For efficient utilization of surface plasmons in surface enhanced Raman scattering (SERS), we investigated the gap modes in flocculates of metal nanoparticles (MNPs), and between MNPs and metal substrates under an external and an attenuated total reflection (ATR) geometry. First, the adsorbed state of thiol molecules and counter ions trapped in solutions were elucidated using flocculation-SERS, in which closely adjacent nanoparticles are formed by using interactions between MNPs and target species. Second, we obtained a pronounced enhancement of 10(5)-10(8) at a nanogap between gold nanoparticles (AuNPs) and various metal substrates even with a large damping under an external geometry. Markedly larger enhancement was obtained for larger AuNPs, by a factor of 10(3) for particles with a radius (r) of 50 nm compared with those of r=15 nm in this geometry. Finally, we attained an additional enhancement factor under an ATR geometry by a coupling of propagating surface plasmons with gap modes.
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 10 5 -10 8 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, fopr example by a factor of 10 3 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.
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