Multilevel Information Encryption Based on Thermochromic Perovskite Microcapsules via Orthogonal Photic and Thermal Stimuli Responses

Increasing modal variations of stimulus-responsive materials ensure the high capacity and confidentiality of information storage and encryption systems that are crucial to information security. Herein, thermochromic perovskite microcapsules (TPMs) with dual-variable and quadruple-modal reversible properties are designed and prepared on the original oil-in-fluorine (O/F) emulsion system. The TPMs respond to the orthogonal variations of external UV and thermal stimuli in four reversible switchable modes and exhibit excellent thermal, air, and water stability due to the protection of perovskites by the core-shell structure. Benefiting from the high-density information storage TPMs, multiple information encryptions and decryptions are demonstrated. Moreover, a set of devices are assembled for a multilevel information encryption system. By taking advantage of TPMs as a "private key" for decryption, the signal can be identified as the corresponding binary ASCII code and converted to the real message. The results demonstrate a breakthrough in high-density information storage materials as well as a multilevel information encryption system based on switchable quadruple-modal TPMs.

Smart colloidal photonic crystal sensors

Smart colloidal photonic crystals (PCs) with stimuli-responsive periodic micro/nano-structures, photonic bandgaps, and structural colors have shown unique advantages (high sensitivity, visual readout, wireless characteristics, etc.) in sensing by outputting diverse structural colors and reflection signals. In this review, smart PC sensors are summarized according to their fabrications, structures, sensing mechanisms, and applications. The fabrications of colloidal PCs are mainly by self-assembling the well-defined nanoparticles into the periodical structure (supersaturation-, polymerization-, evaporation-, shear-, interaction-, and field-induced self-assembly process). Their structures can be divided into two groups: closely packed and non-closely packed nano-structures. The sensing mechanisms can be explained by Bragg's law, including the change in the effective refractive index, lattice constant, and the order degree. The sensing applications are detailly introduced according to the analytes of the target, including solvents, vapors, humidity, mechanical force, temperature, electrical field, magnetic field, pH, ions/molecules, and so on. Finally, the corresponding challenges and the future potential prospects of artificial smart colloidal PCs in the sensing field are discussed.

Reconfigurable Photonic Crystal Reversibly Exhibiting Single and Double Structural Colors

Unlike absorption-based colors of dyes and pigments, reflection-based colors of photonic crystals, so called "structural colors", are responsive to external stimuli, but can remain unfaded for over ten million years, and therefore regarded as a next-generation coloring mechanism. However, it is a challenge to rationally design the spectra of structural colors, where one structure gives only one reflection peak defined by Bragg's law, unlike those of absorption-based colors. Here, we report a reconfigurable photonic crystal that exhibits single-peak and double-peak structural colors. This photonic crystal is composed of a colloidal nanosheet in water, which spontaneously adopts a layered structure with single periodicity (407 nm). After a temperature-gradient treatment, the photonic crystal segregates into two regions with shrunken (385 nm) and expanded (448 nm) periodicities, and thus exhibits double reflection peaks that are blue- and red-shifted from the original one, respectively. Notably, the transition between the single-peak and double-peak states is reversible.

Back-propagation optimization and multi-valued artificial neural networks for highly vivid structural color filter metasurfaces

We introduce a novel technique for designing color filter metasurfaces using a data-driven approach based on deep learning. Our innovative approach employs inverse design principles to identify highly efficient designs that outperform all the configurations in the dataset, which consists of 585 distinct geometries solely. By combining Multi-Valued Artificial Neural Networks and back-propagation optimization, we overcome the limitations of previous approaches, such as poor performance due to extrapolation and undesired local minima. Consequently, we successfully create reliable and highly efficient configurations for metasurface color filters capable of producing exceptionally vivid colors that go beyond the sRGB gamut. Furthermore, our deep learning technique can be extended to design various pixellated metasurface configurations with different functionalities.

Versatile Double Bandgap Photonic Crystals of High Color Saturation

Double bandgap photonic crystals (PCs) exhibit significant potential for applications in various color display-related fields. However, they show low color saturation and inadequate color modulation capabilities. This study presents a viable approach to the fabrication of double bandgap photonic inks diffracting typical secondary colors and other composite colors by simply mixing two photonic nanochains (PNCs) of different primary colors as pigments in an appropriate percentage following the conventional RGB color matching method. In this approach, the PNCs are magnetically responsive and display three primary colors that can be synthesized by combining hydrogen bond-guided and magnetic field (H)-assisted template polymerization. The as-prepared double bandgap photonic inks present high color saturation due to the fixed and narrow full-width at half-maxima of the parent PNCs with a suitable chain length. Furthermore, they can be used to easily produce a flexible double bandgap PC film by embedding the PNCs into a gel, such as polyacrylamide, facilitating fast steady display performance without the requirement of an external magnetic field. This research not only presents the unique advantages of PNCs in constructing multi-bandgap PCs but also establishes the feasibility of utilizing PNCs in practical applications within the fields of anti-counterfeiting and flexible wearable devices.

Magnetically Induced Anisotropic Interaction in Colloidal Assembly

The wide accessibility to nanostructures with high uniformity and controllable sizes and morphologies provides great opportunities for creating complex superstructures with unique functionalities. Employing anisotropic nanostructures as the building blocks significantly enriches the superstructural phases, while their orientational control for obtaining long-range orders has remained a significant challenge. One solution is to introduce magnetic components into the anisotropic nanostructures to enable precise control of their orientations and positions in the superstructures by manipulating magnetic interactions. Recognizing the importance of magnetic anisotropy in colloidal assembly, we provide here an overview of magnetic field-guided self-assembly of magnetic nanoparticles with typical anisotropic shapes, including rods, cubes, plates, and peanuts. The Review starts with discussing the magnetic energy of nanoparticles, appreciating the vital roles of magneto-crystalline and shape anisotropies in determining the easy magnetization direction of the anisotropic nanostructures. It then introduces superstructures assembled from various magnetic building blocks and summarizes their unique properties and intriguing applications. It concludes with a discussion of remaining challenges and an outlook of future research opportunities that the magnetic assembly strategy may offer for colloidal assembly.

Color-Tuning Mechanism of Electrically Stretchable Photonic Organogels

In contrast to nano-processed rigid photonic crystals with fixed structures, soft photonic organic hydrogel beads with dielectric nanostructures possess advanced capabilities, such as stimuli-responsive deformation and photonic wavelength color changes. Recenlty, advanced from well-investigated mechanochromic method, an electromechanical stress approach is used to demonstrate electrically induced mechanical color shifts in soft organic photonic hydrogel beads. To better understand the electrically stretchable color change functionality in such soft organic photonic hydrogel systems, the electromechanical wavelength-tuning mechanism is comprehensively investigated in this study. By employing controllable electroactive dielectric elastomeric actuators, the discoloration wavelength-tuning process of an electrically stretchable photonic organogel is carefully examined. Based on the experimental in-situ response of electrically stretchable nano-spherical polystyrene hydrogel beads, the color change mechanism is meticulously analyzed. Further, changes in the nanostructure of the symmetrically and electrically stretchable organogel are analytically investigated through simulations of its hexagonal close-packed (HCP) lattice model. Detailed photonic wavelength control factors, such as the refractive index of dielectric materials, lattice diffraction, and bead distance in an organogel lattice, are theoretically studied. Herein, the switcing mechanism of electrically stretchable mechanochromic photonic organogels with photonic stopband-tuning features are suggested for the first time.

Regional Control of Multistimuli-Responsive Structural Color-Switching Surfaces by a Micropatterned DNA-Hydrogel Assembly

Structural colors have advantages compared with chemical pigments or dyes, such as iridescence, tunability, and unfading. Many studies have focused on developing the ability to switch ON/OFF the structural color; however, they often suffer from a simple and single stimulus, remaining structural colors, and target selectivity. Herein, we present regionally controlled multistimuli-responsive structural color switching surfaces. The key part is the utilization of a micropatterned DNA-hydrogel assembly on a single substrate. Each hydrogel network contains a unique type of stimuli-responsive DNA motifs as an additional cross-linker to exhibit swelling/deswelling via stimuli-responsive DNA interactions. The approach enables overcoming the existing limitations and selectively programming the DNA-hydrogel to a decrypted state (ON) and an encrypted state (OFF) in response to multiple stimuli. Furthermore, the transitions are reversible, providing cyclability. We envision the potential of our method for diverse applications, such as sensors or anticounterfeiting, requiring multistimuli-responsive structural color switching surfaces.

Background color dependent photonic multilayer films for anti-counterfeiting labeling

A new concept for anti-counterfeiting security films that utilize the humidity from human breath to reveal a QR code on color-tunable one-dimensional (1D) PC films is presented. The 1D PC film was fabricated on a transparent polyethylene terephthalate (PET) substrate via sequential alternate layer deposition of photo-crosslinkable poly(2-vinylnaphthalene-co-benzophenone acrylate) (P(2VN-co-BPA)) and quaternized poly(4-vinylpyridine-co-benzophenone acrylate) (P(4VP-co-BPA)) (P4QP-51%). The films exhibited remarkable color transitions with reliable reversibility and reproducibility. Films placed on a black background exhibited the full visible spectrum color in a high humidity environment. Additionally, films placed on a white background displayed three different composite colors, including yellow, magenta, and cyan. These films with vivid color transitions in a high humidity environment can be applied as anti-counterfeiting films. A hidden QR code was also laser printed on the initial PC film to enhance the film's anti-counterfeiting security capabilities. These colorimetric 1D PC films can be used as anti-counterfeiting labels and for information storage.

Wide-Range Time-Dependent Color-Tunable Light-Response Afterglow Materials via Absorption Compensation for Advanced Information Encryption

Stimuli-responsive luminescent materials with time-dependent color are highly desirable in optical information encryption. In this study, multiple time-dependent color processes are achieved by light-responsive afterglow materials through the strategy of absorption compensation. Based on the single-emission band of light-responsive afterglow materials, the color of samples could show a time-dependent change from colored to colorless over several seconds. The strategy possesses high flexibility such that the stimulus light and emission color of light-responsive afterglow materials can be adjusted conveniently to adapt to various scenes. It is also beneficial to expand the capacity and complexity of information encryption. A three-color, time-resolved anticounterfeiting, and data encryption mode is demonstrated. This facile absorption compensation method based on light-response afterglow materials may promote the development of advanced dynamic information encryption.

Photonic Cholesteric Liquid-Crystal Elastomers with Reprogrammable Helical Pitch and Handedness

The unique combination of the rubber-like property and the photonic helicoidal structure of cholesteric liquid-crystal elastomers (CLCEs) results in one-handed circular polarized light reflection, the wavelength of which is dictated by the Bragg relationship. Herein, a highly stretchable mechanochromic photonic CLCE film was fabricated by cross-linking mesogenic oligomers having thiol terminal groups, which further reacted to form disulfide (-S-S-) linkages. The mechanically stretched photonic CLCE film reflected both right- and left-handed circular polarized lights with a blue-shifted color. The helicoidal pitch and handedness controlled by the applied strain were programmed through a dynamic exchange reaction between the -S-S- linkages, thus realizing the patterning at selective regions. The pattern almost vanished under unpolarized daylight but was visible under circularly polarized light when the patterned photonic CLCE film had been heated above its isotropic temperature. The hidden patterns of the heat-treated CLCE film reappeared under unpolarized daylight when stretched, demonstrating a data encryption ability. These patterned photonic elastomers can be uniquely used in sensors, actuators, soft robotics, flexible displays, data encryption, and anticounterfeiting applications with a mechanochromic camouflage response.

Supramolecular DNA Photonic Hydrogels for On-Demand Control of Coloration with High Spatial and Temporal Resolution

Hydrogels embedded with periodic arrays of nanoparticles display a striking photonic crystal coloration that may be useful for applications such as camouflage, anticounterfeiting, and chemical sensing. Dynamically generating color patterns requires control of nanoparticle organization within a polymer network on-demand, which is challenging. We solve this problem by creating a DNA hydrogel system that shows a 50 000-fold decrease in modulus upon heating by ∼10 °C. Magnetic nanoparticles entrapped within these DNA gels generate a structural color only when the gel is heated and a magnetic field is applied. A spatially controlled photonic crystal coloration was achieved by photopatterning with a near-infrared illumination. Color was "erased" by illuminating or heating the gel in the absence of an external magnetic field. The on-demand assembly technology demonstrated here may be beneficial for the development of a new generation of smart materials with potential applications in erasable lithography, encryption, and sensing.

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.

The untapped potential of magnetic nanoparticles for forensic investigations: A comprehensive review

With a growing interest in precise and sensitive diagnosis for criminal investigations, nanoparticles (NPs) have intrigued scientific minds working in the field of forensic science due to their exceptional properties. Magnetic nanoparticles (MNPs) have emerged as a powerful tool for improving forensic analysis due to their super magnetic behavior combined with smaller dimensions. MNP-based applications can benefit criminologists to solve criminal mysteries with greater precision and pace. This review highlights the different types of MNP-based applications and their developmental and implicational aspects of forensic science. It also renders insight into the future prospects of a splendid blend of nanotechnology and forensic science, leading to a better scientific analysis.

Rapid Responsive Mechanochromic Photonic Pigments with Alternating Glassy-Rubbery Concentric Lamellar Nanostructures

Photonic pigment particles prepared via self-assembly have been suffering from their poor mechanical performances; i.e., they can easily be damaged and lose structural color under a compression force. This greatly limits their uses as mechanochromic pigments. Here, a nanoscale concentric lamellar structure of alternating glassy-rubbery microdomains is successfully created within photonic microparticles through a confined self-assembly and photo-cross-linking strategy. The glassy domain is composed of polystyrene, and cross-linked bottlebrush polydimethylsiloxane served as the supersoft elastic domain. The obtained photonic structure not only shows large deformation and visible color changes under a loaded compression force but also rapidly recovers to its original state in less than 1 s (∼0.16 s) upon unloading. Continuously loading-unloading micro compression test indicates that no obvious damage can be identified after 250 cycles, indicating the high durability of the pigments against deformation. These pigments with different reflected colors are simply obtained using bottlebrush block copolymer formulations with tunable weight percentages of polymer additives. The mechanical robust photonic pigments may be useful in many important applications.

Waterborne fluorescent dual anti-counterfeiting ink based on Yb/Er-carbon quantum dots grafted with dialdehyde nano-fibrillated cellulose

Nanofibrillated cellulose (NFC) is becoming popular in the field of anti-counterfeiting material due to its favorable biocompatibility, renewability, and easy modification properties, which give it great potentials as carrier of carbon quantum dots (CQDs). Herein, we report an effective method to fabricate Yb and Er doped CQDs grafted onto dialdehyde NFC (DANFC). Owning to special rheological properties of NFC, a waterborne fluorescent dual anti-counterfeiting ink was rationally designed and successfully prepared by adding NFC to waterborne ink to form a stable network structure and increase the thixotropy and yield stress. The resulting CQDs exhibited both photoluminescence (PL) and up-conversion luminescence (UCPL), emitting blue and green fluorescence at excitation wavelengths of 370 and 980 nm, respectively. The study provides a novel method to prepare the waterborne fluorescent dual anti-counterfeiting ink based on Yb and Er doped CQDs/DANFC composites, which provides a reference for its application in printing and packaging industry.

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.

Amphiphilic copolymer self-assembly of magnetic nanoparticles for construction of magnetically responsive photonic crystals based on steric hindrance

Herein, a facile, simple and rapid self-assembly of magnetic colloidal nanoparticles (MCNPs) to build magnetically responsive photonic crystals (MRPCs) was devolved. A nonionic amphiphilic random copolymer poly(styrene-co-vinylpyrrolidone) P(St-co-VP) with the monomer molar ratio of 1 : 9 was used not only as an emulsifier for miniemulsion self-assembly of Fe₃O₄ magnetic nanoclusters, but also as the coating material on the magnetic nanoclusters through itself assembly. The self-assembly of the magnetic nanocluster and the polymer coating were completed simultaneously without another polymerization process. The characterization of the MCNPs and the optical properties of the MRPCs were investigated in details. TEM showed that the MCNPs had regular spherical structures with an average diameter of 104.6 nm (RSD = 13.9%, n = 100). P(St-co-VP) self-assembly coating was confirmed by IR and XPS, and thermogravimetric analysis showed that the magnetite content was 76.15%. The large content of magnetite and the thin coating of the copolymer gave MCNPs the high saturated magnetization (M_s) of 52.60 emu g⁻¹. Under an external magnetic field, the MCNPs could assemble MRPCs instantaneously and reversibly. The structural color covered entire visible spectrum by tuning the strength of the external magnetic field. On basis of the steric hindrance from neighboring PVP stretching chains, rather than electrostatic repulsion or solvation layer to counterbalance magnetic attraction, the MRPCs could tolerate the electrolyte as high as 0.10 mol L⁻¹ and the variance of pH from 2.0–12.0. The stability of P(St-co-VP) self-assembly coating was testified through the invariability of the structural color of MRPCs after repeated washing, as well as the recovery of structural color after removing the electrolytes.

One-step self-assembly of magnetic nanoparticles with amphiphilic copolymer for construction of magnetically responsive photonic crystals based on steric hindrance.

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.

Structural color printing with a dielectric layer coated on a nanotextured metal substrate: simulation and experiment

The printing of plasmonic structural colors relies on noble metal nanostructures fabricated on Si, glass, or plastic substrates. This paper presents a simple surface structure for producing vivid structural colors directly from common metal substrates. The structure is formed by texturing the surface of stainless steel (STS) via imprinting and coating it with a dielectric layer. Diverse colors are generated simply by varying the thickness of the dielectric layer. The colors arise from surface plasmon resonance and guided-mode resonance of the incident light, which are excited on the textured STS surface and inside the dielectric layer, respectively. A finite-difference time-domain simulation shows that 500 nm is the optimum texture periodicity with regard to the tunability and vividness of the colors. This is experimentally verified by printing many differently colored images on the surface of STS substrates with a texture period of 500 nm. The proposed structure/method does not require a nanofabrication technique such as electron-beam lithography or focused ion beam etching. The results of the study provide a facile route for producing vivid structural colors on metals, which may find various applications, including surface decoration, product identification, anti-counterfeiting, and perfect absorbers.

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.

Designing Structural-Color Patterns Composed of Colloidal Arrays

Structural coloration provides a great potential for various applications due to unique optical properties distinguished from conventional pigment colors. Structural colors are nonfading, iridescent, and tunable, which is difficult to achieve with pigments. In addition, structural color is potentially less toxic than pigments. However, it is challenging to develop structural colors because elaborate nanostructures are a prerequisite for the coloration. Furthermore, it is highly suggested the nanostructures be patterned at various length scales on a large area to provide practical formats. There have been intensive studies to develop pragmatic methods for producing structural-color patterns in a controlled manner using either colloidal crystals or glasses. This article reviews the current state of the art in the structural-color patterning based on the colloidal arrays. We first discuss common and different features between colloidal crystals and glasses. We then categorize colloidal arrays into six distinct structures of 3D opals, inverse opals, non-close-packed arrays, 2D colloidal crystals, 1D colloidal strings, and 3D amorphous arrays and study various methods to make them patterned from recent key contributions. Finally, we outline the current challenges and future perspectives of the structural-color patterns.

Concept of chiral image storage and selection based on liquid crystals by circular polarization

We demonstrate a liquid crystal (LC)-based optical device with the polarization switching capability, which can store two different chiral images to be selected according to the polarization state of the viewing polarizer. The chiral dual-image device consists of chiral surface patterns for image storage and the LC layer as a tunable phase retarder. Each chiral surface pattern behaves as a helical photonic crystal that reflects circularly polarized light at a specific wavelength. Depending on the applied voltage across the LC layer, either a right-handed or a left-handed circular polarization image appears, and thus one of the two stored images can be selectively read by the polarization state. Our concept of the LC-based chiral image storage and selection provides simplicity in fabrication, flexibility in design, and high optical efficiency. It will be directly applicable for reflective-type 3D displays, color filters, and anti-counterfeiting devices.

Solution-Processable Nanocrystal-Based Broadband Fabry-Perot Absorber for Reflective Vivid Color Generation

Structural reflective colors based on Fabry-Perot (F-P) cavity resonances have attracted tremendous interest for diverse applications, such as color decoration and printing, display, and imaging devices. However, the asymmetric F-P cavity-based reflective colors proposed to date have low color purity and have difficulty to realize a desired vivid color because of a narrow absorption band characteristic in the visible light region. Here, a solution-processed, F-P ultra-broadband light absorber is newly proposed using a high lossy nanoporous material for vivid color generation. An asymmetric metal-insulator-metal structure consists of a high lossy nanoporous metallic film with coupled silver nanocrystals (Ag NCs) as the top layer. The absorbers not only increase the maximum absorption intensity up to ∼98% but also widen the bandwidth by 300 nm, resulting in high color purity in micrometer-scale pixels. Furthermore, the solution-based absorber shows potential to realize a high-resolution display pixel and anticounterfeiting devices having mechanical flexibility using the inkjet printing technology.

Mechanically Excited Multicolor Luminescence in Lanthanide Ions

Mechanoluminescence (ML) featuring photon emission by mechanical stimuli is promising for applications such as stress sensing, display, and artificial skin. However, the progress of utilizing ML processes is constrained by the limited range of available ML emission spectra. Herein, a general strategy for expanding the emission of ML through the use of lanthanide emitters is reported. A lithium-assisted annealing method for effective incorporation of various lanthanide ions (e.g., Tb³⁺ , Eu³⁺ , Pr³⁺ , Sm³⁺ , Er³⁺ , Dy³⁺ , Ho³⁺ , Nd³⁺ , Tm³⁺ , and Yb³⁺ ) into CaZnOS crystals that are identified as one of the most efficient host materials for ML is developed. These doped CaZnOS crystals show efficient and tunable ML spanning full spectrum from violet to near infrared. The multicolor ML materials are used to create encrypted anticounterfeiting patterns, which produce spatially resolvable optical codes under single-point dynamic pressure of a ballpoint pen.

Structurally colored polymer films with narrow stop band, high angle-dependence and good mechanical robustness for trademark anti-counterfeiting

The photonic stop bands of colloidal crystals appear as structural colors, which are potentially useful for display devices, colorimetric sensors, optical filters, paints, and photonic papers. However, low durability and pale colors caused by the undesired scattering of light seriously limit their practical applications. In this article, a polydimethylsiloxane (PDMS)/photonic crystal (PC)/PDMS sandwich structure was designed as a free standing structural colored film with good durability and brilliant color. The monodispersed polystyrene (PS) spheres were self-assembled on the hydrophobic PDMS surface to facilitate the integrity of the assembled structure and then, PDMS with a refractive index of 1.41 was filled in the gaps between the PS spheres (nPS = 1.59), replacing air (nair = 1). The surface was finally covered with a thin layer of PS PCs, forming a continuous and free standing PC film. The continuous feature of the composite PC can greatly improve their mechanical properties. At the same time, the lower index contrast results in narrow reflection peaks for the composite films, which indicates that higher color purity and brightness could be achieved. Clearly distinguished, vivid structural colors can be observed between red to green or green to blue by tuning the viewing angle from 5° to 50° for films composed of PS spheres with diameters of 247 nm or 209 nm, respectively. They can also be easily patterned by spraying methods and embedded as a trademark on clothes. Patterns with different structural colors at different angle can be clearly be observed under sunlight, which makes them potentially useful as security materials.

Bioinspired Stimuli-Responsive Color-Changing Systems

Stimuli-responsive colors are a unique characteristic of certain animals, evolved as either a method to hide from enemies and prey or to communicate their presence to rivals or mates. From a material science perspective, the solutions developed by Mother Nature to achieve these effects are a source of inspiration to scientists for decades. Here, an updated overview of the literature on bioinspired stimuli-responsive color-changing systems is provided. Starting from natural systems, which are the source of inspiration, a classification of the different solutions proposed is given, based on the stimuli used to trigger the color-changing effect.

Eight Inch Wafer-Scale Flexible Polarization-Dependent Color Filters with Ag-TiO2 Composite Nanowires

In this study, 8 in. wafer-scale flexible polarization-dependent color filters with Ag-TiO₂ composite nanowires have been fabricated using nanoimprint and E-beam evaporation. The filters change their color via a simple rotation of the polarizer. In addition, the color of the filter can be controlled by altering the thickness of the Ag and TiO₂ nanowires deposited on the polymer patterns. Polarization-dependent color filters were realized by selective inhibition of transmission using the plasmonic resonance at the insulator/metal/insulator nanostructure interface, which occurs at particular wavelengths for the transverse magnetic polarizations. Special colors, including purple, blue, green, yellow, and pink, could be obtained with high transmission beyond 65% by varying the thickness of the deposited Ag and TiO₂ nanowires on the periodic polymer pattern under transverse magnetic polarization. In addition, a continuous color change was achieved by varying the polarization angle. Last, numerical simulations were implemented in comparison with the experimental results, and the mechanism was explained. We believe that this simple and cost-effective method can be applied to processes such as anticounterfeiting and holographic imaging as well as to color displays.

Inkjet-Printed Multiwavelength Thermoplasmonic Images for Anticounterfeiting Applications

Inkjet printing of thermoplasmonic nanoparticles enables instantaneous, large-area heat pattern generation upon light illumination from distance. By printing multiple metal nanoparticles of different shapes overlaid, we can fabricate multiwavelength thermoplasmonic images, which generate different heat patterns from a single printed image depending on the wavelength choice of light. In this work, we propose a novel multiwavelength thermoplasmonic image printing process that can be used for anticounterfeit technology. With this technology, "printed thermoplasmonic labels" allow fully secured anticounterfeit inspection procedure. Input stimulus of near-infrared or infrared light illumination and output signal reading of thermal patterns can be both completely invisible. Wavelength selective photothermal effect also enables the encryption of the contained information, which adds more complexity and thus higher security.

Controlled Insertion of Planar Defect in Inverse Opals for Anticounterfeiting Applications

Inverse opals have been used for structural coloration and photonic applications owing to their photonic bandgap properties. When the photonic structures contain planar defects, they provide defect modes, which are useful for lasing, sensing, and waveguiding. However, it remains a challenge to insert a planar defect into inverse opals in a reproducible manner. Here, we report a new method for producing planar-defect-inserted inverse opals using sequential capillary wetting of colloidal crystals and creating micropatterns through photolithography. Three cycles of deposition and thermal embedding of colloidal crystals into the underlying film of negative photoresist were performed. In the three cycles, opal, particle monolayer, and opal were sequentially employed, which yielded the monolayer-templated planar defect sandwiched by two inverse opals after particle removal. The planar defect provided a passband whose wavelength can be controlled by adjusting the diameter of particles for the defect layer. Moreover, the defect-inserted inverse opals can be micropatterned by photolithography as the negative photoresist is used as a matrix. The resulting micropatterns deliver a unique spectral code featured by a combination of stop band and defect mode and a graphical code dictated by photolithography, being useful for anticounterfeiting applications.

Structural Color Patterns on Paper Fabricated by Inkjet Printer and Their Application in Anticounterfeiting

Inkjet-printed structural color patterns have attracted great attention in recent years because of their broadly promising applications. However, the patterns are usually fabricated on pretreated plastic substrates. Herein a convenient inkjet printing method was developed to fabricate large-scale computer-designed structural color patterns on photo paper without any treatment using inks containing monodisperse CdS spheres. By this strategy, not only were the single-color and multicolor structural color patterns on paper successfully obtained, but also invisible photonic anticounterfeiting was achieved without any external stimuli. The key point of this anticounterfeiting technique is printing patterns and the background with inks containing uniformed CdS spheres with different diameters but similar intrinsic colors, so that the invisible patterns can be observed clearly by simply changing the viewing angle. The invisible and visible can be realized without the change of intrinsic structure, and the patterns are all solids. The patterns will have long lifetime and good durability, which is beneficial for their practical usage.

Up Scalable Full Colour Plasmonic Pixels with Controllable Hue, Brightness and Saturation

It has long been the interests of scientists to develop ink free colour printing technique using nano structured materials inspired by brilliant colours found in many creatures like butterflies and peacocks. Recently isolated metal nano structures exhibiting preferential light absorption and scattering have been explored as a promising candidate for this emerging field. Applying such structures in practical use, however, demands the production of individual colours with distinct reflective peaks, tunable across the visible wavelength region combined with controllable colour attributes and economically feasible fabrication. Herein, we present a simple yet efficient colour printing approach employing sub-micrometer scale plasmonic pixels of single constituent metal structure which supports near unity broadband light absorption at two distinct wavelengths, facilitating the creation of saturated colours. The dependence of these resonances on two different parameters of the same pixel enables controllable colour attributes such as hue, brightness and saturation across the visible spectrum. The linear dependence of colour attributes on the pixel parameters eases the automation; which combined with the use of inexpensive and stable aluminum as functional material will make this colour design strategy relevant for use in various commercial applications like printing micro images for security purposes, consumer product colouration and functionalized decoration to name a few.

Facile Synthesis of Monodispersed Polysulfide Spheres for Building Structural Colors with High Color Visibility and Broad Viewing Angle

The synthesis and assembly of monodispersed colloidal spheres are currently the subject of extensive investigation to fabricate artificial structural color materials. However, artificial structural colors from general colloidal crystals still suffer from the low color visibility and strong viewing angle dependence which seriously hinder their practical application in paints, colorimetric sensors, and color displays. Herein, monodispersed polysulfide (PSF) spheres with intrinsic high refractive index (as high as 1.858) and light-absorbing characteristics are designed, synthesized through a facile polycondensation and crosslinking process between sodium disulfide and 1,2,3-trichloropropane. Owing to their high monodispersity, sufficient surface charge, and good dispersion stability, the PSF spheres can be assembled into large-scale and high-quality 3D photonic crystals. More importantly, high structural color visibility and broad viewing angle are easily achieved because the unique features of PSF can remarkably enhance the relative reflectivity and eliminate the disturbance of scattering and background light. The results of this study provide a simple and efficient strategy to create structural colors with high color visibility, which is very important for their practical application.

Brilliant Structurally Colored Films with Invariable Stop-Band and Enhanced Mechanical Robustness Inspired by the Cobbled Road

Recently, structural colors have attracted great concentrations because the coloration is free from chemical- or photobleaching. However, the color saturation and mechanical robustness are generally competitive properties in the fabrication of PCs (photonic crystals) films. Besides, the structure of PCs and their derivatives are easy to be invaded by liquids and lead to band gap shifts due to the change of refractive index or periodicity. To solve those problems, we infiltrate polydimethylsiloxane (PDMS) into the intervals between regularly arrayed hollow SiO2 nanospheres, inspired by the cobbled road prepared by embedding stone in the bulk cement matrix. Consequently, the as-prepared PCs films show brilliant colors, invariable stop-bands, and excellent mechanical robustness. Moreover, the water contact angle even reached 166° after a sandpaper abrasion test. The combination of brilliant colors, invariable stop-bands, and excellent robustness is significant for potential application in paint and external decoration of architectures.

Recent advances in the biomimicry of structural colours

Nature has mastered the construction of nanostructures with well-defined macroscopic effects and purposes. Structural colouration is a visible consequence of the particular patterning of a reflecting surface with regular structures at submicron length scales. Structural colours usually appear bright, shiny, iridescent or with a metallic look, as a result of physical processes such as diffraction, interference, or scattering with a typically small dissipative loss. These features have recently attracted much research effort in materials science, chemistry, engineering and physics, in order to understand and produce structural colours. In these early stages of photonics, researchers facing an infinite array of possible colour-producing structures are heavily inspired by the elaborate architectures they find in nature. We review here the recent technological strategies employed to artificially mimic the structural colours found in nature, as well as some of their current and potential applications.

Facile fabrication of magnetically responsive PDMS fiber for camouflage

A new type of photonic crystal PDMS fiber which exhibits tunable structural color upon exposure to external magnetic field is described in this article. The novel magnetic field responsive fiber was prepared from embedding ethylene glycol droplets (containing Fe3O4@C nanoparticles) into PDMS. In the presence of an external magnetic field, Fe3O4@C nanoparticles which dispersed in ethylene glycol droplets formed one dimensional chain-like structures along the magnetic field. As a result, the color of the fiber changes to yellow green. By contrast, when the magnetic field was removed, the color of the fiber will disappear and display its original color. Moreover, this novel PDMS fiber has good mechanical properties and could keep its color under a fixed magnetic field no matter it was stretched or squeezed. This study is expected to have some important applications such as none-powered and functionalized fibers for camouflage.

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

Photonic crystal structures can be created to manipulate electromagnetic waves so that many studies have focused on designing photonic band-gaps for various applications including sensors, LEDs, lasers, and optical fibers. Here, we show that mono-layered, self-assembled photonic crystals (SAPCs) fabricated by using an inkjet printer exhibit extremely weak structural colors and multiple colorful holograms so that they can be utilized in anti-counterfeit measures. We demonstrate that SAPC patterns on a white background are covert under daylight, such that pattern detection can be avoided, but they become overt in a simple manner under strong illumination with smartphone flash light and/or on a black background, showing remarkable potential for anti-counterfeit techniques. Besides, we demonstrate that SAPCs yield different RGB histograms that depend on viewing angles and pattern densities, thus enhancing their cryptographic capabilities. Hence, the structural colorations designed by inkjet printers would not only produce optical holograms for the simple authentication of many items and products but also enable a high-secure anti-counterfeit technique.

Angle-tolerant polarization-tuned color filter exploiting a nanostructured cavity

A polarization-mediated color filter featuring a high angular tolerance is proposed incorporating a metal-dielectric-metal etalon based on a nanostructured cavity, where a one-dimensional subwavelength grating of a high refractive index is embedded in a base layer of a low refractive index. The aim of the nanostructured cavity is mimicking of the equivalent birefringent medium whereby different effective refractive indices are exhibited depending on the incident polarization. As the transmission peak of the etalon is effectively tuned through the tailoring of the refractive index of the cavity, the proposed filter is capable of providing a continuum of vivid output colors through a dynamic control of the polarization. The effective medium theory is chiefly applied for an investigation of the birefringent characteristics of the nanostructured cavity. A dielectric overlay that acts as an anti-reflection coating is specifically adopted for the etalon to enhance the transmission efficiency. The proposed polarization-tuned filter evidently provides a high transmission of ~71% and a high angular tolerance of ~35° in conjunction with a wide polarization-mediated color tuning.

Use of CdS quantum dot-functionalized cellulose nanocrystal films for anti-counterfeiting applications

Structural colors and photoluminescence have been widely used for anti-counterfeiting and security applications. We report for the first time the use of CdS quantum dot (QD)-functionalized cellulose nanocrystals (CNCs) as building blocks to fabricate nanothin films via layer-by-layer (LBL) self-assembly for anti-counterfeiting applications. Both negatively- and positively-charged CNC/QD nanohybrids with a high colloidal stability and a narrow particle size distribution were prepared. The controllable LBL coating process was characterized by scanning electron microscopy and ellipsometry. The rigid structure of CNCs leads to nanoporous structured films on poly(ethylene terephthalate) (PET) substrates with high transmittance (above 70%) over the entire range of visible light and also resulted in increased hydrophilicity (contact angles of ∼40 degrees). Nanothin films on PET substrates showed good flexibility and enhanced stability in both water and ethanol. The modified PET films with structural colors from thin-film interference and photoluminescence from QDs can be used in anti-counterfeiting applications.

Lithographically Encrypted Inverse Opals for Anti-Counterfeiting Applications

Colloidal photonic crystals possess inimitable optical properties of iridescent structural colors and unique spectral shape, which render them useful for security materials. This work reports a novel method to encrypt graphical and spectral codes in polymeric inverse opals to provide advanced security. To accomplish this, this study prepares lithographically featured micropatterns on the top surface of hydrophobic inverse opals, which serve as shadow masks against the surface modification of air cavities to achieve hydrophilicity. The resultant inverse opals allow rapid infiltration of aqueous solution into the hydrophilic cavities while retaining air in the hydrophobic cavities. Therefore, the structural color of inverse opals is regioselectively red-shifted, disclosing the encrypted graphical codes. The decoded inverse opals also deliver unique reflectance spectral codes originated from two distinct regions. The combinatorial code composed of graphical and optical codes is revealed only when the aqueous solution agreed in advance is used for decoding. In addition, the encrypted inverse opals are chemically stable, providing invariant codes with high reproducibility. In addition, high mechanical stability enables the transfer of the films onto any surfaces. This novel encryption technology will provide a new opportunity in a wide range of security applications.

Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting

Photoluminescence (PL), up-conversion PL (UCPL), and phosphorescence are three kinds of phenomena common to light-emitting materials, but it is very difficult to observe all of them simultaneously when they are derived from a single material at room temperature. For the first time, triple-mode emission (that is, PL, UCPL, and room temperature phosphorescence (RTP)) is reported, which relies on a composite of the luminescent carbon dots (CDs) prepared from m-phenylenediamine and poly(vinyl alcohol) (PVA). Moreover, the CDs-PVA aqueous dispersion is nearly colorless and demonstrates promise as a triple-mode emission ink in the field of advanced anti-counterfeiting.