Improvement of flatness and structural regularity of mesostructured silica films having strictly aligned two-dimensional hexagonal structure

Surface flatness and structural regularity of a uniaxially aligned mesostructured silica film (MSF) is significantly improved by a new EISA process which consists of separate water addition to the precursor solution and coating under low humidity (SL process). This process ensures the control of both the state of the siliceous species and the water concentration during the coating process by optimizing the composition of the precursor solution just before the coating. The SL process improves the surface flatness and the regularity of the uniaxially aligned film, which had a large anisotropic roughness. The improved flatness and regularity of the obtained film are evident through reduction of the surface roughness to 1/7 and ten times higher diffraction intensity compared to the film prepared by a reference process using the precursor solution with the same composition. ²⁹Si NMR spectroscopy and in-plane X-ray diffraction (XRD) reveal that the independent control of the state of the siliceous species suppresses the formation of the branched siliceous species while maintaining the optimized water concentration to form the strictly aligned structure. The film with high structural regularity and a flat surface enables new practical precise optical devices utilizing its specific nano-scale anisotropy.