Tomoyuki Kurushima

Photoluminescence and Crystal Structure of Green-Emitting Phosphor CaSc2O4 : Ce3 +

A new green phosphor, Ce 3+ -activated CaSc 2 O 4 , was developed, which shows green luminescence with a peak wavelength of 515 nm under excitation with blue light. Ce 3+ -activated CaSc 2 O 4 can be used as a material for color conversion of white-light-emitting diodes (LEDs), which consist of a blue LED, a green phosphor, and a red phosphor, because the luminescence efficiency of this phosphor is comparable to those of commercial phosphors such as Y 3 Al 5 O 12 :Ce 3+ . The host crystal of this phosphor has an orthorhombic CaFe 2 O 4 structure, and the Ce ion probably exists in an eight-coordinated Ca position. We investigated the dependency of the firing temperature and dopant concentration for luminescence intensity and found that the optimum temperature and concentration were 1600°C and 1 mol % of Ce substituted to the Ca position. The luminescence peak wavelength was shifted toward longer wavelengths by replacing Ca with Mg. In contrast, replacing Ca with Sr resulted in a shift toward shorter wavelengths.

New phosphors for white LEDs: Material Design Concepts

Efficient phosphors for white LEDs have been successfully developed, wherein some Material Design Concepts were utilized to promote our research and development effectively and efficiently. Useful ideas for the development of our red and green phosphors, Sr-rich (Sr,Ca,Eu)AlSiN3, (Ba,Eu)3Si6O12N2 and (Ca,Ce)3(Sc,Mg)2Si3O12, are reviewed.

Synthesis of Eu2 + -Activated MYSi4N7 ( M = Ca , Sr , Ba ) and SrYSi4 − x Al x N7 − x O x ( x = 0 – 1 ) Green Phosphors by Carbothermal Reduction and Nitridation

Rare-earth-activated nitride phosphors, (M,Eu)YSi 4 N 7 (M = Ca,Sr,Ba) and (Sr,Eu)YSi 4-x Al x N 7-x O x (x = 0-1), were synthesized by carbothermal reduction and nitridation. The phosphors show green photoluminescence under excitation with near-UV light and have a potential use for color conversion in white light-emitting diodes. The characterization of the structure, morphology, luminescence, and thermal quenching of the green phosphors is presented.

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.

Synthesis of Y(P,V)O4:Eu3+ red phosphor by spray pyrolysis without postheating

We synthesized europium-activated yttrium phosphovanadate (Y(P,V)O4:Eu3+) red phosphor, which is expected to be applicable in plasma display panels, by spray pyrolysis. The effect of pyrolysis temperature on luminescence characteristics was clarified and the mechanism of this effect is discussed. A precursor solution containing sodium, which would act as a flux, was used in the first experiments. The obtained phosphor showed high luminance under UV excitation but the luminance under vacuum UV (VUV) excitation was very low. The luminance of the phosphor was improved using precursors without sodium. The phosphor synthesized at 1600°C had the same luminance as that synthesized by solid-state reaction under UV excitation. Its luminance under VUV excitation was 93% of the solid-state phosphor. Sodium ions accelerated the crystal growth of phosphor particles, but they decreased the luminance of the phosphor, particularly under VUV excitation.

Trap and nonradiative centers in Ba 3 Si 6 O 12 N 2 :Eu 2+ phosphors observed by thermoluminescence and two-wavelength excited photoluminescence methods

We have studied trap centers and nonradiative (NRR) recombination centers in a Ba3Si6O12N2:Eu2+ (BSON), one of promising materials for efficient and stable phosphors in white LED lamp applications. The energy distribution of four trap centers was obtained by thermo-luminescence (TL) with the excitation energy of 5.59eV. By superposing a below-gap excitation light of 1.77eV and observing the intensity change of the 5d-4f emission of Eu2+ centered at 2.36eV in our two-wavelength excited photoluminescence (TWEPL) measurement, both transient and steady state enhancement were observed. Such peculiar behavior of photo-stimulation is attributed to the coexistence of trap centers and NRR centers: the photoexcitation of electrons from trap centers generates the transient component, while that from NRR centers maintains the steady state component. An optical detection of relatively faint contribution of defects became possible in order to improve further the reliability and efficiency of phosphor materials.