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

Transition metal alloys are an important class of materials in heterogeneous catalysis due in no small part to the often greatly enhanced activity and selectivity they exhibit compared to their monometallic constituents. A host of experimental and theoretical studies have demonstrated that, in many cases, these synergistic effects can be attributed to atomic-scale features of the catalyst surface. Realizing the goal of designing -- rather than serendipitously discovering -- new alloy catalysts thus depends on our ability to predict their atomic configuration under technologically relevant conditions. This dissertation presents original research into the development and use of computational tools to accomplish this objective. These tools are all based on a similar strategy: For each of the alloy systems examined, cluster expansion (CE) Hamiltonians were constructed from the results of density functional theory (DFT) calculations, and then used in Metropolis Monte Carlo (MC) simulations to predict properties of interest. Following a detailed description of the DFT+CE+MC simulation scheme, results for the AuPd/Pd(111) and AuPt/Pt(111) surface alloys are presented. These two systems exhibit considerably different trends in their atomic arrangement, which are explicable in terms of their interatomic interactions. In AuPd, a preference for heteronuclear, Au-Pd interactions results in the preferential formation of Pd monomers and other small ensembles, while in AuPt, a preference for homonuclear interactions results in the opposite. AuPd/Pd(100) and AuPt/Pt(100) were similarly examined, revealing not only the effects of the same heteronuclear/homonuclear preferences in this facet, but also a propensity for the formation of second nearest-neighbor pairs of Pd monomers, in close agreement with experiment. Subsequent simulations of the AuPd/Pd(100) surface suggest the application of biaxial compressive strain as a means increasing the population of this catalytically important ensemble of atoms. A method to incorporate the effects of subsurface atomic configuration is also presented, using AuPd as an example. This method represents several improvements over others previously reported in the literature, especially in terms of its simplicity. Finally, we introduce the dimensionless scaled pair interaction, whereby the finite-temperature atomic configuration of any bimetallic surface alloy may be predicted from a small number of relatively inexpensive calculations. / text

Complex oxides exhibiting a wide variety of novel functional properties such as ferromagnetism and ferroelectricity have been extensively studied during the past decades. Recent advances in the field of oxide heteroepitaxy have made it possible to create and control hybrid oxide heterostructures with abrupt epitaxial interfaces. The oxide heteroepitaxy with the capability of controlling interface composition, strain, length scales, etc. has opened the totally new and exciting scientific avenue and has offered potential device applications to be explored. Epitaxial integration of functional oxides on semiconductor such as Si (001) and Ge(001) is of great interest, as it potentially leads to further technological development of these interesting oxide systems. In this dissertation, using density functional theory we explore physics and chemistry of novel oxide heterostructures and issues related to the integration of functional oxides on semiconductors. Oxide materials that are studied in this dissertation include polar LaAlO₃, high-k dielectric SrTiO₃, photocatalytic anatase TiO₂ and CoO, and strongly correlated magnetic oxide LaCoO₃. / text

Since materials properties are determined by the interactions between the constituent atoms, an accurate description of the inter-atomic interactions is crucial to characterize and control material properties. Particularly, a quantitative understanding of the formation and nature of defects and impurities becomes increasingly important in the era of nanotechnology, as the imperfections largely influence many properties of nanoscale materials. Indeed, due to its technological importance and scientific interest, there have been significant efforts to better understand their behavior in semiconductors and oxides, and their interfaces, yet many fundamental aspects are still ambiguous due largely to the difficulty of direct characterization. Hence, our study has focused on developing a better understanding of atomic-scale defects and impurities using first principles quantum mechanical calculations. In addition, based on the improved understanding, we have attempted to address some engineering problems encountered in the current technology. The first part of this thesis focuses on mechanisms underlying the transient enhanced diffusion of arsenic (As) during post-implantation annealing by examining the interaction of As with vacancies in silicon. In the second part, we address some fundamental features related to plasma-assisted nitridation of silicon dioxide; this study shows that oxygen vacancy related defects play an important role in (experimentally observed) peculiar nitridation at the Si/SiO2 interface during post O2 annealing. In the third part, we examine the interaction between vacancies and dopants in sp2–bonded carbon such as graphene and nanotube, specifically the formation and dynamics of boron-vacancy complexes and their influence on the electrical properties of host materials. In the fourth part, we study the interfacial interaction between amorphous silica (a-SiO2) and graphene in the presence of surface defects in a-SiO2; this study shows possible modifications in the electronic structure of graphene upon the surface defect assisted chemical binding onto the a-SiO2 surface. In the last part, we examine the structural and electronic properties of bismuth vanadate (BiVO4) which is a promising photocatalyst for water splitting to produce hydrogen; this study successfully explains the underlying mechanism of the interesting photocatalytic performance of BiVO4 that has been experimentally found to strongly depend on structural phase and doping. / text

In the first part of this thesis, we present results from extensive molecular dynamics simulations of the collapse transitions of hydrophobic polymers in explicit water. The focus is to understand the roles that curvature and interactions associated with the polymer-water “interface” have on collapse thermodynamics. We show that model hydrophobic polymers can have parabolic, protein-like, temperature-dependent free energies of unfolding. Analysis of the water structure shows that the polymer-water interface can be characterized as soft and weakly dewetted. We also show that an appropriately defined surface tension for the polymer-water interface is independent of the attractive polymer-water interactions. This helped us to develop a perturbation model for predicting the effect of attractions on polymer collapse thermodynamics. In the second part, we explore connections between structure, thermodynamics, and dynamics of inhomogeneous fluids. First, we use molecular dynamics simulations and classical density functional theory (DFT) to study the hard-sphere fluid at approximately 103 equilibrium state points, spanning different confining geometries and particle-boundary interactions. We provide strong empirical evidence that both excess entropy and a new generalized measure of available volume for inhomogeneous fluids correlate excellently with self-diffusivity, approximately independent of the degree of confinement. Next, we study via simulations how tuning particle-wall interactions to flatten or enhance the particle layering of a model confined fluid impacts its self-diffusivity, viscosity, and entropy. Interestingly, interactions that eliminate particle layering can significantly reduce confined fluid mobility, whereas those that enhance layering can have the opposite effect. Excess entropy helps to understand and predict these trends. Finally, we explore the relationships between the effective interparticle interactions, static structure, and tracer diffusivity of a solute in a mixture. We show that knowledge of these relationships can allow one to “tune” the effective interparticle interactions of the solute in a way that increases its tracer diffusivity. One interesting consequence is that the mobility of a hard-sphere solute can be increased by adding a soft-repulsion to its interaction, effectively making it bigger. / text

The chromogenic properties of tungsten trioxide (WO3) have been studied by photoluminescence spectroscopy at 4.2 K in order to characterize the electronic structure of this material and see how this relates to optical responses during chromogenic coloration. Transition processes between electron energy states are often the cause of optical phenomena and it is important to identify such processes in order to understand the chromogenic coloration of tungsten oxide films. Much research work has been devoted to characterize the physical and chemical mechanisms that are responsible for this coloration and this is of fundamental importance to understand the chromogenic behavior. The latest research shows that oxygen vacancies could play an important role in certain coloration processes, but it is still a matter of debate whether these are important for the overall response. This work aims to identify specific transitions that are related to oxygen vacancies by measuring photoluminescence from films with controlled vacancy content. The main goal of the project was to set up an experiment that could measure photoluminescence at liquid helium temperature. This was done by installing and integrating the components included in this experimental set-up. The films had been prepared prior to this work and were deposited on a nanocrystalline CaF2 substrate, which is a material that has a very large band gap and was therefore expected to fully transparent in the UV range. However it was found that the substrate inelastically scattered the UV excitation light, which produced strong signals that overshadowed the photoluminescence and prevented an effective characterization of the electronic structure in the films. Instead, suggestions were given on how to minimize uncertainty factors and overcome the difficulties met in this work. It was also found that the films attain a lasting blue coloration by exposure to UV light in vacuum, and that this might be due to oxygen being desorbed from the film during experiments in vacuum.

In this work, we have implemented an orbital-free density functional theory (OF-DFT) solver using the finite element method. In OF-DFT, the total ground state energy is minimized directly with respect to the electron density, rather than via orbitals like in the standard Kohn-Sham approach. For this to be possible, one needs an approximation of a universal density functional of the non-interacting kinetic energy. Presently available approximations allow for computation with very low computational expense, but which gives inaccurate energies. A stable OF-DFT code can be used as a testbed for new kinetic energy functionals and provide the necessary tool for investigating the accuracy of OF-DFT calculations for complex systems. We have implemented Thomas-Fermi theory with and without nuclear cusp condition, as well as additional exchange terms of Dirac and Amaldi. The program uses an extended version of the steepest descent in order to find the minimizing density in the variational principle. Our results include convergence tests for the hydrogen atom, weak bonding in the H2 molecule, and accurate results for the lightest noble gases (He, Ne, Ar). For heavier atoms (Kr, Xe, Rn), the results are less accurate. In addition, we consider hydrogen in the simple cubic structure without the cusp condition, which is a first attempt to use the code for periodic systems. Lastly, we discuss some possible improvements for the iterative process towards the minimizing density, as well as other possible directions for future development.

Graphene, the first two-dimensional crystal ever found, is a material that has attracted fervent and sustained interest from condensed matter researchers from around the world. It has a unique and unprecedented band structure in a bulk material: the bands near the Fermi level are linear, leading to massless charge carriers that propagate at the speed of light. However, graphene does not possess a band gap, and as such, it cannot be used to process information in any electronic device that uses digital logic. Graphene is oxidized when several different basic functional groups like hydroxyls, carboxyls, and epoxides bond to the hexagonal carbon basal plane to make graphene oxide (GO). The result is a nonstoichiometric and highly disordered system that, according to the results shown in this thesis, consists of zones of densely-packed functional groups interspersed between zones of relatively small functional group concentration. This has been confirmed by DFT calculations presented here, which is the first time that a successful simulation of the GO density of states has been compared to X-ray data. Contrary to many assumptions in the literature, many of the features in the density of states of GO are due not to carbon sites bonded to functional groups, but are due to nearby non-functionalized carbon sites. The band gap of graphene oxide is principally controlled by oxidation level. Reduction, followed by heating, will regenerate the near-Fermi states and close the band gap significantly as has been seen by others. However, heating non-reduced graphene oxide can also result in a much-reduced band gap, which occurs because intercalated water can react with the heated GO sample to remove functional groups by creation and eventual expulsion of carbon dioxide. The band gap of GO is further complicated by stacking effects if it is multilayered, because residual pi-conjugated states in neighboring planes interact. The two major types of stacking in graphite are AA-stacking and AB-stacking. AA-stacking interactions cause the pi * resonance to broaden and push states to lower energy, which means that AA-stacking determines the width of the gap in highly oxidized samples. However, direct oxidation of graphene is not the only way that one alter the electronic structure of GO. Other results presented here also show that non-covalent functionalization of graphene oxide by amorphous solid water is a powerful, reversible way to dramatically change the GO electronic structure.

In this dissertation we discuss the electronic properties, structural configurations, and reaction mechanisms of chemically functionalized graphene and charged atomic metals. In general, we analyze fundamental atomic scale and nanoscale systems with density functional theory in order to investigate chemical reaction energetics for peroxide synthesis as well as methanol production without carbon emission. These systems were found to be tunable via the addition of cationic and anionic charges, change in transition metal type, and modification through chemical functionalization. Furthermore, transition state theory was used to predict an optimal configuration for chemically functionalized graphene, efficient use of anionic atomic gold and palladium for synthesis of water to peroxide, and clean conversion of methane to methanol without carbon dioxide emission utilizing anionic gold.

Density-functional theory has become an indispensible tool for studying matter on the atomic level, being routinely applied across diverse disciplines from solid-state physics to chemistry and molecular biology. Its failure to account for dispersion interactions has spurred intensive research over the past decade. In this thesis, a semilocal density-functional approximation including dispersion is developed by combining standard functionals for exchange and correlation with the nonempirical “exchange-hole dipole moment“ (XDM) dispersion model of Becke and Johnson. With a minimum of empiricism, the method accurately describes all types of noncovalent interactions, from the extremely weak dispersion forces in rare-gas systems to the hydrogen bonding and stacking interactions responsible for the structure and function of biological macromolecules such as DNA and proteins. The method is compatible with a wide variety of standard Gaussian basis sets, and is easily applied to any system that can be modeled with density-functional theory.