Dislocations and vacancies in two-dimensional mixed crystals of spheres and dimers.

In colloidal crystals of spheres, dislocation motion is unrestricted. On the other hand, recent studies of relaxation in crystals of colloidal dimer particles have demonstrated that the dislocation dynamics in such crystals are reminiscent of glassy systems. The observed glassy dynamics arise as a result of dislocation cages formed by certain dimer orientations.

Glassy dislocation dynamics in 2D colloidal dimer crystals.

Although glassy relaxation is typically associated with disorder, here we report on a new type of glassy dynamics relating to dislocations within 2D crystals of colloidal dimers. Previous studies have demonstrated that dislocation motion in dimer crystals is restricted by certain particle orientations. Here, we drag an optically trapped particle through such dimer crystals, creating dislocations.

Confinement-controlled self assembly of colloids with simultaneous isotropic and anisotropic cross-section.

The phase behavior of building blocks with mushroom cap-shaped particle morphology is explored under 2D and quasi-2D confinement conditions. Fast confocal microscopy imaging of the particles sedimented in a wedge cell reveals a range of mono- and bilayer structures partially directed by the isotropic and anisotropic profiles of the particle geometry. The sequence of phases tracked with increasing confinement height includes those reported in spheres, in addition to the more complex rotator and orientation-dependent phases observed for a class of short rod-like colloids.

Asymmetric colloidal dimers under quasi-two-dimensional confinement.

The synthesis and assembly of mildly fused asymmetric polystyrene/silica dimers confined to gap heights intermediate to an in-plane monolayer and an out-of-plane monolayer are explored. Using real-space confocal microscopy, we show that structures evolve from an oblique two-dimensional (2D) phase to a quasi-2D rotator, and finally to an upright hexagonally close-packed monolayer. The existence of the novel quasi-2D state, where out-of-plane motion is allowed, highlights the critical role that confinement dimensionality plays on the nature of ordering in complex colloidal systems.

Restricted dislocation motion in crystals of colloidal dimer particles.

At high area fractions, monolayers of colloidal dimer particles form a degenerate crystal (DC) structure in which the particle lobes occupy triangular lattice sites while the particles are oriented randomly along any of the three lattice directions. We report that dislocation glide in DCs is blocked by certain particle orientations. The mean number of lattice constants between such obstacles is Z[over](exp)=4.

Convectively assembled nonspherical mushroom cap-based colloidal crystals.

Monolayers from mushroom cap-shaped polymer colloids were fabricated by a vertical substrate deposition technique. As confirmed by SEM and autocorrelation analysis, the monolayers show long-range hexagonal packing with particle orientational freedom restricted to either "heads up" or "heads down" alignment with respect to the substrate. The monolayers are modeled as a 2D diffraction grating and were studied with selected area laser diffraction.

Convectively assembled asymmetric dimer-based colloidal crystals.

Monolayer films from polystyrene asymmetric dimer colloidal particles were formed on a silicon substrate using a heat assisted vertical deposition technique. In dilute particle suspensions of systematically varied concentrations, the system maximizes the packing efficiency within a thin meniscus region. Structures with positional order and orientational order in and out of the substrate plane were observed in surface and cross-sectional scanning electron microscopy (SEM) images.

Homogeneous, core-shell, and hollow-shell ZnS colloid-based photonic crystals.

Ordered ZnS-based colloidal crystals from homogeneous, core-shell, and hollow building blocks were prepared via electrosteric colloid stabilization combined with a convective assembly technique. The polyelectrolyte stabilized colloids assembled into face-centered cubic arrays with the (111) face perpendicular to the substrate. Structure-property correlations were made using scanning electron microscopy, scanning transmission electron microscopy, and UV/visible/near-IR spectroscopy.

Chemical bath deposition synthesis of sub-micron ZnS-coated polystyrene.

The synthesis of ZnS-coated polystyrene composite colloids by the decomposition of thioacetamide in the presence of polystyrene seed particles and metal salt is presented. The chemical bath deposition technique incorporates poly(vinyl pyrrolidone) to inhibit particle aggregation during the synthesis so that core-shell particles with sizes in the low sub-micron range were achieved. The shell thickness was well controlled through the reaction time and core size.