Shape is everything in the materials world and colloidal crystals are no exception in this respect. The form of a colloidal crystal determines its property-function relations, especially so in optical applications. We have produced for the first time photonic crystals with varied curvatures using sol-gel inversion of assembly-templated colloidal-crystal opals. The curvature is induced and varied through the formation of an overlayer upon infiltration of the opal microstructures with sol-gel alkoxide precursors. Upon complete conversion of alkoxides into oxides, the overlayer experiences larger shrinkage compared to the infiltrated opals, which causes the overall structure to curl. The highly ordered inverse opal curls can be produced as coils of up to several millimeters in length upon selective removal of the polystyrene (PS) latex. The optical properties of the highly ordered inverse zirconia opal microstructures were studied using UV-vis microspectroscopy.
Beyond bulk and thin-film formats, the morphology of a colloidal crystal can be controlled using a template directing the self-assembly process. One type of template is a lithographically defined surface-relief pattern in a substrate to direct sphere-by-sphere growth into a colloidal-crystal film with a specific orientation. Another involves confined colloidal assembly within a surface-relief pattern, which enables the formation of colloidal-crystal strips with a particular size and shape. Nonplanar confined geometries include cylindrical tubes to produce colloidal-crystal capillary columns and spherical droplets to make colloidal-crystal marbles. Colloidal assembly on the outside of an optical fiber provides colloidal-crystal cladding. These cylindrical, spherical, and tubular formats introduce curvature into colloidal crystals with structural and optical implications.
Colloidal crystals can also function as structure-directing templates to make replica materials called inverted colloidal crystals. Those with a refractive-index contrast greater than COMMUNICATIONS