Electron-Doping-Enhanced Trion Formation in Monolayer Molybdenum Disulfide Functionalized with Cesium Carbonate

We report effective and stable electron doping of monolayer molybdenum disulfide (MoS₂) by cesium carbonate (Cs₂CO₃) surface functionalization. The electron charge carrier concentration in exfoliated monolayer MoS₂ can be increased by about 9 times after Cs₂CO₃ functionalization. The n-type doping effect was evaluated by in situ transport measurements of MoS₂ field-effect transistors (FETs) and further corroborated by in situ ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering measurements. The electron doping enhances the formation of negative trions (i.e., a quasiparticle comprising two electrons and one hole) in monolayer MoS₂ under light irradiation and significantly reduces the charge recombination of photoexcited electron–hole pairs. This results in large photoluminescence suppression and an obvious photocurrent enhancement in monolayer MoS₂ FETs.