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

Devido à inconfiabilidade do modelo EM-X?, no cálculo da energia total, consideramos uma correção na densidade de carga \"muffin-tin\". Com esta correção podemos ajustar a energia total, a partir de parâmetros definidos na teoria. O objetivo deste trabalho é o estudo da curva da energia total associada ao estado excitado do sistema GaAs: AsGa, quando o átomo substitucional de As se desloca na direção . Partindo de cálculos de primeiros princípios (LARGE UNIT CELL APPROACH), reproduzimos a curva da energia total do estado fundamental. A partir dos parâmetros encontrados na correção não \"muffin-tin\" da densidade de carga, calculamos a curva do estado excitado utilizando o conceito de estado de transição de Slater. Nossos resultados mostraram que o efeito Jahn-TeIler não ocorre para defeitos tipo antisítio. Vimos também que a curva do comportamento dos autovalores com o deslocamento do átomo substitucional, está de acordo com cálculos recentes encontrados na literatura. / Since total energy calculations within the Multiple Scattering-X? model are not reliable, a non \"muffin-tin\" correction to the charge density has been considered. With this correction the total energy can be adjusted through parameters defined in the theory. The aim of this work is to study the total energy curve of the excited state of the GaAs: AsGa system when the arsenic substitutional atom is displaced in the direction. As a first step, the ground-state total energy curve obtained from first-principles calculations (LARGE UNIT CELL APPROACH) was reproduced. From the parameters found for the non \"muffin-tin\" charge density corrections, we have calculated the excited-state total energy curve by using the Slater transition-state concept. Our results show that the Jahn-Teller effect is not expected to occur for antisite-like defects. Moreover, the obtained behavior of the eigenvalues with displacement of the substitutional atom is in fairly good agreement with recent theoretical calculations found in the literature.

Devido à inconfiabilidade do modelo EM-X?, no cálculo da energia total, consideramos uma correção na densidade de carga \"muffin-tin\". Com esta correção podemos ajustar a energia total, a partir de parâmetros definidos na teoria. O objetivo deste trabalho é o estudo da curva da energia total associada ao estado excitado do sistema GaAs: AsGa, quando o átomo substitucional de As se desloca na direção . Partindo de cálculos de primeiros princípios (LARGE UNIT CELL APPROACH), reproduzimos a curva da energia total do estado fundamental. A partir dos parâmetros encontrados na correção não \"muffin-tin\" da densidade de carga, calculamos a curva do estado excitado utilizando o conceito de estado de transição de Slater. Nossos resultados mostraram que o efeito Jahn-TeIler não ocorre para defeitos tipo antisítio. Vimos também que a curva do comportamento dos autovalores com o deslocamento do átomo substitucional, está de acordo com cálculos recentes encontrados na literatura. / Since total energy calculations within the Multiple Scattering-X? model are not reliable, a non \"muffin-tin\" correction to the charge density has been considered. With this correction the total energy can be adjusted through parameters defined in the theory. The aim of this work is to study the total energy curve of the excited state of the GaAs: AsGa system when the arsenic substitutional atom is displaced in the direction. As a first step, the ground-state total energy curve obtained from first-principles calculations (LARGE UNIT CELL APPROACH) was reproduced. From the parameters found for the non \"muffin-tin\" charge density corrections, we have calculated the excited-state total energy curve by using the Slater transition-state concept. Our results show that the Jahn-Teller effect is not expected to occur for antisite-like defects. Moreover, the obtained behavior of the eigenvalues with displacement of the substitutional atom is in fairly good agreement with recent theoretical calculations found in the literature.