Luminescence properties of a red phosphor europium tungsten oxide Eu2WO6

Luminescence properties of a red phosphor europium tungsten oxide Eu2WO6

Materials Letters 63 (2009) 1162–1164 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i ...

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Materials Letters 63 (2009) 1162–1164

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Luminescence properties of a red phosphor europium tungsten oxide Eu2WO6 Chuanxiang Qin a, Yanlin Huang a,⁎, Guoqiang Chen b, Liang Shi c, Xuebin Qiao c, Jiuhui Gan a, Hyo Jin Seo c,⁎ a b c

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China College of Textile and Clothing Engineering, Soochow University, Suzhou 215021, China Department of Physics, Pukyong National University, Pusan 608-737, Republic of Korea

a r t i c l e

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Article history: Received 22 January 2009 Accepted 12 February 2009 Available online 21 February 2009 Keywords: Luminescence Phosphors Optical materials and properties Light emitting diodes

a b s t r a c t Eu2WO6 was synthesized by the conventional solid state reaction method. Crystal phase was characterized by the X-ray powder diffraction. The excitation and emission spectra indicate that this phosphor can be effectively excited by near UV (395 nm) and blue light (465 nm), and the emission spectra exhibit a satisfactory red performance at 611 nm, which is due to the characteristic 5D0−7F2 transitions of Eu3+ ions. The luminescence intensities and color purity were investigated by increasing the fired temperature. The phosphor shows stable luminescence and color purity at high temperature. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Since the white light-emitting diodes (LEDs) were available in 1997, much progress has been achieved on blue and near-UV light GaN-based LEDs. Usually, the white light was obtained by combining a 465 nm blue light from the GaN-based LED and yellow light from the phosphor YAG:Ce. This yields cool white LEDs with a high color temperature (Tc) above 4000 K, which does not meet the requirements for ambient lighting [1]. To obtain warm white-LEDs (Tc b 4000 K), the application of an additional red phosphor is required, e.g., CaS:Eu3+, SrY2S4:Eu3+, and Y2O2S:Eu3+. Although this method can get the lumen balance, the luminous efficiency of a white LED complying YAG:Ce decreases from 50 to about 30 lm/W. The red phosphor has exposed some drawbacks: lower efficiency, shorter working lifetime, and instability due to releasing of sulfide gas [2].So it is necessary to find new red phosphors with high efficiency. In recent years, Eu3+-doped tungstates have been paid more attention to develop the red-emitting phosphors [3]. The rare-earth elements can form a series of isomorphous tungstates with general formula of Ln2WO6, and some of them show interesting fluorescent properties [4,5]. However, to our best knowledge, the luminescent properties of Eu2WO6 have not been studied. The present work is to search for novel red phosphors for near UV InGaN chip-based white LEDs. Moreover, it is possible to realize a high activator concentration doped in tungstate without concentration quenching. The phosphor was investigated by X-ray diffraction, exci-

⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Huang), [email protected] (H.J. Seo). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.02.018

tation and emission spectroscopies, temperature dependence of luminescence. 2. Experimental The sample was prepared by solid state reaction at high temperature. The starting materials were Eu2O3 (99.9%) and WO3 (99.9%). The stoichiometric materials were weighed and thoroughly mixed in an agate mortar, then transferred to a corundum crucible and heated at 700, 1000 and 1100 °C for 10 h in air, respectively. The crystal structure as well as the phase purity was identified by recording the powder XRD patterns using Philip's X'Pert diffractometer with Ni filtered CuKα radiation (λ = 1.54056 Å). Excitation and emission spectra were recorded using a Spex-Fluorolog DM3000F spectrofluorimeter and a 450 W xenon lamp as the exciting source. When the samples were heated from 25 to 260 °C, a UV lamp was used to excite the sample to evaluate the temperature limits for the luminescence output. 3. Results and discussion The powder XRD pattern of Eu2WO6 in Fig. 1 shows that the phosphor is of single phase and consistent with JCPDs 23-1071. The obtained pattern was found to be similar to that of the earlier reported for Gd2WO6 [6]. All the peaks in the XRD pattern are indexed with Eu2WO6. No impurity peaks were detected in the experimental range. The excitation spectra were obtained by monitoring the 5D0–7F2 transition of Eu3+ at 611 nm (Fig. 2(a)). The broad band from 200 to 350 nm is ascribed to the charge transfer (CT) transition of O2––W6+ and O2––Eu3+ [7]. The sharp peaks are due to the intra-configurational (f–f) transitions of Eu3+.

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Fig. 1. XRD patterns of Eu2WO6 synthesized by solid-state reaction.

It can be found that the f → f transitions, e.g., 394 nm (7F0–5L6) and 465 nm (7F0–5D2) (which match the emission wavelengths of near-UV and blue LED chips, respectively), are much stronger than the C–T band. This is very different from the other Eu3+-doped phosphors, e.g., YVO4:Eu3+ [8], Y2O2S:Eu3+ [9] and vanadate garnets [10], where usually the CT band is dominated and f–f transitions are hardly to be observed. The reasons need further investigations. This result implies that Eu2WO6 can be effectively excited by the wavelength in near-UV and blue regions. Strong red emission observed under 395 nm excitation (Fig. 2b) was due to the electric dipole transitions of 5D0–7F2. This indicates that Eu3+ occupies a non centro-symmetric site. This is favorable to improve the color purity of the red phosphor. The CIE (Commission Internationale de l'Eclairage, International Commission on Illumination) chromaticity coordinates are calculated to be x = 0.65, y = 0.34, which is closer to the standard of NTSC (x = 0.67, y = 0.33) than that of commercial red phosphor of Y2O2S:Eu3+ (0.622, 0.351) [11]. It is well known that low doping of Eu3+ in a compound leads to weak luminescence, while heavy doping causes quenching of the luminescence. This is known as “concentration quenching”. This process often attributes to energy migration among Eu3+ ions. On the other

Fig. 2. The excitation spectra by monitoring the emission of 611 nm from Eu3+ ions (a); and its emission spectra under the excitation of 395 nm (b).

hand, a low doping ratio not only gives a weak luminescence but also is unfavorable to luminescent applications. However, in Eu3+ concentrated compound Eu2WO6, no concentration quenching can be detected. Attentions have been focused on the spectroscopic properties of rare-earth ions in host lattices. It has been found that some Eu3+ compounds show no concentration quenching at all [12,13], for example, EuP5O14 [14], K5Eu(MoO4)4 [15] etc. This red phosphor is very favorable to suffer from high photon flux during its application without many luminescence quenching. The decay curve of 5D0 → 7F2 luminescence under excitation of 355 nm is shown in Fig. 3. This can be fitted by a single-exponential function as I = Aexp(− t/τ), and the value of lifetime is 0.38 ms. The result shows that the lifetime is short enough for potential applications in displays and lights. Under excitation by a 365 nm UV lamp, emission spectra of the sample were recorded at different temperature (Fig. 4). With increasing temperature, the red emission decreases slowly before 160 °C (decreases by 25%), and quickly to 260 °C (falls by only 40%). In addition, the emission wavelengths show no shifts with increasing temperature. So it is essential that the phosphor can keep stable color

Fig. 3. The decay curves of 5D0 → 7F2 (611 nm) under excitation of 355 nm.

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4. Conclusions Eu2WO6 was prepared by solid-state reaction. The excitation and emission spectra, decay curves and the dependence of luminescence on the temperature were studied. The phosphor shows intense red emission under the excitation at around 400 nm. The color rendering index was calculated to be (0.65, 0.34), which is close to the NTSC standard values. The red phosphor has high quenching temperature and can keep stable color purity with elevated temperature. Due to high emission intensity, a good excitation profile and stable luminescence properties at high temperature, the phosphor Eu2WO6 may be found a possible application on near UV InGaN chip-based white light-emitting diodes. Acknowledgements

Fig. 4. Emission spectra at various temperatures under the excitation of a 365 nm UV lamp. The emission intensities were normalized with respect to the luminescence at 25 °C.

purity at the elevated temperatures. This is favorable for the application in LED devices. The influencing mechanism of temperature on red emission is complicated. Elevated temperature can generate more thermally active phonons, and they may assist electrons at lower energy excited states to jump to higher energy excited states. Under 365-nm excitation (in resonance with the 7F0 → 5L6 transition), the main contributions leading the 611 nm luminescence to change are the thermal-activated distribution of Eu3+ in 7FJ (J = 0, 1, 2) states and that Eu3+ in 7F0 and 7F1 are non-resonantly excited to 5L6 with the assistance of phonons. The temperature-quenching effect of Eu3+ luminescence becomes strong with the elevated temperature, which is generally caused by non-radiative transition and energy transfer processes. The temperature-quenching channels may include the main non-radiative transitions from 5L6 → 5D0 and the energy transfer from one activated center Eu3+ to the other or to the defect centers.

This work was financially supported by the National Natural Science Foundation of China (Grant No.50673071), and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-313-C00312). References [1] Yamada M, Naitou T, Izuno K, Tamaki H, Murazaki Y, Kameshima M, et al. Jpn J Appl Phys 2003;42: L20–3. [2] Neeraj S, Kijima N, Cheetham AK. Chem Phys Lett 2004;387: 2–6. [3] Chuanghung C, Mingfang W, Chishen L, Tengming C. J Solid State Chem 2007;180: 619–27. [4] Blasse G, Bril A. J Chem Phys 1966;45: 2350–6. [5] Templeton DH, Zalkin A. Acta Crystallogr 1963;16: 762–6. [6] Bril A, Blasse G. J Chem Phys 1966;45: 2350–6. [7] Kodaira CA, Brito HF, Felinto MCFC. J Solid State Chem 2003;171: 401–7. [8] Sharma N, Kijima N, Cheetham AK. Solid State Commun 2004;131: 65–9. [9] Chongfeng G, Lin L, Changhong C, Dexiu H, Qiang S. Mater Lett 2008;62: 600–2. [10] Gundiah G, Shimomura Y, Kijima N, Cheetham AK. Chem Phys Lett 2008;455: 279–83. [11] Zhengliang W, Hongbin L, Menglian G, Qiang S. J Alloys Compd 2007;432: 308–12. [12] Blasse G, Bril A. Phys Status Solidi 1967;20: 551–5. [13] Blasse G. J Chem Phys 1967;46: 2583–5. [14] Huber G, Jeser JP, Kruhler WW, Danielmeyer HG. IEEE J Quantum Electron 1974;10: 766–2586. [15] Huber G, Lenth W, Lieberts J, Lutz F. J Lumin 1978;16: 353–60.