Diverse Self-assembly Pathways in Nematic Compartment Network: Topological Percolation and Pathfinding.

The self-assembly of nematic molecules in microcompartments with unambiguously defined surface anchoring is well predictable and is likely to have a single stable topological structure. Here, in contrast, a confined nematic system comprising an array of microcompartments interconnected by channels is demonstrated, exhibiting diverse molecular assembly pathways leading to the formation of four types of topological structures and twelve different patterns randomly distributed. Intercompartment communication via channels plays a crucial role in the diversity of patterns and distributions. It determines the sizes and structures of domains separated by channel defects. The domain structure, which features a pathfinding algorithm and reverse tree structure, can be modelled by an isotropically directed bond percolation with additional restrictions. This system serves as a model for controlled randomness and restricted growth of networks, with potential applications in anticounterfeit protection as a physically unclonable function (PUF) with multiple-level communication protocols.

Green Solvent Selection for All Solution-Processed Inverted Quantum Dot Light Emitting Diode.

Quantum-dot light-emitting diodes (QD-LEDs) have gained attention as potential display technologies. However, the solvents used to dissolve a polymeric hole transport layer (HTL) are hazardous to both humans and the environment. Additionally, intermixing the HTL and QD layers presents a significant challenge when fabricating inverted QD-LEDs. Here, a green solvent selection procedure to achieve good device performance and environmental safety in QD-LEDs is established. This procedure utilizes Hansen solubility parameters and surface roughness to identify a set of solvents that do not lower the device performance by avoiding interlayer mixing or a rough interface. The CHEM21 solvent selection guide is used to screen for environmentally hazardous solvents. Finally, cyclopentanone (CPO) is selected as the optimal HTL solvent from among 16 candidates. Using CPO improves the maximum luminescence by ≈1.6 times and the maximum current efficiency by ≈12.6 times, compared to that of conventional devices using hazardous chlorobenzene. Solvent selection is critical for the fabrication of green and high-performance inverted QD-LEDs, particularly for large display panels that require n-type oxide thin-film transistors.

Photoluminescence enhancement in quantum-dot-polymer films with CO2 micropores through KHCO3 decomposition.

Quantum-dot (QDs) polymer composite films, which are key components in recent display applications, require improved photoluminescence (PL) intensity and color conversion efficiency for better display quality and low power consumption. In this study, we developed a novel approach to improve the photoluminescence (PL) of quantum dot (QDs)-polymer nanocomposite films. This was achieved by incorporating CO2 micropores and scattering particles into QD-embedded photopolymerizable polymer films. CO2 micropores were generated by the decomposition of KHCO3 in the film. The CO2 micropores, along with the partially decomposed KHCO3 microparticles, act as a scattering medium that increases the photon absorbance and improves the PL intensity. The effect of KHCO3 annealing temperature on various optical properties is investigated, and it is found that a large number of uniform micropores are created in the film at an optimal temperature, 110 ℃. Compared to an ordinary QD-polymer film, the PL of the QD-hybrid-foamed polymer film increases by 4.2 times. This method is fast and economically efficient, and provides insights into the design of high-performance optoelectronic devices.

Laser processing of microdroplet structure of liquid crystal in 3D.

Processing of mesoscale structures of soft matter and liquid is of great importance in both science and engineering. In this work, we introduce the concept of laser-assisted micromachining to this field and inject a certain number of microdroplets into a preselected location on the surface of a liquid crystal drop through laser irradiation. The impact of laser energy on the triggered injection is discussed. The sequentially injected microdroplets are spontaneously captured by the defect ring in the host drop and transported along this defect track as micro-cargos. By precisely manipulating the laser beam, the tailored injection of droplets is achieved, and the injected droplets self-assemble into one necklace ring within the host drop. The result provides a bottom-up approach for the in-situ and three-dimensional microfabrication of droplet structure of soft matter using a laser beam, which may be applicable in the development of optical and photonic devices.

Rotation-Time Symmetry Breaking in Frustrated Chiral Nematic Driven by a Pulse-Train Waveform.

We report waveform-induced rotation-time symmetry breaking in liquid crystal director motion. Homeotropic cells filled with a negative dielectric anisotropy chiral nematic exhibit persistent and visually observable waves of director orientation with a time period of at least 30 driving field cycles. Their existence in the space of driving waveform parameters is explored. The possibility of utilizing this system, which exhibits both spatial and temporal long-range order, as a modeling tool for experimental studies on discrete time crystals is discussed.

Multiresponsive Polymer Nanocomposite Liquid Crystals Having Heterogeneous Phase Transitions for Battery-Free Temperature Maintenance Indicators.

Multiresponsive functional materials that respond to more than one external stimulus are promising for novel photonic, electronic, and biomedical applications. However, the design or synthesis of new multiresponsive materials is challenging. Here, this work reports a facile method to prepare a multiresponsive colloidal material by mixing a liquid-crystalline 2D nanocolloid and a functional polymer colloid. For this purpose, electrically sensitive exfoliated α-ZrP 2D nanocolloids and thermosensitive block copolymer colloids that are dispersed well in water are mixed. In the liquid-crystalline nanocomposite, nematic, antinematic, or isotropic assemblies of α-ZrP, nanoparticles can be electrically and selectively obtained by applying electric fields with different frequencies; furthermore, their rheology is thermally and reversibly controlled through thesol-gel-sol transition. The nanocomposite exhibits a solid gel phase within a predesigned gel temperature range and a liquid sol phase outside this range. These properties facilitate the design of a simple display device in which information can be electrically written and thermally stabilized or erased, and using the device, a battery-free temperature maintenance indication function is demonstrated. The proposed polymer nanocomposite method can enrich the physical properties of 2D nanocolloidal liquid crystals and create new opportunities for eco-friendly, reusable, battery-free electro-optical devices.

Hybrid Device of Blue GaN Light-Emitting Diodes and Organic Light-Emitting Diodes with Color Tunability for Smart Lighting Sources.

A lighting device with a wide color-tunable range is still a challenge for lighting based on either organic light-emitting diodes (OLEDs) or inorganic LEDs. In this work, we first proposed a novel hybrid device of organic LEDs and inorganic blue GaN LEDs to achieve full white and other colors. Organic LEDs were stacked with green and red emissive layers and connected with blue GaN LEDs in parallel but in opposite polarity voltage. Under the alternate-current (AC) driving, the hybrid structure can be controlled independently by applying timing variable opposite voltages to emit the light from either blue LEDs or the stacked OLEDs for forming mixed colors. The hybrid device can generate white light, varying in a wide range by changing the amplitude and duty ratio (DR) of AC-driving signals, from cold white to standard white and to warm white (3668-11 833 K). When an AC voltage of (4.80 V, -2.45 V) was applied, the device has a high color gamut of 95.24% National Television System Committee (NTSC) and a high color rendering index (R a) of 92.4%. The novel hybrid device with the blue LED and OLED in opposite polarity exhibits potential applications in smart solid-state lighting, display, and light communication.

Charge Modulation Layer and Wide-Color Tunability in a QD-LED with Multiemission Layers.

Widely tunable color emission from a single pixel is a promising but challenging technology for quantum-dot light-emitting diodes (QD-LEDs). Even a QD-LED pixel with stacked multi-QD layers having different colors is likely to emit a monotonic color because the exciton recombination mostly occurs in 1 or 1.5 QD layers with better charge balance. In this study, an all-solution-processed QD-LED with electrically tunable color emission over a wide color range by introducing a charge modulation layer (CML) is developed. Specifically, the CML acted as a high and narrow energy barrier for electrons between two QD layers, and the electron drift is sensitively controlled via the field-dependent tunneling effect. Therefore, the charge distribution and balance in the two QD layers re-electrically tunable, which enhanced the color tunability. The color tuning range and quantum efficiency are effectively controlled depending on the CML material and thickness. In addition, the color change caused by the solvent effect in a QD-LED with dual QD layers is thoroughly investigated. The proposed method may advance the understanding of QD emission behavior with the use of CML and provide a practical approach for the actual application of color-tunable pixel technology.

Quantum Dot Arrays Fabricated Using In Situ Photopolymerization of a Reactive Mesogen and Dielectrophoresis.

Dielectrophoresis (DEP) is an excellent tool for manipulating small particles within a liquid or gas medium. However, when the size of the particles is too small, such as with quantum dots (QDs), it is difficult to manipulate the particles using DEP because the dielectrophoretic force (FDEP) depends on the volume of the particles and is therefore too weak to achieve particle migration. Herein, we demonstrate a novel method for controlling nanoscale QD particles using DEP by introducing photopolymerized reactive mesogen (RM) bead vehicles. The size of an RM bead is well-controlled by the RM concentration in the medium, and when the size is approximately 0.2 µm or larger, the RM beads can be arbitrarily manipulated using DEP under moderate electric fields. Interestingly, during photopolymerization, QD particles are easily absorbed by polymerized RM beads and most of the QDs are embedded within the RM beads. Hence, we can fabricate periodic QD arrays by manipulating the RM beads containing such dots. In addition, we can fabricate multicolor QD arrays by repeating the processes using different QD particles. The shape of a DEP-assisted QD-RM network pattern can be precisely predicted by calculating the gradient of the square of the electric field (∇E2) and the corresponding FDEP. This new technology may be useful for the fabrication of optical devices, displays, photonic crystal devices, and bioapplications.

All-solution-processed colour-tuneable tandem quantum-dot light-emitting diode driven by AC signal.

We demonstrate a novel structure for a quantum-dot light-emitting diode (QD-LED) with wide-range colour-tuneable pixels, fabricated via full solution processing. The proposed device has a symmetrical structure produced via stacking of an inverted-structure diode with a green QD emission layer (EML) and normal-structure diode with a red QD EML. It is an electron-only device; however, a charge generation layer in the middle of the device generates holes for the formation of excitons. Depending on the polarity of the applied voltage, either the bottom inverted unit or the top normal unit is operated, thereby emitting green or red light, respectively. The working mechanism of the device is investigated via analysis of the charge generation mechanism and carrier transport path. In addition, the colour tunability is verified using a simple alternating current (AC) driving scheme; the duty cycle modulation of the AC signal enables fine colour adjustment over a broad range, from pure green to pure red. Thus, our colour-tuneable QD-LED with vertically stacked independently operated sub-pixels can open a promising pathway towards cost-effective ultra-high-resolution displays.

Generation and manipulation of isotropic droplets in nematic medium using switchable dielectrophoresis.

Dielectrophoresis (DEP) in a medium with anisotropic dielectric susceptibility is very different from typical DEP in an isotropic medium: The direction of particle actuation can be switched depending on the direction of the susceptibility tensor of the medium. However, the understanding of switchable DEP (SDEP) in an anisotropic medium is still in its infant stage. Here, we investigate SDEP using heat-generated isotropic droplets in a nematic liquid crystal (LC) medium. We demonstrate that the location of the generation of isotropic droplets can be partially controlled by controlling the temperature gradient within the LC cell using dielectric loss. The SDEP actuation of isotropic droplets is also highly dependent on the location of the isotropic droplets. Using this method, we fabricated different array patterns of isotropic and nematic phase separations under different applied signals.

Dual-field-induced biaxial nematic ordering of two-dimensional nanoparticles and enhancement of interparticle interactions.

The ordering of 2D biaxial graphene oxide (GO) particles is investigated under the application of orthogonal electric (E) and magnetic fields (B); nematic, antinematic, or biaxial nematic ordering of GO particles is selectively obtained depending on the field conditions. Particularly, a perfect biaxial nematic ordering with the highest birefringence is induced by the dual fields. Unexpectedly, the presence of B enhances the effective polarizability anisotropy, which may attribute to the enhanced steric interparticle interaction. The dual fields induce the microscopic biaxial stacking assembly of GO particles, producing grainy flocs which are not observed in a single-field condition.

Laser-Induced Nanodroplet Injection and Reconfigurable Double Emulsions with Designed Inner Structures.

Microfabrication of complex double emulsion droplets with controlled substructures, which resemble biological cells, is an important but a highly challenging subject. Here, a new approach is proposed based on laser-induced injection of water nanodroplets into a liquid crystal (LC) drop. In contrast to the conventional top-down microfluidic fabrication, this method employs a series of bottom-up strategies such as nanodroplet injection, spontaneous and assisted coalescence, elastically driven actuation, and self-assembly. Each step is controlled precisely by adjusting the laser beam, interfacial tension, and its gradients, surface anchoring, and elasticity of the LC. Whispering gallery mode illumination is used to monitor the injection of droplets. A broad spectrum of double emulsions with a predesigned hierarchical architecture is fabricated and reconfigured by temperature, laser-induced coalescence, and injection. The proposed bottom-up method to produce customized microemulsions that are responsive to environmental cues can be used in the development of drug delivery systems, biosensors, and functional soft matter microstructures.

A self-assembled nanoparticle cluster array fabricated using nematic-isotropic phase separation on a functionalized surface.

The manipulation of a large number of nanoparticles (NPs) is an interesting but challenging task. Here, we demonstrate a new method to fabricate an NP cluster array, in which the shape and size of each NP cluster can be controlled. The method involves the use of the solubility contrast of NPs in the isotropic and nematic liquid crystal (LC) media, and the isotropic-preference difference depending on the types of the surfaces. The former mechanism is used to trap NPs within the isotropic domain, the size of which is simply manipulated by adjusting temperature. The latter mechanism is used to control the location of isotropic pockets in the continuous nematic phase. By controlling the volume and location of the isotropic pocket, one can simply create various types of NP cluster arrays. This method does not involve the use of any external field, and may be applicable to other types of NPs, including ferroelectric or ferromagnetic materials, thereby expanding its applicability.

Switchable dielectrophoresis of defect-free droplets in an anisotropic liquid crystal medium.

Dielectrophoresis (DEP) is widely used in nanoscience and biology to control small particles but its applicability is significantly limited by its one-way impetus characteristics along the square field gradient (∇E2) direction, that is, DEP force, FDEP ∼ ∇E2. Here, switchable DEP (SDEP) using the anisotropic property of a nematic medium is demonstrated; FDEP does not need to be parallel to ∇E2 but is arbitrarily changeable depending on the permittivity tensor orientation of the medium. To effectively demonstrate the SDEP phenomenon, isotropic droplets with infinitesimal surface anchoring in a nematic medium are introduced, in which topological defects of the nematic medium around dispersed objects are effectively eliminated. The experimental behaviours are well explained by theoretical and simulation results. To emphasize the applicability of SDEP, switchable arrays of isotropic droplets and an isotropic pocket carrier system containing micro-particles are demonstrated. The results reveal a new dimension of DEP and provide a novel approach for manipulating nano- or micro-materials in colloids.

Selectivity of Threefold Symmetry in Epitaxial Alignment of Liquid Crystal Molecules on Macroscale Single-Crystal Graphene.

Epitaxial alignment of organic liquid crystal (LC) molecules on single-crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment-layer-free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction. Instead, the first contact geometry of the LC molecules determines the direction. Despite its strong anchoring energy on graphene, the LC alignment direction is readily erasable and rewritable, contrary to previous understanding. In addition, the quality of the threefold symmetric alignment is sensitive to alien residue and graphene imperfections, which can be used to detect infinitesimal impurities or structural defects on the graphene. Based on this unique epitaxial behavior of LCs on SCG, an alignment-layer-free electro-optical LC device and LC alignment duplication, which can result in practical graphene-based flexible LC devices, are realized.

Electrical switching of birefringence in zirconium phosphate colloids with various solvents.

Even though a graphene-oxide (GO) dispersion is attractive for electro-optical switching devices because of its high Kerr coefficient, it has several limitations such as chemical instability and optical loss due to absorption at visible wavelengths. Here we introduce the use of tetrabutylammonium-tethered α-zirconium phosphate (TBA-ZrP) colloid in various solvents for electro-optical switching devices; the TBA-ZrP colloid is chemically stable and optically transparent. We find that the electrical switching behavior of TBA-ZrP is sensitively dependent on the type of solvent. The TBA-ZrP colloid in acetone exhibits the highest effective Kerr coefficient and the fastest switching time, which is related to the unusual behavior of the viscosity of the TBA-ZrP colloid in acetone. Its viscosity is relatively low and less sensitive to concentration compared to ZrP in other solvents. This indicates that the motion of individual nanoparticles is relatively less restricted in acetone. These findings may be useful in developing electro-optical devices using lyotropic liquid crystal colloids.

Electrically and electrohydrodynamically driven phase transition and structural color switching of oligomer tethered 2D colloid.

Two-dimensional (2D) nanoparticles in an oligomer-tethered alpha zirconium phosphate (αZrP) colloid self-assemble to form a cofacial lamellar structure with regular spacing parallel to the surface and exhibit high reflectance and vivid structural colors within the visible frequency spectrum. Here, we demonstrate electrical switching of the structural color reflection by electrical control of the liquid crystalline phase of the αZrP colloid. At low frequency (less than 15 Hz, optimally at 1 Hz), electrohydrodynamic flow in the colloid destroys the photonic crystalline lamellar phase and creates an apparently disordered dynamic state with local nematic orientation. The method using electrohydrodynamic flow is a better approach to erase the photonic crystalline ordering of nanoparticles, than application of a high-frequency field, which has been proposed previously, in terms of the required voltage and color uniformity. The field-induced disordered particle orientation can be spontaneously recovered to the initial photonic crystal state by removing the applied voltage, but this method requires quite a long time and does not work in materials with a high nanoplatelet concentration. On the other hand, by applying a horizontal high-frequency field (approximately 10 kHz), the initial lamellar ordering can be forcibly recovered. In this way, the structural color in the 2D nanoparticle colloid can be repeatedly erased or rewritten by switching the frequency of the applied voltage from 10 kHz to 1 Hz and vice versa, respectively. Our method of switching a 2D colloid using both electrohydrodynamic flow and frequency modulation is expected to be a promising approach to control the photonic crystallinity of colloidal photonic crystals.

Computation of refractive index and optical retardation in stretched polymer films.

Functional polymer films are key components in the display industry, and the theoretical prediction of the optical properties of stretched polymer films is important. In this study, we try to establish the theoretical calculation process without an empirical database to predict the refractive index, including wavelength dispersions and optical retardation of stretched polymer films using several commercial simulation tools. The polarizability tensor and molecular volume for periodic units of polymers are accurately simulated, resulting in the accurate prediction of the mean refractive index and its dispersion for raw polymer materials. The birefringence of stretched films is also calculated to predict reasonably accurate optical properties of stretched films. The simulation method is an effective way that requires a relatively short time and low cost to develop new types of polymer films.

Comment on "Tunable Design of Structural Colors Produced by Pseudo-1D Photonic Crystals of Graphene Oxide" and Thin-Film Interference from Dried Graphene Oxide Film.

The mechanism of the iridescent color reflection from dried thin graphene oxide (GO) film on Si wafer is clarified. Dissimilarly to the photonic crystalline reflection in aqueous GO dispersion, the color reflection in dried GO film originates from the thin film interference. The peak reflection can reach 23% by optimizing the GO thickness and the substrate.