Chulsoo Yoon

Eu2+ luminescence from 5 different crystallographic sites in a novel red phosphor, Ca15Si20O10N30:Eu2+

Because it is impossible to realize broadband-type, red-light-emitting phosphors in the oxide composition pool, both pure nitrides and oxynitrides have been considered as potential red phosphors for use in light emitting diode (LED) applications. However, only a limited number of red phosphors have been discovered. For instance, only a few red phosphors such as CaAlSiN3:Eu2+, Sr2Si5N8:Eu2+, and their variations are worthy of notice. We examined the CaO–Si3N4 binary system to discover novel oxynitride red phosphors. We finally identified a novel oxynitride phosphor, Ca15Si20O10N30:Eu2+, with a cubic structure in the Pa space group. This phosphor showed a broadband-type, red-light emission at UV and blue-light excitations. A continuous-wave-excited, steady-state photoluminescence (PL) and a pulsed-laser-excited, time-resolved PL for the discovered phosphor were investigated in conjunction with a structural analysis of Ca15Si20O10N30:Eu2+. The variation in decay rate with respect to the detection wavelength was examined in detail, and the energy transfer among activators located at 5 different crystallographic sites proved to be responsible.

Combinatorial chemistry of oxynitride phosphors and discovery of a novel phosphor for use in light emitting diodes, Ca1.5Ba0.5Si5N6O3:Eu2+

Solid-state combinatorial chemistry (combi-chem) has been proven to be a versatile tool for the discovery of novel phosphors in the multi-compositional (oxy)nitride search space. Since the advent of a unique solid-state combi-chem technique involving heuristic optimization strategies such as a non-dominated sorting genetic algorithm (NSGA), we have discovered several novel phosphors. The present investigation was focused on a phosphor that was previously discovered using this technique, but was incompletely identified, on pinpointing the optimum processing composition of the discovered phosphor exhibiting promising PL properties with the assistance of particle swarm optimization (PSO), and on identifying the exact structure based on synchrotron X-ray diffraction data. The exact stoichiometry of the discovered, unknown phosphor was determined to be Ca1.5Ba0.5Si5N6O3:Eu2+ and was indexed as a monoclinic lattice in the Cm space group, with lattice parameters a = 7.07033(2) A, b = 23.86709(7) A, c = 4.825304(15) A, and β = 109.0647(2)°. A structural determination process that involved a direct method, simulated annealing, and Rietveld refinement revealed that the exact structure of Ca1.5Ba0.5Si5N6O3:Eu2+ was somewhat a composite structure combining the well known CaSi2N2O2:Eu2+ and BaSi6N8O:Eu2+ structures. The discovered phosphor showed a broad-band type emission of yellow-red light under UV and blue light excitations, which makes it a promising candidate for practical use in LED applications.

Photoluminescence of Ce3 + -Activated β -SiAlON Blue Phosphor for UV-LED

Ce 3+ -doped β-SiAlON oxynitride blue phosphors with a composition of Ce x Si 6―z Al z O z N 8―z (x = 0.005―0.050 and z = 0.25) were synthesized by sintering with gas pressure. Phase purity, microstructure, valence state of Ce ions, and photoluminescence properties were studied. The prepared β-SiAlON:Ce 3+ revealed strong excitation peaks in the near-UV spectral region and intense broad-band blue emissions in the range 470―490 nm. The effects of Ce 3+ doping concentration and temperature dependence on the photoluminescence properties were investigated as well. These experimental results clearly proved that β-SiAlON:Ce 3+ is a good candidate for down-conversion blue phosphor in the application of a UV light emitting diode (LED).

The Composite Structure and Two-Peak Emission Behavior of a Ca1.5Ba0.5Si5O3N6:Eu2+ Phosphor

A Ca1.5Ba0.5Si5O3N6:Eu(2+) phosphor with a monoclinic lattice in the Cm space group exhibiting a composite structure consisting of CaSi2O2N2-like and BaSi6N8O-like structures was examined in terms of structure and luminescence. The luminescent properties of the Ca1.5Ba0.5Si5O3N6:Eu(2+) phosphor could be suitable for light-emitting diode applications since it exhibited a promising yellow (or amber) emission peaking at ∼ 570-590 nm at excitations of 450-460 nm. The present investigation was focused on verifying the composite structure by employing quantum mechanical calculations such as the Hartree-Fock ab initio calculation and a density functional theory calculation along with precise structural and compositional analyses. The two-peak emission behavior ascribed to the composite structure was also examined in terms of continuous wave and time-resolved photoluminescence. In addition, the energy transfer between two activator sites ascribed to the composite structure was examined in detail.

Highly improved reliability of amber light emitting diode with Ca –α–SiAlON phosphor in glass formed by gas pressure sintering for automotive applications

Phosphor in glass (PiG) with 40 wt% of Ca-α-SiAlON phosphor and 60 wt% of Pb-free silicate glass was synthesized and mounted on a high-power blue LED to make an amber LED for automotive applications. Gas pressure sintering was applied after the conventional sintering process was used to achieve fully dense PiG plates. Changes in photoluminescence spectra and color coordination were inspected by varying the thickness of the plates that were mounted after optical polishing and machining. A trade-off between luminous flux and color purity was observed. The commercial feasibility of amber PiG packaged LED, which can satisfy international regulations for automotive components, was successfully demonstrated by examining the practical reliability under 85% humidity at an 85°C condition.