Structural Color for Security Printing: Patterned Robust Colloidal Crystals

Colloidal crystals are periodic arrays of monodisperse particles that exhibit optical stopbands, which can be experimentally observed as a Bragg reflection characterized by a specific Bragg wavelength and width. Precise control of these characteristic parameters is essential for applications in structural color materials, sensors, and tunable photonic crystals. Although the Bragg reflection wavelength can be widely tuned by adjusting the lattice spacing via changes in particle size and concentration, controlling the width over a wide range-such as through expansion-is challenging because it is intrinsically determined by the refractive index contrast between the colloidal particles and their surrounding medium. In this study, the Bragg reflection width of non-close-packed colloidal crystals immobilized in an elastomer film was successfully expanded by adjusting the photoinitiator concentration and ultraviolet light intensity for photopolymerization. Expansion was attributed to the superposition of Bragg reflections at different wavelengths, resulting from spatial variations in the lattice spacings of the non-close-packed colloidal crystals formed during photopolymerization. Owing to the solvent-free and highly flexible nature of the elastomer-immobilized, non-close-packed colloidal crystal film, the Bragg reflection wavelength was readily tuned by mechanical compression while maintaining the expanded Bragg reflection width, thereby advancing the practical applications of structural color materials.

The structural blue color of a Morpho butterfly originates from the diffraction of light and interference effects due to the presence of the microstructures on the wing of the butterfly. Structural color on the surface of a damselfish reversibly changes between green and blue. Inspired by these creatures, we have been trying to prepare high-quality and functional structural color films. We describe our efforts in this Account. A useful technique to prepare such structural color films in colloidal solution is a "lifting" method, which allows us to quickly fabricate brilliant colloidal crystal films. The thicknesses of the films can be controlled by precisely adjusting the particle concentration and the lifting speed. Moreover, in order to prepare a complicated structure, we have used template methods. Indeed, we have successfully prepared the inverse structure of the wing of a Morpho butterfly with this technique. Initially, however, our structural color films had a whitish appearance due to the scattering of light by defects in the colloidal crystal film. Later, we were able to prepare a non-whitish structural color film by doping an appropriate dye in the colloidal particles to absorb the scattering light. In addition to the structural blue color, the wing of the Morpho butterfly has superhydrophobic properties. According to Wenzel's equation, the hydrophobic and hydrophilic properties are enhanced when the roughness of the hydrophobic and hydrophilic surface is increased, respectively. Based on this mechanism, we have successfully prepared structural color films with superhydrophobic properties, as well as with superhydrophilic properties. Another important property that can be seen in nature is tunable structural color, such as the color change that can be seen on the surface of a damselfish. In order to mimic such color change, we have developed several tunable structural color films. In particular, we have successfully prepared phototunable photonic crystals using photoresponsive azobenzene derivatives. In order to apply these structural color films, we developed a technique for patterning them by taking advantage of the wettability of the substrate surface. These materials can be used in the future for self-cleaning pigments and tunable photonic crystals.

The structural color properties of colloidal arrays can be controlled by colloidal assembly via tuning the size, composition, and ordering of colloidal particles and their formation at the macroscale. Controlled assembly and patterning offer many advantages for the technological development of photovoltaics, optics, and lab-on-a-chip, where the ordering of particles can influence the properties and functions of a system. Many techniques have been well established for patterning ordered colloidal arrays (colloidal crystals), and disordered colloidal arrays (colloidal glasses). However, they are time-consuming and require additional steps such as masking, etching, or stamping. The advent of digital manufacturing, in which additive manufacturing is combined with computer-aided design (CAD), can overcome some of the challenges in fabricating structures from colloidal particles. This article presents a review of recent strategies for digitally fabricating 1D (e.g., single line), 2D (e.g., arrays of dots and patterns with lines), and 3D (e.g., dots and balls) colloidal crystals and glasses, including inkjet printing, direct ink writing, electrohydrodynamic jet printing, two-photon lithography, and digital light processing. The requirements of colloidal ink formulations for different 3D printing methods are discussed. The effects of the wettability of the printed ink on the ordering of colloidal particles in the fabricated structures and the resulting structural colors are discussed. Finally, a summary and perspective on future development are presented.

NIPAm-based soft colloidal photonic crystals with high color saturation

Mechanical nondiscoloring and antistretching photonic crystal (PC) films, especially those with stable structure colors during deformation, have great potential applications in wearable display devices, decoration, and packaging. Here, PC films with antistretching and invariant structural colors during deformation were prepared, by combining Zn2+ coordinated elastic material and hydroxypropyl methylcellulose (HPMC) with polystyrene@silica (PS@SiO2) colloidal crystals. The PC films release energy by forming local fractures at a microscopic level during the straining process but the lattice spacing and effective refractive index of the local array do not change. According to the Bragg law, the structure color remains unchanged. The introduction of HPMC gave the PC films excellent tensile properties, and the maximum tensile strength reached 10 MPa. And after 100 times of stretching, bending and compression cycles, the structural color remained unchanged.

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

Hydrophobic photonic crystals (PCs) has been increasingly appreciated as a promising functional material due to their distinct surface characteristic of structural color and hydrophobicity. However, it remains a challenge to fabricate hydrophobic PCs via a one-step process. Inspired by the development of high-performance waterborne coatings, we propose an easy-to-perform and high-efficiency strategy to construct hydrophobic building blocks (diameter of 221, 247, 276 and 305 nm), where the umbelli-form hydrophobic long chain (veova10 Cn > 9) was loaded onto polystyrene (PS) colloidal particles in situ. Taking advantage of the hydrophobic driving force between the colloidal particles, large-scale colloidal photonic crystals (CPCs) film with crack-free morphology was obtained efficiently. The derived CPCs exhibit robust mechanical stability, prominent hydrophobicity and excellent optical properties. In addition, the colloidal latex holds great potential toward PCs coatings on a variety of substrates (glass, plastic and steel) with excellent adhesiveness. Furthermore, we contrive to construct angle-independent structural color films and supraballs, and explore their application in quantum dots (QDs) fluorescence enhancement, which achieved an enhancement effect by more than eight times. From the standpoint of practical applications, we achieved the flexible high-brightness wearable light-emitting diode (LED) displays. This work will lay a foundation for the development of high-efficiency PCs building blocks, and indicate the direction for the meaningful application of CPCs.

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.

Certain oxynitrides (LaTiO2N and TaON) and nitrides (Ta3N5) demonstrate potential as visible-light-driven photocatalysts1. These materials are typically synthesized from oxide precursors by thermal ammonolysis at temperatures up to 950 oC. Accordingly, these oxynitrides and nitrides intrinsically possess an abundance of anion vacancies or defects which act as electron-hole pair recombination centers, thereby limiting photocatalytic performance. In this work, we introduce photonic band gap engineering as a strategy for suppressing electron-hole pair recombination in tantalum nitride thin films, thereby achieving improved photocatalytic performance. Briefly, a series of inverse opal (IO) Ta3N5 thin films with different macropore sizes were fabricated by the colloidal crystal template approach. This kind of approach has been successfully made on TiO2 photocatalysts2. Three batches of monodisperse polymethylmethacrylate (PMMA) colloids of diameters ranging from 230 to 430 nm were first synthesized by the surfactant-free emulsion polymerization of methyl methacrylate. PMMA colloidal crystal thin films were then grown on silica glass slides using a flow-controlled vertical deposition technique. As the meniscus of the suspension moved down the glass slide by pumping up a peristaltic pump, colloidal crystal layer of several microns were deposited. Several drops of TaCl5 solution were placed softly on the edge of the colloidal crystal and immersed into the crystal by capillary phenomena. Subsequently, the interstitial voids in the colloidal crystal films (synthetic opals) were filled with a tantalum hydroxide sol. Ammonolysis of the resulting composite films under an ammonia stream of 200 ml/min at 700ºC yielded IO Ta3N5 thin films, shown in Fig.1. The IO Ta3N5 films possess a 3-dimensionally ordered macroporous structure with periodicity on the scale of visible-light wavelengths, resulting in pseudo photonic band gaps at visible wavelengths. By matching the red-edge of the PBG location with the absorption edge of Ta3N5, electron-hole pair recombination is suppressed and the photocatalytic activity dramatically improved. In this experiment, strong reflection due to [111] photonic bandgap was clearly observed at 543nm for the IO amorphous Ta2O5 film and at 581nm for the IO Ta3N5 film, respectively. Appearance of the IO Ta3N5 film was reddish orange in color but it gave a strong yellow reflection in a certain angle, seen in Fig. 2. Since the optical bandgap of Ta3N5 was estimated to be 2.1eV (=590nm), the IO tantalum nitrides with both optical and bandgaps properly matched could be fabricated. Photocatalystic H2 evolution activity of the IO Ta3N5 under UV or visible ray irradiation will be appeared at the presentation, togather with a comparison with that of Ta3N5 powder. References Tsuyoshi Takata, Chengsi Pan and Kazunari Domen; Sci. Technol. Adv. Mater., 16 (2015) 033506 (18pp).Waterhouse et al., Sci. Rep., 3 (2013) 2849 (5pp). Figure 1

Cephalopods achieve their vivid, dynamic structural coloration through reconfigurable lamellar reflectors, acting as a powerful inspiration for adaptive optical materials. However, synthetic systems that replicate this functionality typically require complex material architectures or multistep fabrication processes. Here, a straightforward strategy is presented for producing magnetically responsive photonic microparticles using linear poly(styrene)‐ b ‐poly(2‐vinylpyridine) block copolymers. Self‐assembly in emulsion droplets yields ellipsoids with stacked lamellar domains that exhibit structural colors spanning the entire visible spectrum. Blending block copolymers with different molecular weights enables precise, continuous control over lamellar periodicity and photonic bandgap spectral position, eliminating the need for chemical modification or the use of swelling agents. Notably, incorporating superparamagnetic iron oxide nanoparticles imparts rapid and reversible magnetic alignment of the microparticles. This orientation aligns the lamellar stacks perpendicular to the magnetic field, resulting in Bragg reflection and angle‐dependent coloration. The reflected wavelength can be tuned by changing the viewing angle, which mimics the dynamic optical responses of biological iridophores. Compared to prior systems, our approach reduces synthetic complexity while maintaining high color intensity and angular tunability. Thus, this work introduces a bio‐inspired materials concept that offers a simple route toward dynamically tunable optical materials for adaptive camouflage, smart coatings, and next‐generation photonic devices.

We have demonstrated for the first time that condensed gel particle suspensions in amorphous-like states display structural color with low angle dependence. This finding is in contrast to the common understanding that a periodic dielectric structure is fundamental to photonic band gap (PBG) production, and it validates the theory that a "tight bonding model" that is applicable to semiconductor systems can also be applied to photonic systems. More practically, this structural colored suspension represents a promising new material for the manufacture of reflective full-color displays with a wide viewing angle and nonfading color materials. This liquid system shows promise as a display material because electronic equipment used for display systems can easily be filled with the liquid in the same way that liquid crystals are currently used.

Structural color comes out of scattering and interference on specific wavelength light by microstructure, it's differ from pigments. If structural color can visualized, it lead to solution of relation between structure and color. In structural color, the color generally evaluated by reflection spectrum. But it's difficult that understand or confirm a color intuition only a graph of reflection spectrum. In this study, structural color is visualized in CIE color system and HSV color system. Reflection spectrums of a male adult Red-flanked bluetail's feather that calculated by finite element method (FEM) are indicated by the two color systems. If absorption is not present, a result color was bluish color. If absorption is present, a result was greenish color. These results agree with a shape of reflection spectrum.

Anti-counterfeiting materials based on colloidal photonic crystals (PCs) have been widely studied due to their inimitable structural color and adjustable photonic band gap (PBG). And the patterned colloidal photonic crystals have been well applied in the field of anti-counterfeiting because of their easy preparation, high performance, and functionalization. The methods for preparing patterned colloidal photonic crystals are outlined, including vertical deposition, spin coating, spraying, and inkjet printing techniques. In this paper, the applications of patterned photonic crystals in the anti-counterfeiting field and their advantages are discussed, the future development of patterned photonic crystals and the challenges they face are also prospected.

Colloidal crystals have been given a lot of attention in the scientific arena because of their versatility. Here we present a drop and slide technique that was adopted to fabricate a 3-D colloidal crystal array from an aqueous solution of monodispersed, highly charged polystyrene spheres. Bragg reflectance spectra were obtained for the array of colloidal crystal at 50% volume fraction of the colloids as a function of time. The time-resolved Bragg reflection spectra observed show a reflection peak maximum gradually moving towards higher energy state during the colloidal crystallization process. The path taken as the wavelength blue-shifts during drying has been hypothesized to be highly dependent on not only the ordering, but also the shape of the liquid-air interface. The study also highlights the crystallization process during drying. The pH effect on reflectance spectra was also studied on the crystal array by introducing various pH solutions into the system. The results of the pH investigation show that the reflectance spectra are blue-shifted as the pH decreases.

Rapid fabrication of colloidal photonic crystals was successfully carried out by spin coating using polymeric beads. To this end, monodisperse polystyrene (PS) nanospheres were synthesized by dispersion polymerization using alcohol as a reaction medium to prepare coating solution. Transmission of visible light at a specific wavelength could be observed, which is measured by transmittance spectra of coating samples. The transmission intensity of light at specific wavelength could be controlled by adjusting rotation speed or the concentration of coating solution, while peak wavelength could be adjusted by the size of PS nanospheres. Inorganic inverse opal like macroporous silica or titania films were fabricated by diffusion of silica or titania nanoparticles through colloidal crystal film, followed by calcination for removal of PS nanospheres. As a demonstrative application of inverse opal films, photocatalytic decomposition of organic dye was performed under UV light illumination by immersing the macroporous titania film in aqueous solution. The rate constant of decomposition reaction could be estimated from transient data of the dye concentration.