Vacancy complexes in carbon and boron nitride nanotubes.

The effect of divacancies on the stability, structural and electronic properties of carbon and boron nitride nanotubes is studied using the ab initio density functional method. V(B)B(N) is more stable in the boron-rich and less stable in the nitrogen-rich growth conditions, and V(N)N(B) is more stable in the nitrogen-rich than in the boron-rich conditions. We find that stoichiometric defects V(B)V(N), V(B)C(N) and V(N)C(B) are stable in both the boron and nitrogen rich environments. The relaxation energy in the V(C)V(C) is lower in the armchair than in the zig-zag and the opposite trend is seen for V(C)B(C) and V(C)N(C). The divacancy is found to be particularly effective in changing the band gap of the semiconducting nanotubes due to the appearance of additional energy levels within the band gap region. For the zig-zag systems, we observe a drastic reduction of the band gap in V(B)B(N), V(N)N(B) and V(N)C(B) and a complete removal of the band gap in V(B)V(N) and V(B)C(N), negating the semiconducting behaviour of the nanotube.