Stability and electronic properties of vacancies and antisites in BC2N nanotubes

The equilibrium geometry, energetic, and electronic properties of antisites and vacancies in BC2N nanotubes are studied by spin density-functional calculations. We investigate these defects in both the zigzag (4,0) and the armchair (3,3) nanotubes. We find that boron and nitrogen, occupying nonequivalent carbon sites (B-CII and N-CI) in both tubes, have the lowest formation energies, showing that they are energetically favorable to form under B-rich and N-rich growth conditions. They also exhibit acceptor and donor properties, suggesting the formation of defect-induced p-type and n-type BC2N nanotubes. In addition, carbon at boron and nitrogen sites (C-B and C-N) also exhibit p-type and n-type properties, respectively, as well as low formation energies. Vacancies are less favorable defects with high formation energies as compared to the most stable antisites. Once a vacancy is formed, a strong reconstruction occurs, resulting in an undercoordinated atom which typically gives rise to deep levels in the band gap, changing the electronic properties of the nanotube. Our results suggest that with suitable growth conditions, it would be possible to synthesize BC2N nanotubes with intrinsic donor and acceptor character by inducing selective antisite defects.