Constructing sub-10-nm gaps in graphene-metal hybrid system for advanced surface-enhanced Raman scattering detection

We prepared a three-dimensional (3D) hybrid structure by assembling dense gold nanoparticles (Au NPs) on graphene supported on electron beam lithography-fabricated silver nanodisk arrays (Ag NDAs). Using the strategy of modulating the structure parameters via EBL system to sufficiently narrow the distances between adjacent disks, we successfully constructed sub-10-nm gaps between the horizontally patterned Ag NDs. Moreover, uniform nanometer-scale graphene gaps were obtained between Au NPs and Ag NDAs. Finite element numerical simulations revealed that the multi-dimensional plasmonic couplings in the 3D Au NP-graphene-Ag NDA system led to an electric field enhancement up to 112 times in graphene defined gaps. As demonstrated by our SERS measurements, the well-designed and fabricated 3D Au NP-graphene-Ag NDA hybrid structure exhibits 3200-fold enhancement of the Raman response of graphene, and sensitive SERS detection with a limit of 0.1 pM for crystal violet molecules, which can be attributed to the extremely strong electric field enhancement and chemical enhancement of graphene. This work represents a step towards high-sensitivity and strong-reproducibility SERS substrate fabrication, and opens a new avenue for rationally designing graphene-plasmonic hybrid structures for SERS sensing.