  The luminescence yield of transition metal dichalcogenide monolayers
frequently suffers from the formation of long-lived dark states, which include
excitons with intervalley charge carriers, spin-forbidden transitions, and a
large center-of-mass momentum located outside the light cone of dispersion
relations. Efficient relaxation from bright exciton states to dark states
suppresses the quantum yield of photon emission. In addition, the radiative
recombination of excitons is heavily influenced by Auger-type exciton-exciton
scattering, which yields another nonradiative relaxation channel at room
temperature. Here, we show that Auger-type scattering is promoted not only
between (bright) excitons but also between excitons and long-lived dark states.
We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BN
over broad time ranges of picoseconds to milliseconds using carefully designed
pump-and-probe techniques. We observed that luminescence quenching associated
with Auger-type scattering occurs on 1-100 microsecond time scales, which thus
correspond to the lifetimes of the relevant dark states. The broad distribution
of the measured lifetimes implies the impact of various types of long-lived
states on the exciton annihilation process.
