Young-Duk Huh

Optical Properties of Three-Band White Light Emitting Diodes

A SrGa 2 S 4 :Eu phosphor was synthesized using a decomposition method without the use of toxic H 2 S gas SrGa 2 S 4 :Eu and ZnCdS:Ag,Cl phosphors were chosen to produce green and red emissions, respectively, under excitation from a blue light emitting diode (LED). A three band white LED was obtained through a combination of nonabsorbed blue emission from a blue LED and green and red emissions from the SrGa 2 S 4 :Eu and ZnCdS:Ag,Cl phosphors. The optical properties of the three band white LED are also discussed.

Analysis of the factors governing the enhanced photoluminescence brightness of Li-doped Y2O3:Eu thin-film phosphors

The authors studied the Li concentration dependence of the internal efficiency and extraction efficiency contributions to the enhanced photoluminescence (PL) brightness of the Li-doped Y2O3:Eu thin films. Experiments conducted with excitation at a wavelength of 254nm showed that adding 20mol% Li2CO3 to conventional Y2O3:Eu thin films improved their PL brightness by a factor of over 7.0. This improvement is attributed to enhanced internal factors (crystallinity, grains, and substitution of interstitial oxygen) and the increased optical volume (thickness), as well as the reduction of photon trapping in the high-index guiding layers of the Y2O3:Eu thin films. They also discuss how to quantitatively determine the contributions of the internal efficiency, the extraction efficiency, and the optical volume to the enhanced PL brightness of the Li-doped Y2O3:Eu thin-film phosphors.

Luminescence Properties of Potential Sr1 − x Ca x Ga2S4 : Eu Green- and Greenish-Yellow-Emitting Phosphors for White LED

We report the optical properties and morphological structure of potential green phosphors (Sr 1-x Ca x Ga 2 S 4 :Eu, x = 0.0-1.0) for application in phosphor-converted three-band white light emitting diodes (LEDs). A series of Sr 1-x Ca x Ga 2 S 4 :Eu phosphors was synthesized using NaBr-aided solid-state reactions in a flowing H 2 S stream. They all crystallized with the orthorhombic PbGa 2 Se 4 phase, and their emissions were efficiently excited up to 480 nm. The optical and morphological suitability of Sr 1-x Ca x Ga 2 S 4 :Eu phosphors is discussed in terms of their application as coatings for altering the color of blue LEDs.

Evaluation of new color metrics: guidelines for developing narrow-band red phosphors for WLEDs

Phosphor-converted white-light-emitting diodes (pc-WLEDs) are rapidly becoming more popular in the lighting industry due to their energy savings, long lifetimes and environmentally friendly characteristics. The color rendering index (CRI, Ra) and the visual energy efficiency (luminous efficacy of radiation, LER) are the critical criteria to be considered when developing novel red phosphors for use in warm white pc-WLEDs to replace incandescent and fluorescent lamps. In this regard, narrow-band red-emitting materials have been intensively developed in terms of the CRI and LER in an effort to complement the red deficiency of the widely commercialized Y3Al5O12:Ce3+ (YAG) phosphor-based pc-WLEDs. However, CRIs are limited in their inability to guarantee good saturated colors of illuminated objects under a warm white color. Instead of using CRI values as criteria, a two-measure system encompassing the color fidelity score (Rf) and the color gamut score (Rg) was developed and adopted as Illuminating Engineering Society of North America (IES) technical memorandum TM-30-2015 for correct evaluations of the color rendition and to guide the optimization of LED light sources. In this review, we summarize the recent findings on novel narrow-band red phosphors, the improved color and visual energy properties of these phosphors, and their ability to improve the optical properties of corresponding warm white pc-WLED lightings. To solve the complex problem of overestimation of high-CRI values, we discuss the ways in which the narrow-band red phosphors affect the new color metrics (Rf, Rg, and color icon) and the LERs of tri-color pc-WLEDs while varying the narrow scale and the blue-shifted peak position of red phosphors. These new color metrics and LER criteria provide guidelines with which many material and chemistry researchers can develop new red phosphors by optimizing the crystal structure, crystal rigidity, local symmetry, the number of available sites for activators, the selection of the host and activator, and other factors.

Preferential intercalation of isomers of anthraquinone sulfonate ions into layered double hydroxides

Intercalation compounds of anthraquinone sulfonate ions into layered double hydroxides (LDHs) have been directly synthesized. From the X-ray diffraction data and the guest size, the orientation of the intercalated anthraquinone sulfonate ions was determined. The intercalated anthraquinone-1,5-disulfonate (AQ15) and anthraquinone-2,6-disulfonate (AQ26) are arranged with their molecular planes and their sulfonate group axes tilted to the perpendicular of the hydroxide layer. Intercalated anthraquinone-2-sulfonate (AQ2) is arranged with the molecular plane and its sulfonate axis also perpendicular to the hydroxide layer with an antiparallel pattern. The product from a mixture of AQ2, AQ15 and AQ26 was also prepared; 86.8% of AQ2 and 13.2% of AQ26 were co-intercalated, whereas AQ15 was not intercalated into the LDH. These results indicate the possibility of a molecular recognition ability of LDH.

Structural effect of a two-dimensional SiO2 photonic crystal layer on extraction efficiency in sputter-deposited Y2O3:Eu3+ thin-film phosphors

We determined the effects of varying the structural parameters of two-dimensional (2D) square lattice SiO2 photonic crystal (PC) arrays on the efficiency of extraction of the photoluminescence (PL) of sputter-deposited Y2O3:Eu3+ thin films, in which the interface between the phosphor layer and the quartz substrate was modified with the 2D SiO2 PC pattern. The structural parameters of the 2D PC pattern were optimized according to their effects on the light extraction efficiency and similar trends were found for both the as-grown and the post-annealed samples; the samples with lattice constants in the range 400−600 nm exhibit the largest enhancement, there is increased enhancement with pattern height increases up to ∼420 nm, and the samples with a filling ratio of ∼0.20 exhibit the maximum enhancement of the extraction efficiency. For 2D PC patterns with a depth of 420 nm, a lattice constant of 600 nm, and a filling ratio of 0.2, the optimum PL extraction efficiencies of as-grown (350 °C) and post-annealed ...

Kinetics of thermal bleaching reaction of the photomerocyanine form of spironaphthooxazine

Abstract The thermal bleaching reactions of photomerocyanine (PMC) form of spironaphthooxazine (SNO) were investigated in various temperatures and solvents using laser flash photolysis. The thermal decay rate of PMC for SNO markedly decreases as temperature decreases and generally increases with solvent polarity. The solvent effect on the rate constant was well explained by using the π ∗ -scale. The activation energy and activation entropy also increase with the solvent polarity and the isokinetic relation has been observed. The combined results are interpreted in terms of dissociation of solvated PMC complex and/or ordered solvent structure around PMC molecules. Based on the experimental results, the potential energy surface along the reaction path in the ground state has been proposed.

Strong perturbation of the guided light within Y2O3:Eu3+ thin-film phosphors coated with two-dimensional air-hole photonic crystal arrays

Two-dimensional (2D) triangular-lattice air-hole nanoarrays of either SiO2 or SiNx were fabricated with nanosphere lithography as photonic crystal layers (PCLs) on sol-gel derived Y2O3:Eu3+ thin-film phosphors (TFPs). The effects of varying the air fill factor of the 2D PCLs on the light extraction efficiency of the TFPs were investigated. The integrated and normally directed photoluminescence extraction efficiencies were found to be improved by coating with 2D SiNx PCLs by factors of above 5.9 and 9.4, respectively; the 2D SiNx PCLs perturb the guided light more effectively than the 2D SiO2 PCLs because of their larger refractive index contrast.

Improved Cathodoluminescence Output Coupling of ZnS:Tb Thin-Film Phosphors Deposited on 2D SiO2 Corrugated Glass Substrate

A simple fabrication method of 2D SiO 2 corrugated layers and their suitability to enhance output coupling efficiency of a ZnS:Tb thin-film phosphor layer in an anode part of field emission display (FED) devices is discussed. For patterning of 2D dotted arrays of SiO 2 films, we used two-step irradiated hologram lithography and reactive ion etching. The formation of a wavelength-scale uniform 2D SiO 2 dotted array was confirmed by morphological analysis. We presented the finite-difference time-domain analysis for calculating the output light resulting from the insertion of a 2D SiO 2 corrugated layer into the interface between the glass substrate and the ZnS:Tb thin-film phosphor. A significant enhancement (a factor of 4.0-4.7) of the output coupling cathodoluminescence (CL) efficiency is reported in the low-voltage (0.5-4.5 kV) characteristics under electron-beam excitation for the ZnS:Tb thin-film phosphor deposited on a 2D SiO 2 corrugated substrate. The computational analysis, CL characterizations, and far-field measurement demonstrated that the 2D SiO 2 corrugated substrate served as diffraction scattering layers, increasing the output coupling efficiency, which is critically important for development of commercial FEDs employing thin-film phosphors.

Preparation with laser ablation and photoluminescence of Y3Al5O12:Ce nanophosphors

We prepared Y3Al5O12:Ce (YAG:Ce) nanophosphors with a mean size of 15 nm by using laser ablation of micron-sized commercial YAG:Ce target phosphor in 1,4-butanediol. The photoluminescence properties of the YAG:Ce nanophosphors were compared with those of micron-sized YAG:Ce phosphor. The elimination of back-scattering effect was confirmed by the optical properties of the YAG:Ce nanophosphors. The thermal stability of the YAG:Ce nanophosphors was also examined for the use of high power white LED. We convinced that the laser ablation method in 1,4-butanediol is an adequate method for the preparation of YAG:Ce nanophosphors for high power white LED with enhancement of the transmittance of the blue light from the InGaN chip.