Surface-enhanced Raman and optical scattering in coupled plasmonic nanoclusters

Optical excitation of small Au nanoparticle (NP) clusters of appropriate wavelength is known to generate intense electromagnetic fields localized uniquely at NP junction sites within the nanoclusters. These intense and localized field hot-sites can induce intense surface-enhanced Raman scattering (SERS) of molecules residing at the junction hot-sites. In this paper, we present a series of electromagnetic simulations, experimental SERS and extinction data obtained from small self-assembled Au NP clusters coated to saturation with a Raman reporter molecule. Our experimental data show that the SERS enhancement factor remains relatively constant despite the heterogeneity of the nanocluster and this is supported by the simulation results. Furthermore, our simulation results show significant variations in the localized electric field intensities of the junction hot-sites in different nanocluster geometries. This explains the observation that increasing the number of hot-sites does not necessarily result in a higher SERS enhancement factor.