Ag thin film is an efficient surface enhanced Raman spectroscopy (SERS) substrate because its quality factor of localized surface plasmon resonance (LSPR) is much larger than other materials, e.g., 10 times larger than that of Au at the wavelength of 500 nm. However, the chemical instability of Ag in ambient environment significantly degrades the SERS properties, and hinders the practical applications. Although conventional protective structures (e.g., silica and alumina barrier) can inhibit the Ag corrosion in ambient, they usually have large light absorption and reduce the plasmonic enhancement.
In this work, we transfer monolayer graphene onto Ag SERS substrate, on one hand, to protect Ag from corrosion, and on the other hand, to prevent the photo-induced damages (photocarbonization, photobleaching and metal-catalyzed reaction) on the probed molecules. Firstly, we comparatively study morphological characteristic of the Ag SERS substrates with and without graphene protective barrier, revealing high corrosion-resistance of monolayer graphene to the oxidizing gas and liquid.
We further demonstrate the graphene coated Ag thin films as stable SERS substrate. After 35-day exposure in air, the graphene coated Ag thin film maintains high SERS sensitivity. Secondly, we systematically study the resistance to photo-induced damages with graphene barrier. Our results show that Ag SERS substrate with graphene coating significantly improves the SERS reproducibility, and simultaneously inhibits metal-catalyzed reaction. In addition, the graphene layer can form strong interaction with the probed R6G molecules through π-π bonding stack, reducing the possibility of photo-induced desorption. In summary, our results show that graphene coating on Ag substrate effectively enhances the resistance to the corrosion and photo-induced damages.