  Spin-forbidden intravalley dark exciton in tungsten-based transition metal
dichalcogenides (TMDCs), owing to its unique spin texture and long lifetime,
has attracted intense research interest. Here, 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 boron nitride
(BN) encapsulated monolayer WSe2 device at low temperature. The unambiguous
evidence of the dark trions is further obtained by directly resolving the
radiation pattern of the dark trions through back focal plane imaging. The dark
trions possess binding energy of ~ 15 meV, and it inherits the long lifetime
and large g-factor from the dark exciton. Interestingly, under the out-of-plane
magnetic field, dressing the dark exciton with one free electron or hole
results in distinctively different valley polarization of the emitted phonon, a
result of the different intervalley scattering mechanism for the electron and
hole. Finally, the lifetime of the positive dark trion can be further tuned
from ~ 50 to ~ 215 ps by controlling the gate voltage. The gate tunable dark
trions ushers in new opportunities for excitonic optoelectronics and
valleytronics.
