Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol in Nanogaps between a Planar Ag Substrate and Pt Nanoparticles

Abstract

To determine how effectively transition metal nanoparticles can couple with a noble metal substrate to induce higher electromagnetic fields, we examined the surface-enhanced Raman scattering (SERS) characteristics of 4-aminobenzenethiol (4-ABT) positioned in the gaps formed by a flat Ag substrate and 20–150 nm Pt nanoparticles. Initially, no Raman peaks of 4-ABT could be identified when 4-ABT was self-assembled on a polished flat Ag substrate; however, Raman peaks could be seen by attaching Pt nanoparticles onto the pendent amine groups. Conversely, a higher Raman signal was observed when larger Pt nanoparticles were attached onto 4-ABT, regardless of the excitation wavelength. Moreover, a higher Raman signal was measured in response to excitation at 488 nm, followed by excitation at 514.5, 568, and 632.8 nm. Very similar size and excitation wavelength dependences were found from the 3-dimensional finite-difference time-domain simulation. Accordingly, the highest enhancement was achieved using ∼150 nm Pt particles at 488 nm excitation with an enhancement factor (EF) of 3.5 × 104 per Pt particle. However, the electromagnetic field enhancement observed was not in conformity with the UV/vis absorbance of ∼150 nm Pt nanoparticles, which suggested that higher EFs would be measured in the order of excitations at 632.8 > 568 > 514.5 > 488 nm. This discrepancy was attributed to the structure of Pt nanoparticles in that the 150 nm Pt particles were actually composed of 7 nm seed particles; thus, the electromagnetic interactions of individual Pt seed particles with the underlying Ag substrate occurred collectively, being more effective at shorter excitation wavelengths.