Mesostructured MTES-Derived Silica Thin Film with Spherical Voids Investigated by TEM: 1. Mesostructure Determination

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 (λL) 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 structuredirecting 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 bodycentered 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.