Substrates do influence the ordering of mesoporous thin films

Abstract

Two kinds of ordered structures, a cylindrical hexagonal and a compact hexagonal arrangement of micelles, were synthesized using either CTAB or a Pluronic block copolymer as templating agents. The ordering of thin films deposited on glass, dielectrics (Si₃N₄, SiTiO_x) and a metal with a native oxide (Ti) were compared. We observed that the ordered texture of the films was greatly influenced by the substrate. In order to differentiate the effects of surface roughness and surface forces, glass substrates were systematically modified by grafting ethylene oxide/propylene oxide (EO/PO) statistical copolymers. Varying the EO content allowed fine tuning of the hydrophilicity of the surface. In this case, the same phase and quality of ordering as on bare glass is observed but with a systematic higher contraction of the ordered structure. This is ascribed to the similar chemical nature between the grafted substrate and the templating molecules, which fasten the formation of the ordered structure.