Aggregates, controlling symmetry

Aggregates, controlling symmetry, 446 Aggregation, molecular orbital modeling, 457-470... 

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

In another demonstration, we found that the small aggregates formed via the TASA process were immediately useful as templates to generate three-dimensional lattices of spherical colloids with symmetries or orientations other than those commonly observed for the spherical systems on flat substrates." For example, when 1% of the sqnare-shaped, tetramer aggregates was mixed with spherical colloids, they were able to direct the spherical colloids into an opaline lattice with its (100) planes oriented parallel to the surface of a flat substrate (Figure 11.1 IB). In this case, the three-dimensional opaline lattice tended to develop a symmetry similar to the added aggregates during the crystallization process. This ability to control the orientation of an opaline lattice provides another way to change the photonic properties of a colloidal crystal. 

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