Research article Received: 7 August 2013,
Revised: 7 November 2013,
Accepted: 16 December 2013
Published online in Wiley Online Library: 3 March 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2634
The effects of charge compensation on photoluminescence properties of a new green-emitting ZnB2O4:Tb3+ phosphor Jie Liu, Zhan-Chao Wu,* Ping Wang, Yong-Mei Mei, Man Jiang and Shao-Ping Kuang ABSTRACT: Charge compensation is an effective way to eliminate charge defects and improve the luminescent intensity of phosphors. In this paper, a new green-emitting phosphor ZnB2O4:Tb3+ was prepared by solid-state reaction at 750°C. The effects of Tb3+ doping content and charge compensators (Li+, Na+ or K+) on photoluminescence properties of ZnB2O4:Tb3+ were investigated. X-ray powder diffraction analysis confirms the sample has cubic structure of ZnB2O4. The excitation and emission spectra indicate that this phosphor can be excited by near ultraviolet light at 378 nm, and exhibits bright green emission with the highest peak at 544 nm corresponding to the 5D4→7F5 transition of Tb3+. The critical quenching concentration of Tb3+ in ZnB2O4 host is 8 mol%. The results of charge compensation show that the emission intensity can be improved by Na+ and K+. Specifically, K+ is the optimal one for ZnB2O4:Tb3+. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: photoluminescence; green-emitting phosphor; charge compensation
Introduction
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White light-emitting diodes (WLEDs) are considered to be the most valuable solid-state light source in the twenty-first century with several advantages such as high efficiency, long lifetime, good stability and adjustable colors (1,2). White light can be obtained by coating a phosphor powder on LED chips in either of two ways. One is based on blue-emitting (450–470 nm) GaN LED chips coated with broad-band yellow-emitting phosphor, e.g., YAG:Ce3+. The other is by mixing blue, green and red phosphors with near ultraviolet (NUV, 350–410 nm) InGaN LED chips. Compared with the first approach, the second one has the following advantages. First, NUV InGaN LED chips can offer higher efficiency solid-state lighting (3,4). Secondly, the WLEDs fabricated by the second approach have a higher color rendering index because all the colors are determined by the phosphors (5,6). Thirdly, the varieties of the phosphors that can be efficiently excited by NUV light are more than those applied on blue LED chips. Therefore, the second fabrication approach is proposed. The phosphors applied on NUV LED chips should meet two criteria. First, the excitation spectrum of phosphors must match the emission spectrum of LED chips. Second, phosphors should have high fluorescence conversion efficiency under UV light excitation. Alkaline earth borates doped with trivalent rare earth ions as phosphors for WLED have attracted much attention because of several advantages such as low synthetic temperature, high stability, high luminescence efficiency and low cost of raw materials. During the past few years, a number of red and green borate phosphors have been synthesized and studied extensively, which can be used for fabricating WLEDs (7–14). However, it can result in charge imbalance and form charge defects in host lattice when the trivalent rare earth ions replace the divalent alkaline earth ions. Generally, charge defects may cause non-radiative
Luminescence 2014; 29: 868–871
transition of luminescence centers and lead to lower luminous efficiency. Therefore, doping charge compensators to eliminate charge defects and improve luminescent intensity of phosphors effectively has aroused a wide interest (15–17). Recently, three new phosphors (BaB2O4:Eu3+, SrB2O4:Eu3+ and SrB2O4:Tb3+) have been reported and the effects of charge compensators (Li+, Na+ or K+) on their photoluminescence spectra were also investigated by our research group (18–20). Preliminary results indicate that chargecompensation effect is related to the radii of charge compensators and the replaced ions. In other words, the more similar the radii of the cations the better effect of charge compensation. Now, a new green-emitting ZnB2O4:Tb3+ phosphor has been synthesized by solid-state methods and its photoluminescence properties were investigated. However, charge compensation research suggests that the charge-compensation effect in this phosphor is not fully consistent with the above rules. So the charge-compensation mechanism for ZnB2O4:Tb3+ is discussed and compared with the other three phosphors discussed in this paper.
Experimental Synthesis The Zn1-xB2O4:Tb3+ x (x = 0.02, 0.04, 0.06, 0.08, 0.10) samples were prepared by conventional solid-state reaction technique. Because * Correspondence to: Z.-C. Wu, State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China. E-mail: [email protected] State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China
Copyright © 2014 John Wiley & Sons, Ltd.
Effects of charge compensation on green ZnB2O4: Tb3+ Phosphor Tb3+ ions were located in Zn2+ sites and some charge defects were built into the lattices, Li+, Na+ or K+ were added as charge compen+ sators. Thus, a series of Zn1-2xB2O4:Tb3+ x, Mx (x = 0.02, 0.04, 0.06, 0.08, 0.10; M = Li, Na, K) samples were also prepared by the same method as a comparison. The starting materials, 5ZnO·2CO3·4H2O (AR), Li2CO3 (AR), Na2CO3 (AR), K2CO3 (AR), B2O3 (AR) and Tb4O7 (99.99%), in appropriate stoichiometric ratio were ground thoroughly in an agate mortar. Then, the mixed powders were sintered in an electric furnace at 750°C for 6 h. Measurements Crystal phase identification was carried out on an X-ray diffractometer (D-MAX2500/PC; RIGAKU Corporation, Tokyo, Japan) using 40 kV, 20 mA and Cu Kα radiation (1.5406 Å). Morphology and size of the calcined particles were observed by field-emission scanning electron microscopy (JSM-6700F; JEOL Corporation, Tokyo, Japan). Excitation and emission spectra of the powdered phosphors were measured by a fluorescence spectrometer (F-2700; HITACHI HighTechnologies Corporation, Tokyo, Japan) and a 450 W xenon lamp was used as the excitation source. All measurements were made at room temperature unless otherwise stated.
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Figure 2. Field-emission scanning electron microscopy image of Zn0.92B2O4:Tb0.08 powders prepared at 750°C.
The results show that the ZnB2O4:Tb3+ phosphor has good crystallinity and a relatively low sintering temperature, which is also consistent with the requirements of energy saving for products in today’s society.
Results and discussion Photoluminescence properties
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Figure 2 shows the field-emission scanning electron microscopy image of Zn0.92B2O4:Tb3+ 0.08 powders prepared at 750°C. It is observed that the microstructure of the phosphor consists of irregular grains with smooth surface and slight sintering phenomenon. The average size of Zn0.92B2O4:Tb3+ 0.08 powders is about 2–6 μm.
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Field-emission scanning electron microscopy images of phosphor powders
The excitation spectrum of Zn0.92B2O4:Tb3+ 0.08 monitored at 544 nm is shown in Fig. 3. It can be seen that the excitation spectrum is composed of a series of sharp lines including the peaks of 317 (7F6→5D0), 338 (7F6→5L7), 350 (7F6→5L9), 367 (7F6→5G5) and 378 nm (7F6→5G6). ZnB2O4:Tb3+ shows intense f→f transition absorption, which may be attributed to the uneven components mixing a small amount of opposite parity wave functions (e.g., 5d) into 4f wave functions. Therefore, the parity selection rule is relaxed (21). Tb3+ with 4f8 configuration has complicated energy levels, so its photoluminescence spectrum consisting of many peaks due to 5 DJ→7FJ´ transitions should be observed. Figure 4 is the emission spectrum of Zn0.92B2O4:Tb3+ 0.08 excited by 378 nm NUV light. There are four major emission peaks at 488, 544, 587 and 621 nm, which are attributed to the 5D4→7F6, 5D4→7F5, 5D4→7F4 and 5D4→7F3 typical transitions of Tb3+, respectively. The strongest emission peak appears at 544 nm because this transition has the largest probability for both electric-dipole and magnetic-dipole induced
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3+ The XRD patterns of Zn0.92B2O4:Tb3+ 0.08 and Zn0.84B2O4:Tb0.08, + + + + + M0.08 (M = Li , Na and K ) are shown in Fig. 1. The primary peaks were found to be in agreement with the Joint Committee on Powder Diffraction Standards (no. 39-1126, ZnB2O4) except for a few weak impurity peaks, which might result from the doped Tb3+. The crystal structure of ZnB2O4 can be refined as the cubic, space group C2/c with a =7.473.
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X-ray diffraction of phosphor powders
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Luminescence 2014; 29: 868–871
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Figure 3. Excitation spectra of Zn0.92B2O4 Tb0.08 (λεm = 544 nm).
Copyright © 2014 John Wiley & Sons, Ltd.
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Figure 1. X-ray diffraction pattern of the samples: (a) ZnB2O4:Tb ; (b) ZnB2O4:Tb , + 3+ + 3+ + Li ; (c) ZnB2O4:Tb , Na ; (d) ZnB2O4:Tb , K .
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Figure 4. Emission spectra of Zn0.92B2O4:Tb0.08 (λex = 378 nm).
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transitions (22). The four typical emission peaks split into different degrees. In particular, the energy level transition of 5D4→7F6 split into two peaks at 487 and 491 nm. The split may result from the crystal field effects, and the extent of the split is related to the characteristics of the ZnB2O4 structure. The effects of the Tb3+ concentration on the emission spectra of ZnB2O4:Tb3+ phosphors were also investigated. The emission spectra of Zn1-xB2O4:Tb3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10) x phosphors excited by 378 nm light and the dependence of the photoluminescence intensity of Zn1-xB2O4:Tb3+ on the Tb3+ x concentration are given in Fig. 5. All the samples with different doping concentrations of Tb3+ display similar emission spectra except for the emission intensity. The photoluminescence intensity increases with the Tb3+ concentration until a maximum intensity is reached, then it decreases. The critical quenching concentration of Tb3+ is defined as the concentration at which the emission intensity begins to decrease. The critical quenching concentration is about 8 mol% in Zn1-xB2O4:Tb3+ x . The low doping concentration leads to weak luminescence due to an insufficient luminescence center. Certainly, excessive doping concentration also causes weak luminescence because of concentration quenching. In ZnB2O4:Tb3+ phosphor, Tb3+ ions are expected to replace Zn2+ ions in the host lattice, and consequently it will be difficult
to keep charge balances in the crystallite sample. Hence, doping univalent charge compensator (Li+, Na+ or K+) is necessary to keep the charge balance. Figure 6 shows the effect of different charge compensators on the emission intensity of ZnB2O4:Tb3+. It is found that the emission intensity of the phosphors changes with different charge compensators. The emission intensity of the phosphor increases by adding K+ and Na+, in particular K+. Contrarily, Li+ reduces the luminescent intensity of ZnB2O4:Tb3+. This result is different from that of similar phosphors in previous reports (18–20). K+, Na+ and Na+ ions are the optimal charge compensators for BaB2O4:Eu3+ (18), SrB2O4:Eu3+ (19) and SrB2O4:Tb3+ (20), respectively. This phenomenon is considered to be due to the similar radii between charge compensator ions (K+, Na+ and Na+) and the replaced ions (Ba2+, Sr2+ and Sr2+), which would lead to a smaller lattice deformation (18–20). The radii of replaced ions Ba2+, Sr2+ and Zn2+ in the host lattice are 135, 113 and 60 pm, respectively. The radii of activator ions Eu3+ and Tb3+ are 95 and 95 pm, respectively. The radii of the charge compensators K+, Na+ and Li+ are 133, 102 and 59 pm, respectively. In the above phosphors, the coordination numbers for Li+ and Zn2+ are 4, while the coordination numbers for other ions are 6 (23,24). In the BaB2O4 or SrB2O4 host lattice, the activator ions Eu3+ or Tb3+ can substitute Ba2+ or Sr2+ easily because the radii of Eu3+ (95 pm) and Tb3+ (95 pm) are smaller than those of Ba2+ (135 pm) and Sr2+ (113 pm). So the more similar the radii sizes of charge compensators and replaced ions, the smaller the deformation of the host lattice, and the better effect of charge compensation. Obviously, this conclusion cannot be simply applied to the ZnB2O4:Tb3+ phosphor. In ZnB2O4: Tb3+, the radius of Tb3+ (95 pm) is larger than that of Zn2+ (60 pm). It is not easy for Tb3+ to substitute Zn2+ in the ZnB2O4 host. When Li+ as charge compensator together with Tb3+ substitute two Zn2+ sites in the ZnB2O4 crystal lattice, Li+ substitutes Zn2+ more easily than Tb3+ because the radius of Li+ is smaller than that of Zn2+ and the radius of Tb3+ is larger than that of Zn2+. That is to say, Li+ competing with Tb3+ leads to Tb3+ replacing Zn2+ more difficultly and the Tb3+-doped concentration decreasing. Therefore, the emission intensity of ZnB2O4:Tb3+, Li+ is weaker than that of ZnB2O4:Tb3+. Contrarily, the radii of Na+ and K+ are larger than in Zn2+ and Tb3+. Tb3+ replaces Zn2+ more easily than Na+ and K+ in this. Therefore, ZnB2O4:Tb3+, M+ (M = K or Na) shows higher emission intensity than ZnB2O4:Tb3+. In addition, the radius of K+ is larger than Na+, so that
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Figure 5. Emission spectra of Zn1-xB2O4:Tbx with varying Tb concentrations (λex = 378 nm): (a) x = 0.02; (b) x = 0.04; (c) x = 0.06; (d) x = 0.08; (e) x = 0.10. 3+ 3+ Inset: dependence of photoluminescence intensity of Zn1-xB2O4:Tbx on Tb doped concentration.
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Figure 6. Effect of different change compensation on the emission intensity of 3+ 3+ 3+ + 3+ + ZnB2O4:Tb : (a) ZnB2O4:Tb ; (b) ZnB2O4:Tb , Li ; (c) ZnB2O4:Tb , Na ; (d) 3+ + ZnB2O4:Tb , K . Inset figure: Four phosphors under ultraviolet light irradiation. Inset table: CIE coordinates of the four phosphors.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014; 29: 868–871
Effects of charge compensation on green ZnB2O4: Tb3+ Phosphor the emission intensity of ZnB2O4:Tb3+, K+ is higher than that of ZnB2O4:Tb3+, Na+. In a word, the effect of charge compensators on ZnB2O4:Tb3+ is still associated with the sizes of ionic radii. Figure 6 shows ZnB2O4:Tb3+ (Fig. 6a), ZnB2O4:Tb3+, Li+ (Fig. 6b), ZnB2O4:Tb3+, Na+ (Fig. 6c) and ZnB2O4:Tb3+, K+ (Fig. 6d) under UV light irradiation. It can be seen that all of the samples show bright green emission. The order of luminescent intensity for the four phosphors is d>c>a>b observed by the naked eye. The sequence is in accordance with results measured by fluorescence spectrometer. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates (inset table in Fig. 6) of these phosphors are also calculated from photoluminescence spectra. It can be seen that all the phosphors show excellent CIE chromaticity coordinates, which are closer to the phase alternation line standard values (x = 0.29, y = 0.60). In particular, the CIE chromaticity coordinates (x = 0.28, y = 0.56) of ZnB2O4:Tb3+, K+ are much closer to phase alternation line standard values than those of other samples. In addition, the degree close to green standard value also meets the sequence of d>c>a>b.
Conclusion A new green-emitting phosphor ZnB2O4:Tb3+ was prepared by conventional solid-state reaction at 750°C and its photoluminescence properties were studied. The phosphor ZnB2O4:Tb3+ exhibits bright green emission under 378 nm NUV light excitation. The excitation bands match well with the NUV GaN-based LED chip, implying that it has good potential as a green-emitting phosphor for application in NUV LEDs. The critical quenching concentration of Tb3+ in ZnB2O4: Tb3+ phosphor is about 8 mol%. The charge compensations of Na+ and K+ improve the luminescent intensity of ZnB2O4:Tb3+, while Li+ brings about a negative effect. K+ is the optimal charge compensator. Surprisingly, the regularity of the charge-compensation effect in ZnB2O4:Tb3+ is completely different from those of similar phosphors in previous reports. The causes for this phenomenon are discussed in depth. The radii sizes of charge compensators, active ions and replaced cations in the host are considered the main factors leading to these differences. This work is useful in the study of the charge compensation mechanism in phosphors with similar structures. Acknowledgments This work was financially supported by the National Natural Science Foundation of the People’s Republic of China (no. 21007029), the Natural Science Foundation of Shandong Province (ZR2012BQ017) and Qingdao Project of Science and Technology (13-1-4-114-jch).
References 1. Li PL, Xu Z, Zhao SL, Zhang FJ, Wang YS. Luminescent characteristics 3+ 3+ and energy transfer of Ca2BO3Cl: Sm , Eu red phosphor. Mater Res Bull 2012;47:3825–9.
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2. Yan XS, Li WW, Sun K. A novel red emitting phosphor CaIn2O4: Eu , 3+ Sm with a broadened near-ultraviolet absorption band for solidstate lighting. Mater Res Bull 2011;46:87–91. 3. Steigerwald DA, Bhat JC, Collins D, Fletcher RM, Holcomb MO, Ludowise MJ, Martin PS, Rudaz SL. Illumination with solid state lighting technology. IEEE J Select Top Quant Elect 2002;8:310–20. 4. Khizar M, Fan ZY, Kim KH, Lin JY, Jiang HX. Nitride deep-ultraviolet light-emitting diodes with microlens array. Appl Phys Lett 2005;86:173504. 5. Guo CF, Ding X, Seo HJ, Ren ZY, Bai JT. Double emitting phosphor 3+ 3+ NaSr4(BO3)3: Ce , Tb for near-UV light-emitting diodes. Opt Laser Technol 2011;43:1351–4. 6. Wu ZC, Liu J, Hou WG, Xu J, Gong ML. A new single-host white2+ light-emitting BaSrMg(PO4)2: Eu phosphor for white-light-emitting diodes. J Alloys Compd 2010;498:139–42. 3+ 7. Wang R, Xu J, Chen C. Luminescent characteristics of Sr2B2O5: Tb , + Li green phosphor. Mater Lett 2012;68:307–9. 8. Zhang JL, Zhang XG, Gong ML, Shi JX, Yu LP, Rong CY, Lian SX. 2+ LiSrBO3: Eu : A novel broad-band red phosphor under the excitation of a blue light. Mater Lett 2012;79:100–2. 9. Nagpure PA, Omanwar SK. Synthesis and photoluminescence study of rare earth activated phosphor Na2La2B2O7. J Lumin 2012;132:2088–91. 10. Lakshmanan A, Bhaskar RS, Thomas PC, Kumar RS, Kumar VS, Jose 3+ MT. A red phosphor for nUV LED based on (Y,Gd)BO3: Eu . Mater Lett 2010;64:1809–12. 11. Xie N, Huang YL, Qiao XB, Shi L, Seo HJ. A red-emitting phosphor of 3+ fully concentrated Eu -based molybdenum borate Eu2MoB2O9. Mater Lett 2010;64:1000–2. 12. Li PL, Wang ZJ, Yang ZP, Guo QL, Li X. Emission features of LiBaBO3: 3+ Sm red phosphor for white LED. Mater Lett 2009;63:751–3. 13. Li PL, Pang LB, Wang ZJ, Yang ZP, Guo QL, Li X. Luminescent charac3+ teristics of LiBaBO3: Tb green phosphor for white LED. J Alloys Compd 2009;478:813–5. 14. Li PL, Wang ZJ, Yang ZP, Guo QL, Fu GS. Luminescent characteristics 3+ 3+ 3+ 3+ 3+ of LiSrBO3: M (M = Eu , Sm , Tb , Ce , Dy ) phosphor for white light-emitting diode. Mater Res Bull 2009;44:2068–71. 15. Zhao X, Ding YF, Li ZS, Yu T, Zou ZG. An efficient charge compen+ 3+ sated red phosphor Sr3WO6: K , Eu — For white LEDs. J Alloys Compd 2013;553:221–4. 16. Puchalska M, Zych E. The effect of charge compensation by means + 3+ of Na ions on the luminescence behavior of Sm -doped CaAl4O7 phosphor. J Lumin 2012;132:826–31. 17. Yan Y, Cao FB, Tian YW, Li LS. Improved luminescent properties of red-emitting Ca0.54Sr0.16Eu0.08Gd0.12(MoO4)0.2(WO4)0.8 phosphor for LED application by charge compensation. J Lumin 2011;131:1140–3. 18. Liu J, Wang XD, Wu ZC, Kuang SP. Preparation, characterization and 3+ red phosphor. photoluminescence properties of BaB2O4: Eu Spectrochim Acta Part A 2011;79:1520–3. 19. Zhao LS, Liu J, Wu ZC, Kuang SP. Optimized photoluminescence of 3+ red-emitting phosphor by charge compensation. SrB2O4: Eu Spectrochim Acta Part A 2012;87:228–31. 20. Wu ZC, Wang P, Liu J, Li C, Zhou WH, Kuang SP. Improved 3+ photoluminescence properties of a new green SrB2O4: Tb phosphor by charge compensation. Mater Res Bull 2012;47:3413–6. 21. Blasse G, Grabmaier BC. Luminescent Materials. Berlin: Springer, 1994. 22. Shionoya S, Yen WM. Phosphor Handbook. New York: CRC Press, 1999. 23. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst 1976;A32:751–67. 3+ 3+ 24. Liu WR, Lin CC, Chiu YC, Yeh, YT, Jang SM, Liu RS. ZnB2O4: Bi , Eu : a highly efficient, red-emitting phosphor. Opt Express 2010;18: 2946–51.
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Luminescence 2014; 29: 868–871
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
Revised: 7 November 2013,
Accepted: 16 December 2013
Published online in Wiley Online Library: 3 March 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2634
The effects of charge compensation on photoluminescence properties of a new green-emitting ZnB2O4:Tb3+ phosphor Jie Liu, Zhan-Chao Wu,* Ping Wang, Yong-Mei Mei, Man Jiang and Shao-Ping Kuang ABSTRACT: Charge compensation is an effective way to eliminate charge defects and improve the luminescent intensity of phosphors. In this paper, a new green-emitting phosphor ZnB2O4:Tb3+ was prepared by solid-state reaction at 750°C. The effects of Tb3+ doping content and charge compensators (Li+, Na+ or K+) on photoluminescence properties of ZnB2O4:Tb3+ were investigated. X-ray powder diffraction analysis confirms the sample has cubic structure of ZnB2O4. The excitation and emission spectra indicate that this phosphor can be excited by near ultraviolet light at 378 nm, and exhibits bright green emission with the highest peak at 544 nm corresponding to the 5D4→7F5 transition of Tb3+. The critical quenching concentration of Tb3+ in ZnB2O4 host is 8 mol%. The results of charge compensation show that the emission intensity can be improved by Na+ and K+. Specifically, K+ is the optimal one for ZnB2O4:Tb3+. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: photoluminescence; green-emitting phosphor; charge compensation
Introduction
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White light-emitting diodes (WLEDs) are considered to be the most valuable solid-state light source in the twenty-first century with several advantages such as high efficiency, long lifetime, good stability and adjustable colors (1,2). White light can be obtained by coating a phosphor powder on LED chips in either of two ways. One is based on blue-emitting (450–470 nm) GaN LED chips coated with broad-band yellow-emitting phosphor, e.g., YAG:Ce3+. The other is by mixing blue, green and red phosphors with near ultraviolet (NUV, 350–410 nm) InGaN LED chips. Compared with the first approach, the second one has the following advantages. First, NUV InGaN LED chips can offer higher efficiency solid-state lighting (3,4). Secondly, the WLEDs fabricated by the second approach have a higher color rendering index because all the colors are determined by the phosphors (5,6). Thirdly, the varieties of the phosphors that can be efficiently excited by NUV light are more than those applied on blue LED chips. Therefore, the second fabrication approach is proposed. The phosphors applied on NUV LED chips should meet two criteria. First, the excitation spectrum of phosphors must match the emission spectrum of LED chips. Second, phosphors should have high fluorescence conversion efficiency under UV light excitation. Alkaline earth borates doped with trivalent rare earth ions as phosphors for WLED have attracted much attention because of several advantages such as low synthetic temperature, high stability, high luminescence efficiency and low cost of raw materials. During the past few years, a number of red and green borate phosphors have been synthesized and studied extensively, which can be used for fabricating WLEDs (7–14). However, it can result in charge imbalance and form charge defects in host lattice when the trivalent rare earth ions replace the divalent alkaline earth ions. Generally, charge defects may cause non-radiative
Luminescence 2014; 29: 868–871
transition of luminescence centers and lead to lower luminous efficiency. Therefore, doping charge compensators to eliminate charge defects and improve luminescent intensity of phosphors effectively has aroused a wide interest (15–17). Recently, three new phosphors (BaB2O4:Eu3+, SrB2O4:Eu3+ and SrB2O4:Tb3+) have been reported and the effects of charge compensators (Li+, Na+ or K+) on their photoluminescence spectra were also investigated by our research group (18–20). Preliminary results indicate that chargecompensation effect is related to the radii of charge compensators and the replaced ions. In other words, the more similar the radii of the cations the better effect of charge compensation. Now, a new green-emitting ZnB2O4:Tb3+ phosphor has been synthesized by solid-state methods and its photoluminescence properties were investigated. However, charge compensation research suggests that the charge-compensation effect in this phosphor is not fully consistent with the above rules. So the charge-compensation mechanism for ZnB2O4:Tb3+ is discussed and compared with the other three phosphors discussed in this paper.
Experimental Synthesis The Zn1-xB2O4:Tb3+ x (x = 0.02, 0.04, 0.06, 0.08, 0.10) samples were prepared by conventional solid-state reaction technique. Because * Correspondence to: Z.-C. Wu, State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China. E-mail: [email protected] State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China
Copyright © 2014 John Wiley & Sons, Ltd.
Effects of charge compensation on green ZnB2O4: Tb3+ Phosphor Tb3+ ions were located in Zn2+ sites and some charge defects were built into the lattices, Li+, Na+ or K+ were added as charge compen+ sators. Thus, a series of Zn1-2xB2O4:Tb3+ x, Mx (x = 0.02, 0.04, 0.06, 0.08, 0.10; M = Li, Na, K) samples were also prepared by the same method as a comparison. The starting materials, 5ZnO·2CO3·4H2O (AR), Li2CO3 (AR), Na2CO3 (AR), K2CO3 (AR), B2O3 (AR) and Tb4O7 (99.99%), in appropriate stoichiometric ratio were ground thoroughly in an agate mortar. Then, the mixed powders were sintered in an electric furnace at 750°C for 6 h. Measurements Crystal phase identification was carried out on an X-ray diffractometer (D-MAX2500/PC; RIGAKU Corporation, Tokyo, Japan) using 40 kV, 20 mA and Cu Kα radiation (1.5406 Å). Morphology and size of the calcined particles were observed by field-emission scanning electron microscopy (JSM-6700F; JEOL Corporation, Tokyo, Japan). Excitation and emission spectra of the powdered phosphors were measured by a fluorescence spectrometer (F-2700; HITACHI HighTechnologies Corporation, Tokyo, Japan) and a 450 W xenon lamp was used as the excitation source. All measurements were made at room temperature unless otherwise stated.
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Figure 2. Field-emission scanning electron microscopy image of Zn0.92B2O4:Tb0.08 powders prepared at 750°C.
The results show that the ZnB2O4:Tb3+ phosphor has good crystallinity and a relatively low sintering temperature, which is also consistent with the requirements of energy saving for products in today’s society.
Results and discussion Photoluminescence properties
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Figure 2 shows the field-emission scanning electron microscopy image of Zn0.92B2O4:Tb3+ 0.08 powders prepared at 750°C. It is observed that the microstructure of the phosphor consists of irregular grains with smooth surface and slight sintering phenomenon. The average size of Zn0.92B2O4:Tb3+ 0.08 powders is about 2–6 μm.
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Field-emission scanning electron microscopy images of phosphor powders
The excitation spectrum of Zn0.92B2O4:Tb3+ 0.08 monitored at 544 nm is shown in Fig. 3. It can be seen that the excitation spectrum is composed of a series of sharp lines including the peaks of 317 (7F6→5D0), 338 (7F6→5L7), 350 (7F6→5L9), 367 (7F6→5G5) and 378 nm (7F6→5G6). ZnB2O4:Tb3+ shows intense f→f transition absorption, which may be attributed to the uneven components mixing a small amount of opposite parity wave functions (e.g., 5d) into 4f wave functions. Therefore, the parity selection rule is relaxed (21). Tb3+ with 4f8 configuration has complicated energy levels, so its photoluminescence spectrum consisting of many peaks due to 5 DJ→7FJ´ transitions should be observed. Figure 4 is the emission spectrum of Zn0.92B2O4:Tb3+ 0.08 excited by 378 nm NUV light. There are four major emission peaks at 488, 544, 587 and 621 nm, which are attributed to the 5D4→7F6, 5D4→7F5, 5D4→7F4 and 5D4→7F3 typical transitions of Tb3+, respectively. The strongest emission peak appears at 544 nm because this transition has the largest probability for both electric-dipole and magnetic-dipole induced
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3+ The XRD patterns of Zn0.92B2O4:Tb3+ 0.08 and Zn0.84B2O4:Tb0.08, + + + + + M0.08 (M = Li , Na and K ) are shown in Fig. 1. The primary peaks were found to be in agreement with the Joint Committee on Powder Diffraction Standards (no. 39-1126, ZnB2O4) except for a few weak impurity peaks, which might result from the doped Tb3+. The crystal structure of ZnB2O4 can be refined as the cubic, space group C2/c with a =7.473.
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Luminescence 2014; 29: 868–871
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Figure 3. Excitation spectra of Zn0.92B2O4 Tb0.08 (λεm = 544 nm).
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Figure 1. X-ray diffraction pattern of the samples: (a) ZnB2O4:Tb ; (b) ZnB2O4:Tb , + 3+ + 3+ + Li ; (c) ZnB2O4:Tb , Na ; (d) ZnB2O4:Tb , K .
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transitions (22). The four typical emission peaks split into different degrees. In particular, the energy level transition of 5D4→7F6 split into two peaks at 487 and 491 nm. The split may result from the crystal field effects, and the extent of the split is related to the characteristics of the ZnB2O4 structure. The effects of the Tb3+ concentration on the emission spectra of ZnB2O4:Tb3+ phosphors were also investigated. The emission spectra of Zn1-xB2O4:Tb3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10) x phosphors excited by 378 nm light and the dependence of the photoluminescence intensity of Zn1-xB2O4:Tb3+ on the Tb3+ x concentration are given in Fig. 5. All the samples with different doping concentrations of Tb3+ display similar emission spectra except for the emission intensity. The photoluminescence intensity increases with the Tb3+ concentration until a maximum intensity is reached, then it decreases. The critical quenching concentration of Tb3+ is defined as the concentration at which the emission intensity begins to decrease. The critical quenching concentration is about 8 mol% in Zn1-xB2O4:Tb3+ x . The low doping concentration leads to weak luminescence due to an insufficient luminescence center. Certainly, excessive doping concentration also causes weak luminescence because of concentration quenching. In ZnB2O4:Tb3+ phosphor, Tb3+ ions are expected to replace Zn2+ ions in the host lattice, and consequently it will be difficult
to keep charge balances in the crystallite sample. Hence, doping univalent charge compensator (Li+, Na+ or K+) is necessary to keep the charge balance. Figure 6 shows the effect of different charge compensators on the emission intensity of ZnB2O4:Tb3+. It is found that the emission intensity of the phosphors changes with different charge compensators. The emission intensity of the phosphor increases by adding K+ and Na+, in particular K+. Contrarily, Li+ reduces the luminescent intensity of ZnB2O4:Tb3+. This result is different from that of similar phosphors in previous reports (18–20). K+, Na+ and Na+ ions are the optimal charge compensators for BaB2O4:Eu3+ (18), SrB2O4:Eu3+ (19) and SrB2O4:Tb3+ (20), respectively. This phenomenon is considered to be due to the similar radii between charge compensator ions (K+, Na+ and Na+) and the replaced ions (Ba2+, Sr2+ and Sr2+), which would lead to a smaller lattice deformation (18–20). The radii of replaced ions Ba2+, Sr2+ and Zn2+ in the host lattice are 135, 113 and 60 pm, respectively. The radii of activator ions Eu3+ and Tb3+ are 95 and 95 pm, respectively. The radii of the charge compensators K+, Na+ and Li+ are 133, 102 and 59 pm, respectively. In the above phosphors, the coordination numbers for Li+ and Zn2+ are 4, while the coordination numbers for other ions are 6 (23,24). In the BaB2O4 or SrB2O4 host lattice, the activator ions Eu3+ or Tb3+ can substitute Ba2+ or Sr2+ easily because the radii of Eu3+ (95 pm) and Tb3+ (95 pm) are smaller than those of Ba2+ (135 pm) and Sr2+ (113 pm). So the more similar the radii sizes of charge compensators and replaced ions, the smaller the deformation of the host lattice, and the better effect of charge compensation. Obviously, this conclusion cannot be simply applied to the ZnB2O4:Tb3+ phosphor. In ZnB2O4: Tb3+, the radius of Tb3+ (95 pm) is larger than that of Zn2+ (60 pm). It is not easy for Tb3+ to substitute Zn2+ in the ZnB2O4 host. When Li+ as charge compensator together with Tb3+ substitute two Zn2+ sites in the ZnB2O4 crystal lattice, Li+ substitutes Zn2+ more easily than Tb3+ because the radius of Li+ is smaller than that of Zn2+ and the radius of Tb3+ is larger than that of Zn2+. That is to say, Li+ competing with Tb3+ leads to Tb3+ replacing Zn2+ more difficultly and the Tb3+-doped concentration decreasing. Therefore, the emission intensity of ZnB2O4:Tb3+, Li+ is weaker than that of ZnB2O4:Tb3+. Contrarily, the radii of Na+ and K+ are larger than in Zn2+ and Tb3+. Tb3+ replaces Zn2+ more easily than Na+ and K+ in this. Therefore, ZnB2O4:Tb3+, M+ (M = K or Na) shows higher emission intensity than ZnB2O4:Tb3+. In addition, the radius of K+ is larger than Na+, so that
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Figure 5. Emission spectra of Zn1-xB2O4:Tbx with varying Tb concentrations (λex = 378 nm): (a) x = 0.02; (b) x = 0.04; (c) x = 0.06; (d) x = 0.08; (e) x = 0.10. 3+ 3+ Inset: dependence of photoluminescence intensity of Zn1-xB2O4:Tbx on Tb doped concentration.
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Figure 6. Effect of different change compensation on the emission intensity of 3+ 3+ 3+ + 3+ + ZnB2O4:Tb : (a) ZnB2O4:Tb ; (b) ZnB2O4:Tb , Li ; (c) ZnB2O4:Tb , Na ; (d) 3+ + ZnB2O4:Tb , K . Inset figure: Four phosphors under ultraviolet light irradiation. Inset table: CIE coordinates of the four phosphors.
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Luminescence 2014; 29: 868–871
Effects of charge compensation on green ZnB2O4: Tb3+ Phosphor the emission intensity of ZnB2O4:Tb3+, K+ is higher than that of ZnB2O4:Tb3+, Na+. In a word, the effect of charge compensators on ZnB2O4:Tb3+ is still associated with the sizes of ionic radii. Figure 6 shows ZnB2O4:Tb3+ (Fig. 6a), ZnB2O4:Tb3+, Li+ (Fig. 6b), ZnB2O4:Tb3+, Na+ (Fig. 6c) and ZnB2O4:Tb3+, K+ (Fig. 6d) under UV light irradiation. It can be seen that all of the samples show bright green emission. The order of luminescent intensity for the four phosphors is d>c>a>b observed by the naked eye. The sequence is in accordance with results measured by fluorescence spectrometer. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates (inset table in Fig. 6) of these phosphors are also calculated from photoluminescence spectra. It can be seen that all the phosphors show excellent CIE chromaticity coordinates, which are closer to the phase alternation line standard values (x = 0.29, y = 0.60). In particular, the CIE chromaticity coordinates (x = 0.28, y = 0.56) of ZnB2O4:Tb3+, K+ are much closer to phase alternation line standard values than those of other samples. In addition, the degree close to green standard value also meets the sequence of d>c>a>b.
Conclusion A new green-emitting phosphor ZnB2O4:Tb3+ was prepared by conventional solid-state reaction at 750°C and its photoluminescence properties were studied. The phosphor ZnB2O4:Tb3+ exhibits bright green emission under 378 nm NUV light excitation. The excitation bands match well with the NUV GaN-based LED chip, implying that it has good potential as a green-emitting phosphor for application in NUV LEDs. The critical quenching concentration of Tb3+ in ZnB2O4: Tb3+ phosphor is about 8 mol%. The charge compensations of Na+ and K+ improve the luminescent intensity of ZnB2O4:Tb3+, while Li+ brings about a negative effect. K+ is the optimal charge compensator. Surprisingly, the regularity of the charge-compensation effect in ZnB2O4:Tb3+ is completely different from those of similar phosphors in previous reports. The causes for this phenomenon are discussed in depth. The radii sizes of charge compensators, active ions and replaced cations in the host are considered the main factors leading to these differences. This work is useful in the study of the charge compensation mechanism in phosphors with similar structures. Acknowledgments This work was financially supported by the National Natural Science Foundation of the People’s Republic of China (no. 21007029), the Natural Science Foundation of Shandong Province (ZR2012BQ017) and Qingdao Project of Science and Technology (13-1-4-114-jch).
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