In this work we propose and demonstrate a solution to the problems which arise when SiO2 monodisperse nanospheres of diameters under 300 nm or over 550 nm are used to obtain opal-based photonic crystals. If the nanospheres are too small, the sedimentation rate is very slow or even may not occur; if they are large enough, no significant order can be achieved because the velocity is too high. This method, based on the electrophoretic phenomenon, allows us to control the sedimentation velocity. Furthermore, other species of importance in this field, such as SiO2 spheres covered with a thick layer of TiO2, do profit from this method.

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