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 $mathrm{MoS}{mathrm{e}}_{2}$ monolayer, we measure ${ensuremath{tau}}_{mathrm{rad}}^{0}=1.8ifmmodepmelsetextpmfi{}0.2phantom{rule{0.16em}{0ex}}mathrm{ps}$ at $T=7phantom{rule{0.16em}{0ex}}mathrm{K}$ 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 $(Tensuremath{lesssim}50phantom{rule{0.16em}{0ex}}mathrm{K})$, 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.