Comparative investigation of Stöber silica Langmuir–Blodgett films as optical model structures



Mono- and multilayered Stöber silica Langmuir–Blodgett (LB) films were investigated by Spectroscopic Ellipsometry (SE) and Optical Waveguide Lightmode Spectroscopy (OWLS) comparatively. The effective refractive index and film thickness were determined from the results of multiparameter fitting of SE and OWLS spectra, and verified by reflectance spectroscopy measurements. LB layers of silica nanospheres are excellent model structures for the experimental verification of the validity of the Effective Medium Approximation (EMA) methods in SE evaluation. Since the structure of the LB films is nearly ideally close-packed, even single layer Maxwell–Garnett EMA offers a sufficiently adequate description of this structure. Deviation from the ideally calculated porosity in the LB monolayer can be interpreted in terms of an average distance between individual spheres (coverage) in the uniform layer (assuming monodisperse particles). In case of the evaluations of OWLS measurements homogeneous single layer model can not be used because of the exponential form of the sensing evanescent field. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)