We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For $\mathrm{MoS}{\mathrm{e}}_2$ monolayer, we measure ${\tau }_{\mathrm{rad}}^0=1.8\pm 0.2\mathrm{ps}$ at $T=7\mathrm{K}$ that we interpret as the intrinsic radiative recombination time. Similar values are found for $\mathrm{WS}{\mathrm{e}}_2$ monolayers. Our detailed analysis suggests the following scenario: at low temperature $\left(T\lesssim 50\mathrm{K}\right)$, the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.

• Received 1 March 2016

DOI:https://doi.org/10.1103/PhysRevB.93.205423