Comparative investigation on synthesis and luminescence of Sr4Al14O25:Eu2+ applied in InGaN LEDs

Journal of Alloys and Compounds 458 (2008) 134–137

Comparative investigation on synthesis and luminescence of Sr4Al14O25:Eu2+ applied in InGaN LEDs Zhanchao Wu, Menglian Gong ∗ , Jianxin Shi, Qiang Su State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China Received 18 November 2006; received in revised form 11 March 2007; accepted 31 March 2007 Available online 5 April 2007

Abstract Blue–green-emitting phosphor Sr4 Al14 O25 :Eu2+ was synthesized with three methods: solid-state reaction (SS), co-precipitation method (CP) and sol–gel method (SG). Bright blue–green-emitting LEDs were fabricated by combining the phosphors with InGaN-based near-ultraviolet (NUV) chips, respectively. The effects of preparation processes on the crystallization, morphology, particle size and photoluminescence, especially on the luminescence properties of phosphor-converted light-emitting diodes (PC-LEDs), were investigated. The results reveal that the relatively regular morphology, small particle size and narrow size distribution are favorable for phosphors to absorb the NUV light from the chip and thus reduce the difference in emission intensity of the three phosphors in LEDs. © 2007 Elsevier B.V. All rights reserved. Keywords: White LEDs; Sr4 Al14 O25 :Eu2+ ; Phosphor; Solid-state; Co-precipitation; Sol–gel

1. Introduction In recent years, the study on RGB phosphors suitable for near-ultraviolet (NUV) excitation has been attracting more and more attention for fabricating white light-emitting diode (LED) [1–4]. Compared with the currently commercial white LED fabricated with a blue chip and a yellow phosphor YAG:Ce3+ , the white LED fabricated with an NUV chip around 390–420 nm and corresponding phosphors has higher efficiency and color rendering index [2–6] because all the colors are determined by the phosphors. Therefore, the wavelength conversion phosphor materials play a crucial role in solid-state lighting innovation. An important new development in the field of luminescent materials is the search for ideal/suitable phosphors for the conversion of the NUV emission from InGaN chips into visible light. Strontium aluminates, doped with Eu2+ , have been studied for a long time for their excellent properties such as high quantum efficiency [7], long persistence of phosphorescence [8] and good stableness [9]. Recently, the luminescence properties of

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the phosphor-converted LEDs (PC-LEDs) fabricated by combining SrAl2 O4 :Eu2+ and Sr4 Al14 O25 :Eu2+ phosphors obtained by solid-state method (SS) with 397 nm InGaN chips, respectively, were investigated and SrAl2 O4 :Eu2+ and Sr4 Al14 O25 :Eu2+ have been approved to be two excellent phosphors applied in UV-LEDs [10,11]. However, large particles and irregular morphologies of samples cause problems in practical application in LEDs, such as difficulty to get good homogeneity by mixing the phosphor and epoxy resin, decreasing the absorption of NUV emitted from InGaN chip. In general, different preparation methods may greatly influence the crystallization, morphology, particle size and luminescence characteristics of phosphor materials. Compared with the samples obtained by conventional SS method, the phosphor materials synthesized by wet chemical method have advantages of low calcinations temperature, small particle size and narrow particle size distribution. In order to optimize the characteristics of Sr4 Al14 O25 :Eu2+ applied in NUV-LED chips, in this article, three experimental methods, SS, co-precipitation method (CP) and sol–gel method (SG) were used to prepare Sr4 Al14 O25 :Eu2+ phosphors, and the effects of preparation processes on the crystallization, morphologies, particle size and luminescence properties were studied.

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2. Experimental 2.1. Synthesis of Sr4 Al14 O25 :Eu2+ by solid-state reaction method The starting materials SrCO3 (AR), Al2 O3 (AR) and Eu2 O3 (99.99%) with molar ratio of 3.92:14:0.08 for Sr:Al:Eu were firstly ground (a little H3 BO3 was added as flux), and then burned in an electric furnace for crystallization at 1350 ◦ C for 3 h under CO atmosphere.

2.2. Synthesis of Sr4 Al14 O25 :Eu2+ by co-precipitation method with ammonium bicarbonate NH4 HCO3 A mixed solution using Sr(NO3 )2 , Al(NO3 )3 and Eu(NO3 )3 solutions with a molar ratio of 3.92:14:0.08 for Sr:Al:Eu was added dropwise to NH4 HCO3 precipitant solution with magnetic stirring. After aging for 15 min, the precipitate was filtered and washed with distilled water. Dried at 100 ◦ C for 8 h, the precursor powder mixed with a little H3 BO3 as flux was then calcined at 1250 ◦ C for 3 h under CO atmosphere.

2.3. Synthesis of Sr4 Al14 O25 :Eu2+ by sol–gel method with citric acid (C6 H8 O7 ·H2 O)

Fig. 1. XRD patterns of 4Sr0.98 Eu0.02 O·7Al2 O3 prepared by different methods.

3.2. Morphology and size of Sr4 Al14 O25 :Eu2+ phosphors A mixed solution with a molar ratio of 3.92:14:0.08 for Sr:Al:Eu using Sr(NO3 )2 , Al(NO3 )3 and Eu(NO3 )3 solutions was added dropwise to the citric acid solution under stirring ([citric acid]:[M] = 3:1). After evaporating the solvent in an 80 ◦ C water-bath, a light yellow transparent viscous gel formed. The gel was then heated at 150 ◦ C for 8 h and finally formed a dry precursor. Subsequently, this solid precursor was calcined at 800 ◦ C for 6 h. The obtained white powder mixed with a little H3 BO3 as flux was fully ground, and then burned at 1150 ◦ C for 3 h in a CO atmosphere.

2.4. Measurements The crystal phase identification was carried out on an X-ray diffractometer (D/max-IIIA diffractometer, RIGAKU Corporation of Japan; 40 kV and 20 mA, ˚ Particles sizes and shapes were observed by field-emission Cu K␣ = 1.5406 A). scanning electron microscopy (FE-SEM, JSM-6330F, JEOL Corporation of Japan). The excitation and emission spectra of the powdered phosphors were measured at room temperature on a Fluorolog-3-21 spectrometer (JOBIN YVON, America) and a 450 W xenon lamp was used as the excitation source. The emission spectra of the blue–green LEDs were recorded on Labsphere Inc. LED-1100.

3. Results and discussion 3.1. X-ray powder diffraction The XRD patterns of the product samples synthesized with SS, CP and SG are shown in Fig. 1 The diffraction peaks can be indexed to the phases of Sr4 Al14 O25 (JCPDS 52-1876), which has been refined to be orthorhombic with space group Pmma ˚ b = 8.487 A ˚ and c = 4.866 A ˚ and cell parameters of a = 24.785 A, 2+ [12]. The result indicates that the doped Eu ions have little influence on the crystalline structure of phosphors due to the similar ionic radii of Eu2+ (0.112 nm) and Sr2+ (0.114 nm) [13]. From the diffraction intensity, it also can be seen that the order of crystallization degree for the phosphors is SS > CP > SG. This sequence is in accordance with firing temperature T (SS) > T (CP) > T (SG).

Fig. 2 shows the FE-SEM images of Sr4 Al14 O25 :Eu2+ phosphors prepared with SS, CP and SG methods. The phosphor prepared by SG appears as relatively regular fine grains with a flaky shape of hexagon, an average size of 400–600 nm and a thickness of about 90 nm, while the particles synthesized by SS and CP, show a heavy agglomerate phenomenon, a big size distribution and irregular shapes with an average size of 1–3 ␮m for CP and 8–10 ␮m for SS. The results indicate that the SG process can effectively control the product size and prevent heavy agglomeration due to the uniformity of the starting reactants, and thus is favorable to formation of superfine phosphors. 3.3. Photoluminescence property of Sr4 Al14 O25 :Eu2+ phosphors The photoluminescence excitation and emission spectra of 4Sr0.98 Eu0.02 O·7Al2 O3 prepared with three methods are similar, so the SS sample is presented as a representative (see Fig. 3). Monitored at 491 nm, a broad excitation band ranging from 300 to 450 nm appears, and the intensity at about 400 nm is high, which means this phosphor is suitable to being excited by an NUV chip. Since the host Sr4 Al14 O25 hardly shows any absorption between 300 and 450 nm, the excitation band is attributed to the transition from 4f7 ground state to the excited sate 4f6 5d1 of the doped Eu2+ ions. Under 367 and 397 nm NUV-light excitation, the 4Sr0.98 Eu0.02 O·7Al2 O3 phosphor shows strong and broad blue–green emission peaking at 491 nm, which attributes to a typical 4f6 5d1 → 4f7 transition of Eu2+ . No emission peaks of Eu3+ were observed in the spectra, indicating that Eu3+ ions in the matrix have been reduced to Eu2+ completely. The effects of different synthesis methods on photoluminescence properties were also investigated. Fig. 4 shows that the emission spectra of 4Sr0.98 Eu0.02 O·7Al2 O3 phosphors prepared by SS (a), CP (b) and SG (c). Excited by 397 nm light, all the 4Sr0.98 Eu0.02 O·7Al2 O3 phosphors emit intense

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Fig. 3. Photoluminescence spectra of 4Sr0.98 Eu0.02 O·7Al2 O3 prepared with SS: (a) excitation spectrum (λem = 491 nm); (b) emission spectrum (λex = 367 nm); (c) emission spectrum (λex = 397 nm).

which is consistent with the decreasing crystallization degree. 3.4. Fabricate LED with Sr4 Al14 O25 :Eu2+ LEDs were fabricated by combining the prepared each 4Sr0.98 Eu0.02 O·7Al2 O3 phosphor and a ∼397 nm-emitting InGaN chip. The emission spectra of the fabricated LEDs under 20 mA forward bias are shown in Fig. 5 The band centering at 397 nm is attributed to the emission of InGaN chips and the broad band peaking at 491 nm is assigned to the emission of 4Sr0.98 Eu0.02 O·7Al2 O3 . Bright blue–green light from the LEDs was observed by naked eyes. The CIE co-ordinates of the LEDs are calculated based on the emission spectra to be (0.149,0.344) for SS, (0.152,0.363) for CP and (0.158,0.375) for SG, respectively.

Fig. 2. FE-SEM images of 4Sr0.98 Eu0.02 O·7Al2 O3 prepared by three methods: (a) SS; (b) CP; (c) SG.

wide-band blue–green light peaking at 491 nm with similar profiles because of the same composition and the same crystalline lattice while the intensity is different with an integrated emission intensity ratio of 4.4:2.1:1.0 for SS:CP:SG,

Fig. 4. Emission spectra of 4Sr0.98 Eu0.02 O·7Al2 O3 prepared by different methods: (a) SS; (b) CP; (c) SG (λex = 397 nm).

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It is important that the remained NUV bands are still intensive, and thus different color phosphors can be co-coated onto the same chip and be excited. The results indicate that all the synthesized 4Sr0.98 Eu0.02 O·7Al2 O3 phosphors are suitable as a blue–green component in NUV InGaN-based LED fabrication, and co-coating of this phosphor with longer wavelength emitting phosphors may be a potential path to fabricate white LEDs. 4. Conclusions

Fig. 5. Emission spectra of the LEDs with 4Sr0.98 Eu0.02 O·7Al2 O3 phosphors prepared by different methods ((a) SS; (b) CP; (c) SG) under a forward bias of 20 mA.

Compared with the photoluminescence intensity ratio for the powdered samples (4.4:2.1:1.0 for SS:CP:SG), the difference in emission intensity ratio for the three PC-LEDs is distinctly reduced (1.5:1.1:1.0 for SS:CP:SG). The absorption intensity for NUV-light emitted from InGaN chip increases in the order: SS < CP < SG. The facts reveal that the light-converted efficiency for the SS sample is the highest, whereas the relatively regular morphology, small particle size and narrow size distribution of the samples synthesized by wet chemical method are favorable to absorb the NUV light from the chip and thus reduce the difference in emission intensity of the three phosphors in LEDs. The photoluminescence intensity of the phosphor obtained by SS is the strongest because of two reasons: first, the crystallization degree of Sr4 Al14 O25 prepared by SS is the best (see Fig. 1); second, high temperature favors the doping Eu2+ ions into Sr4 Al14 O25 lattice. However, too heavy agglomerate phenomenon, too large particles and too irregular morphologies cause disadvantages to the phosphor used for NUV-LEDs in views of the following reasons: (a) it is difficult to get good homogeneity by mixing the phosphor and epoxy resin; (b) interspaces between particles pre-coated on the NUV-LED chips appear more easily, which decrease the absorption for NUV-light emitted from InGaN chip; (c) it is unfavorable for phosphor to give out light because of irregular morphologies. Instead, the phosphor prepared by SG overcomes the above-mentioned disadvantages in a great degree. Therefore, the efficiency of the absorption to NUV light is in a reverse order (SS < CP < SG). These are two competitive factors influencing the luminescence, the crystallization degree and the NUV-light absorption efficiency, and the higher absorption efficiency to NUV-light for CP and SG samples reduces the differences of emission intensity among the three phosphors in LEDs.

Blue–green-emitting phosphor Sr4 Al14 O25 :Eu2+ was synthesized with three methods, SS, CP and SG. Bright blue–green-emitting LEDs were fabricated by combining each phosphor with InGaN-based NUV chip. The results reveal that the light-converted efficiency for SS is the highest, whereas the relatively regular morphology, small particle size and narrow size distribution of the samples synthesized by wet chemical method are favorable to absorb the NUV light emitted from the chip and thus reduce the difference in emission intensity of the three phosphors in LEDs. Acknowledgements This work was funded by research grants from the Guangdong Province Government (Grant No. ZB2003A07) and the Science and Technical Projects of Guangzhou City Government (Grant Nos. 2005Z2-D0061 and 054J205001). References [1] C.H. Kuo, J.K. Sheu, S.J. Chang, Y.K. Su, L.W. Wu, J.M. Tsai, C.H. Liu, R.K. Wu, Jpn. J. Appl. Phys. 42 (2003) 2284. [2] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [3] J.S. Kim, J.Y. Kang, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Parka, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696. [4] Z.L. Wang, H.B. Liang, L.Y. Zhou, M.L. Gong, Q. Su, Chem. Phys. Lett. 412 (2005) 313. [5] J.K. Park, M.A. Lim, C.H. Kim, H.D. Park, J.T. Park, S.Y. Choi, Appl. Phys. Lett. 82 (2003) 683. [6] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [7] B. Smets, J. Rutten, G. Hoeks, J. Electrochem. Soc. 136 (7) (1989) 2119. [8] F.C. Palilla, A.K. Levine, M.R. Tomkus, J. Electrochem. Soc. 115 (6) (1968) 642. [9] Y.H. Lin, Z.T. Zhang, F. Zhang, Z.L. Tang, Q.M. Chen, Mater. Chem. Phys. 65 (2000) 103. [10] Z.C. Wu, J.X. Shi, J. Wang, H. Wu, Q. Su, M.L. Gong, Mater. Lett. 60 (2006) 3449. [11] Z.C. Wu, J.X. Shi, J. Wang, M.L. Gong, Q. Su, J Mater. Sci.: Mater El. (revised). [12] T.N. Nadzhina, E.A. Pobedimskaya, N.V. Belov, Kristallografiya 25 (1980) 938. [13] G. Blasse, Luminescence of Inorganic Solids, Plenum Press, New York, 1978, pp. 457–476.