Excitonic resonances in thin films of WSe 2 : from monolayer to bulk material

International audienceWe present optical spectroscopy (photolumines-cence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe 2 , with thickness ranging from mono-to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe 2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe 2 mono-layer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated