Hybrid Gold–Conductive Metal Oxide Films for Attenuated Total Reflectance Surface Enhanced Infrared Absorption Spectroscopy

Abstract

The use of conductive metal oxide (CMO) films as supporting layers for attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is treated theoretically and experimentally. The greater mid-infrared transparency of thin layers of indium zinc oxide (IZO), as compared to metals, is verified through IR reflectivity measurements and the Drude model. IZO layers sputtered on silicon micromachined internal reflection elements (Si μIREs) are found to have a thin surface layer with slightly different plasma frequency and electronic scattering time compared to the bulk material. The complex permittivity and refractive index of the IZO can be extracted using the Drude model. This allows application of the Bruggeman effective medium theory to calculate the ATR-SEIRAS response of a layer of gold prolate spheroids supported on an IZO film. Calculated ATR-SEIRAS spectra for a 1 nm thick organic film, modeled as a Lorentz oscillator, predict an order of magnitude improvement in absorbance strength using the IZO film as a base layer compared to a conventional, gold covered internal reflection element. These predictions are qualitatively verified by the electrodeposition of gold nanoparticles on an IZO modified Si internal reflection element and the study of the potential controlled adsorption of a pyridine derivative. The IZO/Au layers are found to be very mechanically stable and can withstand large potential perturbations. This is demonstrated through the in situ study of the repeated reductive desorption of a self-assembled monolayer of 4-mercaptobenzoic acid.