Thesis (M.M.S.E.)--University of Delaware, 2008. / Principal faculty advisor: Valeria Gabriela Stoleru, Dept. of Materials Science & Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Orientador: Aguinaldo Robinson de Souza / Banca: Jose Humberto Dias da Silva / Banca: Armando Beltran Flors / Banca: João Batista Lopes Martins / Banca: Nelson Henrique Morgon / Resumo: Este trabalho é resultado de um estudo teórico sobre o GaN e o Semicondutor Magnético Ga1-xMnxN, tanto para bulk (sólido) como para as superfícies nanoestruturadas mais estáveis, dada sua importância para o desenvolvimento como material spintrônico. Analisamos deste material suas propriedades estruturais, energéticas e eletrônicas, a partir de cálculos periódicos baseados na teoria do Funcional da Densidade (DFT), como o funcional híbrido B3LYP, e também apresentamos resultados preliminares do estudo das propriedades magnéticas deste material. Realizamos um estudo pormenorizado das estruturas de bandas e da densidade de estados, do Ga1-xMnxN bulk (x~0,02 a 0,18) quanto em superfícies (x~0,0 a 0,17) para os modelos de supercélula de 32 e 96 átomos bulk e modelos de superfícies com 12 e 24 camadas. Os resultados obtidos nos mostram que para concentrações acima de 6% ocorre um acréscimo na distância de ligação Mn-N na direção c, pelo fato do Mn apresentar um raio atômico superior ao Ga e propriedades físicas e químicas distintas. Para os cálculos com superfícies, foi realizada a substituição do Ga por Mn em diferentes posições relativas na superfície, sub-superfície e core, ocorrendo o aumento da energia total conforme os átomos de Mn se movem para os sítios mais internos da superfície e ao considerar a forma de equilíbrio baseada na estabilidade termodinâmica, os valores das energias superficiais das superfícies (1010) e (1120) do GaN wurtzita são as mais estáveis para a concentração de ~8%. Com o aumento da concentração, ocorre nas estruturas de bandas uma diminuição do gap, tanto para o bulk quanto para as superfícies, porém ele se mantém direto no bulk, com exceção para concentração de 18% e, na superfície (1010), enquanto que na superfície (1120) o gap é indireto... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This work is the result os a theoretical study concerning GaN and the Magnetic Semiconductor Ga1-xMnxN, in both the bulk and the most stable nanoestructured surfaces, due to its importance in the development as spintronic material. We analyze the structural, energetic and electronic properties of this material, by means of periodic calculations based on the Functional Density Theory (DFT), at the hybrid functional B3LYP level, and also present the preliminary results of the study of the magnetic properties of this material. We carried out a detailed study of the band structures and the density of states, for both the Ga1-xMnxN bulk (x~002 a 0,18) and its surfaces (x~0,0 a 0,17) using supercell models constitued by 32 and 96 atoms for the bulk and 12 and 24 atomic layers for the surface slab model. Our results show that for Mn concentrations above 6% there is an increase in the Mn-N bond distance in the c direction, due to the fact that the Mn has an atomic radium greater than that of the Ga and different physical and chemical properties. For the surface calculations, we substituted the Ga for the Mn in different positions relative to the external surface, sub-surface and corre, it was observed that the total energy increased as the Mn atoms moved from the surface layer to the interior sites and when we consider the equilibrium shape based in the thermodynamic stability, the most stable surface energies for the (1010) and (1020) planos of wurtzite GaN are found for the ~8% Mn concentration. When the Mn concentration increases, the band gap decreases for the bulk as well as for the surfaces, the gap being direct for the bulk, except for the 18% concentration, and for the (1010) surface, whereas the gap is found indirect for the (1120) surface for the concentrations 6 and 17%. The analysis of the density... (Complete abstract click electronic access below) / Doutor

No description available.

Diamond has since many years been applied in electronic fields due to its extraordinary properties. Substitutional dopants and surface functionalization have also been introduced in order to improve the electrochemical properties. However, the basic mechanism at an atomic level, regarding the effects of dopants and terminations, is still under debate. In addition, theoretical modelling has during the last decades been widely used for the interpretation of experimental results, prediction of material properties, and for the guidance of future materials. Therefore, the purpose of this research project has been to theoretically investigate the influence of dopants and adsorbates on electronic and geometrical structures by using density functional theory (DFT) under periodic boundary conditions. Both the global and local effects of dopants (boron and phosphorous) and terminations have been studied. The models have included H-, OH-, F-, Oontop-, Obridge- and NH2-terminations on the diamond surfaces. For all terminating species studied, both boron and phosphorous have been found to show a local impact, instead of a global one, on diamond structural geometry and electronic properties. Therefore, the terminating species only affect the DOS of the surface carbon layers. In addition, Oontop-terminated (111) diamond surfaces present reactive surface properties and display metallic conductivity. Moreover, the conductivity of the diamond surface can be dramatically increased by the introduction of a phosphorous dopant in the lattice. The work function of a diamond surface has also been found to be influenced to a large extent by the various adsorbates and the dopant levels. Diamond can also be used as a promising substrate for an epitaxial graphene adlayer. The effects of dopants and terminations on the graphene and diamond (111) interfacial systems have been investigated theoretically in great detail. The interfacial interaction is of the Van der Waal type with an interfacial distance around 3 Å. The interactions between graphene and a terminated diamond substrate were found to be relatively weaker than those for a non-terminated diamond substrate (even with dopants). For all interface systems between graphene and diamond, a diamond-supported graphene adlayer without induced defects can still keep its intrinsic high carrier mobility. A minor charge transfer was observed to take place from the graphene adlayer to a non-terminated diamond substrate (with or without dopants) and to Oontop-, OH- or Obridge-terminated diamond substrates. However, for the situation with an H-terminated diamond surface, the electron transfer took place from the diamond surface to graphene. On the contrary, an interfacial system with a non-terminated diamond surface offers a more pronounced charge transfer than that of the terminated diamond substrates. A small finite band gap at the Dirac point was also observed for the Oontop-terminated diamond-supporting graphene adlayer.

Over the years electronic structure theory has proven to be a powerful method with which one can probe the behaviour of materials, making it possible to describe and predict material properties. The numerical tools needed for these methods are always in need of development, since the desire to calculate more complex materials pushes this field forward. This thesis contains work on both this implementational and developmental aspects. It begins by reviewing density functional theory and dynamical mean field theory, with the aim of merging these two methods. We point out theoretical and technical issues that may occur while doing this. One issue is the Padé approximant, which is used for analytical continuation. We assess the approximant and point out difficulties that can occur, and propose and evaluate methods for their solution. The virial theorem is assessed within the framework of density functional theory merged with many-body methods. We find that the virial theorem is extended from its usual form, and confirm this by performing practical calculations. The unified theory of crystal structure for transition metals has been established a long time ago using early electronic structure calculations. Here we implement the first- principles exact muffin-tin orbitals method to investigate the structural properties of the 6d transition metals. The goal of our study is to verify the existing theory for the mostly unknown 6d series and the performance of the current state-of-the art in the case of heavy d metals. It is found that these elements behave similarly to their lighter counterparts, except for a few deviations. In these cases we argue that it is relativistic effects that cause this anomalous behaviour. Palladium is then studied, taking many-body effects into account. We find that we can reproduce experimental photoemission spectra by these methods, as well as the Fermi surface. The thesis ends with an investigation of the stacking fault energies of the strongly correlated metal cerium. In addition to providing the first ab-initio stacking fault data for the two cubic phases of Ce, we discuss how these results could have an impact on the interpretation of the phase diagram of cerium / <p>QC 20150522</p>

The techniques of valence photoelectron spectroscopy (PES), X-ray diffraction, molecular orbital calculations, and multi-nuclear NMR are combined in a comparison of metal-phosphine bonding in a series of phosphine substituted molybdenum and tungsten metal carbonyl complexes, M(CO)(6-n)(P)(n) [n = 1,2,3,4,6]. The phosphine, P, represents either the mono-dentate phosphine, PMe₃, or one phosphine unit in the diphosphines, Me₂P(CH₂)ₓPMe₂, [x = 1, bis(dimethylphosphino)methane (DMPM); x = 2, 1,2-bis(dimethylphosphino)ethane (DMPE)]. Comparison of PMe₃ and the diphosphines in mono-dentate coordination (i.e. η¹-Mo(CO)₅DMPE) indicates the σ-donor strength is essentially identical for the three phosphines studied. Comparison of PMe₃ and the diphosphines in cis-chelating geometries reveals essentially identical charge at the coordinated phosphorus atoms and nearly identical charge at the metal center for cis-M(CO)₄(PMe₃)₂ and cis-M(CO)₄DMPE despite different local P-M-P bond angles. The X-ray crystal structures reveal a "twist" of the phosphine ligand when in sterically strained coordination geometries. The phosphine twist results in a "bent" metal-phosphine bond and is evaluated based on both electronic and steric considerations. The phosphine twist principle is used in studies on the nature of phosphine ligand electronic effects in the M(CO)(6-n)(P)(n) series at high substitution numbers, n. The PES data of the DMPE complexes for n = 4, cis-Mo(CO)₂(DMPE)₂, and n = 6, Mo(DMPE)₃, show symmetric metal electronic structure, but also a deviation from the previously observed additive behavior of phosphine electronic effects. The PES data for cis-Mo(CO)₂(PMe₃)₄ reveal a symmetric metal electronic structure due to sterically induced ligand-ligand interactions in this metal carbonyl complex. Multi-nuclear NMR data (³¹P and ⁹⁵Mo) are presented and the results discussed in light of the important ligand-ligand interactions observed in the PES studies. In addition, comparison of the NMR results for the mono-dentate and chelating phosphine complexes and the PES metal electronic structures provides a possible contribution to the ring chelate effect that is observed in the ³¹P and ⁹⁵Mo chemical shifts. The ring chelate effect refers to the unexplained relative differences between the ³¹P and ⁹⁵Mo chemical shifts of the cis-(PR₃)₂ complexes and the chelating diphosphine analogues.

No description available.

This thesis is based on theoretical studies of molecular collisions occurring at relatively low to intermediate collision energies. The collisions are called dissociative recombination (DR) and mutual neutralization (MN). In a molecular quantum mechanical picture, both reactions involve many highly excited molecular electronic states that are interacting by non-adiabatic couplings with each other. The molecular complexes involved in the collisions are relatively (diatomic or triatomic systems) composed of relative light atoms. This allows for accurate quantum chemistry calculations and a quantum mechanical description of the nuclear motions. The reactions studied here are the MN reaction in collisions of H++ H-, Li++ F-, and He++ H- and the DR reaction of H2O+. Rotational couplings are investigated in the study of MN reaction for  He++ H . For some reactions, the electronic resonant states have to be considered. These are not bound states, but are states interacting with the ionization continuum. Electronic structure calculations are combined with electron scattering calculations to accurately compute potential energy curves for the resonant states involved in the DR of H2O+ and the MN of  He++ H. From these calculations, the autoionization widths of the resonant states are also obtained. Once the potential energy curves are computed for the systems, the nuclear dynamics are studied either semi-classically, using the Landau-Zener method or quantum mechanically, employing the time-independent and time-dependant Schrödinger equations. Reaction cross section and final states distribution are computed for all the reactions, showing significantly large cross section at low to intermediate collision energies. For the MN processes, studied here, not only total cross sections are calculated but differential cross sections as well. Where possible, comparisons with previous experimental and theoretical results are performed / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.</p>

Advances in experimental techniques such as nanofabrication, characterization and synthesis have resulted in the development of many novel and interesting materials and devices. Surfaces and interfaces play an indispensible role for nanoelectronics development. ZnO and graphene have drawn tremendous research interests in recent years, due to their exceptional merits in electrical, optical and magnetic applications. This thesis attempts to ferret out the current experimental research progress, particularly, the frontiers of ZnO and graphene based surfaces and interfaces, and employs first principles to explore their electronic structures, to acquire mechanistic understanding of experimental findings, and to shed light on rational design of functional devices. / Finally, the magnetic properties of graphene by organic molecule modification are investigated by first principles method. For the first time, we demonstrate that methoxyphenyl group can introduce a delocalized p-type ferromagnetism into graphene sheet, with the Curie temperature (T c) above room temperature. Each aryl group can totally induce 1 muB into molecule/graphene system. Moreover, an around 1.1 eV direct band gap is introduced into both majority and minority spin bands of graphene by methoxyphenyl group modification. Zigzag graphene nanoribbon (GNR) shows strong site-specific magnetism by aryl group adsorption near the edge. At specific site of GNR, each molecule could totally induce 3&sim;4 mu B into molecule/GNR hybrid system. / First, we study the controllable modulation of the electronic structures of ZnO(10 1&macr; 10) surface functionalized by various types of carboxylic acids. The calculated structural results are consistent with the experimental ones attained by the Fourier transform infrared attenuated total reflectance (FT-IR-ATR). Mercapto-acetic acid molecules are found to contribute an abundance of band gap states into ZnO. Mercapto-acetic monolayer functionalized ZnO (10 1&macr; 10) is on the verge of metal-to-insulator transition, which is consistent with the experimental finding of an conductivity increase by 6 orders of magnitude. Mercapto-acetic acid functionalized ZnO (10 1&macr; 10) surface shows a strong configuration-dependence for both electronic structure and adsorption energy. Moreover, mercapto-acetic acid molecule functionalized ZnO also shows facet-dependent characteristic in which the monolayer functionalized ZnO (2 1&macr; 1&macr; 0) does not show metal-to-insulator transition. Acetic acid does not contribute to the band gap states of ZnO (10 1&macr; 10), whereas benzoic acid and 9-anthracenecarboxylic acid do contribute an abundance of band gap states to ZnO(10 1&macr; 10). / Second, we study the band gap opening of graphene bilayer by F4-TCNQ doping and externally applied electric filed effects. With F4-TCNQ concentration of 8.0x1013 molecules/cm2, the electrostatic charge transfer between each F4-TCNQ molecule and graphene is 0.45 e, and the built-in electric field Ebi between the graphene layers could achieve 0.070 V/A. The charge transfer and band gap opening of the F4-TCNQ doped bilayer graphene can be further modulated by externally applied electric field (Eext ). At 0.077 eV/A, the gap opening at the Dirac point ( K) DeltaEK = 306 meV and the band gap Eg 253 meV are around 71% and 49% larger than those of the pristine bilayer under the same Eext. By combining F4-TCNQ molecular doping and Eext, the p-type semiconductor bilayer graphene are attained, with the band gap and hole concentration varied in a wide range. / These four theoretical sub-topics stem from the experimental advances in ZnO and graphene based surfaces and interfaces. They form the mechanistic understanding of the respective surfaces and interfaces down to the molecular level. / Third, the self-assembly mechanism of PTCDA ultrathin films on graphene with the coverage in a range of 0.3&sim;3 monolayers (MLs) are interrogated by first principles method. For alpha modification mode, with critical thickness of 1 ML, the growth of PTCDA on graphene follows the Stranski-Krastanov (SK) growth mode. In contrast, for beta modification mode, the PTCDA can form two complete MLs on graphene substrate. From the thermodynamical viewpoint, alpha modification mode is more stable than beta modification mode. At 1 ML, the PTCDA follows a continuous and planar&dot; packing arrangement on graphene, which is almost unperturbed by typical defects in graphene substrate. This is in consistentcy with the experimental findings. For alpha modification mode with 2 and 3 ML coverage, the bulk-like phases appear. At the same time, the total charge transfer between PTCDA and graphene per 5&check;3x5 super cell at 2 MLs saturates with 0.42e, which is larger than those of 1 or 3 ML coverage. / Tian, Xiaoqing. / Adviser: Jianbin Xu. / Source: Dissertation Abstracts International, Volume: 73-03, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.