Raman Digital Twin of Monolayer Janus Transition Metal Dichalcogenides

TL;DR

The paper addresses the challenge of rapid, in situ identification and monitoring of Janus TMD formation from parent monolayers. It builds a Raman digital twin by performing first-principles DFT calculations to predict vibrational modes and Raman fingerprints for Mo- and W-based 2H Janus TMDs and Td-phase W-based Janus TMDs. It provides distinct Raman signatures for each Janus, demonstrates how symmetry reduction and atomic mass affect the spectra, and introduces a weighted averaging scheme across exchange-correlation functionals to improve agreement with experimental data, achieving RMSE of a few cm^-1. The work delivers a practical database and analysis tool that can guide synthesis, characterization, and quality control of Janus TMDs and is expected to accelerate discovery and integration of Janus 2D materials in devices.

Abstract

Monolayer transition metal dichalcogenides (TMDs) are a key class of two-dimensional (2D) materials with broad technological potential. Their Janus counterparts exhibit unique properties due to broken out-of-plane symmetry and further enrich the functionalities of TMDs. However, experimental synthesis and identification of Janus TMDs remain challenging. It is thus highly desirable to have a rapid, simple, and in situ characterization technique to monitor, in real time, the conversion process from the parent to Janus structure. Raman spectroscopy stands out for such a task as it is a powerful, non-destructive, and very commonly used tool to characterize 2D materials both in situ and ex situ. To realize the full potential of Raman spectroscopy on rapid characterization of Janus TMDs, we present a computational "Raman digital twin" library for various monolayer Janus TMDs in both 2H and Td phases. We focus on group-6 TMDs: MoS$_2$, WS$_2$, MoSe$_2$, WSe$_2$, MoTe$_2$, WTe$_2$ and their Janus variants: MoSSe, MoSTe, MoSeTe, WSSe, WSTe, and WSeTe. Using first-principles density functional theory (DFT), we calculate their vibrational properties and predict distinct Raman fingerprints. These phonon and Raman signatures reflect each material's structural symmetry and atomic composition, enabling clear identification via Raman spectroscopy. Our theoretical work supports experimental efforts by providing benchmarks for material identification, structural analysis, and quality control. The computational library expedites the discovery and development of Janus 2D materials, facilitating tighter integration between theoretical predictions and experimental validation.

Figures (6)

• Figure 1: Side and top views of the optimized crystal structures of (a) MoS$_2$, (b) MoSe$_2$, (c) MoTe$_2$, (d) MoSSe, (e) MoSeTe, and (f) MoSTe monolayers in the 2H phase.
• Figure 2: Polarized Raman spectra of 2H-phase monolayer MoS$_2$, MoSSe, and MoSe$_2$ in the (a) XX and (b) XY configurations. The evolution of characteristic modes is shown, with key Raman modes indicated. Corresponding atomic displacements and calculated frequencies are shown on the bottom.
• Figure 3: Polarized Raman spectra of 2H-phase monolayer MoSe$_2$, MoSeTe, and MoTe$_2$ in the (a) XX and (b) XY configurations. The evolution of characteristic modes is traced from MoSe$_2$ to MoTe$_2$.
• Figure 4: Polarized Raman spectra of 2H-phase monolayer WS$_2$, WSSe, and WSe$_2$ in the (a) XX and (b) XY configurations. The evolution of characteristic vibrational modes is traced from WS$_2$ to WSe$_2$.
• Figure 5: Top and side views of the optimized crystal structures of (a) WTe$_2$, (b) WTeSe, and (c) WSTe monolayers in the Td phase.