Nanospectroscopy of thiacyanine dye molecules adsorbed on silver nanoparticle clusters

•90% of the SERS total enhancement originates from gaps between nanoparticles.•The E field component perpendicular to the cluster surface is the main source of the SERS signal.•The highest enhancement factors are reached at collective NP plasmon resonances.•In the non-resonant case enhancement factors in the range of 102–103 are to be expected.•Citrate anions impede the efficient thiacyanine dye adsorption.