Dark Excitons in Monolayers of Molybdenum based Transition Metal Dichalcogenide

Excitons with binding energies of a few hundreds of meV drive the optical properties of Transition Metal Dichalcogenide (TMD) monolayers. One can expect that the optoelectronic properties will change drastically whether the spin-forbidden dark excitons lie below or above the bright excitons. This exciton fine structure splitting was accurately determined recently for WS2 and WSe2 monolayers using various experimental techniques [1-5]. In contrast the energy of the dark exciton in MoS2 ML has not been measured yet though this was the first member of the TMD family to be established as a direct gap semiconductor in the monolayer form.
Here we have performed magneto-photoluminescence experiments on MoS2 monolayers encapsulated in hexagonal Boron Nitride (hBN) with a magnetic field oriented along the monolayer plane. The high quality of the investigated samples allowed us to determine accurately the bright-dark energy splitting. We have performed similar measurements for other MoX2 monolayers (X=Se or Te) encapsulated in hBN. The different contributions to the exciton fine structure will be discussed.

[1] PRL 119, 047401 (2017)
[2] 2D Mater. 4, 021003 (2017)
[3] Nature Nano 12, 883 (2017)
[4] Nature Nano 12, 856 (2017)
[5] PRL 123, 096803 (2019)