Jumpei Ueda

Analysis of Ce3+ luminescence quenching in solid solutions between Y3Al5O12 and Y3Ga5O12 by temperature dependence of photoconductivity measurement

Photocurrent excitation spectra were measured to investigate the quenching in the garnet solid solutions. Intense photocurrent excitation bands attributed to the lowest 5d1 and the second lowest 5d2 levels were observed in the Ce-doped Y3Al2Ga3O12 (Ce:YAGG) and Y3Ga5O12 (Ce:YGG). Based on the results of temperature dependence of photoconductivity, the 5d1 and 5d2 levels in the Ce:YAGG are found to be located below and within the conduction band, respectively, while both levels in the Ce:YGG are found to be located within its conduction band located at lower energy levels. In addition, the threshold of photoionization from the 4f level of Ce3+ to the conduction band in the Ce:YAGG and Ce:YGG were estimated to be 3.2, and 2.8 eV, respectively. We conclude that the main quenching process in the Ce:YAGG is caused by the thermally stimulated ionization process with activation energy of 90 meV from the 5d1 to the conduction band, and that in the Ce:YGG is caused by the direct ionization process from the 5d levels to the conduction band.

Bright persistent ceramic phosphors of Ce3+-Cr3+-codoped garnet able to store by blue light

We have developed bright persistent phosphors of Ce3+-Cr3+-doped Y3Al5-xGaxO12 (x = 2.5, 3, 3.5) ceramics with green luminescence (Ce3+:5d→4f) via blue-light excitation. The persistent luminance value 5 min after ceasing blue-light excitation for the Ce3+-Cr3+-doped Y3Al2Ga3O12 sample is approximately 3900 times higher than that for a Ce3+-doped Y3Al2Ga3O12 sample and better than that from compact SrAl2O4:Eu2+-Dy3+ powders. The results are consequences of efficient carrier trap formation and effective excitation of the interior of the translucent ceramic samples. A series of Ce3+-Cr3+-doped garnet materials is the best candidate for the persistent phosphor under white-light emitting diode (LED) illumination consisting of blue LEDs and visible phosphors.

Enhancement of Red Persistent Luminescence in Cr3+-Doped ZnGa2O4 Phosphors by Bi2O3 Codoping

Bi2O3 was proved to be an effective codopant to enhance red persistent luminescence in Cr3+-doped ZnGa2O4 spinel. The Cr–Bi-codoped ZnGa2O4 phosphors showed about 10 times higher persistent luminescence intensity than the Cr-singly-doped phosphors. The radiance (in mW sr-1m-2) of persistent luminescence in ZnGa2O4:Cr,Bi phosphors was comparable to that in commercialized SrAl2O4:Eu,Dy phosphors. Increases of Cr3+ absorption and photoluminescence were also observed in the Cr–Br-codoped ZnGa2O4 sample. The obtained results suggest that Bi2O3 may play a critical role in stabilizing Cr3+ in ZnGa2O4 spinel.

Control of electron transfer between Ce3+ and Cr3+ in the Y3Al5−xGaxO12 host via conduction band engineering

Among inorganic compounds doped with lanthanide or transition metal ions for persistent phosphors, the transfer process of the localized electron in the dopant cations to the conduction band (CB) is crucial. In Y3Al5−XGaxO12:Ce3+–Cr3+, we found that the electron produced by photoionization of the Ce3+ ion by UV and blue excitation transfers to the Cr3+ ion through the CB. The electron trapped by Cr3+ is thermally released at different temperatures from 400 K to 150 K in the YAGG host with different Ga content because we managed to decrease the energy gap between the CB and the electron trap by increasing the Ga content. The persistent luminescence mechanism has been explained by constructing the vacuum referred binding energy (VRBE) diagram comprising the Ce3+, Cr2+, valence band (VB) and conduction band (CB) level energies in the YAGG host with different Ga/Al ratios.

Tunable trap depth in Zn(Ga1−xAlx)2O4:Cr,Bi red persistent phosphors: considerations of high-temperature persistent luminescence and photostimulated persistent luminescence

Thermoluminescence peaks for Cr3+ emissions shift to higher temperature due to the partial substitution of Al for Ga sites in ZnGa2O4:Cr,Bi phosphors, which indicates that the electron traps responsible for the red persistent luminescence are tuned to deeper levels by Al-doping. Al-containing phosphors with deeper traps showed a lower persistent luminescence intensity than Al-free samples at 20 °C because the ambient thermal energy is too low to release trapped electrons in deeper levels; however, the Al-containing persistent phosphors perform better at 80 °C with greater thermal energy. At 20 °C, the trapped electrons in the Al-containing phosphors can be efficiently released with the assistance of additional NIR photostimulation. NIR photostimulated red persistent luminescence (NIR-to-red mode) as presented in the Al-containing ZnGa2O4:Cr,Bi phosphors could be a promising technique for new in vivo imaging systems.

Yellow persistent luminescence in Ce3+–Cr3+-codoped gadolinium aluminum gallium garnet transparent ceramics after blue-light excitation

We have developed a yellow persistent phosphor of Ce3+–Cr3+-codoped Gd3Al2Ga3O12 transparent ceramics prepared by a solid-state reaction. The yellow persistent luminescence due to the Ce3+: 5d–4f transition was observed even after 460 nm blue-light excitation as well as after UV excitation. The chromaticity coordinate of the persistent luminescence in the ceramic phosphor is located at , which appears really yellow compared with the color coordinate of the well-known SrAl2O4:Eu2+–Dy3+ or other conventional blue or green persistent phosphors. The luminance values for the transparent ceramic sample 1, 5, and 30 min after the blue excitation was ceased are 731, 63, and 8 mcd/m2, respectively.

Photochromism and white long-lasting persistent luminescence in Bi 3+ -doped ZnGa 2 O 4 ceramics

White long-lasting persistent luminescence covering the whole visible region in Bi3+-doped ZnGa2O4 ceramics is reported. The afterglow luminescence can be observed for several tens of minutes after 360 nm or 280 nm excitation. Photochromism is also observed during ultra-violet excitation. The persistent luminescence and photochromism are considered to originate from electron trapping by defect centers in the ZnGa2O4 crystals. The Bi3+-doped ZnGa2O4 ceramics are expected to be potential white-color afterglow phosphors.

Afterglow Luminescence in Ce3+-Doped Y3Sc2Ga3O12 Ceramics

We report on long-lasting afterglow phosphorescence at around 500 nm in Ce3+-doped yttrium scandium gallium garnet (YSGG) ceramics with the composition of (Y0.995Ce0.005)3Sc2Ga3O12 prepared by solid-state reaction at 1600 °C. The afterglow luminescence was observed for 1 h after the 440 nm excitation in the Ce:YSGG ceramic prepared under vacuum. However, the afterglow decay time decreased after O2 annealing. Therefore, one of the potential traps can be oxygen vacancies. Based on the correlation between the 5d levels and the conduction band, we present an afterglow mechanism and propose a new persistent phosphor under white light emitting diode (LED) illumination.

Band-gap variation and a self-redox effect induced by compositional deviation in ZnxGa2O3+x:Cr3+ persistent phosphors

We made a systematic investigation on the optical properties of non-doped and Cr-doped ZnxGa2O3+x (0.98 ≤ x ≤ 1.02) spinel crystals. Absorption, photoluminescence excitation, and persistent luminescence excitation spectra indicated that the bottom of the conduction band (CB) was affected (i) by compositional deviation from stoichiometry or (ii) by atmosphere control in a similar way. By constructing an energy level diagram, a broad distribution of trap depth in a Cr3+-doped ZnxGa2O3+x sample with a composition of excess Zn or in one prepared in an O2 atmosphere was attributed to lowering of the CB bottom, which resulted in faster decay of persistent luminescence in these two samples. Defect chemical reactions revealed that the anti-site species of Zn′Ga may be the reason for lowering of the CB bottom. A self-redox effect of Cr ions induced by the compositional deviation was found and explained by the defect reactions. The investigation indicated that the composition with slight Zn deficiency is important to obtain a narrow distribution of trap depths and longer persistent luminescence, however the self-reduction effect of Cr ions in the Zn deficient composition also needs to be considered.

Y3Al5−xGaxO12:Cr3+: A novel red persistent phosphor with high brightness

We report on red persistent phosphors of Y3Al5−xGaxO12:Cr3+ garnet (YAGG:Cr3+, x from 0 to 5). In these materials, Cr ions act as both emission centers and electron traps. The trap depth, which is regarded as the energy gap between the bottom of the conduction band and the electron trap, can be optimized by modifying the Ga3+ substitution content (x). After ceasing UV illumination, the persistent luminescence of the YAGG:Cr3+ (x = 3) phosphor was nearly 5 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor. Such novel red persistent phosphors have great potential for an improved in vivo bioimaging application.