Surface-enhanced infrared spectroscopy: A comparison of metal island filmswith discrete and nondiscrete surface plasmons

A study of the surface-enhanced infrared absorption (SEIRA) spectroscopy of para-nitrobenzoic acid (PNBA) adsorbed on thermally evaporated silver film s has been conducted to determine the effect of film architecture on the ma gnitude of the SEIRA enhancement, Ordered arrays of uniformly sized silver nanoparticles, termed periodic particle arrays (PPAs), were prepared on sev eral different infrared transparent substrates (germanium, silicon, and mic a) by nanosphere lithography (NSL), It was found that the ordered arrays de posited by NSL produced sell-defined and intense surface plasmon resonance (SPR) bands in the infrared at frequencies between 1500 and 4000 cm(-1), Th e peak frequency of these infrared SPR bands depended on the array architec ture and the substrate material. By appropriate design of the nanoparticle array, the infrared SPR band can be made to be coincident with the SEIRA se nsitive infrared bands of the PNBA. The trends in the infrared SPR peak fre quencies and band shapes were consistent with predictions from electrodynam ic theory. The SEIRA responses per unit area of deposited metal obtained wi th the PPA-type films were at best comparable to results obtained with diso rdered silver and gold films deposited on the same substrate materials by t hermal evaporation (i.e., in the absence of any NSL masking spheres). The r esults of this study are most consistent with theories and models that attr ibute SEIRA to the dielectric constant and optical extinction spectrum of t he metal film.