical image in Figure 5.10 (b) signal breakdown and no photocurrent is detected when the laser illuminates the device.

Figure 5 . 11

 511 Figure 5.11 Optical images of graphene/InSe/graphene heterostructure under normal conditions (a) and break down condition (b). The sandwiched part is labeled in the black circle line. (c) Comparison of photo-response time before and after breakdown. (d) Optical image of graphene/InSe/Au photodetector and the sandwiched part is labeled by red dash line. (e) The corresponding photocurrent mapping of the device after breakdown. (f) I-V characterization of the device after breakdown.

  InSe/MoS2 heterostructure The InSe/MoS2 heterostructure is prepared by random dry transfer of few-layer InSe onto mechanically exfoliated few-layer MoS2 on SiO2/Si substrate. The optical image of the heterostructure is shown in Figure 5.11 (a). Based on optical contrast the total surface can be divided into five areas which are labeled from A to E. B and C areas are the InSe/MoS2 heterostructure. A and D represent few-layer InSe and few-layer MoS2 respectively. From the corresponding AFM scan shown in Figure 5.11 (b-c), we can determine layer thicknesses: 28.1 nm for InSe and 20.2 nm and 33.9 nm for MoS2 in B and C areas respectively.

Figure 5 . 11

 511 Figure 5.11 Optical image, AFM and Raman shift of InSe/MoS2 heterostructure (a) Optical image which has five areas: A represents InSe, B and C represent InSe/MoS2 heterostructure, D represents MoS2 and E represents SiO2/Si substrate. (b) and (c) The AFM scan and the corresponding height profile. (d-e) Raman shift of InSe, MoS2 and Si spectra from different areas with 532 nm laser (g-h) Raman shift of InSe and Si spectra from different areas with 638 nm laser.

Figure 5 . 12

 512 Figure 5.12 Raman intensity mapping of InSe/MoS2 heterostructure (a) Optical image (b) Intensity mapping of InSe A 1 1g mode with 532 nm laser. (c) Intensity mapping of MoS2 A1g mode with 532 nm laser. (d) Intensity mapping of Si vibration mode with 532 nm laser. (e) Intensity mapping of InSe A 1 1g mode with 638 nm laser. (f) Intensity mapping of Si vibration mode with 638 nm laser.

Figure 5 .

 5 Figure 5.13 (a) Raman intensity of InSe (A1g 1 mode) from different InSe/MoS2 heterostructures as a function of InSe thickness under 532 nm laser. (b) Raman intensity of InSe (A1g 1 mode) from different InSe/MoS2 heterostructures as a function of InSe thickness under 638 nm laser. (c) Intensity Raman mapping of of InSe (A1g 1 mode) as functions of InSe and MoS2 thicknesses under 532 nm laser. (d) Intensity Raman mapping of of InSe (A1g 1 mode) as functions of InSe and MoS2 thicknesses under 638 nm laser. Note: the white lines represent three different thicknesses of InSe/MoS2 heterostructures, the same as in the areas of A, B and C.

  InSe/MoS2 vertical heterostructures (in Chapter 4 we have shown that the two materials are both n-type) are fabricated by using contact or transport layers of Au and graphene respectively so as to rapidly transport photo-excited electrons. The schematics of the two devices are shown in Figure 5.14.

Figure 5 .

 5 Figure 5.14 Left: Schematics of photo-detector based on InSe/MoS2 heterostructure with top Au as transport layer. Right: Schematics of photo-detector based on InSe/MoS2 heterostructure with top graphene as transport layer.

Figure 5 .

 5 Figure 5.15 (c). The photocurrent is negative in the whole device but not uniform at all being essentially concentrated very close to the Au contact at the top of InSe/MoS2 heterostructure.We then characterized the Ids-Vds curves without Vg under different light illumination power as shown in Figure5.15 (d). The curves present the rectifying characteristic of a photodiode.

Figure 5 .

 5 Figure 5.15 Photo-detector based on InSe/MoS2 vertical heterostructure. (a) Optical image of InSe/MoS2 heterostructure (b) Optical image of InSe/MoS2 photo-detector (c) Photocurrent mapping of InSe/MoS2 photo-detector under 523 nm focused laser without bias voltage (d) Ids-Vds characterization under different light conditions illuminated on the whole device (e) Isc and Voc are observed in the focused Ids-Vds characterization (f) Photoresponsivity and EQE as a function of illumination power without bias voltage.

( 2 )

 2 What is the reason for the non-uniform photocurrent distribution? Firstly we analyze the band diagram of InSe/MoS2 heterostructure shown in the left panel of Figure 5.16. Because of the different work functions in InSe (5.2 eV) and MoS2 (4.6 eV), the energy bands will bend when the two materials contact. In our case both InSe and MoS2 are slightly n-doped. Schottky barrier and a built in potential are generated at the interface of the heterostructure, and photo-excited electrons flow from InSe to MoS2 (the current is from source to drain) when light shines on the device thus causing negative photocurrent. The non-uniform distribution of the photocurrent can be understood from the simplified device model shown in the right panel of Figure 5.16. In general, the total resistance of the device can be split up into several contributions: the contact resistance of Au/InSe and MoS2/Au, the sheet resistance of InSe and MoS2 and the resistance at the InSe/MoS2 interface

Figure 5 .

 5 Figure 5.16 Left: Schematics of the band diagram in InSe/MoS2 heterostructure under equivalent condition Right: Schematics of the simplified device model.

Figure 5 . 17

 517 Figure 5.17(a) Optical image of InSe/MoS2 heterostructure with graphene transport layer (b) Optical

  Figure 5.17(a) Optical image of InSe/MoS2 heterostructure with graphene transport layer (b) Optical image of photo-detector based on InSe/MoS2 heterostructure with transport layer of top graphene (c) Photocurrent mapping of InSe/MoS2 photo-detector under 532 nm focused laser without bias voltage (d) Ids-Vds characterization under varying illumination power where the whole device is illuminated. (e-f) Isc and Voc are observed in the focused Ids-Vds characterization.

Figure 5 .

 5 Figure 5.18 (a) Photo-response of the device with back gate at -20 V, 0 V and 10 V respectively. (b) Photo-response time when light is turned on from dark status (c) Photo-response time when light is turned off from illumination status.

Figure 5 .

 5 Figure 5.19 Left: Photoresponsivity of the device under 532 nm and 638 nm light illumination as a function of power without Vds and Vgs. Right: EQE of the device under 532 nm and 638 nm light illumination as a function of power without Vds and Vgs.

Figure 5 .

 5 Figure 5.20 Left: Schematics of the band diagram in InSe/MoS2 heterostructure with graphene transport layer under equivalent condition Right: Schematics of the simplified device model.

  and 4 respectively. This calculation step comes from the article published by M. Buscema et al. and other articles[A1-A3] .

Figure A3 .

 A3 Figure A3. Schematic ray diagram of the InSe/MoS2 heterostructure prepared on SiO2/Si substrate, including multiple reflections at every interface and absorption and emission at a depth X in the InSe layer (dots). Every medium is numbered from 0 to 4. The color of the arrows indicates reflection/transmission in a different medium and the transparency indicates the loss of intensity due to absorption after multiple reflections.

Résumé

  photovoltaïque.

Laser power during Raman scattering experiment: During the Raman characterization

	of 2D materials, appropriate laser power is critical because high laser power can heat the
	lattice and thus damage the 2D samples. However, the 'proper' laser power varies from
	sample to sample. In our graphene measurement, we used 1.2 mW (The diameter of laser
	point is around 1 μm under 100X objective) as 532 nm laser power. However in MoS2
	samples we use only 120 μW. The signals in monolayer graphene or MoS2 are comparable to
	bulk sample for which 5 seconds of acquisition time and 2 times accumulations are performed
	during the experiment. It is worth noting that the experiment gets complex in few-layer III-VI
	samples measurement because they are very sensitive to laser damage and signals in ultrathin
	layers are almost undetectable. In our III-VI samples such as GaS, GaSe and InSe, the
	acquisition time is 60 seconds and the accumulations are 2 times for each sample. Here we
	give an example of sensitivity of few-layer InSe according to different laser power during our
	experiment. The optical images of few-layer InSe which are before laser exposure and after
	laser exposure with the powers of 12 μW, 120 μW and 1.2 mW are shown in Figure 2.8. After
	120 seconds exposing of 12 μW focused laser, the surface of the samples is the same as
	before from the red circle. However for the laser powers of 120 μW and 1.2 mW, one can
	observe a black dot at the focusing point of the laser after exposure, which means the sample
	is damaged by laser heating. Higher power of laser should be avoided during these Raman
	experiments.

Table 4 .

 4

		Detectivity	Rise time	Reference
	1 ]	[Jones]	[s]

1. Comparison of performance of monolayer and few-layer MoS2 photo-detectors.

Perspectives

Novel 2D materials such as MoS2, GaS, GaSe and InSe have drawn much attention in very recent years because graphene has zero band gap and is thus not an ideal material for photoelectric and photovoltaic applications. Up to now, monolayer MoS2 with direct band gap has already been thoroughly investigated in many research areas such as photo-detectors, memory devices and spintronics. 2D III-VI semiconductors like GaS, GaSe and InSe on the other hand are still in the essential stage from characterization of properties to device application since the last three years. Nevertheless it seems clear that 2D III-VI materials have a promising future ahead. For example, the photoresponsivity and EQE of photo-detectors based on few-layer InSe are high and few-layer InSe is a good match with graphene or TMDCs for improved heterostructure device performance. However, there are some critical questions that need work in the future:  On the fundamental side the transition of the electronic structure and the band-gap in going from bulk to few layers is critical for device application. For InSe the band gap transition from direct to indirect seems clear from PL characterization and the first principle calculation. For GaSe, it has a similar band gap transition but less obvious from theory calculation. However, there is no consensus on a possible similar band gap transition in few-layer GaS.

 On the applied side the focus has been on photo-detection of few-layer III-VI materials. More complex devices such as p-n diodes, solar cells and memory devices will certainly be a future research area.

 Spin-orbit coupling and eventual topological effects in these materials is also of great interest with possible applications in spintronics.

 Applications will also depend on the eventual large scale fabrication of these materials with efficient scalable methods like CVD, a method successfully used for graphene and MoS2.

Appendices A1 Simulation part A1.1 Calculation of optical contrast

The optical contrasts of 2D materials such as graphene, MoS2, GaS, GaSe and InSe are characterized from simulation of light reflection and transmission in 2D materials/glass model.

As we showed in chapter 3, the schematic of optical reflection and transmission for 2D layered materials prepared on glass is depicted in Figure A1. For normal light incidence, the intensity of reflected light from the layered materials on top of the glass is given by:

Where

Where

is the relative index of refraction at the interface between air and glass.

The contrast is defined as the relative intensity of reflected light in the presence and absence of 2D layer and can be written as:

Using this formula, we can simulate the optical contrast of 2D materials as a function of incident wavelength. However in the above the spectral sensitivity of the detector (optical microscopy camera) which is important especially for the red, green and blue filters, is not taken into consideration. In the next step, we generalize the model for broadband illumination by taking into account the relative transmission and quantum efficiencies of blue, green and red colors of our optical microscopy camera (figure below): The camera response function we used is:

Finally, the total optical contrast under the mixed light of microscopy camera is calculated by the formula:

We take 𝑆(𝜆) and 𝐶(𝜆) into the formula above, and calculate the contrast values for different layers using MATLAB software.

A1.2 Calculation of light out-coupling in InSe/MoS2 heterostructure

In this section we present the derivation of the optical interference model for InSe/MoS2

van der Waals heterostructure prepared on SiO2 (285 nm)/Si substrate shown in Figure A3.

There are four media including air, InSe, MoS2, SiO2 and Si which are labeled as 0, 1, 2, 3 Firstly, we calculate the effective reflection coefficient at the MoS2/SiO2 interface, each component (r i ) is defined as The total reflection (r ′ ) is the sum overall n r and, taking into account the relationships between the transmission and reflection coefficients expressed before, it can be reduced as: Secondly, the same calculation can now be applied to the interface of InSe/MoS2. The total reflection r ′′ can be reduced as:

Thirdly, we calculate the incident light density at a depth X in InSe layer (Figure A4 (a)).

We have the following components

In 2 = t 01 e -iβ 1 • r " e -1(β 1 -β x ) = t 01 r " e (2iβ 1 -β x ) (A15)

In 3 = t 01 e -iβ 1 • r " e -iβ 1 • r 10 e -iβ x = abs 1 (r 10 r " e -2iβ 1 ) (A16)

In 4 = t 01 e -iβ 1 • r ′ e -iβ 1 • r 10 e -iβ 1 • r " e -1(β 1 -β x ) = abs 2 (r 10 r " e -2iβ 2 ) (A17)

…

In 2n+1 = abs 1 (r 10 r " e -2iβ 2 ) n (A18)

In 2n+2 = abs 2 (r 10 r " e -2iβ 2 ) n (A19)

The total amount of absorbed light is given by the sum over all the n terms and results in:

1+r " r 01 e -i2β 1 (A20)

Fourthly, the same derivation holds for the total amplitude of emitting light from depth X in InSe (Figure A4 (b)) with the appropriate substitution t 01 → t 10 F em = t 10 • e -iβ x +r " e -i(2β 1 -β x )

1+r " r 01 e -i2β 1

Finally, the intensity of the emitting light from InSe at a wavelength λ is given by:

Abstract

Novel two dimensional (2D) semiconductors beyond graphene such as MoS2, GaS, GaSe and InSe are increasingly relevant for emergent applications and devices. In this thesis, we fabricate these 2D samples for photo-detector applications and characterize them with optical microscopy, atomic force microscopy, Raman and photoluminescence (PL) spectroscopy and transmission electron microscopy. Since the interaction of light with the substrate and the ultra-thin photodetector device is critical for its functioning we calculate and measure optical contrast and intensity of light scattered from the device. We also characterize the Raman and PL response as a function of number of layers to study both vibrational properties and the band gap transition. For the device application, we first examine homogenous devices based on few-layer MoS2, GaSe and InSe respectively and find an excellent photoresponsivity in our few-layer MoS2 photo-detector. We then examine several geometries for heterostructure devices, which have the advantage of combining favorable properties of each material to reach better performances. The first example is a graphene/InSe photo-detector where the photoresponsivity increases by four orders of magnitude with respect to a few-layer InSe device while the top graphene layer is also shown to prevent degradation of ultra-thin atomic layers in air. Still more complex graphene/InSe/graphene and graphene/InSe/Au heterostructures show a photovoltaic effect. Finally for the first time, we combine InSe with

MoS2 and obtain a high performance device with fast photo-response, photodiode like behavior, uniform photocurrent distribution and high photovoltaic effect.

Key words: Graphene, InSe, MoS2, few-layer devices, heterostructures, photo-detector