Journal of Alloys and Compounds 387 (2005) 65 69 The trap states in the Sr 2 MgSi 2 O 7 and (Sr,Ca)MgSi 2 O 7 long afterglow phosphor activated by Eu 2+ and Dy 3+ Bo Liu a,, Chaoshu Shi a,b, Min Yin a, Lin Dong a, Zhiguo Xiao c a Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China b National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China c Dalian Luming Light Co. Ltd., Dalian 116025, China Received 6 April 2004; received in revised form 18 June 2004; accepted 18 June 2004 Abstract The excellent long afterglow phosphor Sr 2 MgSi 2 O 7 :Eu 2+,Dy 3+ with the blue light and (Sr,Ca)MgSi 2 O 7 :Eu 2+,Dy 3+ emitting blue green afterglow for more than 20 h under the UV excitation were prepared. The trap states in the samples are studied by thermoluminescence (TL) and photo-stimulated luminescence (PSL). The results of TL and PSL exhibit plenty of traps which are responsible for the long afterglow. TL study shows that a broad band appears at 330 and 357 K for (Sr,Ca)MgSi 2 O 7 :Eu 2+,Dy 3+ with the thermal depth of trap E TL = 0.66 ev and for Sr 2 MgSi 2 O 7 :Eu 2+,Dy 3+ with the thermal depth of trap E TL = 0.75 ev, respectively. PSL spectra reveals that the trapped carries also can be released by optical stimulation with the E PSL = 1.56 1.94 ev. The relation between TL and PSL can be explained by the configuration coordinate mode. 2004 Elsevier B.V. All rights reserved. Keywords: Long afterglow; Thermoluminescence; Photo-stimulated luminescence 1. Introduction The long afterglow Eu 2+ and Dy 3+ activated phosphor is extensively used because of its high luminance and long afterglow [1,2]. The emission of 5d-4f from Eu 2+ is highly efficient and its emission wavelength is strongly dependent on the host lattice so that we can get different color from blue to red. The dopant Dy 3+ acts as a trap-creating ion, which can greatly prolong the afterglow. Silicate host is characteristic with chemical and physical stability, easy preparation and low cost. Therefore the silicate host is attracting more attention in the application of long afterglow phosphors [3 5]. The studied phosphors Sr 2 MgSi 2 O 7 :Eu, Dy (labelled as SB) emitting blue light and (Sr,Ca)MgSi 2 O 7 :Eu, Dy (labelled as SBG) emitting blue green light in this article have a afterglow time longer than 20 h visible to hu- Corresponding author. E-mail address: lbo@ustc.edu.cn (B. Liu). man eye [6]. The detailed afterglow properties are listed in Table 1. The traps in the long afterglow phosphor are very plentiful, which result in the long afterglow. Photo-stimulated luminescence (PSL) is the process in which trapped carries are released by photons and produce the luminescence [7]. The usual photon wavelength used in PSL is from infrared to the green. Thermoluminescence occurs when the trapped carriers are thermally released and then produce luminescence through recombination. TL and PSL techniques are useful tool for evaluating the trap state and understanding mechanism of long afterglow phosphorescence. 2. Experimental The polycrystalline powder samples SB and SBG were prepared by high temperature solid phase reaction using the raw SrCO 3, CaCO 3, MgO, SiO 2,Eu 2 O 3 and Dy 2 O 3 with the 0925-8388/$ see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.061
66 B. Liu et al. / Journal of Alloys and Compounds 387 (2005) 65 69 Table 1 The afterglow properties of the samples Phosphor Color Peak wavelength (nm) Luminous intensity (mcd/m 2 ) 1 min 10 min 30 min 60 min Sr 2 MgSi 2 O 7 :Eu, Dy Blue 466 500 87 28 14 (Sr,Ca)MgSi 2 O 7 :Eu, Dy Blue Green 490 650 121 39 19 D/Mrax-rA Rotating Anode X-ray Diffractometer with Cu K radiation. The luminescence spectra and the photo-stimulated luminescence were measured using 970 CRT fluorescence spectrophotometer made by Shanghai Analytical Instrument Overall Factory. The thermoluminescence measurements were performed from 250 to 450 K by Hg lamp irradiation for 1 min and then a linear heating with rate 0.2 K/s. A TC-100U temperature controller with different program was used for temperature control in the detection. 3. Results and discussion Fig. 1. XRD patterns of Sr 2 MgSi 2 O 7 and (Sr,Ca)MgSi 2 O 7. purity of 99.99%. The raw materials were mixed then sintered at 1300 C for 5 h in a reducing gas flow (5 at.% H 2 +N 2 ). In order to determine the crystal structure of the samples, X- ray diffraction (XRD) measurement was performed using a The XRD patterns of SB and SBG are shown in Fig. 1. The position and the intensity of diffraction peaks of the polycrystalline powder SB are consistent with that of the powder diffraction file (PDF) 75-1736 (Sr 2 MgSi 2 O 7 ). The Sr 2 MgSi 2 O 7 and Ca 2 MgSi 2 O 7 have the same tetragonal crystal system and P42 1 m (No.113) space group but have slight difference in lattice constant, a = b = 7.9957 Å and c = 5.1521 Å for Sr 2 MgSi 2 O 7 and a = b = 7.835 Å and c = 5.01 Å for Ca 2 MgSi 2 O 7 (PDF No. 83-1815). Therefore the 2 at each peak in the XRD patterns for SBG sample is between that of Sr 2 MgSi 2 O 7 and Ca 2 MgSi 2 O 7. Nearly no other phase is observed in the XRD patterns for the two samples. Fig. 2. The emission (right) and excitation (left) spectra of Sr 2 MgSi 2 O 7 :Eu 2+,Dy 3+.
B. Liu et al. / Journal of Alloys and Compounds 387 (2005) 65 69 67 Fig. 3. The emission (right) and excitation (left) spectra of (Sr,Ca)MgSi 2 O 7 :Eu 2+,Dy 3+. Figs. 2 and 3 show the emission spectra of SB and SBG, respectively. The broad bands due to transitions of Eu 2+ between the 8 S 7/2 (4f 7 ) ground state and the excited 4f 6 5d 1 configuration were observed under the ultra violet excitation. The spectrum shape of the two samples is very similar but different in wavelength because of the effect of crystal field in different host. SB emits the blue light band which peaked at 466 nm and SBG emits blue green light with the peak at 490 nm. The excitation spectra also consist of broad bands from 250 nm to visible light which peaked at 363 and 386 nm for SB and SBG, respectively. The effective excitation for the afterglow emis- sion may be extended to green light (approximately 500 nm), although our measurement did not reach here due to the limit of emission wavelength. Such broad excitation band means the samples can effectively and completely absorb the energy of natural light. That is the reason why the samples have strong afterglow after the excitation of visible light. The TL measurement exhibits a broad TL band from 250 to 450 K for the two samples with the peak at 330 and 357 K for SBG and SB, respectively, shown in Fig. 4. The feature of the TL cures is nearly symmetric which means they accord with the second order kinetics. Therefore, it is reasonable to Fig. 4. The TL of the samples.
68 B. Liu et al. / Journal of Alloys and Compounds 387 (2005) 65 69 Fig. 5. The PSL of the samples. deal with the trap parameters using the second order kinetics equation proposed by Garlick and Gibson [8] and Chen [9]. ( I(T ) = n 2 0 s exp E ) kt [ 1 + ( ) n0 s T exp β T 0 ( E ) ] 2 kt dt where n 0 is the concentration of trapped charges at T = 0K,k is Boltzmann s constant, β is the heating rate, s is the frequency factor, and E is the trap depth. The trap depth was obtained with E TL = 0.66 ev and E TL = 0.75 ev for SBG and SB, respectively. The PSL spectra are shown in Fig. 5. The photo stimulation wavelength used in the measurement is longer than their emission wavelength in order to observe the optical effect on the empty trap. Before the measurement of PSL, the samples were excited by ultra-violet in order to fill the traps and then waited for the afterglow decay to be stable. In the PSL spectra the afterglow intensity just was considered as stable background. The PSL spectra of SBG show a complex structure with a main peak at 639 nm (E PSL = 1.94 ev) and some weak peaks extended to 795 nm (E PSL = 1.56 ev). That means that there are plenty of traps, which can be photo stimulated with the energy from 1.56 to 1.94 ev. For given traps, the trapped carries can be released thermally and optically. Accordingly, we can obtain thermal depth and optical depth of traps. Usually, the energy required for photo stimulation is higher than that for thermal stimulation. We tentatively consider that the trap responsible for the TL peak at 330 K is identical with that responsible for the PSL peak at 639 nm for SBG and the trap responsible for the TL peak at 357 K is identical with that responsible for the PSL peak at 625 nm for SB. The depth of an electron or hole trap is the energy necessary for the ionization of the trapped carries. However, there are significantly different energy between the optical ionization and the thermal ionization. In particular, the thermal depth of a trap is the activation energy governed by temperature that is in thermal equilibrium with the crystal lattice. The optical depth of a trap refers to the minimum photon energy sufficient to release the trapped carriers from the trap without the participation of the thermal vibration [10]. The configuration coordinate model for PSL and TL shown in Fig. 6 can illustrate the relation between PSL and TL and the reason why the optical stimulation require higher energy than thermal stimulation. Fig. 5 shows that the many structures exist in PSL spectra and some weak peaks at 710 760 nm, which means the traps are also complicated and seems to consist of many trap levels. Similarly, the TL also shows a broad band, which maybe consist of many trap levels or a trap band with many quasi-continuum of levels. Fig. 6. The scheme of TL and PSL.
B. Liu et al. / Journal of Alloys and Compounds 387 (2005) 65 69 69 4. Conclusion The excellent long afterglow phosphor SB with the blue afterglow and SBG emitting blue green afterglow for more than 20 h under the UV excitation was prepared. The results of TL and PSL exhibit plenty of traps which are responsible for the long afterglow. TL study shows that a broad band appears at 330 and 357 K for SBG with the thermal depth of trap E TL = 0.66 ev and SB with the thermal depth of trap E TL = 0.75 ev, respectively. PSL reveals that the trapped carries also can be released by optical stimulate with the E PSL =1.56 1.94 ev. Acknowledgement This work is supported by Ministry of Science and Technology of China (863 program, Grant No. 2002AA324060). References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670 2673. [2] T. Katsumata, R. Sakai, S. Komuro, T. Orikawa, J. Electrochem. Soc. 150 (5) (2003) H111 H114. [3] Y. Lin, C. Nan, X. Zhou, J. Wu, H. Wang, D. Chen, S. Xu, Mater. Chem. Phys. 82 (3) (2003) 860. [4] L. Yuanhua, T. Zilong, Z. Zhongtai Nan, C. Wen, J. Alloys Compd. 348 (1 2) (2003) 76. [5] J. Ling, C. Chengkang, M. Dali, J. Alloys Compd. 360 (1 2) (2003) 193. [6] X. Luo, J. Duan, G.L.J. Xu, Y. Yang, Z. Xiao, Chin. J. Lumin. 24 (2) (2003) 165. [7] K. Chakrabarti, V.K. Mathur, L.A. Thomas, R.J. Abbundi, J. Appl. Phys. 65 (1989) 2021. [8] G.F.J. Garlick, A.F. Gibson, Proc. Phys. Soc. 60 (1948) 574. [9] R. Chen, J. Electrochem. Soc. 116 (1969) 1254. [10] W. Kuang, M.V. Fok, Proceedings (Trudy) of the P.N. Lebedev Physics Institute, 79, 1976, p. 39.