Mapping the SERS Efficiency and Hot-Spots Localization on Gold Film over Nanospheres Substrates

Noble metal films deposited over two-dimensional arrays of polystyrene nanospheres constitute a confirmed class of efficient and cost-effective substrates for surface enhanced Raman scattering (SERS). In this paper, we perform scanning confocal SERS microscopy to investigate the spatial (lateral) variations of the SERS enhancements on gold films over nanospheres (AuFoN) substrates. By constructing SERS imaging maps with a resolution down to the diffraction limit, the local SERS efficiency is found to vary on two different scales. First, the local SERS efficiency is periodically modulated (intensity ratios of 2-3) by the periodic AuFoN surface topography (as demonstrated by correlation with atomic force microscopy imaging of the same sample area); second, randomly distributed SERS hot-spots are observed, at which the SERS intensity is 1 to 2 orders of magnitude larger than at adjacent regions. Furthermore, these hot-spots exhibit fluctuating behavior, characteristic of single-molecule SERS sensitivity. These results are particularly useful for furthering current understanding of SERS on AuFoN substrates. More generally, the SERS maps provide a direct visual demonstration that in SERS only a fraction of the metallic surface yields the major part of the SERS scattering. The evidence of clear correlations between SERS enhancement and topography can be relevant for the characterization of ordered noble-metal plasmonic structures.