WS2, MoS2 and ZrS2 nanomaterials in various forms, such as nanoflakes, inorganic fullerene-like nanoparticles and nanorattles, were synthesised using two modified conventional techniques: solid-gas reaction and chemical vapour deposition. Both of these techniques are essentially based on reactions between metal oxides/chlorides and sulphur at a relatively low temperature in the range of 350-950°C in H2/Ar. Compared with other common techniques, these techniques are cost effective and environmentally friendly and produce well-crystallised WS2, MoS2 and ZrS2 nanomaterials with controllable sizes and morphologies, arising from the involvement of simple equipment and a H2S-free process. With the solid-gas reaction technique, the formation of WS2 and MoS2 inorganic fullerene like (IF) particles follows a so-called "template growth" mechanism, which implies that the sizes of the final products resemble their metal oxide raw materials. Therefore, because of the usage of WO3 nanoparticles and MoO3 submicron particles as precursors, nanosized WS2 (<100 nm) and submicron-sized MoS2 (approximately 500 nm) particles were generated, respectively. Further investigation of the reaction mechanism reveals that H2 is a vital factor in the formation of WS2 IF nanoparticles. Without H2, WS2 nanoflakes are instead produced, which is attributed to that the formation of WS2 IF nanoparticles based on the synergy between H2 reduction and S sulphidation. Using the CVD technique, WS2 IF nanoparticles with sizes below 100 nm were readily produced. However, the initially formed WS2 IF nanoparticles were poorly crystallised with numerous defects and disconnections, which is consistent with the results of other researchers. In this project, an additional annealing process was introduced to eliminate these defects and disconnections. After this process, well-crystallised WS2 IF nanoparticles were formed, which should exhibit improved mechanical properties and stability. In addition to the WS2 IF nanoparticles, ZrS2 was also prepared using the same route from the reaction of ZrCl4 with S. Unlike the WS2, the generated ZrS2 was in the form of nanoflakes with sizes below 30 nm. Consequently, these nanoflakes exhibited a strong quantum confinement effect and good photocatalytic performance for the decomposition of 4-NP. Based on the investigation of the WO3 sulphidation mechanism, novel W@WS2 and WS2@WS2 nanorattles were designed and first synthesised using a simple gas-solid reaction. The as-synthesised nanorattles were composed of tiny, moveable W/WS2 cores and continuous WS2 shells with much larger sizes. By simply tailoring the processing parameters, several types of nanostructures, including WS2 nanoflakes, IF nanoparticles and nanorattles (with desirable core size and shell thickness) were selectively prepared. Moreover, it was observed for the first time that the as-prepared nanorattles exhibited excellent catalytic activities, which were close to or even better than their much more expensive Au- and Pt-based counterparts.

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.

The ability to form multiple component heterostructures of two-dimensional materials promises to provide access to hybrid materials with tunable properties different from those of the bulk materials or two-dimensional constituents. By taking advantage of the unique properties of different constituents, numerous applications are possible for which none of the individual components are viable. The synthesis of multiple component heterostructures, however, is nontrivial, relying on either the cleaving and stacking of bulk materials in a “scotch tape” type technique or finding coincidentally favorable growth conditions which allow layers to be grown epitaxially on each other in any order. In addition, alloying of miscible materials occurs when the modulation wavelength is small. These synthetic challenges have limited the ability of scientists to fully utilize the potential of multiple component heterostructures. An alternative synthetic route to multiple component heterostructures may be found through expansion of the modulated elemental reactant technique which allows access to metastable products, known as ferecrystals, which are otherwise inaccessible. This work focuses on the expansion of the modulated elemental reactants technique for the formation of ferecrystals containing multiple constituents. As a starting point, the synthesis of the first alloy ferecrystals (SnSe)1.16-1.09([NbxMo1-x]Se2) will be discussed. The structural and electrical characterization of these compounds will then be used to determine the intermixing of the first three component ferecrystal heterojunction ([SnSe]1+δ)([{MoxNb1-x}Se2]1+γ)([SnSe]1+δ)({NbyMo1-y}Se2). Then, by synthesizing ([SnSe]1+δ)m([{MoxNb1-x}Se2]1+γ)1([SnSe]1+δ)m({NbxMo1-x}Se2)1 (m = 0 - 4) compounds with increasing thicknesses of SnSe, the interdiffusion of miscible constituents in ferecrystals will be studied. In addition, by comparison of the ([SnSe]1+δ)m ([{MoxNb1-x}Se2]1+γ)1([SnSe]1+δ)m({NbxMo1-x}Se2)1 (m = 0 - 4) compounds to the ([SnSe]1+δ)m(NbSe2)1 (m = 1 - 8) compounds the electronic interactions of the MoSe2 and NbSe2 layers will be determined. Finally, the effects of different alloying strategies and the interdiffusion of miscible constituents will be further examined by the synthesis of ordered ([SnSe]1.15)1([TaxV1-x]Se2)1([SnSe]1.15)1([VyTa1-y]Se2)1 and ([SnSe]1+δ) ([TaxV1-x]Se2) compounds with the effect of isoelectric doping on the charge density wave transition in (SnSe)1.15(VSe2) also being explored. This work contains previously published and unpublished co-authored material.

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

No description available.

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.