Valley Coherent Hot Carriers and Thermal Relaxation in Monolayer Transition Metal Dichalcogenides

TL;DR

This study demonstrates room temperature valley coherence in monolayer transition metal dichalcogenides using hot photoluminescence, revealing insights into hot carrier dynamics, exciton binding energy, and phonon temperature during energy relaxation.

Contribution

It provides the first direct measurement of valley coherence at room temperature and details the energy relaxation pathways of hot carriers in monolayer TMDs.

Findings

Valley coherence preserved at room temperature in MoS2, MoSe2, WS2, and WSe2.

Lower bound of A exciton binding energy in MoS2 is 0.42 eV.

Carrier temperature and phonon temperature during relaxation are quantitatively extracted.

Abstract

We show room temperature valley coherence with in MoS2, MoSe2, WS2 and WSe2 monolayers using linear polarization resolved hot photoluminescence (PL), at energies close to the excitation - demonstrating preservation of valley coherence before sufficient scattering events. The features of the co-polarized hot luminescence allow us to extract the lower bound of the binding energy of the A exciton in monolayer MoS2 as 0.42 (+/- 0.02) eV. The broadening of the PL peak is found to be dominated by Boltzmann-type hot luminescence tail, and using the slope of the exponential decay, the carrier temperature is extracted in-situ at different stages of energy relaxation. The temperature of the emitted optical phonons during the relaxation process are probed by exploiting the corresponding broadening of the Raman peaks due to temperature induced anharmonic effects. The findings provide a physical picture of photo-generation of valley coherent hot carriers, and their subsequent energy relaxation path ways.