Ag-modified Au nanocavity SERS substrates

Abstract

The engineering of cavity void metallic arrays allows to vary the plasmon-polariton mode energies from the near infrared to the ultraviolet through the tuning of the void height and diameter, and the selection of the appropriate material. Typically Au nanocavity substrates can be grown with better reproducibility, homogeneity, and stability, while Ag structures display significantly larger SERS enhancements. To exploit these two apparently excluding aspects, quality and enhancement, we report a detailed study of 500 nm Au-nanocavity templates modified by the controlled electrochemical deposition of 100 Ag layers, a thickness similar to the visible light skin-depth of bulk Ag. The SERS amplification of the ordered cavity-arrays is determined using 4-mercaptopyridine as a non-electronic resonant molecular probe. The ultrathin Ag layer modification of the Au substrates results in a strong amplification of the SERS signal both in the red and the green part of the spectrum, and in a spectral shift of the Raman resonance scans. These observations are assigned to Ag-induced changes in the plasmon-polariton response of the nanostructure. The reported results provide a general platform for the preparation of renewable SERS-active substrates that combine the durability and higher quality of Au nanotemplates, with the enhanced SERS amplification factors of Ag.