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

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.

arXiv comment8 pages, 4 figures

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• Valley Degree of Freedom in Two-Dimensional van der Waals Materials
• Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations
• Room Temperature Micro-Photoluminescence Studies of Colloidal WS2 Nanosheets
• Inducing room-temperature valley polarization of excitonic emission in transition metal dichalcogenide monolayers
• Directional emission from WS2 monolayer coupled to plasmonic Nanowire-on-Mirror Cavity
• Symmetric Domain Segmentation in WS2 Flakes: Correlating spatially resolved photoluminescence, conductance with valley polarization
• Valley polarization of trions in monolayer MoSe${}_2$ interfaced with bismuth iron garnet
• Robustness of momentum-indirect interlayer excitons in MoS2/WSe2 heterostructure against charge carrier doping