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Microstructure, luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering
Optical Materials 35 (2013) 2183–2187
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Microstructure, luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering Ru-Yuan Yang a, Cheng-Tang Pan b, Kun-Hsien Chen b, Cheng-Yuan Hung c,⇑ a b c
Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung County 912, Taiwan Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung County 804, Taiwan Medical Devices and Opto-electronics Equipment Department, Metal Industries Research & Development Center, Taiwan
a r t i c l e
i n f o
Article history: Received 8 February 2013 Received in revised form 21 May 2013 Accepted 1 June 2013 Available online 28 June 2013 Keywords: Inorganic Compounds Conventional solid-state sintering Luminescence
a b s t r a c t This paper reports the microstructure, luminescence and thermal stability properties of the NaSr1 xPO4:x Tb3+ powders (x = 0.008, 0.01, 0.02, 0.04 and 0.06) via the conventional solid-state sintering at 1200 °C for 5 h. The X-ray diffraction result verifies all diffraction peaks are pure phase of NaSrPO4. The luminescence results show that the NaSrPO4:xTb3+ powders mainly excited at 370 nm have a series of the emissionstates, related to the typical 4f ? 4f intra-configuration forbidden transitions of Tb3+, and a major emission peak of around 546 nm. The concentration quenching of the NaSr1 xPO4:xTb3+ phosphors is appeared at x = 0.02. The decay time values of the NaSr1 xPO4:xTb3+ phosphors for the 5D4 state of the Tb3+ are around 3.30 ms to 3.60 ms. It is also found the chromaticity coordinate of NaSrPO4:Tb3+ phosphor varies with the increase of the concentration of Tb3+ ions from blue to green. Moreover, the thermal stability of the NaSrPO4:xTb3+ phosphors is slightly better than that of conventional YAG phosphors. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, phosphor converted-light emitting diodes (PC-LEDs) become the main stream for the current white LED market [1]. The present method to produce white light adopts the combination of blue LED chip (460 nm) with yellow-emitting phosphor, Y3Al5O12:Ce3+ (YAG:Ce3+), however, it still has some disadvantages, such as low thermal quenching temperature, narrow visible range, and poor color rendering due to the lack of red component [2]. Near-ultraviolet (NUV) InGaN-based LEDs, with a wavelength ranging from 350 to 420 nm, receive more attention because NUV LED can provide a highly efficient solid-state lighting [3]. Thus, new phosphor powders, desirable for color emission using high efficiency NUV excitation, must be searched and developed. Recently, phosphate compounds with the ABPO4 (A = alkaline metals, B = alkaline earth metals) structure were reported and became an important family of luminescent host materials since their tetrahedral rigid three dimensional matrix of phosphate are desired for charge stabilization, thus resulting in excellent thermal stability [4]. ABPO4 doped with different rare-earth ions (Eu3+, Ce3+, Tb3+, Sm3+ etc.), such as KSrPO4:Eu [4,5], KSrPO4:Tb [4,6], KSrPO4:Sm [4], KBaPO4:Eu [7,8], KBaPO4:Tb [7], KBaPO4:Sm [7], NaCaPO4:Eu [9], NaBaPO4:Ce,Tb [10], NaSrPO4:Eu [11–13], ⇑ Corresponding author. Tel.: +886 7 623 5550x315; fax: +886 7 623 5527. E-mail address: [email protected] (C.-Y. Hung).
NaSrPO4:Eu/Tb [14] and NaSrPO4:Eu/Mn [15] could emit different wavelengths, depending on various crystal fields. In which, NaSrPO4 host material has a standard monoclinic crystal structure with lattice parameters as a = 2.041 nm, b = 0.543 nm, c = 1.725 nm, and angle b = 101.76° [12,13]. In past, there have been several investigations of NaSrPO4: Eu phosphors [11–14] for blue emitting. Moreover, NaSrPO4 phosphors co-doped with Eu/ Tb ions were synthesized by a conventional solid-state reaction for blue/green emitting [14]. It is known Tb3+ ion is an important rare earth ion used to generate green emission under NUV excitation. However, microstructure, luminescence and thermal stability properties of the green-emitting NaSrPO4 doped with only Tb3+ ions has not yet been investigated and reported. Therefore, in this paper, the NaSrPO4:Tb3+ phosphors were synthesized in succeed via the solid-state sintering. The effects of different concentrations of Tb3+ ions on microstructure, luminescence and thermal stability characteristics of NaSrPO4:Tb3+ phosphors were investigated and discussed. 2. Experimental procedure 2.1. Samples preparation Na2CO3, SrCO3, NH4H2PO2 and Tb4O7 powders all with a purity of 99.9% were used as the staring materials for NaSr1 xPO4:xTb3+ phosphors with different concentrations of Tb3+ ions (x = 0.008,
0925-3467/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.06.002
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0.01, 0.02, 0.04, 006). The powders were mixed in alcohol as solvent and ball-milled for 3 h with zirconia balls. After drying, the mixed powders were sintered at 1200 °C for 3 h under an air atmosphere with the heating and cooling rate controlled at 10 °C/min using the conventional solid-state reaction in a high-temperature furnace to form the NaSr1 xPO4:xTb3+ phosphors. 2.2. Characterization
Fig. 1. X-ray diffraction patterns of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. The inset is the enlargement of the main diffraction peak of the NaSr1 xPO4:xTb3+ phosphors.
Phases in the synthesized products were identified by X-ray diffraction analysis (Bruker D8 Advance) with Cu Ka radiation of k = 1.5406 Å using a Ni filter. The morphology of phosphor particles were examined by scanning electron microscope SEM (HORIBA EX-200). The luminescence properties including excitation and emission spectra were obtained using spectrofluorimeter (PL, JASCO FP-6000) with 150 W Xenon lamp as radiation source. The temperature-dependent luminescence spectra in the range of 25–350 K were detected by FluoroLog-3 pectrofluorometer (PL, HORIBA JOBIN YVON Fluorolog-3) combined with the heating apparatus.
Fig. 2. SEM images of NaSr1 xPO4:xTb3+ phosphors with (a) x = 0.008, (b) x = 0.01, (c) x = 0.02, (d) x = 0.04 and (e) x = 0.06.
R.-Y. Yang et al. / Optical Materials 35 (2013) 2183–2187
Fig. 3. (a) Excitation spectrum monitored at 546 nm and (b) emission spectrum excited at 354 nm of NaSr1 xPO4:xTb3+ phosphors with Tb3+ doping concentrations of 0.02 and at 370 nm with various concentrations of Tb3+ ions.
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Fig. 4. (a) The decay time curves of the 5D4 state from NaSr1 xPO4:xTb3+ phosphor obtained by excitation at 370 nm and monitored at 546 nm (b) The decay time curves of the 5D3 state from NaSr1 xPO4:xTb3+ phosphor obtained by excitation at 370 nm and monitored at 417 nm.
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. All diffraction peaks are agreed well with JCPDS (No. 33–1282) of NaSrPO4, indicating that Tb3+ ions can replace Sr3+ ions in the Sr3+ site of NaSrPO4 phosphate to form the solid solution and does not change the crystal structure of NaSrPO4, resulting in a single phase. Additionally, the inset of Fig. 1 shows the enlargement of the main diffraction peak of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. It is also found that the 32.74° of diffraction peak shifts slightly toward to the higher angle direction with increasing the Tb3+ ion concentration since the ion radius of Tb3+ (1.02 Å) is smaller than that of Sr2+ (1.29 Å). The result indicates that the ion radius mismatch caused crystal lattice distortion, which is similar to the investigation of Xihua et al. [10]. It is also noted that the intensity of the X-ray diffraction peaks decreased with increasing the Tb content. The result is the same as the investigation reported by Chang et al. [16]. The intensity of the X-ray diffraction peaks would be slightly decreased with the increasing amount of doping concentration of activator due to the losing long-range order domains. It is known phosphor particle’s distribution on luminous efficiency is an important factor for the application of phosphor in W-LEDs [3]. Fig. 2 shows the SEM images of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. The average grain sizes of NaSr1 xPO4:xTb3+ phosphors are
estimated in the range of 2–8 lm. It is known that phosphors with spherical shape and good dispersion have better luminescence. As shown in Fig. 2, although the morphology of the NaSr1 xPO4:xTb3+ phosphors have nearly spherical shape, but also have agglomerates and clusters of several particles. Fig. 3a shows the several excitation peaks are in the spectral region from 300 to 400 nm. It is found that the excitation peaks located at 304 nm, 318 nm, 342 nm, 354 nm, 370 nm, and 375 nm are ascribed to the transition bands of 7F6 ? 3H6, 7F6 ? 5D0, 7 F6 ? 5L7, 7F6 ? 5L9, 7F6 ? 5G5, and 7F6 ? 5G6, respectively [10]. Fig. 3b shows the several emission peaks are in the spectral region from 360 to 650 nm, which are associated with the typical f–f transitions of Tb3+ ion. According to the investigation of Sun et al. [14], for NaSrPO4: Tb3+ phosphor, the 5D3 ? 5D4 transition is typically resonant with the 7F6 ? 7F0 transition. However, due to the cross-relaxation effect from 5D3+7F6 ? 5D3+7F0 transition, the emission of 5D3 ? 7FJ (J = 4–6) transitions are often quenched for high Tb3+ concentration activated phosphors. Therefore, a less intense blue emission is appeared in the region from 360 to 450 nm, which originate from the relaxation of the 5D3 level [10,14]. In a previous work [6], there were two groups appeared in the emission spectra of KSrPO4:Tb3+ phosphors prepared by microwave assisted sintering, including: the blue emissions below 450 nm from 5D3 ? 7F6, 5D3 ? 7F5 and 5D3 ? 7F4 transitions of Tb3+ ion, and the yellowish green and red emissions above 450 nm from
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Table 1 The decay time value and chromaticity (x, y) coordinates of the NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. Tb3+ 5
7
D3– F5 D4–7F5 CIE
5
Concentration (x)
0.008
0.01
0.02
0.04
0.06
s (ms) s (ms)
1.29 3.62 (0.23, 0.28)
1.28 3.59 (0.24, 0.31)
0.96 3.51 (0.27, 0.42)
0.8 3.41 (0.28, 0.41)
0.73 3.30 (0.27, 0.40)
(x, y)
Fig. 5. The CIE1931 chromaticity diagram of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions (x = 0.008, 0.01, 0.02, 0.04 and 0.06).
Fig. 6. Temperature-dependent luminescent properties (excitation at 370 nm) of the NaSr0.98PO4:0.02Tb3+ phosphor measured at temperatures in the range of 25– 300 °C. The inset is the comparison of were temperature-dependent luminescent intensity of NaSr0.98PO4:0.02Tb3+ phosphor and commercially YAG: Ce3+ phosphor (excitation at 460 nm).
5
D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4, and 5D4 ? 7F3 transitions of Tb3+ ion [7,14]. In this study, it is found that the emission spectra at 382 nm, 417 nm, 439 nm, 493 nm, 546 nm, 587 nm, and 624 nm are attributed to the emission-state 5D3 ? 7F6, 5D3 ? 7F5, 5 D3 ? 7F4, 5D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4, and 5D4 ? 7F3 transitions of Tb3+ ions upon excitation at 354 nm, respectively [4]. All the prepared NaSrPO4:Tb3+ phosphors emitted blue–green light
which is mainly contributed by green luminescence from 5D4 to 7 F5 transition of Tb3+ with a wavelength of 546 nm. The emission intensity of the NaSrPO4:Tb3+ phosphor at 546 nm under excitation at 370 nm reaches maximum when the concentration of Tb3+ ion is x = 0.02 and then decreases with the increases of the Tb3+ concentration as shown in the inset of Fig. 3b. Such result indicated NaSrPO4: Tb3+ phosphor exists the concentration quenching effect. Moreover, the shape and the position of the emission peaks are not influenced by the Tb3+ activator concentration in this study. The decay time curves of the 5D4 state obtained by excitation at 370 nm and monitored at 546 nm are shown in Fig. 4a. It is known that the decay behavior can be expressed as I = I0 exp ( t/s), wherein I and I0 are the luminescence intensities at time 0 and t, respectively, and s is the lifetime for exponential component. The calculated values of the decay time are tabulated in Table 1. The decay time values of 5D4–7F5 transition with x = 0.008, 0.01, 0.02, 0.04 and 0.06 are 3.62 ms, 3.59 ms, 3.51 ms, 3.41 ms and 3.30 ms, respectively. Additionally, the decay time curves of the 5D3 state obtained by excitation at 370 nm and monitored at 417 nm are shown in Fig. 4b. The calculated values of the decay time for the 5 D3 state are also tabulated in Table 1. They are shorter than those of 5D4 level, and moreover the decay curves are non-exponentials. The lifetime of the Tb3+ ions slightly decreases as the increase of the Tb3+ ions in this study, which might be due to the energy transfer among Tb3+ ions occurring in a non-radiative manner at higher concentrations of Tb3+ ions in the NaSrPO4 host material [7]. Moreover, it is reported the decay time of electric-dipole allowed transition of f–d is usually not longer than 1 ls [12]. In contrast, the decay time of Tb3+ (5D4) is in the range of milliseconds, because of the forbidden nature of the f–f transition [7,14]. In this study, it is observed that the all specimens were accordance with f–f transitions of Tb3+ ion of milliseconds level, which are agree with the reported by Wang et al. [17]. Typically, color coordinates established by the Commission International de l’Eclairage (CIE) 1931 is used to express the color in a two-dimensional graphical representation of any color perceptible by the human eye on an x–y plot. Fig. 5 shows the CIE chromaticity diagram calculated from Fig. 3. The chromaticity (x, y) coordinates of the prepared NaSr1 xPO4:xTb3+ phosphors with x = 0.008, 0.01, 0.02, 0.04 and 0.06 are (0.23, 0.28), (0.24, 0.31), (0.27, 0.42), (0.28, 0.41) and (0.27, 0.40), respectively, and summarized in Table 1. This result indicates the chromaticity coordinate of NaSrPO4:xTb3+ phosphor varies with the increase of the concentration of Tb3+ions from blue to green. The result which located at the different region of CIE spectrum is similar to that of BaY2ZnO5:Eu3+ phosphors which CIE spectrum are different from blue, white, to red due to different Eu3+-doped concentration [18]. Thus, the NaSr1 xPO4:xTb3+ phosphors may be modulated according to the different applications. The thermal stability of the phosphors is an important factor for the application of phosphor in WLEDs, since it makes considerable influence on the light output as well as color-rendering index [8]. Fig. 6 shows the temperature-dependent luminescence properties of the NaSr0.98PO4:0.02Tb3+ with excitation at 370 nm and monitored at 546 nm, measured at temperatures in the range of 25 to 300 °C. It is obviously that although the emission intensity of the NaSr0.98PO4:0.02Tb3+ phosphor continuously decreases as
R.-Y. Yang et al. / Optical Materials 35 (2013) 2183–2187
temperature increases from 25 °C to 300 °C under excitation at 370 nm, the emission peaks do not shift with increasing temperatures. The reason is that the emission of Tb3+ ions from the f–f transitions within 4f configuration is typically shielded by 5s2 and 5p6 orbits, thus the emission of Tb3+ ions may not be affected by either environment surrounding Tb3+ ions as the increasing temperature or the changes in local host structure [7,15]. Compared to the temperature-dependent luminescence properties of commercial YAG: Ce3+ (excitation at 460 nm and monitored at 550 nm) phosphors [7], as illustrated in the insert of Fig 6, the thermal luminescence stability of NaSr0.98PO4:0.02Tb3+ phosphor was rather slightly better than that of the commercially YAG: Ce3+ phosphor as the temperature is over 200 °C. 4. Conclusion In this paper, we have successfully synthesized NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions (x = 0.008, 0.01, 0.02, 0.04 and 0.06) by conventional solid-state sintering at 1200 °C for 3 h under an air atmosphere. Pure phases of NaSr1 xPO4:xTb3+ phosphors are obtained without the presence of any extraneous phases using conventional solid-state sintering. The average particle sizes are estimated in the range of 2–8 lm. The main of emission peak centering at 546 nm corresponds to 5 D4 ? 7F5 transition, and other small emission peaks at 417 nm, 439 nm, 493 nm, 587 nm, and 624 nm can be assigned to 5 D3 ? 7F5, 5D3 ? 7F4, 5D4 ? 7F6, 5D4 ? 7F4, and 5D4 ? 7F3 transitions, respectively. The optimum concentration of Tb3+ ion is x = 0.02. The lifetime of the Tb3+ ions gradually decreases from 3.62 ms to 3.30 ms as the increase of the Tb3+ ions. It is found that the NaSr1 xPO4:xTb3+ phosphor is color-tunable with variation of the concentration of Tb3+ions from blue to green. The thermal luminescence stability of NaSr0.98PO4:0.02Tb3+ phosphor above 200 °C was better than that of the commercially YAG: Ce3+ phosphor. The above results indicate this NaSrPO4:Tb3+ phosphor is a promising material which could be applied in W-LEDs.
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Acknowledgements The authors would like to thank the financial support from National Science Council, South park administration and Metal Industries Research and Development Center of Taiwan, ROC under Contract Nos. NSC 101-2628-E-020-002-MY3 and 101CE02. The authors would also like to thank National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for the supporting the experimental equipment. References [1] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696–3698. [2] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, W.C. Lai, J.M. Tsai, G.C. Chi, R.K. Wu, IEEE Photonic. Tech. Lett. 15 (2003) 18–20. [3] X.Y. Yang, J. Liu, H. Yang, X.B. Yu, Y.Z. Guo, Y.Q. Zhou, J.Y. Liu, J. Mater. Chem. 19 (2009) 3771–3774. [4] C.C. Lin, R.S. Liu, Y.S. Tang, S.F. Hu, J. Electrochem. Soc. 155 (2008) J248–J251. [5] Y.K. Su, Y.M. Peng, R.Y. Yang, J.L. Chen, Opt. Mater. 34 (2012) 1598–1602. [6] M.H. Weng, R.Y. Yang, Y.M. Peng, J.L. Chen, Ceram. Int. 38 (2012) 1319–1323. [7] C.C. Lin, Y.S. Tang, S.F. Hu, R.S. Liu, J. Lumin. 129 (2009) 1682–1684. [8] S.Y. Zhang, Y. Nakai, T. Tsuboi, Y.L. Huang, H.J. Seo, Inorg. Chem. 50 (2011) 2897–2904. [9] Y.M. Peng, Y.K. Su, R.Y. Yang, Mater. Res. Bull. 48 (2013) 1946–1951. [10] D. Xihua, Z. Weiren, S. Enhai, D. Linlin, F. Xiabing, M. Huachu, J. Rare Earths 30 (2012) 739. [11] D.K. Yim, H.J. Song, I.S. Cho, J.S. Kim, K.S. Hong, Mater. Lett. 65 (2011) 1666– 1668. [12] Y.L. Tung, J.H. Jean, J. Am. Ceram. Soc. 92 (2009) 1860–1862. [13] J.Y. Sun, X.Y. Zhang, J.C. Zhu, Y. Sun, H.Y. Du, Adv. Mater. Res. 502 (2012) 128– 131. [14] J.Y. Sun, J.C. Zhu, X.Y. Zhang, Z.G. Xia, H.Y. Du, J. Lumin. 132 (2012) 2937–2942. [15] Y. Li, H.R. Li, B.T. Liu, J. Zhang, Z.Y. Zhao, Z.G. Yang, Y. Wen, Y.H. Wang, J. Phys. Chem. Solids 74 (2013) 175–180. [16] Y.S. Chang, F.M. Huang, Y.Y. Tsai, L.G. Teoh, J. Lumin. 129 (2009) 1181–1185. [17] F. Wang, H.W. Song, G.H. Pan, L.B. Fan, B.A. Dong, L.N. Liu, X. Bai, R.F. Qin, X.G. Ren, Z.H. Zheng, S.Z. Lu, J. Lumin. 128 (2008) 2013–2018. [18] C.H. Liang, Y.C. Chang, Y.S. Chang, Appl. Phys. Lett. 93 (2008) 211902.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Microstructure, luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering Ru-Yuan Yang a, Cheng-Tang Pan b, Kun-Hsien Chen b, Cheng-Yuan Hung c,⇑ a b c
Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung County 912, Taiwan Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung County 804, Taiwan Medical Devices and Opto-electronics Equipment Department, Metal Industries Research & Development Center, Taiwan
a r t i c l e
i n f o
Article history: Received 8 February 2013 Received in revised form 21 May 2013 Accepted 1 June 2013 Available online 28 June 2013 Keywords: Inorganic Compounds Conventional solid-state sintering Luminescence
a b s t r a c t This paper reports the microstructure, luminescence and thermal stability properties of the NaSr1 xPO4:x Tb3+ powders (x = 0.008, 0.01, 0.02, 0.04 and 0.06) via the conventional solid-state sintering at 1200 °C for 5 h. The X-ray diffraction result verifies all diffraction peaks are pure phase of NaSrPO4. The luminescence results show that the NaSrPO4:xTb3+ powders mainly excited at 370 nm have a series of the emissionstates, related to the typical 4f ? 4f intra-configuration forbidden transitions of Tb3+, and a major emission peak of around 546 nm. The concentration quenching of the NaSr1 xPO4:xTb3+ phosphors is appeared at x = 0.02. The decay time values of the NaSr1 xPO4:xTb3+ phosphors for the 5D4 state of the Tb3+ are around 3.30 ms to 3.60 ms. It is also found the chromaticity coordinate of NaSrPO4:Tb3+ phosphor varies with the increase of the concentration of Tb3+ ions from blue to green. Moreover, the thermal stability of the NaSrPO4:xTb3+ phosphors is slightly better than that of conventional YAG phosphors. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, phosphor converted-light emitting diodes (PC-LEDs) become the main stream for the current white LED market [1]. The present method to produce white light adopts the combination of blue LED chip (460 nm) with yellow-emitting phosphor, Y3Al5O12:Ce3+ (YAG:Ce3+), however, it still has some disadvantages, such as low thermal quenching temperature, narrow visible range, and poor color rendering due to the lack of red component [2]. Near-ultraviolet (NUV) InGaN-based LEDs, with a wavelength ranging from 350 to 420 nm, receive more attention because NUV LED can provide a highly efficient solid-state lighting [3]. Thus, new phosphor powders, desirable for color emission using high efficiency NUV excitation, must be searched and developed. Recently, phosphate compounds with the ABPO4 (A = alkaline metals, B = alkaline earth metals) structure were reported and became an important family of luminescent host materials since their tetrahedral rigid three dimensional matrix of phosphate are desired for charge stabilization, thus resulting in excellent thermal stability [4]. ABPO4 doped with different rare-earth ions (Eu3+, Ce3+, Tb3+, Sm3+ etc.), such as KSrPO4:Eu [4,5], KSrPO4:Tb [4,6], KSrPO4:Sm [4], KBaPO4:Eu [7,8], KBaPO4:Tb [7], KBaPO4:Sm [7], NaCaPO4:Eu [9], NaBaPO4:Ce,Tb [10], NaSrPO4:Eu [11–13], ⇑ Corresponding author. Tel.: +886 7 623 5550x315; fax: +886 7 623 5527. E-mail address: [email protected] (C.-Y. Hung).
NaSrPO4:Eu/Tb [14] and NaSrPO4:Eu/Mn [15] could emit different wavelengths, depending on various crystal fields. In which, NaSrPO4 host material has a standard monoclinic crystal structure with lattice parameters as a = 2.041 nm, b = 0.543 nm, c = 1.725 nm, and angle b = 101.76° [12,13]. In past, there have been several investigations of NaSrPO4: Eu phosphors [11–14] for blue emitting. Moreover, NaSrPO4 phosphors co-doped with Eu/ Tb ions were synthesized by a conventional solid-state reaction for blue/green emitting [14]. It is known Tb3+ ion is an important rare earth ion used to generate green emission under NUV excitation. However, microstructure, luminescence and thermal stability properties of the green-emitting NaSrPO4 doped with only Tb3+ ions has not yet been investigated and reported. Therefore, in this paper, the NaSrPO4:Tb3+ phosphors were synthesized in succeed via the solid-state sintering. The effects of different concentrations of Tb3+ ions on microstructure, luminescence and thermal stability characteristics of NaSrPO4:Tb3+ phosphors were investigated and discussed. 2. Experimental procedure 2.1. Samples preparation Na2CO3, SrCO3, NH4H2PO2 and Tb4O7 powders all with a purity of 99.9% were used as the staring materials for NaSr1 xPO4:xTb3+ phosphors with different concentrations of Tb3+ ions (x = 0.008,
0925-3467/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.06.002
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0.01, 0.02, 0.04, 006). The powders were mixed in alcohol as solvent and ball-milled for 3 h with zirconia balls. After drying, the mixed powders were sintered at 1200 °C for 3 h under an air atmosphere with the heating and cooling rate controlled at 10 °C/min using the conventional solid-state reaction in a high-temperature furnace to form the NaSr1 xPO4:xTb3+ phosphors. 2.2. Characterization
Fig. 1. X-ray diffraction patterns of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. The inset is the enlargement of the main diffraction peak of the NaSr1 xPO4:xTb3+ phosphors.
Phases in the synthesized products were identified by X-ray diffraction analysis (Bruker D8 Advance) with Cu Ka radiation of k = 1.5406 Å using a Ni filter. The morphology of phosphor particles were examined by scanning electron microscope SEM (HORIBA EX-200). The luminescence properties including excitation and emission spectra were obtained using spectrofluorimeter (PL, JASCO FP-6000) with 150 W Xenon lamp as radiation source. The temperature-dependent luminescence spectra in the range of 25–350 K were detected by FluoroLog-3 pectrofluorometer (PL, HORIBA JOBIN YVON Fluorolog-3) combined with the heating apparatus.
Fig. 2. SEM images of NaSr1 xPO4:xTb3+ phosphors with (a) x = 0.008, (b) x = 0.01, (c) x = 0.02, (d) x = 0.04 and (e) x = 0.06.
R.-Y. Yang et al. / Optical Materials 35 (2013) 2183–2187
Fig. 3. (a) Excitation spectrum monitored at 546 nm and (b) emission spectrum excited at 354 nm of NaSr1 xPO4:xTb3+ phosphors with Tb3+ doping concentrations of 0.02 and at 370 nm with various concentrations of Tb3+ ions.
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Fig. 4. (a) The decay time curves of the 5D4 state from NaSr1 xPO4:xTb3+ phosphor obtained by excitation at 370 nm and monitored at 546 nm (b) The decay time curves of the 5D3 state from NaSr1 xPO4:xTb3+ phosphor obtained by excitation at 370 nm and monitored at 417 nm.
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. All diffraction peaks are agreed well with JCPDS (No. 33–1282) of NaSrPO4, indicating that Tb3+ ions can replace Sr3+ ions in the Sr3+ site of NaSrPO4 phosphate to form the solid solution and does not change the crystal structure of NaSrPO4, resulting in a single phase. Additionally, the inset of Fig. 1 shows the enlargement of the main diffraction peak of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. It is also found that the 32.74° of diffraction peak shifts slightly toward to the higher angle direction with increasing the Tb3+ ion concentration since the ion radius of Tb3+ (1.02 Å) is smaller than that of Sr2+ (1.29 Å). The result indicates that the ion radius mismatch caused crystal lattice distortion, which is similar to the investigation of Xihua et al. [10]. It is also noted that the intensity of the X-ray diffraction peaks decreased with increasing the Tb content. The result is the same as the investigation reported by Chang et al. [16]. The intensity of the X-ray diffraction peaks would be slightly decreased with the increasing amount of doping concentration of activator due to the losing long-range order domains. It is known phosphor particle’s distribution on luminous efficiency is an important factor for the application of phosphor in W-LEDs [3]. Fig. 2 shows the SEM images of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. The average grain sizes of NaSr1 xPO4:xTb3+ phosphors are
estimated in the range of 2–8 lm. It is known that phosphors with spherical shape and good dispersion have better luminescence. As shown in Fig. 2, although the morphology of the NaSr1 xPO4:xTb3+ phosphors have nearly spherical shape, but also have agglomerates and clusters of several particles. Fig. 3a shows the several excitation peaks are in the spectral region from 300 to 400 nm. It is found that the excitation peaks located at 304 nm, 318 nm, 342 nm, 354 nm, 370 nm, and 375 nm are ascribed to the transition bands of 7F6 ? 3H6, 7F6 ? 5D0, 7 F6 ? 5L7, 7F6 ? 5L9, 7F6 ? 5G5, and 7F6 ? 5G6, respectively [10]. Fig. 3b shows the several emission peaks are in the spectral region from 360 to 650 nm, which are associated with the typical f–f transitions of Tb3+ ion. According to the investigation of Sun et al. [14], for NaSrPO4: Tb3+ phosphor, the 5D3 ? 5D4 transition is typically resonant with the 7F6 ? 7F0 transition. However, due to the cross-relaxation effect from 5D3+7F6 ? 5D3+7F0 transition, the emission of 5D3 ? 7FJ (J = 4–6) transitions are often quenched for high Tb3+ concentration activated phosphors. Therefore, a less intense blue emission is appeared in the region from 360 to 450 nm, which originate from the relaxation of the 5D3 level [10,14]. In a previous work [6], there were two groups appeared in the emission spectra of KSrPO4:Tb3+ phosphors prepared by microwave assisted sintering, including: the blue emissions below 450 nm from 5D3 ? 7F6, 5D3 ? 7F5 and 5D3 ? 7F4 transitions of Tb3+ ion, and the yellowish green and red emissions above 450 nm from
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Table 1 The decay time value and chromaticity (x, y) coordinates of the NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions. Tb3+ 5
7
D3– F5 D4–7F5 CIE
5
Concentration (x)
0.008
0.01
0.02
0.04
0.06
s (ms) s (ms)
1.29 3.62 (0.23, 0.28)
1.28 3.59 (0.24, 0.31)
0.96 3.51 (0.27, 0.42)
0.8 3.41 (0.28, 0.41)
0.73 3.30 (0.27, 0.40)
(x, y)
Fig. 5. The CIE1931 chromaticity diagram of NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions (x = 0.008, 0.01, 0.02, 0.04 and 0.06).
Fig. 6. Temperature-dependent luminescent properties (excitation at 370 nm) of the NaSr0.98PO4:0.02Tb3+ phosphor measured at temperatures in the range of 25– 300 °C. The inset is the comparison of were temperature-dependent luminescent intensity of NaSr0.98PO4:0.02Tb3+ phosphor and commercially YAG: Ce3+ phosphor (excitation at 460 nm).
5
D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4, and 5D4 ? 7F3 transitions of Tb3+ ion [7,14]. In this study, it is found that the emission spectra at 382 nm, 417 nm, 439 nm, 493 nm, 546 nm, 587 nm, and 624 nm are attributed to the emission-state 5D3 ? 7F6, 5D3 ? 7F5, 5 D3 ? 7F4, 5D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4, and 5D4 ? 7F3 transitions of Tb3+ ions upon excitation at 354 nm, respectively [4]. All the prepared NaSrPO4:Tb3+ phosphors emitted blue–green light
which is mainly contributed by green luminescence from 5D4 to 7 F5 transition of Tb3+ with a wavelength of 546 nm. The emission intensity of the NaSrPO4:Tb3+ phosphor at 546 nm under excitation at 370 nm reaches maximum when the concentration of Tb3+ ion is x = 0.02 and then decreases with the increases of the Tb3+ concentration as shown in the inset of Fig. 3b. Such result indicated NaSrPO4: Tb3+ phosphor exists the concentration quenching effect. Moreover, the shape and the position of the emission peaks are not influenced by the Tb3+ activator concentration in this study. The decay time curves of the 5D4 state obtained by excitation at 370 nm and monitored at 546 nm are shown in Fig. 4a. It is known that the decay behavior can be expressed as I = I0 exp ( t/s), wherein I and I0 are the luminescence intensities at time 0 and t, respectively, and s is the lifetime for exponential component. The calculated values of the decay time are tabulated in Table 1. The decay time values of 5D4–7F5 transition with x = 0.008, 0.01, 0.02, 0.04 and 0.06 are 3.62 ms, 3.59 ms, 3.51 ms, 3.41 ms and 3.30 ms, respectively. Additionally, the decay time curves of the 5D3 state obtained by excitation at 370 nm and monitored at 417 nm are shown in Fig. 4b. The calculated values of the decay time for the 5 D3 state are also tabulated in Table 1. They are shorter than those of 5D4 level, and moreover the decay curves are non-exponentials. The lifetime of the Tb3+ ions slightly decreases as the increase of the Tb3+ ions in this study, which might be due to the energy transfer among Tb3+ ions occurring in a non-radiative manner at higher concentrations of Tb3+ ions in the NaSrPO4 host material [7]. Moreover, it is reported the decay time of electric-dipole allowed transition of f–d is usually not longer than 1 ls [12]. In contrast, the decay time of Tb3+ (5D4) is in the range of milliseconds, because of the forbidden nature of the f–f transition [7,14]. In this study, it is observed that the all specimens were accordance with f–f transitions of Tb3+ ion of milliseconds level, which are agree with the reported by Wang et al. [17]. Typically, color coordinates established by the Commission International de l’Eclairage (CIE) 1931 is used to express the color in a two-dimensional graphical representation of any color perceptible by the human eye on an x–y plot. Fig. 5 shows the CIE chromaticity diagram calculated from Fig. 3. The chromaticity (x, y) coordinates of the prepared NaSr1 xPO4:xTb3+ phosphors with x = 0.008, 0.01, 0.02, 0.04 and 0.06 are (0.23, 0.28), (0.24, 0.31), (0.27, 0.42), (0.28, 0.41) and (0.27, 0.40), respectively, and summarized in Table 1. This result indicates the chromaticity coordinate of NaSrPO4:xTb3+ phosphor varies with the increase of the concentration of Tb3+ions from blue to green. The result which located at the different region of CIE spectrum is similar to that of BaY2ZnO5:Eu3+ phosphors which CIE spectrum are different from blue, white, to red due to different Eu3+-doped concentration [18]. Thus, the NaSr1 xPO4:xTb3+ phosphors may be modulated according to the different applications. The thermal stability of the phosphors is an important factor for the application of phosphor in WLEDs, since it makes considerable influence on the light output as well as color-rendering index [8]. Fig. 6 shows the temperature-dependent luminescence properties of the NaSr0.98PO4:0.02Tb3+ with excitation at 370 nm and monitored at 546 nm, measured at temperatures in the range of 25 to 300 °C. It is obviously that although the emission intensity of the NaSr0.98PO4:0.02Tb3+ phosphor continuously decreases as
R.-Y. Yang et al. / Optical Materials 35 (2013) 2183–2187
temperature increases from 25 °C to 300 °C under excitation at 370 nm, the emission peaks do not shift with increasing temperatures. The reason is that the emission of Tb3+ ions from the f–f transitions within 4f configuration is typically shielded by 5s2 and 5p6 orbits, thus the emission of Tb3+ ions may not be affected by either environment surrounding Tb3+ ions as the increasing temperature or the changes in local host structure [7,15]. Compared to the temperature-dependent luminescence properties of commercial YAG: Ce3+ (excitation at 460 nm and monitored at 550 nm) phosphors [7], as illustrated in the insert of Fig 6, the thermal luminescence stability of NaSr0.98PO4:0.02Tb3+ phosphor was rather slightly better than that of the commercially YAG: Ce3+ phosphor as the temperature is over 200 °C. 4. Conclusion In this paper, we have successfully synthesized NaSr1 xPO4:xTb3+ phosphors with various concentrations of Tb3+ ions (x = 0.008, 0.01, 0.02, 0.04 and 0.06) by conventional solid-state sintering at 1200 °C for 3 h under an air atmosphere. Pure phases of NaSr1 xPO4:xTb3+ phosphors are obtained without the presence of any extraneous phases using conventional solid-state sintering. The average particle sizes are estimated in the range of 2–8 lm. The main of emission peak centering at 546 nm corresponds to 5 D4 ? 7F5 transition, and other small emission peaks at 417 nm, 439 nm, 493 nm, 587 nm, and 624 nm can be assigned to 5 D3 ? 7F5, 5D3 ? 7F4, 5D4 ? 7F6, 5D4 ? 7F4, and 5D4 ? 7F3 transitions, respectively. The optimum concentration of Tb3+ ion is x = 0.02. The lifetime of the Tb3+ ions gradually decreases from 3.62 ms to 3.30 ms as the increase of the Tb3+ ions. It is found that the NaSr1 xPO4:xTb3+ phosphor is color-tunable with variation of the concentration of Tb3+ions from blue to green. The thermal luminescence stability of NaSr0.98PO4:0.02Tb3+ phosphor above 200 °C was better than that of the commercially YAG: Ce3+ phosphor. The above results indicate this NaSrPO4:Tb3+ phosphor is a promising material which could be applied in W-LEDs.
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Acknowledgements The authors would like to thank the financial support from National Science Council, South park administration and Metal Industries Research and Development Center of Taiwan, ROC under Contract Nos. NSC 101-2628-E-020-002-MY3 and 101CE02. The authors would also like to thank National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for the supporting the experimental equipment. References [1] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696–3698. [2] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, W.C. Lai, J.M. Tsai, G.C. Chi, R.K. Wu, IEEE Photonic. Tech. Lett. 15 (2003) 18–20. [3] X.Y. Yang, J. Liu, H. Yang, X.B. Yu, Y.Z. Guo, Y.Q. Zhou, J.Y. Liu, J. Mater. Chem. 19 (2009) 3771–3774. [4] C.C. Lin, R.S. Liu, Y.S. Tang, S.F. Hu, J. Electrochem. Soc. 155 (2008) J248–J251. [5] Y.K. Su, Y.M. Peng, R.Y. Yang, J.L. Chen, Opt. Mater. 34 (2012) 1598–1602. [6] M.H. Weng, R.Y. Yang, Y.M. Peng, J.L. Chen, Ceram. Int. 38 (2012) 1319–1323. [7] C.C. Lin, Y.S. Tang, S.F. Hu, R.S. Liu, J. Lumin. 129 (2009) 1682–1684. [8] S.Y. Zhang, Y. Nakai, T. Tsuboi, Y.L. Huang, H.J. Seo, Inorg. Chem. 50 (2011) 2897–2904. [9] Y.M. Peng, Y.K. Su, R.Y. Yang, Mater. Res. Bull. 48 (2013) 1946–1951. [10] D. Xihua, Z. Weiren, S. Enhai, D. Linlin, F. Xiabing, M. Huachu, J. Rare Earths 30 (2012) 739. [11] D.K. Yim, H.J. Song, I.S. Cho, J.S. Kim, K.S. Hong, Mater. Lett. 65 (2011) 1666– 1668. [12] Y.L. Tung, J.H. Jean, J. Am. Ceram. Soc. 92 (2009) 1860–1862. [13] J.Y. Sun, X.Y. Zhang, J.C. Zhu, Y. Sun, H.Y. Du, Adv. Mater. Res. 502 (2012) 128– 131. [14] J.Y. Sun, J.C. Zhu, X.Y. Zhang, Z.G. Xia, H.Y. Du, J. Lumin. 132 (2012) 2937–2942. [15] Y. Li, H.R. Li, B.T. Liu, J. Zhang, Z.Y. Zhao, Z.G. Yang, Y. Wen, Y.H. Wang, J. Phys. Chem. Solids 74 (2013) 175–180. [16] Y.S. Chang, F.M. Huang, Y.Y. Tsai, L.G. Teoh, J. Lumin. 129 (2009) 1181–1185. [17] F. Wang, H.W. Song, G.H. Pan, L.B. Fan, B.A. Dong, L.N. Liu, X. Bai, R.F. Qin, X.G. Ren, Z.H. Zheng, S.Z. Lu, J. Lumin. 128 (2008) 2013–2018. [18] C.H. Liang, Y.C. Chang, Y.S. Chang, Appl. Phys. Lett. 93 (2008) 211902.