  We investigate the electronic properties of semi-metallic (12,0) carbon
nanotubes in the presence of a variety of mono-, di- and hexa-vacancy defects,
by using first principle DFT combined with non-equilibrium 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, resulting from the curvature effect. As a consequence, the nanotube
conductance is not a monotonic function of the defect size and geometry.
Paradoxically, hexa-vacancy nanotubes have higher conductance than di-vacancy
nanotubes, which is due to the presence of mid-gap states originating from the
defect, thereby enhancing the conductance.
