3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Electronic properties of single walled carbon nanotubes synthesized by laser ablation(2014-07-21) Ncube, SiphephileCurrent research in the field of nano-electronics is directed towards device miniaturization in order to find ways to increase the speed of electronic devices. The work presented in this dissertation is on the electronic transport properties of single walled carbon nanotube (SWNT) ropes synthesized by laser ablation. The measurements were performed on devices with different geometries; namely SWNT mats, metal incorporated (aligned individual and bundled) SWNTs and lastly on aligned pure SWNTs from low temperatures up to room temperature. The work was performed so as to gain an understanding on how best to utilize SWNTs in the semiconductor industry towards miniaturization. Such an understanding would ultimately highlight if SWNTs can be considered as a viable alternative to the current silicon-based technology, which seems to be approaching its physical limit. For a mat of SWNTs, 3D-Variable range hopping is the principal conduction mechanism from 2 K – 300 K. The magneto-resistance was found to be predominantly negative with a parabolic nature which converts to a linear nature as the temperature is increased. The negative MR is a consequence of quantum interference and the positive upturn is attributed to wave function shrinkage at low temperatures as described by the Efros-Shklovskii model. The hopping ranges of the electrons for a SWNT mat increases as the temperature decreases due to manifestation of quantum effects and reduced scattering. It was also found that metal incorporation does not alter the properties of the SWNT significantly. SWNT ropes aligned by di-electrophoresis across a 1 micron gap between gold micro-electrodes, exhibit Tomonaga-Luttinger liquid (TLL) like behaviour, within the 80 K – 300 K temperature range. The effects of confinement and electron-electron interaction unique to one dimension were identified in electronic transport as a non-universal power law dependence of the differential conductance on temperature and source-drain voltage. Ballistic conductance at room temperature was confirmed from the high frequency transport of the SWNT devices. The complex impedance showed some oscillatory behaviour in the frequency range 6 to 30 GHz, as has been predicted theoretically in the Tomonaga-Luttinger Liquid model. The observation of Luttinger Liquid behaviour demonstrates the outstanding nature of these one-dimensional molecular systems. In these devices the charging Coulomb energy of a single particle played a critical role in the overall device performance. This study can be used to understand the nature of dynamics of plasmons which are the charge carriers in a TLL system and how Coulomb interactions can be used to design highly tuneable systems for fabrication of single molecule devices. The incorporation of metal onto individual SWNT ropes does not alter its electronic properties significantly but the properties of the bundled metal incorporated SWNT ropes are altered. This study has found that under optimized conditions SWNTs might be a viable option for incorporation in nano electronics devices. Individual SWNT ropes promise better devices compared to SWNT mats and further work should be done on individual SWNTs.Item Synthesis of carbon nanofibers and their subsequent use as catalyst supports for Fischer-Tropsch synthesis(2014-07-07) Phaahlamohlaka, Tumelo NathanielIn this study the synthesis and use of carbon nanofibers (CNFs) as catalysts supports for Fischer Tropsch synthesis is reported. The synthesis of carbon nanofibers with two distinct morphologies was optimized based on the reports in the literature that the straight (SCNF) and helical (CCNF) carbon nanofibers grow on Cu catalysts with different particle sizes. To selectively grow CNFs with a single morphology Cu catalysts were designed using different synthesis procedures (by using unsupported, coated and silica supported catalysts). The prepared copper oxide (CuO) nanoparticles were characterized by techniques such as TEM, XRD and nitrous oxide chemisorption. These techniques showed that the unsupported and coated CuO catalyst precursors has large particle sizes (range 100-300 nm) and thus had low Cu atomic surface area, while the supported CuO catalysts displayed low particles sizes in the nanoscale regime (<20 nm) and hence had high atomic surface area. Preparation of CNFs was carried out 300 using acetylene (C2H2) gas as the carbon source. Cu catalysts with large particle sizes resulted in straight CNFs and the small supported Cu nanoparticles grew helical CNFs because of the change in the nanoparticle surface energy during adsorption of the acetylene gas and the silica (SiO2) support effects that limited Cu nanoparticles from sintering (i.e. final particles size 60 nm). Soxhlet extraction proved to be an invaluable step in removing adsorbed polycyclic aromatic hydrocarbons. Because of the low thermal stability of these CNFs the materials were then annealed at higher temperatures ranging from 500-1400 in an inert environment (passing N2 gas). The helical CNFs snapped under high temperature annealing ( 900 ) resulting in shorter lengths in comparison to the straight CNFs. BET analysis of the annealed CNFs indicated that the CNFs annealed at 500 and 900 have increased surface area and have a mesoporous pore structure with the surface area ranging from 200-350 m2/g. Raman and Fourier transform IR spectroscopy indicated that the CNFs annealed at 500 and 900 , (which were the main material of interest because of their high surface area and thermal stability) had different hybridized carbon content. CNFs annealed at 500 contained both sp2 and sp3 hybridized carbon while annealing the CNFs at 900 resulted in a complete rehybridization of the carbon content to sp2. The carbon sp3 content in the CNFs annealed at 500 therefore implied that CNFs annealed at this temperature are more defective in comparison to the CNFs annealed at 900 . Since it is well known that material functionalities are affected by the amount of defects present inside the different CNFs were then applied as catalyst supports for Fischer Tropsch synthesis (FTS) to compare the support effects on cobalt active sites. The CNF surfaces were first modified by functionalization using concentrated HNO3 solution. The preparation of the catalyst systems was performed by a simple HDP method using urea. The CNFs and the FT catalysts were characterized using different techniques such as XRD, TEM, BET, TPR and Raman spectroscopy. Reactor studies performed at 220 (P = 8 bar, GHSV= 1200 mL.h-1. ) showed the catalysts had activities with CO conversion ranging from 25-45%. It was observed that catalysts supported on CNFs annealed at 500 displayed higher average activities of about 15% (based on the CO conversions) in relation to the catalysts supported on CNFs annealed at 900 . Catalysts showed minimal water gas shift reaction and high methane selectivity (i.e. 20-30%) which can be attributed to the small Co crystallite sizes and low pressure reaction conditions.Item Chemical vapor growth of nitrogen doped carbon nanotube and graphene materials for application in organic photovoltaic devices.(2014-03-05) Bepete, GeorgeApplication of carbon nanomaterials like fullerene, carbon nanotubes, and graphene in solar cells using solution processable methods presents a great potential to reduce the cost of producing electricity from solar energy. However, carbon nanotubes and graphene materials are predominantly metallic and this limits their function in organic photovoltaic devices (OPVs) where semiconducting behavior is required. Doping of carbon nanomaterials is a well-known method for making them semiconducting. Doping of carbon nanomaterials with nitrogen and boron can tune their properties to suit the requirements for use in photovoltaic applications as n-type and p-type semiconducting materials, respectively. Indeed, the use of nitrogen doped and boron doped carbon nanotubes in organic solar cells together with fullerene acceptors can improve the current density of the OPV devices. Nitrogen doping of carbon nanotubes can be achieved by using nitrogen-containing precursor materials during chemical vapor deposition. However the doping of carbon nanotubes with nitrogen does not automatically make them n-type materials; they remain metallic unless a large amount of quaternary type nitrogen is incorporated in the carbon nanotubes. In this work we have developed a method to control the type of nitrogen that is incorporated in CNTs by using an appropriate synthesis temperature and use of oxygen-containing carbon precursors during the chemical deposition of carbon nanotubes. Quaternary N was incorporated in a CVD process when high temperatures and a high concentration of O in the precursor materials were used. We also showed that the type and amount of N can be changed from pyrrolic and pyridinic-N-oxide to pyridinic N and quaternary N by annealing N doped carbon nanotubes at temperatures above 400°C. At temperatures above 800°C most of the nitrogen is converted to quaternary nitrogen. N-CNT thin films were used in OPVs so as to modify the ITO electrode and transform it into a 3D electrode. The resulting effect was an improved short circuit current density in the devices containing an N-CNT thin film that was placed on top of the ITO electrode. A reduction in efficiency losses in OPVs at increasing light intensity was observed in the NCNT ITO modified electrode OPVs. This is a remarkable finding when considering that one of the main problems hindering commercialization of OPVs is the loss of efficiency at high light intensities. We related these effects to the efficient charge collection by the modified ITO electrode. Incorporation of N-CNTs in the bulk heterojunction layer of the OPV device resulted in poor performance when compared to an OPV device made without N-CNTs. This effect is caused by shorting of the OPVs. We used a method of incorporating N-CNTs whilst minimizing shorting and this showed potential for better performance. A study on the attempted doping of graphene with B to make it a p-type material showed that in the presence of a nitrogen carrier gas, BN instead of B was incorporated in graphene. This remarkable finding enabled us to grow a p-type graphene with a possible a band gap opening. This was corroborated by XPS and Raman spectroscopy studies of the material. This BN doped graphene material showed potential as a possible replacement of PEDOT:PSS as a hole transport material in OPVs. The BN doped graphene material can match the performance of PEDOT:PSS when the level of BN doping in graphene is increased.