Molecular Orientation Analysis of a Single-Monolayer Langmuir−Blodgett Film on a Thin Glass Plate by Infrared Multiple-Angle Incidence Resolution Spectrometry

Abstract

Molecular orientation analysis in a single monolayer deposited on a glass substrate has been a difficult matter, since the glass substrate absorbs infrared rays so strongly that the measurements of infrared spectra are difficult to perform, and the single monolayer is not suitable for X-ray diffraction analysis because no periodical structure is available. When a thin glass is used as the substrate, in particular, the infrared analysis becomes more difficult, since optical fringes appear strongly on the absorption spectra due to the multiple reflections in the glass. In the present study, infrared multiple-angle incidence resolution spectrometry (MAIRS) has been employed to remove the fringes from the spectra of single- and five-monolayer Langmuir-Blodgett (LB) films of cadmium stearate deposited on a thin glass plate. The MAIRS in-plane spectra gave quantitatively reliable infrared transmission spectra for both films with little fringes, which made it possible for the first time to analyze the molecular orientation in the single-monolayer LB film on glass. As a result, it has been revealed that the molecule in the single-monolayer LB film on thin glass exhibits a significantly larger molecular tilt angle than those prepared on other substrates such as gold and germanium.