3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Dynamic electrical transport in carbon nanotubes and nanodiamond films(2014) Chimowa, GeorgeA comprehensive experimental study on alternating current (AC) electrical transport in the three forms of carbon nanotubes (CNTs) and nanodiamond films is presented. It is termed dynamic electrical transport to differentiate it from direct current measurements, which may be referred as static transport. The results and analysis are based on the scattering parameter measurements of a few horizontally aligned single, double, multi-walled carbon nanotubes and nanodiamond films. Which were measured in the frequency range 10 MHz to 65 GHz, at room and cryogenic temperatures using a vector network analyser. The work is motivated by the fact that AC transport in 1D systems has not been fully studied and is not well understood. From direct current measurements, it is known that one dimensional (1D) electrical transport is very different from its two or three dimensional counterpart. This is because adding an electron to a 1D system tends to affect the whole system in ways which to date cannot be fully explained theoretically. CNTs present an ideal platform to study the AC or dynamic transport behaviour of 1D systems because of the high mobility and electrical conductivity at nano-scale. Therefore from the AC complex impedance and conductance, this work demonstrates quantum effects of collectively excited strongly interacting electrons (Luttinger Liquid), which had been predicted theoretically but not observed experimentally using this technique. Ballistic transport at room temperature is also demonstrated by setting the stimulus frequency higher than the scattering rate in the CNTs. A crossover from capacitive to inductive behaviour in the imaginary component of impedance has been shown by improving the CNT-electrode coupling. Furthermore the effect of metal contacts on microwave/ radio frequency transmission is also demonstrated. The results are consolidated by RF simulations, as strong conclusions are drawn. Studies on the dynamic transport in nanodiamond films revealed a crossover from the insulating to semi-metallic regime by nitrogen incorporation. The crossover is explained by considering the changes of the grain boundary morphology. This work shows that AC transport in polycrystalline nanodiamond films is similar to DC transport.Item Microwave assisted techniques for the synthesis of NiSx and GaN semiconductor nanostructures for applications in sensors(2015-02-04) Linganiso, Ella CebisaThe synthesis of good crystalline nanomaterials by green methods is one of the means to preventing global warming. Application of microwave thermal methods and the use of green solvents to synthesize nanomaterials contribute to this goal. Further, the low cost synthesis of nanomaterials contributes to their ease of availability in the market at affordable costs. In this study, different NiSx phases and GaN nanomaterials were obtained by microwave-assisted solution phase synthesis. NiS2, GaN, -NiS, ( & ) NiS and Ni3S2 phases were obtained by using different reagents and applying different reaction parameters. These materials were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence, to evaluate their crystalline phases, morphologies, particle size distribution and optical properties respectively. Hierarchical structures of cubic phase NiS2 and spherical HDA capped nanostructures were synthesized by a MW-assisted hydrothermal technique. The product phase purity was optimized and the effect of precursor concentration and capping agents were discussed. Further, optical properties of bare and HDA capped NiS2 materials are reported. Detailed analysis of the PL properties shown by these materials in the UV-vis range has been given by considering their calculated DOE energy band diagrams. Single phase -NiS nanostructures with uniformly distributed hierarchical networks were synthesized for the first time in this study. The materials were evaluated for thermal stability under an oxidative environment and at temperatures between 150 oC and 600 oC. NiS materials showed stability at 300 oC and NiO formation was observed from 350 oC to 600 oC. The annealing effect on the crystalline size and IR absorption of the annealed samples is reported by XRD and FTIR studied. The EPR properties of the annealed materials were studied and compared to the oxidized materials. The transition temperature of the -NiS was further confirmed by performing electrical measurements on the as-synthesized material. Further, hydrostatic pressure sensing properties, ethanol and tomato VOCs sensing properties of the -NiS/PVA composite based devices were carried out and the results are reported. The ethanol gas sensing properties of the devices prepared showed the highest response when compared to hydrostatic pressure sensing and tomato VOCs gas sensing. UV-blue emitting GaN nanostructures were obtained for the first time by a onestep MW-assisted solvothermal technique. Sensor devices based on the hexagonal wurtzite structures obtained and their PVA composites (GaN/PVA) were prepared with different GaN NPs concentrations. A very high response to hydrostatic pressure was achieved for the devices prepared. The sensitivity of a GaN/PVA composite based device was analyzed for tomato VOCs detection and the results are presented. Binary phase ( & ) synthesis of NiS materials is commonly reported for the synthesis of Ni:S ratio of 1:1 stoichiometry. This is due to the formation of both phases at temperatures lower than 200 oC. Here, the effect of NaOH and the S source was investigated as reaction parameters. It was found that the concentration of OH- ions in solution plays a huge role in the formation of binary phase NiS as well as its morphology distribution in the nanostructures. Hexagonal nanoplatelets, nanorods and nanorodbased flower-like structures were obtained when different reaction parameters were varied in the presence of NaOH. Further, the solubility of different S precursors in the solvent used was studied and found to affect both the morphology and crystalline phase distribution of the products. Preliminary work on the synthesis of Ni3S2 and Se and Te doped Ni3S2 is presented in the last chapter. The crystallite sizes of the materials were determined by use of the Scherrer equation and the elemental composition was confirmed by EDS analysis. The relative humidity gas sensing of the samples materials was determined and sensitivity response of the material to humidity was obtained for the first time.Item Resin-gel synthesis and characterisation of copper and titanium mixed metal oxides nanoparticles(2014-08-21) Dziike, FaraiThe resin-gel method of synthesis successfully produced compounds of mixed metal oxides of copper titanium oxide powders of the form CuxTiyOZ with different compositions. These include Cu3TiO5, Cu3TiO4, Ti3Cu3O, Cu2Ti4O, Cu2Ti2O5 and Cu2TiO3. Heat-treatment of the powders at 300°C, 500°C, 700°C and 900°C for 1 hour was performed to determine the full composition/temperature phase diagram. The target particle size was in the 10- nanometer range, and for most of the samples, this size was achieved. Powder xray diffraction and transmission electron microscopy were the main techniques used to study the crystallization of these materials and their transformation to other polymorphic phases under different temperatures. Phase-match, particle size analysis and TEM imaging determined the properties and characteristics of the respective crystallographic phases of these materials. TEM analysis showed that some powders agglomerated while others exhibited both regular and irregular morphologies and polydisperse particle size distribution. Only a single unique phase was identified, but its structure could not be determined.Item Synthesis and functionalization of gallium nitride nanostructures for gas sensing and catalyst support(2014-01-10) Kente, ThobekaWe report the role of a double step heat treatment process in the synthesis of novel GaN nanostructures (NSs) using a two stage furnace following a catalyst free vapour-solid growth mechanism. Morphological analysis revealed that GaN NSs were composed of rod-like structures with average diameter of 250 nm and accumulated particulates of GaN with diameter of ~ 12 – 16 nm providing enhanced surface area. The wurtzite phase of GaN nanorods of agglomerated nanoclusters was synthesized at temperatures as low as 750 °C. An X-ray photoelectron spectroscopic study confirmed formation of GaN. The surface areas of the GaN NSs were high at ~20 m2/g with respect to that expected for solid nanorod structures. The GaN NSs were of high crystallinity and purity as revealed by structural studies. Raman spectral analysis showed stronger intensity of the A1(LO) mode with respect to that for E2(high) mode indicating the high electronic quality of the sample. A photoluminescence study revealed the dominant presence of a defect band around 1.7-2.1 eV corresponding to nitrogen di-vacancies. Subsequent annealing in NH3 has demonstrated a compensation of the defect state and evolution of a band edge peak with possible hydrogen compensation of surface states. We also report the role of activated carbon on Ga2O3 to make GaN/C nanostructure composites using a single stage furnace. TEM analysis showed that GaN/C nanostructures gave different morphologies with different ratios of GaN/C. The surface areas of these materials showed an increase as the ratio of activated carbon was increased. PXRD showed that a ratio of Ga2O3: C of 1:0.5 (w/w) was sufficient to form GaN. TGA revealed that the ratios of Ga2O3: C of 1:0.5 – 1:2 gave materials that were thermally stable. Raman spectra showed that the material had excellent electronic properties. The material with a Ga2O3/C 1:2 ratios showed a poor gas response due to the change in reference value of resistance with the variation of hydrogen concentration. iv This study also provides the first investigation of GaN as a catalyst support in hydrogenation reactions. The GaN NSs were synthesized via chemical vapour deposition (CVD) in a double stage furnace (750 ºC) while nitrogen doped carbon spheres (NCSs) were made by CVD in a single stage furnace (950 ºC). TEM analysis revealed that the GaN NSs were rod-like with average diameters of 200 nm, while the NCSs were solid with smoother surfaces, and with diameters of 450 nm. Pd nanoparticles (1 and 3% loadings) were uniformly dispersed on acid functionalized GaN NSs and NCSs. The Pd nanoparticles had average diameters that were influenced by the type of support material used. The GaN NSs and NCSs were tested for the selective hydrogenation of cinnamaldehyde in isopropanol at 40 and 60 °C under atmospheric pressure. A comparative study of the activity of the nanostructured materials revealed that the order of catalyst activity was 3% Pd/GaN >3% Pd/NCSs > 1% Pd/NCSs > 1% Pd/GaN. However, 100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1% Pd/GaN at reasonable conversion rates.