Excitons in boron nitride nanotubes: dimensionality effects

TL;DR

This study investigates how the excitonic optical properties of boron nitride nanotubes vary with dimensionality, revealing strong exciton binding energies that depend on system size and explain optical gap observations.

Contribution

First-principles calculations showing the dependence of exciton binding energy on dimensionality in BN systems and explaining optical gap constancy.

Findings

Exciton binding energy varies from 0.7 eV in bulk to over 3 eV in nanotubes.

The excitonic peak position remains nearly constant across different BN systems.

Strongly localized excitons lead to rapid convergence of binding energy with tube diameter.

Abstract

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed "optical gap" constancy for different tubes and bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].