Phonon Assisted Exciton Processes in Two-Dimensional Tungsten Monocarbide

TL;DR

This study uses advanced ab initio methods to analyze phonon-assisted exciton emission in 2D tungsten monocarbide, revealing indirect emission mechanisms, phonon replicas, and rapid non-radiative recombination, with implications for near-infrared photonic devices.

Contribution

It provides the first detailed ab initio analysis of phonon-assisted exciton processes in 2D tungsten monocarbide, highlighting its indirect optical gap and fast non-radiative recombination.

Findings

Photoluminescence involves phonon-assisted indirect emission.

Prominent phonon replicas observed near-infrared wavelengths.

Non-radiative recombination occurs in femtoseconds, much faster than radiative processes.

Abstract

n this study, we utilize a rigorous ab initio-based finite momentum Bethe-Salpeter equation to investigate the photoluminescence emission in two-dimensional hexagonal tungsten carbide (h-WC). This thermodynamically stable monolayer exhibits an indirect optical gap, resulting in phonon-assisted emission. We observe that light absorption is a direct process centered around the direct quasiparticle gap, while light emission is indirect and requires modes between $\Gamma$-$M$ in the phonon dispersion. The emission lines feature prominent phonon replicas at cryogenic temperatures, particularly near-infrared wavelengths (1.09 and 1.17 eV), and we observe exciton thermalization with the crystal beyond 25 K. Additionally, non-radiative recombination is a remarkably fast process, occurring at order of a few femtoseconds (4.8 fs at 0 K and 2.8 fs at 300 K) compared to radiative recombination (2.3 ps at 0 K and 214 ns at 300 K). These optical characteristics of 2D h-WC may facilitate the promise of photon-emitter devices for near-infrared signal communication.