Tunable two-dimensional non-close-packed microwell arrays using colloidal crystals as templates

In this paper we demonstrate a facile and efficient way to fabricate poly(dimethylsiloxane) (PDMS) molds with hexagonal non-close-packed (ncp) arrangements of microwells by casting PDMS prepolymer onto two-dimensional (2D) ncp colloidal crystals. The templates of the 2D ncp colloidal crystals were fabricated via coupling lift-up soft lithography and solvent-swelling. We found that the depths of the microwells together with the lattice spacing can be adjusted by the sphere interstices and chemical composition of the 2D ncp colloidal crystals. The relationship of the surface character of the templates with the depths of the microwells can be explained by the wetting behavior of PDMS prepolymer on the rough surface. Contact angle measurements are consistent with the experimental results of the microwells in depth and agree well with the Cassie-Baxter theory. There are at least three advantages of the approach. First, the depth and distance of the microwells can be controlled. Second, PDMS molds can be easily peeled from the surfaces of the templates, which results in reusing the original templates to make new molds. Third, this method can be applied to other materials, such as photopolymerizable resin or thermosetting resin. The potential application of the microwells is as microlenses to make a pattern or as microvials in bioanalytical techniques.     

Large-scale fabrication of periodic nanostructured materials by using hexagonal non-close-packed colloidal crystals as templates

This letter reports a versatile nonlithographic technique for mass fabricating three types of technologically important materials-polymer microwell arrays, 2D-ordered magnetic nanodots, and semiconductor nanopillar arrays, each with high crystalline qualities and wafer-scale sizes. Spin-coated hexagonal non-close-packed silica colloidal crystals embedded in a polymer matrix are used as starting templates to create 2D polymeric microwell arrays. These through-hole arrays can then be used as second-generation templates to make periodic magnetic nanodots and semiconductor nanopillars. This self-assembly approach is compatible with standard semiconductor microfabrication, and complex micropatterns can be created for potential device applications. The wafer-scale technique may find important applications in biomicroanalysis, high-density magnetic recording media, and microphotonics.     

Fabrication of binary colloidal crystals and non-close-packed structures by a sequential self-assembly method

Binary colloidal films of polystyrene (PS) spheres and silica spheres were fabricated with a sequential growth method using differently sized colloidal particles. In particular, we demonstrate the structures formed by a silica monolayer growing on top of a PS monolayer and a silica multilayer growing on top of a PS monolayer. By removal of the bottom PS layers, non-close-packed hexagonal, pentagonal, and square silica arrays were obtained at the original silica/PS interface. The possible formation mechanism of the non-close-packed structure was discussed, which may be used to explain how 3D colloidal crystals grow on patterned substrates.     

Assembling and manipulating two-dimensional colloidal crystals with movable nanomagnets

We study crystallization of paramagnetic beads in a magnetic field gradient generated by one-dimensional nanomagnets. The pressure in such a system depends on both the magnetic forces and the hydrodynamic flow, and we estimate the flow threshold for disassembling the crystal near the magnetic potential barrier. A number of different defects have been observed which fluctuate in shape or propagate along the crystal, and it is found that the defect density increases away from the nanomagnet. We also study the melting of the crystal/fluid system after removal of the nanomagnet and demonstrate that the bond-oriental order parameter decreases with time. The nanomagnet can be moved in a controlled manner by a weak external magnetic field, and at sufficiently large driving velocities we observe self-healing crack formation characterized by a roughening of the lattice as well as gap formation. Finally, when confined between two oscillating nanomagnets, the colloidal crystal is shown to break up and form dipolar chains above a certain oscillation frequency.     

Supercooled dynamics of grain boundary particles in two-dimensional colloidal crystals

We experimentally investigate the dynamics of particles constituting grain boundaries in a two-dimensional colloidal crystal, using video-microscopy. A clear plateau in the mean square displacement of the grain boundary particles is found, followed by an upswing indicative of cage breaking. The van Hove correlation functions and the non-Gaussian parameter show that grain boundary particle dynamics are highly heterogeneous. Furthermore, we identified clusters of cooperatively moving particles and analyzed the time-dependence of the weight-averaged mean cluster size. We find good correlation between the behavior of the mean square displacement, and the time dependence of the non-Gaussian parameter and the cluster size, as also reported for various supercooled systems. Our results therefore provide experimental support for the similarity between particle dynamics in grain boundaries and in supercooled liquids as suggested by recent computer simulations.     

Fabrication of non-close-packed arrays of colloidal spheres by soft lithography

Ordered 2D non-close-packed sphere arrays with controllable lattice structures have been fabricated by using soft lithography based on the solvent-swelling and mechanical deformation behaviors of PDMS film. The figure shows an SEM image of the ordered quasi-one-dimensional parallel wires of silica spheres on a polymer-coated substrate.     

Effects of stretching on microstructures of polymer-immobilized non-close-packed colloidal crystalline array

Non-close-packed silica colloidal crystalline array was immobilized by polymer, and effects of stretching on the change of the optical properties and microstructure of the colloidal crystalline arrays have been demonstrated. The immobilization was a two-step polymerization process: the first step was with hydrophilic polyethylene glycol acrylate (PEGA) polymer gel, and the second step was with 2-hydroxyethyl acrylate polymer matrix. The structure of the three-dimensional array was maintained during the immobilizing process with lock in periodic order. The peak wavelength of Bragg diffraction of the polymer-immobilized colloidal crystalline array shifted to shorter wavelength with stretching. The peak shift was caused by the compression of the polymer proportional to the stretching ratio, and the compression was homogeneous throughout the polymer-immobilized colloidal crystalline arrays. These results show that by using polymer-immobilized non-close-packed colloidal crystalline array, mechanically tunable photonic crystals can be realized, and they open the possibility of tuning the microstructure of colloidal crystalline array for photonic crystal.     

Wafer-scale periodic nanohole arrays templated from two-dimensional nonclose-packed colloidal crystals

This communication reports a simple yet versatile nonlithographic approach for fabricating wafer-scale periodic nanohole arrays from a large variety of functional materials, including metals, semiconductors, and dielectrics. Spin-coated two-dimensional (2D) nonclose-packed colloidal crystals are used as first-generation shadow masks during physical vapor deposition to produce isolated nanohole arrays. These regular nanoholes can then be used as second-generation etching masks to create submicrometer void arrays in the substrates underneath. Complex patterns with micrometer-scale resolution can be made by standard microfabrication techniques for potential device applications. These 2D-ordered nanohole arrays may find important technological applications ranging from subwavelength optics to interferometric biosensors.     

Recent advances in fabrication of monolayer colloidal crystals and their inverse replicas

Monolayer colloidal crystals(MCCs)are two-dimensional(2D)colloidal crystals consisting of a monolayer of monodisperse colloidal particles arrayed with a 2D periodic order.In recent years,MCCs have attracted intensive interest because they can act as 2D photonic crystals and be used as versatile templates for fabrication of various 2D nanostructure arrays.In this review,we provide an overview of the recent progress in the controllable fabrication of MCCs and their inverse replicas.First,some newly-developed methods for the self-assembly of MCCs based on different strategies including interfacial assembly and convective assembly are introduced.Second,some representative novel methods regarding the fabrication of various functional2D inverse replicas of MCCs,such as 2D arrays of nanobowls,nanocaps,and hollow spheres,as well as 2D monolayer inverse opals(MIOs),are described.In addition,the potential applications of MCCs and their inverse replicas are discussed.     

Large-scale fabrication of wafer-size colloidal crystals, macroporous polymers and nanocomposites by spin-coating

This paper reports a simple spin-coating technique for rapidly fabricating three types of technologically important materials--colloidal crystal, macroporous polymer, and polymeric nanocomposite, each with high crystalline qualities and wafer-scale sizes. Dispersion of monodisperse silica colloids in triacrylate monomers is spin-coated onto a variety of substrates. Shear-induced ordering and subsequent polymerization lead to the formation of three-dimensionally (3D) ordered colloidal crystals trapped inside a polymer matrix. The thickness of as-synthesized colloidal crystal-polymer nanocomposite is highly uniform and can be controlled simply by changing the spin speed and time. Selective removal of the polymer matrix and silica spheres lead to the formation of large-area colloidal crystals and macroporous polymers, respectively. The wafer-scale process is compatible with standard semiconductor microfabrication, as multiple micrometer-sized patterns can be created simultaneously for potential device applications. Normal-incidence transmission spectra in the visible and near-infrared regions show distinct peaks due to Bragg diffraction from 3D ordered structures. The spin-coating process opens a new route to the fundamental studies of shear-induced crystallization, melting and relaxation.     

Influence of humidity on the fabrication of high-quality colloidal crystals via a capillary-enhanced process

Three-dimensional colloidal crystals have attracted a great deal of attention because of their potential use in photonic crystal, sensors, and other applications, but the bottlenecks in fabricating colloidal crystals include longer processing time and the lack of large-area ordered samples. A proposed capillary-enhanced method, which is a novel, efficient process for fabricating high-quality colloidal crystals in 24 h, is reported. It is necessary for increasing the processing rate by elevating the evaporation temperature but commonly resulted in the deposition of less-ordered crystals. However, high-quality colloidal crystals can be obtained in a controlled high-humidity system, resulting from the existence of secondary capillary forces present in high ambient humidity. Furthermore, the effect of secondary capillary forces will be confirmed, and it will increase with increasing humidity levels according to the semiquantitative analysis view of the surface thermodynamic behavior of small particles, including the modified Kelvin and Young-Laplace equations. Therefore, it can fine tune the relative position of the neighboring particles in the microarray and efficiently decrease the number of defects, resulting in the formation of perfect colloidal crystals with the assistance of enhanced secondary capillary forces.     

Rupture and regeneration of colloidal crystals as studied by two-dimensional ultra-small-angle X-ray scattering

The structure of colloidal crystals of silica particles in water was studied by using the two-dimensional (2D) ultra-small-angle X-ray scattering (USAXS) technique. By violent shaking of the dispersion, large (body-centered cubic, bcc) crystals were broken into microcrystals while the lattice structure and lattice constant were preserved. The 2D-USAXS profiles revealed that the [111] direction of bcc microcrystals was parallel to the capillary axis and their orientational distribution with respect to the capillary axis was random. While a prepeak was observed in the one-dimensional USAXS measurements, no such peak was detected by the 2D-USAXS technique. The prepeak was concluded to be due to {110} being rotated by 54.7 degrees (the angle between [001] and [111]) from the capillary axis. The diffraction from the plane was out of the horizontal plane and was observed at a lower angle as a prepeak by detector scanning in the horizontal direction.     

Crystals of crystals: fabrication of encapsulated and ordered two-dimensional arrays of microcrystals

A new technique-crystallization in asymmetric microwells-generates arrays of small crystals with controlled size, orientation, and arrangement in space. These arrays of crystals can be generated in a form completely encapsulated in polymer.     

Effects of medium composition on optical properties and microstructures of non-close-packed colloidal crystalline arrays

The effects of medium composition on the optical properties and microstructures of non-close-packed silica colloidal crystalline arrays have been demonstrated. Water–alcohol mixtures were used as dispersion media for these arrays. Optical properties and microstructures were examined using angle-resolved reflection spectra measurements. The Bragg diffraction peaks of the colloidal crystalline arrays shifted with changing of concentration or hydrocarbon number of alcohol. With an increase in concentration or hydrocarbon number of alcohol, the effective refractive index of the dispersion increased and the interplanar spacing of the colloidal crystalline array decreased. The increase in effective refractive index was caused by an increase in the refractive index of the mixed medium with the change in solvent. The decrease in interplanar spacing of the array was caused by decreased electrostatic repulsions between the silica spheres with decreasing dielectric constant. The current work suggests new possibilities for the control of optical properties and microstructures of colloidal crystalline arrays.     

An improved convective self-assembly method for the fabrication of binary colloidal crystals and inverse structures

We report an improved convective self-assembly method for the fabrication of highly ordered, crack-free binary colloidal crystals (BCCs) and the associated inverse structures in large domains at length scales of several centimeters. With this method, BCCs can be fabricated in a non-close packed pattern and binary inverse opal films can be obtained over a centimeter scale. The presence of tetraethyl orthosilicate (TEOS) sol in the self-assembly system was found to play a significant role in the resultant structures. The pseudostop band positions are adjustable via varying the number ratio of small to large polystyrene (PS) spheres. At a given TEOS-to-PS ratio, the binary inverse opal film thickness was controllable by varying the colloidal volume fraction with an upper thickness threshold (>16 layers).     

Defects of colloidal crystals

Reasons for the appearance of defects of various types in crystalline colloidal structures formed during the self-organization of the ensembles of spherical nanoparticles are analyzed using lyosols of metal nanoparticles stabilized with polymers as examples. Quantitative characteristic of the degree of imperfection of colloidal crystals is proposed and the procedures for the minimization of the degree of imperfection are discussed. Order-disorder phase transitions of colloidal crystals are studied.     

Effects of compression and shearing on the microstructure of polymer-immobilized non-close-packed colloidal crystalline arrays

We have examined the changes in the optical properties and microstructure of polymer-immobilized non-close-packed colloidal crystalline arrays with compression and shearing stress. The optical properties and microstructures of the arrays were measured by angle-resolved reflection spectroscopy. The spectra indicate an increase in the refractive index and a decrease in the interplane spacing with compression, however, indicating an increase in the interplane spacing with shearing stress. These results show that compression decreases the interplane spacing without moving the inner-plane position, while shearing stress increases the interplane spacing by moving the position of the spheres in the same plane.     

Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.