Hierarchical porous silica films with ultralow refractive index

Fabrication of hierarchical porous thin films with controlled pore dimension and shape is a current challenge in materials science. In this work, bimodal hierarchical hybrid organic-inorganic thin films with pores in the meso and macro range have been fabricated. The first order of porosity is obtained through a self-assembly process using organic block copolymers as templates for mesopores; the mesophase in the films is well-ordered and has a tetragonal symmetry. We have introduced fluorinated organic nanoparticles of around 70 nm in the precursor solution to act as template of the second order of porosity. The introduction of the nanoparticles has not affected the organization of the mesophase and after the film deposition the nanoparticles become randomly dispersed within the mesostructured matrix. The organization of the mesophase has been studied in situ by small-angle X-ray scattering using synchrotron light; conventional and high-resolution transmission electron microscopy have been used for direct observation of the porosity morphology. The results indicate that hierarchical porous thin films with a bimodal distribution of the pores are obtained upon postsynthesis calcination at 350 °C. The hierarchical porous thin films have a very low refractive index, 1.14 at 633 nm, lower than the 1.17 value measured for the monomodal mesoporous films. The introduction of templating nanoparticles that are removed upon thermal treatment represents, therefore, a successful strategy to decrease the refractive index in porous films. We have also demonstrated that patterning techniques developed for mesostructured films can be successfully extended to this new typology of hierarchical films. Arrays with bimodal hierarchical pore distribution have been fabricated.