Angle-independent Structural Colors Research Articles

Enhancing color saturation in photonic glasses through optimized absorption

Photonic glasses, isotropically assembled nanoparticles with short-range correlation, can produce angle independent structural colors. They show broader reflectance spectra and lower saturated colors, compared to photonic crystals. Low color saturation creates barriers for photonic glasses to be used for coatings, cosmetics, and colors. Broadband absorbing materials are commonly used to absorb incoherently scattered light to enhance the saturation. However, there is limited understanding on how the absorption quantitatively affects the colors of photonic glasses. To this end, we here use a validated Monte Carlo-based multiple scattering model to investigate how absorption impacts the reflectance spectra in photonic glasses. We show that the color saturation can be maximized with an optimal level of absorption regardless of sample thickness or refractive index contrast between particles and matrix. We quantitatively demonstrate that the multiple scattering is largely reduced with the optimal absorption level and the reflectance is dominantly contributed by the single scattering. The optimal absorption occurs when the sample absorption mean free path is comparable to the transport mean free path, which offers a guidance on how much absorbing material is needed for creating highly saturated photonic glasses. This work will not only pave ways for pushing applications of angle-independent structural colors, but also improve our understanding of light scattering and absorption in short-range correlated disordered systems.

Electrically programmable color and shape morphing in photonic glass elastomers with reduced voltages

Electrochromic materials that integrate structural colors and dielectric elastomer actuators (DEAs) can simultaneously change colors and shapes upon voltages. Typically, operation voltages for these materials are high, reaching several kilovolts. To achieve color changes at lower voltages, we reduce the modulus by lowering silica nanoparticle volume fraction and adding a secondary acrylate monomer, while also ensuring that photonic elastomers maintain recognizable colors. We utilize binary silica nanoparticles to make photonic glass elastomers that show angle-independent structural colors. The photonic glass DEA shows color changes of 85 nm at 800 V with color change sensitivity of 0.11 nm/V, surpassing previous reports. By making a double-layered DEA, we can not only control the shape changes, but also increase the color contrast. This solid-state elastic electrochromic material, offering programmable color change and shape morphing, holds broad applications in wearable devices, camouflage, and flexible displays.

MXenes for Bioinspired Soft Actuators: Advancements in Angle-Independent Structural Colors and Beyond

HighlightsMXene-based soft actuators with angle-independent structural colors have the potential to contribute to various fields, including display technologies, camouflage systems, sensors, and beyond.Bioinspiration has paved the way for developing advanced structural colored soft actuators for biomimetic soft robots.This perspective appraises the development of bioinspired MXene-based soft actuators with angle-independent structural color and beyond in soft robotics.

Angle-Independent Color Change in Thermoresponsive Gel-Immobilized Colloidal Amorphous Film Attached to PET Substrate.

Gel-immobilized colloidal amorphous structures comprise short-range-ordered monodisperse submicrometer particles embedded into a soft polymer gel. They exhibit an angle-independent structural color that is tunable in response to external stimuli via a volume change in the gel, which has significant potential for the development of sensors that respond to stimuli via angle-independent color changes. In this study, the amorphous structure of a charged colloidal suspension in water was immobilized in a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) gel film and simultaneously attached to a polyethylene terephthalate (PET) substrate. The gel film exhibited a uniform angle-independent color that changed in response to changes in temperature (i.e., thermosensitivity). Attachment to the PET substrate suppressed changes in the gel film area and film distortion, despite significant volume changes in the gel. Consequently, the degree of thermosensitivity was enhanced. The PET-attached gel-immobilized colloidal amorphous film was easy to handle and had excellent flexibility, allowing it to wrap around the surfaces of curved objects. These features are advantageous for sensor applications.

Designing color controllable SiO2 amorphous colloidal arrays

Amorphous assembling of SiO2 colloidal spheres is a convenient strategy to obtain amorphous photonic structure, such structure, referred to as SiO2 amorphous colloidal arrays (ACAs), can demonstrate angle-independent structural color owing to constructive interference. To date, an empirical equation for materials researchers to design the color of SiO2 ACAs remains vacant, and several influence factors concerning with the color of SiO2 ACAs are still ignored and uninvestigated. Herein, based on single scattering model, we have proposed an empirical equation to predict the reflection peak of SiO2 ACAs to design their color, and influence factors including background and thickness on the color of SiO2 ACAs were also investigated. By preparing spheres within the designed range as building blocks, SiO2 ACAs with colors varying from blue to red were achieved whose reflection peaks position fits well with the empirical equation. This study can help materials researchers to design and manipulate structural color of SiO2 ACAs while providing insight to understand the origin of structural color of SiO2 ACAs.

Light-based Three-Dimensional Printing of Complex-Shaped Photonic Colloidal Glasses.

Colloidal glasses display angle-independent structural color that is tunable by the size and local arrangement of submicron particles. While films, droplets and microcapsules with isotropic structural color have been demonstrated, the shaping of colloidal glasses in three dimensions remains an open manufacturing challenge. Here, we report a light-based printing platform for the shaping of colloidal glasses into three-dimensional objects featuring complex geometries and vivid structural color after thermal treatment. Rheology, photopolymerization and calcination experiments are performed to design the photoreactive resins leading to printable colloidal glasses. With the help of microscopy, scattering and optical characterization, we show that the photonic properties of the printed objects reflect the locally ordered microstructure of the glass. The capability of the platform in creating three-dimensional objects with isotropic structural color is illustrated by printing lattices and miniaturized sculpture replicas with unique shapes and multimaterial designs. This article is protected by copyright. All rights reserved.

Preparation of para-aramid aerogel fiber structurally colored by light scattering through wet spinning and supercritical drying

This research is an investigation of the preparation conditions for para-aramid aerogel fibers that are structurally colored by Rayleigh scattering, an angle-independent structural color. It is well known that structural color has the angle dependence of colors. However, there is currently a desire for materials with angle-independent structural color. During the fibers’ spinning process, the microstructures of the fibers are generated. The effect of spinning conditions is investigated by measuring the optical properties of the wet gel fibers. The neutralization speed of negatively charged PPTA fibrils has a significant impact on the formation of the dense microstructure. Spinning under the fast neutralization speed condition makes it possible to achieve the blue-colored fiber derived from Rayleigh scattering. The influence of neutralization speed on the microstructure of the fibers is confirmed by FE-SEM characterization. Furthermore, spinning using a narrow inner diameter needle decreases the whiteness component derived from Mie scattering of the fibers, resulting in a more vivid blue color. The angle-independent structurally colored fibers hold great promise for application in clothing, interiors, and industrial materials.

Role of MXenes in advancing soft robotics.

MXenes with their unique electronic, optical, chemical, and mechanical properties have shown great promise in soft robotics. MXene-based soft actuators have been designed to display ultrafast actuations and recovery speeds as well as angle-independent structural colors in response to vapor. Several studies have developed soft actuators by combining MXenes with other materials to mimic the movement of natural organisms. Thus, MXene-based soft actuators have the potential to revolutionize the field of soft robotics and flexible electronics (e.g., wearable devices and artificial muscles). MXene-based artificial muscles have been explored for use in kinetic soft robotics as actuators in microsystems requiring exceptional compliance. MXene-based sensors and actuators have already been developed for human-like sensors and photodetection. However, there are still challenges that need to be addressed in such applications, such as the design of stretchable and compliant robotic skins with a high-level functional integration for soft robotics. The integration of various devices, such as power sources, sensors, and actuators, into soft robotics is another crucial challenge. Despite the excellent stretchability and tensile strength of MXene-based composites, there is a vital need to develop their mechanical and electrochemical features and grant them multi-functionalities. Herein, recent developments pertaining to the applications of MXenes and their composites in soft robotics are discussed with a focus on the important challenges and future perspectives.

Beyond cadmium yellow: CdS photonic crystal pigments with vivid structural colors

This paper reports a method for fabricating CdS photonic crystal pigments through a microfluidic process. The vivid angle-independent structural colors make CdS pigments beyond the traditional cadmium yellow as new pigments towards applications.

Thermal vacuum de-oxygen fabrication of new catalytic pigments: SiO2@TiO2-x amorphous photonic crystals for formaldehyde removal.

Eliminating benzene and formaldehyde pollution is indispensable after the popularity of colorful home decoration in current society. The possibility and advantages of vividly colorful amorphous photonic crystals (APCs) as catalytic pigments were established. Biomimetic synthesis of APCs is an effective approach to obtaining angle-independent structural colors. Herein, we introduce oxygen vacancies through thermal vacuum de-oxygenation to synthesize SiO2@TiO2-x APCs for angle-independent structural colors and enhanced photocatalytic performance in one step. Core-shell nanospheres with controllable particle size were synthesized using a mixed-solvent method as the structural unit of APCs to prepare seven structural colors: red, orange, yellow, green, cyan, blue, and purple. The photocatalytic activity of in situ fabricated SiO2@TiO2-x APCs was conspicuously enhanced by thermal vacuum deoxidation. An amorphous layer formed on the TiO2 nanocrystals provides TiO2-x with excellent spectral response to visible light, transient photocurrent, and surface photovoltage up to 38.44 μA cm-2 and 28.8 mV, respectively. Black TiO2-x absorbs incoherent scattering, causing APCs to generate vividly angle-independent structural colors. The existence of oxygen vacancies in TiO2-x promotes electron activation and a synergistic effect with the photonic local effect of APCs in improving the degradation of formaldehyde by catalytic pigments, effectively protecting the beautiful living environment of human beings.

Bioinspired MXene-Based Soft Actuators Exhibiting Angle-Independent Structural Color

HighlightsDesign and fabrication of MXene-based soft actuators with angle-independent structural color.The nanostructured MXene can not only facilitate the formation of short-range ordered 3D amorphous photonic crystals, but also help significantly improve structural color saturation.The soft actuators exhibit brilliant angle-independent structural color, ultrafast actuation and recovery speeds in response to vapor.

Rapid Color-Switching of MnO2 Hollow-Nanosphere Films in Dynamic Water Vapor for Reversible Optical Encryption.

Drop-casting manganese oxide (MnO2 ) hollow nanospheres synthesized via a simple surface-initiated redox route produces thin films exhibiting angle-independent structural colors. The colors can rapidly change in response to high-humidity dynamic water vapor (relative humidity > 90%) with excellent reversibility. When the film is triggered by dynamic water vapor with a relative humidity of ≈100%, the color changes with an optimal wavelength redshift of ≈60nm at ≈600ms while there is no shift under static water vapor. The unique selective response originates from the nanoscale porosity formed in the shells by randomly stacked MnO2 nanosheets, which enhances the capillary condensation of dynamic water vapor and promotes the change of their effective refractive index for rapid color switching. The repeated color-switching tests over 100 times confirm the durability and reversibility of the MnO2 film. The potential of these films for applications in anti-counterfeiting and information encryption is further demonstrated by reversible encoding and decoding initiated exclusively by exposure to human breath.

A smart large-scale explosive-responsive amorphous photonic crystal sensor based on color analysis method

Leveraging advances in nanoscience and intelligent technology to engineer large-scale and stimuli-response amorphous photonic crystals (APCs) for use in inexpensive and portable sensing system for the detection of explosives. Polymethyl methacrylate (PMMA) colloidal microspheres with uniform particle size and good mono-dispersion were used to prepare APCs by spraying method. The structural color of APCs was angle-independent and changed with different microspheres size. Meanwhile, a robotic arm was designed to achieve an automatic and large-scale preparation of APCs, and this more accurate and cost-effective method not only provides an automatic process for large-scale APCs pattern painting in chemical industry, but also gives chemical sensors an access to be smart devices in life. Then 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and cyclotetramethylenete-tranitramine (HMX) molecularly imprinted APCs (MIAPCs) were fabricated in the same way. The results showed that the response time of MIAPCs was 10 s, and the limit of detection was 0.01 mM with NaOH swollen. Specially, by using the more convenient HSB (Hue-Saturation-Brightness) and CIE (Commission Internationale de L’Eclairage) color analysis methods, combined with principal component analysis and pattern recognition methods, a sensor group with higher selectivity and sensitivity than a single sensor was constructed. A qualitative and semi-quantitative result was realized by responding to the changes of structural color before and after. Compared with traditional PhCs sensor, the APCs sensor has angle-independent structural color, which provides more accurate results and simplifies the detection based on the color analysis. This novel large-scale APCs sensor represents a potential of highly selective and real-time on-site visual explosive detection. In the meanwhile, the automatic APCs casting also offers chemical materials an access to combining with intelligent technology.

Gelatinase-responsive photonic crystal membrane for pathogenic bacteria detection and application in vitro health diagnosis

Selective identification and rapid detection for multiple pathogenic bacteria are important to prevent bacterial infection. Herein, gelatinase-responsive photonic crystal membrane (PCM) is reported as rapid, selective and direct detection platform for multiple pathogenic bacteria. PCMs exhibit angle-independent structural color, and reflection spectra has negligible change after irradiation and storage under different temperature. The ultra-thin response layer makes shorter detection time (30 min) than the reported gelatin-based photonic crystal detection platform (10–12 h). There is obvious positive correlation between reflection spectra change and gelatinase concentration, even if the gelatinase was as low as 4.8 pM. PCM has high stability against multiple interferers, which are beneficial to improve the detection accuracy and reliability. PCM also show “selective” ability to identify typical pathogens from atypical pathogens through specific response to gelatinase, and good selectivity gives PCM direct detection ability without pretreatment. PCM could detect the pathogens bacteria concentration range spanning 7 orders of magnitude from 10 to 107 CFU/mL, and the relative error between gold standard method and the new platform is less than 10%. Furthermore, PCM's in vitro health diagnosis ability was proved by detecting pathogenic bacteria in artificial wound fluid and urine. All the results show that PCM is well promising platform for selective identifying and rapid detecting pathogenic bacteria in vitro diagnosis.

Facile Fabrication of Amorphous Colloidal Array Patterns with Good Mechanical Stability via an Infiltration-Driven Mayer Rod Coating Method

In this study, cationic monodisperse polystyrene (PS) microspheres and polybutyl acrylate (PBA) nanoparticles were synthesized and used in combination to fabricate various steady amorphous colloidal array (ACA) patterns via an infiltration-driven Mayer rod coating method. During the rapid assembly process, PBA nanoparticles were embedded in PS microsphere voids to lock the colloidal array, thereby promoting its mechanical stability. Meanwhile, the fabricated ACA patterns could still exhibit bright angle-independent structural color when moderate PBA nanoparticles introduced. In summary, this study provides a convenient and fast method to fabricate steady ACA patterns, which is a promising potential substitute for traditional ink printing.

초정밀 가공을 이용한 다층 패턴 구조의 각도-독립적 구조색 구현

Structural color refers to a phenomenon, in which white light is influenced by the structure and the color is expressed. It can express various colors depending on the structures and has an advantage in terms of durability compared to the conventional dyeing processes. However, the structural color from a single layer easily varies depending on the light irradiation angle or measurement angle. Hence, numerous studies have been conducted to improve the angle-independency of structural color by fabricating multi-layers with complex nano-patterns. But, they usually require highly controlled processing environments and are not suitable for large-area manufacturing. Therefore, this study aims to develop a fabrication process by utilizing ultra-precision machining and silicon molding. Here, angle-independent structural colors were implemented with multi-layer micro-patterned films. Through the experiments, angle-independency was evaluated by varying the number of silicon films and measurement angles. RGB and HSV values were extracted from the images and applied for quantitative analysis. The suggested fabrication process successfully exhibited the angle-independent structural color with given patterns and film thickness. It is expected that this study can contribute to improving the multi-layer fabrication processes concerning machinability and large-area production.

Bio-inspired non-iridescent structural coloration enabled by self-assembled cellulose nanocrystal composite films with balanced ordered/disordered arrays

Cellulose nanocrystal (CNC) is an interesting polysaccharide nanomaterial that can self-assemble into a chiral nematic liquid crystalline structure in aqueous solution, producing brightly structural color upon selectively reflection of visible light. However, till now, only iridescent structural colors can be created by such colloidal array, making it impossible to be used as colorimetric sensors. Here, inspired by longhorn beetle having short-range ordered but long-range disordered microstructures, it is demonstrated to develop low-/non-angle-dependent structural-color CNC composite films through interference against the long-range orders as a result of introducing positively charged poly (dimethyl diallyl ammonium chloride) (PDDA). By exploring balanced ordered/disordered structures, vivid angle-independent structural colors are realized, enabling accurate cognition for environmental signals, which is crucial to promising applications in sensing, anti-counterfeit, and decoration technology. • Low-/non-angle-dependent structural-color CNC composite films are fabricated. • Balanced ordered/disordered arrays are manipulated by co-assembly with PDDA. • The composite films can be used as colorimetric sensor.

Structural Coloration of Polyester Fabrics with High Colorfastness by Copolymer Photonic Crystals Containing Reactive Epoxy Groups.

Structural color as a revolutionary coloration strategy has been proposed to replace the traditional dyeing and printing process. However, the poor colorfastness and easy crack formation of structural colors on textile fabrics restrict their practical application at present. In this study, poly (tert-butyl acrylate-co-glycidyl methacrylate) (P(t-BA-co-GMA)) copolymers containing reactive epoxy groups with different mass ratios of tert-butyl acrylate (t-BA) and glycidyl methacrylate (GMA) were successfully synthesized, which were used to create structural colors on black polyester fabrics. The results showed that P(t-BA-co-GMA) nanospheres could form crack-free structural colors on polyester fabrics, and the colors vary with the mass ratio of t-BA and GMA to obtain five different colors. The different particle sizes of the different P(t-BA-co-GMA) nanospheres with different refractive indexes and the arrangement of short-range ordered and long-range disordered in microstructures may be the reason of different angle-independent structural colors on polyester fabrics. The P(t-BA-co-GMA) nanosphere structural colors on polyester fabrics possess good abrasion and washing colorfastness. This research provides the experimental basis for the development of crack-free amorphous photonic crystal structural color on fabrics with high colorfastness to promote the practical application of structural color in textile coloration.

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.

Angle-independent responsive organogel retroreflective structural color film for colorimetric sensing of humidity and organic vapors

The angle dependence of photonic crystals (PCs) dramatically limits their practical applications in the colorimetrical sensing of humidity and volatile organic compound (VOC) vapors. In addition, it is challenging for inverse opal PCs to colorimetrically distinguish between vapors with similar refractive indices. Different from the mechanism of PC-based sensors, here, we report an angle-independent polyacrylamide (PAAm) organogel structural color film based on the mechanisms of retroreflection, total internal reflection (TIR) and interference with a shape similar to a single-sided “egg waffle”. During the process of responding to humidity and VOC vapors, the color of the film remains angle-independent in the normal angle range of 0° to 45° under coaxial illumination and observation conditions. At the same time, the film can colorimetrically distinguish between vapors with similar refractive indices, such as methanol and ethanol, which is mainly due to the differences in their polarity and solubility parameters. The film shows good stability, reversibility and selectivity when exposed to vapors. A colorimetric sensor with a new response mechanism is proposed and has the potential to effectively distinguish between vapors with similar refractive indices. Furthermore, this responsive retroreflective structural color film (RRSCF) provides a universal strategy to develop targeted angle-independent structural color sensors by selecting optimized materials.