Optical absorption spectra of monolayer transition metal dichalcogenides at high doping levels

The absorption spectra of semiconducting transition metal dichalcogenides (TMDCs) at monolayer thickness are dominated by excitonic transitions due to the strong Coulomb interactions associated with the materials’ reduced screening and dimensionality. It has been shown that the excitons in these materials can be tuned electrically by using a Si back gate to achieve carrier density up to several times 10¹²cm^-2. Here we extend these studies to higher carrier densities through the use of electric double layer gating technique with the ionic solid conductor LaF₃ as the electrolyte. In this fashion, we are able to electrostatically gate monolayer WSe₂ to a density of a few 10¹³cm^-2. While earlier investigations showed shifts in exciton (and charged exciton) energies and binding energies, the excitonic absorption features remained relatively strong and distinct. At carrier densities in the 10¹³cm^-2 range, however, we find that the excitonic absorption features are no longer observable. The progression of the measured absorption spectra with increasing doping density is compatible with the behavior expected for a Mott transition from the excitonic regime to an electron–hole plasma regime, as was previously reported for the case of strong photodoping of TMDC monolayers.