Dry etching of colloidal crystal films

Two types of non-close-packed colloidal crystal films were prepared by etching the films made of polystyrene nanospheres using a hyperthermal neutral beam of oxygen gas. Etching without sintering above glass transition temperature of the polymer particles resulted in the non-close-packed structure of the nanospheres, in which polystyrene nanospheres in different lattice planes touched each other due to the reduction in the size of the nanospheres that occurred during the etching process. In contrast, a different non-close-packed structure with inter-connecting networks between etched nanospheres was generated by annealing of the colloidal crystal and a subsequent etching process. The photonic bandgap could be tuned during this dry etching of colloidal photonic crystals. This connected open structure could be used as a template for a silica inverse opal by chemical vapor deposition. An alternative dry etching process, reactive ion etching, mainly affected the morphology of particles near the top surface, and only a slight change in the stop band position of the colloidal crystal film was observed.     

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.     

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.     

Fabrication of large scale two-dimensional colloidal crystal of polystyrene particles by an interfacial self-ordering process

Monolayer films of hexagonal close-packed polystyrene (PS) spheres were formed at the air-water interface through a self-ordering process without using Langmuir trough. The contact angle of PS particles on the surface of water was determined by an interfacial swelling method. It was found that the concentration and the nature of surfactant had an obvious influence on the arrangement of PS particles. PS suspension containing Triton X 100 (TX 100) of an appropriate concentration self-assembled into a closely packed monolayer on the surface of water. Sodium dodecyl sulfonate, an anionic surfactant, had a relative weak influence on the arrangement of pre-dried PS particles, in contrast, had an obvious effect on newly synthesized PS particles. Quantitative ultraviolet-visible (UV-vis) absorption spectrometry indicated that about 3% of the added TX 100 was adsorbed on the PS particle surface. Laser diffraction patterns on the monolayer film were used to investigate the lattice orientation. Ultraviolet-visible-near infrared (UV-vis-NIR) spectra of monolayer films of different sized PS particles displayed that the method presented here was universal for preparation of two-dimensional (2D) colloidal crystals.     

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 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.     

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.     

一种制备单分散SiO2空心微球的新方法

在乙醇/氨水介质中, 分别以分散聚合和无皂乳液聚合方法制得的不同粒径聚苯乙烯(PS)微球为模板, 以正硅酸乙酯(TEOS)为前驱体, 通过控制介质中氨水的初始体积, 一步法制得了不同粒径的单分散SiO2空心微球. 整个过程无需添加其它溶剂溶解或高温煅烧的方法来除去模板微球. 对SiO2空心微球进行测试表征, 提出了SiO2空心微球的可能形成机制.     

Three-dimensional colloidal crystals with a well-defined architecture

Monodisperse silica spheres with diameters of 220-1100 nm were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in an alcoholic medium in the presence of water and ammonia. By grafting vinyl or amino groups onto silica surfaces using the coupling agents allyltrimethoxysilane and aminopropyltriethoxysilane, respectively, amphiphilic silica spheres were obtained and could be organized to form a stable Langmuir film at the air-water interface. The controlled transfer of this monolayer of particles onto a solid substrate gave us the ability to build three-dimensional regular crystals with a well-defined thickness and organization. These colloidal crystals diffract light in the UV, the visible, and the near-infrared (NIR) spectral regions, depending on the size of the silica spheres and according to Bragg's law. The depth of the photonic stop band can be tuned by varying the number of deposited layers of particles. By using successive depositions, we could prepare multilayered films with silica spheres of different sizes. The thickness of each slab in the binary crystals can be tuned at the layer level, while the crystalline order of each layer is well preserved.     

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.     

Transferable ordered ni hollow sphere arrays induced by electrodeposition on colloidal monolayer

We report an electrochemical synthesis of two-dimensionally ordered porous Ni arrays based on polystyrene sphere (PS) colloidal monolayer. The morphology can be controlled from bowl-like to hollow sphere-like structure by changing deposition time under a constant current. Importantly, such ordered Ni arrays on a conducting substrate can be transferred integrally to any other desired substrates, especially onto an insulting substrate or curved surface. The magnetic measurements of the two-dimensional hollow sphere array show the coercivity values of 104 Oe for the applied field parallel to the film, and 87 Oe for the applied field perpendicular to the film, which is larger than those of bulk Ni and hollow Ni submicrometer-sized spheres. The formation of hollow sphere arrays is attributed to preferential nucleation on the interstitial sites between PS in the colloidal monolayer and substrate, and growth along PSs' surface. The transferability of the arrays originates from partial contact between the Ni hollow spheres and substrate. Such novel Ni ordered nanostructured arrays with transferability and high magnetic properties should be useful in applications such as data storage, catalysis, and magnetics.     

胶体光子晶体的平面介电常数缺陷

为引入特殊的光学性质,通常需要在三维光子晶体中人为可控地引入缺陷.通过改变局部结构单元的尺寸或介电常数,相应地引入给体或受体掺杂,带来不同的缺陷态.以前文献报道的向胶体光子晶体中引入缺陷,常会因为同时引入尺寸和介电常数掺杂,给掺杂性质的界定带来困难.本文中,我们结合对流自组装法和L-B膜法,在实心二氧化硅微球组成的三维光子晶体内引入尺寸相同的二氧化硅空心球(与实心球相比具有不同折光率)组成的单层平面缺陷,或者在空心球晶体内引入实心球缺陷层,构成实心-空心-实心或空心-实心-空心的三明治结构,在不破坏整体晶格的同时,在三维胶体光子晶体中引入单一的平面介电常数缺陷.     

Facile Fabrication of Large Area Polystyrene Colloidal Crystal Monolayer via Surfactant-free Langmuir-Blodgett Technique

A facile and novel method for the production of a large area of well-ordered polystyrene(PS)colloidal crystal monolayer was established using the surfactant-free Langmuir-Blodgett(LB)technique.The hydrophobic property(film-forming ability)of PS spheres was improved by a thermo-rheology treatment before LB assembly,and a large film was obtained.In contrast to the traditional LB technique,no surfactant was needed in this method,which could eliminate the additional contamination of surfactants in the preparation process and provided the products with versatile applications in nanosphere lithography(NSL)for biosensor,surface plasmon resonance,and surface enhanced Raman spectroscopy.     

胶体晶体自组装排列进展

自组装排列胶体晶体是发展光子晶体等亚微米周期有序结构及新型光电子器件十分重要的环节.高电荷密度单分散胶体球在较弱的离子强度和稀浓度下会自发排列形成紧密堆积的周期性结构(ccp),常常是面心立方(fcc),科学家们以此为基础发展了多种结晶化胶体粒子的方法,包括重力场沉积、电泳沉积、胶体外延技术、垂直沉积、流通池、物理束缚排列及其他的许多方法.目前排列的胶体粒子基本为球形,材料也多为SiO2、PS、PMMA,此外一些复合粒子,主要为核壳粒子的排列这里也稍作介绍,这些方法及其变通的使用可以形成类蛋白石及反蛋白石结构,最终实现光子带隙及其它多种用途。     

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.     

Colloidal crystal microarrays and two-dimensional superstructures: a versatile approach for patterned surface assembly

A simple, fast, and robust approach to colloidal assembly on patterned surfaces was developed. The approach involves the rapid settling and dewetting of suspensions of spherical colloids on lithographically templated surfaces. Using this method, we can quickly and easily fabricate close-packed colloidal crystal microarrays of both silica and polystyrene spheres that range in size from 500 nm to 4.5 microm. The microarrays tend to induce the formation of monolayer colloidal crystals, which can be interconnected and removed from the templates as free-standing colloidal crystal slabs. The same approach can also be used to assemble two-dimensional colloidal crystal superlattices that can adopt a variety of structures. Graphite, kagome, body-centered cubic, open hexagonal, tetragonal, and linear chain structures can all be quickly accessed by adjusting the ratio of the sphere diameter to the template diameter.     

Colloidal interactions between asphaltene surfaces in aqueous solutions

Asphaltene at oil/water interfaces plays a dominant role in the recovery of crude oil. In this study, asphaltene monolayer films were deposited on hydrophobic silicon wafers and silica spheres from oil-water interfaces using a Langmuir interfacial trough. The morphology of the deposited asphaltene films was characterized with an atomic force microscope (AFM). The colloidal forces between the prepared asphaltene films in aqueous solutions were measured with AFM to shed light on the stabilization of water or oil droplets coated with asphaltene films. Factors such as solution pH, KCl concentration, calcium addition, and temperature all showed a strong impact on colloidal forces between the prepared asphaltene films. The findings provided a better understanding of asphaltene interfacial films at an oil/water interface in stabilizing bitumen-in-water and water-in-bitumen emulsions.     

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.     

Self-assembly of spherical colloidal silica along the [100] direction of the FCC lattice and geometric control of crystallite formation

Opalescent starburst patterns were formed on a convex surface by sedimenting colloidal silica spheres with a low silica volume fraction. The opalescent starburst patterns were also formed by the radial expansion of the meniscus when sedimented samples were slowly dried. The dependence of starburst formation on the rate of sedimentation and drying was experimentally investigated. Self-assembled opals fabricated from colloidal silica spheres grew vertically along the [100] direction of the face-centered cubic lattice. Elongated single-crystal crystallites nearly 1 cm wide and a few centimeters long formed in the curve at the bottom of an ordinary laboratory beaker from sedimenting 200- and 290-nm colloidal silica spheres. The elongated crystallites appear to be the result of an increased sedimentation rate, which creates favorable growth conditions. This is caused by the presence of the curve in the beaker and the spheres' inability to expand into the interior of the beaker before being sedimented. A narrowing of crystallites is shown to result from the competition between neighboring crystallites and large crystallites are shown to result from a lack of competition during their formation.     

Sedimentation and drying dissipative structures of colloidal silica (1.2 μm in diameter) suspensions in a glass dish and a polystyrene dish

Sedimentation and drying dissipative structural patterns formed in the course of drying colloidal silica spheres (1.2 μm in diameter) in aqueous suspension have been studied in a glass dish and a polystyrene dish. The broad ring patterns are formed within a short time in suspension state by the convection flow of water and colloidal spheres. The broad ring patterns are not formed when a dish is covered with a cap, which demonstrates the important role of the convectional flow of silica spheres and water accompanied with the evaporation of water on the air-suspension interface. The sedimentary spheres always move by the convectional flow of water, and the broad ring patterns became sharp with time. Broad ring and microscopic fine structures are formed in the solidification processes on the bases of the convectional and sedimentation patterns. Drying patterns of the colloidal suspensions containing sodium chloride are star-like ones, which strongly supports the synchronous cooperative interactions between the salt and colloidal spheres.