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Tb3+ phosphors
Polyhedron 107 (2016) 78–82
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Solid state synthesis and tunable luminescence of LiSrPO4:Eu2+/Mn2+/Tb3+ phosphors Yanyan Cao a, Naidi Liu a, Jie Tian b, Xiao Zhang a,⇑ a b
College of Information Science and Engineering, Hebei North University, Zhangjiakou 075000, China Financial Department, Hebei North University, Zhangjiakou 075000, China
a r t i c l e
i n f o
Article history: Received 2 October 2015 Accepted 11 January 2016 Available online 14 January 2016 Keywords: LiSrPO4 Doping Phosphors Luminescence Energy transfer
a b s t r a c t A series of Eu2+/Mn2+/Tb3+ doped LiSrPO4 phosphors have been synthesized by the solid state reaction. The XRD results show that all phosphors have the pure hexagonal crystal structure, indicating the doping ions have no influence on the phase of LiSrPO4 host. Under the excitation at 365 nm, LiSrPO4/Eu2+ phosphors emit blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+. The Eu2+/Mn2+ or Tb3+ codoped LiSrPO4 phosphors show not only the emission of Eu2+ but also the red emission coming from the 4T2 ? 6A1g transitions of Mn2+ or green emissions induced by the 5D4 ? 7Fj transitions of Tb3+. The increasing Mn2+ or Tb3+ concentration induces the decrease of Eu2+ emission intensity, which suggests the energy from Eu2+ to Mn2+ or Tb3+ in LiSrPO4 host. For LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ phosphor, the white light can be obtained under the 365 nm excitation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction With rapid progress in material design and device fabrication, white light-emitting diode (WLED) is one of the most potential light sources for applications in solid-state lighting. And one of keys to develop WLEDs is to develop phosphors that can be excited effectively by the near ultraviolet to produce white light emitting. One of routes to generate white light is to combine multiple rare ions with red, green and blue or yellow and blue emissions, as well as through the energy transfer between them [1]. Tb3+ ion is regarded as one of promising green-emitting activators due to the 4f–4f transition [2]. The major problem for the Tb3+ ion is the lack of efficient and broad excitation band from near ultraviolet region to visible range, which limits its application in near ultraviolet white LEDs [3]. Mn2+ ion can yield a broad-band orange-red or red emission, which covers almost the whole red emission area [4]. Eu2+ ion is one of the most common activators for phosphors due to the intense and broad excitation and emission bands derived from their dipole allowed 4f–5d electronic transitions. Moreover, for Eu2+ ions, the Stokes shift is relatively small and the decay time is short, making Eu2+ as a popular activator [5]. However, Eu2+ ion can not only act as an effective activator efficiently to absorb near ultraviolet light but also act as an efficient sensitizer for Tb3+ and Mn2+ activators [6]. Therefore, it is highly expected to
⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.poly.2016.01.016 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
obtain a white-light phosphor pumped by near ultraviolet light through energy transfer from Eu2+ to Tb3+ and Mn2+. In recent years, phosphate has attracted much attention as a luminescent host material because of its excellent thermal and charge stabilization. For example, Ba3Y(PO4)3:Sm3+ [7], LiBaPO4: Eu3+ [8], Ba3Bi(PO4)3:Tb3+ [9], LaPO4:Eu3+ [10], YPO4:Eu3+ [11], Ca(3x)Srx(PO4)2:Eu2+ [12], Sr3Bi(PO4)3:Eu2+/Mn2+ [13], BiPO4:Eu3+ [14], and LiSrPO4:Dy3+ [15], have been synthesized by different methods. Among lots of phosphates, the phosphates with ABPO4 formula (A and B are mono and divalent cations, respectively) are in a large family of monophosphates with the different structure types strictly depending on the relative size of the A and B ions [16]. For example, LiSrPO4 can alternatively be viewed as containing octahedral SrO6, tetrahedral PO4, or LiO4 that is connected by an oxygen bridge which cross-link the Sr and Li atoms, to generate a three-dimensional composite framework structure [17]. LiSrPO4 also is a good luminescent host for rare earth ions. Rare ions doped LiSrPO4 phosphors emitting emissions with different colors, such as blue and yellow [15], blue [18,19], blue and green [20], red [21], have been reported. It is also found that Eu2+ and Pr3+ codoped LiSrPO4 can act as solar spectral convertor [22]. However, to the best of our knowledge, there are no reports on the obtaining of white light by codoping rare earth ions into the single LiSrPO4 host. It is well known that white light can be obtained by different methods, such as doping a single rare earth ion into appropriate single-phase hosts, the combination of multiple rare ions with red, green and blue or yellow and blue emission,
Y. Cao et al. / Polyhedron 107 (2016) 78–82
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codoping ion pairs based on the energy transfer mechanism [1]. In this work, we report the luminescence properties of LiSrPO4:Eu2+, LiSrPO4:Eu2+/Mn2+, LiSrPO4:Eu2+/Tb3+, and LiSrPO4:Eu2+/Mn2+/ Tb3+. The results show that the white-light emitting can be obtained by codoping Eu2+, Mn2+ and Tb3+ ions in LiSrPO4 host.
2. Experiments Eu2+ single doped, Eu2+/Mn2+ and Eu2+/Tb3+ co-doped, as well as Eu2+/Mn2+/Tb3+ triple-doped LiSrPO4 phosphors were synthesized by the conventional solid state reaction technique. SrCO3 (strontium carbonate, 99%), LiH2PO4 (lithium phosphate monobasic, 99%), Eu2O3 (europium oxide, 99.99%), Tb4O7 (terbium oxide, 99.99%), and MnCO3 (manganese carbonate, 99.95%) were used as raw materials. All of these materials were purchased from Aladdin and used directly without further purification. In a typical synthesis, the stoichiometric amounts of raw materials were weighted and mixed in an agate mortar by grinding. Then, the mixture was transferred into crucible and precalcined at 600 °C for 3 h in air. Finally, the precalcined powders were regrinded and calcined again under a reducing atmosphere (5%H2/95%N2) at 1300 °C for 4 h. After the system cooled to room temperature naturally, the production was collected and grinded again. The phase purity of the prepared phosphors was investigated by a Rigaku D/max-IIIA X-ray Diffractometer with Cu Ka radiation (k = 1.5406 Å) at 40 kV and 30 mA. The XRD patterns were collected in a range of 20–50°. The luminescence was performed on a Spex Fluorolog-3 Spectrofluorometer equipped with a 450 W xenon lamp as the excitation source.
Fig. 2. Emission spectra of LiSrPO4:Eu2+ phosphors under the excitation at 365 nm.
The XRD patterns of LiSrPO4:9 mol%Eu2+, LiSrPO4:7 mol% Eu2+/9 mol%Mn2+, LiSrPO4:7 mol%Eu2+/9 mol%Tb3+, and LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ phosphors are shown in Fig. 1. All of diffraction peaks fit well with the JCPDS card No. 14-0202, suggesting the formation of a pure hexagonal crystal structure. This result indicates that the dopants have no influence on the crystalline structure of LiSrPO4. Generally, Eu2+, Mn2+, and Tb3+ ions are expected to locate Sr2+ sites in LiSrPO4 host, because of the similar ionic radii of Eu2+ (1.12 Å), Mn2+ (0.93 Å), and Tb3+ (1.09 Å) with that of Sr2+ (1.14 Å).
The emission spectra of LiSrPO4:xmol%Eu2+ (x = 1, 3, 5, 7 and 9) phosphors are shown in Fig. 2. Under the excitation at 365 nm, LiSrPO4:Eu2+ phosphors show intense blue emission bands originating from the 4f65d1 ? 4f7 transitions of Eu2+. It suggests that the phosphors can be excited effectively by the near ultraviolet LED chip. Also, it can be seen that the blue emission increases gradually and reaches the maximum when the doping concentration of Eu2+ reaches a value of 7 mol%. The emission intensity decreases due to the concentration quenching if we further increase the Eu2+ concentrations. On the basis of the Dexter’s energy transfer theory, the concentration quenching is induced by the nonradiative energy migration among the Eu2+ ions [23]. No emission bands of Eu3+ can be observed in the spectra, proving that Eu3+ ions have been reduced to be Eu2+ completely in the matrix crystals. For phosphors with sensitizer and activator, the effective energy transfer occurs when the emission spectrum of sensitizer overlaps with the excitation spectrum of activator [24]. Fig. 3 shows the excitation and emission spectra of LiSrPO4:7 mol%Eu2+, LiSrPO4:5 mol%Mn2+ and LiSrPO4:7 mol%Tb3+ phosphors. Fig. 3A shows the excitation (left) and emission (right) spectra of LiSrPO4:7 mol% Eu2+. The excitation band peaking at 365 nm is induced by the 4f–5d transition of Eu2+ since the host LiSrPO4 hardly shows any
Fig. 1. XRD patterns of LiSrPO4 phosphors doped by different ions.
Fig. 3. Excitation and emission spectra of LiSrPO4:7 mol%Eu2+ (A), LiSrPO4:5 mol% Mn2+ (B) and LiSrPO4:7 mol%Tb3+ (C) phosphors.
3. Result and discussion
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absorption between 250 and 440 nm [18]. Under the excitation at 365 nm, LiSrPO4:7 mol%Eu2+ emits blue light peaking at 464 nm, which originates from the 4f65d1 ? 4f7 transition of Eu2+. Fig. 3B shows the excitation (left) and emission (right) spectra of LiSrPO4:5 mol%Mn2+. The excitation bands in the range of 300–525 nm can be attributed to the d–d transition of Mn2+ [6]. Under the excitation at 468 nm, LiSrPO4:5 mol%Mn2+ show red emission band with a peak at 653 nm, which originates from the 4 T2 ? 6A1g transitions of Mn2+. Fig. 3C shows the excitation (left) and emission (right) spectra of LiSrPO4:7 mol%Tb3+. There are several lines in the region from 300 to 500 nm, which corresponds to f–f transitions of Tb3+ [25]. Under the excitation at 354 nm, LiSrPO4:7 mol%Tb3+ shows emission bands originating from the 5 D4 ? 7Fj (j = 6, 5, 4, and 3) transitions of Tb3+. From Fig. 3, it can be seen clearly that the emission band of Eu2+ overlaps well with both excitation bands of Mn2+ and Tb3+, suggesting the possibility of energy transfer of Eu2+ ? Mn2+ and Eu2+ ? Tb3+. Fig. 4 gives the emission spectra of LiSrPO4:7 mol%Eu2+/ymol% Mn2+ (y = 0, 1, 3, 5, 7 and 9) phosphors under the excitation at 365 nm. The blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+ and the red emission originating from the 4T2 ? 6A1g transitions of Mn2+ can be observed for Eu2+ and Mn2+ codoped LiSrPO4 phosphors. The Eu2+ emission intensity decreases with the increasing Mn2+ concentration, demonstrating the occurrence of energy transfer from Eu2+ to Mn2+ in LiSrPO4 host. The emission intensity of Mn2+ increases with the increasing Mn2+ concentration and reaches the maximum at a value of y = 3. Further increasing Mn2+ concentration will induce the decrease of Mn2+ emission intensity because of the concentration quenching of Mn2+. These results show that the relative intensities of these two emissions can be varied by simply adjusting the amounts of the activators through the principle of energy transfer. In general, there are two types of resonant energy transfer mechanism, namely the exchange interaction and the multipolar interaction [26]. And one of factors for the energy transfer through the exchange interaction is the critical distance between the sensitizer and activator is shorter than 5 Å [27]. The critical distance (RC ) can be estimated 1=3 by the equation of 2 4p3VXN , where X is the combined concentra2+ tion of Eu and critical concentration of Mn2+, V is the volume of the unit cell, and N is the number of available sites of the dopant
the energy transfer mechanism between Eu2+ and Mn2+ should be multipolar interaction. The emission spectra of LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 0, 1, 3, 5, 7 and 9) phosphors under the excitation at 365 nm are shown in Fig. 5. It can be seen that all of Eu2+ and Tb3+ codoped LiSrPO4 phosphors show blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+ and emissions originating from the 5D4 ? 7Fj (j = 6, 5, 4, and 3) transitions of Tb3+. Also, the emission intensity of Tb3+ emission increases with the increasing Tb3+ concentration but the intensity of Eu2+ emission decreases gradually. These results give confirmation of energy transfer from Eu2+ to Tb3+ in the LiSrPO4 host and the tunable luminescence of LiSrPO4: Eu2+/Tb3+ phosphors depending on the Tb3+ concentration. Among the emission bands of Tb3+, the green emission originating from the 5D4 ? 7F5 transition is dominating, which is a magnetic dipole transition with DJ = ± 1 [29]. According the report of Chen et.al, the energy transfer from Eu2+ to Tb3+ is non-radiative [20]. Luminescence decay analysis is one of useful methods to confirm energy transfer in rare earth ions codoping phosphors [30]. Figs. 6 and 7 show the decay curves of LiSrPO4:7 mol%Eu2+/ymol
3
in the unit cell [28]. For LiSrPO4 host, V ¼ 205:36 Å , N ¼ 2, X ¼ 0:1, the obtained value of RC is found to be 6.25 Å. Therefore,
Fig. 5. Emission spectra of LiSrPO4:Eu2+/Tb3+ phosphors under the excitation at 365 nm.
Fig. 4. Emission spectra of LiSrPO4:Eu2+/Mn2+ phosphors under the excitation at 365 nm.
Fig. 6. Decay curves of Eu2+ emission in LiSrPO4:7 mol%Eu2+/ymol%Mn2+ (y = 0, 1, 3, 5, 7 and 9) phosphors.
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Y. Cao et al. / Polyhedron 107 (2016) 78–82 Table 2 CIE chromaticity coordinates for LiSrPO4:Eu2+/Mn2+/Tb3+ phosphors. No.
Samples
1 2 3 4 5 6 7 8 9 10 11 12 13 14
LiSrPO4:7 mol%Eu2+ LiSrPO4:7 mol%Eu2+/1 mol%Mn2+ LiSrPO4:7 mol%Eu2+/3 mol%Mn2+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+ LiSrPO4:7 mol%Eu2+/7 mol%Mn2+ LiSrPO4:7 mol%Eu2+/9 mol%Mn2+ LiSrPO4:7 mol%Eu2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/3 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Tb3+ LiSrPO4:7 mol%Eu2+/7 mol%Tb3+ LiSrPO4:7 mol%Eu2+/9 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/1 mol%Tb3+
CIE (x, y) x
y
0.152 0.175 0.204 0.231 0.239 0.252 0.168 0.174 0.187 0.203 0.228 0.274 0.312 0.294
0.057 0.176 0.170 0.155 0.159 0.163 0.116 0.184 0.249 0.322 0.386 0.181 0.221 0.258
Fig. 7. Decay curves of Eu2+ emission in LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 1, 3, 5, 7 and 9) phosphors.
Table 1 Decay lifetimes and energy transfer efficiencies for LiSrPO4:Eu2+/Mn2+ and LiSrPO4: Eu2+/Tb3+ phosphors. No.
Samples
s (ls)
g (%)
1 2 3 4 5 6 7 8 9 10 11
LiSrPO4:7 mol%Eu2+ LiSrPO4:7 mol%Eu2+/1 mol%Mn2+ LiSrPO4:7 mol%Eu2+/3 mol%Mn2+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+ LiSrPO4:7 mol%Eu2+/7 mol%Mn2+ LiSrPO4:7 mol%Eu2+/9 mol%Mn2+ LiSrPO4:7 mol%Eu2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/3 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Tb3+ LiSrPO4:7 mol%Eu2+/7 mol%Tb3+ LiSrPO4:7 mol%Eu2+/9 mol%Tb3+
0.56 0.49 0.42 0.36 0.32 0.29 0.43 0.39 0.35 0.31 0.26
/ 12.5 25.0 35.7 42.9 48.2 23.2 30.4 37.5 44.6 53.6
Fig. 9. CIE chromaticity coordinates for different phosphors.
Fig. 8. Emission spectra of LiSrPO4:Eu2+/Mn2+/Tb3+ phosphors under the excitation at 365 nm.
%Mn2+ (y = 0, 1, 3, 5, 7 and 9) and LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 1, 3, 5, 7 and 9) phosphors by monitoring the blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+. All decay
curves are well according with the exponential equation of IðtÞ ¼ I0 expðt=sÞ, where IðtÞ and I0 are emission intensities, t is the time, s is the decay time for the exponential components [31]. The reduction of lifetimes for Eu2+ with increasing Mn2+ and Tb3+ concentrations is observed. These results support the energy transfer from Eu2+ to Mn2+ and Tb3+ in LiSrPO4:Eu2+/Mn2+ and LiSrPO4:Eu2+/Tb3+ phosphors. Energy transfer efficiency (g) can be obtained from the decay lifetime by using the equation of g ¼ 1 s=s0 [32], where s and s0 are the lifetimes of sensitizer (Eu2+) with and without the presence of activator (Mn2+ and Tb3+). The lifetimes of 4f65d1 ? 4f7 transitions for Eu2+ and the corresponding energy transfer efficiency in LiSrPO4:Eu2+/Mn2+ and LiSrPO4:Eu2+/Tb3+ phosphors are shown in Table 1. Since Eu2+ ion can act as a sensitizer to enhance the emission intensity of Mn2+/Tb3+ in LiSrPO4 host, the tunable emission should be obtained in Eu2+/Mn2+/Tb3+ codoped LiSrPO4 phosphors under the near ultraviolet excitation. When excited at 365 nm, LiSrPO4: Eu2+/Mn2+/Tb3+ phosphors consist of three major emission bands
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in the entire visible spectral region: the blue emission arising from the 4f65d1 ? 4f7 transitions of Eu2+, the green emission due to the 5 D4 ? 7Fj transitions of Tb3+ and the red emission originating from the d-level spin-forbidden transition of Mn2+, as shown in Fig. 8. Fixing the doping concentrations of Eu2+ and Mn2+ ions, it can be seen that the Tb3+ emission intensity increases with the increasing Tb3+ concentrations, but the Mn2+ emission intensity does not increase nor decrease. This indicates that there has hardly energy transfer between Mn2+ and Tb3+ ions. One of the main purposes of this work is to explore the application potential of the studied phosphors. Therefore, we evaluate the colorimetric performance of these phosphors. The Commission International de L’Eclairage (CIE) chromaticity coordinates are calculated and shown in Table 2 and Fig. 9. For LiSrPO4:7 mol%Eu2+/ ymol%Mn2+ (y = 0, 1, 3, 5, 7 and 9) phosphors, the CIE chromaticity coordinates can be tuned from (0.152, 0.057) to (0.252, 0.163). For LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 0, 1, 3, 5, 7 and 9) phosphors, the CIE chromaticity coordinates can be tuned from (0.152, 0.057) to (0.228, 0.386). And the CIE chromaticity coordinates for LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ are (0.294, 0.258), which are very close to the CIE chromaticity coordinates of white light. 4. Conclusion In summary, we have synthesized a series of Eu2+/Mn2+/Tb3+ doped LiSrPO4 phosphors by the solid state reaction. All phosphors have the pure hexagonal crystal structure, indicating the doping ions have no influence on the phase of LiSrPO4 hosts. The doping concentrations have obvious influence on the emission intensities of phosphors. Under the excitation at 365 nm, LiSrPO4/Eu2+ phosphors emit blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+. It can be found that the critical concentration of Eu2+ for LiSrPO4 is 7 mol%. The Eu2+/Mn2+ or Tb3+ codoped LiSrPO4 phosphors show not only the emission of Eu2+ but also the red emission coming from the 4T2 ? 6A1g transitions of Mn2+ or green emissions induced by the 5D4 ? 7Fj transitions of Tb3+. The increasing Mn2+ or Tb3+ concentration induces the decrease of Eu2+ emission intensity, which suggests the energy from Eu2+ to Mn2+ or Tb3+ in LiSrPO4 host. For LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ phosphor, the white light can be obtained under the 365 nm excitation. Acknowledgement This work is supported by ‘Engineering Technology Research Center of Population Health Informatization in Hebei Province’,
‘Application Technology Research and Development Center of Medical Informatics in Hebei Universities’, Hebei Provincial Education Department Youth Fund Project (No. QN2015148), Zhangjiakou Science and Technology Research and Development Projects (No. 1511075B and No. 1411058I-23), Science and Technology Department of Hebei Province Project (No.15217747D). References [1] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 43 (2014) 1372. [2] S. Wu, X. Dong, J. Wang, Q. Kong, W. Yu, G. Liu, J. Mater. Sci.: Mater. Electron. 25 (2014) 1053. [3] J. Sun, J. Zhu, X. Zhang, Z. Xia, H. Du, J. Lumin. 132 (2012) 2937. [4] G. Zhu, S. Xin, Y. Wen, Q. Wang, M. Que, Y. Wang, RSC Adv. 3 (2013) 9311. [5] S. Lizzo, A.H. Velders, A. Meijerink, G.J. Dirksen, G. Blass, J. Lumin. 65 (1995) 303. [6] X. Chen, P. Dai, X. Zhang, C. Li, S. Lu, X. Wang, Y. Jia, Y. Liu, Inorg. Chem. 53 (2014) 3441. [7] F. Yang, Y. Liu, X. Tian, G. Dong, Q. Yu, J. Solid State Chem. 225 (2015) 19. [8] S. Zhang, Y. Nakai, T. Tsuboi, Y. Huang, H.J. Seo, Chem. Mater. 23 (2011) 1216. [9] F. Yang, H. Ma, Y. Liu, Q. Liu, Z. Yang, Y. Han, Ceram. Int. 39 (2013) 2127. [10] Y. Yang, Mater. Sci. Eng., B 178 (2013) 807. [11] Y. Yang, M. Ding, G. Song, W. Fan, H. Feng, Chem. Phys. Lett. 639 (2015) 67. [12] B. Li, H. Zhang, A. Lan, H. Tang, Superlattices Microstruct. 86 (2015) 425. [13] Y. Liu, A. Lan, Y. Jin, G. Chen, X. Zhang, Opt. Mater. 40 (2015) 122. [14] X. Sun, X. Sun, J. He, X. Li, J. Lv, Ceram. Int. 40 (2014) 7647. [15] J. Sun, X. Zhang, Z. Xia, H. Du, Mater. Res. Bull. 46 (2011) 2179. [16] L. Elammari, M. El Koumiri, I. Zschokke-Gränacher, B. Elouadi, Ferroelectrics 158 (1994) 19. [17] C.C. Lin, Z.R. Xiao, G.-Y. Guo, T.-S. Chan, R.-S. Liu, J. Am. Chem. Soc. 132 (2010) 3020. [18] Z.C. Wu, J.X. Shi, J. Wang, M.L. Gong, Q. Su, J. Solid State Chem. 179 (2006) 2356. [19] Z.C. Wu, J.X. Shi, M.L. Gong, J. Wang, Q. Su, Mater. Chem. Phys. 103 (2007) 415. [20] Y. Chen, J. Wang, X. Zhang, G. Zhang, M. Gong, Q. Su, Sens. Actuators, B 148 (2010) 259. [21] R.Y. Yang, Y.M. Peng, Y.K. Su, J. Electron. Mater. 42 (2013) 2910. [22] Y. Chen, J. Wang, C. Liu, J. Tang, X. Kuang, M. Wu, Q. Su, Opt. Express 21 (2013) 3161. [23] D.L. Dexter, J.H. Schulman, J. Chem. Phys. 22 (1954) 1063. [24] C. Jin, H.X. Ma, Y.F. Liu, Q.B. Liu, G.Y. Dong, Q.M. Yu, J. Alloys Compd. 613 (2014) 275. [25] W. Liu, Z. Hao, X. Zhang, Y. Luo, X. Wang, J. Zhang, Inorg. Chem. 50 (2011) 7846. [26] Y. Li, H. Li, B. Liu, J. Zhang, Z. Zhao, Z. Yang, Y. Wen, Y. Wang, J. Phys. Chem. Solids 74 (2013) 175. [27] B.M. Antipeuko, I.M. Bataev, V.L. Ermolaev, E.I. Lyubimov, T.A. Privalova, Opt. Spectrosc. 29 (1970) 177. [28] Z.-W. Zhang, A.-J. Song, S.-T. Song, J.-P. Zhang, W.-G. Zhang, D.-J. Wang, J. Alloys Compd. 629 (2015) 32. [29] Z. Wang, P. Li, Q. Guo, Z. Yang, Mater. Res. Bull. 52 (2014) 30. [30] V. Naresh, B.H. Rudramadevi, S. Buddhudu, J. Alloys Compd. 632 (2015) 59. [31] J. Li, G. Mo, Y. Bai, A. Bao, J. Mater. Sci.: Mater. Electron. 26 (2015) 7390. [32] P. Paulose, G. Jose, V. Thomas, N. Unnikrishnan, M. Warrier, J. Phys. Chem. Solids 64 (2003) 841.
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Solid state synthesis and tunable luminescence of LiSrPO4:Eu2+/Mn2+/Tb3+ phosphors Yanyan Cao a, Naidi Liu a, Jie Tian b, Xiao Zhang a,⇑ a b
College of Information Science and Engineering, Hebei North University, Zhangjiakou 075000, China Financial Department, Hebei North University, Zhangjiakou 075000, China
a r t i c l e
i n f o
Article history: Received 2 October 2015 Accepted 11 January 2016 Available online 14 January 2016 Keywords: LiSrPO4 Doping Phosphors Luminescence Energy transfer
a b s t r a c t A series of Eu2+/Mn2+/Tb3+ doped LiSrPO4 phosphors have been synthesized by the solid state reaction. The XRD results show that all phosphors have the pure hexagonal crystal structure, indicating the doping ions have no influence on the phase of LiSrPO4 host. Under the excitation at 365 nm, LiSrPO4/Eu2+ phosphors emit blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+. The Eu2+/Mn2+ or Tb3+ codoped LiSrPO4 phosphors show not only the emission of Eu2+ but also the red emission coming from the 4T2 ? 6A1g transitions of Mn2+ or green emissions induced by the 5D4 ? 7Fj transitions of Tb3+. The increasing Mn2+ or Tb3+ concentration induces the decrease of Eu2+ emission intensity, which suggests the energy from Eu2+ to Mn2+ or Tb3+ in LiSrPO4 host. For LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ phosphor, the white light can be obtained under the 365 nm excitation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction With rapid progress in material design and device fabrication, white light-emitting diode (WLED) is one of the most potential light sources for applications in solid-state lighting. And one of keys to develop WLEDs is to develop phosphors that can be excited effectively by the near ultraviolet to produce white light emitting. One of routes to generate white light is to combine multiple rare ions with red, green and blue or yellow and blue emissions, as well as through the energy transfer between them [1]. Tb3+ ion is regarded as one of promising green-emitting activators due to the 4f–4f transition [2]. The major problem for the Tb3+ ion is the lack of efficient and broad excitation band from near ultraviolet region to visible range, which limits its application in near ultraviolet white LEDs [3]. Mn2+ ion can yield a broad-band orange-red or red emission, which covers almost the whole red emission area [4]. Eu2+ ion is one of the most common activators for phosphors due to the intense and broad excitation and emission bands derived from their dipole allowed 4f–5d electronic transitions. Moreover, for Eu2+ ions, the Stokes shift is relatively small and the decay time is short, making Eu2+ as a popular activator [5]. However, Eu2+ ion can not only act as an effective activator efficiently to absorb near ultraviolet light but also act as an efficient sensitizer for Tb3+ and Mn2+ activators [6]. Therefore, it is highly expected to
⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.poly.2016.01.016 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
obtain a white-light phosphor pumped by near ultraviolet light through energy transfer from Eu2+ to Tb3+ and Mn2+. In recent years, phosphate has attracted much attention as a luminescent host material because of its excellent thermal and charge stabilization. For example, Ba3Y(PO4)3:Sm3+ [7], LiBaPO4: Eu3+ [8], Ba3Bi(PO4)3:Tb3+ [9], LaPO4:Eu3+ [10], YPO4:Eu3+ [11], Ca(3x)Srx(PO4)2:Eu2+ [12], Sr3Bi(PO4)3:Eu2+/Mn2+ [13], BiPO4:Eu3+ [14], and LiSrPO4:Dy3+ [15], have been synthesized by different methods. Among lots of phosphates, the phosphates with ABPO4 formula (A and B are mono and divalent cations, respectively) are in a large family of monophosphates with the different structure types strictly depending on the relative size of the A and B ions [16]. For example, LiSrPO4 can alternatively be viewed as containing octahedral SrO6, tetrahedral PO4, or LiO4 that is connected by an oxygen bridge which cross-link the Sr and Li atoms, to generate a three-dimensional composite framework structure [17]. LiSrPO4 also is a good luminescent host for rare earth ions. Rare ions doped LiSrPO4 phosphors emitting emissions with different colors, such as blue and yellow [15], blue [18,19], blue and green [20], red [21], have been reported. It is also found that Eu2+ and Pr3+ codoped LiSrPO4 can act as solar spectral convertor [22]. However, to the best of our knowledge, there are no reports on the obtaining of white light by codoping rare earth ions into the single LiSrPO4 host. It is well known that white light can be obtained by different methods, such as doping a single rare earth ion into appropriate single-phase hosts, the combination of multiple rare ions with red, green and blue or yellow and blue emission,
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codoping ion pairs based on the energy transfer mechanism [1]. In this work, we report the luminescence properties of LiSrPO4:Eu2+, LiSrPO4:Eu2+/Mn2+, LiSrPO4:Eu2+/Tb3+, and LiSrPO4:Eu2+/Mn2+/ Tb3+. The results show that the white-light emitting can be obtained by codoping Eu2+, Mn2+ and Tb3+ ions in LiSrPO4 host.
2. Experiments Eu2+ single doped, Eu2+/Mn2+ and Eu2+/Tb3+ co-doped, as well as Eu2+/Mn2+/Tb3+ triple-doped LiSrPO4 phosphors were synthesized by the conventional solid state reaction technique. SrCO3 (strontium carbonate, 99%), LiH2PO4 (lithium phosphate monobasic, 99%), Eu2O3 (europium oxide, 99.99%), Tb4O7 (terbium oxide, 99.99%), and MnCO3 (manganese carbonate, 99.95%) were used as raw materials. All of these materials were purchased from Aladdin and used directly without further purification. In a typical synthesis, the stoichiometric amounts of raw materials were weighted and mixed in an agate mortar by grinding. Then, the mixture was transferred into crucible and precalcined at 600 °C for 3 h in air. Finally, the precalcined powders were regrinded and calcined again under a reducing atmosphere (5%H2/95%N2) at 1300 °C for 4 h. After the system cooled to room temperature naturally, the production was collected and grinded again. The phase purity of the prepared phosphors was investigated by a Rigaku D/max-IIIA X-ray Diffractometer with Cu Ka radiation (k = 1.5406 Å) at 40 kV and 30 mA. The XRD patterns were collected in a range of 20–50°. The luminescence was performed on a Spex Fluorolog-3 Spectrofluorometer equipped with a 450 W xenon lamp as the excitation source.
Fig. 2. Emission spectra of LiSrPO4:Eu2+ phosphors under the excitation at 365 nm.
The XRD patterns of LiSrPO4:9 mol%Eu2+, LiSrPO4:7 mol% Eu2+/9 mol%Mn2+, LiSrPO4:7 mol%Eu2+/9 mol%Tb3+, and LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ phosphors are shown in Fig. 1. All of diffraction peaks fit well with the JCPDS card No. 14-0202, suggesting the formation of a pure hexagonal crystal structure. This result indicates that the dopants have no influence on the crystalline structure of LiSrPO4. Generally, Eu2+, Mn2+, and Tb3+ ions are expected to locate Sr2+ sites in LiSrPO4 host, because of the similar ionic radii of Eu2+ (1.12 Å), Mn2+ (0.93 Å), and Tb3+ (1.09 Å) with that of Sr2+ (1.14 Å).
The emission spectra of LiSrPO4:xmol%Eu2+ (x = 1, 3, 5, 7 and 9) phosphors are shown in Fig. 2. Under the excitation at 365 nm, LiSrPO4:Eu2+ phosphors show intense blue emission bands originating from the 4f65d1 ? 4f7 transitions of Eu2+. It suggests that the phosphors can be excited effectively by the near ultraviolet LED chip. Also, it can be seen that the blue emission increases gradually and reaches the maximum when the doping concentration of Eu2+ reaches a value of 7 mol%. The emission intensity decreases due to the concentration quenching if we further increase the Eu2+ concentrations. On the basis of the Dexter’s energy transfer theory, the concentration quenching is induced by the nonradiative energy migration among the Eu2+ ions [23]. No emission bands of Eu3+ can be observed in the spectra, proving that Eu3+ ions have been reduced to be Eu2+ completely in the matrix crystals. For phosphors with sensitizer and activator, the effective energy transfer occurs when the emission spectrum of sensitizer overlaps with the excitation spectrum of activator [24]. Fig. 3 shows the excitation and emission spectra of LiSrPO4:7 mol%Eu2+, LiSrPO4:5 mol%Mn2+ and LiSrPO4:7 mol%Tb3+ phosphors. Fig. 3A shows the excitation (left) and emission (right) spectra of LiSrPO4:7 mol% Eu2+. The excitation band peaking at 365 nm is induced by the 4f–5d transition of Eu2+ since the host LiSrPO4 hardly shows any
Fig. 1. XRD patterns of LiSrPO4 phosphors doped by different ions.
Fig. 3. Excitation and emission spectra of LiSrPO4:7 mol%Eu2+ (A), LiSrPO4:5 mol% Mn2+ (B) and LiSrPO4:7 mol%Tb3+ (C) phosphors.
3. Result and discussion
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absorption between 250 and 440 nm [18]. Under the excitation at 365 nm, LiSrPO4:7 mol%Eu2+ emits blue light peaking at 464 nm, which originates from the 4f65d1 ? 4f7 transition of Eu2+. Fig. 3B shows the excitation (left) and emission (right) spectra of LiSrPO4:5 mol%Mn2+. The excitation bands in the range of 300–525 nm can be attributed to the d–d transition of Mn2+ [6]. Under the excitation at 468 nm, LiSrPO4:5 mol%Mn2+ show red emission band with a peak at 653 nm, which originates from the 4 T2 ? 6A1g transitions of Mn2+. Fig. 3C shows the excitation (left) and emission (right) spectra of LiSrPO4:7 mol%Tb3+. There are several lines in the region from 300 to 500 nm, which corresponds to f–f transitions of Tb3+ [25]. Under the excitation at 354 nm, LiSrPO4:7 mol%Tb3+ shows emission bands originating from the 5 D4 ? 7Fj (j = 6, 5, 4, and 3) transitions of Tb3+. From Fig. 3, it can be seen clearly that the emission band of Eu2+ overlaps well with both excitation bands of Mn2+ and Tb3+, suggesting the possibility of energy transfer of Eu2+ ? Mn2+ and Eu2+ ? Tb3+. Fig. 4 gives the emission spectra of LiSrPO4:7 mol%Eu2+/ymol% Mn2+ (y = 0, 1, 3, 5, 7 and 9) phosphors under the excitation at 365 nm. The blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+ and the red emission originating from the 4T2 ? 6A1g transitions of Mn2+ can be observed for Eu2+ and Mn2+ codoped LiSrPO4 phosphors. The Eu2+ emission intensity decreases with the increasing Mn2+ concentration, demonstrating the occurrence of energy transfer from Eu2+ to Mn2+ in LiSrPO4 host. The emission intensity of Mn2+ increases with the increasing Mn2+ concentration and reaches the maximum at a value of y = 3. Further increasing Mn2+ concentration will induce the decrease of Mn2+ emission intensity because of the concentration quenching of Mn2+. These results show that the relative intensities of these two emissions can be varied by simply adjusting the amounts of the activators through the principle of energy transfer. In general, there are two types of resonant energy transfer mechanism, namely the exchange interaction and the multipolar interaction [26]. And one of factors for the energy transfer through the exchange interaction is the critical distance between the sensitizer and activator is shorter than 5 Å [27]. The critical distance (RC ) can be estimated 1=3 by the equation of 2 4p3VXN , where X is the combined concentra2+ tion of Eu and critical concentration of Mn2+, V is the volume of the unit cell, and N is the number of available sites of the dopant
the energy transfer mechanism between Eu2+ and Mn2+ should be multipolar interaction. The emission spectra of LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 0, 1, 3, 5, 7 and 9) phosphors under the excitation at 365 nm are shown in Fig. 5. It can be seen that all of Eu2+ and Tb3+ codoped LiSrPO4 phosphors show blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+ and emissions originating from the 5D4 ? 7Fj (j = 6, 5, 4, and 3) transitions of Tb3+. Also, the emission intensity of Tb3+ emission increases with the increasing Tb3+ concentration but the intensity of Eu2+ emission decreases gradually. These results give confirmation of energy transfer from Eu2+ to Tb3+ in the LiSrPO4 host and the tunable luminescence of LiSrPO4: Eu2+/Tb3+ phosphors depending on the Tb3+ concentration. Among the emission bands of Tb3+, the green emission originating from the 5D4 ? 7F5 transition is dominating, which is a magnetic dipole transition with DJ = ± 1 [29]. According the report of Chen et.al, the energy transfer from Eu2+ to Tb3+ is non-radiative [20]. Luminescence decay analysis is one of useful methods to confirm energy transfer in rare earth ions codoping phosphors [30]. Figs. 6 and 7 show the decay curves of LiSrPO4:7 mol%Eu2+/ymol
3
in the unit cell [28]. For LiSrPO4 host, V ¼ 205:36 Å , N ¼ 2, X ¼ 0:1, the obtained value of RC is found to be 6.25 Å. Therefore,
Fig. 5. Emission spectra of LiSrPO4:Eu2+/Tb3+ phosphors under the excitation at 365 nm.
Fig. 4. Emission spectra of LiSrPO4:Eu2+/Mn2+ phosphors under the excitation at 365 nm.
Fig. 6. Decay curves of Eu2+ emission in LiSrPO4:7 mol%Eu2+/ymol%Mn2+ (y = 0, 1, 3, 5, 7 and 9) phosphors.
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Y. Cao et al. / Polyhedron 107 (2016) 78–82 Table 2 CIE chromaticity coordinates for LiSrPO4:Eu2+/Mn2+/Tb3+ phosphors. No.
Samples
1 2 3 4 5 6 7 8 9 10 11 12 13 14
LiSrPO4:7 mol%Eu2+ LiSrPO4:7 mol%Eu2+/1 mol%Mn2+ LiSrPO4:7 mol%Eu2+/3 mol%Mn2+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+ LiSrPO4:7 mol%Eu2+/7 mol%Mn2+ LiSrPO4:7 mol%Eu2+/9 mol%Mn2+ LiSrPO4:7 mol%Eu2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/3 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Tb3+ LiSrPO4:7 mol%Eu2+/7 mol%Tb3+ LiSrPO4:7 mol%Eu2+/9 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/1 mol%Tb3+
CIE (x, y) x
y
0.152 0.175 0.204 0.231 0.239 0.252 0.168 0.174 0.187 0.203 0.228 0.274 0.312 0.294
0.057 0.176 0.170 0.155 0.159 0.163 0.116 0.184 0.249 0.322 0.386 0.181 0.221 0.258
Fig. 7. Decay curves of Eu2+ emission in LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 1, 3, 5, 7 and 9) phosphors.
Table 1 Decay lifetimes and energy transfer efficiencies for LiSrPO4:Eu2+/Mn2+ and LiSrPO4: Eu2+/Tb3+ phosphors. No.
Samples
s (ls)
g (%)
1 2 3 4 5 6 7 8 9 10 11
LiSrPO4:7 mol%Eu2+ LiSrPO4:7 mol%Eu2+/1 mol%Mn2+ LiSrPO4:7 mol%Eu2+/3 mol%Mn2+ LiSrPO4:7 mol%Eu2+/5 mol%Mn2+ LiSrPO4:7 mol%Eu2+/7 mol%Mn2+ LiSrPO4:7 mol%Eu2+/9 mol%Mn2+ LiSrPO4:7 mol%Eu2+/1 mol%Tb3+ LiSrPO4:7 mol%Eu2+/3 mol%Tb3+ LiSrPO4:7 mol%Eu2+/5 mol%Tb3+ LiSrPO4:7 mol%Eu2+/7 mol%Tb3+ LiSrPO4:7 mol%Eu2+/9 mol%Tb3+
0.56 0.49 0.42 0.36 0.32 0.29 0.43 0.39 0.35 0.31 0.26
/ 12.5 25.0 35.7 42.9 48.2 23.2 30.4 37.5 44.6 53.6
Fig. 9. CIE chromaticity coordinates for different phosphors.
Fig. 8. Emission spectra of LiSrPO4:Eu2+/Mn2+/Tb3+ phosphors under the excitation at 365 nm.
%Mn2+ (y = 0, 1, 3, 5, 7 and 9) and LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 1, 3, 5, 7 and 9) phosphors by monitoring the blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+. All decay
curves are well according with the exponential equation of IðtÞ ¼ I0 expðt=sÞ, where IðtÞ and I0 are emission intensities, t is the time, s is the decay time for the exponential components [31]. The reduction of lifetimes for Eu2+ with increasing Mn2+ and Tb3+ concentrations is observed. These results support the energy transfer from Eu2+ to Mn2+ and Tb3+ in LiSrPO4:Eu2+/Mn2+ and LiSrPO4:Eu2+/Tb3+ phosphors. Energy transfer efficiency (g) can be obtained from the decay lifetime by using the equation of g ¼ 1 s=s0 [32], where s and s0 are the lifetimes of sensitizer (Eu2+) with and without the presence of activator (Mn2+ and Tb3+). The lifetimes of 4f65d1 ? 4f7 transitions for Eu2+ and the corresponding energy transfer efficiency in LiSrPO4:Eu2+/Mn2+ and LiSrPO4:Eu2+/Tb3+ phosphors are shown in Table 1. Since Eu2+ ion can act as a sensitizer to enhance the emission intensity of Mn2+/Tb3+ in LiSrPO4 host, the tunable emission should be obtained in Eu2+/Mn2+/Tb3+ codoped LiSrPO4 phosphors under the near ultraviolet excitation. When excited at 365 nm, LiSrPO4: Eu2+/Mn2+/Tb3+ phosphors consist of three major emission bands
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in the entire visible spectral region: the blue emission arising from the 4f65d1 ? 4f7 transitions of Eu2+, the green emission due to the 5 D4 ? 7Fj transitions of Tb3+ and the red emission originating from the d-level spin-forbidden transition of Mn2+, as shown in Fig. 8. Fixing the doping concentrations of Eu2+ and Mn2+ ions, it can be seen that the Tb3+ emission intensity increases with the increasing Tb3+ concentrations, but the Mn2+ emission intensity does not increase nor decrease. This indicates that there has hardly energy transfer between Mn2+ and Tb3+ ions. One of the main purposes of this work is to explore the application potential of the studied phosphors. Therefore, we evaluate the colorimetric performance of these phosphors. The Commission International de L’Eclairage (CIE) chromaticity coordinates are calculated and shown in Table 2 and Fig. 9. For LiSrPO4:7 mol%Eu2+/ ymol%Mn2+ (y = 0, 1, 3, 5, 7 and 9) phosphors, the CIE chromaticity coordinates can be tuned from (0.152, 0.057) to (0.252, 0.163). For LiSrPO4:7 mol%Eu2+/zmol%Tb3+ (z = 0, 1, 3, 5, 7 and 9) phosphors, the CIE chromaticity coordinates can be tuned from (0.152, 0.057) to (0.228, 0.386). And the CIE chromaticity coordinates for LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ are (0.294, 0.258), which are very close to the CIE chromaticity coordinates of white light. 4. Conclusion In summary, we have synthesized a series of Eu2+/Mn2+/Tb3+ doped LiSrPO4 phosphors by the solid state reaction. All phosphors have the pure hexagonal crystal structure, indicating the doping ions have no influence on the phase of LiSrPO4 hosts. The doping concentrations have obvious influence on the emission intensities of phosphors. Under the excitation at 365 nm, LiSrPO4/Eu2+ phosphors emit blue emission originating from the 4f65d1 ? 4f7 transitions of Eu2+. It can be found that the critical concentration of Eu2+ for LiSrPO4 is 7 mol%. The Eu2+/Mn2+ or Tb3+ codoped LiSrPO4 phosphors show not only the emission of Eu2+ but also the red emission coming from the 4T2 ? 6A1g transitions of Mn2+ or green emissions induced by the 5D4 ? 7Fj transitions of Tb3+. The increasing Mn2+ or Tb3+ concentration induces the decrease of Eu2+ emission intensity, which suggests the energy from Eu2+ to Mn2+ or Tb3+ in LiSrPO4 host. For LiSrPO4:7 mol%Eu2+/5 mol%Mn2+/5 mol%Tb3+ phosphor, the white light can be obtained under the 365 nm excitation. Acknowledgement This work is supported by ‘Engineering Technology Research Center of Population Health Informatization in Hebei Province’,
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