Investigation of the Importance of the Electronic Enhancement Mechanism for Surface-Enhanced Raman Scattering (SERS)

Different mechanisms contribute to the increase of the inelastically scattered signal observed in surface-enhanced Raman spectroscopy (SERS). The so-called electronic (chemical) enhancement mechanism has been studied using a Kretschmann configuration by switching the electronic contribution on and off. For this, the direct coupling of the analyte molecules to the metal surface was prevented with the help of different intermediate layers between molecules and metal consisting of self-assembled monolayers or a metallic oxide layer. It has been observed that for a separation from the metal, despite the only very small (a few nanometers) distance from the metal surface introduced by the interlayers, the ratio between the enhancements determined for in-resonance and preresonance conditions is drastically reduced compared to the situation where the molecules can chemisorb on the metal surface. It was also observed that while in the case of direct contact to the metal surface, the resonance/preresonance enhancement ratio was strongly mode-dependent, such dependence did not occur when the analyte molecule was kept away from direct access to the surface. This confirms the transition to a pure electromagnetic enhancement when the molecules are separated from the metal surface and allows for the characterization of the two mechanisms.