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.