A eletrônica molecular vem sendo investigada intensivamente por mais de vinte anos. Nesse sentido, as pesquisas científicas estão sendo focadas na busca de estruturas que possam ser utilizadas na construção de dispositivos em escalas nanométricas, que possam substituir a tecnologia tradicional do silício. O objetivo principal deste trabalho foi explorar as propriedades físicas de sistemas a base de grafano, um dos mais promissores materiais para serem usados como nanodispositivos. Para isso, foi realizada uma investigação teórica, baseada em cálculos de primeiros princípios, das propriedades estruturais e eletrônicas do grafeno numa forma pura ou com defeitos intrínsecos e extrinsecos. O primeiro grupo de estruturas investigadas foi o grafeno e grafano como nanofolhas constituídas por elementos do grupo IV da tabela periódica (C, SiC, Si, Ge e Sn). Também foram analisadas as mudanças nas propriedades eletrônicas do grafano do grupo IV com a substituição dos átomos de hidrogênio por flúor. A segunda parte do trabalho explorou as propriedades de defeitos estruturais em grafeno, tais como a monovacância, divacância, trivacância e Stone-Wales, e também o grafeno com dopantes (boro e nitrogênio) em diversas configurações. Todos os cálculos foram feitos utilizando métodos ab initio com base na teoria do funcional densidade. Foram estudadas algumas possíveis aplicações para os grupos de estruturas de grafeno investigados, através da análise de algumas de suas propriedades, tais como as densidades de estados próximas ao nível de Fermi e as estruturas de bandas eletrônicas para cada sistema. / The molecular electronics has been investigated for more than twenty years. In this sense, the scientific research has been focused on the search for structures that could be used in nanoelectronic devices that could replace the traditional silicon technology. The major goal of this work is to explore the physical properties of systems based on graphene, one of the most promising materials to be used in nanoelectronics. For that, an ab initio investigation was carried on the structural and electronic properties of graphene in its pristine form and with intrinsic and extrinsic defects. The first investigation explored the properties of group IV nanosheets (of C, SiC, Si, Ge e Sn), and the modifications on their properties as result of hydrogenation or fluorination. The second part of this work explored the physical properties of structural intrinsic defects in graphene, such as monovacancy, divacancy, trivacancy, and Stone-Wales ones. The work also explored the properties of boron and nitrogen dopants. All the calculations were performed using the ab initio methodology, based on the density functional theory.

Nowadays, business intelligence techniques are applied more and more often in different settings including corporations and organizations both in the private and public sector. It is really a broad field which can assist business people to realize the state of their organization and make profitable decisions. In this thesis, I will focus on one of its components, data warehouse, by proposing activity theory as the method to solve the dimension identification problem in data warehouse. Under the background of project IMIS and the involved personnel, who determine the dimension, firstly I study how to use the ER method, “bottom up” method, and activity theory method to identify the dimension in data warehouse, and some relevant knowledge about the three methods. Then, we apply the three methods to identify the dimension. After that, I evaluate the dimension identification results of the three methods according to the feedback from the healthcare organization to get their veracity and integrality. Finally, based on the results of my efforts, I arrive to the conclusion that the activity theory method can be applied to identify the dimension in data warehouse, and with the comparison to the other two traditional methods (ER model and “bottom up”), the activity theory method is more easy and natural to identify the dimension of a dimensional model.

Two dimensional (2D) systems with low carrier density is an outstanding platform for studying a wide spectrum of physics. These include both classical and quantum effects, arising from disorder, Coulomb interactions and even non-trivial topological properties of band-structure. In this thesis, we have explored the physics at low carrier number density in GaAs/AlGaAs heterostructure and bilayer graphene, by investigating in a larger phase space using a variety of electrical measurement tools. A two-dimensional electron system (2DES) formed in a GaAs/AlGaAs heterostructure offers an avenue to build a variety of mesoscopic devices, primarily because its surface gates can very effectively control its carrier density profile. In the first half of the thesis, we study the relevance of disorder in two kinds of devices made in a 2DES. A very strong negative gate voltage not only reduces the carrier density of the 2DES, but also drives it to a disordered state. In this state, we explore a new direction in parameter space by increasing in-plane electric field and investigating its magneto-resistance (MR). At sufficiently strong gate voltage and source-drain bias, we discover a remarkably linear MR. Its enormous magnitude and weak temperature dependence indicate that this is a classical effect of disorder. In another study, we examine a specially designed dual-gated device that can induce low number density in a periodic pattern. By applying appropriate gate voltages, we demonstrate the formation of an electrostatically tunable quantum dot lattice and study the impact of disorder on it. This work is important in paving way for solid state based platform for experimental simulations of artificial solids. The most striking property of bilayer graphene is the ability to open its band gap by a perpendicular electric field, giving the prospects of enabling a large set of de-vice applications. However, despite a band gap, a number of transport mechanisms are still active at very low densities that range from hopping transport through bulk to topologically protected 1D transport at the edges or along 1D crystal dislocations. In the second half of the thesis, we have used higher order statistical moment of resistance/conductance fluctuations, namely the variance of the fluctuations, to complement averaged resistance/conductance, and study and infer the dominant transport mechanism at low densities in a gapped bilayer graphene. Our results show possible evidence of percolative transport and topologically protected edge transport at different ranges of low number densities. We also explore the same phase space by studying its mesoscopic conductance fluctuations at very low temperatures. This is the first of its kind systematic experiment in a dual-gated bilayer graphene device. Its conductance fluctuations have several anomalous features suggesting non-universal behaviour which is at odds with conventional disordered systems.

Abstract The thesis deals with numerical methods designed for the modeling and inversion of geophysical electromagnetic (EM) measurements using a conductive thin plate model. The main objectives are to study the EM induction problem in general and to develop practical interpretation tools for mineral prospecting in particular. The starting point is a linearized inversion method based on the singular value decomposition and a new adaptive damping method. The inversion method is introduced to the interpretation of time-domain EM (TEM) measurements using a thin plate in free-space. The central part of the thesis is a new approximate modeling method, which is based on an integral equation approach and a special lattice model. At first the modeling method is applied to the interpretation of frequency-domain EM (FEM) data using a thin plate in conductive two-layered earth. After this time-domain responses are modeled applying a Fourier-sine transform of broadband FEM computations. The results demonstrate that the approximate computational method can model the geophysical frequency and time-domain EM responses of a thin conductor in conductive host medium with sufficient accuracy, and that the inversion method can provide reliable estimates for the model parameters. The fast forward computation enables interactive interpretation of FEM data and feasible forward modeling of TEM responses. The misfit function mapping and analysis of the singular value decomposition have provided additional information about the sensitivity, resolution, and the correlation behavior of the thin plate parameters.

The study of circulating biomarkers and their association with disease outcomes has become progressively complex due to advances in the measurement of these biomarkers through multiplex technologies. Although the availability of numerous serum biomarkers is highly promising, multiplex assays present statistical challenges due to the high dimensionality of these data. In this dissertation, three studies are presented that address these challenges using L1 penalized regression methods. In the first part of the dissertation, an extensive simulation study is performed for the logistic regression model that compares the Least Absolute Shrinkage and Selection Operator (LASSO) method with five LASSO-type methods given scenarios that are present in serum biomarker research, such as high correlation between biomarkers, weak associations with the outcome, and sparse number of true signals. Results show that choice of optimal LASSO-type method is dependent on data structure and should be guided by the research objective. Methods are then applied to the Tucson Epidemiological Study of Airway Obstructive Disease (TESAOD) study for the identification of serum biomarkers of overweight and obesity. Measurement of serum biomarkers using multiplex technologies may be more variable as compared to traditional single biomarker methods. Measurement error may induce bias in parameter estimation and complicate the variable selection process. In the second part of the dissertation, an existing measurement error correction method for penalized linear regression with L1 penalty has been adapted to accommodate validation data on a randomly selected subset of the study sample. A simulation study and analysis of TESAOD data demonstrate that the proposed approach improves variable selection and reduces bias in parameter estimation for validation data as small as 10 percent of the study sample. In the third part of the dissertation, a measurement error correction method that utilizes validation data is proposed for the penalized logistic regression model with the L1 penalty. A simulation study and analysis of TESAOD data are used to evaluate the proposed method. Results show an improvement in variable selection.

Time series is one of the main research objects in the field of data mining. Visualization is an important mechanism to present processed time series for further analysis by users. In recent years researchers have designed a number of sophisticated visualization techniques for time series. However, most of these techniques focus on the static format, trying to encode the maximal amount of information through one image or plot. We propose the pixel video technique, a visualization technique displaying data in video format. Using pixel video technique, a hierarchal dimension cluster tree for generating the similarity order of dimensions is first constructed, each frame image is generated according to pixeloriented techniques displaying the data in the form of a video.

Ph.D. / In this thesis we undertake a study of formal three-dimensional representational and acceptor methods. In lieu hereof then, we give a short overview of such strategies existing in the literature. Graph and graph grammar theory present us with a powerful two dimensional representational method, and we propose to extend these concepts to the three-dimensional case. We therefore give a short discussion on the theory of graphs and graph grammars. As point of departure, we review the concepts of a structure parameter and structure graph (SG) introduced by us in [BEH,93] and show that these concepts enable us to describe objects in three-dimensional space. We propose various modified graph grammar extensions that generates structure graphs, referred to as Structure Graph Grammar extensions (SGG's) by combining context provisions with the rewriting rules of the various grammar systems. This proposed methodology of ours culminates in the combination of production rule bounded contexts and globally specified contexts, thus defining Structure Graph Grammar extension 7 (SGG-7). We show the applicative value of the three dimensional generative abilities of SGG's by considering the generation of various chemical structural formulae. Brandenburg and Skodinis mentions in [BS,95] that there is a shortcoming in the theory of graph grammars in the sense that in general, there exists no accepting device for graph grammar systems. The following quote from [BS,95,p.336] illustrates this point: "There are no graph automata, which fit to the major classes of graph languages. This is a gap in the theory of graph languages." Regarding the class of languages generated by SGG-7, we propose to fill this gap by introducing an Structure Graph Automaton (SGA) to accept this class of languages.

The present explosion in digital communications and multi-user wireless cellular networks has urged a demand for more effective modulation methods, utilizing the available frequency spectrum more efficiently. To accommodate a large number of users sharing the same available frequency band, one requirement is the availability of large families of spreading sequences with excellent AC and CC properties. Another requirement is the availability of sets of orthogonal basis functions to extend capacity by exploiting all available degrees of freedom (e.g., temporal, frequency and spatial dimesions), or by employing orthogonal multi-code operation in parallel, such as used in the latest 3GPP and 3GPP2 Wide-band Code Division Multiple Access (WCDMA) modulation standards by employing sets of orthogonal Walsh codes to improve the overall data throughput capacity. The generic Direct Sequence Spread Spectrum (DSSS) transmitter developed in this dissertation has originally been designed and implemented to investigate the practicality and usefulness of complex spreading sequences, and secondly, to verify the concept of non-linearly interpolated root-of-unity (NLI-RU) filtering. It was found that both concepts have a large potential for application in point-to-point, and particularly micro-cellular Wireless Local Area Networks (WLANs) and Wireless-Local-Loop (WLL) environments. Since then, several novel concepts and subsystems have been added to the original system, some of which have been patented both locally and abroad, and are outlined below. Consequently, the ultimate goal of this research project was to apply the principles of the generic DSSS transmitter and receiver developed in this study in the implementation of a WLL radio-frequency (RF)-link, and particularly towards the establishment of affordable wireless multimedia services in rural areas. The extended coverage at exceptionally low power emission levels offered by the new design will be particularly useful in rural applications. The proposed WLL concept can for example also be utilized to add a unique mobility feature to for example existing Private Automatic Branch Exchanges (PABXs). The proposed system will in addition offer superior teletraffic capacity compared to existing micro-cellular technologies, e.g., the Digital European Cordless Telephony (DECT) system, which has been consider by Telkom for employment in rural areas. The latter is a rather outdated interim standard offering much lower spectral efficiency and capacity than competitive CDMA-solutions, such as the concept analyzed in this dissertation, which is based on the use of unique large families of spectrally well confined (i.e., band-limited) constant envelope (CE) complex spreading sequences (CSS) with superior correlation properties. The CE characteristic of the new spreading sequences furthermore facilitates the design of systems with superior power efficiency and exceptionally robust performance characteristics (much less spectral re-growth) compared to existing 2G and 3G modulation standards, in the presence of non-linear power amplification. This feature allows for a system with larger coverage for a given performance level and limited peak power, or alternatively, longer battery life for a given maximum communication distance and performance level, within a specified fixed spreading bandwidth. In addition, the possibility to extend the concept to orthogonal multi-code operation provides for comparable capacity to present 3G modulation standards, while still preserving superior power efficiency characteristics in non-linear power amplification. Conventional spread spectrum communication systems employ binary spreading sequences, such as Gold or Kasami sequences. The practical implementation of such a system is relatively simple. The design and implementation of a spread-spectrum communication system employing complex spreading sequences is however considerable more complex and has not been previously presented, nor been implemented in hardware. The design of appropriate code lock loops for CSS has led to a unique design with 3dB performance advantage compared to similar loops designed for binary spreading sequences. The theoretical analysis and simulation of such a system will be presented, with the primary focus on an efficient hardware implementation of all new concepts proposed, in the form of a WLL RF-link demonstrator. / Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

This study investigates learning difficulties involving volumes of solids of revolution (VSOR) at two FET colleges in Gauteng province, in South Africa. The research question for this study was: Why do students have difficulty when learning about volumes of solids of revolution? In order to answer the research question five skill factors were identified as the conceptual framework, subdivided into 11 elements. The five skill factors are: I. Graphing skills and translating between visual graphs and algebraic equations/expressions, II. Three-dimensional thinking, III. Moving between discrete and continuous representations, IV. General manipulation skills and V. Consolidation and general level of cognitive development. Before collecting the main data for this study, a preliminary study and a pilot study were conducted. The data for the main study were then collected in six different investigations. The investigations consisted of two runs of a questionnaire, classroom observations, examination analysis; detailed examination responses and an interview with one student. The results from the questionnaire runs as well as the pilot study reveal that students performed poorly in tasks involving three-dimensional thinking (Skill factor II), moving between discrete and continuous representations (Skill factor III), and consolidation and general level of cognitive development (Skill factor V). Students' performance was satisfactory in tasks involving graphing skills and translating between visual graphs and algebraic equations/expressions (Skill factor I) and general manipulation skills (Skill factor IV). Students were also more competent in solving problems that involved procedural skills than those that required conceptual skills. The challenges that students were faced with in class, evident from the classroom observations allude to the fact that the topic of VSOR is difficult to teach and to learn. It is recommended that VSOR be taught and assessed more conceptually in line with the five skill factors; that curriculum developers must communicate with other stakeholders like industries and other institutions of higher learning and that the Department of Education must provide adequate training for these teachers and liaise with industry in this regard. It is also recommended that the suitability of this topic for the particular cohort of students be reconsidered as it appears to be of too high cognitive demand. / Thesis (PhD)--University of Pretoria, 2012. / Mathematics and Applied Mathematics / unrestricted

A new stereo matching paradigm is introduced as an integrated process of highly discriminating steps, adopting congruously all the fundamental steps of the stereo vision problem. The central objective is the extraction of a disparity map from which the depth map will be derived. A unique representation of the two dimensional (2-D) stereo images into linear, orthogonal, and spatially-varying attributes serve as the mathematical foundation from which the proposed stereo matching method has evolved. The devised attributes contribute equally to the decision making process and provide information on the characterization of a potential match and its validation through a consistency check. A fundamental contribution of this thesis is in creating the possibility for the design of a dimensionally-augmented vision system (2½ -D representation) based on an effective stereo paradigm with realistic computational requirements. In this design configuration, the geometrical mappings between the 3-D real-world measurements with the measurements obtained using the proposed 2½ -D-D representation are established. Computer results for the intended objective of creating highly accurate disparity maps for various scenes with varying complexities clearly demonstrate the soundness of the proposed method both in terms of its matching effectiveness and its realistic computational power requirements. Future objectives point to the development of enhanced algorithms for scene interpretation and understanding based on this augmented representation.