Theoretical study on the physical properties of carbon nanotubes

Abstract

For the last decade, carbon nanotubes have been emerging as one of ideal materials for the building block of the forthcoming nanotechnology, due to their unique electrical and mechanical properties. Depending on detailed wrapping-up methods, their electronic properties show a wide spectrum from metals to large-gap semiconductors with band gaps of 1 eV. In this thesis, we study various physical properties of carbon nanotubes, including electrical properties and their controlling methods, magnetic properties, and transport characteristics, based on the first-principles density-functional theory and the tight-binding model. In carbon nanotubes, calculations of the electronic structure demonstrate that band gap modification such as opening and closure is easily achieved by radial deformations perpendicular to the tube axis. Metallic armchair nanotubes can be semiconductors only when all mirror symmetries are broken during deformation, while surprisingly semiconducting nanotubes can be metals when flattened somewhat. We also find that uniform electric fields perpendicular to the tube axis induce similar band gap modification. We analyze the potential environments in terms of circumferential perturbations on nanotube surfaces. Considering the periodicity of perturbations, we derive selection rules in the subband mixing caused by perturbations. The mixing results in the band gap modification in carbon nanotubes. We also investigate the band-gap modification by radial deformation in BN and $BC_3$ nanotubes. In zigzag BN nanotubes, radial deformations that give rise to transverse pressures of about 10 GPa decrease the gap from 5 to 2 eV, allowing for optical applications in the visible range. Armchair $BC_3$ nanotubes with the gap of about 0.5 eV undergo semiconductor-to-metal transition when collapsed. On the other hand, the band gaps of armchair BN and zigzag $BC_3$ nanotubes are found to be insensitive to radial deformations. To explore possibility of metal-free ferr...