Exciton Effects in Optical Absorption Spectra of Boron-Nitride Nanotubes

Kikuo Harigaya

doi:10.1038/npre.2009.3879.1

Abstract

AbstractExciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changingthe 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 eVat the onsite interaction U=2t with the hopping integral t=1.1 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 almostindependent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallicand semiconducting properties depending on the chiral index. Comparison with recent experiments will be discussed.

Highlights

In the exciton theories with band calculations, the binding energy is near 0.7 eV for the h-BN and BN nanotubes
Exciton effects are considered in zigzag BN nanotubes
Similar magnitudes have been obtained in the recent band calculations

Summary

BN plane

BN nanotube x periodic boundary in the y direction y to make the zigzag (L/2,0) nanotube periodic boundary. Model for nanotubes chiral, zigzag, armchair nanotubes tight binding model with Coulomb interactions site energies interaction strengths. U: onsite repulsion V: nearest neighbor int. Κ-th optical excitation determined by the single-CI method electron-hole excitation from the Hartree-Fock ground state. Exciton effects in C60 up: our theory for C60 down: experiments bold line: C60 in solution dotted line: C60 thin film. Density of states and optical absorption (5,0) zigzag BN nanotube: non-polarized abs. For (U,V)=(2t,1t) site energies: EB = + t , EN = - t as in Yoshioka, Suzuura, Ando, JPSJ 72, 2656 (2003)

Comparison of results

Summary