The interplay between excitons and trions in a monolayer of MoSe2

Abstract

The luminescence and absorption properties of transition metal dichalcogenide mono-layers are widely determined by neutral and charged excitonic complexes. Here, we focus on the impact of a free carrier reservoir on the optical properties of excitonicand trionic complexes in a MoSe2 monolayer at cryogenic temperatures. By applying photodoping via a non-resonant pump laser the electron density can be controlled inour sample, which is is directly reflected in the contribution of excitons and trions to the luminescence signal. We find significant shifts of both the exciton as well as the trion energy in the presence of an induced electron gas both in power- and in time evolution (on the second to minute scale) in our photoluminescence spectra. In particular, in the presence of the photo-doped carrier reservoir, we observe that the splitting between exciton and trion can be enhanced by up to 4 meV. This behaviour is phenomenologically explained by an interplay between an increased screening of excitons via electrons in our system and a modification of the Fermi level. We introduce a simple, but still quantitative treatment of these effects within an variational approach that takes into account both screening and phase space filling effects.