Tunable Structural Color Research Articles

Bioinspired Polypeptide Photonic Films with Tunable Structural Color.

We report a new synthetic strategy of combining N-carboxyanhydride (NCA) chemistry and photonic crystals for the fabrication of polypeptide structural color films. Driven by surface-initiated ring-opening polymerization, the di-NCA derivative of l-cystine (Cys) is introduced to replicate the functionalized colloidal crystal templates and construct freestanding P(Cys) films with tunable structural color. Furthermore, the feasibility of preparing patterned polypeptide photonic films is demonstrated via template microfabrication. Because of the incorporation of l-glutamate (Glu) components, the P(Cys-co-Glu) co-polypeptide films are endowed with a visual color responsiveness toward pH changes. Additionally, the polypeptide photonic films show on-demand degradability. Given the large family of amino acid building blocks, this powerful and versatile approach paves the way for chemical derivatization of multifunctional peptide-based optical platforms.

Chameleon-inspired iridescent structural color textiles with reversible multiple stimulus-responsive functions

• Structural color textile was fabricated by decorating CNCs. • The structural colors can be tuned by varying the proportion of glucose. • The structural colors can also be tuned by a different self-assemble way of CNCs. • Smart textile with reversible multiple stimulus-responsive functions was achieved. • The functions include anti-counterfeiting, RH sensing and water content detection. Over the past millions of years, plants and animals in nature have evolved complex microscopic structures, giving them vivid colors and camouflage that can deceive predators. Among the abundant sources of natural coloration, structural color is dominant in many creatures, which is created by the interaction between light and the nanostructures. Here, inspired by the natural structural color, we report an iridescent cotton fabric (ICF) with reversible multiple stimulus-responsive functions. Cellulose nanocrystal (CNC) could self-assemble into structural color nanostructure on the cotton fabric surface. And the flexible ICFs with tunable structural colors could be achieved by adding different proportions of glucose. Surprisingly, we proposed a novel and eco-friendly method to realize gradient structural colors by changing the self-assembly way of the CNCs onto CF surface without adding superfluous additives. Furthermore, due to the response characteristics of CNC to water molecules, the ICF is endowed with reversible behaviours including anti-counterfeiting, relative humidity sensing and water content detection functions, which make ICF become a smart textile that could be applied in many fields. Taken together, the presented findings are expected to pave the way towards the incorporation of CNCs in textile coloration with the researches focusing on the smart textile materials.

Humidity-Responsive Photonic Crystals with pH and SO2 Gas Detection Ability Based on Cholesteric Liquid Crystalline Networks.

Dynamic photonic crystals with tunable structural colors have been a hot topic in the research of anticounterfeiting devices, decoration, and detection. In this work, we prepared cholesteric liquid crystalline network (CLCN)-based photonic crystals that present humidity- and SO2 gas-responsive behaviors. The covalently cross-linked CLCN film presents humidity-responsive color changes due to the swelling/deswelling of the matrix under different humidity conditions. When treating the CLCN film with SO2 gas, the carboxylic salt converted to the acid and the film was not able to respond to the humidity change anymore. The mechanism of the SO2 gas-gated humidity responsiveness of the CLCN film was characterized. It was found that the acidic gas caused changes of pH, resulting in the conversion of the salt to acid and alteration of the surface property. The influence of concentration of SO2 gas and pH on humidity responsiveness of the CLCN film was investigated. We hope that this method provides inspirations for the design and fabrication of visualized pH and acidic gas detectors.

Photonic crystal films with upconversion luminescence based on the self-assembly of polystyrene encapsulated NaYF4:Ln3+ composite microspheres for dual-mode optical code

The design and fabrication of advanced materials with multiple optical states for constructing security code are of great value in anti-counterfeiting. Herein, novel upconversion photonic crystal (UCPC) films with tunable luminescence and structural color were prepared by vertical deposition of polystyrene encapsulated NaYF4:Ln3+ (CSNPs@SiO2/PS) composite microspheres. The composite microspheres were synthesized by sequential coating technology. Firstly, NaYF4:Ln3+@NaYF4 core/shell nanoparticles (CSNPs) were synthesized and served as upconversion luminescent emission centers. Then, the CSNPs were further coated by a SiO2 shell with pendant double bond to form CSNPs@SiO2-MPS. Afterward, mono-dispersed CSNPs@SiO2/PS composite microspheres were synthesized by emulsion polymerization of styrene in the presence of CSNPs@SiO2-MPS. By tailoring the doped lanthanide ions in CSNPs and adjusting the diameter of the composite microspheres, UCPC films with diverse upconversion fluorescence and structural color can be obtained. As a proof of concept, these different kinds of UCPC films were employed as information coding units to construct dual-mode anti-counterfeiting code. Our protocol provided a new security strategy for information storage.

Magnetic field responsive microspheres with tunable structural colors by controlled assembly of nanoparticles.

Dynamic color tuning has many useful applications in nature for communication, camouflage, mood indication, etc. Structural colors have more advanced applications due to their ability to respond to external stimuli by dynamically changing color. In this work, we proposed an efficient method to prepare magneto-chromatic microspheres with tunable structural color. Through a microfluidic technique, the magneto-chromatic microspheres containing Fe3O4@C magnetic particles were continuously prepared. The size of the microspheres decreases with the increase of PVA solution phase to ETPTA phase flow rate ratio. Furthermore, the microspheres with larger sizes more easily form close packed structures. Microspheres can be constrained in PVA to form a free-standing film after the evaporation of water in PVA solution. The PVA film could display tunable brilliant structural colors when an external magnetic field is applied. Moreover, microspheres with fixed structural colors can also be acquired by polymerizing microspheres under UV light under an external magnetic field.

Cracking enabled unclonability in colloidal crystal patterns authenticated with computer vision.

Colloidal crystals with iridescent structural coloration have appealing applications in the fields of sensors, displays, anti-counterfeiting, etc. A serious issue accompanying the facile chemical self-assembly approach to colloidal crystals is the formation of uncontrolled and irregular cracks. In contrast to the previous efforts to avoid cracking, the unfavorable and random micro-cracks in colloidal crystals were utilized here as unclonable codes for tamper-proof anti-counterfeiting. The special structural and optical characteristics of the colloidal crystal patterns assembled with monodisperse poly(styrene-methyl methacrylate-acrylic acid) core-shell nanospheres enabled multi-anti-counterfeiting modes, including angle-dependent structural colors and polarization anisotropy, besides the physically unclonable functions (PUFs) of random micro-cracks. Moreover, by using the random cracks in the colloidal crystals as templates to guide fluorescent silica nanoparticle deposition, an fluorescent anti-counterfeiting mode with PUFs was introduced. To validate the PUFs of the fluorescent micro-cracks in the colloidal crystals, a novel edge-sensitive template matching approach based on a computer vision algorithm with an accuracy of ∼100% was developed, enabling ultimate security immune to forgery. The computer-vision verifiable physically unclonable colloidal crystals with multi-anti-counterfeiting modes are superior to conventional photonic crystal anti-counterfeiting materials that rely on angle-dependent or tunable structural colors, and the conventional PUF labels in the aspect of decorative functions, which will open a new avenue for advanced security materials with multi-functionality.

Stretchable triboelectric nanogenerator with exteroception-visualized multifunctionality

A liquid crystal (LC)-based triboelectric nanogenerator (TENG) with tunable structural colors and improved water locking effect was designed. The specific applications of this self-powered LC-TENG for optically visualized wearable smart sports assistance and skin-like thermal perception sensing were evaluated.

Active modulation of reflective structural colors.

Dynamically controllable reflective structural colors have been garnering growing attention due to increasing commercial interests in smart displays (e.g., colorimetric labels), colored e-readers, etc. To comply with the requirements of future displays, several strategies have been proposed by various research groups to achieve wide color tuning ranges in the visible regime, high on/off contrast ratios, and fast response times. In this review, we first introduce ways to create optical resonances including plasmonic, photonic, and plasmonic-photonic hybrid structures that form the basis of color generation. We then outline strategies that control the refractive index contrast between the system and the surrounding as a means to actively change the reflective structural colors, backed with representative examples from the literature. Finally, we provide a comparison of dynamic structural colors based on various switching mechanisms summarizing performance metrics that are important for future displays, and conclude with an outlook on current challenges.

Mechanochromic Responses of Cholesteric Liquid Crystal Droplets with Nanoscale Periodic Helical Structures Showing Reversible and Tunable Structural Color

Cholesteric liquid crystal (CLC) droplets with nanoscale periodic helical structures are omnidirectional under planar anchoring, and light is selectively reflected from the droplet center at all directions. However, when CLC droplets are too small, random light scattering dominates. In this regard, we present a facile method to increase the selective Bragg reflection of CLC droplets and demonstrate the unique mechanochromic responses of CLC-droplet-dispersed polymer films. The films are originally opaque and white, when the films are bidirectionally stretched, spherical CLC droplets are squeezed into oblate shape and the point defect at the center of each CLC droplet transitions to a ring defect, greatly increasing the selective Bragg reflection region and resulting in the appearance of structural color. By tuning the helical pitches via the chiral dopant concentration from 1.7 to 3.0 wt %, the desired structural color is achieved. Smart sensors show color changes in response to hydration and dehydration are designed, paving the way for the developments of CLC-based functional materials and smart devices.

Multicolor Photonic Pigments for Rotation-Asymmetric Mechanochromic Devices.

Photonic crystals are extensively explored to replace inorganic pigments and organic dyes as coloring elements in printing, painting, sensing, and anti-counterfeiting due to their brilliant structural colors, chemical stability, and environmental friendliness. However, most existing photonic-crystal-based pigments can only display monochromatic colors once made, and generating multicolors has to start with designing different building blocks. Here, a novel photonic pigment featuring highly tunable structural colors in the entire visible spectrum, made by the magnetic assembly of monodisperse nanorods into body-centered-tetragonal photonic crystals, is reported. Their prominent magnetic and crystal anisotropy makes it efficient to generate multicolors using one photonic pigment by magnetically controlling the crystal orientation. Further, the combination of angle-dependent diffraction and magnetic orientation control enables the design of rotation-asymmetric photonic films that display distinct patterns and encrypted information in response to rotation. The efficient multicolor generation through precise orientational control makes this novel photonic pigment promising in developing high-performance structural-colored materials and optical devices.

Easy preparation of anodic aluminum oxide photonic crystal films with tunable structural colors

Anodic aluminum oxide (AAO) photonic crystal films with Sn nanoparticles electrodeposited at the bottom were prepared using a pseudo-sine-pulse anodization technique followed by an alternating current electrodeposition. Moreover, a pore-widening post-treatment was performed for the Sn electrodeposited films. The phase structure, microstructure and structural color properties of the specimens with different preparation histories were examined. The Sn electrodeposition enabled production of brightly colored AAO photonic crystal films after a short-duration anodization (40 anodization cycles) in a sulfuric acid electrolyte solution at a temperature (around 20 °C) near to room temperature, which is technology-friendly relative to the conventional anodization conditions when using sulfuric acid electrolyte solutions, e.g., after 80–300 anodization cycles at around 0 °C. Moreover, the pore-widening treatment was demonstrated to be convenient and controllable in tuning the porosities and structural color properties of the Sn electrodeposited films. The peak wavelengths of the reflectance spectra were nearly linearly blue-shifted from 681 to 574 nm with increasing pore-widening time from 0 to 8 min, concomitant with a structural color change from bright-red to yellowish-green. Meanwhile, the peak reflectances were linearly enhanced from 59 to 84%, corresponding to an increase of the brightness of the films. The linearly varied peak wavelengths and peak reflectances were related to a linear increase of the porosities of the stem and branched layers in the films with pore-widening time.

Large-Area Fabrication of Structurally Colored and Humidity Sensitive Composite Nanofilm via Ultrasonic Spray-Coating.

Intelligent structural colors have received extensive attention in recent years due to their diverse applications. However, the large-area, uniform, and cost-effective fabrication of ultra-thin structural color films is still challenging. Here, for the first time, we design and employ an ultrasonic spray-coating technique with non-toxic, green nano-silica and polyvinylpyrrolidone as raw materials, to prepare structural color films on silicon wafers. Due to the addition of polyvinylpyrrolidone, the coffee-ring effect during droplet drying is suppressed and uniform composite films are formed. We further perform a detailed study of the influence of various processing parameters including silica/polyvinylpyrrolidone concentration, substrate temperature, nozzle-to-substrate distance, and number of spray-passes on film roughness and thickness. By increasing the number of spray-passes from 10 to 30, the film thickness from 120 to 340 nm is modulated, resulting in different colors, and large-area and uniform colors on commercial round silicon wafers with 15 cm diameter are achieved. The silica/polyvinylpyrrolidone composite films show strong hydrophilicity and are sensitive to humidity changes, leading to quickly tunable and reversible structural colors. Quartz crystal microbalance with dissipation demonstrates water vapor adsorption and condensation on the nanofilm when increasing environmental humidity. Thereby, ultrasonic spray-coating as a novel film fabrication technique provides a feasible scheme for large-area preparation of intelligent structural colors.

Photonic Janus Carbon Fibers with Structural Color Gradient for Multicolored, Wirelessly Wearable Thermal Management Devices

AbstractJanus structure offers materials unique functionalities which are difficult to achieve otherwise with isotropic counterpart, opening up new avenues in diverse application fields. Here, a series of novel photonic Janus carbon fibers (PJCFs) with structural color gradient and excellent mechanical stability are reported by introducing 1D photonic layers with a gradient of periodicity. By virtue of the unique photonic architecture, the PJCFs exhibit a multiple and continuous variation of structural color (i.e., structural color gradient) in a full spectrum. By controlling the thickness of periodic layer, a series of PJCFs with vivid and tunable structural colors ranging from blue, green, orange, to violet‐red can be obtained. Furthermore, their gradient structural colors remain constant after reciprocating rubbing and laundering, showing outstanding mechanical robustness. Notably, benefiting from the inherent electrothermal property and flexibility of CFs, the PJCFs display stable Joule heating performance even under deformation. As proof‐of‐concept, the PJCFs are further integrated with wireless transmission system to construct a multicolored, wirelessly controlled thermal management device, realizing remote and precise temperature manipulation via Bluetooth. This work provides not only a new platform for achieving distinct structural color gradient of CFs, but also a promising strategy toward next‐generation multicolored personal healthcare devices.

Chiral Cellulose Nanocrystal Humidity-Responsive Iridescent Films with Glucan for Tuned Iridescence and Reinforced Mechanics.

The fabrication of biomimetic photonic materials with environmental stimuli-responsive functions from entirely biobased materials is becoming increasingly challenging with the growing demand for biodegradable materials. Herein, the effect of glucan with different molecular weights on the mechanical performance and tunable structural color of iridescent CNC composite films was investigated. The existence of glucan did not influence the self-assembly performance of CNCs, but rather led to an improvement in the mechanical performance, enabling cholesteric CNC composite films with an adjustable structural color. Simultaneously, the iridescent films showed a conspicuous redshift and enlarged initial pitch without obstruction of the chiral structure. In response to environmental humidity, the structural colors of the iridescent composite films can be changed by regulating their chiral nematic structure. In particular, the films demonstrate a reversible structural color change between blue and red at RH between 50 and 98%. The resulting biobased iridescent composite films have potential applications in decorative coating, optical and humidity sensing, and anticounterfeiting.

Highly Flexible, Multicolored, and Multifunctional Single-Fiber-Based Microsensors for UV, Temperature, and Infrared Detection

Flexible and multifunctional microsensors have attracted wide attention owing to their multiple-sensing functions and diverse application fields. Here, we develop a series of highly flexible, multicolored, and multifunctional single-fiber-based (SFB) microsensors for flexible functional electronics. The SFB microsensors are based on the ingenious combination of single ZnO-coated carbon fiber (CF), flexible indium tin oxide/poly(ethylene terephthalate) (ITO/PET) membrane, and typing paper through a facile integration method. Specifically, a series of ZnO-coated CFs are prepared by chemically and coaxially depositing ZnO nanolayers on the CF surface through the atomic layer deposition (ALD) technique. Notably, the resultant SFB microsensors exhibit vivid and tunable structural colors originating from the thin-film interference of ZnO nanolayers as well as optical anisotropy because of the distinct axisymmetric structure of the CF. Benefiting from the optoelectronic properties of the ZnO nanolayer and delicate device structure, the SFB microsensors show a high UV sensitivity (120%) and good mechanical stability at varying bending angles (0–150°) and cycles (600 cycles). Moreover, by harnessing the pyroelectric and photothermal effects, the SFB microsensors can also be utilized to detect ambient temperature changes (0–55 °C) and IR light with high linearity, relatively high sensitivity (0.74% °C–1), and good stability. Owing to these merits, such SFB microsensors are capable of monitoring mouth respiration and nose breathing, implying their applications in wearable health monitors. This work not only integrates different sensing capabilities into a flexible device with reduced complexity but also miniaturizes the size of the device dramatically, holding tremendous prospects for diverse smart microsensing applications.

Tunable Structural Color Images by UV-Patterned Conducting Polymer Nanofilms on Metal Surfaces.

Precise manipulation of light-matter interactions has enabled a wide variety of approaches to create bright and vivid structural colors. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive-index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability of these approaches impede their further development toward flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Here, a novel method is presented to generate structural color images based on monochromic conducting polymer films prepared on metallic surfaces via vapor phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose enables synergistic control of both nanoscale film thickness and polymer permittivity, which generates controllable structural colors from violet to red. Together with grayscale photomasks this enables facile fabrication of high-resolution structural color images. Dynamic tuning of colored surfaces and images via electrochemical modulation of the polymer redox state is further demonstrated. The simple structure, facile fabrication, wide color gamut, and dynamic color tuning make this concept competitive for applications like multifunctional displays.

Dual‐Modal Invisible Photonic Crystal Prints from Photo/Water Responsive Photonic Crystals

Invisible photonic crystal prints (IPCPs) are appealing for anti‐counterfeiting and information protection due to their capability in hiding and showing the encrypted information under specific conditions. Herein, the dual‐modal IPCPs based on responsive photonic crystals (PCs) with tunable photoluminescent (PL) and structural colors are reported. The responsive PCs are prepared by the self‐assembly of dyes doped silica particles into non/swellable polymers. The independent design and control over the optical response of the PL of the particles and structural colors of PCs enable the successful encryption of dual‐modal patterns. Under normal conditions, the as‐fabricated IPCP shows a fake pattern with uniform structural colors and thus hides the encrypted pattern due to the similar particles size, lattice constant, and refractive index of each region. In striking contrast, PL and structural colors‐based new patterns can be instantly and reversibly revealed when the IPCP is exposed to UV illumination or soaked in water. This work provides a new concept in designing and fabricating dual‐modal IPCPs, and facilitates their applications in the fields of color display, antifake package, and multilevel anti‐counterfeiting.

Programmable Color in a Free-Standing Photonic Microgel Film with Ultra-Fast Response.

In this work, a free-standing microgel film with programmable and angle-independent structural color is prepared via a simple but effective method. Dried poly(styrene-N-isopropylacrylamide-acrylic acid) (pStNIPAAmAA) microgels were stabilized by inter-microgel crosslinking, and thus, only microgels were used to build the optical hydrogel. The free-standing microgel film displayed tunable structural color by the swelling/deswelling of the microgels under external stimuli, such as temperature, pH, ionic strength, and organic solvent. Moreover, the structural color of the film is angle-independent for the disordered microgel arrays. It is worth noting that programmable color stripes which have the panther chameleon's ability to change skin color are successfully fabricated by patterning microgels with different thermoresponsivities. More importantly, the microgel film has an ultrafast response to temperature (1.41 s from 20 to 40 °C) and pH (2.24 s from pH 8.3 to pH 2.0), much faster than that of most optical materials reported in previous studies.

Dynamic Structural Colors Based on All‐Dielectric Mie Resonators

AbstractStructural color, which generates vivid colors via the geometrical arrangement of resonant nanostructures, can enhance the recyclability and mechanical stability of colors; structural colors have been intensively studied in recent years, owing to their high resolution, color purity, and stability compared to colors generated by dyes and pigments. Dielectric materials, which possess a suitably high refractive index and low loss in the visible range, are ideal materials for structural colors. However, the static optical performances of most structural colors impose restrictions on their applicability. This article reviews the fundamental physics of dielectric structural color procedures based on Mie resonance; furthermore, it highlights recent progress in tunable structural colors, focusing on their operation mechanisms and practical applications, including the geometric parameters of the nanostructures, optical parameters of the building materials and environments, and properties of the optical stimuli. This article is concluded by presenting an outlook on the future potential of structural colors, as well as the challenges that remain to be addressed.

Bioinspired Tunable Structural Color Film with Janus Wettability and Interfacial Floatability towards Visible Water Quality Monitoring

AbstractSuperwettability materials from existing natural creatures have been widely studied to enable artificial manufacture. Variable wettability states, especially Janus wettability, have attracted particular interest because of the applications in various intelligent systems. However, to date, most of these existing Janus wettability surfaces lack stimuli‐response visualization, which requires the connection of electrical instruments to process and display external stimulus signals. Inspired by the functional performance of lotus leaf and Betta splendens, a multifunctional asymmetric film is designed by using the superhydrophobic/superhydrophilic binary cooperative strategy and tunable structural color feature. Thus, it is demonstrated that the Janus membrane could not only timely report the arrival of the environmental variables via directional migration induced by Marangoni effect, but also quantitively feedback the stimuli through visible structural color variations. These features indicate that the Janus wettability structural color film may open a potential chapter in designing and fabricating the multifunctional robotic environmental detector.