Journal of Alloys and Compounds 476 (2009) 390–392
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Potential red-emitting phosphor for white LED solid-state lighting Liya Zhou ∗ , Jianshe Wei, Jingrong Wu, Fuzhong Gong, Linghong Yi, Junli Huang School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, People’s Republic of China
a r t i c l e
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Article history: Received 21 April 2008 Received in revised form 22 August 2008 Accepted 3 September 2008 Available online 31 October 2008 Keywords: Optical materials Luminescence Phosphors
a b s t r a c t The phosphors Sr1−x MoO4 :Eu3+ x , Kx Sr1−2x MoO4 :Eu3+ x were prepared by solid-state reaction technique at 800 ◦ C. The intense red-emitting phosphor K0.25 Sr0.50 MoO4 :Eu3+ 0.25 with tetragonal structure was obtained. The result indicated that Kx Sr1−2x MoO4 :Eu3+ x phosphors can be excited effectively at 393 nm and 464 nm light. The presences of the K+ ion strengthen the absorption of the phosphors around 400 nm. Compared with Y2 O2 S:0.05Eu3+ , the obtained K0.25 Sr0.50 MoO4 :Eu3+ 0.25 phosphor shows an enhanced red emission under 393 nm excitation. The strong red-emission lines around 616 nm correspond to the forced electric dipole 5 D0 → 7 F2 transitions on Eu3+ . The chromaticity coordinates (x = 0.64, y = 0.35) are close to the standard of NTSC (National Television Standard Committee). The optical properties suggest that K0.25 Sr0.50 MoO4 :Eu3+ 0.25 is an efﬁcient red-emitting phosphor for light emitting diode (LED) applications. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since white light emitting diodes (LEDs) can offer beneﬁts in terms of long lifetime, high luminous efﬁciency, and environmental-friendly characteristics, they are the most important solid-state light sources for substitution the conventional incandescent and ﬂuorescent lamps . In phosphor-converted light-emitting diodes devices, the red/green/blue tricolor phosphors are pumped by UV-InGaN chips (∼400 nm) or blue GaN chips (∼460 nm) and generate white light [2,3]. At present, the most common method to obtain the white light is combining a yellow-emitting phosphor (YAG: Ce3+ ) with a GaN blue LED chip. However, there are some problems for the spectral composition of the light, such as lower color-rendering index, lower luminous efﬁciency [4,5]. With the development of LED chip technology, the emission bands of LED chips shifted from blue light to near UV range because the near UV light can offer higher energy to pump the phosphor . Presently, the main phosphors for near UV-InGaN-based LEDs are BaMgAl10 O17 :Eu2+ for blue, ZnS:(Cu+ , Al3+ ) for green, and Y2 O2 S:Eu3+ for red . However, the efﬁciency of the Y2 O2 S:Eu3+ is much lower than that of the blue and green phosphors, and instability due to releasing of sulﬁde gas . Thus, it is urgent to search for new red phosphors that can be efﬁciently excited around 400 nm and the phosphor must show strong emission under ∼400 nm excitation and with the chromaticity coordinates near the standard of NTSC (National Television Standard Committee).
∗ Corresponding author. Tel.: +86 771 3233718; fax: +86 771 3233718. E-mail address: [email protected]
(L. Zhou). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.005
The scheelite-type (CaWO4 ) metal molybdates are considered to be efﬁcient luminescent hosts [8–10]. SrMoO4 is similar to CaWO4 in crystal structure with tetragonal symmetry and the [MoO4 2− ] oxyanion complex is the principal constitutive element. The central Mo metal ion is surrounded by four O2− ions with tetrahedral coordination. Considering the broad and intense charge-transfer (C-T) absorption bands in the near-UV and excellent thermal and chemical stability of such kind of molybdates [11–13], the Sr1−x MoO4 :Eu3+ x and Kx Sr1−2x MoO4 :Eu3+ x were chosen to be the host lattice in the Eu3+ ion doped phosphors in this paper. Because the lowest excited level (5 D0 ) of the 4f 6 conﬁguration is situated below the 4f5 5d conﬁguration for Eu3+ , and it mainly shows sharp 5 D –7 F red-emission lines around 616 nm when Eu3+ ions occupy 0 2 the lattice sites without centrosymmetry . Therefore, the phosphors with CIE chromaticity coordinates close to the NTSC standard values (x = 0.67, y = 0.33) were achieved in K0.25 Sr0.50 MoO4 :Eu3+ 0.25 phosphor. 2. Experimental The phosphors Sr1−x MoO4 :Eu3+ x (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30), Kx Sr1−2x MoO4 :Eu3+ x (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) were prepared by solid-state reaction technique at high temperature. SrCO3 (A.R. grade), (NH4 )6 Mo7 O24 ·4H2 O (A.R. grade), K2 CO3 (A.R. grade) and Eu2 O3 (99.99%) were used as reagents for sample preparations. Stoichiometric amount of starting materials were mixed homogeneously in an agate mortar and pre-calcined at 500 ◦ C for 3 h, then calcined at 800 ◦ C for 4 h. Powder X-ray diffraction (XRD, 40 kV and 200 mA, Cu K␣ = 1.5406 Å Rigaku/Dmax – 2500) was used to identify the structure of the ﬁnal products. Analysis instrument of laser nanometer granularity (Zetasizer Nano S) was used to observe the distribution and size of the particles. Excitation and emission spectra were recorded on a RF-5301PC ﬂuorescence spectrophotometer use a Xe lamp as the excitation source. All the measurements were carried out at room temperature.
L. Zhou et al. / Journal of Alloys and Compounds 476 (2009) 390–392
Fig. 1. XRD patterns of the Sr0.95 MoO4 :Eu3+ 0.05 (a), K0.15 Sr0.70 MoO4 :Eu3+ 0.15 (b) and K0.25 Sr0.50 MoO4 :Eu3+ 0.25 and (c) phosphors calcined at 800 ◦ C, respectively.
3. Results and discussion 3.1. X-ray diffraction and size distribution characterization The XRD patterns of the typical Sr1−x MoO4 :Eu3+ x and Kx Sr1−2x MoO4 :Eu3+ x are shown in Fig. 1. The powder X-ray diffraction patterns of the samples show that all the phosphors are of single phase and consistent with JCPDS 08-0482 [SrMoO4 ] in Tetragonal structure (space group I41/a(88)). Sr0.95 MoO4 :Eu3+ 0.05 (a), K0.15 Sr0.70 MoO4 :Eu3+ 0.15 (b) and K0.25 Sr0.50 MoO4 :Eu3+ 0.25 (c) prepared in the same temperature did not show distinct differences, indicating that the doped Eu3+ ions and K+ ions did not change the lattice structure. Due to the different valence states and difference of the ion sizes between Mo6+ (0.041 nm) and Eu3+ (0.107 nm), Eu3+ is expected to occupy the Sr2+ site in this phosphor. Fig. 2 shows the particle size distribution of K0.25 Sr0.50 MoO4 :Eu3+ 0.25 phosphor calcined at 800 ◦ C. The particles show a narrow size-distribution and the average diameter of the particles is about 2 m, which is ﬁt to fabricate the solid-lighting devices . 3.2. Photo-luminescent properties Fig. 3 shows the excitation spectra of the Sr0.95 MoO4 :Eu0.05 3+ (a), Sr0.75 MoO4 :Eu3+ 0.25 (b), K0.15 Sr0.70 MoO4 :Eu3+ 0.15 (c) and K0.25 Sr0.50 MoO4 :Eu3+ 0.25 (d) samples calcined at 800 ◦ C by monitoring the
Fig. 2. Particle size distribution of K0.25 Sr0.50 MoO4 :Eu3+ 0.25 phosphor calcined at 800 ◦ C.
of Sr0.95 MoO4 :Eu3+ 0.05 (a), Fig. 3. Excitation spectra (em = 616 nm) Sr0.75 MoO4 :Eu3+ 0.25 (b), K0.15 Sr0.70 MoO4 :Eu3+ 0.15 (c) and K0.25 Sr0.50 MoO4 :Eu3+ 0.25 (d) samples calcined at 800 ◦ C.
emission at 616 nm. Four curves exhibit similar spectroscopic feature, the broad excitation band from 220 nm to 350 nm is ascribed to the O–Mo charge transfer (CT) transition while the lines in 360–500 nm range belong to f–f transitions of Eu3+ ions in the host lattices. The strong excitation band at ∼393 and ∼464 nm attributes to the 7 F0 → 5 L6 and 7 F0 → 5 D2 transitions of Eu3+ , respectively. Comparing the curve a, b with curve c, d, it is obvious that the f–f transitions around 400 nm in curve c and d was strengthen. This may be due to the replacement of partial Sr2+ (4 s2 4p6 ) ions by K+ (3 s2 3p6 ) ions . Fig. 4 represents the emission spectra upon direct excitation the 7 F0 → 5 L6 transition of Eu3+ at 393 nm in the host lattice for Sr0.95 MoO4 :Eu3+ 0.05 (a), Sr0.75 MoO4 :Eu3+ 0.25 (b), K0.15 Sr0.70 MoO4 :Eu3+ 0.15 (c) and K0.25 Sr0.50 MoO4 :Eu3+ 0.25 (d) samples, respectively. The emission spectra are described by the well-known 5 D0 → 7 FJ (J = 0, 1, 2, . . .) emission lines of the Eu3+ ions with the strong emission for J = 2 at 616 nm, which allows that the Eu3+ occupies a center of asymmetry in the host lattice . Other transitions from the 5 DJ excited levels to 7 FJ ground states in the 570–750 nm range are relatively weak. A ratio between the integrated intensity of these two transitions, I0–2 /I0–1 , is used in lanthanide-based systems as a probe of the cation local surroundings . As shown in Fig. 4, the transition 5 D0 → 7 F2 is much stronger than the transition 5 D0 → 7 F1 which suggests that the Eu3+ located in a distorted (or asymmetric) cation environment. This is favorable to improve the color purity of the red phosphor. It can be seen that the luminescence intensity enhances with the increase of the K+ doping ratio because the addition of K+ ions balanced the charge in samples . The relative emission intensities and CIE chromaticity coordinates of Sr1−x MoO4 :Eu3+ x and Kx Sr1−2x MoO4 :Eu3+ x and Y2 O2 S:0.05Eu3+ are listed in Table 1. Usually, a low doping ratio gives weak luminescence while an over-doping ratio perhaps brings quenching of the luminescence. Therefore, the emission intensities of these phosphors under 393 nm excitation were compared. It can be seen that the luminescence intensity of
L. Zhou et al. / Journal of Alloys and Compounds 476 (2009) 390–392
sity is about 2.8 times higher than that of Sr0.75 MoO4 :Eu3+ 0.25 . This can be primarily estimated that the ionic radii of K+ is different from Sr2+ , results in somewhat diverse about the sub-lattice surrounding ﬁeld around the luminescent center Eu3+ ions, which makes them far away in inversion symmetry in SrMoO4 :Eu3+ . Y2 O2 S:0.05Eu3+ was prepared according to the reference . When the excitation wavelength is 393 nm the emission intensity of Y2 O2 S:0.05Eu3+ is only 24% of the K0.25 Sr0.50 MoO4 :Eu3+ 0.25 . The CIE chromaticity coordinates of all the phosphors are calculated and the values of K0.25 Sr0.50 MoO4 :Eu3+ 0.25 are calculated to be x = 0.64, y = 0.35, which is closer to the standard of NTSC than that of Y2 O2 S:0.05Eu3+ . The results suggest that K0.25 Sr0.50 MoO4 :Eu3+ 0.25 is an efﬁcient red-emitting phosphor for light emitting diodes applications. 4. Conclusions
Fig. 4. Emission spectra (ex = 393 nm) of Sr0.95 MoO4 :Eu3+ 0.05 (a), Sr0.75 MoO4 :Eu3+ 0.25 (b), K0.15 Sr0.70 MoO4 :Eu3+ 0.15 (c) and K0.25 Sr0.50 MoO4 :Eu3+ 0.25 (d) samples calcined at 800 ◦ C.
Sr1−x MoO4 :Eu3+ x enhances with the increase of the Eu3+ doping ratio and reaches a maximum at 25 mol% of Eu3+ . When the Eu3+ doping ratio is higher above 25 mol%, the luminescence intensity reduces contrarily. This quenching process often attributes to energy migration among Eu3+ ions. In order to improve the emission efﬁciency of Sr1−x MoO4 :Eu3+ x , the series samples, in which some of the Sr2+ ions were substituted by K+ ions, were investigated. The comparison of emission intensity (5 D0 → 7 F2 transition) under 393 nm excitation of Kx Sr1−2x MoO4 :Eu3+ x recorded under the same experimental measurement conditions is also shown in Table 1. With the increase of the component of K+ and Eu3+ (from x = 0.05 to 0.30), the maximum emission intensity appears at x = 0.25 compositions and its intenTable 1 The CIE chromaticity coordinates and phosphors. Phosphor
Sr0.95 MoO4 :Eu 0.05 Sr0.90 MoO4 :Eu3+ 0.10 Sr0.85 MoO4 :Eu3+ 0.15 Sr0.80 MoO4 :Eu3+ 0.20 Sr0.75 MoO4 :Eu3+ 0.25 Sr0.70 MoO4 :Eu3+ 0.30 K0.05 Sr0.90 MoO4 :Eu3+ 0.05 K0.10 Sr0.80 MoO4 :Eu3+ 0.10 K0.15 Sr0.70 MoO4 :Eu3+ 0.15 K0.20 Sr0.60 MoO4 :Eu3+ 0.20 K0.25 Sr0.50 MoO4 :Eu3+ 0.25 K0.30 Sr0.40 MoO4 :Eu3+ 0.30 Y2 O2 S:0.05Eu3+
D0 → 7 F2 relative emission intensity of
Excitation wavelength (nm)
D0 → 7 F2 relative intensity
393 393 393 393 393 393 393 393 393 393 393 393 393
0.57 0.58 0.57 0.57 0.56 0.55 0.60 0.60 0.61 0.63 0.64 0.64 0.63
0.43 0.42 0.42 0.43 0.43 0.44 0.39 0.40 0.38 0.36 0.35 0.34 0.35
0.3 0.8 1.1 1.3 1.5 1.2 1.2 1.8 2.5 2.8 4.2 3.0 1.0
CIE chromaticity coordinates
A series of red-emitting phosphors Sr1−x MoO4 :Eu3+ x and Kx Sr1−2x MoO4 :Eu3+ x were prepared by solid-state reaction technique, and the average diameter of the K0.25 Sr0.50 MoO4 :Eu3+ 0.25 particles is about 2 m, which is ﬁt to fabricate the solid-lighting devices. The addition of K+ ions balanced the charge in samples, enhanced the luminescent intensity of samples and the luminescent intensity reached the maximum when the doped concentration of K+ and Eu3+ ions was 25 mol%. Compared with Y2 O2 S:0.05Eu3+ , the emission intensity of K0.25 Sr0.50 MoO4 :Eu3+ 0.25 is much stronger than that of Y2 O2 S:0.05Eu3+ and its CIE chromaticity coordinates are closer to NTSC standard values than that of Y2 O2 S:0.05Eu3+ . Upon excitation with near UV light, the phosphor showed strong red-emission lines at 616 nm correspond to the forced electric dipole 5 D0 → 7 F2 transition of Eu3+ . All the results indicated that this red phosphor is a suitable candidate for the fabrication of near UV-InGaN-based LEDs. Acknowledgements This work was ﬁnancially supported by grants from the Science Foundation of Guangxi Province (No. 0731014); the Natural Science Foundation of Guangxi University (X051107), the largescale instrument of Guangxi cooperates and shares the network (496-2007-075), innovation Project of Guangxi Graduate Education (2008105930817M74). References  P.F. Smet, K. Korthout, J.E. Van Haecke, D. Poelman, Mater. Sci. Eng. B 146 (2008) 264–268.  Z.L. Wang, H.B. Liang, M.L. Gong, Q. Su, Opt. Mater. 29 (2007) 896–900.  J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931–2933.  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 Photon. Tech. Lett. 15 (2003) 18–20.  S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2–6.  Z. Ci, Y. Wang, J. Zhang, Y. Sun, Physica B 403 (2008) 670–674.  Z.C. Wu, J.X. Shi, J. Wang, M.L. Gong, Q. Su, J. Solid State Chem. 179 (2006) 2356–2360.  J.W. Yoon, J.H. Ryu, K.B. Shim, Mater. Sci. Eng. B 127 (2006) 154–158.  G.-K. Choi, J.-R. Kim, S.H. Yoon, K.S. Hong, J. Eur. Ceram. Soc. 27 (2007) 3063–3067.  A.W. Sleight, Acta Crystallogr. B 28 (1972) 2899–2902.  X. Li, Z. Yang, L. guan, Q. Guo, S. Huai, P. Li, J. Rare Earth. 25 (2007) 706–709.  Y. Hu, W. Zhuang, H. Ye, D. Wang, S. Zhang, X. Huang, J. Alloy. Comp. 390 (2005) 226–229.  J. Liu, H. Lian, C. Shi, Opt. Mater. 29 (2007) 1591–1594.  R.P. Rao, J. Electrochem. Soc. 143 (1996) 189–197.  Z.L. Wang, H.B. Liang, L.Y. Zhou, Chem. Phys. Lett. 412 (2005) 313–316.  S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999, pp. 190–191.  J.P. Rainho, L.D. Carlos, J. Rocha, J. Lumin. 87–89 (2000) 1083–1086.  K.R. Reddy, K. Annapurna, S. Buddhudu, Mater. Res. Bull. 31 (1996) 1355–1359.