Exciton-to-trion conversion as a control mechanism for valley polarization in room-temperature monolayer WS$_\text{2}$

TL;DR

This study demonstrates that chemical doping in monolayer WS₂ can control exciton-trion conversion, significantly affecting valley polarization at room temperature, with implications for valleytronic applications.

Contribution

It introduces a doping-driven method to manipulate exciton-trion dynamics and valley polarization in monolayer WS₂ at room temperature, supported by a rate equation model.

Findings

Doping converts excitons into trions, enhancing valley polarization.

Doped samples show dominant trion emission with strong valley polarization.

Exciton emission is quenched but remains highly valley-polarized due to conversion into trions.

Abstract

Transition metal dichalcogenide (TMD) monolayers are two-dimensional semiconductors with two valleys in their band structure that can be selectively addressed using circularly polarized light. Their photoluminescence spectrum is characterized by neutral and charged excitons (trions) that form a chemical equilibrium governed by the net charge density. Here, we use chemical doping to drive the conversion of excitons into trions in $\text{WS}_{2}$ monolayers at room temperature, and study the resulting valley polarization via photoluminescence measurements under valley-selective optical excitation. We show that the doping causes the emission to become dominated by trions with a strong valley polarization associated with rapid non-radiative recombination. Simultaneously, the doping results in strongly quenched but highly valley-polarized exciton emission due to the enhanced conversion into trions. A rate equation model explains the observed valley polarization in terms of the doping-controlled exciton-trion equilibrium. Our results shed light on the important role of exciton-trion conversion on valley polarization in monolayer TMDs.