Nanostructure-free Metal-Dielectric Stacks for Raman Scattering Enhancement and Defect Identification in CVD-Grown Tungsten Disulfide (2H-WS₂) Nanosheets

Low-wavenumber Raman (LWR) spectroscopy determines signatures in structural information and layer-to-layer dependency of transition metal dichalcogenides (TMDCs). It supports proper 2D TMDC analysis and subsequent layer verification. The nondestructive nature and ultrafast detection make LWR measurements imperative for layer variations and defect investigations. Interference-enhanced Raman scattering utilizes a metal-dielectric layer to enhance the Raman signal. This has been used to study graphene, C60, and Te. Here, we investigate using Al/Al2O3 coatings to enhance the LWR scattering of different 2H-WS2 layers and understand the structures of these large-area nanosheets. Phase-pure WS2 is synthesized by CVD, and the layers are exfoliated via ultrasonication at 80 kHz. Layers were drop-casted on Al/Al2O3 coatings of different thicknesses of Al2O3 to study differences in bilayers up to a few layers of 2H-WS2. We observe an enhancement of 30 times in the Raman signal (356 cm-1) corresponding to a quarter wave-thick 80 nm Al2O3 and characterize these 2H-WS2 films for their defects. Transfer-matrix calculations confirm the observed behavior to field enhancement. Our work shows how the structures and weaknesses of 2D materials such as WS2 can be better identified, provided an enhancing substrate is chosen.