Exciton Radiative Lifetime in Transition Metal Dichalcogenide Monolayers

TL;DR

This study measures the ultrafast exciton radiative lifetime in transition metal dichalcogenide monolayers, revealing temperature-dependent dynamics and non-equilibrium populations using time-resolved photoluminescence.

Contribution

It provides the first direct measurement of intrinsic exciton radiative lifetime in MoSe2 and WSe2 monolayers with detailed analysis of temperature-dependent exciton dynamics.

Findings

Intrinsic radiative lifetime ~1.8 ps in MoSe2 and WSe2

Temperature influences exciton decay regimes and timescales

Exciton and trion populations are not in thermodynamic equilibrium

Abstract

We have investigated the exciton dynamics in transition metal dichalcogenide mono-layers using time-resolved photoluminescence experiments performed with optimized time-resolution. For MoSe2 monolayers, we measure $\tau_{rad}=1.8\pm0.2$ ps that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe2 mono-layers. Our detailed analysis suggests the following scenario: at low temperature (T $\leq$ 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 PL 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 non-thermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and non-radiative 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.