Ag Nanoparticle-Mediated Raman Scattering of 4-Aminobenzenethiol on a Pt Substrate

Abstract

Raman scattering measurements were conducted for 4-aminobenzenethiol (4-ABT) monolayers assembled on a macroscopically smooth Pt substrate. At the beginning, no Raman peak was detected for 4-ABT on Pt, but upon attaching Ag nanoparticles to the amine groups of 4-ABT on Pt (Ag@4-ABT/Pt), distinct Raman spectra were observed. Considering the fact that almost no Raman peaks are observed when Ag nanoparticles are attached to 4-aminophenylsilane monolayers assembled on a silicon wafer, the Raman spectra observed for Ag@4-ABT/Pt must be surface-enhanced Raman scattering (SERS) spectra, occurring through an electromagnetic (EM) coupling of the localized surface plasmon of Ag nanoparticles with the surface plasmon polariton of the Pt substrate. From the excitation wavelength dependence, we also confirmed the contribution of the charge-transfer enhancement in the SERS spectra of Ag@4-ABT/Pt: it became more important at short-wavelength excitation. Overall, the SERS intensity of Ag@4-ABT/Pt gradually decreased as the excitation wavelength was increased from 488 to 514.5, 568, and 632.8 nm. A similar trend was observed with a finite-difference time-domain calculation, suggesting that the EM coupling should also be strong at short-wavelength excitation. Accordingly, the experimental enhancement factor per Ag nanoparticle was estimated to be as large as 7.9 × 102 under the illumination of 514.5 nm radiation. The present observation clearly demonstrates that the inherent obstacles to the more widespread use of SERS can be overcome by the judicious use of SERS-active nanoparticles directly or indirectly.