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 ad sorbed on a silver island film have been measured for a variety of annealin g conditions over the 15-300 K temperature range. Reversible temperature de pendence of the C-S stretching intensities for an unannealed film similar t o the case of 1-propanethiol reported previously was found, with the intens ity at 15 K being greater than that at 300 K by a factor of similar to 2.5. The preferential SERS enhancement at lower surface temperature, normalized by the value at 300 K, decreased substantially as the silver films were an nealed, For example, the relative enhancement factor at 15 K over 300 K aft er 50 min of annealing at 150 degrees C was similar to 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 sca nning electron microscopy (FE-SEM) and atomic force microscopy (AFM) studie s revealed that the annealing of the films induced changes in the morpholog ies 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 distribution s were calculated using the low-temperature-modified equation for EM enhanc ement [J. Phys. Chem. B 1998, 102, 7203]. The excellent agreement between t he EM theoretical predictions and the experimental findings strongly sugges ted that the observed reversible temperature dependence of the SERS enhance ment and the annealing effect originated from the inherent temperature depe ndence and morphology dependence of the EM mechanism in SERS.