Research article Received: 8 December 2014,
Revised: 1 March 2015,
Accepted: 20 March 2015
Published online in Wiley Online Library: 19 April 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2922
Luminescent properties of Ca3SiO4Cl2 co-doped with Ce3+ and Eu2+ for near-ultraviolet light-emitting diodes Wangqing Shen,a Yiwen Zhub and Zhengliang Wangb* ABSTRACT: Ca3SiO4Cl2 co-doped with Ce3+,Eu2+ was prepared by high temperature reaction. The structure, luminescent properties and the energy transfer process of Ca3SiO4Cl2: Ce3+,Eu2+ were investigated. Eu2+ ions can give enhanced green emission through Ce3+ → Eu2+ energy transfer in these phosphors. The green phosphor Ca2.9775SiO4Cl2:0.0045Ce3+,0.018Eu2+ showed intense green emission with broader excitation in the near-ultraviolet light range. A green light-emitting diode (LED) based on this phosphor was made, and bright green light from this green LED could be observed by the naked eye under 20 mA current excitation. Hence it is considered to be a good candidate for the green component of a three-band white LED. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: luminescent properties; phosphor; light-emitting diodes; Ca3SiO4Cl2
Introduction
Experimental
Phosphor-converted light-emitting diodes (LEDs) are an important kind of solid-state illumination (1–3). Commercial white LEDs are mainly obtained by combining a yellow-emitting phosphor YAG: Ce3+ pumped with a blue-emitting GaN LED chip (4,5). At present, the emission bands of LED chips are shifted to the near-ultraviolet (UV) range and this wavelength can offer a higher efficiency of solid-state lighting (6,7). Conventional phosphors used in fluorescence lighting are not suitable for the near-UV LED chips, because of their poor absorption in the near-UV light region (8). Hence, there is a need to investigate novel phosphors for nearUV LED chips. Alkaline earth halo-silicates are well known hosts for inorganic luminescent materials due to their low synthesis temperature and high luminescence efficiency (9). The compound calcium chlorosilicate Ca3SiO4Cl2 was reported first by Chaterlier (10). Many researchers have investigated its phase structure transformation (10), and the luminescent properties in different crystal structures doped with Eu2+ (9–12). Little information on this host doped with other rare earth ions has been reported. Ce3+ ions are very good luminescent centers as well as sensitizers of luminescence in many hosts (13–17). They can efficiently absorb UV light and transfer the energy to the luminescent centers, thus the emission intensity of the luminescent center would be strengthened. Due to the overlap of spectra between the Ce3+ emission band and the Eu2+ excitation band, the 4f–5d transitions of Ce3+ to Eu2+ are very sensitive to the nature of the host lattice, and the absorption and emission of Ce3+ and Eu2+ are efficient in many hosts (18–20). In this paper, Ca3SiO4Cl2 co-doped with Ce3+,Eu2+ was prepared by high temperature reaction. The luminescent properties and the energy transfer process of Ca3SiO4Cl2:Ce3+,Eu2+ were investigated. Finally, a single green LED was made by combining the green phosphor with ~400 nm-emitting InGaN chips.
Synthesis
Characterizations The structure of these phosphors was recorded by X-ray powder diffraction (XRD) using Cu Kα radiation on a RIGAKU D/max 2200 vpc X-ray diffractometer. Their photoluminescent spectra were recorded on a Cary Eclipse FL1011M003 (Varian) spectrofluorometer and a xenon lamp was used as an excitation source. The electroluminescence of green LEDs was recorded on a LED
* Correspondence to: Z. Wang, Engineering Research Center of Biopolymer Functional Materials of Yunnan, School of Chemistry and Biotechnology, Yunnan Minzu University, Kunming, Yunnan 650500, People’s Republic of China. E-mail: [email protected] a
College of Chemistry and Chemical Engineering, Neijiang Normal University, Neijiang, Sichuan 641112, People’s Republic of China
b
Engineering Research Center of Biopolymer Functional Materials of Yunnan, School of Chemistry and Biotechnology, Yunnan Minzu University, Kunming, Yunnan 650500, People’s Republic of China Abbreviations: LED, light-emitting diode; XRD, X-ray powder diffraction.
Copyright © 2015 John Wiley & Sons, Ltd.
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Luminescence 2015; 30: 1409–1412
The phosphor Ca3SiO4Cl2 was synthesized by a solid-state reaction. The raw materials CaCO3 (A.R. grade), SiO2 (A.R. grade), CaCl2 (A.R. grade), Eu2O3 (99.99% purity), and CeO2 (99.99% purity) were mixed thoroughly and ground together with an agate mortar and pestle. Then the mixture were first pre-fired at 500 °C for 4 h, and then heated at 900 °C for 4 h. The process was carried out in a reducing atmosphere (N2:H2 = 95:5). The green LED was made by combing an InGaN chip (~395 nm) with a mixture of green phosphor and epoxy resin (the mass ratio is 1:1).
W. Shen et al. 700
spectrophotocolorimeter (Everfine PMS-50). All the measurements were performed at room temperature.
Results and discussion
600
b
The XRD patterns of Ca2.997SiO4Cl2:0.003Ce3+ and Ca2.97SiO4Cl2: 0.03Eu2+ are shown in Fig. 1. They are consistent with the Joint Committee on Powder Diffraction Standards ( JCPDS) card 70–2447 [Ca3SiO4Cl2]. This indicates that the as-prepared phosphors share the single phase and the doping of Ce3+ and Eu2+ does not change the crystal structure of Ca3SiO4Cl2. The slight blue shift of diffraction peaks of Ca2.997SiO4Cl2:0.003Ce3+ and Ca2.97SiO4Cl2:0.03Eu2+ is due to the distinct ionic radii between Ca2+ and Eu2+,Ce3+. This host lattice Ca3SiO4Cl2 has a monoclinic structure with space group P21/c, and its lattice parameters are a = 9.782, b = 6.738, and c = 10.799. The doped Eu2+ and Ce3+ will occupy the Ca2+ site in an octahedral site.
Intensity / a.u.
500
XRD patterns of the phosphors
a b c d
a
0.10% 0.15% 0.20% 0.25%
c
400
d 300 200 100 0 250
300
350
Wavelength / nm 3+
Figure 2. The excitation spectra of Ca3(1-x)SiO4Cl2: 3xCe or 0.0025) by monitoring emission at 390 nm.
(x = 0.0010, 0.0015, 0.0020
Luminescent properties of Ca3(1-x)SiO4Cl2: 3xCe3+
Rc ¼ 2
3V 4πx c N
13
700 600
a b c d
b a
500
Intensity / a.u.
Figure 2 shows the excitation spectra of the phosphors Ca3(1-x) SiO4Cl2: 3xCe3+ (x = 0.0010, 0.0015, 0.0020, or 0.0025) by monitoring emission at 390 nm. These four curves are of similar shapes. There are three excitation bands at ~260 nm, ~290 nm and ~330 nm, which are due to the 4f → 5d transitions of Ce3+. When the content of Ce3+ is 0.15%, the excitation intensity is the strongest. Figure 3 shows the emission spectra of Ca3(1 – x)SiO4Cl2:3xCe3+ (x = 0.0010, 0.0015, 0.0020 or 0.0025) under 330 nm excitation. The broad blue emission band is due to the 5d → 4f transition of Ce3+. With the increase in the Ce3+ content, the intensity of the blue emission becomes higher. When the content of Ce3+ is 0.15%, the emission intensity is the strongest. According to Dexter and Schulman (21), the critical concentration of the concentration quenching can be used as a measure of the critical distance (Rc) of energy transfer. The Rc values can be practically calculated using the following equation:
0.10% 0.15% 0.20% 0.25%
c
400
d 300 200 100 0 350
400
450
500
550
600
Wavelength / nm 3+
Figure 3. The emission spectra of Ca3(1 – x)SiO4Cl2:3xCe or 0.0025) under 330 nm.
(x = 0.0010, 0.0015, 0.0020,
where xc is critical concentration, N is the number of Ce3+ ions in the unit cell and V is the volume of the unit cell. In this case, xc is 0.0015, N is 4, and V is 684.17 Å3 (seeing ICSD 38359). The calculated Rc value is about 60 Å for substitution of Ce3+ at the Ca2+ site. From this value, the critical distance (Rc) of energy transfer was larger than the distance (R) between the Ce3+ ions (R value is about 5.41 Å) (seeing ICSD 38359); as it can be seen that R < Rc, it is presumed that energy transfer between Ce3+ ions dominate in the case of Ca3SiO4Cl2:Ce3+ phosphor. Luminescent properties of Ca2.9955–3ySiO4Cl2:0.0045Ce3+,3yEu2+
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3+
Figure 1. The XRD pattern of Ca2.997SiO4Cl2:0.003Ce .
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The excitation and emission spectra of the phosphor Ca2.97 SiO4Cl2:0.03Eu2+ are shown in Fig. 4. There are three excitation bands at ~ 250 nm, ~340 nm and ~440 nm, which are due to the 4f → 5d transitions of Eu2+. These samples show an intense green emission, which is located at 505 nm under 340 nm excitation. This result is in accordance with that reported by Liu et al. (11). One of the conditions usually required for energy transfer is the spectrum overlapping of the donor emission to the acceptor
Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1409–1412
Eu2+ can enhanc green emission through Ce3+ → Eu2+ energy transfer 140
a b
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Em = 505 nm Ex = 340 nm
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0
0
-20 200 250 300 350 400 450 500 550 600 650 700 750
-100 300
Wavelength / nm
400
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700
Wavelength / nm (x = 0.005, 0.010, 0.015,
excitation (19). The excitation spectra of the phosphors Ca2.9955–3y SiO4Cl2:0.0045Ce3+,3yEu2+ (y = 0, 0.002, 0.004, 0.006, 0.008 or 0.010) by monitoring emission at 505 nm are shown in Fig. 5. With the introduction of Eu2+, the excitation spectra of the phosphors are similar to that of Ca2.97SiO4Cl2:0.03Eu2+. The excitation bands of Ce3+ are overlapped by Eu2+. This result shows that Ce3+ can efficiently transfer energy to Eu2+. When the Eu2+ content is 0.6%, the excitation intensity is the strongest. Curve a is the emission spectrum of Ca2.9955SiO4Cl2:0.0045Ce3+, which exhibits an intense blue light (~400 nm) the overlaps the Eu2+ emission. There are two emission peaks in the spectra of Ca2.9955–3y SiO4Cl2:0.0045Ce3+,3yEu2+ (seeing Fig. 6). The emission bands at ~400 nm and 504 nm are due to the 4f 05d1 → 4f15d0 transition of Ce3+ and the 4f 65d → 4f7 transitions of Eu2+, respectively. Eu2+ can give enhanced green emission through Ce3+ → Eu2+ energy transfer. With Eu2+ doping the emission of Ce3+ decreased, and the Eu2+ emission increased. When the content of Eu2+ is 0.6%, the emission intensity of Eu2+ is the strongest. Our aim was to search for novel phosphors for near-UV LED chips, therefore single light-emitting LEDs were made. Figure 7 shows the electroluminescence spectra of the intense green LED made with Ca2.9775SiO4Cl2:0.0045Ce3+,0.018Eu2+ under 20 mA current excitation. It is obvious that there are two emission bands
a b c e d e f f
1000
d c
600
0% 0.2% 0.4% 0.6% 0.8% 1.0%
3+
2+
Figure 6. The emission spectra of Ca2.9955–3ySiO4Cl2:0.0045Ce ,3yEu 0.004, 0.006, 0.008 or 0.010) under 340 nm.
(y = 0, 0.002,
1.0 0.8
Intensity / a.u.
2+
Figure 4. The excitation spectra of Ca3(1 – x)SiO4Cl2:3xEu 0.020, 0.030, or 0.040) by monitoring emission at 505 nm.
Intensity / a.u.
f
0% 0.2% 0.4% 0.6% 0.8% 1.0%
a
20
800
e
400
a
Intensity / a.u.
Intensity / a.u.
b
100
d c
500
120
a b c d e f
0.6 0.4 0.2 0.0 300
400
500
600
700
800
Wavelength / nm Figure 7. The electroluminescence spectra of the LED made with Ca2.9775 3+ 2+ SiO4Cl2:0.0045Ce ,0.018Eu (insets are the photograph of the phosphor under 365 nm excitation and the green LED) under 20 mA current excitation.
in the electroluminescence spectrum. The emission at 395 nm is due to the emission of the near-UV GaN chip. The green emission at ~505 nm is ascribed to the emission of the green phosphor. Bright green light from the LED can be observed by the naked eye (seeing inset of Fig. 7). Its Commission Internationale de l’Eclairage/International Commission on Illumination (CIE) chromaticity coordinates are x = 0.261, y = 0.582.
b
Conclusion
400
a 200 0 200
250
300
350
400
450
500
Wavelength / nm 3+
2+
Luminescence 2015; 30: 1409–1412
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
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Figure 5. The excitation spectra of Ca2.9955–3ySiO4Cl2:0.0045Ce ,3yEu (y = 0, 0.002, 0.004, 0.006, 0.008 or 0.010) by monitoring emission at 390 nm and 505 nm.
A series of phosphors, Ca3(1-x)SiO4Cl2:3xCe3+, Ca2.9955–3ySiO4Cl2: 0.0045Ce3+,3yEu2+, were prepared by solid-state reaction at high temperature, and their structure and photoluminescence properties were investigated. The green phosphor Ca2.9775SiO4Cl2: 0.0045Ce3+,0.018Eu2+ showed intense green emission with broader excitation in the near-UV range. Bright green light from the LED based this phosphor can be observed by the naked eye. Hence it is considered to be a good candidate for the green component of a three-band white LED.
W. Shen et al. 3+
Acknowledgements This work was supported financially by the National Natural Science Foundation of China (21261027).
References 1. Mueller-Mach R, Mueller GO, Krames MR, Trottier T. High-power phosphor-converted light-emitting diodes based on III-nitrides. IEEE J Select Top Quant Elect 2002;8:339–45. 2. Kim JS, Jeon PE, Park YH, Choi JC, Park HL. White-light generation through ultraviolet-emitting diode and white-emitting phosphor. Appl Phys Lett 2004;85:3696–8. 4+ 3. Setlur AA, Heward WJ, Hannah ME, Happek U. Incorporation of Si – 3 3+ N into Ce -doped garnets for warm white LED phosphors. Chem Mater 2008;20:6277–83. 3+ 4. Lee S, Seo SY. Optimization of yttrium aluminum garnet: Ce phosphors for white light-emitting diodes by combinatorial chemistry method. J Electrochem Soc 2002;149:J85–8. 5. Yum JH, Seo SY, Lee S, Sung YE. Y3Al5O12:Ce0.05. Phosphor coatings on gallium nitride for white light emitting diodes. J Electrochem Soc 2003;150:H47–52. 6. Steigerwald DA, Bhat JC, Collins D, Fletcher RM, Holcomb MO, Michael J, Ludowise MJ, Martin PS, Rudaz SL. Illumination with solid state lighting technology. IEEE J Select Top Quant Elect 2002;8:310–20. 7. Khizar M, Fan ZY, Kim KH, Lin JY, Jiang HX. Nitride deep-ultraviolet lightemitting diodes with microlens array. Appl Phys Lett 2005;86(1–3): 173504. 8. Sivakumar V, Varadaraju UV. A promising orange-red phosphor under near UV excitation. Electrochem Solid-State Lett 2006;9:H35–8. 9. Hao ZD, Zhang X, Luo US, Zhang JH. Preparation and 2+ photoluminescence properties of single-phase Ca2SiO3Cl2:Eu bluish-green emitting phosphor. Mater Lett 2013;93:272–4.
10. Liu J, Lian HZ, Shi CS, Sun JY. Eu -doped high-temperature phase Ca3SiO4Cl2 a yellowish orange phosphor for white light-emitting diodes. J Electrochem Soc 2005;152:G880–4. 11. Liu J, Lian HZ, Sun JY, Shi CS. Characterization and properties of green 2+ emitting Ca3SiO4Cl2:Eu powder phosphor for white light-emitting diodes. Chem Lett 2005;34:1340–1. 12. Baginskiy I, Liu RS. Significant improved luminescence intensity of 2+ Eu -doped Ca3SiO4Cl2 green phosphor for white LEDs synthesized through two-stage method. J Electrochem Soc 2009;156:G29–32. 13. Zhang XM, Park B, Choi N, Kim J, Kim GC, Yoo JH. A novel blue-emitting 3+ + Sr3Al2O5Cl2:Ce ,Li phosphor for near UV-excited white-light-emitting diodes. Mater Lett 2009;63:700–2. 14. Zhang XG, Gong ML. Single-phased white-light-emitting NaCaBO3: 3+ 3+ 2+ Ce , Tb , Mn phosphors for LED applications. Dalton Trans 2014; 43:2465–72. 15. Zhang XG, Zhou LY, Pang Q, Shi JX, Gong ML. Tunable Luminescence 3+ 3+ 3+ and Ce → Tb → Eu energy transfer of broadband-excited and 3+ 3+ 3+ narrow line red emitting Y2SiO5:Ce , Tb , Eu phosphor. J Phys Chem C 2014;118:75918. 16. Zhang XG, Zhou LY, Pang Q, Gong ML. Novel broadband excited and 3+ 3+ 3+ linear red-emitting Ba2Y(BO3)2Cl: Ce ,Tb ,Eu phosphor: luminescence and energy transfer. J Am Ceram Soc 2014;97:2124–9. 17. Zhang XG, Gong ML. Photoluminescence properties and energy3+ 2+ transfer of thermal-stable Ce , Mn -codoped barium strontium lithium silicate red phosphors. J Alloys Comp 2011;509:2850–5. 18. Lin H, Liu XR, Pun EYB. Sensitized luminescence and energy transfer in 3+ 2+ Ce and Eu codoped calcium magnesium chlorosilicate. Opt Mater 2002;18:397–401. 3+ 2+ 2+ 19. Ye S, Wang XM, Jing XP. Energy transfer among Ce , Eu , and Mn in CaSiO3. J Electrochem Soc 2008;155:J143–7. 3+ 2+ 20. Lei YZ, Zou Q, Du DH. Energy transfer between Ce and Eu in doped KSrPO4. Appl Phys A 2009;97:635–9. 21. Dexter DL, Schulman JH. Theory of concentration quenching in inorganic phosphors . J Chem Phys 1954;22:1063–70.
1412 wileyonlinelibrary.com/journal/luminescence
Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1409–1412
Revised: 1 March 2015,
Accepted: 20 March 2015
Published online in Wiley Online Library: 19 April 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2922
Luminescent properties of Ca3SiO4Cl2 co-doped with Ce3+ and Eu2+ for near-ultraviolet light-emitting diodes Wangqing Shen,a Yiwen Zhub and Zhengliang Wangb* ABSTRACT: Ca3SiO4Cl2 co-doped with Ce3+,Eu2+ was prepared by high temperature reaction. The structure, luminescent properties and the energy transfer process of Ca3SiO4Cl2: Ce3+,Eu2+ were investigated. Eu2+ ions can give enhanced green emission through Ce3+ → Eu2+ energy transfer in these phosphors. The green phosphor Ca2.9775SiO4Cl2:0.0045Ce3+,0.018Eu2+ showed intense green emission with broader excitation in the near-ultraviolet light range. A green light-emitting diode (LED) based on this phosphor was made, and bright green light from this green LED could be observed by the naked eye under 20 mA current excitation. Hence it is considered to be a good candidate for the green component of a three-band white LED. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: luminescent properties; phosphor; light-emitting diodes; Ca3SiO4Cl2
Introduction
Experimental
Phosphor-converted light-emitting diodes (LEDs) are an important kind of solid-state illumination (1–3). Commercial white LEDs are mainly obtained by combining a yellow-emitting phosphor YAG: Ce3+ pumped with a blue-emitting GaN LED chip (4,5). At present, the emission bands of LED chips are shifted to the near-ultraviolet (UV) range and this wavelength can offer a higher efficiency of solid-state lighting (6,7). Conventional phosphors used in fluorescence lighting are not suitable for the near-UV LED chips, because of their poor absorption in the near-UV light region (8). Hence, there is a need to investigate novel phosphors for nearUV LED chips. Alkaline earth halo-silicates are well known hosts for inorganic luminescent materials due to their low synthesis temperature and high luminescence efficiency (9). The compound calcium chlorosilicate Ca3SiO4Cl2 was reported first by Chaterlier (10). Many researchers have investigated its phase structure transformation (10), and the luminescent properties in different crystal structures doped with Eu2+ (9–12). Little information on this host doped with other rare earth ions has been reported. Ce3+ ions are very good luminescent centers as well as sensitizers of luminescence in many hosts (13–17). They can efficiently absorb UV light and transfer the energy to the luminescent centers, thus the emission intensity of the luminescent center would be strengthened. Due to the overlap of spectra between the Ce3+ emission band and the Eu2+ excitation band, the 4f–5d transitions of Ce3+ to Eu2+ are very sensitive to the nature of the host lattice, and the absorption and emission of Ce3+ and Eu2+ are efficient in many hosts (18–20). In this paper, Ca3SiO4Cl2 co-doped with Ce3+,Eu2+ was prepared by high temperature reaction. The luminescent properties and the energy transfer process of Ca3SiO4Cl2:Ce3+,Eu2+ were investigated. Finally, a single green LED was made by combining the green phosphor with ~400 nm-emitting InGaN chips.
Synthesis
Characterizations The structure of these phosphors was recorded by X-ray powder diffraction (XRD) using Cu Kα radiation on a RIGAKU D/max 2200 vpc X-ray diffractometer. Their photoluminescent spectra were recorded on a Cary Eclipse FL1011M003 (Varian) spectrofluorometer and a xenon lamp was used as an excitation source. The electroluminescence of green LEDs was recorded on a LED
* Correspondence to: Z. Wang, Engineering Research Center of Biopolymer Functional Materials of Yunnan, School of Chemistry and Biotechnology, Yunnan Minzu University, Kunming, Yunnan 650500, People’s Republic of China. E-mail: [email protected] a
College of Chemistry and Chemical Engineering, Neijiang Normal University, Neijiang, Sichuan 641112, People’s Republic of China
b
Engineering Research Center of Biopolymer Functional Materials of Yunnan, School of Chemistry and Biotechnology, Yunnan Minzu University, Kunming, Yunnan 650500, People’s Republic of China Abbreviations: LED, light-emitting diode; XRD, X-ray powder diffraction.
Copyright © 2015 John Wiley & Sons, Ltd.
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Luminescence 2015; 30: 1409–1412
The phosphor Ca3SiO4Cl2 was synthesized by a solid-state reaction. The raw materials CaCO3 (A.R. grade), SiO2 (A.R. grade), CaCl2 (A.R. grade), Eu2O3 (99.99% purity), and CeO2 (99.99% purity) were mixed thoroughly and ground together with an agate mortar and pestle. Then the mixture were first pre-fired at 500 °C for 4 h, and then heated at 900 °C for 4 h. The process was carried out in a reducing atmosphere (N2:H2 = 95:5). The green LED was made by combing an InGaN chip (~395 nm) with a mixture of green phosphor and epoxy resin (the mass ratio is 1:1).
W. Shen et al. 700
spectrophotocolorimeter (Everfine PMS-50). All the measurements were performed at room temperature.
Results and discussion
600
b
The XRD patterns of Ca2.997SiO4Cl2:0.003Ce3+ and Ca2.97SiO4Cl2: 0.03Eu2+ are shown in Fig. 1. They are consistent with the Joint Committee on Powder Diffraction Standards ( JCPDS) card 70–2447 [Ca3SiO4Cl2]. This indicates that the as-prepared phosphors share the single phase and the doping of Ce3+ and Eu2+ does not change the crystal structure of Ca3SiO4Cl2. The slight blue shift of diffraction peaks of Ca2.997SiO4Cl2:0.003Ce3+ and Ca2.97SiO4Cl2:0.03Eu2+ is due to the distinct ionic radii between Ca2+ and Eu2+,Ce3+. This host lattice Ca3SiO4Cl2 has a monoclinic structure with space group P21/c, and its lattice parameters are a = 9.782, b = 6.738, and c = 10.799. The doped Eu2+ and Ce3+ will occupy the Ca2+ site in an octahedral site.
Intensity / a.u.
500
XRD patterns of the phosphors
a b c d
a
0.10% 0.15% 0.20% 0.25%
c
400
d 300 200 100 0 250
300
350
Wavelength / nm 3+
Figure 2. The excitation spectra of Ca3(1-x)SiO4Cl2: 3xCe or 0.0025) by monitoring emission at 390 nm.
(x = 0.0010, 0.0015, 0.0020
Luminescent properties of Ca3(1-x)SiO4Cl2: 3xCe3+
Rc ¼ 2
3V 4πx c N
13
700 600
a b c d
b a
500
Intensity / a.u.
Figure 2 shows the excitation spectra of the phosphors Ca3(1-x) SiO4Cl2: 3xCe3+ (x = 0.0010, 0.0015, 0.0020, or 0.0025) by monitoring emission at 390 nm. These four curves are of similar shapes. There are three excitation bands at ~260 nm, ~290 nm and ~330 nm, which are due to the 4f → 5d transitions of Ce3+. When the content of Ce3+ is 0.15%, the excitation intensity is the strongest. Figure 3 shows the emission spectra of Ca3(1 – x)SiO4Cl2:3xCe3+ (x = 0.0010, 0.0015, 0.0020 or 0.0025) under 330 nm excitation. The broad blue emission band is due to the 5d → 4f transition of Ce3+. With the increase in the Ce3+ content, the intensity of the blue emission becomes higher. When the content of Ce3+ is 0.15%, the emission intensity is the strongest. According to Dexter and Schulman (21), the critical concentration of the concentration quenching can be used as a measure of the critical distance (Rc) of energy transfer. The Rc values can be practically calculated using the following equation:
0.10% 0.15% 0.20% 0.25%
c
400
d 300 200 100 0 350
400
450
500
550
600
Wavelength / nm 3+
Figure 3. The emission spectra of Ca3(1 – x)SiO4Cl2:3xCe or 0.0025) under 330 nm.
(x = 0.0010, 0.0015, 0.0020,
where xc is critical concentration, N is the number of Ce3+ ions in the unit cell and V is the volume of the unit cell. In this case, xc is 0.0015, N is 4, and V is 684.17 Å3 (seeing ICSD 38359). The calculated Rc value is about 60 Å for substitution of Ce3+ at the Ca2+ site. From this value, the critical distance (Rc) of energy transfer was larger than the distance (R) between the Ce3+ ions (R value is about 5.41 Å) (seeing ICSD 38359); as it can be seen that R < Rc, it is presumed that energy transfer between Ce3+ ions dominate in the case of Ca3SiO4Cl2:Ce3+ phosphor. Luminescent properties of Ca2.9955–3ySiO4Cl2:0.0045Ce3+,3yEu2+
1410
3+
Figure 1. The XRD pattern of Ca2.997SiO4Cl2:0.003Ce .
wileyonlinelibrary.com/journal/luminescence
The excitation and emission spectra of the phosphor Ca2.97 SiO4Cl2:0.03Eu2+ are shown in Fig. 4. There are three excitation bands at ~ 250 nm, ~340 nm and ~440 nm, which are due to the 4f → 5d transitions of Eu2+. These samples show an intense green emission, which is located at 505 nm under 340 nm excitation. This result is in accordance with that reported by Liu et al. (11). One of the conditions usually required for energy transfer is the spectrum overlapping of the donor emission to the acceptor
Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1409–1412
Eu2+ can enhanc green emission through Ce3+ → Eu2+ energy transfer 140
a b
600
Em = 505 nm Ex = 340 nm
80 60 40
300
b
200 100
0
0
-20 200 250 300 350 400 450 500 550 600 650 700 750
-100 300
Wavelength / nm
400
500
600
700
Wavelength / nm (x = 0.005, 0.010, 0.015,
excitation (19). The excitation spectra of the phosphors Ca2.9955–3y SiO4Cl2:0.0045Ce3+,3yEu2+ (y = 0, 0.002, 0.004, 0.006, 0.008 or 0.010) by monitoring emission at 505 nm are shown in Fig. 5. With the introduction of Eu2+, the excitation spectra of the phosphors are similar to that of Ca2.97SiO4Cl2:0.03Eu2+. The excitation bands of Ce3+ are overlapped by Eu2+. This result shows that Ce3+ can efficiently transfer energy to Eu2+. When the Eu2+ content is 0.6%, the excitation intensity is the strongest. Curve a is the emission spectrum of Ca2.9955SiO4Cl2:0.0045Ce3+, which exhibits an intense blue light (~400 nm) the overlaps the Eu2+ emission. There are two emission peaks in the spectra of Ca2.9955–3y SiO4Cl2:0.0045Ce3+,3yEu2+ (seeing Fig. 6). The emission bands at ~400 nm and 504 nm are due to the 4f 05d1 → 4f15d0 transition of Ce3+ and the 4f 65d → 4f7 transitions of Eu2+, respectively. Eu2+ can give enhanced green emission through Ce3+ → Eu2+ energy transfer. With Eu2+ doping the emission of Ce3+ decreased, and the Eu2+ emission increased. When the content of Eu2+ is 0.6%, the emission intensity of Eu2+ is the strongest. Our aim was to search for novel phosphors for near-UV LED chips, therefore single light-emitting LEDs were made. Figure 7 shows the electroluminescence spectra of the intense green LED made with Ca2.9775SiO4Cl2:0.0045Ce3+,0.018Eu2+ under 20 mA current excitation. It is obvious that there are two emission bands
a b c e d e f f
1000
d c
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0% 0.2% 0.4% 0.6% 0.8% 1.0%
3+
2+
Figure 6. The emission spectra of Ca2.9955–3ySiO4Cl2:0.0045Ce ,3yEu 0.004, 0.006, 0.008 or 0.010) under 340 nm.
(y = 0, 0.002,
1.0 0.8
Intensity / a.u.
2+
Figure 4. The excitation spectra of Ca3(1 – x)SiO4Cl2:3xEu 0.020, 0.030, or 0.040) by monitoring emission at 505 nm.
Intensity / a.u.
f
0% 0.2% 0.4% 0.6% 0.8% 1.0%
a
20
800
e
400
a
Intensity / a.u.
Intensity / a.u.
b
100
d c
500
120
a b c d e f
0.6 0.4 0.2 0.0 300
400
500
600
700
800
Wavelength / nm Figure 7. The electroluminescence spectra of the LED made with Ca2.9775 3+ 2+ SiO4Cl2:0.0045Ce ,0.018Eu (insets are the photograph of the phosphor under 365 nm excitation and the green LED) under 20 mA current excitation.
in the electroluminescence spectrum. The emission at 395 nm is due to the emission of the near-UV GaN chip. The green emission at ~505 nm is ascribed to the emission of the green phosphor. Bright green light from the LED can be observed by the naked eye (seeing inset of Fig. 7). Its Commission Internationale de l’Eclairage/International Commission on Illumination (CIE) chromaticity coordinates are x = 0.261, y = 0.582.
b
Conclusion
400
a 200 0 200
250
300
350
400
450
500
Wavelength / nm 3+
2+
Luminescence 2015; 30: 1409–1412
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Figure 5. The excitation spectra of Ca2.9955–3ySiO4Cl2:0.0045Ce ,3yEu (y = 0, 0.002, 0.004, 0.006, 0.008 or 0.010) by monitoring emission at 390 nm and 505 nm.
A series of phosphors, Ca3(1-x)SiO4Cl2:3xCe3+, Ca2.9955–3ySiO4Cl2: 0.0045Ce3+,3yEu2+, were prepared by solid-state reaction at high temperature, and their structure and photoluminescence properties were investigated. The green phosphor Ca2.9775SiO4Cl2: 0.0045Ce3+,0.018Eu2+ showed intense green emission with broader excitation in the near-UV range. Bright green light from the LED based this phosphor can be observed by the naked eye. Hence it is considered to be a good candidate for the green component of a three-band white LED.
W. Shen et al. 3+
Acknowledgements This work was supported financially by the National Natural Science Foundation of China (21261027).
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