Latex-templated porous silica films for antireflective applications

Thin films with low-refractive index are of great interest to adjust the optical properties of optical components, e.g. reflectivity. In this respect, sol-gel coatings are very efficient for flat glass functionalisation because of ease of application, low cost and versatility. Mesoporous silica films prepared by surfactant self-assembly have been extensively studied but show poor refractive index stability as capillary condensation of atmospheric water can occur in the pores. One way to tackle this issue is to prepare films with larger pores for which capillary condensation is impossible at ambient humidity. Starting from custom made latex nanoparticles, we successfully prepared sol-gel porous silica film with pore size above 30nm and no microporosity in the silica walls. We are then able to independently and accurately tailor pore size, pore volume fraction and pore surface chemistry, leading to a stable refractive index that can be tuned in a large range (from 1.15 to 1.40 at 600 nm). Pore accessibility as a function of pore size and porous fraction was investigated through ellipsometry-porosimetry for ethanol adsorption, and a transition between open and closed pore structure at decreasing volume fraction was shown. Below the threshold, the films showed a closed porosity structure with a low stable refractive index (down to 1.29 at 600 nm), opening the way to their use for antireflective applications.