[pt] Dicalcogenetos de metais de transição, como dissulfeto de molibdênio, disseleneto de tungstênio, entre outros exibem muitas propriedades interessantes que fazem os materiais desta família boas perspectivas para aplicações futuras, especialmente em diversas applicações da optoeletrônica, desde telecomunicações até medicina, devido à interessante transição de gap de banda indireta para direta que ocorre quando se isola uma única camada destes materiais. Um dos objetivos desta tese é: explorando essa propriedade e modificando a estrutura da dissulfeto de tungstênio usando um método simples e confiável de modificação estrutural, pode-se aprimorar as propriedades dos materiais de forma eficiente, bem como desenvolver novas maneiras de caracterizar esse material e suas modificações, em particular, a determinação de um parâmetro chave para aplicações de WS2, o número de camadas, usando espectroscopia Raman. Neste caso, o tratamento de plasma melhorou a intensidade de fotoemissão em 34 por cento além de p-dopar os cristais monocamada, abrindo portas para a realização de dispositivos baseados em WS2. Além disso, desenvolvemos um método de discriminação de monocamadas que é independente de tensão aplicada, modificação strutural ou dopagem. / [en] Transition metal dichalcogenides, such as molybdenum disulfide, tungsten disulfide, and others exhibit many interesting properties. Such properties make them good prospects for future applications, especially in optoelectronics. From telecommunication to medicine, due to the interesting indirect-to-direct band-gap transition, one isolates a single layer of the material. One of the goals of this thesis is to modify the structure of tungsten disulfide using a simple, reliable, structural modification approach, i.e., plasma treatment. In this way, we were able to enhance the materials luminescence emission intensity up to 34 percent. Also, this treatment pdoped the monolayer structures, opening doors for the realization of devices. Another objective of this work is to develop new ways to characterize this material, particularly the determination of a critical parameter for WS2 applications, the number of layers, using Raman spectroscopy, where we developed a efficient method to discriminate monolayers independently of induced strain, structure modification and doping.

[pt] Materiais nanoestruturados tem um espaço proeminente nos avanços tecnológicos devido a suas propriedades físicas e químicas que podem ser bem diferentes das encontradas no mesmo material em forma de bulk. Na primeira parte desta tese foram investigados, como centros espalhadores em laser aleatório (RL) materiais nanoestruturados naturais, como exemplo disso são o quartzito e a celulose. Nesse contexto, usamos celulose e o quartzito como centros espalhadores de lasers aleatórios, em suspensão com etileno glicol, com o intuito de caracterizar esses sistemas como lasers aleatórios, definindo-se o limiar laser e a variação da largura a meia altura do espectro de emissão laser. São apresentadas também outras configurações do laser aleatório como o filme espesso de celulose e onda guiada com suspensão de quartzito. Uma outra área de pesquisa com grande impulso, após a descoberta do grafeno, é aquela associada aos materiais 2D, tais como os Dicalcogenetos de Metais de Transição (TMDs). Na segunda parte desta tese está relacionada com a caracterização de propriedades ópticas não lineares (ONL) que tem uma enorme importância para aplicações de materiais 2D em dispositivos optoeletrônicos e em nanofotônica. Já na segunda parte dessa tese foram caracterizadas as propriedades de ONL térmica e eletrônica de materiais do tipo TMDs, para a determinação dos valores do índice de refração não linear e o coeficiente de absorção não linear desses materiais, usando a técnica de varredura Z (Z-Scan). / [en] Nanostructured materials have a prominent place in technological advances due to their physical and chemical properties that can be quite different from those found in the same material in bulk form. In the first part of this thesis, random laser scattering centers (RL) were investigated. natural nanostructured materials such as quartzite and cellulose. In this context, we used cellulose and quartzite as scattering centers for random lasers, suspended with ethylene glycol, to characterize these systems defining the laser threshold and full width half maximum (FWHM) of these random lasers. Other random laser configurations are also presented, such as thick cellulose film and guided wave with quartzite suspension. Another area of research with great impetus, after the discovery of graphene, is that associated with 2D materials, such as Transition Metal Dichalcogenides (TMDs). The second part of this thesis is related to the characterization of nonlinear optical properties (NLO) which is of enormous importance for applications of 2D materials in optoelectronic devices and in nanophotonics. In the second part of this thesis, the thermal and electronic NLO properties of TMD-type materials were characterized, to determine the values of the nonlinear refractive index and the nonlinear absorption coefficient of these materials, using the Z- scan technique.

No description available.

The effect of spin-orbit coupling had once been thought to be a minor perturbation to the low energy band structure that could be ignored. Instead, a surge in recent theoretical and experimental efforts have shown spin-orbit interactions to have significant consequences. The main objective of this thesis is to investigate the role of the orbital sector and crystal symmetries in governing the spin texture in materials that have strong spin- orbit interactions. This can be accessed through a combination of spin- and angle-resolved photoemission spectroscopy (ARPES and spin-ARPES), both of which are powerful techniques for probing the one-electron band structure plus interactions, and supported by density functional theory calculations (DFT). We focus first on a globally inversion asymmetric material, the layered semiconductor BiTeI, which hosts a giant spin-splitting of its bulk bands. We show that these spin-split bands develop a previously undiscovered, momentum-space ordering of the atomic orbitals. We demonstrate this orbital texture to be atomic element specific by exploiting resonant enhancements in ARPES. These orbital textures drive a hierarchy of spin textures that are then tied to the constituent atomic layers. This opens routes to controlling the spin-splitting through manipulation of the atomic orbitals. This is contrasted against a material where inversion symmetry is globally upheld but locally broken within each monolayer of a two layer unit cell. Through our ARPES and spin-ARPES measurements of 2H-NbSe2, we discover the first experimental evidence for a strong out-of-plane spin polarisation that persists up to the Fermi surface in this globally inversion sym- metric material. This is found to be intrinsically linked to the orbital character and dimensionality of the underlying bands. So far, previous theories underpinning this (and related) materials' collective phases assume a spin- degenerate Fermi sea. We therefore expect this spin-polarisation to play a role in determining the underlying mechanism for the charge density wave phase and superconductivity. Through these studies, this thesis then develops the importance of global versus local inversion symmetry breaking and uncovers how this is intricately tied to the underlying atomic orbital configuration.

<p dir="ltr">In the realm of two-dimensional (2D) materials, monolayer (ML) transition metal dichalcogenides (TMDCs) have gained significant interest due to their direct bandgap transition, high carrier mobility, strong light-matter interaction, and robust spin and valley degrees of freedom, starkly contrasting their bulk counterparts. Owing to their large surface-to-volume ratio, the integration of ML TMDCs with other various 2D semiconductors and microcavities offers opportunities to study fundamental photo-physics processes at the heterointerfaces, paving the way for implementation of next-generation devices.</p><p dir="ltr">Chapter 1 provides a concise introduction to 2D materials, particularly TMDCs, and their fascinating optical and electronic properties. It examines the role of excitons in 2D materials, and the impact of energy transfer (ET) and charge transfer (CT) on exciton’s properties in TMDC through the construction of 2D van der Waals (vdW) heterostructures and coupling with optical microcavities. This chapter also delves into the potential enhancement of TMDCs’ optical properties by integrating 2D hybrid lead halide perovskites and ultra-thin three-dimensional (3D) halide perovskites with TMDCs. Furthermore, it sets the general context for light-matter interaction, another form of ET, considering both weak and strong coupling regimes.</p><p dir="ltr">Chapter 2 outlines the optical techniques employed to gather data for this work. A focus is placed on ultrafast optical techniques like transient absorption spectroscopy, which allow for direct probing and analysis of ET and CT dynamics at the heterointerface.</p><p dir="ltr">Photoinduced interfacial CT plays a critical role in the field of energy conversion involving vdW heterostructures constructed by inorganic nanostructures and organic materials. However, the control of atomic-scale stacking configurations to modulate charge separation at interfaces remains challenging. Chapter 3 aims to illustrate tunability of interfacial charge separation in a Type-II heterojunction between ML-WS₂ and an organic semiconducting molecule by rational design of relative stacking configurations using 2D perovskites as scaffoldings. This chapter investigates how different molecular stacking, face-to-face versus face-to-edge, affects CT at the heterointerface. Our findings reveal that the CT process heavily depends on the relative stacking configurations at the organic-TMDCs heterointerface, with charge separation being notably slowed down for face-to-edge configuration compared to face-to-face configuration. These investigations open new opportunities for designing efficient charge separation processes in energy conversion applications by judiciously engineering interfaces between organic and inorganic semiconductors, using 2D perovskites as scaffolds.</p><p dir="ltr">Though TMDCs’ large surface-to-volume ratios make them excellent platforms for studying interfacial properties, the presence of bulky ligands on the surface of 2D perovskite poses a challenge, impeding direct interfacial coupling in their heterostructures. Chapter 4 details the fabrication of ML-WS₂ and ultra-thin CH₃NH₃PbX₃ (MAPbX₃, X=Br, I) heterostructures with tunable energy levels, to study the dynamics of CT and ET at these hybrid interfaces. Notably, heterojunctions of WS₂ with pure MAPbBr₃ and MAPbI₃ were elucidated as Type-I and Type-II respectively, using photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements. Transit absorption (TA) spectroscopy investigations unambiguously revealed a rapid ET facilitated by CT in the WS₂/MAPbBr₃ heterostructure, with a time constant of ~20 ps, and a predominantly CT in the WS₂/MAPbI₃ heterostructure with a time constant of ~50 femtosecond (fs). The successful interfacing of low-dimensional perovskites with an extensive array of traditional 2D materials such as TMDCs opens up possibilities for novel optoelectronic properties and applications within the field of 2D material systems. Furthermore, the ultrafast and efficient ET and CT processes hold promise for the creation of advanced energy conversion devices.</p><p dir="ltr">In the last chapter, we successfully fabricated a ML-WS₂ in conjunction with a silver (Ag) nanoparticle (NP) array. Our findings affirmed a weak light-matter coupling between ML-WS₂ and the Ag NP array, as evidenced by angle-resolved photoluminescence spectroscopy. Furthermore, an enhancement in the bright exciton emission from ML-WS₂ was observed at reduced temperatures. The analysis of PL enhancement factor at varying temperatures suggested that an upper bound of the enhancement factor for the bright exciton could reach ~51 or even higher at 7 K, given the imperfect uniformity of the electric filed generated around the NPs. This discovery carries significant implications for the manipulation of excitons in TMDCs and expands their potential applications in the field of optoelectronics.</p>

No description available.

Cette thèse porte sur l’étude du transport de charge et de spin dans les nanostructures 0D, 2D et les hétérostructures 2D-0D de Van der Waals (h-VdW). Les nanocristaux pérovskite de La0.67Sr0.33MnO3 ont révélé des magnétorésistances (MR) exceptionnelles à basse température résultant de l’aimantation de leur coquille indépendamment du coeur ferromagnétique. Les transistors à effet de champ à base de MoSe2 ont permis d’élucider les mécanismes d’injection de charge à l’interface metal/semiconducteur 2D. Une méthode de fabrication des h-VdW adaptés à l’électronique à un électron est rapportée et basée sur la croissance d’amas d’Al auto-organisés à la surface du graphene et du MoS2. La transparence des matériaux 2D au champ électrique permet de moduler efficacement l’état électrique des amas par la tension de grille arrière donnant lieu aux fonctionnalités de logique à un électron. Les dispositifs à base de graphene présentent des MR attribuées aux effets magnéto-Coulomb anisotropiques. / This thesis investigates the charge and spin transport processes in 0D, 2D nanostructures and 2D-0D Van der Waals heterostructures (VdWh). The La0.67Sr0.33MnO3 perovskite nanocrystals reveal exceptional magnetoresistances (MR) at low temperature driven by their paramagnetic shell magnetization independently of their ferromagnetic core. A detailed study of MoSe2 field effect transistors enables to elucidate a complete map of the charge injection mechanisms at the metal/MoSe2 interface. An alternative approach is reported for fabricating 2D-0D VdWh suitable for single electron electronics involving the growth of self-assembled Al nanoclusters over the graphene and MoS2 surfaces. The transparency the 2D materials to the vertical electric field enables efficient modulation of the electric state of the supported Al clusters resulting to single electron logic functionalities. The devices consisting of graphene exhibit MR attributed to the magneto-Coulomb effect.

[pt] O objetivo deste trabalho foi desenvolver métodos eficientes e reprodutíveis de crescimento de monocamadas de WS2, MoS2 e outras heteroestruturas verticais por deposição química em fase de vapor à pressão atmosférica (APCVD). A monocamada separada destes materiais tem grande importância na fabricação de novos dispositivos óticos e Nano eletrônicos. Dispositivos finos e de baixo custo necessitam temperaturas em torno de 800 graus celsius, o que é um problema para aplicações mencionadas acima. Nesta tese, nós propusemos uma nova rota usando APCVD para crescer monocamadas de MoS2 a 550 graus celsius, usando sódio como catalisador. Nós produzimos monocristais e poli cristais controlando a razão de precursores NaNO3/MoO3 e tempo de crescimento. Usando cálculos de primeiros princípios, mostramos que o sódio atua como centro de nucleação para o processo de síntese. A razão de precursores é crucial para diminuir a energia de formação e a temperatura de síntese. Cálculos de primeiros princípios e experimentos concordam que uma razão ideal é em torno de 0.3, proporcionando uma queda de 250 graus celsius na temperatura de crescimento. Nós investigamos as amostras crescidas por APCVD usando espectroscopia de fotoelétrons induzidos por raios-X, microscopia de força atômica, espectroscopia Raman, fotoluminescência e mediadas de transporte. Dicalcogenetos de metais de transição (TMD) dispostos em poucas camadas permitem-nos criar materiais e estudar novos fenômenos físicos. A sequência de empilhamento dos TMDs pode modificar suas propriedades opticas e elétricas. Também sintetizamos poucas camadas de MoS2 e WS2 usando APCVD. Duas e três camadas de WS2, MoS2 e suas heteroestruturas verticais foram caracterizadas através de geração de segundo harmônico (SHG). SHG mostra que as bicamadas crescidas com ângulos de rotação relativos de 0 grau e 60 graus possuem diferentes fases de empilhamento. O SHG do empilhamento bicamada com ângulo relativo de 0 graus aumentos, enquanto para amostras com empilhamento de 60 graus foi zerado. Este comportamento do SHG sugere que duas camadas de MoS2 ou WS2, quando empilhados a 0 graus não possuem simetria de inversão para 3R(AB) entre as camadas inferiores e superiores, enquanto as camadas de 60 graus possuem simetria de inversão (centrossimétricas) e possuem empilhamento na forma 2H(AA). Finalmente, dispositivos foram fabricados em amostras de boa qualidade para a investigação de sua performance elétrica. Os dispositivos mostram comportamento típico tipo-n e sua mobilidade foi estimada a partir das curvas de transporte. A dependência dos modos Raman das nossas amostras de heteroestruturas também foi estudada. Aplicando uma tensão nos dispositivos, o modo A1 mostrou um desvio para o azul e um novo modo surge em 410 cm-1, atribuídos defeitos (D) no cristal. / [en] The aim of this work was to develop reliable and repeatable methods for growing high-quality monolayer MoS2, WS2, and their vertical heterostructure by atmospheric pressure chemical vapor deposition (APCVD) technique. The monolayer of these materials have vital importance in the fabrication of new optical and nanoelectronic devices. Thin and low-cost devices have increased the demand for new synthesis processes. Usually, the synthesis requires temperatures around 800 Celsius degrees, which is an issue for applications mentioned above. In this thesis, we propose a new route using the APCVD technique to grow monolayers of MoS2 at 550 Celsius degrees mediated by sodium as a catalyst. We have produced single crystals and polycrystals by controlling the NaNO3/MoO3 precursor s ratio and growth time. Using first-principles calculations, we find out that sodium is the nucleation site of the growth process. The precursor s ratio is crucial to decrease the energy formation and the synthesis temperature. Firstprinciples calculations and experiments agree with the ideal precursor s rate of 0.3 and with the decrease of the synthesis temperature of 250 Celsius degrees. We investigated the CVD grown sample with X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence spectroscopy, and transport experiments. Few layers of TMDs allow us to create new materials and find new physical phenomena. The stacking sequence in few-layer TMDs can significantly impact on their electrical and optical properties.We also synthesized few layers of MoS2 and WS2 via APCVD. Two and three layers of MoS2, WS2, and their vertical heterostructures were characterized by second harmonic generation (SHG). The SHG shows that the layers in bilayers grow with 0 degrees or 60 degrees has different phase stacking. The SHG from 0 degrees stacked bilayer has increased when compared to monolayer, while the generated signal from bilayer with 60 degrees stacking is zero. This behavior of SHG suggests that the two layers of MoS2 or WS2 when stacked at 0 degrees have no inversion symmetry to 3R(AB) phase stacking between the top layer and the bottom layer. While when stacked with 60 degrees has inversion symmetry (Centrosymmetric) and have 2H(AA0) phase stacking. Finally, the devices were fabricated on good quality samples to investigate their electrical performance. The fabricated devices show typical n-type behavior and mobility was estimated by measuring transport curves. The dependence of Raman modes of our heterostructure device with electron doping was also studied. By applying a voltage across our device the A1 mode shows blueshift and a new mode emerges at ~ 410 cm-1, which is attributed to the defects (D) in the crystal.