Layering transitions in colloidal crystals as observed by diffraction and direct-lattice imaging.

Abstract

Layering transitions of colloidal crystals confined between two smooth glass surfaces have been studied by transmission diffraction of light as well as direct-lattice imaging with an optical microscope. The polystyrene spheres we studied have diameter \ensuremath{\Phi}=0.305 \ensuremath{\mu}m and a surface charge of \ensuremath{\sim}${10}^{4}$ electronic charges, and form three-dimensional (3D) colloidal crystals in completely deionized water at a volume concentration above \ensuremath{\approxeq}0.3%. A 3D suspension of these spheres in completely deionized water forms both body-centered-cubic (bcc) and face-centered-cubic (fcc) crystalline structures as a function of colloid density n\ensuremath{\equiv}(1/${a}_{s}$${)}^{3}$, with lattice constants ranging between 0.7 and 1.5 \ensuremath{\mu}m. When the colloid is confined between glass plates separated by distances D\ensuremath{\sim}0.5--1 \ensuremath{\mu}m, the spheres form a single-layer 2D fluid. For D near 1 \ensuremath{\mu}m, a transition occurs to a single-layer 2D hexagonal crystal. As D increases, the evolution from two- to three-dimensional crystals is observed as a series of structural transitions distinguished by the number of crystal planes between the plates and by the preferred crystal symmetry parallel to the glass boundaries. We present here a study of colloidal crystals confined in a wedge cell that allows diffraction and imaging from the same crystallite. We present diffraction and imaging measurements of structural phases of one- through seven-layer colloidal crystals confined between two smooth glass surfaces, for a range of densities such that 2${a}_{s}$/\ensuremath{\Phi}6. The sequence of structural phases we observe for this range of densities for clean samples is similar but not identical to that observed for ${a}_{s}$/\ensuremath{\Phi}2 by Pieranski, Strzlecki, and Pansu, which was modeled as a hard-sphere system. We also observe differences between the clean thin crystalline phases and ``dirtier'' thin phases to which a stray electrolyte has been added. The use of both diffraction and imaging was found necessary to fully characterize the system.