Silver island films for surface-enhanced infrared absorption spectroscopy: effect of island morphology on the absorption enhancement

Surface-enhanced infrared absorption (SEIRA) spectra of p-nitrobenzoic acid adsorbed on vacuum evaporated silver island films were investigated and the experimental conditions under which the maximum absorption enhancement is observed were explored. The absorption intensity depends on the substrate on which the metal film is evaporated. Low-refractive substrates are optically favorable as the substrate for the SEIRA experiments. The absorption intensity depends also greatly on metal deposition conditions, thickness and evaporation rate of the metal, and temperature and surface properties of the substrate. This is explained by the difference in morphology of the metal islands. By closely observing the metal films by using a scanning electron microscope, it was found that a large enhancement could be obtained when the metal islands are aggregated very densely but are not connected to each other. The largest enhancement factor (peak intensity ratio with and without a metal island film) of about 1000 was observed with a 20 nm thick silver film evaporated on BaF2 at room temperature and at an evaporation rate of 0.01 nm s−1.