Valley polarization and the exciton-trion equilibrium in an atomically thin semiconductor.

Monolayer WS₂ is a transition metal dichalcogenide (TMD) with two direct-bandgap valleys in its band structure that can be selectively addressed using circularly polarized light¹. Its photoluminescence is characterized by excitons and trions that form a chemical equilibrium governed by the net charge density². Chemical doping is a convenient method to achieve high charge densities in TMDs³, which we employ to drive the conversion of excitons into trions. We study the resulting valley polarization under valley-selective optical excitation at room temperature. After doping, the WS₂ emission is 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. We use a rate equation model to explain our observations and shed light on the important role of exciton-trion conversion on valley polarization⁴.

1. Mak, K. et al. Nat. Nanotechnol. 7 (2012)
2. Lien, D. et al. Science 364 (2019)
3. Wang, Y. et al. J. Phys. Chem. Lett. 10 (2019)
4. Carmiggelt, J. et al. Sci Rep 10, 17389 (2020)