Deep levels in the band gap of the carbon nanotube with vacancy-related defects

We study the modification in the electronic structure of the carbon nanotube induced by vacancy-related defects using the first-principles calculation. Three defect configurations which are likely to occur in semiconducting carbon nanotubes are considered. A vacancy-adatom complex is found to bring about a pair of localized states deep inside the energy gap. A pentagon- octagon-pentagon topological defect produced by the divacancy is structurally stable and gives rise to an unoccupied localized state in the gap. We also discuss the character of partially- occupied localized state produced by a substitutional impurity plus a monovacancy.