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

Abstract

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.

Chemical-Vapor-Deposition-Synthesized Two-Dimensional Non-Stoichiometric Copper Selenide (β-Cu2−xSe) for Ultra-Fast Tetracycline Hydrochloride Degradation under Solar Light

“…The CVD route we reported in this paper has a significant advantage compared to other processes. We have previously reported on the CVD route’s reproducibility and scalability, especially when using transition metal foils as the growth substrate [ 45 , 46 , 47 , 51 , 52 , 53 ]. We have started our synthesis from elemental precursors, i.e., Cu foil and Se powder.…”

Enhancement of the E12g and A1g Raman modes and layer identification of 2H‐WS2 nanosheets with metal coatings

“…We followed a similar exfoliation procedure for bulk 2H-WS 2 as in our previous reports, 23,24 using lowboiling-point solvents such as ethanol and isopropanol. The best results for exfoliation were achieved when the bulk 2H-WS 2 was ultrasonicated at 80 kHz and 100% power in a bath sonicator using ethanol.…”

“…This interference effect leads to the enhancement of the Raman signal of the sample. [21][22][23] Additionally, the oxide layer can also act as a spacer layer, which separates the sample from the metal substrate and reduces the quenching effect that can occur when the sample encounters the metal. This further enhances the Raman signal of the sample.…”

“…The incident light that is reflected from the top surface of the oxide layer interferes with the light that is reflected from the metal substrate. This interference effect leads to the enhancement of the Raman signal of the sample 21–23 . Additionally, the oxide layer can also act as a spacer layer, which separates the sample from the metal substrate and reduces the quenching effect that can occur when the sample encounters the metal.…”