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Photoluminescence properties of red europium doped calcium tungstate phosphors for blue-pumped light-emitting diodes
Optik 126 (2015) 1341–1343
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Photoluminescence properties of red europium doped calcium tungstate phosphors for blue-pumped light-emitting diodes W.J. Zhang a , W.L. Feng a,b,∗ , Y.M. Nie a a b
School of Optoelectronic Information, Chongqing University of Technology, Chongqing 400054, China International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 28 February 2014 Accepted 9 April 2015 Keywords: Photoluminescence Phosphor Hydrothermal method CaWO4 :Eu3+
a b s t r a c t A series of stable CaWO4 :Eu3+ phosphors have been prepared by the hydrothermal method. Eu2 O3 (99.99%), CaCl2 (A.R.), Na2 WO4 ·2H2 O (A.R.) are raw materials. The luminescent properties of the phosphors have been investigated by the different contents of the dopants in CaWO4 and the optimum concentration of as-synthesized phophors is 1.5 wt%. The structure, morphology and luminescent properties of the phosphors are characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectroscopy technology. The results showed that introduction of Eu did not change the crystal structure and morphology of CaWO4 :Eu3+ phosphors. The strongest emission at 614 nm is corresponding to 5 D0 → 7 F2 transition of Eu3+ under 464 nm blue light excitation. The result reveals that CaWO4 :Eu3+ is a promising red-emitting phosphor for blue LED applications. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction
2. Experimental
The alkaline earth tungstates or molybdates have attracted more attention because of their potential applications in the solid-state lighting [1–3]. Using the blue LED with a mixture of green–yellow or red–orange phosphors can give white light with an improved color rendering. Many researches have chosen Eu3+ , Sm3+ and Pr3+ as dopant ions in different hosts [4–8], such as SrY2 O4 :Eu3+ [4], Sr4 Al2 O7 :Eu3+ [5], MWO4 :Sm3+ (M = Ca, Sr, Ba) [6], YAl3 (BO3 )4 :Sm3+ [7], SrMoO4 :Pr3+ , B3+ , Li+ [8], etc., to emit red or orange light. In addition, the hydrothermal method is an effective method of synthesis of single crystals that depend on the solubility of raw materials in hot water under high pressure [9,10]. However, the preparation of Eu3+ in CaWO4 matrix by using the hydrothermal method has not been reported. In this paper we report the photoluminescence (PL) of the phosphors CaWO4 :Eu3+ and observed that the phosphors can be excited by the blue light and emit red light. Our results suggest that the studied samples may be good candidates for phosphors pumped by the blue LED. In addition, the present samples would have good thermal stability since they are synthesized at 200 ◦ C.
The samples of CaWO4 :Eu3+ were prepared using a hydrothermal method. The NH4 HCO3 –NH3 ·H2 O is used as the precipitator. The reactants include Na2 WO4 (99.99%), Eu2 O3 (99.99%), CaCl2 (99.9%). According to the nominal compositions of compounds Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%), appropriate amount of starting materials were thoroughly weighed, Na2 WO4 and CaCl2 were respectively dissolved with deionized water under magnetic stirring and heating at 50 ◦ C, Eu2 O3 was dissolved with chlorhydric acid. Then the obtained clear solution was dropped into the mixture of precipitator NH4 HCO3 –NH3 ·H2 O by means of anti-drop with continuous stirring. Precipitated suspension was hydrothermally treated in stainless steel autoclave (1000 ml, filling factor 75%) for 18 h at 200 ◦ C. After the cooling of the autoclave white precipitate was collected through centrifugation. Washing was done up to pH 7. Obtained powders were dried in vacuum at 90 ◦ C for 3 h. To achieve targeted composition, powder was additionally thermally annealed in air at 800 ◦ C for 3 h, after milling, uniform powders were obtained. The crystal structure of the phosphor powders was characterized by X-ray diffraction analysis using an X-ray diffractometer ˚ (XRD; Shimadzu XRD-6000) with Cu K␣ irradiation ( = 1.54056 A). The surface morphology of phosphor was characterized by TESCAN VEGAIILMU scanning electron microscope (SEM). The excitation and emission spectra were measured on a RF-5301 Molecular Fluorescence Spectrometer equipped with xenon lamp as excitation source with 5 nm of the excitation and emission slit. In order to
∗ Corresponding author at: International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China. Tel.: +86 023 62563272. E-mail address: [email protected] (W.L. Feng). http://dx.doi.org/10.1016/j.ijleo.2015.04.015 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
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compare the PL intensity, the amount of the samples was the same. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. Crystal structure of Ca1−x WO4 :xEu3+ The XRD patterns of Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%) are shown in Fig. 1. These peaks of the XRD patterns of the samples agree well with the Joint Committee on Powder Diffraction Standards (JCPDS) file 41-1431 of the host crystal [11], which possesses a pure phase with the scheelite structure of space group I41 /a, indicating that the Eu3+ doped CaWO4 almost cannot change the lattice site of the host crystal. Each calcium or europium ion in the CaWO4 :Eu lattice is surrounded by eight oxygen ions and possessed the tetragonal symmetry structure. 3.2. The morphological analysis A typical SEM image of Ca0.988 Eu0.012 O4 phosphor sample is shown in Fig. 2. It can be seen that Ca0.988 Eu0.012 O4 phosphor sample is made up of uniform flower-like morphology with diameters in the range of 1–3 m. 3.3. Photoluminescence properties Fig. 2. The SEM image of Ca0.985 Eu0.015 WO4 .
The excitation and photoluminescence spectra of Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%) are shown in Fig. 3(a) and (b). In Fig. 3(a), the excitation spectrum demonstrates two broad bands with 220–280 nm and 310–240 nm. The broad band absorption corresponds to an overlap of Eu O charge transfer (CT) band and 1 T1 → 1 A1 transition of WO6 6− complex ion. Additionally, the sharp peaks from 360 to 480 nm are ascribed to the intra-configurational 4f–4f transitions of Eu3+ ions in this host lattice. Six of these sharp absorptions are observed at 362 nm (7 F0 –5 D4 ), 386 nm (7 F0 –5 L7 ), 395 nm (7 F0 –5 L6 ), 422 nm (7 F0 –5 D3 ), 466 nm (7 F0 –5 D2 ) and 474 nm (7 F0 –5 D1 ). With the increasing content of Eu3+ ions, the intensity of absorption band is enhanced until x = 1.5 wt%, and then decreased the content higher than 1.5 wt%. The emission spectra of Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%) excited with 464 nm blue wavelength are shown in Fig. 2(b). Two obvious sharp emission peaks at 591 and 612 nm are observed. The two sharp emission peaks are corresponding to the 5 D –7 F and 5 D –7 F transitions of Eu3+ ion. One can find that the 0 1 0 2 strongest emission occurs when the concentration is 1.5 wt%. At the
Fig. 1. XRD pattern of the Ca1−x WO4 :xEu3+ (x is A: 0.9, B: 1.2, C: 1.5 and D: 1.8 wt%).
Fig. 3. (a) Excitation spectrum of Ca1−x WO4 :xEu3+ (x is A: 0.9, B: 1.2, C: 1.5 and D: 1.8 wt%) (em = 612 nm); (b) Emission spectra of Ca1−x WO4 :xEu3+ (x is A: 0.9, B: 1.2, C: 1.5 and D: 1.8 wt%) (ex = 464 nm).
W.J. Zhang et al. / Optik 126 (2015) 1341–1343
same time, the optical power of excitation is feasible when excited by ex = 464 nm which matches well the emission wavelength of the blue-pumped LED chips. 4. Conclusion In this work, the red phosphors CaWO4 :Eu3+ have been successfully synthesized through hydrothermal method and the PL properties have also been investigated in detail for blue-pumped LED applications. The structure of CaWO4 :Eu3+ is the tetragonal symmetry and the particles of the sample exhibit a good morphology distribution of about 1–3 m and a flower shape. The red emission corresponding to the 5 D0 → 7 F2 was observed under the blue excitation. The characteristic absorption of Eu3+ at 7 F0 → 5 D2 (464 nm) agrees well with the emission wavelengths of blue LED chips. Thus, CaWO4 :Eu3+ may be a good candidate of phosphor for the white LED. In addition, the dependence of the luminescence was studied, and optimum doping concentration for obtaining maximum emitting intensity was confirmed to be at 1.5 wt%. Acknowledgements Project supported by the Natural Science Foundation Project of CQ (grant nos. CSTC2011jjA50015, KJ120826 and KJ130823), the Key Project of Chinese Ministry of Education (no. 212139).
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References [1] W.L. Feng, Y. Jin, Y. Wu, D.F. Li, A.K. Cai, Co-precipitation synthesis and photoluminescence properties of Ba1−x MoO4 :xEu3+ red phosphors, J. Lumin. 134 (2013) 614–617. [2] Z. Wang, H. Liang, M. Gong, Q. Su, Luminescence investigation of Eu3+ activated double molybdates red phosphors with scheelite structure, J. Alloys Compd. 432 (2007) 308–312. [3] W.L. Feng, M.F. Zhao, J.Y. Xue, X.J. Tian, Photoluminescence properties of (Ba1−x Eux )WO4 red synthesized by the coprecipitation/calcination method, J. Alloys Compd. 521 (2012) 146–149. [4] V. Dubey, J. Kaur, S. Agrawal, N.S. Suryanarayana, K.V.R. Murthy, Synthesis and characterization of Eu3+ doped SrY2 O4 phosphor, Optik 124 (2013) 5585–5587. [5] A.N. Yerpude, S.J. Dhoble, Luminescence in trivalent rare earth activated Sr4 Al2 O7 phosphor, Optik 124 (2013) 3567–3570. [6] J. Liao, L. Liu, H. You, H. Huang, W. You, Hydrothermal preparation and luminescence property of MWO4 :Sm3+ (M = Ca, Sr, Ba) red phosphors, Optik 123 (2012) 901–905. [7] E. Cavalli, F. angiuli, I. Kebaïli, M. Dammak, P. Boutinaud, M. Bettinelli, Luminescence dynamics of YAl3 (BO3 )4 :Sm3+ crystals, J. Lumin. 143 (2013) 562–565. [8] F. Zhao, W.L. Feng, X.L. Cheng, Synthesis and luminescent properties of a novel orange SrMoO4 :Pr3+ , B3+ , Li+ phosphor, Acta Opt. Sin. 34 (2014) 0116002. [9] J. Liao, D. Zhou, X. Qiu, S. Liu, H.R. Wen, Charge compensation on the luminescence properties of ZnWO4 :Tb3+ phosphors via hydrothermal synthesis, Optik 124 (2013) 5057–5060. [10] H. Yu, H. Fan, X. Wang, J. Wang, Synthesis and characterization of ZnO microstructures via microwave-assisted hydrothermal synthesis process, Optik 125 (2014) 1461–1464. [11] M. Kay, B. Frazer, I. Almodovar, Neutron diffraction refinement of CaWO4 , J. Chem. Phys. 40 (1964) 504–506.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Photoluminescence properties of red europium doped calcium tungstate phosphors for blue-pumped light-emitting diodes W.J. Zhang a , W.L. Feng a,b,∗ , Y.M. Nie a a b
School of Optoelectronic Information, Chongqing University of Technology, Chongqing 400054, China International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 28 February 2014 Accepted 9 April 2015 Keywords: Photoluminescence Phosphor Hydrothermal method CaWO4 :Eu3+
a b s t r a c t A series of stable CaWO4 :Eu3+ phosphors have been prepared by the hydrothermal method. Eu2 O3 (99.99%), CaCl2 (A.R.), Na2 WO4 ·2H2 O (A.R.) are raw materials. The luminescent properties of the phosphors have been investigated by the different contents of the dopants in CaWO4 and the optimum concentration of as-synthesized phophors is 1.5 wt%. The structure, morphology and luminescent properties of the phosphors are characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectroscopy technology. The results showed that introduction of Eu did not change the crystal structure and morphology of CaWO4 :Eu3+ phosphors. The strongest emission at 614 nm is corresponding to 5 D0 → 7 F2 transition of Eu3+ under 464 nm blue light excitation. The result reveals that CaWO4 :Eu3+ is a promising red-emitting phosphor for blue LED applications. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction
2. Experimental
The alkaline earth tungstates or molybdates have attracted more attention because of their potential applications in the solid-state lighting [1–3]. Using the blue LED with a mixture of green–yellow or red–orange phosphors can give white light with an improved color rendering. Many researches have chosen Eu3+ , Sm3+ and Pr3+ as dopant ions in different hosts [4–8], such as SrY2 O4 :Eu3+ [4], Sr4 Al2 O7 :Eu3+ [5], MWO4 :Sm3+ (M = Ca, Sr, Ba) [6], YAl3 (BO3 )4 :Sm3+ [7], SrMoO4 :Pr3+ , B3+ , Li+ [8], etc., to emit red or orange light. In addition, the hydrothermal method is an effective method of synthesis of single crystals that depend on the solubility of raw materials in hot water under high pressure [9,10]. However, the preparation of Eu3+ in CaWO4 matrix by using the hydrothermal method has not been reported. In this paper we report the photoluminescence (PL) of the phosphors CaWO4 :Eu3+ and observed that the phosphors can be excited by the blue light and emit red light. Our results suggest that the studied samples may be good candidates for phosphors pumped by the blue LED. In addition, the present samples would have good thermal stability since they are synthesized at 200 ◦ C.
The samples of CaWO4 :Eu3+ were prepared using a hydrothermal method. The NH4 HCO3 –NH3 ·H2 O is used as the precipitator. The reactants include Na2 WO4 (99.99%), Eu2 O3 (99.99%), CaCl2 (99.9%). According to the nominal compositions of compounds Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%), appropriate amount of starting materials were thoroughly weighed, Na2 WO4 and CaCl2 were respectively dissolved with deionized water under magnetic stirring and heating at 50 ◦ C, Eu2 O3 was dissolved with chlorhydric acid. Then the obtained clear solution was dropped into the mixture of precipitator NH4 HCO3 –NH3 ·H2 O by means of anti-drop with continuous stirring. Precipitated suspension was hydrothermally treated in stainless steel autoclave (1000 ml, filling factor 75%) for 18 h at 200 ◦ C. After the cooling of the autoclave white precipitate was collected through centrifugation. Washing was done up to pH 7. Obtained powders were dried in vacuum at 90 ◦ C for 3 h. To achieve targeted composition, powder was additionally thermally annealed in air at 800 ◦ C for 3 h, after milling, uniform powders were obtained. The crystal structure of the phosphor powders was characterized by X-ray diffraction analysis using an X-ray diffractometer ˚ (XRD; Shimadzu XRD-6000) with Cu K␣ irradiation ( = 1.54056 A). The surface morphology of phosphor was characterized by TESCAN VEGAIILMU scanning electron microscope (SEM). The excitation and emission spectra were measured on a RF-5301 Molecular Fluorescence Spectrometer equipped with xenon lamp as excitation source with 5 nm of the excitation and emission slit. In order to
∗ Corresponding author at: International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China. Tel.: +86 023 62563272. E-mail address: [email protected] (W.L. Feng). http://dx.doi.org/10.1016/j.ijleo.2015.04.015 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
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compare the PL intensity, the amount of the samples was the same. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. Crystal structure of Ca1−x WO4 :xEu3+ The XRD patterns of Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%) are shown in Fig. 1. These peaks of the XRD patterns of the samples agree well with the Joint Committee on Powder Diffraction Standards (JCPDS) file 41-1431 of the host crystal [11], which possesses a pure phase with the scheelite structure of space group I41 /a, indicating that the Eu3+ doped CaWO4 almost cannot change the lattice site of the host crystal. Each calcium or europium ion in the CaWO4 :Eu lattice is surrounded by eight oxygen ions and possessed the tetragonal symmetry structure. 3.2. The morphological analysis A typical SEM image of Ca0.988 Eu0.012 O4 phosphor sample is shown in Fig. 2. It can be seen that Ca0.988 Eu0.012 O4 phosphor sample is made up of uniform flower-like morphology with diameters in the range of 1–3 m. 3.3. Photoluminescence properties Fig. 2. The SEM image of Ca0.985 Eu0.015 WO4 .
The excitation and photoluminescence spectra of Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%) are shown in Fig. 3(a) and (b). In Fig. 3(a), the excitation spectrum demonstrates two broad bands with 220–280 nm and 310–240 nm. The broad band absorption corresponds to an overlap of Eu O charge transfer (CT) band and 1 T1 → 1 A1 transition of WO6 6− complex ion. Additionally, the sharp peaks from 360 to 480 nm are ascribed to the intra-configurational 4f–4f transitions of Eu3+ ions in this host lattice. Six of these sharp absorptions are observed at 362 nm (7 F0 –5 D4 ), 386 nm (7 F0 –5 L7 ), 395 nm (7 F0 –5 L6 ), 422 nm (7 F0 –5 D3 ), 466 nm (7 F0 –5 D2 ) and 474 nm (7 F0 –5 D1 ). With the increasing content of Eu3+ ions, the intensity of absorption band is enhanced until x = 1.5 wt%, and then decreased the content higher than 1.5 wt%. The emission spectra of Ca1−x WO4 :xEu3+ (x = 0.9, 1.2, 1.5 and 1.8 wt%) excited with 464 nm blue wavelength are shown in Fig. 2(b). Two obvious sharp emission peaks at 591 and 612 nm are observed. The two sharp emission peaks are corresponding to the 5 D –7 F and 5 D –7 F transitions of Eu3+ ion. One can find that the 0 1 0 2 strongest emission occurs when the concentration is 1.5 wt%. At the
Fig. 1. XRD pattern of the Ca1−x WO4 :xEu3+ (x is A: 0.9, B: 1.2, C: 1.5 and D: 1.8 wt%).
Fig. 3. (a) Excitation spectrum of Ca1−x WO4 :xEu3+ (x is A: 0.9, B: 1.2, C: 1.5 and D: 1.8 wt%) (em = 612 nm); (b) Emission spectra of Ca1−x WO4 :xEu3+ (x is A: 0.9, B: 1.2, C: 1.5 and D: 1.8 wt%) (ex = 464 nm).
W.J. Zhang et al. / Optik 126 (2015) 1341–1343
same time, the optical power of excitation is feasible when excited by ex = 464 nm which matches well the emission wavelength of the blue-pumped LED chips. 4. Conclusion In this work, the red phosphors CaWO4 :Eu3+ have been successfully synthesized through hydrothermal method and the PL properties have also been investigated in detail for blue-pumped LED applications. The structure of CaWO4 :Eu3+ is the tetragonal symmetry and the particles of the sample exhibit a good morphology distribution of about 1–3 m and a flower shape. The red emission corresponding to the 5 D0 → 7 F2 was observed under the blue excitation. The characteristic absorption of Eu3+ at 7 F0 → 5 D2 (464 nm) agrees well with the emission wavelengths of blue LED chips. Thus, CaWO4 :Eu3+ may be a good candidate of phosphor for the white LED. In addition, the dependence of the luminescence was studied, and optimum doping concentration for obtaining maximum emitting intensity was confirmed to be at 1.5 wt%. Acknowledgements Project supported by the Natural Science Foundation Project of CQ (grant nos. CSTC2011jjA50015, KJ120826 and KJ130823), the Key Project of Chinese Ministry of Education (no. 212139).
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References [1] W.L. Feng, Y. Jin, Y. Wu, D.F. Li, A.K. Cai, Co-precipitation synthesis and photoluminescence properties of Ba1−x MoO4 :xEu3+ red phosphors, J. Lumin. 134 (2013) 614–617. [2] Z. Wang, H. Liang, M. Gong, Q. Su, Luminescence investigation of Eu3+ activated double molybdates red phosphors with scheelite structure, J. Alloys Compd. 432 (2007) 308–312. [3] W.L. Feng, M.F. Zhao, J.Y. Xue, X.J. Tian, Photoluminescence properties of (Ba1−x Eux )WO4 red synthesized by the coprecipitation/calcination method, J. Alloys Compd. 521 (2012) 146–149. [4] V. Dubey, J. Kaur, S. Agrawal, N.S. Suryanarayana, K.V.R. Murthy, Synthesis and characterization of Eu3+ doped SrY2 O4 phosphor, Optik 124 (2013) 5585–5587. [5] A.N. Yerpude, S.J. Dhoble, Luminescence in trivalent rare earth activated Sr4 Al2 O7 phosphor, Optik 124 (2013) 3567–3570. [6] J. Liao, L. Liu, H. You, H. Huang, W. You, Hydrothermal preparation and luminescence property of MWO4 :Sm3+ (M = Ca, Sr, Ba) red phosphors, Optik 123 (2012) 901–905. [7] E. Cavalli, F. angiuli, I. Kebaïli, M. Dammak, P. Boutinaud, M. Bettinelli, Luminescence dynamics of YAl3 (BO3 )4 :Sm3+ crystals, J. Lumin. 143 (2013) 562–565. [8] F. Zhao, W.L. Feng, X.L. Cheng, Synthesis and luminescent properties of a novel orange SrMoO4 :Pr3+ , B3+ , Li+ phosphor, Acta Opt. Sin. 34 (2014) 0116002. [9] J. Liao, D. Zhou, X. Qiu, S. Liu, H.R. Wen, Charge compensation on the luminescence properties of ZnWO4 :Tb3+ phosphors via hydrothermal synthesis, Optik 124 (2013) 5057–5060. [10] H. Yu, H. Fan, X. Wang, J. Wang, Synthesis and characterization of ZnO microstructures via microwave-assisted hydrothermal synthesis process, Optik 125 (2014) 1461–1464. [11] M. Kay, B. Frazer, I. Almodovar, Neutron diffraction refinement of CaWO4 , J. Chem. Phys. 40 (1964) 504–506.