In this work, wecarry out theoretical study of the silicon surface reconstruction (7x7) grown in the [111] direction, denoted by Si(111) (7x7), using the Density Functional Theory formalism, within the Local Density Approximation (LDA) for the exchange correlation term. The interaction with the valence electrons and the core ion (nucleus more core electrons) was described by norm conserving, non local pseudopotential approach, compatible with the LDA. The one electron Kohn Sham equations was solved self consistently, by expanding the one electron functions in terms of linear combination of numerical atomic orbitals. All the calculations were realized using the Siesta computational code. Using the clean surface reconstruction model of Si(111) (7x7), propose by Takayanagi et al.[1, 2], we modeled the surface as a slab, in which the (7x7) unitary cell contained 200 Si atoms distributed in one adatom layer (Si atoms on the topmost layer) and the other four layers, with 49 H atoms used for saturate the dangling bonds of the most internal layer of the slab. The structural analysis of the Si(111) (7x7) reconstructed surface and the electronic structure analysis showed a good agreement with both the experimental and theoretical results disposable, reproducing correctly the band structure, the surface levels and the metallic character of this surface. The obtained energy per surface atom was 1,132 eV with respect to the bulk energy. It was performed the study of the vacancy energy formation of adatoms on this surface. We find an average value of 1,2 eV of a single vacancy formation of adatom the Si(111) (7x7) surface. The analysis of the energy bands made possible to identify the electronic states due to the adatoms dangling bonds. As a first application of the obtained results to the Si(111) (7x7) clean surface, we investigated the possible states of the physisorption of the chlorobenzene molecules on specific sites on the Si(111) (7x7) surface. This study showed that chlorobenzene molecules interact with the surface for distances around 3,0 Å from the adatoms. We also inferred that the triangular faulted subunit sites are energetically most favorable to the adsorption than the correspondent triangular unfaulted subunit sites. We plotted the adsorption energy curve of the chlorobenzene on the corner adatom at the triangular faulted unitary cell (7x7). The equilibrium distance and the adsorption energy obtained was 3,005 Å and 0,161 eV, respectively. The magnitude of this interaction corresponds to a physics adsorption of the chlorobenzene molecule on the Si(111) (7x7) surface. / Neste trabalho, realizamos o estudo teórico da reconstrução (7x7) da superfície de Silício crescida na direção [111], denotada por Si(111) (7x7), utilizando para isto o formalismo da Teoria do Funcional da Densidade, com a Aproximação da Densidade Local (LDA - LocalDensityApproximation) para o termo de troca correlação. A interação entre os elétrons de valência e o íon de caroço (núcleo mais os elétrons de caroço) foi descrita por meio da Teoria dos Pseudopotenciais não locais de norma conservada, compativel com a LDA. As equações de Kohn Sham de um elétron foram resolvidas autoconsistentemente, expandindo as funções de um elétronemtermos de combinação linear de orbitais atômico numéricos, com base double . Todos os cálculos foram realizados utilizando o código computacional Siesta. Utilizando o modelo de reconstrução da superfície livre de Si(111) (7x7), proposto por Takayanagi et al.[1, 2], modelamos a superfície como um slab, cuja célula unitária (7x7) contém 200 átomos de Si distribuidos em uma camada de adatoms (átomos de Si adsorvidos sobre a superfície propriamente dita) e quatro outras camadas, além dos 49 átomos de H usados para saturar as ligações pendentes da camada mais interna ao material. A análise estrutural da superfície reconstruída Si(111) (7x7) e a análise da estrutura eletrônica mostrou ótima concordância tanto com trabalhos experimentais quanto teóricos, reproduzindo corretamente a estrutura de bandas, os níveis de superfície e o caráter metálico desta superfície. A energia obtida em nosso cálculo por átomo da superfície, com relação à energia bulk, foi de 1,132 eV. Foi feito o estudo da energia de formação de vacâncias do tipo adatom. Encontramos um valor médio de 1,2 eV para a formação de uma única vacância do tipo adatom na superfície Si(111) (7x7). A análise da estrutura de bandas do sistema com vacância possibilitou identificar os estados eletrônicos devido às ligações pendentes dos adatoms. Como uma primeira aplicação dos resultados obtidos para a superfície livre de Si(111) (7x7), investigamos possíveis estados de fisiossorção de moléculas de clorobenzeno sobre sítios específicos sobre a superfície de Si(111) (7x7). Este estudo mostrou que as moléculas de clorobenzeno interagem com a superfície para distâncias de aproximadamente 3,0 Å dos adatoms. Também inferimos que sítios da subunidade triangular faulted são mais favoráveis à adsorção do que sítios correspondentes sobre a subunidade triangula unfaulted. Levantamos a curva da energia de adsorção do clorobenzeno sobre o adatom de canto da subunidade triangular faulted da célula unitária (7x7). A distância de equilíbrio e a energia de adsorção obtidas foram 3,005 Å e 0,161 eV, respectivamente. A ordem de grandeza desta interação corresponde a uma adsorção física da molécula de clorobenzeno sobre a superfície Si(111) (7x7). / Mestre em Física

Este trabalho apresenta o primeiro estudo dos estados aniônicos de baixa energia da para-Benzoquinona (p-BQ) por meio de cálculos de espalhamento elástico de elétrons pela molécula neutra. Foi utilizado o método multicanal de Schwinger implementado com pseudopotenciais de Bachelet, Hamann e Schlüter, na aproximação de núcleos fixos, e nas aproximações estático-troca e estático-troca mais polarização. As seções de choque integrais foram calculadas em duas geometrias do alvo, otimizadas para o estado fundamental da molécula neutra e do ânion. Os estados aniônicos encontrados foram aracterizados e comparados com resultados experimentais, realizados com a técnica de espectroscopia de transferência eletrônica (ETS). Três estados * foram observados, sendo um ligado e dois metaestáveis, em acordo com os experimentos. Uma ressonância de Feshbach foi encontrada nos cálculos, tal qual é observada experimentalmente e sugerida em diversos estudos teóricos de estado ligado. Acredita-se que a aproximação utilizada para descrever estados excitados do alvo poderá ser reconsiderada, em que é sugerida uma proximação mais sofisticada, a ser utilizada em cálculos futuros. Com este estudo, é possível uma melhor compreensão dos resultados experimentais ETS, assim como é permitida uma interpretação de estudos teóricos sobre os estados aniônicos da p-BQ. Ademais, este trabalho possibilita próximos estudos de espalhamento envolvendo quinonas mais complexas. / This work present the first study of low-energy anionic states in para-benzoquinone (p-BQ) by electron scattering calculations. The elastic scattering calculations were done in the fixed-nuclei approximation, with the Schwinger multichannel method implemented with pseudopotentials of Bachelet, Hamann and Schlüter, in the static-exchange and in the static-exchange plus polarization approximations. The integral cross sections were performed in two target geometries optimized for the neutral and anion ground states. The anionic states found were characterized and compared with experimental results obtained by electron transfer spectroscopy (ETS). Three * states were observed, a bound state and two metastable, in good agreement with experimental results. A Feshbach resonance was found in calculations, also it is observed experimentally and it was suggested in several theoretical studies of bound state. It is believed that the approach used to describe the target excited states can be reconsidered, as it is suggested a more sophisticated approach to be used in future calculations. With this study, a better understanding of experimental results ETS is possible, as it allowed an interpretation of theoretical studies of the anionic states of p-BQ. Moreover, this work enables further scattering studies involving more complex quinones.

Nous étudions la dynamique d’anions hydroxyde et fluorure micro-solvatés dans un agrégat d’eau après photo-excitation. Du fait du coût numérique important des calculs ab-initio et de la faible transférabilité des potentiels modèles de la littérature, nous avons développé un nouveau modèle transférable permettant de décrire l’interaction d’un soluté quelconque avec une molécule d’eau. Nous avons également effectué une étude des propriétés statiques de F^-(H2O)n=1−7 et OH−(H2O)n=1−7 qui sert de référence pour la paramétrisation du modèle et fournit une base à l’interprétation des calculs de dynamique. De cette étude, nous avons déduit le nombre de molécules d’eau nécessaires pour stabiliser le premier état excité de F− et OH−. Nous avons aussi déterminé l’impact de la base sur la description des états excités. Enfin, nous avons mis en évidence un motif géométrique favorable à la recombinaison géminée. L’étude de trajectoires sur le premier état singulet excité pour F^-(H2O)3,5 et OH^-(H2O)3,5 montre des différences de comportement entre le fluorure et l’hydroxyde. Pour F^−, l’électron est très diffus et se transfère en une centaine de femtosecondes à l’eau. Dans le cas de OH^−, à cause du dipôle du radical OH qui maintient l’électron, le transfert de charge ne s’effectue que lorsque le radical OH tourne vers l’agrégat d’eau. Cette différence de comportement entre OH^−et F^−offre une piste pour la compréhension du phénomène de recombinaison géminée rapide observé pour OH^−. Nous avons également étudié le spectre d’énergie de détachement vertical de l’électron pour des agrégats d’eau négativement chargés qui constituent l’un des produits finaux de la dynamique des anions photo-excités. Ces résultats ont été discutés et comparés à l’expérience. Nous avons ainsi pu faire correspondre des structures géométriques aux pics des spectres mesurés expérimentalement. Nous discutons aussi du rôle de l’énergie interne des agrégats sur l’allure des spectres via son impact sur l’évaporation de molécules d’eau. / We study the dynamics of micro-solvated hydroxide and fluoride anions in water clusters after photoexcitation. Because ab-initio calculations are numerically expensive and model potentials from literature lack of transferability, we developed a transferable model, which allows us to describe the interaction between any solute and a water molecule. We have studied the statics properties of F^−(H2O)n=1−7 and OH^−(H2O)n=1−7. The results from this study serve as a basis for the parametrization of the model and for the interpretation of dynamics simulation. From this study, we deduced the number of water molecules needed to stabilize an excited state for F^−and OH^−. We investogated the impact of the basis on the description of the excited states At last, we highlight a favorable motif for geminate recombination. Study of trajectories on the first excited singlet states of F^−(H2O)3,5 and OH^−(H2O)3,5 shows differences in the dynamics between OH^−and F^− For F^−, the excess electron is very diffuse and transferred quickly to the water For OH−, because of the OH dipole, the excess electron remains bound to the neutral . OH, until charge transfer takes place when OH rotate to the water cluster. This difference provides a way to understand the fast geminate recombination process observed for OH^−. We also studied vertical detachment spectra of the electron for negatively charged water clusters which are the final products of the anions dynamics on the first excited state. These results are compared to experience from literature. We associate isomers to the experimentally observed peaks. We discuss the effect of internal energy on the shape of the spectra through water molecules evaporation

<p> The theoretical treatment of the induced torque problem is discussed, and equations describing the functional form of the torque amplitude are derived on the basis of a model calculation valid in the high field limit. This functional form is applied to a detailed interpretation of the open orbit structure in gallium, cadmium, zinc and copper. </p> <p> An investigation of the open orbit structure in gallium at l.4°K using this technique has yielded direct information on the connectivity of the sixth-band hole surface. This surface supports a k(c)-trajectory for all field directions in the ab plane, except within 0.1° of the a-axis. A k(a)-trajectory of lower conductivity is reported over a 10° range of field direction centred (32°±2°) from the b-axis in the be plane. These data, in addition to the highly anisotropic amplitude and field dependence of the k(c) -trajectory, require that this surface contact the Brillouin zone boundary at both the k(a) -and k(c) -faces. The present data are compared with available models of the sixth band hole surface, and are found to be in excellent agreement with the predictions of recent pseudopotential calculations . The possibility of magnetic breakdown in the k(c)-trajectory for B|| b-axis is discussed. Finally, a non-linear frequency dependence, and an anisotropic non-quadratic field dependence are understood to occur through the long mean free path and short skin depth parameters in gallium at l.4°K. </p> <p> In cadmium, the induced torque amplitude due to the [0001]-open trajectory tends to saturation at high field intensities for all observation directions. This effect is attributed to magnetic breakdown between the first-and secondhand hole surfaces through the spin·-orbit interaction energy 'gap near the H-symmetry point in the AHL plane. This magnetic breakdown effect is analyzed on the basis of a linear chain model and the theoretical curves of Falicov and Sievert. Detailed analysis indicates the possibility of two separate breakdown probabilities across the HL and HA gaps; for <1010> directions , breakdown fields of 10.8 k0e and (as low as) 0.72 k0e are indicated . A similar range of breakdown fields is indicated for all field directions in the (0001) plane. </p> <p> In zinc, the induced torque technique is used to investigate magnetic breakdown effects in the [0001]-trajectory for specific field directions in the basa l plane. These effects are attributed to the onset of partial breakdown between the monster and cap surfaces near the H-symrnetry points, for field intensities above 16 kOe. The linear chain model is shown to be an inappropriate description of breakdown effects in zinc. An onset field of 16 kOe for <1120> is determined; no breakdown is observed along <lOIO> below 20 kOe. In addition, magnetic breakdown effects occurring in the basal orbits of zinc were investigated. The general monotonic rise in torque amplitude with field intensities is fitted to the theoretical expressions of Falicov, Pippard, and Sievert. The giant quantum oscillations arising through coherence-effect modulation of the breakdown probability are shown consistent with earlier data. </p> <p> The technique is applied to a general survey of the various types of open orbits existing in copper, and the general applicability of the sample torque equations to both compensated and uncompensated metals is demonstrated. </p> / Thesis / Doctor of Philosophy (PhD)

Ce travail de thèse est une recherche sur l'interaction entre des états atomiques excités électroniquement et un environnement non réactif. Nous avons étudié théoriquement et expérimentalement des situations où un atome métallique (K et Ba) est placé dans un environnement de taille finie (agrégat d’argon). La présence de l'environnement affecte les niveaux électroniques de l'atome. En retour, l'excitation de l'atome induit une dynamique de relaxation de l'énergie électronique via les déformations du système atome-agrégat. La partie expérimentale du travail porte sur les deux aspects : spectroscopie et dynamique. Dans les deux cas un premier laser porte l'atome métallique dans un état électronique excité et un second l'ionise. L'observable est le spectre de photoélectrons enregistré après photo ionisation éventuellement complétée par des informations sur les photo-ions qui sont également produits. Cette technique à deux lasers conduit à des mesures de dynamique selon la technique pompe-sonde quand les lasers utilisés sont à impulsion ultracourte (60 fs). L'utilisation de lasers nanosecondes, conduit à des mesures de résonance non résolues temporellement qui donnent des informations spectroscopiques sur la position des niveaux d'énergie du système étudié. D’un point de vue théorique, les états excités du système M-Ar_n ont été calculés ab-initio en utilisant des pseudo-potentiels à grand cœur pour limiter les électrons actifs aux seuls électrons de valence du métal. L’étude d’un métal alcalin (potassium) rend cette méthode particulièrement attractive car un seul électron est actif. Le calcul ab-initio et une simulation Monte-Carlo ont été couplés pour optimiser la géométrie d'agrégats KAr_n (n=1-10) quand K est dans l'état fondamental, excité dans les états 4p ou 5s ou ionisé vers l'état fondamental de l'ion. Des calculs ont également été conduits en collaboration avec B. Gervais (CIMAP, Caen) sur des agrégats KAr_n comportant plusieurs dizaines d'atomes Ar. Des spectres d'absorption ont également été calculés. D’un point de vue expérimental, nous avons pu caractériser les niveaux électroniques excités du potassium et du baryum perturbés par l’agrégat. Dans les deux cas une bande ∏, liante, et une bande ∑, anti-liante, ont été observées. Dans le cas du potassium, nous avons montré que l’excitation dans la bande ∑ conduisait à une éjection de l’agrégat en 1-2 ps alors que pour le baryum, l’état électronique relaxe majoritairement sur l’état ∏ en ≈ 6 ps et ne conduit pas à une éjection. L’interprétation fait appel aux structures et aux potentiels calculés. Une étude équivalente a été conduite sur la molécule de DABCO déposée sur agrégat. Au contraire de K et Ba, le premier état excité de cette molécule a un fort caractère isotrope et diffus, ce qui confère un caractère particulier à la dynamique photoinduite. / This thesis is a study on the interaction between electronically excited atomic states and a non-reactive environment. We have theoretically and experimentally studied situations where a metal atom (Ba or K) is placed in a finite size environment (argon cluster). The presence of the medium affects the electronic levels of the atom. On the other side, the excitation of the atom induces a relaxation dynamics of the electronic energy through the deformation of the cluster. The experimental part of this work focuses on two aspects : the spectroscopy and the dynamics. In both cases a first laser electronically excites the metal atom and the second ionizes the excited system. The observable is the photoelectron spectrum recorded after photoionization and possibly information on the photoion which are also produced. This pump/probe technique, with also two lasers, provide the ultrafast dynamic when the lasers pulses used are of ultrashort (60 fs ). The use of nanosecond lasers leads to resonance spectroscopic measurement, unresolved temporally, which give information on the position of the energy levels of the studied system. From a theoretical point-of-view, the excited states of M-Ar_n were calculated at the ab initio level, using large core pseudo-potential to limit the active electrons of the metal to valence electrons. The study of alkali metals (potassium) is especially well adapted to this method since only one electron is active. The ab-initio calculation and a Monte-Carlo simulation where coupled to optimize the geometry of the KAr_n (n = 1-10) cluster when K is in the ground state of the neutral and the ion, or excited in the 4p or 5s state. Calculations were also conducted in collaboration with B. Gervais (CIMAP, Caen) on KAr_n clusters having several tens of argon atoms. Absorption spectra were also calculated. From an experimental point-of-view, we were able to characterize the excited states of potassium and barium perturbed by the clusters. In both cases a binding ∏-state, and an anti-binding ∑ -state were observed. In the case of potassium, we observered that the excitation on the ∑ -state leads to the ejection of the metal within 1-2 ps while for barium, the excited state relaxes mainly on the ∏-state within 6 ps and does not followed by an ejection. The interpretation of these results uses the structures and potential calculated. A similar study was conducted on the DABCO molecule deposited argon cluster. Instead of K and Ba, the first excited state of this molecule is a diffuse isotropic state, which gives a specific relaxation to the photoinduced dynamics.

Cette thèse est consacrée à l’étude de la dynamique des porteurs de charges sous forte concentration. La méthode Monte Carlo « Full-Band » a été utilisée pour la modélisation du transport et la relaxtion des porteurs de charge dans les semi-conducteurs III-V (GaAs, InAs, GaSb, In0.53Ga0.47As et GaAs0.50Sb0.50). Les structures électroniques ont été calculées par la Méthode des Pseudo-potentiels Non-Locaux Empiriques, ce qui a notamment permis de traiter le cas de l’alliage ternaire GaAs0.50Sb0.50 dans une approche de type Cristal Virtuel, matériau qui souffre d’un manque de caractérisations expérimentales. Dans ces semi-conducteurs polaires fortement dopés, le couplage entre phonons optiques polaires et plasmons a été pris en compte via le calcul de la fonction diélectrique totale incluant les termes associés à l’amortissement dans le système phonon-plasmon auto-cohérents. Ce phénomène de couplage phonon-plasmon, est apparu primordial pour l’analyse de la mobilité des électrons dans GaAs, In0.53Ga0.47As et GaAs0.50Sb0.50 en fonction de la concentration en accepteurs. Dans des semi-conducteurs fortement photo-excités, la relaxation des électrons et des trous a été étudiée en tenant compte du chauffage de la population de phonon (qui ralentit la relaxation des porteurs) avec un modèle Monte Carlo dédié à la dynamique des phonons (Thèse de H. Hamzeh). L’étude a montré que le ralentissement de la relaxation dépend fortement des concentrations de porteurs photo-excités à cause du couplage phonon-plasmon dans ces matériaux. Les processus de génération et recombinaison de porteurs tels que l’absorption optique, la recombinaison radiative, l’ionisation par choc et les recombinaisons Auger, ont été implémentés. Les taux de génération et recombinaison associés sont calculés directement sur les distributions de porteurs modélisées, sans supposer des distributions à l’équilibre. Ces processus sont cruciaux pour l’optimisation de Cellules Solaires à Porteurs Chauds. Le photo-courant de ce type de cellule théorique à haut rendement de 3ème génération avec un absorbeur en In0.53Ga0.47As a été étudié. / The aim of this work is the study of charge carriers dynamic under high carrier concentration regimes. The « Full-Band » Monte Carlo method is used for charge carrier transport/relaxation modeling in III-V semiconductors (GaAs, InAs, GaSb, In0.53Ga0.47As and GaAs0.50Sb0.50). Electronic band structures are calculated with the Non-Local Empirical Pseudopotential Method which enables the study of ternary alloys within a Virtual Crystal approach. This method has been applied to In0.53Ga0.47As and GaAs0.50Sb0.50, the latter being a promising material for Heterojunction Bipolar Transistor applications though it lacks experimental characterizations. In highly doped polar semiconductors, the polar optical phonon – plasmon coupling is accounted for via the calculation of the total dielectric function including self-consistent damping parameters. This coupling appeared crucial for the calculation of minority electron mobilities in highly p-doped GaAs, In0.53Ga0.47As and GaAs0.50Sb0.50. In strongly photo-excited semiconductors, phonon population heating has been included in the study of electrons and holes relaxation. Hot phonon populations, that slow the charge carrier relaxation through the phonon bottleneck effect, have been dealt with a phonon dedicated Monte Carlo model (PhD H. Hamzeh). The study showed that carrier relaxation slowing depends strongly on the photo-excited carrier concentration because of phonon-plasmon coupling in those semiconductors. Charge carrier generation and recombination processes such as photon absorption, radiative recombination, impact ionization and Auger recombinations, have been implemented. The associated generation and recombination rates are directly calculated with the sampled carrier distribution. Thus, the use of coefficients and lifetimes is avoided, and non equilibrium regimes were modeled. Those processes are of prime importance for Hot Carrier Solar Cells optimization. The theoretical photo-current of this kind of 3rd generation solar cell with an In0.53Ga0.47As absorber have been studied.

The structures of AgSin (n = 1 ¡V 16) clusters are investigated using first-principles calculations. Our studies suggest that AgSin clusters with n = 7, 10, and 15 are relatively stable isomers and that these clusters prefer to be exohedral rather than endohedral. Moreover, doping leaves the inner core structure of the clusters largely intact. Additionally, the plot of fragmentation energies as a function of silicon atoms shows that the AgSin are favored to dissociate into one Ag atom and Sin clusters. Alternative pathways exist for n > 7 (except n = 11 and 16) in which the AgSin cluster dissociates into a stable Si7 and a smaller fragment AgSin􀀀7. The AgSi11 and AgSi16 cluster dissociates into a stable Si10 and a small fragment AgSi. Lastly, our analysis indicates that doping of Ag atom significantly decreases the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for n > 7.

The structures of AuSin (n = 1 - 16) clusters are investigated systematically using first-principles calculations. The lowest energy isomers exhibit preference toward exohedral rather than endohedral structure. Our studies suggest that AuSin clusters with n = 5 and 10 are relatively stable isomers. We found no significant alteration in the cluster¡¦s inner core structure for sizes n= 6, 7, 10, 11, 12, 14, and 15 even in the presence of doping. Moreover, analysis of fragmentation energies is presented in detail. Our studies further indicate that doping of Au atom significantly decreases the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for n > 7. Additionally, we report on similar results obtained for CuSin (n = 1 - 16) and AgSin (n = 14, 15, and 16) and compared them with those on AuSin clusters. Next, the low energy isomers for certain sizes of CuSin (n = 10 -16 ) clusters are selected for further optimizations using Gaussian 03 package. We found that for CuSin (n = 12 - 16 ), the endohedral isomers have lower energies than their exohedral counterparts, consistent with a recent study by Janssens et al. [15] in which a similar trend was observed.

The goal of this Thesis is to study the electronic and optical properties of semiconductor nanostructures by employing different theories. The work present in this Thesis is divided into three parts. Part I is devoted to the effective-mass theory and its several applications. A general description of the effective mass theory and several ways of solving the effective-mass Schrodinger equation with an emphasis on the potential morphing method are given in the first chapter. In the following few chapters, we apply these theories in many realistic systems for the study of many properties. They include: i) the binding energy of hydrogentic donor impurity in semiconductor quantum dots under the influence of static electric field and/or magnetic field, ii) the linear and nonlinear optical properties associated with intraband transitions in semiconductor quantum dots, core shell quantum dots and quantum-dot-quantum-ring systems. Part II is devoted to the pseudopotential theory and its several applications. The background theories primarily regarding to the empirical pseudopotential method and configuration interaction approach are described in the first chapter. In the following few chapters, we employ these theories for the study of the electronic and optical properties of many nanostructures of group II-VI materials. The optical properties studied herein include the band gap, Stokes shift, exciton fine structure, optical polarization and absorption spectra. Part III is devoted to the appendix, where twelve published papers are presented. / Στόχος της παρούσας διατριβής είναι η μελέτη των ηλεκτρονικών και οπτικών ιδιοτήτων νανοδομών ημιαγωγών κάνοντας χρήση κατάλληλων υπολογιστικών μεθόδων και τεχνικών. Η διατριβή χωρίζεται σε τρία μέρη. Το πρώτο μέρος εστιάζει στην θεωρία της ενεργούς μάζας (Effective-mass Theory) και τις εφαρμογές της. Στο πρώτο κεφάλαιο παρουσιάζεται το απαραίτητο θεωρητικό υπόβαθρο και δίνεται μία συνοπτική περιγραφή των συνηθέστερων μεθόδων επίλυσης της μονοηλεκτρονιακής εξίσωσης του Schrodinger,δίνοντας ιδιαίτερη έμφαση στην μέθοδο μορφοποίησης δυναμικού (Potential Morphing Method). Στα επόμενα κεφάλαια του πρώτου μέρους οι τεχνικές και μέθοδοι που περιγράφηκαν χρησιμοποιούνται για την μελέτη κρίσιμων ιδιοτήτων και παραμέτρων σε νανοσυστήματα ημιαγωγών. Μεταξύ αυτών είναι: i) η ενέργεια δέσμευσης υδρογονοειδών προσμίξεων τύπου δότη υπό την επίδραση στατικού ηλεκτρικού ή/και μαγνητικού πεδίου, ii) γραμμικές και μη γραμμικές οπτικές ιδιότητες που συνδέονται με intraband μεταβάσεις εντός ζώνης σε κβαντικές τελείες ημιαγωγών, κβαντικές τελείες με δομή πυρήνα-φλοιού και σε μεικτά συστήματα κβαντικής τελείας – κβαντικού δακτυλίου. Το δεύτερο μέρος εστιάζει στην θεωρία των ψευδοδυνάμικών και τις εφαρμογές της. Αρχικά παρουσιάζεται το απαραίτητο θεωρητικό υπόβαθρο της μεθόδου εμπειρικών ψευδοδυναμικών (Empirical Pseudopotential Method) καθώς επίσης και της μεθόδου αλληλεπίδρασης διαμορφώσεων (Configuration Interaction). Στην συνέχεια, οι προαναφερθείσες τεχνικές εφαρμόζονται στην μελέτη των ηλεκτρονικών και οπτικών ιδιοτήτων σε μία πληθώρα νανοδομών ημιαγωγών II-VI. Μεταξύ των ιδιοτήτων αυτών είναι: το ενεργειακό χάσμα, η μετατόπιση Stokes, η λεπτή δομή των εξιτονίων, η οπτική πόλωση και τα φάσματα απορρόφησης. Το τρίτο μέρος της διατριβής περιλαμβάνει το παράρτημα, στο οποίο παρατίθενται οι δώδεκα δημοσιευμένες εργασίες.

The emperical pseudopotential method in the large basis approach was used to calculate the electronic bandstructures of bulk semiconductor materials and layered semiconductor heterostructures. The crucial continuous atomic form factor potentials needed to carry out such calculations were determined by using Levenberg-Marquardt optimization in order to obtain optimal cubic spline interpolations of the potentials. The optimized potentials were not constrained by any particular functional form (such as a linear combination of Gaussians) and had better convergence properties for the optimization. It was demonstrated that the results obtained in this work could potentially lead to better agreement between calculated and empirically determined band gaps via optimization / Physics / M. Sc. (Physics)