Signal Enhancement from Tunable SERS Substrates: Design and Demonstration of Multiple Regimes of Enhancement

The interaction between plasmonic nanoparticle and substrate is of utmost importance for optimal design of surface-enhanced Raman spectroscopy (SERS) substrates. Substrates can either quench or enhance the strength of the localized plasmon resonance of nanoparticles depending on the optical properties. The substrate optical properties were modified by introducing different dielectric films between silver nanoparticle and silicon base substrates. The thickness of the films was varied over a large range (5–200 nm) to observe SERS enhancement because of nonradiative and radiative interactions of the plasmons and the substrate. Energy transfer between the plasmons and the polarization charges in Si was observed for a film thickness of less than 10 nm where SERS intensity followed the permittivity trend of the spacer film. As the spacer thickness increases beyond 40 nm, the effect of Si base substrate subsides and the enhancement/quenching of the SERS signals exhibit an oscillatory behavior with the thickness of the film. The extent of enhancement and quenching can be tuned by optical property of substrate and the excitation wavelength of choice.