Photoluminescence kinetics of dark and bright excitons in atomically thin MoS$_2$

TL;DR

This study investigates the exciton fine structure in MoS2 monolayers and bilayers using time-resolved micro-photoluminescence across temperatures and strain conditions, revealing how strain influences exciton energy splitting and decay dynamics.

Contribution

It provides detailed insights into the energy ordering and decay times of dark and bright excitons in MoS2, highlighting the significant impact of strain on exciton properties.

Findings

Spin-allowed state is lowest in unstrained monolayers.

Dark state is within 2 meV of bright state in monolayers.

Strain increases exciton energy splitting significantly.

Abstract

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. We study time-resolved micro-photoluminescence in MoS$_2$ monolayers and bilayers, both unstrained and compressively strained, in a wide temperature range (10-300 K) to distinguish between exciton states optically allowed and forbidden, both in spin and momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times were found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. Our results unambiguously show that, in an unstrained monolayer, the spin-allowed state is the lowest for a series of A excitons (1.9 eV) with the dark state being < 2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in bilayers (1.5 eV), the spin-forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS$_2$.