The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become
an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap
semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded
two-dimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. Here, we
present temperature-dependent and time-resolved photoluminescence (PL) studies of single-layer MoS2 flakes.
Some of the flakes are covered with oxide layers prepared by atomic layer deposition (ALD). At low temperatures,
we clearly see two PL peaks in the as-prepared flakes without oxide layers, which we may assign to bound and
free exciton transitions. The lower-energy, bound exciton PL peak is absent in the oxide-covered flakes. In
time-resolved PL measurements, we observe very fast photocarrier recombination on the few-ps timescale at low
temperatures, with increasing photocarrier lifetimes at higher temperatures due to exciton-phonon scattering.