C. Douketis et al., Self-affine silver films and surface-enhanced Raman scattering: Linking spectroscopy to morphology, J CHEM PHYS, 113(24), 2000, pp. 11315-11323
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 di
mension similar to2.6, more or less independent of temperature for T less t
han similar to 270 K. The fractal structure is shown to collapse to a more
or less compact structure when the films are annealed to similar to 280 K.
SERS activity ceases at a somewhat lower temperature (similar to 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 low
est deposition temperatures used (24 K). The rise reaches a maximum at a te
mperature 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 consis
tent with recently developed theories of the optical properties of fractal
clusters so long as one includes arguments regarding the optimal dimensions
of the monomers comprising those clusters whose mean value change during a
nnealing. (Fractality, however, is not an essential condition for observing
intense SERS.) The disappearance of the SERS signal before the self-affine
structure of the film collapses is, therefore, attributed to the monomer s
ize having significantly passed its optimal size before the temperature at
which the collapse of the fractal nature of the film is reached. We argue t
hat this collapse is mediated by surface diffusion. The narrow temperature
range over which this occurs is intriguing and worthy of a proper theoretic
al investigation. (C) 2000 American Institute of Physics. [S0021-9606(00)70
539-3].