Chalcogen-Dependent Efficiency of Synthesizing Janus Monolayer Transition Metal Dichalcogenides by Atomic-Layer Substitution

Janus monolayer transition metal dichalcogenides (TMDs) are a unique category of two-dimensional (2D) materials with intriguing properties arising from their out-of-plane asymmetry and inherent electric dipole. Room-temperature (RT) synthesis and patterning of Janus TMDs have recently been realized through an atomic-layer substitution (ALS) approach that steers the reaction pathway in a diverse energy landscape compared to the high-temperature process. In principle, the RT-ALS synthesis of MSSe-type Janus TMDs (M = Mo, W, etc.) can start from either MS₂ or MSe₂, but the associated energy landscapes of these pathways may vary, influencing the reaction efficiency. Herein, we investigate the conversion of Janus MSSe monolayers from MS₂ and MSe₂ prepared by different methods, and our experimental results indicate that the RT-ALS process is more efficient and has a broader reaction window for converting MSe₂ to MSeS than starting the conversion from MS₂. Density functional theory calculations reveal that the reaction energy barrier and overall reaction energy are considerably lower when MSe₂ is employed as the starting material, agreeing with experimental findings. These results improve our understanding of the RT-ALS process, and provide useful guidance for the future design of optimum reaction pathways for various Janus materials with high yield, enhanced uniformity, and controlled dipole orientations.