Gate Tunable Dark Trions in Monolayer WSe2

Monolayer WSe2 is an intriguing material to explore dark exciton physics. We have measured the photoluminescence from dark excitons and trions in ultraclean monolayer WSe2 devices encapsulated by boron nitride. The dark trions can be tuned continuously between negative and positive trions with electrostatic gating. We reveal their spin-triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under a magnetic field. The dark trion binding energies are 14-16 meV, slightly lower than the bright trion binding energies (21-35 meV). The dark trion lifetime (similar to 1.3 ns) is two orders of magnitude longer than the bright trion lifetime (similar to 10 ps) and can be tuned between 0.4 and 1.3 ns by gating. Such robust, optically detectable, and gate tunable dark trions may help us realize trion transport in two-dimensional materials.