Dark trions and biexcitons in WS2 and WSe2 made bright by e-e scattering

TL;DR

This paper demonstrates that intervalley electron-electron scattering in monolayer WS2 and WSe2 makes dark trions and biexcitons partially bright, enabling their observation through radiative recombination with estimated lifetimes around 10 ps.

Contribution

It reveals that electron-electron scattering mixes bright and dark states of trions and biexcitons, making them semi-bright and observable in photoluminescence spectra.

Findings

Intervalley scattering enables brightening of dark complexes.

Estimated radiative lifetime of semi-dark states is ~10 ps.

Temperature-dependent PL spectra show signatures of these complexes.

Abstract

The direct band gap character and large spin-orbit splitting of the valence band edges (at the K and K' valleys) in monolayer transition metal dichalcogenides have put these two-dimensional materials under the spot-light of intense experimental and theoretical studies. In particular, for Tungsten dichalcogenides it has been found that the sign of spin splitting of conduction band edges makes ground state excitons radiatively inactive (dark) due to spin and momentum mismatch between the constituent electron and hole. One might similarly assume that the ground states of charged excitons and biexcitons in these monolayers are also dark. Here, we show that the intervalley K$\leftrightarrows$K' electron-electron scattering mixes bright and dark states of these complexes, and estimate the radiative lifetimes in the ground states of these "semi-dark" trions and biexcitons to be ~ 10ps, and analyse how these complexes appear in the temperature-dependent photoluminescence spectra of WS2 and WSe2 monolayers.