Infrared absorption enhancement of p-cyanobenzoic acid on silver island films deposited on oxidized and hydrogen-terminated Si(100) surfaces

Infrared absorption measurements of thin p-cyanobenzoic acid (PCBA) films have been performed in the Kretschmann ATR geometry in which an island film of silver was deposited in vacuum onto an oxidized or hydrogen-terminated silicon(100) prism surface. It is found that in comparison to the normal ATR measurement (without silver) of PCBA on the oxidized Si(100) surface the absorption intensity of the v(C ≡ N) mode is approximately an order of magnitude enhanced by the presence of a silver island film of 5 nm in mass thickness on that surface but a more enhancement by a factor of 1.7 is obtained with the same thickness of silver on the H-terminated Si(100) surface. The different magnitudes of the absorption enhancement are explained on the basis of the FE-SEM micrographs of silver islands prepared on oxidized and H-terminated Si(100) wafers. A more detailed analysis shows that the enhancement occurs not only for ionized PCBA adsorbed via the carboxylate group on surface silver oxide, but for hydrogen-bonded PCBA presumably condensed on the ionized species. This observation is explained in terms of an electromagnetic field enhancement at the metal island surfaces.