Temperature Dependence and Annealing Effects in Surface-Enhanced Raman Scattering on Chemically Prepared Silver Island Films

The surface-enhanced Raman scattering (SERS) intensities for ethanethiol adsorbed on a silver island film have been measured for a variety of annealing conditions over the 15-300 K temperature range. Reversible temperature dependence of the C-S stretching intensities for an unannealed film similar to the case of 1-propanethiol reported previously was found, with the intensity at 15 K being greater than that at 300 K by a factor of ∼2.5. The preferential SERS enhancement at lower surface temperature, normalized by the value at 300 K, decreased substantially as the silver films were annealed. For example, the relative enhancement factor at 15 K over 300 K after 50 min of annealing at 150 °C was ∼1.2. Such annealing effects on the SERS enhancement were more pronounced at longer wavelengths and during the initial period of annealing. The combined field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) studies revealed that the annealing of the films induced changes in the morphologies of the islands with increases in the radii and decreases in the heights. From the FE-SEM and AFM images the distributions of the aspect ratios of the island hemispheroids for unannealed and annealed films were determined. The average electromagnetic (EM) enhancement factors for such distributions were calculated using the low-temperaturemodified equation for EM enhancement [J. Phys. Chem. B 1998, 102, 7203]. The excellent agreement between the EM theoretical predictions and the experimental findings strongly suggested that the observed reversible temperature dependence of the SERS enhancement and the annealing effect originated from the inherent temperature dependence and morphology dependence of the EM mechanism in SERS.