| Single-walled carbon nanotubes are set apart from other one-dimensional (1D) nanowire materials as they are predicted to display fundamentally distinct electronic properties depending only on diameter and helicity. In this thesis, we present our investigations of the electronic structure of single-walled carbon nanotubes (SWNTs) using scanning tunneling microscopy (STM) and spectroscopy (STS).; We present atomically resolved STM images of SWNTs, whose diameters range between 0.8 and 1.4 nm, and whose chiral angles range between 0 and 30°. Spatially resolved tunneling spectroscopy measurements on these nanotubes revealed two types of electronic behavior, metallic and semiconducting, which depended critically on the nanotube's structural geometry. The energy gaps were found to be independent of chiral angle and to depend inversely with diameter, with a 1 nm-diameter tube having an energy gap of ∼0.7 eV. Extension of the energy range at which STS data were recorded resulted in sharp peaks in the conductance. These peaks in the electronic structure are characteristic of 1D systems, and we compared them with tight-binding calculations on specific tube geometries determined from the resolved STM images.; We also discuss experimental and theoretical investigations of localized structures in SWNTs. We show how reducing a nanotube's length results in the quantum confinement of electrons along the tube length. As the nanotube lengths were reduced to ca. five nm, discrete peaks were observed in the tunneling spectra that correspond to resonant energy levels. The spacing of these levels scales inversely with length. In addition, we characterize with atomic resolution structural defects in as-grown SWNTs, such as mechanical bends and nanotube ends. The presence of these localized structures introduced new, low energy peaks in the density of states. For the capped end, calculations suggest that the new features may arise from specific arrangements of pentagons that close the end.; Finally, we describe how magnetic impurities influence the low-energy properties of carbon nanotubes. A narrow, spectroscopic peak was observed near zero bias in the tunneling spectroscopy over the impurities that suggests strongly evidence of the Kondo effect in a 1D system. Furthermore, in shortened nanotubes, spectroscopic measurements over the magnetic particles exhibited enhanced conductance at the zero bias peak as well as higher order peaks due to finite size. |
