Carbon Nanotube Defects and Carbon Nanotube Composites
A single-walled carbon nanotube (SWCNT) is a one-dimensional (1D) conductor that has been proposed as an ideal element for novel, nanoscale electronics. This dissertation studies the properties of individual SWCNTs in the near-pristine limit where the SWCNT conductor contains one or more defects. The presence of defects significantly modifies the electronic and chemical properties of a SWCNT, with positive and negative impacts on different potential applications. This dissertation completed three different types of experiments to explore these modified properties.
In the first section, SWCNTs with defects were studied during high temperature annealing. Annealing rearranged and diminished the scattering from defects, even to a point where pristine SWCNT conductances were recovered. In the limit of single defects on single SWCNTs, the annealing of one defect was resolved in real time by using electrical conductance as the probe. The work proved that conductance in 1D is sufficiently sensitive to see the annealing of one defect.
The resistance associated with single SWCNT defect was also studied as a function of bias and temperature at low temperature. A singe point defect surrounded on either side by quasi-ballistic, semi-metallic SWCNT was a nearly ideal system for understanding the influence of functional groups on 1D conductors and comparing experiment to theory. Here, transport and local Kelvin Probe Force Microscopy (KPFM) independently demonstrated high-resistance depletion regions over 2.0 micron wide surrounding a point defect in a SWCNT. A defect assisted tunneling through this depletion region via a modified, 1D version of Poole-Frenkel conduction. Given the breadth of theory dedicated to the possible effects of disorder in 1D systems, it was surprising to find that a Poole-Frenkel model could successfully describe defect scattering in SWCNTs.
Finally, the third experiment investigated SWCNTs that had been non-covalently modified with a thin coating of Cu. Bulk CNT/Cu composites have been reported to have surprisingly high conductance and ampacity. Consequently, CNT/Cu composites are a novel conductor with many potential applications. Here, individual SWCNTs were coated with Cu by electrodepotion for electrical studies. Due to SWCNT's hydrophobic and inert surface, achieving conformal Cu coatings was very difficult, but successful results were obtained using both aqueous and non-aqueous Cu electrolytes. The thinnest conformal Cu coatings (40nm) were obtained from electrodeposition in non-aqueous Cu electrolyte. Electrical measurement of Cu-coated SWCNTs revealed a similar temperature dependent to the bulk composite, indicating that the SWCNT plays an essential role in the composite conductance's temperature dependence. However, unlike the preliminary reports, Cu films at these thicknesses could only achieve a fraction of the conductivity of bulk Cu. Therefore, the research was unable to fully test the mechanisms of the improvements reported for bulk CNT/Cu composites.