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Luminescent properties of a new blue long-lasting phosphor Ca2P2O7:Eu2+, Y3+
Materials Chemistry and Physics 113 (2009) 215–218
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Luminescent properties of a new blue long-lasting phosphor Ca2 P2 O7 :Eu2+ , Y3+ Ran Pang a,b , Chengyu Li a,∗ , Su Zhang a,b , Qiang Su a,c a
State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangdong 510275, China b
a r t i c l e
i n f o
Article history: Received 9 May 2008 Received in revised form 4 July 2008 Accepted 8 July 2008 Keywords: Optical materials Defects Luminescence
a b s t r a c t A new pyrophosphate long-lasting phosphor with composition of Ca1.96 P2 O7 :0.02Eu2+ , 0.02Y3+ is synthesized via the high-temperature solid-state reaction method. Its properties are systematically investigated utilizing XRD, photoluminescence, phosphorescence and thermoluminescence (TL) spectra. The phosphor emits blue light that is related to the characteristic emission of Eu2+ due to 5d–4f transitions. For the optimized sample, bright blue long-lasting phosphorescence (LLP) could be observed by naked eyes even 6 h after the excitation source is removed. The TL spectra show that the doping of Y3+ ions greatly enhanced intensity of 335 K peak and created new TL peak at about 373 K that is also responsible for the blue LLP. Based on our study, Y3+ ions are suggested to act as electron traps to improve the performance of the blue phosphorescence of Eu2+ such as intensity and persistent time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Long-lasting phosphorescence (LLP) phosphors are a special kind of energy-storing material, which could store the energy and then release the energy in the form of persistent visible light usually in room temperature. Such phosphors in the form of polycrystalline powder, crystal and glasses etc. have attracted increasing attention for their great practical and potential applications in many fields, e.g., lighting, display, high-energy ray detection, multidimensional optical memory and imaging storage [1–6]. Up to date, most of the LLP materials are based on the Eu2+ activated alkaline earth aluminates and silicates represented by SrAl2 O4 :Eu2+ , Dy3+ and Sr2 MgSi2 O7 :Eu2+ , Dy3+ , respectively. Among those phosphors, the phosphorescence is ascribed to the parity-allowed electronic transition of 4f6 5d1 –8 S7/2 of Eu2+ ion, which is strongly influenced by the host lattice due to crystal-filed effect. In general, other ions, i.e., trivalent rare earth ions, are incorporated in host as auxiliary activators to improve the phosphorescence intensity and persistence time. Those phosphors have been proved to be excellent LLP phosphors with higher brightness, longer persistence time and better chemical and physical stabilities compared with previous sulfide phosphors and used as commercial green and blue LLP phosphors [1,2,6–10]. However, researchers never lost their interests in exploring new LLP phosphors to extend the family of this kind of light-storing materials. In recent years, many novel LLP phosphors
∗ Corresponding author. Tel.: +86 431 85262208; fax: +86 431 85262005. E-mail addresses: [email protected] (C. Li), [email protected] (Q. Su). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.061
based on different hosts have been reported and their mechanisms were discussed [11–16]. But the step of developing new LLP materials with excellent performance is still slow. Actually, LLP is so special a phenomenon due to the thermal stimulated recombination of holes and electrons in traps at room temperature that only the suitable host doped with the right activator could yield considerable phosphorescence. Eu2+ activated the calcium pyrophosphate is an efficient blue phosphor which have extensive applications in the fields of luminescence and biomaterials due to its excellent optical and biological characteristics [17]. Recently, Eu2+ , Mn2+ co-doped Ca2 P2 O7 has been reported to be one of the potential yellow phosphors used in the phosphor mixture for obtaining white light emission from UV devices [18]. Although the optical properties of phosphor Ca2 P2 O7 :Eu2+ was reported long ago by Blasse et al. [19], there are no reports on the LLP phenomenon of this phosphor. In the present article, we developed an intense blue LLP of Eu2+ in Ca2 P2 O7 for the first time by co-doping with Y3+ . The luminescence and defect properties of Ca2 P2 O7 :Eu2+ , Y3+ were systematically investigated by means of photoluminescence spectra, LLP emission and decay curves and thermoluminescence (TL) spectra. Also, the LLP mechanism of this phosphor was discussed in this article. 2. Experimental The phosphor with nominal composition of Ca1.98 P2 O7 :0.02Eu2+ (labeled as CPOE) and Ca1.96 P2 O7 :0.02Eu2+ , 0.02Y3+ (labeled as CPOEY) were synthesized by traditional high-temperature solid-state reaction. CaHPO4 (analytical grade) NH4 H2 PO4 (analytical grade) Eu2 O3 (99.99%) and Y2 O3 (99.99%) were
216
R. Pang et al. / Materials Chemistry and Physics 113 (2009) 215–218
Fig. 1. XRD patterns of CPOE and CPOEY.
explored as staring materials. Stoichiometric mixtures of raw materials were homogeneously ground and then fired in alumina crucible at 600 ◦ C for 2 h. After regrinding, they were sintered at 1250 ◦ C for 3 h in a thermal-carbon reducing atmosphere. After cooling to room temperature naturally, the asobtained sample was ground into powder in an agate mortar for the next measurements. The X-ray power diffraction pattern was measured with a Rigaku-Dmax 2500 X-ray Diffractometer, using Cu K␣ ( = 0.15405 nm) radiation; Photoluminescence (PL) excitation and emission spectra were performed in a Hitachi F-4500 Fluorescence Spectrofluorometer at room temperature equipped with a 150 W xenon lamp as excitation source. The LLP emission spectrum and intensity decay were also measured using the same instrument after the sample was irradiated under 254 nm UV light for 1 min. TL spectra were measured with a 3D-TSL spectra instrument. The sample was place inside a homemade cryostat and heated form 293 to 573 K at the speed of 2 K s−1 . All the measurements expect the TL spectra were performed at room temperature.
3. Results and discussion The X-ray diffraction patterns of the sample CPOE and CPOEY are given in Fig. 1. They are found to be in good agreement with that of ␣-Ca2 P2 O7 registered in JCPDS file 09-0345, indicating that both the CPOE and CPOEY samples are single phase and the co-doping of Eu2+ and Y3+ does not cause any significant change in the host structure. Fig. 2 depicts the excitation and emission spectra of CPOE and CPOEY samples. It is clearly observed that the CPOEY sample shows the same luminescent properties as that of CPOE except
Fig. 2. Photoluminescence excitation and emission spectra of CPOE and CPOEY.
Fig. 3. LLP spectra of CPOE and CPOEY measured 1 min after the excitation source was switch off. Inset: CIE1931 chromaticity diagram.
the luminescence intensity. Monitored at 416 nm, an intense broad band ranging from 200 to 410 nm with two major peaks at 268 and 335 nm can be observed in the excitation spectrum, which is attributed to the 4f–5d parity-allowed transition of Eu2+ . This board excitation band indicates that those phosphors can be well excited by near-UV and visible light. Both the 268 and 335 nm excitation cause the similar emission of Eu2+ in peak shape and position, but the intensity of emission under the 335 nm excitation is much stronger. With excitation wavelength of 335 nm, the emission spectrum exhibits a strong blue band centered at 416 nm, which can be ascribed to the transitions from the lowest component of 5d excitation states to the ground state (8 S7/2 ) of Eu2+ . Moreover, from careful comparison of excitation and emission spectra between CPOE and CPOEY, it is observed that the introduction of Y3+ decreases the emission intensity of Eu2+ in alpha calcium pyrophosphate. Since the Y3+ does not show luminescence and hence will not compete against Eu2+ ion for the excitation energy, the decrease in emission intensity of Eu2+ can be ascribed to the storing of part of excitation energy in the defects induced by the Y3+ doping. It reveals that the Y3+ co-doping plays a role of creating electron traps in CPOEY. The following results will support this assignment. What is interesting of our present work is that we observe bright blue LLP of Eu2+ in CPOEY. Fig. 3 represents the LLP spectrum of CPOEY. As shown in Fig. 3, the LLP spectrum of CPOEY is close in peak shape and location to the steady-state emission spectra presented in Fig. 2, indicating that the persistent phosphorescence arises from Eu2+ ions doped in alpha calcium pyrophosphate. Generally, the color of the phosphorescence is represented by color coordinates which could be calculated from phosphorescence spectrum using chromaticity coordinate calculation method based on the CIE1931 (Commission International de I’Eclairage France) system. From the LLP spectrum of CPOEY, we calculated its phosphorescence chromaticity coordinates, which are (0.168, 0.010). From CIE1931 Chromaticity Diagram in the inset of Fig. 3 one can see that the color of afterglow located at blue region. The phosphorescence intensity decay of CPOEY monitored at wavelength of 416 nm were recorded in a few minutes after the stopping of the 254 nm UV light excitation and shown in Fig. 4. The LLP decay curve consists of a rapid decay component and a very long slow one. The
R. Pang et al. / Materials Chemistry and Physics 113 (2009) 215–218
217
Fig. 5. 3DTL emission spectrum of CPOEY measured form room temperature to 573 K with a heating rate of 2 K s−1 . Fig. 4. LLP Intensity decay of CPOE and CPOEY monitored at the wavelength of 416 nm.
corresponding phosphorescence decay times can be calculated by a curve fitting technique based on the following equation: I = A1 exp
−t 1
+ A2 exp
−t 2
(1)
where I is the phosphorescence intensity; A1 and A2 are constants; t is the time, and 1 and 2 are decay time for exponential components, respectively. The curve fits well with second-order exponential decay and the values of 1 and 2 are analyzed to be 21.8 and 151.8 s. It means that the phosphor could show long persistent phosphorescence. Actually, the blue phosphorescence of this phosphor could be observed by naked eyes even 6 h after excitation source was switched off. In view of the short luminescence decay life of 5d–4f transition of Eu2+ [8], the LLP observed in our phosphor could be attributed to energy exchange processes between traps or traps and emission centers resulting from Eu2+ and Y3+ doping. Moreover, the Eu2+ single doped sample CPOE also shows blue LLP, its LLP emission and decay curves are also demonstrated in Figs. 3 and 4, respectively. The decay curve of CPOE is similar to that of CPOEY, but the LLP initial intensity is much weaker and the blue phosphorescence can only be visible for several minutes in the limit of light perception for naked eyes (0.32 mCd m−2 ). As seen in Figs. 3 and 4, the co-doping of Y3+ ions largely improved the LLP performance of Eu2+ such as brightness and duration. Those results confirm our previous assignment that the Y3+ ions act as electron traps. This assignment explains the decrease in the emission intensity under steady excitation in Fig. 2 and simultaneously accounts for the increase of the LLP emission intensity of Eu2+ in CPOEY in Figs. 3 and 4. TL spectrum is a useful tool to investigate the properties of traps that are responsible for the appearance of LLP. Here, we measured the three-dimension thermoluminescence (3DTL) emission spectrum of CPOEY sample with a 3D-TSL spectra instrument and show it in Fig. 5, from which the origin of phosphorescence emission at different temperature can be obtained. As shown in Fig. 5, all the TL emissions at different temperatures are close in shape and position to the PL emission discussed in Fig. 2, indicating that the TL emissions derive from the 5d–4f transitions of Eu2+ ions. Fig. 6 portrays the TL curves of CPOE and CPOEY. For CPOE, it is clearly observed that there is only a very weak TL peak lying at about 335 K, which might due to intrinsic defects in the CPOE [20]. When Y3+ are doped, the intensity of this peak is greatly enhanced,
Fig. 6. TL curves of CPOE and CPOEY measured form room temperature to 573 K with a heating rate of 2 K s−1 .
and the peak position does not change, which means that the Y3+ doping largely improved the intrinsic defect levels. In addition, a shoulder TL peak located at about 373 K appears in the TL of CPOEY. This new peak, we suppose, is induced by Y3+ doping. The depth of traps in CPOEY can be calculated by processing the TL curve using the general order equation as follows [21,22]:
E
I (T ) = sn0 exp −
× 1+
kT
(l − 1)s ˇ
T
×
T0
E exp − kT
−l/(l−1)
dT
(2)
where E is the activation energy which indicates the trap depth, n0 is the concentration of trapped charges at time t = 0, which greatly affects the luminescent intensity, k is the Botlzmann constant, s is the frequency factor, l is the order of kinetics and ˇ is the heating rate. Here, ˇ = 2 K s−1 . Table 1 gives the calculated results of Table 1 The parameters of the TL curve for CPOEY Peak
E (eV)
s (s−1 )
Tm (K)
n0 (cm−3 )
l
1 2
0.67 0.70
3.82 × 109 5.84 × 108
335 373
2.20 × 105 5.27 × 104
1.96 2.0
218
R. Pang et al. / Materials Chemistry and Physics 113 (2009) 215–218
trap depths and parameters of the TL curve of CPOEY. The activation energy E is 0.67 eV for 335 K peak and 0.70 eV for 373 K peak, respectively, and the trap intensity (n0 ) related to 335 K peak is much stronger than that related to 373 K peak. A suitable depth of the trap is essential necessary for LLP, if the depth of the trapping levels is too low, the phosphor would show a fast decay, which does not last for a long time. On the contrary, if it is too deep the phosphor will no longer show phosphorescence at room temperature. According to our previous research [23], the depth of traps corresponding to TL peaks ranging from 293 to 383 K is suitable for the appearance of LLP. So, both the two TL peaks are responsible for LLP observed in our present work. Based on above results, it is evident that the Y3+ ions play an important role on improving the LLP performance. They not only intensively enhanced the trap intensity of the intrinsic defects but also created new deeper traps that are responsible for LLP, which accounts for the appearance of the bright LLP of CPOEY. On the mechanism of LLP, it is a common viewpoint that during the excitation period the energy of the incident light is partly stored in the form of captured charges. Those captured charges can be thermally released from the traps at room temperature and transferred to doped ions, which results in the characteristic emission of the luminescent ions. Because the release of the captured electrons and traps is durative, the luminescences of the materials show the property of long life. The doped ions substitute the host ions with similar radii yielding defects nearby. These defects serve as hole or electron traps capturing electrons or holes in excitation process and transferring durative energy to the luminescent ions. In the case of CPOEY, the Eu2+ and Y3+ ions are expected to occupy the sites of Ca2+ according to the fact that the radii of Eu2+ (0.130 nm) and Y3+ (0.108 nm) are close to that of Ca2+ (0.118 nm) [24]. Defects were generated by nonequivalent substitution, i.e., two Y3+ ions substituted three Ca2+ ions and created positive defects and a V negative defect, which act as two YCa Ca electron and hole traps. In the process of excitation electrons were pumped to the 5d orbits of Eu2+ ions, part of which were captured and stored in the form of energy. Under by positive defects YCa the thermal activation, these trapped electrons escaped from the traps and transferred to Eu2+ ions followed by the characteristic emission of Eu2+ ions. All discussed above is a general description for the occurrence of LLP of Eu2+ and Y3+ co-activated calcium pyrophosphate. However, the detail of the mechanism is still unclear.
4. Conclusions Eu2+ and Y3+ co-doped Ca2 P2 O7 was synthesized via traditional high-temperature solid-state reaction method. The phosphor emitted blue light that was ascribed to the characteristic d–f emission of Eu2+ . After the excitation source was switched off, bright blue LLP could be observed. Two TL peaks at 335 and 373 K related to two types of defects appeared in TL curve, which were responsible for the LLP in room temperature. The co-doping of Y3+ was proved to improve the performance of the blue phosphorescence of Eu2+ such as intensity and persistent time. Acknowledgement This work was financially supported by National Basic Research Program of China (2007CB935502). References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electonchem. Soc. 143 (1996) 2670. [2] T. Zhang, Q. Su, SID J. 8 (2000) 27. [3] J.R. Qiu, K. Hirao, Solid State Commun. 106 (1998) 795. [4] J.R. Qiu, K. Miura, H. Inpuye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 73 (1998) 1763. [5] C.Y. Li, Q. Su, S.B. Wang, Mater. Res. Bull. 37 (2002) 1443. [6] T.Y. Peng, H.J. Liu, H.P. Yang, C.H. Yan, Mater. Chem. Phys. 85 (2004) 68. [7] Z. Xiao et al., United States Patent U.S. 6,093,346 (2000). [8] N. Kodama, N. Sasaki, M. Yamaga, Y. Masui, J. Lumin. 94/95 (2001) 19. [9] Z.W. Liu, Y.L. Liu, Mater. Chem. Phys. 93 (2005) 129. [10] F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M. Whangbo, A. Garcia, T. Mercier, Chem. Mater. 17 (2005) 3904. [11] J. Wang, S.B. Wang, Q. Su, J. Solid State Chem. 177 (2004) 895. [12] L.Y. Liu, C.Y. Li, S.B. Wang, Q. Su, Appl. Phys. Lett. 88 (2006) 241107. [13] N. Kodama, M. Yamaga, Y. Tanii, J. Qiu, K. Hirao, Appl. Phys. Lett. 75 (1999) 1715. [14] C. Guo, Q. Tang, D. Huang, C. Zhang, Q. Su, J. Phys. Chem. Solid. 68 (2007) 217. [15] Y. Cong, B. Li, B. Lei, W. Li, J. Lumin. 126 (2007) 822. [16] B.F. Lei, B. Li, X.J. Wang, W.L. Li, J. Lumin. 118 (2006) 173. [17] A. Doat, F. Pelle, A. Lebugle, J. Solid State Chem. 178 (2005) 2354. [18] Z.D. Hao, J.H. Zhang, X. Zhang, X.Y. Sun, Y.S. Luo, S.Z. Lu, X.J. Wang, Appl. Phys. Lett. 90 (2007) 261113. [19] G. Blasse, W.L. Wanmaker, J.W. Vrugt, J. Electronchem. Soc. 115 (1968) 673. [20] J. Holsa, T. Aitasalo, H. Jungner, M. Lastusaari, J. Niittykoski, G. Spano, J. Alloys Compd. 374 (2004) 56. [21] S.W.S. McKeever, M. Prokic, P.D. Townsend, Thermoluminescence Dosimetery Materials: Properties and Uses, Nuclear Technology Press, Ashford, UK, 1995, p 182. [22] R. Chen, J. Appl. Phys. 40 (1969) 570. [23] C.Y. Li, Q. Su, J.R. Qiu, Chin. J. Lumin. 24 (2003) 19. [24] R.D. Shannon, Acta Cryst. A32 (1976) 751.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Luminescent properties of a new blue long-lasting phosphor Ca2 P2 O7 :Eu2+ , Y3+ Ran Pang a,b , Chengyu Li a,∗ , Su Zhang a,b , Qiang Su a,c a
State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangdong 510275, China b
a r t i c l e
i n f o
Article history: Received 9 May 2008 Received in revised form 4 July 2008 Accepted 8 July 2008 Keywords: Optical materials Defects Luminescence
a b s t r a c t A new pyrophosphate long-lasting phosphor with composition of Ca1.96 P2 O7 :0.02Eu2+ , 0.02Y3+ is synthesized via the high-temperature solid-state reaction method. Its properties are systematically investigated utilizing XRD, photoluminescence, phosphorescence and thermoluminescence (TL) spectra. The phosphor emits blue light that is related to the characteristic emission of Eu2+ due to 5d–4f transitions. For the optimized sample, bright blue long-lasting phosphorescence (LLP) could be observed by naked eyes even 6 h after the excitation source is removed. The TL spectra show that the doping of Y3+ ions greatly enhanced intensity of 335 K peak and created new TL peak at about 373 K that is also responsible for the blue LLP. Based on our study, Y3+ ions are suggested to act as electron traps to improve the performance of the blue phosphorescence of Eu2+ such as intensity and persistent time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Long-lasting phosphorescence (LLP) phosphors are a special kind of energy-storing material, which could store the energy and then release the energy in the form of persistent visible light usually in room temperature. Such phosphors in the form of polycrystalline powder, crystal and glasses etc. have attracted increasing attention for their great practical and potential applications in many fields, e.g., lighting, display, high-energy ray detection, multidimensional optical memory and imaging storage [1–6]. Up to date, most of the LLP materials are based on the Eu2+ activated alkaline earth aluminates and silicates represented by SrAl2 O4 :Eu2+ , Dy3+ and Sr2 MgSi2 O7 :Eu2+ , Dy3+ , respectively. Among those phosphors, the phosphorescence is ascribed to the parity-allowed electronic transition of 4f6 5d1 –8 S7/2 of Eu2+ ion, which is strongly influenced by the host lattice due to crystal-filed effect. In general, other ions, i.e., trivalent rare earth ions, are incorporated in host as auxiliary activators to improve the phosphorescence intensity and persistence time. Those phosphors have been proved to be excellent LLP phosphors with higher brightness, longer persistence time and better chemical and physical stabilities compared with previous sulfide phosphors and used as commercial green and blue LLP phosphors [1,2,6–10]. However, researchers never lost their interests in exploring new LLP phosphors to extend the family of this kind of light-storing materials. In recent years, many novel LLP phosphors
∗ Corresponding author. Tel.: +86 431 85262208; fax: +86 431 85262005. E-mail addresses: [email protected] (C. Li), [email protected] (Q. Su). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.061
based on different hosts have been reported and their mechanisms were discussed [11–16]. But the step of developing new LLP materials with excellent performance is still slow. Actually, LLP is so special a phenomenon due to the thermal stimulated recombination of holes and electrons in traps at room temperature that only the suitable host doped with the right activator could yield considerable phosphorescence. Eu2+ activated the calcium pyrophosphate is an efficient blue phosphor which have extensive applications in the fields of luminescence and biomaterials due to its excellent optical and biological characteristics [17]. Recently, Eu2+ , Mn2+ co-doped Ca2 P2 O7 has been reported to be one of the potential yellow phosphors used in the phosphor mixture for obtaining white light emission from UV devices [18]. Although the optical properties of phosphor Ca2 P2 O7 :Eu2+ was reported long ago by Blasse et al. [19], there are no reports on the LLP phenomenon of this phosphor. In the present article, we developed an intense blue LLP of Eu2+ in Ca2 P2 O7 for the first time by co-doping with Y3+ . The luminescence and defect properties of Ca2 P2 O7 :Eu2+ , Y3+ were systematically investigated by means of photoluminescence spectra, LLP emission and decay curves and thermoluminescence (TL) spectra. Also, the LLP mechanism of this phosphor was discussed in this article. 2. Experimental The phosphor with nominal composition of Ca1.98 P2 O7 :0.02Eu2+ (labeled as CPOE) and Ca1.96 P2 O7 :0.02Eu2+ , 0.02Y3+ (labeled as CPOEY) were synthesized by traditional high-temperature solid-state reaction. CaHPO4 (analytical grade) NH4 H2 PO4 (analytical grade) Eu2 O3 (99.99%) and Y2 O3 (99.99%) were
216
R. Pang et al. / Materials Chemistry and Physics 113 (2009) 215–218
Fig. 1. XRD patterns of CPOE and CPOEY.
explored as staring materials. Stoichiometric mixtures of raw materials were homogeneously ground and then fired in alumina crucible at 600 ◦ C for 2 h. After regrinding, they were sintered at 1250 ◦ C for 3 h in a thermal-carbon reducing atmosphere. After cooling to room temperature naturally, the asobtained sample was ground into powder in an agate mortar for the next measurements. The X-ray power diffraction pattern was measured with a Rigaku-Dmax 2500 X-ray Diffractometer, using Cu K␣ ( = 0.15405 nm) radiation; Photoluminescence (PL) excitation and emission spectra were performed in a Hitachi F-4500 Fluorescence Spectrofluorometer at room temperature equipped with a 150 W xenon lamp as excitation source. The LLP emission spectrum and intensity decay were also measured using the same instrument after the sample was irradiated under 254 nm UV light for 1 min. TL spectra were measured with a 3D-TSL spectra instrument. The sample was place inside a homemade cryostat and heated form 293 to 573 K at the speed of 2 K s−1 . All the measurements expect the TL spectra were performed at room temperature.
3. Results and discussion The X-ray diffraction patterns of the sample CPOE and CPOEY are given in Fig. 1. They are found to be in good agreement with that of ␣-Ca2 P2 O7 registered in JCPDS file 09-0345, indicating that both the CPOE and CPOEY samples are single phase and the co-doping of Eu2+ and Y3+ does not cause any significant change in the host structure. Fig. 2 depicts the excitation and emission spectra of CPOE and CPOEY samples. It is clearly observed that the CPOEY sample shows the same luminescent properties as that of CPOE except
Fig. 2. Photoluminescence excitation and emission spectra of CPOE and CPOEY.
Fig. 3. LLP spectra of CPOE and CPOEY measured 1 min after the excitation source was switch off. Inset: CIE1931 chromaticity diagram.
the luminescence intensity. Monitored at 416 nm, an intense broad band ranging from 200 to 410 nm with two major peaks at 268 and 335 nm can be observed in the excitation spectrum, which is attributed to the 4f–5d parity-allowed transition of Eu2+ . This board excitation band indicates that those phosphors can be well excited by near-UV and visible light. Both the 268 and 335 nm excitation cause the similar emission of Eu2+ in peak shape and position, but the intensity of emission under the 335 nm excitation is much stronger. With excitation wavelength of 335 nm, the emission spectrum exhibits a strong blue band centered at 416 nm, which can be ascribed to the transitions from the lowest component of 5d excitation states to the ground state (8 S7/2 ) of Eu2+ . Moreover, from careful comparison of excitation and emission spectra between CPOE and CPOEY, it is observed that the introduction of Y3+ decreases the emission intensity of Eu2+ in alpha calcium pyrophosphate. Since the Y3+ does not show luminescence and hence will not compete against Eu2+ ion for the excitation energy, the decrease in emission intensity of Eu2+ can be ascribed to the storing of part of excitation energy in the defects induced by the Y3+ doping. It reveals that the Y3+ co-doping plays a role of creating electron traps in CPOEY. The following results will support this assignment. What is interesting of our present work is that we observe bright blue LLP of Eu2+ in CPOEY. Fig. 3 represents the LLP spectrum of CPOEY. As shown in Fig. 3, the LLP spectrum of CPOEY is close in peak shape and location to the steady-state emission spectra presented in Fig. 2, indicating that the persistent phosphorescence arises from Eu2+ ions doped in alpha calcium pyrophosphate. Generally, the color of the phosphorescence is represented by color coordinates which could be calculated from phosphorescence spectrum using chromaticity coordinate calculation method based on the CIE1931 (Commission International de I’Eclairage France) system. From the LLP spectrum of CPOEY, we calculated its phosphorescence chromaticity coordinates, which are (0.168, 0.010). From CIE1931 Chromaticity Diagram in the inset of Fig. 3 one can see that the color of afterglow located at blue region. The phosphorescence intensity decay of CPOEY monitored at wavelength of 416 nm were recorded in a few minutes after the stopping of the 254 nm UV light excitation and shown in Fig. 4. The LLP decay curve consists of a rapid decay component and a very long slow one. The
R. Pang et al. / Materials Chemistry and Physics 113 (2009) 215–218
217
Fig. 5. 3DTL emission spectrum of CPOEY measured form room temperature to 573 K with a heating rate of 2 K s−1 . Fig. 4. LLP Intensity decay of CPOE and CPOEY monitored at the wavelength of 416 nm.
corresponding phosphorescence decay times can be calculated by a curve fitting technique based on the following equation: I = A1 exp
−t 1
+ A2 exp
−t 2
(1)
where I is the phosphorescence intensity; A1 and A2 are constants; t is the time, and 1 and 2 are decay time for exponential components, respectively. The curve fits well with second-order exponential decay and the values of 1 and 2 are analyzed to be 21.8 and 151.8 s. It means that the phosphor could show long persistent phosphorescence. Actually, the blue phosphorescence of this phosphor could be observed by naked eyes even 6 h after excitation source was switched off. In view of the short luminescence decay life of 5d–4f transition of Eu2+ [8], the LLP observed in our phosphor could be attributed to energy exchange processes between traps or traps and emission centers resulting from Eu2+ and Y3+ doping. Moreover, the Eu2+ single doped sample CPOE also shows blue LLP, its LLP emission and decay curves are also demonstrated in Figs. 3 and 4, respectively. The decay curve of CPOE is similar to that of CPOEY, but the LLP initial intensity is much weaker and the blue phosphorescence can only be visible for several minutes in the limit of light perception for naked eyes (0.32 mCd m−2 ). As seen in Figs. 3 and 4, the co-doping of Y3+ ions largely improved the LLP performance of Eu2+ such as brightness and duration. Those results confirm our previous assignment that the Y3+ ions act as electron traps. This assignment explains the decrease in the emission intensity under steady excitation in Fig. 2 and simultaneously accounts for the increase of the LLP emission intensity of Eu2+ in CPOEY in Figs. 3 and 4. TL spectrum is a useful tool to investigate the properties of traps that are responsible for the appearance of LLP. Here, we measured the three-dimension thermoluminescence (3DTL) emission spectrum of CPOEY sample with a 3D-TSL spectra instrument and show it in Fig. 5, from which the origin of phosphorescence emission at different temperature can be obtained. As shown in Fig. 5, all the TL emissions at different temperatures are close in shape and position to the PL emission discussed in Fig. 2, indicating that the TL emissions derive from the 5d–4f transitions of Eu2+ ions. Fig. 6 portrays the TL curves of CPOE and CPOEY. For CPOE, it is clearly observed that there is only a very weak TL peak lying at about 335 K, which might due to intrinsic defects in the CPOE [20]. When Y3+ are doped, the intensity of this peak is greatly enhanced,
Fig. 6. TL curves of CPOE and CPOEY measured form room temperature to 573 K with a heating rate of 2 K s−1 .
and the peak position does not change, which means that the Y3+ doping largely improved the intrinsic defect levels. In addition, a shoulder TL peak located at about 373 K appears in the TL of CPOEY. This new peak, we suppose, is induced by Y3+ doping. The depth of traps in CPOEY can be calculated by processing the TL curve using the general order equation as follows [21,22]:
E
I (T ) = sn0 exp −
× 1+
kT
(l − 1)s ˇ
T
×
T0
E exp − kT
−l/(l−1)
dT
(2)
where E is the activation energy which indicates the trap depth, n0 is the concentration of trapped charges at time t = 0, which greatly affects the luminescent intensity, k is the Botlzmann constant, s is the frequency factor, l is the order of kinetics and ˇ is the heating rate. Here, ˇ = 2 K s−1 . Table 1 gives the calculated results of Table 1 The parameters of the TL curve for CPOEY Peak
E (eV)
s (s−1 )
Tm (K)
n0 (cm−3 )
l
1 2
0.67 0.70
3.82 × 109 5.84 × 108
335 373
2.20 × 105 5.27 × 104
1.96 2.0
218
R. Pang et al. / Materials Chemistry and Physics 113 (2009) 215–218
trap depths and parameters of the TL curve of CPOEY. The activation energy E is 0.67 eV for 335 K peak and 0.70 eV for 373 K peak, respectively, and the trap intensity (n0 ) related to 335 K peak is much stronger than that related to 373 K peak. A suitable depth of the trap is essential necessary for LLP, if the depth of the trapping levels is too low, the phosphor would show a fast decay, which does not last for a long time. On the contrary, if it is too deep the phosphor will no longer show phosphorescence at room temperature. According to our previous research [23], the depth of traps corresponding to TL peaks ranging from 293 to 383 K is suitable for the appearance of LLP. So, both the two TL peaks are responsible for LLP observed in our present work. Based on above results, it is evident that the Y3+ ions play an important role on improving the LLP performance. They not only intensively enhanced the trap intensity of the intrinsic defects but also created new deeper traps that are responsible for LLP, which accounts for the appearance of the bright LLP of CPOEY. On the mechanism of LLP, it is a common viewpoint that during the excitation period the energy of the incident light is partly stored in the form of captured charges. Those captured charges can be thermally released from the traps at room temperature and transferred to doped ions, which results in the characteristic emission of the luminescent ions. Because the release of the captured electrons and traps is durative, the luminescences of the materials show the property of long life. The doped ions substitute the host ions with similar radii yielding defects nearby. These defects serve as hole or electron traps capturing electrons or holes in excitation process and transferring durative energy to the luminescent ions. In the case of CPOEY, the Eu2+ and Y3+ ions are expected to occupy the sites of Ca2+ according to the fact that the radii of Eu2+ (0.130 nm) and Y3+ (0.108 nm) are close to that of Ca2+ (0.118 nm) [24]. Defects were generated by nonequivalent substitution, i.e., two Y3+ ions substituted three Ca2+ ions and created positive defects and a V negative defect, which act as two YCa Ca electron and hole traps. In the process of excitation electrons were pumped to the 5d orbits of Eu2+ ions, part of which were captured and stored in the form of energy. Under by positive defects YCa the thermal activation, these trapped electrons escaped from the traps and transferred to Eu2+ ions followed by the characteristic emission of Eu2+ ions. All discussed above is a general description for the occurrence of LLP of Eu2+ and Y3+ co-activated calcium pyrophosphate. However, the detail of the mechanism is still unclear.
4. Conclusions Eu2+ and Y3+ co-doped Ca2 P2 O7 was synthesized via traditional high-temperature solid-state reaction method. The phosphor emitted blue light that was ascribed to the characteristic d–f emission of Eu2+ . After the excitation source was switched off, bright blue LLP could be observed. Two TL peaks at 335 and 373 K related to two types of defects appeared in TL curve, which were responsible for the LLP in room temperature. The co-doping of Y3+ was proved to improve the performance of the blue phosphorescence of Eu2+ such as intensity and persistent time. Acknowledgement This work was financially supported by National Basic Research Program of China (2007CB935502). References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electonchem. Soc. 143 (1996) 2670. [2] T. Zhang, Q. Su, SID J. 8 (2000) 27. [3] J.R. Qiu, K. Hirao, Solid State Commun. 106 (1998) 795. [4] J.R. Qiu, K. Miura, H. Inpuye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 73 (1998) 1763. [5] C.Y. Li, Q. Su, S.B. Wang, Mater. Res. Bull. 37 (2002) 1443. [6] T.Y. Peng, H.J. Liu, H.P. Yang, C.H. Yan, Mater. Chem. Phys. 85 (2004) 68. [7] Z. Xiao et al., United States Patent U.S. 6,093,346 (2000). [8] N. Kodama, N. Sasaki, M. Yamaga, Y. Masui, J. Lumin. 94/95 (2001) 19. [9] Z.W. Liu, Y.L. Liu, Mater. Chem. Phys. 93 (2005) 129. [10] F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M. Whangbo, A. Garcia, T. Mercier, Chem. Mater. 17 (2005) 3904. [11] J. Wang, S.B. Wang, Q. Su, J. Solid State Chem. 177 (2004) 895. [12] L.Y. Liu, C.Y. Li, S.B. Wang, Q. Su, Appl. Phys. Lett. 88 (2006) 241107. [13] N. Kodama, M. Yamaga, Y. Tanii, J. Qiu, K. Hirao, Appl. Phys. Lett. 75 (1999) 1715. [14] C. Guo, Q. Tang, D. Huang, C. Zhang, Q. Su, J. Phys. Chem. Solid. 68 (2007) 217. [15] Y. Cong, B. Li, B. Lei, W. Li, J. Lumin. 126 (2007) 822. [16] B.F. Lei, B. Li, X.J. Wang, W.L. Li, J. Lumin. 118 (2006) 173. [17] A. Doat, F. Pelle, A. Lebugle, J. Solid State Chem. 178 (2005) 2354. [18] Z.D. Hao, J.H. Zhang, X. Zhang, X.Y. Sun, Y.S. Luo, S.Z. Lu, X.J. Wang, Appl. Phys. Lett. 90 (2007) 261113. [19] G. Blasse, W.L. Wanmaker, J.W. Vrugt, J. Electronchem. Soc. 115 (1968) 673. [20] J. Holsa, T. Aitasalo, H. Jungner, M. Lastusaari, J. Niittykoski, G. Spano, J. Alloys Compd. 374 (2004) 56. [21] S.W.S. McKeever, M. Prokic, P.D. Townsend, Thermoluminescence Dosimetery Materials: Properties and Uses, Nuclear Technology Press, Ashford, UK, 1995, p 182. [22] R. Chen, J. Appl. Phys. 40 (1969) 570. [23] C.Y. Li, Q. Su, J.R. Qiu, Chin. J. Lumin. 24 (2003) 19. [24] R.D. Shannon, Acta Cryst. A32 (1976) 751.