Surface-enhanced Raman scattering from mildly roughened surfaces: influence of metal grain size on signal

The intensity of surface enhanced Raman scattering (SERS) from benzoic acid and benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a strong dependence on the rate at which the Ag film was deposited. Slowly deposited Ag films give a superior SERS response, by up to a factor of 10, to quickly deposited films, with films deposited at an intermediate rate yielding intermediate results. Careful examination using STM indicates that little distinction can be made between the differently prepared films in terms of surface roughness. By contrast TEM measurements reveal that the average metal grain dimension in slowly deposited Ag films is about 3-4 times greater than that in their quickly deposited counterparts. On the premise that the fundamental excitation of importance to the enhancement mechanism is the surface plasmon polariton (SPP) it is argued that the contrast in Raman scattering efficiency is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities. Corollary data on elastic SPP-photon scattering obtained in a related experiment are also presented.

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