The interplay between excitons and trions in a monolayer ofMoSe2

The luminescence and absorption properties of transition metal dichalcogenide monolayersare widely determined by neutral and charged excitonic complexes. Here, we focus on theimpact of a free carrier reservoir on the optical properties of excitonic and trioniccomplexes in a MoSe2 monolayer at cryogenic temperatures. By applyingphotodoping via a non-resonant pump laser, the electron density can be controlled in oursample, which is directly reflected in the contribution of excitons and trions to theluminescence signal. We find significant shifts of both the exciton and trion energies inthe presence of an induced electron gas both in power- and in time evolution (on thesecond to minute scale) in our photoluminescence spectra. In particular, in the presenceof the photo-doped carrier reservoir, we observe that the splitting between excitons andtrions can be enhanced by up to 4 meV. This behaviour is phenomenologically explained byan interplay between an increased screening of excitons via electrons in our system and amodification of the Fermi level. We introduce a simple but still quantitative treatment ofthese effects within a variational approach that takes into account both screening andphase space filling effects.