Ask those who have tried to produce photonic bandgap materials in the lab with colloidal suspensions, and you're likely to hear about the cracks that appear as the crystals dry. A discovery by a team of researchers at the University of Illinois may go a long way in solving that problem.The self-organizing process known as nanoparticle haloing creates stabler colloidal systems. Charged nanoparticles (red) added to the solution envelop larger, uncharged particles (blue) and control their spacing. Courtesy of Jennifer A. Lewis.When colloidal crystals are grown in an aqueous environment, gaps can form between the particles. As the assembly dries, these spaces collapse and create defects, such as cracks, which affect the desired periodicity. In the July 31 issue of Proceedings of the National Academy of Sciences, Jennifer A. Lewis and her colleagues at Illinois and at Carnegie Mellon University in Pittsburgh report a new technique for regulating the stability of colloidal suspensions that may enable engineers to tailor the interactions of colloidal particles.The process introduces highly charged nanoparticles to a mixture of larger, uncharged particles. In solution, the nanoparticles repel one another and arrange themselves in a halolike configuration around the uncharged ones. This self-organizing process imparts stability to the larger particles, which would attract one another by van der Waals forces under normal conditions.The process, which the researchers call nanoparticle haloing, offers the ability to tune the separation of these particles and therefore to create either more or less rigid structures. Theoretically, this makes it possible to custom-design a colloidal crystal that can better handle thermal and capillary forces.Lewis and her Illinois colleague Paul V. Braun have begun proof-of-concept work. They are assembling the structures on patterned substrates via colloidal epitaxy, a technique pioneered by one of their collaborators, Pierre Wiltzius of Lucent Technologies Inc.'s Bell Labs in Murray Hill, N.J.Index of refraction for photonicsThe next step will be to experiment with different microspheres and nanoparticles to realize the desired combination. They also hope to identify nanoparticles with the required index of refraction for applications in photonics.