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

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 {Φ}=0.305 {μ}m and a surface charge of {~}10⁴ electronic charges, and form three-dimensional (3D) colloidal crystals in completely deionized water at a volume concentration above {}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{≡}(1/aₛ{)}³$, with lattice constants ranging between 0.7 and 1.5 {μ}m. When the colloid is confined between glass plates separated by distances D{~}0.5--1 {μ}m, the spheres form a single-layer 2D fluid. For D near 1 {μ}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 2aₛ/{Φ}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 aₛ/{Φ}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.