The enhancement of photoluminescence characteristics of Eu-doped barium strontium silicate phosphor particles by co-doping materials

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1 Journal of Alloys and Compounds 402 (2005) The enhancement of photoluminescence characteristics of Eu-doped barium strontium silicate phosphor particles by co-doping materials Hee Sang Kang a, Seung Kwon Hong b, Yun Chan Kang b,, Kyeong Youl Jung c, Yong Gun Shul d, Seung Bin Park a a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon , Republic of Korea b Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul , Republic of Korea c Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong-gu, Daejeon , Republic of Korea d Department of Chemical Engineering, Yonsei University, 134, Shincheon-dong, Seodaemun-gu, Seoul , Republic of Korea Received 19 March 2005; accepted 8 April 2005 Available online 4 June 2005 Abstract Green light emitting (Ba,Sr) 2 SiO 4 :Eu phosphor particles with high photoluminescence intensity under long wavelength ultraviolet (UV) were prepared by spray pyrolysis from colloidal spray solution. Yttrium, cerium and holmium components were introduced as co-doping materials to improve the photoluminescence characteristics of (Ba,Sr) 2 SiO 4 :Eu phosphor particles in the spray pyrolysis. The photoluminescence intensities of co-doped (Ba,Sr) 2 SiO 4 :Eu phosphor particles were about % of (Ba,Sr) 2 SiO 4 :Eu phosphor particles without co-dopant. The highest photoluminescence intensity was achieved when the doping concentration of yttrium was about 1.7 times of the doping concentration of europium. The photoluminescence intensity of the sieved phosphor particles using 20 m sieve was comparable to that of the original (Ba,Sr) 2 SiO 4 :Eu phosphor particles Elsevier B.V. All rights reserved. Keywords: Phosphor; Spray pyrolysis; Silicate; Flux material 1. Introduction Phosphor-combined light emitting diode (LED) is widely studied as backlight of the liquid crystal display (LCD) and lamp for lighting [1 3]. In order to apply phosphor-combined LED for a light source instead of the conventional lamp, it is essential to develop a LED lamp which emits white light. Pasting fluorescent materials over an ultraviolet (UV) LED chip is considered as the efficient process because the process can produce full color. In order to successfully apply phosphor-combined UV LED to the lighting, fluorescent materials emitting blue, green and red lights should have high efficiency under long wavelength UV [3 9]. Corresponding author. Tel.: ; fax: address: yckang@konkuk.ac.kr (Y.C. Kang). Green light emitting (Ba,Sr) 2 SiO 4 :Eu phosphor was known to be suitable for UV LED phosphor because it has short decay time and high luminescence characteristics under long wavelength UV. Especially, the chromaticity of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles could be controlled by changing the ratio of barium and strontium of the host material. (Ba,Sr) 2 SiO 4 :Eu phosphor particles were prepared and optimized by the conventional solid state reaction method [8 13]. In order to improve the photoluminescence characteristics of (Ba,Sr) 2 SiO 4 :Eu phosphor, several rareearth materials were employed as co-dopant in the solid state reaction method [14]. Aluminum, yttrium and gadolinium co-dopants improved the photoluminescence intensities of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles in the solid state reaction method. The morphology and luminescence characteristics of the phosphor particles are affected by the phosphor preparation process. Spray pyrolysis is efficient /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 for the preparation of multicomponent oxide phosphor particles because of advantages in controlling the composition and morphology of the phosphor particles [15 18]. In this work, the photoluminescence characteristics of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles were improved in the spray pyrolysis process by optimizing the co-doping system. In the spray pyrolysis, yttrium, cerium and holmium as co-dopants influenced the excitation and emission spectra of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles. H.S. Kang et al. / Journal of Alloys and Compounds 402 (2005) Experimental (Ba,Sr) 2 SiO 4 :Eu phosphor particles with the composition of Ba x Sr 0.5 SiO 4 :Eu ,Re x (Re = Y, Ce and Ho) were prepared by spray pyrolysis. The as-prepared (Ba,Sr) 2 SiO 4 :Eu,Re phosphor particles were obtained by spray pyrolysis. An ultrasonic spray generator with six vibrators that have a frequency of 1.7 MHz was used to produce a large amount of droplets. The general flow diagram of the spray pyrolysis process is given elsewhere [16]. The spray solutions were prepared from barium nitrate, strontium nitrate, europium oxide and fumed silica. The overall solution concentration was 0.3 M. The as-prepared particles obtained by spray pyrolysis at 900 C were post-treated at 1200 C for 3 h under 10% H 2 /N 2 mixture gas. The morphology of phosphor particles was investigated by scanning electron microscopy (PHILIPS XL 30S FEG). X-ray diffraction patterns of phosphor particles were obtained using an X-ray diffractometer (RIGAKU DMAX-33 X-ray) with Ni filtered Cu K radiation (λ = C). Diffraction patterns were taken over the range of 20 2θ 70 with a scan rate of 5 2θ/min. The X-ray diffractometer was operated at 40 kv and 40 ma. The photoluminescence spectra of (Ba,Sr) 2 SiO 4 :Eu phosphor particles after the post-treatment were measured using a spectrofluorophotometer (Perkin-Elmer LS50-B) under the excitation of a 410 nm UV produced by a Xe flash lamp. Fig. 1. PL emission spectra of Ba x Sr 0.5 SiO 4 :Eu ,Y x phosphor particles. doping concentration of yttrium. The phosphor particles had a broad emission spectrum between 460 and 580 nm and had the maximum peak intensity at 508 nm. However, photoluminescence intensities of the prepared phosphor particles were affected by the co-doping concentrations of yttrium into the phosphor particles. The maximum photoluminescence intensity of (Ba,Sr) 2 SiO 4 :Eu phosphor particles with yttrium codopant was 143% of that of the phosphor particles without co-dopant. The excitation spectra of (Ba,Sr) 2 SiO 4 :Eu phosphor particles prepared from spray solutions with different doping concentrations of yttrium are shown in Fig. 2. The prepared (Ba,Sr) 2 SiO 4 :Eu phosphor particles had broad excitation wavelength ranging from 220 to 420 nm. Yttrium used as codopant changed the spectrum of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles. The excitation spectrum of (Ba,Sr) 2 SiO 4 :Eu phosphor particles had higher intensity at the wavelength below 263 nm than those of the (Ba,Sr) 2 SiO 4 :Eu,Y 3. Results and discussions (Ba,Sr) 2 SiO 4 :Eu 2+ phosphor particles co-doped with rareearth materials were prepared by spray pyrolysis. The photoluminescence intensities of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles obtained at the optimum preparation conditions were measured under excitation wavelength of 410 nm. The as-prepared particles obtained by spray pyrolysis at 900 C from spray solutions with different co-dopant materials were post-treated at 1200 C for 3 h under reducing atmosphere. Several co-dopants such as Y, Ce and Ho were effective in order to improve the photoluminescence intensities of (Ba,Sr) 2 SiO 4 :Eu phosphor particles in the spray pyrolysis. Fig. 1 shows the photoluminescence spectra of (Ba,Sr) 2 SiO 4 :Eu phosphor particles co-doped with yttrium. The prepared (Ba,Sr) 2 SiO 4 :Eu phosphor particles had the same photoluminescence spectra, irrespective of the co- Fig. 2. PL excitation spectra of Ba x Sr 0.5 SiO 4 :Eu ,Y x phosphor particles.

3 248 H.S. Kang et al. / Journal of Alloys and Compounds 402 (2005) Fig. 3. Photoluminescence intensities of Ba x Sr 0.5 SiO 4 :Eu ,Ce x phosphor particles as a function of cerium content. Fig. 4. Photoluminescence intensities of Ba x Sr 0.5 SiO 4 :Eu ,Ho x phosphor particles as a function of holmium content. phosphor particles. On the other hand, the excitation spectrum of (Ba,Sr) 2 SiO 4 :Eu,Y phosphor particles had higher intensity at the wavelength above 263 nm than those of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles. Developed UV LED had high power output in the wavelength between 380 and 410 nm. Therefore, (Ba,Sr) 2 SiO 4 :Eu,Y is more appropriate than (Ba,Sr) 2 SiO 4 :Eu as green phosphor in the application to phosphor-combined UV LED. The effect of cerium co-dopant on the photoluminescence intensities of (Ba,Sr) 2 SiO 4 :Eu phosphor particles prepared by spray pyrolysis is shown in Fig. 3. The low doping concentration of cerium improved the photoluminescence intensity of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles. The optimum doping concentration of cerium was about five times the doping concentration of europium for showing the maximum photoluminescence intensity of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles. The maximum photoluminescence intensity of (Ba,Sr) 2 SiO 4 :Eu,Ce phosphor particles was 141% of that of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles without co-dopant. The high doping concentration of cerium abruptly decreased the photoluminescence intensity of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles. Fig. 4 shows the effect of the holmium co-dopant on the photoluminescence intensity of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles prepared by spray pyrolysis. Holmium was less effective in improvement of the photoluminescence intensity of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles than yttrium and cerium components in the spray pyrolysis. The maximum photoluminescence intensity of the phosphor particles with holmium co-dopant was 127% of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles without co-dopant. Fig. 5 shows the morphology of the (Ba,Sr) 2 SiO 4 :Eu phosphor particle with and without yttrium co-dopant. The as-prepared particles obtained by spray pyrolysis were posttreated at the post-treatment temperature of 1200 C under reducing atmosphere. The as-prepared particles obtained by spray pyrolysis had spherical shape and non-aggregation characteristics. However, the crystal structure of the Fig. 5. SEM photographs of post-heat treated (Ba,Sr) 2 SiO 4 :Eu 2+, co-dopant phosphor particles: (a) no co-dopant and (b) co-dopant.

4 H.S. Kang et al. / Journal of Alloys and Compounds 402 (2005) Fig. 7. Photoluminescence intensities of Ba x Sr 0.5 SiO 4 :Y phosphor particles after sieving. is shown in Fig. 7. The photoluminescence intensity of the sieved phosphor particles using 20 m sieve was comparable to that of the original (Ba,Sr) 2 SiO 4 :Eu phosphor particles. 4. Conclusions Fig. 6. SEM photographs of (Ba,Sr) 2 SiO 4 :Eu 2+, co-dopant phosphor particles for effect of sieving: (a) before sieving and (b) after using 20 m sieve. (Ba,Sr) 2 SiO 4 :Eu phosphor was not obtained. Post-treatment under reducing atmosphere produced the phase pure (Ba,Sr) 2 SiO 4 :Eu phosphor particles with high photoluminescence intensity. The morphology of the as-prepared particles was destroyed after post-treatment at the post-treatment temperature of 1200 C. The post-treated (Ba,Sr) 2 SiO 4 :Eu phosphor particles had large size and non-spherical shape. Yttrium co-dopant did not change the morphology of the (Ba,Sr) 2 SiO 4 :Eu phosphor. Fig. 6 shows the SEM photographs of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles before and after sieving using 20 m sieve. In this work, milling process was not performed to reduce the mean particle size of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles prepared by spray pyrolysis. Some of the (Ba,Sr) 2 SiO 4 :Eu particles prepared by spray pyrolysis had large size and aggregated morphology before sieving. On the other hand, the sieved particles had regular morphology and narrow particle size distribution below 20 m. The relative photoluminescence intensities of the sieved phosphor particles compared with that of the original (Ba,Sr) 2 SiO 4 :Eu phosphor particles obtained before sieving The photoluminescence characteristics of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles were improved in the spray pyrolysis process by optimizing the co-doping system. The codopants such as yttrium, cerium and holmium improved the photoluminescence characteristics of the (Ba,Sr) 2 SiO 4 :Eu phosphor particles prepared by spray pyrolysis from a colloidal spray solution. The co-doped (Ba,Sr) 2 SiO 4 :Eu phosphor particles had the good excitation spectrum ranging from 260 to 430 nm. Thus, the co-doped (Ba,Sr) 2 SiO 4 :Eu phosphor particles prepared by spray pyrolysis could be applied as green phosphor to long wavelength UV LED. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, Germany, [2] P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64 (1997) 427. [3] 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 Photonic Tech. Lett. 15 (1) (2003) 18. [4] S. Bender, R. Franke, E. Hartmann, V. Lansmann, M. Jansen, J. Hormes, J. Non-Cryst. Solids 298 (2002) 99. [5] M. Peng, Z. Pei, G. Hong, Q. Su, J. Mater. Chem. 13 (2003) [6] Y. Hu, W. Zhuang, H. Ye, S. Zhang, Y. Fang, X. Huang, J. Lumin. 111 (2005) 139. [7] Y.F. Lin, Y.H. Chang, B.S. Tsai, J. Alloys Compd. 377 (2004) 277. [8] T.L. Barry, J. Electrochem. Soc. 115 (11) (1968) [9] S.H.M. Poort, W. Janssen, G. Blasse, J. Alloys Compd. 260 (1997) 93. [10] J.M. Fields Jr., P.S. Dear, J.J. Brown Jr., J. Am. Ceram. Soc. 55 (12) (1972) 585.

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