Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes

TL;DR

This study investigates exciton effects in boron-nitride nanotubes, revealing that their optical properties are largely independent of nanotube geometry, contrasting with carbon nanotubes.

Contribution

First detailed theoretical analysis of exciton effects in BN nanotubes using the extended Hubbard model and configuration interaction technique.

Findings

Optical gap of (5,0) BN nanotube is about 6 eV.

Exciton binding energy is approximately 0.50 eV.

Optical properties are nearly independent of nanotube chirality.

Abstract

Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.