First-principles study of small-radius single-walled BN nanotubes

The structural, electronic, and vibrational properties of small-radius single-walled BN nanotubes are studied using the density functional method with the local density approximation. The results show that the chirality preference of BN nanotubes observed in experiments may be explained from the relative stability of the corresponding BN strips. Compared with the armchair BN strips, the zigzag BN strips have larger binding energies and thus may be more easily formed. The smallest stable BN nanotube is found to be the (5,0) zigzag nanotube. The energy gap of small zigzag BN nanotubes decreases rapidly with the decrease of radius. The phonon dispersions of BN nanotubes are calculated and the frequency of the radial breathing mode is found to be inversely proportional to the nanotube radius.