An efficient blue-emitting phosphor LiCaPO4 :Eu2+ for white LEDs

An efficient blue-emitting phosphor LiCaPO4 :Eu2+ for white LEDs

Solid State Communications 150 (2010) 1493–1495 Contents lists available at ScienceDirect Solid State Communications journal homepage: www.elsevier...

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Solid State Communications 150 (2010) 1493–1495

Contents lists available at ScienceDirect

Solid State Communications journal homepage: www.elsevier.com/locate/ssc

An efficient blue-emitting phosphor LiCaPO4 :Eu2+ for white LEDs Chuiming Wan a , Jianxin Meng a,b,∗ , Fengjin Zhang a , Xiaoling Deng a , Chuangtao Yang a a

Department of Chemistry, Jinan University, Guangzhou 510632, People’s Republic of China

b

Institute of Nanochemistry, Jinan University, Guangzhou 510632, People’s Republic of China

article

info

Article history: Received 17 December 2009 Received in revised form 15 April 2010 Accepted 24 May 2010 by P. Sheng Available online 1 June 2010

abstract A new blue-emitting phosphor LiCaPO4 :Eu2+ was synthesized by solid state reaction at a relatively low temperature of 900 °C. It gives a single intense emission band centering at 470 nm, which corresponds to the 4f6 5d1 → 4f7 transition of Eu2+ . The dependence of luminescence intensities on Eu2+ concentration was investigated. The phosphor, with a single excitation band extending from 250 to 400 nm, could be efficiently excited by near-ultraviolet light-emitting diodes and is believed to be a promising blueemitting phosphor for white light-emitting diodes. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved.

Keywords: A. LiCaPO4 :Eu2+ Phosphor C. Luminescence D. White light-emitting diodes

White light-emitting diodes (W-LEDs) offer benefits such as high luminous efficiency, low energy consumption, long lifetime, and environment friendly and so on. They are tipped to be the next generation solid state lighting, in the replacement of conventional incandescent and fluorescent lamps which are Hg pollutants, frangible and high energy consumption [1,2]. The commercially available W-LEDs based on phosphors have been produced since 1996, which combine a blue light-emitting GaN chip with an yellow phosphor YAG:Ce3+ . However, colors of these W-LEDs have not been fully satisfactory due to the limited white hue tunability, color rendering index, and correlated color temperature [3]. To overcome these disadvantages, another approach has been suggested which utilizes red/green/blue tricolor phosphors excited by near-ultraviolet light-emitting diodes (NUVLEDs) to generate white light. This strategy yields light with better spectral characteristics, excellent color rendering and high color tolerance [4]. Therefore, searching for phosphors excited by NUVLED chips has drawn great attention. Recently, phosphors of phosphate matrix have been an attractive category of W-LEDs phosphors. SrZn2 (PO4 )2 :Eu2+ , Mn2+ have been prepared at temperature less than 1000 °C and could act as a good single-phase white-lighting phosphor for NUVLED [5,6]. Liu et al. [7] and Wu et al. [8] reported that Eu2+ doped LiSrPO4 shows excellent hydrolytic stability and charge stabilization comparable

∗ Corresponding author at: Department of Chemistry, Jinan University, Guangzhou 510632, People’s Republic of China. Tel.: +86 20 85220223. E-mail addresses: [email protected], [email protected] (J. Meng).

to commercial phosphor BaMgAl10 O17 :Eu2+ (BAM). Tang et al. [9] reported that KSrPO4 :Eu2+ shows higher thermally stable luminescence than YAG:Ce3+ at temperature higher than 225 °C. To the best of our knowledge, there was no report on the research of LiCaPO4 :Eu2+ for potential application as a W-LED phosphor. In this letter, LiCaPO4 :Eu2+ blue phosphor is prepared by solid state reaction and its luminescent properties are investigated. The phosphor can be prepared at relatively low temperature of 900 °C. It also gives much stronger relative fluorescence intensity and has a better color rendering index, and exhibits a great potential as a blue phosphor for NUVLED based W-LEDs. Starting materials of Li2 CO3 (A.R.), (NH4 )2 HPO4 (A.R.), CaCO3 (A.R.) and Eu2 O3 (4N) were weighed in proper stoichiometric ratio, mixed in an agate mortar, placed in a corundum crucible, presintered at 600 °C for 3 h in air and re-sintered under a reductive atmosphere (5%H2 + 95%N2 ) at 900 °C for 3 h to yield phosphors. As a comparison, another two recently reported phosphors for WLEDs, LiSrPO4 :Eu2+ and NaCaPO4 :Eu2+ , were also prepared at their optimized conditions. LiSrPO4 :Eu2+ were prepared by sintering at 1300 °C for 3 h, and NaCaPO4 :Eu2+ at 900 °C for 3 h. This optimized condition is accordant with reference [8,10]. Phase purities of all the samples were checked by powder X-ray diffraction (XRD) (Rigaku, D/max-IIIA analysis with Cu Kα radiation operated at 36 kV and 20 mA. λ = 0.15406 nm, scanning speed 8°/min). Excitation and emission spectra were measured using Hitachi F-4500 fluorospectrometer equipped with 150 W Xenon lamps (operate voltage 400 V, scanning speed 2400 nm/min, in slit 5 nm, out slit 2.5 nm). Diffuse reflection spectrum was measured

0038-1098/$ – see front matter Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.05.037

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Fig. 1. XRD patterns of LiCaPO4 :Eu2+ sintered at different temperatures.

Fig. 2. Excitation and emission spectra of LiSrPO4 :Eu2+ , LiCaPO4 :Eu2+ and NaCaPO4 :Eu2+ .

using Varian Cary5000 UV–VIS–NIR spectrophotometer. CIE coordinate was measured using AvaSpec-2048 spectrometer. All measurements were performed at room temperature. Fig. 1 shows XRD patterns of LiCaPO4 :Eu2+ . XRD patterns for sample sintered at 800 °C agree well with JCPDS card No.14-0403 except several weak CaP2 O6 peaks. Pure phase LiCaPO4 formed when sintered at temperature range from 900 to 950 °C. It can be concluded that doping of Eu2+ do not cause significant change in host structure of LiCaPO4 . As ionic radii of Eu2+ (0.110 nm) and Ca2+ (0.099 nm) are similar, Eu2+ is expected to occupy Ca2+ site preferentially. Emission and excitation spectra of LiCaPO4 :Eu2+ are presented in Fig. 2. The emission spectrum exhibits a symmetrical band between 450–500 nm with a peak at 470 nm. This intense blueemission corresponds to the allowed 4f6 5d1 → 4f7 transition of Eu2+ that substitute Ca2+ sites in the host lattice. The excitation spectrum shows a single broadband ranging from 300 to 450 nm, which indicate that the phosphor is suitable for a color converter of NUVLED based W-LEDs. Emission and excitation spectrum of two recently reported phosphors of phosphate matrix, LiSrPO4 :Eu2+ and NaCaPO4 :Eu2+ , were also presented in Fig. 2 for comparison. It can be concluded that LiCaPO4 :Eu2+ has the highest emission intensity, which is about 2.5 times that of LiSrPO4 :Eu2+ and about 5 times that of NaCaPO4 :Eu2+ . Although both LiSrPO4 :Eu2+ and LiCaPO4 :Eu2+ give blue emissions, the position of emission band slightly shift from 450 nm for LiSrPO4 :Eu2+ to 470 nm for LiCaPO4 :Eu2+ . In general, blue phosphor with emission peak at 470 nm has optimal color rendering index [11]. The CIE chromaticity

Fig. 3. Diffuse reflection spectra of LiSrPO4 :Eu2+ , LiCaPO4 :Eu2+ and NaCaPO4 : Eu2+ .

Fig. 4. Relationship between emission intensity and Eu2+ concentration.

coordination of LiCaPO4 :Eu2+ phosphor is measured to be x = 0.124 and y = 0.175, which falls into blue region in the CIE 1931. As a result, it could be concluded that LiCaPO4 :Eu2+ is a better blue phosphor with phosphate matrix which could be conveniently applied in fabricating NUVLED based W-LEDs. Fig. 3 shows the diffuse reflection spectra of LiSrPO4 :Eu2+ , LiCaPO4 :Eu2+ and NaCaPO4 :Eu2+ . LiCaPO4 :Eu2+ gives two absorption bands at 310–410 and 250–300 nm, corresponding well to its excitation spectrum, which are attributed to the 4f7 → 4f6 5d1 electronic transitions of Eu2+ . The average absorbency of LiCaPO4 :Eu2+ is 47% between 310 and 410 nm, which indicates that LiCaPO4 :Eu2+ can efficiently absorb near-ultraviolet emission from NUVLED. The higher absorbency of LiCaPO4 :Eu2+ , 50% at 400 nm, than those of LiSrPO4 :Eu2+ and NaCaPO4 :Eu2+ , which are 36% and 38% at 400 nm, respectively, might partly contribute to their remarkably different emission intensities. Fig. 4 shows emission intensity of LiCaPO4 :xEu2+ with different Eu2+ concentrations. The emission intensity increased initially with Eu2+ concentration and reached the maximum at x = 0.07, and concentration quenching occurs at a higher Eu2+ concentration. Concentration quenching is believed mainly caused by the non-radiative energy transfer among Eu2+ ions, which is usually occurs as a result of an interaction exchange or a multipole–multipole interaction [12,13]. The optimal Eu2+ concentration in LiCaPO4 :xEu2+ is similar with that in LiSrPO4 :Eu2+ and NaCaPO4 :Eu2+ , which is 7% and 5%, respectively. Effects of sintering temperature and sintering time on relative fluorescence intensity of LiCaPO4 :Eu2+ 7% are also investigated in

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matches well with the emission of NUVLED. The phosphor gives a single intense blue band centering at 470 nm upon near-ultraviolet excitation. Diffuse reflection spectrum indicate that LiCaPO4 :Eu2+ can efficiently absorb near-ultraviolet emission from NUVLED. Phosphors sintered at 900 °C for 3 h give the strongest luminescence intensity. The present phosphors can be prepared at a much lower temperature than the recently reported blue phosphors, LiSrPO4 :Eu2+ , and have a better color rendering index. It is believed that LiCaPO4 :Eu2+ is a promising blue phosphors for fabricating NUVLED based W-LEDs. Acknowledgement The authors acknowledge the financial support from the Natural Science Foundation of China (Grant No. 30670523), and the Natural Science Foundation of Guangdong Province (Grant No. 05200555).

Fig. 5. Effects of sintering temperature and time on relative PL intensity of LiCaPO4 :Eu2+ .

detail as shown in Fig. 5. The fluorescence intensity firstly increases with temperature, reaches the maximum at 900 °C, and greatly reduced at higher temperature. It is believed that pure phase LiCaPO4 could give the strongest emission, since pure phase LiCaPO4 could not be achieved at temperature lower than 900 °C, as shown in Fig. 1. Decrease of fluorescence intensity for samples sintered at higher temperature may be owing to the vaporizing of Li+ ions, which may cause minor impurity phases in the phosphors. The optimal sintered temperature of LiCaPO4 :Eu2+ is also remarkably lower than that of LiSrPO4 :Eu2+ phosphor [8], which is as high as 1300 °C. The sintered time was also optimized and phosphors sintered for 3 h at 900 °C give the strongest luminescence intensity. In conclusion, an efficient blue-emitting phosphor LiCaPO4 :Eu2+ is synthesized by solid state reaction and its luminescent properties are investigated. The excitation spectrum of the phosphor

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