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Abstract
Understanding the effect of electron-transfer quenching on the photoluminescence (PL) of phosphors is important but often underestimated. Here we study the PL of two Pr3+-doped phosphors, MgGeO3:Pr and MgSiO3:Pr, which share analogous chemical formulas but exhibit different PL performances. Ultraviolet and red emissions dominate the PL of MgSiO3:Pr at room temperature, while the ultraviolet emission is absent in MgGeO3:Pr. A photoionization experiment, which is based on thermoluminescence excitation spectroscopy, reveals that an impurity-trapped exciton state quenches the 4f5d state of Pr3+ in MgGeO3. We believe that the present experimental approach, as well as the associated physical insight on the electron-transfer quenching, is generally applicable for many existing phosphors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phosphors activated by trivalent praseodymium ion (Pr3+) show potential of applications as red luminescent materials [1–4] or scintillators [5–8] etc. The luminescence performance of Pr3+ varies considerably in different phosphors [9,10], which may exhibit sharp-line emission from the 1S0, 3P0 or 1D2 state (intra-configurational 4f2→4f2 transition, hereafter referred to as f→f) and/or broad-band emission from the lowest 4f5d state (inter-configurational 4f5d→4f2 transition, hereafter referred to as d→f). Besides the specific emission spectral profiles, Pr3+ has a tendency to be oxidized in some phosphors [11–13]. That is, upon appropriate excitation, electron transfer may take place between the excited state of Pr3+ and the delocalized state of host. In the past years, although substantial attention had been paid to develop phosphors activated by Pr3+, the research on the relationship between electron transfer and PL is relatively lacking, mainly focusing on the study of metal-to-metal charge-transfer state [14–16]. Thus, further investigation on the effect of electron transfer on photoluminescence will be interesting from a scientific point of view, as well as in view of the possible application.
The photoluminescence (PL) of Pr3+ in MgGeO3 has been reported before [17], featuring a dominating red emission from the 1D2 level upon excitation at room temperature. When the Ge4+ ion in MgGeO3:Pr is replaced by Si4+ ion, a novel silicate phosphor system MgSiO3:Pr is presented. The two phosphors share analogous chemical formulas, but exhibit different luminescence performances. According to previous reports, the bottom of the conduction band of MgGeO3 with a d10 ion (Ge4+) is at lower energy than in the case of silicate [18,19]. Taking into account the fact that the luminescence properties of Pr3+ ion in phosphors are strongly dependent on the energy position of the delocalized conduction band, it seems interesting to study the PL of Pr3+, as well as the possible electron transfer between the Pr3+ and conduction-band state, in the MgGeO3 and MgSiO3.
In this study, we carry out PL investigations on MgGeO3:Pr and MgSiO3:Pr phosphors. The Pr3+ ion exhibits different emission performances in the two phosphors. Our spectroscopic experiment provides a clear argument on the existence of impurity-trapped exciton (ITE) state at room temperature in MgGeO3:Pr. Accordingly, we ascribe the absence of ultraviolet emission (d→f transition) in MgGeO3:Pr to an electron-transfer quenching caused by the ITE state.
2. Experimental
The MgGeO3:0.1%Pr3+ and MgSiO3:0.1%Pr3+ phosphors were synthesized by a solid-state reaction method. Raw materials, involving MgO, GeO2, SiO2 and Pr6O11 powders, were mixed in stoichiometric proportions. The products were obtained by sintering for 3 h in air at different temperatures: the MgGeO3:Pr was sintered at 1250 °C, and the MgSiO3:Pr was sintered at 1300 °C.
PL spectra were measured with a Horiba FluoroMax-Plus spectrofluorometer equipped with a R928P photomultiplier tube (200–850 nm). An Energetiq EQ-99XFC light source coupled with a Mini-Chrom monochromator (Edmund Optics, 190-650 nm) was used to excite the samples. Appropriate optical filters were used to avoid stray light in spectral measurements. Thermoluminescence measurements were conducted using a SL08-L TL Reader (Guangzhou Rongfan Science and Technology Co., Ltd). Temperature dependence of PL emission spectra were recorded using the spectrofluorometer equipped with a Cryo-77 cryostat (Orient Koji Scientific). Before each measurement, the phosphor was heat-treated at 700 K to empty traps.
3. Results and discussion
The PL emission spectrum of MgGeO3:Pr upon 254 nm excitation at room temperature is shown in Fig. 1(a). It consists of dominating sharp-line emission peaks with a maximum at 626 nm, which is due to the transition from the 1D2 state to the 3H4 ground state multiplet. In addition, a weak emission band peaking at around 420 nm has been observed. In the case of MgSiO3:Pr, upon exposing to a 254 nm illumination, besides a red emission from the 1D2 state, there is an intense ultraviolet emission band, which arises from the d→f transition [solid-line curve in Fig. 1(b)]. Whereas, it is generally acknowledged that the d→f emission of Pr3+ features multi-band spectral shape in ultraviolet phosphors [20]. To present a complete d→f emission spectral shape of the MgSiO3:Pr, we record the ultraviolet emission spectrum over 245−400 nm upon 220 nm excitation, as shown by the dashed-line curve in Fig. 1(b). The PL excitation spectrum of MgSiO3:Pr by monitoring the red emission at 607 nm is given in the left-hand side of Fig. 1(b). There is a broad band peaking at 258 nm, which conforms to the ‘‘mirror image’’ of the ultraviolet emission band, indicating that such an excitation originates from the lowest 4f5d state of Pr3+. Notably, Fig. 1(a) shows that the MgGeO3:Pr shares a similar ultraviolet excitation band, whose mirror-imaged emission is visually absent in the corresponding emission spectrum.
Fig. 1. Photoluminescence (PL) emission and excitation spectra recorded at room temperature. (a) In MgGeO3:Pr, the emission spectrum is recorded under 254 nm excitation. The excitation spectrum is obtained by monitoring the 626 nm emission. (b) In MgSiO3:Pr, upon 254 nm excitation, besides the red emission, there is an intense broad-band emission in the ultraviolet region. A complete spectral shape of the ultraviolet emission is also recorded (dashed-line curve) under 220 nm excitation. The monitoring wavelength for the excitation spectrum is 607 nm.
The apparent difference between the emissions of MgGeO3:Pr and MgSiO3:Pr inspires us to investigate their luminescence mechanism. According to Blasse’s study [21], there is generally no d→f transition emission of Pr3+ at room temperature in phosphors containing d10 ions (e.g., In3+, Ga3+ or Ge4+). A similar effect has been observed when the Al3+ ion in Y3Al5O12:Pr is replaced by Ga3+ with 3d10 configuration. While the Y3Al5O12:Pr shows efficient d→f transition emission, the Y3Ga5O12:Pr does not [22,23]. The absence of d→f transition in Y3Ga5O12:Pr has been be ascribed to an electron-transfer quenching. That is, the lowest level of the 4f5d configuration is situated in the conduction band of Y3Ga5O12, so that a photoionization of the Pr3+ occurs. Accordingly, a low-lying ITE state is proposed to account for the quenching of the lowest 4f5d level in Y3Ga5O12:Pr. The hole of the ITE is on the ionized Pr3+ ion, while the electron is delocalized over the neighboring d10 ions. Like in Y3Ga5O12:Pr, we consider that the MgGeO3:Pr system has somewhere a low-lying ITE state, which may quench the d→f transition emission.
To gain insight of the ITE state in MgGeO3:Pr, we conduct photoionization experiments based on thermoluminescence excitation spectroscopy. The measurements have been carried out by illuminating the MgGeO3:Pr phosphor at room temperature with monochromatic light whose wavelength is tuned between 300–400 nm in 10 nm step. If the excitation energy is high enough to promote the 4f2 ground state electron to a delocalized state, the delocalized electron is possibly captured by electron traps and subsequently released during heating, followed by the recombination with the ionized luminescent center [24]. Figure 2(a) shows a set of thermoluminescence curves monitored at 626 nm; each is recorded from 300 to 473 K.These thermoluminescence curves show similar spectral shapes but with varied intensities.
Fig. 2. Photoionization of MgGeO3:Pr. (a) Thermoluminescence (TL) curves for the 626 nm emission. The thermoluminescence curves are recorded after illuminating the phosphor at room temperature with different excitation wavelengths between 300–400 nm in 10 nm step. (b) Thermoluminescence excitation spectrum, i.e., integrated thermoluminescence intensity as a function of the excitation wavelength. The onset of electron delocalization energy shown in the thermoluminescence creation spectrum (∼340 nm) is an indication of photoionization threshold of the phosphor at room temperature. A room-temperature PL emission spectrum of the phosphor is also presented, which is the same as the one in Fig. 1(a).
According to previous studies [24−26], the photoionization energy of phosphors can be determined from a thermoluminescence excitation spectrum (i.e., a plot of thermoluminescence integrated intensity versus the illumination wavelength), as shown by the red ball curve in Fig. 2(b). At wavelengths shorter than 340 nm the trap filling occurs and the thermoluminescence intensity increases rapidly with decreasing wavelength. It means that 340 nm is the onset of photoionization threshold at room temperature, revealing the delocalized character of excited state in MgGeO3:Pr. The thermoluminescence excitation spectrum conforms to the “mirror image” of the weak emission band [Fig. 2(b)], providing strong evidence that the weak emission band in MgGeO3:Pr originates from an ITE composition associated with ionization properties. Notably, upon 300–400 nm illumination on the MgSiO3:Pr phosphor, as expected, no thermoluminescence signal can be detected.
To visually illustrate the effect of ITE state on the emission characteristics of Pr3+, we present simplified configuration coordinate diagrams in Fig. 3. For clarity, only (4f2) 1D2, (4f2) 3H4, the lowest 4f5d level, as well as the ITE state have been drawn. Lack of d→f transition in MgGeO3:Pr means that the ITE state may non-radiatively quench the lowest 4f5d level. Alternatively, if the ITE state is absent (i.e., in MgSiO3:Pr), the d→f emission occurs.
Fig. 3. Schematic configuration coordinate diagrams. In MgGeO3:Pr, d→f transition is quenched by the ITE state, which is excited upon high-energy illumination (e.g., ultraviolet light). In MgSiO3:Pr, d→f emission occurs since the ITE state is absent.
The PL properties are further studied by means of exciting the two phosphors upon 254 nm illumination at different temperatures. In MgGeO3:Pr, the weak emission band covering 300-600 nm arises from the ITE state, and the dominating red emission is ascribed to the 1D2→3H4 transition, as shown in Fig. 4(a), in which the 1D2 PL intensity varies only slightly from 100 to 500 K. That is understandable since the emission transition from 1D2 state is the dominating radiative channel in MgGeO3:Pr. On the contrary, increasing thetemperature, MgSiO3:Pr is found to boost the 1D2 PL intensity while suppressing the d→f transition pathway, as shown in Fig. 4(b). The increase of 1D2 emission with increasing temperature may stem from a thermally activated crossover [27]. That is, upon 254 nm excitation, the 3PJ/1I6 states may be fed through intersystem crossing from the 4f5d state. Subsequently, the 1D2 may be efficiently populated by multi-phonon relaxation. Competition between the d→f radiative transition and a non-radiative depletion of the 4f5d state (e.g., thermally activated crossing over the energy barrier of the 4f5d state) seems to play an important role in the mechanism.
Fig. 4. PL emission spectra recorded at 100, 300 and 500 K. (a) Upon 254 nm excitation, MgGeO3:Pr exhibits dominating red emission with similar emission intensity at different temperatures. The emission intensity of ITE state decreases along with the temperature. (b) For MgSiO3:Pr, illuminating the phosphor using 254 nm light, both the d→f emission intensity and the 1D2 emission intensity are temperature dependent. The spectra are plotted on the same intensity scale in (a) and (b), respectively.
4. Conclusion
In summary, we have carried out spectroscopic investigations on the PL performances of MgGeO3:Pr and MgSiO3:Pr. The two phosphors exhibit different emission spectral profiles. By combining the thermoluminescence excitation spectroscopy and the temperature-dependent PL measurements, we conclude that the ITE state quenches the d→f emission and feeds the 1D2 emitting state in MgGeO3:Pr. Consequently, the study provides a point of view toward further understanding the effect of electron-transfer quenching on the PL of phosphors.
Funding
National Natural Science Foundation of China (11774046, 51732003); Department of Science and Technology of Jilin Province (20180414082GH).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. P. T. Diallo, P. Boutinaud, R. Mahiou, and J. C. Cousseins, “Red luminescence in Pr3+-doped calcium titanates,” Phys. Status Solidi A 160(1), 255–263 (1997). [CrossRef]
2. P. Boutinaud, E. Pinel, M. Oubaha, R. Mahiou, E. Cavalli, and M. Bettinelli, “Making red emitting phosphors with Pr3+,” Opt. Mater. 28(1-2), 9–13 (2006). [CrossRef]
3. X. Yang, J. Liu, H. Yang, X. Yu, Y. Guo, Y. Zhou, and J. Liu, “Synthesis and characterization of new red phosphors for white LED applications,” J. Mater. Chem. 19(22), 3771–3774 (2009). [CrossRef]
4. T. C. Liu, B. M. Cheng, S. F. Hu, and R. S. Liu, “Highly stable red oxynitride β-SiAlON:Pr3+ phosphor for light-emitting diodes,” Chem. Mater. 23(16), 3698–3705 (2011). [CrossRef]
5. H. Yamada, A. Suzuki, Y. Uchida, M. Yoshida, H. Yamamoto, and Y. Tsukuda, “A scintillator Gd2O2S:Pr,Ce,F for x-ray computed tomography,” J. Electrochem. Soc. 136(9), 2713–2716 (1989). [CrossRef]
6. M. Nikl, A. Yoshikawa, A. Vedda, and T. Fukuda, “Development of novel scintillator crystals,” J. Cryst. Growth 292(2), 416–421 (2006). [CrossRef]
7. A. M. Srivastava, “Inter- and intraconfigurational optical transitions of the Pr3+ ion for application in lighting and scintillator technologies,” J. Lumin. 129(12), 1419–1421 (2009). [CrossRef]
8. M. Nikl, A. Yoshikawa, K. Kamada, K. Nejezchleb, C. R. Stanek, J. A. Mares, and K. Blazek, “Development of LuAG-based scintillator crystals–A review,” Prog. Cryst. Growth Charact. Mater. 59(2), 47–72 (2013). [CrossRef]
9. A. M. Srivastava, “Aspects of Pr3+ luminescence in solids,” J. Lumin. 169, 445–449 (2016). [CrossRef]
10. F. Liu and X. J. Wang, “Effects of non-4f states on Pr3+ luminescence in phosphors,” Chin. J. Lumin. 38(1), 1–2 (2017). [CrossRef]
11. G. Wittmann and R. M. Macfarlane, “Photon-gated photoconductivity of Pr3+:YAG,” Opt. Lett. 21(6), 426–428 (1996). [CrossRef]
12. E. van der Kolk, P. Dorenbos, C. W. E. van Eijk, S. A. Basun, G. F. Imbusch, and W. M. Yen, “5d electron delocalization of Ce3+ and Pr3+ in Y2SiO5 and Lu2SiO5,” Phys. Rev. B 71(16), 165120 (2005). [CrossRef]
13. N. Kristianpoller, D. Weiss, N. Khaidukov, V. Makhov, and R. Chen, “Thermoluminescence of some Pr3+ doped fluoride crystals,” Radiat. Meas. 43(2-6), 245–248 (2008). [CrossRef]
14. G. Blasse, “Quenching of luminescence by electron transfer,” Chem. Phys. Lett. 56(3), 409–410 (1978). [CrossRef]
15. P. Boutinauda, R. Mahiou, E. Cavalli, and M. Bettinelli, “Red luminescence induced by intervalence charge transfer in Pr3+-doped compounds,” J. Lumin. 122-123, 430–433 (2007). [CrossRef]
16. Y. Gao, F. Huang, H. Lin, J. Zhou, J. Xu, and Y. Wang, “A novel optical thermometry strategy based on diverse thermal response from two intervalence charge transfer states,” Adv. Funct. Mater. 26(18), 3139–3145 (2016). [CrossRef]
17. Y. Liang, F. Liu, Y. Chen, X. Wang, K. Sun, and Z. Pan, “Red/near-infrared/short-wave infrared multi-band persistent luminescence in Pr3+-doped persistent phosphors,” Dalton Trans. 46(34), 11149–11153 (2017). [CrossRef]
18. Y. Katayama, T. Kayumi, J. Ueda, P. Dorenbos, B. Vianad, and S. Tanabe, “The role of Ln3+ (Ln = Eu,Yb) in persistent red luminescence in MgGeO3:Mn2+,” J. Mater. Chem. C 5(34), 8893–8900 (2017). [CrossRef]
19. L. Lin, C. Shi, Z. Wang, W. Zhang, and M. Yin, “A kinetics model of red long-lasting phosphorescence in MgSiO3:Eu2+,Dy3+,Mn2+,” J. Alloys Compd. 466(1-2), 546–550 (2008). [CrossRef]
20. G. Blasse, J. P. M. van Vliet, J. W. M. Verwey, R. Hoogendam, and M. Wiegel, “Luminescence of Pr3+ in scandium borate (ScBO3) and the host lattice dependence of the Stokes shift,” J. Phys. Chem. Solids 50(6), 583–585 (1989). [CrossRef]
21. G. Blasse, C. de Mello Donega, N. Efryushina, V. Dotsenko, and I. Berezovskaya, “Luminescence of Pr3+ in indium borate (InBO3),” Solid State Commun. 92(8), 687–688 (1994). [CrossRef]
22. P. Dorenbos, “Electronic structure and optical properties of the lanthanide activated RE3(Al1-xGax)5O12 (RE = Gd,Y,Lu) garnet compounds,” J. Lumin. 134, 310–318 (2013). [CrossRef]
23. A. Katelnikovas, H. Bettentrup, D. Dutczak, A. Kareiva, and T. Justel, “On the correlation between the composition of Pr3+ doped garnet type materials and their photoluminescence properties,” J. Lumin. 131(12), 2754–2761 (2011). [CrossRef]
24. F. Liu, R. S. Meltzer, X. Li, J. D. Budai, Y. S. Chen, and Z. Pan, “New localized/delocalized emitting state of Eu2+ in orange-emitting hexagonal EuAl2O4,” Sci. Rep. 4(1), 7101 (2015). [CrossRef]
25. F. You, A. J. J. Bos, Q. Shi, S. Huang, and P. Dorenbos, “Thermoluminescence investigation of donor (Ce3+,Pr3+,Tb3+) acceptor (Eu3+,Yb3+) pairs in Y3Al5O12,” Phys. Rev. B 85(11), 115101 (2012). [CrossRef]
26. J. Ueda, P. Dorenbos, A. J. J. Bos, A. Meijerink, and S. Tanabe, “Insight into the thermal quenching mechanism for Y3Al5O12:Ce3+ through thermoluminescence excitation spectroscopy,” J. Phys. Chem. C 119(44), 25003–25008 (2015). [CrossRef]
27. J. Ueda, A. Meijerink, P. Dorenbos, A. J. J. Bos, and S. Tanabe, “Thermal ionization and thermally activated crossover quenching processes for 5d-4f luminescence in Y3Al5-xGaxO12:Pr3+,” Phys. Rev. B 95(1), 014303 (2017). [CrossRef]
