Excitation Wavelength Dependence of Combined Surface- and Graphene-Enhanced Raman Scattering Experienced by Free-Base Phthalocyanine Localized on Single-Layer Graphene-Covered Ag Nanoparticle Arrays

Hybrid systems constituted by plasmonic nanostructures and single-layer graphene (SLG) as well as their employment as platforms for surface-enhanced Raman scattering (SERS) of the molecular species have recently become a subject of interest. By contrast, only a few studies were targeted specifically on the combination of SERS with graphene-enhanced Raman scattering (GERS) of aromatic molecules. In this paper, we have investigated the mechanisms of combined SERS + GERS by micro-Raman spectral mapping of the hybrid system constituted by annealed Ag nanoparticles (NPs) on the glass substrate overdeposited first by SLG and, subsequently, by a monolayer (ML) of free-base phthalocyanine (H2Pc) molecules, as well as of glass/SLG/H2Pc(ML) and of graphite/H2Pc(ML) reference systems. Raman mapping was performed at multiple excitation wavelengths spanning the 532-830 nm range and was complemented by surface plasmon extinction and transmission electron microscopy images of the Ag NP platform. Observation of SERS + GERS in the aforementioned hybrid system was established by the determination of GERS, SERS, and SERS + GERS enhancement factors. By construction and the mutual comparison of GERS + SERS and GERS excitation profiles of H2Pc vibrational modes, operation of two mechanisms of GERS additively with the electromagnetic SERS enhancement in SERS + GERS of H2Pc in the glass/Ag NPs/SLG/H2Pc(ML) hybrid system has been ascertained. Finally, achievement of the same level of the weak negative doping of SLG by Ag NPs in the probed hybrid system and by glass in the reference system has been established as a necessary condition for the proper evaluation of mechanisms of combined SERS and GERS, and evidence for the fulfillment of this condition in the hybrid systems reported here was provided.