Band shapes in the infrared spectra of thin organic films on metal nanoparticles

The shape of bands in surface-enhanced infrared spectra of molecules near the surface of metal nanoparticles is shown to depend on whether or not the nanoparticles have exceeded the percolation threshold. If the nanoparticles are in the form of independent metal islands, the absorption bands of molecules within 5 nm of the surface are symmetrical. Immediately after the percolation threshold has been exceeded, the bands start to assume a dispersive shape. The greatest enhancement is found immediately after the percolation threshold has been exceeded. This effect was shown both for the metals that are most commonly applied to surface-enhanced infrared absorption (SEIRA) studies, namely silver and gold, but also to zinc for which surface enhancement has not previously been reported and appears to be applicable to all metals.