Exciton radiative lifetime in transition metal dichalcogenide monolayers

We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoSe2 monolayer, we measure tau(0)(rad) = 1.8 +/- 0.2 ps at T = 7K that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T less than or similar to 50 K), 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.