Evaporation-controlled self-assembly of silica surfactant mesophases

Surfactant templated silica mesophases belong to the class of self-assembled materials that exhibit long range ordered two-dimensional (2D) hexagonal, three-dimensional (3D) hexagonal, or 3D cubic mesostructures when the composition of the initial deposited solution and its aging time have been optimized. Thin films of such mesophases with a thickness typically less than 300 nm are now routinely obtained by dip coating. In this study a reference solution was used with a chemical composition leading to the formation of thin films with a 3D hexagonal (P6(3)/mmc) mesostructure by dip coating. Thick films (about 10 mum thick) were alternatively prepared by a evaporation-controlled self-assembly (ECSA) process and studied with grazing incidence small-angle X-ray scattering (GISAXS). In the ECSA process, the mesostructure was developed under slower evaporation conditions than for dip-coated films by finely tuning the ethanol/H2O evaporation rate of the reference solution horizontally deposited on a silicon wafer. It is shown that the evaporation rate of the solvent is one of the key parameters that control the final mesostructure and can, under certain conditions, promote the formation of the cubic mesophase. The ability to precisely control such a structural arrangement on the mesoscale is of major interest for making sensor arrays, nanoreactors, photonic and fluidic devices, and low dielectric constant films.