High-quality Janus MoSSe and WSSe monolayers enabled by hBN-supported chalcogen substitution.

Janus transition metal dichalcogenide (TMD) monolayers offer distinctive physical properties and device applications because their broken out-of-plane mirror symmetry induces an intrinsic out-of-plane dipole. High crystal quality is essential for accessing their intrinsic excitonic physics, yet improving Janus TMD quality remains challenging due to structural degradation and strain introduced during chalcogen substitution. To address this challenge, we develop a hexagonal boron nitride (hBN)-supported chalcogen substitution process that yields crack-free Janus TMD monolayers with improved optical uniformity. MoSe2 and WSe2 monolayers are first grown on hBN substrates and then converted into Janus MoSSe and WSSe monolayers via room-temperature H2 plasma treatment, respectively. Compared with conventional SiO2-supported samples, the hBN-supported process yields Janus monolayers with reduced inhomogeneous lattice strain, owing to negligible in-plane interactions with the substrate. The hBN-supported samples exhibit narrow photoluminescence (PL) linewidths of 40-50 meV at room temperature, which further decrease to ~9 meV at 5 K. This linewidth reduction enables quantitative analysis of the trion binding energy and exciton-phonon interaction and provides a practical route to accessing intrinsic properties of Janus TMDs.