Electronic structure of colloidal 2H-MoS2 mono- and bilayers determined by spectroelectrochemistry

We investigate the electronic structure of mono- and bilayers of colloidal 2H-MoS2 nanosheets synthesized by wet-chemistry using potential-modulated absorption spectroscopy (EMAS), differential pulse voltammetry (DPV) and electrochemical gating (ECG) measurements. We report the energetic positions of the conduction and valence band edges of the direct and indirect bandgap and observe strong bandgap renormalization effects, charge screening of the exciton as well as intrinsic n-doping of the as-synthesized material. We find two distinct transitions in the spectral regime associated with the C exciton, which overlap into a broad signal upon filling the conduction band. In contrast to the oxidation, the reduction of the nanosheets is largely reversible, enabling potential applications for reductive electrocatalysis. This work demonstrates that EMAS is a highly sensitive tool for determining the electronic structure of thin films with few nanometer thickness and that colloidal chemistry affords high-quality transition metal dichalcogenide nanosheets with an electronic structure comparable to that of exfoliated samples.