Tunable Structural Color Research Articles

Arrays of Bowl-Shaped Janus Particle Film with Structured Colors.

Structural colors generated via total internal reflection (TIR) using nanostructure-free micro-concave shapes have garnered increasing attention. However, the application of large micro-concave structures for structural coloration remains limited. Herein, a flexibly tunable structural color film fabricated by casting polydimethylsiloxane (PDMS) on an array of large poly(glycidyl methacrylate) (PGMA) bowl-shaped particles is reported. The resultant film exhibits tunable red to green structural colors with changing observation angles. Moreover, the color can be further tailored by altering the shape of the film itself. The incorporation of the PDMS layer not only facilitates a shift in the locus of TIR from the bottom surface to the top concave surface of the particles, thereby enabling the generation of structural color, but also confers enhanced flexibility to the film. Further decoration with silver nanoparticles imparts antimicrobial properties, yielding a novel antimicrobial coating material with structural colors. The simple and cost-effective strategy for the production of structural color films provides potential applications in antimicrobial coatings, enabling accessible and customizable structural coloration using big-size micro-concave particles.

Direct Ink Writing of Cephalopod Skin‐Like Core‐Shell Fibers From Cholesteric Liquid Crystal Elastomers and Dyed Solutions

AbstractDirect ink writing (DIW) of core‐shell structures allows for patterning hollow or composite structures for shape morphing and color displays. Cholesteric liquid crystal elastomers (CLCEs) with liquid crystal mesogens assembled in a helix superstructure are attractive for generating tunable iridescent structural colors. Here, by fine‐tuning the rheology of the core and shell materials, respectively, this study creates droplets or a continuous filament in the core from the precursors of polydimethylsiloxane (PDMS) or poly(vinyl alcohol), whereas CLCE forms the outer shell. By introducing a dye in the droplets, the skin structures of cephalopods, consisting of chromatophores and iridocytes, are mimicked for enhanced color saturation, lightness, and camouflage. After removal of the core material, a CLCE hollow fiber is obtained, which can switch colors upon mechanical stretching and pneumatic actuation, much like papilla along with iridocytes. Further, liquid crystal mesogens assembled in the bulk of the fiber are in polydomain. Thus, the skin appears opalescent at room temperature, much like how leucophores enhance reflectins. Upon heating above the nematic to isotropic transition temperature, the skin becomes transparent. Lastly, a cephalopod model is constructed, where different parts of the model can change colors independently based on different mechanisms.

Size-Dependent Magnetic Responsiveness of a Photonic Crystal of Graphene Oxide Nanosheets.

A magnetically responsive photonic crystal of colloidal nanosheets can exhibit a controllable structural color, offering diverse potential applications. In this study, we systematically investigated how the lateral sizes of graphene oxide (GO) nanosheets affect their magnetic responsiveness in a photonic system. Contrary to the prediction that larger lateral sizes of nanosheets would be more responsive to an applied magnetic field based on the magnetic energy of anisotropic materials, we discovered that GO nanosheets with larger lateral sizes in the photonic system scarcely responded to a 12 T magnetic field. The lack of magnetic response may be due to the strongly restricted rotational motion of GO nanosheets by mutual electrostatic forces. In contrast, GO nanosheets with medium lateral sizes readily responded to the 12 T magnetic field, forming a uniaxially oriented structure that resulted in a vivid structural color. However, smaller GO nanosheets displayed a less vivid structural color, possibly because of less structural ordering of GO nanosheets. Finally, we found that the photonic crystal of GO nanosheets with optimized lateral sizes responded effectively to the 12 T magnetic field across various GO concentrations, resulting in a vivid and tunable structural color.

Facile Synthesis of Soft Core-Shell Nanospheres for Constructing Multiresponsive Photonic Crystal Security Patterns.

Responsive photonic crystals (RPCs) presenting tunable structural colors under external stimuli are widely applied in the fields of dynamic displays, sensors, and anticounterfeiting. However, the development of multiresponsive photonic crystal (MRPC) films possessing versatile variable optical properties remains a significant challenge due to the tedious synthetic procedure of multifunctional building blocks and complex assembly processes, thereby constraining their extensive applications. In the present work, a series of soft nanospheres with a hydrophobic cores and responsive hydrophilic shells have been synthesized by a facile one-step surfactant-free emulsion polymerization method. The MRPC patterns were then prepared by depositing soft nanosphere emulsions onto the 3D-printed substrates with a topological structure followed by drying at room temperature. The shells of soft nanospheres deformed and fused with each other, resulting in the formation of transparent MRPC film patterns. The MRPC patterns exhibited brilliant structural color in a wet state but lost the color again after complete drying. Such a reversible structural color was ascribed to the change of the refractive index (RI) of the hydrophilic shell layers of nanospheres according to wetting/drying state shifting. Moreover, the on-demand designed MRPC patterns could rapidly respond to external stimuli of temperature, pH, and organic solvents in a reversible manner, and multichannel encrypted security labels were also demonstrated. We postulate that our facile and feasible approach can be applied to the systematic design and large-scale production of MRPC patterns for a variety of applications.

Infrared Stealth Coating with Tunable Structural Color Based on ZnO Spheres.

It is important to develop low infrared (IR) emissive coating with tunable structure color to improve the infrared-visible stealth performance of military equipment. In this work, uniform ZnO spheres are used as building units to construct photonic structures with both bright adjustable structure color and low IR emissivity due to the relatively high refractive index and low IR emissivity of ZnO. The vivid tunable structural colors are provided by the photonic bandgap of ZnO photonic crystals (PCs) or the quasi-bandgap of amorphous photonic crystals (APCs), respectively. Both ZnO PCs and APCs exhibited low IR emissivity in 3-5µm. The IR emissivity of 255nm ZnO PCis 0.483 and the IR emissivity of 255nm ZnO APC is 0.492 at 25°C. With the increase of temperature, the IR emissivity offurther decreased to 0.295 and 0.312 at 300°C. These structures can be applied to a variety of surfaces, and all these structures have good thermal and light stability as well. This work may open a simple and effective way to fabricate materials with good infrared-visible stealth performance, expanding the application of ZnO PCs and APCs coatings in the camouflage area.

Bio‐Inspired Cellulose Composites With Multicolor Separation via Electro‐Thermal and Magneto‐Thermal Techniques for Multifunctional Applications

AbstractBiomimetic optical devices based on cellulose nanocrystals with tunable structural colors have received significant attention recently. However, the limited ability to control multicolor separation beyond simple single‐color modulation restricts its practical applications. Here, a diversified multicolor separation strategy for the cholesteric phase cellulose composite (CPCC) is presented. The CPCC is prepared by integrating a high‐concentration self‐assembled hydroxypropyl cellulose with a cross‐linked poly(acrylic acid‐acrylamide) (P(AA‐AM)) network. By adjusting the cross‐linking degree of P(AA‐AM) in CPCC, Thermally induced multicolor separation is achieved from a homochromatic state at room temperature to multicolor patterned display at elevated temperatures. Furthermore, by utilizing the electric heating effect of conductive carbon oil, the multicolor separation under low voltage is achieved, and based on this, pixelated electro‐thermochromic displays are developed. Additionally, magneto‐thermal multicolor separation induced by introducing FeNi3 nanoparticles is investigated with efficient magnetic induction heating ability into CPCC. The multicolor separation effect is further improved by pixelated distribution of FeNi3 nanoparticles and adjusting the concentration of FeNi3 in each pixel. Finally, thermochromism, electro‐thermochromic, and magneto‐thermochromic functionalities are integrated into the CPCC, achieving advanced multilevel information encryption. This multilevel approach of controlling multicolor separation significantly enhances the functionality of nanocellulose in various potential photonic applications.

Tunable temperature-responsive photonic ionogels with dual signals output

Photonic ionogels with dual electrical and optical output have been intensively studied. However, tunable temperature-responsive photonic ionogel assembled by thermosensitive nanogels has not been studied yet. Herein, an innovative approach to fabricate photonic ionogels has been developed for smart wearable devices with tunable temperature sensitivity and structural color. Firstly, poly(isopropylacrylamide-r-phenylmaleanilic acid) P(NIPAm-r-NPMA) nanogels self-assemble into photonic crystals in 2-hydroxyethyl acrylate (HEA), water, and the ionic liquid of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. And then robust photonic ionogels are developed through a polymerization of 2-hydroxyethyl acrylate crosslinked by poly(ethylene glycol) diacrylate (PEGDA). The incorporation of the ionic liquid, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, enhances the mechanical strength of photonic ionogels and tunes the temperature-sensitivity of the ionogels, making them adaptable to various environmental conditions. The findings demonstrate that these ionogels can serve dual functions in smart wearable devices, combining electrical and optical signal outputs due to the conductivity of the ionic liquid and structural color from the nanogel assembly. The resultant photonic ionogels exhibit exceptional substrate adhesion, mechanical stability, and fast resilience. More significantly, the nanogels within these ionogels serve as the building blocks of photonic crystals (PCs) endow with angle-independent coloration and enhance stretchability beyond 200 %, while the stretchability of the ionogles without the nanogels is only about 100 %. Our photonic ionogels with tunable temperature-sensitivity and dual outputs will open an avenue to the development of the innovative smart wearable devices.

Expanded polarization tunable structural colors with two-way transmission and reflection enabled by Al-HfO2 bilayer structure

Expanded polarization tunable structural colors with two-way transmission and reflection enabled by Al-HfO2 bilayer structure

Ethanol-responsive structural colors with multi-level information encryption based on the patterned three-layer inverse opal photonic crystal

Stimulus-responsive inverse opal photonic crystals (IOPCs) with tunable structural colors show significant promise in information security. To improve upon the traditional bilayer structure with limited color information and single decoding mode, this work developed an ethanol-responsive structure with multi-level information encryption ability by inserting a functional layer into two shielding layers (red Layer A with a photonic stop band (PSB) at 640 nm and green Layer C with a PSB at 530 nm). The functional layer was composed of colorless Layer B, a quick response (QR) code pattern made of TiO2 nanoparticles, and a dense polymer. Due to the isolation of distinct layers, different reflectance values, and different PSB positions of the three-layer IOPC, the structural color of Layer B could only be “turned on” by wetting the entire structure when its PSB redshifted from 360 nm to 460 nm. Specifically, when either side was individually wetted, the PSB of Layer A or C redshifted to 825 nm or 685 nm, and the color of the QR code was dominated by the unwetted red or green layer. After the entire structure had been soaked, the blue QR code was decoded. Meanwhile, when the detecting angle increased from 5° to 60°, the PSBs of Layers B and C in the wetted three-layer IOPC blueshifted from 460 nm to 365 nm and from 685 nm to 540 nm, respectively, which resulted in a cascade decoding process with a single- or mixed-color output. This structure provides a good foundation for multi-level information encryption.

Active Full‐Color Generation Based on a Liquid Crystal‐Integrated Plasmonic Metasurface

Structural colors based on metasurfaces outperform traditional pigments and dyes in terms of nonfading, high spatial resolution, and high stability, usually incorporating active materials for tunability. Liquid crystals (LCs) are suitable for tunable structural color design due to their large birefringence and fast modulation by external stimuli. However, most LC‐integrated structural colors focus on tailoring the polarization angle of incident light to generate two colors and their mixing. Herein, a scheme of full‐color generation based on a plasmonic metasurface integrated with LCs utilizing the combination of the polarization angle rotation effect of the twisted‐nematic LCs and the refractive index modulation through the realignment of the LCs near the metasurface is demonstrated. Based on the proposed structural color method, full‐color generation of a record color gamut of 60.7% standard Red Green Blue region, equivalent to 43% National Television Standards Committee area, in the LC‐integrated metasurface, has been numerically realized by tuning the bias voltage of the LCs in reflection. The achieved color gamut is nearly 4 times wider than the previously reported result. The proposed active full‐color generation metasurface shows great potential in applications for low‐power reflective color display, anticounterfeiting, and optical encoding.

Smart manufacturing of ZIF-8 photonic crystals: Catalytic formaldehyde degradation and colorful eco-friendly coatings

Formaldehyde and other harmful gases cause environmental pollution and threaten human health. Environment-friendly coatings is an effective strategy to address these environmental and health concerns. In this study, a large-scale amorphous photonic crystals (APC) based on modified ZIF-8 (Hxh-ZIF-8) with photocatalytic degradation of formaldehyde and tunable structural color were designed for eco-friendly coatings by programmable smart manufacturing. As APC unit, Hxh-ZIF-8 nanoparticles with controllable diameters were synthesized utilizing solvothermal method and the −N = C = O group was introduced with thermal oxidation. The results of photocatalytic degradation revealed that H5h-ZIF-8, through thermal oxidation, exhibited a formaldehyde degradation rate 9.8 times than that of ZIF-8. The main reactive components in photocatalysis were holes (h+) and superoxide radicals (·O2–), showcasing both stability and reproducibility. The color and brightness of the APC were adjusted by tuning the Hxh-ZIF-8 diameter. The angle-independent APC ranging from red to purple were fabricated with 177, 165 and 235 nm Hxh-ZIF-8 nanoparticles in various proportions with programmable smart manufacturing. In this study, a large-scale ZIF-8-based APC was designed with programmable smart manufacturing for environment-friendly coating, which combined environmental protection technology with structural color, and paved a new strategy for the development of multifunctional materials in air quality management.

Dual-Mode Hydrogels with Structural and Fluorescent Colors toward Multistage Secure Information Encryption.

Constructing an anti-counterfeiting material with non-interference dual optical modes is an effective way to improve information security. However, it remains challenging to achieve multistage secure information encryption due to the limited stimulus responsiveness and color tunability of the current dual-mode materials. Herein, a dual-mode hydrogel with both independently tunable structural and fluorescent colors toward multistage information encryption, is reported. In this hydrogel system, the rigid lamellar structure of poly(dodecylglyceryl itaconate) (pDGI) formed by shear flow-induced self-assembly provides the restricted domains wherein monomers undergo polymerization to form a hydrogel network, producing structural color. The introduction of fluorescent monomer 6-acrylamidopicolinate (6APA) as a complexation site provides the possibility of fluorescent color formation. The hydrogel's angle-dependent structural color can be controlled by adjusting the crosslinking density and water content. Additionally, the fluorescence color can be modulated by adjusting the ratio of lanthanide ions. Information of dual-mode can be displayed separately in different channels and synergistically overlayed to read the ultimate message. Thus, a multistage information encryption system based on this hydrogel is devised through the programed decryption process. This strategy holds tremendous potential as a platform for encrypting and safeguarding valuable and authentic information in the field of anti-counterfeiting.

Omnidirectional color wavelength tuning of stretchable chiral liquid crystal elastomers

Wavelength-tunable structural colors using stimuli-responsive materials, such as chiral liquid crystals (CLCs), have attracted increasing attention owing to their high functionality in various tunable photonic applications. Ideally, on-demand omnidirectional wavelength control is highly desirable from the perspective of wavelength-tuning freedom. However, despite numerous previous research efforts on tunable CLC structural colors, only mono-directional wavelength tuning toward shorter wavelengths has been employed in most studies to date. In this study, we report the ideally desired omnidirectional wavelength control toward longer and shorter wavelengths with significantly improved tunability over a broadband wavelength range. By using areal expanding and contractive strain control of dielectric elastomer actuators (DEAs) with chiral liquid crystal elastomers (CLCEs), simultaneous and omnidirectional structural color-tuning control was achieved. This breakthrough in omnidirectional wavelength control enhances the achievable tuning freedom and versatility, making it applicable to a broad range of high-functional photonic applications.

A Tunable Hydrophilic-Hydrophobic, Stimulus Responsive, and Robust Iridescent Structural Color Bionic Film with Chiral Photonic Crystal Nanointerface.

Bio-inspired in nature, using nanomaterials to fabricate the vivid bionic structural color and intelligent stimulus responsive interface as smart skin or optical devices are widely concerned and remain a huge challenge. Here, the bionic flexible film is designed and fabricated with chiral nanointerface and tunable hydrophilic-hydrophobic by the ultrasonic energy perturbation strategy and crosslinking of the cellulose nanocrystals (CNC). An intelligent nanointerface with adjustable hydrophilic and hydrophobic properties is constructed by the supramolecular assembly using a smart ionic liquid molecule. The bionic flexible film possessed the variable hydrophilic-hydrophobic, stimulus responsive, and robust iridescent structural color. The reflective wavelength and the helical pitch of the film can be easily modulated through the ultrasonic energy perturbation strategy. The bionic flexible film by covalent cross-linking has excellent robustness, good elasticity and flexibility. The tunable brilliant structural color of the chiral nanointerface is attributed to the surface charge change of the CNC photonic crystal, which is disturbed by ultrasonic energy perturbation. The bionic flexible film with tunable structure color has intelligent hydrophilic and hydrophobic stimulus response properties. The chiral bionic materials have potential applications in smart skin, optical devices, bionic materials, robots, anti-counterfeiting, colorful displays, and stealth materials.

Electrochromic Passive Matrix Display Utilizing Diode‐Like Redox Reactions on Indium‐Tin‐Oxide

Recent years have shown many advances in the development of tunable structural colors by combining nanostructures with electrochromic materials. One main goal is to develop energy saving color displays that rely on ambient light instead of being emissive. However, all displays need to be pixelated to show arbitrary images and few studies have addressed the challenge of preparing and controlling individual electrochromic pixels. Herein, a very simple method to reach this milestone by using passive matrix addressing is presented, which requires no additional electronic components in the pixels. It is shown that the common transparent conductor indium tin oxide (ITO) in non‐aqueous electrolytes exhibits the diode‐like behavior (threshold in voltage in relation to current and coloration) necessary to prevent significant cross‐talk between pixels. The chemical nature of the redox activity that enables this behavior is attributed to omnipresent oxygen and the formation of superoxide ions. Additionally, it is shown that a gel‐like electrolyte can be prepared by optical lithography, which makes the concept compatible with patterning of pixels at high resolution. This method for preparing pixelated displays should be compatible with practically any type of electrochromic surface in both reflective and transmissive configurations. Also, the counter electrode maintains excellent transparency since it simply consists of ITO. The results should prove very useful as the research field of tunable structural colors moves from proof‐of‐concept to real devices.

Dual layer chessboard metasurface sandwiched by a spin-on-carbon for spectral modulation.

Metasurfaces, composed by metals and dielectrics in periodical order with subwavelength pitches, are of great importance for their unique ability to abruptly manipulate optical fields. So far, all the reported metasurfaces are constructed by thermally deposited metals and dielectric films, based on semiconductor processes which are expensive and time-consuming. Inspired by the outstanding dry etch property of spin-on-carbon (SOC) as the interlayer material in CMOS technology, this paper proposes to utilize the SOC as the dielectric layer in a chessboard metasurface with dual layer of gold to form an array of local surface plasmonic resonators (localized surface plasmon resonance). Finite difference and time domain (FDTD) method is used to investigate the spectral characteristics in reflectance of the metasurface in both visible and short wavelengths of infrared light. Electron beam lithography is applied to generate the nanoscale chessboard pattern on ZEP520A, followed by a conventional oxygen-based plasma etch to form high aspect ratio nanopillar arrays in SOC with the feature width under 50 nm, and ended by a thermal deposition of gold to form self-aligned dual layer local surface plasmonic resonators (LSPRs). The measured reflectance spectra agree with the simulated. A wealth of optical properties, such as coupling induced modulations of spectra by LSPRs, are revealed and analyzed. These special modes result in tunable structural colors and wavelength-selective antireflection ability. To the best of our knowledge, this is the first time that SOC is applied in the construction of metasurfaces, which has great potential for next generation nanophotonic devices.

Aiegen-based fluorescent polymer films with tunable Janus luminescence and structural color

Fluorescent polymer films (FPFs) doped with aggregation-induced emission luminogens (AIEgens) have attracted great attention of scientists for their potential to solve the aggregation-caused quenching effect of traditional fluorescent molecules. Here, to rich the information and enhance the function of FPFs, AIEgens (DCDPP-2TPA) doped FPF was placed in the middle of two different inverse opal photonic crystals (IOPCs), whose photonic band gaps matched with the FPF's red and blue region respectively. Owing to the regulation effect of the IOPCs, tunable Janus luminescence and structural color depending on the viewing angle were achieved. Through designing complex patterns in FPFs, encrypted information can be decoded simply by ultraviolet light. More importantly, the decoded information displays different luminescent colors on each side, and the luminescence on each side is tunable with viewing angles. The “sandwich” structure can be used in most daily environments. Its unique property of variable Janus emissions and structural colors endow the structure with highly safe anti-counterfeiting performance.

Sustainable Cellulose-Derived Organic Photonic Gels with Tunable and Dynamic Structural Color.

Structural color is a fascinating optical phenomenon arising from intricate light-matter interactions. Biological structural colors from natural polymers are invaluable in biomimetic design and sustainable construction. Here, we report a renewable, abundant, and biodegradable cellulose-derived organic gel that generates stable cholesteric liquid crystal structures with vivid structural colors. We construct the chromatic gel using a 68 wt % hydroxypropyl cellulose (HPC) matrix, incorporating distinct polyethylene glycol (PEG) guest molecules. The PEGs contain peculiar end groups with tailored polarity, allowing for precise positioning on the HPC helical backbone through electrostatic repulsion between the PEG and HPC chains. This preserves the HPC's chiral nematic phase without being disrupted. We demonstrate that the PEGs' polarity tunes the HPC gel's reflective color. Additionally, gels with variable polarities are highly sensitive to temperature, pressure, and stretching, resulting in rapid, continuous, and reversible color changes. These exceptional dynamic traits establish the chiral nematic gel as an outstanding candidate for next-generation applications across displays, wearables, flexible electronics, health monitoring, and multifunctional sensors.

Synergistic color-changing and conductive photonic cellulose nanocrystal patches for sweat sensing with biodegradability and biocompatibility.

Given the ongoing requirements for versatility, sustainability, and biocompatibility in wearable applications, cellulose nanocrystal (CNC) photonic materials emerge as excellent candidates for multi-responsive wearable devices due to their tunable structural color, strong electron-donating capacity, and renewable nature. Nonetheless, most CNC-derived materials struggle to incorporate color-changing and electrical sensing into one system since the self-assembly of CNCs is incompatible with conventional conductive mediums. Here we report the design of a conductive photonic patch through constructing a CNC/polyvinyl alcohol hydrogel modulated by phytic acid (PA). The introduction of PA significantly enhances the hydrogen bonding interaction, resulting in the composite film with impressive flexibility (1.4 MJ m-3) and progressive color changes from blue, green, yellow, to ultimately red upon sweat wetting. Interestingly, this system simultaneously demonstrates selective and sensitive electrical sensing functions, as well as satisfactory biocompatibility, biodegradability, and breathability. Importantly, a proof-of-concept demonstration of a skin-adhesive patch is presented, where the optical and electrical dual-signal sweat sensing allows for intuitive visual and multimode electric localization of sweat accumulation during physical exercises. This innovative interactive strategy for monitoring human metabolites could offer a fresh perspective into the design of wearable health-sensing devices, while greatly expanding the applications of CNC-based photonic materials in medicine-related fields.

Rapid fabrication of tunable structural color patterns by spray-coating.

Structural color, a color generated based on physical principles, has broad applications such as displays, optical sensors, and anti-counterfeiting. Traditional methods for producing structural colors are often complex and time-consuming, whereas spray-coating colloidal self-assembly offers a simple and controllable alternative. However, due to the high-pressure atomization process, colloidal inks often form amorphous photonic structures (APSs), making it challenging to precisely control the assembly of colloidal particles on substrates to achieve ordered structures. By rationally designing the composition of colloidal mixed solutions, controlling particle concentration, and adjusting evaporation temperatures, it is possible to effectively regulate the assembly of colloidal particles and obtain angle-dependent iridescent colors. This work proposes a simple spray-coating process that enables the control of both ordered and disordered structures, with tunable optical properties, suitable for colloidal patterning on various substrates. This method not only simplifies the fabrication of photonic crystals (PCs) but also has broad potential, particularly in anti-counterfeiting, where it enables the creation of hard-to-replicate structured patterns with enhanced security.