Singlet and triplet trions in WS$_2$ monolayer encapsulated in hexagonal boron nitride

TL;DR

This study investigates singlet and triplet trions in WS$_2$ monolayers encapsulated in hexagonal boron nitride, revealing their energy separation, polarization properties, and temperature-dependent emission characteristics.

Contribution

It provides the first detailed analysis of both singlet and triplet trions in WS$_2$ monolayers, highlighting their energy separation and polarization behavior.

Findings

Small energy separation between singlet and triplet trions.

Triplet trion emission preserves helicity better than singlet.

Singlet trions are observable at room temperature, triplet trions mainly at low temperatures.

Abstract

Embedding a WS$_2$ monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron-electron exchange interaction for the trion triplet in a WS$_2$ monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures.