Tunable Colors in Opals and Inverse Opal Photonic Crystals

Colloidal photonic crystals and materials derived from colloidal crystals can exhibit distinct structural colors that result from incomplete photonic band gaps. Through rational materials design, the colors of such photonic crystals can be tuned reversibly by external physical and chemical stimuli. Such stimuli include solvent and dye infiltration, applied electric or magnetic fields, mechanical deformation, light irradiation, temperature changes, changes in pH, and specific molecular interactions. Reversible color changes result from alterations in lattice spacings, filling fractions, and refractive index of system components. This review article highlights the different systems and mechanisms for achieving tunable color based on opaline materials with close‐packed or non‐close‐packed structural elements and inverse opal photonic crystals. Inorganic and polymeric systems, such as hydrogels, metallopolymers, and elastomers are discussed.     

Rapid and Large‐Scale Fabrication of Full Color Woodpile Photonic Crystals via Interference from a Conformal Multilevel Phase Mask

The practical use of photonic crystals with structural colors requires technology capable of rapidly producing large‐area, three‐dimensional (3D) periodic nanostructures. Until now, the fabrication of 3D photonic crystals has relied mainly on additive manufacturing and colloidal self‐assembly. These technologies have provided a useful academic platform based on precisely controlled 3D periodicity but have not evolved into mass production technology. Here, optical lithography for the rapid fabrication of large‐area 3D photonic crystals with structural colors is introduced. The key strategy is to incorporate two orthogonal line gratings (periodicity: 300 nm) made of an elastomer to create a conformal multilevel phase mask. When the mask is irradiated with a 355 nm laser, the five beam interference is established in the proximity region. The interlayer thickness between the two orthogonal line gratings controls the phase difference, which is closely related to the symmetry of the resulting 3D interference pattern. The interlayer thickness is designed to produce a woodpile structure with a planar periodicity of 300 nm and a vertical periodicity of 716 nm. The pattern area of the woodpile photonic crystal is expanded to 1 in². Red, green, and blue colors are experimentally realized by controlling the vertical shrinkage of the photoresist.     

Structural Color Boosted Electrochromic Devices: Strategies and Applications

Electrochromic devices (ECDs) have attracted substantial attention on account of their unique characteristics and prosperous applications. However, their monotonous color modification remains a momentous challenge that significantly restricts their application scope. Integrating structural colors that arise from the interaction between light and periodic micro/nanostructures with electrochromic materials (ECMs) turns out to be an effective approach to enhancing the comprehensive performance of ECDs more than the coloration effectiveness. This review provides an overview on the recent burgeoning development of structural color ECDs regarding strategies and applications. The performance parameters of ECDs and physical principles regarding the generation of structural colors are introduced. Then, strategies that introduce structural colors into ECDs are discussed according to the category of chromogenic micro/nanostructures, namely, gratings, (single-layer and multilayer) thin films, photonic crystals, plasmonic metasurfaces, and composite structures. The current state-of-the-art research activities of structural color ECDs for display application including segmented and pixelated devices are also presented. Additionally, the future perspective of structural color ECDs with respect of major challenges and potential solutions is proposed.     

Macroporous Hydrogels for Fast and Reversible Switching between Transparent and Structurally Colored States

Colloidal crystals have been used for creating stimuli‐responsive photonic materials. Here, macroporous hydrogels are designed, through a simple and reproducible protocol, that rapidly and reversibly switch between highly transparent and structurally colored states. The macroporous hydrogels are prepared by film‐casting photocurable dispersions of silica particles in hydrogel‐forming resins and selectively removing silica particles. The silica particles spontaneously form a nonclose‐packed array due to repulsive interparticle interaction, which form the regular array of cavities after removal. However, the cavities are randomly collapsed by drying, losing a long‐range order and rendering the materials highly transparent. When the hydrogels are swollen by either water, ethanol, or the mixture, the regular array is restored, which develops brilliant structural colors. This switching is completed in tens of seconds and repeatable without any hysteresis. The resonant wavelength depends on the composition of the water–ethanol mixture, where the dramatic shift occurs in one‐component‐rich mixtures due to the composition of the hydrogel. Micropatterns can be designed to have distinct domains of the macroporous hydrogels, which are transparent at the dried state and disclose encrypted graphics and unique reflectance spectra at the wet state. This class of solvent‐responsive photonic hydrogels is potentially useful for alcohol sensors and user‐interactive anti‐counterfeiting materials.     

Extreme Optical Properties Tuned Through Phase Substitution in a Structurally Optimized Biological Photonic Polycrystal

Biological photonic structures evolved by insects provide inspiring examples for the design and fabrication of synthetic photonic crystals. The small scales covering the beetle Entimus imperialis are subdivided into irregularly shaped domains that mostly show striking colors, yet some appear colorless. The colors originate from photonic crystals consisting of cuticular material and air, which are geometrically separated by a triply periodic D‐surface (diamond). The structure and orientation of the photonic crystals are charactized and it is shown that in colorless domains SiO₂ substitutes the air. The experimental results are incorporated into a precise D‐surface structure model used to simulate the photonic band structure. The study shows that the structural parameters in colored domains are optimized for maximum reflectivity by maximizing the stop gap width. The colorless domains provide a biological example of how the optical appearance changes through alteration of the refractive index contrast between the constituting phases.     

Fluorescent Photochromic α-Cyanodiarylethene Molecular Switches: An Emerging and Promising Class of Functional Diarylethene

Fluorescent photochromic molecules that exhibit distinct light-triggered changes in their emission colors are highly desirable for the fabrication of smart soft materials and advanced photonic devices. α-Cyanodiarylethenes, that is, α-cyano-functionalized diarylethenes, as alternative “non-azo” Z/E photochromic molecular switches, are popular choices due to their unique characteristics such as their aggregation-induced emission or aggregation-induced-enhanced emission behavior in their self-assembled states, and visible changes in fluorescence colors during Z/E photoisomerization. In recent years, the development of fluorescent photochromic α-cyanodiarylethene-based compounds including α-cyanostilbenes, dicyanodistyrylbenzenes, and diaryldicyanoethenes, has mainly focused on molecular design, photochemical and photophysical behavior in solution, and smart soft matter technologies. In this review, recent significant achievements in light-responsive systems based on the Z/E photoisomerization of fluorescent photochromic α-cyanodiarylethene switches that span the range from liquid crystals to gels and finally to self-assembled nanostructures, are highlighted. The smart soft materials constructed from α-cyanodiarylethene molecular switches find use in a plethora of areas, including display, sensing, encrypting, actuating, and biomedical imaging applications, among others. The review concludes with a brief perspective on some major challenges and opportunities for the future development of light-responsive smart soft photonic materials.     

Beyond Seashells: Bioinspired 2D Photonic and Photoelectronic Devices

The discovery of novel materials that possess extraordinary optical properties are of special interest, as they inspire systems for next‐generation solar energy harvesting and conversion devices. Learning from nature has inspired the development of many photonic nanomaterials with fascinating structural colors. 2D photonic nanostructures, inspired by the attractive optical properties found on the inner surfaces of seashells, are fabricated in a facile and scalable way. The shells generate shining clusters for preying on phototactic creatures through interaction with incident solar light in water. By alternately depositing graphene and 2D ultrathin TiO₂ nanosheets to form 2D–2D heterostructures and homostructures, seashell‐inspired nanomaterials with well‐controlled parameters are successfully achieved. They exhibit exceptional interlayer charge transfer properties and ultrafast in‐plane electron mobility and present fascinating nacre‐mimicking optical properties and significantly enhanced light‐response behavior when acting as photoelectrodes. A window into the fabrication of novel 2D photonic structures and devices is opened, paving the way for the design of high‐performance solar‐energy harvesting and conversion devices.     

Simple Holographic Patterning for High‐Aspect‐Ratio Three‐Dimensional Nanostructures with Large Coverage Area

Using the vertical standing wave phenomena commonly regarded as a deterrent in holographic lithography, multifaceted three‐dimensional (3D) nanostructures are fabricated on polymeric photoresist materials using a simple two‐beam interferometer. Large‐area 3D nanostructures with high aspect ratios (greater than 10) are readily produced using this methodology, including grating, pillar and pore patterns. Furthermore, manipulation of the lithography process conditions results in unique sidewall profiles of the nanostructures. Such 3D holographic control even produces highly porous polymer membranes composed of 3D interconnected pore networks, which resembles the 3D photonic crystal compound nanostructures that were previously attainable only with limited pattern coverage area using complex multibeam holographic lithography processes. Such well‐tailored high‐aspect‐ratio 3D nanostructures with large pattern coverage area further enable the fabrication of novel nanostructures for functionalized materials via various additive and subtractive pattern transfer techniques such as etching, deposition, and molding. In particular, direct molding followed by thermal decomposition process leads to the synthesis of hierarchical titanium oxide nanostructures of tunable 3D geometry, which would be of great significance in applications of photonic crystals, photovoltaic solar cells, and photocatalyst in water decontamination.     

Direct Growth of Vertically Orientated Nanocavity Arrays for Plasmonic Color Generation

Plasmonic structural colors, arising from resonance interactions between photons and metallic nanostructures, have been developed rapidly for high‐end applications. However, common structural color materials and fabrication methods usually have open plasmonic nanostructures and limited scalability, respectively. Here, a new scheme based on Ag nanowire arrays/SiO₂ composite metamaterial films with subwavelength enclosed nanostructures involved that combine a dielectric gap layer and a metal mirror is presented. The whole stacked structure can be simply prepared only via magnetron sputtering without any other procedures. Specifically, by changing deposition parameters, the geometry size and sub‐10 nm periodic parameters of the structure unit cell array can be finely tuned in a controllable and reproducible way. By experiments and simulations, it is demonstrated how interwire coupled plasmonic transverse modes in vertically orientated nanocavity arrays control multiple nanocavity standing‐wave resonances at visible wavelengths, generating three primary colors‐included bright and saturated colors across a wide gamut. Large‐area and uniform structural colors, whether on rigid or flexible substrates, show angle‐insensitive and air‐stable features. In a wider perspective, this work suggests that the material scheme and fabrication advances represent a robust platform for plasmonic color designing, theory exploring, and large‐scale manufacturing.     

Designing Multicolor Micropatterns of Inverse Opals with Photonic Bandgap and Surface Plasmon Resonance

Structural coloration provides unique features over chemical coloration, such as nonfading, color tunability, and high color brightness, rendering it useful in various optical applications. To develop the structural colors, two different mechanisms of coloration–photonic bandgap (PBG) and surface plasmon resonance (SPR)–have been separately utilized. In this work, a new method is suggested to create structurally colored micropatterns by regioselectively employing SPR in a single film of inverse opal with PBG. The inverse opals are prepared by thermal embedding of opal into a negative photoresist and its subsequent removal. The inverse opals have a hexagonal array of open pores on the surface which serves as a template to make SPR‐active nanostructures through a directional deposition of gold, a perforated gold film and an array of curved gold disks are formed. With a shadow mask lithographically prepared, the gold is regioselectively deposited on the surface of the inverse opal, which results in two distinct regions of gold‐free inverse opal with PBG and gold nanostructure with SPR. As PBG and SPR develop their own structural colors respectively, the resultant micropatterns exhibit pronounced dual colors. More importantly, the micropatterns show the distinguished optical response for evaporation of volatile liquids that occupy the pores.     

Highly Brilliant Noniridescent Structural Colors Enabled by Graphene Nanosheets Containing Graphene Quantum Dots

The most important properties of noniridescent structural colors of amorphous photonic structures (APS) are sufficient color brightness and saturation, which are difficult to be optimized simultaneously. Herein, highly saturated and brilliant noniridescent structural colors are achieved by introducing graphene nanosheets, which contain a fraction of graphene quantum dots (GQDs), into the short‐range ordered APS. The effective modulation of the photoluminescence (PL) of GQDs by the selective enhancement of absorption at the blue pseudo photonic bandgap edges of the APS boosts the PL with wavelength matching that of the photonic bandgap and thus enables high structural color brightness; the uniform light absorption of graphene nanosheets in the whole visible spectra contributes to the high color saturation. Furthermore, by using APS films with short‐range order as templates, a brilliant colorful humidity sensor is demonstrated. Compared with the conventional sensing platform based on photonic crystals, the humidity sensor with brilliant noniridescent structural colors is more convenient by avoiding the confusing color dependence on the viewing angles. The improvement in the structural color brightness of the APS films by facile graphene doping will facilitate their practical applications in fields of decorations, packaging, pigments, sensors, displays, or other color‐related areas.     

Spray Synthesis of Photonic Crystal Based Automotive Coatings with Bright and Angular-Dependent Structural Colors

It is a challenge to prepare photonic crystal (PC) automotive coatings via spraying colloidal solutions because the fast fabrication tends to produce amorphous photonic crystals with faint colors. Here, a two-step spraying process followed by thermal curing is developed to prepare waterproof PC coatings with bright, uniform, and angular-dependent structural colors. The solvents in PC paint are studied to achieve a quick formation of liquid PC intermediate, which transformed into a highly crystalline PC coating. Thanks to the narrow and intense reflections, the as-made coatings present colors with high saturation, including the red/yellow/green/blue colors achieved by tuning the particle size, and many spectral/non-spectral colors via the mixing of base colors. Meanwhile, patterned or fully covered PC coatings with good chemical and mechanical stability can be prepared on different substrates, where the shiny colors changing with the viewing angles or the surface curvatures offer a new choice for personalized automotive coatings.     

Light‐Induced Color Change in the Sapphirinid Copepods: Tunable Photonic Crystals

Light‐induced tunable photonic systems are rare in nature, and generally beyond the state‐of‐the‐art in artificial systems. Sapphirinid male copepods produce some of the most spectacular colors in nature. The male coloration, used for communication purposes, is structural and is produced from ordered layers of guanine crystals separated by cytoplasm. It is generally accepted that the colors of the males are related to their location in the epipelagic zone. By combining correlative reflectance and cryoelectron microscopy image analyses, together with optical time lapse recording and transfer matrix modeling, it is shown that male sapphirinids have the remarkable ability to change their reflectance spectrum in response to changes in the light conditions. It is also shown that this color change is achieved by a change in the thickness of the cytoplasm layers that separate the guanine crystals. This change is reversible, and is both intensity and wavelength dependent. This capability provides the male with the ability to efficiently reflect light under certain conditions, while remaining transparent and hence camouflaged under other conditions. These copepods can thus provide inspiration for producing synthetic tunable photonic arrays.     

Biopolymer Templated Glass with a Twist: Controlling the Chirality,Porosity, and Photonic Properties of Silica with Cellulose Nanocrystals

A detailed investigation of the formation and properties of mesoporous silica templated by the chiral nematic liquid crystal phase of cellulose nanocrystals (CNCs) is presented. Under appropriate conditions, CNCs co‐assemble with silica up to loadings of ≈60 wt% to give composite films with periodic chiral nanostructures. The periodicity of these films can be readily controlled to obtain materials that selectively reflect light with wavelengths ranging from ≈400–1400 nm. The co‐assembly of CNCs and silica into ordered chiral nematic structures is demonstrated to occur within a narrow window of pH and is affected by aging: a slow rate of silica condensation appears to be vital for the formation of well‐ordered materials. CNCs can be removed from the composite films by calcination or acid hydrolysis to give high surface area chiral nematic mesoporous silica (CNMS) with tunable pore diameters. The combination of mesoporosity and chiral nematic ordering in CNMS enables it to be used in a unique way for refractometric sensing applications. It is shown that, when using circular dichroism (CD) signals to monitor the chiral photonic properties of CNMS, variations in refractive index can be detected based on changes of both CD signal intensity and peak position with good sensitivity.     

Constrained Order in Nanoporous Alumina with High Aspect Ratio: Smart Combination of Interference Lithography and Hard Anodization

With only two matched processing steps, the fabrication of thick nanoporous alumina membranes with mono‐oriented, perfect hexagonal packing of pores, and precise control of all structural parameters over large areas is demonstrated. The cylindrical pores are uniform in shape and widely tunable in their dimensions and spatial distribution, with aspect ratios as high as 500. In brief, electropolished aluminum is first patterned using three‐beam interference lithography in a single step and then anodized in a hard regime. The periodic concavities in the aluminum surface guide the pore nucleation, and the self‐ordering phenomenon guarantees the maintenance of the predefined arrangement throughout the entire layer. In contrast to other methods, the interpore distance can be easily adjusted, the porous layer is not limited in thickness, no prefabricated stamps are involved, and the periodic pattern can be easily reproduced without risk of degradation. The approach overcomes the time, cost, and scale limitations of other existing processes. These membranes are well‐suited for the templated fabrication of perfectly ordered arrays of highly uniform 1D nanostructures. Thus, the application fields of these functional membranes are diverse: magneto‐optical and opto‐electronic devices, photonic crystals, solar cells, fuel cells, and chemical and biochemical sensing systems, to name a few.     

Thermo-Adaptive Block Copolymer Structural Color Electronics

Flexible electronics that enable the visualization of thermal energy have significant potential for various applications, such as thermal diagnosis, sensing and imaging, and displays. Thermo-adaptive flexible electronic devices based on thin 1D block copolymer (BCP) photonic crystal (PC) films with self-assembled periodic nanostructures are presented. By employing a thermo-responsive polymer/non-volatile hygroscopic ionic liquid (IL) blend on a BCP film, full visible structural colors (SCs) are developed because of the temperature-dependent expansion and contraction of one BCP domain via IL injection and release, respectively, as a function of temperature. Reversible SC control of the bi-layered BCP/IL polymer blend film from room temperature to 80 °C facilitates the development of various thermo-adaptive SC flexible electronic devices including pixel arrays of reflective-mode displays and capacitive sensing display. A flexible diagnostic thermal patch is demonstrated with the bi-layered BCP/IL polymer blend enabling the visualization of local heat sources from the human body to microelectronic circuits.     

Designing Multicolored Photonic Micropatterns through the Regioselective Thermal Compression of Inverse Opals

Colloidal assemblies develop pronounced structural colors due to the selective diffraction of light. Micropatterns with multiple structural colors are appealing for the use in a variety of photonic applications. Here, a lithographic approach is reported, which provides a high level of control over the size, shape, and color of a micropattern using the anisotropic shrinkage of inverse opals made of a negative photoresist heated to high temperatures. Shrinkage occurs uniformly across the thickness of the film, leading to a blueshift in the structural color while maintaining a high reflectivity across the full visible spectrum. The rate of shrinkage is determined by the annealing temperature and the photoresist crosslinking density. The rate can, therefore, be spatially modulated by applying UV radiation through a photomask to create multicolor micropatterns from single‐colored inverse opals. The lateral dimensions of the micropattern features can be as small as the thickness of the inverse opal.     

Prospects for the pulsed electrodeposition of zinc-oxide hierarchical nanostructures

Studies into the effect of the conditions of pulsed electrodeposition upon the structural and sub-structural parameters, morphology, and optical properties of ZnO-crystallite arrays make it possible to establish those parameters optimal for the formation of ZnO nanorods oriented normally to the substrate surface. These parameters are as follows: an electrolyte temperature of 70–85°C, duty cycle of 40%, and a pulse-repetition frequency of 2 Hz. The nanorod dimensions can be varied by heating or cooling the electrolyte within the above-indicated limits; as a result, small-sized nanorods can be electrically deposited on the surface of larger nanorods to form hierarchical nanostructures. By varying the duty cycle, it is possible to modify the surface morphology of the arrays up to the formation of mesoporous ZnO networks. In combination with ZnO nanorods, such networks are capable of forming hierarchical nanostructures with large specific areas.     

Aerosol‐Synthesis of Mesoporous Organosilica Nanoparticles with Highly Reactive,Superacidic Surfaces Comprising Sulfonic Acid Entities

Combining high internal surface area with tailor‐made surface properties is pivotal for granting advanced functional properties in many areas like heterogeneous catalysis, electrode materials, membranes, or also biomimetics. In this respect, organic‐inorganic hybrid nanostructures and in particular mesoporous organosilica materials are ideal systems. Here, the preparation of mesoporous solids via a new sol–gel building block comprising sulfonic acid (R‐SO₃H) is described. The degree of organic modification is not only maximal (100%), it is also proven that the novel material exhibits superacid properties. Furthermore, an aerosol assisted method is applied for generating this material in the form of mesoporous, spherical nanoparticles with substantial colloidal stability. Highly acidic, high surface area materials, like prepared here, are promising candidates for numerous future applications like in heterogeneous catalysis or for proton conducting membranes. However, first experiments addressing the antibacterial effect of the sulfonic‐acid, mesoporous organosilica materials are shown. It is demonstrated that the superacid character is required for exhibiting sufficient antifouling activity.     

Broadband photonic structures for quantum light sources

Quantum light sources serve as one of the key elements in quantum photonic technologies. Such sources made from semiconductor material, e.g., quantum dots (QDs), are particularly appealing because of their great potential of scalability enabled by the modern planar nanofabrication technologies. So far, non-classic light sources based on semiconductor QDs are currently outperforming their counterparts using nonlinear optical process, for instance, parametric down conversion and four-wave mixing. To fully exploring the potential of semiconductor QDs, it is highly desirable to integrate QDs with a variety of photonic nanostructures for better device performance due to the improved light-matter interaction. Among different designs, the photonic nanostructures exhibiting broad operation spectral range is particularly interesting to overcome the QD spectral inhomogeneity and exciton fine structure splitting for the generations of single-photon and entangled photon pair respectively. In this review, we focus on recent progress on high-performance semiconductor quantum light sources that is achieved by integrating single QDs with a variety of broadband photonic nanostructures i.e. waveguide, lens and low-Q cavity.