We analyse the influence of uncharged N vacancies (neutral F centres), created either under conditions of AlN nanotube growth or by its soft irradiation, on the atomic and electronic structure. Periodic one-dimensional (1D) density functional theory (DFT) calculations on models of defective single-walled nanotubes (SW NTs) allow us to analyse how NT chirality and concentration of F centres change their properties compared to the corresponding defect-free nanotubes. We have simulated reconstruction around periodically repeated F centres on 1 nm AlN SW NTs with armchair- and zigzag-type chiralities. To achieve the limit of an isolated vacancy for both chiralities, we have considered different inter-defect distances repeated along the axes of these nanotubes. For dF–F≥20 Å, the interaction between defects is found to be negligible, since energy dispersion does not exceed 0.02 eV. We also analyse the influence of F centres on the energy cost required to wrap up AlN graphitic nanosheets (NSs) of both chiralities into the corresponding 1 nm thick SW NTs. The electronic properties of defective NS and NTs of both chiralities have been compared with those for defective AlN bulk three-dimensional (3D) structures of wurtzite and zinc-blend. The presence of N vacancies in various aluminium nitride structures (including SW NTs) results in the appearance of defect energy levels in the band gap with the prevailing contribution from 3s and 3p atomic orbitals of the nearest Al atoms. We have found that the larger the concentration of F centres is, the smaller the maximal energy gap between defect levels is, i.e. an increase in defect content in AlN NTs results in their higher conductivity. Our DFT calculations of point defects on SW NTs have been correlated with theoretical prediction for electrical breakdown voltage induced on a defective nanotube upon removing a single bond.
J. Phys.: Condens. Matter 19 (2007) 395021 (18pp)

Abstract. For theoretical simulation on AlN nanotubes (NTs) of different chiralities (armchair-and zigzag-type) and uniform diameters, we have considered their single-walled (SW) 1D periodic models. For this aim, we have performed ab initio DFT calculations on AlN SW NTs using formalism of the localized Gaussian-type atomic functions as implemented in CRYSTAL-03 computer code. We have shown that the smaller the diameter of AlN single-walled nanotube is, the closer its electronic and structural properties to AlN bulk. We have analysed an influence of N vacancies (neutral F centres) created by either soft irradiation of nanotubes or under experimental conditions of their growth, on the atomic and electronic structure of AlN SW NTs. We have found the small inward relaxation of the Al nearest neighbours and the N next-nearest neighbours around each point defect formed on 1 nm AlN NTs of both chiralities. Presence of N vacancy in both types of nanotubes has resulted in appearance of the two defect energy levels in their band gaps consisting of mainly 3s and 3p states of the nearest Al atoms.
Journal of Physics: Conference Series 93 (2007) 012005