Dynamical properties of ultrathin armchair boron nitride nanotubes using density functional theory

Abstract

Using density functional theory, we studied electronic and vibrational properties of small (n, n; n = 2–7) single walled boron nitride nanotubes (BNNTs) in armchair configuration with diameter ranging from 3 Å to 9.8 Å. We used plane wave pseudo potential method and generalized gradient approximation as exchange correlation functional. Armchair BNNT is a semiconductor with a large indirect band gap ∼4–5 eV unlike armchair carbon nanotubes (CNT) which are metallic. As n varies from 2 to 7 i.e. with increasing diameter, there is no major variation in band-gap. For even number of n, the lowest π* is at Z point and singly degenerate, while for odd n, it is doubly degenerate. Density of states show sharp singularities following E−1/2 trend which is characteristics of 1D structure. Phonon dispersion curves for all BNNTs show no imaginary frequencies throughout the Brillouin zone confirming the dynamical stability of BNNTs with small diameter. Phonon frequencies are found to be diameter dependent. Results of the electronic and vibrational properties using ab initio calculations are comparable to the previous experimental and theoretical studies with highlighting variation in properties as change in diameter of nanotube.