Ab initio study of confinement and surface effects in hexagonal AlN nanotubes

Thickness dependence of electronic and optical properties of AlN nanotubes is studied using density functional theory. It is found that the surface atoms induce their electronic states near the upper valence band edge as well as the lower conduction band edge. Moreover, the absorption spectrum of AlN nanotubes is shown to be dependent on the ratio of surface to bulk atoms. The effect of piezoelectric field on the optical properties of AlN nanotubes is also computed. We showed that in addition to the shifts towards higher energies, as it is usual in bulk AlN, one can also obtain shifts towards lower energies according to frequencies.