Ultralow-refractive-index optical thin films built from shape-tunable hollow silica nanomaterials.

Shape-tunable hollow silica nanomaterials, including hollow silica nano-spheres and nano-tadpoles, were synthesized with a one-step soft-templating method. A possible particle growth mechanism was proposed. Films were built from these shape-tunable hollow silica nanomaterials with refractive indices as low as 1.045, which is, to the best of our knowledge, the third-lowest value ever reported so far. For the first time, the refractive indices of films were tuned by changing the morphology of building blocks. These films are intrinsically hydrophobic. Moreover, the process used to prepare these nanomaterials without the need for expensive equipment or any post-treatment is well suited for industrial production on large surfaces.