Intermixed WS₂+MoS₂ Nanodisks/Graphene van der Waals Heterostructures for Surface-Enhanced Raman Spectroscopy Sensing

This work explores a rationale design of intermixed WS2 nanodiscs (WS2NDs) and MoS2 nanodiscs (MoS2NDs) on graphene (WS2NDs+MoS2NDs/graphene) for ultrasensitive surface-enhanced Raman spectroscopy (SERS) beyond the sensitivity limit of the SERS substrates based on metallic plasmonic nanostructures. On the individual WS2NDs and MoS2NDs, localized surface plasmonic resonance (LSPR) is enabled by the dipole–dipole interaction at the WS2NDs/graphene and MoS2NDs/graphene van der Waals (vdW) interfaces. The intermixed WS2NDs+MoS2NDs/graphene allows superposition of the LSPR effects from the two types of plasmonic NDs. The enhanced SERS sensitivity is illustrated in the boosted graphene Raman peaks by ∼14-fold on the WS2NDs+MoS2NDs/graphene, in contrast to ∼7.6-fold on the counterparts with single types of the NDs. Furthermore, the SERS enhancement factors of the test molecules (fluorescent Rhodamine 6G or R6G) of 5 × 10–5 M concentration Raman spectra (normalized to that on graphene) are ∼16.4 and 8.1 considering the R6G 613 cm–1 peak intensities were sensed on the WS2NDs+MoS2NDs/graphene and MoS2–NDs/graphene (or WS2NDs/graphene), respectively. In addition, the WS2NDs+MoS2NDs/graphene SERS substrate exhibits remarkably high SERS sensitivity as high as (5–7) × 10–13 M, which is about 2 or more orders of magnitude greater than that reported on the AuNPs/graphene and other plasmonic metal nanostructures/2D materials SERS substrates. This result demonstrates a promising bottom-up approach in designing high-sensitivity SERS substrates based entirely on 2D atomic materials.