An Efficient Route to Mesoporous Silica Films with Perpendicular Nanochannels

The fabrication of mesoporous silica films with well-defined morphologies offers tremendous opportunity for device structures. In particular the generation of a mesoporous silica film with cylindrical nanochannels of prescribed diameters oriented normal to the substrate holds great promise for applications in catalysis, bio-molecular separations, sensors, photonics, synthesis of aligned metallic nanowires and carbon nanotubes and for guest-host applications with electrically and/or optically active species. However fabricating well-defined, robust films by conventional evaporation-induced cooperative self-assembly approaches has proven to be extremely challenging. The general approach to mesoporous materials is based on the discovery that metal oxide precursors and structure-directing surfactants or amphiphilic block copolymers can selfassemble cooperatively to yield ordered composites from aqueous alcohol solutions via controlled solvent evaporation. The organic surfactant can be removed to produce an inorganic mesoporous network. The size and shape of the pore are governed by the molecular mass and composition of the block copolymer, the nature of the precursor, solution acidity, temperature and concentration. The ability to control pore size, morphology and orientation in these materials has been the subject of significant research since their initial discovery. While evaporation-induced cooperative self-assembly is a remarkably powerful approach, it has a number of limitations. One is poor control over orientational alignment of the cylindrical pores. Simultaneous structural evolution from cooperative block copolymer self-assembly and precursor condensation coupled with preferential affinity of one of the components of the sol solution for the interfaces (substrate/film or film/air) leads to a parallel orientation of the cylindrical pores with respect to the substrate. Magnetic fields have been used as an external agent to align the cylindrical channels along the field direction. However, the direct preparation of mesoporous silica film with perpendicularly oriented cylindrical channels on conductive or non-conductive substrates without using any external agents has remained elusive. Recently it has been reported that porous anodic aluminum oxide membranes can be used as orientation inducing scaffolds to generate vertically aligned mesoporous silica channels within the pores of the membranes. However, the preparation of the aluminum oxide scaffold is tedious and its use as a support produces a composite film containing only a small fraction of mesoporous silica channels. In this work, we demonstrate an efficient route to robust mesoporous silica films with nanochannels oriented normal to the substrate surface by a direct block copolymer template replication process. Recently, we described a method to prepare robust mesoporous materials by a phase-selective silica deposition within a microphase separated block copolymer template dilated with supercritical carbon dioxide. The use of supercritical fluid as a processing solvent enables facile transfer and reaction of precursors within the polymer film without disturbing the template order. The template can be removed to obtain a mesoporous inorganic replica. Unlike the conventional cooperative self assembly approach, our strategy for mesoporous metC O M M U N IC A TI O N