Avoiding cracks in self-assembled photonic band-gap crystals

Abstract
Thin colloidal crystals (or synthetic opals) composed of Stöber silica spheres typically develop cracks when they are utilized to obtain photonic band-gap crystals (or inverted opals). We find that, by sintering the silica spheres prior to assembly of the opal, these cracks can be avoided. We report the effects of temperature and duration of the heat treatment on 850 nm silica spheres using electron microscopy, thermogravimetry, and light scattering. We also find a large dependence of the refractive index of the silica on the temperature of the heat treatment. This may allow tuning of the refractive index of silica spheres.
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