Relevance of gap mode Raman spectroscopy to various substrates

In order to clarify the relevance of gap mode Raman spectroscopy in metal nanoparticles/gap/metal substrates, we investigated the effect of dielectric constants of substrates, and the size of gold nanoparticles (AuNPs). First, we obtained pronounced enhancement of 10⁵-10⁸ in Raman scattering of thiol molecules on various metal substrates with large damping, and even on silicon substrates. Also, significantly larger enhancement was obtained for larger AuNPs, for example by a factor of 10³ for larger particles with a radius (r) of 50 nm compared with those with r = 15 nm. These experimental observations can be explained by a coupling of dipole in AuNPs formed at localized surface plasmon resonance, and image dipole induced in the substrates. The observed properties in gap mode Raman spectroscopy are consistent with those anticipated by theoretical calculations using finite difference time domain (FDTD) method. (author)