Colorimetric Recording of Thermal Conditions on Polymeric Inverse Opals

Recording thermal conditions, i.e., temperature and time, is of great importance for various applications. Although thermometers can measure temperature and record its temporal change with electronic devices, they are nondisposable and not patch‐type, restricting their uses. Here, photonic films are designed that record thermal condition through irreversible structural deformation and intuitively report it with color patterns. The photonic films are inverse opals made of negative photoresist on a solid support, where the cross‐linking density of the photoresist is regioselectively adjusted. The photonic films show a gradual blueshift of structural color upon heating due to anisotropic compression of the inverse opal, of which the rate depends on temperature and cross‐linking density. For single cross‐linking density, thermal input is quantified from the color change in the form of coupled temperature and time. With multiple cross‐linking densities in a single film, the multicolor pattern is developed, from which the temperature and time are decoupled and separately estimated for isothermal condition.     

A Supramolecular Nanocomposite as a Near‐Infrared‐Transmitting Optical Filter for Security and Forensic Applications

Visibly opaque but near‐infrared (NIR)‐transparent materials are an essential component for night‐vision photography, security imaging, and forensic applications. Herein, the development of a novel supramolecular black dye from a diketopyrrolopyrrole (DPP)‐based low‐molecular‐weight organogelator is described. In the solution state, the monomer of DPP–Amide exhibits a deep green color with a broad absorption in the visible region due to firm intramolecular charge transfer from the donor to the acceptor unit. Interestingly, due to the synergistic effect of H‐bonding and π‐stacking, DPP–Amide can form a black organogel in toluene with complete spectral coverage from 300 to 800 nm, and transmits beyond 850 nm. In the gel state, complete visible‐spectrum coverage is achieved due to the simultaneous formation of both H‐ and J‐type aggregates, which is confirmed via absorption studies. To create a free‐standing NIR‐transmitting elastomeric black filter, nanoscopic molecular aggregates of DPP–Amide (0.15 wt%) are embedded into a poly(dimethylsiloxane) matrix. This nanocomposite possesses high NIR transparency with good thermal and photostability for practical applications. Finally, the use of the developed material for NIR photography, security, and forensic‐related applications is demonstrated.     

Nanoengineering of Particle Surfaces

The creation of core–shell particles is attracting a great deal of interest because of the diverse applicability of these colloidal particles; e.g., as building blocks for photonic crystals, in multi‐enzyme biocatalysis, and in drug delivery. This review presents the state‐of‐the‐art in strategies for engineering particle surfaces, such as the layer‐by‐layer deposition process (see Figure), which allows fine control over shell thickness and composition.     

Robust Full-Spectral Color Tuning of Photonic Colloids

Creation of color through photonic morphologies manufactured by molecular self-assembly is a promising approach, but the complexity and lack of robustness of the fabrication processes have limited their technical exploitation. Here, it is shown that photonic spheres with full-color tuning across the entire visible spectrum can be readily and reliably achieved by the emulsification of solutions containing a block copolymer (BCP) and two swelling additives. Solvent diffusion out of the emulsion droplets gives rise to 20–150 µm-sized spheres with an onion-like lamellar morphology. Controlling the lamellar thickness by differential swelling with the two additives enables color tuning of the Bragg interference-based reflection band across the entire visible spectrum. By studying five different systems, a set of important principles for manufacturing photonic colloids is established. Two swelling additives are required, one of which must exhibit strong interactions with one of the BCP blocks. The additives should be chosen to enhance the dielectric contrast, and the formation kinetics of the spheres must be sufficiently slow to enable the emergence of the photonic morphology. The proposed approach is versatile and robust and allows the scalable production of photonic pigments with possible future applications in inks for cosmetics and arts, coatings, and displays.     

Metallosupramolecular Photonic Elastomers with Self‐Healing Capability and Angle‐Independent Color

Photonic elastomers that can change colors like a chameleon have shown great promise in various applications. However, it still remains a challenge to produce artificial photonic elastomers with desired optical and mechanical properties. Here, the generation of metallosupramolecular polymer‐based photonic elastomers with tunable mechanical strength, angle‐independent structural color, and self‐healing capability is reported. The photonic elastomers are prepared by incorporating isotropically arranged monodispersed SiO₂ nanoparticles within a supramolecular elastomeric matrix based on metal coordination interaction between amino‐terminated poly(dimethylsiloxane) and cerium trichloride. The photonic elastomers exhibit angle‐independent structural colors, while Young's modulus and elongation at break of the as‐formed photonic elastomers reach 0.24 MPa and 150%, respectively. The superior elasticity of photonic elastomers enables their chameleon‐skin‐like mechanochromic capability. Moreover, the photonic elastomers are capable of healing scratches or cuts to ensure sustainable optical and mechanical properties, which is crucial to their applications in wearable devices, optical coating, and visualized force sensing.     

Flexible and Responsive Chiral Nematic Cellulose Nanocrystal/Poly(ethylene glycol) Composite Films with Uniform and Tunable Structural Color

The fabrication of responsive photonic structures from cellulose nanocrystals (CNCs) that can operate in the entire visible spectrum is challenging due to the requirements of precise periodic modulation of the pitch size of the self‐assembled multilayer structures at the length scale within the wavelength of the visible light. The surface charge density of CNCs is an important factor in controlling the pitch size of the chiral nematic structure of the dried solid CNC films. The assembly of poly(ethylene glycol) (PEG) together with CNCs into smaller chiral nematic domains results in solid films with uniform helical structure upon slow drying. Large, flexible, and flat photonic composite films with uniform structure colors from blue to red are prepared by changing the composition of CNCs and PEG. The CNC/PEG(80/20) composite film demonstrates a reversible and smooth structural color change between green and transparent in response to an increase and decrease of relative humidity between 50% and 100% owing to the reversible swelling and dehydration of the chiral nematic structure. The composite also shows excellent mechanical and thermal properties, complementing the multifunctional property profile.     

Biomaterial‐Based “Structured Opals” with Programmable Combination of Diffractive Optical Elements and Photonic Bandgap Effects

Naturally occurring iridescent systems produce brilliant color displays through multiscale, hierarchical assembly of structures that combine reflective, diffractive, diffusive, or absorbing domains. The fabrication of biopolymer‐based, hierarchical 3D photonic crystals through the use of a topographical templating strategy that allows combined optical effects derived from the interplay of predesigned 2D and 3D geometries is reported here. This biomaterials‐based approach generates 2D diffractive optics composed of 3D nanophotonic lattices that allow simultaneous control over the reflection (through the 3D photonic bandgap) and the transmission (through 2D diffractive structuring) of light with the additional utility of being constituted by a biocompatible, implantable, edible commodity textile material. The use of biopolymers allows additional degrees of freedom in photonic bandgap design through directed protein conformation modulation. Demonstrator structures are presented to illustrate the lattice multifunctionality, including tunable diffractive properties, increased angle of view of photonic crystals, color‐mixing, and sensing applications.     

Modulation of Multiscale 3D Lattices through Conformational Control: Painting Silk Inverse Opals with Water and Light

Structural proteins from naturally occurring materials are an inspiring template for material design and synthesis at multiple scales. The ability to control the assembly and conformation of such materials offers the opportunity to define fabrication approaches that recapitulate the dimensional hierarchy and structure–function relationships found in nature. A simple and versatile directed assembly method of silk fibroin, which allows the design of structures across multiple dimensional scales by generating and tuning structural color in large‐scale, macro defect‐free colloidally assembled 3D nanostructures in the form of silk inverse opals (SIOs) is reported. This approach effectively combines bottom‐up and top‐down techniques to obtain control on the nanoscale (through silk conformational changes), microscale (through patterning), and macroscale (through colloidal assembly), ultimately resulting in a controllable photonic lattice with predefined spectral behavior, with a resulting palette spanning almost the entire visible range. As a demonstration of the approach, examples of “multispectral” SIOs, paired with theoretical calculations and analysis of their response as a function of changes of lattice constants and refractive index contrast are illustrated.     

Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next‐generation optoelectronics since they can be stacked layer‐by‐layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice‐mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h‐BN/p‐MoTe₂/graphene/n‐SnS₂/h‐BN p–g–n junction, fabricated by a layer‐by‐layer dry transfer, demonstrates high‐sensitivity, broadband photodetection at room temperature. The combination of the MoTe₂ and SnS₂ of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built‐in electric field for high‐efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5?7‐layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W^?1 with fast photoresponse and specific detectivity up to ≈10¹³ Jones in the ultraviolet–visible–near‐infrared spectrum. This result suggests that the vdW p–g–n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh‐sensitivity and broadband photonic detectors.     

Ge2Sb2Te5‐Based Tunable Perfect Absorber Cavity with Phase Singularity at Visible Frequencies

The metal‐dielectric stacks‐based asymmetric Fabry–Perot (F–P) cavity systems have recently attracted much interest from the scientific community for realizing perfect absorption over the spectral bands from visible to infrared since they possess a lithography‐free design that is cost‐effective and scalable. This study experimentally demonstrates an asymmetric F–P cavity system for achieving tunable wide angle perfect absorption and phase singularity. The proposed system shows tunable multiband perfect absorption in the visible spectral region by incorporating an ultrathin layer of phase change material such as Ge₂Sb₂Te₅ (GST) in the stack. The system shows multi‐narrowband perfect absorption with a maximum of 99.8% at a specific incident angle and polarization state when the GST is in amorphous phase; however, the absorption bands blueshift and broaden after switching to the crystalline phase. More importantly, the proposed scheme shows tunable phase singularity at the reflection‐less point. The obtained tunable perfect absorption and abrupt phase change are solely due to the presence of a highly absorbing ultrathin layer of GST in the stack. Experimental results are validated using an analytical simulation model based on a transfer matrix method. The proposed scheme could find potential applications in active photonic devices such as phase‐sensitive biosensors and absorption filters.     

Colloidal photonic crystal pigments with low angle dependence

Poly(methyl methacrylate) (PMMA)-based colloidal photonic crystals have an incomplete photonic band gap (PBG) and typically appear iridescent in the visible range. As powders, synthetic PMMA opals are white, but when infiltrated with carbon black nanoparticles, they exhibit a well-defined color that shows little dependence on the viewing angle. The quantity of black pigment determines the lightness of the color by controlling scattering. The combined effects of internal order within each particle and random orientation among the particles in the powder are responsible for this behavior. These pigments were employed as paints, using a mixture of polyvinyl acetate as a binder and deionized water as the solvent, and were applied to wood and paper surfaces for color analysis.     

Rigorous calculations and fabrication by self-assembly techniques of 2D subwavelength structures of gold for photonic applications

The use of metal 2D subwavelength structures (SWSs) is a promising solution for all those applications where a selective emission from a thermal source is desirable, e.g., photovoltaic and blackbody emission. The investigation of the SWS's photonic bandgap properties is challenging, especially for the infrared and visible spectra, where the fabrication difficulties have always represented an obstacle. In this paper, the anodization of aluminum films as a self-assembly method for the SWS fabrication is proposed. A rigorous calculation of 2D SWSs of gold having high absorptivity in the visible and low absorptivity in the NIR, their fabrication by DC-sputtering deposition through anodic porous alumina templates, and their optical and topographic characterization are presented.     

Tunable Design of Structural Colors Produced by Pseudo‐1D Photonic Crystals of Graphene Oxide

It is broadly observed that graphene oxide (GO) films appear transparent with a thickness of about several nanometers, whereas they appear dark brown or almost black with thickness of more than 1 μm. The basic color mechanism of GO film on a sub‐micrometer scale, however, is not well understood. This study reports on GO pseudo‐1D photonic crystals (p1D‐PhCs) exhibiting tunable structural colors in the visible wavelength range owing to its 1D Bragg nanostructures. Striking structural colors of GO p1D‐PhCs could be tuned by simply changing either the volume or concentration of the aqueous GO dispersion during vacuum filtration. Moreover, the quantitative relationship between thickness and reflection wavelength of GO p1D‐PhCs has been revealed, thereby providing a theoretical basis to rationally design structural colors of GO p1D‐PhCs. The spectral response of GO p1D‐PhCs to humidity is also obtained clearly showing the wavelength shift of GO p1D‐PhCs at differently relative humidity values and thus encouraging the integration of structural color printing and the humidity‐responsive property of GO p1D‐PhCs to develop a visible and fast‐responsive anti‐counterfeiting label. The results pave the way for a variety of potential applications of GO in optics, structural color printing, sensing, and anti‐counterfeiting.     

强散射作用产生的艳丽方向性结构色

通过光与微纳结构相互作用产生颜色的结构生色材料一直是显示、防伪和刺激响应材料领域研究的热点.与光子晶体生色的镜面反射特性相比,基于散射的结构生色具有更宽的可视角,因此受到越来越多的关注.无序结构产生的宽散射光谱往往导致其结构色纯度和亮度降低,而有序结构在可见光区一般具有强的反射和衍射作用,因此很少有研究者关注有序结构的...     

Improving Defect‐Based Quantum Emitters in Silicon Carbide via Inorganic Passivation

Defect‐based color centers in wide‐bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite‐vacancy pairs (CAV centers) in 4H‐SiC, which serve as single‐photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect‐based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers.     

Inkjet Printing of Patterned,Multispectral, and Biocompatible Photonic Crystals

Patterning of photonic crystals to generate rationally designed color‐responsive materials has drawn considerable interest because of promising applications in optical storage, encryption, display, and sensing. Here, an inkjet‐printing based strategy is presented for noncontact, rapid, and direct approaches to generate arbitrarily patterned photonic crystals. The strategy is based on the use of water‐soluble biopolymer‐based opal structures that can be reformed with high resolution through precise deposition of fluids on the photonic crystal lattice. The resulting digitally designed photonic lattice formats simultaneously exploit structural color and material transience opening avenues for information encoding and combining functions of optics, biomaterials, and environmental interfaces in a single device.     

Tetragonal Single‐Crystalline Boron Nanowires with Strong Anisotropic Light Scattering Behaviors and Photocurrent Response in Visible‐Light Regime

Boron is a narrow‐bandgap (1.56 eV) semiconductor with high melting‐point, low‐density, large Young's modulus and very high refractive index (3.03) close to silicon. Therefore, boron nanostructures is expected to possess strong visible‐light scattering properties. However, photonic and optoelectronic properties of the boron nanostructures are seldom studied until now. In this paper, we have successfully prepared single‐crystalline boron nanowire (BNW) arrays with high‐density on Si substrate. All the BNWs are found to possess strong light‐scattering behaviors in the visible regime. Most of all, the scattered light is found to polarize along the longitudinal direction of the nanowire. They also have excellent second‐harmonic generation (SHG) properties under ultrafast laser irradiation. Further optoelectronic measurements show that an individual BNW device exhibits notable photocurrent responses in the visible‐light range at ambient conditions, which can be attributed to the strong coupling effect between individual BNW and the visible light. The maximum photoresponsivity of an individual BNW can reach up to 12.12 A W^–1 at a voltage of 10 V, and the response time is only 18 ms. Therefore, it unveils that the BNWs have a promising future in visible‐light communications and detections.     

Alpha Lead Oxide (α‐PbO): A New 2D Material with Visible Light Sensitivity

Even though transition metal dichalcogenides (TMDCs) are deemed to be novel photonic and optoelectronic 2D materials, the visible band gap being often limited to monolayer, hampers their potential in niche applications due to fabrication challenges. Uncontrollable defects and degraded functionalities at elevated temperature and under extreme environments further restrict their prospects. To address such limitations, the discovery of a new 2D material, α‐PbO is reported. Micromechanical as well as sonochemical exfoliation of 2D atomic sheets of α‐PbO are demonstrated and its optical behavior is investigated. Spectroscopic investigations indicate layer dependent band gaps. In particular, even multilayered PbO sheets exhibit visible band gap > 2 eV (direct) which is rare among semiconducting 2D materials. The emission lifetime of multilayer PbO atomic sheets is 7 ns (dim light) as compared to the monolayer which gives 2.5 ns lifetime and an intense light. Density functional theory calculations of layer dependent band structure of α‐PbO matches well with experimental results. Experimental findings suggest that PbO atomic sheets exhibit hydrophobic nature, thermal robustness, microwave stability, anti‐corrosive behaviour and acid resistance. This new low‐cost, abundant and robust 2D material is expected to find many applications in the fields of electronics, optoelectronics, sensors, photocatalysis and energy storage.     

Pressure‐Responsive Hierarchical Chiral Photonic Aerogels

Pressure‐responsive chiral photonic aerogels are fabricated by combining liquid crystal self‐assembly and ice‐templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure‐induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent‐sensitive ink that shows quick color response to different solvents is designed using the pressure‐responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.     

The Langmuir‐Blodgett Approach to Making Colloidal Photonic Crystals from Silica Spheres

The area of colloidal photonic crystal research has attracted enormous attention in recent years as a result of the potential of such materials to provide the means of fabricating new or improved photonic devices. As an area where chemistry still predominates over engineering the field is still in its infancy in terms of finding real applications being limited by ease of fabrication, reproducibility and ‘quality’‐ for example the extent to which ordered structures may be prepared over large areas. It is our contention that the Langmuir‐Blodgett assembly method when applied to colloidal particles of silica and perhaps other materials, offers a way of overcoming these issues. To this end the assembly of silica and other particles into colloidal photonic crystals using the Langmuir‐Blodgett (LB) method is described and some of the numerous papers on this topic, which have been published, are reviewed. It is shown that the layer‐by‐layer control of photonic crystal growth afforded by the LB method allows for the fabrication of a range of novel, layered photonic crystals that may not be easily assembled using any other approach. Some of the more interesting of these structures, including so‐called heterostructured photonic crystals comprising of layers of spheres having different diameters are presented and their optical properties described. Finally, we offer our comments as to future applications of this interesting technology.