Raman Scattering of 4-Aminobenzenethiol Sandwiched between Au Nanoparticles and a Macroscopically Smooth Au Substrate: Effect of Size of Au Nanoparticles

In order to obtain information on the nature of the hot sites in surface-enhanced Raman scattering, we have fabricated a model nanostructure composed of 4-aminobenzenethiol (4-ABT) that has further been sandwiched between An nanoparticles and a macroscopically smooth Au substrate. In the absence of An nanoparticles, that is, when 4-ABT is adsorbed solely onto a planar Au substrate, no Raman peak is identified at all using 632.8 nm radiation as the excitation source. When Au nanoparticles are attached onto the amine groups of 4-ABT on a planar Au substrate, Raman peaks are readily observed, however. What is intriguing is that a more intense Raman signal is obtained with increasing size of the Au nanoparticles at least with dimensions of under similar to 100 nm. Much the same trend is confirmed from the finite-difference time-domain calculation. This is thought to suggest that the electromagnetic coupling of the localized surface plasmon of Au nanoparticles with the surface plasmon polariton of the underneath smooth Au Substrate occurs more favorably with larger-sized nanoparticles leading to the molecules trapped in the gap exhibiting strong Raman peaks.