Defects And The Modulation Of Electronic Properties In Carbon Nanotubes

Using the tight-binding Green's function method, we systematically investigated the defects in carbon nanotubes and their physical effects on the electronic properties of the systems.After analyzing the spatial distribution of the defect states, we studied the interactions between the defects. It is found that the distance of effective interaction, characterized by its decay factor with increasing distance, might be as long as several nanometers, far beyond the sizes of the localized defect states. There is a kind of long range interaction between the defects. In defected one-dimensional semiconductor systems, even if the defect-states located in the band gap are several nanometers apart, they will interact with each other through low barrier quantum tunneling. This mechanism is confirmed by a monoatomic chain model.We investigated the quantum conductance of the metallic single-walled carbon nanotubes with different configurations of double defects: double Stone-Wales defects and double monovacancies. It is found that although there are various configurations of double defects, there are only a few types of electronic properties. Some different configurations of double defects correspond to equivalent electronic properties. By use of the degenerate perturbation theory, the physics behind this equivalence is revealed by analyzing the energy band structures.The circumferential interference is found to be an essential mechanism in determining the transport properties of the carbon nanotubes. The quantum conductance can be largely reduced by interference when there is mismatch between the Fermi wave vector and the atomic lattice along the circumferential direction. This helps to understand the experimental observations that even very low concentration of defects may largely reduce the quantum conductance. We further investigated the carbon nanotubes with complex multi-defects. It is found that the quantum conductance depends on the relative positions of the neighboring defects, and a quantized conductance is revealed for the carbon nanotubes with periodical defects.