Surface-Enhanced Raman Scattering on Silver Nanoparticles in Different Aggregation Stages

Abstract Enhancement factors of surface‐enhanced Raman scattering (SERS) are compared for crystal violet attached to 15–60‐nm‐sized colloidal silver particles in various aggregation stages using 830 nm and 407 nm laser excitation. Enhancement factors on the order of 106–107 have been measured at 407 nm excitation for molecules adsorbed on spatially‐isolated silver spheres exhibiting strong, sharp, single‐sphere plasmon resonance at 395 nm. Extremely large SERS enhancement factors on the order of 1014 are obtained at near infrared excitation for molecules on clusters built from colloidal particles. The SERS enhancement factor is demonstrated to be independent of the size of the clusters. The increase of SERS enhancement factors by 7–8 orders of magnitude by the formation of aggregates, as well as the extremely small density of “hot sites” exhibiting this high level of enhancement, provide further strong arguments for the electromagnetic origin of the hot spots and for strongly enhanced local optical fields as a key effect in SERS at an extremely high enhancement level.