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.

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.

Palladium nanoparticles have been electrodeposited on the surfaces of conductive indium tin oxide (ITO) modified silicon internal reflection elements. The resulting films are shown to be excellent platforms for attenuated...

No abstract

No abstract