Tunable Design of Structural Colors Produced by Pseudo‐1D Photonic Crystals of Graphene Oxide

It is broadly observed that graphene oxide (GO) films appear transparent with a thickness of about several nanometers, whereas they appear dark brown or almost black with thickness of more than 1 μm. The basic color mechanism of GO film on a sub‐micrometer scale, however, is not well understood. This study reports on GO pseudo‐1D photonic crystals (p1D‐PhCs) exhibiting tunable structural colors in the visible wavelength range owing to its 1D Bragg nanostructures. Striking structural colors of GO p1D‐PhCs could be tuned by simply changing either the volume or concentration of the aqueous GO dispersion during vacuum filtration. Moreover, the quantitative relationship between thickness and reflection wavelength of GO p1D‐PhCs has been revealed, thereby providing a theoretical basis to rationally design structural colors of GO p1D‐PhCs. The spectral response of GO p1D‐PhCs to humidity is also obtained clearly showing the wavelength shift of GO p1D‐PhCs at differently relative humidity values and thus encouraging the integration of structural color printing and the humidity‐responsive property of GO p1D‐PhCs to develop a visible and fast‐responsive anti‐counterfeiting label. The results pave the way for a variety of potential applications of GO in optics, structural color printing, sensing, and anti‐counterfeiting.     

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.     

Bioinspired Hydrogel Interferometer for Adaptive Coloration and Chemical Sensing

Living organisms ubiquitously display colors that adapt to environmental changes, relying on the soft layer of cells or proteins. Adoption of soft materials into an artificial adaptive color system has promoted the development of material systems for environmental and health monitoring, anti‐counterfeiting, and stealth technologies. Here, a hydrogel interferometer based on a single hydrogel thin film covalently bonded to a reflective substrate is reported as a simple and universal adaptive color platform. Similar to the cell or protein soft layer of color‐changing animals, the soft hydrogel layer rapidly changes its thickness in response to external stimuli, resulting in instant color change. Such interference colors provide a visual and quantifiable means of revealing rich environmental metrics. Computational model is established and captures the key features of hydrogel stimuli‐responsive swelling, which elucidates the mechanism and design principle for the broad‐based platform. The single material–based platform has advantages of remarkable color uniformity, fast response, high robustness, and facile fabrication. Its versatility is demonstrated by diverse applications: a volatile‐vapor sensor with highly accurate quantitative detection, a colorimetric sensor array for multianalyte recognition, breath‐controlled information encryption, and a colorimetric humidity indicator. Portable and easy‐to‐use sensing systems are demonstrated with smartphone‐based colorimetric analysis.     

Bioinspired Heterogeneous Structural Color Stripes from Capillaries

As an important characteristic of many creatures, structural colors play a crucial role in the survival of organisms. Inspired by these features, an intelligent structural color material with a heterogeneous striped pattern and stimuli‐responsivity by fast self‐assembly of colloidal nanoparticles in capillaries with a certain diameter range are presented here. The width, spacing, color, and even combination of the structural color stripe patterns can be precisely tailored by adjusting the self‐assembly parameters. Attractively, with the integration of a near‐infrared (NIR) light responsive graphene hydrogel into the structural color stripe pattern, the materials are endowed with light‐controlled reversible bending behavior with self‐reporting color indication. It is demonstrated that the striped structural color materials can be used as NIR‐light‐triggered dynamic barcode labels for the anti‐counterfeiting of different products. These features of the bioinspired structural color stripe pattern materials indicate their potential values for mimicking structural color organisms, which will find important applications in constructing intelligent sensors, anti‐counterfeiting devices, and so on.     

Super‐Elastic Magnetic Structural Color Hydrogels

Structural color hydrogels are promising candidates as scaffold materials for tissue engineering and for matrix cell culture and manipulation, while their super‐elastic features are still lacking due to the irreconcilable interfere of the precursor and the self‐assembly unit. This hinders many of their practical biomedical applications where elasticity is required. Herein, hydrophilic and size‐controllable Fe₃O₄@poly(4‐styrenesulfonic acid‐co‐maleic acid) (PSSMA)@SiO₂ magnetic response photonic crystals are fabricated as the assembly units of the structural color hydrogels by orderly packing of core–shell colloidal nanocrystal clusters via a two‐step facile synthesis approach. These units are capable of responding instantaneously to an external magnetic field with resistance to interference of ions, thus, by integrating super‐elastic hydrogels, super‐elastic magnetic structural color hydrogels can be achieved. The structural color arises from the dynamic ordering of the magnetic nanoparticles through the contactless control of external magnetic field, allowing regional polymerization of hydrogels via changing orientation and strength of external magnetic field. These regionally polymerized super‐elastic magnetic structural color hydrogels can work as anti‐counterfeiting labels with super‐elastic identification, which may be widely used in the future.     

3D Helix Engineering in Chiral Photonic Materials

Engineering the helical structure of chiral photonic materials in three dimensions remains a challenge. 3D helix engineered photonic materials are fabricated by local stratification in a photopolymerizable chiral nematic liquid crystal. The obtained chiral photonic materials reflect both handedness of circular polarized light and show super‐reflectivity. Simulations match the experimentally observed photonic properties and reveal a distorted helical structure. 3D engineered polymer films can be made that reflect both left‐ and right handed circular and linear polarized light dependent and exhibit a changing color contrast upon altering the polarization of incident light. Hence, these 3D engineered photonic materials are of interest for new and emerging applications ranging from anti‐counterfeit labels and data encryption to aesthetics and super‐reflective films.     

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.     

非晶无序光子晶体结构色机理及其应用

结构色是一种由光学尺度的微纳结构与光相互作用形成干涉、衍射或散射而产生颜色的物理生色效应。与化学生色不同,结构色由于没有色素或者染料的参与,因此没有颜色褪色的现象,同时能够避免使用染料和色素带来的环境污染。目前结构色材料受到研究者和应用开发人员的广泛关注,大量的研究发现结构色可以来源于光子晶体与非晶光子晶体两种结构。光子晶体由规整的周期性结构组成,产生的颜色鲜艳却具有明显的角度依赖性。而非晶光子晶体,其"自身缺陷"导致的短程有序结构具备了各向同性的光子带隙、非虹彩效应、光局域化等特点,赋予了材料柔和亮丽不随角度变化的显色效果,可控的激光效应以及优良的发光效率,从而更能满足材料领域对光散射和光传输等方面的特殊需求。对非晶光子晶体的概念和结构,与可见光作用产生颜色的原理,以及制备非晶光子晶体的不同方法(平板刻蚀法、胶体颗粒自组装法、模板法、相分离法)做了详细的讨论,并对非晶光子晶体产生的结构色效应在光电器件、功能涂料和纺织材料等多个领域中的应用进行了展望。     

Pressure‐Responsive Hierarchical Chiral Photonic Aerogels

Pressure‐responsive chiral photonic aerogels are fabricated by combining liquid crystal self‐assembly and ice‐templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure‐induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent‐sensitive ink that shows quick color response to different solvents is designed using the pressure‐responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.     

机械力致变色光子晶体材料研究进展

目的 总结光子晶体材料在机械力致变色方面的研究现状,基于机械力致变色光子晶体的特点进行展望,为进一步研究和应用提供参考.方法 基于机械力的施加方式,从拉伸变色和压缩变色两个方面系统介绍了机械力致变色光子晶体的制备方法、光学性能以及机械性能,并分析比较了这两种光子晶体材料的应用现状和前景.结论 机械力致变色光子晶体在结构上有多种形式,包括嵌入胶体阵列的弹性体光子晶体、胶体交联光子晶体、层状光子晶体、链状光子晶体等,近些年这些光子晶体已经可以达到明亮的结构色和应变能力.机械力致变色光子晶体因其形式的多样性和良好的光学、机械性能,使其在应力检测、人体运动状态监测、防伪、显示等方面得以应用,具有很大的实际应用潜力.     

Single‐Stimulus‐Induced Modulation of Multiple Optical Properties

Stimuli‐responsive smart optical materials hold great promise for applications in active optics, display, sensing, energy conversion, military camouflage, and artificial intelligence. However, their applications are greatly restricted by the difficulty of tuning different optical properties within the same material, especially by a single stimulus. Here, magnetic modulations of multiple optical properties are demonstrated in a crystalline colloidal array (CCA) of magnetic nanorods. Small‐angle X‐ray scattering studies reveal that these nanorods form an unusual monoclinic crystal in concentrated suspensions. The CCA exhibits optical anisotropy in the form of a photonic bandgap and birefringence, thus enabling magnetic tuning of the structural color and transmittance at a rate of 50 Hz. As a proof‐of‐concept, it is further demonstrated that the fabrication of a multifunctional device for display, anticounterfeiting, and smart‐window applications based on this multiple magneto‐optical effect. The study not only provides a new model system for understanding colloidal assembly, but also opens up opportunities for new applications of smart optical materials for various purposes.     

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.     

Structural Coloration with Nonclose‐Packed Array of Bidisperse Colloidal Particles

Colloidal crystals and glasses have their own photonic effects. Colloidal crystals show high reflectivity at narrowband, whereas colloidal glasses show low reflectivity at broadband. To compromise the opposite optical properties, a simple means is suggested to control the colloidal arrangement between crystal and glass by employing two different sizes of silica particles with repulsive interparticle potential. Monodisperse silica particles with repulsive potential spontaneously form crystalline structure at volume fraction far below 0.74. When two different sizes of silica particles coexist, the arrangement of silica particles is significantly influenced by two parameters: size contrast and mixing ratio. When the size contrast is small, a long‐range order is partially conserved in the entire mixing ratio, resulting in a pronounced reflectance peak and brilliant structural color. When the size contrast is large, the long‐range order is rapidly reduced along with mixing ratio. Nevertheless, a short‐range order survives, which causes low reflectivity at a broad wavelength, developing faint structural colors. These findings offer an insight into controlling the colloidal arrangements and provide a simple way to tune the optical property of colloidal arrays for structural coloration.     

Triple‐Mode Emissions with Invisible Near‐Infrared After‐Glow from Cr3+‐Doped Zinc Aluminum Germanium Nanoparticles for Advanced Anti‐Counterfeiting Applications

Materials exhibiting persistent luminescence (PersL) have great prospect in optoelectronic and biomedical applications such as optical information storage, bio‐imaging, and so on. Unfortunately, PersL materials with multimode emission properties have been rarely reported, although they are expected to be very desirable in multilevel anti‐counterfeiting and encryption applications. Herein, Cr³⁺‐doped zinc aluminum germanium (ZAG:Cr) nanoparticles exhibiting triple‐mode emissions are designed and demonstrated. Upon exposure to steady 254 nm UV light, the ZAG:Cr nanoparticles yield steady bluish‐white emission. After turning off the UV light, the emission disappears quickly and the mode switches to transient near‐infrared (NIR) PersL emission at predominantly 690 nm. The transient NIR PersL emission which arises from Cr³⁺ is induced by non‐equivalent substitution of Ge⁴⁺. After persisting for 50 min, it can be retriggered by 980 nm photons due to the continuous trap depth distribution of ZAG:Cr between 0.65 and 1.07 eV. Inspired by the triple‐mode emissions from ZAG:Cr, multifunctional luminescent inks composed of ZAG:Cr nanoparticles are prepared, and high‐security labeling and encoding encryption properties are demonstrated. The results indicate that ZAG:Cr nanoparticles have great potential in anti‐counterfeiting and encryption applications, and the strategy and concept described here provide insights into the design of advanced anti‐counterfeiting materials.     

Tunable structural color in organisms and photonic materials for design of bioinspired materials

In this paper, the key topics of tunable structural color in biology and material science are overviewed. Color in biology is considered for selected groups of tropical fish, octopus, squid and beetle. It is caused by nanoplates in iridophores and varies with their spacing, tilting angle and refractive index. These examples may provide valuable hints for the bioinspired design of photonic materials. 1D multilayer films and 3D colloidal crystals with tunable structural color are overviewed from the viewpoint of advanced materials. The tunability of structural color by swelling and strain is demonstrated on an example of opal composites.     

Optical properties caused by periodical array structure with colloidal particles and their applications

High quality opal films have been attracting much attention due to their novel properties and applications, such as smart materials with structural color, novel photonic/optical devices and three-dimensional photonic crystals. In this article, the author reported a colloidal crystal consisting of cubic closely packed (ccp) polystyrene particles and filled with polydimethylsiloxane (PDMS) elastomer. The array of ccp (111) planes diffracts light of selective wavelengths according to Bragg’s law. The PS-PDMS hybrid opal films exhibit dynamic tuning structural colors. The lattice distance of ccp (111) planes is variable by swelling PDMS elastomer with hydrophobic liquid or by applying mechanical deformation. The hybrid opal films have potential applications in wide fields, for example, in smart sensing materials, color imaging without pigments and strain mapping of plastic deformation.     

Magnetically Responsive Elastomer–Silicon Hybrid Surfaces for Fluid and Light Manipulation

Stimuli‐responsive surfaces with tunable fluidic and optical properties utilizing switchable surface topography are of significant interest for both scientific and engineering research. This work presents a surface involving silicon scales on a magnetically responsive elastomer micropillar array, which enables fluid and light manipulation. To integrate microfabricated silicon scales with ferromagnetic elastomer micropillars, transfer printing‐based deterministic assembly is adopted. The functional properties of the surface are completely dictated by the scales with optimized lithographic patterns while the micropillar array is magnetically actuated with large‐range, instantaneous, and reversible deformation. Multiple functions, such as tunable wetting, droplet manipulation, tunable optical transmission, and structural coloration, are designed, characterized, and analyzed by incorporating a wide range of scales (e.g., bare silicon, black silicon, photonic crystal scales) in both in‐plane and out‐of‐plane configurations.     

Refractive Index-Adaptive Nanoporous Chiral Photonic Crystal Film for Chemical Detection Visualized via Selective Reflection

Photonic crystals (PC) are of great importance in technology, especially in optics and photonics. In general, the structural color of PCs responds to external stimuli primarily by changing their periodicity. Herein, the authors report on refractive index (RI) adaptive PCs. Cross-linked cholesteric films with interconnected nanopores exhibit a very low RI without light scattering. Transparent PC films with maximum reflectance in the ultravoilet (UV) region respond to various chemicals by changing the reflective color of the PC. The authors demonstrate its unique colorimetric chemical detections of hazardous organic liquids. Loading various chemicals into nanopores significantly shifts the structural color into the visible range depending on the chemical's RI. These results are unique in that the structural color of photonic films is mediated by RI changes rather than periodicity changes. In principle, nanoporous photonic crystal films can detect the RI of a chemical substance by its unique color. In contrast to volumetric changes, this sensing mechanism offers several advantages, including durability, excellent sensitivity, fast response time, and wide detection range. These results provide useful insight into stimulus-responsive PCs. The structural color of PC films can be effectively tuned by adjusting average RIs instead of changing periodicity.     

Enhanced light trapping in dye-sensitized solar cell by coupling to 1D photonic crystal and accounting for finite coherence length

Theoretical analysis of dye-sensitized solar cell integrated with photonic crystals based on coupled coherent and geometrical optics formalism has presented. The effect of structural parameters, such as thickness of layer, number of unit cells and light incident angle on the optical properties and photo-current magnification for the proposed photonic crystal based dye-sensitized solar cell have studied. The angular response of the cell in terms of light harvesting efficiency and cumulative photo-generation rate has also analysed. A strategy has presented to enhance the performance of the cell under oblique incidence. The effect of number of photonic crystal unit cells has also analysed in view of percentage enhancement in cumulative photo-generation rate. This work provides new insight into the design and tailoring of the photonic crystals to enhance the light harvesting efficiency in the solar cells.     

Biomaterial‐Based “Structured Opals” with Programmable Combination of Diffractive Optical Elements and Photonic Bandgap Effects

Naturally occurring iridescent systems produce brilliant color displays through multiscale, hierarchical assembly of structures that combine reflective, diffractive, diffusive, or absorbing domains. The fabrication of biopolymer‐based, hierarchical 3D photonic crystals through the use of a topographical templating strategy that allows combined optical effects derived from the interplay of predesigned 2D and 3D geometries is reported here. This biomaterials‐based approach generates 2D diffractive optics composed of 3D nanophotonic lattices that allow simultaneous control over the reflection (through the 3D photonic bandgap) and the transmission (through 2D diffractive structuring) of light with the additional utility of being constituted by a biocompatible, implantable, edible commodity textile material. The use of biopolymers allows additional degrees of freedom in photonic bandgap design through directed protein conformation modulation. Demonstrator structures are presented to illustrate the lattice multifunctionality, including tunable diffractive properties, increased angle of view of photonic crystals, color‐mixing, and sensing applications.