Excitons in Boron Nitride Nanotubes: Dimensionality Effects

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 hexagonal BN.

Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride

“…On the experimental side, the challenge is the fabrication of high quality single crystals which has been achieved only recently 2 . On the theoretical side, it is well established that the band-structure of hBN displays an indirect gap 3,4 and the optical absorption spectrum is dominated by correlation effects leading to a strong Frenkel-type excitonic peak at 5.8 eV [4][5][6] in agreement with the experimental findings 1 . However, the origin of the fine structure of the absorption and luminescence spectra around this peak is still under debate [7][8][9] .…”

Optical properties of bulk semiconductors and graphene/boron nitride: The Bethe-Salpeter equation with derivative discontinuity-corrected density functional energies

“…The LDA-ALDA spectrum shows a broad absorption peak with an onset at 4.5 eV in good agreement with literature. 56 The GLLBSC-ALDA spectrum is essentially identical to the LDA-ALDA, but blue shifted by the difference in the band gap. For the BSE calculation, the Brillouin zone was sampled on a non--centered 32 × 32 Monkhorst-Pack grid, and 70 unoccupied bands were included to obtain the screened interaction W .…”

“…The optical properties of layered BN sheets as well as BN nanotubes have been studied extensively both experimentally 55 and theoretically. 13,56 Upon adsorption of graphene onto a h-BN sheet, a small band gap of around 10-200 meV, depending on the configuration and interplane distance, emerges. 57 The ground state electronic properties, including the role of dispersive forces, and the band structure have been studied.…”

“…These exciton energies agree well with the value of 6.2, 7.0, and 7.4 eV obtained with the GW-BSE scheme. 56 For the graphene/h-BN interface, we studied the structure where one C atom is on top of a B atom and the other C atom is above the center of the BN ring, as shown in Figs. 4(a) and 4(b).…”

Vibrational Properties of Hexagonal Boron Nitride: Inelastic X-Ray Scattering andAb InitioCalculations

“…Since h-BN is a relevant material for UV lasing, it is imperative to resolve the existing controversies and critically assess the quality of the ab initio calculations. Furthermore, we need exact information on the lattice dynamics in order to properly understand the contributions due to electron-phonon coupling observed in the cathodoluminescence and absorption spectra of h-BN [7] and BN nanotubes [15,16].In this Letter we address the above discrepancies in the vibrational properties by reporting inelastic x-ray scattering (IXS) measurements and first principles calculations of the phonon dispersion relations of bulk h-BN. We also report ab initio calculations of the phonon dispersion of a monolayer of h-BN on a nickel substrate.…”