Remote Plasma-Enabled Vertical Asymmetry Engineering of 2D PtSe₂ 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 (PtSe₂) via one-step chalcogen exchange. Using a single-reactor H₂S 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 (PtS₂). Janus PtSSe monolayers derived from exfoliated PtSe₂ exhibit an out-of-plane dipole and piezoresponse, with enhanced surface-enhanced Raman scattering relative to corresponding PtSe₂ and PtS₂ monolayers. Extending this approach from monolayers to four-layer (4L) PtSe₂ grown by metal-organic chemical vapor deposition, selective sulfurization produces Janus PtSSe/3L-PtSe₂ heterostructures with improved hydrogen evolution reaction kinetics relative to 4L-PtSe₂ and fully sulfurized 4L-PtS₂. 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.