Inkjet Printing Based Mono-layered Photonic Crystal Patterning for Anti-counterfeiting Structural Colors

Electrically Responsive Photonic Crystals with Enhanced Suspension Stability and Color Saturation for Electrophoretic Displays and Smart Windows

Electrophoretic displays (EPDs) based on photonic crystals show great potential due to their reduced eye fatigue and low power consumption. However, the current image quality and service life of this system still face great challenges. In this work, we fabricated a new kind of electrically responsive photonic crystal (ERPC) device based on PSMA@SiO2 liquid colloidal crystals (LCCs) for EPDs. By introduction of the PSMA core with lower density and higher refractive index, the suspension stability and color saturation of PSMA@SiO2 LCCs were greatly enhanced compared with those of bare SiO2 LCCs. The PSMA@SiO2 LCCs showed brilliant colors, wide color tuning range (∼200 nm), and good reversibility under low voltages (<4 V). Interestingly, the transparency of PSMA@SiO2 LCCs could also be obviously regulated by an electric field, which was different from the traditional ways that change the thickness of PCs or contrast of refractive index (Δn) between the nanospheres and matrix. This transparency modulation offered a novel idea for the transmittance control of smart windows. As a proof of concept, we fabricated a new type of patterned ERPC device to demonstrate their potential in electrophoretic displays and smart windows with controllable transmittance under an electric field.

Electrically triggered photonic crystal anti-counterfeiting tags with multi-level response fabricated by regioselective modification of ITO electrode surface

Anticounterfeiting materials based on the photonic crystal (PC) have attracted great interest due to their unique visual effects originating from the changeable structural colors under various external stimuli. However, there still are challenges to improving the anticounterfeiting performance by enhancing the complexity and diversity of the color changes. Here, we fabricated an electrically triggered anticounterfeiting tag by encapsulating the responsive PC with the surface-modified and patterned ITO electrode. The degree of Au deposition or chemical etching in different regions of the ITO was precisely controlled to achieve multi-level differentiated electrical responses, which made the invisible pattern of the tag at 0 V be "revealed in multicolor form" or "gradually revealed" under increasing voltages. The tag possessed two working modes, more diversified visual effects, good usability, and reversibility, which let it become a potentially useful material for anti-counterfeiting applications in the future.

Inkjet Printed Photonic Cellulose Nanocrystal Patterns

Naturally-sourced cellulose nanocrystals (CNCs) are elongated, birefringent nanoparticles that can undergo cholesteric self-assembly in water to produce vibrant, structurally colored films. As such, they are an ideal candidate for use as sustainable and cost-effective inks in the printing of scalable photonic coatings and bespoke patterns. However, the small volume and large surface area of a sessile CNC drop typically leads to rapid evaporation, resulting in microfilms with a coffee-stain-like morphology and very weak coloration. Here, it is demonstrated that inkjet printing of CNC drops directly through an immiscible oil layer can immediately inhibit water loss, resulting in reduced internal mass flows and greater time for cholesteric self-assembly. The color of each microfilm is determined by the initial composition of the drop, which can be tuned on-demand by exploiting the overprinting and coalescence of multiple smaller drops of different inks. This enables the production of multicolored patterns with complex optical behaviors, such as angle-dependent color and polarization-selective reflection. Finally, the array can be made responsive to stimuli (e.g., UV light, polar solvent) by the inclusion of a degradable additive. This suite of functional properties promotes inkjet-printed photonic CNC arrays for smart colorimetric labeling or optical anticounterfeiting applications.

Communicating Supraparticles to Enable Perceptual, Information-Providing Matter

Materials are the fundament of the physical world, whereas information and its exchange are the centerpieces of the digital world. Their fruitful synergy offers countless opportunities for realizing desired digital transformation processes in the physical world of materials. Yet, to date, a perfect connection between these worlds is missing. From the perspective, this can be achieved by overcoming the paradigm of considering materials as passive objects and turning them into perceptual, information-providing matter. This matter is capable of communicating associated digitally stored information, for example, its origin, fate, and material type as well as its intactness on demand. Herein, the concept of realizing perceptual, information-providing matter by integrating customizable (sub-)micrometer-sized communicating supraparticles (CSPs) is presented. They are assembled from individual nanoparticulate and/or (macro)molecular building blocks with spectrally differentiable signals that are either robust or stimuli-susceptible. Their combination yields functional signal characteristics that provide an identification signature and one or multiple stimuli-recorder features. This enables CSPs to communicate associated digital information on the tagged material and its encountered stimuli histories upon signal readout anywhere across its life cycle. Ultimately, CSPs link the materials and digital worlds with numerous use cases thereof, in particular fostering the transition into an age of sustainability.

Reconfigurable Grating Diffraction Structural Color in Self-Assembled Colloidal Crystals

Self-assembled colloidal crystals display structural colors due to light diffracted from their microscale, ordered structure. This color arises due to Bragg reflection (BR) or grating diffraction (GD); the latter mechanism is much less explored than the former. Here the design space for generating GD structural color is identified and its relative advantages are demonstrated. Electrophoretic deposition is used to self-assemble crystals with fine crystal grains from colloids of diameter 1.0 µm. The structural color in transmission is tunable across the full visible spectrum. The optimum optical response-represented by both color intensity and saturation-is observed at low layer number (≤5 layers). The spectral response is well predicted by Mie scattering of the crystals. Taken together, the experimental and theoretical results demonstrate that vivid grating colors with high color saturation can be produced from thin layers of micron-sized colloids. These colloidal crystals extend the potential of artificial structural color materials.

Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

Butterfly-Inspired Tri-State Photonic Crystal Composite Film for Multilevel Information Encryption and Anti-Counterfeiting

Counterfeiting is a worldwide issue and has long troubled legitimate businesses, while nowadays anti-counterfeiting materials and technology are still insufficient to combat the escalating counterfeit behaviors. Inspired by hindwing structure of Troides magellanus, a new kind of anti-counterfeiting material taking advantage of both physical and chemical structures to display multiple optical states is prepared. The chemical units (luminescent lanthanide) are blended with physical units (monodispersed colloidal particles) and mediating molecules, which are then assembled into a photonic crystal structure at room temperature in less than 10 s through a new assembly technique called molecule-mediated shear-induced assembly technique (MSAT). The as-prepared photonic crystal films feature three unique optical states, each displaying structural, fluorescent, and phosphorescent color under different lighting conditions, which integrates colors from both physical and chemical origins. Furthermore, by incorporating different luminescent materials into different parts of the photonic crystal pattern, a high-level information encryption system is designed to be capable of carrying three distinct types of information. Thanks to this powerful tool of MSAT, it is now possible to assemble different-sized, even irregular non-spherical units with monodispersed spherical units into high-quality photonic crystal films, which provides easy access to incorporating new features into photonic crystal systems.

Multilayer Smart Holographic Label with Integrated RFID for Product Security and Monitoring

Counterfeiting presents a major economic problem and an important risk for the public health and safety of individuals and countries. To make the counterfeiting process more difficult, and to ensure efficient authentication, a solution would be to attach anti-counterfeit labels that include a radio frequency identification (RFID) element to the products. This can allow real-time quality check along the entire supply chain. In this paper we present the technology optimized to obtain a multilayer holographic label with a high degree of security, patterned on a thin zinc sulfide film of a semi-transparent holographic foil rather than on the standard substrate for diffractive optical elements (metallized foil). The label is applied onto the product surface or packaging for anti-counterfeit protection. The developed multilayer structure contains various elements such as: a holographic background, nanotext-type elements, holographic elements, and an RFID antenna. The employed semi-transparent holographic foil offers the RFID antenna the possibility to transmit the electromagnetic signal through the label and thus to maximize the antenna footprint, achieving up to 10 m reading distance, with a 6 cm × 6 cm label, much smaller than the commercial standard (minimum 10 cm × 10 cm).

Recent Advances in Printable Flexible Optical Devices: From Printing Technology and Optimization Strategies to Perspectives

Recently, flexible optical devices have triggered booming developments in various research fields, including display equipment, sensors, energy conversion, and so on, due to their high compatibility, portability, and wearability. With the advantages of strong design ability, high precision, and high integration, printing technologies have been recognized as promising methods to realize flexible optical devices. In this Perspective, recent progress on printing strategies for fabricating flexible optical devices are introduced systematically. First, through adjusting the composition of inks, selecting flexible substrates, and controlling external stimulation, fabrication of flexible optical devices based on inkjet printing is illustrated. Then, flexible optical devices fabricated by template-induced printing, 3D printing, slot-die printing, and screen printing are summarized. Finally, prospects and future development directions based on printing technology for flexible optical devices are proposed.

Refractory Metals and Oxides for High-Temperature Structural Color Filters

Refractory metals have recently garnered significant interest as options for photonic applications due to their superior high-temperature stability and versatile optical properties. However, most previous studies only consider their room-temperature optical properties when analyzing these materials' behavior as optical components. Here, we demonstrate structural color pixels based on three refractory metals (Ru, Ta, and W) for high-temperature applications. We quantify their optical behavior in an oxygenated environment and determine their dielectric functions after heating up to 600 °C. We use in situ oxidation, a fundamental chemical reaction, to form nanometer-scale metal oxide thin-film bilayers on each refractory metal. We fully characterize the behavior of the newly formed thin-film interference structures, which exhibit vibrant color changes upon high-temperature treatment. Finally, we present optical simulations showing the full range of hues achievable with a simple two-layer metal oxide/metal reflector structure. All of these materials have melting points >1100 °C, with the Ta-based structure offering high-temperature stability, and the Ru- and W-based options providing an alternative for reversible color filters, at high temperatures in inert or vacuum environments. Our approach is uniquely suitable for high-temperature photonics, where the oxides can be used as conformal coatings to produce a wide variety of colors across a large portion of the color gamut.

Large-scale vivid metasurface color printing using advanced 12-in. immersion photolithography

Nanostructures exhibiting optical resonances (so-called nanoantennas) have strong potential for applications in color printing and filtering with sub-wavelength resolution. While small scale demonstrations of these systems are interesting as a proof-of-concept, their large scale and volume fabrication requires deeper analysis and further development for industrial adoption. Here, we evaluate the color quality produced by large size nanoantenna arrays fabricated on a 12-in. wafer using deep UV immersion photolithography and dry etching processes. The color reproduction and quality are analyzed in context of the CIE color diagram, showing that a vivid and vibrant color palette, almost fully covering the sRGB color space, can be obtained with this mass-manufacturing-ready fabrication process. The obtained results, thus, provide a solid foundation for the potential industrial adoption of this emerging technology and expose the limits and challenges of the process.

Inkjet Printing of Structurally Colored Self-Assembled Colloidal Aggregates

Structurally colored materials offer increased stability, high biocompatibility, and a large variety of colors, which can hardly be reached simultaneously using conventional chemical pigments. However, for practical applications, such as inkjet printing, it is vital to compartmentalize these materials in small building blocks (with sizes ideally below 5 μm) and create "ready-to-use" inks. The latter can be achieved by using photonic balls (PB): spherical aggregates of nanoparticles. Here, we demonstrate, for the first time, how photonic ball dispersions can be used as inkjet printing inks. We use solvent drying techniques to manufacture structurally colored colloidal aggregates. The as-fabricated photonic balls are dispersed in pentanol to form ink. A custom-made inkjet printing platform equipped with an industrial printhead and recirculation fluidic system is used to print complex structurally colored patterns. We increase color purity and suppress multiple scattering by introducing carbon black as a broadband light absorber.

Dual-Colored Janus Microspheres with Photonic and Plasmonic Faces

Photonic and plasmonic colors, stemming from nanostructures of dielectric materials and metals, are promising for pigment-free coloration. In particular, nanostructures with structural colors have been employed in stimuli-responsive Janus microparticles to provide active color pixels. Here, the authors report a simple strategy to produce electro-responsive Janus microspheres composed of photonic and plasmonic faces for active color change. The photonic microspheres are first prepared by self-assembly of silica particles in emulsion droplets of photocurable resin. The silica particles form 3D crystalline arrays in the interior and 2D hexagonal arrays on the interface. The emulsion droplets are photocured and the silica particles are selectively removed to make porous photonic microspheres with hexagonal arrays of dimples on the surface. Directional deposition of gold or aluminum on the photonic microsphere develops plasmonic color on the top hemisphere while maintaining photonic color on the bottom hemisphere. Moreover, the metal deposited on one side renders the Janus microspheres electro-responsive. Therefore, the photonic and plasmonic colors are switchable by the orientation control of the Janus microspheres with an external electric field. The photonic and plasmonic colors are independently adjustable by employing two different sizes of silica particles in core-shell emulsion drops.

Recent Progress in Responsive Structural Color

Structural color has been regarded as an ideal alternative to pigments because of the advantages of environmental friendliness, resistance to fading, and dynamic regulation. Responsive structural color can give real-time visible feedback to external stimuli and thus has great prospects in many applications, such as displays, sensing, anticounterfeiting, information storage, and healthcare monitoring. In this Perspective, we elucidate basic concepts, controllable fabrications, and promising applications of responsive structural colors. In particular, we systematically summarize the general regulation mode of all kinds of responsive structural color systems. First, we introduce the basic chromogenic structures as well as the regulation modes of responsive structural color. Second, we present the fabrication methods of patterned structural color. Then, the promising applications of responsive structural color systems are highlighted in detail. Finally, we present the existing challenges and future perspectives on responsive structural colors.

Structural color printing via polymer-assisted photochemical deposition

Structural color printings have broad applications due to their advantages of long-term sustainability, eco-friendly manufacturing, and ultra-high resolution. However, most of them require costly and time-consuming fabrication processes from nanolithography to vacuum deposition and etching. Here, we demonstrate a new color printing technology based on polymer-assisted photochemical metal deposition (PPD), a room temperature, ambient, and additive manufacturing process without requiring heating, vacuum deposition or etching. The PPD-printed silver films comprise densely aggregated silver nanoparticles filled with a small amount (estimated <20% volume) of polymers, producing a smooth surface (roughness 2.5 nm) even better than vacuum-deposited silver films (roughness 2.8 nm) at ~4 nm thickness. Further, the printed composite films have a much larger effective refractive index n (~1.90) and a smaller extinction coefficient k (~0.92) than PVD ones in the visible wavelength range (400 to 800 nm), therefore modulating the surface reflection and the phase accumulation. The capability of PPD in printing both ultra-thin (~5 nm) composite films and highly reflective thicker film greatly benefit the design and construction of multilayered Fabry-Perot (FP) cavity structures to exhibit vivid and saturated colors. We demonstrated programmed printing of complex pictures of different color schemes at a high spatial resolution of ~6.5 μm by three-dimensionally modulating the top composite film geometries and dielectric spacer thicknesses (75 to 200 nm). Finally, PPD-based color picture printing is demonstrated on a wide range of substrates, including glass, PDMS, and plastic, proving its broad potential in future applications from security labeling to color displays.

Upconversion Nanoparticle-Integrated Bilayer Inverse Opal Photonic Crystal Film for the Triple Anticounterfeiting

Optical anticounterfeiting plays a vital role in information security because it can be recognized by the naked eye and is difficult to imitate. Herein, a hydrophilic modified upconversion nanoparticle (M-UCNP)-integrated bilayer inverse opal photonic crystal (IOPC) film was designed in which the luminescent M-UCNPs were deposited on the surface of the optimized bilayer structure with double photonic stop bands. The structure which can modulate light to produce structural colors can also enhance the upconversion luminescence (UCL) to improve the anticounterfeiting effect synergistically. On the one hand, the reflection colors from green to blue were observed in the specular angles on the front (540-layer) of the film. Meanwhile, the scattering colors under nonspecular angles from red to blue on the back (808-layer) appeared in the natural light. On the other hand, the bilayer structure in which the 808-layer functions as a "secondary excitation source" to improve the intensity of the excitation light on M-UCNPs and the 540-layer reflects the emission light of the M-UCNPs to enhance the UCL intensity endows the film with good night vision ability. Finally, the dual-mode structural colors and enhanced UCL of the patterned film work together to realize triple anticounterfeiting in banknotes.

Bioinspired Supramolecular Photonic Composites: Construction and Emerging Applications

Natural organisms have evolved fascinating structural colors to survive in complex natural environments. Artificial photonic composites developed by imitating the structural colors of organisms have been applied in displaying, sensing, biomedicine, and many other fields. As emerging materials, photonic composites mediated by supramolecular chemistry, namely, supramolecular photonic composites, have been designed and constructed to meet emerging application needs and challenges. This feature article mainly introduces the constructive strategies, properties, and applications of supramolecular photonic composites. First, constructive strategies of supramolecular photonic composites are summarized, including the introduction of supramolecular polymers into colloidal photonic array templates, co-assembly of colloidal particles (CPs) with supramolecular polymers, self-assembly of soft CPs, and compounding photonic elastomers with functional substances via supramolecular interactions. Supramolecular interactions endow photonic composites with attractive properties, such as stimuli-responsiveness and healability. Subsequently, the unique optical and mechanical properties of supramolecular photonic composites are summarized, and their applications in emerging fields, such as colorful coatings, real-time and visual motion monitoring, and biochemical sensors, are introduced. Finally, challenges and perspectives in supramolecular photonic composites are discussed. This feature article provides general strategies and considerations for the design of photonic materials based on supramolecular chemistry. This article is protected by copyright. All rights reserved.

Carbon Dots Embedded in Cellulose Film: Programmable, Performance-Tunable, and Large-Scale Subtle Fluorescent Patterning by in Situ Laser Writing

Fluorescent patterns with multiple functions enable high-security anti-counterfeiting labels. Complex material synthesis and patterning processes limit the application of multifunctional fluorescent patterns, so the technology of in situ fluorescent patterning with tunable multimodal capabilities is becoming more necessary. In this work, an in situ fluorescent patterning technology was developed using laser direct writing on solid cellulose film at ambient conditions without masks. The fluorescent intensity and surface microstructure of the patterns could be adjusted by programmable varying of the laser parameters simultaneously. During laser direct writing, carbon dots are generated in situ in a cellulose ester polymer matrix, which significantly simplifies the fluorescent patterning process and reduces the manufacturing cost. Interestingly, the tunable fluorescent intensity empowers the fabrication of visual stereoscopic fluorescent patterns with excitation dependence, further improving its anti-counterfeiting performance. The obtained fluorescent patterns still show ultrahigh optical properties after being immersed in an acid/base solution (pH 5-12) over one month. In addition, the anti-UV performance of the obtained laser-patterned film with transmittance around 90% is comparable to that of commercial UV-resistant films. This work provided an advanced and feasible approach to fabricating programmable, performance-tunable, subtle fluorescent patterns in large-scale for industrial application.

Reconfigurable Optical Physical Unclonable Functions Enabled by VO2 Nanocrystal Films

Optical physical unclonable function (PUF) is one of the most promising hardware security solutions, which has been proven to be resistant to machine learning attacks. However, the disordered structures of the traditional optical PUFs are usually deterministic once they are manufactured and therefore exhibit fixed challenge-response behaviors. Herein, a reconfigurable PUF (R-PUF) is proposed and demonstrated by using the reversible phase transition behavior of VO₂ nanocrystals combined with TiO₂ disordered nanoparticles. Both the simulation and experiment results show that the near-infrared laser speckle pattern of the R-PUF can be almost completely altered after the phase transition of VO₂ nanocrystals, resulting in a reconfigurable and reproducible optical response. The similarity of the response speckles shows an obvious hysteresis loop during the rise and drop of temperature, providing a simple way to regulate and control the response behaviors of the R-PUF. More importantly, the hysteretic characteristic provides a new dimension to describe the challenge-response behavior of the R-PUF besides the laser speckle, providing an effective way to improve the security and encoding capacity of the optical PUFs. The proposed R-PUF can be employed as a promising security primitive for high robustness and high-security authentication and encryption.

Bio-inspired structural colors and their applications

Structural colors, generated by the interaction of interference, diffraction, and scattering between incident light and periodic nanostructured surfaces with features of the same scale with incident visible light wavelengths, have recently attracted intense interest in a wide range of research fields, due to their advantages such as various brilliant colors, long-term stability and environmental friendliness, low energy consumption, and mysterious biological functions. Tremendous effort has been made to design structural colors and considerable progress has been achieved in the past few decades. However, there are still significant challenges and obstacles, such as durability, portability, compatibility, recyclability, mass production of structural-color materials, etc., that need to be solved by rational structural design and novel manufacturing strategies. In this review, we summarize the recent progress of bio-inspired structural colors and their applications. First, we introduce several typical natural structural colors displayed by living organisms from fundamental optical phenomena, including interference, diffraction grating, scattering, photonic crystals effects, the combination of different phenomena, etc. Subsequently, we review recent progress in bio-inspired artificial structural colors generated from advanced micro/nanoscale manufacturing strategies to relevant biomimetic approaches, including self-assembly, template methods, phase conversion, magnetron sputtering, atomic layer deposition, etc. Besides, we also present the current and potential applications of structural colors in various fields, such as displays, anti-counterfeiting, wearable electronics, stealth, printing, etc. Finally, we discuss the challenges and future development directions of structural colors, aiming to push forward the research and applications of structural-color materials.

Shaping in the Third Direction; Synthesis of Patterned Colloidal Crystals by Polyester Fabric-Guided Self-Assembly

A polyester fabric with rectangular openings was used as a sacrificial template for the guiding of a sub-micron sphere (polystyrene (PS) and silica) aqueous colloid self-assembly process during evaporation as a patterned colloidal crystal (PCC). This simple process is also a robust one, being less sensitive to external parameters (ambient pressure, temperature, humidity, vibrations). The most interesting feature of the concave-shape-pattern unit cell (350 μm × 400 μm × 3 μm) of this crystal is the presence of triangular prisms at its border, each prism having a one-dimensional sphere array at its top edge. The high-quality ordered single layer found inside of each unit cell presents the super-prism effect and left-handed behavior. Wider yet elongated deposits with ordered walls and disordered top surfaces were formed under the fabric knots. Rectangular patterning was obtained even for 20 μm PS spheres. Polyester fabrics with other opening geometries and sizes (~300–1000 μm) or with higher fiber elasticity also allowed the formation of similar PCCs, some having curved prismatic walls. A higher colloid concentration (10–20%) induces the formation of thicker walls with fiber-negative replica morphology. Additionally, thick-wall PCCs (~100 μm) with semi-cylindrical morphology were obtained using SiO₂ sub-microspheres and a wavy fabric. The colloidal pattern was used as a lithographic mask for natural lithography and as a template for the synthesis of triangular-prism-shaped inverted opals.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Development of Mechanically Reliable and Transparent Photochromic Film Using Solution Blowing Spinning Technology for Anti-Counterfeiting Applications

Photochromic materials have attracted broad interest to enhance the anti-counterfeiting of commercial products. In order to develop anti-counterfeiting mechanically reliable composite materials, it is urgent to improve the engineering process of both the material and matrix. Herein, we report on the development of anti-counterfeiting mechanically reliable nanocomposites composed of rare-earth doped aluminate strontium oxide phosphor (RESA) nanoparticles (NPs) immobilized into the thermoplastic polyurethane-based nanofibrous film successfully fabricated via the simple solution blowing spinning technology. The generated photochromic film exhibits an ultraviolet-stimulated anti-counterfeiting property. Different films of different emissive properties were generated using different total contents of RESA. Transmission electron microscopy was utilized to investigate the morphological properties of RESA NPs to display a particle diameter of 3-17 nm. The morphologies, compositions, optical transmittance, and mechanical performance of the produced photochromic nanofibrous films were investigated. Several analytical methods were employed, including energy-dispersive X-ray spectroscopy, scanning electron microscopy, and Fourier-transform infrared spectrometry. The fibrous diameter of RESA-TPU was in the range of 200-250 nm. In order to ensure the development of transparent RESA-TPU film, RESA must be prepared in the nanosized form to allow better dispersion without agglomeration in the TPU matrix. The luminescent RESA-TPU film displayed an absorbance intensity at 367 nm and two emission intensities at 431 and 517 nm. The generated RESA-TPU films showed an enhanced hydrophobicity without negatively influencing their original appearance and mechanical properties. Upon irradiation with ultraviolet light, the transparent nanofibrous films displayed rapid and reversible photochromism to greenish-yellow without fatigue. The produced anti-counterfeiting films demonstrated stretchable, flexible, and translucent properties. As a simple sort of anti-counterfeiting substrates, the current novel photochromic film provides excellent anti-counterfeiting strength at low-cost as an efficient method to develop versatile materials with high mechanical strength to create an excellent market as well as adding economic and social values.

Fabrication of robust and reusable mold with nanostructures and its application to anti-counterfeiting surfaces based on structural colors

In this study, we report a method to fabricate molds and flexible stamps with 2D photonic crystal structures. This includes self-assembly of polystyrene particles into monolayer, oxygen reactive ion etching, thin film (chromium (Cr)) deposition, and polydimethylsiloxane replication. By tuning the thickness of Cr layer, reusable master molds with nano bumps or nano concaves could be prepared selectively. We showed that the replicated flexible stamps out of these molds exhibited structural colors. Characteristics of the colors depended on viewing angle, brightness of background and light source. And the colors even faded out when the background is white or when the stamp was bent. By using this feature, possible strategies for anti-counterfeiting applications have been suggested in this study. Since the molds are reusable and the fabrication method is simple and cost-effective, this study is expected to contribute to nano devices for industries in future.

Luminescent Erbium-Doped Silicon Thin Films for Advanced Anti-Counterfeit Labels

The never-ending struggle against counterfeit demands the constant development of security labels and their fabrication methods. This study demonstrates a novel type of security label based on downconversion photoluminescence from erbium-doped silicon. For fabrication of these labels, a femtosecond laser is applied to selectively irradiate a double-layered Er/Si thin film, which is accomplished by Er incorporation into a silicon matrix and silicon-layer crystallization. The study of laser-induced heating demonstrates that it creates optically active erbium centers in silicon, providing stable and enhanced photoluminescence at 1530 nm. Such a technique is utilized to create two types of anti-counterfeiting labels. The first type is realized by the single-step direct laser writing of luminescent areas and detected by optical microscopy as holes in the film forming the desired image. The second type, with a higher degree of security, is realized by adding other fabrication steps, including the chemical etching of the Er layer and laser writing of additional non-luminescent holes over an initially recorded image. During laser excitation at 525 nm of luminescent holes of the labels, a photoluminescent picture repeating desired data can be seen. The proposed labels are easily scalable and perspective for labeling of goods, securities, and luxury items.

Nonintrusively Adjusting Structural Colors of Sealed Two-Dimensional Photonic Crystals: Immediate Transformation between Transparency and Intense Iridescence and Their Applications

Responsive photonic crystals (PCs), which can adjust structural colors in response to external stimuli, show great potential applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting. In contrast, conventional structure-intrusive adjustment manners that external stimuli directly interact with the ordered arrays may lead to structural damage or longer response time. Here, a noninvasive adjustment of the structural colors of two-dimensional (2D) PCs (2D-PCs) is explored based upon diffraction theory. Sealed 2D-PCs and 2D inverse opal photonic crystal (IOPC) flexible devices are prepared. They are highly transparent in air but immediately exhibit intense viewing angle-dependent structural colors after being dipped in water. The mechanism of transparent-iridescent immediate transformation is explained by Bragg's law. The design mechanism is examined by numerical simulation and spectral shifts in different external media. We demonstrate its applications in the fields of information encryption and anticounterfeiting by using the transparent-iridescent immediate transformation of sealed 2D-PC patterns and 2D IOPC free-standing films sealed on the product surface. Because of the strong contrast between transparency and intense iridescence, reversible and immediate transformation, and durability, sealed 2D-PCs and 2D IOPC flexible devices designed by the noninvasive adjustment strategy will lead to a variety of new applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting.

Artificial Structural Colors and Applications

Structural colors are colors generated by the interaction between incident light and nanostructures. Structural colors have been studied for decades due to their promising advantages of long-term stability and environmentally friendly properties compared with conventional pigments and dyes. Previous studies have demonstrated many artificial structural colors inspired by naturally generated colors from plants and animals. Moreover, many strategies consisting of different principles have been reported to achieve dynamically tunable structural colors. Furthermore, the artificial structural colors can have multiple functions besides decoration, such as absorbing solar energy, anti-counterfeiting, and information encryption. In the present work, we reviewed the typical artificial structural colors generated by multilayer films, photonic crystals, and metasurfaces according to the type of structures, and discussed the approaches to achieve dynamically tunable structural colors.

Designable structural coloration by colloidal particle assembly: from nature to artificial manufacturing

Structural color attracts considerable scientific interests and industrial explorations in various fields for the eco-friendly, fade-resistant, and dynamic advantages. After the long-period evolution, nature has achieved the optimized color structures at various length scales, which has inspired people to learn and replicate them to improve the artificial structure color. In this review, we focus on the design of artificial structural colors based on colloidal particle assembly and summarize the functional bioinspired structure colors. We demonstrate the design principles of biomimetic structural colors via the precise structure engineering and typical bottom-up methods. Some main applications are outlined in the following chapter. Finally, we propose the existing challenges and promising prospects. This review is expected to introduce the recent design strategies about the artificial structure colors and provide the insights for its future development.

Color matching ability of resin composites incorporating supra-nano spherical filler producing structural color

OBJECTIVE

The aim of this study was to evaluate the optical properties of supra-nano spherical fillers with different diameters and the color matching ability of resin composites (RC) incorporating these fillers.

METHODS

Two types of SiO₂-ZrO₂ nano fillers with different diameters (150nm and 260nm) were used. The size distribution of each filler was measured and filler morphology was observed. The colors and spectral reflection spectra were measured by a spectral reflectometer. Experimental RCs incorporating ϕ150-nm/ϕ260-nm filler (D150RC/D260RC) were prepared. For the base dentin part, disc specimens (Estelite Astelia: A1B, A2B, A3B, A3.5B, or A4B) were prepared with a cylindrical cavity. Estelite Astelia with NE shade was layered on top as the enamel layer. Disk specimens with different cavity depths were prepared using A3B shade. Experimental RC was used to fill the cavity, and spectral reflection spectrums were obtained and analyzed. Filtek Supreme Ultra (FSU) with A3B shade was used (n=10) as a control.

RESULTS

Both ϕ150-nm and ϕ260-nm nano fillers showed uniform spherical shape and exhibited no aggregation. The maximum peaks of the spectral reflection spectra of the ϕ150-nm and ϕ260-nm nano fillers were 380nm and 580nm, producing structural colors close to blue and yellow, respectively. The spectral reflection spectrum of FSU had a broad peak at 540nm, and D150RC had a significant peak at 420nm. The D260RC specimen had a broad peak at 680nm. The peaks of D150RC and D260RC significantly decreased in accordance with the shift in base RC shade from A1B to A4B. There was no significant difference in the peak of the reflection spectral spectra among different cavity depths of D260RC. These results suggest that the experimental RC could reflect base RC colors via the matrix resin, and the amount of transmitted light from the base RC was not much different with cavity depth.

SIGNIFICANCE

D260RC producing structural color demonstrated a broad spectrum and reduction in brightness and chromatic value by adapting to surrounding restorative materials, suggesting its ability to enhance the chameleon (blending) effects to improve color matching. D260RC showed better color matching ability than resin composite containing uniformly sized ϕ150-nm SiO₂-ZrO₂ supra-nano spherical filler.

Photonic Plasticines with Uniform Structural Colors, High Processability, and Self-Healing Properties

Despite the vast variety of colloidal superstructures available in soft matter photonics, it remains challenging to balance the trade-off between their optical microstructures and material processability. By synergizing colloidal photonics and dynamic chemistry, a type of photonic "plasticine" with characteristics of uniform structural colors, high processability, and self-healing is demonstrated. Specifically, a boronate ester bond-based macromonomer is first prepared through complexation between the diols of polyvinyl alcohol and the boronic acid group of 3-(acrylamido) phenylboronic acid in the presence of concentrated silica colloids. Upon photopolymerization, the dynamic photonic plasticine is formed in situ as the result of the crosslinking of the boronate ester bonded networks. The randomly packed colloids inside the plasticine compose the amorphous photonic crystals, giving rise to angle-independent structural colors that would not compromise during subsequent processing steps; the reversible nature of the boronate ester bonds endows the plasticine with self-adaptable and self-healing properties. Further, the plasticine is also compatible with common shaping methods, that is, cutting, molding, and carving, and thus can be facilely processed into 3D structural colored objects, holding great potentials in fields such as bio-encoding, optical filters, anti-counterfeiting, etc.

Photonics in nature and bioinspired designs: sustainable approaches for a colourful world

Biological systems possess nanoarchitectures that have evolved for specific purposes and whose ability to modulate the flow of light creates an extraordinary diversity of natural photonic structures. In particular, the striking beauty of the structural colouration observed in nature has inspired technological innovation in many fields. Intense research has been devoted to mimicking the unique vivid colours with newly designed photonic structures presenting stimuli-responsive properties, with remarkable applications in health care, safety and security. This review highlights bioinspired photonic approaches in this context, starting by presenting many appealing examples of structural colours in nature, followed by describing the versatility of fabrication methods and designed coloured structures. A particular focus is given to optical sensing for medical diagnosis, food control and environmental monitoring, which has experienced a significant growth, especially considering the advances in obtaining inexpensive miniaturized systems, more reliability, fast responses, and the use of label-free layouts. Additionally, naturally derived biomaterials and synthetic polymers are versatile and fit many different structural designs that are underlined. Progress in bioinspired photonic polymers and their integration in novel devices is discussed since recent developments have emerged to lift the expectations of smart, flexible, wearable and portable sensors. The discussion is expanded to give emphasis on additional functionalities offered to related biomedical applications and the use of structural colours in new sustainable strategies that could meet the needs of technological development.

Printing of Colorful Cellulose Nanocrystalline Patterns Visible in Linearly Polarized Light

This study addresses the inkjet printing approach for fabrication of cellulose nanocrystalline (CNC) patterns with tunable optical properties varied by the thickness of deposited layers. In particular, forming functional patterns visible only in linearly polarized light is of the primary interest. The possibility of controlling the bright iridescent color response associated with the birefringence in the chiral anisotropic structure of inkjet-printed layers of CNC with sulfo-groups (s-CNC) has been thoroughly investigated. In this connection, we have elaborated an appropriate synthesis sequence for deriving printable inks in the form of sedimentation-stable s-CNC colloids with various concentrations of solid phase and experimentally determined the optimal regimes of their inkjet printing. For this purpose, the rheological parameters and s-CNC particle concentration have also been optimized. The study is accomplished with a comprehensive optical characterization of the deposited s-CNC layers with variable thickness, drying conditions, and the polarization state. The experimental results demonstrate the feasibility of inkjet printing technology to perform the precise fabrication of optically active s-CNC patterns with variable optical properties. These results are particularly relevant for applications requiring special conditions of color demonstration in security printing for such as anticounterfeiting applications, polygraphy decoration printing, and color photo filters.

Robust Structurally Colored Coatings Composed of Colloidal Arrays Prepared by the Cathodic Electrophoretic Deposition Method with Metal Cation Additives

Structurally colored coatings composed of colloidal arrays of monodisperse spherical particles have attracted great attention owing to their versatile advantages, such as low cost, resistance to fading, and low impacts on the environment and human health. However, the weak mechanical stability is considered to be a major obstacle for their practical applications as colorants. Although several approaches based on the addition of polymer additives to enhance the adhesion of particles have been reported, the challenge remains to develop a strategy for the preparation of structurally colored coatings with extremely high robustness using a simple process. Here, we have developed a novel approach to fabricate robust structurally colored coatings by cathodic electrophoretic deposition. The addition of a metal salt, i.e., Mg(NO₃)₂, to the coating dispersion allows SiO₂ particles to have a positive charge, which enables the electrophoresis of SiO₂ particles toward the cathode. At the cathode, Mg(OH)₂ codeposits with SiO₂ particles because OH^- ions are generated by the decomposition of dissolved oxygen and NO₃^- ions. The mechanical stability of the colloidal arrays obtained by this process is remarkably improved because Mg(OH)₂ facilitates the adhesion of the particles and substrates. The brilliant structural color is maintained even after several cycles of the sandpaper abrasion test. We have also demonstrated the coating on a stainless steel fork. This demonstration reveals that our approach enables a homogeneous coating on a complicated surface. Furthermore, the high durability of the coating is clarified because the coating did not peel off even when the fork was stuck into a plastic eraser. Therefore, the coating technique developed here will provide an effective method for the pervasive application of the structural color as a colorant.

Putting Ink into Polyion Micelles: Full-Color Anticounterfeiting with Water/Organic Solvent Dual Resistance

Anticounterfeiting paintings are usually with limited colors and easy blurring and need to be dispersed in an environmentally unfriendly organic solvent. We report a set of water-based polyion micellar inks to solve all these problems. Upon complexation of reversible coordination polymers formed with rare earth metal ions Eu³⁺ and Tb³⁺ and the aggregation-induced emission ligand tetraphenylethylene-L₂EO₄ with oppositely charged block polyelectrolyte P2MVP₂₉-b-PEO₂₀₅, we are able to generate polyion micelles displaying three elementary emission colors of red (R) (Φ_Eu³⁺ = 24%), green (G) (Φ_Tb³⁺ = 7%), and blue (B) (Φ_TPE = 9%). Full-spectrum emission and white light emission (0.34, 0.34) become possible by simply mixing the R, G, and B micelles at the desired fraction. Strikingly, the micellar inks remain stable even after soaking in water or organic solvents (ethyl acetate, ethanol, etc.) for 24 h. We envision that polyion micelles would open a new paradigm in the field of anticounterfeiting.

Inkjet Printing Humidity Sensing Pattern Based on Self-Organizing Polystyrene Spheres

This study is devoted to the development of photonic patterns based on polystyrene spheres (PSS) incorporated in chitosan hydrogels by inkjet printing. Using this method, high-resolution encrypted images that became visible only in high humidity were obtained. Inks based on PSS with carboxylic groups on the surface were made, and their rheological parameters (viscosity, surface tension, and ζ-potential) were optimized according to the Ohnesorge theory. The obtained value of the ζ-potential indicated the stability of the synthesized colloidal inks. The dependences of the printing parameters on the concentration of ethylene glycol in PSS dispersion, the drop spacing, the shape of the printed pattern, waveform, the temperature of the printing process, and the degree of ordering of the PSS-based photonic crystal were investigated. The scanning electronic microscope (SEM) images confirmed that the optimal self-organization of PSS was achieved at the following values of 0.4% weight fraction (wt%) carboxylic groups, the drop spacing of 50 μm, and the temperature of the printing table of 25 °C. High-resolution microstructures were obtained by drop-on-demand printing with a deposited drophead diameter of 21 μm and an accuracy of ±2 μm on silicon and glass substrates. The deposition of chitosan-based hydrogels on the obtained polystyrene photonic crystals allowed reversibly changing the order of the diffraction lattice of the photonic crystal during the swelling of the hydrogel matrix, which led to a quick optical response in the daylight. The kinetics of the appearance of the optical response of the obtained coating were discussed. The simplicity of production, the speed of image appearance, and the ability to create high-resolution patterns determine the potential applications of the proposed systems as humidity sensors or anticounterfeiting coatings.

Multimodal and Covert-Overt Convertible Structural Coloration Transformed by Mechanical Stress

Most materials and devices with structurally switchable color features responsive to external stimuli can actively and flexibly display various colors. However, realizing covert-overt transformation behavior, especially switching between transparent and colored states, is more challenging. A composite laminate of soft poly(dimethylsiloxane) (PDMS) with a rigid SiO₂ -nanoparticle (NP) structure pattern is developed as a multidimensional structural color platform. Owing to the similarity in the optical properties of PDMS and SiO₂ NPs, this device is fully transparent in the normal state. However, as their mechanical strengths differ considerably, upon compressive loading, a buckling-type instability arises on the surface of the laminate, leading to the generation of 1D or 2D wrinkled patterns in the form of gratings. Finally, an application of the device in which quick response codes are displayed or hidden as covert-overt convertible colored patterns for optical encryption/decryption, showing their remarkable potential for anticounterfeiting applications, is demonstrated.

A simple technique for the fabrication of P(St-BA-AA) colloidal crystal microdots on ink-jet paper

The generation of three-dimensional superb packages of compact hexagonal periodic assembly with multifaceted layers was demonstrated using Poly (styrene-butylcrylate-acrylic acid) on glass substrates. The synthesized P(St-BA-AA) microspheres were used to formulate a fast drying UV curable terpolymer microspheres printing ink for the generation of colloidal crystal microdots on ink-jet paper with remarkable colours. The terpolymer microspheres and their formulated terpolymer ink undergo self-assembly to form blue monochromatic and viewing angle dependent tunable colours, thus affirming the photonic nature of the generated colloidal crystal films. Unlike other printing techniques which usually make use of specialized tools, this study generated a well ordered coloured tunable assembly of spherical shaped core-shell colloidal crystal microdots on the surface of an inkjet-paper by manually writing the as-synthesized P(St-BA-AA) UV curable printing ink microdots on it. The TEM analysis showed core and shell sizes of 198/50nm and 176/30nm for P(St-BA-AA)1 and P(St- BA-AA)2 respectively. Whenever the prepared terpolymer microspheres are used within the heating and transition temperatures of 383 °C and 110 °C, their thermal stability is retained. This simple technique of generating crystals microdots on inkjet paper may find use in optical devices, security applications and other colour coating applications.

Optical Nanomaterials and Enabling Technologies for High-Security-Level Anticounterfeiting

Optical nanomaterials have been widely used in anticounterfeiting applications. There have been significant developments powered by recent advances in material science, printing technologies, and the availability of smartphone-based decoding technology. Recent progress in this field is surveyed, including the availability of optical reflection, absorption, scattering, and luminescent nanoparticles. It is demonstrated that advances in the design and synthesis of lanthanide-doped upconversion nanoparticles will lead to the next generation of anticounterfeiting technologies. Their tunable optical properties and optical responses to a range of external stimuli allow high-security level information encoding. Challenges in the scale-up synthesis of nanomaterials, engineering of assessorial devices for smart-phone-based decryption, and alignment to the potential markets which will lead to new directions for research, are discussed.

Printing a Wide Gamut of Saturated Structural Colors Using Binary Mixtures, With Applications in Anticounterfeiting

Use of colloidal suspensions to generate structural colors has the potential to reduce the use of toxic metals or organic pigments in inkjet printing, coatings, cosmetics, and other applications, and is a promising avenue to create large-scale nanostructures that produce long-lasting colors. However, expanded use of structural colors requires a reduction in coffee-ring effects during printing, which currently requires intricately patterned substrates or high particle concentrations, and diversification of colors to compete with conventional printing inks. Here, we treat substrate surfaces with cold plasma to facilitate spontaneous assembly of particles into colloidal nanostructures, reducing the need for highly concentrated particle suspensions. Moreover, by employing binary mixtures, we can tune the lightness of the hue produced or change the hue itself, allowing us to cover wider regions of color space. We demonstrate the use of this cold-plasma approach on a variety of substrates, favoring substrate diversity on which printing can be performed. This methodology enables creation of high-resolution, complex designs and opens a path for extending the limits of anticounterfeiting applications by using binary mixtures.

Structural Color Materials for Optical Anticounterfeiting

The counterfeiting of goods is growing worldwide, affecting practically any marketable item ranging from consumer goods to human health. Anticounterfeiting is essential for authentication, currency, and security. Anticounterfeiting tags based on structural color materials have enjoyed worldwide and long-term commercial success due to their inexpensive production and exceptional ease of percept. However, conventional anticounterfeiting tags of holographic gratings can be readily copied or imitated. Much progress has been made recently to overcome this limitation by employing sufficient complexity and stimuli-responsive ability into the structural color materials. Moreover, traditional processing methods of structural color tags are mainly based on photolithography and nanoimprinting, while new processing methods such as the inkless printing and additive manufacturing have been developed, enabling massive scale up fabrication of novel structural color security engineering. This review presents recent breakthroughs in structural color materials, and their applications in optical encryption and anticounterfeiting are discussed in detail. Special attention is given to the unique structures for optical anticounterfeiting techniques and their optical aspects for encryption. Finally, emerging research directions and current challenges in optical encryption technologies using structural color materials is presented.

In-Plane Direct-Write Assembly of Iridescent Colloidal Crystals

Materials made by directed self-assembly of colloids can exhibit a rich spectrum of optical phenomena, including photonic bandgaps, coherent scattering, collective plasmonic resonance, and wave guiding. The assembly of colloidal particles with spatial selectivity is critical for studying these phenomena and for practical device fabrication. While there are well-established techniques for patterning colloidal crystals, these often require multiple steps including the fabrication of a physical template for masking, etching, stamping, or directing dewetting. Here, the direct-writing of colloidal suspensions is presented as a technique for fabrication of iridescent colloidal crystals in arbitrary 2D patterns. Leveraging the principles of convective assembly, the process can be optimized for high writing speeds (≈600 µm s^-1 ) at mild process temperature (30 °C) while maintaining long-range (cm-scale) order in the colloidal crystals. The crystals exhibit structural color by grating diffraction, and analysis of diffraction allows particle size, relative grain size, and grain orientation to be deduced. The effect of write trajectory on particle ordering is discussed and insights for developing 3D printing techniques for colloidal crystals via layer-wise printing and sintering are provided.

Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

Colour patterns based on micro-nano structure have attracted enormous research interests due to unique optical switches and smart surface applications in photonic crystal, superhydrophobic surface modification, controlled adhesion, inkjet printing, biological detection, supramolecular self-assembly, anti-counterfeiting, optical device and other fields. In traditional methods, many patterns of micro-nano structure are derived from changes of refractive index or lattice parameters. Generally, the refractive index and lattice parameters of photonic crystals are processed by common solvents, salts or reactive monomers under specific electric, magnetic and stress conditions. This review focuses on the recent developments in the fabrication of micro-nano structures for patterns including styles, materials, methods and characteristics. It summarized the advantages and disadvantages of inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology, etc. This review will provide a summary on designing micro-nano patterns and details on patterns composed of photonic crystals by surface wrinkles technology and plasmonic micro-nano technology. In addition, colour patterns as switches are fabricated with good stability and reproducibility in anti-counterfeiting application. Finally, there will be a conclusion and an outlook on future perspectives.

Generation of highly integrated multiple vivid colours using a three-dimensional broadband perfect absorber

The colour printing technology based on interactions between geometric structures and light has various advantages over the pigment-based colour technology in terms of nontoxicity and ultrasmall pixel size. The asymmetric Fabry-Perot (F-P) cavity absorber is the simplest light-interacting structure, which can easily represent and control the colour by the thickness of the dielectric layer. However, for practical applications, an advanced manufacturing technique for the simultaneous generation of multiple reflective colours is required. In this study, we demonstrate F-P cavity absorbers with micropixels by overcoming the difficulties of multi-level pattern fabrication using a nanoimprinting approach. Our asymmetric F-P cavity absorber exhibited a high absorption (approximately 99%) in a wide visible light range upon the incorporation of lossy metallic materials, yielding vivid colours. A high-resolution image of eight different reflective colours was obtained by a one-step process. This demonstrates the potential of this technology for device applications such as high-resolution colour displays and colour patterns used for security functions.

Inkjet Printing of Patterned, Multispectral, and Biocompatible Photonic Crystals

Patterning of photonic crystals to generate rationally designed color-responsive materials has drawn considerable interest because of promising applications in optical storage, encryption, display, and sensing. Here, an inkjet-printing based strategy is presented for noncontact, rapid, and direct approaches to generate arbitrarily patterned photonic crystals. The strategy is based on the use of water-soluble biopolymer-based opal structures that can be reformed with high resolution through precise deposition of fluids on the photonic crystal lattice. The resulting digitally designed photonic lattice formats simultaneously exploit structural color and material transience opening avenues for information encoding and combining functions of optics, biomaterials, and environmental interfaces in a single device.

Selective photonic printing based on anisotropic Fabry-Perot resonators for dual-image holography and anti-counterfeiting

We present the photonic printing that can display different color images depending on the optical polarization of incident light. The dynamic selection among different images becomes possible by using anisotropic Fabry-Perot resonators that incorporate a layer of liquid crystal molecules aligned by directional molecular registration (DMR) as polarization-dependent color pixels. Using the new device platform, we demonstrate a prototype of an anticounterfeiting label with inherent anti-replicability that results from the molecular-level origin of security images. In addition, this concept is extended to polarization-selective holography. Our molecular-level approach enables to develop a new class of security labels and holographic storage media.

Recent Advances in Colloidal Photonic Crystal-Based Anti-Counterfeiting Materials

Colloidal photonic crystal (PC)-based anti-counterfeiting materials have been widely studied due to their inimitable structural colors and tunable photonic band gaps (PBGs) as well as their convenient identification methods. In this review, we summarize recent developments of colloidal PCs in the field of anti-counterfeiting from aspects of security strategies, design, and fabrication principles, and identification means. Firstly, an overview of the strategies for constructing PC anti-counterfeiting materials composed of variable color PC patterns, invisible PC prints, and several other PC anti-counterfeiting materials is presented. Then, the synthesis methods, working principles, security level, and specific identification means of these three types of PC materials are discussed in detail. Finally, the summary of strengths and challenges, as well as development prospects in the attractive research field, are presented.