An Effective Medium Theory Description of Surface-Enhanced Infrared Absorption from Metal Island Layers Grown on Conductive Metal Oxide Films

The influence of a thin film of a supporting layer of conductive metal oxide (CMO) on the surface-enhanced infrared spectra generated from metal island layers is studied using an effective medium (EM) approximation and compared to experimental results. Gold island films electrodeposited on indium tin oxide (ITO) coated on an internal reflection element (IRE) give rise to asymmetric (bimodal or derivative-looking) line shapes and have strong surface-enhanced infrared absorption spectroscopy (SEIRAS) intensities for adsorbed cyanate using both p- (transverse magnetic) and s- (transverse electric) plane-polarized light. The dependence of the SEIRAS intensity on the angle of incidence is very different compared to metal films directly deposited on the surface of the IRE, as larger magnitude SEIRAS intensities are observed at angles close to the critical angle. The role of additional enhancement effects from possible plasmonic phenomena arising in the ITO layer is shown not to contribute to the ATR-SEIRAS mechanism. The observed spectra are modeled using an EM treatment of the gold island film, and a good qualitative and semiquantitative agreement is found between the calculated and experimental results. Using the Fresnel equations and EM-determined optical constants, the reflectivity of the interface is shown to be highly dependent on the volume fraction of gold in the enhancing layer, and asymmetric and even inverted bands arise near the metal percolation threshold. Electric field analysis shows that the low free charge carrier density (relative to metals) of ITO gives rise to field distributions similar to those of an absorbing dielectric layer on the surface of an IRE.