Thesis (MScIng)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Thesis (Ph. D.)--Baylor University, 2006. / Includes bibliographical references (p. 133-144).

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 103-121).

Growth and evolution of germanium (Ge) nanocrystals embedded into a silicon oxide (SiO₂) system have been studied based on the Ge content of co-sputtered Ge-SiO₂ films using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy. It was found that when the proportion of Ge relative to Ge oxide is 20%, TEM showed that annealing the samples at 800°C for 60 min resulted in the formation of a denuded region between the silicon/silicon oxide (Si/SiO₂) interface and a band of Ge nanocrystals towards the surface of the film. By introducing a 20nm thick thermal oxide barrier on top of the silicon (Si) substrate on which the film is deposited, no denuded region in the bulk of this sample is observed. It is proposed that this barrier is effective in reducing both Ge diffusion into the Si substrate and Si diffusion from the substrate into the film. Si diffusing from the Si substrate reduces the Ge oxide into Ge which can subsequently diffuse into the Si substrate. However, the oxide barrier is able to confine the Ge within the oxide matrix so that the denuded region in the bulk of the film cannot form. However the reduction in diffusion should be more significant for Ge as its diffusion coefficient is lower than Si due to its larger size. It is suggested that the denuded region consists of amorphous Ge diffusing towards the Si/SiO₂ interface. When the Ge content is increased to slightly more than 70%, TEM showed that Ge nanocrysyals formed after annealing at 800°C for only 30 min for samples with and without the oxide barrier. There is no denuded region between the Ge nanocrystals band and the Si/SiO₂ interface for both samples but it was observed that coarsening effects were more prominent in the film deposited on top of the oxide barrier. The reduction effect of Si on Ge oxide should not play a significant role in these samples as the Ge content is high. / Singapore-MIT Alliance (SMA)

Data representations in low dimensions such as results from unsupervised dimensionality reduction methods are often visually interpreted to find clusters of observations. To identify clusters the result must be appreciably clustered. This property of a result may be called "clusteredness". When judged visually, the appreciation of clusteredness is highly subjective. In this paper we suggest an objective way to assess clusteredness in data representations. We provide a definition of clusteredness that captures important aspects of a clustered appearance. We characterize these aspects and define the extremes rigorously. For this characterization of clusteredness we suggest an index to assess the degree of clusteredness, coined the OPTICS Cordillera. It makes only weak assumptions and is a property of the result, invariant for different partitionings or cluster assignments. We provide bounds and a normalization for the index, and prove that it represents the aspects of clusteredness. Our index is parsimonious with respect to mandatory parameters but also exible by allowing optional parameters to be tuned. The index can be used as a descriptive goodness-of-clusteredness statistic or to compare different results. For illustration we use a data set of handwritten digits which are very differently represented in two dimensions by various popular dimensionality reduction results. Empirically, observers had a hard time to visually judge the clusteredness in these representations but our index provides a clear and easy characterisation of the clusteredness of each result. (authors' abstract) / Series: Discussion Paper Series / Center for Empirical Research Methods

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The mobility of trace metals and radionuclides released into aquatic and terrestrial environments by mining, industrial processes, and municipal waste disposal practices is an area that deserves significant scientific, public health, and regulatory attention. Indirect microbial interaction offers one potential mechanism for immobilizing these contaminants. For example, many metals, such as uranium and chromium, are less soluble once bound as iron oxide precipitates, thus inhibiting the spread of soluble heavy metals and radionuclides within groundwater and halting them from contaminating additional areas. Microbial iron oxidation is known to contribute to the immobilization of heavy metals and radionuclides in contaminated sites. A safe, cost-effective decontamination method for these materials is the association of radionuclides with iron oxides formed via microbial pathways, thus precipitating them out of solution and providing a promising technique for in situ bioremediation. Nitrate-dependent iron-oxidizing bacteria have been shown to play an important role in the retention of soluble uranium by forming iron oxides that absorb onto soluble U(VI) particles, rendering them immobile as U(VI)-iron oxides. Dechloromonas aromatica strain RCB is a β-proteobacterium that has been cultured and extensively studied in our laboratory and is capable of reducing perchlorate and anaerobically oxidizing benzene, humic acids, and ferrous iron. In addition, a newly-isolated β-proteobacterium, Diaphorobacter sp. strain TPSY, is of interest due to its ability to anaerobically oxidize humic acids, uranium, and ferrous iron. Thus, these two strains have enormous bioremediative potential and are prime candidates for in situ bioremediation. Microarray analysis was used to identify genes induced under iron-oxidizing conditions and RNA dot blotting was used to confirm mRNA expression in D. aromatica. As a follow-up, RNA arbitrarily primed (RAP)-PCR, a method used to randomly reverse-transcribe RNA into cDNA, was also used to identify expression that might not have been observed in the microarray. Genes that were identified from both microarray and RAP-PCR experiments include several hypothetical proteins, lipoproteins, and transmembrane proteins located in major operons, as well as genes annotated as signal transduction enzymes, c-type cytochromes, and proteins involved in chemotaxis, flagellar, and pilus development. Suicide vectors were used to create deletion mutations targeting the genes of interest. Additionally, transposon mutagenesis was used in Diaphorobacter sp. TPSY to identify any potential iron-oxidizing mutants. Out of seven TPSY mutants deficient in iron oxidation, four were identified as genes that encode an inner membrane protein, a signal transduction protein, a putative lipoprotein downstream of a cytochrome c, and a regulatory DNA-binding protein. Mutants were confirmed for their inability to oxidize iron by measuring Fe(II) concentrations over time with a ferrozine assay. The identification of genes involved in microbial anaerobic nitrate-dependent iron oxidation will prove to be a valuable asset when designing and assessing bioremediative strategies.

This research deals with the synthesis and characterization of titanium diboride (TiB2) from novel carbon coated precursors. This work provides information on using different boron sources and their effect on the resulting powders of TiB2.The process has two steps in which the oxide powders were first coated with carbon by cracking of a hydrocarbon gas, propylene (C3H6) and then, mixed with boron carbide and boric acid powders in a stoichiometric ratio. These precursors were treated at temperatures in the range of 1200-1400° C for 2 h in flowing Argon atmosphere to synthesize TiB2.The process utilizes a carbothermic reduction reaction of novel carbon coated precursor that has potential of producing high-quality powders (sub-micrometer and high purity). Single phase TiB2 powders produced, were compared with commercially available titanium diboride using X-ray diffraction and Transmission electron microscopy obtained from boron carbide and boric acid containing carbon coated precursor.

In this paper, I provide an interpretation of Frege's logicist project, drawing a connection between it and his idiosyncratic view of truth.

This article provides a framework for assessing and quantifying "clusteredness" of a data representation. Clusteredness is a global univariate property defined as a layout diverging from equidistance of points to the closest neighboring point set. The OPTICS algorithm encodes the global clusteredness as a pair of clusteredness-representative distances and an algorithmic ordering. We use this to construct an index for quantification of clusteredness, coined the OPTICS Cordillera, as the norm of subsequent differences over the pair. We provide lower and upper bounds and a normalization for the index. We show the index captures important aspects of clusteredness such as cluster compactness, cluster separation, and number of clusters simultaneously. The index can be used as a goodness-of-clusteredness statistic, as a function over a grid or to compare different representations. For illustration, we apply our suggestion to dimensionality reduced 2D representations of Californian counties with respect to 48 climate change related variables. Online supplementary material is available (including an R package, the data and additional mathematical details).