Narrowed bandgaps and stronger excitonic effects from small boron nitride nanotubes

The bandgap of boron nitride nanotubes (BNNTs) is generally considered to be independent on tube radius and chirality. However, we have observed that the bandgaps of BNNTs do depend on the tube size. Photoluminescence excitation spectroscopy with variable photon energies in vacuum ultraviolet (VUV) range revealed that the bandgap becomes smaller when the tube diameter decreases. This is consistent with red-shifted luminescent emissions. The strong interactions between excitons and phonons are possibly responsible for the bandgap narrowing as the function of nanotube size.