Size-dependent electronic and optical properties of GaN nanotubes studied using LDA calculations

Abstract

The electronic structures and optical properties of GaN nanotubes are studied by using density functional theory with the generalized gradient approximation. Our calculations find that the threefold-coordinated N and Ga atoms at lateral facets in a GaN nanotube give rise to defect states near the top of valence band and the bottom of conduction band, respectively. Moreover, the calculated imaginary parts of dielectric functions, ${ϵ}_{2}$, show two significant peaks that, respectively, originate from the surface atoms and the bulk atoms. We find that the former peak shows a redshift and the latter shows a blueshift with respect to the major peak in the ${ϵ}_{2}$ curve of a wurtzite GaN. It is observed from the ${ϵ}_{2}$ curves that, as the thickness of GaN nanotubes increases, the intensity of the peak associated with the threefold-coordinated atoms decreases, while that associated with the bulk atoms is enhanced relatively. When the thickness of a GaN nanotube is large enough, the former peak vanishes nearly and the latter one tends to the case of a wurtzite GaN. We therefore predict that the absorption spectra of realistic GaN nanotubes with large thickness are most probably associated with the electronic states of the bulk portions of the tubes.