Methylated silica surfaces having tapered nipple-dimple nanopillar morphologies as robust broad-angle and broadband antireflection coatings

Abstract

In this work, mechanically, chemically, and thermally resistant broadband and broad-angle antireflection coatings were prepared on 10 cm diameter glass substrates combining sol−gel deposition with nanoimprint lithography. The coatings are composed of water-repellent methylated silica (Si₄O₇Me₂) and exhibit a transverse refractive index gradient created by tapered, nipple-dimple, subwavelength nanostructures, featuring a record vertical aspect ratio of ∼1.7. The structure is composed of hexagonal arrays of nanopillars (∼200 nm height, ∼120 nm width) and holes (∼50 nm depth, ∼100 nm width) with a 270 nm pitch. The corresponding effective refractive index is between 1.2 and 1.26, depending on the fabrication conditions. Total transmission for double-face nanoimprint wafers reaches 96−97% in the visible range; it is limited by specular reflection and mostly by the intrinsic diffusion of the glass substrate. The antireflective effect is effective up to an ∼60° incidence angle. We address the robustness of the inorganic-based coating in various realistic and extreme conditions, comparing them to the organic perfluoropolyether (PFPE) counterpart (master reference). The sol−gel system is extremely stable at high temperature (up to 600 °C, against 200 °C for the polymer reference). Both systems showed excellent chemical stability, except in strong alkaline conditions. The inorganic nanostructure showed an abrasion resistance of more than 2 orders of magnitude superior to the polymer one with less than 20% loss of antireflective performance after 2000 rubbing cycles under an ∼2 N cm⁻² pressure. This difference springs from the large elastic modulus of the sol−gel material combined with an excellent adhesion to the substrate and to the specific nipple-dimple conformation. The presence of holes allows maintaining a refractive index gradient profile even after tearing out part of the nanopillar population. Our results are relevant to applications where transparent windows with broadband and broad-angle transmission are needed, such as protective glasses on photovoltaic cells or C-MOS cameras.