Effects Of Oxygen Adsorption And Topological Defects On Electronic Transport Properties Of Carbon Nanotubes

Nano-electronics have been attached importance in the scientific research as traditional micro-electronic technology is approaching fundamental limits and the new development of micro-fabrication technology. In particular, Single-wall carbon nanotubes (SWCNTs) have attracted great interest due to their quasi-one-dimensional structure and unique electronic properties. Both experiments and theory have shown that the electronic properties of SWCNTs are extremely affected by the effects of surface adsorption and topological defects. In this thesis, we study the effects of oxygen adsorption and topological defects on the electronic transport properties in detail.The binding energies, optical properties and energy band structures of the oxygen adsorption on semiconducting SWCNTs are studied by the density functional theory calculations. Through four different relaxed physical adsorption models, it is found that the most stable adsorption site is one in which the oxygen molecule sits above the center of the carbon ring. The binding energies and optical absorption peaks of the stable cases are consistent with experimental results. The band structures show that there appears an oxygen doping subband at the Fermi level which indicates p-type conducting behavior. The phenomenon of oxygen photoinduced desorption depending on the frequency of incident light has been explained theoretically. Otherwise, in the stable chemisorption case, the width of bandgap became narrow, energy band splitting, and there is non-absorption in the near infrared and visible light region.Using the density-functional theory combining non-equilibrium Green's function technique, the density of states and transport properties of (5,5) metallic SWCNT with Stone-Wales defects have been studied. The results show that defect states appearing both in the valence and conduction band induce the corresponding electrons scattering because of the deformation of Stone-Wales defects (two pentagon-heptagon pair topological defects). As the effect of external perturbation of SWCNT rope, a pseudogap of about 0.1 eV at the Fermi level appears, and it makes a deep valley in the transmission probability curve.Finally, the electronic structure and the transport properties of the (7,0)-(6,0) SWCNT heterojunction connecting with one pentagon-heptagon pair topological defect have been studied. The local density of states (LDOS) indicate that there are metal-induced gap states near the interface which decay exponentially and localized defectstates both at the top of valence band and the bottom of conduction band. At low temperature range, phase coherent effect between these localized states makes DOS regularly oscillate, which influences electronic transmission probability extremely. The low temperature behaviors of localized defect states could explain the experimental phenomenon of abnormal resistance vs. temperature coefficient. When bias applied to the heterojunction, the different scattering effects between the two defect states cause the nonlinear and asymmetrical characteristics of the current-voltage curve.