Structural Color Materials Research Articles

Polymerization-induced ultrafast and massive growth of colloidal photonic crystals on hydrogel surface

Structural color materials based on colloidal photonic crystals (CPCs) have been extensively investigated since they can be prepared efficiently. Commonly, photonic hydrogels are prepared by a polymerizing pre-gel solution, in which colloids are self-assembled by passive methods. Except for magnetic colloids, the process of forming colloidal photonic crystals in a pre-gel solution requires hours to days, and it is almost impossible to assemble the colloids directly in hydrogels. Here, we present a mild strategy to prepare hydrogels with surface structural color, ordered colloidal arrays are formed at the air-hydrogel interface actively during the polymerization process, and bright structural color forms in minutes on a large scale. Polymer exclusion is the key driving force for the assembly of the colloids, other factors, such as evaporation flux, can also promote the colloids to assemble. Significantly, heterogeneous structural color patterns can be produced easily by exposing the hydrogel surface to air sequentially using a mask during the polymerization process. The simple and fast method provides a general route for preparing structural colored surfaces, which may be suitable for various applications.

Preparation of structural colors from lignin: Improving the homogeneity between different raw materials by solvent precipitation fractionation

Structural color materials prepared by lignin colloidal spheres (LCSs) have attracted great interest due to their biocompatibility, biodegradability, multifunctionality, and ability to weaken incoherent scattering. However, the lignin raw materials originating from different biomass resources have distinct heterogeneity leading to a big difference in the size of resultant LCSs via self-assembly, which is a major challenge for the preparation of stable structural colors. In this work, we employ the solvent precipitation method to fractionate the crude lignin samples obtained from corncob, softwood, hardwood, and bamboo, respectively. After solvent precipitation fractionation, the self-assembly behaviors of different lignin samples become similar. The LCSs prepared by fractionated lignin samples show quite similar sizes (212, 202, 215, and 210 nm) compared with the LCSs prepared by crude lignin samples (341, 83, 128, and 253 nm). The quantitative atomic force microscope force spectroscopy demonstrates the improved homogeneity of LCSs prepared by fractionated lignin samples is attributed to similar long-ranged intermolecular forces in self-assembly. By employing LCSs as building blocks, the lignin-based pigments with stable structural colors are successfully prepared. The presented colors are independent of the biomass resource types, which benefits the product stability and large-scale production of lignin-based structural color materials.

Stealthy and hyperuniform isotropic photonic band gap structure in 3D.

In photonic crystals, the propagation of light is governed by their photonic band structure, an ensemble of propagating states grouped into bands, separated by photonic band gaps. Due to discrete symmetries in spatially strictly periodic dielectric structures their photonic band structure is intrinsically anisotropic. However, for many applications, such as manufacturing artificial structural color materials or developing photonic computing devices, but also for the fundamental understanding of light-matter interactions, it is of major interest to seek materials with long range nonperiodic dielectric structures which allow the formation of isotropic photonic band gaps. Here, we report the first ever 3D isotropic photonic band gap for an optimized disordered stealthy hyperuniform structure for microwaves. The transmission spectra are directly compared to a diamond pattern and an amorphous structure with similar node density. The band structure is measured experimentally for all three microwave structures, manufactured by 3D laser printing for metamaterials with refractive index up to . Results agree well with finite-difference-time-domain numerical investigations and a priori calculations of the band gap for the hyperuniform structure: the diamond structure shows gaps but being anisotropic as expected, the stealthy hyperuniform pattern shows an isotropic gap of very similar magnitude, while the amorphous structure does not show a gap at all. Since they are more easily manufactured, prototyping centimeter scaled microwave structures may help optimizing structures in the technologically very interesting region of infrared.

Thermochromic structural color based on pressure storage-release toward emerging applications

Conventional thermochromic photonic crystal (TPC) hydrogels and liquids have gained prominence due to their unique capability of adjusting their colors by temperature (T). However, challenges including non-recordable structural colors, poor stability, and limited color tunability significantly limit their advanced applications. To address these issues, a versatile pressure-stored TPC (PSTPC) with repeatable fixable compressed non-close-packing structures has been fabricated based on combining phase-changing materials and elastic photonic crystals. Distinguishing from conventional TPCs, PSTPCs possess 1) recordable colors with excellent stability under normal conditions; 2) adjustable colors from red to blue by simply altering storable pressure; and 3) a T-triggered color change by releasing the pressure. These PSTPCs have been experimentally proved to be ideal candidates for various applications, including hot warning labels, ink-free rewritable papers, and elaborate anti-counterfeiting tags. This work offers a new perspective for designing and fabricating combined materials-based stimulus-responsive structural color materials with advanced multifunctional applications.

Structural Color Materials with Color Mixing Effect Using Noble Metal‐Free Plasmonic Particles in SiO2–ZrN System (Advanced Optical Materials 19/2024)

Structural Color Materials with Color Mixing Effect Using Noble Metal‐Free Plasmonic Particles in SiO₂–ZrN System (Advanced Optical Materials 19/2024)

Synthesis of ZrN–SiO2 core–shell particles by a sol–gel process and use as particle‐based structured coloring material

AbstractZrN–SiO2 core–shell particles were prepared, where the ZrN core nanoparticles and SiO2 shell were designed to exhibit localized surface plasmon resonances (LSPRs) and structural coloring. The heating of ZrO2 nanoparticles with Mg3N2 under a nitrogen gas flow produced ZrN nanoparticles with a diameter in the range of 10–20 nm. The dispersion of ZrN nanoparticles in water exhibited an absorption maximum at approximately 700 nm owing to LSPRs. An SiO2 shell was formed on the ZrN nanoparticles using a sol–gel process. Scanning transmission electron microscopy confirmed the formation of ZrN–SiO2 core–shell particles containing ZrN particles with a diameter of approximately 10 nm. The SiO2 shell thickness was controlled by varying the reaction time to form SiO2. The use of particles as a structural component of a structural color material owing to the high uniformity of the size of obtained core–shell particles was investigated. The obtained ZrN–SiO2 core–shell particles were arrayed on a glass substrate using a layer‐by‐layer method. The particle‐stacked film of the ZrN–SiO2 core–shell particles exhibited the maximum reflection depending on the particle size of the SiO2 shell.

Research Progress of Bioinspired Structural Color in Camouflage.

Bioinspired structural color represents a burgeoning field that draws upon principles, strategies, and concepts derived from biological systems to inspire the design of novel technologies or products featuring reversible color changing mechanisms, with significant potential applications for camouflage, sensors, anticounterfeiting, etc. This mini-review focuses specifically on the research progress of bioinspired structural color in the realm of camouflage. Firstly, it discusses fundamental mechanisms of coloration in biological systems, encompassing pigmentation, structural coloration, fluorescence, and bioluminescence. Subsequently, it delineates three modulation strategies-namely, photonic crystals, film interference, and plasmonic modulation-that contribute to the development of bioinspired structural color materials or devices. Moreover, the review critically assesses the integration of bioinspired structural color materials with environmental contexts, with a particular emphasis on their application in camouflage. Finally, the paper outlines persisting challenges and suggests future development trends in the camouflage field via bioinspired structural color.

Recent advances in structural color materials based on flexible cellulose nanocrystals

Recent advances in structural color materials based on flexible cellulose nanocrystals

Structural Color Materials with Color Mixing Effect Using Noble Metal‐Free Plasmonic Particles in SiO2–ZrN System

AbstractStructurally colored materials with a color mixing effect are developed using SiO2–ZrN core–shell particles, where ZrN nanoparticles used as shells exhibited localized surface plasmon resonances (LSPRs). ZrN nanoparticles (10–30 nm) are attached to monodispersed SiO2 particles by modifying SiO2 particles with polymers. The attached ZrN nanoparticles exhibited a plasmon resonance absorption band at 700 nm. These SiO2–ZrN core–shell particles have a highly monodisperse particle size and assemble into a particle‐stacked film with Bragg diffraction light in the visible spectrum. The particle‐stacked film of the SiO2–ZrN core–shell particles exhibited the maximum reflection depending on the particle size of the SiO2 core. The ratio of the reflection intensity in the long‐wavelength region corresponding to the ZrN absorption to that in the short‐wavelength region of the particle‐stacked film containing the ZrN shell is less than that of the ratio in only SiO2 particle‐stacked film. The results reveal that these particle‐stacked films exhibit a “color mixing effect” that combines specific wavelength absorption through plasmon resonance and specific wavelength diffraction owing to the periodic structure without using precious metals.

Non-iridescent magnetite photonic crystal films and pigments with enhanced magnetic coupling effect

Non-iridescent structural color films and pigments with novel enhanced microwave absorption characteristics are successfully fabricated at room temperature for the first time. The quasi-uniform magnetite nanoparticles (CV = 0.10 ∼ 0.20) with high refractive index (n = 2.42) and broad light absorption provide evident low-angle-independent property for the structural color pigments. More importantly, a universal technical protocol is developed for preparation of photonic crystals with inorganic nanoparticles, i.e., through designing of the binding strength between nanoparticles and mediated molecules, the slurry modulus is increased to 105 Pa, which effectively drives the nanoparticles to rapidly assemble into closely packed ordered arrays by our-designed powerful assembly technique. Additionally, the structural color materials are subjected as photonic crystal pigments (∼20 wt%) embedding in a transparent polymer matrix to form color coatings. Remarkably, the photonic crystal pigments exhibit strong magnetic coupling effect and enhanced microwave absorption performance due to the regular arrangement, with superior reflection loss (RLmin) values of −14.97 dB (increase by 65.8 % compared with randomized magnetic nanoparticles), and the effective absorption bandwidth (EAB) of 2.15 GHz. This new kind of photonic crystal pigment, combining visual color and radar wave absorption, provides a novel idea for developing of multi-wave-band high-performance stealth materials.

Facile Access to High Solid Content Monodispersed Microspheres via Dual-Component Surfactants Regulation toward High-Performance Colloidal Photonic Crystals.

Monodispersed microspheres play a major role in optical science and engineering, providing ideal building blocks for structural color materials. However, the method toward high solid content (HSC) monodispersed microspheres has remained a key hurdle. Herein, a facile access to harvest monodispersed microspheres based on the emulsion polymerization mechanism is demonstrated, where anionic and nonionic surfactants are employed to achieve the electrostatic and steric dual-stabilization balance in a synergistic manner. Monodispersed poly(styrene-butyl acrylate-methacrylic acid) colloidal latex with 55wt% HSC is achieved, which shows an enhanced self-assembly efficiency of 280% compared with the low solid content (10wt%) latex. In addition, Ag-coated colloidal photonic crystal (Ag@CPC) coating with near-zero refractive index is achieved, presenting the characteristics of metamaterials. And an 11-fold photoluminescence emission enhancement of CdSe@ZnS quantum dots is realized by the Ag@CPC metamaterial coating. Taking advantage of high assembly efficiency, easily large-scale film-forming of the 55wt% HSC microspheres latex, robust Ag@CPC metamaterial coatings could be easily produced for passive cooling. The coating demonstrates excellent thermal insulation performance with theoretical cooling power of 30.4W m-2, providing practical significance for scalable CPC architecture coatings in passive cooling.

Structural color‐based physical unclonable function

AbstractCommodity fraud poses significant economic and public health risks while jeopardizing market stability. A promising avenue for addressing this issue involves the incorporation of physical unclonable function (PUF) in anti‐counterfeiting labels for commodity authentication purposes. PUFs are a large number of unbreakable security labels generated through a random process, which exhibit unique physical pattern responses that are impervious to replication. In particular, a novel kind of a PUF model, called structural color‐based PUFs, combing the structural color characteristics of angle‐dependent stability and brightness with unclonable property, offers unassailable encryption capabilities and serves as a formidable safeguard against forgery. This review undertakes a comprehensive summary of recent advancements in PUF technology leveraging structural color materials. Moreover, it provides a systematic description of the recognition and authentication technology employed in optical structural color PUFs. Finally, a prospective summary and outlook is proposed to explain existing challenges, and highlight potential developments in anti‐counterfeiting technology incorporating structural color PUF labels.

Effect of annealing treatment on optical properties of structural color films

Structural color materials based on periodic micro and nano structures present unique color rendering mechanism and optical properties, and have a wide range of applications in display, packaging, sensing and other fields. However, it is still facing the problems of constructing large area, good consistency and low cost periodic structures. In this paper, the structural color film was prepared by annealing treatment on a periodic template. The effects of coating time, annealing time and annealing process on the optical properties of the sample such as hue and saturation were analyzed. It was confirmed that the sample with 300 nm polystyrene microsphere template presented orange yellow with incident angle of 30° before annealing, then turned to be light pink under 800 °C for 120 min annealing process and orange red under 800 °C for 180 min in air. Additionally, the sample with 420 nm polystyrene microsphere template showed cyan before annealing, and yellow-green under 800 °C for 120 min annealing treatment and orange-yellow under 800 °C for 180 min. The periodicity of the annealed gold microspheres was still limited by the template period with different diameters. Morever, when the coating and annealing time were the same, the higher the annealing temperature was, the higher the saturation get. Under the same coating time and annealing temperature, the saturation was higher with short annealing time, and the hue blue-shifted as the incident angle increased from 15° to 45°. While fixing the annealing time and temperature, the saturation increased gradually as the coating time increasing from 10S to 40S, and the color of the sample red-shifted.

3D-Printed Biomimetic Structural Colors.

Resolution control and expansibility have always been challenges to the fabrication of structural color materials. Here, a facile strategy to print cholesteric liquid crystal elastomers (CLCEs) into complex structural color patterns with variable resolution and enhanced expansibility is reported. A volatile solvent is introduced into the synthesized CLC oligomers, modifying its rheological properties and allowing direct-ink-writing (DIW) under mild conditions. The combination of printing shear flow and anisotropic deswelling of ink drives the CLC molecules into an ordered cholesteric arrangement. The authors meticulously investigate the influence of printing parameters to achieve resolution control over a wide range, allowing for the printing of multi-sized 1D or 2D patterns with constant quality. Furthermore, such solvent-cast direct-ink-writing (DIW) strategy is highly expandable and can be integrated easily into the DIW of bionic robots. Multi-responsive bionic butterfly and flower are printed with biomimetic in both locomotion and coloration. Such designs dramatically reduced the processing difficulty of precise full-color printing and expanded the capability of structural color materials to collaborate with other systems.

Multicomponent structural color membrane based on soft lithography array for high-sensitive Raman detection

Inspired by ordered photonic crystals and structural color materials in nature, we successfully prepared hydroxypropyl cellulose (HPC) photonic films with ordered surface arrays by double-imprint soft lithography. Then we introduced another important material of the cellulose family, cellulose nanocrystals (CNC), which has liquid crystal nature and birefringent properties of the particles, into the system to realize the single-point shrinkage of the film array and the control of structural color. Through multi-component doping and concentration control, we further optimized the multi-scale structure of the materials, and obtained HPC/CNCs composite photonic films with excellent properties in color, stability and flexibility, whose elastic modulus and tensile properties are significantly higher than those of single-component. Further loading of SiO2@PDA enhances the color saturation and realizes the in-situ reduction of metal ions on the film surface. This plasma film can track a variety of substances with high sensitivity and long-term stability, showing potential application prospects in the field of surface-enhanced Raman scattering (SERS), which provides a potential possibility for chiral structures to be used in the field of biosensor detection and circularly polarized luminescence.

Emerging interactively stretchable electronics with optical and electrical dual-signal feedbacks based on structural color materials

Emerging interactively stretchable electronics with optical and electrical dual-signal feedbacks based on structural color materials

Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles.

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

Self-Destructive Structural Color Liquids for Time-Temperature Indicating.

Vaccines are undoubtedly a powerful weapon in our fight against global pandemics, as demonstrated in the recent COVID-19 case, yet they often face significant challenges in reliable cold chain transport. Despite extensive efforts to monitor their time-temperature history, current time-temperature indicators (TTIs) suffer from limited reliability and stability, such as difficulty in avoiding human intervention, inapplicable to subzero temperatures, narrow tracking temperature ranges, or susceptibility to photobleaching. Herein, we develop a class of structural color materials that harnesses dual merits of fluidic nature and structural color, enabling thermal-triggered visible color destruction based on triggering agent-diffusion-induced irreversible disassembly of liquid colloidal photonic crystals for indicating the time-temperature history of the cold chain transport. These self-destructive structural color liquids (SCLs) exhibit inherent irreversibility, superior sensitivity, tunable self-destructive time (minutes to days), and a wide tracking temperature range (-70 to +37 °C). Such self-destructive SCLs can be conveniently packaged into flexible TTIs for monitoring the storage and exposure status of diverse vaccines via naked-eye inspection or mobile phone scanning. By overcoming the shortcomings inherent in conventional TTIs and responsive photonic crystals, these self-destructive SCLs can increase their compatibility with cold chain transport and hold promise for the development and application of the next-generation intelligent TTIs and photonic crystals.

Reconfigurable Grating Diffraction Structural Color in Self-Assembled Colloidal Crystals.

Self-assembled colloidal crystals display structural colors due to light diffracted from their microscale, ordered structure. This color arises due to Bragg reflection (BR)or grating diffraction (GD); the latter mechanism is much less explored than the former. Here the design space for generating GD structural color is identified and its relative advantages are demonstrated.Electrophoretic deposition is used to self-assemble crystals with fine crystal grains from colloids of diameter 1.0µm. The structural color in transmission is tunable across the full visible spectrum. The optimum optical response-represented by both color intensity and saturation-is observed at low layer number (≤5layers). The spectral response is well predicted by Mie scattering of the crystals. Taken together, the experimental and theoretical results demonstrate that vivid grating colors with high color saturation can be produced from thin layers of micron-sized colloids. These colloidal crystals extend the potential of artificial structural color materials.

Efficient preparation of single-sided structural color fabrics with asymmetric wettability, angle-dependence and patternability based on liquid photonic crystals

Liquid photonic crystals (LPCs) with structural colors have become the most promising materials for preparing structural color fabrics, which have received extensive attention in textile printing and dyeing. However, the tedious and uncontrollable preparation process for LPCs and its expensive equipment for structural color fabrics with asymmetric wettability limit the practical application of structural color materials. Herein, inspired by the Janus wettability of the unidirectional moisture transport fabric, a simple and efficient approach is reported to fabricate single-sided structural color fabrics (SSCFs) with asymmetric wettability and angle dependence. To obtain asymmetrically wetted substrate fabrics, a low-cost, high-temperature assisted spraying strategy is used to build a hydrophobic layer on one side of the fabric. The ready-to-use LPCs with target mass fraction are accurately prepared by re-dispersing the dry poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) microsphere powder and are uniformly scraped onto the hydrophilic side of the fabric to prepare SSCFs. The as-prepared samples show a brilliant and iridescent structural color, and their color is adjusted by varying the size of the P(St-MMA-AA) microspheres. The water contact angle of the front side (with structural colors) of SSCFs treated by sintering and water bath is 37°, which facilitates further encapsulation to improve the color fastness of SSCFs. Meanwhile, the photonic crystals on the fabric remain ordered. This work provides an alternative, low-cost and practical preparation scheme for the structural coloring of textiles based on LPCs.