No description available.

The intent of this thesis is to improve upon previously proposed tight-binding models for one dimensional black phosphorus, or phosphorene. Previous models offer only a qualitative analysis of the band structure of phosphorene, and fail to fully realize critical elements in the electronic band structure necessary for transport calculations. In this work we propose an improved tight-binding model for phosphorene by including up to eight nearest-neighbor interactions. The efficacy of the model is verified by comparison with DFT-HSE06 calculations, and the anisotropy of the effective masses in the armchair and zigzag directions is considered.

No description available.

Silicon nitride is a bulk and a coating material exhibiting excellent mechanical properties. The understanding of the complex processes at the nanometre scale gained through experimental research will be enhanced by the existence of a computationally efficient and accurate model that is able to describe the mechanical properties of silicon nitride. Such a model has yet to be proposed. In this thesis we present a transferable reduced tight-binding (TB) model for the silicon nitride system. More precisely, this model consists of a reduced TB model for elemental silicon, a reduced TB model for elemental nitrogen, and a reduced TB model for silicon nitride. These models are developed within the framework of coarse-graining the electronic structure from density functional theory (DFT) to tight binding (TB) to bond-order potentials (BOPs), and can therefore be used in the future as the stepping stone to develop BOPs for the application in large scale simulations. The bond integrals employed in the reduced TB models are obtained directly from mixed-basis DFT projections of wave functions onto a minimal basis of atom-centred orbitals. This approach reduces the number of overall parameters to be fitted and provides models which are transferable through the different coarse-graining levels. We provide an example by using the same bond integrals in the reduced TB model for silicon and the preliminary bond-based BOP for silicon. DFT binding energies of ground state and metastable crystal structures are used as the benchmark to which the TB and BOP repulsive parameters are fitted. In addition to model development, we present an improved methodology when going from TB to reduced TB. By weighting all four σ TB bond integrals equally, we provide a new parameterisation (Eqs. (2.73) and (2.74)) and show that the quality of the silicon reduced TB model can be increased by choosing one of the reduced TB parameters to be distance invariant. The ingredients, the development methodology, and the quality of each of the four models are discussed in a separate chapter. The quality of the reduced TB models and BOP is demonstrated by comparing their predictions for the binding energies, heats of formation, elastic constants, and defect energies with DFT and experimental values.

Made available in DSpace on 2016-06-02T20:16:43Z (GMT). No. of bitstreams: 1 1994.pdf: 1761798 bytes, checksum: 183ce1c471cf1079a1302b309c923a68 (MD5) Previous issue date: 2002-04-11 / Financiadora de Estudos e Projetos / In the present work the Drude weight is calculated to study the capacity of conduction at finite temperatures in one-dimensional rings with the potencial given by the sequences of different degrees of randomness and threaded by a time independent magnetic flux. In this context, the tight-binding approximation is used to solve the Schrödinger equation,also permitting the potencials, given by substitution sequences, to be implemented in a simple way. We study four situations in which the potencial is given by periodic and random sequence. The transport properties exhibited by the system in each case are to reflect the degree of randomness presented by these potencials, although in a nontrivial manner if one considers a hierarchy of disorder based on the Fourier transform of the sequences. The role played by temperature in the capacity of conduction, as compared with the role played by the potencials, is also described. / No presente trabalho, através do cálculo do peso de Drude estudamos a capacidade de condução a temperaturas finitas, em anéis unidimensionais com potenciais dados por seqüências de diferentes graus de desordem e atravessados por fluxo magnético independente do tempo. Neste contexto, utilizamos o modelo tight-binding na resolução da equação de Schrödinger, o que permite inserir os potenciais, dados por seqüências de substituição, de maneira simples. Estudamos quatro casos em que esse potencial seja dado por seqüências não periódicas. Os resultados são comparados com os dois casos limites de organização, a saber, as seqüências periódica e desordenada. Observa-se que o grau de desordem apresentado por estes potenciais reflete-se nas propriedades de transporte exibidas pelo sistema em cada caso, embora de maneira não trivial se considerada uma hierarquia de desordem baseada na transformada de Fourier das seqüências. Descreve-se também a influência da temperatura, frente à influência dos potenciais, na capacidade de condução.

Made available in DSpace on 2015-03-03T15:16:23Z (GMT). No. of bitstreams: 1 LeonardoM.pdf: 1880413 bytes, checksum: c785ee7cbc933eb3ac782dc9ac382e6c (MD5) Previous issue date: 2009-12-18 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / We use a tight-binding formulation to investigate the transmissivity and the currentvoltage (I_V) characteristics of sequences of double-strand DNA molecules. In order to reveal the relevance of the underlying correlations in the nucleotides distribution, we compare theresults for the genomic DNA sequence with those of arti_cial sequences (the long-range correlated Fibonacci and RudinShapiro one) and a random sequence, which is a kind of prototype of a short-range correlated system. The random sequence is presented here with the same _rst neighbors pair correlations of the human DNA sequence. We found that the long-range character of the correlations is important to the transmissivity spectra, although the I_V curves seem to be mostly inuenced by the short-range correlations. We also analyze in this work the electronic and thermal properties along an _-helix sequence obtained from an _3 peptide which has the uni-dimensional sequence (Leu-Glu-Thr- Leu-Ala-Lys-Ala)3. An ab initio quantum chemical calculation procedure is used to obtain the highest occupied molecular orbital (HOMO) as well as their charge transfer integrals, when the _-helix sequence forms two di_erent variants with (the so-called 5Q variant) and without (the 7Q variant) _brous assemblies that can be observed by transmission electron microscopy. The di_erence between the two structures is that the 5Q (7Q) structure have Ala ! Gln substitution at the 5th (7th) position, respectively. We estimate theoretically the density of states as well as the electronic transmission spectra for the peptides using a tight-binding Hamiltonian model together with the Dyson's equation. Besides, we solve the time dependent Schrodinger equation to compute the spread of an initially localized wave-packet. We also compute the localization length in the _nite _-helix segment and the quantum especi_c heat. Keeping in mind that _brous protein can be associated with diseases, the important di_erences observed in the present vi electronic transport studies encourage us to suggest this method as a molecular diagnostic tool / Nesta tese, investigamos a transmissividade e as caracter?sticas de corrente como fun??o da diferen?a de potencial, no contexto da liga??o forte, em seq??ncias de dupla fita do DNA. Com o intuito de investigar a relev?ncia das correla??es subjacentes nas distribui??es dos nucleot?deos, comparamos os resultados de uma seq??ncia gen?mica do DNA com duas seq??ncias artificiais (Fibonacci e Rudin-Shapiro, que apresentam correla??o de longo alcance) e uma seq??ncia aleat?ria, prot?tipo de sistemas de correla??o de curto alcance. A seq??ncia aleat?ria utilizada apresenta a mesma correla??o de pares de primeiros vizinhos que a seq??ncia do DNA humano. Observamos que a caracter?stica de correla??o de longo alcance ? importante para o espectro de transmissividade, apesar das curvas IXV serem mais influenciadas por correla??es de curto alcance. Neste trabalho, analisamos tamb?m as propriedades t?rmicas e eletr?nicas de uma seq?encia α-h?lice, obtida de um pept?deo α3, o qual apresenta a seguinte seq??ncia unidimensional (Leu-Glu-Thr-Leu-Ala-Lys-Ala)3 (estrutura prim?ria). C?lculos ab initio qu?nticos s?o utilizados para obter as energias dos orbitais moleculares mais altos (HOMO, highest occupied molecular orbital), bem como suas integrais de transfer?ncias de cargas quando a seq??ncia α-h?lice forma uma estrutura fibrosa (variante 5Q) e n?o fibrosa (variante 7Q), as quais podem ser observadas atrav?s de microscopia eletr?nica de transmiss?o. A diferen?a entre as duas estruturas ? que a estrutura 5Q (7Q) apresenta a substitui??o Ala → Gln na 5a (7a) posi??o, respectivamente. N?s estimamos, teoricamente, a densidade de estado bem como o espectro de transmiss?o eletr?nico dos pept?deos, utilizando um Hamiltoniano no formalismo da liga??o-forte juntamente com a equa??o de Dyson. Al?m disso, n?s resolvemos a equa??o de Schr?dinger dependente do tempo para obter o espalhamento de um pacote de onda inicialmente localizado. N?s calculamos tamb?m o comprimento de localiza??o e, por fim, o calor espec?fico qu?ntico. Vale lembrar que a forma??o de prote?nas fibrosas podem estar associadas ? doen?as, de forma que as importantes diferen?as observadas no estudo das propriedades eletr?nicas de transporte nos encorajam a sugerir este m?todo como uma ferramenta de diagn?stico molecular

Im Folgenden werden Transferprozesse in Nanohybridsystemen theoretisch untersucht. Diese Hybridsysteme sind vielversprechende Kandidaten für neue optoelektronische Anwendungen und erfahren daher ein erhebliches Forschungsinteresse. Jedoch beschränken sich Arbeiten darüber hauptsächlich auf experimentelle Untersuchungen und kaum auf die dazugehörige theoretische Beschreibung. Bei den theoretischen Betrachtungen treten entscheidende Limitierungen auf. Es werden entweder Details auf der atomaren Ebene vernachlässigt oder Systemgrößen betrachtet, die wesentlich kleiner als im Experiment sind. Diese Thesis zeigt, wie die bestehenden Theorien verbessert werden können und erweitert die bisherigen Untersuchungen durch die Betrachtung von vier neuen hoch relevanten Nanohybridsystemen. Das erste System ist eine Nanostruktur, die aus einem Au-Kern und einer CdS-Schale besteht. Beim zweiten System wurde eine ZnO/Para-Sexiphenyl Nanogrenzfläche untersucht. Die zwei anderen Systeme beinhalten jeweils einen CdSe-Nanokristall, der entweder mit einem Pheophorbide-a-Molekül oder mit einem röhrenförmigen Farbstoffaggregat wechselwirkt. In allen Systemen ist der Anregungsenergie-Transfer ein entscheidender Transfermechanismus und steht im Fokus dieser Arbeit. Die betrachteten Hybridsysteme bestehen aus zehntausenden Atomen und machen daher eine individuelle Berechnung der einzelnen Subsysteme sowie deren gegenseitiger Wechselwirkung notwendig. Die Halbleiter-Nanostrukturen werden mit der Tight-Binding-Methode und der Methode der Konfigurationswechselwirkung beschrieben. Für das molekulare System wird die Dichtefunktionaltheorie verwendet. Die dazugehörigen Rechnungen wurden von T. Plehn ausgeführt. Das metallische Nanoteilchen wird durch quantisierte Plasmon-Moden beschrieben. Die verwendeten Theorien ermöglichen eine Berechnung von Anregungsenergietransfer in Nanohybridsystemen von bisher nicht gekannter Systemgröße und Detailgrad. / In the following, transfer phenomena in nanohybrid systems are investigated theoretically. Such hybrid systems are promising candidates for novel optoelectronic devices and have attracted considerable interest. Despite a vast amount of experimental studies, only a small number of theoretical investigations exist so far. Furthermore, most of the theoretical work shows substantial limitations by either neglecting the atomistic details of the structure or drastically reducing the system size far below the typical device extension. The present thesis shows how existing theories can be improved. This thesis also expands previous theoretical investigations by developing models for four new and highly relevant nanohybrid systems. The first system is a spherical nanostructure consisting of an Au core and a CdS shell. By contrast, the second system resembles a finite nanointerface built up by a ZnO nanocrystal and a para-sexiphenyl aggregate. For the last two systems, a CdSe nanocrystal couples either to a pheophorbide-a molecule or to a tubular dye aggregate. In all of these systems, excitation energy transfer is an essential transfer mechanism and is, therefore, in the focus of this work. The considered hybrid systems consist of tens of thousands of atoms and, consequently, require an individual modeling of the constituents and their mutual coupling. For each material class, suitable methods are applied. The modeling of semiconductor nanocrystals is done by the tight-binding method, combined with a configuration interaction scheme. For the simulation of the molecular systems, the density functional theory is applied. T. Plehn performed the corresponding calculations. For the metal nanoparticle, a model based on quantized plasmon modes is utilized. As a consequence of these theories, excitation energy transfer calculations in hybrid systems are possible with unprecedented system size and complexity.

Carbon nanotubes have been envisioned to become a very important material in various applications. This is due to the unique properties of carbon nanotubes which can be exploited in applications on length scales spanning from the nano world to our macroscopic world. For example, the electronic properties of carbon nanotubes makes them utterly suitable for nano electronics while the strength of them makes them suitable for reinforcements in plastics. Both of these applications do however require... mer the ability for systematic production of carbon nanotubes with certain properties. This is called selective carbon nanotube growth and today this has not been achieved with total success. In the work presented in the thesis several different computational methods have been applied in our contribution to the systematic search for selective carbon nanotube growth. Put in a context of previous knowledge about carbon nanotube growth our results provide valuable clues to which parameters that control the carbon nanotube growth. In association with the latest results we even dare to, with all modesty, speculate about a plausible control mechanism. The studies presented in the thesis addressed different stages of carbon nanotube growth, spanning from the properties affecting the initiation of the growth to the parameters affecting the termination of the growth. In some more detail this included studies of the melting temperatures of nanoscaled metal clusters. The expected size dependence of the melting temperatures was confirmed and the melting temperatures of clusters on substrates were seen to depend both on the material and shape of the surface. As this constitute the premises prior to the carbon nanotube growth it was followed by studies of the interaction between carbon nanotubes and metal clusters of different size and constitution. This was done using different computational methods and at different temperatures. It soon became apparent that the clusters adapted to the carbon nanotube and not vice versa. This held true irrespectively of the constitution of the cluster, that is for both pure metal and metal carbide. It was also seen that there exist a minimum cluster size that prevent the carbon nanotube end from closing. Closure of the carbon nanotube end is likely to lead to the termination of the growth which lead to studies of other reasons for growth termination, e.g., Ostwald ripening of the catalyst particles. This was investigated with the result that the rate of the Ostwald ripening may depend on both the chirality and diameter of the carbon nanotubes. It is suggested that this may provide some answers to the controlled growth of carbon nanotubes. / <p>Disputationen sker fredagen den 3 december 2010, kl. 10:15, Kollektorn, Kemivägen 9</p>

La thèse s’inscrit dans le contexte de sûreté nucléaire relatif à l'hydruration des gainages combustibles en Zircaloy, en modélisant, à l’échelle atomique, les phénomènes d’ordre chimique hydrogène - lacune atomique, sur le sous-réseau interstitiel tétraédrique des hydrures de zirconium CFC. Une telle démarche s’est déclinée en deux étapes : en premier lieu, le développement d’un modèle énergétique atomistique à la fois précis et peu coûteux numériquement, puis l’implémentation d’approches thermostatistiques de type Monte-Carlo à l’aide de ce modèle. En prenant pour point de départ un Hamiltonien de Liaisons Fortes (TB), la construction du modèle énergétique a reposé sur la dérivation d’interactions multiatomiques entre atomes d’hydrogène, à l’aide de la méthode des perturbations généralisée (GPM) basée sur une représentation de l’état de désordre interstitiel dans l’Approximation du Potentiel Cohérent (CPA). La démarche a permis de réduire l'énergie d'ordre à un modèle d'Ising effectif dérivé des liaisons fortes (TBIM), basé sur les interactions de paires effectives entre atomes d’hydrogène. Le TBIM a ensuite été validé, en comparant les énergies de structures ordonnées d’une part reconstruites en TBIM, et d’autre part obtenues par des calculs directs d’énergie totale effectués soit en Liaisons Fortes, soit par des méthodes ab initio (DFT). L'implémentation d'une approche Monte-Carlo canonique par le TBIM a permis de caractériser les différentes transitions ordre-désordre, et d'établir un diagramme de phase de l’ordre chimique hydrogène - lacune atomique, sur le sous-réseau interstitiel tétraédrique des hydrures de zirconium CFC. / The thesis addresses the nuclear safety issue of Zircaloy fuel cladding hydruration, by modelling, at atomistic scale, chemical ordering processes between hydrogen and atomic vacancies on tetrahedral interstitial sublattice of CFC zirconium hydrides. This has been achieved into two steps : first the development of an atomistic energetic model sufficiently precise and not too much CPU time consuming, and secondly its implementation in Monte-Carlo thermostatistical simulations. Starting from a Tight-Binding (TB) Hamiltonian, the energetic model has been derived from the calculation of multiatomic interactions between hydrogen atoms, using the Generalized Perturbation Method (GPM) applied to an interstitial disorder described within the Coherent Potential Approximation (CPA). The path allows us to reduce the ordering energy to a Tight-Binding Ising Model (TBIM), based on effective pairwise interactions between hydrogen atoms. The TBIM has been validated by comparing ordering energies of ordered structures either reconstructed using TBIM, or directly obtained from total energy calculations perfor- med both within TB and ab initio (DFT) methods.By implementing a canonical Monte-Carlo with TBIM, we obtain different order-disorder phase transitions, and a phase diagram of H-vacancy chemical ordering, on the tetrahedral interstitial sublattice of CFC zirconium hydrides.

The electronical and structural properties of TlSe-type chain-like crystals are the main topic of this study. A computational method which is Tight Binding method is introduced and used to obtain the electronic band structure of TlSe and TlInSe2 . For both materials the partial and total density of states are calculated. The results are compared with the other theoretical results.