First-principles study of luminescence in hexagonal boron nitride single layer: exciton-phonon coupling and the role of substrate

TL;DR

This study uses a new ab initio method to analyze how atomic vibrations and substrate interactions influence luminescence in monolayer hexagonal boron nitride, revealing the roles of excitons and phonons in its emission spectra.

Contribution

The paper introduces a novel ab initio approach to account for exciton-phonon interactions in luminescence spectra, specifically applied to monolayer hBN and its substrate effects.

Findings

Luminescence spectra depend strongly on substrate interaction.

Both direct and phonon-assisted emission channels are present.

Substrate changes the nature of hBN from direct to indirect.

Abstract

Hexagonal boron nitride (hBN) is a wide band gap material with both strong excitonic light emission in the ultraviolet and strong exciton-phonon coupling. Luminescence experiments performed on the recently synthesized monolayer form (m-hBN) present emission spectra that differ from one another, with some suggesting a coexistence between phonon-assisted and direct emission channels. Motivated by these results, we investigated the optical response of (m-hBN) using a new \textit{ab initio} approach that takes into account the effects of atomic vibrations on the luminescence spectra. We construct the dynamical exciton-phonon self-energy, then use it to perturbatively correct the optical response functions and test this approach on bulk hBN as a benchmark. Within our approach we are able to estimate the renormalisation of the direct peak induced by phonon-assisted transitions, and this allows us to accurately describe spectra where both processes are present. We found that the emission signal of m-hBN is strongly dependent on its interaction with the substrate, which changes its nature from direct to indirect material and modifies the screening felt by the electrons. We attribute the m-hBN emission signal to the bright direct excitons and consider the likelihood of phonon replicas appearing.