Exciton Dynamics in Monolayer Transition Metal Dichalcogenides

TL;DR

This paper reviews experimental and theoretical studies on exciton and valley dynamics in monolayer transition metal dichalcogenides, highlighting the complex mechanisms affecting exciton lifetimes and spin/valley relaxation.

Contribution

It provides a comprehensive overview of current understanding and identifies key areas where further experiments are needed to clarify exciton recombination mechanisms.

Findings

Exciton decay times range from femtoseconds to nanoseconds.

Valley depolarization times can exceed one nanosecond.

Electron-hole exchange influences spin and valley dynamics.

Abstract

Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population decay and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond. At present, however, a consensus on the microscopic mechanisms governing exciton radiative and non-radiative recombination is lacking. The strong exciton oscillator strength resulting in up to 20% absorption for a single monolayer points to ultrafast radiative recombination. However, the low quantum yield and large variance in the reported lifetimes suggest that non-radiative Auger-type processes obscure the intrinsic exciton radiative lifetime. In either case, the electron-hole exchange interaction plays an important role in the exciton spin and valley dynamics. In this article, we review the experiments and theory that have led to these conclusions and comment on future experiments that could complement our current understanding.