Sun Woog Kim

Site engineering concept of Ce 3+ -activated novel orange-red emission oxide phosphors

Abstract: Novel Sr6(Y1-xCex)2Al4O15 (0.005 ≤ x ≤ 0.20) phosphors were synthesized in a single phase form by a conventional solid-state reaction method. These phosphors can be achieved the emission color tunable from blue to orange-red by controlling the Ce3+ doping site in the Sr6Y2Al4O15 lattice and exhibit orange-red emission centered on 600 nm by blue light irradiation as the Ce3+ concentration was increased. To the best of our knowledge, this is the first report of Ce3+ doping oxide phosphor exhibiting orange-red emission centered on 600 nm under blue light excitation.

Unusual, broad red emission of novel Ce3+-activated Sr3Sc4O9 phosphors under visible-light excitation

Most conventional white light emitting diodes (white-LEDs) that are widely used as a new lighting system in next generation lights with Y3Al5O12:Ce3+ (YAG:Ce3+)-based phosphors have a low colour rendering index (CRI) because the YAG:Ce3+ phosphor shows a weak emission intensity in the red spectral region. Therefore, discovering a red-emitting phosphor with a high-efficiency is quite important to enhance the CRI of white-LEDs. In this study, we successfully discovered a novel, red-emitting Ce3+-activated Sr3Sc4O9 phosphor that can be excited by blue-light irradiation at 425 nm. A crystal structure of the host material was first determined by Rietveld refinement, which indicated that it should be isostructural with Ba3Ln4O9 (Ln = Sc, Y and Dy-Lu). As the Ce3+ content increased, the X-ray diffraction patterns shifted to a lower angle, which suggested that the Ce3+ ion could substitute Sc in the Sr3Sc4O9 host. Under UV and blue-light excitation, the Ce3+-activated Sr3Sc4O9 phosphor exhibited a broad emission band with a maximum peak at 620 nm, and its full width half maximum (FWHM) was 180 nm (4530 cm−1). The highest emission intensity was obtained for Sr3(Sc0.997Ce0.003)4O9, and the internal quantum efficiency of this phosphor under excitation at 425 nm was 53%. To the best of our knowledge, the emission band of the Ce3+-activated Sr3Sc4O9 phosphor is the longest wavelength for a reported Ce3+-activated oxide phosphor.

Novel Soft Chemical Synthesis Methods of Ceramic Materials

We report novel soft chemical synthesis method, solid hydratethermal reaction (SHR) method as a new soft chemistry. This method is very simple and can synthesize the ceramic materials just by storing the mixture of raw materials added a small amount of water in a reactor at low temperature below 373 K. For example, nanosize YVO4 (under 100 nm in diameter) was obtained using the SHR method.

Synthesis and the luminescent properties of NaAlSiO4:Eu2+ phosphor using SiO powder as a silica source

A yellow-emitting silicate phosphor, NaAlSiO4:Eu2+, was synthesized through a new reducing technique, using SiO powder as a silica source, during the preparation process. The addition of SiO powder as a silica source significantly enhanced the Eu2+ content, which led to an increase in the photoluminescence emission intensity of the NaAlSiO4:Eu2+ phosphor. The optimization of the composition resulted in obtaining the maximum emission intensity for NaAlSiO4:7 mol% Eu2+.

Synthesis of blue-emitting (K 1−xNa x)Mg 4(PO 4)3:Eu 2+ phosphors

Blue-emitting (K 1−xNa x)Mg 4(PO 4)3:Eu 2+ phosphors were synthesized through a conventional solid-state reaction method, and their photoluminescence properties were characterized. These phosphors exhibited the typical emission band assigned to the transition from 4f65d1 to 4f7 of Eu2+, and the emission intensity was effectively enhanced by Na +-doping into the host lattice. The highest blue emission peak intensity was obtained for (K 0.65Na 0.35)Mg 4(PO 4)3:1 mol%Eu2+, where the relative emission peak intensity was 33% of that for the commercial blue-emitting BaMgAl 10O 17:10 mol%Eu 2+ phosphor. In addition, these phosphors have excellent thermal stability in the temperature quenching effect.

Determination of the crystal structure and photoluminescence properties of NaEu1−xGdx(MoO4)2 phosphor synthesized by a water-assisted low-temperature synthesis technique

A single-phase red-emitting NaEu(MoO4)2 phosphor with nanosized particles was synthesized using a water-assisted solid state reaction (WASSR) method, a new low-temperature synthesis method developed by our group. NaEu(MoO4)2 exhibits a monoclinic structure with a space group C2/c (no. 15), which is composed of a Na/EuO8 dodecahedron with the site occupancy of Na/Eu = 1 : 1 and an MoO4 tetrahedron. To enhance the emission intensity of the NaEu(MoO4)2 phosphor, Gd3+ was doped into the Eu3+ sites and NaEu1−xGdx(MoO4)2 (0 0.30. This indicates that the optimum composition of the phosphor for achieving a high emission intensity that is ∼3.8 times higher than that of the NaEu(MoO4)2 (x = 0) phosphor is NaEu0.70Gd0.30(MoO4)2. These phosphors exhibit a granular particle morphology with the particle size of the phosphor being ∼200 nm.

Color Tuning of Oxide Phosphors

In white-LED application, the development of novel phosphors with high luminescence efficiency and high-emission color purity is extremely important because the performance of white LEDs is directly affected by the luminescence properties of the phosphors. In this chapter, therefore, we review some Eu2+- and Ce3+-activated phosphor materials and discuss the luminescence properties of these phosphors from the viewpoint of the relationship between the emission-band position and the crystallographic environment including the local site symmetry of the dopant site in the host lattice. We also propose a material-design concept to develop long wavelength-emission phosphors.

Synthesis of Nano-Sized Materials Using Novel Water Assisted Solid State Reaction Method

We report synthesis of nanosized oxide materials using a novel water assisted solid state reaction (WASSR) method. This novel soft chemical synthesis method is very simple and can synthesize nanoparticle materials just by storing or mixing raw materials added a small amount (typically 10wt%) of water in a reactor at low temperature below 373 K. Combinations of raw materials have a significant influence on the reaction rate.

Nanophosphors synthesized by the water-assisted solid-state reaction (WASSR) method: Luminescence properties and reaction mechanism of the WASSR method

ABSTRACT Ceramic materials have been widely used in various applications and are significantly important in our daily life. The various methods for synthesizing ceramics powder materials have been already reported, and the synthesis method is known to considerably affect the characteristics and particle morphology of ceramic powder materials. We have recently developed a novel soft chemical synthesis method, named water-assisted solid-state reaction (WASSR) method, which can be synthesized at low temperatures below 100°C. We further demonstrated the effect and availability of this method on the synthesis process for ceramic oxide materials. In this paper, we review some results for the nanophosphors synthesized by the WASSR method and discuss the reaction mechanism of this method.

Luminescence of Phosphor Balls Prepared Using Melt Quenching Synthesis Method

Well-grown M3MgSi2O8(M = Ca, Sr and Ba):Eu2+ phosphors were synthesized by novel melt quenching synthesis method. The luminescence property of the Sr3MgSi2O8:Eu2+ phosphor synthesized by the melt synthesis method is comparable to those of the sample synthesized by a conventional solid state reaction. Sintered phosphor balls with excellent luminescent characteristics were obtained.