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Exciton radiative lifetime in transition metal dichalcogenide monolayers

Abstract : We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measureτ 0 rad = 1.8 ± 0.2 ps at T = 7 K 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 ≲ 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.
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Cédric Robert, D. Lagarde, F. Cadiz, G. Wang, Benjamin Lassagne, et al.. Exciton radiative lifetime in transition metal dichalcogenide monolayers. Physical Review B: Condensed Matter and Materials Physics, American Physical Society, 2016, 93 (20), ⟨10.1103/physrevb.93.205423⟩. ⟨hal-01980741⟩

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