Combining experiments on luminescent centres in hexagonal boron nitride with the polaron model and ab initio methods towards the identification of their microscopic origin

TL;DR

This study combines ab initio calculations and polaron models to identify the microscopic origin of luminescent centres in hexagonal boron nitride, revealing three carbon-based defect types responsible for emission features.

Contribution

It introduces a novel theoretical approach integrating ab initio and polaron models to accurately identify defect origins in hBN luminescent centres.

Findings

Identification of three specific carbon-based defects as luminescent centres

Excellent agreement between calculated and experimental photoluminescence excitation maps

Confirmation of phonon-assisted excitation mechanism with 170 meV phonon energy

Abstract

The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent centres with emission energies of 2 eV which exhibit pronounced phonon sidebands. We investigate the microscopic origin of these luminescent centres by combining ab initio calculations with non-perturbative open quantum system theory to study the emission and absorption properties of 26 defect transitions. Comparing the calculated line shapes with experiments we narrow down the microscopic origin to three carbon-based defects: $\mathrm{C_2C_B}$, $\mathrm{C_2C_N}$, and $\mathrm{V_NC_B}$. The theoretical method developed enables us to calculate so-called photoluminescence excitation (PLE) maps, which show excellent agreement with our experiments. The latter resolves higher-order phonon transitions, thereby confirming both the vibronic structure of the optical transition and the phonon-assisted excitation mechanism with a phonon energy 170 meV. We believe that the presented experiments and polaron-based method accurately describe luminescent centres in hBN and will help to identify their microscopic origin.