Hyoung Sun Yoo

Luminescence Properties and Energy Transfer of Site-Sensitive Ca6−x−yMgx−z(PO4)4:Euy2+,Mnz2+ Phosphors and Their Application to Near-UV LED-Based White LEDs

On the basis of the structural information that the host material has excellent charge stabilization, blue-emitting Ca(6-x-y)Mg(x)(PO(4))(4):Eu(y)(2+) (CMP:Eu(2+)) phosphors were synthesized and systematically optimized, and their photoluminescence (PL) properties were evaluated. Depending upon the amount of Mg added, the emission efficiency of the phosphors could be enhanced. The substitution of Eu(2+) affected their maximum wavelength (lambda(max)) and thermal stability because the substitution site of Eu(2+) could be varied. To obtain single-phase two-color-emitting phosphors, we incorporated Mn(2+) into CMP:Eu(2+) phosphors. Weak red emission resulting from the forbidden transition of Mn(2+) could be enhanced by the energy transfer from Eu(2+) to Mn(2+) that occurs because of the spectral overlap between the photoluminescence excitation (PLE) spectrum of Mn(2+) and the PL spectrum of Eu(2+). The energy transfer process was confirmed by the luminescence spectra, energy transfer efficiency, and decay curve of the phosphors. Finally, the optimized Ca(6-x-y)Mg(x-z)(PO(4))(4):Eu(y)(2+),Mn(z)(2+) (CMP:Eu(2+),Mn(2+)) phosphors were applied with green emitting Ca(2)MgSi(2)O(7):Eu(2+) (CMS:Eu(2+)) phosphors to ultraviolet (UV) light emitting diode (LED)-pumped white LEDs. The CMS:Eu(2+)-mixed CMP:Eu(2+), Mn(2+)-based white LEDs showed an excellent color rendering index (CRI) of 98 because of the broader emission band and more stable color coordinates than those of commercial Y(3)Al(5)O(12):Ce(3+) (YAG:Ce(3+))-based white LEDs under a forward bias current of 20 mA. The fabricated white LEDs showed very bright natural white light that had the color coordinate of (0.3288, 0.3401), and thus CMP:Eu(2+),Mn(2+) could be regarded as a good candidate for UV LED-based white LEDs.

Luminescent and structural properties of (Sr(1-x),Ba(x))3MgSi2O8:Eu2+: effects of Ba content on the Eu2+ site preference for thermal stability.

A blue-color-emitting phosphor, (Sr(1-x),Ba(x))(3)MgSi(2)O(8):Eu(2+) (SB(x)MS:Eu(2+)), with various Ba contents, was synthesized, and its thermal stability was evaluated. Depending upon the Ba content, the dominant emission wavelength of the SB(x)MS:Eu(2+) phosphor could be tuned, and good photoluminescence (PL) properties were obtained under an excitation by a 147 nm source. The PL behavior showed that the thermal stability of SB(x)MS:Eu(2+) baked at 500 degrees C was dependent upon the Ba content. On the basis of the results of electron spin resonance on Eu(2+) and Rietveld refinement against neutron powder diffraction data of the phosphor, it was found that the improved thermal stability of SB(x)MS:Eu(2+) could be ascribed to Eu(2+) occupying preferred Sr sites among three possible locations depending upon the amount of Ba substitution. It is also inferred from this observation that the average interatomic distances between Eu(2+) and O (d(Eu-O)) of the different Sr sites may play an important role in the thermal stability of the phosphor.

Effects of Eu2 + Concentration Variation and Ce3 + Codoping on Photoluminescence Properties of BaGa2S4 : Eu2 + Phosphor

Ba1�x�2yEuxCeyLiyGa2S4 phosphor samples were synthesized by a solid-state reaction method. The lattice parameter of the Ba1�xEuxGa2S4 phosphor samples was linearly decreased from 12.654 to 12.492 A, and the main emission wavelength was shifted from 501 to 530 nm with increase of the Eu 2+ ion concentration x from 0.05 to 0.50. The relative photoluminescence PL intensity under 450 nm excitation was increased with increase of the x from 0.05 to 0.25 due to the excitation band extended to the longer wavelength. However, when x 0.30, the PL intensity was decreased due to nonradiative transitions among the Eu 2+ ions and the formation of EuGa2S4 impurity phase. The Commission International de l’Eclairge chromaticity coordinates were also largely shifted from the bluish-green emitting region x = 0.143, y = 0.506 to the green emitting one x 2.414, y 0.658 by increasing the x from 0.05 to 0.50. The energy transfer from the Ce 3+ ion to the Eu 2+ ion enhanced the PL intensity of the Eu 2+ ion emission under near ultraviolet and blue excitation. The PL intensity of the Ba0.900Eu0.050Ce0.025Li0.025Ga2S4 phosphor sample was higher by 24% than that of the Ba0.95Eu0.05Ga2S4 phosphor sample under 450 nm excitation.

Synthesis of Phosphor Photonic Crystals by Self-Assembly of SiO2 / Y2O3 : Tb3 + Core/Shell Particles and Its Photoluminescence Properties

3+ core/shell particles were prepared by a heterogeneous precipitation coating of the phosphor shell on the surface of the core particles. The shell thickness and the calculated crystallite size were increased with the core size and, thus, the photoluminescence PL intensity was also increased. The phosphor photonic crystals PCs were fabricated by self-assembly of the core/shell particles. The PL intensity of the phosphor PCs was higher by around 2.0 times than that of the randomly stacked reference sample due to the high packing density and quality. The suppression of the PL emission peaks by the overlap with the stopband and the enhancement of the emission peak at the blue edge of the stopband were observed. The color purity of the phosphor PCs was definitely improved by the design of the stopband position.

Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)(3)phen phosphor and its enhanced photoluminescence.

Luminescence films were prepared by infiltration of the tris(dibenzoylmethane) mono(1, 10-phenanthroline) europium incorporated ormosil into colloidal SiO(2) photonic crystal templates. Because a stopband of the template was not overlapped with the PL excitation and emission bands, the stopband did not suppress the PL intensity. The PL intensity of the infiltrated film into the template was about 13.1 times higher than that of the plane film prepared without the template. Three major terms, which are the mass term, the scattering term, and the crystallinity term, were considered as factors that improve the PL intensity. The relative ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2.

Organic wavelength‐converting‐film‐based hybrid planar white light‐emitting diodes

— In this study, organic wavelength-converting films (WCFs) applied to InGaN blue LED-based hybrid planar WLED has been fabricated. The organic dye layer in the WCF was formed between the upper and bottom polymer sheets by using a simple roll-laminating technique. Subsequently, the hybrid planar WLEDs have been fabricated based upon these films. The luminous efficiency of green WCF-based hybrid planar WLEDs with a single blue LED chip was 34.6 lm/W and that of red-WCF-assisted green WCF-based hybrid planar WLEDs was 27.3 lm/W under 20 mA. The use of WCF to fabricate hybrid planar WLEDs showed better stability than that of directly coating organic color-convergence materials (CCMs) on the LED chips. It only decreased to about 10% of the initial wavelength-converting intensity after 1 hour of continual operation at 20 mA.

Effects of controlling crystallites of core/shell SiO 2 /Y 2 O 3 : Eu 3+ on its photoluminescence properties

Recently, core/shell particles have been extensively investigated in many applications because of the fact that their properties can be widely and easily tailored by changing their sizes, morphologies, compositions, and structures. Monodisperse spherical core/shell phosphor is one of the most interesting practical applications of the core/shell particles. Because the monodisperse spherical phosphor particles have many advantages such as high mobility and good dispersion properties in paste state, high packing density, and low light scattering on their surfaces, it is ideal for a high definition display [1–3]. In addition, because it is relatively easy to separately control crystallite size of the core/shell particles from their particle size, it is possible to separately investigate effects of the crystallite/particle sizes on the photoluminescence (PL) properties of a phosphor.