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MRS BULLETIN/OCTOBER 1998
particles with lattice constants ranging from a few nanometers to a few microns have potential applications as optical computing elements, chemical sensors, microwave components, and templates for fabricating quantum electronic systems—to name just a few. The articles in this month's issue highlight the hierarchy of scientific issues that must be addressed before colloidal crystals can reach their potential as industrial materials. This hierarchy begins with the colloidal particles' interactions and dynamics— the microscopic processes underlying colloidal crystallization. J.C. Crocker and D.G. Grier discuss some surprising new insights into the colloidal domain's inner workings that have been revealed by the first direct measurements of colloidal interactions. These interactions balance against thermal forces and external influences such as gravity to determine what configurations the particles will adopt. It seems however that much remains to be learned about how colloidal particles interact before these interactions can be fully exploited for producing colloidal materials to order. Prompting colloidal particles to organize themselves into useful arrangements is the next level of the hierarchy. The secret to making colloidal crystals is to create populations of particles with very tightly controlled size distributions. Ensembles of such uniformly sized spheres can undergo phase transitions to a rich variety of ordered and disordered, solid and fluid states. C.A. Murray charts these natural yet still quite mysterious processes in her survey of colloidal phase transitions. Of particular interest in her work is the influence that confining sur-
faces have on the selection and evolution of order. This influence might remind the attentive reader of the effect of substrates during solidification of conventional materials. This resemblance is far from coincidental because colloidal phase transitions are thought to be closely analogous to their conventional counterparts. The analogy promises to yield profound new insights into the general mechanisms of structural phase transitions because colloidal crystals' structure and dynamics can be studied with detail unattainable in conventional atomic or molecular materials. A. van Blaaderen carries this analogy one step further by crystallizing colloidal spheres on a lithographically defined substrate. This is the mesoscopic equivalent of epitaxial growth. The hope here is to impress extremely long-range threedimensional order on growing colloidal crystals by properly crafting their surface layers. A high degree of crystalline perfection will be as necessary to the photonic exploitation of colloidal crystals as has been the case in the electronic exploitation of silicon. Not only does this approach to ordering work, but preliminary studies with specially prepared composite colloidal particles show clear progress toward the goal of fabricating a photonic bandgap material—the optical equivalent of a semiconductor. The final step in the hierarchy involves m
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