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CAETANO, Ewerton Wagner Santos. Cálculo ab initio de propriedades optoeletrônicas da L-alanina. 2005. 481 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2005. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-06T21:54:46Z No. of bitstreams: 1 2005_tese_ewscaetano.pdf: 51179784 bytes, checksum: 17f997f7ac957b09b76e07735c26d8d7 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-07T14:23:59Z (GMT) No. of bitstreams: 1 2005_tese_ewscaetano.pdf: 51179784 bytes, checksum: 17f997f7ac957b09b76e07735c26d8d7 (MD5) / Made available in DSpace on 2015-05-07T14:23:59Z (GMT). No. of bitstreams: 1 2005_tese_ewscaetano.pdf: 51179784 bytes, checksum: 17f997f7ac957b09b76e07735c26d8d7 (MD5) Previous issue date: 2005 / Ab initio calculations for the molecule and crystal of L-alanine are presented. Geometry optimizations were performed for three L-alanine molecular conformers using three different methods: Hartree-Fock approximation with a 6-31++G(d,p) basis, Hartree-Fock approximation with a 6-311++G(3d,3p) basis and Density Functional Theory (B3LYP exchange-correlation functional) with a 6-31++G(d,p) basis. From the geometry optimizations, total energies, dipole and quadrupole moments, polarizabilities, thermodynamical properties, normal modes, Raman, infrared and VCD spectra, energy levels and frontier molecular orbitals were obtained and comparisons with experimental results were carried out. For the L-alanine crystal, geometry optimization was performed within the framework of DFT (LDA and GGA), and band structures, effctive masses, dielectric function, absorption, reflectivity and refraction index were calculated. Those results were employed to investigate the photoluminescence spectra of undoped and Mn-doped L-alanine crystals. The ab initio results allow assignment of the photoluminescence peaks in the visible region to lattice-related processes of exciton nature associated with polaron levels and defect trapping centers for carriers. By evaluating the vertical and adiabatic transitions between the ground state and excited states of a single L-alanine molecule in the zwitterion form, the very thin photoluminescence peak in the ultraviolet region is assigned to intramolecular transitions in the weakly interacting L-alanine molecules forming the crystals, being a signature of the type of the crystalline amino acid. In the case of Mn-doped alanine crystal, the integrated photoluminescence intensity is shown to be dramatically quenched by the intersticial manganese, with a decrease greater than 65% for higher doping. Preliminar ab initio calculations indicate that this quenching is related to the change of exciton energy levels due to the charge redistribution in the unit cell when manganese ions are present. / São apresentados aqui cálculos de primeiros princípios realizados para a obtenção de propriedades ópticas e eletrônicas da molécula e do cristal de L-alanina. No caso da molécula, foi efetuada a otimização da geometria para três conformações e usando três métodos diferentes: aproximação de Hartree-Fock na base 6-31++G(d,p), aproximação de Hartree-Fock na base 6-311++G(3d,3p) e teoria do funcional da densidade (funcional de troca e correlação B3LYP) na base 6-31++G(d,p). A partir da otimização das geometrias para diferentes conformações, foram obtidas a energia total, momentos de dipolo e quadrupolo, polarizabilidades, propriedades termodinâmicas, modos normais de vibração, espectros Raman, infravermelho e VCD, níveis de energia eletrônicos e orbitais HOMO e LUMO, com comparações com resultados experimentais ao longo do trabalho. Para o cristal de L-alanina, a geometria dos átomos na célula unitária foi otimizada usando as aproximações LDA e GGA, determinando-se a estrutura de bandas, massas efetivas, função dielétrica, absorção, refletividade e índice de refração. Estes dados são usados na investigação do espectro de luminescência do cristal de L-alanina puro e dopado com manganês. Os cálculos ab initio permitem associar os picos de fotoluminescência no visível a processos de natureza excitônica relacionados a níveis de polaron e impurezas aprisionadoras de portadores. Calculando as transições vertical e adiabática entre os estados excitados e o estado fundamental de uma molécula de L-alanina simples na forma zwitteriônica, o pico estreito da fotoluminescência no ultravioleta é atribuído a transições intramoleculares nas moléculas de L-alanina fracamente interagentes dentro do cristal, o que deixa uma assinatura molecular específica do aminoácido. Já o estudo da fotoluminescência integrada do cristal dopado com Mn2+ revela um forte quenching da luminescência provocado pela presença do íon metálico nos interstícios da célula unitária. Resultados de cálculos ab initio preliminares indicam que tal quenching esta relacionado ao modo como a distribuição de energias excitônicas muda quando o íon e inserido num interstício da célula unitária.

The advance in transparent electronics requires high-performance transparent conducting oxide materials. The microscopic properties of these materials are sensitive to the presence of defects and interfaces and thus fundamental understanding is required for materials engineering. In this thesis, first principles density functional theory is used to investigate the possibility of tuning the structural, electronic and magnetic properties of tin oxide by means of defects and interfaces. Our aim is to reveal unique properties and the parameters to control them as well as to explain the origin of unique phenomena in oxide materials. The stability of native defect in tin monoxide (SnO) under strain is investigated using formation energy calculations. We find that the conductivity (which is controlled by native defects) can be switched from p-type to either n-type or undoped semiconducting by means of applied pressure. We then target inducing magnetism in SnO by 3d transition metal doping. We propose that V doping is efficient to realize spin polarization at high temperature. We discuss different tin oxide interfaces. Metallic states are found to form at the SnO/SnO2 interface with electronic properties that depend on the interface terminations. The origin of these states is explained in terms of charge transfer caused by chemical bonding and band alignment. For the SnO/SnO2 heterostructure, we observe the formation of a two dimensional hole gas at the interface, which is surprising as it cannot be explained by the standard polar catastrophe model. Thus, we propose a charge density discontinuity model to explain our results. The model can be generalized to other polar-polar interfaces. Motivated by technological applications, the electronic and structural properties of the MgO (100)/SnO2 (110) interface are investigated. Depending on the interface termination, we observe the formation of a two dimensional electron gas or spin polarized hole gas. Aiming to identify further key parameters, we examine O deficient LaAlO3/SrTiO3 (110) and (001) superlattices under hydrostatic pressure. Presence of O vacancies results in formation of a two-dimensional electron gas, for which we observe a distinct spatial pattern of carrier density that depends strongly on the amount of applied pressure.

Flexible electronics have garnered much interest over the past several decades. Hybrid organic-inorganic materials, such as metal-organic frameworks, offer a unique opportunity to encompass the effective electronic properties of the inorganic material and the flexible nature of the organic with the potential of enhancing other desirable properties, such as the contributing multiferroicity. Using a first principles approach, the goal of this thesis is to serve as a guide for identifying potential ferroelectric and multiferroic metal-organic frameworks. This is done through a screening method of metal-organic frameworks based on their geometry; certain symmetry operators cannot be present in a ferroelectric material. We report the theoretical spontaneous polarization for several dozens of MOFs in which ferroelectricity has not previously been tested, and further we discuss the likelihood that these materials could be engineered to have either increased polarization or added ferromagnetism, the latter of which would lead to multiferroicity.

Extreme conditions allow us to reveal unusual material properties. At the same time an experimental approach is di-cult under such conditions. Capabilities of a theoretical approach based on simplied models are limited. This explainsa wide application of computer simulations at extreme conditions. My thesis is concerned with computer simulations undersuch a conditions. I address such problems as melting, solidsolid phase transitions, shockwave impact on material properties and chemical reactions under extreme conditions. We addressed these problems to facilitate simulations of phase transitions to provide some interpretation of experimental data andexplain enigmatic phenomena in interior of the Earth. / <p>QC 20160615</p>