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Photoluminescence Saturation and Exciton Decay Dynamics in Transition Metal Dichalcogenide Monolayers

We report a photoluminescence (PL) and transient reflection spectroscopy study of exciton dynamics in monolayer transition transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that the 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, > 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 mechanism and PL saturation behavior of MoSe2 and MoS2 monolayers can be explained by the difference in their exciton lifetime.