Jae Soo Yoo

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.

Second-harmonic generation (SHG) and photoluminescence properties of noncentrosymmetric (NCS) layered perovskite solid solutions, CsBi1−xEuxNb2O7 (x = 0, 0.1, and 0.2)

Solid solutions of noncentrosymmetric (NCS) Dion–Jacobson phases, CsBi1−xEuxNb2O7 (x = 0, 0.1, and 0.2), have been synthesized through standard solid-state reactions. Powder X-ray diffraction analysis suggests that the solid solutions crystallize in the NCS polar orthorhombic space group, P21am (no. 26). CsBi1−xEuxNb2O7 reveal layered perovskite structures that are composed of corner-shared distorted NbO6 octahedra and the stereoactive A-cation, Bi3+. Powder second-harmonic generation (SHG) measurements of the reported materials show that CsBiNb2O7, CsBi0.9Eu0.1Nb2O7, and CsBi0.8Eu0.2Nb2O7 possess SHG efficiencies 50, 30, and 25 times that of α-SiO2, respectively. The decrease in the SHG for Eu3+-doped compounds is attributable to the deficiency of the net moment generated from the polyhedra of the polarizable lone pair cation, Bi3+. The stronger electric dipole transition bands compared to those of the magnetic dipole transitions in the photoluminescence emission spectra of CsBi1−xEuxNb2O7 confirm the asymmetric coordination environment of Bi3+/Eu3+ sites in the framework.

Solid-state synthesis, structure, second-harmonic generation, and luminescent properties of noncentrosymmetric BaSi7N10:Eu2+ phosphors

Eu-doped BaSi7N10 phosphors were synthesized by high-temperature solid-state reactions and their structural and optical characteristics were investigated. The X-ray diffraction analysis shows that the phosphor crystallizes in the monoclinic noncentrosymmetric space group, Pc (no. 7). The second-harmonic generation measurements on BaSi7N10 using 1064 nm radiation reveal frequency doubling efficiency similar to that of KH2PO4 (KDP). With respect to the doped BaSi7N10, the SHG efficiencies decrease with Eu2+ doping. The particle size-dependent SHG characteristics revealed that BaSi7N10 is a type-1 nonphase matchable material. The phosphor shows photoluminescence (PL) emission at 475 nm, which could be attributed to the 4f6 5d → 4f7 transition in the Eu2+ ion and the peak emission wavelength does not shift with more Eu2+ addition. The photoluminescent excitation (PLE) spectra show broad excitation in the range 250–400 nm peaking at 332 nm. The temperature-dependent PL emission exhibits small thermal quenching of the phosphor with thermal activation energy ΔE = 0.18 eV for the Ba0.92Eu0.08Si7N10 phosphor. The phosphor exhibits good surface morphology with uniformly hexagonal-shaped particles with an average size of 15 μm. The purity of emission was gauged by CIE coordinates (x, y) and the coordinates for the phosphor exhibiting maximum luminescence intensity were (0.18, 0.29).

Effect of ZnCdSe/ZnSe Core/Shell Quantum Dots on the Efficiency of Organic Photovoltaic Cells

Efficiency of Organic Photovoltaic Cells Kyoung Soon Choi, Yensil Park, Jae Soo Yoo,* Soo Young Kim, Heesun Yang, Gwan Ho Jung, and Jong-Lam Lee* School of Chemical Engineering and Materials Science, Chung-Ang University, Dongjak-gu, Seoul 156-756, Korea Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea

Photoconversion Mechanisms and the Origin of Second-Harmonic Generation in Metal Iodates with Wide Transparency, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) and NaLa(IO3)4:Ln3+ (Ln = Sm and Eu)

Four new metal iodates, namely, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu), and a series of NaLa(IO3)4:Ln3+ (Ln = Sm and Eu) solid solutions were synthesized through hydrothermal reactions. The structures of the title compounds are similar to that of NaY(IO3)4 crystallizing in the acentric monoclinic space group Cc. The iodate materials reveal layered structures composed of LnO8 square antiprisms and IO3 polyhedra, in which each layer is connected by the I···O interactions. NaLa(IO3)4 suggests a great potential as a matrix for optical source attributed to its acentricity and broad transparency from visible to mid-IR region. The photoluminescence properties depending on the concentration of Sm3+ reveal that NaLa(IO3)4:Sm3+ undergoes a self-quenching relaxation over 7 mol % of Sm3+ by dipole-quadrupole interactions. Attributable to the asymmetric coordination environment of Ln3+, stronger electric dipole transitions compared to magnetic dipole transitions were observed for both compounds. In addition, the materials exhibit strong second-harmonic generation (SHG) responses and are type I phase-matchable. The structural origin of the SHG properties for the reported iodates is elucidated.

Reduced graphene oxide enwrapped phosphors for long-term thermally stable phosphor converted white light emitting diodes.

The long-term instability of the presently available best commercial phosphor-converted light-emitting diodes (pcLEDs) is the most serious obstacle for the realization of low-cost and energy-saving lighting applications. Emission from pcLEDs starts to degrade after approximately 200 h of operation because of thermal degradation of the phosphors. We propose a new strategy to overcome this thermal degradation problem of phosphors by wrapping the phosphor particles with reduced graphene oxide (rGO). Through the rGO wrapping, we have succeeded in controlling the thermal degradation of phosphors and improving the stability of fabricated pcLEDs. We have fabricated pcLEDs with long-term stability that maintain nearly 98% of their initial luminescence emission intensity even after 800 h of continuous operation at 85 °C and 85% relative humidity. The pcLEDs fabricated using SrBaSi2O2N2:Eu2+ phosphor particles wrapped with reduced graphene oxide are thermally stable because of enhanced heat dissipation that prevents the ionization of Eu2+ to Eu3+. We believe that this technique can be applied to other rare-earth doped phosphors for the realization of highly efficient and stable white LEDs.

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.