Motoyuki Shigeiwa

Photoluminescence and Crystal Structure of Green-Emitting Ca3Sc2Si3O12 : Ce3 + Phosphor for White Light Emitting Diodes

We have developed a new oxide-based phosphor Ca 3 Sc 2 Si 3 O 12 :Ce 3+ , which is applicable as a green-emitting color converter for white light emitting diodes (LEDs). This phosphor absorbs blue light around 450 nm and emits green luminescence, with a peak wavelength around 505 nm. It is a promising candidate for application in LEDs as quenching of the phosphor at 150°C was smaller than that of Y 3 Al 5 O 12 :Ce yellow phosphor. A white LED with high color rendering was fabricated by combining this phosphor with a blue GaN LED and a red phosphor. The luminescence of this phosphor is derived from the 5d-4f transition of the Ce ion and the luminescence decay curve fit a single exponential function. This phosphor has a garnet-type host crystal structure. X-ray absorption fine structure analysis showed that Ce ions replaced the Ca position of the host crystal as Ce 3+ .

Singlet oxygen generation by two-photon excitation of porphyrin derivatives having two-photon-absorbing benzothiadiazole chromophores

Porphyrin derivatives with four two-photon absorbing 2,1,3-benzothiadiazole chromophores at the meso positions provide large two-photon absorption cross-sections up to 735 GM as well as large quantum yields of one-photon singlet oxygen sensitization of ca. 0.7, leading to generation of singlet oxygen by two-photon excitation of 800 nm. The two-photon singlet oxygen sensitization is discussed on the basis of a parameter of the normalized oxygen luminescence intensity obtained from the two-photon experiment.

Redshift of Green Photoluminescence of Ca3Sc2Si3O12 : Ce3 + Phosphor by Charge Compensatory Additives

The luminescence wavelength of Ca 3 Sc 2 Si 3 O 12 :Ce 3+ , which is a green-emitting phosphor under blue light excitation, was moved toward longer wavelength by addition of Mg. This modification is very valuable for commercial applications because it increases the brightness of the phosphor by increasing the overlap with the visibility curve. Magnesium is likely incorporated into the Sc position of the host crystal because the ionic radius of Mg is close to that of Sc. The substitution probably compensates for the excess positive charge induced by incorporation of Ce into the Ca position. The content of Ce in the host crystal was increased by increasing the amount of Mg in the raw material mixture; the inverse relationship was also observed. The spectral shift was also observed by adding Na or Li, which probably substituted the Ca position and compensated the excess charge induced by Ce in the Ca position. The change in luminescence spectra occurred irrespective of the type of charge compensatory additive used. Therefore, the spectral modification was mainly due to the increase in Ce 3+ activator. Modification of the luminescence decay characteristics by the additives was also examined. The decay curve of the phosphor containing Mg showed a nonexponential profile.

Development of Photochromic Two-Photon Absorption Dyes

ABSTRACT We synthesized three photochromic two-photon absorptive diarylethene derivatives with styrylbenzene unit and measured two-photon absorption cross-sections at 800 nm by using an open aperture Z-scan method with 120 femtosecond pulses. It was found that two of the dyes have large values (δ = 52 GM) equivalent to AF-50.

XAFS Analysis of Local Structure around Ce in Ca3Sc2Si3O12:Ce Phosphor for White LEDs

We have studied the local structure around Ce atom in Ca3Sc2Si3O12 host crystal, which has been developed as a new green phosphor for white light emitting diodes (LEDs). As the local structure and chemical environment of the dopant atom are very important to improve the performance of the phosphor, we have used XAFS to get chemical and structural information around the Ce dopant. The XANES spectrum of the Ce LIII‐edge reveals that the Ce atom is trivalent in Ca3Sc2Si3O12. There are two kinds of possible Ce substitution sites, Ca site and Sc site, in garnet type Ca3Sc2Si3O12 crystal structure. The Ce atom is found to be at the Ca site in the host crystal by the comparison of the Fourier transform of Ce K‐edge EXAFS spectrum with those of Ca and Sc K‐edge EXAFS spectra. The theoretical analysis with FEFF also clarified the Ce substitution at the Ca site. Furthermore, the result of the analysis indicates the structural disorder around Ca and Si atoms at 3.75 A. It is possible that there are some defects arou...

Analysis of the Local Structure around Eu and Mn Ions in Alkaline‐Earth Silicate Phosphors for White Light Illumination

M2SiO4‐based phosphors (M: alkaline‐earth metal) that emit red to blue light are expected to offer high color rendering to white light‐emitting diodes (LEDs) in combination with blue or near‐UV excitation sources. It is very important for the complete control of the emission color to understand the crystal field around the active elements (rare‐earth and transition metals). XAFS spectroscopy is applied to a (Ba,Ca)2SiO4:Eu,Mn phosphor at Eu L3‐ and Ba, Ca, Eu, Mn K‐edges to elucidate the local environments of Eu and Mn. Eu L3‐ and Mn K‐edge XANES spectra showed that Eu and Mn are both divalent, like Ba and Ca. K‐edge EXAFS spectra indicated that the local structures of Eu and Mn are similar to those of Ba and Ca, respectively. However, the curve‐fitting analysis showed that the first coordination shell of Eu has two Eu–O bonds that are both shorter than the Ba–O bond. FEFF calculations were also performed based on a BaCaSiO4 model constructed from the crystal structure of KNaSO4. They suggested that Eu su...