Highly Enhanced Light-Outcoupling Efficiency in ITO-Free Organic Light-Emitting Diodes Using Surface Nanostructure Embedded High-Refractive Index Polymers

We develop the high-performance internal light-outcoupling (HRLOC) system based on the high-refractive index polyimide (PI) and metal oxide nanoparticles for organic light-emitting diodes (OLEDs) with silver nanowires (AgNWs). The spontaneously formed nanobump structures, high refractive index, and light-scattering properties of HRLOC significantly enhance the light-extraction efficiency of OLEDs. Not only do the outcoupling structures improve the light-extraction efficiency, but also remarkably enhance the electrical properties of OLEDs. HRLOC leads to the regular and smooth formation of AgNWs, resulting in the improvement of the electrical properties of devices by preventing electrical shorts and leakage currents. The power efficiency of the AgNW-based OLEDs with PI is improved by a factor of 1.31 compared to the reference device with indium tin oxide (ITO) transparent electrode at a luminance of 20 000 cd/m². The efficiency is further improved by incorporating TiO₂ nanoparticles into the PI matrix by a factor of 1.69. To our knowledge, the optically and electrically enhanced OLEDs show one of the highest enhancement factors reported for ITO-free OLEDs with internal outcoupling structures. In addition, the outcoupling structures are solution processable, thermally stable, and can be scaled up to 200 × 200 mm² for large-area applications. We believe that the light-outcoupling structures developed here have great potential for efficient, low-cost, and flexible ITO-free OLEDs

Rapid, Microwave-Assisted Synthesis of Cubic, Three-Dimensional, Highly Porous MOF-205 for Room Temperature CO₂ Fixation via Cyclic Carbonate Synthesis

A dual-porous, three-dimensional, metal–organic framework [Zn₄O­(2,6-NDC)­(BTB)_4/3] (MOF-205, BET = 4200 m²/g) has been synthesized using microwave power as an alternative energy source for the first time, and its catalytic activity has been exploited for CO₂–epoxide coupling reactions to produce five-membered cyclic carbonates under solvent-free conditions. Microwave synthesis was performed at different time intervals to reveal the formation of the crystals. Significant conversion of various epoxides was obtained at room temperature, with excellent selectivity toward the desired five-membered cyclic carbonates. The importance of the dual porosity and the synergistic effect of quaternary ammonium salts on efficiently catalyzed CO₂ conversion were investigated using various experimental and physicochemical characterization techniques, and the results were compared with those of the solvothermally synthesized MOF-205 sample. On the basis of literature and experimental inferences, a rationalized mechanism mediated by the zinc center of MOF-205 for the CO₂–epoxide cycloaddition reaction has been proposed

Atomic Layer Etching Mechanism of MoS₂ for Nanodevices

Among the layered transition metal dichalcogenides (TMDs) that can form stable two-dimensional crystal structures, molybdenum disulfide (MoS₂) has been intensively investigated because of its unique properties in various electronic and optoelectronic applications with different band gap energies from 1.29 to 1.9 eV as the number of layers decreases. To control the MoS₂ layers, atomic layer etching (ALE) (which is a cyclic etching consisting of a radical-adsorption step such as Cl adsorption and a reacted-compound-desorption step via a low-energy Ar⁺-ion exposure) can be a highly effective technique to avoid inducing damage and contamination that occur during the reactive steps. Whereas graphene is composed of one-atom-thick layers, MoS₂ is composed of three-atom-thick S_(top)Mo_(mid)S_(bottom) layers; therefore, the ALE mechanisms of the two structures are significantly different. In this study, for MoS₂ ALE, the Cl radical is used as the adsorption species and a low-energy Ar⁺ ion is used as the desorption species. A MoS₂ ALE mechanism (by which the S_(top), Mo_(mid), and S_(bottom) atoms are sequentially removed from the MoS₂ crystal structure due to the trapped Cl atoms between the S_(top) layer and the Mo_(mid) layer) is reported according to the results of an experiment and a simulation. In addition, the ALE technique shows that a monolayer MoS₂ field effect transistor (FET) fabricated after one cycle of ALE is undamaged and exhibits electrical characteristics similar to those of a pristine monolayer MoS₂ FET. This technique is also applicable to all layered TMD materials, such as tungsten disulfide (WS₂), molybdenum diselenide (MoSe₂), and tungsten diselenide (WSe₂)