Modification on luminescent properties of SrAl 2 O 4 :Eu 2+, Dy 3+ phosphor by Yb 3+ ions doping

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1 Journal of Luminescence 128 (28) Modification on luminescent properties of SrAl 2 O 4 :Eu 2+, Dy 3+ phosphor by Yb 3+ ions doping Ren Chen, Yinhai Wang, Yihua Hu, Zhengfa Hu, Cheng Liu School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 516, People s Republic of China Received 2 June 27; received in revised form 27 November 27; accepted 29 November 27 Available online 14 December 27 Abstract The modification effect of the doping of Yb 3+ ions, as an auxiliary activator, onto the luminescent properties of SrAl 2 O 4 :Eu 2+,Dy 3+ phosphor was studied for the first time. The phosphorescent nanoparticles were prepared by the combustion method. The experimental results indicate that the appropriate doping of Yb 3+ ions largely improves phosphorescence of the phosphors with more intense luminescence, higher brightness, and no change in emission spectrum peaked at 513 nm. Meanwhile the decay speed of the phosphor nanoparticles rises increasingly with the doping ratio of Yb 3+ ions, whereas an excessive Yb 3+ ions doping leads to the disappearance of the pure monoclinic phase of SrAl 2 O 4 and the appearance of the weak diffraction lines of the YbAlO 3 phase. The phosphorescent mechanism of the phosphors could be well understood based on the hole, thermally released from the trap levels of Dy 3+ and Yb 3+. r 28 Elsevier B.V. All rights reserved. PACS: k; Keywords: SrAl 2 O 4 :Eu 2+,Dy 3+ ; Auxiliary activator of Yb 3+ ; Long afterglow; Combustion method 1. Introduction The green-luminescent SrAl 2 O 4 :Eu 2+,Dy 3+ phosphor has drawn many research interests since it was first developed by Matsuzawa et al. [1], because of its excellent luminescent properties, such as high luminescent intensity, high quantum efficiency and long-lasting phosphorescence, and high chemical stability. By now, SrAl 2 O 4 :Eu 2+,Dy 3+ phosphor has been extensively studied [1 5]. Matsuzawa et al. [1] and Yamamoto et al. [2] explained that the longlasting phosphorescence mechanism of Eu 2+, Dy 3+ codoped strontium aluminates phosphor resulted from the holes thermally released from the trap levels of Dy 3+, which had the optimum trap depth at the room temperature [2]. Whereas, recently, Dorenbos [3,4] proposed an alternative model that involves the 5d electrons of the Eu 2+ to conduction band states and a subsequent trapping by Dy 3+. Peng s group synthesized the SrAl 2 O 4 :Eu 2+, Dy 3+ phosphor and studied its phosphorescence [5]. In Corresponding author. Tel.: ; fax: address: huyh@gdut.edu.cn (Y. Hu). their work, the SrAl 2 O 4 :Eu 2+,Dy 3+ phosphor nanoparticles were prepared by glycine-nitrate solution combustion synthesis at 5 1C, followed by heating the resultant combustion ash at 11 1C in a weak reductive atmosphere of active carbon. A shift to the short wavelength in emission spectrum was found when compared with those obtained by the solid-state reaction synthesis method. Compared with traditional sulfide phosphors, SrAl 2 O 4 : Eu 2+,Dy 3+ phosphor possesses safer, chemically stable, and very bright and long-lasting phosphorescence that leads to a large field of applications, such as luminous paint in highway, airport, buildings, and ceramic products, as well as in warning signs and exit marks, etc. More advanced and practical applications make higher requirements to the features of this kind of phosphorescent materials in luminescent intensity and brightness, so it is necessary to further improve their luminescent properties. It is known that doping with different ions is a good way to modify the luminescent properties of phosphorescent materials. Katsumata et al. [6] reported the characterization of trap levels in the long-lasting phosphors of SrAl 2 O 4 :Eu 2+ doped with Nd, Sm, Gd, Dy, and Y, and /$ - see front matter r 28 Elsevier B.V. All rights reserved. doi:1.116/j.jlumin

2 R. Chen et al. / Journal of Luminescence 128 (28) that the long-lasting phosphorescence was influenced strongly by the depth and density of the traps. Aizaw et al. [7] reported the long afterglow phosphorescent sensor materials of SrAl 2 O 4 :Eu 2+ doped with various rare earth ions, and that the life time and intensity of the long afterglow phosphorescence of SrAl 2 O 4 :Eu 2+ changed along with different doping auxiliary activators (Y. La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), and they were dominated by the thermal excitation of trapped carrier. Yang et al. [8] reported the phosphorescent characteristics and mechanism of Eu 3+ and Cu 2+ codoped SrAl 2 O 4, and that the excitation and emission spectra of Cu 2+ and Eu 3+ co-doped phosphors are significantly different from those of SrAl 2 O 4 :Eu and SrAl 2 O 4 :Cu. The phosphorescent characteristics of these materials could be strongly influenced by the doping of rare earth ions. The Yb 3+ ions doping, as auxiliary activators, has been studied extensively [9,1]. However, till now, there is no report about modifying the luminescent properties of SrAl 2 O 4 :Eu 2+, Dy 3+ phosphor by using the doping of Yb 3+ ions. In our work, SrAl 2 O 4 :Eu 2+,Dy 3+ phosphors doped with various Yb 3+ ions were successfully synthesized by the combustion method and their luminescence was measured and analyzed. The experiment results show that the initial intensity of the phosphors and the afterglow time can be modified by various doping ratios of Yb 3+ ions. 2. Experimental procedures The additional Yb 3+ -doped SrAl 2 O 4 :Eu 2+,Dy 3+ phosphor nanoparticles were prepared by combustion method from the starting materials of Sr(NO 3 ) 2, Al(NO 3 ) 3 9H 2 O, Eu 2 O 3 (3N), Dy 2 O 3 (3N), Yb 2 O 3 (3N), H 3 BO 3, and CO (NH 2 ) 2. Firstly, according to Sr.97 x Al 2 O 4 :Eu 2+.1, Dy 3+.2, Yb 3+ x, stoichiometric composition of Sr(NO 3 ), Al(NO 3 ).9H 2 O, CO(NH 2 ) 2, and H 3 BO 3 were dissolved in the de-ionized water to form the precursor solution. Secondly, the other solution of Eu(NO 3 ) 3, Dy(NO 3 ) 3, and Yb(NO 3 ) 3 in HNO 3 would be made to offer the Eu 3+, Dy 3+ and Yb 3+ ions, subsequently mixing both of them together and stirring the mixture solution for about 2 h to form a homogeneous solution at a constant temperature of 8 1C. All starting reagents were of the analytical purity. After that, the precursor solution was put into a crucible and introduced into a muffle furnace maintained at 6 1C. Initially, it was extremely necessary to perform the dehydration by boiling the solution, followed by evolving decomposition under the presence of large amounts of gases (oxides of carbon, nitrogen, and ammonia). Then the reagent was spontaneously ignited and underwent combustion to produce white and green foamy and voluminous ash. The whole process was maintained within less than 1 min. In order to investigate to what extent the molar ratio of Yb 3+ ions doping could affect the phosphorescence of Yb 3+ -doped SrAl 2 O 4 :Eu 2+, Dy 3+ phosphor nanoparticles, we have synthesized six kinds of samples with different molar ratios of Yb 3+ ions at.,.5,.1,.15,.2, and.5 by using the same combustion method and for simplicity we ordered the samples as,,,,, and, respectively. The X-ray diffraction (XRD) patterns were measured on a Y-2 X-ray diffractometer using Cu Ka (l= A ) as an excitation source, operated at 3 kv voltage and 2 ma current. The emission spectra were collected by a RF-531 fluorescence spectrofluorometer. However, the decay profiles of afterglow and the initial intensity of phosphorescence of the nanoparticles were measured on a GSZF-2A single-photon counter system. 3. Results and discussion 3.1. X-ray diffraction patterns The XRD of the SrAl 2 O 4 :Eu 2+,Dy 3+ phosphors that were ground completely in the agate mortar with various molar ratios of Yb 3+ ions doping are shown in Fig. 1. As can be seen, the phosphorescence of,, and are from pure single-phase compounds, being in good agreement with the JCPDS data file (No ), all of the XRD peaks came from the monoclinic-phase SrAl 2 O 4, and no traces of rare earth metal ions were observed. However,for,, and, especially, the XRD spectra were from both of the SrAl 2 O 4 phase compounds, and a small quantity of YbAlO 3 phase, corresponding to the JCPDS data file (No ). It suggests that the XRD patterns of the phosphors with a little amount of Yb 3+ ions doping maintain the pure SrAl 2 O 4 monoclinic phase. Since the valence state of Yb in the by-product is trivalent, it is most likely that the Yb ions lie in the trivalent state in the SrAl 2 O 4 system YbAlO 3 : SrAl 2 O 4 : θ (degree) 431 Fig. 1. XRD patterns of the phosphors (Sr.97 x Al 2 O 4 :Eu 2+.1, Dy 3+.2, and Yb 3+ x, x is.,.5,.1,.15,.2,.5, respectively, corresponding to,,,,, ).

3 1182 ARTICLE IN PRESS R. Chen et al. / Journal of Luminescence 128 (28) The average crystal size was determined from the XRD pattern parameters according to the Scherrer equation [11]: D hkl ¼ kl/(b cos y), where D hkl is the size of the crystal particles, k is equal to.89, b is the full-width at halfmaximum (FWHM), and y is the diffraction angle. From the FWHM data of the diffraction peaks, the crystal diameter D 211 of,,,,, and were calculated to be 36.9, 33.9, 33.9, 33.9, 33.9, and 36.9 nm, respectively, there is no remarkable change in the size among the six samples along with the increasing Yb 3+ ions doping. Relative PL Intensity (arb.units) min 1 min 3 min 5 min 1 min 3.2. Emission spectra of phosphorescence The phosphorescence spectra of SrAl 2 O 4 :Eu 2+, Dy 3+ phosphors grown from various Yb 3+ ions doping are showed in Fig. 2 that were obtained by exciting the samples at the wavelength 355 nm, and exhibit a broadband emission from Eu 2+ accompanied with the peak at l ¼ 513 nm. The emission is due to the 5d 4f transition of Eu 2+ ions [12,13]. The position of the emission peak in the phosphorescence almost shows no change, regardless of the diversity of the Yb 3+ ions doping, implying that no obvious change occurs in the crystal field effect on 5d electron states of Eu 2+ ions, even though the molar ratio of Yb 3+ ions doping varies. In addition, the appearance of new YbAlO 3 phase did not influence the emission peak either. Fig. 3 displays typical time-resolved phosphorescence spectra of SrAl 2 O 4 :Eu 2+, Dy 3+ with.1 molar ratio doping of Yb 3+ ions (). The phosphorescence spectra were measured for every 1 min UV excitation using l ¼ 355 nm. The emission peak at l ¼ 513 nm in the afterglow phosphorescence remained the same without any change, indicating that the 5d 4f transition of Eu 2+ ions fully dominates during the decay of afterglow phosphorescence. Relative PL intensity (arb.units) Wavelength (nm) Fig. 2. Emission spectra of the phosphors (Sr.97 x Al 2 O 4 :Eu 2+.1, Dy 3+.2, Yb 3+ x, x is.,.5,.1,.15,.2,.5, respectively, corresponding to,,,,, and ) excited by l ¼ 355 nm UV lights Wavelength (nm) Fig. 3. Time-resolved phosphorescence spectra of Sr.96 Al 2 O 4 :Eu 2+.1, Dy 3+.2, and Yb 3+.1 phosphor which were examined in,1,3,5,1 min after the l ¼ 355 nm UV light excitation for 1 min. Relative Intensity (arb.units) 5x1 5 4x1 5 3x1 5 2x1 5 1x Yb 3+ doping (molar ratio) Fig. 4. The initial intensity of the phosphors (Sr.97 x Al 2 O 4 :Eu 2+.1,Dy 3+.2, and Yb 3+ x, x is.,.5,.1,.15,.2,.5, respectively, corresponding to,,,,, ) were examined immediately after the UV excitation for 1 min The initial intensity of the phosphors The initial intensity of the phosphors with different molar ratio doping of Yb 3+ ions were recorded simultaneously by just removing the UV excitation source after 1 min duration of UV excitation. As is shown in Fig. 4, the initial intensity of the phosphors rose dramatically at first and then decreased gradually when the molar ratio of Yb 3+ ions doping was over.1. The initial photoluminescence of the phosphor with.1 molar ratio Yb 3+ ions doping is about two times as intense as that of the no Yb 3+ -doped phosphor. The phosphorescent intensity of the phosphors is dependent on the concentrations of the auxiliary activators. Therefore, doping of Yb 3+ ions as auxiliary activators may promote the trapping density that is

4 R. Chen et al. / Journal of Luminescence 128 (28) positive to the phosphorescent intensity. However, Yb 3+ auxiliary activators are of limited solubility, about.1molar ratio, in SrAl 2 O 4 system. As can be seen from the XRD patterns, an excessive doping of Yb 3+ ions could result to the by-product of YbAlO 3 product; this new YbAlO 3 phase perhaps causes the drop of the initial intensity Decay curves of phosphorescence The decay curves of the afterglow phosphorescence are shown in Fig. 5, recorded by removing of UV source after 1 min irradiation of UV light. All of them show both a rapid decay and subsequential long-lasting phosphorescence. The decay behavior can be analyzed by curve fitting as referred to Refs. [14,15], relying on the following double exponential equation: I ¼ I 1 exp t t 1 þ I 2 exp t t 2, (1) where I represents the phosphorescent intensity; I 1 and I 2 are constants; t is the time; t 1 and t 2 are the decay constants, deciding the decay rate for the rapid and slow exponentially decay components, respectively. The fitting results of parameters of t 1 and t 2 are listed in Table 1. A trend can be observed that the decay constants of the phosphors decrease gradually with the increasing of Yb 3+ ions doping to SrAl 2 O 4 :Eu 2+,Dy 3+ phosphors. It is known that the carriers trapped at different trapping level are responsible for the various decay components, because the recombination between the excited electron and trapped hole is influenced greatly by the trap depth. Aitasalo et al. [16] estimated the trap depths of the thermoluminescence (TL) main peaks of the CaAl 2 O 4 :Eu 2+,R 3+ materials using the heating rate method. They found that the trap depth for CaAl 2 O 4 :Eu 2+,Dy 3+ was about.61 ev and the trap depth for CaAl 2 O 4 :Eu 2+, Yb 3+ was about.45 ev, which was a little shallower than Relative intensity (arb.units) 4x1 5 3x1 5 2x1 5 1x time (second) Fig. 5. Decay curves of the phosphors (Sr.97 x Al 2 O 4 :Eu 2+.1, Dy 3+.2, and Yb 3+ x, x is.,.5,.1,.15,.2,.5, respectively, corresponding to,,,,, ), after the UV excitation for 1 min. Table 1 Fitting result for SrAl 2 O 4 :Eu 2+,Dy 3+ phosphors with various Yb 3+ ions doping Samples Decay constants (t, s) the former. We also propose that similar phenomena should exist in SrAl 2 O 4 system, and the decreasing of the decay constants could be accounted for as following: the depth of the carrier trapping level of Yb 3+ is shallower than that of the Dy 3+ amongst the SrAl 2 O 4 :Eu 2+,Dy 3+, Yb 3+ phosphor, and thus the carrier trapped by Yb 3+ is thermally easily excited at room temperature leading to more rapid decay of the phosphor. Abbruscato [17] carried out the Hall-effect measurements for the SrAl 2 O 4 :Eu 2+ phosphors under UV excitation and identified the conducting species to be a hole. Matsuzawa et al. [1] conducted the photocurrent measurement of the SrAl 2 O 4 :Eu 2+, Dy 3+ phosphors under nonuniform illumination using l ¼ 365 nm UV light, they discovered that the charged species was a hole and the photoconductivity of SrAl 2 O 4 :Eu 2+,Dy 3+ phosphor was due to the motion of holes in the valence band. It can be thus inferred that the conducting species in SrAl 2 O 4 :Eu 2+, Dy 3+,Yb 3+ are mostly holes. In the long afterglow phenomena, three different kinds of centers could be involved: emitting center of Eu 2+ ; deep hole trapping level of Dy 3+ and shallow-hole trapping level of Yb 3+. Therefore, the phosphorescent mechanism of the phosphors system can be interpreted as following: upon the exposure of phosphors to the ¼ 355 nm UV lights, the direct excitation of 4f 5d transition of Eu 2+ happens and plenty of holes are created; some of the free holes are released thermally to the valence band, meantime, one part of the released holes travel through the valence band, and are trapped by Dy 3+ and Yb 3+. When the excitation source is removed, the excited Eu 1+ is relaxed to the metastable state (Eu 1+ )*. The trapped holes are released thermally to the valence band and move to recombine with the excessive electrons in the metastable state (Eu 1+ )* sites leading to the long afterglow. The schematic representation of luminescent mechanism is shown in Fig Conclusion t 1 (s) t 2 (s) (.) (.5) (.1) (.15) (.2) (.5) SrAl 2 O 4 :Eu 2+,Dy 3+ nanophosphors with various Yb 3+ ions doping were synthesized successfully by the combustion method. The crystal structures are almost the same under a small amount of Yb 3+ ions doping, whereas the

5 1184 ARTICLE IN PRESS R. Chen et al. / Journal of Luminescence 128 (28) Relaxation YbAlO 3 phase will appear when the Yb 3+ ions doping is excessive. All the peaks in emission spectra are almost the same, irrelevant of doping of Yb 3+ ions in our experiment. The luminescent properties of the phosphors can be greatly modified by the doping of Yb 3+ ions. The phosphors doped with an appropriate amount of Yb 3+ result in a very intense and bright initial phosphorescent emission. Acknowledgments Conduction Band Eu Excitated state Metastable state Yb traps Velence Band Dy traps Fig. 6. The schematic graph of long afterglow phosphorescent mechanism for these phosphors. This work was supported by the National Nature Science Foundation of China under the Grant no The authors are grateful to Professor Y.M. Zhao (South China University of Technology) and Professor L. Luo (Guang Dong University of Technology) for their useful discussions. References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (8) (1996) 267. [2] H. Yamamoto, T. Matsuzawa, J. Lumin (1997) 287. [3] P. Dorenbos, J.Electrochem. Soc. 152 (25) H17. [4] P. Dorenbos, J. Lumin. 122/123 (27) 315. [5] T.Y. Peng, H.P. Yang, X.L. Pu, B. Hu, Z.CH. Jiang, CH.H. Yan, Mater. Lett. 58 (24) 352. [6] H. Aizawa, T. Katsumata, J. Takahashi, K. Matsunaga, S. Komuro, T. Morikawa, Rev. Sci. Instrum. 74 (3) (23) [7] T. Katsumata, S. Toyomane, A. Tonegawa, Y. Kanai, U. Kaneyama, K. Shakuno, R. Sakai, S. Komuro, T. Morikawa, J. Cryst. Growth (22) 361. [8] P. Yang, M.K. Lu, C.F. Song, D. Xu, D.R. Yuan, G.M. Xia, S.W. Liu, Inorg. Chem. Commun. 5 (22) 919. [9] J. Holsa, H. Jungner, M. Lastusaari, J. Niittykoski, J. Alloys Compd. 323/324 (21) 326. [1] C. Guo, Q. Tang, D. Huang, C. Zhang, Q. Su, J. Phys.Chem. Solids 68 (27) 217. [11] Z.L. Fu, S.H. Zhou, Y.N. Yu, S.Y. Zhang, Chem. Phys. Lett. 395 (24) 285. [12] T. Katsumata, T. Nabae, K. Sasajima, T. Matsuzawa, J. Cryst. Growth 183 (1998) 361. [13] Y. Lu, Y. Li, Y. Xiong, D. Wang, Q. Yin, Microelectron. J. 35 (24) 379. [14] R. Sakai, T. Katsumata, S. komuro, T. Morikawa, J. Lumin. 85 (1 3) (1999) 149. [15] Z.X. Yuan, C.K. Chang, D.L. Mao, W.J. Ying, J. Alloys Compd. 377 (24) 268. [16] T. Aitasalo, P. Deren, J. Holsa, H. Jungner, J.-C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, W. Strek, J.Solid State Chem. 171 (23) 114. [17] V. Abbruscato, J.Electrochem. Soc. 118 (1971) 93.