In this dissertation work, the study focuses on large areal growth of MoS2 monolayers and a study of the structural, optical and electrical properties of such monolayers before and after transfer using a polymer-lift off technique. This work will discuss the issue of contact resistance and the effect of defects (both intrinsic and extrinsic) on the overall quality of the monolayer films. The significance of this dissertation work is that a reproducible strategy for monolayer MoS2 film growth and quantification of areal coverage as well as the detrimental effects of processing on device performance is presented.

The first-principles density functional theory (DFT) was implemented to investigate the structural, electronic and magnetic properties of vanadium (V) substituted and chalcogen (Se) vacancies in tungsten diselenide (WSe 2 ) monolayer, novel two dimensional (2D) monolayer (ML) structures in binary compounds ZnX (X= As, Sb, and Bi), and novel 2D electrides on transition metal-rich mono-oxide or chalcogenides, based on Perdew-Burke-Ernzerhof (PBE) exchange functional employed in Vienna Ab-Initio Simulation Packages (VASP). The inherent defect in 2D transition metal dichalcogenides (TMDCs) contains unavoidable substitutional defects and a certain amount of chalcogen vacancies. This type of defect affects the electronic and magnetic properties of 2D-TMDCs. To account for this fact, we demonstrated using DFT that the V-doped WSe 2 monolayer exhibits long-range ferromagnetic order. Further, the chalcogenide (Se) vacancies clustered around V-atom enhance the ferromagnetic properties of the system consistent with experimental findings. This dissertation explores the important role of Se-vacancies in the magnetic properties of the V-doped WSe 2 monolayer and proposes a method to enhance the magnetic properties of such 2D non-magnetic van der Waal (vdW) materials. In the second study, we have attempted theoretically to engineer the monolayer structure in II-V binary compounds ZnX with orthorhombic symmetry. We proved the dynamical stability of the bulk and ML structures manifested by the absence of imaginary frequencies in phonon dispersion curves. Our calculations on the density of states (DOS), and band structures using GGA indicate the increasing value of bandgap as well as the transition from indirect to direct bandgap while going from bulk to monolayer structure of ZnX. Our theoretical calculations will represent an archetype of novel 2D semiconductors on ZnX. Next, we have tailored using DFT, the structural and electronic properties of the 2D electrides that belong to transition metal-rich mono-oxide and chalcogenides with hexagonal (Hf 2 X; X = O, S, Se, Te), and orthorhombic (Ti2S and Zr2S) symmetry thereby introducing novel electrides to the electride family. The Bader charge analysis, electron localization function (ELF), projected DOS, and the calculated value of low work functions provides sufficient theoretical shreds of evidence to prove these materials as electrides.

Monolayer two-dimensional (2D) materials have been regarded as a hot topic in the fields of condensed matter physics, materials science, and chemistry due to their unique physical, chemical, and electronic properties. However, the research on the preparation method and properties understanding of the 2D monolayer are inadequate. In this dissertation, taking 2D nickel-iron layered double hydroxides (NiFe LDHs) and molybdenum disulfide (MoS2) as examples, the practicability of the direct synthesis of NiFe LDHs monolayer and the thermal enhancement catalytic performance of 2D MoS2 monolayer (MoS2 ML) are discussed. First, a one-pot synthetic strategy (bottom-up method) is presented to synthesize 2D NiFe-based LDHs monolayers, including NiFe, Co-, Ru-, doped, and Au-modified NiFe LDHs. The prerequisite and universality of this strategy are investigated and confirmed. The features of LDHs are characterized by advanced technologies. The obtained LDH bulks own a large interlayer spacing up to 8.2 Å, which can be facilely exfoliated into monolayers in water by hand-shaking within 10 s. As a result, the as-prepared NiFe-based LDH monolayers display a good electrocatalytic oxygen evolution reaction (OER) performance. This facile strategy paves the way for designing easily exfoliated LDHs for highly active catalysts and energy conversion devices based on other monolayer LDHs. Second, with gold-modified tape, 2D MoS2 ML is exfoliated from the bulk crystal through a micromechanical exfoliation method (top-down strategy). The thermal effects of MoS2 ML are confirmed by Raman and photoluminescence (PL) spectra. Moreover, an on-chip MoS2 ML hydrogen evolution reaction (HER) reactor is designed and fabricated. The thermal effects generate efficient electron transfer in the MoS2 ML and at the electrolyte-catalyst (MoS2 ML) interface, leading to an enhanced HER performance. Compared to the results obtained at room temperature, the MoS2 ML shows a direct thermal enhanced HER performance at higher temperatures. In summary, the findings and understandings, the direct synthesis and direct thermal enhancement catalytic performance, of 2D monolayers offer a guideline for synthesizing and catalyst application of other 2D monolayers.