Theoretical models to integrate pores size distribution and to analyze permeation of a droplet into coated paper are proposed. Droplet permeation into porous media plays an important role in the image formation of inkjet printing. In this study, pores size distribution of porous media of different coated papers was investigated using nitrogen adsorption-desorption method. Based on the pores size distribution, theoretical model for estimating permeation amount into coated paper of pico-scale droplet was proposed, with the consideration of Lucas-Washburn equation. In order to verify the theoretical model, experiments of droplet observation with pure water and several propylene glycol (PG) aqueous solutions were conducted. For some paper samples, results of theoretical calculation and experiments generally agree well. However, there are cases where the theoretical predictions show discrepancy. Possible reasons are considered to be deviation of pores size distribution. Moreover, the accuracy of estimated evaporation volume by Hu and Larson's Equation, and the complex shape of the pores in coated layer also contribute to the errors. The effect of these factors still needs to be further explored.

The development of efficient red light-emitting diodes (LEDs) based on GaN is pivotal to ultra-compact, full-color, and high-resolution micro-LED (μ-LED) displays. In red LEDs using Eu-doped GaN (GaN : Eu), the peak position of the emission is extremely stable with respect to ambient temperature and injected current. The output power of the red LED has already proceeded to 1 mW due to the intrinsic and extrinsic control of Eu emission. The effect of carrier sidewall-related non-radiative recombination on photoluminescence quantum efficiency is negligible due to the limited carrier diffusion length of GaN : Eu. We also demonstrate a monolithic vertical stack full-color LED consisting of GaN : Eu and InGaN quantum wells.

In recent years, metal halide perovskites have emerged as one of the most promising light-emitting materials owing to their high color purity and tunability. In addition, their favorable optoelectrical properties have enabled their use in a wide variety of applications, such as photovoltaics and light-emitting diodes (LEDs). In particular, bright luminescent colloidal nanocrystals (NCs) show considerable potential for LEDs applications. However, the low stability and durability of perovskite NCs hamper their commercialization. Therefore, post-synthetic surface modification of perovskite NCs has become an important strategy for achieving not only excellent optical properties but also high-efficiency LEDs. Techniques such as ligand exchange, anion exchange, metal doping, and novel purification processes have been demonstrated to passivate surface defects and remove impurities from the NCs. In this review, we summarize our recent work on post-synthetic surface modification treatments of perovskite NCs toward achieving highly efficient LEDs.

Along with the development of organic light-emitting diodes (OLEDs), there has been continuous development of light amplification by stimulated emission of radiation (laser) technology based on organic materials over the last 30 years. Although indications of current injection lasing using organic materials have been reported by some groups, extensive studies must still be conducted with the aim of reducing the lasing threshold to provide a robust system that is resistant to high-power excitation. One of the major causes of the poor stability of existing systems is triplet accumulation. To address this issue, triplet excitons must be removed rapidly from the active device layers. This review focuses on a unique laser system composed of a lasing dye with a small singlet-triplet energy gap ΔE_ST and a triplet scavenging host that allows true continuous-wave (CW) organic lasing operation under optical pumping conditions. Furthermore, efficient suppression of the electroluminescence (EL) roll-off that occurs in OLEDs during high current injection is demonstrated, indicating the potential for application of this technology to organic semiconductor laser diodes (OSLDs).

Various optical device applications of surface plasmon (SP) resonance have been reported, in particular, we have discovered that plasmonics can significantly improve the luminous efficiency of materials, and applicable to high-efficiency light-emitting devices include light-emitting diodes (LEDs). In order to develop efficient plasmonic LEDs at a practical level, tuning the SP resonance wavelength is one of the most important issues as well as device development. Based on the SP resonance, the artificial materials whose optical properties can be greatly modulated and controlled by metallic nanostructures are called plasmonic metamaterials. Many types of plasmonic metamaterials have been fabricated dependent on top-down microfabrication techniques such as electron beam lithography. Here, we present a new plasmonic metamaterial based on random nanostructures that can be fabricated easily, inexpensively, and over a large area by a bottom-up approach and enable tuning of the SP resonance in the deep ultraviolet (UV) and near infrared (IR) wavelength regions.

In this review, principle and experimental for electrogenerated chemiluminescence (ECL) are briefly introduced. ECL involves electrode reactions (heterogeneous electron transfer), diffusion, homogeneous electron transfer, formation of excited states, and emission process in solution. ECL behaviors are frequently discussed based on thermodynamics, i.e. , whether the emissive excited states can be formed via homogeneous electron transfer or not. Here, ECL mechanism in mixed system, which was revealed based on the homogeneous electron transfer process and electronic structure of several photoluminescent materials, are focused. It is demonstrated that kinetic analysis is useful to discuss the ECL efficiency quantitatively. The rate constant of the homogeneous electron transfer depends on the energy gap between the initial (a pair of oxidized and reduced forms of photoluminescence material) and final states (singlet and triplet excited states), and the gap can arbitrarily be controlled in a mixed ECL system, which contains coreactant systems.

Light-emitting electrochemical cells (LECs) are one of the simplest light-emitting devices. Numerous LECs have been demonstrated using various organic materials, including fluorescent polymers. Recently, advancements in device technologies have allowed the realization of LECs using inorganic materials, especially with low-dimensional materials that are typically not easy to apply in light-emitting devices. This review focuses on the current progress in the device structures, operating mechanisms, and advantages of LECs that enable the utilization of diverse inorganic materials. In particular, recent advances in low-dimensional-material LECs, such as those employing quantum dots and two-dimensional materials, are highlighted. Owing to their advantages, low-dimensional-material LECs, including chiral light-emitting devices, offer significant functionalities and device utilities, exhibiting promise for new optoelectronic applications with emergent materials.