Latex-templated silica films: Tailoring porosity to get a stable low-refractive index

Porous sol-gel silica films are prepared using different PMMA latex nanoparticles, 30-80 nm in diameter, as sacrificial templates. By changing the size and the content of the latex particles in the deposited silica sol, it was possible for the first time to tailor the porosity of the sol-gel films (uniform pore size and porous fraction from 0.10 to 0.74) independently of their thickness. This is the consequence of a low microporosity in the silica walls as shown by the correspondence between the measured porous fraction after calcination and the starting latex volume fraction. No ethanol capillary condensation occurs in these films before high partial pressure (above 0.9), leading to a stable refractive index that can be tuned in a large range (from 1.15 to 1.40 at 600 nm). A porosity percolation transition with the opening of the extrinsic pore interconnection was observed at a pore fraction threshold of about 0.40 for different sizes of the initial template. 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.