Surface Reconstruction of the <111> Face of FCC Colloidal Crystals

Using diffraction of visible light, we have observed a complex phase diagram of surface reconstruction on the 111> face of 3D FCC colloidal crystals near clean glass surfaces. The colloidal crystals are comprised of monodisperse polystyrene latex spheres of diameter 0.305microns and surface charge ~ 2 x 10 sup 4 electrons in water suspension. Very sharp superlattice diffraction patterns exist in bulk samples that have equilibrated for several days without vibration. The superlattice periods vary from ~ 20 - ~ 2.5 nearest neighbor separations as the bulk density changes by about 25%. This occurs in a regular repeating progression over a large range of bulk density with nearest neighbor separation varying from 1.7 to 2.8 diameters. We believe the driving force for the reconstruction is slightly higher equilibrium density adjacent to the glass surface due to the existence of a smaller Coulomb screening length there. The only other know 111> FCC surface reconstruction is that of Au, which has a surface density about 4% greater than that of the bulk.