Atomic vacancy defects in the electronic properties of semi-metallic carbon nanotubes

We investigate the electronic properties of semimetallic (12,0) carbon nanotubes in the presence of a variety of monovacancy, divacancy, and hexavacancy defects, by using first principle density functional theory combined with nonequilibrium Green's function technique. We show that defect states related to the vacancies hybridize with the extended states of the nanotubes to modify the band edge, and change the energy gap. As a consequence, the nanotube conductance is not a monotonic function of the defect size and geometry. Paradoxically, tetravacancy and hexavacancy nanotubes have higher conductance than divacancy nanotubes, which is due to the presence of midgap states originating from the defect, thereby enhancing the conductance.