Synthesis and characterization of ordered arrays of topological defects in mesoporous silica films

Defects are commonly found throughout crystalline solids and often have a pronounced effect on mechanical, optical, chemical, and transport properties.¹ Liquid-crystalline materials also contain defects resulting in topologies where the director field is undefined at a point or on a curve at the defect core.² In both the crystalline and liquid-crystalline state, defects typically disrupt long-range order, rendering the material isotropic at large length scales. Acid synthesis of mesoporous silica proceeds through a liquid-crystalline stage³ in which defects determine the channel orientation of the resulting porous material. Previously, hexagonal phase mesoporous silica films have been grown with channels dominantly oriented parallel to the substrate.^4-6 Here, we report that ordered arrays of topological defects can be generated leading to mesoporous silica films with long-range orientation and rotational order with approximately half of the channels oriented perpendicular to the substrate by growing the films epitaxially on a crystalline substrate in a shear flow field. Two-dimensional X-ray diffraction patterns recorded from films grown on mica are 'single-crystal-like,' exhibiting 6-fold symmetry that is rotationally invariant over the entire film.