Raman spectrum of Janus transition metal dichalcogenide monolayers WSSe and MoSSe

TL;DR

This study provides a detailed Raman spectral analysis of Janus TMD monolayers WSSe and MoSSe, identifying vibrational modes and defect-related features, thereby aiding future material synthesis and application research.

Contribution

It offers the first comprehensive Raman spectral characterization of Janus TMD monolayers, combining experimental data with theoretical calculations and symmetry analysis.

Findings

Identified four first-order Raman modes in WSSe and MoSSe

Detected defect-activated phonon processes correlating with crystal quality

Provided a framework for assessing defect concentration via Raman spectroscopy

Abstract

Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolved Raman data with calculations of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quantitative assessment of the defect concentration and, thus, the crystal quality of the materials synthesized. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.