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Blue photoluminescence and long lasting phosphorescence properties of a novel chloride phosphate phosphor: Sr5(PO4)3Cl:Eu2+
Journal of Luminescence 147 (2014) 229–234
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Blue photoluminescence and long lasting phosphorescence properties of a novel chloride phosphate phosphor: Sr5(PO4)3Cl:Eu2 þ Chuanqiang Wu, Jiachi Zhang n, Pengfei Feng, Yiming Duan, Zhiya Zhang n, Yuhua Wang Key Laboratory for Magnetism Magnetic Materials of the Ministry of Education, Tianshui Street 222#, Lanzhou University, Lanzhou 730000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 September 2013 Received in revised form 6 November 2013 Accepted 13 November 2013 Available online 22 November 2013
A novel blue emitting long lasting phosphorescence phosphor Sr5(PO4)3Cl:Eu2 þ is synthesized by solid state method at 1223 K in reducing atmosphere. The afterglow emission spectrum shows one broad band centered at 441 nm due to the 5d–4f transition of Eu2 þ at six coordinated Sr(II) sites and the color coordinates are calculated to be (0.149, 0.095) which is close to the light blue region. The excitation band is in 240–430 nm and partly overlaps the solar irradiation on Earth's surface. The long lasting phosphorescence of the optimal sample doping by 0.1 mol%Eu2 þ can be recorded for about 1040 s (0.32 mcd/m2). Thermoluminescence shows that there are at least three types of traps corresponding to peaks at 340 K, 382 K, 500 K, respectively. The filling and fading experiments reveal that the traps in Sr5(PO4)3Cl:Eu2 þ are independent. The shallow traps (340 K) essentially contribute to the visible long lasting phosphorescence, while the deep traps (382 K and 500 K) are proved to be very stable. Thus, the Sr5(PO4)3Cl:Eu2 þ material shows potential applications as not only a long lasting phosphorescence phosphor, but also an optical storage material. & 2013 Elsevier B.V. All rights reserved.
Keywords: Luminescence Optical materials Optical properties
1. Introduction Long lasting phosphorescence (LLP) is an interesting optical phenomenon where, a material excited by high energy radiation produces visible luminescent emission for an appreciable time at room temperature even after the irradiation light sources have been removed [1–3]. In recent years, the LLP phosphors have attracted wide attention for possible applications in many fields, such as emergency lighting, safety indication, billboards, interior decoration [4–8]. The present commercial LLP phosphors are SrAl2O4:Eu2 þ , Dy3 þ (green), CaAl2O4:Eu2 þ , Nd3 þ (blue) and Y2O2S:Eu3 þ , Ti3 þ , Mg2 þ (red), respectively [9]. Nowadays, these commercial LLP phosphors still have some shortcomings such as the low moisture resistance of aluminates and the poor stability of oxysulfide and the LLP mechanism is still not clear. For fundamental reasons, it is necessary to explore the possibilities of new systems that can be used as LLP phosphors. Alkaline earth chloride phosphates have been studied for years and it is generally considered as an ideal host with low synthesis temperature and excellent stability [10–12]. On the other hand, the luminescence of Eu2 þ activated phosphors usually results from the allowed electric dipole transition (4f7 to 4f65d1) and thus the efficient emission of Eu2 þ can be expected in many hosts [13].
n
Corresponding authors. Tel.: þ 86 931 8912772; fax: þ 86 931 8913554. E-mail addresses: [email protected] (J. Zhang), [email protected] (Z. Zhang). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.055
In 1992, Capobianco et al. first reported the near-infrared luminescence spectroscopy of the Mn5 þ ion in Sr5(PO4)3Cl and then Misra et al. subsequently studied the Mn5 þ -doped Sr5(PO4)3Cl as a solidstate-laser material [10,11]. Sr5(PO4)3Cl:Eu2 þ , Mn2 þ was investigated as a near-ultraviolet excited LEDs phosphor by Guo et al. in 2003 [14]. However, to the best of our knowledge, the visible long lasting phosphorescence of Sr5(PO4)3Cl:Eu2 þ has been never reported. In this work, the blue LLP is observed in the Sr5(PO4)3Cl:Eu2 þ material synthesized by solid state reaction at 1223 K in reducing atmosphere. The PL and LLP properties of Sr5(PO4)3Cl:Eu2 þ material are investigated in detail. The filling and fading experiments are carried out for revealing the traps in Sr5(PO4)3Cl:Eu2 þ material. According to the results, the long lasting phosphorescence mechanism for the independent traps of Sr5(PO4)3Cl:Eu2 þ is proposed. 2. Experimental The Sr5(PO4)3Cl:Eu2 þ samples were prepared by a solid state reaction with SrCO3 (A.R.), SrCl2 6H2O (A.R.), (NH4)2HPO4 (A.R.) and Eu2O3 (99.99%). After the raw materials were prepared in stoichiometric ratios and thoroughly homogenized (all grinding was performed using an agate pestle and mortar), the mixture was transferred into an alumina crucible and then loaded into a tube furnace. The samples were sintered at 1223 K for 3 h in a reducing atmosphere of H2 (5%) and N2 (95%) to complete the reaction.
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After firing, the samples were cooled to room temperature within the furnace and then ground again using an agate mortar. The sample phases were identified by powder diffraction (XRD) with Ni-filtered CuK a radiation at scanning step of 0.021. Photoluminescence (PL) spectra were recorded using a FLS-920T spectrophotometer (Edinburgh Instruments Ltd, Edinburgh, U.K.) with a 450 W xenon arc lamp (Xe900) as the light source. LLP decay curve measurements were taken with a PR305 LLP instrument (Zhejiang University Sensing Instrument Co. Ltd., Hangzhou, China). The thermoluminescence (TL) curves were measured using a FJ-417A TL meter (Beijing Nuclear Instrument Factory, Beijing, China). All samples were first exposed to radiation using a UV lamp (254 nm) for 10 min and then heated from room temperature to 673 K at a rate of 1 K/s. All measurements were carried out at room temperature except for TL measurements.
3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of a typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample obtained in reducing atmosphere and the corresponding standard PDF card is also given for comparison. As shown in Fig. 1, all peaks of the obtained Sr5(PO4)3Cl:Eu2 þ sample can be well indexed by the standard PDF card (PDF#701007) of Sr5(PO4)3Cl. No extra peak is observed, indicating that the obtained sample is a single phase and the doping of Eu2 þ ion does not induce the second phase. Accordingly, the obtained Sr5(PO4)3Cl:Eu2 þ sample belongs to a hexagonal system with the former of P63/m space-group symmetry. The crystal structure and spheres for Sr2 þ ions in the Sr5(PO4)3Cl crystal can be depicted by “Diamond software” in Fig. 2 [15]. It shows that there are two inequivalent cationic sites in Sr5(PO4)3Cl, Sr(I) seven coordinated with five oxygen atoms and two chlorine atoms and Sr(II) six coordinated with six oxygen atoms [10–14]. Considering that the ionic radius of Eu2 þ given by 0.112 nm is almost equal to that of Sr2 þ ions (0.114 nm), Eu2 þ ions will replace the two types of Sr2 þ sites statistically in the unit cell. Fig. 3 exhibits the PL excitation spectrum ((a) λem ¼467 nm) and PL emission spectrum ((b) λex ¼254 nm) of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample. Upon excitation at 254 nm, it is found that the emission spectrum of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample consists of an asymmetric broad band in region 400–550 nm and it can be well fitted into two components at 426 nm and 467 nm, respectively. It is well known that the emission of Eu2 þ is strongly dependent on the crystal field strength of the host matrix and thus it can vary from ultraviolet
Fig. 1. The X-ray diffraction pattern of a typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample.
Fig. 2. The crystal structure and spheres for Sr2 þ ions in the Sr5(PO4)3Cl crystal.
to red region [16]. Considering the different surrounding coordination of the two Sr2 þ (Eu2 þ ) sites, the emission components at 426 nm and 467 nm can be ascribed to the electric-dipole-allowed 4f65d1 to 4f7 transition of the Eu2 þ ions at Sr(I) and Sr(II), respectively [17,18]. Monitored by the emission at 467 nm, the excitation spectrum exhibits an unresolved broad band in the range from 240 nm to 430 nm, which can be assigned to the host absorption and the transitions from the ground state 4f7 to the crystal-field split components of 4f65d1 configuration [19]. It is well known that the solar spectrum received on Earth's surface is in 300–1200 nm. This result shows that the excitation spectrum of Sr5(PO4)3Cl:Eu2 þ material partly overlaps with the near-ultraviolet (NUV) portion of the solar radiation and thus the LLP can be activated by the NUV part of the sunlight. However, we have to say that the LLP of Sr5(PO4)3Cl:Eu2 þ material is difficulty to be activated in visible light of sunlight and thus the further improvement is still necessary. Fig. 4a shows the LLP emission spectrum of a typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample and the PL spectrum is also given for comparison. Evidently, only one emission band at 441 nm is observed and it can be also ascribed to the 5d–4f transition of Eu2 þ . This result indicates that only the six coordinated Eu2 þ centers at Sr (II) sites contribute to the LLP of Sr5(PO4)3Cl:Eu2 þ . The CIE color coordinates of PL (marked with ■) and LLP (marked with □) can be calculated to be (0.154, 0.029), (0.149, 0.095), respectively, as shown as in Fig. 4b. Since the LLP intensity keeps decreasing during the measurement of LLP spectrum, the LLP band should shift to short wavelength region and thus the color coordinates of LLP should be more close to the light blue region. Fig. 5 presents the LLP decay curves of Sr5(PO4)3Cl:xmol%Eu2 þ (x ¼0.03, 0.05, 0.10, 0.15, 0.20) recorded immediately after removing the UV irradiation source (254 nm). It is found that the LLP of the optimal Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample could be recorded as long as 1040 s (0.32 mcd/m2). Since the eye sensitivity is about 100 times better than this value (0.32 mcd/m2), we can actually observe a much longer LLP by using the “dark-adapted eye” in complete darkness. Naturally, it would be of interest to find out the LLP processes or models by fitting LLP decay curves. Previously, many researches fit the LLP decay curves by using function of exponential terms in linear-linear plot to obtain the fitting results such as I0, A and τ [20]. As a result, the decay curve is often divided into multiple exponential components, generally called “fast”, “medium” or “slow”. However, it should be given on a logarithmic scale and this is the only way to see the different components in fact [21]. For the optimal Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample, it turns out that the fitting of the decay within the almost all time
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Fig. 3. The PL excitation spectrum ((a) λem ¼467 nm) and PL emission spectrum ((b) λex ¼ 254 nm) of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample.
Fig. 4. The LLP and PL emission spectrum (a) and CIE chromaticity coordinates (b) of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample.
range of the measurement is also not possible even by using a function of three exponential terms The failure of decay fitting suggests that retrapping process which competes with recombination for LLP is essentially not negligent in this case. In other words, the carrier (electron or hole) that escapes a trap may be captured by another trap and thus it is difficult to exponentially fit the decay behavior. In addition, the retrapping process will inevitably decrease the LLP intensity and slow the LLP decay speed. In general, the thermoluminescence (TL) technique is a very useful tool for revealing the nature of defects produced in insulators or semiconductors by UV light. Fig. 6a presents the TL glow curves of Sr5(PO4)3Cl:x%Eu2 þ (x ¼0.03, 0.05, 0.10, 0.15, 0.20). It can be seen that the Sr5(PO4)3Cl:Eu2 þ material possesses the significantly more complex TL glow curve and it consists of two
intense peaks located at about 340 K, 382 K and one small band at around 500 K. This result reveals that there are at least three types of traps with different depths in Sr5(PO4)3Cl:Eu2 þ material. According to Clabau et al., Eu2 þ can exist in oxidized state Eu3 þ so that the electron promoted to a 5d orbital under excitation may potentially be trapped by anion vacancies [22]. Therefore, the traps involved in LLP of Sr5(PO4)3Cl:Eu2 þ material may be due to oxygen vacancies or clusters which are created during the synthesis process at high temperature [22,23]. Fig. 6b shows the TL intensity of P1 (340 K) and P2 (382 K) peaks as a function of the Eu2 þ contents. It is found that the integral TL intensity increases gradually and then decreases when the Eu2 þ content is more than 0.1 mol% due to the concentration quenching effect [5]. Additionally, the TL profiles are not clearly changed by increasing
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Eu2 þ doping content and it suggests that no new trap has been induced by doping Eu2 þ . Accordingly, the irradiation time has a significant effect on the filling of traps and this effect is useful to investigate the trap in materials [23]. Fig. 7a and b show the TL glow curves of the optimal Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample recorded immediately after UV irradiation (254 nm) for different times. It can be seen that, the integral TL intensity of both P1 and P2 peaks increases at the same time by increasing the UV irradiation time. The TL intensity of P1 and P2 peaks as a function of the irradiation time is exhibited in Fig. 7c. The TL intensity at 382 K (P2) due to deep traps is always higher than that at 340 K (P1) due to shallow traps. This result indicates that the filling sequence of the traps in Sr5(PO4)3Cl:Eu2 þ material is not related to the depth of traps and thus it can be concluded that the three types of traps in Sr5(PO4)3Cl:Eu2 þ material are independent in fact. Therefore, the shallow and deep traps can intercept the free carriers at the same time as shown as in Fig. 7c.
Fig. 5. The LLP decay curves of Sr5(PO4)3Cl:xmol%Eu2 þ (x ¼0.03, 0.05, 0.10, 0.15, 0.20) recorded immediately after removing the UV irradiation source (254 nm) and the LLP time as a function of the Eu2 þ contents (inset).
The fading of thermoluminescence is due partly to the bleaching of the trapped carriers by thermal energy and partly by other processes. The measurement of the TL glow curves with different delay time after ceasing the UV irradiation may also give important information on the trap structure in materials [23]. As shown in Fig. 8a and b, the TL intensity of P1 (340 K) due to the shallow traps decreases very quickly and keeps only 77% of its initial intensity after 600 s. At the same time, it is found that the deep traps at 382 K (P2) are inactive and their TL intensities do not decrease clearly. This result indicates that the shallow traps corresponding to the P1 (340 K) peak should be mainly responsible for the visible LLP of Sr5(PO4)3Cl:Eu2 þ material. On the other hand, although the P1 peak has almost disappeared after 18,000 s (5 h), we still do not observe clear reduction of the TL peak at high temperature (P2 and P3) as shown as in Fig. 8c. This result suggests that some deep stable traps exist in Sr5(PO4)3Cl:Eu2þ material. Therefore, the Sr5(PO4)3Cl:Eu2þ material also shows the potential application as an optical storage material. Based on the above results as well as previous work, a simple LLP mechanism of Sr5(PO4)3Cl:Eu2þ material is depicted in Fig. 9. It is well known that the ground level (4f7) of Eu2þ is higher than the Fermi level but lower than the bottom of conduction band (CB), while the excited levels (4f65d1) are partly overlapped with the CB [24]. The LLP process in Sr5(PO4)3Cl:Eu2þ material consists of four main steps: (1) with the irradiation at 254 nm, the electrons are excited to higher levels (4f65d1) and some holes are left at the same time. (2) Because all traps in Sr5(PO4)3Cl:Eu2 þ material are independent, the excited electrons can be captured by both shallow (P1) and deep (P2) traps at the same time and thus the excitation energy is efficiently stored. (3) The electrons are subsequently released from traps in temperature order under the action of thermal activation at an appropriate temperature and then transferred to the emission centers, followed by the recombination and finally resulting in the LLP. Finally, it is important to say that the Sr5(PO4)3Cl:Eu2þ material still shows some shortcomings: Despite of the low synthesis temperature and good stability, the LLP time of Sr5(PO4)3Cl:Eu2þ material is still not long enough when compared with the present commercial LLP phosphors. Therefore, it is necessary to further improve the LLP performance of Sr5(PO4)3Cl:Eu2 þ material. Considering the rich distribution of deep traps in this material, we plan to adjust the depth of trap by proper co-doping method. Generally, the introduction of
Fig. 6. The TL glow curves (a) of Sr5(PO4)3Cl:x%Eu2 þ (x¼ 0.03, 0.05, 0.10, 0.15, 0.20) and the TL intensity of P1 (340 K) and P2 (382 K) peaks as a function of the Eu2 þ contents (b).
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Fig. 7. The filling experiments: the TL glow curves of Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample recorded immediately after UV irradiation (254 nm) for different times ((a) and (b)) and the TL intensity of P1 and P2 peaks as a function of the irradiation time (c).
Fig. 8. The fading experiments: the TL glow curves of Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample after UV irradiation for 10 min and then delaying for different time ((a) and (b)) and the TL intensity of P1 and P2 peaks as a function of the delaying time (c).
negatively charged defect can decrease the depth of electron traps (positively charged) in this case and thus the co-doping of some Re3 þ ions can create negative [VSr″] defects which may be useful for the improvement of the LLP of Sr5(PO4)3Cl:Eu2þ material. We hope to give readers good news in our following work.
4. Conclusions We first obtain a novel long lasting phosphorescence phosphor Sr5(PO4)3Cl:Eu2 þ by conventional solid state method. The blue
long lasting phosphorescence originates from the 5d–4f transition of Eu2 þ at six coordinated Sr(II) sites and can be properly recorded for 1040 s (0.32 mcd/m2). The excitation spectrum of Sr5(PO4)3Cl: Eu2 þ partly overlaps the solar irradiation. The decay curve can not be exponentially fitted even using a function of three exponential terms due to the efficient retrapping process. At least three types of independent traps corresponding to peaks at about 340 K, 382 K and 500 K exist in Sr5(PO4)3Cl:Eu2 þ material. It reveals that the shallow traps (340 K) are mainly responsible for the visible long lasting phosphorescence. The deep traps (382 K and 500 K) are proved to be able to hold the carriers more steadily. Therefore, the
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References
Fig. 9. The simple LLP mechanism of Sr5(PO4)3Cl:Eu2 þ material.
Sr5(PO4)3Cl:Eu2 þ may be used as not only a long lasting phosphorescence phosphor, but also an optical storage material.
Acknowledgements This work was supported by the National Nature Science Young Foundation of China (No. 10904057, 51202099), the Fundamental Research Funds for Central Universities (Nos. Lzjbky-2011125 and Lzjbky-2013-182), the Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education (No. LZUMMM2013007), the National Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 201310730111, 201310730104).
[1] Y.H. Jin, Y.H. Hu, L. Chen, X.J. Wang, G.F. Ju, Z.F. Mu, J. Lumin. 138 (2013) 83. [2] L.R. Shi, J.C. Zhang, H.H. Li, P.F. Feng, X. Liu, Z.Q. Fu, Y.H. Wang, J. Alloys Compd. 579 (2013) 82. [3] C.E. Cui, G.W. Jiang, P. Huang, L. Wang, D. Liu, J. Lumin. 145 (2014) 665. [4] J.C. Zhang, Q.S. Qin, M.H. Yu, M.J. Zhou, Y.H. Wang, J. Lumin. 132 (2012) 23. [5] J.C. Zhang, R. Hu, Q.S. Qin, D. Wang, B.T. Liu, Y. Wen, M.J. Zhou, Y.H. Wang, J. Lumin. 132 (2012) 2590. [6] F.W. Kang, Y.H. Hu, H.Y. Wu, Z.F. Mu, G.F. Ju, C.J. Fu, N.N. Li, J. Lumin. 132 (2013) 887. [7] D. Kulesza, A. Wiatrowska, J. Trojan-Piegza, T. Felbeck, R. Geduhn, P. Motzek, E. Zych, U. Kynast, J. Lumin. 133 (2013) 51. [8] X.H. Xu, Q.L. He, L.T. Yan, J. Alloys Compd. 574 (2013) 22. [9] P.F. Feng, J.C. Zhang, C.Q. Wu, X. Liu, Y.H. Wang, Mater. Chem. Phys. 141 (2013) 495. [10] J.A. Capobianco, G. Cormier, M. Bettinelli, R. Moncorgé, H. Manaa, J. Lumin. 54 (1993) 325. [11] S.K. Misra, L.E. Misiak, J.A. Capobianco, J. Phys. Condens. Matter 6 (1994) 3955. [12] L. Li, R.X. Yang, Z. Du, K.S. Zou, X.S. Zhang, Chin. Sci. Bull. 48 (2003) 1558. [13] X. Zhao, Z.S. Li, T. Yu, Z.G Zou, RSC Adv. 3 (2013) 1965. [14] C. Guo, L. Luan, X. Ding, F. Zhang, F.G. Shi, F. Guo, L. Liang, Appl. Phys. B 95 (2009) 779. [15] J.S. Zhang, B.J. Chen, J.S. Sun, X.P. Li, L.H. Cheng, H.Y. Zhong, J. Phys. D: Appl. Phys. 45 (2012) 325105. [16] V.R. Bandi, B.K. Grandhe, K.W. Jang, S.S. Kim, D.S. Shin, Y.I. Lee, J.M. Lim, T.K. Song, J. Lumin. 131 (2011) 2414. [17] G. Lakshminarayana, J. Qiu, M.G. Brik, I.V. Kityk, J. Phys. Condens. Matter 20 (2008) 335106. [18] Y. Gandhi, M.G. Brik, P.R. Rao, N. Veeraiah, J. Alloys Compd. 508 (2010) 278. [19] R.J. Yu, C.F. Guo, T. Li, Y. Xu, Curr. Appl. Phys. 13 (2013) 880. [20] B.F. Lei, H.R. Zhang, W.J. Mai, S. Yue, Y.L. Liu, S.Q. Man, Solid State Sci. 13 (2011) 525. [21] K.V.D. Eechhout, A.J.J. Bos, D. Poelman, P.F. Smet, Phys. Rev. B: Condens. Matter 87 (2013) 045126. [22] F. Clabau, X. Rocquefelte, T.L. Mercier, P. Deniard, S. Jobic, M.H. Whangbo, Chem. Mater. 18 (2006) 3212. [23] J.C. Zhang, M.H. Yu, Q.S. Qin, H.L. Zhou, M.J. Zhou, X.H. Xu, Y.H. Wang, J. Appl. Phys. 108 (2010) 123518. [24] P. Dorenbos, Chem. Mater. 17 (2005) 6452.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Blue photoluminescence and long lasting phosphorescence properties of a novel chloride phosphate phosphor: Sr5(PO4)3Cl:Eu2 þ Chuanqiang Wu, Jiachi Zhang n, Pengfei Feng, Yiming Duan, Zhiya Zhang n, Yuhua Wang Key Laboratory for Magnetism Magnetic Materials of the Ministry of Education, Tianshui Street 222#, Lanzhou University, Lanzhou 730000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 September 2013 Received in revised form 6 November 2013 Accepted 13 November 2013 Available online 22 November 2013
A novel blue emitting long lasting phosphorescence phosphor Sr5(PO4)3Cl:Eu2 þ is synthesized by solid state method at 1223 K in reducing atmosphere. The afterglow emission spectrum shows one broad band centered at 441 nm due to the 5d–4f transition of Eu2 þ at six coordinated Sr(II) sites and the color coordinates are calculated to be (0.149, 0.095) which is close to the light blue region. The excitation band is in 240–430 nm and partly overlaps the solar irradiation on Earth's surface. The long lasting phosphorescence of the optimal sample doping by 0.1 mol%Eu2 þ can be recorded for about 1040 s (0.32 mcd/m2). Thermoluminescence shows that there are at least three types of traps corresponding to peaks at 340 K, 382 K, 500 K, respectively. The filling and fading experiments reveal that the traps in Sr5(PO4)3Cl:Eu2 þ are independent. The shallow traps (340 K) essentially contribute to the visible long lasting phosphorescence, while the deep traps (382 K and 500 K) are proved to be very stable. Thus, the Sr5(PO4)3Cl:Eu2 þ material shows potential applications as not only a long lasting phosphorescence phosphor, but also an optical storage material. & 2013 Elsevier B.V. All rights reserved.
Keywords: Luminescence Optical materials Optical properties
1. Introduction Long lasting phosphorescence (LLP) is an interesting optical phenomenon where, a material excited by high energy radiation produces visible luminescent emission for an appreciable time at room temperature even after the irradiation light sources have been removed [1–3]. In recent years, the LLP phosphors have attracted wide attention for possible applications in many fields, such as emergency lighting, safety indication, billboards, interior decoration [4–8]. The present commercial LLP phosphors are SrAl2O4:Eu2 þ , Dy3 þ (green), CaAl2O4:Eu2 þ , Nd3 þ (blue) and Y2O2S:Eu3 þ , Ti3 þ , Mg2 þ (red), respectively [9]. Nowadays, these commercial LLP phosphors still have some shortcomings such as the low moisture resistance of aluminates and the poor stability of oxysulfide and the LLP mechanism is still not clear. For fundamental reasons, it is necessary to explore the possibilities of new systems that can be used as LLP phosphors. Alkaline earth chloride phosphates have been studied for years and it is generally considered as an ideal host with low synthesis temperature and excellent stability [10–12]. On the other hand, the luminescence of Eu2 þ activated phosphors usually results from the allowed electric dipole transition (4f7 to 4f65d1) and thus the efficient emission of Eu2 þ can be expected in many hosts [13].
n
Corresponding authors. Tel.: þ 86 931 8912772; fax: þ 86 931 8913554. E-mail addresses: [email protected] (J. Zhang), [email protected] (Z. Zhang). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.055
In 1992, Capobianco et al. first reported the near-infrared luminescence spectroscopy of the Mn5 þ ion in Sr5(PO4)3Cl and then Misra et al. subsequently studied the Mn5 þ -doped Sr5(PO4)3Cl as a solidstate-laser material [10,11]. Sr5(PO4)3Cl:Eu2 þ , Mn2 þ was investigated as a near-ultraviolet excited LEDs phosphor by Guo et al. in 2003 [14]. However, to the best of our knowledge, the visible long lasting phosphorescence of Sr5(PO4)3Cl:Eu2 þ has been never reported. In this work, the blue LLP is observed in the Sr5(PO4)3Cl:Eu2 þ material synthesized by solid state reaction at 1223 K in reducing atmosphere. The PL and LLP properties of Sr5(PO4)3Cl:Eu2 þ material are investigated in detail. The filling and fading experiments are carried out for revealing the traps in Sr5(PO4)3Cl:Eu2 þ material. According to the results, the long lasting phosphorescence mechanism for the independent traps of Sr5(PO4)3Cl:Eu2 þ is proposed. 2. Experimental The Sr5(PO4)3Cl:Eu2 þ samples were prepared by a solid state reaction with SrCO3 (A.R.), SrCl2 6H2O (A.R.), (NH4)2HPO4 (A.R.) and Eu2O3 (99.99%). After the raw materials were prepared in stoichiometric ratios and thoroughly homogenized (all grinding was performed using an agate pestle and mortar), the mixture was transferred into an alumina crucible and then loaded into a tube furnace. The samples were sintered at 1223 K for 3 h in a reducing atmosphere of H2 (5%) and N2 (95%) to complete the reaction.
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After firing, the samples were cooled to room temperature within the furnace and then ground again using an agate mortar. The sample phases were identified by powder diffraction (XRD) with Ni-filtered CuK a radiation at scanning step of 0.021. Photoluminescence (PL) spectra were recorded using a FLS-920T spectrophotometer (Edinburgh Instruments Ltd, Edinburgh, U.K.) with a 450 W xenon arc lamp (Xe900) as the light source. LLP decay curve measurements were taken with a PR305 LLP instrument (Zhejiang University Sensing Instrument Co. Ltd., Hangzhou, China). The thermoluminescence (TL) curves were measured using a FJ-417A TL meter (Beijing Nuclear Instrument Factory, Beijing, China). All samples were first exposed to radiation using a UV lamp (254 nm) for 10 min and then heated from room temperature to 673 K at a rate of 1 K/s. All measurements were carried out at room temperature except for TL measurements.
3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of a typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample obtained in reducing atmosphere and the corresponding standard PDF card is also given for comparison. As shown in Fig. 1, all peaks of the obtained Sr5(PO4)3Cl:Eu2 þ sample can be well indexed by the standard PDF card (PDF#701007) of Sr5(PO4)3Cl. No extra peak is observed, indicating that the obtained sample is a single phase and the doping of Eu2 þ ion does not induce the second phase. Accordingly, the obtained Sr5(PO4)3Cl:Eu2 þ sample belongs to a hexagonal system with the former of P63/m space-group symmetry. The crystal structure and spheres for Sr2 þ ions in the Sr5(PO4)3Cl crystal can be depicted by “Diamond software” in Fig. 2 [15]. It shows that there are two inequivalent cationic sites in Sr5(PO4)3Cl, Sr(I) seven coordinated with five oxygen atoms and two chlorine atoms and Sr(II) six coordinated with six oxygen atoms [10–14]. Considering that the ionic radius of Eu2 þ given by 0.112 nm is almost equal to that of Sr2 þ ions (0.114 nm), Eu2 þ ions will replace the two types of Sr2 þ sites statistically in the unit cell. Fig. 3 exhibits the PL excitation spectrum ((a) λem ¼467 nm) and PL emission spectrum ((b) λex ¼254 nm) of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample. Upon excitation at 254 nm, it is found that the emission spectrum of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample consists of an asymmetric broad band in region 400–550 nm and it can be well fitted into two components at 426 nm and 467 nm, respectively. It is well known that the emission of Eu2 þ is strongly dependent on the crystal field strength of the host matrix and thus it can vary from ultraviolet
Fig. 1. The X-ray diffraction pattern of a typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample.
Fig. 2. The crystal structure and spheres for Sr2 þ ions in the Sr5(PO4)3Cl crystal.
to red region [16]. Considering the different surrounding coordination of the two Sr2 þ (Eu2 þ ) sites, the emission components at 426 nm and 467 nm can be ascribed to the electric-dipole-allowed 4f65d1 to 4f7 transition of the Eu2 þ ions at Sr(I) and Sr(II), respectively [17,18]. Monitored by the emission at 467 nm, the excitation spectrum exhibits an unresolved broad band in the range from 240 nm to 430 nm, which can be assigned to the host absorption and the transitions from the ground state 4f7 to the crystal-field split components of 4f65d1 configuration [19]. It is well known that the solar spectrum received on Earth's surface is in 300–1200 nm. This result shows that the excitation spectrum of Sr5(PO4)3Cl:Eu2 þ material partly overlaps with the near-ultraviolet (NUV) portion of the solar radiation and thus the LLP can be activated by the NUV part of the sunlight. However, we have to say that the LLP of Sr5(PO4)3Cl:Eu2 þ material is difficulty to be activated in visible light of sunlight and thus the further improvement is still necessary. Fig. 4a shows the LLP emission spectrum of a typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample and the PL spectrum is also given for comparison. Evidently, only one emission band at 441 nm is observed and it can be also ascribed to the 5d–4f transition of Eu2 þ . This result indicates that only the six coordinated Eu2 þ centers at Sr (II) sites contribute to the LLP of Sr5(PO4)3Cl:Eu2 þ . The CIE color coordinates of PL (marked with ■) and LLP (marked with □) can be calculated to be (0.154, 0.029), (0.149, 0.095), respectively, as shown as in Fig. 4b. Since the LLP intensity keeps decreasing during the measurement of LLP spectrum, the LLP band should shift to short wavelength region and thus the color coordinates of LLP should be more close to the light blue region. Fig. 5 presents the LLP decay curves of Sr5(PO4)3Cl:xmol%Eu2 þ (x ¼0.03, 0.05, 0.10, 0.15, 0.20) recorded immediately after removing the UV irradiation source (254 nm). It is found that the LLP of the optimal Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample could be recorded as long as 1040 s (0.32 mcd/m2). Since the eye sensitivity is about 100 times better than this value (0.32 mcd/m2), we can actually observe a much longer LLP by using the “dark-adapted eye” in complete darkness. Naturally, it would be of interest to find out the LLP processes or models by fitting LLP decay curves. Previously, many researches fit the LLP decay curves by using function of exponential terms in linear-linear plot to obtain the fitting results such as I0, A and τ [20]. As a result, the decay curve is often divided into multiple exponential components, generally called “fast”, “medium” or “slow”. However, it should be given on a logarithmic scale and this is the only way to see the different components in fact [21]. For the optimal Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample, it turns out that the fitting of the decay within the almost all time
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Fig. 3. The PL excitation spectrum ((a) λem ¼467 nm) and PL emission spectrum ((b) λex ¼ 254 nm) of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample.
Fig. 4. The LLP and PL emission spectrum (a) and CIE chromaticity coordinates (b) of the typical Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample.
range of the measurement is also not possible even by using a function of three exponential terms The failure of decay fitting suggests that retrapping process which competes with recombination for LLP is essentially not negligent in this case. In other words, the carrier (electron or hole) that escapes a trap may be captured by another trap and thus it is difficult to exponentially fit the decay behavior. In addition, the retrapping process will inevitably decrease the LLP intensity and slow the LLP decay speed. In general, the thermoluminescence (TL) technique is a very useful tool for revealing the nature of defects produced in insulators or semiconductors by UV light. Fig. 6a presents the TL glow curves of Sr5(PO4)3Cl:x%Eu2 þ (x ¼0.03, 0.05, 0.10, 0.15, 0.20). It can be seen that the Sr5(PO4)3Cl:Eu2 þ material possesses the significantly more complex TL glow curve and it consists of two
intense peaks located at about 340 K, 382 K and one small band at around 500 K. This result reveals that there are at least three types of traps with different depths in Sr5(PO4)3Cl:Eu2 þ material. According to Clabau et al., Eu2 þ can exist in oxidized state Eu3 þ so that the electron promoted to a 5d orbital under excitation may potentially be trapped by anion vacancies [22]. Therefore, the traps involved in LLP of Sr5(PO4)3Cl:Eu2 þ material may be due to oxygen vacancies or clusters which are created during the synthesis process at high temperature [22,23]. Fig. 6b shows the TL intensity of P1 (340 K) and P2 (382 K) peaks as a function of the Eu2 þ contents. It is found that the integral TL intensity increases gradually and then decreases when the Eu2 þ content is more than 0.1 mol% due to the concentration quenching effect [5]. Additionally, the TL profiles are not clearly changed by increasing
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Eu2 þ doping content and it suggests that no new trap has been induced by doping Eu2 þ . Accordingly, the irradiation time has a significant effect on the filling of traps and this effect is useful to investigate the trap in materials [23]. Fig. 7a and b show the TL glow curves of the optimal Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample recorded immediately after UV irradiation (254 nm) for different times. It can be seen that, the integral TL intensity of both P1 and P2 peaks increases at the same time by increasing the UV irradiation time. The TL intensity of P1 and P2 peaks as a function of the irradiation time is exhibited in Fig. 7c. The TL intensity at 382 K (P2) due to deep traps is always higher than that at 340 K (P1) due to shallow traps. This result indicates that the filling sequence of the traps in Sr5(PO4)3Cl:Eu2 þ material is not related to the depth of traps and thus it can be concluded that the three types of traps in Sr5(PO4)3Cl:Eu2 þ material are independent in fact. Therefore, the shallow and deep traps can intercept the free carriers at the same time as shown as in Fig. 7c.
Fig. 5. The LLP decay curves of Sr5(PO4)3Cl:xmol%Eu2 þ (x ¼0.03, 0.05, 0.10, 0.15, 0.20) recorded immediately after removing the UV irradiation source (254 nm) and the LLP time as a function of the Eu2 þ contents (inset).
The fading of thermoluminescence is due partly to the bleaching of the trapped carriers by thermal energy and partly by other processes. The measurement of the TL glow curves with different delay time after ceasing the UV irradiation may also give important information on the trap structure in materials [23]. As shown in Fig. 8a and b, the TL intensity of P1 (340 K) due to the shallow traps decreases very quickly and keeps only 77% of its initial intensity after 600 s. At the same time, it is found that the deep traps at 382 K (P2) are inactive and their TL intensities do not decrease clearly. This result indicates that the shallow traps corresponding to the P1 (340 K) peak should be mainly responsible for the visible LLP of Sr5(PO4)3Cl:Eu2 þ material. On the other hand, although the P1 peak has almost disappeared after 18,000 s (5 h), we still do not observe clear reduction of the TL peak at high temperature (P2 and P3) as shown as in Fig. 8c. This result suggests that some deep stable traps exist in Sr5(PO4)3Cl:Eu2þ material. Therefore, the Sr5(PO4)3Cl:Eu2þ material also shows the potential application as an optical storage material. Based on the above results as well as previous work, a simple LLP mechanism of Sr5(PO4)3Cl:Eu2þ material is depicted in Fig. 9. It is well known that the ground level (4f7) of Eu2þ is higher than the Fermi level but lower than the bottom of conduction band (CB), while the excited levels (4f65d1) are partly overlapped with the CB [24]. The LLP process in Sr5(PO4)3Cl:Eu2þ material consists of four main steps: (1) with the irradiation at 254 nm, the electrons are excited to higher levels (4f65d1) and some holes are left at the same time. (2) Because all traps in Sr5(PO4)3Cl:Eu2 þ material are independent, the excited electrons can be captured by both shallow (P1) and deep (P2) traps at the same time and thus the excitation energy is efficiently stored. (3) The electrons are subsequently released from traps in temperature order under the action of thermal activation at an appropriate temperature and then transferred to the emission centers, followed by the recombination and finally resulting in the LLP. Finally, it is important to say that the Sr5(PO4)3Cl:Eu2þ material still shows some shortcomings: Despite of the low synthesis temperature and good stability, the LLP time of Sr5(PO4)3Cl:Eu2þ material is still not long enough when compared with the present commercial LLP phosphors. Therefore, it is necessary to further improve the LLP performance of Sr5(PO4)3Cl:Eu2 þ material. Considering the rich distribution of deep traps in this material, we plan to adjust the depth of trap by proper co-doping method. Generally, the introduction of
Fig. 6. The TL glow curves (a) of Sr5(PO4)3Cl:x%Eu2 þ (x¼ 0.03, 0.05, 0.10, 0.15, 0.20) and the TL intensity of P1 (340 K) and P2 (382 K) peaks as a function of the Eu2 þ contents (b).
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Fig. 7. The filling experiments: the TL glow curves of Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample recorded immediately after UV irradiation (254 nm) for different times ((a) and (b)) and the TL intensity of P1 and P2 peaks as a function of the irradiation time (c).
Fig. 8. The fading experiments: the TL glow curves of Sr5(PO4)3Cl:0.1 mol%Eu2 þ sample after UV irradiation for 10 min and then delaying for different time ((a) and (b)) and the TL intensity of P1 and P2 peaks as a function of the delaying time (c).
negatively charged defect can decrease the depth of electron traps (positively charged) in this case and thus the co-doping of some Re3 þ ions can create negative [VSr″] defects which may be useful for the improvement of the LLP of Sr5(PO4)3Cl:Eu2þ material. We hope to give readers good news in our following work.
4. Conclusions We first obtain a novel long lasting phosphorescence phosphor Sr5(PO4)3Cl:Eu2 þ by conventional solid state method. The blue
long lasting phosphorescence originates from the 5d–4f transition of Eu2 þ at six coordinated Sr(II) sites and can be properly recorded for 1040 s (0.32 mcd/m2). The excitation spectrum of Sr5(PO4)3Cl: Eu2 þ partly overlaps the solar irradiation. The decay curve can not be exponentially fitted even using a function of three exponential terms due to the efficient retrapping process. At least three types of independent traps corresponding to peaks at about 340 K, 382 K and 500 K exist in Sr5(PO4)3Cl:Eu2 þ material. It reveals that the shallow traps (340 K) are mainly responsible for the visible long lasting phosphorescence. The deep traps (382 K and 500 K) are proved to be able to hold the carriers more steadily. Therefore, the
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References
Fig. 9. The simple LLP mechanism of Sr5(PO4)3Cl:Eu2 þ material.
Sr5(PO4)3Cl:Eu2 þ may be used as not only a long lasting phosphorescence phosphor, but also an optical storage material.
Acknowledgements This work was supported by the National Nature Science Young Foundation of China (No. 10904057, 51202099), the Fundamental Research Funds for Central Universities (Nos. Lzjbky-2011125 and Lzjbky-2013-182), the Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education (No. LZUMMM2013007), the National Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 201310730111, 201310730104).
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