Janus transition metal dichalcogenides (TMDs) have recently attracted attention as key building blocks for nanoscrolls and moiré superlattices, due to their asymmetric structure and intrinsic built-in electric field [1,2] . High-quality crystals are essential for accurately probing their intrinsic properties. However, improving the quality of Janus TMDs remains a major challenge, primarily because defects are introduced through chalcogen substitution processes. Here, we report the fabrication and characterization of optically uniform Janus TMD monolayers on hexagonal boron nitride (hBN) substrates. MoSe2 and WSe2 monolayers were directly grown on hBN by chemical vapor deposition, and subsequently converted to Janus MoSSe and WSSe monolayers by H2 plasma treatment at room temperature. This process yields crack-free Janus TMD monolayers with suppressed lattice strain, unlike samples prepared on SiO2. These hBN-supported samples exhibit a narrow photoluminescence (PL) linewidth of approximately 9 meV at 5 K. The reduced linewidth enables the precise investigation of excitonic properties such as trion binding energies and exciton-phonon scattering. These results highlight the critical role of TMD-substrate interactions in preserving material quality during chalcogen substitution and provide valuable insights into the intrinsic physical properties of Janus TMD-based nanostructures.
Fig. 1. (a) Schematic illustrations of Janus MoSSe, its nanoscrolls, and moiré superlattices. [1,2]
