A novel Surface Enhanced Raman Scattering (SERS) substrate based on Ag nanoparticle (SNP) monolayer films covered with vacuum deposited Ag caps is described. By varying the deposition angle, caps of different sizes can be fabricated. This method results in reproducible SERS from different Raman-active molecules adsorbed on the substrate. The two prevailing models currently used to explain the SERS phenomenon are the local field enhancement mechanism (LFEM) and the chemical enhancement mechanism (CM). LFEM states that molecules within the local field produced by surface plasmons experience greater excitation by incident electromagnetic radiation leading to an increase in Raman scattering events. The CM depends upon specific interaction between the molecules and the metal surface. This interaction may lead to a resonant charge-transfer complex which can produce SERS-like, resonance Raman scattering.

This work provides experimental evidence to support an alternative model for the SERS mechanism based on plasmon-induced electronic coupling between the molecules and the metal surface. This coupling causes the conduction electrons from the metal to penetrate into adsorbed molecules. As a result, the plasmon resonance becomes modulated with the vibrational modes of the molecules and the entire system scatters Stokes-shifted light. This model is termed a plasmon-induced, electronic coupling SERS mechanism.