Constructing sensitive SERS substrate with a sandwich structure separated by single layer graphene

We report a sandwich-structured surface-enhanced Raman scattering substrate by integrating gold nanoparticles and electron beam lithography-fabricated hexagon-shaped silver nanohole arrays separated by single layer graphene. In the sandwich configuration, the graphene interlayer plays multiple roles for promoting the surface-enhanced Raman scattering performances: (i) acting as a supporting layer to avoid the dropping of the top layer of gold nanoparticles; (ii) an intrinsic spacer to create nanometer-scale gaps between gold nanoparticles and hexagon-shaped silver nanohole arrays; (iii) serving as a protective layer to suppress the oxidation of the bottom metal silver; (iv) functioning as a molecule harvester for adsorbing the target analyte. By decreasing inter-hole distances of hexagon-shaped silver nanohole arrays, we have obtained sub-10 nm-wide gaps neighboring the adjacent silver holes. Finite element numerical simulations demonstrated that the gold nanoparticle-graphene-hexagon-shaped silver nanohole array hybrid system generates a substantial amount of hot spots with strong electric field enhancement. The well-designed and fabricated gold nanoparticle-monolayer graphene-hexagon-shaped silver nanohole array sandwich structure exhibits high sensitivity with the limit of detection at sub-picomolar level and good reproducibility with a standard deviation of 6.2% for crystal violet. The work endows great potentials to the rational design and fabrication of two-dimensional material-plasmonic hybrids for surface-enhanced Raman scattering applications.