Direct observation and gate manipulation of dark trions in monolayer WSe₂

Dark trions, the bound states between a dark exciton and an electron (hole), are intriguing entities with novel applications, because their long lifetime and finite net charge allow us to efficiently control the excitonic dynamics by electric field. Detection of dark trions is, however, exceedingly challenging due to their optical inactivity. Prior research required indirect detection techniques, such as altering their spins with in-plane magnetic field and coupling to surface plasmons. Here we report the direct observation and gate manipulation of intrinsic dark trions in monolayer WSe₂. By using ultraclean WSe₂ devices encapsulated by boron nitride, we can directly resolve the weak photoluminescence of spin-forbidden dark trions and continuously tune between negative and positive charged dark trions with electrostatic gating. We also reveal their spin triplet configuration and distinct valley emission by their characteristic Zeeman splitting under magnetic field. The dark trions exhibit large binding energy (14-16 meV) and narrow line width (2.5 meV), signifying their high stability and long lifetime. Such robust and directly detectable dark trions provide a crucial component to realize electrically controllable trion transport in two-dimensional materials.