Single colloidal crystals

Abstract

Colloidal suspensions of highly charged, monodisperse polymer spheres exhibit long-range (crystalline) translational ordering in appropriate conditions of charge, number density, counterion concentration and temperature^1–5. These ‘colloidal crystals’, which can conveniently be made to have lattice parameters comparable to or greater than optical wavelengths, offer unique opportunities for the study of the collective static and dynamic behaviour of strongly interacting spherical particles. For example, such systems may readily be probed by relatively simple but powerful light scattering spectroscopic techniques^6,7. Furthermore, the ordering itself offers intrinsic advantages, both experimental (for example, fluctuations normally observable only about the k-space origin appear about Bragg spots, with reduced stray light and multiple scattering effects), and theoretical (the simplicity of calculating on the basis of a known rather than only statistically defined structure is well appreciated from experience with atomic solids and liquids). To exploit these ordered structures fully, reliable methods of producing orientated single crystals suitable for light scattering and other optical studies must be developed. For light scattering from bulk samples the primary requirement is that the ratio of interparticle spacing to diameter, a/d, be sufficiently large. This reduces interference by multiple scattering and renders the particle scattering factor nearly constant through those scattering angles [|k|<(2→5)2π/a] where the most useful information concerning crystal structure and dynamics appears. We now describe a technique whereby single body-centred cubic (b.c.c.) colloidal crystals, well suited for light scattering and having particular orientations, may be produced.