Optimum massthickness of Ag-nanoparticle film for surface enhanced Raman scattering

Raman scattering from a few monolayers of benzoic acid (BA) enhanced by a thin layer of Ag-nanoparticles (AGNs) was studied, using a series of AGN films having different massthickness covering a range from 5 to 50 nm. Surface images taken using an atomic force microscope clearly reveal each individual AGN of average size ∼25 nm. Extinction spectra of the AGN films exhibit a surface-plasma resonance peak at ∼380 nm with a long tail extending to the visible region. When emphasizing the electromagnetic effect, the attenuated total reflection configuration was adopted to obtain an enhanced Raman spectrum. An optimum massthickness for enhancing Raman signal was found to be ∼15 nm. A semi-empirical formula was also developed to estimate the Raman signals from a few monolayers of BA, hence the enhancement factor. The estimated enhancement factor is in the order of 104 for the optimum case. The enhancement is believed to originate from the excitation of surface plasmons (SPs).