Electronic structure of radially deformed BN andBC3nanotubes

We investigate the band-gap modification by radial deformation in BN and ${\mathrm{BC}}_3$ nanotubes through first-principles pseudopotential density-functional calculations. In zigzag BN nanotubes, radial deformations that give rise to transverse pressures of about 10 GPa decrease the gap from 5 to 2 eV, allowing for optical applications in the visible range. When armchair ${\mathrm{BC}}_3$ nanotubes with the gap of about 0.5 eV are collapsed down to the interlayer distance of 3.5 Å, a gap closure occurs due to the lowering of the unoccupied $\pi$ band. On the other hand, the band gaps of armchair BN and zigzag ${\mathrm{BC}}_3$ nanotubes are found to be insensitive to radial deformations. This different behavior between zigzag and armchair nanotubes is attributed to the different characteristics of states near the gap.