To elucidate the persistent luminescence mechanism of the Ce3+-Cr3+-codoped Y3Al2Ga3O12 garnet (YAGG), the valence states of Ce and Cr ions before and during UV charging were investigated by X-ray absorption near edge structure (XANES) spectroscopy. In the XANES spectra for Ce LIII and Cr K edges of YAGG:Ce3+-Cr3+ under UV illumination, the valence states of Ce4+ and Cr2+ were detected, but not in the XANES spectra of YAGG:Ce3+ and YAGG:Cr3+. We conclude that the combination of Ce3+ and Cr3+ causes the valence state change into Ce4+ and Cr2+ by UV illumination, which is the mechanism of persistent luminescence.
© 2017 Optical Society of America
Persistent phosphors show continuous luminescence for a long duration ranging from several minutes to a few hours even after the excitation light source is removed, as opposed to phosphors that show luminescence with a decay time on the nanosecond and millisecond scale because of intrinsic ion transitions. Persistent phosphors have found application in luminous paints in indoor and outdoor environments, for emergency signs, clock dials, and glow-in-the-dark road marks [1–4].
In 2014, we successfully developed new persistent ceramic phosphors with a new combination of lanthanide luminescent centers and transition metal trap centers in Y3Al5-xGaxO12(YAGG): Ce3+-Cr3+ (x = 2.5, 3, 3.5). The phosphors show Ce3+:5d-4f green persistent luminescence (λem = 510 nm) for several hours after blue-light excitation [5–8]. This phosphor can absorb blue light via Ce3+:4f-5d1 transition with a large cross section and the excited electron can transfer to the conduction band (CB) easily due to the small energy gap between the 5d1 level and the bottom of CB. In our previous work, we suggested that the Ce3+ ion loses an electron and the Cr3+ ion captures the electron under excitation, which is the main mechanism of persistent luminescence [5, 6]. However, the origin of the electron donors and acceptors in YAGG:Ce3+-Cr3+ persistent phosphors have never been directly demonstrated.
To elucidate whether the origin of the electron donors and acceptors related to the Ce and Cr ions, X-ray absorption spectroscopy (XAS) becomes a smart tool. So far, the nature of the electron donors and acceptors were investigated by using XAS in many persistent phosphors, such as aluminosilicate glasses doped with Eu2+ , (Ca,Sr)2MgSi2O7:Eu2+-R3+ (R = Yb, Dy, Ce) , (Ca,Sr)2MgSi2O7:Eu2+-R3+ (R = Yb, Dy, Ce) [10, 11], CaAl2O4:Eu2+-R3+(R = Yb, Sm, Nd, Ce)  and SrAl2O4:Eu2+-Dy3+ . Carlson and Hölsä et al. reported that the valence state of Eu2+ changes into Eu3+ in Sr2MgSi2O7:Eu2+ and the relative amount of Eu3+ increases with increasing the exposure time of X-ray in the X-ray absorption near-edge structure (XANES) spectra . Also, for the most famous, brightest and longest persistent phosphor of SrAl2O4:Eu2+-Dy3+, Korthout and Smet et al. investigated the XANES spectra of the Eu LIII and Dy LIII edges after short and long X-ray charging. They concluded that a partial Eu2+ ion change into the Eu3+ ion by X-ray charging while the valence state of Dy3+ remains unchanged . In this study, to investigate the valence change of the Y3Al2Ga3O12 persistent phosphors with new cation combination of Ce3+ and Cr3+, the XANES spectra were measured with and without UV charging during measurement.
Polycrystalline ceramics of Ce3+(0.5%) singly-doped, Ce3+(0.5%)-Cr3+(0.05%)-codoped and Cr3+(0.2%) singly-doped Y3Al2Ga3O12 were synthesized by solid state reaction. The chemicals Y2O3 (99.99%), Al2O3 (99.99%), Ga2O3 (99.99%), CeO2 (99.99%), Cr2O3 (99.9%) were used as starting materials. The powders were mixed by ball milling (Fritsch, Premium Line P-7) with ethanol and the obtained slurry was dried and pulverized. The dried powder was pressed into pellets sized 10 mm-ϕ × 2 mm thickness. The pellets were sintered at 1600 °C in air. The persistent luminescence spectra were measured by a radiance-meter (B&W Tek Glacier X) after ceasing 460 nm excitation for 5min. The X-ray absorption spectroscopy (XAS) was performed at the beamline BL9A of Photon Factory (KEK, Japan). The photon flux density of X-ray at 7 keV is 4×1012 photons/sec/mm2. The Ce LIII and Cr K X-ray absorption near edge structure (XANES) were recorded for all the samples in fluorescence mode with/without UV (250nm-400nm) illumination of a Xe lamp (Asahi Spectra, Max 302). The reason why the sample was illuminated by UV is to cause the ionization of Ce3+. As references, standard Ce(NO3)3, CeO2, Cr2O3 samples were also measured. The Ce LIII and Cr K XANES for the reference samples were recorded in the transmission mode.
3. Results and discussion
Figure 1 (inset) shows the sample picture under w-LED illumination and after ceasing the w-LED illumination. Under the w-LED illumination, the body color of both Ce3+-singly doped and Ce3+-Cr3+ co-doped samples are yellow due to the strong Ce3+: 4f-5d absorption in the blue region. After ceasing the w-LED illumination, the only Ce3+-Cr3+ co-doped sample shows green persistent luminescence. The green persistent luminescence peaked at 510 nm is attributed to the transition from the lowest 5d1 state to the 4f ground state of Ce3+ as shown in Fig. 1. The persistent luminescence spectrum matches with the sensitivity curve of the photopic vision of human eyes. Thus, the persistent luminescence of YAGG:Ce3+-Cr3+ is suitable for the luminous paint application such as emergency signs and road markers. The persistent intensity at 5 min after ceasing 460 nm excitation in YAGG:Ce3+-Cr3+ is 3900 times higher than that in YAGG:Ce3+ . This result strongly indicates that the Cr3+ ion acts as an efficient carrier trapping center.
To check the possibility that Ce3+ ions act as electron donors, the XANES spectra for the Ce LIII edge of the YAGG samples under different conditions are investigated as shown in Fig. 2(a). Figure 2(b) shows reference spectra of Ce(NO3) and CeO2 for Ce3+ and Ce4+, respectively. In Fig. 2(a), red and green solid lines show the XANES spectra for the YAGG:Ce3+-Cr3+ sample with and without UV illumination. In these XANES spectra, two main peaks were observed at 5725eV and 5735 eV. Compared with the reference XANES spectra in Fig. 2(b), the peaks at 5725 eV and 5735 eV are mainly contributed by the X-ray absorption of Ce3+ and Ce4+, respectively. The valence state of Ce3+ and Ce4+ can be well distinguished because the main XANES peak is different from each other. From the previous results of thermoluminescence excitation spectrum for YAGG:Ce3+-Cr3+, it was confirmed that this persistent phosphor can be charged by UV illumination . It is expected that the Ce3+ and Ce4+ XANES peak intensities decrease and increase, respectively, by UV illumination. However, two XANES spectra with and without UV for Ce LIII edge in YAGG:Ce3+-Cr3+ are almost same to each other. This is probably because the strong X-ray irradiation also causes ionization of Ce3+ ion (charging process) without UV illumination. As an evidence of the efficient ionization, strong persistent luminescence was observed after one cycle of XANES measurement without UV illumination. In order to check the relationship between charging process and valence state change, the XANES spectrum of Ce LIII edge for YAGG:Ce3+ with UV illumination was measured as shown by a blue solid line of Fig. 2(a). In the XANES spectrum for YAGG:Ce3+, the XANES peak intensity of Ce3+ and Ce4+ becomes much stronger and weaker, respectively, compared with YAGG:Ce3+-Cr3+ as shown in Fig. 2(a). From the XANES spectrum, it is found that the valence state of Ce ion in YAGG:Ce3+ is almost trivalent even under UV illumination. Indeed, the XANES spectrum of YAGG:Ce3+ without UV illumination is completely same to that with UV illumination. These results indicate that Ce3+ does not change into Ce4+ in YAGG:Ce3+ by UV illumination although the photon energy of UV is large enough for the excitation of Ce3+:4f-electron to the conduction band based on the photoconductivity measurement and the vacuum referred binding energy (VRBE) for YAGG:Ce3+ [6, 14, 15]. Even though the 4f electron is ionized by UV illumination, the ionized electron could be bounded by Ce ion. However, in YAGG:Ce3+-Cr3+, a part of Ce3+ changes into Ce4+ from the XANES spectra with UV illumination as shown in Fig. 2(a). The valence state change from Ce3+ to Ce4+ can be caused by the existence of Cr3+ electron traps which attract the ionized electrons from Ce3+.
In order to check the valence state changing of Cr ion, the XANES spectrum of Cr K edge for YAGG:Ce3+-Cr3+ and YAGG:Cr3+ with and without UV illumination were measured as shown in Fig. 3(a). Figure 3(b) shows the reference XANES spectra of Cr2O3 and CrCl2  for Cr3+ and Cr2+ ions. From the reference spectra, the energy of XANES edge related to the Cr2+-compound is much lower than that related to the Cr3+-compound. As with the XANES for Ce LIII, the XANES spectral shape for Cr K edge in YAGG:Ce3+-Cr3+ does not change significantly with and without UV illumination. This result also shows the charging process occur by X-ray irradiation during XANES measurement. On the other hand, the XANES spectrum for Cr K edge in YAGG:Cr3+ with UV illumination shows a different spectral shape compared with YAGG:Ce3+-Cr3+. When Ce3+ coexists in YAGG:Cr3+, the intensity of XANES edge in the range from 5985 eV to 6000 eV increases and the intensity at around 6007 eV decreases. From this result, the concentration ratio of Cr2+ respect to Cr3+ in YAGG:Ce3+-Cr3+ is much higher than that in YAGG:Cr3+. If there is the electron donor of Ce3+ which can be ionized by UV illumination, a number of Cr3+ ions can capture the ionized electrons and change into the Cr2+ ions. We, therefore, conclude that the combination of Ce3+ and Cr3+ causes the valence state change to Ce4+ and Cr2+ efficiently by UV illumination, which is the charging mechanism of persistent luminescence.
In summary, the valence states of Ce and Cr ions before and during charging were investigated by X-ray absorption near edge structure (XANES) spectra. To investigate the valence state change for Ce3+ and Cr3+ by UV illumination, the XANES spectra for Ce LIII and Cr K edges were measured with and without UV illumination. However, the XANES spectra do not change significantly by UV illumination in both Ce and Cr edges. This is probably because the strong X-ray irradiation also causes the release of the electron for Ce3+ and the capture of the electron for Cr3+ without UV illumination. On the other hands, in the XANES spectrum for Y3Al2Ga3O12:Ce3+, the XANES peak intensity of Ce3+ and Ce4+ becomes much stronger and weaker, respectively, compared with Y3Al2Ga3O12:Ce3+-Cr3+. This is because some part of Ce3+ can change into Ce4+ because the Cr3+ ions can capture electrons. In addition, when the Ce3+ ion co-exists in Y3Al2Ga3O12:Cr3+, the XANES peak of Cr2+ can be detected by UV illumination. These results show that the 4f electron of Ce3+ is excited by UV to the conduction band and act as electron donors. And then, the Cr3+ ion captures the ionized electron and changes into Cr2+. Therefore, we conclude that the Ce3+ is photo-ionized and Cr3+ captures the ionized electron for the charging process of Y3Al2Ga3O12:Ce3+-Cr3+ persistent phosphors.
Japan Society for the Promotion of Science KAKENHI (16K05934 and 16H06441).
The XANES measurements have been performed under the approval of the Photon Factory Program Advisory Committee (No. 2014G600).
References and links
1. T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+,Dy3+,” J. Electrochem. Soc. 143(8), 2670–2673 (1996). [CrossRef]
2. K. Van den Eeckhout, P. F. Smet, and D. Poelman, “Persistent luminescence in Eu2+-doped compounds: a review,” Materials (Basel) 3(4), 2536–2566 (2010). [CrossRef]
3. K. Van den Eeckhout, D. Poelman, and P. Smet, “Persistent luminescence in non-Eu2+-doped Compounds: A Review,” Materials (Basel) 6(7), 2789–2818 (2013). [CrossRef]
5. J. Ueda, K. Kuroishi, and S. Tanabe, “Bright persistent ceramic phosphors of Ce3+-Cr3+-codoped garnet able to store by blue light,” Appl. Phys. Lett. 104(10), 101904 (2014). [CrossRef]
6. J. Ueda, P. Dorenbos, A. J. J. Bos, K. Kuroishi, and S. Tanabe, “Control of electron transfer between Ce3+ and Cr3+ in the Y3Al5-xGaxO12 host via conduction band engineering,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(22), 5642–5651 (2015). [CrossRef]
7. J. Ueda, “Analysis of optoelectronic properties and development of new persistent phosphor in Ce3+-doped garnet ceramics,” J. Ceram. Soc. Jpn. 123(1444), 1059–1064 (2015). [CrossRef]
8. J. Xu, J. Ueda, K. Kuroishi, and S. Tanabe, “Fabrication of Ce3+–Cr3+ co-doped yttrium aluminium gallium garnet transparent ceramic phosphors with super long persistent luminescence,” Scr. Mater. 102(0), 47–50 (2015). [CrossRef]
9. J. Qiu, M. Kawasaki, K. Tanaka, Y. Shimizugawa, and K. Hirao, “Phenomenon and mechanism of long-lasting phosphorescence in Eu2+-doped aluminosilicate glasses,” J. Phys. Chem. Solids 59(9), 1521–1525 (1998). [CrossRef]
10. Z. Qi, C. Shi, M. Liu, D. Zhou, X. Luo, J. Zhang, and Y. Xie, “The valence of rare earth ions in R2MgSi2O7:Eu, Dy (R = Ca, Sr) long-afterglow phosphors,” Phys. Status Solidi, A Appl. Res. 201(14), 3109–3112 (2004). [CrossRef]
11. S. Carlson, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, J. Niittykoski, and R. Valtonen, “X-ray absorption study of rare earth ions in Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials,” Opt. Mater. 31(12), 1877–1879 (2009). [CrossRef]
12. J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, E. Welter, and D. A. Zajac, “Valence and environment of rare earth ions in CaAl2O4:Eu2+,R3+ persistent luminescence materials,” Spectrochim. Acta B At. Spectrosc. 65(4), 301–305 (2010). [CrossRef]
13. K. Korthout, K. Van den Eeckhout, J. Botterman, S. Nikitenko, D. Poelman, and P. F. Smet, “Luminescence and X-ray absorption measurements of persistent SrAl2O4:Eu,Dy powders: Evidence for valence state changes,” Phys. Rev. B 84(8), 085140 (2011). [CrossRef]
14. J. Ueda, S. Tanabe, and T. Nakanishi, “Analysis of Ce luminescence quenching in solid solutions between Y3Al5O12 and Y3Ga5O12 by temperature dependence of photoconductivity measurement,” J. Appl. Phys. 110(5), 053102 (2011). [CrossRef] [PubMed]
15. P. Dorenbos, “Electronic structure and optical properties of the lanthanide activated RE3(Al1−xGax)5O12 (RE=Gd, Y, Lu) Garnet Compounds,” J. Lumin. 134(0), 310–318 (2013). [CrossRef]
16. K. L. Fujdala and T. D. Tilley, “Thermolytic molecular precursor routes to Cr/Si/Al/O and Cr/Si/Zr/O catalysts for the oxidative dehydrogenation and dehydrogenation of propane,” J. Catal. 218(1), 123–134 (2003). [CrossRef]