Excitons in boron nitride single layer

TL;DR

This study combines ab initio and tight-binding methods to analyze excitonic effects in boron nitride single layers, revealing strongly localized excitons that deviate from hydrogenic models.

Contribution

It provides a detailed, quasi-quantitative analysis of excitons in BN single layers using a simplified band structure and a combined computational approach.

Findings

Ground state exciton is tightly bound and localized.

Excitons deviate from hydrogenic models, showing unique localization.

Other excitons resemble those in transition metal dichalcogenides.

Abstract

Boron nitride single layer belongs to the family of 2D materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining \textit{ab initio} calculations and a tight-binding-Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence ($\pi$) and conduction ($\pi^*$) bands the tight-binding analysis becomes quasi quantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground state exciton is not a genuine Frenkel exciton, but a very localized "tightly-bound" one. The other ones are similar to those found in transition metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.