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.     

非紧密堆积型光子晶体凝胶的胀缩行为

制备了Bragg衍射峰波长位置在700～800 nm之间的非紧密堆积型光子晶体凝胶(NCPPCGs). 通过自然干燥失水, 复水浸泡, 不同浓度的乙醇溶液浸泡和不同pH缓冲溶液浸泡等处理, 研究了NCPPCGs的溶胀、收缩(胀缩)行为和Bragg衍射峰迁移的关系; 在凝胶自然干燥失水过程中, Bragg衍射峰蓝移, 复水浸泡后, Bragg衍射峰快速红移至干燥前的位置; 凝胶经不同浓度的乙醇溶液浸泡后, 凝胶体积改变, 并且乙醇浓度升高时, Bragg衍射峰蓝移; 经水解处理的NCPPCGs具有pH 响应特性, 当其浸泡在pH为2.2～9.6的缓冲溶液中时, 随pH值的增大Bragg衍射峰红移, pH为9.6～10.6时, Bragg衍射峰蓝移. 外界因素导致NCPPCGs的Bragg衍射峰位置的迁移覆盖整个可见光区域.     

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.     

Optical response of mesoporous synthetic opals to the adsorption of chemical species

We have demonstrated the fabrication of a colloidal crystalline array (synthetic opal) from monodispersed mesoporous silica spheres (MMSS) and the control of its optical response simply by changing the amount of benzene vapor adsorbed into the pores of MMSS. It was revealed that the refractive index of the colloidal crystal of MMSS showed an 11.7% increase by taking advantage of benzene adsorption, and thereby, the structural color changed reversibly. We also conducted the same measurement on silica spheres without mesopores and observed no change in the refractive index or the structural color. This optical response gives rise to the possibility of using MMSS colloidal crystals not only for controlling light reflection but also as sensing devices based on color change due to vapor adsorption. We have also incorporated an organic dye, the porphyrin derivative alpha,beta,chi,delta,-tetrakis(1-methylpyridinium-4-yl)porphyrin rho-toluenesulfonate (TMPyP), into the pores of MMSS. By adopting an electrophoretic deposition process in ethanol, periodic arrays fabricated from TMPyP-MMSS conjugates with absolute zeta-potentials near zero were obtained. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths due to an increase in the refractive index with increasing amounts of TMPyP adsorbed in the pores. The current work demonstrates the new possibility of creating colloidal crystals from MMSS with mesopores filled with various kinds of adsorbates to control the optical response effectively.     

Close-packed colloidal crystalline arrays composed of polystyrene latex coated with titania nanosheets

We have demonstrated that polystyrene latex coated with titania nanosheets can be fabricated into a close-packed colloidal crystalline array, and that these coated colloidal spheres can be used to control the peak position of optical stop bands through the coating. The titania-nanosheets-coated polystyrene latex was prepared by the layer-by-layer (LBL) assembly coating process, involving alternating lamination of cationic polyelectrolytes and anionic titania nanosheets on monodisperse polystyrene latex particles. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths with the coating of titania nanosheets. This red shift was caused by an increase in refractive index upon coating, as revealed by angle-resolved reflection spectra measurements. The current work suggests new possibilities for the creation of advanced colloidal crystals having tunable optical properties from tailored colloidal spheres.     

Tuning the effective width of the optical stop band in colloidal photonic crystals

The optical stop band in colloidal crystals is characterized by the central frequency and bandwidth. Although the former is known to be highly tunable by changing the lattice constant, the latter is basically determined by the refractive index contrast between the particles and the background medium that is intrinsic to the materials. In this study, we show that the effective bandwidth in gelled colloidal crystals can also be tuned by controlling the fabrication conditions. Single-domain gelled colloidal crystals were prepared by photopolymerization under various photoirradiation conditions. It was observed that the width of the stop band in the transmission or reflectance spectrum could be expanded by simply adjusting the irradiation time.     

Close-packed colloidal crystalline arrays composed of silica spheres coated with titania

Titania coated monodisperse silica spheres have been synthesized and fabricated as a close-packed colloidal crystalline array. We have demonstrated that the coated colloidal sphere can be used to control the peak position of the optical stop band through variation of the coating thickness. The titania coated silica spheres were prepared by the layer-by-layer assembly coating process, which reciprocally laminates the cationic polyelectrolyte and the anionic titania nanosheets on a monodisperse silica spheres, and were sintered to change the titania nanosheets to anatase. The Bragg diffraction peak of the colloidal crystalline array shifted to the long wavelength region with an increase of thickness of the titania layer. Angle-resolved reflection spectra measurements clarified that the red shift was caused by increasing of the refractive index with increase of the thickness of the layer. The current work suggests new possibilities for the creation of advanced colloidal crystalline arrays with tunable optical properties from tailored colloidal spheres.     

Continuous modification of upconversion luminescence of fluorescent dye in the crystalline colloidal arrays

The crystalline colloidal arrays with controllable photonic bandgaps were prepared by the change of volume fraction of the polystyrene microspheres. Upconversion emission property of fluorescent dye has investigated in crystalline colloidal array, and continuous modification of the upconversion emission of fluorescent dye was observed. A significant suppression of upconversion emission of dye in the range of the photonic bandgap as well as enhancement at the bandgap edge was obtained in the crystalline colloidal arrays. In addition, upconversion emission of dye was also enhanced when the excited light overlapped with the long or short bandgap edge of the crystalline colloidal arrays, which is due to slow photons effect near the edges of a photonic bandgap. The continuous modification and enhancement of upconversion emission may be important for the development of low-threshold upconversion lasers and displays.     

Vapor detection enabled by self-assembled colloidal photonic crystals

Here we report the sensitive and reversible detection of vapors by using self-assembled colloidal photonic crystals. The condensation of various vapors in the interstitials of silica colloidal photonic crystals leads to red-shift and amplitude reduction of optical stop bands. A linear relationship between wavelength shift and vapor partial pressure has been observed for a variety of vapors including ethanol, water, and toluene. Importantly, the sensitivity of colloidal photonic crystal-based vapor detectors can be improved by nearly two orders of magnitude by using a new full-peak analysis technique that takes advantage of the manifest amplitude reduction of optical stop bands during vapor condensation. Optical simulation based on a scalar-wave approximation model shows that the predicted optical responses during vapor condensation in colloidal photonic crystals agree well with experimental results. The condensation of vapors between submicrometer-scale microspheres, a topic that has received little examination, has also been investigated by both experiments and theoretical calculations. Predictions based on a modified Kelvin equation match with the experiments for a wide range of vapor partial pressures.     

Fabrication of photonic crystals with nigrosine-doped poly(MMA-co-DVB-co-MAA) particles

A convenient approach was developed to fabricate monodisperse nigrosine-doped poly(methyl methacrylate-co-divinylbenzene-co-methacrylic acid) nanoparticles with different cross-linkage by soap-free emulsion polymerization at boiling status and swelling process. The dye-doped nanoparticles were used for the fabrication of colloidal crystal films and beads. It was found that nigrosine dye in the nanoparticles can efficiently depress the light scattering inside the colloidal crystal films and eliminate the iridescent effect in the photonic beads. These results make the colloidal crystals useful in photonic paper, bioassay, and so on.     

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.     

胶态磁组装分子印迹光子晶体及其对L-苯丙氨酸的响应性

将胶态磁组装光子晶体与分子印迹技术结合, 通过磁场诱导快速、 可逆地组装得到一种灵敏度高、 选择性强且响应速度快的胶态磁组装分子印迹光子晶体(CMA-MIPCs), 并将其用于L-苯丙氨酸(L-Phe)分子的响应性研究. 结果表明, 细乳液聚合法制得的L-Phe磁性分子印迹纳米粒子(MMIPs)具有规则的球形形貌和明显的核-壳结构, 平均粒径为104.3 nm. CMA-MIPCs对L-Phe分子的识别可直接通过光学信号进行表达, 当L-Phe的浓度从6.0×10^-7 mol/L增加至6.0×10^-4 mol/L时, CMA-MIPCs的衍射色发生从紫色到深黄色的明显变化, 最大衍射峰位置红移181 nm, 响应过程仅需1 min. CMA-MIPCs对L-Phe的结构类似物L-酪氨酸(L-Tyr)和L-色氨酸(L-Trp)均无响应性, 表明CMA-MIPCs具有良好的选择性.     

Photochemical formation of silver-gold (Ag?Au) composite colloids in solutions containing sodium alginate

Photochemical formation of colloidal silver, colloidal gold and silver-gold (Ag-Au) composite colloids under mild conditions has been studied. Irradiation of either aqueous AgCIO₄ or HAuCI₄ solution in the presence of sodium alginate (SA) with 253.7 nm light yielded colloidal silver or gold, whose particle diamter was 10-30 nm or 40-60 nm, respectively. The Ag-Au composite colloids consisting of mixtures of silver and gold domains (particle diameter 30-150 nm) have been prepared and their extinction spectra have been examined on the basis of a conventional Mie theory in combination with an effective medium theory to estimate the optical constants of these colloids. It has been shown that the extinction spectra of the Ag-Au composite colloids are completely different from those of Ag-Au alloy colloids, in that the former have two extinction maxima close to the colloidal extinction bands of pure silver and gold, in contrast to a single extinction maximum of the latter. The importance of natural, high-molecular carboxylic acids such as alginic acid in the photochemical formation of metal colloids and thin films has been stressed.     

High-speed fabrication of patterned colloidal photonic structures in centrifugal microfluidic chips

In this paper, we report a rapid and facile method for fabricating colloidal photonic crystals inside microchannels of radially symmetric microfluidic chips which were made using soft-lithography. As the suspension of monodisperse silica or polystyrene latex spheres was driven to flow through the channels under the action of centrifugal force, the colloidal spheres were quickly assembled into face centered cubic arrangement which had a few photonic stop bands. The soft-microfluidic channels and cells confined the colloidal crystals into designed patterns. The optical reflectance was modulated by the refractive-index mismatch between the colloidal particles and the solvent in the interstices between the particles. Therefore, the present microfluidic chips with built-in colloidal photonic crystals can be used as in-situ optofluidic microsensors for high throughput screening or light filters in integrated adaptive optical devices.     

Preparations and optical properties of ordered arrays of submicron gel particles: interconnected state and trapped state

Two types of thermosensitive opal-structured hydrogel systems, "interconnected" and "trapped" gel particle arrays, were newly developed by extremely simple methods using silica colloidal crystal as a template. Although both systems diffract visible light following Bragg's law combined with Snell's law, the temperature dependences of their optical properties were quite different. The "interconnected" array exhibited a reversible change in the peak values of the reflection spectra, mainly determined by the swelling ratio of the hydrogel, as a function of the water temperature. Since the swelling ratio is dominant over the peak value, we can observe water temperature through the color of the interconnected type of gel membrane. The "trapped" array revealed a reversible change in the peak intensity of the reflection spectra with the change in temperature, whereas no change in the peak position was observed. We can interpret this phenomenon in the following ways. As the rise in temperature causes a decrease in the water content of the NIPA gel particles, the gel particles becomes stickier on the cavity wall of polystyrene PPM. This may induce a disturbance in the ordered array of the gel particles and form many layers of rough surfaces in the inverse opal structure of the PPM. This situation may lead to the stronger diffused reflection of light from the gel particles, resulting in the decrease in peak intensity at higher temperatures.     

基于胆甾相液晶的可调制光子晶体

将胆甾相液晶填充进胶体晶体内部空隙, 通过胆甾相液晶与胶体晶体的耦合, 构建了一种新型可调制液晶光子晶体. 填充于胶体晶体内部的胆甾相液晶织构呈现典型的手性近晶相(S)特征. 由于胆甾相液晶具有特定的选择性反射, 当胶体晶体的带隙处于胆甾相液晶的反射波长范围之内, 则随着温度的改变, 胶体晶体的带隙与胆甾相液晶的带隙同时发生蓝移. 在一定温度条件下, 胆甾相液晶的带隙将与胶体晶体的带隙发生耦合, 实现了光子晶体带隙在单峰与双峰之间的可逆切换.     

Synthesis of Highly Charged, Monodisperse Polystyrene Colloidal Particles for the Fabrication of Photonic Crystals

We have developed a series of emulsion polymerization recipes for the synthesis of highly charged, monodisperse polystyrene colloids of diameters between 100-400 nm. These spherical colloidal particles were crosslinked with divinyl benzene and functionalized with 1-allyloxy-2-hydroxypropane sulfonate. These highly charged, monodisperse colloidal particles readily self-assemble into robust three-dimensionally ordered crystalline colloidal arrays (CCAs). These CCAs operate as photonic crystals that Bragg diffract light in the ultraviolet, visible, and infrared regions of the spectrum. Copyright 2000 Academic Press.     

Photonic band structures of colloidal crystals measured with angle-resolved reflection spectroscopy

We acquired angle- and polarization-resolved reflection spectra from a colloidal crystal made of polystyrene spheres along the two perpendicular directions corresponding to the LU and LW directions in the first Brillouin zone of an fcc lattice. Dispersion relations between the reflection peak positions and the wave vectors of the incident light were obtained from the measured spectra and compared with calculated photonic band structures. For the first stop band region in the spectra, the behavior of the reflection peak due to Bragg diffraction agreed with the calculated band structure and revealed some differences induced by the polarization and crystalline orientations. The spectral features observed in the higher energy regions also revealed these differences. In addition, dispersion relationships between the peak positions and the wave vectors were obtained from the results of fitting each spectrum with several Gaussian curves, compared with the calculated photonic band structures. The relationships obtained for the LU direction almost matched the calculated band structure, while the relationships obtained for the LW direction revealed the features of the mixed band structure calculated for the two perpendicular directions. These results indicate that angle- and polarization-resolved reflection spectroscopy has the potential to experimentally analyze the photonic band structures of actual photonic crystals.     

流动控制沉积法制备PMMA叠层光子晶体薄膜及其光学性能研究

采用流动控制沉积法, 通过调控泵速和聚甲基丙烯酸甲酯(PMMA)胶体微球溶液的浓度, 制备出微球排列高度有序且薄膜紧密附着于基底的高质量光子晶体薄膜. 获得了制备高质量PMMA光子晶体薄膜的组装条件范围, 发现在该条件范围内, 当泵速或胶体微球溶液浓度一定时, PMMA光子晶体薄膜的厚度随胶体微球溶液浓度的增加或泵速的降低而增加. 研究了组装条件对PMMA光子晶体薄膜光学性能的影响, 发现光子禁带位置随光子晶体薄膜厚度增加或减少而红移或蓝移. 在此基础上, 控制组装条件得到了不同尺寸微球堆叠而成的叠层光子晶体薄膜, 并研究了其光学性能的变化规律. 结果显示, 叠层光子晶体薄膜的光子禁带峰为各层叠层光子晶体禁带峰的简单叠加, 且峰强度受光入射角方向影响.     

Endowing Zeolite LTA Superballs with the Ability to Manipulate Light in Multiple Ways

Advances in zeolites research emerging from interdisciplinary efforts have opened new opportunities beyond conventional applications. Colloids drive much current research owing to their distinct collective behaviors, but so far, using zeolites as a colloidal building block to construct ordered superstructures remains unexplored. Herein we show that self-assembly of colloidal zeolite LTA superball (ZAS) by tilted-angle sedimentation forms macroscopic films with micro-mesoporosity and 3D long-range periodicity featuring a photonic band gap (PBG) that is tunable through the superball geometry and responds reversibly to chemical vapors. Remarkably, self-assembly of ZAS at elevated temperature forms 3D chiral photonic crystals that enable negative circular dichroism, selective reflection of right-handed circularly polarized (CP) light and left-handed CP luminescence based on PBG. We present a novel class of functional colloids and zeolite-based photonic crystals with the ability to manipulate light in several ways.