Influence of F centres on structural and electronic properties of AlN single-walled nanotubes

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 1nm 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 d_F-F≥20, the interaction between defects is found to be negligible, since energy dispersion does not exceed 0.02eV. 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 1nm 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.