(Invited) Investigation of Therml Ionization Quenching and Design of Persistent Luminescnece in Ce3+-Doped Garnet Phosphors

Abstract

Ce3+-doped garnet materials have attracted a great deal of attention in the w-LED application because of their various optical properties. However, the mechanism of their thermal quenching has not clearly been elucidated yet. In this study, the two possibilities of thermally activated ionization and crossover were investigated as cause of quenching process of 5d luminescence in Ce3+-doped Y3Al5-xGaxO12. Based on the photoconductivity and thermoluminescence measurements, the quenching is found to be caused by thermal ionization. The thermal ionization quenching is a disadvantage for w-LED application, but for persistent phosphors, it is a required process. Using the thermal ionization process by blue light excitation, the blue light chargeable persistent phosphors were successfully developed. For the synthesis of polycrystalline ceramics of Y3Al5-xGaxO12(YAGG):Ce3+(0.5 %), Y3Al5-xGaxO12:Ce3+(0.5 %) -Cr3+(0.05% ), the Y2O3, Al2O3, Ga2O3, CeO2 and Cr2O3were used as starting materials. The mixed powder was pressed into a pellet and sintered at 1400~1600°C. The optoelectronic, photoluminescence and persistent luminescence properties in the obtained samples were investigated. From the PL spectra measurement of Ce3+-singly doped YAGG, the PL intensity decreases with increasing Ga content, x, and no photoluminescence was observed in the x=5 sample. In our photoconductivity measurement, the intense photocurrent excitation bands attributed to the transitions from the ground 4f level to the 5d1 and 5d2, which are the lowest and the 2nd lowest 5d levels, were observed at 300K in the x=3 and 5 samples doped with Ce3+. This result shows that the luminescence quenching is caused by thermal ionization process in the x=3 and 5 samples1). However, the photoconductivity by blue excitation was not observed in the x=0 sample (YAG:Ce3+) at 300K. For checking thermal ionization process of YAG:Ce3+ at high temperatures, thermoluminescence(TL) excitation spectra were measured. In the TL excitation (TLE) spectrum at room temperature there is no Ce3+:5d1 band (the lowest excited 5d level). However, in the TLE spectrum at 573 K, which corresponds to the onset temperature of luminescence quenching, a TLE band due to the Ce3+:5d1 excitation was observed at around 450 nm. This is the evidence that the luminescence quenching of YAG:Ce3+ at high temperatures is caused by the thermal ionization2). Using the thermal ionization process, we tried to prepare blue light chargeable persistent phosphors. In the Ce3+-singly doped YAGG phosphors, there is no appropriate electron traps for persistent phosphors at ambient temperature. We discovered that Cr3+ ions act efficiently as electron traps in the garnet materials. The Y3Al2Ga3O12:Ce3+–Cr3+ showed Ce3+:5d–4f green persistent luminescence (λem = 505 nm) for several hours after blue-light excitation3). The TL glow peak temperature of Y3Al2Ga3O12:Ce3+–Cr3+ is located at around 300K. By increasing the Ga content of Y3Al5-xGaxO12:Ce3+-Cr3+, the TL glow peak temperature are changed from 400 K to 150 K for glow curves measured at 10 K min-1. The corresponding trap depths vary from 0.41 eV to 1.2 eV4). A trap depth distribution with a width that changes with Ga content was observed in Y3Al5-xGaxO12:Ce3+–Cr3+. The width is minimum for x = 0, x = 3, and x = 5 which was explained by the statistics in the distribution of the Ga ions on the sites around the Cr defect. From simulating the distribution we conclude that Ga first start to occupy the tetrahedral sites and above x = 3 also the octahedral sites are being occupied. 1) J. Ueda, S. Tanabe, and T. Nakanishi, J. Appl. Phys., 110,053102 (2011). 2) J. Ueda, P. Dorenbos, A. J. J. Bos, A. Meijerink, and S. Tanabe, J. Phys. Chem. C, 119,25003-25008 (2015). 3) J. Ueda, K. Kuroishi, and S. Tanabe, Appl. Phys. Lett., 104,033519 (2014). 4) J. Ueda, P. Dorenbos, A. J. J. Bos, K. Kuroishi, and S. Tanabe, J. Mater. Chem. C, 3, 5642-5651 (2015).