Tunable Structural Color Research Articles

Generation of Reflection Colors from Metal–Insulator–Metal Cavity Structure Enabled by Thickness-Dependent Refractive Indices of Metal Thin Film

Tunable structural colors have diverse applications ranging from displays and photovoltaics to surface decoration and art. A metal–insulator–metal (MIM) cavity structure formed by thin continuous l...

Printing Flowers? Custom-Tailored Photonic Cellulose Films with Engineered Surface Topography

Wrought by nature's wondrous hand, surface topographies are discovered on all length scales in living creatures and serve a variety of functions. Inspired by floral striations, here we developed a scalable means of fabricating custom-tailored photonic cellulose films that contained both cholesteric organization and microscopic wrinkly surface topography. Free-standing films were prepared by molding cellulose nanocrystal ink onto an oriented wrinkled template through evaporation-assisted nanoimprinting lithography, yielding morphology-induced light scattering at a short wavelength as well as optically tunable structural color derived from the helical cellulose matrix. As a result, the interplay between the two photonic structures, grating-like surface and chiral bulk, led to selective scattering of circularly polarized light with specific handedness. Moreover, the wrinkled surface relief on cholesteric cellulose films could be precisely controlled, enabling engineered printing of microscopic patterned images.

Iridescence-controlled and flexibly tunable retroreflective structural color film for smart displays.

Structural color materials, which use nano- or microstructures to reflect specific wavelengths of ambient white light, have drawn much attention owing to their wide applications ranging from optoelectronics, coatings, to energy-efficient reflective displays. Although various structural color materials based on specular or diffuse reflection have been demonstrated, neither efficient retroreflective structural colors nor iridescent and non-iridescent colors to different observers simultaneously were reported by existing artificial or natural structural color materials. Here, we show that by partially embedding a monolayer of polymer microspheres on the sticky side of a transparent tape, the spontaneously formed interferometric structure on the surface of air-cushioned microspheres can lead to unique structural colors that remain non-iridescent under coaxial illumination and viewing conditions, but appear iridescent under noncoaxial illumination and viewing conditions. Our findings demonstrate a smart, energy-efficient, and tunable retroreflective structural color material that is especially suitable for nighttime traffic safety and advertisement display applications.

Tunable, Cost‐Effective, and Scalable Structural Colors for Sensing and Consumer Products

AbstractThe cost‐effective colorimetric detection of chemicals can be a potential substitute for expensive spectrometers. Here, a structural color sensor is presented that can distinguish seven different organic solvents through a timed sharp color change. The color sensor is based on interference effects in a metal–insulator–metal Fabry–Perot (FP) cavity with polydimethylsiloxane (PDMS) serving as the dielectric layer. By tuning the cross‐linker to monomer ratio of PDMS and employing a porous nickel (Ni) top layer, the PDMS films experience shrinkage that results in substantial color changes. The time‐dependent color change can be used to indicate the type of solvent that the sensor was exposed to. When the sensor is immersed in a solvent, the PDMS diffuses through the porous Ni film at different rates depending on the solvent, resulting in a shrinkage of the PDMS and resonance shift of the FP cavity. Materials used in this single‐use sensor are cost effective (≈0.28 USD m−2), and the fabrication process requires no lithography and is scalable. Moreover, the FP configuration produces vibrant colors that cover ≈50% of the standard Red Green Blue (sRGB) color space and is suited for decorative purposes and consumer products as well.

Synthesis of strawberry-like Fe3O4@SiO2@Ag composite colloidal particles for constructing responsive photonic crystals

Well-defined Fe3O4@SiO2 composite colloidal particles decorated with Ag nanoparticles were successfully synthesized through a simple chemical approach. These particles self-assembled into 1D chain-like photonic structures under the induction of magnetic field and diffract tunable structural colors. The hue and brightness of the structural colors are related to Ag nanoparticles deposited on the particles surface and its brightness increases with the increase of Ag concentration. This phenomenon could be the result of the superposition of photonic bandgap formed by photonic crystals and possible plasmonic property, as well as capability in absorbing stray light of Ag nanoparticles. As-prepared Fe3O4@SiO2@Ag colloidal particles have great potential to be applied in many crucial areas such as color displaying, biological detection and anti-counterfeiting technology.

Hand Painting of Noniridescent Structural Multicolor through the Self-Assembly of YOHCO3 Colloids and Its Application for Anti-Counterfeiting.

YOHCO3 colloidal particles with tunable size, composition, and optical properties were prepared, and they were used for the fabrication of amorphous photonic crystals? (APCs) patterns through direct hand painting. YOHCO3 colloids were synthesized by a seeding growth method, in which the colloid size could be controlled by altering the seed amounts and the composition and optical properties can be altered via the doping of Eu3+. APCs? films with bright, permanent, and tunable structural colors were prepared by the self-assembly of YOHCO3 colloids of different sizes. Multicolor patterns can be obtained quickly and efficiently by hand painting with the dispersion of YOHCO3 colloids as ink. An APCs? pattern assembled from YOHCO3:Eu colloids is also fabricated, and the pattern shows blue structural color under natural light and bright red colors under illumination of UV light. The facile synthesis procedure, simple assembly process, and unique optical properties of the APCs make it valuable for practical applications such as structural color-based printing and anticounterfeiting.

Designing Responsive Photonic Crystal Patterns by Using Laser Engraving.

Soft photonic crystals are periodic nanostructures that have attracted much attention for their applications in sensors, owing to their tunable structural colors in response to external stimuli. Patterned photonic crystals provide a novel strategy for constructing high-performance photonic materials with unique structures and functions. In this work, laser engraving is used for the first time to design patterns on a layered photonic hydrogel. This approach is based on the integration of laser power and chemical modifications to embed different polymer composites (polyelectrolyte and neutral polymers) along a prescribed laser-printed path. The polyelectrolyte and neutral composites show differential swelling or shrinking, causing a mechanical instability in the layered hydrogel. The resultant soft polymeric materials appear as synchronous tuning in the photonic band gaps in response to external stimuli. This approach is favorable for designing responsive photonic crystals with controllable optical properties and 3D shape transformation. Moreover, it is of great use in developing advanced photonic crystals for applications in sensors, soft actuators, and drug release.

Structural Color Circulation in a Bilayer Photonic Crystal by Increasing the Incident Angle.

Photonic crystals (PCs) have been widely applied in the anticounterfeiting field according to their easily tunable and angle-dependent structural colors. However, most studies are now focused on single-layer PCs assembled from monodisperse colloidal spheres, which have only one bandgap. Here, we prepared bilayer photonic crystal films by choosing 250 and 330 nm silica spheres as the bottom and top layer, respectively. The effect of the incident angle on the bandgap of PCs was investigated and the results showed that the bandgap of the bilayer PCs was incident angle dependent-the structure exhibited two strong bandgaps within small incident angles, while as the incident angle increases, both the bandgaps blue-shifted and more importantly, the bandgap of the bottom layer disappeared with a further increase in the incident angle. Furthermore, with the delicate design of the thickness of the top layer, this bilayer structure selectively displayed the structural colors of the bottom layer, overlap colors of both the top and the bottom layer, and the color of only the top layer, respectively. By changing the incident angle, the color circulation from green to magenta, orange, yellow, and green again was realized. The realization of the controllable color tunability further motived us towards the patterning of the bilayer PCs, which showed promising potential in the anticounterfeiting field.

Cascade‐Microphase‐Separation‐Induced Hierarchical Photonic Structures in Supramolecular Organogel for Deformation‐Insensitive Structural Colors

AbstractPhotonic materials with tunable structural colors in response to various stimuli have enabled diverse sensing and displaying applications. However, the changeable colors caused by mechanical deformations may severely confuse the optical readouts for the upcoming wearable photonics. Herein, an extremely stretchable supramolecular organogel is demonstrated with deformation‐insensitive structural colors through an unconventional cascade‐microphase‐separation process among the silica nanoparticles (SiO2 NPs), silicone oils, and hydrogen‐bonded polymer gel. The resultant sparse and abundant microscale SiO2 NP aggregates with proper sizes, shapes, and random distributions in the gel matrix serve as rigid structural color domains and secure invariant colors to the whole soft sample under extreme mechanical deformations. Moreover, the dynamic hydrogen bonding of the organogel matrix offers the nanocomposites shear‐thinning and easy‐molding property, making them ideal for color paints in versatile coating applications. The deformable organogel nanocomposites have great potentials in various fields requiring stable colors and open up a new avenue for the development of wearable photonic devices.

Correction: Preparation of a colloidal photonic crystal containing CuO nanoparticles with tunable structural colors

[This corrects the article DOI: 10.1039/C5RA21035F.].

Structurally colored protease responsive nanoparticle hydrogels with degradation-directed assembly.

A tunable protease responsive nanoparticle hydrogel (PRNH) that demonstrates large non-iridescent color changes due to a degradation-directed assembly of nanoparticles is reported. Structurally colored composites are fabricated with silica particles, 4-arm poly(ethylene glycol) norbornene (4PEGN), and a proteolytically degradable peptide. When placed in a protease solution, the peptide crosslinks degrade causing electrostatic binding and adsorption of the polymer to the particle surface which leads to the assembly of particles into compact amorphous arrays with structural color. The particle surface charge and size is investigated to probe their effect on the assembly mechanism. Interestingly, only PRNHs with highly negative particle surface charge exhibit color changes after degradation. Ultra-small angle X-ray scattering revealed that the particles become coated in polymer after degradation, producing a material with less order compared to the initial state. Altering the particle diameter modulates the composites' color, and all sizes investigated (178-297 nm) undergo the degradation-directed assembly. Varying the amount of 4PEGN adjusts the swollen PRNH color and has no effect on the degradation-directed assembly. Taken together, the effects of surface charge, particle size, and polymer concentration allow for the formulation of new design rules for fabricating tunable PRNHs that display vivid changes in structural color upon degradation.

Kerker‐Conditioned Dynamic Cryptographic Nanoprints

AbstractHigh‐quality and tunable structural color printing with feasible fabrication is a key goal in recent decades, holding the possibility for full‐color microdisplay and impeccable cryptographic nanoprints. Plasmonic approach suffers from the inevitable material absorption restricting the color spectrum with suppressed lightness. Recent progress in dielectric nanophotonics offers low‐loss solution for color rendering, but dynamic functionality is still in infancy of changing colors with unchanged portraits, where more sophisticated physical mechanisms are in demand. Here, a strategy using full Kerker's conditions is reported to design low‐loss silicon metasurfaces which can generate broad spectrum of colors beyond the standard Red Green Blue (sRGB) gamut. The combination of the two Kerker's conditions enables dynamic and bright color palettes and full‐color microdisplay. Meanwhile, the asymmetric unit‐cell can guarantee polarization‐angle‐controlled reflective response with greatly sharpened spectra by complete control of the Kerker's effects. As a proof‐of‐concept, polarization‐encrypted images are demonstrated where completely distinguished pieces of information can be decoded exclusively only in predetermined polarization angle. They serve as highly‐secured cryptographic nanoprints that can be integrated with conventional smartphone technology. The complementary‐metal‐oxide‐semiconductor (CMOS)‐compatible metasurfaces can provide a new roadmap of anti‐counterfeiting tagging, advanced optical data storage, and tunable high‐contrast microprinting for real‐life applications.

Self-assembly of colloidal Poly(St-MMA-AA) core/shell photonic crystals with tunable structural colors of the full visible spectrum

This study used the copolymer emulsion method to synthesize colloidal monodisperse core/shell microspheres composed of the poly(styrene-co-methyl methacrylate-co-acrylic acid) [Poly(St-MMA-AA)]. Colloidal photonic crystal (CPhC) films have prepared by using polymer composite microspheres and successfully fabricated via natural sedimentation. The synthesized results of polymer composite microsphere exhibited uniform and good morphology. The resultant of CPhC color films demonstrated vividly structural colors due to 3D CPhC nanostructures. The structural colors over full visible spectra could be tuned by controlling the filling factor of crystals and diameter of microspheres according to the Bragg law. Additionally, the structural color of Poly(St-MMA-AA) CPhC films has measured under the standard colorimetric system. This study demonstrated a approach to fabricate tunable structural color for numerous applications.

Coloration of Liquid-Metal Soft Robots: From Silver-White to Iridescent

Gallium-based room-temperature liquid metals are becoming increasingly attractive and outstanding candidates for designing soft robots because of their remarkable electrical conductivity, superior flexibility, excellent stability, and low toxicity. However, the color of liquid metals is limited to shiny silver-white with high reflectivity, which is not helpful for camouflage, like that found in natural soft animals such as cephalopods. Herein, a biomimetic chromatic liquid-metal soft robot with tunable structural colors is reported. Colors ranging from white to gold and black appear on the surface of liquid metal when placed on a graphite substrate and mixed with Al foil in an electrolyte solution. A stable liquid-metal functional material with a rainbow-like appearance is realized under the regulation of an electric field. Further composition and structure characterization reveals that it is a nanoscale Ga2O3 film that displays the multicolor characteristic. The nanostructural film indicates that light scattering of Ga2O3 occurs when the liquid metal is on the graphite surface, and thin-film interference triggers iridescence when the liquid metals are subjected to electrolysis, respectively. These results provide a route to create kaleidoscopic and colorful liquid metals, which are expected to have diverse applications, especially in reinforcing soft robot design with intelligent camouflage function.

Tunable reflective color filters based on asymmetric Fabry-Perot cavities employing ultrathin Ge2Sb2Te5 as a broadband absorber.

We demonstrated a tunable structural color filter based on an asymmetric Fabry-Perot cavity employing germanium antimony tellurium alloy Ge2Sb2Te5 (GST) as a switchable ultrathin lossy layer. The color tunability and switch mechanism of our designed structure were investigated by both simulation and analytical approaches. Both numerical simulations and analytical results show that the tunable reflective colors can be generated through the reversible phase transition of GST from amorphous to crystalline. Additionally, the generated colors possess high brightness, high saturation, and a wide gamut. Our designed structure will inspire phase-transition-based systems' potential applications in colorimetric sensing, smart windows, full-color printing and displays, anti-counterfeiting, and data encryption.

Flexible, self-standing and patternable P(MMA-BA)/TiO2 photonic crystals with tunable and bright structural colors

To meet the requirement for flexible and transferrable optical equipment in life, developing flexible and self-supported photonic crystals (PCs) is substantially meaningful. Herein, we presented a facile and eco-friendly method to construct flexible, self-standing organic/inorganic hybrid 1DPCs with bright and tunable structural colors. 4-stack nanoparticle-based poly(methyl methacrylate-butyl acrylate) (P(MMA-BA)/TiO2) PCs are fabricated on low-surface-energy polypropylene substrates and covered with polydimethylsiloxane (PDMS) precursor afterwards. Through thermal treatment, the PCs together with cured PDMS can be easily lifted off without other post-treatment, and the color hue of the transferred 1DPC films holds good consistency with that before transfer. Tension test shows the self-standing PC film has good flexibility and better toughness than bare PDMS film. Based on this simple operation, patterned PC films can be achieved as long as placing a mask before covering the precursor. Due to organic vapor sensitivity of P(MMA-BA), this colorful self-supported PC structure shows distinguishable color change when exposing to different vapors, realizing naked-eye recognition of the solvents with the flexible organic/inorganic hybrid 1DPC device.

Cost‐Effective, Flexible, Hydrophobic, and Tunable Structural Color Polymeric Bragg Reflector Metastructures

AbstractThe synthesis of flexible photonic materials, by sustainable and scalable methods, is proved to be a difficult task for the materials science and industrial communities. Alongside, the modern society has also grown a strong dependence on polymeric materials, demanding superpolymers that combine functionality and cost with superior after‐use properties. The path for accomplishing this aim is made possible by mimicking nature through the merging of self‐ordered nanostructures and a commodity thermoplastic, resorting to basic fabrication infrastructure. This work presents the development of a flexible material that exhibits tunable structural color due to its 3D polyethylene based nanonetwork. These nanonetworks are hydrophobic, and change color depending on the refractive index of the material filling their voids. This developed flexible metamaterial is projected to open opportunities for the fabrication of economically affordable (around 0.008 € cm−2) and solvent‐free photonic nanostructures with multipurpose applications such as sensing, energy saving, clothing, and photovoltaics, among others.

Shear thickening fluid with tunable structural colors

We report a kind of shear thickening fluid (STF) which exhibits non-iridescent structural colors (defined as colorful shear thickening fluid, c-STF). The color of the c-STF can be tuned in a whole visible light range via changing the particle size, the filling fraction and temperature. The color of this c-STF is characterized by reflection and transmission spectrum and the inner structure was observed via confocal microscope. The relationship between the structure transitions, color, and rheological properties are systematically investigate. The measurements show that the occurrence of photonic band gap is responded for the short-range geometric order. The semitransparent colorful STF create the possibility to observe the transition of inner structure in shear thickening (ST) phenomenon, which will attribute to further study the mechanism of ST. And the tuned colors also extend the potential applications such as full-color displays with a wide viewing angle and color-tunable soft armor.

Instantaneous Magnetically Assembled and Hydrophilic Photonic Crystals with Controlled Diffraction Colors

The time-consuming assembling process of the traditional photonic crystals (PCs) and non-water-dispersibility of the reported magnetic responsive PCs (MRPCs) have greatly limited the application especially in the biotechnological fields. Herein, the hydrophilic and size-controllable Fe3O4@poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA)@SiO2 MRPCs were fabricated by orderly assembling of the core–shell colloidal nanocrystal clusters via a two-step facile synthesis approach. Due to the rich carboxyl and hydroxyl groups of PSSMA, the obtained MRPCs have excellent properties of hydrophilicity, high surface charge which presented magnetically tunable photonic structural colors, and rapid reflection signal in aqueous solution under external magnetic field within 1 s. The diffraction color of the MRPCs in the entire visible range could be tuned by adjusting the magnetic strength or the nanoparticle size, which bring a clear change of the structure color from brilliant red to modena by the naked eye. Thus, th...

Confined Growth of ZIF‐8 Nanocrystals with Tunable Structural Colors

AbstractZeolitic imidazolate frameworks (ZIF‐8) have promising applications as sensors or catalysts due to their highly porous crystalline structures. While most of the previous studies are based on ZIF‐8 crystals either in isolated particles in aqueous environments or in a compact colloidal form, here a facile method is reported to achieve wafer‐based isolated ZIF‐8 nanocrystals to facilitate their integration in microsystems and as surface coatings for catalysis. The fabrication process includes the growth of compact zinc oxide film by atomic layer deposition that functions as the Zn source for the ZIF‐8 synthesis, and the dispersion of gold nanoparticles as inhibitors for the following crystallization transformation of ZIF‐8 crystals. By choosing the concentration of gold nanoparticles, the density of ZIF‐8 nanocrystals can be controlled and the sizes of individual ZIF‐8 crystals can be scaled down to ≈100 nm. A wide range of structural colors generated by the ZIF‐8 nanocrystals is also observed, which can be attributed to the size‐dependent resonant scattering as verified by finite‐difference time‐domain simulations and classical Mie theory. The scalable fabrication of wafer‐based ZIF‐8 nanocrystals empowered with tunable optical properties paves a new way to explore the promising applications in nanophotonics and bionanotechnology.