  Here, we show that deep trapped "dark" exciton states are responsible for the
surprisingly long lifetime of band-edge photoluminescence in acid-treated
single-layer MoS2. Temperature-dependent transient photoluminescence
spectroscopy reveals an exponential tail of long-lived states extending
hundreds of meV into the band gap. These sub-band states, which are
characterized by a 4 microsecond radiative lifetime, quickly capture and store
photogenerated excitons before subsequent thermalization up to the band edge
where fast radiative recombination occurs. By intentionally saturating these
trap states, we are able to measure the "true" 150 ps radiative lifetime of the
band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature.
These experiments reveal the dominant role of dark exciton states in
acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at
room temperature in trap states below the band edge. We hypothesize that these
states are associated with native structural defects, which are not introduced
by the superacid treatment; rather, the superacid treatment dramatically
reduces non-radiative recombination through these states, extending the exciton
lifetime and increasing the likelihood of eventual radiative recombination.
