There is much interest in the electronic potential of ‘nano’-semiconductors. The avenue of research pursued in this project was in inorganic analogues of graphene, namely metal chalcogenides MxEy (M = metal, E = S, Se, Te, x ≠ y = integer value). Thin films of these materials have been used in solar cells, ambient thermoelectric generators and IR detectors, due to their interesting properties, such as: optoelectronics, magnetooptic, piezoelectric, thermoelectric and photovoltaic, as well as electrical conductivity. The key issues with the use of these materials are the formation of controlled films, especially in terms of stoichiometry, crystallinity and uniformity, and also the precursor system used. The aim of this research was to synthesise and isolate novel precursor compounds for use in the deposition of metal sulfide thin films (for use with molybdenum and tungsten). The potential viability of the compounds as single source precursors (ssp) was judged following ThermoGravimetric Analysis (TGA). The compounds were also subjected to analysis using NMR (1H, 13C and 31P where applicable), infrared and UV-Vis spectroscopy, as well as elemental analysis. Cadmium sulfide (CdS) is one of the key direct band gap II-VI semiconductors, having vital optoelectronic applications for laser light-emitting diodes, and optical devices based on non-linear properties. The ratio of these films should ideally be 1:1, however, during the formation of cadmium sulfide films, particularly at elevated temperatures, a common problem encountered is the production of sulfur deficient films. These films have a formula consistent with 〖Cd〗_x S_y, where x is an integer value greater than y, but the sulfur deficiency is generally no greater than 10 %. In order to correct this sulfur deficiency, it was decided to investigate deposition making use of both a ssp and an additional sulfur source, with the aim of producing uniform films with 1:1 Cd:S.Molybdenum disulfide films have been deposited previously from multi source precursors and more recently using ssp. In this project MoS2 was deposited using novel ssps in both LP and AACVD on a variety of substrates with the aim of producing uniform thin films and assessing any differences in the morphology of the deposition. This work was continued with the deposition of WS2 and MoxW1-xS2 from ssps which had not been reported previously. The films deposited were analysed using XRD, SEM, EDX (when available) and Raman spectroscopy.

IV Resumo Título: Nanoestruturas de Dissulfeto de Molibdênio: Síntese e caracterização para produção de Hidrogênio Mestrando: André Luís Silveira Fraga Orientador: Prof. Marcos José Leite Santos Palavras Chave: nanoestruturas de MoS2, nanopartículas de ouro, semicondutores, produção de hidrogênio. Neste trabalho é apresentada a síntese e caracterização de nanoestruturas de MoS2 e nanoestruturas de MoS2 decoradas com nanopartículas de ouro. O MoS2 foi obtido através de rota hidrotermal a 200 °C durante períodos de síntese de 2, 6, 12 e 24 horas. Como precursores foram utilizados molibdato de sódio, ácido 3-mercaptopropiônico, cisteamina e L-cisteína. Para avaliar o efeito da presença dos ligantes nas estruturas, as amostras de MoS2 foram tratadas térmicamente a temperaturas de 250, 550 e 750 °C, em atmosfera de argônio. Com o objetivo de avaliar o efeito da presença de nanopartículas de ouro nas propriedades fotocatalíticas do material, foi realizada a síntese in situ de nanopartículas de ouro aderidas às estruturas de MoS2. Os materiais foram caracterizados através das técnicas de difração de raios X (DRX), microscopia eletrônica de transmissão (MET), microscopia eletrônica de varredura (MEV) e espectroscopia do ultravioleta e visível (UV-Vis). As áreas superficiais e quantidade de poros foram avaliadas através das técnicas de BET (Brunauer, Emmett and Teller) e DFT (density functional theory). O precursor ácido 3-mercaptopropiônico resultou na formação de aglomerados de nanofolhas com cerca de 500 nm de diâmetro na sua maior dimensão. Ao usar cisteamina e L-cisteína foram obtidas nanoestruturas com formato de nanoflores com cerca de 300 nm de diâmetro formadas por pétalas com cerca de 30 nm. Um resultado interessante foi a rápida formação das nanoflores na presença de L-cisteína. As estruturas de nanoflores apresentaram produção de hidrogênio de até 9,6 mmol/gh. / In this work the synthesis and characterization of MoS2 nanostructures and MoS2 nanostructures decorated with gold nanoparticles is presented. The materials were obtained by hydrothermal route at 200 °C during synthesis periods of 2, 6, 12 and 24 hours. Sodium molybdate was used as Molybdenium precursor and 3-mercaptopropionic acid, cysteamine and L-cysteine as sulfur precursors. To evaluate the effect of ligands on the structures, the MoS2 samples were thermally treated at 250, 550 and 750 °C under argon atmosphere. The effect of gold nanoparticles on the photocatalytic properties of the material was evaluated by obtaining and materials with gold nanoparticle adhered to the MoS2 structures. The materials were characterized by X-ray diffraction (XRD) techniques, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and ultraviolet and visible spectroscopy (UV-Vis). The surface areas and amount of pores were evaluated using BET (Brunauer, Emmett and Teller) and DFT (density functional theory) techniques. The precursor 3-mercaptopropionic acid resulted in the formation of nano-foil agglomerates of about 500 nm in diameter. On the other hand, when using cysteamine and L-cysteine, flower-shaped nanostructures of about 300 nm in diameter formed by petals of about 30 nm were obtained. An interesting result was the rapid formation of nanoflores in the presence of L-cysteine. Nanoflower structures showed hydrogen production up to 9.6 mmol / gh.

Two dimensional (2D) materials can withstand an order of magnitude more strain than their bulk counterparts, which results in dramatic changes to electrical, thermal and optical properties. These changes can be harnessed for technological applications such as tunable light emitting diodes or field effect transistors, or utilized to explore novel physics like exciton confinement, pseudo-magnetic fields (PMFs), and even quantum gravity. However, current techniques for straining atomically thin materials offer limited control over the strain field, and require bulky pressure chambers or large beam bending equipment. This dissertation describes the development of micro-electromechanical systems (MEMS) as a platform for precisely controlling the magnitude and orientation of the strain field in 2D materials. MEMS are a versatile platform for studying strain physics. Mechanical, electrical, thermal and optical probes can all be easily incorporated into their design. Further, because of their small size and compatibility with electronics manufacturing methods, there is an achievable pathway from the laboratory bench to real-world application. Nevertheless, the incorporation of atomically thin crystals with MEMS has been hampered by fragile, non-planer structures and low friction interfaces. We have innovated two techniques to overcome these critical obstacles: micro-structure assisted transfer to place the 2D materials on the MEMS gently and precisely, and micro-riveting to create a slip-free interface between the 2D materials and MEMS. With these advancements, we were able to strain monolayer molybdenum disulfide (MoS2) to greater than 1\% strain with a MEMS for the first time. The dissertation develops the theoretical underpinnings of this result including original work on the theory of operation of MEMS chevron actuators, and strain generated PMFs in transition metal dichalcogenides, a large class of 2D materials. We conclude the dissertation with a roadmap to guide and inspire future physicists and engineers exploring strain in 2D systems and their applications. The roadmap contains ideas for next-generation fabrication techniques to improve yield, sample quality, and add capabilities. We have also included in the roadmap proposals for experiments such as a speculative technique for realizing topological quantum field theories that mimics recent theoretical wire construction methods.

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Hydrogen is a carrier energy gas that can be utilized as a clean energy source instead of oil and natural gas. Splitting the water into hydrogen and oxygen is one of the most favorable methods to generate hydrogen. The catalytic properties of molybdenum disulfide (MoS2) could be valuable in this role, particularly due to its unique structure and ability to be chemically modified, enabling its catalytic activity to be further enhanced or made comparable to that of Pt-based materials. In general, these modification strategies may involve either structural engineering of MoS2 or enhancing the kinetics of charge transfer, including by confining to single metal atoms and clusters or integrating with a conductive substrate. We present the results of synergetic integration of MoS2 films with a Si-heterojunction solar cell for generating H2 via the photochemical water splitting approach. The results of the photochemical measurements demonstrated an efficient photocurrent of 36. 3 mA cm-2 at 0 V vs. RHE and an onset potential of 0.56 V vs. RHE. In addition to 25 hours of continuous photon conversion to H2 generation, this study points out that the integration of the Si-HJ with MoS2 is an effective strategy for enhancing the internal conductivity of MoS2 towards efficient and stable hydrogen production. Moreover, we studied the effect of doping an atomic scale of Pt on the catalytic activity of MoS2. The electrochemical results indicated that the optimum single Pt atoms loading amount demonstrated a distinct enhancement in the hydrogen generating, in which the overpotential was minimized to -0.0505 V to reach a current density of 10 mA cm−2 using only 10 ALD cycles of Pt. The Tafel slopes of the ALD Pt/ML-MoS2 electrodes were in the range of 55–120 mV/decade, which indicates a fast improvement in the HER velocity as a result of the increased potential. Stability is another important parameter for evaluating a catalyst. The same (10 ALD cycles) Pt/ML-MoS2 electrode was able to continuously generate hydrogen molecules at for 150 hours. These superior results demonstrate that the low conductivity of semiconductive MoS2 can be enhanced by anchoring the film with Pt SAs and clusters, leading to sufficient charge transport and a decrease in the overpotential.

abstract: Layered chalcogenides are a diverse class of crystalline materials that consist of covalently bound building blocks held together by van der Waals forces, including the transition metal dichalcogenides (TMDCs) and the pnictogen chalcogenides (PCs) among all. These materials, in particular, MoS2 which is the most widely studied TMDC material, have attracted significant attention in recent years due to their unique physical, electronic, optical, and chemical properties that depend on the number of layers. Due to their high aspect ratios and extreme thinness, 2D materials are sensitive to modifications via chemistry on their surfaces. For instance, covalent functionalization can be used to robustly modify the electronic properties of 2D materials, and can also be used to attach other materials or structures. Metal adsorption on the surfaces of 2D materials can also tune their electronic structures, and can be used as a strategy for removing metal contaminants from water. Thus, there are many opportunities for studying the fundamental surface interactions of 2D materials and in particular the TMDCs and PCs. The work reported in this dissertation represents detailed fundamental studies of the covalent functionalization and metal adsorption behavior of layered chalcogenides, which are two significant aspects of the surface interactions of 2D materials. First, we demonstrate that both the Freundlich and Temkin isotherm models, and the pseudo-second-order reaction kinetics model are good descriptors of the reaction due to the energetically inhomogeneous surface MoS2 and the indirect adsorbate-adsorbate interactions from previously attached nitrophenyl (NP) groups. Second, the covalent functionalization using aryl diazonium salts is extended to nanosheets of other representative TMDC materials MoSe2, WS2, and WSe2, and of the representative PC materials Bi2S3 and Sb2S3, demonstrated using atomic force microscopy (AFM) imaging and Fourier transform infrared spectroscopy (FTIR). Finally, using AFM and X-ray photoelectron spectroscopy (XPS), it is shown that Pb, Cd Zn and Co form nanoclusters on the MoS2 surface without affecting the structure of the MoS2 itself. The metals can also be thermally desorbed from MoS2, thus suggesting a potential application as a reusable water purification technology. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

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This dissertation presents studies electrodeposited MoS2 and metal-doped MoS2 thin films, and their performance for energy storage, catalysis, and biosensor applications. Ni-doped MoS2 thin films were fabricated by electrodeposition from electrolytes containing both MoS42- and varying concentrations of Ni2+, followed by annealing at 400 ºC for 2 h in an Ar atmosphere. The film resistivity increased from 11.3 µΩ-cm for un-doped MoS2 to 32.8 µΩ-cm for Ni-doped MoS2 containing 9 atom% Ni. For all Ni dopant levels studied, only the x-ray diffraction (XRD) pattern expected for MoS2 is observed, with the average grain size increases with increasing Ni content. Ni-doped MoS2 thin films were tested for their activity towards the hydrogen evolution reaction (HER) in 0.5M H2SO4. Tafel equation fits reveal that the catalytic activity for HER, as measured by the exchange current density, increases up to 6 atom% Ni, and then decreases slightly for 9 atom% Ni. Ni-doped MoS2 thin films were also tested in 1.0 M Na2SO4 for use within electrochemical supercapacitors, and the capacitance per unit area increases by 2-3x for 9 atom% Ni-doped MoS2 relative to un-doped MoS2. The highest specific capacitance obtained for Ni-doped MoS2 during galvanostatic charge-discharge measurements is ~300 F/g

2D materials like graphene and MoS_2 are atomically thin, extremely strong and flexible, making them attractive for integration into strain engineered devices. Strain on these materials can change physical properties, as well as induce exotic physics, not typically seen in solid-state systems. Here, we probe the novel physics arising from distorted lattices of 2D materials, strained by nanopillars indentation and microelectromechanical systems (MEMS), using Raman and photoluminescence (PL) spectroscopy. From nanopillars strained multilayer MoS_2, we observe exciton and charge carrier funneling due to strain, inducing dissociation of excitons in to free electron-hole pairs in the indirect material. Using MEMS devices, we were able to dynamically strain monolayer and multilayer graphene. Multilayer graphene under MEMS strain showed signatures of loss in Bernal stacking due to shear of the individual layers, indicating that MEMS can be used to tune the layer commensuration with tensile strain. We further explore simulation of pseudo-magnetic fields (PMFs) generated in monolayer graphene strained by MEMS, using machine learning, to accelerate and optimize the strength and uniformity of the PMF in new graphene geometries. Nanopillars provide non-uniform, centrally biaxial strain to multilayer MoS_2 transferred on top. Raman E^1_2g and PL redshift across the pillar confirms 1-2% strain in the material. We also observe a softening in the A_1g Raman mode and an enhancement in the overall PL with an increase in radiative trions, under strain. The changes in these charge-dependent features indicates funneling of charge carriers and neutral excitons to the apex of the pillar, as strain locally deforms the band structure of the conduction and valence bands. DFT calculations of the band structure in bilayer MoS_2 under biaxial strain shows the conduction band is lowered, further increasing the indirectness of multilayer MoS_2. This should cause the PL intensity to decrease, whereas we observe an increase in MoS_2 PL intensity under strain. We theorize that this is due to a dissociation of excitons into free electron-hole pairs. The increase in charge carrier densities due to strain leads to a renormalization of the local band structure and increased dielectric screening, supporting free electron-hole recombination at the K-point without momentum restrictions. In turn, electron-hole recombination occurs around the K-point inducing a high intensity PL, which opens attractive opportunities for utilization in optoelectronic devices. MEMS chevron actuators can dynamically strain 2D materials, which we demonstrate through uniaxial strain in CVD and exfoliated graphene. We use a novel microstructure assisted transfer technique which can deterministically place materials on non-planar surfaces like MEMS devices. Building on previously reported 1.3% in monolayer MoS2 from our group, we report tunable 0.3% strain in CVD monolayer graphene and 1.2% strain in multilayer exfoliated graphene using MEMS chevron actuators, detected by Raman spectroscopy. The asymmetric-to-symmetric strain evolution of the 2D phonon line shape in multilayer graphene is evidence of changes in interlayer interactions, caused by shearing between layers. This demonstrates that MEMS can be used to tune the commensuration in few layer 2D materials, which is a promising avenue towards Moiré engineering. Using machine learning, we also simulate optimal monolayer graphene geometries for generating strong, uniform pseudo-magnetic fields by MEMS strain. The coupled use of finite-element methods, variational auto-encoder, and auxiliary neural network accelerates the search for PMFs in strained graphene, while optimizing the graphene shape for fabrication through electron-beam lithography. Our experimental and simulated work creates a road-map for rapid advancement in zero-field quantum Hall effect devices using graphene-integrated MEMS actuators.