Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

C Robert,B Han,A Delhomme,K Watanabe,X Marie,P Kapuscinski,T Taniguchi,M Potemski,M R Molas,B Urbaszek,M Bartos,T Amand,C Faugeras

doi:10.1038/s41467-020-17608-4

Abstract

Excitons with binding energies of a few hundreds of meV control the optical properties of transition metal dichalcogenide monolayers. Knowledge of the fine structure of these excitons is therefore essential to understand the optoelectronic properties of these 2D materials. Here we measure the exciton fine structure of MoS2 and MoSe2 monolayers encapsulated in boron nitride by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The experiments performed in transverse magnetic field reveal a brightening of the spin-forbidden dark excitons in MoS2 monolayer: we find that the dark excitons appear at 14 meV below the bright ones. Measurements performed in tilted magnetic field provide a conceivable description of the neutral exciton fine structure. The experimental results are in agreement with a model taking into account the effect of the exchange interaction on both the bright and dark exciton states as well as the interaction with the magnetic field.

Highlights

Excitons with binding energies of a few hundreds of meV control the optical properties of transition metal dichalcogenide monolayers
First we have investigated the effect of an in-plane magnetic field (Voigt configuration) on the low-temperature photoluminescence spectra in MoS2 monolayer
The measured value of Δ = 14 meV in MoS2 ML generates several remarks and interrogations. (i) First it demonstrates the key role played by the exciton exchange energy contribution to the bright-dark energy splitting

Summary

Results

Mixing of exciton states in magnetic field. First, let us recall that the point symmetry group of a TMD monolayer is D3h. We observe at low energy, typically 14 meV below XB, an additional peak which shows up above ~12 T This feature, interpreted as the brightened spin-forbidden dark exciton, has been reproduced on several samples and spot positions (see data in Supplementary Note 1 principle), this line should correspond to both brightened gray and dark excitons, but the inhomogeneous linewidth in our samples is too broad (the linewidth of XD is 5 meV) to enable us to distinguish the expected small splitting δ between gray and dark states. The measured quadratic behavior is a strong indication that the low-energy line corresponds to Magnetic field Bll (T)

20 Laser hBN

Discussion

Methods