colloids

research: crystalline colloidal arrays

norris nature
Though synthetic colloidal crystals were identified early in the 20th century with the emulsion polymerization of styrene, there has been a resurgence of colloidal crystals in research in order to establish fundamental insights into colloidal forces and self-assembly. In addition, these highly ordered structures have been exploited as precursors for the next generation of advanced materials. If the dimensionally periodic dielectric structure of the crystal exhibits a sufficient dielectric contrast between the particles and its interstitial spaces and has a periodicity that is on the order of the wavelength of light, the colloidal crystal may act as a 3-dimensional diffraction grating for visible light and “opalesce”. The attached upper-right figure demonstrates such a periodic structure with a blue opalescence (Nature 414, 289-293 (2001)). These systems have been labeled photonic crystals since they can exhibit a photonic bandgap (or more typically, a stop band). Colloidal crystals have been exploited in this field since they may undergo self-assembly at a nanometer length scales, resulting in spatial periodicities that may range from ca. 100 - 1000 nm; a number of review articles on colloidally-based photonic bandgap materials has been recently presented that discusses the underlying physics of these interesting structures.
opaline tiger paw
In general, there are two approaches for manipulating monodisperse spherical colloidal particles for the generation of photonic crystals. One approach involves the assembly of the particles into close-packed arrays through sedimentation and typically relies on non-specific particle-particle “hard sphere” packing to induce order. The attractive aspects of this assembly approach is both its simplicity and versatility. The second approach utilizes the long range electrostatic repulsive interactions of charged colloidal spheres suspended in a liquid medium to procure order. These latter systems will often adopt a minimum energy crystal structure with either bcc or fcc symmetry. The attached left figure demonstrates such an electrostatically stabilized "colloidal crystal" that has been templated into a Clemson University tiger paw. The Foulger group continues to pursue a number of different research thrusts into crystalline colloidal arrays that are presented by a review of our publications.
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