No description available.

In this work a systematic method for determining correlated wavefunctions of extended systems in the ground state as well as in excited states is presented. It allows to fully exploit the power of quantum-chemical programs designed for correlation calculations of finite molecules. Using localized Hartree-Fock (HF) orbitals (both occupied and virtual ones), an effective Hamiltonian which can easily be transferred from finite to infinite systems is built up. Correlation corrections to the matrix elements of the effective Hamiltonian are derived from clusters using an incremental scheme. To treat the correlation effects, multireference configuration interaction (MRCI) calculations with singly and doubly excited configurations (SD) are performed. This way one is able to generate both valence and conduction bands where all correlation effects in the excited states as well as in the ground state of the system are taken into account. An appropriate size-extensivity correction to the MRCI(SD) correlation energies is developed which takes into account the open-shell character of the excited states. This approach is applicable to a wide range of polymers and crystals. In the present work trans-polyacetylene is chosen as a test system. The corresponding band structure is obtained with the correlation of all electrons in the system being included on a very high level of sophistication. The account of correlation effects leads to substantial shifts of the "center-of-mass" positions of the bands (valence bands are shifted upwards and conduction bands downwards) and a flattening of all bands compared to the corresponding HF band structure. The method reaches the quantum-chemical level of accuracy. Further an extention of the above approach to excitons (optical excitations) in crystals is developed which allows to use standard quantum-chemical methods to describe the electron-hole pairs and to finally obtain excitonic bands.

In this work a systematic method for determining correlated wavefunctions of extended systems in the ground state as well as in excited states is presented. It allows to fully exploit the power of quantum-chemical programs designed for correlation calculations of finite molecules. Using localized Hartree-Fock (HF) orbitals (both occupied and virtual ones), an effective Hamiltonian which can easily be transferred from finite to infinite systems is built up. Correlation corrections to the matrix elements of the effective Hamiltonian are derived from clusters using an incremental scheme. To treat the correlation effects, multireference configuration interaction (MRCI) calculations with singly and doubly excited configurations (SD) are performed. This way one is able to generate both valence and conduction bands where all correlation effects in the excited states as well as in the ground state of the system are taken into account. An appropriate size-extensivity correction to the MRCI(SD) correlation energies is developed which takes into account the open-shell character of the excited states. This approach is applicable to a wide range of polymers and crystals. In the present work trans-polyacetylene is chosen as a test system. The corresponding band structure is obtained with the correlation of all electrons in the system being included on a very high level of sophistication. The account of correlation effects leads to substantial shifts of the "center-of-mass" positions of the bands (valence bands are shifted upwards and conduction bands downwards) and a flattening of all bands compared to the corresponding HF band structure. The method reaches the quantum-chemical level of accuracy. Further an extention of the above approach to excitons (optical excitations) in crystals is developed which allows to use standard quantum-chemical methods to describe the electron-hole pairs and to finally obtain excitonic bands.

A aproximação da densidade local para a teoria do funcional da densidade tem, no passado, levado a resultados conflitantes para a dimerização de cadeias infinitas do trans-poliacetileno (trans-PA). Estes resultados variaram deste forte dimerização, próximo dos resultados experimentais, até fraca dimerização, ou até mesmo nenhuma dimerização. Desde que a aproximação da densidade local usualmente descreve transições de fase estruturais corretamente, esta situação insatisfatória no caso do trans-PA clama por uma investigação detalhada. Neste trabalho estudamos o problema descrevendo a molécula como um modelo de Hubbard-Peierls unidimensional. Estabelecemos uma teoria do funcional da densidade e construímos uma aproximação da densidade local para este modelo. Em acordo com os cálculos ab initio encontramos que este tipo de aproximação não descreve adequadamente a dimerização. Propomos então um Formalismo alternativo, baseado em funcionais da matriz-densidade. Neste formalismo a energia de troca-correlação é escrita como um funcional da densidade de carga (como no método tradicional) e do parâmetro de ordem para a transição. Desta maneira obtemos um tratamento aperfeiçoado para a fase dimerizada. Nossos resultados sugerem que uma descrição adequada de sistemas espacialmente correlacionados dentro da teoria do funcional da densidade requer uma nova classe de funcionais, que vai além da aproximação da densidade local por levar explicitamente em conta parâmetros de ordem espaciais de longo alcance. / The local-density approximation to density-functional theory has, in the past, lead to conflicting results for the dimerization of infinite trans-polyacetylene (trans-PA) chains. These results range from strong dimerization, close to the one observed experimentally, to weak dimerization, or even no dimerization at all. Since the local-density approximation usually describes structural phase transitions correctly, this unsatisfactory situation in the case of trans-PA calls for a detailed investigation. In this work we study the problem by describing the polyacetylene molecule as a one-dimensional Hubbard-Peierls model. We set up a density-functional theory for this model, and construct a local-density approximation. In agreement with the ab initio calculations we find that this type of approximation does not consistently describe the dimerization. We therefore propose an alternative formalism, based on density-matrix functionals. In this formalism the exchange-correlation energy is written as a functional of the charge density (as in the traditional method) and the order parameter for the dimerization transition. In this way we achieve an improved treatment of the dimerized phase. Our results suggest that a reliable description of spatially correlated systems within density-functional theory requires a new type of functionals, going beyond the local-density approximation by explicitly accounting for long-range spatial order.

A aproximação da densidade local para a teoria do funcional da densidade tem, no passado, levado a resultados conflitantes para a dimerização de cadeias infinitas do trans-poliacetileno (trans-PA). Estes resultados variaram deste forte dimerização, próximo dos resultados experimentais, até fraca dimerização, ou até mesmo nenhuma dimerização. Desde que a aproximação da densidade local usualmente descreve transições de fase estruturais corretamente, esta situação insatisfatória no caso do trans-PA clama por uma investigação detalhada. Neste trabalho estudamos o problema descrevendo a molécula como um modelo de Hubbard-Peierls unidimensional. Estabelecemos uma teoria do funcional da densidade e construímos uma aproximação da densidade local para este modelo. Em acordo com os cálculos ab initio encontramos que este tipo de aproximação não descreve adequadamente a dimerização. Propomos então um Formalismo alternativo, baseado em funcionais da matriz-densidade. Neste formalismo a energia de troca-correlação é escrita como um funcional da densidade de carga (como no método tradicional) e do parâmetro de ordem para a transição. Desta maneira obtemos um tratamento aperfeiçoado para a fase dimerizada. Nossos resultados sugerem que uma descrição adequada de sistemas espacialmente correlacionados dentro da teoria do funcional da densidade requer uma nova classe de funcionais, que vai além da aproximação da densidade local por levar explicitamente em conta parâmetros de ordem espaciais de longo alcance. / The local-density approximation to density-functional theory has, in the past, lead to conflicting results for the dimerization of infinite trans-polyacetylene (trans-PA) chains. These results range from strong dimerization, close to the one observed experimentally, to weak dimerization, or even no dimerization at all. Since the local-density approximation usually describes structural phase transitions correctly, this unsatisfactory situation in the case of trans-PA calls for a detailed investigation. In this work we study the problem by describing the polyacetylene molecule as a one-dimensional Hubbard-Peierls model. We set up a density-functional theory for this model, and construct a local-density approximation. In agreement with the ab initio calculations we find that this type of approximation does not consistently describe the dimerization. We therefore propose an alternative formalism, based on density-matrix functionals. In this formalism the exchange-correlation energy is written as a functional of the charge density (as in the traditional method) and the order parameter for the dimerization transition. In this way we achieve an improved treatment of the dimerized phase. Our results suggest that a reliable description of spatially correlated systems within density-functional theory requires a new type of functionals, going beyond the local-density approximation by explicitly accounting for long-range spatial order.

Neste trabalho à feita uma revisÃo das principais propriedades da molÃcula de poliacetileno sob o ponto de vista da teoria de campos. A seguir, estuda-se duas cadeias de poliacetileno acopladas usando o mesmo formalismo e verifica-se que esse sistema apresenta um gap em sua estrutura de bandas. Esse gap de energia à calculado em termos de uma massa efetiva quantizada que depende do acoplamento entre as cadeias de poliacetileno. Quando o acoplamento cessa o gap desaparece e recupera-se o resultado prÃvio de uma simples cadeia nÃo dimerizada. Mostra-se que entÃo hà uma quebra da simetria quiral. O comportamento do poliacetileno acoplado tambÃm à analisado quando existem sÃlitons em sua estrutura. Um formalismo à desenvolvido para mostrar que hà um fenÃmeno oscilatÃrio de natureza quÃntica anÃlogo Ãs oscilaÃÃes de Bloch. A condutividade do sistema à tambÃm calculada. / We review in this work the principal properties of single polyacetylene chain from a theoretical field formalism. Then, we study a coupled polyacetylene chain using the same formalism and verify that this system presents a gap in its electronic band structure. This energy gap is calculated in terms of a quantized effective mass that depends on the coupling between the polyacetylene chains. As the coupling decreases the gap vanishes and we restore the previous results of one single polyacetylene chain not dimerized. We show that there is a chiral broken symmetry. We study also the behavior of coupled polyacetilene in presence of solitons. A formalism is developed to show that there is an oscillatory phenomenon of quantum character analogous to Bloch oscillations. The conductivity of the system is also calculated.

xvii, 159 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Experimental studies on mix-conducting hetero-ionic junctions of anionically (PA A ) and cationically (PA C ) functionalized polyacetylene ionomers, as well as each individual ionomer, in thin-film sandwich configurations are reported for the purpose of better understanding the interaction between ionic and electronic charge transports in mixed ionic-electronic conductor (MIEC) systems. The transport of ions in both individual ionomers as well as their hetero-ionic junction was investigated via small-amplitude AC impedance spectroscopy in the absence of significant interference from the electronic charge transport. Modeling of the impedance results reveal important information about the materials such as: ion conductivity, activation energy of ion conduction, ion hopping frequency, dielectric constant, interfacial capacitance, and estimates of effective ion density. Electrochemical injection of electronic charge carriers into PA A and PA C from gold electrodes was monitored to determine the applied potentials needed to drive hole and electron injection into each ionomer. It is found that for both ionomers, the onset voltages for unipolar and bipolar charge injection are similar, and holes can be injected at close to zero bias. The responses of the complete Au|PA A |PA C |Au hetero-ionic junction, as well as each constituent ionomer layer in Au|Ionomer|Au configuration, to various stepping biases were investigated through current-voltage and impedance measurements to study the origin of the asymmetric current-voltage response observed in the hetero-ionic junction. Analysis of the results reveal a working mechanism of a mix-conducting junction that is fundamentally different from that of a purely electronic pn junction. When illuminated with light, the Au|PA A |PA C |Au junction exhibits unidirectional photovoltage and photocurrent with the PA A side at higher potential, while the Au|PA A |Au and Au|PA C |Au samples exhibit symmetric photoresponses. The efficiency of photocurrent generation in the Au|PA A |PA C |Au junction was found to be strongly dependent on the direction of illumination and on the sample thickness. These observations can be explained by the difference in the mobility of holes and electrons and the existence of a built-in ionic space charge region at the PA A |PA C interface. A mechanism of photoresponse unique to MIEC junctions was proposed, and the magnitude of built-in potential was estimated. / Committee in charge: J David Cohen, Chairperson, Physics; Mark Lonergan, Advisor, Chemistry; Roger Haydock, Member, Physics; David Strom, Member, Physics; David Tyler, Outside Member, Chemistry

No description available.

In this thesis, we report the various aspects of fabricating a Cold Atmospheric Plasma system, which can be used for Plasma Enhanced Chemical Vapour Deposition. The greatest advantage of this system is its vacuum free operation, which provides a cost e effctive alternative over conventional high vacuum systems. We have designed a reactor geometry for such a plasma system, in which, the contamination due to ambient air is kept at a minimum value using a low flow of Ar (500 sccm). Towards this end, we have modeled and simulated the flow pattern of Ar gas entering the reactor geometry and have studied its e effectiveness in removing air from the plasma zone. We have fabricated such a geometry and studied the contamination at different flow rates of Ar by observing the plasma optical emission. Further, the aspect of lamentation in atmospheric pressure plasma has been studied and we have identified a few process parameters which can convert a filamentary discharge to a diffused glow. Subsequently, a complete system was developed, including an in-house built high voltage power supply, to generate a plasma with low contamination and less number of laments. We have also carried out plasma diagnostics, specifically to estimate the Electron Energy Distribution Function (EEDF) of the plasma, by analysing the radiation emitted from an Ar plasma, acquired using an Optical Emission Spectroscope. The peaks in the spectrum were curve flatted with Voigt pro les and their widths and intensities were mapped to the electron number density and the EEDF of the plasma, using the mathematical models for Stark broadening and Corona population respectively. An optimization routine based on Nelder-Mead simplex algorithm was run to estimate the optimal values of these plasma parameters that produced a good match between the simulated spectrum and the experimentally acquired one. This analysis estimated that the value of electron number density in our plasma was in the range 0:82 1017 cm 3 to 3:56 1017 cm 3 and the electron temperature was in the range 0.36 eV -0.39 eV . It also predicted that the EEDF closely approximated a Maxwellian distribution. As a proof of concept, the fabricated reactor was used to deposit thin films of Polyacetylene over microscopic cover glass slides by polymerizing Acetylene gas in the cold plasma. Deposition rates as high as 1 m=min, were obtained during thin lm deposition of the polymer. The polymeric structure of the lm was studied using NMR and FT-IR. XPS measurement revealed 5% O2 inclusion in the samples. XRD showed no distinguishable peak, indicating the amorphous state of the films. The surface morphology investigated using SEM revealed highly porous broid kind of structures, which appeared to be agglomeration of particles with sizes in the order of few micrometers. P-type Polyacetylene lms were fabricated by doping them with 5.3% by atomic concentration of I2 vapours. The UV-Visible spectroscopy study revealed a bandgap of 2.05 eV for undoped and 1.49 eV for the doped Polyacetylene samples. The lms exhibited an increase in conductivity by two orders of magnitude; from 3:6 10 13 1cm 1 to 3:5 10 11 1cm 1 for un-doped and doped Polyacetylene samples respectively.

No description available.