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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Three-dimensional reconstruction of topological deformation in chiral nematic microspheres using fluorescence confocal polarizing microscopy.

Chiral nematic droplets exhibit abundant topological defect structures, which have been intensively studied, both theoretically and experimentally. However, to observe and reconstruct the exact shape of three-dimensional (3D) defect structures has been a challenging task. In this study, we successfully reconstruct the 3D defect structures within a CLC microsphere with long helical pitches by combining polarized optical microscopy (POM) and laser scanning type fluorescence confocal polarizing microscopy (FCPM). The obtained confocal stack images provide us with the vertical location of disclination defects, to allow reconstruction of the full 3D structures. The reconstructed 3D structures can be viewed from different directions, providing a better understanding of the topological structure. Moreover, the defect lines are identified to be + 1 defects, different from the previous prediction. Thus, FCPM provides an excellent tool to study the complex topological configuration in microspheres, and fosters its potential applicability in new devices based on topologically structured soft media.

Guided Electro-Optical Switching of Small Graphene Oxide Particles by Larger Ones in Aqueous Dispersion.

Although the large Kerr coefficient of aqueous graphene oxide (GO) dispersions is quite attractive for electro-optical applications with low power consumption, the maximum birefringence of GO dispersions is not sufficiently high for actual display applications. Here we report that adding a small amount of larger GO particles (about 4 µm) into a high-concentration dispersion of small GO (about 0.2 µm) can improve the electro-optical sensitivity to an electric field and also the maximum birefringence. Large GOs induce the ordering of small particles and enhance the electro-optical switching. Large GOs have higher polarizability and are easily driven under an applied electric field, and the rotational motion of large GO particles leads to switching of surrounding small GO particles, improving the electro-optical performance. The binary mixture can overcome the limitations of pure dispersions of large GO or small GO particles; the former has high interparticle interaction, and the latter has low sensitivity to an electric field.

Electro-optical switching of graphene oxide liquid crystals with an extremely large Kerr coefficient.

The sensitive response of the nematic graphene oxide (GO) phase to external stimuli makes this phase attractive for extending the applicability of GO and reduced GO to solution processes and electro-optic devices. However, contrary to expectations, the alignment of nematic GO has been difficult to control through the application of electric fields or surface treatments. Here, we show that when interflake interactions are sufficiently weak, both the degree of microscopic ordering and the direction of macroscopic alignment of GO liquid crystals (LCs) can be readily controlled by applying low electric fields. We also show that the large polarizability anisotropy of GO and Onsager excluded-volume effect cooperatively give rise to Kerr coefficients that are about three orders of magnitude larger than the maximum value obtained so far in molecular LCs. The extremely large Kerr coefficient allowed us to fabricate electro-optic devices with macroscopic electrodes, as well as well-aligned, defect-free GO over wide areas.

Manipulation of structural color reflection in graphene oxide dispersions using electric fields.

Aqueous graphene oxide (GO) dispersions with a photonic crystal structure are carefully prepared to produce structural color reflection. We fabricate a simple reflective GO cell with a unique electrode design and demonstrate that the resulting structural color reflection is electrically erasable and rewritable. GO concentration and the direction of the electric field are vital factors in the development of the device. The resulting device works well, although it exhibits a rather slow response time; in particular, the spontaneous recovery time from dark to bright color reflection requires tens of minutes. Through the application of a horizontal electric field parallel to the substrate, the recovery time can be improved, resulting in a recovery period of 3 min. Although many unsolved issues remain, the findings in GO dispersion may provide a new possibility for color filter-less bi-stable color displays with low power consumption.

Optimization of particle size for high birefringence and fast switching time in electro-optical switching of graphene oxide dispersions.

In order to use graphene oxide (GO) dispersions for electro-optical applications, both a high GO concentration and a high electrical sensitivity are essential; however, these have not been achieved to date. Here, we report that by optimizing the mean size of GO particles to approximately 0.5 µm, one can obtain a high GO concentration of up to 2 wt% and high electrical sensitivity simultaneously. By reducing the mean GO-particle size, the interparticle interaction and the rotational viscosity can be significantly reduced, and a high-concentration isotropic phase can be obtained. As a result, the maximum birefringence increases and the dynamic response becomes faster. However, further decrease of the mean size below 0.1 µm causes a decrease in the anisotropy of electrical polarizability, resulting in the reduction of the electrical sensitivity of GO dispersions.

Complementarity between fluorescence and reflection in photoluminescent cholesteric liquid crystal devices.

The combination of photoluminescence (PL) and cholesteric liquid crystal (CLC) provides interesting complementary features for an optimized display application. Distortion of the Bragg lattice of CLCs decreases selective reflection but increases fluorescence intensity; recovery of a uniform lattice in turn results in increased reflection and decreased fluorescence. This complementary relationship between the fluorescence and the Bragg reflection gives rise to self-compensations for color shifts due to either dynamic slow response of CLC helix or mismatch of oblique incidence of light with respect to the helical axis. These color shifts have long been intrinsic unsolved limitations of conventional CLC devices. Thus, the complementary coupling between the fluorescence and the CLC Bragg reflections plays an important role in improving the color performance and the quality of moving images.

Glossiness representation using binocular color difference.

We demonstrate that a binocular color difference can be used to express the surface glossiness of an object on 3D display devices without being accompanied by a specular reflection pattern. A simple image with a binocular color difference provides a similar surface appearance impression to a real object that has the same binocular color difference. It is found that human binocular perception is likely to interpret binocular color difference as spectral reflectance rather than as transparency. Binocular glossiness is caused not only by a binocular lightness difference but also by a chromatic or hue difference.

Reflective-emissive photoluminescent cholesteric liquid crystal display.

We fabricated a photoluminescent cholesteric liquid crystal (PL-CLC) cell for a display application that can be used to display high-quality moving pictures under all ambient conditions including dark and sunlit conditions. The PL-CLC cell is switchable between the reflective mode under bright conditions and the emissive mode in the dark. The effective reflectance of the PL-CLC is higher than that of a conventional CLC device by more than 30%, and the contrast ratios were approximately 10 and 7 in the reflective and emissive modes, respectively. We directly compared the proposed PL-CLC cell with conventional LCD and CLC cells under sunlit, office, and dark environments and confirmed that the PL-CLC cell exhibited superior visibility under all ambient conditions.

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.