Electronic and optical properties of zigzag and armchair BeO nanotubes exploiting DFT

Abstract

We have discussed the electronic bandgap and optical spectra of zigzag and armchair beryllium oxide nanotubes utilizing PBEsol exchange-correlation energy functionals in the framework of density functional theory (DFT) as implemented in the SIESTA code. The wholly occupied valence bands in both zBeONTs and aBeONTs are separated into two bands, formed mainly by a small admixture of Be 2s-2p states with 2s and 2p orbitals of oxygen, confirming ionic bonding between beryllium and oxygen atoms. The bandgap of BeONTs increases with diameter, reaching its peak in monolayer beryllium oxide. The data indicate that the y and z polarizations of the static refractive index (n₀(y) and n₀(z)) of the simulated nanotubes increase by diameter. For the polarization perpendicular to the tube axis, the index of refraction is lower than that of the polarization parallel to the tube axis (z-polarization). Comparing the static refractive index of carbon nanotubes with our data, one can conclude that the refractive index of CNTs is higher due to the higher density of carbon (2.26 gr/cm³) in comparison with beryllium (1.85 gr/cm³) and oxygen (1.43 gr/cm³). Moreover, the highest peaks of optical absorption are predicted to be around $\Delta$ E∼9–9.5 eV. Armchair BNNTs have higher optical absorption than zigzag BNNTs due to differences in band structures and symmetry. As beryllium oxide nanotubes increase, their absorption behavior converges, and the distinction between armchair and zigzag nanotubes diminishes. The absorption coefficient peak is at 21.5 eV in bulk BeO.