Hidden Role of Electron Accumulation in Driving Room-Temperature Topotactic Substitution for Janus 2D Semiconductors

Understanding atomic substitution in Janus transition metal dichalcogenides (TMDs) is crucial for controlling intrinsic out-of-plane dipoles and electronic asymmetry. Although room-temperature Janus conversion has been reported, the microscopic origin of selective chalcogen replacement remains unclear. Here, we develop an in situ optical–electrical platform enabling correlated Raman and field-effect transistor measurements during plasma-assisted atomic-layer substitution under vacuum. We identify a plasma-electron-assisted room-temperature topotactic reaction, in which electrons accumulate near the TMD surface and at the MoSe2/SiO2 interface, weaken metal–chalcogen bonds, and facilitate hydrogen-assisted substitution. Density functional theory calculations show that excess electron doping lowers bond formation energies. Ultraviolet illumination further enhances electron accumulation and accelerates substitution kinetics. This work experimentally verifies a charge-mediated topotactic substitution mechanism and establishes plasma-electron-assisted synthesis as a route for room-temperature Janus TMD formation.