Brightening of dark excitons in monolayers of semiconducting transition metal dichalcogenides

Abstract

We present low temperature magneto-photoluminescence experiments which demonstrate the brightening of dark excitons by an in-plane magnetic field B applied to monolayers of different semiconducting transition metal dichalcogenides. For both WSe2 and WS2 monolayers, the dark exciton emission is observed at ∼50 meV below the bright exciton peak and displays a characteristic doublet structure which intensity is growing with B 2 , while no magnetic field induced emission peaks appear for MoSe2 monolayer. Our experiments also show that the MoS2 monolayer has a dark exciton ground state with a dark-bright exciton splitting energy of ∼100

Dark excitations in monolayer transition metal dichalcogenides

“…Stronger doping may affect the band structure and such effects are not accounted for in our calculations. Interestingly, the experimental results of Molas et al [21] for WSe 2 reveal some differences with earlier measurements. In particular, the bright trion is found to have a binding energy of more than 50 meV.…”

“…However, basic optical properties such as photoluminescence intensities, exciton decay rates, valley scattering, etc., will depend strongly on the interplay between the bright and dark states in the low-energy part of the spectra [17][18][19]. In recent experiments by Zhang et al [20] and Molas et al [21], it has been shown that dark excitations can be experimentally probed by applying an in-plane magnetic field. Due to the resulting mixture of dark and bright states, they become visible in photoluminescence.…”

“…Finally, we qualitatively discuss the possibility of observing the different excitations in photoluminescence experiments [21]. For example, for the formation of a negative trion, an exciton and an extra electron are required.…”

“…In contrast to this, TMDCs including W will show an inverted trend, i.e., the dark states should be more likely than bright states. However, the weight of dark states in photoluminescence will also depend on the mixing to bright states via the applied in-plane magnetic field [20,21].…”

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

“…Here, charge tunable samples [66] or experiments in transverse magnetic fields [67,68] are expected to give a better indication on the dark-bright order in future experiments on these narrow linewidth samples. The temperature dependence of the PL peak position follows a standard hyperbolic cotangent relation [69]:…”