Effect of dielectric spacer layers on SERS with Au nanoparticle arrays on silicon substrates

The optical response of a plasmonic nanostructure is often highly dependent on the nature of the substrate supporting it. To study the effect of the substrate on surface enhanced Raman scattering (SERS), we have fabricated a series of SERS substrates consisting of a hexagonal array of Au nanoparticles self assembled on block copolymer films, a silicon oxide (dielectric) layer and a silicon substrate. The inter-particle distance and the dielectric layer thickness were controlled. The SERS Enhancement Factors (EF) were calculated by comparing the Raman spectra of 4-aminothiophenol adsorbed on the surface of the Au nanoparticles and in a standard solution. The SERS EF were found to be strongly affected by the inter-particle distance and silicon oxide thickness. Changing the inter-particle spacing induced a 10$^{\mathrm{2}}$ variation in the EF while changing the oxide thickness increased the range of EF values by an additional factor of 10. Maximal enhancement factors were found with oxide layer thicknesses between 50 nm and 100 nm beneath the 30 nm polymer film. This geometry both improved the resonance condition with the probe laser and reduced the absorption by the substrate. This work illustrates that optimization of plasmonic-based sensors should consider both the metallic and the surrounding structures.