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 an 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 renormalization 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.
P. Lechifflart, F. Paleari, D. Sangalli, and C. Attaccalite, First-principles study of luminescence in hexagonal boron nitride single layer: Exciton-phonon coupling and the role of substrate, Phys. Rev. Materials 7, 024006 (2023)
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