Provided herein are dual- and multi-layer metal film over nanostructure substrates. These Dual-FON and Multi-FON SERS substrates comprise a rough nanostructured layer and two or more SERS-active metal film layers deposited thereon with a layer of dielectric between the metal film layers. Also provided is a method of increasing the intensity of a Raman signal during surface enhanced Raman spectroscopy using the SERS substrates and a method of fabricating these Dual- or Multi-FON SERS substrates.