Kyung Pil Kim

Control of Spectral Properties of Strontium-Alkaline Earth-Silicate-Europium Phosphors for LED Applications

SiO 4 -based phosphors doped by Eu 2 + were studied for use in white light emitting diodes (LEDs). Variations of excitation and emission spectra by changes in the alkaline earth ions in host lattice were studied. It was found that the Sr 2 SiO 4 :Eu -Ba 2 SiO 4 :Eu system was excellent for white LED applications with excitation in the 380-465 nm region. Specially, the yellow light intensity from (Sr,Ba)SiO 4 :Eu was comparable to yttrium aluminum gamet:Ce phosphors for excitation at 465 nm (blue-emitting LED). The peak excitation wavelength could be controlled by the Ba/Sr ratio in the host lattice. Also, it was found that (Sr,Mg)SiO 4 :Eu phosphors efficiently emit blue light at 405 nm excitation.

Tuning the Optical Properties of ( Sr , Ba ) 3Si6O3N8 : Eu Phosphor for LED Application

Highly efficient green-emitting (Sr,Ba) 3 Si 6 O 3 N 8 :Eu phosphors are prepared by solid-state reaction. Their optical properties are adjusted to be suitable for white light-emitting diode (LED) application. It is found that there are two ways to efficiently tune the excitation and emission spectra of this material system. One method is to adjust the ratio of Sr 2+ ions to Ba 2+ ions in the host lattice. The other method is to control the europium concentration. The luminescent intensity of the phosphor sample is optimized at Sr 2.8 Ba 0.2 Si 6 O 3 N 8 :Eu 0.2 . In this case, a synergy effect in the photoluminescence intensity is observed when substituting Ba 2+ for Sr 2+ ions while keeping the Sr 3 Si 6 O 3 N 8 crystal structure as the dominant phase. The excitation spectrum of the phosphor sample is broad due to the 4f-5d transitions, which is favorable to InGaN chip application. Also, an emission wavelength is easily tuned to the peak wavelength of 538 nm with the full width of half-emission maximums of 77 nm. Temperature-dependent photoluminescence intensity exhibits a low thermal quenching. We believe that this green oxynitride phosphor can be used as a solid-state lighting source.

Optical Characteristics of Polymorphic Y2 − x Si2O7 : Eu x 3 + Crystal for Lamp Application

We synthesized polymorphs of Y 1-x Si 2 O 7 :Eu x by a solid-state reaction and then examined the red-emitting Y 2-x Si 2 O 7 :Eu x (0 ≤ x ≤ 2) phosphors for their potential use in cold-cathode fluorescent lamps (CCFLs). The ultimate goal was to use polymorphs of Y 2 Si 2 O 7 to generate an emission shift to a deep red color. We found that all the crystalline phases of Y 2-x Si 2 O 7 :Eu x (α, β, γ and δ) could be made by the solid-state reaction. They are adjusted by changing the concentration of Eu ions and altering the annealing temperature. The 5 D 0 → 7 F 2 transition (peak wavelength of 626 nm) of Eu 3+ ion in the Y 2 Si 2 O 7 lattice is desirable for CCFL application. The maximal luminance at 626 nm was obtained with Y 1.84 S 2 O 7 :Eu 0.16 in the a phase. However, for the CCFL application, it is necessary to solve the side peak resulting from the 5 D 0 → 7 F 1 transition.

Effect of gadolinium incorporation on optical characteristics of YNbO4:Eu3+ phosphors for lamp applications

Eu3+-doped (Y,Gd)NbO4 phosphor was synthesized by solid-state reaction for possible application in cold cathode fluorescent lamps.A broad absorption band with peak maximum at 272 nm was observed which was due to the charge transfer between Eu3+ ions and neighboring oxygen anions.A deep red emission at the peak wavelength of 612 nm was observed which could be attributed to the 5D0→7F2 transition in Eu3+ ions.The highest luminance for Y1x-yGdyNbO4:Eux3+ under 254 nm excitation was achieved at Eu3+ concentration of 18 mol.％(x=0.18) and Gd3+ concentration of 8.2 mol.％ (v=0.082).The luminance of Y0.738Gd0.082NbO4:Eu3+0.18 was higher than that of a typical commercial phosphor Y2O3:Eu3+ and the CIE chromaticity coordinate was (0.6490,0.3506),which was deeper than that of Y2O3:Eu3+.The particle size of the synthesized phosphors was controlled by the NaCl flux and particle size as high as 8μm with uniform size distribution of particles was obtained.

Study on the Valence State of Eu Ions in Sr3Al2O6:Eu Phosphor

Europium-activated strontium aluminate phosphor, Sr3Al2O6:Eu2+, was prepared by solid-state reaction. The main concerns on the formation of a stable Sr3Al2O6 phase and the complete reduction of europium ions in this crystal phase, which is considered controversial in the literature and is available for white light-emitting-diode application, were addressed. It was found that the single phase of Sr3Al2O6 could be well formed and the valence state of europium ions depended on treatment temperature, flux, reduction gas, and the resultant stiffness of the host lattice. The redshift emission of Eu2+ in the Sr3Al2O6 host was not marked in our experimental regime. However, the green emission of Sr3Al2O6:Eu2+ increased gradually with the mole fraction of Al2O3 to SrCO3 in the starting materials owing to the complete formation of the Sr3Al2O6 phase.

Optical Properties of Blue-Light-Emitting (Ca,Sr)Mg2Si3O9:Eu2+ Phosphor

For light-emitting diode (LED) excitation at 400 nm, the optical properties of a Eu2+-activated CaO–SrO–MgO–SiO2 material system were investigated. All the materials were synthesized by solid state reaction. In particular, (Ca,Sr)Mg2Si3O9:Eu2+, which has the same crystal structure as CaMgSi2O6, was found to be promising as a blue-light-emitting phosphor for near UV LED application. The luminance intensity was optimized by controlling the Eu2+ concentration and the composition of the host lattice. The ratio of calcium ions to strontium ions was a convenient parameter for adjusting the maximum excitation peak to 400 nm, which is favorable for near UV LED excitation. The highest luminance intensity of Ca1-x-ySryMg2Si3O9:Eux2+ under 405 nm excitation was achieved at the Eu2+ concentration of x=0.01 and a Sr2+ concentration of y=0.3. The luminance intensity of (Ca,Sr)Mg2Si3O9:Eu2+ was found to be superior to that of a commercial blue-light-emitting BaMgAl10O17:Eu2+ phosphor, which is used for near-UV LED excitation.