Dae-Ho Yoon

Tailoring of Deep‐Red Luminescence in Ca2SiO4:Eu2+

We report a new dicalcium silicate phosphor, Ca(2-x)Eu(x)SiO4, which emits red light in response to blue-light excitation. When excited at 450u2005nm, deep-red emission at 650u2005nm was clearly observed in Ca1.2Eu0.8SiO4, the external and internal quantum efficiencies of which were 44u2009% and 50u2009%, respectively. The red emission from Ca(2-x)Eu(x)SiO4 was strongly related to the peculiar coordination environments of Eu(2+) in two types of Ca sites. The red-emitting Ca2SiO4:Eu(2+) phosphors are promising materials for next-generation, white-light-emitting diode applications.

The design of a ceramic phosphor plate with functional materials for application in high power LEDs

Here we report the design of a ceramic phosphor plate made up of different functional materials, such as SiO2 and MgO, and Lu3Al5O12:Ce3+ (LuAG:Ce) for the fabrication of remote phosphors in high-power LED applications. Remote phosphors can be applied directly to LEDs for thermal dissipation. The amounts of SiO2 and MgO were controlled to optimize the photoluminescence (PL) intensities. The addition of MgO was more effective, and the optimized content of MgO was between 5 and 5.5 wt%. Ceramic phosphor plates with MgO showed better properties than others with regard to thermal stability, performance degradation and CIE colour coordinate changes. MgO is expected to be an effective functional material for the fabrication of ceramic phosphor plates.

Characteristics of indium incorporation in InGaN/GaN multiple quantum wells grown on a-plane and c-plane GaN

We investigated the characteristics of InGaN-based multiple quantum wells (MQWs) grown on a-plane and c-plane GaN templates, which were grown by metal-organic chemical vapor deposition onto r-plane and c-plane sapphire, respectively. A shorter photoluminescence peak wavelength and peaks with larger full-width at half-maximum are observed for MQWs grown on an a-plane GaN template compared with a c-plane GaN template, despite the same growth conditions used. A growth model based on the atomic configuration of the growing surfaces is proposed to explain the difference in optical emission properties and indium incorporation between a-plane and c-plane MQWs.

A homojunction of single-crystalline β-Ga2O3 nanowires and nanocrystals

A homojunction of single-crystalline ?-Ga2O3 nanowires and nanocrystals was realized. Ga2O3 nanowires were synthesized at about 850?950??C using a mixture of Ga2O3 and graphite powder with vapor phase epitaxy. It was found that the ?-Ga2O3 nanocrystals form a single unit with the nanowires and are part of the same single crystal with a perfect lattice. Growth of ?-Ga2O3 nanocrystals follows the growth direction of the main ?-Ga2O3 nanowires. The distance between neighboring atoms in ?-Ga2O3 nanowires was 0.522?nm, which is similar to the theoretical value for bulk ?-Ga2O3.

Green-emitting Lu3Al5O12:Ce3+ phosphor as a visible light amplifier for dye-sensitized solar cells

A down convertor Lu3Al5O12:Ce3+ (LuAG) phosphor was used as a visible light amplifier for dye-sensitized solar cells (DSSCs). LuAG particles emitting green light with a wavelength of 470 to 650 nm, which is capable of stimulating Ru-dye molecules, enhanced the light-harvesting properties of a DSSC. The average short-circuit current density (Jsc) of the DSSC containing LuAG down conversion layers was approximately 9.6% higher than that of the mesoporous (mp) TiO2-based DSSC. Therefore, the average power conversion efficiency (PCE) for the DSSC with LuAG was 7.5% under AM 1.5 conditions, which was 8.7% higher than that for the mp-TiO2 based DSSC. The superior photovoltaic properties of the DSSC containing LuAG down conversion layers under UV light illumination revealed the effective wavelength down-converting properties of the LuAG down conversion layers.

Photoluminescence properties of Eu2+-doped Ba3Si6O9N4 phosphors synthesized by a gas reduction nitridation method for white light emitting diodes

A series of Eu2+-activated Ba3Si6N9O4 blue-green phosphors was synthesized using a gas reduction nitridation method with NH3 gas. The x-ray diffraction (XRD) pattern of the synthesized phosphors sintered at 1300 °C was well matched to the reference peaks. The synthesized Ba3Si6N9O4:Eu2+ phosphors absorbed light from the 350–470 nm region. The emission spectrum (monitored at λem = 494 nm) increased with increasing maintenance time using NH3 gas. The highest emission intensity was observed when the maintenance time was 12 h. In the XRD patterns, the relative intensity of the phosphors shows a maximum at a maintenance time of 12 h. On the other hand, maintaining for 12 h showed the best result. Also the Ba3Si6N9O4 phosphor exhibited strong thermal properties compared to the commercial phosphor. The green-emitting Ba3Si6N9O4:Eu2+ phosphors obtained could be applied as white light emitting diodes.

Enhancement of thermal quenching properties of a yellow-emitting SiO2-coated Y3Al5O12:Ce3+ phosphor for white light-emitting diode applications

To improve the phosphor thermal properties of phosphor-converted white light emitting diodes, the Y3Al5O12:Ce3+ phosphor was coated with SiO2. The photoluminescence properties of the SiO2-coated Y3Al5O12:Ce3+ phosphor showed excellent thermal properties at 460 nm. The temperature dependence of photoluminescence was measured from 20 to 180 °C. The SiO2-coated phosphor showed improved thermal quenching properties compared to pristine phosphors. We concluded that the SiO2-coated phosphor limits the thermal quenching properties. This result suggests that the SiO2-coated YAG:Ce3+ phosphor could be considered as a good candidate for white light-emitting diode applications.

Mesoporous amorphous binary Ru–Ti oxides as bifunctional catalysts for non-aqueous Li–O2 batteries

Mesoporous amorphous binary Ru-Ti oxides were prepared as bifunctional catalysts for non-aqueous Li-O2 batteries, and their electrochemical performance was investigated for the first time. A Li-O2 battery with mesoporous amorphous binary Ru-Ti oxides exhibited a remarkably high capacity of 27100 mAh g-1 as well as a reduced overpotential. A GITT analysis suggested that the introduction of amorphous TiO2 to amorphous RuO2 was responsible for the enhanced kinetics toward both the oxygen reduction reaction and oxygen evolution reaction. Excellent cyclic stability up to 230 cycles was achieved, confirming the applicability of the new bifunctional catalyst in non-aqueous Li-O2 batteries.

In situ synthesis of amorphous titanium dioxide supported RuO2 as a carbon-free cathode for non-aqueous Li–O2 batteries

Amorphous TiO2 supported crystalline RuO2 (a-TiO2/c-RuO2 hybrid) was prepared as a carbon-free cathode by in situ sol–gel synthesis, and its electrochemical performance in non-aqueous Li–O2 batteries was investigated for the first time. The a-TiO2/c-RuO2 hybrid enhanced battery performance, and this enhancement was attributed to the crystallinity of the TiO2. It was found that amorphous TiO2 is more electrochemically active toward the discharge/charge processes than crystalline TiO2. The a-TiO2/c-RuO2 hybrid exhibited good reversibility as well as cyclic stability, making it a promising carbon-free cathode material for non-aqueous Li–O2 batteries.

Fabrication and analysis of luminous properties of ceramic phosphor plate for high-power LED

LEDs are considered to be an alternative for enhancement of energy efficiency, applied for numerous areas such as display, automotive headlight not only lights. Yellow phosphor is generally utilized with blue LEDs to generate WLED, Y Al O :C e is typically used as the yellow phosphor. The phosphor, mixed with epoxy resin, has been used by being spread and hardened on the blue LED chip. This paste-based packaging gives rise to problems of degradation of phosphor by heat and decrease of luminous efficiency. Although phosphor plate is used instead of the epoxy-phosphor mixture to solve these problems, loss of luminous efficacy by total internal reflection inside the plate also should be solved. In this study, we coated the side of the plate with silver as one of the solution.