In dieser Arbeit wurde eine Tandemsolarzelle aus III-V-Halbleitern auf der Gitterkonstanten von InP mit einem neuartigen Tunnelkontakt entwickelt. Für die Entwicklung der monolithischen Präparation wurden insbesondere kritische Hetero-Grenzflächen im Bereich des Tunnelkontaktes mit oberflächensensitiven Messmethoden untersucht. Die Tandemsolarzelle bestand aus Einzelsolarzellen mit Absorberschichten aus InGaAs (E_g=0,73eV) und InGaAsP (E_g=1,03eV), deren Serienverschaltung mit einem Tunnelkontakt erfolgte, der aus einer n-InGaAs- und einer p-GaAsSb-Schicht bestand. Die Halbleiterschichten wurden mit metallorganischer Gasphasenepitaxie (MOCVD) einkristallin auf einem InP(100)-Substrat gitterangepasst präpariert. Insbesondere wurde der Einfluss der Präparation von InGaAs-Oberflächen auf die Schärfe der InGaAs/GaAsSb-Grenzfläche in-situ mit RAS und nach einem kontaminationsfreien Transfer ins UHV mit UPS, XPS und LEED untersucht. Dabei konnten erstmals drei verschiedene Rekonstruktionen der MOCVD-präparierten InGaAs-Oberfläche beobachtet werden, die von der Heiztemperatur abhängig waren: eine As-reiche (4x3)-, eine InGa-reiche (2x4)- und eine ebenfalls InGa-reiche (4x2)/c(8x2)-Rekonstruktion. Danach erfolgte die Untersuchung des Wachstums von dünnen GaAsSb-Schichten auf diesen drei InGaAs-Oberflächen. Anhand des Sb/As-Verhältnisses im GaAsSb konnte die Präparation auf der (4x3)-rekonstruierten Oberfläche als die schlechteste beurteilt werden. Abschließend wurden Tandemsolarzellen mit verschieden dicken Absorberschichten der InGaAsP-Topzelle gefertigt. Der höchste Wirkungsgrad einer hier hergestellten Tandemsolarzelle betrug 7,3% unter einem gefilterten Sonnenspektrum, das eine GaAs-basierte Tandemsolarzelle mit großen Bandlücken (E_g>1,4eV) simulierte. Die Kombination einer solchen Tandemsolarzelle mit der hier entwickelten InGaAs/InGaAsP-Tandemsolarzelle hat das Potential, für konzentriertes Sonnenlicht eine Konversionseffizienz von deutlich über 40% zu erreichen. / A monolithic low band gap tandem solar cell made up of III-V semiconductors lattice matched to InP and including a novel tunnel junction was developed. Critical hetero interfaces were investigated in detail, in particular the ones related to the tunnel diode. The tandem solar cell was composed of single junction cells with InGaAs (E_g=0.73eV) and InGaAsP (E_g=1.03eV) absorber layers. The serial connection of the subcells was realized by using a tunnel junction including n-InGaAs and p-GaAsSb layers. Metal organic vapor phase epitaxy (MOVPE) was used to prepare the III-V layers lattice matched on InP(100) substrates. In particular, the influence of the preparation of the InGaAs surface on the sharpness of the InGaAs/GaAsSb interface was investigated in-situ by Reflection Anisotropy Spectroscopy (RAS). After a contamination free transfer to UHV the samples were analyzed by UPS, XPS and LEED. Three different surface reconstructions of MOVPE-prepared InGaAs were determined for the first time: an As-rich (4x3)-, an InGa-rich (2x4) and an also InGa-rich (4x2)/c(8x2)-reconstructed surface. In a second step, the growth of thin GaAsSb layers on the three different InGaAs surfaces was studied. The Sb/As-ratio in the GaAsSb layer indicated that the preparations on the InGa-rich surfaces result in a sharper interface. Finally, tandem solar cells with different thicknesses for the absorber layer of the top cell were produced. The highest efficiency obtained for the tandem solar cell was 7.3%, when measured under a filtered solar spectrum to simulate the operation below a GaAs-based tandem solar cell (E_g>1.4eV). The combination of a high band gap tandem solar cell with the InGaAs/InGaAsP tandem solar cell developed here is estimated to reach under a concentrated solar spectrum a total efficiency of more than 40% after further optimization steps.

Le raffinement de nuage de points et la reconstruction de surface sont deux problèmes fondamentaux dans le traitement de la géométrie. La plupart des méthodes existantes ont été ciblées sur les données de capteur de distance et se sont avérées être mal adaptées aux données multi-vues. Dans cette thèse, deux nouvelles méthodes sont proposées respectivement pour les deux problèmes avec une attention particulière aux données multi-vues. La première méthode permet de lisser les nuages de points provenant de la reconstruction multi-vue sans endommager les données. Le problème est formulé comme une optimisation non-linéaire sous contrainte et ensuite résolu par une série de problèmes d’optimisation sans contrainte au moyen d’une méthode de barrière. La seconde méthode effectue une triangulation du nuage de points d’entrée pour générer un maillage en utilisant une stratégie de l’avancement du front pilotée par un critère de l’empilement compact de sphères. L’algorithme est simple et permet de produire efficacement des maillages de haute qualité. Les expérimentations sur des données synthétiques et du monde réel démontrent la robustesse et l’efficacité des méthodes proposées. Notre méthodes sont adaptées aux applications qui nécessitent des informations de position précises et cohérentes telles que la photogrammétrie et le suivi des objets en vision par ordinateur / Point cloud refinement and surface reconstruction are two fundamental problems in geometry processing. Most of the existing methods have been targeted at range sensor data and turned out be ill-adapted to multi-view data. In this thesis, two novel methods are proposed respectively for the two problems with special attention to multi-view data. The first method smooths point clouds originating from multi-view reconstruction without impairing the data. The problem is formulated as a nonlinear constrained optimization and addressed as a series of unconstrained optimization problems by means of a barrier method. The second method triangulates point clouds into meshes using an advancing front strategy directed by a sphere packing criterion. The method is algorithmically simple and can produce high-quality meshes efficiently. The experiments on synthetic and real-world data have been conducted as well, which demonstrates the robustness and the efficiency of the methods. The developed methods are suitable for applications which require accurate and consistent position information such photogrammetry and tracking in computer vision

A reconstrução de superfícies a partir de nuvens de pontos faz parte de um novo paradigma de modelagem em que modelos computacionais para objetos reais são reconstruídos a partir de dados amostrados sobre a superfície dos mesmos. O principal problema que surge nesse contexto é o fato de que não são conhecidas relações de conectividade entre os pontos que compõe a amostra. Os objetivos do presente trabalho são estudar métodos implícitos para a reconstrução de superfícies e propor algumas melhorias pouco exploradas por métodos já existentes. O uso de funções implícitas no contexto da reconstrução conduz a métodos mais robustos em relação a ruídos, no entanto, uma das principais desvantagens de tais métodos está na dificuldade de capturar detalhes finos e sharp features. Nesse sentido, o presente trabalho propõe o uso de abordagens adaptativas, tanto na poligonalização de superfícies quanto na aproximação de superfícies. Além disso, questões relativas à robustez das soluções locais e à qualidade da malha também são abordadas. Por fim, o método desenvolvido é acoplado aumsoftware traçador de raios afimde se obterumamaneira de modelar cenas tridimensionais utilizando nuvens de pontos, além dos objetos gráficos tradicionais. Os resultados apresentados mostram que muitas das soluções propostas oferecem um incremento à qualidade dos métodos de reconstrução anteriormente propostos / Surface reconstruction from point clouds is part of a new modeling paradigm in which computational models for real objects are reconstructed from data sampled from their surface. The main problem that arises in this context is the fact that there are no known connectivity relationships amongst the points that compose the sample. The objectives of the present work are to study implicit methods for surface reconstruction and to propose some improvements scarcely explored by previous work. The use of implicit functions in the context of surface reconstruction leads to less noise sensitive methods; however, one major drawback of such methods is the difficulty in capturing fine details and sharp features. Towards this, the present work proposes the use of adaptive approaches, not only in the polygonization but also in the surface approximation. Besides, robustness issues in local solutions and mesh quality are also tackled. Finally, the developed method is embedded in a ray tracer software in order to set a basis for modeling tridimensional scenes using point sets, in addition to traditional graphic objects. The presented results show that a great deal of the proposed solutions offer a quality increase to the reconstruction method previously proposed

Submitted by Silvana Teresinha Dornelles Studzinski (sstudzinski) on 2015-10-27T14:34:35Z No. of bitstreams: 1 Fabrício Müller da Silva_.pdf: 105910980 bytes, checksum: 4ce66a9d5fff9a2b2a97835c54dac355 (MD5) / Made available in DSpace on 2015-10-27T14:34:35Z (GMT). No. of bitstreams: 1 Fabrício Müller da Silva_.pdf: 105910980 bytes, checksum: 4ce66a9d5fff9a2b2a97835c54dac355 (MD5) Previous issue date: 2015-08-31 / Nenhuma / Este trabalho apresenta um algoritmo para simplificação de nuvens de pontos baseado na inclinação local da superfície amostrada pelo conjunto de pontos de entrada. O objetivo é transformar a nuvem de pontos original no menor conjunto possível, mantendo as características e a topologia da superfície original. O algoritmo proposto reamostra de forma adaptativa o conjunto de entrada, removendo pontos redundantes para manter um determinado nível de qualidade definido pelo usuário no conjunto final. O processo consiste em um particionamento recursivo do conjunto de entrada através da Análise de Componentes Principais (PCA). No algoritmo, PCA é aplicada para definir as partições sucessivas, para obter uma aproximação linear (por planos) em cada partição e para avaliar a qualidade de cada aproximação. Por fim, o algoritmo faz uma escolha simples de quais pontos serão mantidos para representar a aproximação linear de cada partição. Estes pontos formarão o conjunto de dados final após o processo de simplificação. Para avaliação dos resultados foi aplicada uma métrica de distância entre malhas de polígonos, baseada na distância de Hausdorff, comparando a superfície reconstruída com a nuvem de pontos original e aquela reconstruída com a nuvem filtrada. Os resultados obtidos com o algoritmo conseguiram uma taxa de até 95% de compactação do conjunto de dados de entrada, diminuindo o tempo total de execução do processo de reconstrução, mantendo as características e a topologia do modelo original. A qualidade da superfície reconstruída com a nuvem filtrada também é atestada pela métrica de comparação. / This paper presents a simple and efficient algorithm for point cloud simplification based on the local inclination of the surface sampled by the input set. The objective is to transform the original point cloud in a small as possible one, keeping the features and topology of the original surface. The proposed algorithm performs an adaptive resampling of the input set, removing unnecessary points to maintain a level of quality defined by the user in the final dataset. The process consists of a recursive partitioning in the input set using Principal Component Analysis (PCA). PCA is applied for defining the successive partitions, for obtaining the linear approximations (planes) for each partition, and for evaluating the quality of those approximations. Finally, the algorithm makes a simple choice of the points to represent the linear approximation of each partition. These points are the final dataset of the simplification process. For result evaluation, a distance metric between polygon meshes, based on Hausdorff distance, was defined, comparing the reconstructed surface using the original point clouds and the reconstructed surface usingthe filtered ones. The algorithm achieved compression rates up to 95% of the input dataset,while reducing the total execution time of reconstruction process, keeping the features and the topology of the original model. The quality of the reconstructed surface using the filtered point cloud is also attested by the distance metric.

In this work the electronic and structural properties of anatase TiO2 (100) surface and gold adsorption have been investigated by using the first-principles calculations based on density functional theory (DFT). TiO2 is a wide band-gap material and to this effects it finds numerous applications in technology such as, cleaning of water, self-cleaning, coating, solar cells and so on. Primarily, the relation between the surface energy of the anatase (100)-1x1 phase and the TiO2-layers is examined. After an appropriate atomic layer has been chosen according to the stationary state of the TiO2 slab, the adsorption behavior of the Au atom and in the different combinations are searched for both the surface and the surface which is supported by a single Au atom/atoms. It has been observed that a single Au atom tends to adsorb to the surface which has an impurity of Au atom or atoms. Although, the high metal concentration on the surface have increased the strength of the adsorption, it is indicated that the system gains a metallic property which is believed to cause problems in the applications. In addition, the gold clusters of the dimer (Au2) and the trimer (Au3) have been adsorbed on the surface and their behavior on the surface is investigate. It is observed that the interaction between Au atoms in the atomic cluster each other is stronger than that of gold clusters and the surface.

This thesis describes theory and algorithms for use with Radial Basis Functions (RBFs), emphasising techniques motivated by three particular application areas. In Part I, we apply RBFs to the problem of interpolating to integral data. While the potential of using RBFs for this purpose has been established in an abstract theoretical context, their use has been lacking an easy to check sufficient condition for finding appropriate parent basic functions, and explicit methods for deriving integral basic functions from them. We present both these components here, as well as explicit formulations for line segments in two dimensions and balls in three and five dimensions. We also apply these results to real-world track data. In Part II, we apply Hermite and pointwise RBFs to the problem of surface reconstruction. RBFs are used for this purpose by representing the surface implicitly as the zero level set of a function in 3D space. We develop a multilevel piecewise technique based on scattered spherical subdomains, which requires the creation of algorithms for constructing sphere coverings with desirable properties and for blending smoothly between levels. The surface reconstruction method we develop scales very well to large datasets and is very amenable to parallelisation, while retaining global-approximation-like features such as hole filling. Our serial implementation can build an implicit surface representation which interpolates at over 42 million points in around 45 minutes. In Part III, we apply RBFs to the problem of animation control in the area of motion synthesis---controlling an animated character whose motion is entirely the result of simulated physics. While the simulation is quite well understood, controlling the character by means of forces produced by virtual actuators or muscles remains a very difficult challenge. Here, we investigate the possibility of speeding up the optimisation process underlying most animation control methods by approximating the physics simulator with RBFs.

Many computer vision applications such as image filtering, segmentation and stereovision can be formulated as optimization problems. Recently discrete, convex, globally optimal methods have received a lot of attention. Many graph-based methods suffer from metrication artefacts, segmented contours are blocky in areas where contour information is lacking. In the first part of this work, we develop a discrete yet isotropic energy minimization formulation for the continuous maximum flow problem that prevents metrication errors. This new convex formulation leads us to a provably globally optimal solution. The employed interior point method can optimize the problem faster than the existing continuous methods. The energy formulation is then adapted and extended to multi-label problems, and shows improvements over existing methods. Fast parallel proximal optimization tools have been tested and adapted for the optimization of this problem. In the second part of this work, we introduce a framework that generalizes several state-of-the-art graph-based segmentation algorithms, namely graph cuts, random walker, shortest paths, and watershed. This generalization allowed us to exhibit a new case, for which we developed a globally optimal optimization method, named "Power watershed''. Our proposed power watershed algorithm computes a unique global solution to multi labeling problems, and is very fast. We further generalize and extend the framework to applications beyond image segmentation, for example image filtering optimizing an L0 norm energy, stereovision and fast and smooth surface reconstruction from a noisy cloud of 3D points

Oxide ultra-thin film surfaces have properties and structures that are significantly different from the terminations of the corresponding bulk crystals. For example, surface structures of epitaxial ultra-thin oxide films are highly influenced by the crystallinity and electronegativity of the metal substrates they grown on. Some enhanced properties of the novel reconstructions are related to catalysis, sensing and microelectronics, which has resulted in an increasing interest in this field. Ultra-thin TiO_x films were grown on Au(111) substrates in this work. Two well-ordered structures within monolayer coverage - honeycomb (HC) and pinwheel - were generated and investigated. Special attention has been paid to the uniform (2 x 2) Ti₂O₃ HC phase including its regular structure and imperfections such as domain boundaries (DBs) and point defects. Linear DBs with long-range repeating units have been observed; density functional theory (DFT) modelling has been used to simulate their atomic structures and calculate their formation energies. Rotational DBs/defects show up less frequently, however a six-fold symmetrical 'snowflake' DB loop stands out. Two types of point defects have been discovered and assigned to Ti vacancies and oxygen vacancies/hydroxyl groups. Their diffusion manners and pairing habits have been discussed within an experimental context. The results of growing NbO_x ultra-thin films on Au(111) are also presented in this thesis. An identical looking (2 x 2) HC structure to the Ti₂O₃ ultra-thin film has been formed; a stoichiometry of Nb2O3 is suggested. Another interesting reconstruction is a hollow triangle structure. Various sizes have been found, and sides of these equilateral triangles all show a double-line feature aligned along the { 1 ₁⁻ } directions of the Au(111) lattice. Chemical composition characterisations of NbO_x thin films are still required as is DFT modelling. Experimental techniques used in this thesis include scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Ultra-thin oxide films were created by physical vapour deposition (PVD) in ultra-high vacuum (UHV) systems.

Image-based 3D reconstruction refers to the capture and virtual reconstruction of real scenes, through the use of ordinary camera sensors. A common approach is the use of the algorithms Structure from Motion, Multi-view Stereo and Poisson Surface Reconstruction, that fares well for many types of scenes. However, a problem that this pipeline suffers from is that it often falters when it comes to texture-less surfaces and areas, such as those found in man-made environments. Building facades, roads and walls often lack detail and easily trackable feature points, making this approach less than ideal for such scenes. To remedy this weakness, this thesis investigates an expanded approach, incorporating line segment detection and line cloud generation into the already existing point cloud-based pipeline. Texture-less objects such as building facades, windows and roofs are well-suited for line segment detection, and line clouds are fitting for encoding 3D positional data in scenes consisting mostly of objects featuring many straight lines. A number of approaches have been explored in order to determine the usefulness of line clouds in this context, each of them addressing different aspects of the reconstruction procedure.

Métodos de representação de superfícies a partir de pontos não-organizados se mantêm como uma das principais vertentes científicas que aquecem o estado-da-arte em Computação Gráfica e, significativamente, estão sendo reconhecidos como uma ferramenta interessante para definição de interfaces móveis no contexto de simulações numéricas de escoamento de fluidos. Não é difícil encontrar motivos para tais fatos: pelo lado da computação gráfica, por exemplo, a manipulação de conjuntos de pontos massivos com geometrias complexas e sujeitos a informações ruidosas ainda abre margem para novas metodologias. Já no âmbito da mecânica dos fluidos, onde os dados não são originados de \\emph tridimensionais, mas sim de interfaces entre fluidos imiscíveis, mecanismos de representação de superfícies a partir de pontos não-organizados podem apresentar características computacionais e propriedades geométricas que os tornem atrativos para aplicações em simulação de fenômenos físicos. O objetivo principal dessa tese de doutorado foi, portanto, o desenvolvimento de técnicas de representação de superfícies a partir de pontos não-organizados, que sejam capazes de suprir restrições de importantes trabalhos prévios. Nesse sentido, primeiramente focalizamos a elaboração de técnicas baseadas em formulações de mínimos-quadrados-móveis e de uma técnica robusta de partição da unidade implícita adaptativa em duas vias. Além de mecanismos de representação de superfícies a partir de pontos não-organizados, também propusemos um método promissor para representação de interfaces em simulação numérica de escoamento de fluidos multifásicos. Para isso, embasamo-nos numa abordagem Lagrangeana (livre-de-malhas), fundamentada no método dos mínimos-quadrados-móveis algébricos e apresentamos diversos resultados numéricos, estudos de convergências e comparações que evidenciam o potencial dessa metodologia para simulações numéricas de fenômenos físicos. Apesar de a contribuição principal deste trabalho ser o desenvolvimento de métodos para representação de superfícies a partir de pontos não-organizados, a experiência que adquirimos no desenvolvimento dessas técnicas nos conduziu à elaboração de mecanismos para representação de dados volumétricos não-organizados. Por conta disso, apresentamos dois mecanismos de representação a partir de dados volumétricos não-organizados com o intuito de serem aplicáveis a informações oriundas de malhas contendo células arbitrárias, isto é, propusemos a definição de um método de rendering unificado / Surface reconstruction from unorganized points has been one of the most promising scientific research areas in Computer Graphics. In addition, it has been used successfully for the definition of fluid interface in numerical simulation of fluid flow. There are several reasons to that fact: for instance, considering Computer Graphics, we have the handling of out-of-core data from complicated geometries and subject to noisy information that brings out opportunities for the development of new techniques. Further, considering Numerical Fluid Mechanics, where the input data does not come from tridimensional scanners, but from fluid interfaces, schemes that define the surface from unorganized points can offer geometrical and computational properties useful to numerical fluid flow simulation. The main goal of this project was the development of novel techniques for reconstructing surfaces from unorganized points with the capability to overcome the main drawbacks of important previous work. To that end, first we focused on the development of techniques based on moving-least-squares and on a robust twofold partition of unity Implicits. Added to the development of surface reconstruction from unorganized points, we proposed a novel scheme for defining fluid flow interfaces. We approach a meshless Lagrangian based on algebraic moving-least-squares surfaces. In addition, we presented several numerical results, convergence tests and comparisons, which state the power of the method to numerical simulation of physical phenomena. Although our main contributions were focused on surface reconstruction from points, we proposed methods to function reconstruction from unorganized volumetric data. Thus, we present two schemes to represent volumetric data from arbitrary meshes, i.e., a unified rendering scheme