Photoluminescence saturation and exciton decay dynamics in transition metal dichalcogenide monolayers

We report a photoluminescence (PL) and transient reflection spectroscopy study of the exciton dynamics in monolayer transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs at an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that nonlinear exciton-exciton annihilation is the dominant exciton decay process in monolayer MoSe2 in contrast to the previously-reported linear exciton decay in monolayer MoS2. In addition, the exciton lifetime in MoSe2, which is greater than 125 ps, is more than an order of magnitude longer than the several-ps exciton lifetime in MoS2. We find that the dramatically different exciton decay mechanisms and PL saturation behaviors of the MoSe2 and the MoS2 monolayers can be explained in terms of the difference in their exciton lifetimes.