Light‐Patterned Crystallographic Direction of a Self‐Organized 3D Soft Photonic Crystal

Uniform and patterned orientation of a crystallographic direction of ordered materials is of fundamental significance and of great interest for electronic and photonic applications. However, such orientation control is generally complicated and challenging with regard to inorganic and organic crystalline materials due to the occurrence of uncontrollable dislocations or defects. Achieving uniform lattice orientation in frustrated liquid‐crystalline phases, like cubic blue phases, is a formidable task. Taming and tailoring the ordering of such soft, cubic lattices along predetermined or desired directions, and even imparting a prescribed pattern on lattice orientation, are more challenging, due to the entropy‐domination attribute of soft matter. Herein, we disclose a facile way to realize designed micropatterning of a crystallographic direction of a soft, cubic liquid‐crystal superstructure, exhibiting an alternate uniform and random orientation of the lattice crystallographic direction enabled by a photoalignment technique. Because of the rewritable trait of the photoalignment film, the pattern can be erased and rewritten on‐demand by light. Such an oriented soft lattice sensitively responds to various external stimuli such as temperature, electric field, and light irradiation. Furthermore, advanced reflective photonic applications are achieved based on the patterned crystallographic orientation of the cubic blue phase, soft lattice.     

Visible‐Light‐Induced Self‐Organized Helical Superstructure in Orientationally Ordered Fluids

Light‐induced phenomena occurring in nature and in synthetic materials are fascinating and have been exploited for technological applications. Here visible‐light‐induced formation of a helical superstructure is reported, i.e., a cholesteric liquid crystal phase, in orientationally ordered fluids, i.e., nematic liquid crystals, enabled by a visible‐light‐driven chiral molecular switch. The cyclic‐azobenzene‐based chiral molecular switch exhibits reversible photoisomerization in response to visible light of different wavelengths due to the band separation of n–π* transitions of its trans‐ and cis‐isomers. Green light (530 nm) drives the trans‐to‐cis photoisomerization whereas the cis‐to‐trans isomerization process of the chiral molecular switch can be caused by blue light (440 nm). It is observed that the helical twisting power of this chiral molecular switch increases upon irradiation with green light, which enables reversible induction of helical superstructure in nematic liquid crystals containing a very small quantity of the molecular switch. The occurrence of the light‐induced helical superstructure enables the formation of diffraction gratings in cholesteric films.     

Light‐Driven Reversible Transformation between Self‐Organized Simple Cubic Lattice and Helical Superstructure Enabled by a Molecular Switch Functionalized Nanocage

Self‐organized stimuli‐responsive smart materials with adjustable attributes are highly desirable for a plethora of device applications. Simple cubic lattice is quite uncommon in soft condensed matter due to its lower packing factor. Achieving a stable simple cubic soft lattice and endowing such a lattice with dynamic reconstruction capability solely by a facile light irradiation are of paramount significance for both fundamental studies and engineering explorations. Herein, an elegant stable self‐organized simple cubic soft lattice, i.e., blue phase II, in a chiral liquid crystal (LC) system is disclosed, which is stable down to room temperature and exhibits both reversible lattice deformation and transformation to a helical superstructure, i.e., cholesteric LC, by light stimulation. Such an amazing trait is attained by doping a judiciously designed achiral photoresponsive molecular switch functionalized polyhedral oligomeric silsesquioxane nanocage into a chiral LC host. An unprecedented reversible collapse and reconstruction of such a high symmetric simple cubic blue phase II driven by light has been achieved. Furthermore, a well‐defined conglomerate micropattern composed of simple cubic soft lattice and helical superstructure, which is challenging to fabricate in organic and inorganic crystalline materials, is produced using photomasking technology. Moreover, the promising photonic application based on such a micropattern is demonstrated.     

Photoalignment and Surface‐Relief‐Grating Formation are Efficiently Combined in Low‐Molecular‐Weight Halogen‐Bonded Complexes

It is demonstrated that halogen bonding can be used to construct low‐molecular‐weight supramolecular complexes with unique light‐responsive properties. In particular, halogen bonding drives the formation of a photoresponsive liquid‐crystalline complex between a non‐mesogenic halogen bond‐donor molecule incorporating an azo group, and a non‐mesogenic alkoxystilbazole moiety, acting as a halogen bond‐acceptor. Upon irradiation with polarized light, the complex exhibits a high degree of photoinduced anisotropy (order parameter of molecular alignment > 0.5). Moreover, efficient photoinduced surface‐relief‐grating (SRG) formation occurs upon irradiation with a light interference pattern, with a surface‐modulation depth 2.4 times the initial film thickness. This is the first report on a halogen‐bonded photoresponsive low‐molecular‐weight complex, which furthermore combines a high degree of photoalignment and extremely efficient SRG formation in a unique way. This study highlights the potential of halogen bonding as a new tool for the rational design of high‐performance photoresponsive suprastructures.     

Defect‐Free,Highly Uniform Washable Transparent Electrodes Induced by Selective Light Irradiation

A simple route to fabricate defect‐free Ag‐nanoparticle–carbon‐nanotube composite‐based high‐resolution mesh flexible transparent conducting electrodes (FTCEs) is explored. In the selective photonic sintering‐based patterning process, a highly soft rubber or thin plastic substrate is utilized to achieve close and uniform contact between the composite layer and photomask, with which uniform light irradiation can be obtained with diminished light diffraction. This well‐controlled process results in developing a fine and uniform mesh pattern (≈12 μm). The mesh patternability is confirmed to be dependent on heat distribution in the selectively light‐irradiated film and the pattern design for FTCE could be adopted for more precise patterns with desired performance. Moreover, using a very thin substrate could allow the mesh to be positioned closer to the strain‐free neutral mechanical plane. Due to strong interfacial adhesion between the mesh pattern and substrate, the mesh FTCE could tolerate severe mechanical deformation without performance degradation. It is demonstrated that a transparent heater with fine mesh patterns on thin substrate can maintain stability after 100 repeated washing test cycles in which a variety of stress situations occurring in combination. The presented highly durable FTCE and simple fabrication processes may be widely adoptable for various flexible, large‐area, and wearable optoelectronic devices.     

Organohalide Lead Perovskites: More Stable than Glass under Gamma‐Ray Radiation

Organohalide metal perovskites have emerged as promising semiconductor materials for use as space solar cells and radiation detectors. However, there is a lack of study of their stability under operational conditions. Here a stability study of perovskite solar cells under gamma‐rays and visible light simultaneously is reported. The perovskite active layers are shown to retain 96.8% of their initial power conversion efficiency under continuous irradiation of gamma‐rays and light for 1535 h, where gamma‐rays have an accumulated dose of 2.3 Mrad. In striking contrast, a glass substrate shows obvious loss of transmittance under the same irradiation conditions. The excellent stability of the perovskite solar cells benefits from the self‐healing behavior to recover its efficiency loss from the early degradation induced by gamma‐ray irradiation. Defect density characterization reveals that gamma‐ray irradiation does not induce electronic trap states. These observations demonstrate the prospects of perovskite materials in applications of radiation detectors and space solar cells.     

Dynamic Control of Light Direction Enabled by Stimuli‐Responsive Liquid Crystal Gratings

The ability to control light direction with tailored precision via facile means is long‐desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next‐generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli‐controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.     

A Fluid Liquid‐Crystal Material with Highly Polar Order

An anomalously large dielectric permittivity of ≈10⁴ is found in the mesophase temperature range (MP phase) wherein high fluidity is observed for a liquid‐crystal compound having a 1,3‐dioxane unit in the mesogenic core (DIO). In this temperature range, no sharp X‐ray diffraction peak is observed at both small and wide Bragg angles, similar to that for a nematic phase; however, an inhomogeneous sandy texture or broken Schlieren one is observed via polarizing optical microscopy, unlike that for a conventional nematic phase. DIO exhibits polarization switching with a large polarization value, i.e., P = 4.4 µC cm^?2, and a parallelogram‐shaped polarization–electric field hysteresis loop in the MP phase. The inhomogeneously aligned DIO in the absence of an electric field adopts a uniform orientation along an applied electric field when field‐induced polarization switching occurs. Furthermore, sufficiently larger second‐harmonic generation is observed for DIO in the MP phase. Second‐harmonic‐generation interferometry clearly shows that the sense of polarization is inverted when the +/? sign of the applied electric field in MP is reversed. These results suggest that a unidirectional, ferroelectric‐like parallel polar arrangement of the molecules is generated along the director in the MP phase.     

Facile preparation of Ag3PO4 rhombic dodecahedron microcrystals with enhanced catalytic activities under visible light irradiation

Rhombic dodecahedron shaped Ag₃PO₄ microcrystals were prepared by hydrothermal treatment of as-precipitated Ag₃PO₄ products. The phase formation and morphology of the synthesized products were characterized by powder X-ray diffraction and field emission scanning electron microscopy. Experimental results revealed that hydrothermal reaction time can effectively influence the formation of rhombic dodecahedron-shaped Ag₃PO₄ microcrystals. The possible formation mechanism for rhombic dodecahedrons morphology was studied. Moreover, the rhombic dodecahedron-shaped Ag₃PO₄ microcrystals such as 48 h hydrothermally treated sample exhibited higher catalytic activity than as-precipitated and 12 h hydrothermally treated samples under visible light irradiation for the degradation of methylene blue. The enhanced photocatalytic activities of rhombic dodecahedron-shaped Ag₃PO₄ microcrystals is attributed to the existence of most exposed {110} facets.     

Type‐I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects

Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type‐II band alignment and interlayer exciton. However, the studies of type‐I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX₂ (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer‐dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type‐I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX₂ homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type‐I homojunction based 2D semiconductor materials.     

Synergies of Electrochemical Metallization and Valance Change in All‐Inorganic Perovskite Quantum Dots for Resistive Switching

The in‐depth understanding of ions' generation and movement inside all‐inorganic perovskite quantum dots (CsPbBr₃ QDs), which may lead to a paradigm to break through the conventional von Neumann bottleneck, is strictly limited. Here, it is shown that formation and annihilation of metal conductive filaments and Br^? ion vacancy filaments driven by an external electric field and light irradiation can lead to pronounced resistive‐switching effects. Verified by field‐emission scanning electron microscopy as well as energy‐dispersive X‐ray spectroscopy analysis, the resistive switching behavior of CsPbBr₃ QD‐based photonic resistive random‐access memory (RRAM) is initiated by the electrochemical metallization and valance change. By coupling CsPbBr₃ QD‐based RRAM with a p‐channel transistor, the novel application of an RRAM–gate field‐effect transistor presenting analogous functions of flash memory is further demonstrated. These results may accelerate the technological deployment of all‐inorganic perovskite QD‐based photonic resistive memory for successful logic application.     

Effective Light Directed Assembly of Building Blocks with Microscale Control

Light‐directed forces have been widely used to pattern micro/nanoscale objects with precise control, forming functional assemblies. However, a substantial laser intensity is required to generate sufficient optical gradient forces to move a small object in a certain direction, causing limited throughput for applications. A high‐throughput light‐directed assembly is demonstrated as a printing technology by introducing gold nanorods to induce thermal convection flows that move microparticles (diameter = 40 µm to several hundreds of micrometers) to specific light‐guided locations, forming desired patterns. With the advantage of effective light‐directed assembly, the microfluidic‐fabricated monodispersed biocompatible microparticles are used as building blocks to construct a structured assembly (≈10 cm scale) in ≈2 min. The control with microscale precision is approached by changing the size of the laser light spot. After crosslinking assembly of building blocks, a novel soft material with wanted pattern is approached. To demonstrate its application, the mesenchymal stem‐cell‐seeded hydrogel microparticles are prepared as functional building blocks to construct scaffold‐free tissues with desired structures. This light‐directed fabrication method can be applied to integrate different building units, enabling the bottom‐up formation of materials with precise control over their internal structure for bioprinting, tissue engineering, and advanced manufacturing.     

M‐shaped Grating by Nanoimprinting: A Replicable,Large‐Area,Highly Active Plasmonic Surface‐Enhanced Raman Scattering Substrate with Nanogaps

Plasmonic nanostructures separated by nanogaps enable strong electromagnetic‐field confinement on the nanoscale for enhancing light‐matter interactions, which are in great demand in many applications such as surface‐enhanced Raman scattering (SERS). A simple M‐shaped nanograting with narrow V‐shaped grooves is proposed. Both theoretical and experimental studies reveal that the electromagnetic field on the surface of the M grating can be pronouncedly enhanced over that of a grating without such grooves, due to field localization in the nanogaps formed by the narrow V grooves. A technique based on room‐temperature nanoimprinting lithography and anisotropic reactive‐ion etching is developed to fabricate this device, which is cost‐effective, reliable, and suitable for fabricating large‐area nanostructures. As a demonstration of the potential application of this device, the M grating is used as a SERS substrate for probing Rhodamine 6G molecules. Experimentally, an average SERS enhancement factor as high as 5×10⁸ has been achieved, which verifies the greatly enhanced light–matter interaction on the surface of the M grating over that of traditional SERS surfaces.     

Electron irradiation damage in TiC

Electron irradiation experiments were carried out on (1 0 0) TiC_0.93 thin foils inside an electron microscope operated at 100 kV at room temperature. Both graphite rings and extra diffraction spots were observed as irradiation continued. The extra diffraction spots correspond to an ordered cubic superstructure having a composition near TiC_0.5, as determined by Goretzki. These extra spots were disrupted with continued irradiation. These results are discussed in the light of displacement damage in TiC.     

Mastering the Photothermal Effect in Liquid Crystal Networks: A General Approach for Self‐Sustained Mechanical Oscillators

Chemical networks and molecular switches dominate the area of research geared toward macroscopic motion of materials. A counter‐intuitive approach to create self‐sustained oscillation by light irradiation of ordinary photostabilizers in splay‐aligned liquid‐crystalline networks made from commercial mesogens is developed. Photostabilizers or any molecules that are able to quickly dissipate the absorbed light through heat, by vibrational and/or rotational modes, can reach self‐oscillating macroscopic motion where self‐shadowing plays a critical role. The mechanical self‐oscillation is linked to temperature oscillations and the asymmetric response over the film thickness. Only a localized responsive zone, acting as hinge, activates the oscillation of a beam‐shaped device. The outcome of this research is extended from UV to near‐IR actuation, making bulk applications to convert sunlight into mechanical work within reach.     

Compartmentalized Jet Polymerization as a High‐Resolution Process to Continuously Produce Anisometric Microgel Rods with Adjustable Size and Stiffness

In the past decade, anisometric rod‐shaped microgels have attracted growing interest in the materials‐design and tissue‐engineering communities. Rod‐shaped microgels exhibit outstanding potential as versatile building blocks for 3D hydrogels, where they introduce macroscopic anisometry, porosity, or functionality for structural guidance in biomaterials. Various fabrication methods have been established to produce such shape‐controlled elements. However, continuous high‐throughput production of rod‐shaped microgels with simultaneous control over stiffness, size, and aspect ratio still presents a major challenge. A novel microfluidic setup is presented for the continuous production of rod‐shaped microgels from microfluidic plug flow and jets. This system overcomes the current limitations of established production methods for rod‐shaped microgels. Here, an on‐chip gelation setup enables fabrication of soft microgel rods with high aspect ratios, tunable stiffness, and diameters significantly smaller than the channel diameter. This is realized by exposing jets of a microgel precursor to a high intensity light source, operated at specific pulse sequences and frequencies to induce ultra‐fast photopolymerization, while a change in flow rates or pulse duration enables variation of the aspect ratio. The microgels can assemble into 3D structures and function as support for cell culture and tissue engineering.     

Light‐Driven ZnO Brush‐Shaped Self‐Propelled Micromachines for Nitroaromatic Explosives Decomposition

Self‐propelled micromachines have recently attracted lots of attention for environmental remediation. Developing a large‐scale but template‐free fabrication of self‐propelled rod/tubular micro/nanomotors is very crucial but still challenging. Here, a new strategy based on vertically aligned ZnO arrays is employed for the large‐scale and template‐free fabrication of self‐propelled ZnO‐based micromotors with H₂O₂‐free light‐driven propulsion ability. Brush‐shaped ZnO‐based micromotors with different diameters and lengths are fully studied, which present a fast response to multicycles UV light on/off switches with different interval times (2/5 s) in pure water and slow directional motion in aqueous hydrogen peroxide solution in the absence of UV light. Light‐induced electrophoretic and self‐diffusiophoretic effects are responsible for these two different self‐motion behaviors under different conditions, respectively. In addition, the pH of the media and the presence of H₂O₂ show important effects on the motion behavior and microstructure of the ZnO‐based micromotors. Finally, these novel ZnO‐based brush‐shaped micromotors are demonstrated in a proof‐of‐concept study on nitroaromatic explosive degradation, i.e., picric acid. This work opens a completely new avenue for the template‐free fabrication of brush‐shaped light‐responsive micromotors on a large scale based on vertically aligned ZnO arrays.     

Iodine Vacancy Redistribution in Organic–Inorganic Halide Perovskite Films and Resistive Switching Effects

Organic–inorganic halide perovskite (OHP) materials, for example, CH₃NH₃PbI₃ (MAPbI₃), have attracted significant interest for applications such as solar cells, photodectors, light‐emitting diodes, and lasers. Previous studies have shown that charged defects can migrate in perovskites under an electric field and/or light illumination, potentially preventing these devices from practical applications. Understanding and control of the defect generation and movement will not only lead to more stable devices but also new device concepts. Here, it is shown that the formation/annihilation of iodine vacancies (V_I's) in MAPbI₃ films, driven by electric fields and light illumination, can induce pronounced resistive switching effects. Due to a low diffusion energy barrier (≈0.17 eV), the V_I's can readily drift under an electric field, and spontaneously diffuse with a concentration gradient. It is shown that the V_I diffusion process can be suppressed by controlling the affinity of the contact electrode material to I^? ions, or by light illumination. An electrical‐write and optical‐erase memory element is further demonstrated by coupling ion migration with electric fields and light illumination. These results provide guidance toward improved stability and performance of perovskite‐based optoelectronic systems, and can lead to the development of solid‐state devices that couple ionics, electronics, and optics.     

Polarization conversion in conical diffraction by metallic and dielectric subwavelength gratings

Subwavelength metallic and dielectric diffraction gratings which rotate the linear polarization of incident light by 90 degrees are examined. Using rigorous diffraction theory in total-internal-reflection configuration, it is shown that full conversion from incident transverse electric field to transverse magnetic zero-order field can be achieved with both dielectric and metallic elements, but dielectric gratings provide higher efficiency and are thus preferable. The fabrication aspects and constraints are discussed in detail and the behavior of the gratings over broad wavelength bands is presented.