Le graphène possède un gaz bidimensionnel de porteurs de charge stable et exposé à l'environnement sans aucune protection. Par conséquent, ses performances électriques sont extrêmement sensibles aux conditions environnementales, notamment aux impuretés chargées et aux corrugations imposées par le substrat sous-jacent. Ces éléments ont une contribution majeure dans la dégradation des propriétés de transport électronique du matériau.L'objectif de cette thèse est d'explorer par diverses techniques des méthodes pour atténuer ces effets par optimisation de son environnement direct.La première méthode consiste à reporter le graphènesur une couche neutre d'un cristal de nitrure de bore hexagonal (BN). Diverses techniques de fabrication d'empilement de Graphène sur BN sont présentées, notamment la croissance directe de graphène sur un cristal de BN exfolié sur un substrat catalytique qui aboutit à la formation d'empilements de structure bien contrôlée. Les échantillons sont mesurés à très basse température. Les effets de localisation faible mesurés par magnéto-transport montrent une amélioration nette des performances notamment de la longueur de cohérence et de la mobilité électronique par rapport à un échantillon de référence constitué du même ruban de graphène déposé sur substrat conventionnel de silicium oxydé.La deuxième technique consiste à isoler le graphène de son support par surgravure de la silice et suspension du graphène sous la forme d'une membrane autosupportée et tenue par ses extrémités. Après avoir introduit des techniques de fabrication spécifiques, les mesures de transport et le couplage à des modes de vibration mécanique sont étudiés température variable. Ces données permettent notamment une mesure du coefficient d'expansion thermique du graphène. / Charge carriers in graphene form stable two-dimensional gases which are fully exposed to the environment. As a consequence, the electrical performance of graphene is strongly affected by surface charged impurities as well as topographic perturbations inherited from the underlying substrate.This thesis addresses several methods to circumvent that issue.The first method consists in embedding graphene in an optimized environment by depositing graphene onto some neutral and crystalline material. Novel 2D insulating materials such as hexagonal boron nitride buffer layer (BN) appears as ideal substrates to get rid of detrimental effect of interfacial charges and corrugation. Several fabrication schemes of Graphene/BN stacks are shown including some direct in-situ growth of graphene on BN crystal using an innovative proximity-driven chemical vapour growth based on BN exfoliation on copper. In order to explore the effects of the improved substrate on the transport properties of graphene, we have performed low temperature magneto-transport studies on these stacks. We present a direct comparison of weak localization signals with those acquired on a graphene/silica reference device. A clear increase of the coherence length is shown on Graphene/BN stacks together with improved electronic mobility and charge neutrality.Removing the substrate and suspending graphene is another approach for optimization of the graphene environment which forms the second topic covered in this thesis. After introducing an improved recipe for preserving the quality of graphene throughout an elaborate fabrication process, we probe the room- and low-temperature performance of the nano-electro-mechanical devices based on doubly clamped suspended graphene ribbons. The obtained data are used for characterizing the thermal expansion of CVD graphene.

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In this work the synthesis of MgCdS ternary semiconductor nanocrystal alloys has been carried out by aqueous route through a bottom-up approach, using conventional hydrothermal heating as well as in situ onto graphene matrices. In the synthesis of MgCdS nanocrystals, the effect of each reaction parameter on the spectroscopic properties was studied aiming to understand the possibilities to control the optical properties. Emission spectra of MgCdS samples obtained in the experiments designed to optimize reaction parameters exhibited a single emission band reflecting nanocrystal growth, with quantum yields as high as 85%. Based on the presence of two bands in absorption spectra as well as atomic absorption spectrometry (AAS) data it was possible to propose that nanocrystals are composed of Cd and Mg. Concerning the structural architecture, it has been proposed that nanocrystals show a core-shell structure with a diffuse interface. Data from AAS also showed that the final composition of nanocrystals is generally different from the initial reaction Cd:Mg proportion, as the metal precursors have distinct reactivities. Morphological analyses by transmission electron microscopy (TEM) of nanocrystals evidenced the predominance of spherical shapes and sizes below 4 nm. Studies of the formation of nanocrystal alloys with Mg1-xCdxS and Cd1-xMgxS composition, by ion exchange from the binary components MgS and CdS helped the discussion of spectroscopic behavior of the ternary system MgCdS. It was possible to confirm that the introduction of a second cation (Cd2+ or Mg2+) into each binary structure (MgS or CdS) is consistent with the observation of two absorption bands and only one emission band. The addition of graphene derivatives during the synthesis of MgCdS nanocrystals was carried out aiming to improve the properties of the materials, as well as providing a physical support to the nanocrystals, favoring future applications. The presence of graphene induced shifts in the emission bands to larger wavelengths concomitant with intensity reduction, which can be taken as evidence of interactions between the materials. The morphologies of composites were characterized by typical graphene sheets decorated with spherical nanocrystals. / Neste trabalho foram realizadas as sínteses de nanocristais (NCs) semicondutores ternários de MgCdS via síntese aquosa através da metodologia bottom-up, assistida por tratamento térmico hidrotermal convencional, além da síntese in situ de nanocompósitos de MgCdS em matrizes de grafeno. A síntese do nanocristal de MgCdS foi avaliada através do efeito da variação de cada parâmetro de síntese sobre as propriedades espectroscópicas do material, a fim de se compreender as possibilidades de controle das propriedades ópticas. Os espectros de emissão dos NCs de MgCdS, referente ao estudo de otimização dos parâmetros de síntese, apresentaram uma única banda de emissão intensa que reflete o crescimento do nanocristal, com rendimentos quânticos de fotoluminescência elevados, chegando a 85%. Com base na presença de duas bandas de absorção no espectro de UV-visível, bem como de dados de espectrofotometria de absorção atômica (AAS), pode-se inferir que os nanocristais são compostos pelos metais de Cd e Mg, propondo-se a hipótese de uma arquitetura caroço-casca com interface difusa. Os dados obtidos através de AAS mostraram também que, como os precursores tem reatividades distintas, a composição dos materiais formados tende a diferir da proporção Cd2+:Mg2+ utilizada na reação. As análises morfológicas realizadas por microscopia eletrônica de transmissão (TEM) permitiram verificar o contorno esférico e uniforme das nanoesferas e estimar o tamanho dos nanocristais, sendo abaixo de 4 nm. Estudos de formação de ligas do tipo Mg1-xCdxS e Cd1-xMgxS, por troca iônica a partir dos componentes binários MgS e CdS permitiram compreender melhor os dados espectroscópicos dos nanocristais formados introduzindo ambos precursores simultaneamente. Confirmou-se que a introdução do segundo cátion (Cd2+ ou Mg2+) em cada estrutura binária (MgS ou CdS) de fato causa a formação de duas bandas de absorção e somente uma de emissão. A implementação de derivados de grafeno na síntese do MgCdS foi realizada a fim de aprimorar as propriedades gerais do material, bem como de propiciar um suporte físico aos nanocristais de MgCdS, favorecendo aplicações. A presença do grafeno na síntese do nanocristal proporcionou deslocamento da banda de emissão para maiores comprimentos de onda com redução da intensidade luminescente, evidenciando interações entre os materiais. As morfologias dos compósitos apresentam folhas de grafeno decoradas com nanocristais esféricos. / São Cristóvão, SE

Orientadores: Fernando Iikawa, Odilon Divino Damasceno Couto Junior / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-29T15:53:03Z (GMT). No. of bitstreams: 1 MirandaLaHera_VladimirRoger_M.pdf: 7239733 bytes, checksum: 1e843140547afd4fb68e666db8a03911 (MD5) Previous issue date: 2015 / Resumo: Neste trabalho foram estudadas as propriedades ópticas de nanofios (NWs) semicondutores InAsP/InP na fase Wurzita, crescidos pelo método Vapor-Liquid-Solid (VLS) no sistema Chemical Beam Epitaxy (CBE). As medidas ópticas foram realizadas por espectroscopia de fotoluminescência em ensembles e nanofios individuais, por macro e micro-fotoluminescência. O interesse pelas ligas de InAsP é que elas apresentam energia do gap na faixa de infravermelho próximo, uma faixa espectral utilizada para a tecnologia de telecomunicações bem como em fabricação de detetores de compostos de carbono nocivos, pois eles apresentam absorção óptica nessa faixa. Além disso, InAsP na forma de estruturas na escala nanométrica, em particular, em NWs, além de ter a fase cúbica estável, que ocorre em todos os fosfetos e arsenetos de compostos III-V, a fase hexagonal wurtzita é também observada. A maioria de suas propriedades na estrutura wurtzita não ainda foi investigada em detalhes. Os nanofios heteroestruturados contendo InAsP e InP na fase wurtzita não são bem conhecidos também. O ponto principal desta tese é, portanto, investigar as propriedades ópticas desses compostos na forma de nanofios, que estão na fase wurtzita, e estudar a emissão óptica destas heteroestruturas, onde envolve o efeito de confinamento quântico, o qual pode ser utilizado para sintonizar o comprimento de onda da emissão. Investigamos NWs de ligas de InAsP com três composições diferentes, mudando o fluxo de arsina e fosfina, que foram crescidos usando três tamanhos de nanopartículas de catalizadores de Au diferentes de 2, 5 e 20 nm. Observamos que a forma do nanofio depende do tamanho de nanopartículas de Au. Para menores tamanhos, obteve-se uma forma de torre, enquanto que para o maior, a forma de agulha. A concentração de P é de cerca de 50% estimada por espectroscopia de fotoluminescência e de energia dispersiva de raios-X. A emissão óptica é de cerca de 1.5 µm, adequada para aplicação em dispositivos de telecomunicações. Nos NWs heteroestruturados de InAsP/InP, investigamos as amostras com tempos de inclusão diferentes de InAsP (2, 5, 10, 20 e 40 s) no InP, e elas foram crescidas com diferentes tamanhos de nanopartículas de Au (2, 5 e 20 nm) utilizadas como catalisador. Nessas amostras, todos os nanofios apresentam a forma de uma agulha. Os espectros de macro e micro-fotoluminescência mostram fortes emissões ópticas atribuídas à camada de InAsP e variam entre 800-1000 nm. A energia de emissão depende da quantidade de InAsP de acordo com o efeito de confinamento quântico. Também, observamos várias linhas estreitas nos espectros de micro-fotoluminescência de nanofios individuais atribuídos aos estados localizados das camadas InAsP. Essas linhas são provenientes de duas regiões, sendo uma delas da camada de InAsP axial catalisado e uma segunda, da camada lateral de InAsP devido ao crescimento epitaxial. Este resultado mostra que os NWs de InAsP/InP apresentam alta qualidade cristalina e são sistemas promissores para a aplicação em dispositivos ópticos / Abstract: In this work, we studied the optical properties of wurtzite phase InAsP alloy nanowires (NWs) and InAsP/InP heterostructure nanowires grown by Vapor-Liquid-Solid (VLS) method in a Chemical Beam Epitaxy (CBE) system. The optical measurements were carried out by photoluminescence spectroscopy in ensemble and single NWs by macro and micro-photoluminescence techniques. The interest for InAsP alloys is that they present gap energy in the near infra-red, a spectral range commonly used for the telecommunication technology as well as for harmful carbon compounds detection sensors, since their optical absorption is in the same energy range. Furthermore, the InAsP in nanoscale structures, in particular, in NWs, in addition to the stable cubic phase, which occurs in all other phosphide and arsenide III-V compounds, hexagonal wurtzite phase is also observed. Most of the properties of their wurtzite structure has not been investigated in details yet. The heterostructure NWs containing InAsP and InP in wurtzite phase are not deeply known as well. The main purpose of this thesis is, therefore, to investigate the optical properties of this compounds in NW forms, which present wurtzite phase, and to study the optical emission from the heterostructures, where the quantum confinement effect can also be used to tune the emission wavelength. We investigated InAsP alloy NWs with three compositions changing the arsine and phosphine flux and they are grown using three sizes of Au-nanoparticle catalyst, 2, 5 and 20 nm. We note that the NW shape depends on the Au-nanoparticle size. For small size, a tower-like shape was observed, while for large one, the needle-like one. The P content of the samples is around the 50 % estimated by the photoluminescence and by Energy-Dispersive X-ray spectroscopy. The optical emission is around 1.5 µm, appropriate for telecommunication device applications. For InAsP/InP heterostructure NWs, we investigated samples with different InAsP time deposition (2, 5, 10, 20 and 40 s) onto the InP and they were grown with different Au-nanoparticle size (2, 5 and 20 nm) used as catalyst. In these samples, all nanowires present needle-like shape. The macro and micro-photoluminescence spectra show strong optical emissions in 800-1000 nm range attributed to the InAsP layer emissions. The emission energy depends on the amount of InAsP according to the quantum confinement effect. We observed several sharp lines in the micro-photoluminescence spectra of single NWs attributed to the localized states of the InAsP layers. They come from two regions, one of them from the axial catalyst InAsP layer and second one, from the lateral InAsP epitaxial growth layer. The result shows that the InAsP/InP heterostructures NWs grown by VLS method in the CBE system present high crystal quality and are promising structure for optical device applications / Mestrado / Física / Mestre em Física / 1247651/2013 / CAPES

Le sujet de cette thèse est à l’interface entre l’étude de composés à base de nitrure et des structures émergeantes formées par les matériaux bidimensionnels (2D) d’épaisseur atomique. Ce travail se consacre sur l’hybridation des propriétés électriques et optiques des semi-conducteurs à larges bandes interdites que sont les nitrures et des performances mécaniques, électriques et optiques des matériaux lamellaires, récemment isolé à l’échelle d’un plan atomique, qui sont aujourd’hui considérées avec attention aux regards de futures applications et d’études plus fondamentales. En particulier, une étude des propriétés électroniques, optiques et structurelles d’hétérostructures composées de plusieurs matériaux lamellaires et d’interfaces entre matériaux 2D et 3D a été réalisé par des moyens de microscopie et de spectroscopie tel que la spectroscopie Raman, de photoémission et d’absorption.Ce manuscrit traite dans un premier temps des propriétés structurelles et électroniques du nitrure de bore hexagonal (h-BN), matériau isolant aux propriétés optiques exotiques et essentiel dans la future intégration de ce type de matériaux 2D permettant de mettre en valeur leurs propriétés intrinsèques.En utilisant le graphène comme substrat les problèmes de mesures par photoémission rencontrés pour des matériaux isolant ont pu être surmonté dans le cas du h-BN et une étude des défauts structurels a pu être réalisée. Par conséquent, les premières mesures directes de la structure de bande électronique de plusieurs plans de h-BN sont présentées dans ce manuscrit.Dans un second temps, une approche d’intégration de ces matériaux 2D différente a été étudiée en formant une hétérostructure 2D/3D. L’interface de cette hétérojonction, composée d’un plan de disulfure de molybdène (MoS2) de dopage intrinsèque N associé à 300 nm de nitrure de gallium (GaN) intentionnellement dopé P à l’aide de magnésium, a été caractérisée. Un transfert de charge du GaN vers le MoS2 a pu être identifié suggérant un contrôle des propriétés électroniques de ce type de structure par le choix de matériaux.Ces travaux ont permis de révéler les diagrammes de bandes électroniques complet des structures étudiées a pu être obtenu permettant une meilleur compréhension de ces systèmes émergeants. / This thesis is at the interface between the study of nitride based compounds and the emerging structures formed by atomically thin bi-dimensional (2D) materials. This work consists in the study of the hybridization of the properties of large band gap materials from the nitride family and the mechanical, electronic and optical performances of layered materials, recently isolated at the monolayer level, highly considered due to their possible applications in electronics devices and fundamental research. In particular, a study of electronics and structural properties of stacked layered materials and 2D/3D interfaces have been realised with microscopic and spectroscopic means such as Raman, photoemission and absorption spectroscopy.This work is firstly focused on the structural and electronic properties of hexagonal boron nitride (h-BN), insulating layered material with exotic optical properties, essential in in the purpose of integrating these 2D materials with disclosed performances. Using graphene as an ideal substrate in order to enable the measure of insulating h-BN during photoemission experiments, a study of structural defects has been realized. Consequently, the first direct observation of multilayer h-BN band structure is presented in this manuscript. On the other hand, a different approach consisting on integrating bi-dimensional materials directly on functional bulk materials has been studied. This 2D/3D heterostructure composed of naturally N-doped molybdenum disulphide and intentionally P-doped gallium nitride using magnesium has been characterised. A charge transfer from GaN to MoS2 has been observed suggesting a fine-tuning of the electronic properties of such structure by the choice of materials.In this work present the full band alignment diagrams of the studied structure allowing a better understanding of these emerging systems.

Le contrôle optique des propriétés physiques d’un matériau suscite l’intérêt des scientifiques pour des enjeux aussi bien fondamentaux qu’appliqués. L’axe de recherche original que nous avons développé dans le cadre de ce travail de thèse visait à la réalisation et l’étude d’hétérostructures moléculaires photo-magnétiques dans des gammes de température susceptibles d’applications. L’approche proposée consistait à élaborer des hétérostructures de type multiferroïque constituées de deux phases, l’une piézomagnétique et l’autre photo-strictive. L’idée était d’optimiser le couplage, d’origine élastique, entre ces propriétés pour permettre l’observation d’effets photo-magnétiques à des températures plus élevées que celles rapportées pour les matériaux monophasés. La couche photo-strictive peut se déformer sous irradiation lumineuse, générant des contraintes biaxiales dans la couche magnétique. Si celle-ci présente une forte réponse piézomagnétique, son aimantation peut être fortement modifiée, notamment au voisinage du point de Curie, allant jusqu’à un éventuel décalage de la température critique sous contrainte. Les composés moléculaires analogues du Bleu de Prusse, de formule générique AxM[M’(CN)6]y . zH2O (où A est un alcalin et M,M’ des métaux de transition), semblaient particulièrement adaptés à l’élaboration de telles hétérostructures. Nous avons utilisé le composé Rb0,5Co[Fe(CN)6]0,8 . zH2O pour la phase photo-strictive, au coeur, et Rb0,2Ni[Cr(CN)6]0,7 . z’H2O ou K0,2Ni[Cr(CN)6]0,7 . z’H2O pour la phase magnétique, en coquille. Ces deux phases présentent un désaccord paramétrique de 5,3%.L’objectif principal de ce travail de thèse était de comprendre et de contrôler le couplage élastique entre le cœur et la coquille. Nous avons ainsi dans un premier temps mis en évidence l’existence de ce couplage, la présence de la coquille modifiant les propriétés de photo-commutation du cœur et la déformation du réseau cristallin du cœur étant partiellement transmise à la coquille, induisant des modifications structurales et magnétiques de la coquille. Nous nous sommes dans un second temps intéressés à différents paramètres pouvant influence le couplage. D’abord en étudiant des paramètres géométriques, en faisant varier la taille des particules de cœur, l’épaisseur de la coquille et la microstructure de la coquille. Nous avons à cette occasion mis en évidence les facteurs régissant la croissance des particules de cœur et de la coquille. Ces études ont révélé que le rapport volumique entre le cœur et la coquille contrôlait la qualité du couplage, et que des modifications de la microstructure avait une influence à la fois sur les propriétés de photo-commutation du cœur, mais aussi sur la réponse de la coquille. Enfin, nous avons étudié des coquilles de nature chimique différente pour changer le désaccord paramétrique entre le cœur et la coquille. Il en ressort qu’en diminuant le désaccord, on améliore le couplage, mais cela se traduit notamment par une rétroaction de la coquille plus forte. Si cette rétroaction devient trop importante, le réseau du cœur ne peut plus se déformer. Il s’agit donc de trouver un compromis entre force du couplage et force de la rétroaction de la coquille. Finalement, nous avons mis en évidence le fait que l’on ne peut pas simplement associer l’effet de la coquille à un effet de pression hydrostatique, mais que le couplage des réseaux cristallins joue un rôle important dans la synergie entre les deux phases. / The optical control of the physical properties of a material has drawn considerable attention during the past few years for a fundamental point of view and for applications. The originality of the project developed during this thesis was based on the synthesis and the study of photo-magnetic heterostructures in a temperature range convenient for applications. The approach consisted of developing multiferroic-like heterostructures that associate a piezomagnetic phase and a photo-strictive phase. The idea was to exploit the coupling of elastic origin between these properties, to allow the observation of photo-magnetic effects at temperatures higher than those reported for single-phase materials. The photo-strictive phase can deform under light irradiation, generating biaxial strain in the magnetic phase. If the piezomagnetic response of the latter is high enough, its magnetization could be modulated, especially at the vicinity of the Curie temperature, with a possible shift of the critical temperature under stress. In this project, we focused on molecular solids based on polycyanometallates, namely Prussian blue analogues, whose generic formula is AxM[M’(CN)6]y . zH2O (where A is an alkali metal and M,M transition metals). We used the compound Rb0,5Co[Fe(CN)6]0,8 . zH2O for the photo-strictive phase and Rb0,2Ni[Cr(CN)6]0,7 . z’H2O or K0,2Ni[Cr(CN)6]0,7 . z’H2O for the magnetic phase. These two phases have a lattice mismatch of 5.3%The main objective of this work was to understand and to control the elastic coupling between the core and the shell. We first highlighted the existence of this coupling, the presence of the shell changing the photo-switching properties of the core, and the deformation of the crystalline lattice of the core inducing structural and magnetic modifications in the shell. Then, we focused on the study of different parameters which can have an impact on the behavior of the heterostructures under light irradiation. We showed that the volumic ratio between the core and the shell is a key factor to control the efficiency of the coupling. The microstructure of the shell can also play an important role, but is not as well understood. In the end, we studied other Prussian blue analogs shells in order to change the lattice mismatch between the core and the shell. We could evidence that by reducing the lattice mismatch we tend to increase the coupling, but if this coupling is to strong, the retroaction of the shell hinders completely the dilatation of the core lattice. The idea is also to find a compromise between the strength of the coupling and the strength of the shell retroaction. In the end, we proved that we cannot associate the effect of the shell to an hydrostatic pressure, but that the coupling of the crystalline lattices play an important role in the synergy between the two phases.

No description available.

<p>Since the successful exfoliation of graphene, two-dimensional (2D) materials have attracted a lot of scientific interest due to their electronic, chemical, and mechanical properties. Due their reduced dimensionality, these 2D materials exhibit superior mechanical and optoelectronic properties when compared to their bulk counterparts. Within the family of 2D materials, the ultrathin transition metal dichalcogenides (TMDs) such as Tungsten diselenide and Molybdenum disulphide have gained significant attention due to their chemical versatility and tunability. Furthermore, it is possible to leverage the distinct characteristic properties of these 2D materials, which are held together by van der Waals forces, by stacking different 2D layers on top of each other resulting in van der Waals (vdW) heterostructures. Due to the absence of feasible methods to effectively deform the crystal structures of these 2D materials and vdW heterostructures, their mechanical properties have not been thoroughly understood. The atomistic simulations can effectively capture the material behavior at the nanoscale level and help us not only not only understand the mechanical properties of these materials but also aid in the development of tailored processes to tune the material properties for the design of novel metamaterials. Using atomistic simulations, we develop the process - property relationships which can guide the direction of experimentation efforts, thereby making the process of discovering and designing new metamaterials efficient. </p><p>In this work, we have used laser shock nanostraining technique which is a scalable approach to modulate the optomechanical properties of 2D materials and vdW materials for practical semiconductor industry applications. The deformation mechanisms of 2D materials such as graphene, boron nitride (BN) and TMDs such as WSe₂ and MoS₂ are examined by employing a laser shocking process. We report studies on crystal structure deformation of multilayered WSe₂ and monolayer graphene at ultra-high strain rate using laser shock . The laser shocking process generates high pressure at GPa level, causing asymmetric 3D straining in graphene and a novel kinked-like locking structure in multilayered WSe₂. The deformation processes and related mechanical behaviors in laser shocked 2D materials are examined using atomistic simulations. Moiré heterostructures can be obtained by introducing a twist angle between these 2D layers, which can result into vdW materials with different properties, thereby adding an additional degree of freedom in the process-property design approach. We were able to successfully create a tunable stain profile in 2D materials and vdW heterostructures to modulate the local properties such as friction, and bandgap by controlling the level of laser shock, twist angle between the 2D layers and by applying appropriate laser shock pressure . We thus extend this knowledge to further explore the pathways of strain modulation using a combination of laser shocking process, moiré engineering, and strain engineering in 2D materials consisting of graphene, BN, and MoS₂ and to develop the process - property relationships in vdW materials. </p><p>In summary, this research presents a systematic understanding of the effect of laser shocking process on the van der Waals materials and demonstrates the modulation of mechanical and opto-electronic property using laser nanostraining approach. This understanding provides us with opportunities for deterministic design of 2D materials with controllable properties for semiconductor and nanoelectronics applications.</p>

As the family of two−dimensional(2D) materials has grown, two−dimensional heterostructure devices have emerged as great alternatives to replace conventional electronic materials and enable new functionality such as flexible and bendable electronics. The fabrication and performance of these devices depend critically on the understanding and ability to manipulate the mechanical interplay between the stacked materials. In this dissertation, we investigate adhesive interactions and determine the shear modulus of heterostructure devices made from Molybdenum Disulfide (MoS2). MoS2 has been attracting attention recently due to its semiconductor nature (having a direct band gap of 1.9 eV) along with its exceptional mechanical strength and flexibility. As the first step of our research, we suspended MoS2 flakes grown through chemical vapor deposition (CVD) over substrates made of metal (gold, titanium, chromium), semiconductor (germanium, silicon), insulator (silicon oxide), and semi-metal (graphite). Then, by creating pressure differences across the membrane, we forced MoS2 to bulge upward until we observe separation from the surface of the substrates. We demonstrated that MoS2 on graphite has the highest work of separation within the tested surface materials. Furthermore, we measured considerable adhesion hysteresis between the work of separation and the work of adhesion. We proposed that surface roughness and chemical interactions play a role in surface adhesion and separation of 2D materials. These experiments are critical to guiding the future design of electrical and mechanical devices based on 2D materials. Next, we measured the effective shear modulus of MoS2/few−layer graphene (FLG) heterostructures by employing a blister test. Again, by introducing a pressure differential across the suspended MoS2 membrane over the FLG substrate, the MoS2/FLG heterostructure peeled off from the silicon oxide surface once the critical pressure is exceeded. Incorporating a modified free energy model and Hencky’s axisymmetric membrane solution, we determine the average effective shear modulus of the heterostructure. This is the first experimental measurement of the shear modulus of heterostructure devices using a blister test and this platform can be extended to determine the shear modulus of other 2D heterostructures as well. / 2024-05-24T00:00:00Z

<p>Tuning the structural and electronic properties of layered materials is critical for the development of thin, flexible semiconductors that are capable of overcoming Moore’s law. Intercalation of transition metals (TMs) into the interlayer gaps of a two-dimensional host material is one of the most promising methods toward modifying the electronic properties without disrupting the chemical bonds within the layers. Previous studies have shown that the intercalation of TMs into Bi2Se3, SnS2, TaS2, and NbS2 altered the electronic, optical, and magnetic properties of the material due to orbital hybridization between the d-orbitals of the intercalant and the bands of the host material. However, the synthesis of intercalated 2D materials using compositionally-limited because the process is driven by a charge transfer reaction from the intercalant to the conduction band of the host material, which is difficult to achieve on group VI TMDs (MoS2, WS2) with high energy conduction bands. As a result, only metal atoms that are highly reducing, like alkali metals, can be effectively intercalated into WS2. Meanwhile, alkali metal-intercalated WS2 materials are unstable under ambient conditions, which significantly limits further device application. In this dissertation, we developed a solution-phase synthetic method to successfully intercalate a broad range of redox-active TM cations into WS2 and access a variety of intercalation morphologies. With these different intercalated structures, the electronic properties of WS2 can be systematically adjusted.</p> <p>First, we synthesized vanadium-intercalated WS2, and structural characterization reveals that solvated vanadium cations are uniformly intercalated in WS2, which significantly increases the interlayer spacing from 6.2 Å to 14.2 Å. Raman and X-ray absorption spectroscopy (XAS) experiments indicate a strong interaction between the vanadium intercalants and the WS2 basal plane. Electronic transport measurements show that the vanadium-intercalated WS2 is an n-type semiconductor with room-temperature conductivity of 12 S/cm, 2 orders of magnitude higher than pristine WS2. The electronic properties can be further tuned by varying the concentration of V intercalants.</p> <p>We further synthesized TM-intercalated WS2 using 17 different metal precursors, varying the identity, reduction potential, charge density, and ionic radius in order to determine the key properties that influence intercalation. With detailed structural characterization, we determined that both charge density and reduction potential of the precursor are critical toward achieving selective intercalation over secondary nucleation. The strength of the host-guest interaction is also dependent on the transition metal identity. With the strongest interaction between the TM intercalants and WS2 basal plane, FeCl3-WS2 has the lowest work function of 4.97 eV and the highest conductivity of 110 S/cm.</p>

Ever since the discovery of van der Waals materials, the condensed matter community has developed a wide spectrum of techniques to probe various phases in these fascinating materials. Among these phases, correlated phenomena are of great importance to physicists, and recent progress on moiré heterostructures offers a highly flexible and tunable platform to study them. It has been established in previous works that twisted WSe₂, a type of semiconductor in the van der Waals family, has great potential in hosting a large number of correlated phases and phase transitions. However, it is believed that commensurability plays a critical role in the stability of correlations. In this thesis, we demonstrate correlated physics in twisted WSe₂ beyond commensurate fillings, as well as their magnetic field dependence, via electric transport measurements. At modest magnetic fields, a Stoner-like instability in the system near van Hove singularities causes a reconstruction of the Fermi surface. On the other hand, at extremely high magnetic fields, the system exhibits reentrant insulating behaviors that are possibly due to the presence of strong excitonic interactions. Furthermore, correlated topological states are observed away from half-filling in the imbalanced excitonic metallic regime. This wide range of tunability once again proves moiré heterostructures as a promising platform to simulate quantum correlation effects on a lattice.