Seit ihrer Erfindung in 1994 haben sich Quantenkaskadenlaser (QCL) zu der Standard-Halbleiterlaserquelle im mittleren und weiten Infrarotspektrum entwickelt. Diese unipolaren Laser basieren auf der Populations-Inversion zwischen quantisierten sub-Bändern in Halbleiterheterostrukturen. Ein gutes theoretisches Modell ist essenziell für die Optimierung und weitere Entwicklung von neuen QCL Laserquellen. Eine einfache Methode, Elektronentransport in QCL zu beschreiben, stützt sich auf ein phänomenologisches Modell für die Streuraten zwischen elektronischen sub-Bändern. Das Hauptziel dieser Arbeit ist die Entwicklung eines kompakten Ansatzes für Streuraten für die effiziente Vorhersage der temperaturabhängigen Charakteristika von QCLs im mittleren Infrarotspektrum. Die Arbeit beginnt mit einem kurzen Überblick über Halbleiterheterostrukturen und die wichtigsten Streumechanismen für Übergänge zwischen sub-Bändern in QCLs. Dabei sind elastische Übergänge sowie Phononenstreuung für die Übergangsraten zwischen verschiedenen sub-Bändern relevant. Außerdem werden die notwendigen Modellierungstechniken für Simulationsprozesse in QCLs mit einem selbst-konsistenten Streuraten-Modell vorgestellt. In dieser Arbeit wurde ein vereinfachtes Modell für vertikalen Elektronentransport zwischen sub-Bändern bei der Temperatur von Flüssigstickstoff entwickelt. Die Übergangsrate ist in diesem Ansatz das Produkt des Überlappintegrals der quadrierten Moduli der einhüllenden Funktion und einem phänomenologischen Faktor, der von der Übergangsenergie abhängt. Der Übergangsfaktor wird für verschiedene Übergangsmechanismen einzeln hergeleitet, und eine Erweiterung des Modells auf einen breiten Temperaturbereich wird vorgestellt. Schließlich analysieren wir die sogenannte T0-Charakteristik für einige Designs der aktiven Region, die aus Rechnungen mit vorhandenen temperaturabhängigen Modellen und experimentellen Daten gewonnen wurden. / Since their invention in 1994, quantum cascade lasers (QCLs) have become the standard semiconductor laser source for the mid- and far-infrared spectral range. These unipolar devices are based on the population inversion between quantized subbands in biased semiconductor heterostructures. A useful theoretical model is essential for the optimization and further development of new QCL sources. A simple method for describing the electron transport in QCL is based on scattering rates between electron subbands. These can be described easiest using a phenomenological model with experimental or empirical parameters. The main goal of this work is development of compact description of scattering processes in the frame of scattering-rate approach for the reliable prediction of temperature dependent characteristics of mid-infrared quantum cascade lasers. We start this work with a brief overview of semiconductor heterostructures and main intersubband scattering mechanisms for quantum cascade lasers. The resulting transition rates from initial states to another subbands are described by phonons and elastic scattering. Additionally, necessary modeling techniques are considered for simulation processes in QCLs using self-consistent scattering-rate model. Based on original work we introduce a simplified model for vertical electron transport between separated subbands at liquid nitrogen temperatures. In this approach the transition rate is written as the product of the overlap integral for the squared moduli of the envelope functions and a phenomenological factor that depends on the transition energy. The approach is reviewed and extended for a broad temperature range. There, the transition factor is derived and written for different scattering mechanisms separately. Then we analyze “so-called” T0 characteristic for a number of active region designs received from the calculations by present temperature dependent model and the experimental data.

In this thesis, a first-principles methodology to investigate the impact of electron-phonon interactions on the charge-carrier mobilities in organic molecular crystals has been developed. Well-known organic materials such as oligoacene and oligothienoacene derivatives were studied in detail. The nature of the intramolecular vibronic coupling in oligoacenes and oligothienoacenes was studied using an approach that combines high-resolution gas-phase photo-electron spectroscopy measurements with first-principles quantum-mechanical calculations. The electron interactions with optical phonons in oligoacene single crystals were investigated using both density functional theory and empirical force field methods. The low-frequency optical modes are found to play a significant role in dictating the temperature dependence of the charge-transport properties in the oligoacene crystals. The microscopic charge-transport parameters in the pentathienoacene, 1,4-diiodobenzene, and 2,6-diiodo-dithieno[3,2-<i>b</i>:2',3'-<i>d</i>]thiophene crystals were also investigated. It was found that the intrinsic charge transport properties in the pentathienoacene crystal might be higher than that in two benchmark high-mobility organic crystals, i.e., pentacene and sexithienyl. For 1,4-diiodobenzene crystal, a detailed quantum-mechanical study indicated that its high mobility is primarily associated with the iodine atoms. In the 2,6-diiododithieno[3,2-<i>b</i>:2',3'-<i>d</i>]thiophene crystal, the main source of electronic interactions were found along the π-stacking direction. For negatively charged carriers, the halogen-functionalized molecular crystals show a very large polaron binding energy, which suggests significantly low charge-transport mobility for electrons.

Les multicouches de graphène en empilement rhomboédrique sont considérés comme une phase prometteuse du carbone. Cela est due à la particularité de cette phase de pouvoir exhiber des états à forte corrélation électronique comme le magnétisme ou la supraconductivité à haute température critique. Ce qui est due, a son tour, à l’occurrence d’un état de surface avec une dispersion d’énergie électroniques quasi-nulle à proximité du niveau de Fermi. Malgré que le graphite Bernal soit la forme la plus stable du graphite, des échantillons a trois et quatre couches de graphène en empilement rhomboédrique ont pu être synthétisés. Plus récemment, des flocons d’épaisseur dépassant les 17 couches ont été isolés et provisoirement attribués à des séquences d’empilement rhomboédrique. Cette attribution à été faite via des expériences de spectroscopie Raman sous champ magnétique, bien que l’empreinte Raman des multicouche de graphène en empilement rhomboédrique est actuellement inconnue. Même le cas simple du spectre Raman résonnant à deux phonons (le pic 2D) du graphite Bernal n’est pas totalement compris. Dans ce travail de thèse, nous fournissons une description ab-initio complète du pic Raman 2D dans les systèmes de graphène à trois et quatre couches pour tous les empilements possibles, ainsi que pour le graphite Bernal, rhomboédrique et une alternance de graphite Bernal et rhomboédrique. / Multi-layer graphene with rhombohedral ABC stacking is considered as a promising carbon phase possibly displaying correlated states like magnetism or high-T c superconductivity due to the occurrence of an ultraflat electronic surface band at the Fermi level. Despite Bernal graphite being the most stable form of graphite, three and four layers graphene samples with rhombohedral stacking can be synthesized. Recently, flakes of thickness up to 17 layers were tentatively attributed ABC sequences although the Raman fingerprint of rhombohedral multilayer graphene is currently unknown and the 2D two-phonon resonant Raman spectrum of Bernal graphite not completely theoretically understood. Here we provide a complete first principles description of the 2D Raman peak in three and four layer graphene for all possible stackings, as well as for bulk Bernal, rhombohedral and an alternation of Bernal and rhombohedral graphite, that can be seen as a periodic sequence of ABA and ABC trilayers. Calculations for several laser energies are performed and we give practical prescriptions are proposed to identify long range sequences of ABC multi-layer graphene flakes.

In dieser Doktorarbeit untersuchen wir den Einfluss von Elektron-Phonon-Wechselwirkungen (EPW) auf die Bandlueckenrenormierung in kristallinen Festkoerpern bei endlichen Temperaturen. Das Hauptziel besteht darin, den Einfluss der Kernbewegung und der thermischen Ausdehnung des Gitters auf die Bandstruktur in einer Vielzahl von Materialien zu quantifizieren. Zu diesem Zweck wird der Temperatureinfluss auf das EPW in harmonischen Naeherungen unter Verwendung der stochastischen Abtastmethode und vollstaendig anharmonisch durch Durchführung von ab initio Molekulardynamiksimulationen (aiMD). Die Bandluecke bei endlichen Temperaturen wird aus der thermodynamisch gemittelten Spektralfunktion extrahiert, die unter Verwendung der Bandentfaltungstechnik berechnet wird. Waehrend die Verwendung von aiMD bereits fuer Berechnungen von EPW verwendet wurde, wurde die Kombination von aiMD und Bandentfaltung zur Behandlung der Bandluecken renormalisierung erst kuerzlich verwendet. In dieser Doktorarbeit haben wir eine verbesserte Bandentfaltungstechnik verwendet, um die Berechnung effektiv zu verwalten. Diese verbesserte Methode enthaelt mehrere methodische Neuerungen, die dazu dienen, den Rechenaufwand zu verringern und das statistische Rauschen in den Endergebnissen zu minimieren. Die aktualisierte Methode wurde gruendlich bewertet, dokumentiert und mit einer benutzerfreundlichen Oberflaeche gestaltet. Wir praesentieren eine umfassende Untersuchung der numerischen Aspekte der thermodynamischen Mittelung, der Schaetzung von Fehlerbalken und der Bewertung der Konvergenz in Bezug auf die Groesse der Simulationssuperzelle. Unser etabliertes Protokoll ermoeglicht die Berechnung der Bandlückenrenormierung bei endlichen Temperaturen, was in guter Uebereinstimmung mit frueheren theoretischen Studien und experimentellen Daten steht. / In this thesis, we investigate the influence of electron-phonon interactions (EPI) on the band gap renormalization in crystalline solids at finite temperatures. The main goal is to identify the impact of the nuclear motion and the lattice thermal expansion on the band structure in a wide range of materials. For this purpose, the temperature influence on the EPI is calculated in the harmonic approximations by utilizing the stochastic sampling methodology and fully anharmonically, by performing ab initio molecular dynamics simulations (aiMD). The band gap at finite temperatures is extracted from the thermodynamically averaged spectral function, which is calculated using band-unfolding technique. While utilization of aiMD was already used for calculations of EPI the combination of aiMD and band-unfolding to treat the band gap renormalization was used only recently. In this thesis, we employed an improved band unfolding technique in order to effectively manage the calculations. This improved method incorporates several methodological innovations that serve to mitigate computational cost and minimize statistical noise in the final results. The updated method was thoroughly benchmarked, documented, and designed with a user-friendly interface. We present a comprehensive examination of the numerical aspects of thermodynamic averaging, the estimation of error bars, and the evaluation of convergence with respect to the size of the simulation supercell. Our established protocol enables the calculation of band gap renormalization at finite temperatures, which is in good agreement with prior theoretical studies and experimental data.

Nano-manufacturing is receiving significant attention in industry due to the ever-growing interest in nanotechnology in research institutions. It is hypothesized that single-step or direct-write nano-scale machining might be achieved by coupling nano-probe field emission with radiation transfer. A laser may be used to heat a workpiece within a microscopic region that encloses an even smaller nanoscopic region subjected to a focused electron beam. The electron-beam supplies marginal heat sufficient to remove a minute volume of material by evaporation or sublimation. Experimentally investigating this hypothesis requires an estimate of the power needed in the electron-beam. To this end, a detailed numerical study is conducted to study the possibility of using the nano-probe field emission for nano-machining. The modeling effort in this case is divided into two parts. The first part deals with the electron-beam propagation inside a target workpiece. The second part considers the temperature increase due to the energy transfer between the electron-beam and the workpiece itself. A Monte Carlo/Ray Tracing technique is used in modeling the electron-beam propagation. This approach is identical to that of a typical Monte Carlo simulation in radiative transfer, except that proper electron scattering properties are employed. The temperature distribution inside a gold film is predicted using the heat conduction equations. Details of the various numerical models employed in the simulation and a series of representative results will be presented in this dissertation.

No description available.

No description available.

Despite over 100 years of scientific research, a full understanding of superconductivity remains elusive. While it is known that the electron-phonon interaction is responsible for the formation of Cooper pairs in conventional superconductors, many superconductors exhibit behaviour suggestive of more exotic pairing mechanisms. In this thesis, two novel superconducting materials are considered, monolayer transition metal dichalcogenide, MoS₂, and iron-based superconductor, LaFeAsO_1−xF_x. The former is ideal for the study of the electron-phonon interaction, as it not only has potential applications as an atomically thin transistor, but also displays a dome-shaped superconductive state as a function of electron doping. In the latter, the superconductive state emerges from a magnetic parent compound upon flourine doping. Its high critical temperature is thought to be enhanced by magnetic fluctuation rather than being purely phonon-mediated. By using novel first-principles techniques, the electron-phonon interaction in electron doped single-layer MoS₂ is investigated. The superconducting gap is calculated using the Migdal-Eliashberg theory, and by considering the electronic structure and lattice dynamics in this material, an explanation is provided for the experimentally observed doping-dependent critical temperature in this material. The origin of the doping-induced transition from a magnetic phase to a nonmagnetic phase in LaFeAsO_1−xF_x is determined. A new model to capture the effects of the fluorine dopants is developed, which has implications for the electron-phonon interaction in this material.

The technological interest in a material depends very much on its electrical, magnetic, optical and/or mechanical properties. In carbon nanotubes the atoms form a cylindrical structure with a diameter of the order 1 nm, but the nanotubes can be up to several hundred micrometers in length. This makes carbon nanotubes a remarkable model for one-dimensional systems. A lot of efforts have been dedicated to manufacturing carbon nanotubes, which is expected to be the material for the next generation of devices. Despite all the attention that carbon nanotubes have received from the scientific community, only rather limited progress has been made in the theoretical understanding of their physical properties. In this work, we attempt to provide an understanding of the electron-phonon and electron-defect interactions in semiconducting zigzag carbon nanotubes using a tight-binding approach. The electronic energy dispersion relations are calculated by applying the zone-folding technique to the dispersion relations of graphene. A fourth-nearest-neighbour force constant model has been applied to study the vibrational modes in the carbon nanotubes. Both the electron-phonon interaction and the electron-defect interaction are formulated within the tight-binding approximation, and analyzed in terms of their quantum mechanical scattering rates. Apart from the scattering rates, their components in terms of phonon absorption, phonon emission, backscattering and forward scattering have been determined and analyzed. The scattering rates for (5,0), (7,0), (10,0), (13,0) and (25,0) carbon nanotubes at room temperature and at 10K are presented and discussed. The phonon scattering rate is dependent on the lattice temperature in the interval 0-0.17 eV. We find that backscattering and phonon emission are dominant over forward scattering and phonon absorption in most of the energy interval. However, forward scattering and phonon absorption can be comparable to backscattering and phonon emission in limited energy intervals. The phonon modes associated with each peak in the electron-phonon scattering rates have been identified, and the similarities in the phonon scattering rates between different nanotubes are discussed. The dependence of the defect scattering rate on the tube diameter is similar to that of the phonon scattering rate. Both the phonon and the defect scattering rates show strong dependence on the tube diameter (i.e., the scattering rate decreases as a function of the index of the nanotube). It is observed that the backscattering and forward scattering for electrons interacting with defects occur with same frequency at all energies, in sharp contrast to the situation for phonon scattering. It is demonstrated that the differences in the scattering rate between different tubes are mainly due to the differences in their band structures.

Processos fotofísicos em polímeros conjugados estão intimamente relacionados com a conformação dos segmentos moleculares. Filmes finos de polímeros conjugados apresentam uma anisotropia intrínseca resultante da conformação dos segmentos moleculares tornando esses materiais atrativos em estudos fotofísicos pela emissão polarizada apresentada quando excitado via radiação eletromagnética ou sob aplicação de uma diferença de potencial elétrico. Neste trabalho procuramos correlacionar o espectro de emissão fotoluminescente de filmes finos de um derivado do polifluoreno, nominalmente poli(9,9-dioctilfluorenil-2-7-diil) terminado com dimetilfenil, com sua anisotropia molecular. Filmes finos mecanicamente estirados sofrem um processo de reordenamento molecular induzindo a emissão de luz polarizada predominantemente na direção de estiramento. O estiramento ocasiona ainda um aumento no comprimento de conjugação efetivo dos segmentos moleculares influenciando no acoplamento elétron-fônon. Através da técnica de elipsometria, foi possível determinar os estados de polarização da luz (através dos parâmetros de Stokes) e medidas de fotoluminescência estacionária dependente da temperatura nos possibilitou aferirmos sobre o acoplamento elétron-fônon a partir do Princípio de Franck-Condon. Medidas de fotoluminescência de excitação (PLE) determinou que o espectro da PL consiste da sobreposição espectral de duas espécies emissoras: a espécie isolada e a espécie agregada. Para baixas temperaturas a PL apresenta picos de emissão bem definidos como resultado da dinâmica molecular do PFO correspondendo ao favorecimento de emissão da espécie isolada. Para temperaturas acima da temperatura de transição \'beta\' (~270 K), a emissão da espécie agregada é favorecida, ocorrendo uma possível transferência de energia da espécie isolada para a agregada. O estiramento induz um aumento do comprimento de conjugação, refletido na diminuição do fator de Huang-Rhys, \'S IND. ISO\'POT. LO\'|140 K = 0,40 para amostra não estirada e \'S IND. ISO\'POT.2LO\'| 140 K = 0,19 para a amostra com a maior taxa de estiramento, tornando o espectro mais resolvido. Amostras não estiradas sob excitação paralela ao estiramento apresentaram polarização total de emissão P = 3,4% linearmente paralela ao estiramento e anisotropia de fluorescência de r = 0,025 e amostras com estiramento L = 2Lo apresentaram P = 46,1% de emissão polarizada ao longo da direção de estiramento e uma anisotropia de fluorescência de r = 0,27. A emissão polarizada mostrou ser independente da temperatura. A anisotropia de fluorescência mostrou ser fortemente dependente do estiramento e da anisotropia para temperaturas acima de 340 K, temperatura característica de um inicio de transição de fase do PFO. / Photophysics processes in conjugated polymer are closely related with the molecular segments conformation. Conjugated polymers thin films has shown an intrinsic anisotropy due to the molecular segments conformation making this materials attractive in photophysics studies by its polarized emission when stimulated by light or biased. In this work, we correlated the photoluminescence spectra of a derivative PFO polymer thin films, namely poly(9,9-dioctylfluorenyl-2,7-diyl) end capped with dimethylphenyl, with the molecular anisotropy. Mechanically stretched thin films undergo a molecular rearrangement process of inducing emission of light predominantly polarized in the direction of stretch. The stretching also causes an increase in the effective conjugation length of the molecular segments influencing the electron-phonon coupling. By ellipsometry technique, it was possible to determine the polarization states of light (by the Stokes parameters) and temperature dependent stationary photoluminescence measurements enabled us to get the electron-phonon coupling from the Franck-Condon principle. Measurements of photoluminescence excitation (PLE) have determined that the PL spectrum consists of spectral overlap of the two emitting species: the isolated and aggregated species. At low temperatures the PL emission peaks has presented well-defined as a result of PFO molecular dynamics favoring the emission of the isolated species. For temperatures above the transition beta temperature (270 K), the emission of aggregated species is favored, causing a possible energy transfer isolated to aggregate species. The stretching induces an increase in the conjugation length, reflected in the decreasing Huang-Rhys factor \'S IND. ISO\' POT. LO\'|140 K = 0,40 to non-stretched samples and \'S IND. ISO\' POT. 2Lo\'| 140 K = 0,19 for the sample with the highest draw ratio, making the spectrum more resolved. Unstretched samples under polarized excitation parallel to the stretching showed total polarized emission P = 3,4% linearly parallel to the stretching and fluorescence anisotropy of r = 0,025 and the L = 2Lo samples showed P = 46,1% of polarized emission along the direction of stretching and fluorescence anisotropy r = 0,27. The polarized emission was found to be independent of temperature. The fluorescence anisotropy was found to be strongly dependent of stretching rates and for temperatures above 340 K, a characteristic onset temperature of phase transition of the PFO.