Enhancement of the E¹_2g and A_1g Raman modes and layer identification of 2H-WS₂ nanosheets with metal coatings

Raman spectroscopy in transition metal dichalcogenides (TMDCs) helps determine their structural information and layer dependency. Because it is non-destructive and fast, it is an archetypal spectroscopic technique to investigate the structure and defects in TMDCs. In our earlier study, we used a metal-dielectric coating to enhance Raman signal of WS₂ because the Raman Spectra measured from WS₂ coated on the standard Si/SiO₂ was significantly lower. This metal-dielectric coating allowed access to the otherwise unavailable E¹_2g and A_1g modes of WS₂. In this study, we compare the Raman spectra of WS₂ on a Si/SiO₂ to that of metal layers (Au [200 nm] and Al [200 nm]). A significant enhancement in the Raman signal of 2-3L WS₂ is observed for both the Au and Al coatings. Although 200 nm Au coating enhances the Raman Signal better than the 10 nm Au coating, it does not resolve the other signature vibrations other than E¹_2g and A_1g. Whereas, coating based on Al (200 nm) and Al (10 nm) both enhance the Raman signal of WS₂. In the case of Al coating, we report well-resolved vibrational modes such as the ones below <300 cm⁻¹in a few layered WS₂. This enhancement allows us to quantify the number of layers and also investigate the presence of defects. Photoluminescence measurements on 2–3L and 15L WS₂ on Al (200 nm) confirm this.