3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Electrical transport properties of nitrogen doped carbon microspheres(2014-07-22) Wright, William PatrickA suite of four samples of nitrogen doped carbon microspheres, each with a di erent level of nitrogen dopant, was synthesised in a horizontal chemical vapour deposition reaction. The samples were characterized using scanning electron microscopy, Raman spectroscopy and electron paramagnetic resonance spectroscopy. Scanning electron microscopy showed that microspheres were produced by the reaction. Raman spectroscopy con rmed the graphitic nature of the samples. Electron paramagnetic resonance spectroscopy determined that nitrogen was present in the graphitic lattice and was used as a non-destructive technique to measure the amount of substitutional nitrogen present in the samples. In order to perform electrical transport measurements an automated magneto-transport measurement station was developed in the laboratory. This transport station was computer controlled and contained all of the necessary hardware and software required to perform magneto-electrical transport measurements. Variable temperature electrical transport measurements were performed on all samples to determine their conductive properties. Resistance measurements showed that two of the samples were semiconductors while the other two samples displayed a transition to metallic behaviour at higher temperatures. This transition can be ascribed to the thermal desorption of nitrogen dopant. Models were tted to the data and the semiconducting behaviour is best explained by a model of uctuation induced tunnelling while the metallic behaviour is best explained by a quasi-1 dimensional metallic term based on electron-phonon interactions. The IV characteristics of two of the samples display increasing non-linearity of the current's voltage dependence with decreasing temperature. The other two samples exhibit this behaviour at lower temperatures while higher temperature IV data displays a current saturation with increasing voltage. The same models used to explain the resistance measurements can be used to explain the IV characteristics data extremely well. The magnetoresistance data taken with the direction of current ow orientated both parallel and perpendicular to the eld, show a transition from negative to positive magnetoresistance with decreasing temperature. The results of these experiments are inconclusive, as a theoretical model of magnetoresistance in systems that conduct via uctuation induced tunnelling is not well de ned. A comparison between the resistance measurements of all four samples was made to determine the e ect of nitrogen doping on the samples' electronic transport properties. The result of this comparison was indeterminate. This was due to samples with identical nitrogen dopant levels displaying vastly di erent conductive properties and indicates that very strict synthesis conditions need to be adhered to in order to ensure sample quality. Resistance measurements were rerun on the two samples that displayed purely semiconducting behaviour to investigate the possibility of atmospheric doping. It was found that the samples now displayed a transition to metallic behaviour and a reduced resistance. These results are suggestive of atmospheric doping by oxygen and water vapour.Item Boron and nitrogen doped carbons for photochemical degradation reactions.(2014-06-19) Tetana, Zikhona NobuntuUnable to load abstract.Item Examining the effect of pH on the structure and stability of CLIC1 with E228L and E85L CLIC1 variants(2013-08-01) Cross, Megan OliviaThe chloride intracellular channel CLIC1 is an anion channel protein that has been implicated in a number of physiological processes. It is fascinating in that it is synthesised as a soluble monomer that is able to reversibly bind membranes without the aid of a membrane-targeting tag or receptor. CLIC1 membrane binding is promoted by low pH and involves separation of the N- and C-domains and subsequent refolding of the N-domain, which traverses the membrane as an α-helix. At the low pH of a membrane surface, pH 5.5, soluble CLIC1 demonstrates decreased conformational stability and forms a partially unfolded intermediate state under mild denaturing conditions. In this study, these pH-effects are proposed to occur as a result of low pH-induced protonation of two conserved glutamate residues, Glu85 and Glu228. Both are involved in domain-maintaining interactions and are proposed to form part of an electrostatic network of pH-sensitive residues. At low pH, protonation of these glutamates would break their electrostatic interactions, allowing separation of the domains. To investigate this possibility, Glu228 and Glu85 were mutated to leucine residues. Each variant protein was then investigated at pH 7.0 and pH 5.5 and results were compared to the wild-type. Secondary and tertiary structures were examined using far-UV circular dichroism and fluorescence spectroscopy, respectively. Conformational flexibility was investigated with limited thermolysin proteolysis. Stability was studied using thermal and urea-induced equilibrium unfolding. The unfolding intermediate state was detected using ANS binding and its structure was characterised. While neither residue substitution caused global structural perturbations, both destabilised the structure and promoted intermediate formation at pH 5.5. This was particularly evident for the E85L variant, which also formed a significant intermediate population at pH 7.0. It was concluded that the interactions of Glu228 and Glu85 are involved in maintaining the CLIC1 native state. Additionally, the lack of pH-dependence of intermediate formation in the E85L variant suggested that Glu85 is likely to function as a pH-sensor. It is thus involved in the „priming‟ of the CLIC1 structure for the conformational changes that may lead to membrane binding.Item Magnetic properties of nitrogen- doped carbon nanospheres(2013-03-07) Dubazane, Makhosonke BerthwellElectron spin resonance (ESR) was used to characterize a suite of carbon nanospheres (CNS) samples with varying nitrogen concentrations at room temperature. The CNS were produced using two different reactors (vertical and horizontal) under different preparatory conditions. Resonance spectra of samples produced from the vertical reactor showed resonance lines- a narrow paramagnetic component, and broader component. They were attributed to nitrogen paramagnetic impurities and carrier spins, respectively. Samples produced in the horizontal reactor revealed stronger line spectra that were narrower and Dysonian in shape. The nitrogen content of the samples produced by the horizontal reactor was determined through ESR analysis which involves integration of the resonance peak, and normalizing to the mass of the sample. The relative g-shift was also measured by using a DPPH reference sample. Room temperature power saturation experiments were performed on samples produced from the horizontal reactor with the aim of estimating the spin relaxation times. Two samples from the horizontal reactor were further investigated at low temperatures (4 K- 320 K) at a constant microwave power. The resonance parameters investigated were linewidth, asymmetry ratio and amplitude, and possible spin-lattice relaxation mechanisms were investigated. The variation of the amplitude with temperature was investigated using two models: (1) a model based on lattice vibrations, and (2) a model based on nanographites assembly (considered interaction between carrier and localized spins). At low temperatures both models have amplitude that changes inversely with temperature in accordance with Curie law. At high temperatures (T > 200 K) a model based on nanographites assembly provide an alternative; it describes the rise in the signal amplitude in terms of thermally activated paramagnetic electrons from non-magnetic ground state to excited state at energy . Analysis of linewidth and asymmetry ratio data confirmed that the spin-lattice relaxation governed by thermal activated electrons is a dominant relaxation mechanism at high temperatures.