Self-assembled Colloidal Crystals Research Articles

Photoelectrochemical Behavior of Hierarchically Structured Si/WO3 Core–Shell Tandem Photoanodes

WO3 thin films have been deposited in a hierarchically structured core-shell morphology, with the cores consisting of an array of Si microwires and the shells consisting of a controlled morphology WO3 layer. Porosity was introduced into the WO3 outer shell by using a self-assembled microsphere colloidal crystal as a mask during the deposition of the WO3 shell. Compared to conformal, unstructured WO3 shells on Si microwires, the hierarchically structured core-shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected. The use of structured substrates also improved the growth rate of microsphere-based colloidal crystals and suggests strategies for the use of colloidal materials in large-scale applications.

Colloidal Transfer Printing

Many fields of research have adopted self-assembly of colloidal spheres as an easy and reliable method to produce macroscopic structures with nanoscale periodicity. The field of soft lithography in particular has used colloidal self-assembly to fabricate lithographic masks and templates. We developed a colloidal lithography method that uses the colloidal assembly directly to produce submicrometer topographic and chemical surface patterns. The method does not require any specialized equipment, making it particularly useful in biological and chemical laboratories without lithography expertise. The technique involves the curing and solvent removal of a self-assembled colloidal crystal from an inorganic surface. The result is a triangular array of polymer features with submicrometer periodicity that covers square centimeters of surface area. The feature size and spacing is easily controlled, and the features serve as reactive sites for biomolecule immobilization.

Statistical characterization of self-assembled colloidal crystals by single-step vertical deposition

We have statistically characterized the self-assembly of multi-layer polystyrene colloidal crystals, using the technique of vertical deposition, with parameters chosen to produce thick layers of self-assembled crystals in one deposition step. The size distribution of domains produced with this technique was seen to follow a log-normal distribution, hinting that aggregation or fragmentation phenomena play a role. In addition, using a lithographically directed self-assembly method, we have shown that the size of multi-layer, continuous crack-free domains in lithographically defined areas can be many times larger than in the surrounding areas. In a single deposition step, we have produced continuous colloidal crystal films of 260nm diameter polystyrene spheres approximately 30–40 layers thick, with a controllable lateral size of 80–100μm without lithography, and as high as 250μm with the lithographic template. This method allows us to suppress the domain size fluctuations and produces mesoscopically thick colloidal crystals of selected size at a selected location.

Preparation of a Three-Dimensional Ordered Macroporous Carbon Nanotube/Polypyrrole Composite for Supercapacitors and Diffusion Modeling

Three-dimensional ordered macroporous (3DOM) carbon nanotube (CNT)/polypyrrole (PPy) composite electrodes for supercapacitor application were prepared through cyclic voltammetric copolymerization from a solution containing both acid-treated CNTs and pyrrole monomers. A self-assembled SiO2 colloidal crystal was used as the sacrificial template. After electrochemical copolymerization, the template was removed, and a 3DOM CNT/PPy composite electrode was obtained. The specific capacitance of the composite reached 427 F g–1 at the scanning rate of 5 mV s–1, and it is calculated that ion diffusion contributed approximately 30% to the specific capacitance of the composite. A mathematical model of mass transport was proposed to evaluate the ion diffusion capability on the surfaces of 3DOM, nanoporous, and planar films. The calculation results showed that the flux (i.e., ion flux per unit length) of 3DOM film was larger than that of planar film, while the flux of nanoporous film was close to that of planar film. The model indicates that 3DOM film is favorable for ion transportation, while nanoporous film does the opposite. The model partially explains the reason why the specific capacitance of the prepared 3DOM CNT/PPy composite is far above the specific capacitance values of other reported CNT/PPy composites, even the nanoporous CNT/PPy composite.

3D Ordered Macroporous Ge/Al and Ge/Si Bilayer Films Made by Electrodeposition from Ionic Liquids

Abstract Three dimensionally ordered macroporous (3DOM) Ge/Al and Ge/Si bilayer films have been made via ordered polystyrene (PS) templates by electrodeposition from ionic liquids (IL), at room temperature. For this purpose, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]Tf2N) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]Tf2N) with aluminium, germanium and silicon chlorides dissolved in were used as electrolytes. Self-assembled PS colloidal crystals were made by the vertical deposition method and used as templates. Germanium was chosen as the first layer, then after washing and drying Al and Si layers were deposited subsequently. SEM results verify that the 3DOM Ge/Al and Ge/Si have a different morphology, and EDX was used to analyze the composition of the samples. Novel 3DOM bilayer films made from reactive elements could have some potential as anode host material in future generation lithium ion batteries and can be regarded as the first step to make 3D ordered macroporous bilayer films.

Monolayer colloidal mask with tunable interstice size for nanosphere lithography

Nanosphere lithography (NSL), which utilizes self-assembled monolayer colloidal crystal (MCC) as a mask to fabricate ordered nanostructures over large areas, is a high through-put and low-cost method. In this work, polystyrene (PS) MCC with tunable interstice size is prepared by a combination of self-assembly of PS nanospheres at the air–water interface and chemical vapor deposition (CVD) of siloxane polymers. Water vapor around the MCC reacts with the silicon tetrachloride (SiCl4) steam introduced by nitrogen carrier gas to form siloxane polymers. Consequently, it induces the formation of a siloxane polymers layer coating on the surface of each PS sphere, which results in the shrinkage of the interstice between neighboring PS spheres. In this way, the deformation of colloidal spheres during thermal annealing could be avoided. Accordingly, extra cracks arising from the constraint of spheres deformation on a rigid substrate are eliminated. Furthermore, the interstice size of the colloidal mask can be precisely controlled by manipulating the CVD conditions. In addition, the colloidal mask could be transferred onto any kind of substrates including rigid, flexible, flat, or curved solid substrates after CVD coating. The method provides a facile approach to the fabrication of interstice-size-tunable, high-quality colloidal masks for NSL application.

Optical studies of random disorder of colloidal photonic crystals and its evolution in evaporation induced self-assembly

Self-assembled photonic structures have been under theoretical and experimental study for decades, whereas previous theories on optical properties were mainly concerned with perfect structure or some certain limited kinds of disordered photonic crystals (PCs), making them unsuitable for characterizing the real self-assembled PCs. In order to improve our understanding of the mechanism of self-assembly and provide more crucial clues to further grow perfect crystals, we extended previous widely used scalar wave approximation (SWA), making it be able to characterize long-range disorder (β) and short-range disorder (α) in PCs synthetically in a simple and effective way. Excellent agreement with in situ observed reflectance of evaporation induced self-assembled colloidal photonic crystals (CPCs) was obtained, demonstrating that the introduction of the parameters α and β in SWA can successfully characterize the disorder in self-assembled CPCs. Furthermore, extended SWA was further used to study the disorder formation in self-assembly, and it was found that during growing stage both β and α drop down, whereas in drying stage β stays nearly unchanged while α increases significantly. It turned out that the growing stage of self-assembly is a stage when the structure transforms from disordered to ordered one, and growth induced disorder mainly arises in drying stage. The results obtained provide an insight into the growth mechanisms of self-assembly and theoretical basis for characterizing optical properties of disordered PCs.

Magnetic Assembly Route to Colloidal Responsive Photonic Nanostructures

Responsive photonic structures can respond to external stimuli by transmitting optical signals. Because of their important technological applications such as color signage and displays, biological and chemical sensors, security devices, ink and paints, military camouflage, and various optoelectronic devices, researchers have focused on developing these functional materials. Conventionally, self-assembled colloidal crystals containing periodically arranged dielectric materials have served as the predominant starting frameworks. Stimulus-responsive materials are incorporated into the periodic structures either as the initial building blocks or as the surrounding matrix so that the photonic properties can be tuned. Although researchers have proposed various versions of responsive photonic structures, the low efficiency of fabrication through self-assembly, narrow tunability, slow responses to the external stimuli, incomplete reversibility, and the challenge of integrating them into existing photonic devices have limited their practical application. In this Account, we describe how magnetic fields can guide the assembly of superparamagnetic colloidal building blocks into periodically arranged particle arrays and how the photonic properties of the resulting structures can be reversibly tuned by manipulating the external magnetic fields. The application of the external magnetic field instantly induces a strong magnetic dipole-dipole interparticle attraction within the dispersion of superparamagnetic particles, which creates one-dimensional chains that each contains a string of particles. The balance between the magnetic attraction and the interparticle repulsions, such as the electrostatic force, defines the interparticle separation. By employing uniform superparamagnetic particles of appropriate sizes and surface charges, we can create one-dimensional periodicity, which leads to strong optical diffraction. Acting remotely over a large distance, magnetic forces drove the rapid formation of colloidal photonic arrays with a wide range of interparticle spacing. They also allowed instant tuning of the photonic properties because they manipulated the interparticle force balance, which changed the orientation of the colloidal assemblies or their periodicity. This magnetically responsive photonic system provides a new platform for chromatic applications: these colloidal particles assemble instantly into ordered arrays with widely, rapidly, and reversibly tunable structural colors, which can be easily and rapidly fixed in a curable polymer matrix. Based on these unique features, we demonstrated many applications of this system, such as structural color printing, the fabrication of anticounterfeiting devices, switchable signage, and field-responsive color displays. We also extended this idea to rapidly organize uniform nonmagnetic building blocks into photonic structures. Using a stable ferrofluid of highly charged magnetic nanoparticles, we created virtual magnetic moments inside the nonmagnetic particles. This "magnetic hole" strategy greatly broadens the scope of the magnetic assembly approach to the fabrication of tunable photonic structures from various dielectric materials.

Three-Dimensional Structure and Defects in Colloidal Photonic Crystals Revealed by Tomographic Scanning Transmission X-ray Microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.

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.

Three-dimensionally ordered macroporous silicon films made by electrodeposition from an ionic liquid

Three dimensionally ordered macroporous (3DOM) silicon films have been made via ordered polystyrene (PS) templates by electrodeposition from an ionic liquid (IL). For this purpose, the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) with SiCl(4) dissolved in it was used as an electrolyte and the electrodeposition of macroporous silicon could be achieved at room temperature (~20 °C). Self-assembled PS colloidal crystals with different diameters were used as templates. Scanning electron microscopy and X-ray photoelectron spectroscopy confirm the quality of the samples, and the optical transmission measurement demonstrates that the 3DOM silicon film has a bandgap in the near infrared regime. Such a material has the potential to make 3DOM silicon feasible for electrical and optical applications.

Enhanced light diffraction from self-assembled double-layer colloidal crystals

We demonstrate that double-layer colloidal photonic crystals function as wavelength- and polarization-insensitive blazed transmission gratings with efficiencies as high as 63% in the high-energy spectral region. This phenomenon, which is known as specular-resonance-enhanced diffraction, has been reported for a small-scale cluster of microspheres with a limited size parameter. By performing accurate characterization employing large-area colloidal crystal films self-assembled by an inclined dip-coating method with a controlled number of layers, this peculiar and efficient diffraction was demonstrated to be a universal property of double-layer colloidal crystal films.

Highly Ordered Mixed Protein Patterns Over Large Areas from Self‐Assembly of Binary Colloids

A novel single step method is developed for patterning multiple proteins within self-assembled binary colloidal crystal patterns. Self-assembly takes place by confining the suspensions in which either one or both types of particles are coated separately with proteins prior to mixing. This method offers precise control over the spacing between the proteins and the pattern size over large areas without any involvement of expensive lithographic techniques.

Three-dimensional actuators transformed from the programmed two-dimensional structures via bending, twisting and folding mechanisms

Combining the physical principle of actuators with the basic concept of photonic crystals, colour-tunable three-dimensional (3D) photonic actuators were successfully fabricated. By controlling the d-spacings and the refractive index contrasts of the self-assembled 3D colloidal photonic crystals, colours of the photonic actuators were tuned. Various shapes of these 3D actuating objects were constructed by transforming the programmed 2D structures via bending, twisting and folding mechanisms. These 2D structures were first programmed by breaking the symmetry. The selective swellings were then applied as driving forces to control the shapes and colours of the photonic actuators. Scroll photonic actuators had been first demonstrated by bending the traditional 2D cantilever structure (K.-U. Jeong, et al., J.Mater.Chem., 2009, 19, 1956). By breaking the symmetry of a cantilever structure perpendicular to its long axis, polypeptide-/DNA-like 3D helical photonic actuators were obtained from the programmed 2D structure via twisting processes. Both left- and right-handed scrolls and helices with various colours can be achieved by changing the polarity of solvents. Different types of 3D actuators, such as cube, pyramid and phlat ball, were also demonstrated via the folding mechanism. The reversible 3D photonic actuators transformed from the programmed 2D structures via the bending, twisting and folding mechanisms may be applied in the field of mechanical actuators, and optoelectronic and bio-mimetic devices.

Colloidal Self‐Assembly Meets Nanofabrication: From Two‐Dimensional Colloidal Crystals to Nanostructure Arrays

Self-assembly of colloidal microspheres or nanospheres is an effective strategy for fabrication of ordered nanostructures. By combination of colloidal self-assembly with nanofabrication techniques, two-dimensional (2D) colloidal crystals have been employed as masks or templates for evaporation, deposition, etching, and imprinting, etc. These methods are defined as "colloidal lithography", which is now recognized as a facile, inexpensive, and repeatable nanofabrication technique. This paper presents an overview of 2D colloidal crystals and nanostructure arrays fabricated by colloidal lithography. First, different methods for fabricating self-assembled 2D colloidal crystals and complex 2D colloidal crystal structures are summarized. After that, according to the nanofabrication strategy employed in colloidal lithography, related works are reviewed as colloidal-crystal-assisted evaporation, deposition, etching, imprinting, and dewetting, respectively.

Fast response photonic crystal pH sensor based on templated photo-polymerized hydrogel inverse opal

Polymer hydrogels can exhibit large reversible volume changes in response to external stimuli, and thus are regarded as excellent materials for chemical sensors. In this report, we demonstrate a mechanically robust and fast response photonic crystal pH sensor fabricated by templated photo-polymerization of hydrogel monomers within the interstitial space of a self-assembled colloidal photonic crystal. Throughout a rigorous optimization of the photo-polymerization, pH sensors showing a response time of less than 10 s upon a pH change were fabricated. Repeated pH changes revealed that the sensor has a long lifetime (>6 months) without degradation of the response time or reproducibility in pH-driven color change.

Highly monodisperse vinyl functionalized silica spheres and their self-assembled three-dimensional colloidal photonic crystals

A one-step method was presented for the controlled synthesis of highly monodisperse vinyl functionalized silica spheres (VFSSs), which could be self-assembled into three-dimensional (3D) fcc colloidal photonic crystals on a horizontal glass slide by a gravity sedimentation process. One precursor (vinyltriethoxysilane) and one solvent (water) were used in the experiment. The size of VFSSs could be adjusted from 420 to 650 nm with relative standard deviation below 2% by controlling the dripping speed of diluted ammonia catalyst, without changing the concentration of precursor/catalyst. SEM and TEM images illustrate the copiousness in quantity and the uniformity in size/shape of the VFSSs. 29Si nuclear magnetic resonance (NMR) spectrum confirms the surfaces of VFSSs have abundant vinyl groups (–CH CH 2) which connect to the silicon atoms. Optical characterizations of the transmission spectra reveal that the periodic arrays exhibit a photonic band gap in the (1 1 1) direction of the fcc lattice in consistent with the Bragg diffraction theory. This work has also been extended to prepare other monodisperse surface-modified silica spheres and fabricate organic functionalized colloidal templates.

Self-Assembled Colloidal Crystals in Capillary with Its Fiber Junction

Silica microspheres self-assembled in glass capillary are investigated. Monodisperse silica microsphere dispersions in diameter 320 nm are self-organized into a bulk cylindrical colloidal crystal by evaporation induced nucleation and crystallization. The resulting colloidal crystals are characterized by optical microscopy and scanning electronic microscopy (SEM), and the SEM images show these crystals dominate in fcc lattice with its (111) crystallographic axis as longitudinal. The colloidal crystal filled capillary is packaged into a heat-shrink plastic tube and a fiber measurement system is designed to measure the optical property of colloidal bulk in capillary. It is found that an appreciable bandgap appears at wavelength 686 nm from the transmission spectroscopy, which is consistent with the theoretical estimation. A considerable photonic band gap of up to –10 dB and a steep photonic band edge of up to 0.25 dB/nm indicate that silica microspheres are promising for implementing optical filter applications in fiber systems.

Ultrasmall-angle X-ray scattering analysis of photonic crystal structure

The results of an ultrasmall-angle X-ray scattering study of iron(III) oxide inverse opal thin films are presented. The photonic crystals examined are shown to have fcc structure with amount of stacking faults varying among the samples. The method used in this study makes it possible to easily distinguish between samples with predominantly twinned fcc structure and nearly perfect fcc stacking. The difference observed between samples fabricated under identical conditions is attributed to random layer stacking in the self-assembled colloidal crystals used as templates for fabricating the inverse opals. The present method provides a versatile tool for analyzing photonic crystal structure in studies of inverse opals made of various materials, colloidal crystals, and three-dimensional photonic crystals of other types.

Structure transitions of polymer-mediated colloidal crystals

The self-assembled colloidal crystals with structure transitions were realized experimentally on substrates with polymer brushes. Such polymer-mediated colloidal crystals were found to experience a series of in-layer structure transitions including stable columnar, polycrystalline, mixed square-hexagonal, square, hexagonal structures when the mass ratio of colloidal particles to free polymers was adjusted from 0.125 to 10. Among these structures, a large area interfacial square structure was proved to be a body-centered tetragonal (BCT) structure in three dimensions in the polymer-mediated colloidal crystals.