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The persistent energy transfer of Eu2+ and Mn2+ and the thermoluminescence properties of long-lasting phosphor Sr3MgSi2O8:Eu2+, Mn2+, Dy3+
Optical Materials 33 (2011) 1781–1785
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
The persistent energy transfer of Eu2+ and Mn2+ and the thermoluminescence properties of long-lasting phosphor Sr3MgSi2O8:Eu2+, Mn2+, Dy3+ Yu Gong, Yuhua Wang ⇑, Xuhui Xu, Yanqin Li, Shuangyu Xin, Liurong Shi Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 6 April 2011 Received in revised form 2 June 2011 Accepted 14 June 2011 Available online 18 July 2011 Keywords: Persistent energy transfer Thermoluminescence Long-lasting phosphor
a b s t r a c t Eu2+, Mn2+ and Dy3+ co-doped long-lasting phosphors Sr3MgSi2O8 were prepared by a solid-state reaction under a reductive atmosphere. Fluorescence spectra demonstrated that the weak red emission resulting from the forbidden transition of Mn2+ could be enhanced by the energy transfer from Eu2+ to Mn2+. The energy transfer between Eu2+ and Mn2+ was systematically investigated. The phosphorescence spectra revealed that Eu2+ could persistently transfer its energy to Mn2+ after removing the excitation source. The duration of Mn2+ can prolong to more than 2 h. The thermoluminescence spectra were used to characterize the ability of the trap to trapping the carriers. By the analysis of the ionization potentials, the roles of Mn2+ and Dy3+ in the afterglow process were discussed. A possible afterglow mechanism was presented and discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Long-lasting phosphorescence (LLP) materials can be widely used in areas such as safety indication and emergency lighting. Many studies have been done on the synthesis technique, longlasting phosphorescence properties, and mechanism of various rare-earth-doped crystals and glasses [1,2]. Up to now, the colors of the commercial LLPs are limited to blue (CaAl2O4:Eu2+, Nd3+) and yellow–green (SrAl2O4:Eu2+, Dy3+), there is still needed for the red-light LLPs for the tricolor. Although some sulfides and oxysulfides exhibit good red phosphorescence, their poor chemical stabilities are undesirable for practical application. Thereby, the achievement of oxide red LLPs is a challenging goal. Recently, many works indicate that the afterglow color can be specially designed according to the requirement of applications on the basis of the concept persistent energy-transfer (PET) proposed by Yen et al. [3]. Based on the PET of Ce3+ to Tb3+, Ce3+ to Mn2+, and Eu2+ to Cr3+ some persistent phosphors with green, yellow, and red emissions have been reported in aluminates [4–6]. Owing to the forbidden transition (4T1–6A1) required by the selection rule [7], the Mn2+ singly doped phosphor generally cannot show considerable red emission. However, Eu2+ can be used as an efficient sensitizer that transfers energy to Mn2+ in several host lattices. Wang et al. once reported the red color LLP materials through PET of Eu2+ to Mn2+ in MgSiO3:Eu2+, Mn2+, Dy3+ [8], and BaMg2Si2O7:Eu2+, Mn2+, Dy3+ [9].
New applications for the LLPs have been presented lately, such as radiation detection [10] as well as sensors for structural damage, fracture of materials [11], and temperature [12], which requires the host materials of LLPs more steady than the aluminates host. For the third generation LLPs, the alkaline silicate phosphors have more advantages on chemical stability, heat stability, and excellent weather resistance compared with sulfides and aluminates phosphors [13,14]. M3MgSi2O8 (M = Ca, Sr or Ba) phosphors have been reported as early as 1957 [15]. In recent years, the M3MgSi2O8 compounds have attracted much attention as a promising host material for Eu2+-doped blue phosphor. Jung and Seo carried out VUV excitation studies on (Ba, Sr)3MgSi2O8:Eu2+ phosphors for plasma display panel (PDP) [16]. Kim et al. reported the energy transfer from Eu2+ to Mn2+ in Sr3MgSi2O8 for solid-state lighting applications [17,18]. In this work, a novel LLP material Sr3MgSi2O8:Eu2+, Mn2+, Dy3+ was synthesized by a solid state reaction. The PET from Eu2+ to Mn2+ was systematically investigated and the effects of PET for the red luminescence and phosphorescence of Mn2+ were investigated. Through the analysis of the thermoluminescence (TL) glow curves and the ionization potentials (IPs), the role of Dy3+ and Mn2+ in the afterglow process has been brought forward and a possible afterglow mechanism was discussed. 2. Experimental section 2.1. Materials and synthesis
⇑ Corresponding author. Tel.: +86 931 8912772; fax: +86 931 8913554. E-mail address: [email protected] (Y. Wang). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.06.015
A high-temperature solid-state synthesis procedure was employed to synthesize the powder sample Sr2.99xMg1ySi2O8:xEu2+,
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yMn2+, 0.01Dy3+. Stoichiometric amounts of starting materials SrCO3 (analytical reagent), Mg(NO3)26H2O (analytical reagent), H2SiO3 (analytical reagent), Eu2O3 (99.99%), Dy2O3 (99.99%), and Mn(CH3COO)26H2O (99.9%) with the desired chemical formula and doped concentration was mixed in an agate mortar with a small amount of H3BO3 (99.9%) as flux, thoroughly ground, and calcined at 900 °C for 2 h. Subsequently, the calcined materials were re-ground in the mortar and re-fired at 1300 °C for 4 h in the reducing atmosphere (the mixture of 5% H2 and 95% N2) to synthesize the phosphor.
lamp) as the light source. The decay curves were performed by a PR305 Phosphorophotometer after the samples were irradiated by UV light (365 nm) for 15 min. TL curves were measured on a FJ-427A TL meter (Beijing Nuclear Instrument Factory). The sample weight was kept constant (5 mg). Prior to the TL measurements, powder samples were first exposed by 354 nm. Different delay times (30 s, 30 min, 60 min and 90 min) were employed after the end of irradiation. The actual TL measurement then heated from room temperature to 400 °C with the rate of 1 °C/s. All the measurements were performed at room temperature except TL spectra.
2.2. Characterization
3. Results
The phases of as-prepared phosphor samples were identified by powder X-ray diffraction (XRD) analysis (Rigaku D/max-2400/pc with Ni-filter Cu Ka radiation). The photoluminescence (PL) spectra and photoluminescence excitation spectra (PLE) were obtained using FLS920T spectrofluorometer with Xe 900 (450 W xenon arc
3.1. X-ray diffraction analyses The XRD patterns for the series samples are performed to verify the phase purity and the results are shown in Fig. 1b. All the observed peaks can be indexed to the pure orthorhombic phase of Sr3MgSi2O8 (space group of P21/a) [19] and match well with JCPDS card 10-0075 (Fig. 1a) in the doping concentration ranges investigated in this study. This observation indicates that a series of samples used in this study are chemical and structural Sr3MgSi2O8. As shown in Fig. 1b, the XRD patterns move to high angle with the increase of the Mn2+ concentration, which means Mn2+ have already doped into the lattice. 3.2. Luminescent properties of Sr3MgSi2O8:Eu2+, Dy3+ and Sr3MgSi2O8: Mn2+, Dy3+
Fig. 1. The XRD patterns of (a) Sr3MgSi2O8 (JCPDS File No. 10-0075) and (b) Sr2.99xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+ (x = 0, 0.005; 0 6 y 6 0.2).
The normalized excitation and emission spectra of Sr3MgSi2O8:Eu2+, Dy3+ and Sr3MgSi2O8:Mn2+, Dy3+ are displayed in Fig. 2. The excitation spectrum shows a strong absorption band in the UV region which is corresponding to the transition from 8S7/2 to t2g and eg of Eu2+ as exhibited in Fig. 2a. Sr3MgSi2O8:Eu2+, Dy3+ reveals an emission band centered at 457 nm which ascribes to the transition from t2g to 8S7/2 [20]. Fig. 2b represents the normalized PL and PLE spectra for Mn2+, Dy3+ co-activated Sr3MgSi2O8. The excitation spectrum of Mn2+ consists of several bands owing to the transitions from 6A1(6S) to 4E(4D), 4T2(4D), [4A1(4G),4E(4G)], 4T2(4G) and 4T1(4G) [21]. The weak red emission
Fig. 2. The normalized excitation (dash lines) and emission (solid lines) spectra of (a) Sr2.985MgSi2O8:0.005Eu2+, 0.01Dy3+; and (b) Sr2.99Mg0.95Si2O8:0.05Mn2+, 0.01Dy3+.
Y. Gong et al. / Optical Materials 33 (2011) 1781–1785
Fig. 3. The emission spectra of Sr3xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+ phosphors with (a) x = 0.005, y = 0; (b) x = 0, y = 0.05; (c) x = 0.005, y = 0.05; (d) x = 0.005, y = 0.10; (e) x = 0.005, y = 0.15; and (f) x = 0.005, y = 0.20 excited by 354 nm. The inset shows the excitation spectrum of Sr2.985MgSi2O8:0.005Eu2+, 0.05Mn2+ monitored at 670 nm.
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Fig. 4. The decay curves of Sr3xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+ phosphors with (a) x = 0.005, y = 0; (b) x = 0.005, y = 0.05; (c) x = 0.005, y = 0.10; (d) x = 0.005, y = 0.15; (e) x = 0.005, y = 0.20; and (f) x = 0, y = 0.05. The inset shows the relationship between the initial intensity of the phosphorescence and the concentration of Mn2+ after shut off the lamp-house for 10 s.
is due to the spin-forbidden 4T1(4G) ? 6A1(6S) transition of Mn2+. As the d-d transition of Mn2+ is forbidden in spin and parity, so its excitation transition is difficult to pump and the intensity of emission is very weak. 3.3. The energy transfer from Eu2+ to Mn2+ in Sr3MgSi2O8:Eu2+, Mn2+, Dy3+ Fig. 3 exhibits the emission spectra of Sr3MgSi2O8:Eu2+, Mn2+, Dy phosphors with different doping concentrations, which are excited at 354 nm, corresponding to the optimal excitation wavelength of the energy donor Eu2+. The emission bands which belong to Eu2+ and Mn2+ are found. Interestingly, the emission intensity of Mn2+ increases as the concentration of Mn2+ increases while the emission intensity of Eu2+ decreases. And when the sample is monitored by 670 nm (in the inset of Fig. 3) which is the characteristic emission of Mn2+, the excitation bands ascribed to both Eu2+ and Mn2+ are found, indicating that Mn2+ is essentially excited through Eu2+. These results show that efficient energy transfer from Eu2+ to Mn2+ happens. Furthermore, a significant spectral overlap between the emission band of Eu2+ (Fig. 2a) and the excitation peak of Mn2+ (Fig. 2b) has been found. According to the Dexter’s theory [22], the mechanism of energy transfer (ET) basically requires a spectral overlap between the donor emission and the acceptor excitation. The result also proves that Eu2+ may transfer the energy to Mn2+. 3+
3.4. The persistent energy transfer from Eu2+ to Mn2+ in Sr3MgSi2O8: Eu2+, Mn2+, Dy3+ Fig. 4 presents the decay curves of Sr3xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+. Though the duration of the phosphor decreases as the concentration of Mn2+ increases, the initial intensity of the phosphorescence increases nearly by two times when y = 0.10 as shown in the inset of Fig. 4. In order to find the effect of Eu2+ and Mn2+ in the phosphorescence process, the phosphorescence spectra of Sr2.985Mg0.90Si2O8: 0.005Eu2+, 0.10Mn2+, 0.01Dy3+ with different delay time after shutting the lamp-house were measured (Fig. 5). The phosphorescence spectra are the same as the fluorescence spectra in Fig. 3, which means both the Eu2+ and Mn2+ play the role of emission center in the phosphorescence process. As the delay time increases from
Fig. 5. The phosphorescence spectra of Sr2.985Mg0.90Si2O8:0.005Eu2+, 0.10Mn2+, 0.01Dy3+ after shut off the lamp-house for (a) 5 min; (b) 10 min; (c) 30 min; and (d) 1 h; the inset shows the relative phosphorescence intensity of IMn2+/IEu2+ after shut off the lamp-house for different times.
5 min to 1 h, both the phosphorescence intensity of Eu2+ and Mn2+ decreases, however, the decay rate of the blue (Eu2+) band is much faster than the red (Mn2+) band as shown in the inset of Fig. 5. For Mn2+ single doped sample, the decay time is only 20 min, however, for the Eu2+ and Mn2+ co-doped sample the decay time prolongs to more than 2 h, as shown in Fig. 4. The above description indicates that a persistent energy transfer from Eu2+ to Mn2+ happens during the phosphorescence process. As described in Figs 4 and 5, through persistent energy transfer from Eu2+ to Mn2+, both the decay time and the phosphorescence intensity of Mn2+ largely increase, which means this kind of phosphor has a great potential to be a promising candidate for red light long-lasting phosphor. 3.5. The thermoluminescence of the sample The extraordinarily long afterglow could be attributed to the energy exchange processes between traps or traps and emission
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Fig. 6. Thermoluminescence glow curves of Sr2.985Mg1xSi2O8:0.005Eu2+, xMn2+, 0.01Dy3+ (a) x = 0; (b) x = 0.05; (c) x = 0.10; (d) x = 0.15; and (e) x = 0.20.
centers. In most cases, the information regarding to the trap and the trapping level can be obtained by TL curve analysis. The intensity of the TL glow curve is related to the amount of trapped charge carriers, and the maximum temperature of the TL peak provides information on the depth of the charge carrier trap. The shallow traps have contribution for the initial intensity of the afterglow, and the deep traps make contribution for the duration [23,24]. As describe in Fig. 6, the TL glow curves show concentration dependence. For Eu2+ and Mn2+ co-doped samples, the TL intensity of the high temperature tail (ca. 200 °C) is decreased and finally disappeared completely as the content of Mn2+ increases, which indicates the concentration of the deep trap decreased by co-doped of
Mn2+ and this is deleterious for the lengthening of the persistent luminescence. However, the TL intensity of the low temperature tail (ca. 50 °C) increased first and then decreased, as the content of Mn2+ increases, which suggests that the concentration of the shallow traps can be increased when the contents of Mn2+ are in a suitable range, and this is favorable for the initial intensity (curve b and c). The fading of thermoluminescence is partly due to the release of the trapped carriers by the thermal energy. Important information on the trap structure in the material can be achieved through measuring the TL glow curves with different delay time after ceasing the UV irradiation. As shown in Fig. 7a and b, the intensity of the TL bands decreases as the delay time increases. However, the fading rate of the low temperature tail is much quicker than that of the high temperature tail which caused by the release of the carriers from the shallow trap is much easier than the deep trap. The position of the TL glow peak at low temperature tail seems to move slightly to higher temperature with longer delay time, which means there are at least two traps at ca. 50 °C. On the other hand, comparing with Fig. 7a and b, no TL peak is found at high temperature scope in Fig. 7c and d. And the TL peaks at low temperature tail are faded completely in Fig. 7c and d when the delay time is 90 and 30 min respectively. It is the reason why the duration of Sr2.985Mg0.85Si2O8:0.005Eu2+, 0.15Mn2+, 0.01Dy3+ and 2+ 2+ 3+ Sr2.985Mg0.80Si2O8:0.005Eu , 0.20Mn , 0.01Dy is much shorter than that of the others (Fig. 4). 3.6. The afterglow mechanism of the sample We now consider the mechanism of the long-persistence phosphorescence in these crystals. Oxygen vacancy (V o€ ) usually appears in the oxide host when a sample is sintered in reducing
Fig. 7. Comparison of the thermoluminescence glow curves of Sr2.985Mg1xSi2O8:0.005Eu2+, xMn2+, 0.01Dy3+ for (a) x = 0.05; (b) x = 0.10; (c) x = 0.15; and (d) x = 0.20 with different delay times after ceasing the UV irradiation.
Y. Gong et al. / Optical Materials 33 (2011) 1781–1785
atmosphere [25]. And the electron freed from Eu2+ upon UV radiation may be easily trapped by the nearby V o€ to form V o_ (or V xo after trapping of the second electron), while the hole left behind may become intercepted in the vicinity of the activator forming the [Eu2+h]. The electron can migrate from one trap to another with the aid of acquiring (or releasing) thermal energy, which will prolong the duration of the phosphor. Finally, the electron that trapped by the V o_ (or V xo ) can be freed to [Eu2+-h] by thermal energy leading the long- persistence phosphorescence. Of course, the hole created to [Eu2+-h] can be trapped by hole traps, too. The possible hole traps are the Dy3+ ions and VSr. Alvani et al. [26] and Lin et al. [27] proposed that Dy3+ acts as the hole trap and dominates the long-persistence properties of Sr3MgSi2O8 and Ca3MgSi2O8. Nevertheless, the moving of holes is even harder than that of electrons [28,29] and the Dy4+ is not steadily subsist in the phosphor [30], we suggest that only a few hole traps nearby the [Eu2+-h] can trap the holes, thereby, the dominate reason for the long-persistence phosphorescence is not the hole trap but the electron trap (V o€ ). It is common to assume that point defects in solids are randomly distributed. However, defect migration may be significant during synthesis at high temperature and cannot be neglected even at room temperature if the material is co-doped with other ions. According to the Whangbo et al.0 s point [2], the Ln3+ (rare-earth ions) co-dopants whose ionization potentials (IPs) are lower than that of the host alkaline-earth cation could be capable to improve the phosphorescence of the alkaline-earth aluminates, silicates, or aluminosilicates doped with Eu2+, as the ability to attract and stabilize anion vacancies would be enhanced by Ln3+ cations. The IPs of Dy3+ is 41.5 eV which is lower than that of Sr2+ (43.71 eV) [31]. It is suggested that Dy3+ in our system can pull the V o€ to Eu2+, and it is more comfortable for the V o€ to capture the electron freed from Eu2+. Thus, in the Sr3MgSi2O8:Eu2+, Mn2+, Dy3+, Dy3+ plays a role of changing the trap number and the trap depth of the V o€ , which results in the prolong of the duration. The transition-metal-ion luminescent center Mn2+ (3d5) can be ionized into Mn3+ easily [2]. After removing the light source, the 3d orbital of Mn3+ is spatially extended enough to favor a direct radiative transition for the electron which trapped by V o_ (or V xo ) to their empty 3d levels, inducing to a red long-persistence phosphorescence, which may debase the capability of V o€ to put aside the electron. The above process may lead to an increase of the shallow trap concentration and a decrease of the deep trap concentration as shown in Fig. 7b and c. However, when the doping concentration of Mn2+ is high (Fig. 7d and e), the distance between Eu2+ and Mn2+ becomes shorter, the electron freed from Eu2+ (or Mn2+) upon UV photons may not be trapped by Vo but Mn3+ (or [Eu2+-h]) and then releases through a direct radiative transition, which may decrease the concentration of the trap (Fig. 7d and e). Meanwhile, the energy will be released during the persistent energy transfer from Eu2+ to Mn2+ by non-radiation transition, which may decrease the afterglow properties too. Further investigations are necessary to identify the traps in the phosphor and to clarify the mechanism of the long-lasting phosphorescence in the polycrystal. 4. Conclusions Novel long-lasting phosphor with highly chemical stabilities is achieved successfully in the alkaline silicate. The energy transfers
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from Eu2+ to Mn2+. After shutting the lamp-house, the decay rate of the emission band of Eu2+ is much higher than that of Mn2+, which implies Eu2+ persistently transfers the energy to Mn2+. The electron trap plays a dominant role for the long-persistence phosphorescence. Mn2+ increases the shallow traps and decreases the deep traps in the sample leading to the initial intensity of the phosphorescence increasing and the duration of the phosphor decreasing. Through co-doping Eu2+ and Mn2+, the duration prolongs for more than 2 h compared with Mn2+ single doped sample. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 10874061), the National Science Foundation for Distinguished Young Scholars (No. 50925206), and the Research Fund for the Doctoral Program of Higher Education (No. 200807300010). References [1] J. Qiu, K. Miura, H. Inouye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 73 (1998) 1763–1765. [2] F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, M.-H. Whangbo, Chem. Mater. 18 (2006) 3212–3220. [3] W.M. Yen, D. Jia, W. Jia, X.J. Wang, US Patent No. 0164277 A1, 2004. [4] D. Jia, R.S. Meltzer, W.M. Yen, W. Jia, X. Wang, Appl. Phys. Lett. 80 (2002) 1535– 1537. [5] D. Jia, X.J. Wang, W. Jia, W.M. Yen, J. Appl. Phys. 93 (2003) 48. [6] R.X. Zhong, J.H. Zhang, X. Zhang, S.Z. Lu, X.J. Wang, Appl. Phys. Lett. 88 (2006) 201916. [7] W.J. Yang, T.M. Chen, Appl. Phys. Lett. 88 (2006) 101903. [8] X.J. Wang, D.D. Jia, W.M. Yen, J. Lumin. 34 (2003) 102–103. [9] S. Ye, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, X.J. Wang, J. Appl. Phys. 101 (2007) 063545. [10] M. Kowatari, D. Koyama, Y. Satoh, K. Iinuma, S. Uchida, Methods Phys. Res. A 480 (2002) 431–439. [11] C.-N. Xu, T. Watanabe, M. Akiyama, X.-G. Zheng, Appl. Phys. Lett. 74 (1999) 2414–2416. [12] H. Aizawa, T. Katsumata, J. Takahashi, K. Matsunaga, S. Komuro, T. Morikawa, E. Toba, Electrochem. Solid-State Lett. 5 (2002) H17–H19. [13] T.L. Barry, J. Electrochem. Soc. 115 (1968) 1181. [14] Z.G. Xiao, Z.Q. Xiao, Chinese Pat. 98105078.6, 1998. [15] H.A. Klasens, A.H. Hoekstra, A.P.M. Cox, J. Electrochem. Soc. 104 (1957) 93. [16] Ha-Kyun Jung, Kyung Soo Seo, Opt. Mater. 28 (2006) 602. [17] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, Appl. Phys. Lett. 84 (2004) 2931–2933. [18] L. Ma, D. Wang, Z. Mao, Q. Liu, Appl. Phys. Lett. 93 (2008) 144101. [19] T. Aitasalo, A. Hietikko, J. Hölsä, M. Lastusaari, J. Niittykoski, T. Piispanen, Z. Kristallogr, Suppl. 26 (2007) 461–466. [20] Ki Hyuk Kwon, Won Bin Im, Ho Seong Jang, Hyoung Sun Yoo, Young Jeon Duk, Inorganic Chem. 48 (2009) 11525–11532. [21] L. Wang, X. Liu, Z. Hou, C. Li, P. Yang, J. Lin, J. Phys. Chem. C 112 (48) (2008) 18882–18888. [22] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [23] P. Dorenbos, A.J.J. Bos, Radiat. Measure. 43 (2008) 39–145. [24] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon, Oxford, 1981. pp. 159–165. [25] J. Qiu, K. Hirao, Solid State Commun. 106 (1998) 795. [26] A.A. Sabbagh Alvani, M.F. oztarzadehb, A.A. Sarabi, J. Lumin. 114 (2005) 131– 136. [27] Y. Lin, Z. Zhang, Z. Tang, X. Wang, Z. Zheng, J. Eur. Ceram. Soc. 21 (2001) 683. [28] H. Hosono, T. Kinoshita, H. Kawazoe, M. Yamazaki, Y. Yamamoto, N. Sawanobori, J. Phys.: Condens. Matter 10 (1998) 9541. [29] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [30] G. Meijer, J. Electrochem. Soc. (28 March) (1958). [31] J. Emsley, The Elements, Clarendon Press, Oxford, 1991. pp. 184.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
The persistent energy transfer of Eu2+ and Mn2+ and the thermoluminescence properties of long-lasting phosphor Sr3MgSi2O8:Eu2+, Mn2+, Dy3+ Yu Gong, Yuhua Wang ⇑, Xuhui Xu, Yanqin Li, Shuangyu Xin, Liurong Shi Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 6 April 2011 Received in revised form 2 June 2011 Accepted 14 June 2011 Available online 18 July 2011 Keywords: Persistent energy transfer Thermoluminescence Long-lasting phosphor
a b s t r a c t Eu2+, Mn2+ and Dy3+ co-doped long-lasting phosphors Sr3MgSi2O8 were prepared by a solid-state reaction under a reductive atmosphere. Fluorescence spectra demonstrated that the weak red emission resulting from the forbidden transition of Mn2+ could be enhanced by the energy transfer from Eu2+ to Mn2+. The energy transfer between Eu2+ and Mn2+ was systematically investigated. The phosphorescence spectra revealed that Eu2+ could persistently transfer its energy to Mn2+ after removing the excitation source. The duration of Mn2+ can prolong to more than 2 h. The thermoluminescence spectra were used to characterize the ability of the trap to trapping the carriers. By the analysis of the ionization potentials, the roles of Mn2+ and Dy3+ in the afterglow process were discussed. A possible afterglow mechanism was presented and discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Long-lasting phosphorescence (LLP) materials can be widely used in areas such as safety indication and emergency lighting. Many studies have been done on the synthesis technique, longlasting phosphorescence properties, and mechanism of various rare-earth-doped crystals and glasses [1,2]. Up to now, the colors of the commercial LLPs are limited to blue (CaAl2O4:Eu2+, Nd3+) and yellow–green (SrAl2O4:Eu2+, Dy3+), there is still needed for the red-light LLPs for the tricolor. Although some sulfides and oxysulfides exhibit good red phosphorescence, their poor chemical stabilities are undesirable for practical application. Thereby, the achievement of oxide red LLPs is a challenging goal. Recently, many works indicate that the afterglow color can be specially designed according to the requirement of applications on the basis of the concept persistent energy-transfer (PET) proposed by Yen et al. [3]. Based on the PET of Ce3+ to Tb3+, Ce3+ to Mn2+, and Eu2+ to Cr3+ some persistent phosphors with green, yellow, and red emissions have been reported in aluminates [4–6]. Owing to the forbidden transition (4T1–6A1) required by the selection rule [7], the Mn2+ singly doped phosphor generally cannot show considerable red emission. However, Eu2+ can be used as an efficient sensitizer that transfers energy to Mn2+ in several host lattices. Wang et al. once reported the red color LLP materials through PET of Eu2+ to Mn2+ in MgSiO3:Eu2+, Mn2+, Dy3+ [8], and BaMg2Si2O7:Eu2+, Mn2+, Dy3+ [9].
New applications for the LLPs have been presented lately, such as radiation detection [10] as well as sensors for structural damage, fracture of materials [11], and temperature [12], which requires the host materials of LLPs more steady than the aluminates host. For the third generation LLPs, the alkaline silicate phosphors have more advantages on chemical stability, heat stability, and excellent weather resistance compared with sulfides and aluminates phosphors [13,14]. M3MgSi2O8 (M = Ca, Sr or Ba) phosphors have been reported as early as 1957 [15]. In recent years, the M3MgSi2O8 compounds have attracted much attention as a promising host material for Eu2+-doped blue phosphor. Jung and Seo carried out VUV excitation studies on (Ba, Sr)3MgSi2O8:Eu2+ phosphors for plasma display panel (PDP) [16]. Kim et al. reported the energy transfer from Eu2+ to Mn2+ in Sr3MgSi2O8 for solid-state lighting applications [17,18]. In this work, a novel LLP material Sr3MgSi2O8:Eu2+, Mn2+, Dy3+ was synthesized by a solid state reaction. The PET from Eu2+ to Mn2+ was systematically investigated and the effects of PET for the red luminescence and phosphorescence of Mn2+ were investigated. Through the analysis of the thermoluminescence (TL) glow curves and the ionization potentials (IPs), the role of Dy3+ and Mn2+ in the afterglow process has been brought forward and a possible afterglow mechanism was discussed. 2. Experimental section 2.1. Materials and synthesis
⇑ Corresponding author. Tel.: +86 931 8912772; fax: +86 931 8913554. E-mail address: [email protected] (Y. Wang). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.06.015
A high-temperature solid-state synthesis procedure was employed to synthesize the powder sample Sr2.99xMg1ySi2O8:xEu2+,
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yMn2+, 0.01Dy3+. Stoichiometric amounts of starting materials SrCO3 (analytical reagent), Mg(NO3)26H2O (analytical reagent), H2SiO3 (analytical reagent), Eu2O3 (99.99%), Dy2O3 (99.99%), and Mn(CH3COO)26H2O (99.9%) with the desired chemical formula and doped concentration was mixed in an agate mortar with a small amount of H3BO3 (99.9%) as flux, thoroughly ground, and calcined at 900 °C for 2 h. Subsequently, the calcined materials were re-ground in the mortar and re-fired at 1300 °C for 4 h in the reducing atmosphere (the mixture of 5% H2 and 95% N2) to synthesize the phosphor.
lamp) as the light source. The decay curves were performed by a PR305 Phosphorophotometer after the samples were irradiated by UV light (365 nm) for 15 min. TL curves were measured on a FJ-427A TL meter (Beijing Nuclear Instrument Factory). The sample weight was kept constant (5 mg). Prior to the TL measurements, powder samples were first exposed by 354 nm. Different delay times (30 s, 30 min, 60 min and 90 min) were employed after the end of irradiation. The actual TL measurement then heated from room temperature to 400 °C with the rate of 1 °C/s. All the measurements were performed at room temperature except TL spectra.
2.2. Characterization
3. Results
The phases of as-prepared phosphor samples were identified by powder X-ray diffraction (XRD) analysis (Rigaku D/max-2400/pc with Ni-filter Cu Ka radiation). The photoluminescence (PL) spectra and photoluminescence excitation spectra (PLE) were obtained using FLS920T spectrofluorometer with Xe 900 (450 W xenon arc
3.1. X-ray diffraction analyses The XRD patterns for the series samples are performed to verify the phase purity and the results are shown in Fig. 1b. All the observed peaks can be indexed to the pure orthorhombic phase of Sr3MgSi2O8 (space group of P21/a) [19] and match well with JCPDS card 10-0075 (Fig. 1a) in the doping concentration ranges investigated in this study. This observation indicates that a series of samples used in this study are chemical and structural Sr3MgSi2O8. As shown in Fig. 1b, the XRD patterns move to high angle with the increase of the Mn2+ concentration, which means Mn2+ have already doped into the lattice. 3.2. Luminescent properties of Sr3MgSi2O8:Eu2+, Dy3+ and Sr3MgSi2O8: Mn2+, Dy3+
Fig. 1. The XRD patterns of (a) Sr3MgSi2O8 (JCPDS File No. 10-0075) and (b) Sr2.99xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+ (x = 0, 0.005; 0 6 y 6 0.2).
The normalized excitation and emission spectra of Sr3MgSi2O8:Eu2+, Dy3+ and Sr3MgSi2O8:Mn2+, Dy3+ are displayed in Fig. 2. The excitation spectrum shows a strong absorption band in the UV region which is corresponding to the transition from 8S7/2 to t2g and eg of Eu2+ as exhibited in Fig. 2a. Sr3MgSi2O8:Eu2+, Dy3+ reveals an emission band centered at 457 nm which ascribes to the transition from t2g to 8S7/2 [20]. Fig. 2b represents the normalized PL and PLE spectra for Mn2+, Dy3+ co-activated Sr3MgSi2O8. The excitation spectrum of Mn2+ consists of several bands owing to the transitions from 6A1(6S) to 4E(4D), 4T2(4D), [4A1(4G),4E(4G)], 4T2(4G) and 4T1(4G) [21]. The weak red emission
Fig. 2. The normalized excitation (dash lines) and emission (solid lines) spectra of (a) Sr2.985MgSi2O8:0.005Eu2+, 0.01Dy3+; and (b) Sr2.99Mg0.95Si2O8:0.05Mn2+, 0.01Dy3+.
Y. Gong et al. / Optical Materials 33 (2011) 1781–1785
Fig. 3. The emission spectra of Sr3xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+ phosphors with (a) x = 0.005, y = 0; (b) x = 0, y = 0.05; (c) x = 0.005, y = 0.05; (d) x = 0.005, y = 0.10; (e) x = 0.005, y = 0.15; and (f) x = 0.005, y = 0.20 excited by 354 nm. The inset shows the excitation spectrum of Sr2.985MgSi2O8:0.005Eu2+, 0.05Mn2+ monitored at 670 nm.
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Fig. 4. The decay curves of Sr3xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+ phosphors with (a) x = 0.005, y = 0; (b) x = 0.005, y = 0.05; (c) x = 0.005, y = 0.10; (d) x = 0.005, y = 0.15; (e) x = 0.005, y = 0.20; and (f) x = 0, y = 0.05. The inset shows the relationship between the initial intensity of the phosphorescence and the concentration of Mn2+ after shut off the lamp-house for 10 s.
is due to the spin-forbidden 4T1(4G) ? 6A1(6S) transition of Mn2+. As the d-d transition of Mn2+ is forbidden in spin and parity, so its excitation transition is difficult to pump and the intensity of emission is very weak. 3.3. The energy transfer from Eu2+ to Mn2+ in Sr3MgSi2O8:Eu2+, Mn2+, Dy3+ Fig. 3 exhibits the emission spectra of Sr3MgSi2O8:Eu2+, Mn2+, Dy phosphors with different doping concentrations, which are excited at 354 nm, corresponding to the optimal excitation wavelength of the energy donor Eu2+. The emission bands which belong to Eu2+ and Mn2+ are found. Interestingly, the emission intensity of Mn2+ increases as the concentration of Mn2+ increases while the emission intensity of Eu2+ decreases. And when the sample is monitored by 670 nm (in the inset of Fig. 3) which is the characteristic emission of Mn2+, the excitation bands ascribed to both Eu2+ and Mn2+ are found, indicating that Mn2+ is essentially excited through Eu2+. These results show that efficient energy transfer from Eu2+ to Mn2+ happens. Furthermore, a significant spectral overlap between the emission band of Eu2+ (Fig. 2a) and the excitation peak of Mn2+ (Fig. 2b) has been found. According to the Dexter’s theory [22], the mechanism of energy transfer (ET) basically requires a spectral overlap between the donor emission and the acceptor excitation. The result also proves that Eu2+ may transfer the energy to Mn2+. 3+
3.4. The persistent energy transfer from Eu2+ to Mn2+ in Sr3MgSi2O8: Eu2+, Mn2+, Dy3+ Fig. 4 presents the decay curves of Sr3xMg1ySi2O8:xEu2+, yMn2+, 0.01Dy3+. Though the duration of the phosphor decreases as the concentration of Mn2+ increases, the initial intensity of the phosphorescence increases nearly by two times when y = 0.10 as shown in the inset of Fig. 4. In order to find the effect of Eu2+ and Mn2+ in the phosphorescence process, the phosphorescence spectra of Sr2.985Mg0.90Si2O8: 0.005Eu2+, 0.10Mn2+, 0.01Dy3+ with different delay time after shutting the lamp-house were measured (Fig. 5). The phosphorescence spectra are the same as the fluorescence spectra in Fig. 3, which means both the Eu2+ and Mn2+ play the role of emission center in the phosphorescence process. As the delay time increases from
Fig. 5. The phosphorescence spectra of Sr2.985Mg0.90Si2O8:0.005Eu2+, 0.10Mn2+, 0.01Dy3+ after shut off the lamp-house for (a) 5 min; (b) 10 min; (c) 30 min; and (d) 1 h; the inset shows the relative phosphorescence intensity of IMn2+/IEu2+ after shut off the lamp-house for different times.
5 min to 1 h, both the phosphorescence intensity of Eu2+ and Mn2+ decreases, however, the decay rate of the blue (Eu2+) band is much faster than the red (Mn2+) band as shown in the inset of Fig. 5. For Mn2+ single doped sample, the decay time is only 20 min, however, for the Eu2+ and Mn2+ co-doped sample the decay time prolongs to more than 2 h, as shown in Fig. 4. The above description indicates that a persistent energy transfer from Eu2+ to Mn2+ happens during the phosphorescence process. As described in Figs 4 and 5, through persistent energy transfer from Eu2+ to Mn2+, both the decay time and the phosphorescence intensity of Mn2+ largely increase, which means this kind of phosphor has a great potential to be a promising candidate for red light long-lasting phosphor. 3.5. The thermoluminescence of the sample The extraordinarily long afterglow could be attributed to the energy exchange processes between traps or traps and emission
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Fig. 6. Thermoluminescence glow curves of Sr2.985Mg1xSi2O8:0.005Eu2+, xMn2+, 0.01Dy3+ (a) x = 0; (b) x = 0.05; (c) x = 0.10; (d) x = 0.15; and (e) x = 0.20.
centers. In most cases, the information regarding to the trap and the trapping level can be obtained by TL curve analysis. The intensity of the TL glow curve is related to the amount of trapped charge carriers, and the maximum temperature of the TL peak provides information on the depth of the charge carrier trap. The shallow traps have contribution for the initial intensity of the afterglow, and the deep traps make contribution for the duration [23,24]. As describe in Fig. 6, the TL glow curves show concentration dependence. For Eu2+ and Mn2+ co-doped samples, the TL intensity of the high temperature tail (ca. 200 °C) is decreased and finally disappeared completely as the content of Mn2+ increases, which indicates the concentration of the deep trap decreased by co-doped of
Mn2+ and this is deleterious for the lengthening of the persistent luminescence. However, the TL intensity of the low temperature tail (ca. 50 °C) increased first and then decreased, as the content of Mn2+ increases, which suggests that the concentration of the shallow traps can be increased when the contents of Mn2+ are in a suitable range, and this is favorable for the initial intensity (curve b and c). The fading of thermoluminescence is partly due to the release of the trapped carriers by the thermal energy. Important information on the trap structure in the material can be achieved through measuring the TL glow curves with different delay time after ceasing the UV irradiation. As shown in Fig. 7a and b, the intensity of the TL bands decreases as the delay time increases. However, the fading rate of the low temperature tail is much quicker than that of the high temperature tail which caused by the release of the carriers from the shallow trap is much easier than the deep trap. The position of the TL glow peak at low temperature tail seems to move slightly to higher temperature with longer delay time, which means there are at least two traps at ca. 50 °C. On the other hand, comparing with Fig. 7a and b, no TL peak is found at high temperature scope in Fig. 7c and d. And the TL peaks at low temperature tail are faded completely in Fig. 7c and d when the delay time is 90 and 30 min respectively. It is the reason why the duration of Sr2.985Mg0.85Si2O8:0.005Eu2+, 0.15Mn2+, 0.01Dy3+ and 2+ 2+ 3+ Sr2.985Mg0.80Si2O8:0.005Eu , 0.20Mn , 0.01Dy is much shorter than that of the others (Fig. 4). 3.6. The afterglow mechanism of the sample We now consider the mechanism of the long-persistence phosphorescence in these crystals. Oxygen vacancy (V o€ ) usually appears in the oxide host when a sample is sintered in reducing
Fig. 7. Comparison of the thermoluminescence glow curves of Sr2.985Mg1xSi2O8:0.005Eu2+, xMn2+, 0.01Dy3+ for (a) x = 0.05; (b) x = 0.10; (c) x = 0.15; and (d) x = 0.20 with different delay times after ceasing the UV irradiation.
Y. Gong et al. / Optical Materials 33 (2011) 1781–1785
atmosphere [25]. And the electron freed from Eu2+ upon UV radiation may be easily trapped by the nearby V o€ to form V o_ (or V xo after trapping of the second electron), while the hole left behind may become intercepted in the vicinity of the activator forming the [Eu2+h]. The electron can migrate from one trap to another with the aid of acquiring (or releasing) thermal energy, which will prolong the duration of the phosphor. Finally, the electron that trapped by the V o_ (or V xo ) can be freed to [Eu2+-h] by thermal energy leading the long- persistence phosphorescence. Of course, the hole created to [Eu2+-h] can be trapped by hole traps, too. The possible hole traps are the Dy3+ ions and VSr. Alvani et al. [26] and Lin et al. [27] proposed that Dy3+ acts as the hole trap and dominates the long-persistence properties of Sr3MgSi2O8 and Ca3MgSi2O8. Nevertheless, the moving of holes is even harder than that of electrons [28,29] and the Dy4+ is not steadily subsist in the phosphor [30], we suggest that only a few hole traps nearby the [Eu2+-h] can trap the holes, thereby, the dominate reason for the long-persistence phosphorescence is not the hole trap but the electron trap (V o€ ). It is common to assume that point defects in solids are randomly distributed. However, defect migration may be significant during synthesis at high temperature and cannot be neglected even at room temperature if the material is co-doped with other ions. According to the Whangbo et al.0 s point [2], the Ln3+ (rare-earth ions) co-dopants whose ionization potentials (IPs) are lower than that of the host alkaline-earth cation could be capable to improve the phosphorescence of the alkaline-earth aluminates, silicates, or aluminosilicates doped with Eu2+, as the ability to attract and stabilize anion vacancies would be enhanced by Ln3+ cations. The IPs of Dy3+ is 41.5 eV which is lower than that of Sr2+ (43.71 eV) [31]. It is suggested that Dy3+ in our system can pull the V o€ to Eu2+, and it is more comfortable for the V o€ to capture the electron freed from Eu2+. Thus, in the Sr3MgSi2O8:Eu2+, Mn2+, Dy3+, Dy3+ plays a role of changing the trap number and the trap depth of the V o€ , which results in the prolong of the duration. The transition-metal-ion luminescent center Mn2+ (3d5) can be ionized into Mn3+ easily [2]. After removing the light source, the 3d orbital of Mn3+ is spatially extended enough to favor a direct radiative transition for the electron which trapped by V o_ (or V xo ) to their empty 3d levels, inducing to a red long-persistence phosphorescence, which may debase the capability of V o€ to put aside the electron. The above process may lead to an increase of the shallow trap concentration and a decrease of the deep trap concentration as shown in Fig. 7b and c. However, when the doping concentration of Mn2+ is high (Fig. 7d and e), the distance between Eu2+ and Mn2+ becomes shorter, the electron freed from Eu2+ (or Mn2+) upon UV photons may not be trapped by Vo but Mn3+ (or [Eu2+-h]) and then releases through a direct radiative transition, which may decrease the concentration of the trap (Fig. 7d and e). Meanwhile, the energy will be released during the persistent energy transfer from Eu2+ to Mn2+ by non-radiation transition, which may decrease the afterglow properties too. Further investigations are necessary to identify the traps in the phosphor and to clarify the mechanism of the long-lasting phosphorescence in the polycrystal. 4. Conclusions Novel long-lasting phosphor with highly chemical stabilities is achieved successfully in the alkaline silicate. The energy transfers
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from Eu2+ to Mn2+. After shutting the lamp-house, the decay rate of the emission band of Eu2+ is much higher than that of Mn2+, which implies Eu2+ persistently transfers the energy to Mn2+. The electron trap plays a dominant role for the long-persistence phosphorescence. Mn2+ increases the shallow traps and decreases the deep traps in the sample leading to the initial intensity of the phosphorescence increasing and the duration of the phosphor decreasing. Through co-doping Eu2+ and Mn2+, the duration prolongs for more than 2 h compared with Mn2+ single doped sample. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 10874061), the National Science Foundation for Distinguished Young Scholars (No. 50925206), and the Research Fund for the Doctoral Program of Higher Education (No. 200807300010). References [1] J. Qiu, K. Miura, H. Inouye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 73 (1998) 1763–1765. [2] F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, M.-H. Whangbo, Chem. Mater. 18 (2006) 3212–3220. [3] W.M. Yen, D. Jia, W. Jia, X.J. Wang, US Patent No. 0164277 A1, 2004. [4] D. Jia, R.S. Meltzer, W.M. Yen, W. Jia, X. Wang, Appl. Phys. Lett. 80 (2002) 1535– 1537. [5] D. Jia, X.J. Wang, W. Jia, W.M. Yen, J. Appl. Phys. 93 (2003) 48. [6] R.X. Zhong, J.H. Zhang, X. Zhang, S.Z. Lu, X.J. Wang, Appl. Phys. Lett. 88 (2006) 201916. [7] W.J. Yang, T.M. Chen, Appl. Phys. Lett. 88 (2006) 101903. [8] X.J. Wang, D.D. Jia, W.M. Yen, J. Lumin. 34 (2003) 102–103. [9] S. Ye, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, X.J. Wang, J. Appl. Phys. 101 (2007) 063545. [10] M. Kowatari, D. Koyama, Y. Satoh, K. Iinuma, S. Uchida, Methods Phys. Res. A 480 (2002) 431–439. [11] C.-N. Xu, T. Watanabe, M. Akiyama, X.-G. Zheng, Appl. Phys. Lett. 74 (1999) 2414–2416. [12] H. Aizawa, T. Katsumata, J. Takahashi, K. Matsunaga, S. Komuro, T. Morikawa, E. Toba, Electrochem. Solid-State Lett. 5 (2002) H17–H19. [13] T.L. Barry, J. Electrochem. Soc. 115 (1968) 1181. [14] Z.G. Xiao, Z.Q. Xiao, Chinese Pat. 98105078.6, 1998. [15] H.A. Klasens, A.H. Hoekstra, A.P.M. Cox, J. Electrochem. Soc. 104 (1957) 93. [16] Ha-Kyun Jung, Kyung Soo Seo, Opt. Mater. 28 (2006) 602. [17] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, Appl. Phys. Lett. 84 (2004) 2931–2933. [18] L. Ma, D. Wang, Z. Mao, Q. Liu, Appl. Phys. Lett. 93 (2008) 144101. [19] T. Aitasalo, A. Hietikko, J. Hölsä, M. Lastusaari, J. Niittykoski, T. Piispanen, Z. Kristallogr, Suppl. 26 (2007) 461–466. [20] Ki Hyuk Kwon, Won Bin Im, Ho Seong Jang, Hyoung Sun Yoo, Young Jeon Duk, Inorganic Chem. 48 (2009) 11525–11532. [21] L. Wang, X. Liu, Z. Hou, C. Li, P. Yang, J. Lin, J. Phys. Chem. C 112 (48) (2008) 18882–18888. [22] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [23] P. Dorenbos, A.J.J. Bos, Radiat. Measure. 43 (2008) 39–145. [24] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon, Oxford, 1981. pp. 159–165. [25] J. Qiu, K. Hirao, Solid State Commun. 106 (1998) 795. [26] A.A. Sabbagh Alvani, M.F. oztarzadehb, A.A. Sarabi, J. Lumin. 114 (2005) 131– 136. [27] Y. Lin, Z. Zhang, Z. Tang, X. Wang, Z. Zheng, J. Eur. Ceram. Soc. 21 (2001) 683. [28] H. Hosono, T. Kinoshita, H. Kawazoe, M. Yamazaki, Y. Yamamoto, N. Sawanobori, J. Phys.: Condens. Matter 10 (1998) 9541. [29] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [30] G. Meijer, J. Electrochem. Soc. (28 March) (1958). [31] J. Emsley, The Elements, Clarendon Press, Oxford, 1991. pp. 184.