Electrically Tunable Soft-Solid Block Copolymer Structural Color

Dynamic Photonic Janus Colloids with Axially Stacked Structural Layers

Diblock copolymer (dBCP) particles capable of dynamic shape and color changes have gained significant attention due to their versatility in programmable shapes and intricate nanostructures. However, their application in photonic systems remains limited due to challenges in achieving a sufficient number of defect-free photonic layers over a tens-of-micrometer scale. In this study, we present a pioneering demonstration of photonic dBCP particles featuring over 300 axially stacked photonic layers with responsive color- and shape-transforming capabilities. Our approach leverages the complex interplay between the macrophase separation of multiple incompatible components and the microphase separation of dBCP from solvent-evaporative microemulsions. Specifically, continuous phase separation of silicone oil from polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), triggered by solvent evaporation, promotes the anisotropic growth of PS-b-P2VP layers. This results in the formation of Janus colloids, where an oil droplet merges with a nanostructured polymer cone and lamellar structures align along the long axis of the cone. We highlight the capability to precisely adjust the particle morphology and the corresponding orientation, dispersion, and structural color window by modulating both the molecular weight of PS-b-P2VP and the volume ratio between PS-b-P2VP and silicone oil. Furthermore, reversible swelling/deswelling of photonic colloids is visualized and correlated with their structural colors. Finally, we demonstrate the potential of this study by presenting a multicolor-patterned array of photonic colloids, highlighting the possibilities for applications in smart photonic ink and devices.

Interactive structural color displays of nano-architectonic 1-dimensional block copolymer photonic crystals: FOCUS ISSUE REVIEW

For changing environmental circumstances, interactive structural color (SC) observation is a promising strategy to store and express external information. SCs based on self-assembled block copolymer (BCP) photonic crystals have been a research focus due to their facile and diverse nanostructures relying on the volume ratio of blocks. Their unique nano-architectonics can reflect incident light due to constructive interference of the two different dielectric constituents. Their excellent ability to change nano-architectonics in response to external stimuli (i.e. humidity, temperature, pH, and mechanical force) allows for a programmable and stimuli-interactive BCP SC display. In this review, recent advances in programmable and stimuli-interactive SC displays with the 1-dimensional self-assembled BCP nano-architectonics are comprehensively discussed. First, this review focuses on the development of programmable BCP SCs that can store various information. Second, stimuli-interactive BCP SCs capable of responding reversibly to external stimuli are also addressed. Particularly, reversible BCP SC changes are suitable for rewritable displays and emerging human-interactive BCP SC displays that detect various human information through changes in electric signals with the simultaneous alteration of the BCP SCs. Based on previously reported literature, the current challenges in this research field are further discussed, and the perspective for future development is presented in terms of material, nano-architectonics, and process.

Dual Responsive Dependent Background Color Based on Thermochromic 1D Photonic Crystal Multilayer Films

In this paper, we present dual responsive one-dimensional (1D) photonic crystal (PC) multilayer films that utilize a high-humidity environment and temperature. Dual responsive 1D PC multilayer films are fabricated on precoated thermochromic film by sequential alternate layer deposition of photo-crosslinkable poly(2-vinylnaphthalene-co-benzophenone acrylate) (P(2VN-co-BPA)) as a high refractive index polymer, and poly(4-vinylpyrollidone-co-benzophenone acrylate) P(4VP-co-BPA) as a low refractive index polymer. The thermochromic film shows a vivid color transition from black to white at 28 °C. Three different colors of thermochromic 1D PC multilayer films are prepared by thickness modulation of P(4VP-co-BPA) layers, and the films on a black background exhibit visible spectrum color only in a high-humidity environment (over 90% relative humidity (RH)). For the three films placed on a hands display, three different composite colors are synthesized by the reflection of light, including yellow, magenta, and cyan, due to the changing of backgrounds from black to white with temperature. Additionally, the films show remarkable color transitions with reliable reversibility. The films can be applied as anti-counterfeiting labels and can be used for smart decoration films. To the best of our knowledge, this is the first report of dual response colorimetric films that change color in various ways depending on temperature and humidity changes, and we believe that it can be applied to various applications.

Thin film block copolymer self-assembly for nanophotonics

The nanophotonic engineering of light-matter interactions has profoundly changed research behind the design and fabrication of optical materials and devices. Metasurfaces-arrays of subwavelength nanostructures that interact resonantly with electromagnetic radiation-have emerged as an integral nanophotonic platform for a new generation of ultrathin lenses, displays, polarizers and other devices. Their success hinges on advances in lithography and nanofabrication in recent decades. While existing nanolithography techniques are suitable for basic research and prototyping, issues of cost, throughput, scalability, and substrate compatibility may preclude their use for many metasurface applications. Patterning via spontaneous self-assembly of block copolymer thin films offers an enticing alternative for nanophotonic manufacturing that is rapid, inexpensive, and applicable to large areas and diverse substrates. This review discusses the advantages and disadvantages of block copolymer-based nanopatterning and highlights recent progress in their use for broadband antireflection, surface enhanced Raman spectroscopy, and other nanophotonic applications. Recent advances in diversification of self-assembled block copolymer nanopatterns and improved processes for enhanced scalability of self-assembled nanopatterning using block copolymers are also discussed, with a spotlight on directions for future research that would enable a wider array of nanophotonic applications.

Visualization of nonsingular defect enabling rapid control of structural color

Stimuli-interactive structural color (SC) of a block copolymer (BCP) photonic crystal (PC) uses reversible alteration of the PC using external fluids and applied forces. The origin of the diffusional pathways of a stimulating fluid into a BCP PC has not been examined. Here, we directly visualize the vertically oriented screw dislocations in a one-dimensional lamellar BCP PC that facilitate the rapid response of visible SC. To reveal the diffusional pathway of the solvent via the dislocations, BCP lamellae are swollen with an interpenetrated hydrogel network, allowing fixation of the swollen state and subsequent microscopic examination. The visualized defects are low-energy helicoidal screw dislocations having unique, nonsingular cores. Location and areal density of these dislocations are determined by periodic concentric topographic nanopatterns of the upper surface-reconstructed layer. The nonsingular nature of the interlayer connectivity in the core region demonstrates the beneficial nature of these defects on sensing dynamics.

Fabrication of multi-responsive photonic crystals based on selenium-containing copolymers

Responsive photonic crystals (PCs) have attracted great interest due to their adjustable structure color.

Poly(vinylpyridine)-Containing Block Copolymers for Smart, Multicompartment Particles

Multicompartment particles generated by the self-assembly of block copolymers (BCPs) have received considerable attention due to their unique morphologies and functionalities. A class of important building blocks for multicomponent particles...

Dynamically Tuneable Reflective Structural Coloration with Electroactive Conducting Polymer Nanocavities

Dynamic control of structural colors across the visible spectrum with high brightness has proven to be a difficult challenge. Here, this is addressed with a tuneable reflective nano-optical cavity that uses an electroactive conducting polymer (poly(thieno[3,4-b]thiophene)) as spacer layer. Electrochemical doping and dedoping of the polymer spacer layer provides reversible tuning of the cavity's structural color throughout the entire visible range and beyond. Furthermore, the cavity provides high peak reflectance that varies only slightly between the reduced and oxidized states of the polymer. The results indicate that the polymer undergoes large reversible thickness changes upon redox tuning, aided by changes in optical properties and low visible absorption. The electroactive cavity concept may find particular use in reflective displays, by opening for tuneable monopixels that eliminate limitations in brightness of traditional subpixel-based systems.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Bioinspired Electro-Responsive Multispectral Controllable Dye-Doped Liquid Crystal Yolk-Shell Microcapsules for Advanced Textiles

Liquid crystal microcapsules have attracted increasing attention due to their sophisticated structures and adjustable multifunctional features. However, the synthesis of a microscale substrate with wide electromagnetic waveband modulation characteristics and good photoelectric stabilization is still limited and challenging. Herein, a new breed of microcapsules containing dye-doped liquid crystals in a yolk-shell configuration with VTES (vinyl-trim-ethyl-silane)-modified Fe₃O₄@SiO₂ is created. It exhibits an unexpected color enhancement effect, reversible electrochromic performance, and excellent magnetically controllable characteristics. Additionally, a multispectral (visible light, near-infrared light, and high-frequency electromagnetic wave) electro-responsive fabric based on the proposed microcapsules was developed to explore its application in wearable sensors. The present work opens an avenue toward the fabrication of microscale microencapsulated soft materials with a continuous and stable yolk-shell structure. Moreover, it will expand the application regimes of liquid crystal materials in smart windows and advanced textiles.

Controllable Radical Polymerization of Selenide Functionalized Vinyl Monomers and Its Application in Redox Responsive Photonic Crystals

Selenium-containing monomer (p-phenylseleno) styrene (p-PhSeSt) is polymerized by reversible addition-fragmentation chain transfer polymerization. Polymer, (P(p-PhSeSt)), with controlled molecular weight and narrow molecular weight is obtained. The selenide moiety in obtained P(p-PhSeSt) can be selectively oxidized to selenoxide or selenone groups by H₂ O₂ or NaClO, respectively. These oxidized groups can be further reduced to selenide by Na₂ S₂ O₄ . The structure changing of polymers during such redox cycle is characterized by nuclear magnetic resonance, X-ray photoelectron spectroscopy, and size exclusion chromatography. Properties, such as thermal performance, glass transition temperature, water contact angles, and refractive indices, of the resulting polymers are systematically investigated before and after oxidation. In addition, SiO₂ inverse opal photonic crystal (IOPC) is fabricated by sacrificial polymer colloidal template method. Owing to changes of the RIs of P(p-PhSeSt) after selective oxidation, the predictable change of PC bandgap as a redox-responsive PC sensor is successfully realized, which provides new perspectives for modulating photonic crystals.

Transparent and UV Blocking Structural Colored Hydrogel for Contact Lenses

Usually, materials with perfect structures possess excellent properties, but it is not always the case. Here, a new approach is reported to construct structural colored hydrogel films with excellent ultraviolet (UV) blocking performance for contact lenses. The theoretical simulation predicts that with perfect periodic structures, the hydrogel films can strongly reflect incident light in a narrow visible wavelength range and thus exhibit extraordinarily brilliant colors. However, such hydrogel films cannot effectively block UV light. By slightly breaking the structural periodicity (quasi-periodic structure), strong diffuse scattering or pseudoabsorption of light can occur for all of the wavelengths shorter than a structural characteristic length, leading to perfect UV blocking. According to the theoretical prediction, a structural colored hydrogel film with nearly periodic polystyrene sphere arrays in poly(hydroxyethyl methacrylate) hydrogel matrix is fabricated; this hydrogel film possesses brilliant colors and perfect UV blocking, and the core particle composition and size have been investigated in detail for the optimized properties of contact lenses. Meanwhile, the cell proliferation assay proves the cytocompatibility of the hydrogel for real application. Regarding its unique optical characteristics, the as-prepared structural colored hydrogel shows great promise in the fields of UV-protective equipment, medical device, soft robot, sensor, and so on.

3D touchless multiorder reflection structural color sensing display

The development of a lightweight, low-power, user-interactive three-dimensional (3D) touchless display in which a human stimulus can be detected and simultaneously visualized in noncontact mode is of great interest. Here, we present a user-interactive 3D touchless sensing display based on multiorder reflection structural colors (SCs) of a thin, solid-state block copolymer (BCP) photonic crystal (PC). Full-visible-range SCs are developed in a BCP PC consisting of alternating lamellae, one of which contains a chemically cross-linked, interpenetrated hydrogel network. The absorption of a nonvolatile ionic liquid into the domains of the interpenetrated network allows for further manipulation of SC by using multiple-order photonic reflections, giving rise to unprecedented visible SCs arising from reflective color mixing. Furthermore, by using a hygroscopic ionic liquid ink, a printable 3D touchless interactive display is created where 3D position of a human finger is efficiently visualized in different SCs as a function of finger-to-display distance.

Highly Invisible Photonic Crystal Patterns Encrypted in an Inverse Opaline Macroporous Polyurethane Film for Anti-Counterfeiting Applications

Invisible photonic crystal (PC) pattern with encrypted and discoverable information is potentially useful for anti-counterfeiting labels, but it is still a big challenge to realize strict invisibility, fast response, and convenient triggering. Here, a new kind of soaking-revealed invisible PC pattern is fabricated by the regional coating of "ethylene glycol-ethanol" ink on a collapsed inverse opaline macroporous polyurethane (IOM-PU) film, followed by a quick thermal treatment. During the above process, wet heating retains the collapsed but recoverable IOM structure, but dry heating disables the recovery of ordered IOM structure due to the adhesion of macropore walls, which render the "pattern" and the "background" with different optical responses to the solvent. In the dry state, the pattern was invisible because both the collapsed IOM-PU film and the adhesive PU film are colorless and transparent. Once the sample is soaked in ethanol-water mixtures, the invisible pattern appears immediately because only the "wet-heated" region recovers the ordered macroporous structure and shows color, which forms a significant contrast in color to the "dry-heated" region. Compared to the previously invisible PC pattern, the current material has many superior properties, such as high invisibility, large color contrast in showing, excellent recyclability, and good toughness in bending and stretching.

High-Performance and Multifunctional Colorimetric Humidity Sensors Based on Mesoporous Photonic Crystals and Nanogels

Colorimetric sensors, as a key branch of the application of photonic crystals (PCs), brings enthusiasm to scientists to do research. Here, simple mesoporous and structurally colored one-dimensional photonic crystals (1DPCs) constructed by alternating assembly of poly(acrylamide- N,N'-methylene bis(acrylamide)) (P(AM-MBA)) nanogels and TiO₂ nanoparticles are reported as high-performance colorimetric humidity sensors. The sensors with bright colors display rapid response to relative humidity (RH) change and reach sensing balance in 0.5 s. By varying RH from 47.0% to 89.3%, stopband of a sensor changes from 426 to 668 nm, almost spanning the whole visible range. Meanwhile, visual sensing of RH possesses good reversibility and repeatability. Moreover, the sensors with delicate patterns are facilely fabricated by partial UV photodegradation of the polymer layers with nano TiO₂ as catalyst. The delicate patterns and backgrounds show different colors and change color simultaneously and quickly by varying the ambient humidity. Accurate QR code pattern is also realized on the PC sensor; it is found successful reading of the data is only achieved by increasing RH to realize high color contrast between the code and background. Given their excellent properties, the porous hybrid PCs are promising as high-performance humidity sensors with potential display, decoration, information-storage, and encryption functions.

One-Step Anionic Copolymerization Enables Formation of Linear Ultrahigh-Molecular-Weight Block Copolymer Films Featuring Vivid Structural Colors in the Bulk State

Ultrahigh-molecular-weight (UHMW) tapered block copolymers (BCPs) consisting of polyisoprene- block-poly(4-methylstyrene) featuring overall molar masses in the range of 1101-2033 kg mol^-1 ( M_w) are synthesized via a convenient one-step anionic copolymerization protocol. The obtained UHMW BCPs are investigated by differential scanning calorimetry, size exclusion chromatography, and ¹H NMR spectroscopy. Microphase separation for the UHMW BCPs in the bulk state is investigated by transmission electron microscopy (TEM) measurements and scanning electron microscopy (SEM), revealing well-ordered lamellar and spherical domains with large domain sizes in the range of 100-200 nm. Excellent order and periodicity are observed for lamellar morphologies over large film areas of 90 × 120 μm. Because of this high order of the underlying domains and the different refractive indices of the block segments, vivid structural colors can be observed in the bulk state. Structural colors of BCP films are investigated by angle-dependent UV/vis measurements, revealing intensive reflection colors according to Bragg's law of diffraction. The optical characteristics are directly correlated to TEM and SEM results. Moreover, colored BCP films featuring spherical domains for one block segment with domain sizes of 97-122 nm revealed blue structural colors stemming from disordered particle scattering.

Generation of intensity-tunable structural color from helical photonic crystals for full color reflective-type display

A new concept of intensity-tunable structural coloration is proposed on the basis of a helical photonic crystal (HPC). The HPCs are constructed from a mixture of chiral reactive mesogens by spin-coating, followed by the photo-polymerization. A liquid crystal (LC) layer, being homogeneously aligned, is prepared on the HPCs to serve as a tunable waveplate. The electrical modulation of the phase retardation through the LC layer directly leads to the intensity-tunable Bragg reflection from the HPCs upon the incidence of the polarized light. The bandwidths of the structural colors are found to be well preserved regardless of the applied voltage. A prototype of a full color reflective-type display, incorporated with three primary color units, is demonstrated. Our concept of decoupling two mutually independent functions, the intensity modulation by the tunable waveplate and the color reflection by the HPCs provides a simple and powerful way of producing a full color reflective-type display which possesses high color purity, high optical efficiency, the cycling durability, and the design flexibility.

Printable and Rewritable Full Block Copolymer Structural Color

Structural colors (SCs) of photonic crystals (PCs) arise from selective constructive interference of incident light. Here, an ink-jet printable and rewritable block copolymer (BCP) SC display is demonstrated, which can be quickly written and erased over 50 times with resolution nearly equivalent to that obtained with a commercial office ink-jet printer. Moreover, the writing process employs an easily modified printer for position- and concentration-controlled deposition of a single, colorless, water-based ink containing a reversible crosslinking agent, ammonium persulfate. Deposition of the ink onto a self-assembled BCP PC film comprising a 1D stack of alternating layers enables differential swelling of the written BCP film and produces a full-colored SC display of characters and images. Furthermore, the information can be readily erased and the system can be reset by application of hydrogen bromide. Subsequently, new information can be rewritten, resulting in a chemically rewritable BCP SC display.

Photonic Devices Out of Equilibrium: Transient Memory, Signal Propagation, and Sensing

Soft photonic materials are important for sensors, displays, or energy management and have become switchable under static equilibrium conditions by integration of responsive polymer features. The next step is to equip such materials with the ability for autonomously dynamic and self-regulating behavior, which would advance their functionality and application possibilities to new levels. Here, this study shows the system integration of a nonlinear, biocatalytic pH-feedback system with a pH-responsive block copolymer photonic gel, and demonstrates autonomous transient memories, remotely controlled signal propagation, and sensing. This study utilizes an enzymatic switch to program the lifetime of the reflective state of a photonic gel, and induces propagation of pH-waves extinguishable by illumination with UV-light. The described combination of nonlinear chemistry and responsive photonic gels opens pathways toward out-of-equilibrium photonic devices with active and autonomous behavior useful for sensing, computation, and communication.

Recent advances in the biomimicry of structural colours

Nature has mastered the construction of nanostructures with well-defined macroscopic effects and purposes. Structural colouration is a visible consequence of the particular patterning of a reflecting surface with regular structures at submicron length scales. Structural colours usually appear bright, shiny, iridescent or with a metallic look, as a result of physical processes such as diffraction, interference, or scattering with a typically small dissipative loss. These features have recently attracted much research effort in materials science, chemistry, engineering and physics, in order to understand and produce structural colours. In these early stages of photonics, researchers facing an infinite array of possible colour-producing structures are heavily inspired by the elaborate architectures they find in nature. We review here the recent technological strategies employed to artificially mimic the structural colours found in nature, as well as some of their current and potential applications.