Excitonic Transport and Intervalley Scattering in Exfoliated MoSe2 Monolayer Revealed by Four-Wave-Mixing Transient Grating Spectroscopy

TL;DR

This study uses four-wave-mixing transient grating spectroscopy to reveal excitonic transport, rapid intervalley scattering, and exciton-exciton annihilation in high-quality monolayer MoSe2, providing detailed insights into its excitonic dynamics.

Contribution

It introduces a novel spectroscopic approach to disentangle complex excitonic processes and accurately characterize excitonic properties in monolayer MoSe2.

Findings

Long-lived excitons (~230 ps)

Fast intervalley scattering (<120 fs)

Exciton diffusion constant ~1.4 cm²/s

Abstract

Exciton intervalley scattering, annihilation and relaxation dynamics, and diffusive transport in a monolayer transition metal dichalcogenides are central to the functionality of devices based on them. This motivated us to investigate these properties in exfoliated high-quality monolayer MoSe2 using heterodyned nonlinear four-wave-mixing transient grating spectroscopy. While free exciton excitations are found to be long-lived (~230 ps), an extremely fast intervalley scattering ( less than 120 fs) is observed leading to a negligible valley polarization, consistent with steady state photoluminescence measurements. The exciton population decay shows an appreciable contribution from exciton-exciton annihilation reactions with an annihilation constant of ~ 0.01 cm2s-1, which in addition leads to an extra contribution to the transient grating response. The underlying excitonic dynamics were numerically modeled by including exciton-exciton annihilation, also in the diffusion equation, which allows extraction of the diffusion constant, D ~1.4 cm2s-1. Our results provide a method that allows for the disentanglement of the intricate dynamics involving many-body annihilation processes and a detailed characterization of the excitonic properties of monolayer MoSe2