Asymmetric trions in monolayer transition metal dichalcogenides

TL;DR

This paper reveals that the optical trion peak in monolayer transition metal dichalcogenides is influenced by asymmetric wave functions and dark excitons, challenging the traditional interpretation of trion binding energy.

Contribution

It introduces a theoretical model showing the significance of dark excitons and wave function asymmetry in understanding trion energies in 2D semiconductors.

Findings

The measured trion energies in MoSe2 and WSe2 are explained by the model.

Dark excitons are more strongly bound than bright excitons.

Wave function asymmetry affects the optical spectra interpretation.

Abstract

Exciton spectroscopy serves as a sensitive probe of electronic states in two-dimensional semiconductors. A prominent feature in optical spectra is the trion peak arising from the binding of a charge carrier to an exciton. The splitting between the exciton and trion peaks is usually interpreted as the trion binding energy, but we theoretically show that this view is incomplete. Since dark excitons are more strongly bound than the bright exciton, the trion wave function is asymmetric and a large contribution to the measured splitting is the difference between the bright and dark exciton binding energies. Our model quantitatively explains the measured trion energies in MoSe2 and WSe2, demonstrating the importance of the internal structure of the exciton for the interpretation of the optical response of transition metal dichalcogenides.