Remote Plasma‐Enabled Vertical Asymmetry Engineering of 2D PtSe 2 via Phase‐Preserving Chalcogen Exchange

Vertical asymmetry in two‐dimensional (2D) transition metal dichalcogenides induces intrinsic dipoles, symmetry‐breaking functionalities, and interface‐specific electronic reconstruction. However, existing Janus synthesis processes rely on sequential treatments, high‐temperature substitution, or solid‐phase precursors. These approaches often cause phase transitions, lattice degradation, or defect formation, limiting scalability and device performance. Here, we introduce a remote plasma‐enabled strategy for phase‐preserving vertical asymmetry engineering in 2D Platinum Diselenide (PtSe 2 ) via one‐step chalcogen exchange. Using a single‐reactor H 2 S remote plasma, treatment at near‐room‐temperature (∼50°C) selectively substitutes top‐layer chalcogen atoms, forming Janus PtSSe with preserved crystallinity and minimal lattice perturbation. Increasing the temperature to 200–300°C enables complete conversion to crystalline 2D Platinum Disulfide (PtS 2 ). Janus PtSSe monolayers derived from exfoliated PtSe 2 exhibit an out‐of‐plane dipole and piezoresponse, with enhanced surface‐enhanced Raman scattering relative to corresponding PtSe 2 and PtS 2 monolayers. Extending this approach from monolayers to four‐layer (4L) PtSe 2 grown by metal‐organic chemical vapor deposition, selective sulfurization produces Janus PtSSe/3L‐PtSe 2 heterostructures with improved hydrogen evolution reaction kinetics relative to 4L‐PtSe 2 and fully sulfurized 4L‐PtS 2 . Our results establish remote plasma‐driven chalcogen exchange as a generalizable platform for symmetry‐controlled 2D materials, enabling scalable engineering of vertical polarity, interfacial charge distribution, and functional responses.