Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their unique electrical, optical, mechanical, and thermal properties not found in their 3D counterparts. They can be obtained by mechanical, chemical, or electrochemical exfoliation. However, these strategies lack uniformity and produce defect-rich samples, making it impossible for large-scale device fabrication. Chemical vapor deposition (CVD) method emerges as the viable candidate to create atomically thin specimens at the technologically relevant scale. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge. The spatial inhomogeneity and the associated grain boundaries between randomly oriented domains culminate to the deleterious quality of TMDs, breaking of the long-range crystalline periodicity and introduction of insidious strain. Recent research efforts have therefore dedicated to obtaining the single crystallinity of 2D materials by controlling the orientation and dimensionality to obtain a large-scale and grain boundary-free monolayer films for Si-comparable electron mobility and overcoming the scaling limitation of traditional Si-based microelectronics,. In the first part of this thesis, orientation and dimensionality controlling of TMDs are discussed. To this end, we systematically study the growth of stereotypical molybdenum disulfide (MoS2) monolayer on a c-plane sapphire with CVD to elucidate the factors controlling their orientation. We have arrived at the conclusion that the concentration of precursors- that is, the ratio between sulfur and molybdenum oxide, plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. Later, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer, and single-crystalline MoS2 nanoribbons on β-gallium (iii) oxide (β-Ga2O3) (100) substrates. LDE MoS2 nanoribbons have spatial uniformity over a long-range and transport characteristics on par with those seen in exfoliated benchmarks. In the second part, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast as a result of the residual 3-fold astigmatism in modern scanning transmission electron microscopy (STEM) and its unintended impact on violating the power-law dependence of contrast on coordination modes between the transition metal and chalcogenide atoms.

In this dissertation, I present the results of my research regarding hybrid semiconductor interfaces between organic and inorganic semiconductors. Using photoemission spectroscopy, I elucidate the important role of defect-induced electronic states within the inorganic semiconductor phase. These states significantly affect both the energy level alignment and the charge carrier dynamics at the hybrid interface. I demonstrate that the behavior of these hybrid semiconductor interfaces is complex and not well characterized by current models for organic semiconductor interfaces. Specifically, I show that hybrid interfaces host unique electronic phenomena that depend sensitively on the surface structure of the inorganic semiconductor. I also demonstrate new applications of photoemission spectroscopies that enable the direct analysis of important properties of inorganic semiconductors, including charge carrier behavior near hybrid interfaces and the electronic character of defect-induced energy levels. The research presented here focuses on two different n-type inorganic semiconductors, tin disulfide (SnS₂) and zinc oxide (ZnO). SnS₂ is a layered transition metal dichalcogenide that presents an atomically flat and inert surface, ideal for sensitively probing electronic interactions at the hybrid interface. To probe the electronic structure of the SnS₂ surface, I used a variety of organic molecules, including copper phthalocyanine, vanadyl naphthalocyanine, chloro-boron subphthalocyanine, and C₆₀. ZnO has a complex surface structure that can be modified by simple experimental procedures; it was therefore used as a tunable semiconductor substrate where the effects of altered electronic structure can be observed. By carefully studying the origin of hybrid interfacial interactions, these research projects provide a first step in explicitly elucidating the fundamental mechanisms that determine the electronic properties of hybrid interfaces.

Two-dimensional (2D) layered materials such as MoS2 have been recognized as high on-off ratio semiconductors which are promising candidates for electronic and optoelectronic devices. In addition to the use of individual 2D materials, the accelerated field of 2D heterostructures enables even greater functionalities. Device designs differ, and they are strongly controlled by the electronic band alignment. For example, photovoltaic cells require type II heterostructures for light harvesting, and light-emitting diodes benefit from multiple quantum wells with the type I band alignment for high emission efficiency. The vertical tunneling field-effect transistor for next-generation electronics depends on nearly broken-gap band alignment for boosting its performance. To tailor these 2D layered materials toward possible future applications, the understanding of 2D heterostructure band alignment becomes critically important. In the first part of this thesis, we discuss the band alignment of 2D heterostructures. To do so, we firstly study the interlayer coupling between two dissimilar 2D materials. We conclude that a post-anneal process could enhance the interlayer coupling of as-transferred 2D heterostructures, and heterostructural stacking imposes similar symmetry changes as homostructural stacking. Later, we precisely determine the quasi particle bandgap and band alignment of the MoS2/WSe2 heterostructure by using scan tunneling microscopy/spectroscopy (STM/S) and micron-beam X-ray photoelectron spectroscopy (μ-XPS) techniques. Lastly, we prove that the band alignment of 2D heterojunctions can be accurately predicted by Anderson’s model, which has previously failed to predict conventional bulk heterostructures. In the second part of this thesis, we develop a new Chemical Vapor Deposition (CVD) method capable of precisely controlling the growth area of p- and n-type transition metal dichalcogenides (TMDCs) and further form lateral or vertical 2D heterostructures. This method also allows p- and n-type TMDCs to separately grow in a selective area in one step. In addition, we demonstrate a first bottom-up 2D complementary inverter based on hetero-TMDCs.

abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are effective components in optoelectronic devices due to their tunable and attractive electric, optical and chemical properties. Combining different 2D TMDCs into either vertical or lateral heterostructures has been pursued to achieve new optical and electronic properties. Chemical treatments have also been pursued to effectively tune the properties of 2D TMDCs. Among many chemical routes that have been studied, plasma treatment is notable for being rapid and versatile. In Wang’s group earlier work, plasma treatment of MoS2 and WS2 resulted in the formation of MoO3 and WO3 nanosheets and nanoscrolls. However, plasma treatment of 2D TMDC heterostructures have not been widely studied. In this dissertation, MoS2/WS2 vertical and lateral heterostructures were grown and treated with air plasma. The result showed that the vertical heterostructure and lateral heterostructures behaved differently. For the vertical heterostructures, the top WS2 layer acts as a shield for the underlying MoS2 monolayer from oxidizing and forming transition metal oxide nanoscrolls, as shown by Raman spectroscopy and atomic force microscopy (AFM). On the contrary, for the lateral heterostructures, the WS2 that was grown surrounding the MoS2 triangular core served as a tight frame to stop the propagation of the oxidized MoS2, resulting a gradient of crack distribution. These findings provide insight into how plasma treatment can affect the formation of oxide in heterostructure, which can have further application in nanoelectronic devices and electrocatalysts. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019

Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂. / text

The use of low-friction coatings on machine elements is steadily increasing, and they are expected to play an important role in the reduction of fuel consumption of future motorized vehicles. Many low-friction coatings function by transformation of the outermost coating layer into tribofilms, which then cover the coating surface and its counter surface. It is within these tribofilms that sliding takes place, and their properties largely determine the performance. The role of the coating is then not to provide low friction, but to supply support and constituents for the tribofilm. In this thesis, the formation of such tribofilms has been studied for a number of different low-friction coatings. The sensitivity of the tribofilm formation towards changes in the tribological system, such as increased surface roughness, varied surrounding atmosphere and reduced availability of the tribofilm constituents has been given special attention. For TaC/aC coatings, the formation of a functioning tribofilm was found to be a multi-step process, where wear fragments are formed, agglomerated, compacted and eventually stabilized into a dense film of fine grains. This formation is delayed by a moderate roughening of the coated surface. Coatings based on tungsten disulphide (WS2) are often able to provide exceptionally low friction, but their use is restricted by their poor mechanical properties and sensitivity to humidity. Large improvements in the mechanical properties can be achieved by addition of for example carbon, but the achievable hardness is still limited. When titanium was added to W-S-C coatings, a carbidic hard phase was formed, causing drastically increased hardness, with retained low friction. Titanium oxides in the tribofilms however caused the friction to be high initially and unstable in the long term. In a study of W-S-N coatings, the effects of humidity and oxygen were studied separately, and it was found that the detrimental role of oxygen is larger than often assumed. Low friction tribofilms may form by rearrangement of coating material, but also by tribochemical reactions between constituents of the coating and its counter surface. This was observed for Ti-C-S coatings, which formed WS2 tribofilms when sliding against tungsten counter surfaces, leading to dramatic friction reductions.

This thesis explores structural and electronic properties of layered materials at the nanometre scale. Room temperature and low temperature ultrahigh vacuum scanning probe microscopy (scanning tunneling microscopy, scanning tunneling spectroscopy, atomic force microscopy) is used as the primary characterization method. The main findings in this thesis are: (a) observations of the atomic lattice and imaging local lattice defects of semiconducting ReS2 by scanning tunneling microscopy, (b) measurement of the electronic band gap of ReS2 by scanning tunneling spectroscopy, and (c) scanning tunneling microscopy study of 1T-TaS2 lattice and chemically functionalizing its defects with magnetic molecules.

Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS2 films. An understanding of these effects is critical for integrating MoS2 into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS2 films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS2. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS2 for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS2 films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.

abstract: A highly uniform and repeatable method for synthesizing the single-layer transition metal dichalcogenides (TMDs) molybdenum disulfide, MoS2, and tungsten disulfide, WS2, was developed. This method employed chemical vapor deposition (CVD) of precursors in a custom built cold-wall reaction chamber designed to allow independent control over the growth parameters. Iterations of this reaction chamber were employed to overcome limitations to the growth method. First, molybdenum trioxide, MoO3, and S were co-evaporated from alumina coated W baskets to grow MoS2 on SiO2/Si substrates. Using this method, films were found to have repeatable coverage, but unrepeatable morphology. Second, the reaction chamber was modified to include a pair of custom bubbler delivery systems to transport diethyl sulfide (DES) and molybdenum hexacarbonyl (MHC) to the substrate as a S and Mo precursors. Third, tungsten hexacarbonyl (WHC) replaced MHC as a transition metal precursor for the synthesis of WS2 on Al2O3, substrates. This method proved repeatable in both coverage and morphology allowing the investigation of the effect of varying the flow of Ar, varying the substrate temperature and varying the flux of DES to the sample. Increasing each of these parameters was found to decrease the nucleation density on the sample and, with the exception of the Ar flow, induce multi-layer feature growth. This combination of precursors was also used to investigate the reported improvement in feature morphology when NaCl is placed upstream of the substrate. This was found to have no effect on experiments in the configurations used. A final effort was made to adequately increase the feature size by switching from DES to hydrogen sulfide, H2S, as a source of S. Using H2S and WHC to grow WS2 films on Al2O3, it was found that increasing the substrate temperature and increasing the H2S flow both decrease nucleation density. Increasing the H2S flow induced bi-layer growth. Ripening of synthesized WS2 crystals was demonstrated to occur when the sample was annealed, post-growth, in an Ar, H2, and H2S flow. Finally, it was verified that the final H2S and WHC growth method yielded repeatability and uniformity matching, or improving upon, the other methods and precursors investigated. / Dissertation/Thesis / Doctoral Dissertation Physics 2019

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