Jongsu Kim

New Green Phosphor (Ba1.2Ca0.8-xEux)SiO4 for White-Light-Emitting Diode

A novel promising green phosphor, (Ba1.2Ca0.8-xEux)SiO4, has been developed for low- color-temperature solid-state lighting when used in combination with near-ultraviolet light-emitting diodes. It has a unique crystal structure (hexagonal, T phase) with five different cation sites among all possible (Ba,Sr,Ca)2SiO4:Eu2+ compounds. It exhibits a broad absorption band in the near-ultraviolet region of approximately 400 nm and an intense broad green emission. It shows a slightly higher thermal quenching temperature (175 °C) than conventional yellow (Ba,Sr,Ca)2SiO4:Eu2+ phosphors (150 °C).

Fabrication of a Functionally Graded Copper-Zinc Sulfide Phosphor

Functionally graded materials (FGMs) are compositionally gradient materials. They can achieve the controlled distribution of the desired characteristics within the same bulk material. We describe a functionally graded (FG) metal-phosphor adapting the concept of the FGM; copper (Cu) is selected as a metal and Cu- and Cl-doped ZnS (ZnS:Cu,Cl) is selected as a phosphor and FG [Cu]-[ZnS:Cu,Cl] is fabricated by a very simple powder process. The FG [Cu]-[ZnS:Cu,Cl] reveals a dual-structured functional material composed of dense Cu and porous ZnS:Cu,Cl, which is completely combined through six graded mediating layers. The photoluminescence (PL) of FG [Cu]-[ZnS:Cu,Cl] is insensitive to temperature change. FG [Cu]-[ZnS:Cu,Cl] also exhibits diode characteristics and photo reactivity for 365u2009nm -UV light. Our FG metal-phosphor concept can pave the way to simplified manufacturing of low-cost and can be applied to various electronic devices.

Acquisition and analysis of Terahertz Time Domain imaging and sensing

We present results of an acquisition of Terahertz imaging and its analysis in terms of frequency based on a Time Domain Terahertz Spectroscopy System. THz radiation was generated using a photoconductive antenna based on LT-GaAs and was detected by the electro-optical sampling in a ZnTe crystal. Broadband THz pulses were obtained in the range from 0.1 THz to 4 THz, with a peak intensity at 1 THz. The full transmission spectrum for different materials was measured, and frequency-dependent imaging characteristics were analyzed.

Synthesis and optical properties of ZnO nanowires for nanophotonics

High quality ZnO nanowires were synthesized by chemical vapor deposition using both a catalyst-assisted vapor-liquid-solid and a catalyst-free vapor-solid deposition approach. and their optical properties studied using photoluminescence and Raman spectroscopy combined with confocal laser scanning microscopy. Strong UV near band edge along with defect related visible luminescence emissions were observed and their relative intensity compared. We report here the growth of ZnO nanowires by chemical vapor deposition using both a catalyst-assisted vapor-liquid-solid and a catalyst-free vapor-solid deposition approach. The nanowires were characterized through scanning electron microscopy, x-ray diffraction, optical absorption, micro-photoluminescence, confocal Raman spectroscopy, and Terahertz time domain spectroscopy.

Excellent Brightness with Shortening Lifetime of Textured Zn2SiO4:Mn2+ Phosphor Films on Quartz Glass

Green-emissive textured Zn2SiO4:Mn2+ phosphor films were fabricated by the thermal diffusion of ZnO:Mn on quartz glass. The Zn2SiO4:Mn2+ phosphor films became textured along several hexagonal directions and their chemical composition was continuously graded at the interface. The decay time of Mn2+ was as short as 4.4 ms, and the optical transition probability of the films defined as the inverse of decay time showed a strong correlation with film texture degree as a function of annealing temperature. The brightest Zn2SiO4:Mn2+ film showed a photoluminescent brightness as high as 65% compared with a commercial Zn2SiO4:Mn2+ phosphor powder screen and a maximum absolute transparency of 70%. These excellent optical properties are explained by the combination of the unique textured structure and continuous grading of the Zn2SiO4:Mn2+ chemical composition at the interface.

Super-bright and short-lived photoluminescence of textured Zn2SiO4:Mn2+ phosphor film on quartz glass

Green-emissive textured Zn2SiO4:Mn2+ phosphor film was fabricated by a thermal diffusion of ZnO:Mn on quartz glass. The characterization has been performed in terms of Mn2+ ions concentration (Mn/Zn=1~9 mol %). As an increase of Mn2+ ions concentration in the Zn2SiO4:Mn2+ phosphor film, the emission peak was red shifted from 519 nm to 526 nm, and the decay time to 10% of the maximum intensity was shorter from 20 ms to 0.5 ms. All annealed Zn2SiO4:Mn2+ phosphor films became textured along some hexagonal directions on the amorphous quartz glass. The brightest Zn2SiO4:Mn2+ film at optimal Mn2+ concentration of 5 % showed the photoluminescence brightness of 65 % and the shortened decay time of 4.4 ms in comparison with a commercially Zn2SiO4: Mn2+ powder phosphor screen. The excellencies can be attributed to a unique textured structure.