Two-dimensional combinatorial investigation of raman and fluorescence enhancement in silver and gold sandwich substrates

We present a two-dimensional combinatorial investigation of resonant Raman and fluorescence enhancement in silver and gold sandwich structures. Gold and silver, separated by a thin alumina spacer layer, were deposited in two orthogonal gradients, with a thickness from a few hundred to a few nanometers, covering the range from island films through percolation to continuous films. Resonant Raman spectra of Rhodamine 6G adsorbed on the substrate surface were recorded in a 13 × 13 matrix using an automated scanning stage. The most Raman-active substrates were composed of silver (8 nm)-on-alumina (7 nm)-on-gold (5 nm). They consist of both gold and silver discontinuous nanoparticle films separated by alumina, forming a labyrinthine network structure. Gold-on-alumina-on-silver substrates also displayed increased activity compared with that of goldon-alumina substrates. Maximum fluorescence intensity was observed on silver films nominally 35 nm thick covered by 8 nm of alumina. The efficacy of the combinatorial method to correlate multiple aspects of the measurements and reduce uncertainties is demonstrated.