Full-Multiscale Spontaneous Organization for Optically Anisotropic Titania Films

Titania films with a completely controlled hierarchical structure, at microscopic, mesoscopic, and macroscopic scales, are successfully prepared by carefully combining "top-down" and "bottom-up" nanoprocesses. The titania films are composed of regularly arranged anatase nanocrystals, which form a 2D hexagonal mesostructure with cylindrical mesopores. Furthermore, the cylindrical mesopores are aligned in one direction in the plane of the film over the whole area. Thus, hierarchical structural regularities over multiple length scales, i.e., atomic (10^-10 m), mesoscopic (10^-8 m), and macroscopic (10^-2 m) scales, are achieved. The mesoporous titania films with a controlled alignment are prepared via sol-gel chemistry using the self-assembly process of amphiphilic molecules combined with a lithographically prepared anisotropic substrate with a fine wavy cross-section. The carefully designed sol-gel process using Pluronic P123 as a structure-directing agent allows the retention of the aligned mesoporous structure as well as the formation of crack-free films even after the crystallization of titania. The anisotropic mesoporous structure with pore walls composed of high-refractive-index crystalline titania exhibits remarkable optical anisotropy, birefringence. This full-multiscale structural control of an inorganic material, from atomic to centimeter scales, affords distinguished functionalities to artificially prepared nanomaterials, paving the way for creating new values.