Increasing Charge Transfer between Graphene and R6G for Surface-Enhanced Raman Spectroscopy Application by Tuning Fermi Energy of Graphene Using Platinum Nanoparticles and Ultraviolet Irradiation.*

ABSTRACT: Surface-enhanced Raman spectroscopy (SERS), which relies on chemical (CM) and electromagnetic (EM) mechanisms, is a crucial analytical instrument with extremely high sensitivity. Because the CM depends on an effective charge transfer between the probe molecules and SERS substrates, optimizing the CM enhancement requires engineering the molecule attachment and the energy level alignment at the molecule/substrate interface.  Here we report an increased CM of Rhodamine 6G (R6G) on graphene SERS substrates by using a combination of atomic layer deposition (ALD) decorating of Pt nanoparticles (Pt-NPs) and C-band ultraviolet (UVC) irradiation.  The former showed a 270% improvement, which is explained by the activation of the graphene surface, p-doping for better attachment of R6G molecules and charge transfer, and the surface's transition from hydrophobic to hydrophilic and the down-shift of the Fermi energy (p-doping) following UVC exposure. By further p-doping graphene, which modifies the graphene's Fermi energy to encourage charge (hole) transfer at the R6G/graphene interface, the Pt-NPs decoration adds an extra enhancement of 250%. R6G SERS sensitivity has increased to 5x10^-9 M as a result of the UVC irradiation and Pt-NPs decorating.

Figure 1. (a) Schematic representation of a graphene SERS substrate with UVC irradiation and Pt-NP (grey spheres) decoration for SERS CM enhancement. (b) Raman spectra of R6G molecules at different concentrations in the range  of 5×10^-6 M to 5×10^-9on  Pt-NPs/graphene substrate with UVC irradiation for 8 minutes. The marks of # and * donate Si and graphene, respectively