Particle and substrate charge effects on colloidal self-assembly in a sessile drop

By direct video monitoring of dynamic colloidal self-assembly during solvent evaporation in a sessile drop, we investigated the effect of surface charge on the ordering of colloidal spheres. The in situ observations revealed that the interaction between charged colloidal spheres and substrates affects the mobility of colloidal spheres during convective self-assembly, playing an important role in the colloidal crystal growth process. Both ordered and disordered growth was observed depending on different chemical conditions mediated by surface charge and surfactant additions to the sessile drop system. These different self-assembly behaviors were explained by the Coulombic and hydrophobic interactions between surface-charged colloidal spheres and substrates.     

Colloidal crystals of core-shell type spheres with poly(styrene) core and poly(ethylene oxide) shell

Elastic modulus and crystal growth kinetics have been studied for colloidal crystals of core–shell type colloidal spheres (diameter = 160–200 nm) in aqueous suspension. Crystallization properties of three kinds of spheres, which have poly(styrene) core and poly(ethylene oxide) shell with different oxyethylene chain length (n = 50, 80 and 150), were examined by reflection spectroscopy. The suspensions were deionized exhaustively for more than 1 year using mixed bed of ion-exchange resins. The rigidities of the crystals range from 0.11 to 120 Pa and from 0.56 to 76 Pa for the spheres of n = 50 and 80, respectively, and increase sharply as the sphere volume fraction increase. The g factor, parameter for crystal stability, range from 0.029 to 0.13 and from 0.040 to 0.11 for the spheres of n = 50 and 80, respectively. These g values indicate the formation of stable crystals, and the values were decreased as the sphere volume fraction increased. Two components of crystal growth rate coefficients, fast and slow, were observed in the order from 10⁻³ to 10¹ s⁻¹. This is due to the secondary process in the colloidal crystallization mechanism, corresponding to reorientation from metastable crystals formed in the primary process and/or Ostwald-ripening process. There are no distinct differences in the structural, kinetic and elastic properties among the colloidal crystals of the different core–shell size spheres, nor difference between those of core–shell spheres and silica or poly(styrene) spheres. The results are very reasonably interpreted by the fact that colloidal crystals are formed in a closed container owing to long-range repulsive forces and the Brownian movement of colloidal spheres surrounded by extended electrical double layers, and their formation is not influenced by the rigidity and internal structure of the spheres.     

Effects of liquid bridge between colloidal spheres and evaporation temperature on fabrication of colloidal multilayers

For the application of colloidal crystal films as "photonic band gap" materials, their domain size and thickness are significant. The substrate withdrawing speed, the colloidal suspension volume fraction, and the colloidal suspension temperature have been studied for the domain size and thickness controls of colloidal crystals in this study. Stable dispersions of monodispersed polystyrene spheres with a diameter of 245 nm were synthesized according to a general emulsion polymerization for colloidal crystal films. By experimental results and the theoretical relationship between the number of layers and other parameters, we could know that the water bridge between colloidal spheres (which is formed by capillary force) influences the number of colloidal crystal layers significantly.     

Crystallization of colloidal crystal on hydrogel surface

A dip-coating method to fabricate wet and dry type of colloidal crystal films was developed. The wet type of colloidal crystal film was fabricated by lifting an agarose-hydrogel-coated substrate out of an aqueous suspension containing monodisperse polymer spheres and the dry type of colloidal crystal film was derived by following desiccation of the wet film. Monodisperse spheres formed ordered structures in the both type of the films, which contributed sharp reflection peaks. Brilliant colors were observed when the reflection peaks fell in the visible region. Formation mechanism of the colloidal crystal and their optical properties were discussed.     

Colloidal crystals of core–shell-type spheres in deionized aqueous suspension

The structure, crystal growth kinetics and rigidity of colloidal crystals of core–shell-type latex spheres (diameters 280–330 nm) with differences in shell rigidity have been studied in aqueous suspension, mainly by reflection spectroscopy. The suspensions were deionized exhaustively for more than 2 years using mixed-bed ion-exchange resins. The five kinds of core–shell spheres examined form colloidal crystals, where the critical sphere concentrations, _c, of crystallization (or melting) are high and range from 0.01 to 0.06 in volume fraction. Nearest-neighbor intersphere distances in the crystal lattice agree satisfactorily with values calculated from the sphere diameter and concentration. The crystal growth rates are between 0.1 and 0.3 s^–1 and decrease slightly as the sphere concentration increases, indicating that the crystal growth rates are from the secondary process in the colloidal crystallization mechanism, corresponding to reorientation from metastable crystals formed in the primary process and/or Ostwald-ripening process. The rigidities of the crystals range from 2 to 200 Pa, and increase sharply as the sphere concentration increases. The g factor, the parameter for crystal stability, is around 0.02 irrespective of the sphere concentration and/or the kind of core–shell sphere. There are no distinct differences in the structural, kinetic and elastic properties among the colloidal crystals of the different core–shell-type spheres, showing that the internal sphere structure does not affect the properties of the colloidal crystals. The results show that colloidal crystals form in a closed container owing to long-range repulsive forces and the Brownian movement of colloidal spheres surrounded by extended electrical double layers and that their formation is not influenced by the rigidity and internal structure of the spheres.     

Inward-growing self-assembly of colloidal crystal films on horizontal substrates

Colloidal crystal films have been fabricated on solid substrates with a horizontal deposition method. Scanning electron microscope images showed that the colloidal crystal films exhibit ordered face-centered cubic structures in large domains. Optical measurements demonstrated the presence of photonic band gap along the crystallographic [111] direction. The fabrication method described in this paper allows one to rapidly fabricate colloidal crystal films of different thicknesses, which can be controlled by varying colloidal suspension concentration or volume. In addition, the method also works well for growing colloidal crystal films on a hydrophilic solid substrate with a rough surface. Furthermore, the fabrication of colloidal crystal heterostructures has been demonstrated. An inward-growing mechanism responsible for self-assembly of colloidal spheres on horizontal substrates has been proposed to interpret the observed experimental results.     

Thermo-sensitive colloidal crystals of silica spheres in the presence of large spheres with poly (N-isopropyl acrylamide) shells

Thermo-sensitive colloidal crystals are prepared simply by mixing colloidal silica spheres and large thermo-sensitive gel spheres. The thermo-reversible change in the lattice spacing of colloidal crystals of monodisperse silica spheres (CS82, 103 nm in diameter) depends on the size of the admixed temperature-sensitive gel spheres. For spheres with sizes less and greater than that of the silica spheres, the lattice spacing upon temperature increase above the lower critical solution temperature of poly(N-isopropyl acrylamide) decreases (cf. Okubo et al. Langmuir 18:6783, 2002) and increases, respectively. A mechanism, which is able to explain these experimental findings, is proposed. Moreover, crystal growth rates and the rigidities of the thermo-sensitive colloidal crystals are studied.     

Reversible control of light reflection of a colloidal crystal film fabricated from monodisperse mesoporous silica spheres

In this letter, we report a novel method for controlling the light reflection of a colloidal crystal. Highly monodisperse mesoporous silica spheres have been successfully organized into a hexagonally close-packed colloidal crystal film. Just by introducing water vapor into the fabricated colloidal film, the structural color and reflection spectra were changed dramatically because of water vapor adsorption occurring in the mesoporous channels. This phenomenon can be observed reversibly over five cycles. We are convinced that this is the first report on controlling the light reflection of a colloidal crystal film dynamically by taking advantage of adsorption properties inherent to mesoporous silica spheres.     

Fabrication of spherical colloidal crystals using electrospray

We demonstrated the use of electrohydrodynamic atomization to prepare uniform-sized emulsion droplets in which equal spheres of silica or polystyrene were dispersed. The size of the emulsion droplets was easily controlled by the electric field strength and the flow rate, independently of the diameter of the nozzles. During the evaporation of solvent in the droplets, spherical colloidal crystals were formed by self-assembly of the monodisperse colloidal spheres. The diameter of the spherical colloidal crystals was in the range of 10-40 microm. Depending on the stability of colloidal particles, the morphology of the self-assembled structure was varied. In particular, silica spheres in ethanol droplets were self-assembled into compactly packed silica colloidal crystals in spherical shapes, whereas polystyrene latex spheres in toluene droplets self-assembled into spherical colloidal crystal shells with hollow cores. The silica colloidal assemblies reflected diffraction colors according to the three-dimensionally ordered arrangement of silica spheres.     

Colloidal crystals formed by aqueous suspensions of monodispersed silica, polystyrene, and poly(methyl methacrylate) colloidal spheres

Colloidal crystals consisted of silica, polystyrene, and poly(methyl methacrylate) monodispersed suspensions; deionized sufficiently in water at the same condition; were formed; and their properties were compared changing sphere diameter and volume fraction systematically. The size of these colloidal crystals was maximized at their critical sphere concentration irrespective of their sphere size. The Bragg peak wavelengths of these colloidal crystals were uniquely determined only by the sphere diameter and volume fraction for all kinds of colloidal spheres used in this work. The larger the sphere volume fraction, the larger the crystal growth rates, and there were no significant differences among the colloidal spheres. The rigidity of colloidal crystals increased in proportion to the number density of spheres. Consequently, the crystallization mechanism and properties of colloidal crystals formed by these spheres are not dependent on the kind of spheres, but they are dependent only on the sphere diameter and number density.     

Doped colloidal photonic crystal structure with refractive index chirping to the [111] crystallographic axis

A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the face-centered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.     

Fabrication of hollow colloidal crystal cylinders and their inverted polymeric replicas

We fabricated colloidal crystals on a fiber by a dip-coating method. The self-assembly of monodisperse colloidal particles was affected by the curvature of the fiber (the reciprocal of the fiber radius). As the fiber became smaller in diameter, fewer layers of the colloidal spheres were coated for a given lift-up speed. The hollow colloidal crystal cylinders were used as a template for creating macroporous structure having three-dimensionally interconnected air cavities. Specifically, the polymer precursor was infiltrated into the colloidal crystal template and the macroporous polymer structures were obtained after the selective etching of colloidal particles.     

基于温敏水凝胶的可调胶体晶体制备

基于单分散胶体粒子悬浊液在温敏水凝胶表面可以形成湿润型胶体晶体的现象, 利用温敏水凝胶对水的控释作用制备了温度敏感的可调制胶体晶体. 在室温下利用提拉法在温敏水凝胶聚N-异丙基丙烯酰胺(PNIPAAm)表面制备湿润型胶体晶体膜. 由于胶体粒子的有序排列, 胶体晶体显示出一个尖锐的反射峰. 当温度上升到34 ℃以上时, 由于PNIPAAm水凝胶中的水被释放, 导致胶体晶体中粒子浓度降低, 粒子间距增加; 反射峰发生红移. 这些特性可以通过温度变化进行调制.     

Kinetics of colloidal alloy crystallization of binary mixtures of monodispersed polystyrene and/or colloidal silica spheres having different sizes and densities in microgravity using aircraft

 Crystal growth rates in colloidal alloy crystallization of binary mixtures of monodispersed polystyrene and/or silica spheres having different sizes and densities are studied in microgravity by parabolic flights of an aircraft. The crystal growth rates are obtained by time-resolved reflection spectroscopy with a continuous circulating-type stopped-flow-cell system. The growth rates of alloy crystallization increase substantially in microgravity up to about 1.7 times those in normal gravity, which is in contrast to the retarding microgravity effect on the crystallization of single-component spheres. The disappearance of the segregation effect in microgravity is the main cause for the enhancing effect. The absence of convection of the suspension and the lack of downward sedimentation of colloidal spheres are also important. Received: 19 July 1999/Accepted in revised form: 1 September 1999     

G1 dendrimers-mediated evolution of silver nanostructures from nanoparticles to solid spheres

With the control of G1 poly(amidoamine) (PAMAM), an evolutionary course of stable colloidal silver from discrete nanoparticles to solid spheres through ultraviolet irradiation reduction of silver nitrate solutions was observed by transmission electron microscopy (TEM). The morphologies of the products depend on the Ag+ concentration. A mechanism of globular assembly was put forward to interpret the evolution of the nanostructures. Powder X-ray diffraction (XRD), electron diffraction (ED) patterns, and X-ray photoelectron spectroscopy (XPS) indicate the presence of cubic symmetry silver. XPS and Fourier transform infrared (FT-IR) spectroscopy confirm that dendrimers have participated in the stabilization and control of Ag nanostructures. In the UV-vis spectra, the intense surface plasmons are centered at 425 and 430 nm corresponding to the shapes of dots and solid spheres, respectively. The solid spheres exhibit excellent catalytic efficiency on the reduction of 2,7-dicholoroflurescein (DCF).     

Metal patterning on silicon surface by site-selective electroless deposition through colloidal crystal templating

Site-selective Cu deposition on a Si substrate was achieved by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. Uniformly sized nano/microstructures were produced on the substrate using a monolayer coating of colloidal spheres instead of a conventional resist. The Cu patterns obtained were of two different types: networklike honeycomb and isolated-island patterns with a minimum period of 200 nm. Each ordered pattern with the desired intervals was composed of clusters of Cu nanoparticles with a size range of 50-100 nm. By the present method, it is possible to control the periodicity of metal arrays by changing the diameter of the colloidal spheres used as an initial mask and to adjust the shape of the metal patterns by changing the mask structure for electroless plating.     

胶体晶体模板法制备三维有序大孔复合氧化物*

胶体晶体模板法是制备三维有序大孔（3DOM）复合氧化物材料的有效方法。制备过程一般包括3个步骤：首先,将单分散微球堆积成三维有序排列的胶体晶体;其次,将液态前驱体填充到胶体晶体的间隙,并在原位转化为固体骨架;最后,将微球去除,在原来微球间的空隙位置得到固体骨架,原来微球占据的位置则成为相互连接的孔穴。其中,胶体晶体模板的组装、前驱体的填充以及模板的去除都是制备3DOM复合氧化物的关键影响因素。本文针对这几个控制因素对胶体晶体模板法制备3DOM复合氧化物的影响进行了概述,并对孔结构的表征以及材料在催化和电极材料等方面的应用作了简单介绍。     

Opal and inverse opal photonic crystals: Fabrication and characterization

Three-dimensional photonic crystals made of close-packed polymethylmethacrylate (PMMA) spheres or air spheres in silica, titania and ceria matrices have been fabricated and characterized using SEM, XRD, Raman spectroscopy and UV–Vis transmittance measurements. The PMMA colloidal crystals (opals) were grown by self-assembly from aqueous suspensions of monodisperse PMMA spheres with diameters between 280 and 415 nm. SEM confirmed the PMMA spheres crystallized uniformly in a face-centred cubic (fcc) array, and UV–Vis measurements show that the colloidal crystals possess pseudo photonic band gaps in the visible and near-IR regions. Inverse opals were prepared by depositing silica (SiO₂), titania (TiO₂) or ceria (CeO₂) in the voids of the PMMA colloidal crystals using sol-gel procedures, then calcining the resulting structure at 550 °C to remove the polymer template. The resulting macroporous materials showed fcc ordering of air spheres separated by thin frameworks of amorphous silica, nanocrystalline titania or nanocrystalline ceria particles, respectively. Optical measurements confirmed the photonic nature of the inverse opal arrays. UV–Vis data collected for the opals and inverse opals obeyed a modified Bragg’s law expression that considers both diffraction and refraction of light by the photonic crystal architectures. The versatility of the colloidal crystal template approach for the fabrication of macroporous oxide structures is demonstrated.     

SiO2胶体晶体垂直沉积自组装方法的改进

采用垂直沉积技术及相应的改进方法,使用化学合成的400 nm单分散二氧化硅微球自组装制备了胶体晶体薄膜。通过扫描电镜与分光光度计对样品的微观结构与透过光谱进行了表征,并对比研究了不同的垂直沉积方法对胶体晶体的影响。结果表明,通过温度与流量控制两种改进手段,均能制备具有六方密堆结构周期排列的胶体晶体薄膜。在垂直沉积过程中适当的升高温度有利于降低胶体粒子的用量,而通过流量控制的垂直沉积技术则可以有效缩短自组装时间。通过调节蠕动泵改变液面与基板的相对运动速度,或者调控温度改变胶体溶液的蒸发速率,可在材料表面形成单层或多层的胶体晶体薄膜。改进的垂直沉积技术将有望应用于快速沉积大面积、高质量的胶体晶体材料。     

Fabrication of BaTiO3 Inverse Opal Photonic Crystal

The colloidal crystal template or opal with a closed-packed face centered cubic (fcc) lattice, was prepared from monodisperse polystyrene (PS) spheres by gravity sedimentation. The template was used for the generation of photonic crystal. The template provided void space for infiltration of liquid precursor composed of titanium butyloxide, barium acetate, ethanol, and acetic acid. The opal composite was hydrolyzed, dried, sintered by heating for completely removing PS spheres to form BaTiO3 photonic crystals with inverse opal structure. The PS spheres were replaced by air spheres, which interconnected each other through the windows on the BaTiO3 wall.So both the BaTiO3 wall and air void constitute continuous phases.