Water vapor adsorption and liquid penetration into thermally oxidized porous silicon films were studied by variable angle spectroscopic ellipsometry. Characterization of the surfaces in air combined with multilayer optical modeling showed the existence of a gradient in the volume porosity and bulk-like silicon skeleton in the layers. Films oxidized at 300°C contained a thin oxide layer and further oxidation at 800°C resulted in an almost total oxidation of the silicon skeleton, which yielded an increased hydrophilicity and an almost complete pore filling by water both from saturated atmospheres and from a liquid phase. Total water adsorption per gram adsorbent at saturation was 0.50 cm3/g for the 300°C samples and 0.45 cm3/g for the totally oxidized films. Oxidation at 800°C resulted in a surface area of 72 m2/g determined by a modified Brunauer-Emmett-Teller procedure.
Porous silicon Fabry-Pérot optical filters with reflectivity peaks in the visible and near infrared spectral range have been manufactured electrochemically and characterized with variable angle spectroscopic ellipsometry. Generalized ellipsometry and the Bruggeman effective medium approximation were employed to study the anisotropy of the samples including determination of the tilt of the optic axis relative to the sample normal. At a wavelength of 700 nm, the difference between the real parts of the ordinary and extraordinary indexes of refraction of the low and high index sublayers were 0.05 and 0.07, respectively. In addition, the effect of temperature on the spectral redshifts caused by capillary condensation of water and toluene vapors in the porous material are investigated. It is demonstrated that a temperature decrease causes the spectral shifts to occur at lower partial pressures, indicating a more effective vapor capture at lower temperatures. Finally, it is shown that this phenomenon can be used to increase the ellipsometric response to very low vapor concentrations and thus improve the sensitivity of the technique in gas sensor applications.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.