Microstructural Characterization of Polystyrene-block-poly(ethylene oxide)-Templated Silica Films with Cubic-Ordered Spherical Mesopores

Philipps University of Marburg, Marburg, Hesse, Germany
Langmuir (Impact Factor: 4.46). 09/2003; 19(18). DOI: 10.1021/la034798m


Abstract: We report the synthesis and characterization of mesostructured thin silica films derived from methyltriethoxysilane (MTES) and/or tetraethyl orthosilicate (TEOS) silica precursors and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers via the solvent evaporation-induced self-assembly (EISA) process. It is found that the meso- and microstructure of the calcined films consists of cubic-ordered arrays of spherical mesopores of 5-7 nm in diameter, interconnected with a small number (4% by volume) of PEO-induced micropores of ~1 nm in diameter, as determined by new experimental results of transmission electron microscopy, N2 sorption, gas permeation, and grazing incidence small-angle X-ray scattering studies. The present comprehensive study of these novel closed-pore films should provide a general methodology for microstructural characterization of other related porous films prepared by similar self-assembly processes.

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    ABSTRACT: Abstract: The mesostructure of a hydrophobic silica thin film with spherical voids arranged in an ordered array was investigated and resolved thoroughly by transmission electron microscopy (TEM). The novel TEM approach involved sample tilting to get different zone axes and simulation, as well as the calculation of plane spacings and angles for comparison of the experimental values with the corresponding theoretical ones. The camera constant (L) was calibrated carefully by the diffraction pattern of the silicon substrate, on which the closed-cell mesostructured thin film was developed. This film was fabricated by solvent evaporation-induced sol-gel and self-assembly processes, with methyl triethoxysilane [MTES, Si(OCH2CH3)3CH3] as the silica precursor and a polystyrene-block-poly(ethylene oxide) diblock as the structure-directing agent; pyrolysis was performed carefully to remove the template. The present TEM exploration concludes that the mesostructure is exclusively body-centered cubic (bcc) rather than face-centered cubic(fcc); this conclusion was based on the assessment of the experimental tilting angles and the theoretical angles employed to get one zone axis from another, as well as on the evaluation of the experimental and simulated TEM images. The bcc mesostructure with a slight distortion was further assigned to be body-centered tetragonal (bct) with the preferred cell parameters of a = 13.5 nm and c = 13.0 nm; this assignment was based on the comparison between the experimental and the calculated data, including the plane spacings and the angles between various planes. The present self-assembled MTES-derived silica thin film with closed cells arranged in a slightly distorted bcc array is believed to be the first and is a promising candidate to function as a low-dielectric-constant (k) insulating material needed for future semiconductor devices.
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