Monolayers at solid–solid interfaces probed with infrared spectroscopy

The sensitivities of infrared spectra of thin adsorbate layers measured in either transmission, internal reflection or external reflection can be greatly increased if a light incidence medium with a high refractive index such as an IR-transparent solid material is used. This increase in sensitivity is due to the strong enhancement of the perpendicular electric field in a thin layer of low refractive index sandwiched between two high refractive index materials. Based on model calculations of a hypothetical sample layer, the influence and optimization of experimental parameters such as incidence angle, sample layer thickness and optical contact between layers are investigated. Under optimized conditions, this enhancement can exceed a factor of 100 when compared to conventional surface IR techniques. In addition, the spectra of sandwiched sample layers are governed by a uniform surface selection rule, such that only the perpendicular vibrational components are enhanced, and they permit a straightforward, substrate-independent analysis of surface orientations. Experimental examples of monolayer spectra of long-chain hydrocarbon compounds adsorbed onto gold and silicon substrates and contacted with a germanium crystal used as the incidence medium demonstrate the simple experimental realization and unprecedented sensitivity of this sandwich technique, and they offer novel insights into the chemistry and structure of monolayers confined and compressed between two solid surfaces. Figure IR reflection spectrum of a monolayer of a fatty acid methyl ester sandwiched between silicon and germanium.