Magnetite 3D colloidal crystals formed in the early solar system 4.6 billion years ago

Three-dimensional colloidal crystals made of ferromagnetic particles, such as magnetite (Fe(3)O(4)), cannot be synthesized in principle because of the strong attractive magnetic interaction. However, we discovered colloidal crystals composed of polyhedral magnetite nanocrystallites of uniform size in the range of a few hundred nanometers in the Tagish Lake meteorite. Those colloidal crystals were formed 4.6 billion years ago and thus are much older than natural colloidal crystals on earth, such as opals, which formed about 100 million years ago. We found that the size of each individual magnetite particle determines its morphology, which in turn plays an important role in deciding the packing structure of the colloidal crystals. We also hypothesize that each particle has a flux-closed magnetic domain structure, which reduces the interparticle magnetic force significantly.     

微凝胶胶体晶体研究进展

Poly(N-isopropylacrylamide)(PNIPAM)微凝胶粒子是一种软的胶体粒子.和单分散的SiO_2、PS、PMMA等硬的胶体粒子一样,单分散的PNIPAM微凝胶粒子也可以自组装成为高度有序的胶体晶体.微凝胶粒子软物质的特性及其对外部刺激的响应性赋予其不同于硬球的组装行为.微凝胶胶体晶体的高度有序结构及其刺激响应性使其在诸多领域有重要用途.本文分别介绍了三维及二维微凝胶胶体晶体组装的研究进展,并对已开发的基于微凝胶胶体晶体的应用进行了总结.     

Evolution of interparticle capillary forces during drying of colloidal crystals

Photonic crystals are periodic structures that have the capability to manipulate the photons in the same way as semiconductors do for electrons. The self-assembly strategy that utilizes colloidal crystals as a template to form photonic crystals has received a great deal of recent research interest because it is simple and cost-effective. Experimental studies and theoretical analysis have speculated that capillary forces play a pivotal role in forming the colloidal crystals during the crystal growth process and that particularly during the drying stage the changing of the magnitude of capillary forces is critical to the resultant microstructure. This paper presents a computational analysis of the changing capillary forces, which may throw light on a refined strategy for controlling colloidal crystal growth.     

带电悬浮粒子胶体晶体的固定化

采用光固化技术, 以丙烯酰胺单体与亚甲基双丙烯酰胺交联剂在紫外光的照射下发生光聚合反应, 嵌入聚苯乙烯胶体晶体, 实现了胶体晶体的固定化. 结合反射光谱和Kossel衍射技术研究对照了固定化前后胶体晶体的变化, 实验结果表明, 通过这种水凝胶固定化的胶体晶体保存了未固定前悬浮液中胶体晶体的结构. 但固定化后的胶体晶体的晶面间距和晶体的尺寸都略微减小. 通过对固定化后的水凝胶长时间的反射光谱观测, 发现固定化后胶体晶体在Milli-Q水中起初会发生溶胀, 经过2-5天溶胀-消溶胀过程达到平衡, 平衡后的水凝胶胶体晶体十分稳定, 可以长时间保持胶体晶体的结构. 因此, 胶体晶体固定化不但极大地提高了悬浮液中胶体晶体的抗剪切能力, 还克服了悬浮液中胶体晶体对离子、外界干扰的敏感性, 扩大了胶体晶体的实际应用价值.     

Nonspherical colloidal crystals fabricated by the thermal pressing of colloidal crystal chips

Nonspherical colloids and their ordered arrays may be more attractive in applications such as photonic crystals than their spherical counterparts because of their lower symmetries, although such structures are difficult to achieve. In this letter, we describe the fabrication and characterization of colloidal crystals constructed from nonspherical polyhedrons. We fabricated such nonspherical colloidal crystals by pressing spherical polymer colloidal crystal chips at a temperature slightly lower than the glass-transition temperature (T(g)) of these polymer colloids. During this process, the polymer microspheres were distinctively transformed into polyhedrons according to their crystal structures, whereas the long-range order of the 3D lattice was essentially preserved. Because a working temperature lower than T(g) effectively prevented the colloidal crystals from fusing into films, the spherical colloidal crystals were transformed greatly under pressure, which lead to obvious change in the optical properties of colloidal crystals. Besides their special symmetry and optical properties, these nonspherical colloidal crystals can be used as templates for 2D or 3D structures of special symmetry, such as 2D nano-networks. We anticipate that this fabrication technique for nonspherical colloidal crystals can also be extended to nonspherical porous materials.     

Growth of columnar hydrogel colloidal crystals in water-organic solvent mixture

A novel emulsion method has been demonstrated to grow columnar hydrogel colloidal crystals by mixing an aqueous suspension of poly-N-isopropylacrylamide-co-allylamine microgels with organic solvent, driven by the coalescence of micelles consisting of organic oil droplets coated by many microgels. This method leads to microgel colloidal crystals of several centimeters growing from the top to the bottom along the gravity direction. Both temperature and polymer concentration play critical roles for the formation of columnar crystals. A phase diagram has been determined, and it can be used as a guide to selectively grow different crystals, including columnar crystals and randomly oriented crystals, and enable the coexistence of columnar crystals and randomly oriented crystals.     

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.     

Microcontact printing of colloidal crystals

Patterned two-dimensional (2D) colloidal crystals have been transferred by a modified mucp technique that was based on the use of polymer film as "glue" to provide an efficient interaction between the microsphere "ink" and substrate. The versatility of this method has been demonstrated by the patterning of colloidal crystal on a nonplanar substrate and heterogeneously structured colloidal crystal film. The table of contents graphic shows an SEM image of the ordered parallel lines of 2D colloidal crystals on a polymer-coated glass tube with a 3.7 mm radius of curvature.     

Colloidal crystals as templates for porous materials

Close-packed colloidal crystals are promising precursors for novel materials, but only after appropriate methods are developed to fix their structure. A wide range of advanced materials has recently been synthesized by replicating the structure of colloidal crystals into durable solid matrices. Such materials with structured pores have promise as photonic crystals, catalysts, and membranes, and in a variety of other applications. This paper reviews the methods used in the formation of these materials and likely future trends in the field.     

Opal and inverse opal fabricated with a flow-controlled vertical deposition method

In this work, an improved vertical deposition method, namely, a flow-controlled vertical deposition (FCVD) method, was used to grow colloidal crystals with large spherical colloids in water solvent and to infiltrate the colloidal crystals. Using the FCVD method, latex spheres as large as 2 microm can be fabricated into colloidal crystals in water. In addition, the method works very well for controlling surface morphologies of silica-infiltrated opals. Furthermore, fabrication of colloidal crystal heterostructures was demonstrated.     

Optical intensity gradient by colloidal photonic crystals with a graded thickness distribution

Gradient colloidal crystals with a thickness gradient were prepared by the vertical deposition technique with vertically graded concentration suspensions. The thickness of the gradient colloidal crystal gradually changes at different positions along the specific gradient direction of the crystal. The thickness gradient was determined by the concentration gradient, depending on the initial colloidal concentration and the settling time. The optical transmission intensity at the dip wavelength can be tuned by changing the thickness of the colloidal crystals. The gradient colloidal crystals lead to a gradient of optical intensity at the dip in transmission light. The gradient of optical intensity at the dip increases as the thickness gradient of the colloidal crystal increases.     

Colloidal crystals from surface-tension-assisted self-assembly: a novel matrix for single-molecule experiments

In this work, we develop a new method of creating colloidal crystals with cavities for the entrapment and long-term observation of single biomolecules. Colloidal crystals are first fabricated using surface-tension-assisted self-assembly. Surface tension helps to reduce the interparticle distance between dispensed colloids. Subsequently, the colloids are used as a matrix in which single fluorescently tagged molecules can be tracked using fluorescence microscopy. This method has a high efficiency of self-assembly for small volumes (4 microL) of colloidal suspensions (polystyrene colloids with diameters of 1000, 500, 200, and 100 nm) at low concentration (1% w/w). The spatial hindrance effect on the diffusion of molecules and their entrapment is discussed on the basis of fluorescence correlation spectroscopy results from the diffusion of molecules with different hydrodynamic radii in the cavities of colloidal crystals formed from micrometer- to nanometer-sized polystyrene spheres. Single horseradish peroxidase molecules turning over fluorescent products are tracked over a few seconds. This shows that colloidal crystals can be used to test the function of single molecules of enzymes and protein under controlled spatial confinement.     

Multiple electrokinetic actuators for feedback control of colloidal crystal size

We report a feedback control method to precisely target the number of colloidal particles in quasi-2D ensembles and their subsequent assembly into crystals in a quadrupole electrode. Our approach relies on tracking the number of particles within a quadrupole electrode, which is used in a real-time feedback control algorithm to dynamically actuate competing electrokinetic transport mechanisms. Particles are removed from the quadrupole using DC-field mediated electrophoretic-electroosmotic transport, while high-frequency AC-field mediated dielectrophoretic transport is used to concentrate and assemble colloidal crystals. Our results show successful control of the size of crystals containing 20 to 250 colloidal particles with less than 10% error. Assembled crystals are characterized by their radius of gyration, crystallinity, and number of edge particles, and demonstrate the expected size-dependent properties. Our findings demonstrate successful ensemble feedback control of the assembly of different sized colloidal crystals using multiple actuators, which has broad implications for control over nano- and micro- scale assembly processes involving colloidal components.     

Three-dimensional colloidal crystal-assisted lithography for two-dimensional patterned arrays

In this paper, we report our recent work on preparing two-dimensional patterned microstructure arrays using three-dimensional colloidal crystals as templates, namely, colloidal crystal-assisted lithography. Two alternative processes are described and involved in colloidal crystal-assisted lithography. One is based upon imprinting the polymer films with three-dimensional silica colloidal crystals, and the other is based upon chemically depositing Ag microstructures on Au substrates covered by polymer colloidal crystals. By varying the experimental conditions in the colloidal crystal-assisted lithography process, we can intentionally control the morphologies of the resulting microstructures. The resultant Ag-coated Au substrates can be used as surface-enhanced Raman scattering substrates, and they would provide an ideal system for the mechanism study of surface-enhanced Raman scattering. We expect that colloidal crystal-assisted lithography will be a versatile approach which can be applied to patterning other materials such as functional molecules, polymers, oxides, and metals.     

Automated preparation method for colloidal crystal arrays of monodisperse and binary colloid mixtures by contact printing with a pintool plotter

Photonic crystals and photonic band gap materials with periodic variation of the dielectric constant in the submicrometer range exhibit unique optical properties such as opalescence, optical stop bands, and photonic band gaps. As such, they represent attractive materials for the active elements in sensor arrays. Colloidal crystals, which are 3D gratings leading to Bragg diffraction, are one potential precursor of such optical materials. They have gained particular interest in many technological areas as a result of their specific properties and ease of fabrication. Although basic techniques for the preparation of regular patterns of colloidal crystals on structured substrates by self-assembly of mesoscopic particles are known, the efficient fabrication of colloidal crystal arrays by simple contact printing has not yet been reported. In this article, we present a spotting technique used to produce a microarray comprising up to 9600 single addressable sensor fields of colloidal crystal structures with dimensions down to 100 mum on a microfabricated substrate in different formats. Both monodisperse colloidal crystals and binary colloidal crystal systems were prepared by contact printing of polystyrene particles in aqueous suspension. The array morphology was characterized by optical light microscopy and scanning electron microscopy, which revealed regularly ordered crystalline structures for both systems. In the case of binary crystals, the influence of the concentration ratio of the large and small particles in the printing suspension on the obtained crystal structure was investigated. The optical properties of the colloidal crystal arrays were characterized by reflection spectroscopy. To examine the stop bands of the colloidal crystal arrays in a high-throughput fashion, an optical setup based on a CCD camera was realized that allowed the simultaneous readout of all of the reflection spectra of several thousand sensor fields per array in parallel. In agreement with Bragg's relation, the investigated arrays exhibited strong opalescence and stop bands in the expected wavelength range, confirming the successful formation of highly ordered colloidal crystals. Furthermore, a narrow distribution of wavelength-dependent stop bands across the sensor array was achieved, demonstrating the capability of producing highly reproducible crystal spots by the contact printing method with a pintool plotter.     

Slow vertical deposition of colloidal crystals: a Langmuir-Blodgett process?

For an evaporating colloidal suspension in which the evaporation velocity exceeds the sedimentation velocity, particles will accumulate at the solvent-air interface. If neither diffusion nor convection can disperse this accumulation, it is expected to grow into a colloidal multilayer several microns thick. We observe that the thickness of colloidal crystals vertically deposited from 1 mum diameter polystyrene latex suspensions of 0.002 < or = phi < or = 0.008 increases linearly with distance in the growth direction and that these thickness profiles are consistent with their growth from a horizontal colloidal layer accumulated beneath the solvent-air interface. We describe a means for performing vertical deposition at growth rates slower than the evaporation rate by adding solvent to the bottom of the colloidal suspension and observe that halving the growth rate of vertical deposition increases both the thickness and the reflectivity of the resulting colloidal crystals, effects indistinguishable from those of doubling the concentration of the colloidal suspension, data also consistent with the colloidal crystals' growth from a horizontal layer of particles beneath the interface. If sufficiently little reorganization is involved as particles move from this horizontal layer to the vertically deposited colloidal crystal, slow vertical deposition of polymer microspheres might be thought of as the Langmuir-Blodgett transfer of a horizontal colloidal crystal onto a vertical substrate. Colloidal crystals deposited using both high concentration and slowed growth can have peak IR reflectance in excess of 80%, exceeding most published values. These observations provide a conceptual framework for engineering vertically deposited colloidal crystals that combine thickness with good optical performance.     

Generalized fabrication of two-dimensional non-close-packed colloidal crystals

In this paper we report a generalized templating approach for fabricating wafer-scale, two-dimensional, non-close-packed (ncp) colloidal crystals. Polymer nanocomposites consisting of monolayer ncp colloidal crystals prepared by a spin-coating process are used as sacrificial templates. After removal of the colloidal silica templates, the voids in the polymer matrix are infiltrated with other materials. By plasma-etching the polymer matrix, wafer-scale ncp colloidal crystals from a variety of functional materials can be made. This technique is scalable and compatible with standard microfabrication. Two-component colloidal arrays with complex micropatterns can also be fabricated by combining microfabrication with this templating approach. Normal-incidence reflectivity spectra of replicated titania ncp arrays agree well with theoretical prediction using Scalar Wave Approximation.     

Nanoparticle-mediated epitaxial assembly of colloidal crystals on patterned substrates

We have studied the assembly of 3-D colloidal crystals from binary mixtures of colloidal microspheres and highly charged nanoparticles on flat and epitaxially patterned substrates created by focused ion beam milling. The microspheres were settled onto these substrates from dilute binary mixtures. Laser scanning confocal microscopy was used to directly observe microsphere structural evolution during sedimentation, nanoparticle gelation, and subsequent drying. After microsphere settling, the nanoparticle solution surrounding the colloidal crystal was gelled in situ by introducing ammonia vapor, which increased the pH and enabled drying with minimal microsphere rearrangement. By infilling the dried colloidal crystals with an index-matched fluorescent dye solution, we generated full 3-D reconstructions of their structure including defects as a function of initial suspension composition and pitch of the patterned features. Through proper control over these important parameters, 3-D colloidal crystals were created with low defect densities suitable for use as templates for photonic crystals and photonic band gap materials.     

Video-tape observation of the crystal growth and morphology of colloidal single crystals

Crystal growth and morphology of colloidal crystals of silica spheres (81.2 nm in diameter) are observed directly on a video-tape camera. Crystal growth from the round-shaped, small single crystals to the angular-shaped ones is clear. It is observable that the single crystals are packed densely and separated from each other with the grain boundaries. The morphology of colloidal crystals is quite similar to that of typical crystals such as metals, proteins, and ice.     

Microwave-assisted self-organization of colloidal particles in confining aqueous droplets

Monodisperse aqueous emulsion droplets encapsulating colloidal particles were produced in the oil phase, and controlled microwave irradiation of the aqueous drop phase created spherical colloidal crystals by so-called evaporation-induced self-organization of the colloidal particles. Unlike usual colloidal crystals, colloidal crystals in spherical symmetry (or photonic balls) possessed photonic band gaps for the normal incident light independent of the position all over the spherical surface. While the consolidation of colloidal particles in emulsion droplets in an oven took several hours, the present microwave-assisted evaporation could reduce the time for complete evaporation to a few tens of minutes. Under the microwave irradiation, the aqueous phase in emulsions was superheated selectively and the evaporation rate of water could be controlled easily by adjusting the microwave intensity. The result showed that the packing quality of colloidal crystals obtained by the microwave-assisted self-organization was good enough to show photonic band gap characteristics. The reflectance of our photonic balls responded precisely to any change in physical properties including the size of colloidal particles, refractive index mismatch, and angle of the incident beam. In particular, for polymeric particles, the photonic band gap could be tuned by the intensity of microwave irradiation, and the reflection color was red-shifted with stronger microwave irradiation. Finally, for better photonic band gap properties, inverted photonic balls were prepared by using the spherical colloidal crystals as sacrificial templates.