Unveiling the origin of anomalous low-frequency Raman mode in CVD-grown monolayer WS₂

Substrates provide the necessary support for scientific explorations of numerous promising features and exciting potential applications in two-dimensional (2D) transition metal dichalcogenides (TMDs). To utilize substrate engineering to alter the properties of 2D TMDs and avoid introducing unwanted adverse effects, various experimental techniques, such as high-frequency Raman spectroscopy, have been used to understand the interactions between 2D TMDs and substrates. However, sample–substrate interaction in 2D TMDs is not yet fully understood due to the lack of systematic studies by techniques that are sensitive to 2D TMD–substrate interaction. This work systematically investigates the interaction between tungsten disulfide (WS₂) monolayers and substrates by low-frequency Raman spectroscopy, which is very sensitive to WS₂–substrate interaction. Strong coupling with substrates is clearly revealed in chemical vapor deposition (CVD)-grown monolayer WS₂ by its low-wavenumber interface mode. It is demonstrated that the enhanced sample–substrate interaction leads to tensile strain on monolayer WS₂, which is induced during the cooling process of CVD growth and could be released for monolayer WS₂ sample after transfer or fabricated by an annealing-free method such as mechanical exfoliation. These results not only suggest the effectiveness of low-frequency Raman spectroscopy for probing sample–substrate interactions in 2D TMDs, but also provide guidance for the design of high-performance devices with the desired sample–substrate coupling strength based on 2D TMDs.