Structure of Mesoporous Silica Thin Films Prepared by Contacting PEO₁₀₆−PPO₇₀−PEO₁₀₆ Films with Vaporized TEOS

Abstract

Highly ordered mesoporous silica thin films have been prepared on silicon substrates by contacting PEO106−PPO70−PEO106 (Pluronic F127) triblock copolymer films with hydrolyzed tetraethyl orthosilicate (TEOS) followed by calcination. 2D grazing angle of incidence small angle X-ray scattering (GISAXS) patterns analyzed in the context of the distorted wave Born-approximation (DWBA) show that the films are (111)-oriented and possess rhombohedral symmetry with lattice constants a = 16.8 nm and α = 70°. Further, high resolution field emission scanning electron microscope (FESEM) observations showed that the films have a lamellar structure supported with periodically arranged pillars. To our knowledge this is the first report of a rhombohedral mesophase obtained using Pluronic F127. Additionally, the pore connectivity in the films here differs from previously reported rhombohedral films. Here the films are capped with a dense layer of silica and appear to not have significant mesoporosity in the direction perpendicular to the substrate. As a result of this structure, after silylation the films have a low relative dielectric constant of ∼1.86. In addition, by comparing the X-ray diffraction (XRD) patterns with the GISAXS analysis, we show how using Bragg's law to calculate d-spacings from XRD data can significantly underestimate the d-spacing. Taking into account the effects of refraction, we report a modified expression of Bragg's law that may be used to recover accurate d-spacings from XRD data.