3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Magnetotransport properties of rare earth element modified carbon nanotubes(2018) Ncube, SiphephileFunctionalization and filling of carbon nanotubes has been tailored over years to modify the exceptional properties of the 1-dimensional (1D) conductor for magnetic properties based applications. Hence such a system exploits the spin and charge property of the electron, analogous to a quantum conductor coupled to magnetic impurities which poses an interesting scenario for the study of Kondo physics and related phenomena. A study of the low temperature electronic transport and magnetic properties of carbon nanotubes modified with gadolinium derivatives is presented in this thesis. The methods of modification used are chemical functionalization and capillary filling. The presence of gadolinium in the nanostructures extends the functionality of the nanotubes from conventional electronics to spintronics. Filled and functionalized multiwalled carbon nanotubes are characterized as well as filled double walled carbon nanotubes. This system gives a chance to study the interaction of a ballistic conductor with magnetic impurities. Multiwalled carbon nanotubes functionalized with a gadolinium based supramolecular complex show enhanced magnetic properties and unexpected electronic behaviour that has not been observed in this material before. A newly developed synthesis technique has been employed for the synthesis and it is found that the functionalization method of the nanocomposite enhances the strength of magnetic interaction leading to a large effective moment of 15.79 μB and non-superparamagnetic behaviour unlike what has been previously reported. Saturating resistance at low temperatures is fitted with the numerical renormalization group formula verifying the Kondo effect for magnetic impurities on a metallic electron system. Magnetoresistance shows that devices fabricated from aligned gadolinium functionalized MWNTs exhibit spin-valve switching behaviour of up to 8%. The electronic transport properties of MWNTs filled with GdCl3 nanomagnets clearly shows the co-existence of Kondo correlation and cotunelling within the superparamagnetic limit. The Fermi liquid description of the Kondo effect and the interpolation scheme are fitted to the resistance-temperature dependence yielding the onset of the Kondo scattering temperature and a Kondo temperature for this nanocomposite, respectively. Cotunneling of conduction electrons inhibiting a Kondo type interaction has been verified from the exponential decay of the intensity of the fano shaped non zero bias anomalous conductance peaks which also show strong resonant features observed only in GdCl3 filled MWNT devices. Hence these features are explained in terms of magnetic coherence and spin-flip effects along with the competition between the Kondo effect and co-tunneling. The properties of doublewalled carbon naotubes filled with GdCl3 are also presented. They show superparamagnetic behaviour and zero bias anomalies similar to what was observed in Gd filled MWNTs. This work is the first on such lanthanide modified CNT hybrid bundle devices. The study raises a new possibility of tailoring magnetic interactions for spintronic applications in carbon nanotube systems. It highlights the possibility of enhancing magnetic interactions in carbon systems through chemical modification. Furthermore, the study demonstrates the rich physics that might be useful for developing spin-based quantum computing elements based on 1D channels.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.