Defect formation and electron transport in carbon nanotubes

In this thesis, we present electron transport measurements performed on single-walled carbon nanotubes. In the first set of experiments, we focused on inter-tube interactions in devices with various nanotube configurations. We measured transport in ropes of carbon nanotubes, studying the effects of inter-tube coupling on the overall conduction through a rope. Low-temperature measurements yielded Coulomb peaks that were found to cluster in a quasi-periodic fashion. By comparing the two periodicities observed in the gate sweeps of our samples, we were able to extract a rough estimate of tube-tube capacitive coupling. In order to study interactions between two individual nanotubes, we used an atomic force microscope tip to push pairs of single nanotubes into contact or into close proximity. In these different configurations, we were able to directly measure coupling between the two tubes.; We have found that a significant fraction of metallic tubes grown by various methods of synthesis unexpectedly exhibit gate-dependent resistances at room temperature. Scanned gate measurements were used to obtain spatially resolved images of electrostatic modulation of conduction in nanotubes. We show that this behavior originates from resonant electron scattering by intrinsic defects in tubes.; In the next set of experiments, we studied various mechanisms of defect formation. We used an atomic force microscope tip to mechanically buckle tubes at a specific site. Introduction of new scattering centers by this technique was verified by scanned gate microscopy. By forming two closely spaced defects on metallic nanotubes, we defined quantum dots less than 100 nm in length. AFM manipulation was also used to induce varying degrees of local elongation in nanotubes. Using this technique, we were able to induce strains well above the theoretically predicted threshold (6%--12%) for the formation of structural defects. Scanned gate microscopy then allowed us to observe the introduction of new scattering centers in nanotubes strained at levels consistent with these predictions.