Effective Medium Approximation for Infrared Surface-Enhancing Substrates

Surface-enhanced infrared absorption spectroscopy (SEIRAS) is an increasingly popular analytical technique for studying molecular adsorption, finding applications in catalysis, biosensing and battery research. Crucial to its operation is a textured metal film, which can be produced by physical vapour deposition, galvanic electroless deposition and more recently, electrochemical reduction onto a conductive metal oxide layer. All methods suffer from poorly defined substrates which are difficult to reproduce with a consistent response. A model which predicts the optical response of a SEIRAS film could guide the development of better deposition procedures. This work aims to implement a model utilizing the Bruggeman effective medium approximation (EMA) and the Fresnel equations to calculate SEIRAS spectra for a range of morphologies and validate the calculations with experimental spectra. The developed model reproduces the asymmetric line shapes seen in experimental measurements, which arise from the Fresnel equations and a small change in the refractive index upon molecular adsorption onto a moderately reflective substrate. The Bruggeman model demonstrates that composite metal-dielectric films can acquire a permittivity function yielding moderate reflection under the conditions of a SEIRAS experiment. The angle-dependent behaviour is explained well by electric field strength calculations. The Bruggeman EMA also successfully explains the general trends observed in SEIRAS films and appears to be a useful tool to optimize the surface morphology for maximal enhancement without yielding bimodal spectra. Further work incorporating percolation theory as well as nucleation and growth models may be able to link the crucial fill-factor parameter to an experimentally accessible variable