Synthesis and luminescence properties of the red phosphor CaZrO 3 :Eu 3+ for white light-emitting diode application

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1 Cent. Eur. J. Phys. 9(4) DOI: /s Central European Journal of Physics Synthesis and luminescence properties of the red phosphor CaZrO 3 :Eu 3+ for white light-emitting diode application Research Article Junli Huang 1,2,3, Liya Zhou 1,2, Yuwei Lan 1,2, Fuzhong Gong 1,2, Qunliang Li 1,2, Jianhua Sun 1,2 1 Key laboratory of new processing technology for nonferrous metals and materials, Ministry of Education, Guangxi University, Nanning, , China 2 School of Chemistry and Chemical Engineering, Guangxi University, Nanning, , China 3 Research and Quality Detection Center, Gongdong Evergreen Group Co. Ltd., Zhanjiang, , China Received 5 July 2010; accepted 29 November 2010 Abstract: PACS (2008): Hx, Jy Keywords: Eu 3+ -doped CaZrO 3 phosphor with perovskite-type structure was synthesized by the high temperature solid-state method. The samples were characterized by X-ray diffraction, scanning electron microscopy, fluorescence spectrophotometer and UV-vis spectrophotometer, respectively. XRD analysis showed that the formation of CaZrO 3 was at the calcinations temperature of 1400 C. The average diameter of CaZrO 3 with 4 mol% doped-eu 3+ was 2 µm. The PL spectra demonstrated that CaZrO 3 :Eu 3+ phosphor could be excited effectively in the near ultraviolet light region (397 nm) and emitted strong red-emission lines at 616 nm corresponding to the forced electric dipole 5 D 0 7 F 2 transitions of Eu 3+. Meanwhile, the light-emitting diode was fabricated with the Ca 0.96 ZrO 3 :Eu 3+ phosphor, which can efficiently absorb 400 nm irradiation and emit red light. Therefore Ca 0.96 ZrO 3 :Eu 3+ may have applications for a near ultraviolet InGaN chip-based white light-emitting diode. phosphor luminescence light-emitting diode Versita Sp. z o.o. 1. Introduction Since becoming commercially available in 1997, the white light-emitting diode (LED) has been considered as the zhouliyatf@163.com (Corresponding author) new generation solid-state light and has attracted much attention for its many advantages such as high light efficiency, good stability, adjustable colors and environmental friendliness [1 4]. At the moment, one of the most convenient approaches to achieving white light is the combination of a near-uv LED chip ( nm) with red, green and blue phosphors [5]. The red, green and blue phosphor materials currently commercially available for near-uv 975

2 Synthesis and luminescence properties of the red phosphor CaZrO 3 :Eu 3+ for white light-emitting diode application GaN-based LED are Y 2 O 2 S:Eu 3+, ZnS:(Cu +, Al 3+ ) and BaMgAl 10 O 17 :Eu 2+, respectively [6]. Unfortunately, the brightness of the Y 2 O 2 S:Eu 3+ red phosphor is much weaker than that of the green and blue phosphors. In order to obtain white light, the white LEDs are usually composed of a phosphor mixture of 80% red, 10% green and 10% blue. In additional to the above mentioned facts, sulfide-based phosphors are chemically unstable. Therefore, new and stable red phosphors are expected to be excited efficiently with intense emission under stimulation in the near UV irradiation range around 400 nm. Recently, more investigations have focused on luminescent properties of rare earth ion doped perovskite-type oxides, based on the fact that they were extremely stable and could withstand extreme environmental conditions, which could be an outstanding property for applications [7]. Thus a number of ABO 3 -like compounds with the perovskite structure such as A (A = Ca, Sr, Ba)TiO 3 [8 11], SrTiO 3 :Al, Pr [12], CaZrO 3 :Ln (Ln = Pr, Tm, Eu) [13 15] and BaZrO 3 :Eu [16, 17] had been studied and showed very interesting luminescent properties. As an ABO 3 -like compound, calcium zirconate (CaZrO 3 ) has an orthorhombic perovskite-type structure [18]. No studies on Eu 3+ doped-cazro 3 prepared by solid-state reaction have been reported so far. In this study, the solid-state reaction technique was explored to synthesize CaZrO 3 :Eu 3+ phosphor. The luminescence properties and the effects of doping concentration on PL were also investigated. tometer. The UV-vis absorption spectrum was recorded on a UV-2501PC UV-Vis spectrophotometer (Shimadzu in Japan). The LED parameters were measured on an EVERFINE PMS-80 UV-VIS-IR spectrophotometer. All measurements of the phosphors were carried out at room temperature. 3. Results and discussion 3.1. XRD and size-distribution characterization XRD patterns of Ca 0.96 ZrO 3 :Eu 3+ calcined at 1400 C for 5 hours are shown in Fig. 1. Compared to the standard card of JCPDS (CaZrO 3 ) with the lattice constants: a = , b = , c = (space group Pnma [62]), the 2θ angles of the main diffraction peaks of the Ca 0.96 ZrO 3 :Eu 3+ XRD pattern, such as 22.10, 30.98, 31.48, 31.94, 45.12, 50.84, 55.44, and 65.76, were consistent with those of JCPDS (CaZrO 3 ). Therefore, the CaZrO 3 phase was dominant for a calcination temperature of 1400 C. Eu 3+ -doping did not change the lattice of the powder. 2. Experimental 2.1. Preparation of CaZrO 3 :Eu 3+ phosphor The CaZrO 3 :Eu 3+ phosphors were prepared by the high temperature solid-state method. Eu 2 O 3 (99.99%), CaCO 3 (A.R.) and ZrO 2 (A.R.) were used as initiating materials. Stoichiometric amounts for each compound were accurately weighed, mixed and ground in an agate mortar. Then the mixture was calcined at 1400 C for 5 hours in air and cooled to room temperature Characterization of CaZrO 3 :Eu 3+ phosphor Powder X-ray diffraction (XRD, 40 kv and 30 ma, Cu K α = Å Rigaku/Dmax-2500X in Japan) was used to identify the crystal phase of the final products. Scanning electron microscopy (SEM, HITACHI S-3400N) was used to observe the particle morphology. Room temperature excitation and emission spectra were recorded on a HITACHI F-2500 fluorescence spectropho- Figure 1. The XRD pattern of Ca 0.96 ZrO 3 :Eu 3+ for 5 h. calcined at 1400 C Fig. 2 shows the SEM image of the Ca 0.96 ZrO 3 :Eu 3+ phosphor. As shown in Fig. 2, there was a slight sintering phenomenon due to the high calcination temperature. The shape of the particles is irregular with an average diameter of about 2 µm, which indicates that these micro crystalline phosphors can result in high luminescent intensities and the particles are fit to be used in fabricating the solid-lighting devices [19, 20]. 976

3 Junli Huang, Liya Zhou, Yuwei Lan, Fuzhong Gong, Qunliang Li, Jianhua Sun (λ ex = 397 nm) and 1.9 (λ ex = 277 nm), respectively, which indicated that the Ca 0.96 ZrO 3 :Eu 3+ phosphors can be excited efficiently in the range of near UV. Figure 2. The SEM image of Ca 0.96 ZrO 3 :Eu C for 5 h. phosphor calcined at 3.2. Photoluminescence properties of CaZrO 3 :Eu 3+ Fig. 3 shows the excitation (λ em = 616 nm) and emission (λ ex = 397 and 277 nm) spectra of Ca 0.96 ZrO 3 :Eu 3+. As shown in Fig. 3, the charge transfer band (CTB) which corresponds to an electron transfer from an oxygen 2p orbital to an empty 4f orbital of a europium ion was observed in the region of nm. Sharp lines in the nm range are intra-configurational 4f 4f transitions of Eu 3+ in the host lattices, and the strong excitation at 397 nm is assigned to 7 F 0 5 L 6 transitions of Eu 3+. Meanwhile, the emission spectrum involves the characteristic emission lines originating from 5 D 2 7 F 3 (513 nm), 5 D 2 7 F 3 (536 nm) and 5 D 0 7 F J (J = 0, 1, 2,...) transitions of Eu 3+. The intensity of the emission peak at 616 nm corresponding to the electric dipole transition 5 D 0 7 F 2 of Eu 3+ is much stronger than that of the peak at 593 nm assigned to the magnetic dipole transition 5 D 0 7 F 1 of Eu 3+, which allows the Eu 3+ to occupy the non-inversion symmetry sites in the host lattices. Eu 3+ (0.095 nm) might occupy the Ca 2+ (0.099 nm) site of CaZrO 3 lattice for the similar ionic radius. The coordination of Ca ions in CaZrO 3 is and CaZrO 3 has an orthorhombic distorted perovskite structure, in which the point symmetry at the Ca site is C 1 with no inversion center. Some kinds of defects (oxygen interstitials or displacements) might be introduced for charge compensation. Compared with the CaZrO 3 :Eu 3+ prepared by the Pechini method in cited Ref. [15], the obtained CaZrO 3 :Eu 3+ phosphor showed a bright red emission under 397 nm excitation. The ratio between the integrated intensity of 5 D 0 7 F 2 to 5 D 0 7 F 1 transitions for Ca 0.96 ZrO 3 :Eu 3+ was calculated to be 2.1 Figure 3. Excitation (λ em = 616 nm) and emission (λ ex = 397 and 277 nm) spectra of Ca 0.96 ZrO 3 :Eu 3+ phosphor. Figure 4. Emission spectra of Ca 1 x ZrO 3 :Eu 3+ x phosphors excited at 397 nm with different contents of doped-eu 3+ at room temperature. Inset: Emission spectra of Ca 1 x ZrO 3 :Eu 3+ x phosphors with different Eu 3+ doping ratios. The luminescence intensity of phosphors is always dependent on the doping concentration. Fig. 4 shows the effect of Eu 3+ -doped content on the intensity at the wavelength of 616 nm for CaZrO 3 :Eu 3+ powders with 397 nm excitation. It can be found that the emission intensity increases with the increase of Eu 3+ content and reaches the maximum at 4 mol%, and then decreases with 5 mol% doped-content, which indicates the concentration quenching. This quenching process is often attributed to energy migration among Eu 3+ ions. Usually, an over-doping concentration will result in the enhancement of non-radiative 977

4 Synthesis and luminescence properties of the red phosphor CaZrO 3 :Eu 3+ for white light-emitting diode application relaxation between the neighboring Eu 3+ ions. As shown in this study, the optimal mole concentration of Eu 3+ in Ca 1 x ZrO 3 :Eu 3+ x phosphors is 4 mol%. The UV-vis absorption spectrum of Ca 0.96 ZrO 3 :Eu 3+ phosphor is shown in Fig. 5. The broad band from 200 nm to 350 nm in the UV-vis absorption spectrum revealed the charge transfer band (CTB) of Eu 3+ -O 2 was effectively excited by the ultraviolet light. The weak absorption peaks at nm and nm are due to the f f electron transition of Eu 3+ ions, which was in accordance with the findings of excitation spectrum analysis. Figure 6. Emission spectra of the original 395 nm-emitting InGaN chip (a) and the red-emitting LEDs with Ca 0.96 ZrO 3 :Eu 3+ (b) under 20 ma forward bias. Inset: The photograph of the red LED-based Ca 0.96 ZrO 3 :Eu Conclusions Figure 5. UV-vis absorption spectrum of Ca 0.96 ZrO 3 :Eu 3+ phosphor obtained from the solid-state reaction. Inset: detail of the absorption peaks at nm and nm. Eu 3+ -doped CaZrO 3 phosphors were prepared by the solid-state reaction. Upon excitation with near UV light (397 nm), the phosphor showed strong red-emission lines at 616 nm, corresponding to the forced electric dipole 5 D 0 7 F 2 transition of Eu 3+. The average diameter of the particles was about 2 µm. The UV-vis absorption also illustrated that the charge transfer band (CTB) of Eu 3+ -O 2 was effectively excited by the ultraviolet light. The fabricated LED further confirmed that the Ca 0.96 ZrO 3 :Eu 3+ phosphors can efficiently absorb 400 nm radiation and emit red light. All results indicate that this red phosphor could be a great candidate for the production of near UV InGaN based light emitting diodes Fabrication of LED with the Ca 0.96 ZrO 3 :Eu 3+ phosphor A LED was fabricated by coating the Ca 0.96 ZrO 3 :Eu 3+ phosphor onto a 395 nm-emitting InGaN chip. The emission spectra of the original 395 nm-emitting InGaN chip (a) and the red-emitting LED with Ca 0.96 ZrO 3 :Eu 3+ (b) under 20 ma forward bias were shown in Fig. 6. The band 395 nm was attributed to the emission of the InGaN chip and the sharp peaks at 592, 616, 656 and 702 nm were generated by the absorption of the coated phosphor of Ca 0.96 ZrO 3 :Eu 3+. Bright red light from the LED was observed by LED test. Since the Ca 0.96 ZrO 3 :Eu 3+ can absorb the 400 nm excitation energy efficiently, it is considered as a good candidate for the red component of trichromatic WLED application. Acknowledgments This project is supported by open foundation of the key lab of new processing technology for nonferrous metals and materials (GXKFJ09-12). References [1] Y.H. Wang et al., Physica B 403, 2071 (2008) [2] L.Y. Zhou, J.S. Wei, F.Z. Gong, J.L. Huang, L.H. Yi, J. Solid State Chem. 181, 1337 (2008) [3] Z.L. Wang et al., J. Lumin. 128, 147 (2008) [4] C.Q. Zhu, S.G. Xiao, J.W. Ding, X.L. Yang, R.F. Qiang, Mater. Sci. Eng. B-Adv. 150, 95 (2008) [5] Z.L. Wang et al., Chem. Phys. Lett. 412, 313 (2005) 978

5 Junli Huang, Liya Zhou, Yuwei Lan, Fuzhong Gong, Qunliang Li, Jianhua Sun [6] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387, 2 (2004) [7] Y.X. Pan et al., J. Solid State Chem. 174, 69 (2003) [8] P. Boutinaud, E. Pinel, M. Dubois, A.P. Vink, R. Mahiou, J. Lumin. 111, 69 (2005) [9] S. Okamoto, H. Yamamoto, Appl. Phys. Lett. 78, 655 (2001) [10] H.X. Zhang et al., Appl. Phys. Lett. 77, 609 (2000) [11] J. Amami et al., J. Lumin , 383 (2006) [12] J.K. Park, H. Ryu, H.D. Park, S.Y. Choi, J. Eur. Ceram. Soc. 21, 535 (2001) [13] E. Pinel, P. Boutinaud, R. Mahiou, J. Alloy. Compd. 380, 225 (2004) [14] H.W. Zhang, X.Y. Fu, S.Y. Niu, Q. Xin, J. Lumin. 128, 1348 (2008) [15] H.W. Zhang, X.Y. Fu, S.Y. Niu, Q. Xin, J. Alloy. Compd. 459, 103 (2008) [16] J. Alarcon, D. van der Voort, G. Blasse, Mater. Res. Bull. 27, 467 (1992) [17] X.H. Liu, X.D. Wang, Opt. Mater. 30, 626 (2007) [18] H.J.A. Koopmans, G.M.H. van de Velde, P.J. Gellings, Acta Crystallogr. C 39, 1323 (1983) [19] B. Vengala Rao, S. Buddhudu, Mater. Chem. Phys. 111, 65 (2008) [20] R.P. Rao, J. Electrochem. Soc. 143, 189 (1996) 979