Photonic Films Research Articles

Bio‐Inspired Wrinkled Photonic Elastomer with Superior Controllable and Mechanically Stable Structure for Multi‐Mode Color Display

AbstractColor is an excellent carrier of information between organisms. Photonic elastomers that convert abstract mechanical information into intuitive color information have shown great potential in various application fields. However, angular correlation of structural colors and monotonous color conversion mode affect the accuracy of material information reading and the applicability in diverse application scenarios. Here, inspired by a kind of bright blue spider‐ Poecilotheria metallica, a new wrinkled photonic elastomer structure is reported. Through wrinkling stretchable 1D photonic crystals (1DPCs), photonic elastomers with both one‐directional and omni‐directional angle‐independent brilliant structural colors are realized. It is worth noting that with the hybrid 1DPC and polydimethylsiloxane substrate support, the wrinkled photonic structure and structural color remain stable after 1000 strain cycles, and mechanochromic sensitivity can reach 3.25 nm/%, indicating strong structural stability and sensitive deformation performance under stress. More importantly, by comprehensively manipulating micro‐wrinkle structure and lattice spacing of the photonic films, the structural color can achieve delayed discoloration and invisible‐visible reversible switching performance only through a single strain direction. The proposed wrinkled photonic elastomers have a broad application value in the fields of visual strain sensing, wearable devices, and information encryption. Furthermore, it provides a new strategy for color regulation of photonic materials.

Chiral semiconductor photonic thin film with tunable circularly polarized luminescence

Inorganic nanowires and semiconductor quantum dots are co-assembled into chiral photonic crystals, generating circularly polarized luminescence with a significant dissymmetry factor and tunable peak position, intensity, and sign by controlling the photonic crystals' bandgap, pitch number, and handedness.

Nature-inspired construction of iridescent CNC/Nano-lignin films for UV resistance and ultra-fast humidity response

Exploration of functional materials based on sustainable and renewable biomolecules has been of much interest. Herein, nature-inspired photonic films were proposed by incorporation of bio-based lignin nanoparticles (LNPs) into chiral nematic cellulose crystals (CNCs). Evaporation induced self-assembly (EISA) formed oriented and layered structure of the nanocomposites iridescent films with enlarged helix pitches by intercalation of higher amounts of LNPs. Decreased crystallite sizes and expanding layer gaps indicated the homogeneous distribution and hydrophobic interactions between CNCs and LNPs. Distinguished UV absorption capabilities with over 90 % shielding capabilities in UVB region and increased hydrophobicity with the contact angle of 75° were achieved for the composite films due to the presence of hydrophobic lignin. The proposed optical films also showed outstanding cytocompatibility owing to all-natural components introduced into the materials, which may display great potentials in many fields such as stimuli sensing, anti-counterfeiting and wearable devices.

Kerr nonlinear medium assisted double-face absorbers for differential manipulation via an all-optical operation.

Recently, light absorbers have attracted great attentions due to their promising in applications in functional optoelectronic devices. Herein, we theoretically propose and numerically demonstrate a new absorber platform, which consists of a 280-nm-thick photonic nonlinear waveguide film covering on the metal grating structure. Strong reflection inhibition and absorption enhancement is achieved in both the forward and backward directions, which indicates potential novel performances since the previous reports only achieved absorption in one side due to the using of opaque metal film substrate or the reflective mirror. The anti-reflection bands or the absorption peaks at the shorter and longer wavelength ranges are related to the excitation of the propagating surface plasmon resonance by the slit-assisted grating and the cavity mode by the slit in the metal film. Strong differential manipulation is realized for the double-face absorbers via the all-optical operation. Moreover, the operation wavelengths for the double-face light absorber can be modified strongly via using an asymmetric dielectric medium for the coating films. These new findings pave approaches for subtractive lightwave modulation technology, selective filtering, multiplex sensing and detection, etc.

A Surface Diffusion Barrier Strategy toward Water-Resistant Photonic Materials for Accurate Detection of Ethanol.

Photonic materials that enable visual detection of chemicals have shown great potential for wide applications in chemical, environmental, biotechnological, and food industries, but until now, using hydrophilic photonic materials for tracing water-soluble chemicals remains a major challenge due to the strong water interference. Here, we demonstrate a two-step co-assembly and subsequent surface coating strategy to develop an ethanol-sensitive and anti-water interference photonic crystal film. By using citric acid as a co-assembly phase, high ethanol sensing is realized because of the strong intermolecular affinity. By controlling the thickness of the outer polyvinyl butyral layer, selective ethanol penetration but a water barrier is enabled. Notably, the composite photonic films are free-standing, highly flexible, and controllably structurally colored. We further present using the composite film to quantitatively trace ethanol/water mixtures and potentially track drunk driving as a colorimetric sensor. The heuristic two-step modification strategy proposed in this work not only overcomes the limitation of water interference for hydrophilic colorimetric sensors but also provides references to develop more new photonic materials with water resistance that need to be applied in water/humid environments.

Biomimetic Chiral Photonic Materials with Tunable Metallic Colorations Prepared from Chiral Melanin-like Nanorods for UV Shielding, Humidity Sensing, and Cosmetics.

Many biological species combine the helical organization of cellulose or chitin microfibrils with broadband light absorption of black melanin to produce brilliant structural colors with metallic and glossy effects and other diverse functions. In this work, based on core-shell CNC@PDA chiral nanorods consisting of cellulose nanocrystals (CNCs) as the core and melanin-like polydopamine (PDA) as the shell that can form well-defined chiral liquid crystal phases, we report chiral photonic materials that closely mimic the unique coloration mechanisms and functionalities mastered by several biological species. The photonic films formed by such single CNC@PDA nanorods have brilliant iridescent structural colors originating from selective reflection of circularly polarized lights by the helical organization of CNC@PDAs across the films. Furthermore, the colors of such films have background-independent brightness, high visibility, and metallic effects that arise from the light absorption of the PDA component. Especially, the color ranges and metallic effects of the films can be conveniently tuned by varying the thickness of the PDA shell. In addition, the UV absorption and hygroscopic properties of PDA endow these CNC@PDA films with efficient broadband UV shielding and sensitive humidity-induced dynamic color changes. Due to the mussel-like superior adhesion of PDA, CNC@PDA-based photonic coatings can be formed conformably onto diverse kinds of substrates. A shiny eye shadow with viewing angle-dependent colorful patterns was used to demonstrate the potential applications. With combinations of multiple unique properties in one photonic material fabricated from a single building block, these CNC@PDA-based films are expected to have potential applications in cosmetics, UV protection, anticounterfeiting, chiral reflectors, etc.

Structurally Colored Cellulose Nanocrystal Films as Transreflective Radiative Coolers

Radiative cooling forms an emerging direction in which objects are passively cooled via thermal radiation to cold space. Cooling materials should provide high thermal emissivity (infrared absorptance) and low solar absorptance, making cellulose an ideal and sustainable candidate. Broadband solar-reflective or transparent coolers are not the only systems of interest, but also more pleasingly looking colored systems. However, solutions based on wavelength-selective absorption generate not only color but also heat and thereby counteract the cooling function. Intended as coatings for solar cells, we demonstrate a transreflective cellulose material with minimal solar absorption that generates color by wavelength-selective reflection, while it transmits other parts of the solar spectrum. Our solution takes advantage of the ability of cellulose nanocrystals to self-assemble into helical periodic structures, providing nonabsorptive films with structurally colored reflection. Application of violet-blue, green, and red cellulose films on silicon substrates reduced the temperature by up to 9 °C under solar illumination, as result of a combination of radiative cooling and reduced solar absorption due to the wavelength-selective reflection by the colored coating. The present work establishes self-assembled cellulose nanocrystal photonic films as a scalable photonic platform for colored radiative cooling.

Chiral self-assembly of cellulose nanocrystals is driven by crystallite bundles

The transfer of chirality across length-scales is an intriguing and universal natural phenomenon. However, connecting the properties of individual building blocks to the emergent features of their resulting large-scale structure remains a challenge. In this work, we investigate the origins of mesophase chirality in cellulose nanocrystal suspensions, whose self-assembly into chiral photonic films has attracted significant interest. By correlating the ensemble behaviour in suspensions and films with a quantitative morphological analysis of the individual nanoparticles, we reveal an inverse relationship between the cholesteric pitch and the abundance of laterally-bound composite particles. These ‘bundles’ thus act as colloidal chiral dopants, analogous to those used in molecular liquid crystals, providing the missing link in the hierarchical transfer of chirality from the molecular to the colloidal scale.

Cellulose nanocrystal chiral photonic micro-flakes for multilevel anti-counterfeiting and identification

Counterfeiting and forgery have caused enormous economic loss and many safety concerns, and many approaches have been employed for anti-counterfeiting. However, most of these approaches suffer intrinsic issues such as replicability, environmental sustainability, or the requirement of professional equipment to authenticate. Cellulose nanocrystal (CNC) chiral photonic films prepared by evaporation induced self-assembling display iridescent structural colors, circular dichroism, and unique fingerprint texture in microsize, which are promising for anti-counterfeiting. However, the application of CNC films in anti-counterfeiting are severely limited by their brittleness and nonuniformity. Herein, CNC chiral photonic micro-flakes facilely prepared via ultrasonication of CNC films preserve the optical characteristics of CNC chiral photonic films, and can be mixed with polymer matrices to fabricate flexible films or coatings for practical anti-counterfeiting and identification. The anti-counterfeiting characteristics of CNC chiral photonic micro-flakes can be authenticated by an inspection with naked eyes, using 3D glasses, and further with a polarized optical microscope. Therefore, CNC chiral photonic micro-flakes provide a non-replicable, environmentally friendly, easy-to-authenticate, and practical approach for multilevel anti-counterfeiting and identification.

Photonic Crystal-Enhanced Photodynamic Antibacterial Therapy

Photonic Crystal-Enhanced Photodynamic Antibacterial Therapy

The Limited Palette for Photonic Block-Copolymer Materials: A Historical Problem or a Practical Limitation?

Block-copolymer self-assembly has proven to be an effective route for the fabrication of photonic films and, more recently, photonic pigments. However, despite extensive research on this topic over the past two decades, the palette of monomers and polymers employed to produce such structurally colored materials has remained surprisingly limited. In this Scientific Perspective, the commonly used block-copolymer systems reported in the literature are summarized (considering both linear and brush architectures) and their use is rationalized from the point of view of both their historical development and physicochemical constraints. Finally, the current challenges facing the field are discussed and promising new areas of research are highlighted to inspire the community to pursue new directions.

The Limited Palette for Photonic Block-Copolymer Materials: A Historical Problem or a Practical Limitation?

Block‐copolymer self‐assembly has proven to be an effective route for the fabrication of photonic films and, more recently, photonic pigments. However, despite extensive research on this topic over the past two decades, the palette of monomers and polymers employed to produce such structurally colored materials has remained surprisingly limited. In this Scientific Perspective, the commonly used block‐copolymer systems reported in the literature are summarized (considering both linear and brush architectures) and their use is rationalized from the point of view of both their historical development and physicochemical constraints. Finally, the current challenges facing the field are discussed and promising new areas of research are highlighted to inspire the community to pursue new directions.

Bioinspired Polypeptide Photonic Films with Tunable Structural Color.

We report a new synthetic strategy of combining N-carboxyanhydride (NCA) chemistry and photonic crystals for the fabrication of polypeptide structural color films. Driven by surface-initiated ring-opening polymerization, the di-NCA derivative of l-cystine (Cys) is introduced to replicate the functionalized colloidal crystal templates and construct freestanding P(Cys) films with tunable structural color. Furthermore, the feasibility of preparing patterned polypeptide photonic films is demonstrated via template microfabrication. Because of the incorporation of l-glutamate (Glu) components, the P(Cys-co-Glu) co-polypeptide films are endowed with a visual color responsiveness toward pH changes. Additionally, the polypeptide photonic films show on-demand degradability. Given the large family of amino acid building blocks, this powerful and versatile approach paves the way for chemical derivatization of multifunctional peptide-based optical platforms.

3D Porous cellulose/Si-Al inorganic polymer photonic film with precisely structure-enhanced solar reflectivity for daytime radiative cooling

Porous structural polymer materials have the potential to achieve excellent passive daytime radiative cooling (PDRC). However, traditional porous polymer materials have irregularly internal structures that cannot effectively scatter sunlight, resulting in a low reflectivity in the solar spectrum. In this work, we designed a low-cost and eco-friendly structural film that transformed the cellulose acetate and Si-Al inorganic polymer particles into a regular three-dimensional (3D) network using the solvent exchange method. As a result, regularly distributed cavities with evenly size ~3 µm self-formed with the leaving of solvent, which significantly enhanced the solar reflectance of the structural PDRC film to about 98%. Meanwhile, a high infrared emissivity in atmospheric window was also 83%. To prove the concept, the structural film was used for the outdoor PDRC process, and 6.56 ℃ cooling capacity could be produced even under the noon solar irradiance (~1600 W/m2) and sub-ambient temperature (~34.6 ℃), as well as 2.38 ℃ below sub-ambient temperature at night. In addition, the mechanical strength and self-cleaning surface were integrated into the photonic film which extended its environment adaptability effectively. It was believed that this regularly porous strategy induced by, the hybrid of inorganic polymer would inspire the design of structural PDRC materials and the large-scale passive cooling technology.

Wearable transdermal microneedle patch based on photonic crystal hydrogel for glucose monitoring

Daily blood glucose monitoring is the cornerstone of diabetes management. Research efforts have been made on developing non-invasive or minimally invasive point-of-care testing system that can minimize the inconvenience of such daily test bringing to diabetes. Hereby, we develop a wearable and portable microneedle based biosensor for in situ glucose detection in an enzyme free and minimally invasive way. The photonic hydrogel film integrated inside the microneedle platform is functionalized with phenylboronic acid and displays glucose responsiveness. Presence of glucose in the chemical matrix of interstitial fluids can be distinguished with naked-eyes via the color changes of the hydrogel film. Quantification of glucose is achieved by analyzing color of hydrogel film with the facilitation of a mobile phone; thereby fast and direct diagnosis is enabled. Extraction of interstitial fluids and sensing of glucose within the psychological range was successfully demonstrated on porcine skin as human skin model. The developed microneedle platform provides a promising tool for detecting other health related analytes in an enzyme-free way with minimal invasiveness.

Photonic Cellulose Films with Vivid Structural Colors: Fabrication and Selectively Chemical Response

Recent advances in structural-color cellulose nanocrystal (CNC) materials have been made toward chemical sensing applications; however, such materials lack sufficient color chroma for naked-eye observation, and their selective recognition to given chemicals as well as the corresponding mechanism has rarely been reported. Here, a dopamine-infiltration and post-polymerization approach is proposed to construct vivid structural-color composite films. The chiral nematic structure of CNC enables the structural coloration, while the strong light absorption of the polymeric co-phase, polydopamine (PDA) enhances the color chroma and visibility. By controlling the PDA amount, the composite films can detect organic solvents quantitatively and selectively via visible color changes. From the viewpoint of the compatibility and similitude principle, notably, a critical solubility parameter distance (R0) between PDA and "active" solvents is defined with a three-dimensional Hansen solubility sphere; this well constructs a rule for the sensing selectivity of the chemochromic composite films. The findings pave the foundation for the design of colorimetric sensors with specifically testing objects.

Rapid Assembly of Magnetoplasmonic Photonic Arrays for Brilliant, Noniridescent, and Stimuli-Responsive Structural Colors.

There are usually trade-offs between maximizing the color saturation and brightness and minimizing the angle-dependent effect in structural colors. Here, a magnetic field-induced assembly for the rapid formation of scalable, uniform amorphous photonic arrays (APAs) featuring unique structural colors is demonstrated. The magnetic field plays a fundamental role in photonic film formation, making this assembly technology versatile for developing structural color patterns on arbitrary substrates. The synergistic combination of surface plasmonic resonance of the Ag core and broadband light absorption of high refractive index (RI) Fe3 O4 shell in hybrid magnetoplasmonic nanoparticles (MagPlas NPs) enables breaking the trade-offs to produce brilliant, noniridescent structural colors with high tunability and responsiveness. These features enable the fabrication of various types of highly sensitive and reliable colorimetric sensors for naked-eye detection without sophisticated instruments. Furthermore, large-scale structural color patterns are effortlessly achieved, demonstrating the high potential of the present approach for full-spectrum displays, active coatings, and rewritable papers.

Circularly Polarized Luminescent Chiral Photonic Films Based on the Coassembly of Cellulose Nanocrystals and Gold Nanoclusters.

In this work, we studied the formation and properties of composite films coassembled by cellulose nanocrystals (CNCs) and bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs). The influences of the BSA-AuNC concentration on the structure and optical properties of CNC-based composite films were further studied. It was found that the composite film retained the chiral nematic structure and optical activity. The self-assembled CNC and BSA-AuNC helical superstructures can produce strong, left-handed, circularly polarized luminescence with dissymmetry factors up to 0.287. Meanwhile, the third component, polyethylene glycol, was introduced without affecting the structural color and fluorescence characteristics of the composite film to enhance the flexibility of the film. The simplicity of the film preparation, the abundance of CNCs, and the flexibility and stability of the composite films pave the way for the production of functional materials with integrated functions.

Fast Self‐Assembly of Scalable Photonic Cellulose Nanocrystals and Hybrid Films via Electrophoresis

Nano-enabled, bio-based, functional materials are key for the transition to a sustainable society as they can be used, owing to both their performance and nontoxicity, to gradually replace existing nonrenewable engineering materials. Cellulose nanocrystals (CNCs), produced by acid hydrolysis of cellulosic biomass, have been shown to possess distinct self-assembly, optical, and electromechanical properties, and are anticipated to play an important role in the fabrication of photonic, optoelectronic, and functional hybrid materials. To facilitate CNCs' technological viability, a method suitable for industrial exploitation is developed to produce photonic films possessing long-range chirality on conductive, rigid, or flexible, substrates within a few minutes. The approach is based on electrophoretic deposition (EPD)-induced self-assembly of CNCs, where photonic films of any size can be produced by controlling CNC surface properties and EPD parameters. CNC film coloration can be determined by the CNC aqueous suspension characteristics, while their reflected intensity can be tuned by changing the duration and number of electrodeposition cycles. EPD-induced self-assembly of CNCs is compatible with in situ reduction of gold precursors without the need to use additional reducing agents (some of which are considered toxic), thereby allowing the preparation of hybrid photonic films with tunable plasmonic response in a one-pot process.

Voltage-tunable elastomer composites that use shape instabilities for rapid structural color changes.

Structurally colored materials can switch colors in response to external stimuli, which makes them potentially useful as colorimetric sensors, dynamic displays, and camouflage. However, their applications are limited by the angular dependence, slow response, and absence of synchronous control in time and space. In addition, out-of-plane deformation from shape instability easily occurs in photonic films, leading to inhomogeneous colors in photonic-crystal materials. To address these challenges, we combine structurally colored photonic glasses and dielectric elastomer actuators. We use an external voltage signal to tune color changes quickly (much less than 0.1 s). The photonic glassses produce colors with low angular dependence, so that their colors are homogeneous even when they become curved due to voltage-triggered instabilities (buckling or wrinkling). As proof of concept, we present a pixelated display in which segments can be independently and rapidly turned on and off. This wide-angle, instability-tolerant, color-changing platform could be used in next-generation soft and curved color displays, camouflage with both shape and color changes, and multifunctional sensors.