Hydrophobic Barriers for Directing Physarum polycephalum Propulsion and Navigation.

Physarum polycephalum (P. polycephalum) is a unicellular protist with unique properties, such as learning and remembering in its cultured environment without a brain or central nervous system. The organism has been extensively used in morphology, taxis, and positive feedback dynamics studies. However, the lack of standardization of materials and substrate designs used in P. polycephalum studies has significantly limited conducting such studies, increasing the cost and time. In this study, we introduce a method to control the direction and migration of P. polycephalum by drawing hydrophobic lines and patterns. Our study succeeded in controlling the movement of P. polycephalum by setting a variety of hydrophobic designs such as complete barrier, single-slit barrier, taper barrier, dumbbell barrier, and one-side-opened rectangular barrier, suggesting the effectiveness of the hydrophobic barrier in regulating the propulsion and navigation of the organisms. Moreover, we demonstrated that utilizing such geometric constraints can reduce the experimental time required for toxicity testing based on P. polycephalum by more than 300%. Our techniques open new possibilities for studying the biophysical properties and behaviors of P. polycephalum, while also facilitating toxicity testing.

How Colloidal Lithography Limits the Optical Quality of Plasmonic Nanohole Arrays.

Colloidal lithography utilizes self-assembled particle monolayers as lithographic masks to fabricate arrays of nanostructures by combination of directed evaporation and etching steps. This process provides complex nanostructures over macroscopic areas in a simple, convenient, and parallel fashion without requiring clean-room infrastructure and specialized equipment. The appeal of the method comes at the price of imperfections impairing the optical quality, especially for arrayed nanostructures relying on well-ordered lattices. Imperfections are often generically mentioned to rationalize the discrepancy between experimental and simulated resonances. Yet, little attention is given to detailed structure-property relationships connecting typical defects directly with the optical properties. Here, we use a correlative approach to connect nano- and microscopic defects occurring from the colloidal lithography process with the resulting local optical properties. We use nanohole arrays as a common plasmonic structure known to be sensitive to lattice imperfections. Correlative optical and electron microscopies reveal the individual role of packing order, organic impurities, and solid polymer bridges. Our findings show that simple cleaning processes with solvents and oxygen plasma already improve the optical quality but also highlight how well-controlled self-assembly processes are required for predictable optical properties of such nanostructures.

Luminance enhancement of top-emitting blue organic light emitting diodes encapsulated with silicon nitride thin films by a double-layer nano-structure.

Even though it is in high demand to introduce a nano-structure (NS) light extraction technology on a silicon nitride to be used as a thin film encapsulation material for an organic light-emitting diode (OLED), only an industry-incompatible wet method has been reported. This work demonstrates a double-layer NS fabrication on the silicon nitride using a two-step organic vapor phase deposition (OVPD) of an industry-compatible dry process. The NS showed a wrinkle-like shape caused by coalescence of the nano-lenses. The NS integrated top-emitting OLED revealed 40 percent enhancement of current efficiency and improvement of the luminance distribution and color change according to viewing angle.

Organic/Inorganic Hybrid Thin-Film Encapsulation Using Inkjet Printing and PEALD for Industrial Large-Area Process Suitability and Flexible OLED Application.

We present herein the first report of organic/inorganic hybrid thin-film encapsulation (TFE) developed as an encapsulation process for mass production in the display industry. The proposed method was applied to fabricate a top-emitting organic light-emitting device (TEOLED). The organic/inorganic hybrid TFE has a 1.5 dyad structure and was fabricated using plasma-enhanced atomic layer deposition (PEALD) and inkjet printing (IJP) processes that can be applied to mass production operations in the industry. Currently, industries use inorganic thin films such as SiNx and SiOxNy fabricated through plasma-enhanced chemical vapor deposition (PECVD), which results in film thickness >1 µm; however, in the present work, an Al2O3 inorganic thin film with a thickness of 30 nm was successfully fabricated using ALD. Furthermore, to decouple the crack propagation between the adjacent Al2O3 thin films, an acrylate-based polymer layer was printed between these layers using IJP to finally obtain the 1.5 dyad hybrid TFE. The proposed method can be applied to optoelectronic devices with various form factors such as rollables and stretchable displays. The hybrid TFE developed in this study has a transmittance of 95% or more in the entire visible light region and a very low surface roughness of less than 1 nm. In addition, the measurement of water vapor transmission rate (WVTR) using commercial MOCON equipment yielded a value of 5 × 10-5 gm-2 day-1 (37.8 °C and 100% RH) or less, approaching the limit of the measuring equipment. The TFE was applied to TEOLEDs and the improvement in optical properties of the device was demonstrated. The OLED panel was manufactured and operated stably, showing excellent consistency even in the actual display manufacturing process. The panel operated normally even after 363 days in air. The proposed organic/inorganic hybrid encapsulant manufacturing process is applicable to the display industry and this study provides basic guidelines that can serve as a foothold for the development of various technologies in academia and industry alike.

Surface wrinkle formation by liquid crystalline polymers for significant light extraction enhancement on quantum dot light-emitting diodes.

We propose an optimal outcoupling structure of a quantum-dot light-emitting diode (QLED) and present material properties based on numerical calculations via the ray-tracing method, in which light extraction properties are obtained according to the surface wrinkles on a substrate. After analyzing the designed microstructure elements, the optimal model was derived and applied to the QLEDs; consequently, the outcoupling efficiency enhanced by 31%. The liquid crystalline polymer forming the random surface wrinkles not only achieves an excellent light extraction through plasma crosslinking but also facilitates large-area processes. We propose an optical design rule for high-efficiency QLED design by analyzing the electro-optical efficiency, emission spectrum, and angular radiation pattern of the optical device.

Thin-film transistor-driven vertically stacked full-color organic light-emitting diodes for high-resolution active-matrix displays.

Thin-film transistor (TFT)-driven full-color organic light-emitting diodes (OLEDs) with vertically stacked structures are developed herein using photolithography processes, which allow for high-resolution displays of over 2,000 pixels per inch. Vertical stacking of OLEDs by the photolithography process is technically challenging, as OLEDs are vulnerable to moisture, oxygen, solutions for photolithography processes, and temperatures over 100 °C. In this study, we develop a low-temperature processed Al2O3/SiNx bilayered protection layer, which stably protects the OLEDs from photolithography process solutions, as well as from moisture and oxygen. As a result, transparent intermediate electrodes are patterned on top of the OLED elements without degrading the OLED, thereby enabling to fabricate the vertically stacked OLED. The aperture ratio of the full-color-driven OLED pixel is approximately twice as large as conventional sub-pixel structures, due to geometric advantage, despite the TFT integration. To the best of our knowledge, we first demonstrate the TFT-driven vertically stacked full-color OLED.

Ultraflexible and transparent electroluminescent skin for real-time and super-resolution imaging of pressure distribution.

The ability to image pressure distribution over complex three-dimensional surfaces would significantly augment the potential applications of electronic skin. However, existing methods show poor spatial and temporal fidelity due to their limited pixel density, low sensitivity, or low conformability. Here, we report an ultraflexible and transparent electroluminescent skin that autonomously displays super-resolution images of pressure distribution in real time. The device comprises a transparent pressure-sensing film with a solution-processable cellulose/nanowire nanohybrid network featuring ultrahigh sensor sensitivity (>5000 kPa-1) and a fast response time (<1 ms), and a quantum dot-based electroluminescent film. The two ultrathin films conform to each contact object and transduce spatial pressure into conductivity distribution in a continuous domain, resulting in super-resolution (>1000 dpi) pressure imaging without the need for pixel structures. Our approach provides a new framework for visualizing accurate stimulus distribution with potential applications in skin prosthesis, robotics, and advanced human-machine interfaces.

Visible-Light-Induced Trifluoromethylation of Unactivated Alkenes with Tri(9-anthryl)borane as an Organophotocatalyst.

Tri(9-anthryl)borane was successfully applied as an organophotocatalyst for the visible-light-induced trifluoromethylation of unactivated alkenes with CF3I. The mild reaction conditions tolerated a variety of functional groups, and the reaction could be extended to perfluoroalkylations with C3F7I and C4F9I. Mechanistic studies revealed that the photoredox catalysis involves an oxidative quenching pathway.

Facile extraction of cellulose nanocrystals.

A simple process for extracting cellulose nanocrystals (CNCs) is proposed that only uses high-pressure homogenization (HPH) controlling a process temperature. The proposed process was assessed and compared with normal production through acidic hydrolysis. Temperature-controlled HPH produced CNCs with high crystallinity, which linearly increased with increasing process temperature over 20 passes. The CNCs had uniform widths and lengths in the ranges of 4-14 nm and 60-320 nm, respectively. Undesirable chemical reaction can be avoided with the proposed process because no chemical was used to promote the CNC extraction. This method is an efficient and sustainable green approach to CNC production.

Color-tunable organic light-emitting diodes with vertically stacked blue, green, and red colors for lighting and display applications.

We demonstrate independently and simultaneously controlled color-tunable organic light-emitting diodes (OLEDs) with vertically stacked blue, green, and red elements. The blue, green, and red elements were placed at the bottom, middle, and top positions, respectively, forming color-tunable OLEDs. The independently driven blue, green, and red elements in the color-tunable OLEDs exhibited low driving voltages of 5.3 V, 3.0 V, and 4.6 V, as well as high external quantum efficiencies of 11.1%, 10.9%, and 9.6%, respectively, at approximately 1000 cd/m2. Each element in the color-tunable OLEDs showed high-purity blue, green, and red colors with little parasitic emission owing to the delicately designed device structure resultant from optical simulations. The color-tunable OLEDs could produce any colors inside the triangle formed with blue (0.136, 0.261), green (0.246, 0.697), and red (0.614, 0.386) Commission Internationale de l'éclairage (CIE) 1931 color coordinates. In addition, the correlated color temperatures (CCTs) of white colors in the color-tunable OLED can be easily changed from the warm white to the cool white by controlling the red, green, and blue emissions simultaneously. The white colors in the color-tunable OLED have the CIE 1931 color coordinate of (0.304, 0.351), with a CCT of 6289 K and (0.504, 0.440), with a CCT of 2407K at the driving voltage of 5 V (blue), 2.8 V (green), 4.4 V (red), and 4.6 V (blue), 3 V (green), 5 V (red), respectively. Furthermore, the white color in the color-tunable OLED exhibited a high color rendering index (~88.7) due to vertically stacked three color system. Moreover, we successfully fabricated a large-sized, 14 × 12 pixel array of the color-tunable OLEDs to demonstrate lighting and display applications, respectively.

Light-Induced Fluorescence Modulation of Quantum Dot-Crystal Violet Conjugates: Stochastic Off-On-Off Cycles for Multicolor Patterning and Super-Resolution.

Photoswitching or modulation of quantum dots (QDs) can be promising for many fields that include display, memory, and super-resolution imaging. However, such modulations have mostly relied on photomodulations of conjugated molecules in QD vicinity, which typically require high power of high energy photons at UV. We report a visible light-induced facile modulation route for QD-dye conjugates. QD crystal violets conjugates (QD-CVs) were prepared and the crystal violet (CV) molecules on QD quenched the fluorescence efficiently. The fluorescence of QD-CVs showed a single cycle of emission burst as they go through three stages of (i) initially quenched "off" to (ii) photoactivated "on" as the result of chemical change of CVs induced by photoelectrons from QD and (iii) back to photodarkened "off" by radical-associated reactions. Multicolor on-demand photopatterning was demonstrated using QD-CV solid films. QD-CVs were introduced into cells, and excitation with visible light yielded photomodulation from "off" to "on" and "off" by nearly ten fold. Individual photoluminescence dynamics of QD-CVs was investigated using fluorescence correlation spectroscopy and single QD emission analysis, which revealed temporally stochastic photoactivations and photodarkenings. Exploiting the stochastic fluorescence burst of QD-CVs, simultaneous multicolor super-resolution localizations were demonstrated.

SnS4(4-), SbS4(3-), and AsS3(3-) Metal Chalcogenide Surface Ligands: Couplings to Quantum Dots, Electron Transfers, and All-Inorganic Multilayered Quantum Dot Sensitized Solar Cells.

Three inorganic capping ligands (ICLs) for quantum dots (QDs), SnS4(4-), SbS4(3-) and AsS3(3-), were synthesized and the energy levels determined. Proximity between the ICL LUMO and QD conduction level governed the electronic couplings such as absorption shift upon ligand exchange, and electron transfer rate to TiO2. QD-sensitized solar cells were fabricated, using the ICL-QDs and also using QD multilayers layer-by-layer assembled by bridging coordinations, and studied as a function of the ICL ligand and the number of QD layers.

Layer-by-layer-assembled quantum dot multilayer sensitizers: how the number of layers affects the photovoltaic properties of one-dimensional ZnO nanowire electrodes.

Layer cake: Multilayered CdSe quantum dot (QD) sensitizers are layer-by-layer assembled onto ZnO nanowires by making use of electrostatic interactions to study the effect of the layer number on the photovoltaic properties. The photovoltaic performance of QD-sensitized solar cells critically depends on this number as a result of the balance between light-harvesting efficiency and carrier-recombination probability.

Preparation of multilayered CdSe quantum dot sensitizers by electrostatic layer-by-layer assembly and a series of post-treatments toward efficient quantum dot-sensitized mesoporous TiO2 solar cells.

A multilayer of CdSe quantum dots (QDs) was prepared on the mesoporous surface of a nanoparticulate TiO(2) film by a layer-by-layer (LBL) assembly using the electrostatic interaction of the oppositely charged QD surface for application as a sensitizer in QD-sensitized TiO(2) solar cells. To maximize the absorption of incident light and the generation of excitons by CdSe QDs within a fixed thickness of TiO(2) film, the experimental conditions of QD deposition were optimized by controlling the concentration of salt added into the QD-dissolved solutions and repeating the LBL deposition a few times. A proper concentration of salt was found to be critical in providing a deep penetration of QDs into the mesopore, thus leading to a dense and uniform distribution throughout the whole TiO(2) matrix while anchoring the oppositely charged QDs alternately in a controllable way. A series of post-treatments with (1) CdCl(2), (2) thermal annealing, and (3) ZnS-coating was found to be very critical in improving the overall photovoltaic properties, presumably through a better connection between QDs, effective passivation of QD's surface, and a high impedance of recombination, which were proved by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) experiments. With a proper post-treatment of multilayered QDs as a sensitizer, the overall power conversion efficiency in the CdSe QD-sensitized TiO(2) solar cells could reach 1.9% under standard illumination condition of simulated AM 1.5G (100 mW/cm(2)).

Layer-by-Layer Quantum Dot Assemblies for the Enhanced Energy Transfers and Their Applications toward Efficient Solar Cells.

Two different quantum dots (QDs) with an identical optical band gap were prepared: one without the inorganic shell and short surface ligands (BQD) and the other with thick inorganic shells and long surface ligands (OQD). They were surface-derivatized to be positively or negatively charged and were used for layer-by-layer assemblies on TiO2. By sandwiching BQD between OQD and TiO2, OQD photoluminescence showed seven times faster decay, which is attributed to the combined effect of the efficient energy transfer from OQD to BQD with the FRET efficiency of 86% and fast electron transfer from BQD to TiO2 with the rate of 1.2 × 10(9) s(-1). The QD bilayer configuration was further applied to solar cells, and showed 3.6 times larger photocurrent and 3.8 times larger photoconversion efficiency than those of the device with the OQD being sandwiched by BQD and TiO2. This showcases the importance of sophisticated control of QD layer assembly for the design of efficient QD solar cells.