Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In the present study, the implementation and parameterization of the dispersion correction functions, denominated DM1 and DM2, was carried out to the quantum semiempirical methods RM1 and PM6, in the program Mopac2007. The aim of this study is to improve the description of weak interactions by the methods above mentioned with the addition of na energy correction. The DM1 methodology contains 7 parameters while the DM2 model presents 13 parameters for the correction function. Unlike previous work, the correction was used to adjust not only the energy, but also the geometric parameters. The parameterization was performed using a training set composed by 10 structures from S22 database and 24 structures from S66 database. The results show that RM1-DM1 and RM1- DM2 improved the total response function when compared to RM1, the RM1- DM1 is better at describing the geometries of the complexes on the other hand, the RM1-DM2 is more efficient in reproducing the dispersion energy. The PM6- DM1 and PM6-DM2 methods, for the test set, had a smaller response function than PM7 and PM6 and their extensions with correction of dispersion forces. The best response function for PM6-DM1 and PM6-DM2 is related to the description of the geometries of the complexes, with the dispersion energy being slightly higher than that obtained by the PM6-DH2 method. The semiempirical methods discussed here, together with the theory of density functional, with the most popular B3LYP functional, were used to calculate the geometry of cyclodextrin/guest complexes. The results obtained, with semiempirical methods, showed that AM1, PM3, PM6-DM1 and PM6-DM2 presented the lowest values of RMSD and variability for a set of 43 cyclodextrins. The DFT methodology was used to calculate the geometry of a set of 9 cyclodextrins/guest complexes, the result was compared to semiempirical methods. The best DFT result was obtained with the 6-31G base, it is similar to obtained by the RM1 method. For this set, the methods AM1, PM6-DM1 and PM6-DM2 presented the best results in the description of the geometries. / No presente estudo foi realizada a implementação e parametrização das funções de correção de dispersão, denominadas DM1 e DM2, aos métodos semiempíricos quânticos RM1 e PM6, no programa Mopac2007. O estudo tem o como finalidade melhorar a descrição de interações fracas pelos métodos supracitados com a adição de uma correção de energia. A metodologia DM1 contém 7 parâmetros enquanto, o modelo DM2 apresenta 13 parâmetros para a função de correção. Diferente de trabalhos anteriores a correção foi usada para ajustar, não somente, a energia, mas também parâmetros geométricos. A parametrização foi executada usando um conjunto treinamento que usa 10 estruturas do banco de dados S22 e 24 estruturas do banco de dados S66. Os parâmetros obtidos através do conjunto treinamento foram aplicados ao conjunto teste, o qual é formado pelas 54 estruturas que não fizeram parte do conjunto treinamento. Os resultados mostram que RM1-DM1 e RM1-DM2 melhoraram a função resposta total quando comparado ao RM1, sendo o RM1-DM1 melhor em descrever as geometrias dos complexos e RM1-DM2 mais eficiente em reproduzir a energia de dispersão. Os métodos PM6-DM1 e PM6-DM2 para o conjunto teste apresentaram uma função resposta menor do que o PM7 e PM6 e suas extensões com correção das forças de dispersão. A melhor função resposta para PM6-DM1 e PM6-DM2 está relacionada a descrição das geometrias dos complexos, sendo a energia de dispersão um pouco maior do que a obtida pelo método PM6-DH2. Os métodos semiempíricos, aqui abordados, junto com a teoria do funcional da densidade, com o funcional mais popular B3LYP, foram usados para calcular a geometria de complexos ciclodextrina/convidado. Os resultados obtidos com os métodos semiempíricos mostraram que os métodos AM1, PM3, PM6-DM1 e PM6-DM2 apresentam os menores valores de RMSD e variabilidade para um conjunto de 43 ciclodextrinas. A metodologia DFT foi usada para calcular a geometria para um conjunto de 9 complexos ciclodextrinas/convidado, o resultado foi comparado aos métodos semiempíricos. O melhor resultado DFT foi obtido com a base 6-31G, esse é semelhante ao obtido pelo método RM1. Para esse conjunto os métodos AM1, PM6-DM1 e PM6-DM2 apresentaram os melhores resultados na descrição das geometrias. / São Cristóvão, SE

The continuous migration to smaller feature sizes puts high demands on materials and technologies for future ultra-large-scale integrated circuits. Particularly, the copper-based interconnect system will reach fundamental limits soon. Their outstanding properties make metallic carbon nanotubes (CNTs) an ideal material to partially replace copper in future interconnect architectures. Here, a low contact resistance to existing metal lines is crucial. The present thesis contributes to the theory and numerical description of electronic transport in metallic CNTs with metal contacts. Different theoretical approaches are applied to various contact models and electrode materials (Al, Cu, Pd, Ag, Pt, Au) are compared. Ballistic transport calculations are based on the non-equilibrium Greens function formalism combined with tight-binding (TB), extended Hückel theory (EHT) and density functional theory (DFT). Simplified contact models allow a qualitative investigation of both the influence of geometry and CNT length, and the strength and extent of the contact on the transport properties. In addition, such simple contact models are used to compare the influence of different electronic structure methods on transport. It is found that the semiempirical TB and EHT are inadequate to quantitatively reproduce the DFT-based results. Based on this observation, an improved set of Hückel parameters is developed, which remedies this insufficiency. A systematic investigation of different contact materials is carried out using well defined atomistic metal-CNT-metal structures, optimized in a systematic way. Analytical models for the CNT-metal interaction are proposed. Based on that, electronic transport calculations are carried out, which can be extended to large systems by applying the computationally cheap improved EHT. The metal-CNT-metal systems can then be ranked by average conductance: Ag ≤ Au < Cu < Pt ≤ Pd < Al. This corresponds qualitatively with calculated contact distances, binding energies and work functions of CNTs and metals. To gain a deeper understanding of the transport properties, the electronic structure of the metal-CNT-metal systems and their respective parts is analyzed in detail. Here, the energy resolved local density of states is a valuable tool to investigate the CNT-metal interaction and its influences on the transport. / Die kontinuierliche Verkleinerung der Strukturgrößen stellt hohe Anforderungen an Materialen und Technologien zukünftiger hochintegrierter Schaltkreise. Insbesondere die Leistungsfähigkeit kupferbasierte Leitbahnsystem wird bald an fundamentale Grenzen stoßen. Aufgrund ihrer hervorragenden Eigenschaften könnten metallische Kohlenstoffnanoröhren (engl. Carbon Nanotubes, CNTs) Kupfer in zukünftigen Leitbahnsystemen teilweise ersetzen. Dabei ist ein geringer Kontaktwiderstand mit vorhandenen Leitbahnen von entscheidender Bedeutung. Die vorliegende Arbeit liefert grundlegende Beiträge zur Theorie und zur numerischen Beschreibung elektronischer Transporteigenschaften metallischer CNTs mit Metallkontakten. Dazu werden verschiedene theoretische Ansätze auf diverse Kontaktmodelle angewandt und eine Auswahl von Elektrodenmaterialen (Al, Cu, Pd, Ag, Pt, Au) verglichen. Die Beschreibung ballistischen Elektronentransports erfolgt mittels des Formalismus der Nichtgleichgewichts-Green-Funktionen in Kombination mit Tight-Binding (TB), erweiterter Hückel-Theorie (EHT) und Dichtefunktionaltheorie (DFT). Vereinfachte Kontaktmodelle dienen der qualitativen Untersuchung des Einflusses von Geometrie und Länge der Nanoröhren, sowie von Stärke und Ausdehnung des Kontaktes. Darüber hinaus erlauben solch einfache Modelle mit geringem numerischen Aufwand den Einfluss verschiedener Elektronenstrukturmethoden zu untersuchen. Es zeigt sich, dass die semiempirischen Methoden TB und EHT nicht in der Lage sind die Ergebnisse der DFT quantitativ zu reproduzieren. Ausgehend von diesen Ergebnissen wird ein verbesserter Satz von Hückel-Parametern generiert, der diesen Mangel behebt. Die Untersuchung verschiedener Kontaktmaterialien erfolgt an wohldefinierten atomistischen Metall-CNT-Metall-Strukturen, welche systematisch optimiert werden. Analytische Modelle zur Beschreibung der CNT-Metall-Wechselwirkung werden vorgeschlagen. Darauf aufbauende Berechnungen der elektronischen Transporteigenschaften, können mit Hilfe der verbesserten EHT auf große Systeme ausgedehnt werden. Die Ergebnisse ermöglichen eine Reihung der Metall-CNT-Metall-Systeme hinsichtlich ihrer Leitfähigkeit: Ag ≤ Au < Cu < Pt ≤ Pd < Al. Dies korrespondiert qualitativ mit berechneten Kontaktabständen, Bindungsenergien und Austrittarbeiten der CNTs und Metalle. Zum tieferen Verständnis der Transporteigenschaften erfolgt eine detaillierte Analyse der elektronischen Struktur der Metall-CNT-Metall-Systeme und ihrer Teilsysteme. Dabei erweist sich die energieaufgelöste lokale Zustandsdichte als nützliches Werkzeug zur Visulisierung und zur Charakterisierung der Wechselwirkung zwischen CNT und Metall sowie deren Einfluss auf den Transport.

In this work, the structural, electronic, and optical properties of CdS nanoparticles with sizes up to 4nm have been calculated using density-functional theory (DFT). Inaccuracies in the description of the unoccupied states of the applied density-functional based tight-binding method (DFTB) are overcome by a new SCF-DFTB method. Density-functional-based calculations employing linear-response theory have been performed on cadmium sulfide nanoparticles considering different stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), particle shapes (spherical, cuboctahedral, tetrahedral), and saturations (unsaturated, partly saturated, completely saturated). For saturated particles, the calculated onset excitations are strong excitonic. The quantum-confinement effect in the lowest excitation is visible as the excitation energy decreases towards the bulk band gap with increasing particle size. Dangling bonds at unsaturated surface atoms introduce trapped surface states which lie below the lowest excitations of the completely saturated particles. The molecular orbitals (MOs), that are participating in the excitonic excitations, show the shape of the angular momenta of a hydrogen atom (s, p). Zincblende- and wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctions strongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Finally, a very good agreement of the calculated data to experimentally available spectra and excitation energies is found.