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Study of photoluminescence, radioluminescence and decay performances of Pr3+ co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics
Journal of Luminescence 153 (2014) 453–457
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Study of photoluminescence, radioluminescence and decay performances of Pr3 þ co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics Cen Jiang, Qiuhong Yang n, Ye Yuan, Qing Lu, Shenzhou Lu, Yunhan Li School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 November 2013 Received in revised form 18 February 2014 Accepted 25 February 2014 Available online 11 March 2014
High optical-quality Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics were prepared by the solid-state reaction method and sintered in H2 atmosphere. Photoluminescence (PL) and radioluminescence (RL) intensities were enhanced by co-doping minor Pr3 þ content due to the energy transfer (ET), and quenched with the increase of Pr3 þ content due to concentration quenching of excess Pr3 þ . The possible ET process had been demonstrated. Decay time was also shortened by co-doping Pr3 þ . The results showed that Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics are promising scintillation materials. & 2014 Elsevier B.V. All rights reserved.
Keywords: Eu/Pr co-doped ceramics Photoluminescence Radioluminescence Decay time
1. Introduction Nowadays, the requirements, e.g. high density, good stability, high X-ray absorption and light output, short decay time, low afterglow (o1% after 3/100 ms), good spectral match to photodetectors (500–1000 nm) and low cost [1–2], of scintillation materials used in medical devices are ever-increasing. CsI:T1 and CdWO4 are the dominant crystal scintillators currently used in detectors in commercial X-CT (X-ray computed tomography, hereinafter referred to as X-CT or CT) scanners. However, observation of their properties listed in Table 1 shows that CdWO4 has low light yield and CsI:T1 has high afterglow and radiation damage. Furthermore, it is extremely difficult to grow high-quality and large-size single crystals due to high melting point. For many studies, ceramics have several advantages over single crystals, e.g. easier preparation at lower sintering temperature and lower cost, high stability, possibility to synthesize new compositions and highly doped center ions, etc. [5–7]. Cubic Y2O3 crystal is a promising host due to its low phonon energy ( 380 cm 1), chemical durability, thermal stability, high thermal conductivity ( 13.6 W/km) and broad optical spectra [8–10]. Scintillation ceramics, as an alternative to single crystals, have been promising materials for X-CT since the first scintillation ceramic Eu:(Y,Gd)2O3 was commercially used in X-CT [4,11]. CdWO4 crystal has been
n
Corresponding author. Tel.: þ 86 21 66137125. E-mail address: [email protected] (Q. Yang).
http://dx.doi.org/10.1016/j.jlumin.2014.02.035 0022-2313/& 2014 Elsevier B.V. All rights reserved.
replaced by Eu:(Y,Gd)2O3 ceramics in GE Medical System's CT products [12] for lower afterglow, lower radiation damage (as shown in Table 1), and superiorities from ceramics. Though the decay time (1000 μs), to some degree, meets the demand of today's CT X-ray detectors (decay time value should be less than 1000 μs), Eu:(Y,Gd)2O3 ceramics need improvement for shorter decay time and lower afterglow. It has been reported that the afterglow can be strongly decreased by co-doping with Pr2O3 in Eu:(Y,Gd)2O3 ceramics [13]. In addition, Pr3 þ has a much shorter decay time, so it can play an important role in shortening decay time in the co-doped system. However, co-doping Pr3 þ with Eu3 þ was reported to strongly weaken the emission intensity in some matrices [14,15]. Considering the concentration quenching of Pr3 þ , introduction of minor Pr3 þ is advisable for the emission intensity. Additionally, La2O3 can be used as a good additive into Y2O3 for high optical quality according to our previous studies [16,17]. In this paper, x at% Pr codoping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0–1, x starts from a relatively low level) transparent ceramics were fabricated. The effect of Pr3 þ on photoluminescence (PL) properties, decay time and radioluminescence (RL) properties was investigated in detail.
2. Experimental procedure x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0, 0.005, 0.07, 0.15 and 1) transparent ceramics were fabricated by the solid-state reaction method and sintered in H2 atmosphere. High purity
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C. Jiang et al. / Journal of Luminescence 153 (2014) 453–457
Table 1 Scintillation properties of scintillators for medical X-CT detector [3,4]. Materials
Density (g cm–3)
ρZ4 ( 106)
Relative light output (%)a
Decay time (μs)
Afterglow (% after 3/100 ms)
Radiation damage (%)b
CdWO4 CsI:T1 Eu0.06Y1.34Gd0.6O3
7.9 4.5 5.9
134 38 44
30 100 70
5 1 1000
0.02 0.3 o0.01
2.9 þ 13.5 o 1.0
a b
Measured with photodiodes for X-ray tube voltage of 60 kVp. Percent relative change in light output after dose of 450 Roentgens of 140 kVp X-rays.
Pr6O11, Eu2O3, Y2O3 and La2O3 (99.99%) nanopowders were weighed according to the desired composition. The concentration of La2O3 is 10 at% and Eu2O3 and Pr6O11 is at the cost of Y2O3. The starting powders were mixed in absolute ethyl alcohol for 5 h with zirconia balls, then dried and calcined in a muffle furnace at 1200 1C for 10 h. Afterwards, the powders were ball re-milled for 5 h, dried, sieved, and isostatically pressed into disks with 15 mm in diameter and 4 mm in thickness at 200 MPa/cm2. Finally specimens were sintered at 1605–1680 1C for 45 h in H2 atmosphere. The sintered specimens were cut and double polished with 2 mm in thickness for optical test and spectral analysis. The microstructure was observed with an optical microscope (Model BX60, OLYPMUS, Japan). Raman spectra were collected with a Raman spectrometer (HORIBA JOBIN YVON HR800UV) at room temperature under the excitation of 514.5 nm. The phase composition was identified by powder X-ray diffraction (Model D/Max2550, Rigaku, Japan). Photoluminescence spectra (λexc ¼270 nm) and photoluminescence lifetime were measured at room temperature with a spectrophotometer (Model FLS920, Edinburgh instrument, Britain) using Xe light as pump source. Radioluminescence spectra were collected using the custom made 5000M fluorometer, Horiba Jobin Yvon, equipped with an X-ray tube (steady-state tube, 40 kV, Mo anode).
Fig. 1. Microscopic photograph of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics.
3. Results and discussion Fig. 1 shows the microscopic photograph of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics. The specimens have a compact structure, and no second phase or other impurities are observed except for a few pores. The grain size is between 20 and 40 μm. The optical in-line transmittance of 0.07 at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics is displayed in Fig. 2. The highest transmittance reaches about 71%. The photo of specimens is presented in the inset of Fig. 2. The color of the specimens varies from light red to yellow when Pr3 þ content x varies from 0 to 1. The letters under the specimen can be clearly seen. The stable form of cubic bixbyite structure in (Y0.9La0.1)O3 host which has been reported before [18] ensures a high transparency in our specimens. Fig. 3 shows the Raman spectra of x at% Pr co-doping 5 at% Eu: (Y0.9La0.1)2O3 ceramics. The spectrum of Y2O3 is in accordance with those reported in the literature [19,20]. The bands of (Y0.9La0.1)2O3 ceramics are similar with those of Y2O3 though all bands of (Y0.9La0.1)2O3 ceramics are broadened. This reveals that La2O3, as a sintering aid, has no impact on the crystal structure, only causing a distorted structure. The bands at 700 cm 1 corresponding to 533 nm can be ascribed to the emission of Eu3 þ . The X-ray diffraction pattern of Eu:(Y0.9La0.1)2O3 was collected and presented in Fig. 4. The XRD pattern of Eu:(Y0.9La0.1)2O3 is in good agreement with the pattern of Y2O3 in JCPDS card (43-1036). Raman spectra of the specimens co-doped with Pr3 þ are also similar with that of the specimen single-doped with Eu3 þ , which indicates that Pr3 þ has no impact on the crystal structure.
Fig. 2. In-line optical transmission spectra of 0.07 at% Pr co-doping 5 at% Eu: (Y0.9La0.1)2O3 transparent ceramics. Inset is the photograph of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics (left: x ¼0, right: x ¼1). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
The effect of Pr3 þ content on luminescence properties was studied by emission spectra. Fig. 5 shows the room temperature PL emission spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x ¼0–1) ceramics excited by 270 nm. The predominant 612 nm red emission can be assigned to the 5D0–7F2 electric dipole transition of Eu3 þ [21]. The other weak emission peaks (581, 588, 594, 600, 631, 652, and 663 nm) are related to other 5D0–7Fj transitions (as marked in Fig. 5) [22]. Compared with the specimen without Pr3 þ , the specimens with Pr3 þ shows no predominant emissions from Pr3 þ at around 631 and 622 nm for C2 and S6 sites, respectively. Thus, Pr3 þ probably is more likely to act as a sensitizer in the co-doped system. The quenched emission in Eu3 þ doped materials with increased 3þ Pr concentration had been reported by Mhlongo [14] and
C. Jiang et al. / Journal of Luminescence 153 (2014) 453–457
Fig. 3. Raman spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x ¼0–1) ceramics at room temperature (λexc ¼ 514.5 nm).
Fig. 4. X-ray diffraction patterns of Eu:(Y0.9La0.1)2O3.
Fig. 5. PL emission spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0–1) ceramics at room temperature (λexc ¼ 270 nm).
Xie [15] already. In their work, the effects of Pr3 þ concentration on emission properties were investigated at a relatively high concentration level of Pr3 þ . Our work, however, began at extremely low concentration level of Pr3 þ . It can be seen from Fig. 5 that the emission intensity increases in the first stage of increasing Pr3 þ content and then decreases when the Pr3 þ content continuously increases from 0.07 at% to 1 at%. The result suggests that minor
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Fig. 6. Schematic energy transfer pathway from Pr3 þ to Eu3 þ .
Fig. 7. PL decay curves of 5D0–7F2 emission (Eu3 þ ) in x at% Pr co-doping 5 at% Eu: (Y0.9La0.1)2O3 (x ¼0–1) ceramics at room temperature.
content of Pr3 þ is favorable for enhancing the emission intensity of Eu3 þ . The dominant reason is the energy transfer (ET) from Pr3 þ to Eu3 þ . The possible ET mechanisms are demonstrated based on the energy level diagram of Eu3 þ and Pr3 þ , as shown in Fig. 6. Under excitation at 270 nm, the excitation energy is absorbed through charge-transfer (CT) transition of Eu3 þ –O2 , and then transferred to Eu3 þ to relax radiatively from 5D0 to 7F2 (612 nm red light). However, the absorbed energy by Eu3 þ ion is partly consumed by the non-radiative relaxation. Pr3 þ can make up for this loss. Pr3 þ can also be efficiently excited under excitation in a wide UV range. Hence, Pr3 þ is excited into 4f5d state by 270 nm, followed by a non-radiative relaxation to the 3P0 level. The subsequent energy released from 3P0-3H6 is well-matched to 7F1–5D0 of Eu3 þ and promote the state of Eu3 þ to higher level of 5D0 (step ①). On the other hand, the energy from 3P0 (Pr3 þ ) can also be transferred to 5 D1 (Eu3 þ ) and then relax to 5D0 (step ②). These two steps efficiently increase the population of 5D0 and release energy through the transition of 5D0-7F2, which helps to enhance the 612 nm red emission of Eu3 þ . On the other hand, obvious dropping of the emission intensity is also observed in our experiment when Pr3 þ content x comes to 1. This is due to the concentration quenching of Pr3 þ ions. The PL decay time of the 5D0 excited state of Eu3 þ was measured for the 612 nm emission and their decay curves are shown in Fig. 7. The curves are fitted to two exponentials according to the following
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C. Jiang et al. / Journal of Luminescence 153 (2014) 453–457
equation: IðtÞ ¼ y0 þ A1 e t=τ1 þ A2 e t=τ2 , where τ1 and τ2 are fast and slow decay times. Fitting results are summarized in Table 2. The results suggest that Eu3 þ ions occupy two different sites of C2 and S6 in cubic Y2O3 lattice. Additionally, it can be found that fast decay
Table 2 Dual-exponential fitting results of decay time. Pr3 þ content x (at%)
0 0.005 0.07 0.15 1
Fast decay
Slow decay
τ1 (μs)
Proportion (%)
τ2 (μs)
Proportion (%)
749.39 939.50 871.90 650.05 220.28
48.52 97.00 94.16 62.78 53.37
1187.96 2753.68 1799.02 1114.91 629.93
51.48 3.00 5.84 37.22 46.63
Fig. 8. RL spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x ¼0–1) ceramics under the X-ray excitation (40 kV, 30 mA).
plays a significantly dominant role in Eu/Pr co-doped specimens, particularly when x¼ 0.005 and 0.07. The overall PL decay time is shown in Fig. 9(c). The drastic decrease in decay time shows that Pr3 þ can reduce the lifetime significantly and thus help lower the afterglow effectively. Xie [15] reported a decay time of 1300 μs in Lu2O3 with Eu3 þ and Pr3 þ content of 5 and 0 mol%, and a decay time of 670 μs when Pr3 þ content reached 0.5 mol%. Obviously, (Eu, Pr):(Y, La)2O3 materials own shorter decay time. RL spectra were also collected while x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0–1) ceramics were irradiated at room temperature with X-rays from Mo target operated at 40 kV and 30 mA. The smoothing for the spectra was conducted. The smoothed radioluminescence (RL) spectra are presented in Fig. 8. The position and shape for PL and RL are similar. The intensive peak corresponding to the 5D0-7F2 transition of Eu3 þ gives a bright red luminescence that matches to photodetectors. Minor Pr3 þ still works on the enhancement of emission intensity and the concentration quenching of Pr3 þ is still present when x ¼1. However, the tendency of intensity for PL and RL shows some difference. The strongest luminescence is obtained when x ¼0.005 instead of x¼ 0.07. The emission for x¼0.15 is weaker than for x¼ 0 in PL spectra; however, much more efficient emissions are observed for x¼ 0.15 in RL spectra. Thus, we can find that the difference in the excitation mechanism between UV and X-ray results in different performances of PL and RL inspite of their similar emission mechanism. RL cannot be replaced by PL to evaluate scintillation properties of scintillation materials. Therefore, it is improper to use PL as a substitute to study the scintillation properties in previous work from a few other researchers. The overall PL and RL intensities together with the overall decay time are shown in Fig. 9. It can be found that minor Pr3 þ content can reduce the PL decay time and at the same time enhance both PL and RL emission intensities. The enhancement of RL emission intensity under the excitation of X-ray and the shortening of the decay time are of great benefit for application in today's CT X-ray detectors.
Fig. 9. The overall (a) RL intensity, (b) PL intensity and (c) PL decay time of the specimens with the increasing Pr3 þ content.
C. Jiang et al. / Journal of Luminescence 153 (2014) 453–457
4. Conclusion High optical-quality Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics were prepared by conventional ceramics processing. PL and RL intensities were enhanced when minor Pr3 þ content was codoped, owing to the energy transfer (ET) from Pr3þ to Eu3þ . The quenched emission is also observed in PL and RL spectra due to concentration quenching of excess Pr3þ . The difference in intensity tendency between PL and RL spectra indicates that it is improper to investigate the scintillation properties by using PL instead of RL in previous work from a few other researchers. In addition, Pr3þ is found to have a great impact on the shortening of the PL decay time and thus helps lower the afterglow. Given overall properties, Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics exhibit the potential to act as scintillation materials for X-CT scanner. Acknowledgment We would like to acknowledge Dr. Danping Chen and Master Chunmei Tang from Institute of Optics and Fine Mechanics in Shanghai for their kind help on RL spectra measurement under the X-ray excitation. References [1] B.C. Grabmaier, J. Lumin. 60–61 (1994) 967. [2] W.W. Moses, Scintillator requirements for medical imaging, in: Proceedings of the International Conference on Inorganic Scintillators and their Applications: SCINT99, Moscow, Russia. LBNL-4580, 1999.
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[3] Carel W.E. van Eijk, Phys. Med. Biol. 47 (2002) 85. [4] C. Greskovich, S. Duclos, Annu. Rev. Mater. Sci. 27 (1997) 69. [5] Y. Futami, T. Yanagida, Y. Fujimoto, J. Pejchal, M. Sugiyama, S. Kurosawa, Y. Yokota, A. Ito, A. Yoshikawa, T. Goto, Radiat. Meas. 55 (2013) 136. [6] A. ikesue, Y.L. Aung, Nat. Photonics 2 (2008) 721. [7] R.M. Krsmanovic, Ž. Antić, B. Bartova, M.G. Brik, M.D. Dramicanin, Ceram. Int. 38 (2012) 1303. [8] L. Laversenne, Y. Guyot, C. Goutaudier, M.T. Cohen Adad, G. Boulon, Opt. Mater. 16 (2001) 475. [9] R.H. Hoskins, B.H. Soffer, Appl. Phys. Lett. 4 (1964) 22. [10] U. Griebner, V. Petrov, K. Petermann, V. Peters, Opt. Express 12 (2004) 3125. [11] C. Greskovich, D. Cusano, D. Hoffman, R.J. Riedner, Am. Ceram. Soc. Bull. 71 (1992) 1120. [12] S.J. Duclos, C.D. Greskovich, R.J. Lyons, J.S. Vartuli, D.M. Hoffman, R.J. Riedner, M.J. Lynch, Nucl. Instrum. Methods Phys. Res. A 505 (2003) 68. [13] W. Kostler, A. Winnacker, W. Rossner, J. Phys. Chem. Solids 56 (1995) 907. [14] G.H. Mhlongo, M.S. Dhlamini, O.M. Ntwaeaborwa, H.C. Swart, K.T. Hillie The influence of Pr3 þ co-doping on the photoluminescence and cathode luminescence properties of SiO2:Eu3 þ /Tb3 þ , in: Proceedings of the 56th Annual Conference of the South African Institute of Physics (SAIP2011), St. George Hotel, Pretoria, July 12–15, 2011. [15] J. Xie, L.Y. Deng, Y. Shi, J.J. Xie, H.J. Qiu, G.X. Song, J. Lumin. 131 (2011) 970. [16] H.J. Zhang, Q.H. Yang, H.X. Zhou, J. Xu, Chin. J. Inorg. Chem. 25 (2009) 578. [17] H.J. Zhang, Q.H. Yang, S.Z. Lu, J. Chin., Ceram. Soc. 37 (2009) 476. [18] Q.H. Yang, C.G. Dou, J. Ding, X.M. Hu, Appl. Phys. Lett. 91 (2007) 111918. [19] A. Ubaldini, M.M. Carnasciali, J. Alloys Compd. 454 (2008) 374. [20] H. Guo, Y.M. Qiao, Opt. Mater. 31 (2009) 583. [21] Y.K. Kim, H.K. Kim, G. Cho, D.K. Kim, Nucl. Instrum. Methods Phys. Res. B 225 (2004) 392. [22] C.N. Wang, Y. Li, W.P. Zhang, M. Yin, Spectrochim. Acta Part A 75 (2010) 8.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Study of photoluminescence, radioluminescence and decay performances of Pr3 þ co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics Cen Jiang, Qiuhong Yang n, Ye Yuan, Qing Lu, Shenzhou Lu, Yunhan Li School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 November 2013 Received in revised form 18 February 2014 Accepted 25 February 2014 Available online 11 March 2014
High optical-quality Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics were prepared by the solid-state reaction method and sintered in H2 atmosphere. Photoluminescence (PL) and radioluminescence (RL) intensities were enhanced by co-doping minor Pr3 þ content due to the energy transfer (ET), and quenched with the increase of Pr3 þ content due to concentration quenching of excess Pr3 þ . The possible ET process had been demonstrated. Decay time was also shortened by co-doping Pr3 þ . The results showed that Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics are promising scintillation materials. & 2014 Elsevier B.V. All rights reserved.
Keywords: Eu/Pr co-doped ceramics Photoluminescence Radioluminescence Decay time
1. Introduction Nowadays, the requirements, e.g. high density, good stability, high X-ray absorption and light output, short decay time, low afterglow (o1% after 3/100 ms), good spectral match to photodetectors (500–1000 nm) and low cost [1–2], of scintillation materials used in medical devices are ever-increasing. CsI:T1 and CdWO4 are the dominant crystal scintillators currently used in detectors in commercial X-CT (X-ray computed tomography, hereinafter referred to as X-CT or CT) scanners. However, observation of their properties listed in Table 1 shows that CdWO4 has low light yield and CsI:T1 has high afterglow and radiation damage. Furthermore, it is extremely difficult to grow high-quality and large-size single crystals due to high melting point. For many studies, ceramics have several advantages over single crystals, e.g. easier preparation at lower sintering temperature and lower cost, high stability, possibility to synthesize new compositions and highly doped center ions, etc. [5–7]. Cubic Y2O3 crystal is a promising host due to its low phonon energy ( 380 cm 1), chemical durability, thermal stability, high thermal conductivity ( 13.6 W/km) and broad optical spectra [8–10]. Scintillation ceramics, as an alternative to single crystals, have been promising materials for X-CT since the first scintillation ceramic Eu:(Y,Gd)2O3 was commercially used in X-CT [4,11]. CdWO4 crystal has been
n
Corresponding author. Tel.: þ 86 21 66137125. E-mail address: [email protected] (Q. Yang).
http://dx.doi.org/10.1016/j.jlumin.2014.02.035 0022-2313/& 2014 Elsevier B.V. All rights reserved.
replaced by Eu:(Y,Gd)2O3 ceramics in GE Medical System's CT products [12] for lower afterglow, lower radiation damage (as shown in Table 1), and superiorities from ceramics. Though the decay time (1000 μs), to some degree, meets the demand of today's CT X-ray detectors (decay time value should be less than 1000 μs), Eu:(Y,Gd)2O3 ceramics need improvement for shorter decay time and lower afterglow. It has been reported that the afterglow can be strongly decreased by co-doping with Pr2O3 in Eu:(Y,Gd)2O3 ceramics [13]. In addition, Pr3 þ has a much shorter decay time, so it can play an important role in shortening decay time in the co-doped system. However, co-doping Pr3 þ with Eu3 þ was reported to strongly weaken the emission intensity in some matrices [14,15]. Considering the concentration quenching of Pr3 þ , introduction of minor Pr3 þ is advisable for the emission intensity. Additionally, La2O3 can be used as a good additive into Y2O3 for high optical quality according to our previous studies [16,17]. In this paper, x at% Pr codoping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0–1, x starts from a relatively low level) transparent ceramics were fabricated. The effect of Pr3 þ on photoluminescence (PL) properties, decay time and radioluminescence (RL) properties was investigated in detail.
2. Experimental procedure x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0, 0.005, 0.07, 0.15 and 1) transparent ceramics were fabricated by the solid-state reaction method and sintered in H2 atmosphere. High purity
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C. Jiang et al. / Journal of Luminescence 153 (2014) 453–457
Table 1 Scintillation properties of scintillators for medical X-CT detector [3,4]. Materials
Density (g cm–3)
ρZ4 ( 106)
Relative light output (%)a
Decay time (μs)
Afterglow (% after 3/100 ms)
Radiation damage (%)b
CdWO4 CsI:T1 Eu0.06Y1.34Gd0.6O3
7.9 4.5 5.9
134 38 44
30 100 70
5 1 1000
0.02 0.3 o0.01
2.9 þ 13.5 o 1.0
a b
Measured with photodiodes for X-ray tube voltage of 60 kVp. Percent relative change in light output after dose of 450 Roentgens of 140 kVp X-rays.
Pr6O11, Eu2O3, Y2O3 and La2O3 (99.99%) nanopowders were weighed according to the desired composition. The concentration of La2O3 is 10 at% and Eu2O3 and Pr6O11 is at the cost of Y2O3. The starting powders were mixed in absolute ethyl alcohol for 5 h with zirconia balls, then dried and calcined in a muffle furnace at 1200 1C for 10 h. Afterwards, the powders were ball re-milled for 5 h, dried, sieved, and isostatically pressed into disks with 15 mm in diameter and 4 mm in thickness at 200 MPa/cm2. Finally specimens were sintered at 1605–1680 1C for 45 h in H2 atmosphere. The sintered specimens were cut and double polished with 2 mm in thickness for optical test and spectral analysis. The microstructure was observed with an optical microscope (Model BX60, OLYPMUS, Japan). Raman spectra were collected with a Raman spectrometer (HORIBA JOBIN YVON HR800UV) at room temperature under the excitation of 514.5 nm. The phase composition was identified by powder X-ray diffraction (Model D/Max2550, Rigaku, Japan). Photoluminescence spectra (λexc ¼270 nm) and photoluminescence lifetime were measured at room temperature with a spectrophotometer (Model FLS920, Edinburgh instrument, Britain) using Xe light as pump source. Radioluminescence spectra were collected using the custom made 5000M fluorometer, Horiba Jobin Yvon, equipped with an X-ray tube (steady-state tube, 40 kV, Mo anode).
Fig. 1. Microscopic photograph of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics.
3. Results and discussion Fig. 1 shows the microscopic photograph of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics. The specimens have a compact structure, and no second phase or other impurities are observed except for a few pores. The grain size is between 20 and 40 μm. The optical in-line transmittance of 0.07 at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics is displayed in Fig. 2. The highest transmittance reaches about 71%. The photo of specimens is presented in the inset of Fig. 2. The color of the specimens varies from light red to yellow when Pr3 þ content x varies from 0 to 1. The letters under the specimen can be clearly seen. The stable form of cubic bixbyite structure in (Y0.9La0.1)O3 host which has been reported before [18] ensures a high transparency in our specimens. Fig. 3 shows the Raman spectra of x at% Pr co-doping 5 at% Eu: (Y0.9La0.1)2O3 ceramics. The spectrum of Y2O3 is in accordance with those reported in the literature [19,20]. The bands of (Y0.9La0.1)2O3 ceramics are similar with those of Y2O3 though all bands of (Y0.9La0.1)2O3 ceramics are broadened. This reveals that La2O3, as a sintering aid, has no impact on the crystal structure, only causing a distorted structure. The bands at 700 cm 1 corresponding to 533 nm can be ascribed to the emission of Eu3 þ . The X-ray diffraction pattern of Eu:(Y0.9La0.1)2O3 was collected and presented in Fig. 4. The XRD pattern of Eu:(Y0.9La0.1)2O3 is in good agreement with the pattern of Y2O3 in JCPDS card (43-1036). Raman spectra of the specimens co-doped with Pr3 þ are also similar with that of the specimen single-doped with Eu3 þ , which indicates that Pr3 þ has no impact on the crystal structure.
Fig. 2. In-line optical transmission spectra of 0.07 at% Pr co-doping 5 at% Eu: (Y0.9La0.1)2O3 transparent ceramics. Inset is the photograph of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics (left: x ¼0, right: x ¼1). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
The effect of Pr3 þ content on luminescence properties was studied by emission spectra. Fig. 5 shows the room temperature PL emission spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x ¼0–1) ceramics excited by 270 nm. The predominant 612 nm red emission can be assigned to the 5D0–7F2 electric dipole transition of Eu3 þ [21]. The other weak emission peaks (581, 588, 594, 600, 631, 652, and 663 nm) are related to other 5D0–7Fj transitions (as marked in Fig. 5) [22]. Compared with the specimen without Pr3 þ , the specimens with Pr3 þ shows no predominant emissions from Pr3 þ at around 631 and 622 nm for C2 and S6 sites, respectively. Thus, Pr3 þ probably is more likely to act as a sensitizer in the co-doped system. The quenched emission in Eu3 þ doped materials with increased 3þ Pr concentration had been reported by Mhlongo [14] and
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Fig. 3. Raman spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x ¼0–1) ceramics at room temperature (λexc ¼ 514.5 nm).
Fig. 4. X-ray diffraction patterns of Eu:(Y0.9La0.1)2O3.
Fig. 5. PL emission spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0–1) ceramics at room temperature (λexc ¼ 270 nm).
Xie [15] already. In their work, the effects of Pr3 þ concentration on emission properties were investigated at a relatively high concentration level of Pr3 þ . Our work, however, began at extremely low concentration level of Pr3 þ . It can be seen from Fig. 5 that the emission intensity increases in the first stage of increasing Pr3 þ content and then decreases when the Pr3 þ content continuously increases from 0.07 at% to 1 at%. The result suggests that minor
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Fig. 6. Schematic energy transfer pathway from Pr3 þ to Eu3 þ .
Fig. 7. PL decay curves of 5D0–7F2 emission (Eu3 þ ) in x at% Pr co-doping 5 at% Eu: (Y0.9La0.1)2O3 (x ¼0–1) ceramics at room temperature.
content of Pr3 þ is favorable for enhancing the emission intensity of Eu3 þ . The dominant reason is the energy transfer (ET) from Pr3 þ to Eu3 þ . The possible ET mechanisms are demonstrated based on the energy level diagram of Eu3 þ and Pr3 þ , as shown in Fig. 6. Under excitation at 270 nm, the excitation energy is absorbed through charge-transfer (CT) transition of Eu3 þ –O2 , and then transferred to Eu3 þ to relax radiatively from 5D0 to 7F2 (612 nm red light). However, the absorbed energy by Eu3 þ ion is partly consumed by the non-radiative relaxation. Pr3 þ can make up for this loss. Pr3 þ can also be efficiently excited under excitation in a wide UV range. Hence, Pr3 þ is excited into 4f5d state by 270 nm, followed by a non-radiative relaxation to the 3P0 level. The subsequent energy released from 3P0-3H6 is well-matched to 7F1–5D0 of Eu3 þ and promote the state of Eu3 þ to higher level of 5D0 (step ①). On the other hand, the energy from 3P0 (Pr3 þ ) can also be transferred to 5 D1 (Eu3 þ ) and then relax to 5D0 (step ②). These two steps efficiently increase the population of 5D0 and release energy through the transition of 5D0-7F2, which helps to enhance the 612 nm red emission of Eu3 þ . On the other hand, obvious dropping of the emission intensity is also observed in our experiment when Pr3 þ content x comes to 1. This is due to the concentration quenching of Pr3 þ ions. The PL decay time of the 5D0 excited state of Eu3 þ was measured for the 612 nm emission and their decay curves are shown in Fig. 7. The curves are fitted to two exponentials according to the following
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equation: IðtÞ ¼ y0 þ A1 e t=τ1 þ A2 e t=τ2 , where τ1 and τ2 are fast and slow decay times. Fitting results are summarized in Table 2. The results suggest that Eu3 þ ions occupy two different sites of C2 and S6 in cubic Y2O3 lattice. Additionally, it can be found that fast decay
Table 2 Dual-exponential fitting results of decay time. Pr3 þ content x (at%)
0 0.005 0.07 0.15 1
Fast decay
Slow decay
τ1 (μs)
Proportion (%)
τ2 (μs)
Proportion (%)
749.39 939.50 871.90 650.05 220.28
48.52 97.00 94.16 62.78 53.37
1187.96 2753.68 1799.02 1114.91 629.93
51.48 3.00 5.84 37.22 46.63
Fig. 8. RL spectra of x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x ¼0–1) ceramics under the X-ray excitation (40 kV, 30 mA).
plays a significantly dominant role in Eu/Pr co-doped specimens, particularly when x¼ 0.005 and 0.07. The overall PL decay time is shown in Fig. 9(c). The drastic decrease in decay time shows that Pr3 þ can reduce the lifetime significantly and thus help lower the afterglow effectively. Xie [15] reported a decay time of 1300 μs in Lu2O3 with Eu3 þ and Pr3 þ content of 5 and 0 mol%, and a decay time of 670 μs when Pr3 þ content reached 0.5 mol%. Obviously, (Eu, Pr):(Y, La)2O3 materials own shorter decay time. RL spectra were also collected while x at% Pr co-doping 5 at% Eu:(Y0.9La0.1)2O3 (x¼ 0–1) ceramics were irradiated at room temperature with X-rays from Mo target operated at 40 kV and 30 mA. The smoothing for the spectra was conducted. The smoothed radioluminescence (RL) spectra are presented in Fig. 8. The position and shape for PL and RL are similar. The intensive peak corresponding to the 5D0-7F2 transition of Eu3 þ gives a bright red luminescence that matches to photodetectors. Minor Pr3 þ still works on the enhancement of emission intensity and the concentration quenching of Pr3 þ is still present when x ¼1. However, the tendency of intensity for PL and RL shows some difference. The strongest luminescence is obtained when x ¼0.005 instead of x¼ 0.07. The emission for x¼0.15 is weaker than for x¼ 0 in PL spectra; however, much more efficient emissions are observed for x¼ 0.15 in RL spectra. Thus, we can find that the difference in the excitation mechanism between UV and X-ray results in different performances of PL and RL inspite of their similar emission mechanism. RL cannot be replaced by PL to evaluate scintillation properties of scintillation materials. Therefore, it is improper to use PL as a substitute to study the scintillation properties in previous work from a few other researchers. The overall PL and RL intensities together with the overall decay time are shown in Fig. 9. It can be found that minor Pr3 þ content can reduce the PL decay time and at the same time enhance both PL and RL emission intensities. The enhancement of RL emission intensity under the excitation of X-ray and the shortening of the decay time are of great benefit for application in today's CT X-ray detectors.
Fig. 9. The overall (a) RL intensity, (b) PL intensity and (c) PL decay time of the specimens with the increasing Pr3 þ content.
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4. Conclusion High optical-quality Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics were prepared by conventional ceramics processing. PL and RL intensities were enhanced when minor Pr3 þ content was codoped, owing to the energy transfer (ET) from Pr3þ to Eu3þ . The quenched emission is also observed in PL and RL spectra due to concentration quenching of excess Pr3þ . The difference in intensity tendency between PL and RL spectra indicates that it is improper to investigate the scintillation properties by using PL instead of RL in previous work from a few other researchers. In addition, Pr3þ is found to have a great impact on the shortening of the PL decay time and thus helps lower the afterglow. Given overall properties, Eu/Pr co-doped (Y0.9La0.1)2O3 transparent ceramics exhibit the potential to act as scintillation materials for X-CT scanner. Acknowledgment We would like to acknowledge Dr. Danping Chen and Master Chunmei Tang from Institute of Optics and Fine Mechanics in Shanghai for their kind help on RL spectra measurement under the X-ray excitation. References [1] B.C. Grabmaier, J. Lumin. 60–61 (1994) 967. [2] W.W. Moses, Scintillator requirements for medical imaging, in: Proceedings of the International Conference on Inorganic Scintillators and their Applications: SCINT99, Moscow, Russia. LBNL-4580, 1999.
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