Convective assembly of antireflective silica coatings with controlled thickness and refractive index

Convective assembly at high volume fraction was used to deposit silica nanoparticle coatings onto glass and silicon substrates. By allowing control of the film structure and thickness, this technique provides a means for making large-scale coatings with antireflective properties. The reflectance was reduced by 50% for silicon (at 600 nm) and by 70% for single glass/air surface. Microstructural investigations using SEM, AFM, profilometry, and ellipsometry provided good correlation to the observed macroscopic optical properties. By virtue of the coatings' uniformity, the reflectance and transmission spectra from both substrates could be modeled well by classical reflection relations, using a volume-averaged refractive index. Data analysis showed that the relatively high packing fraction in nanocoatings made from monodisperse spheres is responsible for the limit on antireflective capabilities. To overcome this restriction, low-density silica coatings were made from binary colloidal mixtures of different diameter SiO2 particles. The packing fraction of these coatings was further optimized to yield 88% maximal reduction in the reflectance of glass surfaces. The technique is simple, inexpensive, and scalable.