Synthesis of 2H-Phase Janus MoSeTe Monolayers and Their Second-Harmonic Generation

Janus MoSeTe, with intrinsic structural asymmetry and strong spin–orbit coupling, is promising for valleytronics and nonlinear optics. While its properties have been widely predicted theoretically, controlled experimental synthesis has remained elusive. Here, we report the successful synthesis of 2H-phase Janus MoSeTe monolayers, bridging theory and experiment. Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning transmission electron microscopy confirm the Janus structure. The extremely weak photoluminescence indicates an indirect bandgap. We further study its second-harmonic generation (SHG) properties, finding that broken out-of-plane mirror symmetry enhances the second-order nonlinear optical response to ∼1.7× that of monolayer MoSe2. When MoSe2 is stacked on Janus MoSeTe with a ∼30° twist, forming a van der Waals heterostructure, SHG is further enhanced to ∼2.7× monolayer MoSe2 and exhibits anisotropy (maximum-to-minimum intensity ratio ∼1.34). These results provide a route for synthesizing novel 2D Janus TMDs and establish Janus MoSeTe-based systems as a platform for tunable second-order nonlinear optics.