Colloidal epitaxy produces optically useful crystallization
A template for directing colloidal particles to form large three-dimensional arrays with adjustable crystal orientation has been developed by Alfons van Blaaderen of Utrecht University (the Netherlands) with Pierre Wiltzius and Rene Ruel of Bell Laboratories (Murray Hill, NJ). The template--made from a 500-nm-thick polymer layer, of poly(methylmethacrylate), imprinted by electron-beam lithography with a 400 ¥ 400 array of holes--directed crystallization of separately synthesized silica spheres suspended in dimethylformamide. After slow sedimentation, used to control layer-by-layer growth, the spheres formed a face-centered cubic crystalline structure the size of the template.
Colloidal particles do not naturally make the large single crystals needed for optical applications, explains Wiltzius. The particles typically settle in hexagonally packed layers that stack randomly. This colloidal epitaxy method is a simple way of directing the particles to form a crystalline structure. To be useful for optical applications, the particles must be arranged in a well-ordered three-dimensional array, and their diameter must be comparable to the wavelength of visible or infrared light. The process opens u¥novel ways to design materials based on colloidal crystals, such as optical filters, switches, gratings, and even laser host materials, says Wiltzius. Future research will determine the sedimentation rate and volume fraction dependence on the epitaxial structures as well as feasibility of using dielectric particles with higher indexes of refraction.
