Chekesha M. Liddell

ZnS‐Based Photonic Crystals

Recent studies of the application of II-VI materials to the fabrication of photonic crystals are reported. Modeling studies show the potential of ZnS photonic crystals to provide a new method of controlling the emission characteristics of this material system so as to enhance color, intensity and decay time. The same structures are also shown to possess giant refraction and dispersion properties that can be used to control (collect, focus and steer) light. The fabrication of these period structures is addressed. Two approaches are being considered: the fabrication of two-dimensional ZnS photonic crystals by conventional electron beam lithography and the formation of three-dimensional ZnS photonic crystals by cost-effective self-assembly methods. ZnS and related II-VI compounds are very attractive for applications in photonic crystal devices operating in the visible and near IR region due to their high indices of refraction and large bandgaps that make them highly transparent in the visible. We report recent studies on the self-assembly of nanoparticles and subsequent ZnS infiltration techniques that can be used to control the emission and out-coupling properties of phosphors embedded in a photonic crystal.

Anisotropic Magnetic Colloids: A Strategy to Form Complex Structures Using Nonspherical Building Blocks

Microscopic magnetic particles dispersed in a solvent—or dipolar colloidal fluids—commonly assemble into chains due to a directional attractive interparticle potential. These chained structures can impart optical anisotropy (i.e., birefringence) to dipolar fluids, and have been demonstrated as effective matrix materials in the rapid separation of DNA using microfluidic electrophoresis. In the special case of magnetorheological (MR) systems, which require external fields to induce dipoles in polarizable colloids, an abrupt microstructural transition from isolated (unpolarized) particles into oriented chains can produce dramatic changes in the viscous behavior. This responsive property makes MR fluids attractive as field-controllable damping fluids in hydraulic valves, shock absorbers, brakes, and so on. Dipolar fluids also display interesting ordering phenomena beyond the 1D case. For instance, nickel-coated glass microspheres were shown to pack onto square, oblique, triangular, and even quasicrystalline lattices with five-fold symmetry in 2D. Binary particle mixtures assembled into ‘‘flower’’-like aggregates and superlattices in a planar configuration. In 3D, dipolar fluids were predicted to form rich mesophases as a function of the dipole moment strength, particle volume fraction, and the relative ‘‘hard’’ or ‘‘soft’’ nature of the colloidal interactions. Examples include the body-centered-tetragonal (bct) and body-centered-orthorhombic (bco) crystals, along with a broad fluid–bct coexistence phase. Yethiraj and van Blaaderen demonstrated these phases experimentally using a system with tunable long-range

Tetrastack: Colloidal diamond-inspired structure with omnidirectional photonic band gap for low refractive index contrast

Omnidirectional photonic band gaps opening at low values of refractive index contrast have been found for a nonspherical colloid-based photonic crystal structure. A mechanically stable design is described for the diamondlike photonic crystal composed of colloidal tetrahedra. The proposed tetrastack structure displays omnidirectional 2–3 band gap over a large range of filling fractions, refractive index contrasts, and building block orientations. The threshold refractive index for the inverted tetrastack structure was 1.94. A gap width of 25.3% relative to the center frequency was obtained for an inverted tetrastack with a 0.21 filling fraction of silicon.

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. In the later case, the major particle axis reorients with respect to the substrate. Closest packing considerations provide rationale for the observed 1Delta (hexagonal)-1Buckled-1Sides (rotator)-2square (square)-2Delta (hexagonal)-2Sides (rotator) structural transitions with height.

Magnetically responsive and hollow colloids from nonspherical core-shell particles of peanut-like shape

Monodisperse peanut-shaped particles having various chemical compositions, architectures (core–shell and hollow) and properties (optical and magnetic) were prepared from hematite templates. The colloids enable shape programming alone, optical manipulation, or magnetic field approaches to assembly.

Synthesis and assembly of nonspherical hollow silica colloids under confinement

Hard peanut-shaped colloids were synthesized and organized into a degenerate crystal (DC), a phase previously observed only in simulations. In this structure, particle lobes tile a triangular lattice while their orientations uniformly populate the three underlying crystalline directions.

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. We find a two-stage relaxation response where initially dislocations glide until encountering particles that cage their motion. Subsequent relaxation occurs logarithmically slowly through a second process where dislocations hop between caged configurations. Finally, in simulations of sheared dimer crystals, the dislocation mean squared displacement displays a caging plateau typical of glassy dynamics. Together, these results reveal a novel glassy system within a colloidal crystal.

Rotator and crystalline films viaself-assembly of short-bond-length colloidal dimers

Nonspherical particles of pear-like and spherocylinder shape were organized into diverse two-dimensional (2D) structures, including the orientationally disordered rotator. Dry films with hexagonal, oblique, and centered rectangular symmetry were obtained by using convective assembly to condense and confine the system in a thin meniscus region. Monte Carlo simulations confirmed the transition from fluid to rotator simply as a function of system density and short-bond-length particle morphology.

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)=4.6+/-0.2 in experimentally observed DC grains and Z[over](sim)=6.18+/-0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these obstacles is observed to proceed through dislocation reactions. We estimate that the energetic cost of dislocation pair separation via such reactions in an otherwise defect free DC grows linearly with final separation, hinting that the material properties of DCs may be dramatically different from those of 2-D crystals of spheres.

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.