Block Copolymer Photonic Crystals Research Articles

Reconfigurable dual-mode optical encryption enabled by block copolymer photonic crystal with micro-imprinted holographic metasurface

Dual-mode optical encryption based on holographic metasurfaces and color components is of great attraction because of their enhanced information security and storage; however, the realization of independently as well as reversibly encodable holographic metasurfaces and color components remains unreported. Herein, we present reconfigurable dual-mode encryptions of structural colors (SC) and holograms, achieved through stimuli-responsive block copolymer (BCP) photonic crystals (PCs) with micro-imprinted holographic metasurfaces. Holographic images appear when the micro-imprinted BCP PCs, consisting of self-assembled alternating lamellae of two dielectrics, are exposed to an incident laser. A characteristic SC develops in the visible range when the imprinted film is immersed in a liquid agent that can swell one of the dielectrics, allowing for dual-mode holographic and SC encodings in the solid and liquid states, respectively. The dual-mode optical encoding is reconfigured. The holographic image can be erased and replaced with another micropattern, while preserving the SC. Moreover, an SC, set by crosslinking of the swellable lamellae, is reset by chemical de-crosslinking and subsequent transient re-crosslinking, enabling the SC reconfigurability of the BCP PC film. A prototype of a high-security reconfigurable dual encryption has been developed, wherein true information is decrypted when holographic passwords are confirmed with full-color visible SC passwords.

Detection of extremely low- and high-concentration organic acids by a highly-aligned lamellar block copolymer photonic crystal film

Detection of organic acids was carried out using lamella-forming polystyrene-block-poly (4-vinylpyridine) (PS–P4VP) block copolymer (BCP) photonic crystals. PS-P4VP photonic crystal thin films featuring highly-aligned lamellar microstructures parallel to the substrate were prepared through solvent annealing method by chloroform. When the lamella-forming PS-P4VP films were immersed in low-concentration organic acids, including formic acid (FA), acetic acid (AA), and propionic acid (PA), red-shifting reflectance wavelengths could be observed with the increase of the acid concentration. In contrast to the AA and PA systems, the lamella-forming PS-P4VP thin film was able to detect extremely low-concentration FA (5 × 10−5 vol%) due to its strongly protonated ability. When the PS-P4VP thin film was immersed in high-concentration FA and AA solutions, the longer and red-shifting reflectance wavelengths were observed with the increase of the acid concentration. In addition, the unexpected blue-shifting reflectance wavelength was observed when the film was immersed in a high-concentration PA solution. This is attributed to the alternation of segregation strength resulted from the compatibility effect. As a result, well-aligned lamellar PS-P4VP photonic crystal thin films are highly sensitive to the stimuli of organic acids for detecting extremely low- and high-concentration organic acids due to the change of protonated ability and compatibility, respectively.

Self-powered finger motion-sensing structural color display enabled by block copolymer photonic crystal

Self-powered user-interactive displays that facilitate the visualization of human information acquired by sensors are of great interest in emerging human–machine interface technology with efficient energy consumption. Herein, a self-powered motion-sensing display capable of simultaneously detecting and visualizing finger motions is presented. Our device is based on a one-dimensional photonic crystal of an interpenetrated hydrogel network block copolymer (IHN-BCP) consisting of alternating water-absorbable and non-absorbable lamellae. Triboelectrification is achieved as a function of relative humidity from 30% to 80%. The direct visualization of the humidity is also achieved through the humidity-dependent structural color of the photonic crystal in the full visible range. Furthermore, the humidity-responsive triboelectrification and structural color of our IHN-BCP photonic crystal facilitates the development of a self-powered finger motion-sensing display where diverse gestures of a finger with natural humidity are quantitatively recognized, such as vertical and sliding motion of the finger with simultaneous visualization of the motions in both contact and non-contact modes.

Thermo‐Adaptive Block Copolymers: Thermo‐Adaptive Block Copolymer Structural Color Electronics (Adv. Funct. Mater. 11/2021)

In article number 2008548, Seunggun Yu, Cheolmin Park, and co-workers demonstrate thermo-adaptive flexible electronic devices, based on thin self-assembled block copolymer photonic crystal films. The reversible structural color of the thermo-responsive photonic crystal film facilitates the development of a sensing display as well as diagnostic thermal patches, making it a step forward in emerging thermotronics suitable for a variety of human-interactive flexible photo-electronics.

Trapping Structural Coloration by a Bioinspired Gyroid Microstructure in Solid State.

In theory, gyroid photonic crystals in butterfly wings exhibit advanced optical properties as a result of their highly interconnected microstructures. Because of the difficulties in synthesizing artificial gyroid materials having periodicity corresponding to visible wavelengths, human-made visible gyroid photonic crystals are still unachievable by self-assembly. In this study, we develop a physical approach-trapping of structural coloration (TOSC)-through which the visible structural coloration of an expanded gyroid lattice in a solvated state can be preserved in the solid state, thereby allowing the fabrication of visible-wavelength gyroid photonic crystals. Through control over the diffusivity and diffusive distance for solvent evaporation, the single-molecular-weight gyroid block copolymer photonic crystal can exhibit desired structural coloration in the solid state without the need to introduce any additives, namely, evapochromism. Also, greatly enhanced reflectivity is observed arising from the formation of porous gyroid nanochannels, similar to those in butterfly wings. As a result, TOSC facilitates the fabrication of the human-made solid gyroid photonic crystal featuring tunable and switchable structural coloration without the synthesis to alter the molecular weight. It appears to be applicable in the fields of optical communication, energy, light-emission, sensors, and displays.

Self-assembled block copolymer photonic crystal for selective fructose detection

The use of one-dimensional photonic crystals fabricated from a self-assembled lamellar block copolymer as a sensitive and selective fructose sensor is investigated. The polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) films are functionalized with 2-(bromomethyl)phenylboronic acid. The boronic acid moiety confined within the lamellar morphology can reversibly bind to sugars such as fructose, imparting the photonic properties of the PS-b-P2VP film. The films exhibit a detection limit of 500μM in water and 1mM in phosphate buffered saline. Exposure to a 50mM solution of fructose invokes a highly visible color change from blue to orange. The films are also able to selectively recognize and respond to fructose in competitive studies in the presence of glucose, mannose and sucrose.

Colorimetric humidity sensors based on block copolymer photonic gels

The measurement of humidity is demanded in wide range of areas, including agriculture, horticulture, meteorological services, food processing, air-conditioning, and electronic processing. While electronic humidity sensors are commercialized, other types of humidity sensors that can be fabricated thin, light-weight and cheap are still required for many applications. Comparing with their conventional electronic counterparts, colorimetric humidity sensors based on photonic crystals offer specific advantages, such as small size and weight, cheap, and exemption of electromagnetic interference. Photonic crystals, artificial materials with a periodic refractive index, provide remarkable control of light and offer exciting prospects for next-generation optical sensors. Suitably designed photonic crystals respond to environmental changes with a visual color change and can act as a colorimetric sensor, requiring no power source and no read-out system other than the human eye. For example, colloidal crystal arrays (CCAs) have been functionalized with hydrogels for constructing tunable photonic crystals, and their unique responsive optical properties have been utilized for transducing chemical or biological signals directly into optical signals. While simple, this sensing approach is intuitively appealing and is explored as chemical indicators and biological assay techniques. Self-assembly of block copolymers have also been investigated for creating various photonic structures. Their abundant morphologies including lamellae, cylinders, gyroids and spheres were exploited to create 1D, 2D, and 3D photonic crystals. Since the band structures are highly dependent on both the refractive index and the periodicity of the dielectric layers, the photonic band gaps of block copolymer photonic crystals can be further modulated when a domain is swollen by selective solvents. Our group recently reported highly tunable photonic gels based on polystyrene-b-quaternized poly(2-vinyl pyridine) (PS-b-QP2VP). Selective swelling of PS-b-QP2VP block copolymer lamellae leads to increase both the domain spacing and the refractive index contrast between the layers, and which results in the shift of the photonic band position to longer wavelengths. The swollen photonic gels found many applications including as an active component of reflective displays, memory and biosensors. Herein, we report block copolymer photonic gels as colorimetric humidity sensor films where the measurement sensitivity and humidity range can be modulated by simple ion-exchanging or by varying molecular weight of block copolymers. The block copolymer photonic gel films were prepared as previously described. Briefly, in-plane oriented lamellar films were first prepared by spin casting PS57-b-P2VP57 or PS190-b-P2VP190 solution (6 wt% in propylene glycol monomethyl ether acetate, the subscript number represent the number-average molecular weight of each block, Mn×10) onto glass followed by annealing the films in chloroform vapor at 50 C for 2 days. The thickness of the film was controlled to ~1 μm. The P2VP blocks were then quaternized using iodomethane (5 vol% in hexane) at 50 C for 72 h. Finally, the counter anions pairing with pyridinium were exchanged with a variety of anions including iodide, bromide, chloride and acetate by immersing the films into aqueous salt solutions. The optical properties of PS-b-QP2VP block copolymer photonic gel films were measured on the UV-vis spectrometer by reflection mode while continuously varying humidity from 20% to 100%. Humidity was controlled by using the mixtures of ethylene glycol and water. For photonic stop bands (PSB) measurements, photonic gel films were placed in the chamber containing various compositions of ethylene glycol and water, and allowed to equilibrate for 10 min before measurement. Environmental scanning electron microscopy (ESEM) images were obtained a Philips XL30 ESEM. The relative humidity in the vicinity of the specimen was controlled by varying the vacuum level of the specimen chamber in ESEM.

Photoresponsive Block Copolymer Photonic Gels with Widely Tunable Photosensitivity by Counter‐Ions

Block copolymer photonic crystals comprising polyelectrolyte hydrogels exhibit strong reflective multicolors in response to near-UV radiation. Due to unique volume transition of swollen gels, the photonic gels show high photosensitivity, and which can be widely tunable by exchanging counter-anions. Multicolor photonic patterns created by photolithography can be repeatedly fixated and reactivated by sequentially exchanging counter-anions with different photosensitivity.

Electrically tunable block copolymer photonic crystals with a full color display

Electrically tunable full color display photonic crystals (PCs) made of a thin film of block copolymer, polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP), have been developed. The phase separation of PS and P2VP blocks leads to a one-dimensional periodic lamellar structure parallel to the surface of the thin film. In a solvent mixture of water and ethanol, each P2VP layer is swollen and partially charged. Applying a low voltage (<2.5 V) to such a thin film can lead to the repeatable expansion or contraction of each P2VP layer, depending on the direction and strength of the electrical potential. Such a variation of the lamellar periodicity leads to a full color display over a wide range from ultraviolet to near infrared. The advantages of such PCs are their facile preparation, low driving voltage and good durability, which make them potentially useful in information technologies.

Two-dimensional block copolymer photonic crystals

A high molecular weight polystyrene-block-polyisoprene copolymer was synthesized by anionic polymerization of styrene and isoprene monomers. Polystyrene cylindrical domains in a hexagonal lattice with relatively large periodicity and controlled orientation have been produced through roll casting of the polystyrene-block-polyisoprene copolymer with a total molecular weight of 1.0×10 6 g/mol. The large periodicity and effective processing lead to reflectivity of about 70% in the visible regime, and generate a two-dimensional photonic crystal with a partial band gap.

Acoustic excitations in a self-assembled block copolymer photonic crystal.

High resolution Brillouin light scattering can sensitively detect acoustic phonons in concentrated solutions of a high molecular weight poly(styrene-b-isoprene) symmetric copolymer in toluene. This block copolymer lamellar forming system also possesses a photonic stop band in the visible spectrum. Based on the low but finite contrast in mechanical properties between the styrene and isoprene components and taking into account the geometrical characteristics of the layered microstructure, we calculate the acoustic band structure and represent the observed acousticlike and opticlike phonons.

Bicontinuous Cubic Block Copolymer Photonic Crystals

Three‐dimensional photonic crystals are fabricated from block copolymers, forming a double gyroid network structure. Further processing produces three‐dimensionally nanoporous, freestanding materials (see Figure), and allows dielectric enhancement and infiltration, which dramatically improve optical and material properties. These etched materials provide a path to highly effective photonic crystals via self‐assembly.