Buried interfaces of bulk, solid materials play a key role in novel molecular electronic structures such as molecular junctions, molecular switches or monolayer transistors, where two bulk electronic materials (metal, semiconductor or insulator) are joined by a single layer of molecules as the active component. Little is known, however, about the composition and structure of these interfaces in relation to their electronic properties because of the lack of suitable characterization techniques. It is shown in this study, that infrared spectroscopy has a unique sensitivity for thin surface layers sandwiched between two bulk, solid phases of high refractive index and can therefore be used to probe the static composititon and structure of such interfaces as well as reactions triggered by joining two solid surfaces.
As model systems organic monolayers with different terminal functional groups, adsorbed on metall or semiconductor surfaces, were used. They were contacted with an IR-transparent, high refractive index material such as silicon or germanium. The IR-beam is directed through this high index phase to the interface and yields strongly enhanced spectra of the sandwiched sample layer. A comparison of these spectra of the "compressed" monolayer films to the spectra of the same, uncompressed films in contact with air reveal both structural differences as well as specific chemical interactions of the terminal functional group with the opposing solid surface, ranging from weak bond polarizations to chemical bonding reactions.