Brightened emission of dark trions in transition-metal dichalcogenide monolayers

TL;DR

This paper reveals that electron-electron and hole-hole interactions can brighten dark trion emissions in transition-metal dichalcogenide monolayers, providing new insights into their recombination processes and spectral features.

Contribution

It demonstrates that dark trion emission can be brightened through second-order electron-electron and hole-hole interactions, supported by magneto-photoluminescence evidence and a proposed mechanism involving the $b3$ valence band.

Findings

Brightened emission of dark trions observed in doped monolayers.

Electron-electron and hole-hole interactions enable second-order recombination.

The new emission peak appears in the near infrared, about 500 meV below excitons.

Abstract

The optical emission spectra of semiconducting transition-metal dichalcogenide monolayers highlight fascinating recombination processes of charged excitons (trions). When charge tunable WSe$_2$ monolayers are moderately doped with electrons, a strong luminescence peak emerges just below the well-understood spectral lines associated with the recombination of negatively charged bright and dark trions. Despite previous investigations, its origin remains elusive. Here, we demonstrate that this luminescence peak is the result of electron-electron assisted recombination that brightens the dark trion emission. Supporting evidence for this second-order recombination process comes from identifying the brightened emission of positively charged dark trions when the monolayer is electrostatically doped with holes. Remarkably, the discovered hole-hole assisted luminescence peak emerges in the near infrared, about 500 meV below the well-studied spectral region of excitons and trions. In magneto-photoluminescence experiments we find that the $g$-factor of this new transition ($g$ = +4) has an opposite sign compared to the well-known $g$-factor of neutral or charged excitons. This allows us to propose a mechanism of brightening of the positively charged dark trion involving the $\Gamma$ valence band.