Plasmonic surface-enhanced Raman scattering of multilayer HfS2 by Ag nanoprisms, Au nanorods, and nanospheres

Layered HfS2 is a promising wide-bandgap semiconductor for next-generation optoelectronic and nanophotonic light-sensitive devices. It exhibits a rich Raman scattering spectrum featuring a dominant A1g mode and several low-intensity modes. At the same time, the Raman intensity of HfS2 is almost indistinguishable when the layer number is low. In this report, surface-enhanced Raman scattering (SERS) of multilayer HfS2 flakes is approached by their decoration with plasmonic silver and gold nanoparticles (NPs) of spherical and nonspherical shapes, which have absorption bands overlapping that of HfS2. Ag nanotriangles (NTs or nanoprisms) support three localized surface plasmon resonance (SPR) modes located at 566, 408 and 333 nm, and Au nanorods (NRs) have two modes located at 920 and 517 nm. Conversely, Ag and Au nanospheres (NSs) own a single SPR mode at 395 and 513 nm. The decoration of HfS2 flakes with Ag NTs, Ag NSs, Au NRs, and Au NSs enables SERS with enhancement factors of 4.9, 1.4, 3.3, and 2.5, respectively. The highest intensity is obtained using nonspherical Ag NTs, whose strong in-plane dipole SPR plasmon mode well fits the wavelength of Raman excitation of 532 nm. Even though the SPR of Au NPs coincides with the excitation wavelength, Au generates somewhat less intense plasmon field compared to that of Ag NPs. Naturally, a stronger light-matter interaction with nonspherical NPs empowers their stronger SPR and SERS due to the generation of plasmonic hot spots at their tips and sharp edges. These results provide fundamental insights into the light-matter coupling in semiconductor/metal-NP plasmonic hybrid systems as well as their SERS application.