Electronic localization in organic conjugated polymers.

Conjugated polymers are promising organic materials for the development of\ud photovoltaic devices, and the main opto-electronic features are currently under intense\ud study. Concerning the effects induced by disorder effects, conjugation break and\ud localization are supposed to be determinant in the observed properties. As expected the\ud localization of electronic states has dramatic counterparts in the charge carrier mobility\ud and in the exciton formation/dissociation processes

Optimal Hartree-Fock Exact Exchange fraction to describe small organic oligomers.

Electronic conduction by conjugated polymers has attracted attention because their physical and chemical properties are important in the development of new devices. To characterize long polymers chains, it is important to understand the properties of small molecules, similar to the monomers units. We perform quantum calculations for pairs of oligomers - Stilbene (C14H12) and Azobenzene (C12H10N2); Fluorene (C12H10) and Carbazole (C12H9N) - that have similar structures, with nitrogen substitution in clue positions of the molecule. For all molecules, the optimized atomic geometry was found using Density Functional Theory with Generalized Gradient Approximation (GGA) PBE including van der Waals corrections based on Hirshfeld distribution. For these optimized structures, we perform DFT calculations including several fractions (from 0 to 100%) of Hartree-Fock exact exchange, using the hybrid functional PBEh, in order to verify what is the best choice to mitigate the self-interaction errors in the computed Kohn-Sham energies. The results are compared with theoretical GW previous calculations performed by our group. All calculations were performed via the FHI-Aims package

Confinement effects on Titanium Oxide nanostructures.

Titanium oxide is the most commonly used material for dye-sensitized solar cells. In such devices it is usually found as a nanostructured film, made by the agglomeration of several nanoparticles. The effects of nano-dimensionality in comparison to crystalline materials are however still poorly understood. We show two different effects, in the electrical and optical spectra of TiO2 anatase, due to confinement.CNPqFAPES

Spectroscopic characterization of schiff base-copper complexes immobilized in smectite clays

Herein, the immobilization of some Schiff base-copper(II) complexes in smectite clays is described as a strategy for the heterogenization of homogeneous catalysts. The obtained materials were characterized by spectroscopic techniques, mostly UV/Vis, EPR, XANES and luminescence spectroscopy. SWy-2 and synthetic Laponite clays were used for the immobilization of two different complexes that have previously shown catalytic activity in the dismutation of superoxide radicals, and disproportionation of hydrogen peroxide. The obtained results indicated the occurrence of an intriguing intramolecular redox process involving copper and the imine ligand at the surface of the clays. These studies are supported by computational calculations

Theory Deep Levels Silicon

Estudamos neste trabalho a estrutura eletrônica de defeitos localizados em silício, responsáveis pela introdução de níveis profundos na faixa proibida do semicondutor. Utilizamos para tanto modelos de aglomerados moleculares dentro do formalismo do Espalhamento Múltiplo com aproximação local X para o potencial de troca (MS-X). Um tratamento adequado para os orbitais de superfície foi usado. Os defeitos estudados foram de dois tipos: defeitos simples (monovacância Si:V e oxigênio substitucional Si:O) e pares de defeitos vizinhos na rede (divacância Si : V IND 2) e par de átomos de fósforo Si: P IND 2). Os defeitos foram estudados em diferentes estados de carga (Si:O e Si : O POT -\', Si: P IND 2 e Si: P IND 2 POT + Si: V POT 0 IND 2, Si: V IND 2 POT + e Si: V POT IND 2) e no caso da divacância e do centro P IND 2 POT + foram incluídos efeitos de polarização de spin. Encontramos para todos os defeitos estudados indícios da ocorrência de efeitos Jahn-Teller. Incluímos análises das distorções dos primeiros vizinhos ao defeito nos sistemas Si: O POT , P IND 2 POT + e V POT IND 2. O modelo adotado mostrou-se capaz de descrever satisfatoriamente a estrutura eletrônica dos defeitos estudados, fornecendo resultados quantitativos que podem ser comparados diretamente com a experiência.In this work we studied the electronic structure of deep-level defects in silicon. To do this we use molecular cluster models within the formalism of the Multiple Scattering method, in the local density functional approximation (X). The surface orbitals of the cluster are treated in a convenient way. The defects studied here were of two kinds: simple defects (single vacancy Si:V and oxygen substitutional Si:0), and pairs of defects occupying neighbouring sites in the lattice (divacancy Si :V2 and the pair of phosphoru- atoms Si :P2 ). The defects were studied in different charge states (Si:O and Si:O-, Si:P2 and Si:P+, Si:V20, SI:V2-) and for the divacancy and the center Si:P2+ the calculations were carried out to the spin-polarized limit. For all defects studied we found evidence as to the possible occurrence of Jahn-Teller effects. Analysis of nearest-neighbours distortions were included for the systems Si:O- , Si:V2+ and Si:V2-. The electronic structure of the defects studied here was satisfactorily described by the model we adopted, and quantitative results are given, that can be compared straightforwardly with experimental results

Theory Deep Levels Silicon

Estudamos neste trabalho a estrutura eletrônica de defeitos localizados em silício, responsáveis pela introdução de níveis profundos na faixa proibida do semicondutor. Utilizamos para tanto modelos de aglomerados moleculares dentro do formalismo do Espalhamento Múltiplo com aproximação local X para o potencial de troca (MS-X). Um tratamento adequado para os orbitais de superfície foi usado. Os defeitos estudados foram de dois tipos: defeitos simples (monovacância Si:V e oxigênio substitucional Si:O) e pares de defeitos vizinhos na rede (divacância Si : V IND 2) e par de átomos de fósforo Si: P IND 2). Os defeitos foram estudados em diferentes estados de carga (Si:O e Si : O POT -\', Si: P IND 2 e Si: P IND 2 POT + Si: V POT 0 IND 2, Si: V IND 2 POT + e Si: V POT IND 2) e no caso da divacância e do centro P IND 2 POT + foram incluídos efeitos de polarização de spin. Encontramos para todos os defeitos estudados indícios da ocorrência de efeitos Jahn-Teller. Incluímos análises das distorções dos primeiros vizinhos ao defeito nos sistemas Si: O POT , P IND 2 POT + e V POT IND 2. O modelo adotado mostrou-se capaz de descrever satisfatoriamente a estrutura eletrônica dos defeitos estudados, fornecendo resultados quantitativos que podem ser comparados diretamente com a experiência.In this work we studied the electronic structure of deep-level defects in silicon. To do this we use molecular cluster models within the formalism of the Multiple Scattering method, in the local density functional approximation (X). The surface orbitals of the cluster are treated in a convenient way. The defects studied here were of two kinds: simple defects (single vacancy Si:V and oxygen substitutional Si:0), and pairs of defects occupying neighbouring sites in the lattice (divacancy Si :V2 and the pair of phosphoru- atoms Si :P2 ). The defects were studied in different charge states (Si:O and Si:O-, Si:P2 and Si:P+, Si:V20, SI:V2-) and for the divacancy and the center Si:P2+ the calculations were carried out to the spin-polarized limit. For all defects studied we found evidence as to the possible occurrence of Jahn-Teller effects. Analysis of nearest-neighbours distortions were included for the systems Si:O- , Si:V2+ and Si:V2-. The electronic structure of the defects studied here was satisfactorily described by the model we adopted, and quantitative results are given, that can be compared straightforwardly with experimental results

Atomistic molecular dynamics study of interface formation: Al on poly(p-phenylene vinylene)

We model interface formation by metal deposition on the conjugated polymer poly-para-phenylene vinylene, studying direct aluminum and layered aluminum-calcium structures Al/PPV and Al/Ca/PPV. To do that we use classical molecular dynamics simulations, checked by ab initio density-functional theory calculations, for selected relevant configurations. We find that Al not only migrates easily into the film, with a strong charge transfer to the neighboring chains, but also promotes rearrangement of the polymer in the interfacial region to the hexagonal structure. On the other hand, our results indicate that a thin Ca layer is sufficient to protect the film and maintain a well-defined metal/polymer interface, and that also a thin Al capping layer may protect the whole from environmental degradation

Effects of torsional disorder on poly-para-phenylene

We study the effect of thermal disorder on the electronic structure of one-dimensional poly-para-phenylene (PPP). In a real chain the torsion angles between rings are bound to be distributed over a range of values, which depend on temperature, and thus the chain is intrinsically disordered. In this study we simulated this kind of thermally induced off-diagonal disorder through the simple Huckel method. We base our Hamiltonian on ab initio results for the effect of temperature on torsion angles, and the effect of torsion angles on the energy gap. We analyze the electronic structure of 200-monomer-long chains focusing on the density of states, and the associated localization character (measured by the inverse participation ratio). Our results contrast with the usually assumed Gaussian-shaped density of localized states for disordered systems. (C) 2009 Elsevier B.V. All rights reserved.CNPqConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESPFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP