Graphene, the thinnest material with a stable 2D structure, is a potential alternative for silicon-based electronics. However, zero band gap of graphene causes a poor on-off ratio of current thus making it unsuitable for logic operations. This problem prompted scientists to find other suitable 2D materials. Creating vacancy defects or synthesizing hybrid 2D planar interfaces with other 2D materials, is also quite promising for modifying graphene properties. Experimental productions of these materials lead to the formation of possible defects and impurities with significant influence in device properties. Hence, a detailed understanding of the effects of impurities and defects on the properties of 2D systems is quite important. In this thesis, detailed studies have been done on the effects of impurities and defects on graphene, hybrid graphene/h-BN and graphene/graphane structures, silicene and transition metal dichalcogenides (TMDs) by ab-initio density functional theory (DFT). We have also looked into the possibilities of realizing magnetic nanostructures, trapped at the vacancy defects in graphene, at the reconstructed edges of graphene nanoribbons, at the planar hybrid h-BN graphene structures, and in graphene/graphane interfaces. A thorough investigation of diffusion of Fe adatoms and clusters by ab-initio molecular dynamics simulations have been carried out along with the study of their magnetic properties. It has been shown that the formation of Fe clusters at the vacancy sites is quite robust. We have also demonstrated that the quasiperiodic 3D heterostructures of graphene and h-BN are more stable than their regular counterpart and certain configurations can open up a band gap. Using our extensive studies on defects, we have shown that defect states occur in the gap region of TMDs and they have a strong signature in optical absorption spectra. Defects in silicene and graphene cause an increase in scattering and hence an increase in local currents, which may be detrimental for electronic devices. Last but not the least, defects in graphene can also be used to facilitate gas sensing of molecules as well as and local site selective fluorination.

In this thesis the layered, two-dimensional material MoTe2 is examined experimentally for its optoelectronic properties, using a field effect transistor device configuration. MoTe2 experiences a strong light matter interaction, which is highly dependent on the conditions of the measurement, and the wavelength of light used. Light is able to: produce a photocurrent in MoTe2, desorb adsorbates from the surface, and even controllably thin by a single layer at a time. A theoretical study on MoTe2 also provides insights on the source of some of these interesting light matter interactions. MoTe2 is found to be a fast and responsive photodetector when illuminated with red laser light in ambient conditions, with increases in current stemming from the photovoltaic effect. Due to the generated charge carriers from the photovoltaic effect, conductivity can increase by increasing the Fermi energy of the material, or by a photogating effect where excited charges are trapped and behave as an artificial gate for the field effect transistor. The mechanisms of charge trapping are experimentally investigated due to their prevalence in the photodetection mechanisms. A theoretical study points towards the existence of two types of trap states, in not just MoTe2 but all transition metal dichalcogenides, with shallow traps closer to the valence band edge (τ ~ 500 s) and deeper traps (τ ~ 1000 s), further away from the valence band edge. MoTe2, under the effects of higher energy photons from blue and green lasers, showed different photocurrent mechanisms to red light. From the increased energy of the photons, photo-desorption of adsorbates on the surface of MoTe2 occurred causing a decrease in the overall current, in a rarely seen photocurrent mechanism. Again, both shallow and deep traps are evident from the experimental measurements, with the shallow traps being removed when illuminated by higher energy photons. Finally, a humidity assisted photochemical layer-by-layer etching process was developed with an in-situ Raman spectroscopy system, able to thin MoTe2 by a single layer at a time with 200 nm spatial resolution. MoTe2 FETs were created with thinned channels to examine the effect of the thinning technique on optoelectronic properties. Some improvement in optoelectronic performance (higher responsivity, higher mobility) was seen for the thinned channel devices, with great improvement observed for monolayer MoTe2.

The relative ease of tuning the properties of two-dimensional materials compared to their three-dimensional counterparts offers great potential to achieve previously inaccessible multifunctional devices. In this Dissertation, we use strain engineering as a non-destructive way to control the properties of two-dimensional materials, employing density functional theory and chemical vapor deposition. In the first part of the Dissertation, density functional theory is used to investigate the effect of biaxial strain on the structural, electronic, and magnetic properties of pristine and Janus Cr-trihalide monolayers. We find that the broken inversion symmetry of the Janus monolayers X3-Cr2-Y3 (X, Y = Cl, Br, and I) enhances their functionality by making the magnetic anisotropy tunable by strain and inducing an out-of-plane electric polarization. A very negative magnetic anisotropy energy of ̶ 3.77 meV per formula unit is realized in the Cl3-Cr2-I3 monolayer under ̶ 5% strain. In the second part of the Dissertation, we perform a comprehensive investigation of thermally strained monolayer MoS2, both theoretically and experimentally, to tune the sulfur vacancy density. Due to a dominant role of the intralayer electrostatic interaction, compressive (tensile) biaxial strain decreases (increases) the sulfur vacancy formation energy and, thus, increases (decreases) the probability of creating sulfur vacancies. This fundamental relationship opens a new venue for defect engineering of transition metal dichalcogenides.

Two-dimensional (2D) materials show novel electronic, optical and chemical properties and have great potential in devices such as field-effect transistors (FET), photodetectors and gas sensors. This thesis focuses on scanning tunneling microscopy and spectroscopy (STM/STS) investigation of interfaces and defects 2D transition metal dichalcogenides (TMDCs). The first part of the thesis focuses on the synthesis of 2D TiSe2 with chemical vapor transport (CVT). By properly choosing the growth condition, Sub-10 nm TiSe2 flakes were successfully obtained. A 2 × 2 charge density wave (CDW) was clearly observed on these ultrathin flakes by scanning tunneling microscopy (STM). Accurate CDW phase transition temperature was measured by transport measurements. This work opens up a new approach to synthesize TMDCs. The second part of the thesis focuses on monolayer vacancy islands growing on TiSe2 surface under electrical stressing. We have observed nonlinear area evolution and growth from triangular to hexagonal driven by STM subjected electrical stressing. Our simulations of monolayer island evolution using phase-field modeling and first-principles calculations are in good agreement with our experimental observations. The results could be potentially important for device reliability in systems containing ultrathin TMDCs and related 2D materials subject to electrical stressing. The third part of the thesis focuses on point defects in 2D PtSe2. We observed five types of distinct defects from STM topography images and measured the local density of states (LDOS) of those defects from scanning tunneling spectroscopy (STS). We identified the types and characteristics of these defects with the first-principles calculations. Our findings would provide critical insight into tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering or varying growth condition in few-layer 1T-PtSe2 and other related 2D materials. / Doctor of Philosophy / Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted more and more attentions. When the thickness of a layered material thinned to one or few atoms, it shows interesting properties different from its bulk phase. Due to the reduced dimensionality, interfaces and defects in 2D materials will significantly affect the electronic property and chemical activity. However, such nanometer scale features are several orders of magnitude smaller than the wavelength of visible light, which is the limit of resolution for optical microscope. Scanning tunneling microscope (STM) is widely used in study of 2D materials not only because it can provide the topography and local electronic information at atomic scale, but also because of the possibility of directly fabricate atomic scale structure on the surface. The first part of the thesis focuses on the synthesis of 2D TiSe2 with chemical vapor transport (CVT). TiSe2 belongs to the transition metal dichalcogenides (TMDCs) family, showing a sandwiched layered structure. When the temperature goes down to 200K, a 2 × 2 superlattice called charge density wave (CDW) will show up, which is clearly observed in our STM images. The second part of the thesis focuses on monolayer vacancy islands growing on TiSe2 surface controlled by electrical stressing. During continuous STM scanning, we have observed nonlinear area growth of the vacancy islands. The shape of those islands transfers from triangular to hexagonal. We successfully simulated such growth using phase-field modeling and first-principles calculations. The results could be potentially important for device reliability in systems containing ultrathin TMDCs and related 2D materials subject to electrical stressing. The third part of the thesis focuses on defects in 2D PtSe2. We observed five types of distinct defects in our STM topography images. By comparing them with DFT-calculated simulation images, we identified the types and characteristics of these defects. Our findings would provide critical insight into tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering in few-layer 1T-PtSe2 and other related 2D materials.

2-Dimensional (2D) materials are characterised by atomic thickness and significantly larger edge-lengths, producing particles which are highly confined in 1 direction. Reducing a material to one or few atomic layers gives rise to structural and electronic properties that deviate significantly from those of the bulk crystal. For this reason 2D nanosheets have been investigated for potential application in sensing, catalysis, capacitance, photovoltaics and for flexible circuits (among others).Despite rapid progress in understanding the synthesis and properties of 2D nanosheets in recent years, there remain significant problems surrounding the development of scalable production methods, understanding and tuning fundamental properties, and controlling the size and monodispersity of semiconductor crystals. In addition, new materials with novel properties are constantly sought in order to meet specific requirements. Although the tools developed over the last 12 years can often be applied to the fabrication of these materials, understanding their behaviour and limitations is ongoing. The following thesis discusses the routes to the fabrication of 2-dimensional materials and explores the production of MoS2, black phosphorus and tin(II) sulfide nanosheets. The aim of each piece of work is determined by the level of development of the field; MoS2 nanosheets have been known for several years and therefore the work presented was motivated by a desire to impart size control for specific applications. The study of phosphorene and 2D tin(II) sulfide is in its infancy; as such the focus remains on scalable nanosheet exfoliation and developing an understanding of their properties. The following studies on phosphorene report the exfoliation of nanosheets in organic and aqueous surfactant solutions and an investigation of the stability and breakdown products of the resulting colloidal suspensions. The stabilisation of phosphorene in aqueous media paves the way for its use in biological systems. Band-gap tuning in IV-VI analogues of phosphorene is demonstrated by size-selection of exfoliated SnS nanosheets. Although the physical characteristics of nanosheets and their incorporation into devices receive some attention, this thesis will focus mainly on the synthetic aspects of 2D materials research.

Two-dimensional (2D) materials, referring to materials being just one atom thick, prove to be attractive not only in fundamental research but also in applications. Graphene, a single layer of carbon atoms arranged in hexagonal rings, is just the first among other materials (including hexagonal boron nitride and molybdenum disulfide) that could be isolated into mono-atomic layers. The presented thesis investigates proton transport through atomically thin two-dimensional materials. While the electronic, optical and mechanical properties of graphene and other 2D materials have been intensely researched over the past decade, much less is known on the interaction of these crystals with protons. It has been reported that most of the defect free two dimensional materials are impermeable to nearly all gases, molecules and ions. Whether proton, the smallest positively charged ion, could transport through two dimensional materials at a low energy level remains unknown. This work investigates proton transport through 2D materials, including graphene, hexagonal boron nitride and molybdenum disulfide, in two different systems: Nafion/Pd solid system and liquid/liquid interface system, both of which provided consistent results. Our results suggest that proton can transport through the interatomic spacings in the lattice of single layer BN and graphene, while single layer MoS2 is impermeable to protons. Single layer BN is the most conductive to protons among the 2D materials investigated in this thesis. Lower proton conductance of graphene is due to its delocalized π electrons while proton impermeability of MoS2 is due to the three atomic layers structure. Moreover, proton transfer is greatly facilitated by the deposition of platinum nanoparticles on the proton conductive 2D membranes to such a degree that platinum decorated BN seems to present negligible resistance to the transfer of protons through its lattice.

This thesis explores the substrate-dependent charge carrier dynamics of large area graphene fi lms. Using terahertz spectroscopy, we measure conductivity spectra of graphene supported by seven distinct substrates, and extract their transport properties (carrier scattering time, doping density, and carrier mobility) within the Drude model. We find that graphene supported by distinct substrates exhibit signi cantly different transport properties. We propose that graphene fi lms supported by substrates with less charged impurities exhibit longer scattering times, and enhanced carrier mobilities, emphasizing the importance of the graphene-substrate interaction for optimization of device performance. These results will be signi cant for the effective integration of graphene into future technologies where an optimal carrier mobility is desired.

In this work, we demonstrate the mechanism for etching exfoliated graphene on SiO2 and other technological important substrates (Si, SiC and ITO), using low-energy electron sources. Our mechanism is based on helium ion sputtering and vacancy formation. Helium ions instead of incident electrons cause the defects that oxygen reacts with and etches graphene. We found that etching does not occur on low-resistivity Si and ITO. Etching occurs on higher resistivity Si and SiC, although much less than on SiO2. In addition, we studied the degradation mechanism of MoS2 under ambient conditions using as-grown and preheated mono- and thicker-layered MoS2 films. Thicker-layered MoS2 do not exhibit the growth of dendrites that is characteristic of monolayer degradation. Dendrites are observed to stop at the monolayer-bilayer boundary. Raman and photoluminescence spectra of the aged bilayer and thicker-layered films are comparable to those of as-grown films. We found that greater stability of bilayers and thicker layers supports a previously reported mechanism for monolayer degradation involving Förster resonance energy transfer. As a result, straightforward and scalable 2D materials integration, or air stable heterostructure device fabrication may be easily achieved. Our proposed mechanisms for etching graphene and ambient degradation of MoS2 could catalyze research on realizing new devices that are more efficient, stable, and reliable for practical applications.

This work is dedicated to electron transport in atomically thin crystals. We explore hydrodynamic effects in the electron liquid of graphene and perform a comprehensive study of electronic and optical properties of a novel 2D semiconductor - indium selenide(InSe). Graphene hosts a high quality electron system with weak phonon coupling such that electron-electron scattering can be the dominant process responsible for the establishment of local equilibrium of the electronic system above liquid nitrogen temperatures. Under these conditions, charge carriers are expected to behave as a viscous fluid with a hydrodynamic behaviour similar to classical gases or liquids. In this thesis, we aimed to reveal this hydrodynamic behaviour of the electron fluid by studying transport properties of high-quality graphene devices. To amplify the hydrodynamic effects, we used a special measurement geometry in which the current was injected into the graphene channel and the voltage was measured at the contact nearest to the injector. In this geometry we detected a negative signal which is developed as a result of the viscous drag between adjacent fluid layers, accompanied by the formation of current vortices. The magnitude of the signal allowed us to perform the first measurement of electron viscosity. In order to understand how an electron liquid enters the hydrodynamic regime we studied electron transport in graphene point contacts. We observed a drop in the point contact resistance upon increasing temperature. This drop was attributed to the interaction-induced lubrication of the point contact boundaries that was found to be strong enough to prevent momentum relaxation of charge carriers. The viscosity of the electron fluid was measured over a wide range of temperatures and at different carrier densities. Experimental data was found to be in good agreement with many-body calculations. In this work we also studied transport properties of two-dimensional InSe. We observed high electron mobility transport, quantum oscillations and a fully developed quantum Hall effect. In optical studies, we revealed that due to the crystal symmetry a monolayer InSe features suppressed recombination of electron-hole pairs.

Two dimensional transition metal dichalcogenides provide a host of unique optoelectronic properties, attributed to their two dimensional nature and unique band structure, making them promising for future optoelectronics device applications. In the work presented in this thesis, we focus on the theoretical understanding and modelling of the optoelectronic properties of monolayer transition metal dichalcogenides, their heterostructures and multilayers. We studied the relaxation rates of photo-excited carriers leading to the formation of electron-hole pairs and their subsequent radiative recombination, resulting in emission of light. We find sub-ps relaxation times, attributed to the strong coupling of carriers with optical phonons, allowing the efficient formation of strongly bound multi-particle complexes such as excitons, trions and biexcitons, which can recombine radiatively if allowed by selection rules. We classify the various complexes according to their optical activity, and predict using diffusion quantum Monte Carlo calculations the resulting photoluminescence spectra in these materials. We proposed a novel, material specific, Auger process in WS2 and WSe2 involving dark excitons, which dominates over radiative processes for relatively low carrier densities, providing an explanation to the observed low quantum efficiencies in these materials. In the same pair of materials, we have shown how the ground state dark trions and biexcitons can become bright and recombine radiatively through an electron-electron intervalley scattering process, resulting in new observable lines in the photoluminescence spectra of these materials. The ability to form van der Waals heterostructures of two or more layers of these materials, allows for new degrees of freedom to be explored and utilised. The heterobilayer system made of MoSe2/WSe2 has a type-II band alignment, allowing for the formation of interlayer bound complexes with carriers localized on opposite layers. We studied the bound complexes formed in this bilayer system, localized on donor impurities. We used quantum Monte Carlo methods to obtain binding energies and wave functions, and calculated the radiative rates and doping dependent photoluminescence spectra of these complexes for closely aligned layers, and asymptotic behaviour for strongly misaligned layers. Finally, we studied few-layers of 2H-stacked transition metal dichalcogenides. The van der Waals quantum well structure results in the splitting of the conduction and valence bands into multiple subbands with energy spacings covering densely the infrared to far-infrared spectral range. We developed a hybrid k.p-tight binding model parameterised by DFT calculations of monolayer and bulk crystals of the studied materials. We used the model to describe the subband dispersions, transition energies, phonon induced broadening and resulting absorption lineshapes for both p-doped and n-doped few-layer films.