Self-affine silver films and surface-enhanced Raman scattering: Linking spectroscopy to morphology

The relationship between the self-affine structure of cold-deposited films and the surface-enhanced Raman (SERS) intensity of benzene adsorbed on the films is examined. Based on variable temperature STM studies the structure of cold-deposited silver films is shown to be self-affine with a fractal dimension ∼2.6, more or less independent of temperature for T less than ∼270 K. The fractal structure is shown to collapse to a more or less compact structure when the films are annealed to ∼280 K. SERS activity ceases at a somewhat lower temperature (∼250 K) for all the films examined. SERS enhancements rise by factors as great as 8 as a function of annealing temperature over and above their values at the lowest deposition temperatures used (24 K). The rise reaches a maximum at a temperature that depends both on the deposition temperature of the films and the excitation wavelength. (Such an annealing effect on the SERS intensity has been known for many years.) We suggest that the observations are consistent with...