High-quality Colloidal Crystals Research Articles

Investigation of the regulatory mechanism of monolayer colloidal crystals prepared by gas-liquid-solid three-phase interfacial self-assembly method

In recent years, the gas-liquid interface self-assembly method has emerged as a recognized and efficient technique for preparing two-dimensional monolayer colloidal crystals. However, challenges such as limited preparation area, dependence on complex equipment, and high costs persist in the conventional preparation process. This paper proposes a straightforward, rapid, and cost-effective method to implement a gas-liquid-solid three-phase interfacial self-assembly strategy. This approach enhances the gas-liquid interfacial self-assembly method, allowing for the preparation of large-area, high-quality single-layer colloidal crystals (>10 cm2) without using any surfactant. The gas-liquid-solid three-phase interfacial self-assembly process was analyzed in detail, and it was found that the main factors affecting the quality of the monolayer colloidal crystals were the volume ratio of PS colloidal solution to ethanol and the self-assembly temperature. It was shown that the optimal self-assembly conditions were achieved after dispersion by ultrasonic dispersion in a water bath at 32 °C–30 °C at a volume ratio of 1:2.5 of PS colloidal solution to ethanol. In addition, particles of various sizes can be prepared by this method to produce tightly arranged monolayer films that can be transferred to a variety of different substrates with great versatility. The demonstrated efficiency of this self-assembly method underscores its significant potential for application in large-scale manufacturing and practical industrialization.

Engineering particles for sensing applications via in-situ synthesizing carbon dots@SiO2 photonic crystals

Particle engineering has been one of the hotspots in both stimuli-responsive and smart nanomaterials. However, efficient and facile implementation of robust covalent interaction in particle engineering remains a great challenge. Herein, we firstly propose an easy-to-perform and versatile bottom-up particle engineering strategy for covalent coupling between SiO2 particle and carbon dots (CDs) in situ. Robust covalent interactions enable the nano-micro coupling of CDs and SiO2 particles, and nearly twofold fluorescence enhancement of CDs was obtained. Moreover, hydrogen bond-driven assembly of the carbon dots@SiO2 (CDs@SiO2) particles was demonstrated via molecular dynamics (MD) simulations, which guaranteed the high-quality colloidal photonic crystals (CPCs) without coffee ring effect. In addition, the optical coupling between CPCs and CDs ensures brighter and more saturated structural colors, as well as enhanced fluorescence for microfluidics-derived bi-optical CDs@SiO2 CPC supraballs. Based on this, bi-optical signal detection chip towards veterinary drug is developed. The in-situ covalent coupling strategy developed in this work might open an innovative avenue towards the particle engineering, which will also provide a methodological reference for the design of functional inorganic particles.

Magnetic Field-Assisted Fast Assembly of Microgel Colloidal Crystals.

Compared with the colloidal crystals (CCs) of hard spheres, large-scale, high-quality CCs of soft microgel spheres are easier to be assembled because they are more tolerant to defects. However, to assemble microgel CCs, a microgel dispersion should first be concentrated and then allowed to crystallize, which is tedious and time-consuming. Herein, we demonstrated that a magnetic poly(N-isopropylacrylamide) (PNIPAM) microgel with an Fe3O4 core and a PNIPAM shell can be assembled into CCs quickly by simply applying an external magnetic field to the diluted microgel dispersions. The resulting CCs are highly ordered as revealed by their iridescent color, laser diffraction pattern, and confocal characterization. They display a sharp Bragg peak on their reflection spectra, which shifts to lower wavelength when heated because of the thermosensitivity of the PNIPAM shell. The magnetic assembly is not only simple and fast but also allows control of the CC structure in both horizontal and vertical directions. Using spatially varying magnetic fields, patterned microgel CCs were facilely assembled. More importantly, magnetic microgel spheres with different sizes can be assembled in a layer-by-layer manner by adding them sequentially, and the thickness of each layer can be simply controlled by the amount of spheres added.

Colloidal Photonic Crystals of Reusable Hydrogel Microparticles for Sensor and Laser Applications.

Although a wide variety of techniques have been developed to date for the fabrication of high-quality colloidal photonic crystals (CPCs) using monodisperse silica and polystyrene microparticles, poly(N-isopropylacrylamide) (PNIPA) hydrogel microparticles have rarely been utilized for the preparation of active CPCs despite the intriguing feature of temperature-responsive volume changes. This report describes the promising potential abilities of PNIPA hydrogel microparticles for sensor and laser applications. Monodisperse PNIPA hydrogel microparticles were synthesized by emulsion polymerization, and the microparticle diameter was finely controlled by adjusting the surfactant concentration. Such hydrogel microparticles spontaneously formed uniform CPCs with visible Bragg reflection even in fluid suspensions. The addition of small amounts of ionic substances into the centrifuged and deionized CPC suspensions enabled the on-demand color switching between Bragg reflection and white turbidity with temperature, leading to temperature- and ion-sensing applications. Moreover, our expanding experiments successfully demonstrated the optically excited laser action with a single and narrow peak from CPC suspensions with light-emitting dyes by the photonic band gap effect. After the light-emitting dyes were simply removed from the CPC suspensions by centrifugation, the purified PNIPA hydrogel microparticles were permanently reusable as the CPC laser microcavities to generate the laser action at other wavelengths using different dyes. This study contributes the circular economy concept using reusable hydrogel microparticles for the realization of a sustainable society.

Controlling the quality of SiO2 colloidal crystals by temperature ramping on the vertical convective self-assembly method

Traditionally, the vertical convective self-assembly method (VCSA) uses time-invariant temperature conditions to obtain high-quality colloidal crystal (CC) films, leaving unexplored possible effects due to temperature variations during the elaboration process. Here, the quality of colloidal crystal films grown by the VCSA method when applying temperature ramps is investigated. This study is conducted by combining experimental measurements and simulations of concentration profile and meniscus displacement, using the temperature dependent Kynch’s theory. The quality of the elaborated CC films is evaluated by analyzing the resulting optical Bragg diffraction. Colloidal crystal films deposited under controlled parameters showed that high-quality films can be achieved by positioning the meniscus into the uniform zone under temperature ramping, as a definite relation between sedimentation velocity and evaporation rate, in agreement with modelling results.

Accelerated annealing of colloidal crystal monolayers by means of cyclically applied electric fields

External fields are commonly applied to accelerate colloidal crystallization; however, accelerated self-assembly kinetics can negatively impact the quality of crystal structures. We show that cyclically applied electric fields can produce high quality colloidal crystals by annealing local disorder. We find that the optimal off-duration for maximum annealing is approximately one-half of the characteristic melting half lifetime of the crystalline phase. Local six-fold bond orientational order grows more rapidly than global scattering peaks, indicating that local restructuring leads global annealing. Molecular dynamics simulations of cyclically activated systems show that the ratio of optimal off-duration for maximum annealing and crystal melting time is insensitive to particle interaction details. This research provides a quantitative relationship describing how the cyclic application of fields produces high quality colloidal crystals by cycling at the fundamental time scale for local defect rearrangements; such understanding of dynamics and kinetics can be applied for reconfigurable colloidal assembly.

Hydrophobic fluorinated colloidal photonic crystals for heterogeneous aggregated cluster encoding and energy-saving applications

The construction of high-quality colloidal photonic crystals (CPCs) for optoelectronics, energy and biological applications, featuring the viable and effective self-driven assembly route, still remains a challenge. Herein, we demonstrate the efficient self-assembly of hydrophobic fluorine-containing poly(trifluoroethyl methacrylate) (PTFEMA) CPCs via hydrophobic force for heterogeneous aggregated cluster encoding and photoluminescence enhancement of liquid crystal display (LCD). The CPC films with periodic arranged structure self-assembled through the hydrophobic terminals (-CF3) display tunable brilliant structural color and large-area crack-free morphology. Moreover, taking advantage of PTFEMA emulsion and superparamagnetic Fe3O4 nanoparticle suspension as two discontinuous phases, Janus microbeads are fabricated by the triphase microfluidic technology, which are subsequently constructed into heterogeneous aggregated clusters that perform multiple structural color signals for the optical encoding. Eventually, the PTFEMA CPC film serves as the photoluminescence enhanced film to significantly elevate the photoluminescence (PL) intensity of CdSe@ZnS quantum dots by ca. 11 folds in an LCD device. The novel PTFEMA CPCs may offer the alternative opportunities for optical encoding towards sensing, anticounterfeiting and information storage, as well as the energy-saving optoelectronic devices.

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications.

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

Elastomer-immobilized tunable colloidal photonic crystal films with high optical qualities and high maximum strain

High-quality elastomer-immobilized colloidal crystal films with low particle volume fractions demonstrated a full-colour change from red to blue with high uniformity, achieved by varying the mole fraction of monomers in the hydrogel or stretching.

Convex-Meniscus-Assisted Self-Assembly at the Air/Water Interface to Prepare a Wafer-Scale Colloidal Monolayer Without Overlap.

Self-assembly at the air/water interface (AWI) has proven to be an efficient strategy for fabricating two-dimensional (2D) colloidal monolayers, which was widely used as the template for nanosphere lithography in nanophononics, optofluidics, and solar cell studies. However, the monolayers fabricated at the AWI usually suffer from a small domain area and quasi-double layer structure caused by submerged particles. To overcome this, we proposed an improved protocol to prepare 2D colloidal monolayers free of overlapping nanospheres at the AWI. Utilizing the stable suspension infusion to the water surface, a convex meniscus, whose height is related to viscous force, was formed adjoining the three-phase boundary. As a result of the resistance of the convex meniscus, the polystyrene nanospheres in the initial suspension directly self-assembled into a preliminary monolayer, which proved effective in preventing nanospheres' sinking and increasing the colloidal crystal domain size. An optimal parameter for transferring the monolayer was also developed based on the numerical simulation results. Finally, a wafer-scale monolayer, covered with less than one nanosphere per 100 μm × 100 μm area, was achieved on the desired substrate with an average domain size attaining centimeter scale. The high-quality 2D colloidal crystal may further promote the application of nanosphere lithography, especially in the fields that require a defect-free template.

Crucial Impact of Hydrophilicity on the Self-Assembled 2D Colloidal Crystals Using Langmuir-Blodgett Method.

Large-scale close-packed two-dimensional (2D) colloidal crystal with high coverage is indispensable for various promising applications. The Langmuir-Blodgett (LB) method is a powerful technique to prepare 2D colloidal crystals. However, the self-assembly and movement of microspheres during the whole LB process are less analyzed. In this study, we clarify the crucial impact of hydrophilicity of the microspheres on their self-assembly in the LB process and on the properties of the prepared 2D colloidal crystals. The characteristic surface pressure-area isotherms of the microspheres have been analyzed and adjusted by only counting the quantity of the microspheres on the water surface, which leads to more accurate results. The critical surface pressures for hydrophilic and hydrophobic microspheres are about 61 and 46 mN/m, respectively. The decrease of the surface hydrophilicity of microspheres facilitates their self-assembly on the water surface, which further leads to higher coverage and less defects of the 2D colloidal crystals. A coverage of as high as 97% was obtained using hydrophobic microspheres. Entropy and intersphere capillary forces drive the self-assembly and transportation of the microspheres, respectively. Caused by the diffraction of visible light, opposite contrasts at local adjacent regions on the surface of the 2D colloidal crystals have been observed. The understanding of self-assembly of the microspheres during the LB process paves the way to fabricate the high-quality 2D colloidal crystals for various applications such as photonic papers and inks, stealth materials, biomimetic coatings, and related nanostructures.

Entropy-Induced Self-Assembly of Colloidal Crystals with High Reflectivity and Narrow Reflection Bandwidth.

Cracks and defects, which could result in lower reflectivity and larger full width at half maximum (FWHM), are the major obstacles for obtaining highly ordered structures of colloidal crystals (CCs). The high-quality CCs with high reflectivity (more than 90%) and 9.2 nm narrow FWHM have been successfully fabricated using a fixed proportion of a soft matter system composed of silica particles (SPs), polyethylene glycol diacrylate (PEGDA), and ethanol. The influences of refractivity difference, volume fractions, and particle dimension on FWHM were illuminated. Firstly, we clarified the influences of the planar interface and the bending interface on the self-assembly. The CCs had been successfully fabricated on the planar interface and presented unfavorable results on the bending interface. Secondly, a hard sphere system consisting of SPs, PEGDA, and ethanol was established, and the entropy-driven phase transition mechanism of a polydisperse system was expounded. The FWHM and reflectivity of CCs showed an increasing trend with increasing temperature. Consequently, high-quality CCs were obtained by adjusting temperatures (ordered structure formed at 90 °C and solidified at 0 °C) based on the surface phase rule of the system. We acquired a profound understanding of the principle and process of self-assembly, which is significant for preparation and application of CCs such as optical filters.

Vivid structural colors from long-range ordered and carbon-integrated colloidal photonic crystals.

A facile strategy to prepare high-quality colloidal photonic crystals (PCs) with good visibility is proposed. Based on a high refractive-index material (zinc sulfide), highly monodispersed colloidal particles are successfully produced and assembled into long-range ordered crystalline colloidal arrays. The carbon-based materials are in situ incorporated with the long-range ordered colloidal PCs, which endows PCs with the combined characteristics to simultaneously achieve an intense photonic stop band and excellent control of incoherent light scattering. Owing to these merits, the obtained ZnS colloidal PCs have demonstrated strong brightness with the maximum reflectivity of 98%. Moreover, the coloration, saturation, and viewing angle are all improved. This study provides a straightforward and cost-effective strategy to create structural colors with high-quality visibility, which is expected to facilitate future applications of colloidal PCs.

Construction of Hydrogen-Bond-Assisted Crack-Free Photonic Crystal Films and Their Performance on Fluorescence Enhancement Effect

The ability to fabricate high-quality colloidal photonic crystal (CPC) films in large areas is critical in many applications, ranging from flexible displays, security devices to optical enhancement. Herein, a large-scaled CPC film with crack-free structure and uniform optical performance is prepared via a hydrogen-bond-assisted method. The crack-free CPC film is ascribed to the intermolecular hydrogen bonds between carboxyl (COOH) moieties of poly(styrene-methyl methacrylate acrylic acid) microspheres and isocyanate (NCO) moieties of polyurethane. Furthermore, the as-prepared CPC film is applied as a Bragg reflection mirror for fluorescence enhancement. It is demonstrated that a fourfold enhancement of fluorescence signal is achieved when the stopband of CPCs overlaps with the emission wavelength of quantum dots. This simple method of preparing large-area and crack-free CPC film is promising for developing efficient optical devices.

Dependence of crystallization of Brownian particles by sedimentation on the force direction

The formation of a large close-packed colloidal crystal with the fcc structure was observed during the sedimentation of colloidal particles in an inverted pyramidal pit [S. Matsuo et al., Appl. Phys. Lett. 82, 4285 (2003)]. Carrying out Brownian dynamics simulations, we confirmed that large grains with the fcc structure are formed when the apex angle of the inverted pyramidal container is suitable and the force direction is parallel to the the center axis [Y. Kanatsu and M. Sato, J. Phys. Soc. Jpn. 84, 044601 (2015)]. To form a high-quality colloidal crystal without defects, it is important to investigate in detail how the quality of a colloidal crystal is affected by the force direction and container shape. In this paper, we focus on the effect of the force direction on crystal quality and investigate how the ratio of the number of the hcp structured particles, Nhcp, to that of fcc structured particles, Nfcc, is affected by the force direction. In our simulation, the ratio of Nfcc to Nhcp is hardly changed when the force direction deviates from the central axis: Nfcc is much larger than Nhcp irrespective of the force direction. Thus, our results show that the crystal structure is insensitive to the force direction in forming a colloidal crystal by sedimentation in an inverted pyramidal container.

Large-scale high-quality 2D silica crystals: dip-drawing formation and decoration with gold nanorods and nanospheres for SERS analysis

High-quality colloidal crystals (CCs) are important for use in photonic research and as templates for large-scale plasmonic SERS substrates. We investigated how variations in temperature, colloid concentration, and dip-drawing parameters (rate, incubation time, etc) affect the structure of 2D CCs formed by highly monodisperse silica nanoparticles (SiNPs) synthesized in an l-arginine solution and regrown by a modified Stöber method. The best quality 2D CCs were obtained with aqueous 12 wt% colloids at a temperature of 25 °C, an incubation time of 1 min, and a drawing rate of 50 mm min−1. Assembling of gold nanorods (GNRs) on 2D CCs resulted in the formation of ring-like chains with a preferential tail-to-tail orientation along the hexagonal boundaries. To the best of our knowledge, this is the first time that such nanostructures have been prepared. Owing to the preferential tail-to-tail packing of GNRs, 2D SiNP CC + GNR substrates demonstrated an analytical SERS enhancement of about 8000, which was 10 to 15 times higher than that for self-assembled GNRs on a silicon wafer. In addition, the analytical SERS enhancement was almost 60 times lower after replacing the nanorods in 2D SiNP CC + GNR substrates with 25 nm gold nanospheres.

Facile Synthesis of Highly Ordered Macroporous CeO<sub>2</sub> Film for Methylene Blue Degradation

Negative-charged PS microspheres were prepared through emulsifier-free emulsion polymerization method. High-quality PS colloidal crystals were obtained via the horizontal deposition method. Hierarchically ordered macroporous CeO2sheets were fabricated on fluorine-doped tin oxide (FTO) substrates using PS colloidal crystals as template by a continuous infiltration process. The structure and morphology of the products were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and nitrogen adsorption-desorption. The characterized results showed that macroporous CeO2sheets replicated well three-dimensionally (3D) ordered structure derived from PS colloidal crystals template and possessed a relatively large specific surface area. Moreover, the synthesized hierarchical CeO2sheets showed the highest photocatalytic activity under solar irradiation and also exhibited remarkable ability to remove methylene blue, suggesting they are promising catalyst for wastewater treatment.

Fabrication of Wafer-Size Monolayer Close-Packed Colloidal Crystals via Slope Self-Assembly and Thermal Treatment

We developed a simple and general approach for constructing a wafer-scale monolayer, close-packed polystyrene (PS), and SiO2 sphere arrays, namely colloidal crystals, which have significant potential in various applications. The method combines slope self-assembly and thermal treatment to achieve large-area and high-quality colloidal crystal with a proper slant angle (θ) and latex concentration (volume fraction, φ). The dependence of the structures of colloidal crystals on a dispersion system was also investigated. Moreover, a theoretical analysis of the slope self-assembly method was proposed. In addition, we applied the method to assemble PS spheres on different kinds of substrates, which indicates that the method is a versatile and reliable way to fabricate monolayer colloidal crystals.

Fabrication of Three-Dimensionally Ordered Macroporous TiO2 Films with Enhanced Photovoltaic Conversion Efficiency

Negative-charged polystyrene (PS) microspheres were prepared through a soap-free emulsion polymerization method using potassium persulfate as initiator. Three-dimensionally ordered macroporous TiO2 films were fabricated using the high-quality PS colloidal crystals templates obtained via a horizontal deposition method. The as-prepared macroporous TiO2 films were applied as the photoanodes in dye-sensitized solar cell (DSSC). The microstructure of the products were characterized by X-ray diffractometer, fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption–desorption analyzer. The results showed that the macroporous TiO2 films replicated well the 3D ordered structure derived from PS colloidal crystal templates and revealed a relatively large specific surface area (69.99 m2/g), which could increase the capacity of TiO2 film anode for absorbing dyes and scattering light. The photocurrent–voltage (I–V) characteristics of DSSC were measured by a digital source meter under simulated solar light. The results indicated that the introduction of an ordered macroporous TiO2 interfacial layer increased the photovoltaic conversion efficiency, which was improved by 68 % from 3.61 to 6.08 %, as compared to a device using a bare P25 TiO2 photoanode. Our results showed that the hierarchically ordered macroporous TiO2 bilayer films photoanode for DSSC could be helpful to improve the photovoltaic conversion efficiency.

Dual broadband near-infrared perfect absorber based on a hybrid plasmonic-photonic microstructure

High performance light absorber with a broad bandwidth is particularly desirable for near-infrared photodetection and optical interconnects. Here we demonstrate a dual broadband perfect absorber in the near-infrared regime, which is based on a hybrid plasmonic-photonic microstructure. Such a microstructure is fabricated by self-assembling a monolayer colloidal crystal on an optically opaque metal film followed by depositing a thin metallic half-shell on the top of the colloidal particles. Both experimental and numerical simulation results show that the simply designed absorbers have dual broadband with absorption exceeding 90% in the near-infrared region with the absorption bands being scalable by tuning the size of the colloidal particles. Moreover, the absorption efficiency shows an extremely slight dispersion for the incident angles up to 50 degrees, benefit from the high symmetry as well as the highly modulated plasmonic microstructures that lead to a weak polarization dependence of these two absorption bands. The relative ease of growing high-quality colloidal crystals and the low cost of fabricating such plasmonic-photonic microstructures with high reproducibility could promise applicability of the light absorber in the field of photodetectors, thermal emitters and photovoltaics.