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"The objective of this project was to extend fundamentally the current kp theory by applying the Burt-Foreman formalism, rather than the conventional Luttinger-Kohn formalism, to a number of novel nanostructure geometries. The theory itself was extended in two ways. First in the application of the Burt-Foreman theory to computing the momentum matrix elements. Second in the development of a new formulation of the multiband kp Hamiltonian describing cylindrical quantum dots. A number of new and interesting results have been obtained. The computational implementation using the finite difference method of the Burt-Foreman theory for two dimensional nanostructures has confirmed that a non-uniform grid is much more efficient, as had been obtained by others in one dimensional nanostructures. In addition we have demonstrated that the multiband problem can be very effectively and efficiently solved with commercial software (FEMLAB). Two of the most important physical results obtained and discussed in the dissertation are the following. One is the first ab initio demonstration of possible electron localization in a nanowire superlattice in a barrier material, using a full numerical solution to the one band kp equation. The second is the demonstration of the exactness of the Sercel-Vahala transformation for cylindrical wurtzite nanostructures. Comparison of the subsequent calculations to experimental data on CdSe nanorods revealed the important role of the linear spin splitting term in the wurtzite valence band."

Since the discovery of graphene, a new era of physics called "Two Dimensional (2D)Materials" has emerged. Group IV and Group III Selenides such as SnSe and InSe arepromising members of the 2D family. Structure of Group IV selenides is unique and highlysensitive to pressure and temperature. To further tweaking their properties by structuralchanges, thorough understanding of how the structure relates to the electronic bands is veryimportant. Based on the results from DFT calculations, I carefully analyzed electronic bandstructures of layered SnSe with various interlayer stacking. The first part of this dissertationdiscussed the possible stacking-dependent indirect-direct transition of bilayer SnSe.By further analysis, these results reveal that the directionality of interlayer interactionsdetermine the critical features of their electronic band structures. Further, it demonstratedthat such changes can be achieved by substitutional chemical doping. Using a multi-scalemodeling approach by combining the result of DFT and Boltzmann Transport Theory, Idiscussed the electron transport properties of co-doped SnSe, a class of thermodynamicallyand dynamically stable structures. The second part discussed on charge transfer across InSe/Gas interface, which showsbi-polar transport properties. This finding is in a good agreement with the recent experimentalobservations. Fundamental understanding of charge transfer in few-layer InSe /gasinterfaces at the atomic level is expected to pave the path for designing gas sensing devices.

Polímeros conjugados semicondutores tem atraído grande interesse nas últimas décadas devido às possíveis aplicações como componentes ativos em aplicações optoeletrônicas. A adequação destes semicondutores orgânicos para a fabricação de dispositivos depende do entendimento e controle de propriedades eletrônicas básicas: gap fundamental (Eg) e potencial de ionização (IP). Nesse contexto, estudos teóricos baseados em cálculos de primeiros princípios tem se mostrado muito úteis, uma vez que possibilitam a simulação de processos físicos em condições ideais, onde se pode analisar as propriedades eletrônicas de polímeros desconsiderando efeitos do ambiente ou desordem estrutural. A Teoria do Funcional da Densidade (DFT) tem se tornado o método mais comum para o cálculo da estrutura eletrônica do estado fundamental de uma ampla variedade de materiais orgânicos complexos. Embora cálculos DFT baseados na diferença de energias totais tem sido aplicados com sucesso para estimar IPs de moléculas pequenas, este método falha nas propriedades de sistemas conjugados longos. Realmente, a capacidade preditiva da DFT padrão com respeito as propriedades espectroscópicas é frequentemente limitada, entretanto o tratamento adequado das excitações eletrônicas através de abordagens de muitos corpos é ainda muito difícil para materiais orgânicos complexos. Funcionais híbridos que misturam uma fração () de troca exata (EX) não-local ao correspondente semi-local representam uma boa alternativa, embora a quantidade ideal de EX seja, em geral, dependente do sistema. Neste trabalho, adotamos um esquema não-empírico baseado na aproximação G0W0 para identificar o valor ótimo de para o funcional híbrido PBE no qual a correção de autoenergia para o orbital mais alto ocupado (HOMO) de Kohn-Sham generalisado é minimizado. Estudamos, com base nessa estratégia, a dependência com o comprimento das propriedades eletrônicas básicas em uma família de oligômeros conjugados 1D de trans-poliacetileno (TPA). Nossos cálculos mostram que a fração EX ótima (dependente do tamanho) incorporada ao PBEh reproduz com precisão os IPs experimentais determinados em fase gasosa, / Semiconducting conjugated polymers have attracted considerable interest over the past decades due to the promising applications as active components for optoelectronic applications. The suitability of such organic semiconductors for device fabrication relies on quantitative understanding and control of basic electronic properties: fundamental gap (Eg) and ionization potential (IP). In this context, theoretical studies based on first principles approaches have proven useful, through simulating physical processes in ideal conditions, in which one might analyse the electronic properties of polymers apart from the effects of the surrounding environment or structural disorder. Density Functional Theory (DFT) has become an usual choice for calculating the ground state electronic structure of a wide variety of complex organic materials. Although DFT calculations based on total energy differences have been successfully applied to estimate IPs of small molecules, they fail for properties of long conjugated systems. Indeed, the predictive ability of standard DFT with respect to spectroscopic properties is often limited, however a proper treatment of the electronic excitations through many-body approaches is still very difficult for complex organic materials. Hybrid functionals that mix a fraction (_) of nonlocal exact exchange (EX) with the semilocal counterpart represent a good alternative, although the ideal amount of EX is usually system dependent. In this work, we adopt a non-empirical scheme based on the G0W0 approximation to identify the optimum _ value for the PBE hybrid functional for which the self-energy correction to the generalized Kohn-Sham highest occupied molecular orbital (HOMO) is minimized. Based on this strategy we study the size dependence of the basic electronic properties in a family of 1D _-conjugated oligomers of trans-polyacetylene (TPA). Our calculations demonstrate that the size dependent optimal EX fraction incorporated in PBEh accurately reproduces IPs from experimental gas phase data, although no particular constraint has been imposed a priori. Furthermore, we note that the optimum _-value decreases exponen tially with chain length going from _ w0.85 for the smaller oligomer (ethylene, n=1) up to _ w0.75 extrapolated for an isolated TPA chain. The accuracy of our optimized PBEh in predicting IPs and Eg is superior to other conventional mean field approaches, as demonstrated for a selected set of conjugated molecules such as acenes and phenylenes. As a result, we can obtain good estimations for the energy barriers of electron transfer in organic/organic interfaces. On the other extreme, we analyse the influence of exact exchange on the electronic structure of the prototypical metal system gold (Au), commonly used as electrode in organic devices. In this case, we confirm the expected result that the insertion of even a small fraction of EX into PBE functional distorts the Au band structure, worsening the description of electronic properties compared to regular PBE. We then proceed to analyse the factibility of studying polymer/metal interface systems using pure DFT. Our calculations reveal that the result is too system-dependent: for the TPA/Au(111) interface, an artificial charge transfer takes place at interface due to an underestimation of the IPs of the conjugated system inherent to the underlying DFT approximation. Finally, our study emphasizes the importance of a physically motivated choice of EX fraction in hybrid functionals for accurately predicting both ionization potentials and fundamental gaps of organic semiconductors relevant for nanoelectronics.

Notre thèse consiste en l’étude des relations entre la morphologie microscopique et les propriétés optoélectroniques de films minces de matériaux organiques pi-conjugués. En particulier, nous avons porté notre attention sur des oligomères, polymères et copolymères pi-conjugués à base de thiophène et/ou de fluorène, particulièrement intéressants pour des applications dans des dispositifs optiques et électroniques « plastiques ». Nous avons montré que l’assemblage des molécules peut mener à des (nano)structures de taille et de forme spécifiques, par exemple des structures fibrillaires (unidimensionnelles), des plaquettes (bidimensionnelles), ou des agrégats non-texturés, en fonction de l’architecture moléculaire, du solvant et du substrat utilisés dans la préparation des films minces. La modélisation moléculaire nous a permis d’une part de proposer des modèles d’assemblages de molécules au sein des différentes structures, et d’autre part de mieux comprendre l’importance des interactions molécule-molécule et molécule-surface. Nous avons ainsi pu établir les relations entre l’ordre structural au sein de ces dépôts minces et les propriétés optiques et électroniques, en étudiant notamment la photoluminescence à l’état solide ou le transport de charges dans des transistors à effet de champ. Nous avons également eu recours à des techniques de lithographie "douce" pour contrôler l’assemblage des molécules conjuguées, ce qui a permis d’améliorer les performances des dispositifs électroniques. Globalement, les résultats obtenus apportent une meilleure compréhension des relations entre l’organisation des molécules conjuguées à l’état solide et les propriétés optoélectroniques des matériaux conjugués semiconducteurs.

En este trabajo se han realizado cálculos de primeros principios para estudiar las propiedades físicas de superficies y materiales laminares. Los cálculos se basan en la obtención de las propiedades electrónicas por medio de la Teoría del Funcional de la Densidad, con la que se obtienen la energía y fuerzas atómicas para cada sistema estudiado. De esta forma, se realizaron cálculos de optimización estructural y de dinámica molecular, que proporcionan información sobre las estructuras de equilibrio y la dinámica atómica en función de las condiciones externas (tales como presión y temperatura). Los cálculos realizados se han centrado en diferentes sistemas con interés experimental, siempre en estrecha colaboración con distintos grupos experimentales. En el campo de superficies, se han estudiado problemas relacionados con el crecimiento de láminas delgadas de Cobre para metalización de circuitos microelectrónicos, la adsorción de fullerenos sobre superficies de Germanio, y la medición de ondas de densidad de carga mediante microscopía de efecto túnel en bronces azules. En materiales laminares, el trabajo se ha centrado en comprender el efecto de la presión sobre las propiedades estructurales y electrónicas de semiconductores laminares III-VI, así como en explicar la aparición de nuevas fases no-laminares para varios de estos sistemas a altas presiones. / In this work we have made first-principles calculations to study the physical properties of surfaces and layered materials. The calculations are based on obtaining the electronic properties through the Density Functional Theory, with which the energy and atomic forces for each system studied are derived. Thus, structural optimization calculations and molecular dynamics were carried out. They provide information about the equilibrium structure and the atomic dynamic as a function of external conditions (such as temperature and pressure). Calculations have focused on different systems with experimental interest, always in strong collaboration with different experimental groups. In the field of surfaces we have been studied problems associated with the growth of thin films of copper for metallization of microelectronic circuits, the adsorption of fullerenes on Germanium surfaces, and the measurement of charge density waves by scanning tunneling microscopy in bronze blue. In layered materials, the work has focused on understanding the effect of pressure on the structural and electronic properties of layered semiconductor III-VI, as well as explaining the emergence of new no-layered phases for several of these systems at high pressures.

Niobium (Nb) is a fascinating element, that when it is in a solid state has remarkable properties. This is believed to be a result of its electronic configuration that has partially filled 4d and 5s sub-shells. Nb has a melting temperature of 2750 K, a high strength at high temperature, and a good wear resistance. Because of these properties, Nb is used as material for components of rockets and jet engines, and for strengthening steel. In the phonon dispersion relations, Kohn anomalies are experimentally observed to weaken with increased temperature, which is related to the superconducting properties of Nb. I include anharmonicity when I calculate the thermodynamic properties of Nb and relate this to the electronic structure. In this thesis I show that anharmonicity can not be neglected when considering thermodynamic properties of Nb. I observe broadening in the electronic band structure with increasing temperature, correlated with the gradual weakening of the Kohn anomalies in the phonon dispersion relations. Kohn anomaly in the phonon dispersion relation can be observed at 300 K and is completely absent at 1200 K. The observation of the Kohn anomaly's disappearance in the calculations is of great importance because it cannot be repeated by approaches that do not include anharmonic effects, meaning that properties that are directly related to phonon dispersion, like elastic constants, can be calculated more accurately with this approach.

This thesis concerns experimental studies of nanocrystalline tungsten trioxide thin films. Functional properties of WO3 have interesting applications in research areas connected to energy efficiency and green nanotechnology. The studies in this thesis are focused on characterization of fundamental electronic and optical properties in the semiconducting transition metal oxide WO3. The thesis includes also applied studies of photocatalytic and photoelectrochemical properties of the material.     All nanocrystalline WO3 thin films were prepared using DC magnetron sputtering. It was found that structures like hexagonal and triclinic phase with different properties can be produced with sputtering technique. Thin film deposition has been performed using different process parameters with emphasis on sputter pressure and films that mainly consist of monoclinic γ-phase, with small contributions of ε-phase. Changes in the pressure are shown to affect the number of oxygen vacancies in the WO3 thin film, with close to stoichiometric WO3 formed at high pressures (30 mTorr), and slightly sub-stochiometric WO3-x, x = 0.005 at lower pressures (10 mTorr). Both stoichiometric and sub-stoichiometric thin films have been characterized by several structural, optical and electronic techniques.    The electronic structure and especially band gap states have been explored and optical properties of WO3 and WO3-x have been studied in detail. The band gap has been determined to be in the range 2.7-2.9 eV. Absorption due to polaron absorption (W5+  -W6+), oxygen vacancy sites (Vo -W6+), and due to differently charged oxygen vacancy states in the band gap have been determined by spectrophotometry and photoluminescence spectroscopy, in good agreement with resonant inelastic x-ray spectroscopy and theoretical calculations. The density of electronic states in the band gap was determined from cyclic voltammetry measurements, which correlate with O vacancy concentration as compared with near infrared absorption.      By combining different experimental methods a thorough characterization of the band gap states have been possible and this opens up the opportunity to tailor the WO3 functionalities. WO3 has been shown to be visible active photocatalyst, and a promising electrode material as inferred from photo-oxidation and water splitting measurements, respectively. Links between device performance in photoelectrochemical experiments, charge transport and the electronic structure have been elucidated.