Recently, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) materials have drawn significant attention due to their unique optical and electrical properties1, 2. This offers unique opportunities for the next-generation electronic and optoelectronic devices3. Specifically, recent innovations in the big-data-driven prediction of new 2D materials, integration of new device architectures, interfacial engineering of contacts between semiconductor/metals and semiconductor/dielectrics as well as encapsulation in hexagonal boron nitride4, 5 have further propelled the electrical mobility to be on a par with or even beyond the silicon (Si) counterpart. These strategies hold tantalizing prospects on extending the Moore's law. Yet, there is still room for improvement before 2D TMDCs become truly technologically relevant. The challenge lies in the full validation of the intrinsic charge transport that is associated with the specific nature and ordered arrangement of atoms in the atomically thin crystal lattice. This requires, the controlled stitch of both metals and chalcogenides in an atom-by-atom fashion. To this end, a variety of synthetic approaches have been developed, this includes but not limited to chemical vapor deposition (CVD) 6, 7, mechanical exfoliation8 and solution-based exfoliation9. Among which, CVD shows better controllability over thicknesses, geometric shapes, sizes, and qualities through manipulation of the growth factors, e.g., growth temperature, pressure, precursor ratio, and gas carrier. These complex growth environments will significantly confound the scalability, crystallinity, defect density, and reproducibility of the CVD approach. Therefore, an impetus exists to gain fundamental insights into the universal growth mechanism that is currently lacking and therefore curbs the realization o the controlled epitaxy of high-mobility three-atom-thick semiconducting TMDCs films with wafer-scale-homogeneity. In this thesis, a mechanistic study toward revealing the epitaxy growth mechanism is established to include 1) epitaxy growth of multilayer, 2) epitaxy growth of heterostructures, and 3) epitaxy growth of high quality (exceedingly low defect density) of 2D TMDCs materials through a controlled CVD strategy.

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In this thesis, we contribute to the understanding of electronic and optical properties of 2-dimensional materials, with a strong focus on graphene-based nanostructures \cite{graphene_book}. The thesis is structured into eight chapters, starting with an introduction and ending with a conclusion. In chapter 2, we present the methods used throughout this thesis. We start by introducing the tight-binding model to understand the single-particle properties of graphene, bilayer graphene, and graphene quantum dots. We then introduce configuration interaction, the Hubbard model, the Bethe-Salpeter equation, and Hartree-Fock as tools for tackling the interacting problem and correlated electron systems. We also discuss numerical methods, including techniques for addressing the numerical complications that arise when working with the many-body problem such as the calculation of Coulomb matrix elements. In chapter 3, we present a new approach to the energy spectra of $p_z$ electrons in small hexagonal graphene quantum dots. This approach is analytical, and allows us to predict the dependence of the energy gap on size and edge type. In chapter 4, we describe a proposal of a quantum simulator of an extended bipartite highly tunable Hubbard model with broken sublattice symmetry inspired by graphene. We predict the electronic and magnetic properties of a small simulator. The proposed simulator, allows us to study the ground state of the Hubbard Hamiltonian for a broad range of regimes accessible due to the high tunability of the simulator. In chapter 5, we study the electronic properties of quasi 2-dimensional quantum dots made of topological insulators using HgTe. We show that in a square HgTe quantum dot one set of material parameters defines the topologically nontrivial case, in which topologically protected edge states are found, and another set of parameters defines a topologically trivial regime corresponding to a trivial insulator without edge states. In chapter 6, we examine excitons in AB-stacked gated bilayer graphene (BLG) quantum dots (QDs). We confine both electrons and holes using gates and demonstrate that excitons can exist in the BLG QD. We predict absorption to occur in the terahertz regime and find that low-energy excitons are dark. In chapter 7, we determine the many-body states of massive Dirac Fermions confined in a bilayer graphene lateral gated quantum dot. Tuning the strength of Coulomb interactions versus the single-particle level spacing we predict the existence of spontaneously spin and valley symmetry-broken states of interacting massive Dirac Fermions.

Atomically thin 2D materials have gained the interest of the scientific community in the past decade due to their exotic electronic and optoelectronic properties, thus emerging as potential candidates for the next generation of nano-devices. Quantum confinement in one of the dimensions is the primary reason for these exotic properties. However, it has been seen that these properties are widely inconsistent, and they are controlled by variety of factors such as material synthesis, device fabrication, testing environment, etc. Due to low dimensional nature of these materials, defects are inevitable. These defects typically originate from either the presence of bulk impurities or interface between sample and substrate. These defects manifest as mid-gap states in semiconductor channel and act as trapping centers for charge carriers, thus often referred to as trap states. The presence of trap states is not necessarily a detrimental thing. In this dissertation, I will focus on the role these trap states play in the emergence of a few electronic and optoelectronic properties.High responsivity (R) in photodetectors based on 2D materials is mainly associated with a presence of photogating effect in which trap states dynamics plays a crucial role. Photogating also results in fractional power (γ) dependence of the photocurrent (Iph) on an effective illumination intensity (Peff). Chapter 2 presents photoconductivity studies of few layers of rhenium diselenide (ReSe2) based field-effect transistors (FETs) over a wide range of applied gate voltages (-48 V ≤ Vg ≤ 60 V) and temperature (20 K ≤ T ≤ 300 K). A very high responsivities ≈ 16500 A/W and external quantum efficiency (EQE) ~ 106 % (at 140 K, Vg = 60 V and Peff = 0.2 nW) was obtained. Investigating R and γ at various gate voltages and over a wide range of temperatures leads to a strong correlation between R and γ. Such correlations indicate the importance of trap states and photogating in governing high responsivities in these materials. It is expected that thicker samples will aid in photoconduction by effectively increasing photon absorption. In chapter 3, a layer dependent study of optoelectronic properties of indium selenide (InSe) based FETs shows that responsivity decreases for thicker InSe devices. In these devices, photogating remains constant (similar γ) and responsivity depends predominately upon field-effect mobility (μFE). Interlayer resistance regulates the mobility and (consequentially) responsivity. Thus, mobility dominates the responsivity and trap states play second fiddle. The presence of metal−insulator transition (MIT) in two-dimensional (2D) systems leads to tunable material properties by regulating parameters such as charge carrier density. Chapter 4 shows our observation on MIT in the 2D copper indium selenide (CuIn7Se11) flakes by electrostatic doping via the SiO2 back gate. A temperature and gate voltage dependence of conductivity (σ) of CuIn7Se11 FET shows clear evidence of the metallic and insulating phase. Evidence of 2D variable-range hopping (VRH) and percolation critical conductivity confirms the presence of charge density inhomogeneity originating from trap states. The low effective mass and high dielectric of copper indium selenide systems result in a lower critical charge carrier density required for percolation-driven MIT, attended by conventional SiO2 dielectric gate. Even though findings reported in this dissertation are performed on specific materials, fundamental understandings can be easily extrapolated to other 2D systems. Understanding the role of trap states will provide valuable insights for the design and development of high-performance devices using 2D materials.

Atomically thin two-dimensional (2D) materials come in all necessary flavors to make semiconductor devices: conductors, semiconductors, and insulators. Graphene, transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (hBN) are the quintessential building blocks. The van der Waals nature of the bonds in 2D films allows the ability to stack materials to achieve novel properties because of their exceptional mechanical, electronic, and optical properties and interactions, which enables various applications of 2D materials in transistors, biosensors, light-emitting devices, and photodetectors. Spectroscopic measurements such as Raman and photoluminescence (PL) reveal a wealth of information since 2D materials are affected by their environment and other local perturbations, e.g., strain and charge doping. My research focused on developing efficient and noninvasive optical methods to evaluate and characterize the properties of 2D materials. In particular, we investigated strain-tunable properties, the effects and signature of charge doping, and the environmental screening properties of graphene and TMDCs. Identifying the charge density and impurities in graphene is vital for graphene-based applications, which require high-quality graphene. I developed an effective optical method to determine the doping level and the local charge density variations in graphene before any fabrication process. This method differentiates charge density variations in graphene via the Raman 2D peak asymmetry that manifests at low charge 1-25 × 1010 cm-2. We explore the effect of charge inhomogeneity, "charge puddles", within the laser spot using simulated Raman 2D spectra, revealing a different signature for large or small charge puddles. Our work provides a simple and noninvasive optical method for estimating the doping level, local charge density variation, and transport properties of graphene, with up to two orders of magnitude higher precision than previously reported optical methods. Strain is another crucial factor that significantly impacts the properties of 2D materials. We studied the charge distribution and radiative efficiency of excitonic complexes in strained monolayer TMDCs, especially WSe2. Straining and electrostatic gating are combined to investigate the dynamics of quasi-particles in WSe2. We found that negative trions accumulate while positive trion emission is near zero, indicating that both conduction and valence bands are bent downwards in the strained area. Finite element analysis of strain distribution and density functional theory calculations of band structures of WSe2 support the experimental results. Hence, localized strain allows locally separating electrons and holes in WSe2 and manipulating light-matter interaction for applications in novel strained-engineered optoelectronics. I applied machine learning and deep learning techniques to improve the efficiency and accuracy of data processing and analysis since traditional methods require domain expertise and have the potential to introduce artifacts. I categorized the wealth of information and data by applying machine learning to spectroscopic information to separate different influences, e.g., strain, charge doping, and dielectric environment. We developed deep learning models to classify graphene Raman spectra according to different charge densities and dielectric environments. To improve the accuracy and generalization of all models, we use data augmentation through additive noise and peak shifting. Using a convolutional neural net (CNN) model, we demonstrated the spectra classification with 99% accuracy. Our approach has the potential for fast and reliable estimation of graphene doping levels and dielectric environments. The proposed model paves the way for achieving efficient analytical tools to evaluate the properties of graphene. / 2022-11-23T00:00:00Z

This dissertation will have three chapters. In chapter one, a comprehensive review on defects in two dimensional materials will be presented. The aim of this review is to elaborate on different types of defects in two dimensional (2D) materials like graphene and transition metal dichalcogenides (TMDs). First, different types of point and line defects, e.g. vacancies, anti-sites, guest elements, adatoms, vacancy clusters, grain boundaries, and edges, in these materials are categorized in terms of structure. Second, interactions among defects are discussed in terms of their rearrangement for low-energy configurations. Before studying the electronic and magnetic properties of defective 2D materials, some of the structures are considered in order to see how defect structure evolves to a stable defect configuration. Next, the influence of defects on electronic and magnetic properties of 2D materials is discussed. Finally, the dynamic behavior of defects and 2D structures under conditions such as electron beam irradiation, heat treatment, and ambient conditions, is discussed. Later as a case study, defects in a two dimensional transition metal dichalcogenide will be presented. Among two-dimensional (2D) transition metal dichalcogenides (TMDs), platinum diselenide (PtSe2) stands at a unique place in the sense that it undergoes a phase transition from type-II Dirac semimetal to indirect-gap semiconductor as thickness decreases. Defects in 2D TMDs are ubiquitous and play crucial roles in understanding and tuning electronic, optical, and magnetic properties. Here intrinsic point defects in ultrathin 1T-PtSe2 layers grown on mica were investigated through the chemical vapor transport (CVT) method, using scanning tunneling microscopy and spectroscopy (STM/STS) and first-principles calculations. Five types of distinct defects were observed from STM topography images and the local density of states of the defects were obtained. By combining the STM results with first-principles calculations, the types and characteristics of these defects were identified, which are Pt vacancies at the topmost and next monolayers, Se vacancies in the topmost monolayer, and Se antisites at Pt sites within the topmost monolayer. Our study shows that the Se antisite defects are the most abundant with the lowest formation energy in a Se-rich growth condition, in contrast to cases of 2D molybdenum disulfide (MoS2) family. 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. Also, in order to investigate the layer dependency of vibrational and electronic properties of two dimensional materials, 2M-WS2 material was selected. Raman spectroscopy and DFT calculation proved that all Raman active modes have a downshift when material is thinned to few layers (less than 5 layers). It was proven that there is a strong interaction between layers such that by decreasing the number of layers, the downshift in Raman active modes is mostly for the ones which belong to out-of-plane atomic movements and the most downshift is for the Ag2 Raman active mode. Also, I investigated the effect of number of layers on the band structure and electronic properties of this material. As the number of layers decreases, band gap does not change until the materials is thinned down to only a single monolayer. For a single monolayer of 2M-WS2, there is an indirect band gap of 0.05eV; however, with applying in-plane strain to this monolayer, the material takes a metallic behavior as the strain goes beyond ±1%. / Doctor of Philosophy / Graphite (consisting of graphene as building blocks) and TMDS in bulk form are layered and with exfoliation one can reach to few layers which is called two-dimension. Two dimensional materials like graphene have been used in researches vastly due to their unique properties, e.g. high carrier mobility, and tunable electronic properties. Transition metal dichalcogenides (TMDs) with a general formula of MX2, where M represents transition metal elements (groups 4-10) and X represents chalcogen elements (S, Se or Te), are another family of two-dimensional materials which have been extensively studied in the past few years. Besides exfoliation, there are also synthesis methods to produce two dimensional materials, e.g. chemical vapor deposition and chemical vapor transport. Normally, after synthesizing these materials, researchers investigate structure and electronic properties of these materials. There might be some atoms which no longer exist in the structure; hence, those are replaced by either vacancies or other elements which all of them are called defects. In chapter 1, defects in graphene and transition metal dichacolgenides were investigated, carefully. Later, dynamic behavior of defects in these materials were investigated and finally, the effect of defects on the electronic properties of the two dimensional materials were investigated. Chapter two talks about a case study which is two dimensional 1T-PtSe2. In this chapter, 5 different kinds of defects were studied using scanning tunneling microscopy and spectroscopy investigations and density functional theory was used to prove our assumptions of the origin of defects. Also, another thing which is investigated by researcher is that how atoms in two dimensional materials vibrate and how the number of layers in the two dimensional material influences vibrations of atoms. Other than this, electronic properties of these materials is dependent upon the number of layers. When these materials are synthesized, there is a stress applied to the material due the mismatch between the material and its substrate, so it is worth investigating the effect of stress (strain) on the structure, and electronic properties of the material of interest. For this purpose, 2M-WS2 was exfoliated on Si/SiO2 substrate and the layer dependency of its vibrational modes was investigated using Raman spectroscopy and density functional theory calculation. Also, in order to investigate the influence of stress (strain) on the electronic properties of two dimensional 2M-WS2, a single monolayer of this materials underwent a series of strains in density functional theory calculations and the effect of strain on the electronic properties of this material was investigated.