Direct Observation of Gate-Tunable Dark Trions in Monolayer WSe₂

Spin-forbidden intravalley dark excitons in tungsten-based transition-metal dichalcogenides (TMDCs) have attracted intense research interest. It has been discovered that tungsten-based TMDCs such as WSe₂ and WS₂ have a unique bandstructure, in which the spin-orbit coupling also induces the splitting of the conduction band. The resulted ground state of the exciton, however, is a spin-triplet state as the spin-forbidden dark exciton. We show that we can control the dark exciton electrostatically by dressing it with one free electron or free hole, forming the dark trions. The existence of the dark trions is suggested by the unique magneto-photoluminescence spectroscopy pattern of the monolayer WSe₂ device. And the existence of the dark trions is unambiguously demonstrated by directly resolving the radiation angle of the dark trions through back focal plane imaging. The dark trions possess binding energy of ∼15 meV, and inherit the long lifetime and large g-factor from the dark exciton. The dark trions open the door to new possibilities of valleytronics and excitonic applications.