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Designing novel nanostructures which exploit the self-assembly capabilities of biomolecules yields a promising approach to control matter at the nanoscale. Here, the homochiral molecular self-assemblies of the methionine and tyrosine amino acids on the monocrystalline Ag(111) and Cu(111) surfaces are characterized by means of scanning tunneling microscopy (STM) and spectroscopy (STS), helium atom scattering (HAS), x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) in ultrahigh vacuum (UHV). On Ag(111), methionine self-assembles into supramolecular chains following the <110> substrate axis, forming regular nanogratings with tunable periodicity. Within the nanowires, a zwitterionic dimerization scheme is revealed. STS shows that the biomolecular nanostructures act as tunable one-dimensional quantum resonators for the surface state electrons. Zero-dimensional electronic confinement is achieved by positioning single iron atoms in the molecular trenches. This shows a novel approach to control the dimensionality of surface state electrons. The nanogratings were exploited to steer the spontaneous one-dimensional ordering of cobalt and iron atoms. For T > 15 K, the metal species self-align into homogeneously distributed chains in between the biomolecular trenches with ~25 Å interatomic distace. For Co, the dynamics of the self-alignment was monitored, revealing a reduced mobility in comparison with isolated Co atoms on bare Ag(111). On Cu(111), the self-assembly of methionine is influenced by the substrate reactivity and its temperature during molecular deposition. For T < 273 K, the biomolecules assemble in anisotropic extended clusters oriented with a -10° rotation off the <110> substrate orientations, whereas above 283 K a regularly ordered 1D phase arises with a +10° rotation off these high-symmetry axis. XPS reveals a structural transformation triggered by a thermally activated deprotonation of the zwitterionic ammonium group. On Ag(111), tyrosine self-assembles above a critical temperature into linear structures primarily following the substrate crystalline symmetry. A zwitterionic non-covalent molecular dimerization is demonstrated, NEXAFS data providing evidence of a non-flat adsorption of the phenyl ring. This recalls the geometrical pattern of methionine on Ag(111) and supports a universal self-assembling scheme for amino acids on close-packed noble metal surfaces, the different mesoscopic ordering being determined by the side chain reactivity.

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2009" Ivana Adamovic. 12/19/2004. Report is also available in paper and microfiche from NTIS.

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1930" Heather Marie Netzloff. 12/19/2004. Report is also available in paper and microfiche from NTIS.

Les oxydes d’iridium (iridates) ont attiré particulière attention au cours de la dernière décennie grâce à l’identification d’un état isolant de Mott induit par l’action conjointe du champ cristallin, de la corrélation électronique et du couplage spin-orbite. Cet état a été intensément investigué et des phases et excitations nouvelles ont été prédites théoriquement et aussi individuées expérimentalement. Sans doute, la diffusion inélastique résonante de rayons X (RIXS) est l’une des techniques les plus adoptée pour mesurer les excitations à basse énergie des iridates. En effet, la section efficace relativement large de la spectroscopie RIXS au seuil L3 de l’iridium et la bonne résolution en énergie ont encouragé l’emploi de cette technique. Cette thèse se pose un double objectif : concevoir des développements instrumentaux ayant pour but d’améliorer les possibilités offertes par la spectroscopie RIXS, et appliquer le RIXS afin d’étudier la physique à basse énergie de certains iridates.Le principal projet de développement instrumental est un nouveau spectromètre RIXS avec résolution en polarisation. L’analyse de la polarisation des rayons X diffusés permet d’obtenir des informations sur la symétrie et donc la nature des excitations. Cependant, elle est peu employée à cause de problèmes techniques qui naissent quand l’on veut préserver aussi la résolution en énergie et l’efficience du spectromètre. Même si le polarimètre RIXS projeté n’est pas encore disponible, le schéma optique a été vérifié et validé. Le polarimètre aura une résolution en énergie et une efficience équivalentes à celles des spectromètres RIXS courants. Le second développement technique comprend l’équipement permettant de réaliser des mesures RIXS à basse température et haute pression. Ces équipements ont permis d’investiguer l’évolution en pression des excitations magnétique du composé Sr3Ir2O7 en dessous de la température de Néel et jusqu’au 12 GPa. Les mesures peuvent aider l’affinage des modèles magnétiques courants pour ce système. Ces mesures démontrent que les excitations magnétiques peuvent être acquises en haute pression par la spectroscopie RIXS, démontrant ainsi la possible utilisation de cette technique dans ce nouveau domaine.Cette thèse comprend aussi des ultérieurs travails expérimentaux. Le premier considère le composé CaIrO3, dont la structure cristalline est constituée par des octaèdres partageant un sommet et une arête dans deux directions orthogonales. Du coup, les interactions magnétiques sont très différentes selon la direction cristallographique. Spécifiquement, la suppression du couplage de type Heisenberg dans la direction où les octaèdres partagent une arête produit des interactions magnétiques principalement unidimensionnelles. La caractéristique de ceux-ci est la présence d’un continuum d’excitations avec une dépendance en énergie et quantité de mouvement typique, qui a été révélé par la spectroscopie RIXS. Les excitations électroniques de CaIrO3 ont aussi des caractéristiques propres. Enfin, j’ai étudié les propriétés électroniques du composé Rb2[IrF6]. Des calculs récents proposaient que ce système possède un état d’isolant de Mott similairement aux oxydes d’iridium. Les mesures RIXS ont aidé à éclairer les propriétés électroniques de ce composé. La solidité des propriétés électroniques a été vérifiée par rapport à la substitution du métal alcalin ou de l’halogène, et à l’application de pression.Cette thèse accroit l’importance de l’utilisation de la spectroscopie RIXS dans des domaines qui étaient précédemment inexplorés. L’analyse de la polarisation des rayons X diffusés sera avantageuse dans les cas où la nature des excitations ne peut être établie sans ambiguïté. L’équipement développé pour réaliser les mesures RIXS en conditions extrêmes permet d’étudier la dynamique électronique et magnétique dans des phases de la matière inaccessibles jusqu’à aujourd’hui. / Iridium oxides (iridates) have raised notable attention in the last decade due to the identification of a Mott insulating state realized by the joint action of crystal field, electron correlation, and spin-orbit coupling. Such state has been intensively investigated and novel quantum phases and excitations have been theoretically predicted and experimentally found. Undoubtedly, one of the most employed techniques to elucidate the low-energy physics of iridates is resonant inelastic X-ray scattering (RIXS). At the iridium L3 edge, it benefits from a particularly good energy resolution, which matches the energy scales of the relevant excitations, and from a favorable inelastic cross-section. The aim of the present thesis is twofold: conceive challenging instrumental upgrades that contribute to the advancement of the technique itself, and apply RIXS to inspect the magnetic and electronic properties of selected iridates.The main instrumental development concerns the design of a new RIXS spectrometer with polarization resolution. Polarization analysis of the scattered X-rays provides useful information about the symmetry and thus the nature of an excitation. However, it is rather unexploited because of severe technical challenges when energy resolution and efficiency must be preserved. The designed RIXS spectrometer with polarization analysis capabilities is still under construction, however the optical scheme has been validated by preliminary tests. Full polarization analysis is expected without degradation of energy resolution or efficiency with respect to current state-of-the-art RIXS spectrometers. Additional technical developments include sample-environment equipment to perform RIXS experiments in low-temperature and high-pressure conditions. The equipment has been successfully utilized to investigate the magnetic dynamics of the bilayer-perovskite Sr3Ir2O7 below its Néel temperature and up to 12 GPa. Our measurements provide additional observations that may sharpen the challenge to theoretical understanding of the magnetic dynamics of this material. Moreover, we demonstrate for the first time that RIXS experiments of the magnetic dynamics can be extended to unexplored thermodynamic conditions.Besides instrumental advances, additional experimental work has been carried out in order to study the magnetic and electronic excitations of the post-perovskite CaIrO3. Owing to its peculiar crystal structure, featuring both edge- and corner-sharing octahedra, the magnetic interactions of CaIrO3 are very different along orthogonal directions. In particular, the inhibition of the Heisenberg coupling along the edge-sharing direction induces one-dimensional magnetic behavior with characteristic fractional spinon-like excitations, which have been detected by RIXS. Electronic excitations are also found to have particular properties. Finally, I have focused on the electronic structure of Rb2[IrF6], which was theoretically predicted to realize a Mott insulating state similar to the one of iridium oxides. RIXS measurements helped to elucidate the electronic properties of this compound. The robustness of the electronic state has been tested against substitutions of the alkali metal and halogen, and application of physical pressure.The present work extends the potential of the RIXS technique to domains previously unexplored, i.e. polarization analysis of the scattered X-rays and high-pressure low-temperature experiments. I hope that the instrumental upgrades and applications of RIXS discussed in this thesis will further promote the technique as a powerful and reliable tool to characterize elementary excitations in correlated-electron systems.

Au cours de cette thèse, nous avons étudiés théoriquement les propriétés structurelles et électroniques des super-réseaux d'oxyde de manganèse en structure perovskiteö au moyen de calculs ab initio.Les oxydes de manganèse au lanthane, donnés avec la formule générique La(1-x)A(x)MnO(3) (LAMO) (A un élément divalent), constituent une classe importante d'oxydes de manganèse en raison de leurs diverses propriétés, telles que l'effet de magnétorésistance colossale, leur riche diagramme de phase en fonction du dopage, de la température ou de champs externes, et leur grande température Curie. Ces propriétés peuvent être exploitées dans de nombreuses applications technologiques potentielles telles que les valves de spin ou les injecteurs de spin. Le contrôle des propriétés de ces matériaux peut se faire par dépôt sous forme de films minces ou comme blocs de construction dans des super-réseaux. Lorsque x = 1/3, le La(1-x)A(x)MnO(3) massif est ferromagnétique et métallique grâce au mécanisme de double échange dans la electrons 3d de Mn. Lorsque Mn est dans un état de valence mixte, les orbitales eg (dx2-y2 and dz2) sont partiellement occupées et peuvent se délocaliser sur les atomes de Mn voisins, seulement si ceux-ci sont alignés ferromagnétiquement. Dans des films très minces, puisque la direction perpendiculaire au substrat, c, n'a que quelques cellules unitaires d'épaisseur, seules les interactions dans le plan (ab) sont importantes pour les propriétés thermodynamiques. En agissant sur la géométrie de la couche LAMO, on peut ainsi maximiser l'occupation de l'orbitales dx2-y2 et augmenter l'échange magnétique et la température Curie associée.Notre but était donc de concevoir de nouveaux matériaux avec un ordre orbital 3d spécifique afin d'assurer les propriétés magnétiques souhaitées.Dans cette thèse, nous avons travaillé sur deux types de super-réseaux. Le premier était constitué de deux oxydes de manganèse antiferromagnétiques, non dopés, LaMnO(3) et SrMnO(3), c'est-à-dire des super-réseaux [LaMnO(3)]n/[SrMnO(3)]m. Nous avons étudié l'état fondamental magnétique pour différentes valeurs n et m afin d'expliquer les résultats expérimentaux surprenants. Le deuxième type de super-réseaux que nous avons étudiés est composé de couches métalliques LAMO en alternance avec des couches isolantes. En effet, les super-réseaux avec des interfaces métal-isolant ont un grand potentiel dans les applications de valves de spin. Ainsi, nous avons d'abord considéré des super-réseaux entre composés ferromagnétiques-métalliques et ferroélectriques-isolantes [LAMO](3)/[BTiO(3)](3) (A = Sr ou Ba, B = Ba ou Pb). Dans ces super-réseaux, les propriétés magnétiques sont malheuresement réduites en raison de la délocalisation d'électrons dz2 à l'interface entre Mn et Ti. Dans de tels super-réseaux , nous avons clarifié le rôle de la polarisation des couches ferroélectriques et le rôle des mouvements antiferrodistortifs dans les couches de manganite. Enfin, de manierè à 'éviter que la délocalisation ait lieu à l'interface, nous avons conçu un autre super-réseau avec interface métal-isolant dans lequel nous avons remplacé l'isolant (BTiO(3)) par un oxyde simple (BO): [LAMO]n/[BO]p superlattices (A = Sr ou Ba, B = Ba, Sr ou Mg et n = 3 ou 6, p = 6 ou 2). Dans ces nouveaux super-réseaux, nous avons réussi à promouvoir les occupations des orbitales dx2-y2 dans les interfaces assurant un fort moment magnetique à l'interface et a priori une fort temperature de Curie. Nous avons également montré une faible corrélation entre la conductivité électrique et l'ordre orbitaire. / This thesis theoretically studies structural and electronic properties of perovskite manganese oxide superlattices by means of ab-initio calculations.Lanthanum manganese oxides, given with the generic formula La1−xAxMnO3 (LAMO) (A a divalent element), are an important class of perovskite manganese oxides due to their various exotic properties, such as giant and colossal magnetoresistance effect, rich phase diagrams with respect to doping, temperature or external fields, and intinsic large Curie temperature. These properties can be exploited in many potential technological applications such as spin valves or spin injectors. Controlling the properties of these materials can be done through deposition as thin films or as building blocks in superlattices. When x = 1/3, bulk La1−xAxMnO3 is ferromagnetic and metallic due to the double-exchange mechanism in the Mn 3d shell. When Mn is in a mixed valence state, the eg orbitals (dx2-y2 and dz2) are partially occupied, and can delocalize on neighboring Mn atoms only if the latter are ferromagnetically aligned. In very thin films, since the direction perpendicular to the substrate, c, is only a few unit cell thick, only in-plane (ab) interactions are important for the thermodynamic properties. By acting on the LAMO layer geometry, one can thus maximize the dx2-y2 occupancy and increase the magnetic exchange and related Curie temperature.Our aim was thus to design new materials with desired 3d orbital order so that to ensure desired magnetic properties.In this thesis, we worked on two types of superlattices. The first one was made of two undoped antiferromagnetic manganese oxides LaMnO3 and SrMnO3, i.e. [LaMnO3]n/[SrMnO3]m superlattices. We investigated the magnetic ground state for different n and m values in order to explain suprising experimental results. The second type of superlattices is composed of metallic LAMO layers with alternated insulating layers. Indeed, the superlattices with metal-insulator interfaces have a great potential in spin valves applications. Thus, we first considered the ferromagnetic-metallic/ferroelectric-insulating [LAMO]3/[BTiO3]3 superlattices (A= Sr or Ba, B=Ba or Pb) where the magnetic properties are reduced due to delocalization of dz2 electrons at the interface from Mn to Ti. In such superlattices, we clarified the role of the polarization of the ferroelectric layers and the role of the antiferrodistortive motions in the manganite layers. In order to prevent the delocalization taking place at the interface, we designed another superlattice with metallic-insulator interface where we replaced the insulator (BTiO3) with a simple oxide (BO); that is [LAMO]n/[BO]p superlattices (A= Sr or Ba, B=Ba, Sr or Mg and n=3 or 6, p=6 or 2). Within this new superlattice, we successully managed to promote dx2-y2 orbital occupancies at the interfaces which ensures a large magnetic moment at the interfaces and an expected large Curie temperature. We also showed the weak correlation between electrical conductivity and orbital ordering.

Made available in DSpace on 2016-06-02T20:16:47Z (GMT). No. of bitstreams: 1 3834.pdf: 2869870 bytes, checksum: cda4c3f94950b35242f2e16dff60bd42 (MD5) Previous issue date: 2011-08-09 / Universidade Federal de Sao Carlos / The nanoscopic structures with ring topology, or quantum rings, have attracted the interest due to their unique rotational symmetry and the possibility of checking fundamental quantum phenomena. Among them, the study of Aharonov-Bohm interference effects appears with special emphasis. Analytical calculations of the electronic structure of quantum rings were performed for the electron confined in one-dimensional and threedimensional potentials. These calculations were complemented by the electronic structure simulation of the valence band using the k.p method, main objective this work. This theoretical contribution is a part of a collaboration with experimental groups of growth and spectroscopy which deal with problems related to the manipulation of electronic states and spin properties of quantum rings. The ground states of both electrons and holes in quantum dots (quasi-zero-dimensional systems) have zero angular momentum (in the absence of magnetic fields and low fields) and exhibit a diamagnetic response of the spin states in the presence of an external magnetic field. In non-magnetic quantum dots, the spin properties are mainly attributed to the electron spin and the heavy hole. On the other hand, it is assumed that the light holes have a minor role in the properties of the exciton ground state. Our results show that the interband coupling may lead to the angular momentum hybridization of the electronic states, even in the ground state and the the light hole may assume a relevant role. The adaptation and improvement of calculations of the electronic structure using the k.p methods were two of the main objectives of this work. The theoretical tools developed aim to contribute to the establishment of protocols for optimal use and application of such systems. / As estruturas nanoscópicas de topologia anelar, ou anéis quânticos, têm atraído o interesse devido a sua simetria rotacional única e à possibilidade de verificar fenômenos quânticos fundamentais. Dentre eles, o estudo de efeitos relacionados à interferência do tipo Aharonov-Bohm aparece com especial ênfase. Cálculos analíticos da estrutura eletrônica dos anéis quânticos foram realizados para os casos de um elétron confinado em um potencial unidimensional e tridimensional. Estes cálculos foram complementados com a simulação da estrutura eletrônica da banda de valência utilizando o método k.p, que é o objetivo principal do trabalho. Esta contribuição teórica forma parte de uma colaboração com grupos experimentais de crescimento e de espectroscopia onde são tratados problemas relacionados à manipulação de estados eletrônicos e de spin em sistemas quase-zero dimensionais de topologia anelar. O estado fundamental do elétron e do buraco em pontos quânticos (sistemas quase-zero-dimensionais) possui tipicamente momento angular zero (na ausência de campos magnéticos e para campos suficientemente baixos) e exibe uma resposta diamagnética dos estados desdobrados de spin uma vez que um campo magnético externo é aplicado. Nos pontos quânticos não magnéticos as propriedades de spin são fundamentalmente atribuídas aos elétrons e ao buraco pesado. Em contrapartida, se assume que os buracos leves possuem um papel menor nas propriedades do estado fundamental do éxciton. Nossos resultados mostram que o acoplamento entre bandas pode gerar estados com hibridização do momento angular, inclusive o estado fundamental e o papel do buraco leve acaba sendo relevante. A adaptação e melhoramento do cálculo da estrutura eletrônica utilizando métodos k.p foram dois dos objetivos fundamentais deste trabalho. As ferramentas teóricas desenvolvidas visam contribuir para o estabelecimento de protocolos para o uso e aplicação otimizada de tais sistemas.

Dans cette thèse nous avons évalué l'effet de la substitution partielle du cobalt par des éléments non-magnétiques de type p (M) sur les propriétés structurales et magnétiques des composés RCo5. Les échantillons ont été synthétisées par fusion dans un four à induction, puis caractérisés en utilisant des nombreuses techniques expérimentales: diffraction de rayons X et de neutrons, microscopie électronique à balayage, magnétométrie, susceptométrie en courant alternatif et spectroscopie photoélectronique par rayons X.Tous les composés de type RCo5-xMx (R= Pr, Sm, Tb, Er et Tm; M= Si, Ge, Al, Ga; x=0,5 et x=1) étudiés gardent la structure cristalline de type CaCu5 des composés de départ RCo5, mais les paramètres de la maille cristalline sont modifiés par la présence des éléments M. Le domaine de stabilité thermique des phases RCo5-xMx est notablement modifié par la présence d'élément métalloïde en substitution sur les sites du cobalt. Il peut être étendu pour Al et Ga et réduit pour Si et Ge. Les atomes de l'élément M sont localisés préférentiellement sur le site cristallographique Co 3g. La solubilité des éléments Ge et Si dans la structure RCo5 est trouvée être inférieure à celle des métalloïdes ayant un électron de moins tels que Al et Ga.La substitution M/Co a une influence importante sur les propriétés magnétiques des composés RCo5. La température d'ordre et l'aimantation spontanée diminuent significativement après le remplacement partiel du cobalt par l'élément métalloïde. Ces modifications sont induites par l'évolution des interactions d'échange en particulier Co-Co mais aussi par la réduction de l'aimantation du Co liée à sa sensibilité au voisinage atomique et magnétique local. La direction de facile aimantation à la température ambiante est préservée le long de l'axe cristallographique c pour tous les échantillons étudiés. Dans quelques composés RCo5-xMx (R = Sm, Er et Tm) la substitution de M au cobalt change le mécanisme de coercitivité par rapport aux composés de départ RCo5. Une coercitivité élevée a été détectée à basse température, surtout pour les composés SmCo4Al et SmCo4Ga, qui présentent aussi des champs d'anisotropie énormes, bien supérieurs à ceux de phase SmCo5.Les expériences XPS sur les composés de type RCo5-xMx ont détecté la réduction de la densité d'états électroniques au niveau de Fermi par rapport aux composés de type RCo5. La bande 3d du cobalt est remplie lors de la substitution M/Co et conduit à un moment magnétique du cobalt réduit. / In this work we have evaluated the effect of the partial substitution of cobalt with non-magnetic p-type elements (M) on the structural and magnetic properties of RCo5 compounds. The samples were prepared by alloying in an induction furnace and were characterized using various experimental techniques: X-ray and neutron diffraction, scanning electron microscopy, magnetometry, AC susceptometry and X-ray photoelectron spectroscopy.All of the studied RCo5-xMx compounds (R=Pr, Sm, Tb, Er and Tm; M=Si, Ge, Al and Ga; x=0.5 and x=1) maintain the CaCu5 crystal structure of the RCo5 compounds, however the lattice parameters are modified due to the presence of M elements. The thermal stability range of the RCo5-xMx phases is modified significantly due to the Co site substitutions. The thermal stability increases for Al and Ga substitutions and decreases when M is Si or Ge. The M atoms were found to preferentially occupy the Co 3g site. The solubility of Ge and Si in the RCo5 structure is inferior to that of metalloid elements with one less electron, such as Al and Ga. The M/Co substitution has an important influence on the magnetic properties of RCo5 compounds. The ordering temperature and the spontaneous magnetization are significantly reduced after the partial substitution of cobalt by the metalloid elements. These changes are induced in particular by the evolutions of the Co-Co exchange interactions and also by the reduction of the Co magnetization due to the sensitivity of Co to the local atomic and magnetic vicinity. The easy magnetization direction at room temperature is preserved along the c-axis for all of the studied samples. In several RCo5-xMx (R = Sm, Er and Tm) compounds the substitution of Co with M atoms changes the coercivity mechanism compared to RCo5. A higher coercivity was found at low temperatures, especially for SmCo4Al and SmCo4Ga compounds, which also show huge values of the anisotropy field, well above the ones found in SmCo5.The XPS measurements on RCo5-xMx compounds show a reduction of the density of states at the Fermi level compared to the RCo5 compounds. There is a filling of the Co 3d band following the M/Co substitution, leading to a lower Co moment.

Desde sua descoberta por S. Iijima em 1991, os nanotubos de carbono têm sido considerados um dos materiais nanoestruturados mais promissores para o desenvolvimento de novos dispositivos eletrônicos em escala nanoscópica. Devido _a sua alta razão entre a área superficial e o volume, esse material se destaca para aplicações como sensores de gás. No presente trabalho, estudamos através de simulações computacionais, a possibilidade de nanotubos de carbono com defeitos de nitrogênio (os chamados nanotubos CNx), poderem ser usados como sensores de moléculas gasosas. Na primeira parte do trabalho foram realizados cálculos de estrutura eletrônica baseados na Teoria do Funcional da Densidade (DFT) para diferentes sistemas formados pelo nanotubo e pela molécula. Através de cálculos de energia de ligação, foi possível identificar quais gases poderiam ou não serem adsorvidos à superfície do nanotubo. Dentre as moléculas investigadas, o monóxido de carbono e a amônia mostraram ser as mais facilmente adsorvidas ao nanotubo. Na segunda parte, foram realizados cálculos das propriedades de transporte utilizando o formalismo das funções de Green fora do equilíbrio (NEGF) recursivo. Foi possível concluir que os nanotubos estudados poderiam ser usados para detectar o monóxido de carbono e a amônia. Todavia, em relação à seletividade, os resultados indicaram que não parece possível distinguir essas duas moléculas, caso o sistema fosse inserido em um ambiente contendo uma mistura desses gases. Ainda, foram feitas simulações de nanotubos contendo defeitos aleatoriamente distribuídos, de forma a levar em conta os fatores de desordem característicos de sistemas mais realistas. / Since their discovery by S. Iijima in 1991, carbon nanotubes have been considered as one of the most promising nanostructured materials for the development of new nanoscopic electronic devices. Due to its high surface area to volume ratio, this material stands out as a candidate for possible gas sensoring applications. In this thesis, we have studied, by means of computational simulations, the possibility of using carbon nanotubes containing nitrogen defects (the so-called CNx nanotubes) as gas sensors. In the first part, we have performed electronic structure calculations based on Density Functional Theory (DFT) of several systems to address the possible binding of different molecules to the nanotube surface. Our results indicate that, among the molecules which were investigated, carbon monoxide and ammonia adsorb more easily to the nanotube surface. In the second part of this thesis, we have performed calculations of the transport properties by means of non-equilibrium Green\'s function formalism (NEGF). The results have shown that the nitrogen-defect carbon nanotubes could be used to detect, mainly carbon monoxide and ammonia molecules. On the other hand, when dealing with the selectivity of this system, it seems to be not possible to distinguish these gases, in the case of inserting the system in a environment containing a mixture of these molecules. Finally, we have simulated carbon nanotubes with defects randomly distributed along its length, in order to take into account disordering factors usually found in more realistic nanosensors.

Utilizou-se um Hamiltoniano KP (6x6) do tipo Kane para, se estudar a estrutura de bandas e níveis de Landau para heteroestruturas de semicondutores zincblende dos grupos III-V e II-VI. Os efeitos do acoplamento entre as bandas de condução e valência, da mistura dos estados da banda de valência, da não-parabolicidade dos níveis, da total degenerescência dos níveis, do warping e das descontinuidades das massas efetivas nas heterointerfaces são levados em conta. Mostrou-se que a interação entre as bandas de condução e valência não pode ser desprezada, mesmo para semicondutores de gap largo, como citado em trabalhos existentes na literatura. Para um estudo sistemático do modelo, utilizou-se um poço quântico de GaAs Ga(Al)As e então aplicou-se o modelo a um sistema de semicondutores semi-magnéticos (poço quântico de CdTe Cd(Mn)Te). / A Kane-like (6x6) KP Hamiltonian is used to study the subband structure and Landau levels for group III-V and group II-VI zincblende semiconductor heterostructures. The effects of conduction-valence band coupling, valence band states mixing, nonparabolicity of the levels, the full degeneracy of the levels, warping and effective masses discontinuities at the heterointerfaces are taken into account. It is shown that the interaction between conduction-valence bands cannot be neglected, even so the semicondutctor have wide gap, as claimed in previous work in the literature. GaAs-Ga(Al)As quantum well was used as a model for a systematic study of the effects of each effective KP parameters. Then, it was applied to the study the subband structure of semi-magnetic semiconductor system (a quantum well of CdTe-Cd(Mn)Te.