Shiro Funahashi

Structure and luminescence of a novel orange-yellow-emitting Ca1.62Eu0.38Si5O3N6 phosphor for warm white LEDs, discovered by a single-particle-diagnosis approach

An orange-yellow-emitting Ca1.62Eu0.38Si5O3N6 phosphor was discovered by a single-particle-diagnosis approach. The crystal structure was analysed by single crystal X-ray diffraction, revealing that it crystallizes in a monoclinic system with the space group of Cm (No. 8), cell parameters of a = 7.0595(2) A, b = 23.7504(6) A, and c = 9.6345(2) A. Both the single crystal and the powder sample show similar luminescence properties. The excitation spectrum ranges from UV to blue light regions, enabling its use in UV, or blue LED driven white-LEDs. The new phosphor exhibits a two-peak emission behavior, with a very weak blue emision band centered at about 470 nm and a strong orange-yellow emission band centred at 592– 600 nm, which correspond to the emissions of two different Eu2+ sites in the structure. Subsequently, an efficient energy transfer between two different Eu2+ sites was discussed. In addition, the new phosphor shows high thermal stability. At 150 °C, the luminescence intensity of the single crystal remains 84% of that measured at room temperature, with high activation energy for thermal quenching (0.240 eV). The results suggest that the new orange-yellow phosphor has potential application in vivid indoor lighting or displays.

Significant colour tuning via energy transfer in Eu2+ solely doped La2.5Ca1.5Si12O4.5N16.5

Eu2+ doped La2.5Ca1.5Si12O4.5N16.5 powders have been prepared via a solid-state reaction synthesis and have been studied as a function of the Eu concentration. A large shift of the position of the Eu2+ 5d–4f emission band from 495 to 575 nm was observed with increasing Eu concentration, which changes the emission colour of the phosphors from blue-green for low Eu concentrations towards orange for highly concentrated samples. Similarly, the maximum in the excitation spectrum shifted from 388 to 511 nm. The shifts are due to the two distinct crystallographic sites on which the Eu2+ ions are substituted, which give rise to a high and a low energy emission band. Increased energy transfer at higher Eu concentration from the high to the low energy sites results in a relative increase of the low energy site emission. Temperature dependent luminescence measurements reveal a slight blue shift of the luminescence with increasing temperature due to energy back-transfer and show that the emission is temperature stable with half of the luminescence maintained up to 600 K.

Substitutional disorder in Sr2−yEuyB2−2xSi2+3xAl2−xN8+x (x≃ 0.12, y≃ 0.10)

A novel nitride, Sr2-yEuyB2-2xSi2+3xAl2-xN8+x (x ≃ 0.12, y ≃ 0.10) (distrontium europium diboron disilicon dialuminium octanitride), with the space group P62c, was synthesized from Sr3N2, EuN, Si3N4, AlN and BN under nitrogen gas pressure. The structure consists of a host framework with Sr/Eu atoms accommodated in the cavities. The host framework is constructed by the linkage of MN4 tetrahedra (M = Si, Al) and BN3 triangles, and contains substitutional disorder described by the alternative occupation of B2 or Si2N on the (0, 0, z) axis. The B2:Si2N ratio contained in an entire crystal is about 9:1.

New phosphors Eu2+ and Ce3+ doped Sr4-x(Si,Al)19+x(N,O)29+x for white LED applications (Conference Presentation)

Light-emitting diodes (LEDs) have been steadily consolidating their share in the lighting and display market. Phosphor-converted (pc) white LED becomes the preferred way to generate white light especially for general lighting, as it is much cheaper and simpler than RGB system. Phosphors are essential to high color quality and luminous efficacy. However, the number of commercially available phosphors is very limited. Therefore, developing new phosphors suitable for various white LED applications is very important. Recently, our group developed the single-particle-diagnosis approach [1-2] to discover new phosphors, with which a tiny luminescent microcrystalline particle down to 5-10 μm can be selected from powder mixtures. In this work, we report a new green emitting Sr-sialon:Eu phosphor discovered by this approach. The crystal structure was solved and refined from single crystal X-ray diffraction data. Sr-sialon:Eu crystallizes in the trigonal space group P3m1 (no. 156) with a = b = 12.1054 A, c = 4.8805 A and Z = 1, and consists of a network of corner sharing (Si,Al)(N,O)4 tetrahedra. Upon doping with Eu2+, the emission band can be tuned from 487 nm to 541 nm with fwhm = 96-124 nm. Ce3+ doped Sr-sialon phosphor shows strong blue emission around 435 nm with a fwhm ≈ 90 nm after 355 nm light excitation. The blue luminescence exhibits a small thermal quenching behavior at high temperature. These performances show that the new Eu2+ and Ce3+ doped Sr-sialon phosphors are promising for white LED applications. [1] N. Hirosaki, T. Takeda, S. Funahashi and R.-J. Xie, Chemistry of Materials, 2014, 26, 4280-4288. [2] T. Takeda, N. Hirosaki, S. Funahashi and R.-J. Xie , Materials Discovery, 2015, 1, 29-37.