Janus Monolayer Transition-Metal Dichalcogenides

TLDR

The Janus SMoSe monolayer was produced by sulfurizing monolayer MoSe₂, replacing the top selenium layer with sulfur, and its structure was confirmed by Raman, photoluminescence, TEM, XPS, and TOF‑SIMS. The synthesized Janus SMoSe monolayer exhibits a sandwiched S–Mo–Se crystal structure and shows high basal‑plane hydrogen evolution reaction activity driven by intrinsic defects and strain, as confirmed by DFT.

Abstract

The crystal configuration of sandwiched S–Mo–Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.