Colloidal Crystal Structures Research Articles

Fabrication of Centimeter-Sized Single-Domain Two-Dimensional Colloidal Crystals in a Wedge-Shaped Cell under Capillary Forces

Self-assembly of colloidal spheres confined within cells of different shapes formed with two slides under capillary forces are studied. It is found that by controlling the shape of the cell the curvature of the drying front can result in a significant effect on the self-organization process. A curved drying front formed within parallel slides is always associated with growth of colloidal crystal structures with a high density of disorder. We demonstrate that single-domain two-dimensional colloidal crystals with centimeter size can be grown under capillary forces under a straight drying front formed in a wedge-shaped cell. These findings are demonstrated by laser diffraction, microscopy imaging methods and off-normal optical transmission measurements. The present growth method should be of importance in expanding colloidal crystal applications in angle-resolved nanosphere lithography, as well as in preparation of high-quality quasi-three-dimensional plasmonic crystals.

Inverted Colloidal Crystal Scaffolds for Uniform Cartilage Regeneration

A uniform distribution of chondrocytes in 3D scaffolds is a critical challenge to cartilage regeneration. This study aims to resolve the problem by showing uniformly distributed chondrogenesis in chitin/chitosan matrix with pores of inverted colloidal crystal (ICC) structure. The results revealed that the effect of solvent on the regularity of colloidal crystal arrays was in the order of ethanol > ethylene glycol > acetone. When the concentration of chitin/chitosan gel was <2.5%, the porosity of ICC scaffolds with pure ethanol was approximately 84%. The highly porous freeform scaffolds produced random and unconnected pores, in general. The viability of bovine knee chondrocytes (BKCs) in ICC constructs was >92%. Over 4 weeks of cultivation, the percentage of biodegradation of ICC scaffolds with pure ethanol was approximately 34%. The order in the produced BKCs, glycosaminoglycans (GAGs), and collagen was freeform constructs > ICC constructs with pure ethanol > ICC constructs with 95% acetone. However, the spatial distribution of BKCs in ICC constructs was more uniform than that in freeform constructs. In addition, BKCs could secrete GAGs and type II collagen in the core of ICC constructs, indicating the maintenance of phenotypic chondrocytes and their metabolism. The ICC constructs with well-controlled pore regularity and unique topography can generate uniform tissue-engineered cartilage.

Photonic band structure of three-dimensional colloidal crystals with field-induced lattice structure transformation

By utilizing the electrorheological effect, three-dimensional colloidal crystals can be produced, whose lattice structure can be changed from the body-centered-tetragonal lattice to other lattices under the application of electric fields. This paper calculates photonic band structures of such crystals with lattice structure transformation, and demonstrates the existence of complete band gaps for some intermediate lattices. Thus, it becomes possible to use the electrorheological effect to achieve photonic crystals with desired photonic gap properties resulting from tunable structures.

Directed self-assembly of colloidal crystals by dielectrophoretic ordering observed with small angle neutron scattering (SANS)

We report the first in situ measurements of directed self-assembly (DSA) of colloidal crystal structures by dielectrophoresis (DEP) observed with Small Angle Neutron Scattering (SANS). Development of a new sample environment allows for real time observation of electric field driven crystallization of sub-micron particles. SANS measurements for charge-stabilized colloidal dispersions of two different particle sizes as a function of electric field strength and frequency identify the order-disorder transition. The results validate a proposed master scaling curve for particle ordering developed for micron particles and thereby extends the scaling range to include sub-micron particles. This new experimental method enables the possibility of studying electric field induced alignment and ordering of colloidal nanoparticles.

Shape and Feature Size Control of Colloidal Crystal-Based Patterns Using Stretched Polydimethylsiloxane Replica Molds

In this work, we fabricated various patterns using colloidal crystals as master molds via the soft lithography method. Even though colloidal crystals consist of spherical colloidal particles, nonspherical shaped patterns such as rectangular or elongated hexagonal shaped patterns can be fabricated using a stretched polydimethylsiloxane (PDMS) replica mold. The pattern shape and feature size can be easily controlled by changing the stretching axis and ratio of the PDMS replica mold. The deformations of the PDMS mold were simulated using the finite element method, and they are consistent with experimental results. The elongated patterns were used as templates to offer new types of colloidal crystal superlattice structures. A proposed pattern-control method will significantly expand the usefulness and diversity of micro/nanopatterning technology.

Design of 2D Binary Colloidal Crystals in a Nematic Liquid Crystal

In this paper, we examine directed self-assembly in a 2D binary system of dipolar and quadrupolar colloidal particles with normal surface boundary conditions, dispersed in the nematic liquid crystal. Using the laser tweezers, we assembled a large variety of stable 2D colloidal crystal structures. In all analyzed structures, the particles, their surface treatment and the cell conditions were the same, which gives us the ability to systematically follow the evolution of colloidal assembly when many particles are present. We present an analogy between molecular self-assembly and organization of colloidal microspheres in liquid crystalline medium to extend the strategy for designing colloidal crystalline structures of different level of complexity.

Formation of Patterned Arrays of Polystyrene Colloidal Crystal Structures on Flexible Functional Substrates

This contribution presents the first preparation of patterned arrays of highly ordered polystyrene colloidal crystal structures on flexible functional substrates. The formation of patterned arrays of colloidal crystals over large areas (5x1 cm) with periodic line patterns ranging in pitch from 25 to 450 microm is demonstrated. The protocol developed to achieve this is applicable to a wide-range of substrates and is inherently scalable. Interestingly, directed colloidal deposition was found to be more susceptible to fluctuations in the deposition conditions than bulk deposition. The conditions required for directed deposition were systematically investigated, and the success of the optimized protocol was illustrated by the deposition of ordered structures on a range of functionalized rigid and flexible substrates. These advances-low-cost production on flexible functional substrates and the fabrication of structures of controlled geometry-address two of the major challenges in developing devices using colloidal crystal structures.

Fabrication of Ordered Nanostructured Arrays Using Poly(dimethylsiloxane) Replica Molds Based on Three‐Dimensional Colloidal Crystals

AbstractHexagonally arrayed structures of colloidal crystals with uniform surface are a good candidate for master molds to be used in soft lithography. Here, the fabrication of periodically arrayed nanostructures using poly(dimethylsiloxane) (PDMS) molds based on three‐dimensionally (3D) ordered colloidal crystals is reported. A robust, high‐quality 3D colloidal‐crystal master molds is prepared using the colloidal suspension containing a water‐soluble polymer. The surface patterns of the 3D colloidal crystals can then be transferred onto a polymer film via soft lithography, by means of the replication of the surface pattern with PDMS. Various hexagonally arrayed nanostructure patterns can be fabricated, including close‐packed and non‐close‐packed 2D arrays and honeycomb structures by the structural modification of the 3D colloidal‐crystal templates. The replicated hexagonally arrayed structures can also be used as templates for producing colloidal crystals with 2D superlattices.

An alternative method to obtain direct opal photonic crystal structures

We describe the protocol that we have elaborated in order to obtain monosize polystyrene spheres. Starting from these spheres a simple and effective method, based on spin-coating technique, was developed to realize colloidal photonic crystal structures. The process produces compact 3D arrays of polystyrene microspheres (opals) that are organized into crystalline lattices. This process offers the ability to rapidly form 3D photonic crystals using inexpensive instrumentation, which makes it attractive for an array of applications. Process parameters, fabricated structures, and their experimental characterization are presented.

Layer-by-layer templated growth of colloidal crystals with packing and pattern control

Layer-by-layer deposition of polymer colloidal particles on patterned electrode surfaces under an applied electric field is discussed. Colloidal crystals with a low packing density, grown on hexagonally patterned electrode substrates from single-sized colloidal particles, are presented. A nucleation mechanism of colloidal layer formation determined the grain size of the colloidal crystals. Alternating deposition of large (L) and small (S) colloidal particles with radii ratios of R S/ R L = 0.41 and 0.49 on patterned surfaces with hexagonal and square types of pattern led to the formation of 2D and 3D binary colloidal crystals with LS, LS 2 and LS 3 stoichiometries. Planar defects in form of colloidal monolayers composed of colloidal particles of smaller sizes were easily introduced into colloidal crystals using the layer-by-layer deposition technique. This method allows one to fabricate stable non-close-packed colloidal crystal structures in a direct deposition process.

Experimental determination of the band structure of photonic crystals of colloidal silica spheres

A photonic band structure of colloidal crystals of silica spheres is analytically determined by a band model with three fitting parameters: the sphere size, the effective refractive index, and the band-gap. Optical properties of the crystals annealed at various temperatures were characterized by a procedure similar to X-ray diffraction technique, and the width of photonic band-gap measured from the transmission spectra experimentally servers as an additional check on the validation of the model. The photonic band structures defined by the band-gap, the refractive index, and the Brillouin zone are obviously superior to the use of the Bragg's expression involving simple zone folding.

Immobilization of Charged Colloidal Crystals in a Polymer Matrix

We immobilized colloidal crystals of charged particles in a poly (acrylamide) matrix by photoinduced polymerization. A reflection spectrum and Kossel-line diffraction were employed to trace and compare changes in the colloidal crystal structure before and after immobilization processing. Our experiments showed that immobilized colloidal crystals successfully retained the structure of colloidal crystals unless the sizes and the lattice spacings of the immobilized colloidal crystals decreased slightly. By observing the structure of immobilized crystals in Milli-Q water we confirmed that the lattice spacings of the crystals varied for several days initially during immobilization because of gel swelling or de-swelling. After reaching a balance (2-5 d), the immobilized colloidal crystals are found to be stable in Milli-Q water. Our study thus explores potential applications of colloidal crystals such as their use in photonic materials.

Poly(lactic-co-glycolic acid) Bone Scaffolds with Inverted Colloidal Crystal Geometry

Controllability of scaffold architecture is essential to meet specific criteria for bone tissue engineering implants, including adequate porosity, interconnectivity, and mechanical properties to promote bone growth. Many current scaffold manufacturing techniques induce random porosity in bulk materials, requiring high porosities (>95%) to guarantee complete interconnectivity, but the high porosity sacrifices mechanical properties. Additionally, the stochastic arrangement of pores causes scaffold-to-scaffold variation. Here, we introduce a biodegradable poly(lactic-co-glycolic acid) (PLGA) scaffold with an inverted colloidal crystal (ICC) structure that provides a highly ordered arrangement of identical spherical cavities. Colloidal crystals (CCs) were constructed with soda lime beads of 100-, 200-, and 330-mum diameters. After the CCs were annealed, they were infiltrated with 85:15 PLGA. The method of construction and highly ordered structure allowed for ease of control over cavity and interconnecting channel diameters and for full interconnectivity at lower porosities. The scaffolds demonstrated high mechanical properties for PLGA alone (>50 MPa), in vitro biocompatibility, and maintenance of osteoblast phenotype, making them promising for a highly controllable bone tissue engineering scaffold.

Patterned Colloidal Photonic Domes and Balls Derived from Viscous Photocurable Suspensions

Many-body colloidal systems have been studied intensively on account of their phase behavior and photonic characteristics. Direct visualization of colloids using optical microscopy makes it possible to analyze colloidal crystal structures with ease; this allows such systems to be used as ‘‘visible’’ models of atomic or molecular assemblies. The periodic modulation of the refractive index within a colloidal crystal induces photonic bandgaps at wavelengths comparable to the periodicity or lattice constant. Generally, colloidal crystals are prepared by the self-organization of colloidal particles, and therefore the photonic bandgap properties can readily be controlled by adjusting the size and concentration of the particles. More importantly, the fabrication of colloidal photonic crystals via self-organization is simple and cheap compared with the conventional lithography-based approach. However, evaporation-induced self-assembly, the most popular method for fabricating close-packed colloidal crystals, requires long times and subtle fabrication conditions, and inevitably produces crystals with cracks due to non-uniformities in the capillary force. Although dispersion of charged colloidal particles in an aqueous medium and subsequent infiltration of a non-ionic monomer can produce crack-free non-close packed crystals, this approach still requires complicated processes to achieve large scale production. Recently, we developed a novel and simple method for spontaneous crystallization of charged colloidal particles in a photocurable resin. In this method, the viscous suspension is brought into contact with a template and crystallization occurs, starting at the interface between the colloidal suspension and the template. More importantly, the high viscosity of the colloidal suspension and

Orientationally correlated colloidal polycrystals without long-range positional order

We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of-but distinct from-two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns.

Structural Analysis of “Semisoft” Colloidal Crystals by Confocal Laser Scanning Microscopy

Semisoft colloidal crystals, which are formed by the work of steric or excluded-volume interactions among the concentrated polymer brushes (CPBs) afforded on spherical particles, were studied with respect to their crystalline structure as a function of graft chain length, while the graft density was approximately fixed at a high value. Rhodamine-labeled, low-polydispersity silica particles (average diameter 590 nm) were synthesized using the sol−gel chemistry based on the Stober method and then grafted with a shell of well-defined poly(methyl methacrylate) high-density brush by surface-initiated atom transfer radical polymerization. The internal structure of colloidal crystals formed in a liquid suspension of those hybrid particles was directly (in situ) observed by confocal laser scanning microscopy in fluorescence mode. The crystalline structure was of close-packed type in all cases, and the nearest-neighbor interparticle distance in the crystal increased with increasing graft chain length. The crystals...

Fabrication of ZnO colloidal photonic crystal by spin-coating method

We describe a simple and effective method, based on spin coating, developed for the fabrication by self-assembly of ZnO colloidal photonic crystal structures. The scanning electron microscope images indicate ordered structures with few defects. All the ordered structures exhibit a stop band in optical transmission experiments. The position of the stop band scales nicely with the particles size, indicating that it is an intrinsic feature of the photonic crystals. With decrease of ZnO particles size, the positions of the band are shifted to the shorter wavelength.

Structure of Polydisperse Inverse Ferrofluids: Theory and Computer Simulation

By using theoretical analysis and molecular dynamics simulations, we investigate the structure of colloidal crystals formed by nonmagnetic microparticles (or magnetic holes) suspended in ferrofluids (called inverse ferrofluids), by taking into account the effect of polydispersity in size of the nonmagnetic microparticles. Such polydispersity often exists in real situations. We obtain an analytical expression for the interaction energy of monodisperse, bidisperse, and polydisperse inverse ferrofluids. Body-centered tetragonal (bct) lattices are shown to possess the lowest energy when compared with other sorts of lattices and thus serve as the ground state of the systems. Also, the effect of microparticle size distributions (namely, polydispersity in size) plays an important role in the formation of various kinds of structural configurations. Thus, it seems possible to fabricate colloidal crystals by choosing appropriate polydispersity in size.

Binary Colloidal Crystals with a Wide Range of Size Ratios via Template-Assisted Electric-Field-Induced Assembly

Here we present a simple but general method for constructing complex binary colloidal crystals with an almost full range of size ratios, which is referred to as "template-assisted electric-field-induced assembly (TAEFIA)". The dependence of the structures of binary colloidal crystals on size ratio (gamma) and volume fraction (varphi) of the colloidal suspension was investigated. Binary colloidal crystals with gamma ranges from 0.10 to 0.91 were fabricated, and an attempt to fabricate a triple colloidal crystal via TAEFIA was presented. We suggest that TAEFIA is a versatile way to grow colloidal crystals with binary or more complex structures.

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. The confining effect of the meniscus height drove the formation of the resulting oblique and hexagonal lattices with controlled orientation. The crystals exhibited features similar to the planes of the boron nitride and zinc sulfide atomic structures. The diffraction properties of both colloidal crystal structures were demonstrated via selected area diffraction for laser light in the visible region.