  Thin colloidal crystals (or synthetic opals) composed of Stober 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.
