Neste trabalho, apresentamos estudos sobre o magnetotransporte em um sistema de bicamadas com uma rede de antipontos triangulares em campos magnéticos baixos sob a aplicação de campos elétricos externos, que são produzidos por voltagens de porta. A bicamada é feita em poços quânticos largos (wide quantum well) de alta densidade eletrônica, formado em heteroestruturas semicondutoras de AlxGa1xAs=GaAs. Oscila- ções magneto-inter-sub-banda (MIS) são observadas em poços quânticos largos de alta densidade eletrônica com duas sub-bandas ocupadas. Estas são originadas pelo espalhamento inter-sub-bandas e tem um máximo para campos magnéticos B que satisfazem a condição de alinhamento entre os leques dos níveis de Landau de cada sub-banda. Oscila- ções de comensurabilidade são observadas na magnetoresistência que é sensível ao arranjo do potencial dos antipontos. A aplicação de campos elétricos faz diminuir o número de oscilações na magnetoresistência para campos magnéticos compreendidos entre 0; 1T e 0; 4T, observando-se uma transição das oscilações MIS aos efeitos de comensurabilidade. Aplicando voltagens de porta podemos variar a amplitude do potencial dos antipontos. / In this work, we present studying about magnetotransport in a bilayer system with triangular antidot lattices in low magnetic elds under the application of external electric eld. The bilayer forms inside a wide quantum well of high electron density in semiconductor heterostructures formed by AlxGa1xAs=GaAs. Magneto-inter-subband (MIS) oscillations are observed in a wide quantum wells of high electron density with two subbands occupied, and they are caused by intersubband scattering and have a maximum for a magnetic eld B that satises the alignment condition between the staircase of Landau level. Commensurability oscillations are observed in magnetoresistance, which is sensitive to the potential of antidot arrangements. The application of electric elds decrease the number of oscillations in the magnetoresistance for magnetic elds between 0; 1T and 0:4T, showing a transition of MIS oscillations to commensurability oscillations. We varied the amplitude of the potential of the antidots applying dierent gate voltages.

Orientador: Bernardo Laks / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-12T11:04:39Z (GMT). No. of bitstreams: 1 Marcal_Nei_D.pdf: 86929566 bytes, checksum: 04c8aa7c4176175d973ff0990de45bad (MD5) Previous issue date: 2008 / Resumo: Antes da década de 70, todos os materiais poliméricos eram considerados como isolantes e suas aplicações tecnológicas levavam em conta esta característica. De lá para cá, uma nova classe desses materiais, os polímeros conjugados, determinaram uma nova forma de aplicação de sistemas poliméricos baseados em suas propriedades elétricas e de ótica não-linear. Um maior estímulo surgiu a partir do experimento de Mac Diarmid, Heeger e Shirakawa [1] que, expondo o Poliacetileno a agentes oxidantes, demonstraram ser possível obter um sistema no estado metálico. Atualmente encontramos filmes de Poliacetileno com condutividade elétrica da ordem do cobre (105 S/cm). O Poliacetileno, quando no regime metálico, i.e., sob alta dopagem, apresenta algumas características de metal comum: alta condutividade elétrica (cresce 13 ordens de grandeza), susceptibilidade de Pauli finita e absorção no infravermelho. Já outras propriedades como a presença de modos vibracionais localizados no infravermelho e o não comportamento da condutividade com o inverso da temperatura evidenciam ser este um material não usual. Estes polímeros conjugados que apresentam uma extensiva delocalização de elétrons são considerados semicondutores orgânicos com gap de energia relativamente pequeno, da ordem de 1,5 a 2,0 eV. O comportamento semicondutor e as propriedades decorrentes entre os elétrons e a luz têm originado a construção de vários dispositivos semicondutores e optoeletrônicos [2, 7, 3]. Problemas técnicos como estabilidade ao ambiente, processabilidade e solubilidade destes materiais provocaram a produção de uma nova classe de materiais poliméricos que foi obtida por polimerização eletroquímica [57, 58, 59] cuja estrutura molecular trata-se de sistemas que introduzem grupos vinilas (V) entre anéis de tiofeno (T). Experimentos de voltametria cíclica, espectroscopia de absorção ótica e ressonância eletrônica de spin indicam que esses sistemas possuem potencial de ionização e gap de energia menores que o apresentado pelo Politiofeno. Estudos com oligômeros de tiofeno (T) com vinilenos (V) sugerem a possibilidade de escolha desse material como alternativa ao politiofeno. O objetivo deste trabalho foi investigar teoricamente a influência do grupo vinila (V) sobre as propriedades eletrônicas nestes polímeros, reproduzir os resultados experimentais e determinar qual proporção de vinilenos (V) e tiofenos (T) que provoque o menor gap de energia de forma que quando sobre dopagem possibilite uma transição isolante metal. Desta maneira, primeiramente, determinamos as geometrias dos sistemas de interesse utilizando métodos semi-empíricos. Posteriormente investigamos a estrutura eletrônica dos polímeros de tiofeno (T) com vinilenos (V), sendo que estes polímeros foram estudados para o caso neutro e na presença de defeitos conformacionais do tipo pólaron e bipólaron. Finalizamos o estudo investigando as absorções ópticas UV-vis dos sistemas de interesse através de cálculos semi-empíricos utilizando o código ZINDO/S. / Abstract: Before the 1970s, all polymeric materials were considered insulators; therefore their technological applications would take this trait into account. Since then, a new development on these materials, the conjugated polymers, determined new applications for polymeric systems based in their electrical and nonlinear optical properties. Greater interest arose from the experiment by Mac Diarmid, Heeger and Shirakawa [1] who, by using polyacetylene and oxidizing agents, showed that it is possible to obtain a system in the metallic state. Nowadays it is possible to find polyacetylene films with electrical conductivity of the order of copper (10-5 S/cm). Polyacetylene, when in its metallic behavior, i.e., under high dopage, presents some characteristics of real metal: high electrical conductivity (increased by 13 orders of magnitude), finite Pauli susceptibility and infrared absorption. On the other hand, other properties such as the presence of vibrational modes localized on infrared and the odd behavior of conductivity versus the inverse of temperature make clear that this is a unusual material. These conjugated polymers, presenting an extensive delocalization of electrons, are considered organic semiconductors with relatively low energy gap, of the order of 1.5 to 2.0 eV. The semiconductive behavior and the resulting properties of the interaction between electrons and light have been the drive for the manufacturing of several semiconductor and optoelectronic devices [2, 7, 3]. Technical problems, such as environmental stability, processability and solubility of these materials, gave rise to the production of a new kind of polymeric materials that were obtained by electrochemical polymerization [57, 58, 59], in which the molecular structure is a system that introduces vinylene groups (V) between tiophene rings (T). Experiments involving cyclic voltametry, optical absorption spectrometry and spin electronic ressonance indicate that these systems have ionization potential and energy gap smaller than those presented by Polythiophene. Studies with thiopene oligomers (T) with vinylene (V) suggest this material can be chosen as an alternative to Polytiophene. The goal of this work is to theoretically investigate the in uence of the vinyle group (V) on the electronic properties on these polymers, reproduce experimental results and determine what is the vinylene (V) to thiophene (T) rate that causes the smallest energy gap, such that doping will produce a insulator-metal transition. Therefore, we first determine the target systems' geometry using semi-empirical methods. Then we investigate the electronic structure of the tiophene (T) and vinylene (V) polymers both for neutral systems and in the presence of conformational defects of polaron and bipolaron types. We nalized the study by investigating the UV-vis optical absorption of the target systems through semi-empirical calculations using ZINDO/S code. / Doutorado / Estrutura Eletronica de Atomos e Moleculas ; Teoria / Doutor em Ciências

Orientador: Jose Mario Martinez / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Matematica, Estatistica e Computação Cientifica / Made available in DSpace on 2018-08-04T03:19:31Z (GMT). No. of bitstreams: 1 Francisco_JulianodeBem_D.pdf: 963435 bytes, checksum: 88cedcb7cfb40d6a63f0ce67f0abb970 (MD5) Previous issue date: 2005 / Resumo: Algoritmos robustos e numericamente viáveis para resolver problemas de otimização têm sido cada vez mais solicitados em problemas práticos que aparecem em engenharia, química, física, entre outras áreas. Com isso em mente, este trabalho apresenta um novo método globalmente convergente baseado em região de confiança para resolver sistemas não-lineares indeterminados (mais incógnitas do que equações) com restrições de caixa, podendo, portanto, ser aproveitado para a fase de viabilidade nos algoritmos baseados em restauração periódica. É mostrado que esse método apresenta, sob certas hipóteses, convergência localmente quadrática. Em uma outra parte deste trabalho é apresentado um novo algoritmo globalmente convergente, o qual se baseia em região de confiança, para resolver problemas de otimização do tipo min f(x); s:a: x 2 D; onde f : Rn ! R é assumida para ser continuamente diferenciável e D C Rn, um subconjunto fechado arbitrário. Em vez de considerar a região de confiança explicitamente nos subproblemas, esse método introduz um parâmetro de regularização que busca imitar a região de confiança. Com essa caracterização, os subproblemas consistem em minimizar um modelo quadratico de f sujeito ao subconjunto D. Uma importante aplicação desse novo algoritmo aparece em química quântica e resultará em um novo algoritmo globalmente convergente, robusto e numericamente viável para calcular estruturas eletrônicas de átomos e moléculas / Abstract: Abstract Robust and numerically feasible algorithms for solving optimization problems have been demanded for solving practice problems that appear in Engineering, Chemistry, Physics and others. This work present a new globally convergent method based on trust regions for solving box-constrained underdetermined nonlinear systems (more unknowns than equations), that can be used on the feasibility fase of algorithms based on periodic restoration. Under some assumptions, it will be proved locally quadratic convergence. In other part of this work, a new globally convergent algorithm is introduced, based on trust regions, for solving the optimization problem min f(x); s:t: x 2 D; where f : Rn ! R is continuously dierentiable and D C Rn is an arbitrary closed subset. Instead of considering explicitly the trust region on the subproblems, the method introduces a regularization parameter that mimics the trust region. With this characterization, the subproblems consist on minimizing a quadratic model of f subject to D. numerically feasible globally convergent algorithm for electronic structure calculations is obtained. / Doutorado / Matematica Aplicada / Doutor em Matemática Aplicada

Abstract This thesis presents computational first-principles investigations of nuclear magnetic resonance (NMR) parameters in metal-containing nanosystems. Special attention is paid to the relativistic effects observed in the vicinity of heavy elements. Small transition metal complexes are used to assess the feasibility of a quasirelativistic density functional theory (DFT) approach for calculating nuclear magnetic shielding tensors of increasingly heavy metal nuclei, followed by applications of the concept to larger systems. Nuclear magnetic shielding constants, shielding anisotropies, and chemical shifts with respect to metal ions are calculated in dimethyl and water complexes of the group-12 transition metals 67Zn, 111/113Cd, and 199/201Hg, using Hartree–Fock and DFT methods with relativistic corrections from the Breit–Pauli Perturbation Theory (BPPT). Four-component relativistic Dirac–Hartree–Fock and correlated, nonrelativistic ab initio calculations are used to benchmark the BPPT and DFT methods, respectively. The DFT/BPPT approach, combined with Monte Carlo simulations at finite temperatures, is subsequently used to calculate the chemical shift of a guest 129Xe inside a tetrahedral, iron-based cage. Complementing experiments, the encapsulation of xenon is verified, and empirically elusive details are revealed about the guest dynamics. Finally, the full shielding tensors of 31P and 195Pt and the indirect spin–spin coupling constants between the two nuclei are studied in five crystalline platinum(II) dialkyldithiophosphato complexes, concentrating on the solid-state chemical shift anisotropy and asymmetry parameters of phosphorus and platinum. The NMR parameters are calculated using DFT and the two-component zerothorder regular approximation (ZORA) for relativistic effects, combining molecular and solid-state models to incorporate indispensable contributions due to spin–orbit and crystal lattice corrections for the shielding tensors. Four-component matrix-Dirac–Kohn–Sham shielding calculations are used to benchmark the ZORA method. Qualitative, in cases nearly quantitative agreement is obtained with experiments, allowing the validation of the X-ray structures of the complexes, as well as a deeper analysis of the differences between them, including the major contributions to the NMR parameters. The results presented here demonstrate that computational NMR, a branch of relativistic quantum chemistry, is applicable and useful in studying nanoscale systems containing heavy elements, such as transition metals. Approximations are necessary to enable the treatment of large and complex targets, but sufficient accuracy is achieved for supplementing experiments with reliable and useful data that provides additional insight and analysis possibilities.

Abstract This thesis presents computational studies of both the static and dynamic parameters of the nuclear magnetic resonance (NMR) spectroscopy of gaseous xenon. First, state-of-the-art static magnetic resonance parameters are computed in small xenon clusters by using methods of quantum chemistry, and second, time-dependent relaxation phenomena are investigated via molecular dynamics simulations at different experimental conditions. Based on the underlying quantum and classical mechanics concepts, computational methods represent a procedure complementary to experiments for investigating the properties of atoms, molecules, clusters and solids. Static NMR spectral parameters, chemical shift, shielding anisotropy and asymmetry parameter, nuclear quadrupole coupling, and spin-rotation coupling, are calculated using different electronic structure methods ranging from the uncorrelated Hartree-Fock method to correlated second-order Møller-Plesset many-body perturbation, complete/restricted active space multiconfiguration self-consistent field, and to coupled-cluster approaches. The bond length dependence of these properties is investigated in the xenon dimer (Xe2). A well-characterized property in experimental NMR, the second virial coefficient of nuclear shielding, is theoretically calculated by a variety of methods and convincingly verified against experimental findings. Here, it is mandatory to include effects from special relativity as well as electron correlation. As a side result, a purely theoretical potential energy curve for Xe2, comparable to best experimental ones, is calculated. A pairwise additive scheme is established to approximate the NMR properties in differently coordinated sites of xenon clusters Xen (n = 2 - 12). Especially the pairwise additive chemical shift values are found to be in close agreement with quantum-chemical results and only a small scaling factor close to unity is needed for the correct behavior. Finally, a dynamical magnetic resonance property, the experimental nuclear spin-lattice relaxation rate R1 of monoatomic Xe gas due to the chemical shift anisotropy (CSA) mechanism is validated from first principles. This approach is based on molecular dynamics simulations over a large range of temperatures and densities, combined with the pairwise additive approximation for the shielding tensor. Therein, the shielding time correlation function is seen to reflect the characteristic time scales related to both interatomic collisions and cluster formation. For the first time, the physics of gaseous xenon is detailed in full in the context of CSA relaxation.

The investigation and control of molecular properties is currently a dynamic research field. Here I present molecular level studies of porphyrin molecules adsorbed on metal surfaces via Low Temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS), supported by complementary X-ray absorption experiments. Intermolecular and molecule-surface interactions of tetrapyrdil porphyrin (TPyP) on Ag(111) and Cu(111) were investigated. TPyP self-assembles on Ag(111) over a wide sample temperature range into large, highly-ordered 2D chiral domains. By contrast, adsorption of TPyP on the more reactive Cu(111) leads to temperature dependent assemblies, governed decisively by the strong substrate influence. The increased metal-surface interactions on Cu(111) are accompanied by a conformational distortion of the porphyrin macrocycle. The TPyP’s pyridil groups were coordinated with single iron molecules, forming metal-organic complexes. Furthermore, the porphyrin’s macrocycle was metalated by exposing a layer of well-ordered TPyP to an iron atom beam, demonstrating a novel approach towards the fabrication of metallo-tetraaryl porphyrins performed in two dimensions under ultrahigh vacuum conditions. This method was similarly used to form lanthanideporphyrinates by coordinating tetraphenyl porphyrin (TPP) macrocycles with cerium. The influence of the metal center on the porphyrins’ electronic structure was investigated via STS for TPP, TPyP,Fe−TPyP, Fe−TPP, Ce−TPP, and Co−TPP, whereby the inhomogenous electron density distribution associated with individual frontier orbitals were imaged via dI/dV mapping. The symmetry and form of the molecular orbitals could be directly correlated to the saddle-shaped conformational adaptation for the case of Co −TPP. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis is concerned with the chemistry and physics of C(100) surfaces of diamond. The polished and cleaned C(100) surface is examined by surface microscopy (Atomic-force Microscopy), electron diffraction (Low-energy Electron Diffraction) and photoemission (X-ray Photoelectron Spectroscopy and Ultra-violet Photoelectron Spectroscopy). Results are presented on the presence of oxygen, nitrogen and hydrogen/deuterium on the C(100) surface. Finally, the valence band structure of diamond is probed by angle-resolved photoemission. We have confirmed by AFM that the grooves from the soft polishing process are present on a polished C(100) surface and found sporadic traces of hard polish on a surface polished in the soft polishing direction. XPS studies have verified heating cycles by electron beam bombardment as a suitable cleaning procedure for pure reconstructed C(100) surfaces. By allowing the crystal to cool slowly, the first experimental evidence of quarter-order LEED spots have been found, which suggest that buckled dimerisation might have occurred similar to those on Si(100) and Ge(100). We present the first experimental electron spectroscopy results for a nitrogen impurity in diamond by showing the N KLL Auger spectrum. An attempt to smooth a C(100) surface of diamond by an atomic hydrogen plasma did not succeed. AFM studies showed no evidence for the surface smoothing reported in other studies, but the results enable us to explain the different plasma published in the literature. The valence band of diamond is investigated by off-normal ARUPS. The features observed are consistent with possible transitions, which are determined using bulk band structure calculations and comparison with the experimental binding energies.

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Orientador: Augusto Batagin Neto / Resumo: Polímeros orgânicos conjugados são considerados materiais de grande relevância para aplicações tecnológicas variadas, principalmente devido às suas propriedades optoeletrônicas únicas e métodos utilizados em sua síntese. Nesse contexto, os derivados de polipirrol (PPy) têm sido amplamente empregados. A grande variabilidade de síntese desse material permite a produção de uma série de derivados com propriedades distintas, permitindo sua aplicação em diversas áreas. Neste trabalho, cálculos de estrutura eletrônica foram realizados para avaliar a influência de grupos laterais nas propriedades estruturais, ópticas, eletrônicas e de reatividade de derivados de PPy, em especial para aplicações como sensores químicos. Os cálculos foram feitos para sistemas oligoméricos aplicando a teoria do funcional da densidade. Estudos de preliminares foram conduzidos utilizando dois funcionais distintos para otimização de geometria e avaliação de propriedades optoeletrônicas. Estudos comparativos da alternância de comprimento de ligação, distribuição espacial e energética dos orbitais de fronteira, gaps eletrônicos, energias de ligação de éxcitons, espectros de absorção óptica, densidade eletrônica de estados e reatividade local foram conduzidos para cada derivado e a influência dos grupos laterais foi discutida em termos de suas propriedades de inserção/retirada de elétrons. Um conjunto de regras simples (equações lineares) foi proposto para a predição de propriedades optoeletrônicas de derivado... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Conjugated organic polymers have been considered interesting materials for varied technological applications, mainly due to their unique optoelectronic properties and variety of methods employed in their synthesis. In this context, polypyrrole (PPy) derivatives have been widely employed. The great versatility of synthesis of this material allows the production of a number of derivatives with distinct properties, allowing their application in several areas. In this report, aiming to guide the design of compounds with specific features, electronic structure calculations were conducted to evaluate the influence of side groups in the structural, optical and electronic properties of PPy derivatives, specially for application in chemical sensors. The calculations were carried out for oligomeric systems in the framework of the density functional theory. Preliminary benchmark studies were conducted by employing two distinct functionals for geometry optimization and evaluation of optoelectronic properties. Comparative studies of the bond length alternation, spatial and energetic distribution of the frontier orbitals, electronic gaps, exciton binding energies, optical absorption spectra, electronic density of states and local reactivity were conducted for each derivative and the influence of the side groups was discussed in terms of their electron donation/withdrawing properties. A set of simple rules (linear equations) was proposed for the prediction of optoelectronic properties of PP... (Complete abstract click electronic access below) / Mestre