First-principles Study Of The Doping And Structural Effects On The Properties Of Carbon Nanotubes

Carbon nanotubes, as low dimensional structures, have unique electronic properties, and are considered to have great potential applications in both nanotechnology and nanoscale engineering. In this dissertation, we investigate the doping and structural effects on the properties of carbon nanotubes by using the first-principles methods which are based on density functional theory.The interactions of Ti and Zr with the tips of open-ended single-wall carbon nanotubes have been studied. It is found that Ti or Zr atoms can saturate the dangling bonds of a tip to make it closed. Zr displays much stronger interaction with the contacted carbon atoms than Ti. The Fermi energies of the hybrid systems increase dramatically, and the peak values of the density of states near the Fermi levels increase significantly. The field emission properties of the carbon nanotubes can be improved.The adsorption of a single Ti atom on single-wall silicon carbide nanotubes (SWSiCNTs) and the corresponding adsorption of hydrogen molecules to the Ti atom have also been investigated. We present and discuss the most energetically favorable sites for a single Ti atom adsorbed on the outside of SWSiCNTs. Ti and C atoms form a strong chemical bond while the interaction between Ti and Si is rather weak. Up to four hydrogen molecules can be attached to the Ti atom and all of them are not dissociative.The electronic structure of contact between Ti surface and a semiconducting carbon nanotube is also investigated. The nanotube placed on the Ti surface exhibits obvious cross section distortion and strong chemical bonds form between C and Ti. Our results suggest that the nanotube may become metallic and the electrons can be transferred from the nanotube to the Ti electrode without any electrostatic potential barrier. The results are quite different from those of semiconducting carbon nanotube on Al surface, given that Al and Ti have similar work functions. This difference can explain the unexpected experimental difference between Al-nanotube contact and Ti-nanotube contact. The electronic properties of a hybrid carbon material, imperfect fullerenes covalently bonded to defective single-walled carbon nanotubes (the NanoBud), are also investigated. Our results suggest that NanoBuds of zigzag carbon nanotubes are semiconductors, no matter whether the zigzag nanotubes are metallic or semiconducting. Local states appear in the band gap of the NanoBuds near the Fermi level. NanoBuds of armchair carbon nanotubes remain metallic. These properties may have great potential applications in future nanodevice design.