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Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 879–883
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor Qiyue Shao , Huijuan Li, Kewei Wu, Yan Dong, Jianqing Jiang Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
a r t i c l e in fo
abstract
Article history: Received 18 June 2008 Received in revised form 16 March 2009 Accepted 19 March 2009 Available online 2 April 2009
Sodium europium double tungstate [NaEu(WO4)2] phosphor was prepared by the solid-state reaction method. Its crystal structure, photoluminescence properties and thermal quenching characteristics were investigated aiming at the potential application in the field of white light-emitting diodes (LEDs). The influences of Sm doping on the photoluminescence properties of this phosphor were also studied. It is found that this phosphor can be effectively excited by 394 or 464 nm light, which nicely match the output wavelengths of near-ultraviolet (UV) or blue LED chips. Under 394 or 464 nm light excitation, this phosphor exhibits stronger emission intensity than the Y2O2S:Eu3+ or Eu2+-activated sulfide phosphor. The introduction of Sm3+ ions can broaden the excitation peaks at 394 and 464 nm of the NaEu(WO4)2 phosphor and significantly enhance its relative luminance under 400 and 460 nm LEDs excitation. Furthermore, the relative luminance of NaEu(WO4)2 phosphor shows a superior thermal stability compared with the commercially used sulfide or oxysulfide phosphor, and make it a promising red phosphor for solid-state lighting devices based on near-UV or blue LED chips. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.m Keywords: Tungstate Phosphor Photoluminescence Thermal quenching White LEDs
1. Introduction White light-emitting diodes (LEDs), as the promising solidstate lighting sources to replace conventional incandescent and fluorescent lamps, have attracted much attention due to their high reliability, long lifetime, low energy consumption and environment-friendly characteristics [1]. Phosphor-converted LED technique is an important approach to produce solid-state illumination devices. An excellent approach using the combination of a GaN-based blue LED and a cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor was first proposed in 1996 [2], and nowadays it is still the most widely used method to realize the white LEDs. However, such white LEDs have low colorrendering index (CRI) because of their weak emission intensity in red spectral region. As a possible solution, a separate red-emitting phosphor can be used to compensate the red deficiency in light output. The red phosphor for blue LED chips is commercially still limited to Eu2+-activated alkaline-earth sulfide, which has poor chemical stability and low luminescence efficiency [3,4]. Another type of phosphor-converted white LEDs can be fabricated by employing blue/green/red tricolor phosphors excited by a
Corresponding authors. Tel.: +86 25 52090630; fax: +86 25 52090634.
E-mail addresses: [email protected] (Q. Shao), [email protected] (J. Jiang). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.03.016
near-ultraviolet (UV) (360–410 nm) InGaN-based LED, which can compete for applications requiring high quality of light. The commonly used red-emitting phosphor for near-UV LED is Y2O2S:Eu3+. The Y2O2S:Eu3+ phosphor is chemically unstable and not desirable in efficiency, compared with blue (BaMgAl10O17:Eu2+) and green (ZnS:Cu+, Al3+) phosphors [5]. Hence, there is an urgent need to search a suitable red phosphor for fabrication of phosphorconverted white LEDs. The promising red phosphor should be excited efficiently by the blue light around 460 nm or the near-UV light around 400 nm. As interesting candidates for red-emitting phosphors, Eu3+activated double molybdates or tungstates ALn(MO4)2 (A ¼ Li+, Na+, K+; Ln ¼ trivalent rare-earth ions; M ¼ Mo,W) have attracted much attention [5–11]. ALn(MO4)2 materials adopt a scheelite-like (CaWO4) structure, exhibit excellent thermal and chemical stability, and therefore are considered to be efficient host candidates. Eu3+ ions were frequently used as activators for the red phosphors, which mainly show very sharp 5D0–7F2 red emission lines around 612 nm as Eu3+ ions occupy the lattice sites without inversion symmetry. In ALn(MO4)2 materials, Mo6+or W6+ is coordinated by four oxygen atoms in a tetrahedral site, and the rare-earth/sodium ions occupy eight-coordinated sites. The tetrahedral Mo6+ and W6+ have similar ionic radius (0.41 and 0.42 A˚), and make it possible to prepare solid solutions of the type ALn(WO4)2x(MoO4)x [5]. The photoluminescence properties of scheelite-type NaLn0.95Eu0.05(WO4)2x(MoO4)x (Ln ¼ Gd, Y, Bi)
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solid solutions were studied by Neeraj et al. [5]. These materials show intense red emission by exciting at 394 nm using the sharp 7 F0-5L6 line of Eu3+ ions, and could be used as red phosphors for white lighting devices utilizing near-UV LED chips. Wang et al. [6] studied double molybdates NaLa1xEux(MoO4)2 with various Eu content, and no concentration quenching of Eu3+ ions was observed in the series of samples. Furthermore, Wang et al. prepared composition-optimized double molybdate (Li0.333Na0.334K0.333)Eu(MoO4)2, which shows broadened excitation band in near-UV range and intense red emission with appropriate CIE chromaticity coordinates [7]. Sivakumar and Varadaraju [8] reported a red-emitting tungstomolybdate phosphor with composition of AgGd0.95Eu0.05(WO4)2x(MoO4)x, and this material can be excited efficiently by near-UV and blue (i.e., 394 and 465 nm) light. The tungstomolybdate phosphors with the composition of KEu(WO4)2x(MoO4)x [9] and LiEu(WO4)2x(MoO4)x [10] have also been reported recently, and are considered to be potential redemitting phosphors for white LEDs. Motivated by the above-stated studies and the attempts to develop suitable red phosphors for white LEDs, sodium europium double tungstate (NaEu(WO4)2 phosphor was prepared by the solid-state reaction method in this paper. Its crystal structure and photoluminescence properties were investigated aiming at the potential application in the field of phosphor-converted white LEDs. The influences of sintering temperature, Sm doping and atmosphere temperature on the photoluminescence properties of NaEu(WO4)2 phosphor were also studied.
Fig. 1. XRD patterns of NaEu(WO4)2 sintered at different temperature from 700 to 1200 1C.
2. Experimental The NaEu(WO4)2 samples were prepared by the solid-state reaction technique at high temperature. The starting materials NaHCO3 (A. R. grade), WO3 (99.9% purity), Eu2O3 (99.99% purity) and Sm2O3 (99.99% purity) were weighted by stoichiometric ratio. NaF (A. R. grade) at a certain mass ratio was added as the flux to improve the chemical reaction. After these powders were mixed thoroughly, the homogeneous mixture was filled into a little alumina crucible and calcined in a muffle furnace for 4 h at the temperature from 700 to 1200 1C. The crystal structure of the final products was examined by powder X-ray diffraction (XRD) using Cu Ka radiation. Photoluminescence excitation and emission spectra were recorded by a Hitachi F-7000 fluorescence spectrophotometer at room temperature. The temperature-dependent luminance of the phosphors was recorded by a self-made measuring system, which includes a temperature-controlled heating plate and a fluo-brightness meter (Zhejiang University Sensing Instruments Co., Ltd., China). Two near-UV LEDs (lem ¼ 385, 400 nm) and a blue LED (lem ¼ 460 nm) were used as excitation sources for temperature-dependent luminance measurements.
3. Results and discussion 3.1. Crystal structure and photoluminescence properties of NaEu(WO4)2 The XRD patterns of the NaEu(WO4)2 phosphors fired at different temperature are shown in Fig. 1. After being sintered at 700 1C for 4 h, a white product can be obtained. XRD data indicate that the product sintered at 700 1C is not a single phase. In addition to strong diffraction peaks corresponding to scheelitelike phase, a peak at 2y ¼ 33.021 ascribed to cubic Eu2O3 phase and two unidentifiable peaks at 2y ¼ 21.861 and 36.561 can also been observed. It indicates that the formation of scheelite-like
Fig. 2. Excitation spectrum for the 615 nm emission of the NaEu(WO4)2 phosphor sintered at 1100 1C for 4 h.
NaEu(WO4)2 is incomplete at this temperature. With the sintering temperature increasing, the obtained products vary from white to pink in color. The samples sintered at 800–1100 1C have a single crystalline phase with scheelite-like structure, and their XRD patterns are consistent with that given in JCPDS card 25–0828 [Na0.5Gd0.5MoO4]. When the firing temperature increases up to 1200 1C, a weak diffraction peak related to an unknown phase can be observed at 2y ¼ 37.701. It indicates that an excessively high sintering temperature (X1200 1C) is unfavorable for the formation of NaEu(WO4)2 with scheelite-like structure. Fig. 2 represents the excitation spectrum of the NaEu(WO4)2 phosphor sintered at 1100 1C for 4 h. The excitation spectrum for monitoring the 5D0-7F2 emission (615 nm) of Eu3+ ions consists of a broad band and some sharp lines. The broad excitation band centered at 280 nm can be attributed to the charge-transfer transition arising from oxygen to tungsten [5,10]. In the range from 325 to 550 nm, the sample shows characteristic intraconfigurational 4f–4f transitions of Eu3+: sharp 7F0-5L6 transition for 394 nm, 7F0-5D2 transition for 464 nm and the 7F1-5D1 transition for 535 nm [10]. So the NaEu(WO4)2 phosphor can be excited efficiently by near-ultraviolet (394 nm) or blue (464 nm)
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light, nicely in agreement with the output wavelength of near-UV or blue LED chips in phosphor-converted white LED. Fig. 3 shows the emission spectrum of NaEu(WO4)2 sintered at 1100 1C for 4 h under 394 nm light excitation. The spectrum essentially consists of several sharp lines with wavelength ranging from 580 to 710 nm, which are associated with the 5D0-7FJ (J ¼ 1, 2, 3, 4) transitions from the excited levels of Eu3+ to the ground state. The major emission of NaEu(WO4)2 was found at 615 nm (5D0-7F2), which corresponds to red emission. In addition, three weak emission lines at 590 nm (5D0-7F1), 654 nm (5D0-7F3) and 702 nm (5D0-7F4) can also be observed [5]. Other transitions of Eu3+ from the 5DJ excited levels to 7FJ ground states are too weak to be distinguishable. Fig. 3 also represents the emission spectrum of a commercial Y2O2S:Eu3+ phosphor under the same excitation intensity at 394 nm. Its strongest emission peak is located at 626 nm. The emission peak (615 nm) intensity of NaEu(WO4)2 is about 8.5 times higher than that (626 nm) of Y2O2S:Eu3+. The integral emission intensity, which is proportional to the quantum efficiency, of NaEu(WO4)2 is about 5.0 times higher than that of Y2O2S:Eu3+. The emission spectrum of NaEu(WO4)2 excited at 460 nm is shown in Fig. 4, and the sample was sintered at 1100 1C for 4 h. It is obvious that the spectrum is identical with that excited by 394 nm light. Eu2+-activated alkaline earth binary sulfides is one of commonly used red phosphors for blue LED chips. For comparison, the emission spectrum of a commercial sulfide phosphor is also presented in Fig. 4, and the excitation
Fig. 3. Emission spectra of NaEu(WO4)2 sintered at 1100 1C for 4 h and a commercial Y2O2S:Eu3+ phosphor (excited at 394 nm).
Fig. 4. Emission spectra of NaEu(WO4)2 sintered at 1100 1C for 4 h and a commercial sulfide phosphor (excited at 460 nm).
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wavelength is similarly at 460 nm. The sulfide phosphor is activated by Eu2+ ions and exhibits broad-band emission in the vicinity of 635 nm. Under the same excitation intensity at 460 nm, the NaEu(WO4)2 is 7.2 times higher than sulfide phosphor on the relative intensity of the highest peaks. The integral emission intensity of NaEu(WO4)2 is higher by 16% than that of Eu2+activated sulfide phosphor. As the excitation wavelength turns to 464 nm, the integral emission intensity of NaEu(WO4)2 is even 5.2 times higher than that of sulfide phosphor. These results imply that NaEu(WO4)2 is an promising red phosphor for white LEDs based on blue or near-UV chips. Fig. 5 shows the influences of sintering temperature on the photoluminescence properties of NaEu(WO4)2. It is observed that the 5D0-7F2 (615 nm) emission intensity under 394 or 464 nm excitation is gradually increased with increasing the sintering temperature from 700 to 1100 1C. Sintering at 1200 1C can induce a hard aggregation and the formation of impurity phase (as shown in Fig. 1), and therefore suppresses the luminescence of NaEu(WO4)2. The optimum sintering temperature is 1100 1C to obtain NaEu(WO4)2 with higher photoluminescence efficiency. 3.2. Photoluminescence properties of NaEu1xSmx(WO4)2 Sm3+ ion can be used as an efficient sensitizer to enhance the light emission of Eu3+-activated phosphors [6,11]. Therefore, NaEu1xSmx(WO4)2 phosphors with various x values were also prepared by sintering at 1100 1C for 4 h. The influences of Sm content on their luminescent properties were investigated. Fig. 6(a)–(f) presents the excitation spectra of NaEu1xSmx(WO4)2 phosphors monitored at 615 nm, and x value varies from 0 to 0.1 at an interval of 0.02. In order to clarify the energy transfer between Eu3+ and Sm3+ ions, Sm-doped scheelite-like material NaY0.94Sm0.06(WO4)2 was also prepared, which on excitation at 404 nm exhibits four main emission bands around 565, 599, 607 and 646 nm (not shown). These emission bands can be attributed the 4G5/2-6H5/2, 4 F3/2-6H9/2, 4G5/2-6H7/2, 4F3/2-6H11/2 transitions of Sm3+ ions [12]. The excitation spectrum of NaY0.94Sm0.06(WO4)2 monitoring 599 nm emission is also presented in Fig. 6(g), consisting of some broad bands and sharp lines. The broad band centered at about 268 nm can be attributable to the charge-transfer transition between O2 and Sm3+ [13]. The strongest excitation peak is located at 404 nm, assigned to the 6H5/2-4K11/2 transition of Sm3+ ion [11]. In the wavelength range from 460 to 480 nm, the excitation peaks of Sm3+ ion are overlapped, and therefore a broader excitation band is formed.
Fig. 5. 5D0-7F2 emission (615 nm) intensity of NaEu(WO4)2 as a function of sintering temperature under 394 or 464 nm excitation.
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Fig. 6. Excitation spectra of NaEu1xSmx(WO4)2 (x ¼ 0–0.1) (monitoring 615 nm emission) and NaY0.94Sm0.06(WO4)2 (monitoring 599 nm emission). (a) x ¼ 0; (b) x ¼ 0.02; (c) x ¼ 0.04; (d) x ¼ 0.06; (e) x ¼ 0.08; (f) x ¼ 0.1; (g) NaY0.94Sm0.06 (WO4)2. All the samples were sintered at 1100 1C for 4 h. The curve (g) was magnified for 20 times for clarity.
In the excitation spectra of NaEu1xSmx(WO4)2 (x ¼ 0.02–0.1), the excitation peak at 404 nm can be obviously detected because of the introduction of Sm3+ ions. As mentioned above, it is from the 6H5/2-4K11/2 transition of Sm3+ ions. The 404 nm excitation peak of Sm3+ ions overlaps with that of Eu3+ ions at 394 nm and can induce the broadening of excitation band around 400 nm, compared with the NaEu(WO4)2 phosphor without Sm doping. With the increasing of Sm content the intensity of 404 nm peak is enhanced, and then decreases due to the concentration quenching effects as x value is larger than 0.8. It is noted that the excitation peaks around 464 nm are also broadened with the introduction of Sm3+ ions. This is related to the broad excitation band of Sm3+ ions in the wavelength range from 460 to 480 nm, as shown in Fig. 6(g). These results indicate that the introduction of Sm3+ ions can broaden the excitation bands of NaEu(WO4)2 phosphors, therefore, can enhance their energy absorption and emission intensity under near-UV or blue light irradiation. The emission spectra of Sm3+-doped NaEu(WO4)2 show characteristic spectral lines of Eu3+ emission under 394 or 464 nm excitation. No Sm3+ emission was observed even under direct excitation of the 6H5/2-4K11/2 transition (404 nm) of Sm3+ ions. The strongest emission line is still at 615 nm, due to the 5 D0-7F2 transition of Eu3+ ions. It implies that the absorbed energy of Sm3+ ions can be efficiently transferred to the Eu3+ ions. The 615 nm emission intensity of NaEu1xSmx(WO4)2 is shown in Fig. 7(a) as a function of Sm content, under the excitation of 394, 404 and 464 nm light, respectively. On excitation at 404 nm, the irradiation energy is mostly absorbed by Sm3+ ions and the emission intensity increases as x varies from 0.02 to 0.06. As x40.06, the emission intensity decreases due to the concentration quenching effects. By exciting at 394 or 464 nm, the intensity of light emission at 615 nm basically decreases with the increase of Sm content. The results obtained by us are not in agreement with that reported by Wang et al. [11]. They claimed that 4 mol% Sm doping could enhance the emission intensity of sodium europium molybdates under the excitation of 395 nm light. For the measurements of emission spectra, narrow-band light with a full-width at half-maximum (FWHM) of 2.5 nm was used as the excitation source by setting the slit width in fluorescence spectrophotometer. As discussed above, the excitation peaks at 394 and 464 nm are definitely from the energy level transition of Eu3+ ions. As the samples are directly excited by the narrow-band
Fig. 7. (a) The 615 nm emission intensity and (b) the relative luminance of NaEu1xSmx(WO4)2 as a function of x value by the different excitation sources. All the samples were sintered at 1100 1C for 4 h.
light source at 394 or 464 nm, the excitation energy is mostly absorbed by the Eu3+ ions. With the increase of Sm content, the Eu content in phosphors decreases and therefore the 615 nm emission intensity was suppressed. However, the broadened excitation peaks at 400 and 460 nm in Sm-doped NaEu(WO4)2 can enhance the energy absorption as a broad-band excitation source is used. Fig. 7(b) shows the relative luminance of NaEu1xSmx(WO4)2 as a function of x value. Three types of LED were used as the light source, whose the peak wavelength is located at 460, 400 and 385 nm, respectively. These LEDs have broad-band emission spectra with FWHM values of 20–30 nm. Doping certain amount of Sm can enhance the relative luminance of the NaEu(WO4)2 phosphor. It can be found with increasing Sm content the relative luminance under 400 nm LED excitation increases from 100 (x ¼ 0) to 134 (x ¼ 0.06). Excited by 460 and 385 nm LEDs, the relative luminance of the NaEu(WO4)2 phosphor can be increased by 14% (x ¼ 0.06) and 8% (x ¼ 0.04), respectively. 3.3. Temperature-dependent photoluminescence properties of NaEu(WO4)2 In addition to high brightness and efficiency, the thermal quenching property is also one of the most important technological parameters for phosphors applied in white LEDs [14,15], especially in high-power white LED devices. The promising phosphors should perform well and retain their brightness and
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4. Conclusions In this paper, the NaEu(WO4)2 red phosphor with a scheelitelike structure has been synthesized by the solid-stated reaction technique at high temperature. This material exhibits more efficient luminescence efficiency under excitation of near-UV or blue light radiation, in comparison with the commercially used sulfide or oxysulfide phosphors. Doping certain amount of Sm3+ can broaden the excitation band of NaEu(WO4)2, and therefore can enhance its emission intensity under excitation of near-UV or blue LEDs. Thermal quenching data indicate that the luminance of the NaEu(WO4)2 phosphor exhibits an excellent thermal stability in the temperature region from 30 to 150 1C. All the results indicate that the NaEu(WO4)2 material is a potential red phosphor for the fabrication of solid-state lighting devices based on near-UV or blue LED chips. Fig. 8. Thermal quenching characteristics of (a) the NaEu(WO4)2 phosphor under excitation of 460 nm LED, (b) the NaEu(WO4)2 phosphor under excitation of 400 nm LED, (c) the commercial Eu2+-activated sulfide phosphor under excitation of 460 nm LED and (d) the commercial Y2O2S:Eu3+ phosphor under excitation of 400 nm LED.
chromatic properties at the elevated temperatures of the LED junction (4100 1C) [16]. The thermal quenching properties of the prepared NaEu(WO4)2 phosphor were investigated in the temperature range from 30 to 200 1C. The obtained results were presented in Fig. 8, and the LEDs emitting at 460 and 400 nm were used as the excitation sources. For comparison, the temperature quenching data of the commercially used sulfide and oxysulfide standards were also presented in Fig. 8. By exciting of 460 nm LED, the relative luminance of Eu2+-activated sulfide phosphor increases slightly in the range 30–100 1C, and then decreases rapidly with the increasing temperature. The relative luminance of sulfide falls by 10% at 150 1C, and by 34% at 200 1C. In contrast, the NaEu(WO4)2 phosphor exhibits an excellent thermal stability. By exciting of 460 nm LED, its relative luminance is quite constant up to 160 1C, and falls only by 5% at 200 1C. As a 400 nm LED was used as an excitation source, at 150 1C the NaEu(WO4)2 phosphor still can retain 98% of its luminance at room temperature. The relative luminance of the Y2O2S:Eu3+ increases at elevated temperatures because its excitation maximum moves to a longer wavelength [16]. This is disadvantageous for practical application in the field of white LEDs, since it will alter the chromatic parameters of the device as a function of temperature [16].
Acknowledgements This work was financially supported by a research grant from the Ministry of Education of China (Grant no. 707029) and the Natural Science Foundation of Jiangsu Province (Grant no. BK2008317). References [1] M.S. Shur, A. Zukauskas, Proc. IEEE 93 (2005) 1691. [2] S. Nakamura, S. Perton, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Laser, Springer, Berlin, Germany, 1997. [3] C.F. Guo, D.X. Huang, Q. Su, Mater. Sci. Eng. B 130 (2006) 189. [4] Y.S. Hu, W.D. Zhuang, H.Q. Ye, D.H. Wang, S.S. Zhang, X.W. Huang, J. Alloy. Compd. 390 (2005) 226. [5] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [6] Z. Wang, H. Liang, M. Gong, Q. Su, Electrochem. Solid-State Lett. 8 (2005) H33. [7] Z. Wang, H. Liang, L. Zhou, H. Wu, M. Gong, Q. Su, Chem. Phys. Lett. 412 (2005) 313. [8] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H168. [9] T. Kim, S. Kang, J. Lumin. 122–123 (2007) 964. [10] C.H. Chiu, M.F. Wang, C.S. Lee, T.M. Chen, J. Solid State Chem. 180 (2007) 619. [11] Z.L. Wang, H.B. Liang, M.L. Gong, Q. Su, Opt. Mater. 29 (2007) 896. [12] J.P. Feist, A.L. Heyes, Meas. Sci. Technol. 11 (2000) 942. [13] K. Yao, M.W. Wang, S.X. Liu, L.D. Zhang, W.J. Li, J. Rare Earths 24 (2006) 524. [14] J.H. Ryu, Y.G. Park, H.S. Won, S.H. Kim, H. Suzuki, J.M. Lee, C. Yoon, M. Nazarov, D.Y. Noh, B. Tsukerblatc, J. Electrochem. Soc. 155 (2008) J99. [15] R.J. Xie, N. Hirosaki, Sci. Technol. Adv. Mater. 8 (2007) 588. [16] G. Gundiah, Y. Shimomura, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 455 (2008) 279.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor Qiyue Shao , Huijuan Li, Kewei Wu, Yan Dong, Jianqing Jiang Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
a r t i c l e in fo
abstract
Article history: Received 18 June 2008 Received in revised form 16 March 2009 Accepted 19 March 2009 Available online 2 April 2009
Sodium europium double tungstate [NaEu(WO4)2] phosphor was prepared by the solid-state reaction method. Its crystal structure, photoluminescence properties and thermal quenching characteristics were investigated aiming at the potential application in the field of white light-emitting diodes (LEDs). The influences of Sm doping on the photoluminescence properties of this phosphor were also studied. It is found that this phosphor can be effectively excited by 394 or 464 nm light, which nicely match the output wavelengths of near-ultraviolet (UV) or blue LED chips. Under 394 or 464 nm light excitation, this phosphor exhibits stronger emission intensity than the Y2O2S:Eu3+ or Eu2+-activated sulfide phosphor. The introduction of Sm3+ ions can broaden the excitation peaks at 394 and 464 nm of the NaEu(WO4)2 phosphor and significantly enhance its relative luminance under 400 and 460 nm LEDs excitation. Furthermore, the relative luminance of NaEu(WO4)2 phosphor shows a superior thermal stability compared with the commercially used sulfide or oxysulfide phosphor, and make it a promising red phosphor for solid-state lighting devices based on near-UV or blue LED chips. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.m Keywords: Tungstate Phosphor Photoluminescence Thermal quenching White LEDs
1. Introduction White light-emitting diodes (LEDs), as the promising solidstate lighting sources to replace conventional incandescent and fluorescent lamps, have attracted much attention due to their high reliability, long lifetime, low energy consumption and environment-friendly characteristics [1]. Phosphor-converted LED technique is an important approach to produce solid-state illumination devices. An excellent approach using the combination of a GaN-based blue LED and a cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor was first proposed in 1996 [2], and nowadays it is still the most widely used method to realize the white LEDs. However, such white LEDs have low colorrendering index (CRI) because of their weak emission intensity in red spectral region. As a possible solution, a separate red-emitting phosphor can be used to compensate the red deficiency in light output. The red phosphor for blue LED chips is commercially still limited to Eu2+-activated alkaline-earth sulfide, which has poor chemical stability and low luminescence efficiency [3,4]. Another type of phosphor-converted white LEDs can be fabricated by employing blue/green/red tricolor phosphors excited by a
Corresponding authors. Tel.: +86 25 52090630; fax: +86 25 52090634.
E-mail addresses: [email protected] (Q. Shao), [email protected] (J. Jiang). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.03.016
near-ultraviolet (UV) (360–410 nm) InGaN-based LED, which can compete for applications requiring high quality of light. The commonly used red-emitting phosphor for near-UV LED is Y2O2S:Eu3+. The Y2O2S:Eu3+ phosphor is chemically unstable and not desirable in efficiency, compared with blue (BaMgAl10O17:Eu2+) and green (ZnS:Cu+, Al3+) phosphors [5]. Hence, there is an urgent need to search a suitable red phosphor for fabrication of phosphorconverted white LEDs. The promising red phosphor should be excited efficiently by the blue light around 460 nm or the near-UV light around 400 nm. As interesting candidates for red-emitting phosphors, Eu3+activated double molybdates or tungstates ALn(MO4)2 (A ¼ Li+, Na+, K+; Ln ¼ trivalent rare-earth ions; M ¼ Mo,W) have attracted much attention [5–11]. ALn(MO4)2 materials adopt a scheelite-like (CaWO4) structure, exhibit excellent thermal and chemical stability, and therefore are considered to be efficient host candidates. Eu3+ ions were frequently used as activators for the red phosphors, which mainly show very sharp 5D0–7F2 red emission lines around 612 nm as Eu3+ ions occupy the lattice sites without inversion symmetry. In ALn(MO4)2 materials, Mo6+or W6+ is coordinated by four oxygen atoms in a tetrahedral site, and the rare-earth/sodium ions occupy eight-coordinated sites. The tetrahedral Mo6+ and W6+ have similar ionic radius (0.41 and 0.42 A˚), and make it possible to prepare solid solutions of the type ALn(WO4)2x(MoO4)x [5]. The photoluminescence properties of scheelite-type NaLn0.95Eu0.05(WO4)2x(MoO4)x (Ln ¼ Gd, Y, Bi)
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solid solutions were studied by Neeraj et al. [5]. These materials show intense red emission by exciting at 394 nm using the sharp 7 F0-5L6 line of Eu3+ ions, and could be used as red phosphors for white lighting devices utilizing near-UV LED chips. Wang et al. [6] studied double molybdates NaLa1xEux(MoO4)2 with various Eu content, and no concentration quenching of Eu3+ ions was observed in the series of samples. Furthermore, Wang et al. prepared composition-optimized double molybdate (Li0.333Na0.334K0.333)Eu(MoO4)2, which shows broadened excitation band in near-UV range and intense red emission with appropriate CIE chromaticity coordinates [7]. Sivakumar and Varadaraju [8] reported a red-emitting tungstomolybdate phosphor with composition of AgGd0.95Eu0.05(WO4)2x(MoO4)x, and this material can be excited efficiently by near-UV and blue (i.e., 394 and 465 nm) light. The tungstomolybdate phosphors with the composition of KEu(WO4)2x(MoO4)x [9] and LiEu(WO4)2x(MoO4)x [10] have also been reported recently, and are considered to be potential redemitting phosphors for white LEDs. Motivated by the above-stated studies and the attempts to develop suitable red phosphors for white LEDs, sodium europium double tungstate (NaEu(WO4)2 phosphor was prepared by the solid-state reaction method in this paper. Its crystal structure and photoluminescence properties were investigated aiming at the potential application in the field of phosphor-converted white LEDs. The influences of sintering temperature, Sm doping and atmosphere temperature on the photoluminescence properties of NaEu(WO4)2 phosphor were also studied.
Fig. 1. XRD patterns of NaEu(WO4)2 sintered at different temperature from 700 to 1200 1C.
2. Experimental The NaEu(WO4)2 samples were prepared by the solid-state reaction technique at high temperature. The starting materials NaHCO3 (A. R. grade), WO3 (99.9% purity), Eu2O3 (99.99% purity) and Sm2O3 (99.99% purity) were weighted by stoichiometric ratio. NaF (A. R. grade) at a certain mass ratio was added as the flux to improve the chemical reaction. After these powders were mixed thoroughly, the homogeneous mixture was filled into a little alumina crucible and calcined in a muffle furnace for 4 h at the temperature from 700 to 1200 1C. The crystal structure of the final products was examined by powder X-ray diffraction (XRD) using Cu Ka radiation. Photoluminescence excitation and emission spectra were recorded by a Hitachi F-7000 fluorescence spectrophotometer at room temperature. The temperature-dependent luminance of the phosphors was recorded by a self-made measuring system, which includes a temperature-controlled heating plate and a fluo-brightness meter (Zhejiang University Sensing Instruments Co., Ltd., China). Two near-UV LEDs (lem ¼ 385, 400 nm) and a blue LED (lem ¼ 460 nm) were used as excitation sources for temperature-dependent luminance measurements.
3. Results and discussion 3.1. Crystal structure and photoluminescence properties of NaEu(WO4)2 The XRD patterns of the NaEu(WO4)2 phosphors fired at different temperature are shown in Fig. 1. After being sintered at 700 1C for 4 h, a white product can be obtained. XRD data indicate that the product sintered at 700 1C is not a single phase. In addition to strong diffraction peaks corresponding to scheelitelike phase, a peak at 2y ¼ 33.021 ascribed to cubic Eu2O3 phase and two unidentifiable peaks at 2y ¼ 21.861 and 36.561 can also been observed. It indicates that the formation of scheelite-like
Fig. 2. Excitation spectrum for the 615 nm emission of the NaEu(WO4)2 phosphor sintered at 1100 1C for 4 h.
NaEu(WO4)2 is incomplete at this temperature. With the sintering temperature increasing, the obtained products vary from white to pink in color. The samples sintered at 800–1100 1C have a single crystalline phase with scheelite-like structure, and their XRD patterns are consistent with that given in JCPDS card 25–0828 [Na0.5Gd0.5MoO4]. When the firing temperature increases up to 1200 1C, a weak diffraction peak related to an unknown phase can be observed at 2y ¼ 37.701. It indicates that an excessively high sintering temperature (X1200 1C) is unfavorable for the formation of NaEu(WO4)2 with scheelite-like structure. Fig. 2 represents the excitation spectrum of the NaEu(WO4)2 phosphor sintered at 1100 1C for 4 h. The excitation spectrum for monitoring the 5D0-7F2 emission (615 nm) of Eu3+ ions consists of a broad band and some sharp lines. The broad excitation band centered at 280 nm can be attributed to the charge-transfer transition arising from oxygen to tungsten [5,10]. In the range from 325 to 550 nm, the sample shows characteristic intraconfigurational 4f–4f transitions of Eu3+: sharp 7F0-5L6 transition for 394 nm, 7F0-5D2 transition for 464 nm and the 7F1-5D1 transition for 535 nm [10]. So the NaEu(WO4)2 phosphor can be excited efficiently by near-ultraviolet (394 nm) or blue (464 nm)
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light, nicely in agreement with the output wavelength of near-UV or blue LED chips in phosphor-converted white LED. Fig. 3 shows the emission spectrum of NaEu(WO4)2 sintered at 1100 1C for 4 h under 394 nm light excitation. The spectrum essentially consists of several sharp lines with wavelength ranging from 580 to 710 nm, which are associated with the 5D0-7FJ (J ¼ 1, 2, 3, 4) transitions from the excited levels of Eu3+ to the ground state. The major emission of NaEu(WO4)2 was found at 615 nm (5D0-7F2), which corresponds to red emission. In addition, three weak emission lines at 590 nm (5D0-7F1), 654 nm (5D0-7F3) and 702 nm (5D0-7F4) can also be observed [5]. Other transitions of Eu3+ from the 5DJ excited levels to 7FJ ground states are too weak to be distinguishable. Fig. 3 also represents the emission spectrum of a commercial Y2O2S:Eu3+ phosphor under the same excitation intensity at 394 nm. Its strongest emission peak is located at 626 nm. The emission peak (615 nm) intensity of NaEu(WO4)2 is about 8.5 times higher than that (626 nm) of Y2O2S:Eu3+. The integral emission intensity, which is proportional to the quantum efficiency, of NaEu(WO4)2 is about 5.0 times higher than that of Y2O2S:Eu3+. The emission spectrum of NaEu(WO4)2 excited at 460 nm is shown in Fig. 4, and the sample was sintered at 1100 1C for 4 h. It is obvious that the spectrum is identical with that excited by 394 nm light. Eu2+-activated alkaline earth binary sulfides is one of commonly used red phosphors for blue LED chips. For comparison, the emission spectrum of a commercial sulfide phosphor is also presented in Fig. 4, and the excitation
Fig. 3. Emission spectra of NaEu(WO4)2 sintered at 1100 1C for 4 h and a commercial Y2O2S:Eu3+ phosphor (excited at 394 nm).
Fig. 4. Emission spectra of NaEu(WO4)2 sintered at 1100 1C for 4 h and a commercial sulfide phosphor (excited at 460 nm).
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wavelength is similarly at 460 nm. The sulfide phosphor is activated by Eu2+ ions and exhibits broad-band emission in the vicinity of 635 nm. Under the same excitation intensity at 460 nm, the NaEu(WO4)2 is 7.2 times higher than sulfide phosphor on the relative intensity of the highest peaks. The integral emission intensity of NaEu(WO4)2 is higher by 16% than that of Eu2+activated sulfide phosphor. As the excitation wavelength turns to 464 nm, the integral emission intensity of NaEu(WO4)2 is even 5.2 times higher than that of sulfide phosphor. These results imply that NaEu(WO4)2 is an promising red phosphor for white LEDs based on blue or near-UV chips. Fig. 5 shows the influences of sintering temperature on the photoluminescence properties of NaEu(WO4)2. It is observed that the 5D0-7F2 (615 nm) emission intensity under 394 or 464 nm excitation is gradually increased with increasing the sintering temperature from 700 to 1100 1C. Sintering at 1200 1C can induce a hard aggregation and the formation of impurity phase (as shown in Fig. 1), and therefore suppresses the luminescence of NaEu(WO4)2. The optimum sintering temperature is 1100 1C to obtain NaEu(WO4)2 with higher photoluminescence efficiency. 3.2. Photoluminescence properties of NaEu1xSmx(WO4)2 Sm3+ ion can be used as an efficient sensitizer to enhance the light emission of Eu3+-activated phosphors [6,11]. Therefore, NaEu1xSmx(WO4)2 phosphors with various x values were also prepared by sintering at 1100 1C for 4 h. The influences of Sm content on their luminescent properties were investigated. Fig. 6(a)–(f) presents the excitation spectra of NaEu1xSmx(WO4)2 phosphors monitored at 615 nm, and x value varies from 0 to 0.1 at an interval of 0.02. In order to clarify the energy transfer between Eu3+ and Sm3+ ions, Sm-doped scheelite-like material NaY0.94Sm0.06(WO4)2 was also prepared, which on excitation at 404 nm exhibits four main emission bands around 565, 599, 607 and 646 nm (not shown). These emission bands can be attributed the 4G5/2-6H5/2, 4 F3/2-6H9/2, 4G5/2-6H7/2, 4F3/2-6H11/2 transitions of Sm3+ ions [12]. The excitation spectrum of NaY0.94Sm0.06(WO4)2 monitoring 599 nm emission is also presented in Fig. 6(g), consisting of some broad bands and sharp lines. The broad band centered at about 268 nm can be attributable to the charge-transfer transition between O2 and Sm3+ [13]. The strongest excitation peak is located at 404 nm, assigned to the 6H5/2-4K11/2 transition of Sm3+ ion [11]. In the wavelength range from 460 to 480 nm, the excitation peaks of Sm3+ ion are overlapped, and therefore a broader excitation band is formed.
Fig. 5. 5D0-7F2 emission (615 nm) intensity of NaEu(WO4)2 as a function of sintering temperature under 394 or 464 nm excitation.
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Fig. 6. Excitation spectra of NaEu1xSmx(WO4)2 (x ¼ 0–0.1) (monitoring 615 nm emission) and NaY0.94Sm0.06(WO4)2 (monitoring 599 nm emission). (a) x ¼ 0; (b) x ¼ 0.02; (c) x ¼ 0.04; (d) x ¼ 0.06; (e) x ¼ 0.08; (f) x ¼ 0.1; (g) NaY0.94Sm0.06 (WO4)2. All the samples were sintered at 1100 1C for 4 h. The curve (g) was magnified for 20 times for clarity.
In the excitation spectra of NaEu1xSmx(WO4)2 (x ¼ 0.02–0.1), the excitation peak at 404 nm can be obviously detected because of the introduction of Sm3+ ions. As mentioned above, it is from the 6H5/2-4K11/2 transition of Sm3+ ions. The 404 nm excitation peak of Sm3+ ions overlaps with that of Eu3+ ions at 394 nm and can induce the broadening of excitation band around 400 nm, compared with the NaEu(WO4)2 phosphor without Sm doping. With the increasing of Sm content the intensity of 404 nm peak is enhanced, and then decreases due to the concentration quenching effects as x value is larger than 0.8. It is noted that the excitation peaks around 464 nm are also broadened with the introduction of Sm3+ ions. This is related to the broad excitation band of Sm3+ ions in the wavelength range from 460 to 480 nm, as shown in Fig. 6(g). These results indicate that the introduction of Sm3+ ions can broaden the excitation bands of NaEu(WO4)2 phosphors, therefore, can enhance their energy absorption and emission intensity under near-UV or blue light irradiation. The emission spectra of Sm3+-doped NaEu(WO4)2 show characteristic spectral lines of Eu3+ emission under 394 or 464 nm excitation. No Sm3+ emission was observed even under direct excitation of the 6H5/2-4K11/2 transition (404 nm) of Sm3+ ions. The strongest emission line is still at 615 nm, due to the 5 D0-7F2 transition of Eu3+ ions. It implies that the absorbed energy of Sm3+ ions can be efficiently transferred to the Eu3+ ions. The 615 nm emission intensity of NaEu1xSmx(WO4)2 is shown in Fig. 7(a) as a function of Sm content, under the excitation of 394, 404 and 464 nm light, respectively. On excitation at 404 nm, the irradiation energy is mostly absorbed by Sm3+ ions and the emission intensity increases as x varies from 0.02 to 0.06. As x40.06, the emission intensity decreases due to the concentration quenching effects. By exciting at 394 or 464 nm, the intensity of light emission at 615 nm basically decreases with the increase of Sm content. The results obtained by us are not in agreement with that reported by Wang et al. [11]. They claimed that 4 mol% Sm doping could enhance the emission intensity of sodium europium molybdates under the excitation of 395 nm light. For the measurements of emission spectra, narrow-band light with a full-width at half-maximum (FWHM) of 2.5 nm was used as the excitation source by setting the slit width in fluorescence spectrophotometer. As discussed above, the excitation peaks at 394 and 464 nm are definitely from the energy level transition of Eu3+ ions. As the samples are directly excited by the narrow-band
Fig. 7. (a) The 615 nm emission intensity and (b) the relative luminance of NaEu1xSmx(WO4)2 as a function of x value by the different excitation sources. All the samples were sintered at 1100 1C for 4 h.
light source at 394 or 464 nm, the excitation energy is mostly absorbed by the Eu3+ ions. With the increase of Sm content, the Eu content in phosphors decreases and therefore the 615 nm emission intensity was suppressed. However, the broadened excitation peaks at 400 and 460 nm in Sm-doped NaEu(WO4)2 can enhance the energy absorption as a broad-band excitation source is used. Fig. 7(b) shows the relative luminance of NaEu1xSmx(WO4)2 as a function of x value. Three types of LED were used as the light source, whose the peak wavelength is located at 460, 400 and 385 nm, respectively. These LEDs have broad-band emission spectra with FWHM values of 20–30 nm. Doping certain amount of Sm can enhance the relative luminance of the NaEu(WO4)2 phosphor. It can be found with increasing Sm content the relative luminance under 400 nm LED excitation increases from 100 (x ¼ 0) to 134 (x ¼ 0.06). Excited by 460 and 385 nm LEDs, the relative luminance of the NaEu(WO4)2 phosphor can be increased by 14% (x ¼ 0.06) and 8% (x ¼ 0.04), respectively. 3.3. Temperature-dependent photoluminescence properties of NaEu(WO4)2 In addition to high brightness and efficiency, the thermal quenching property is also one of the most important technological parameters for phosphors applied in white LEDs [14,15], especially in high-power white LED devices. The promising phosphors should perform well and retain their brightness and
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4. Conclusions In this paper, the NaEu(WO4)2 red phosphor with a scheelitelike structure has been synthesized by the solid-stated reaction technique at high temperature. This material exhibits more efficient luminescence efficiency under excitation of near-UV or blue light radiation, in comparison with the commercially used sulfide or oxysulfide phosphors. Doping certain amount of Sm3+ can broaden the excitation band of NaEu(WO4)2, and therefore can enhance its emission intensity under excitation of near-UV or blue LEDs. Thermal quenching data indicate that the luminance of the NaEu(WO4)2 phosphor exhibits an excellent thermal stability in the temperature region from 30 to 150 1C. All the results indicate that the NaEu(WO4)2 material is a potential red phosphor for the fabrication of solid-state lighting devices based on near-UV or blue LED chips. Fig. 8. Thermal quenching characteristics of (a) the NaEu(WO4)2 phosphor under excitation of 460 nm LED, (b) the NaEu(WO4)2 phosphor under excitation of 400 nm LED, (c) the commercial Eu2+-activated sulfide phosphor under excitation of 460 nm LED and (d) the commercial Y2O2S:Eu3+ phosphor under excitation of 400 nm LED.
chromatic properties at the elevated temperatures of the LED junction (4100 1C) [16]. The thermal quenching properties of the prepared NaEu(WO4)2 phosphor were investigated in the temperature range from 30 to 200 1C. The obtained results were presented in Fig. 8, and the LEDs emitting at 460 and 400 nm were used as the excitation sources. For comparison, the temperature quenching data of the commercially used sulfide and oxysulfide standards were also presented in Fig. 8. By exciting of 460 nm LED, the relative luminance of Eu2+-activated sulfide phosphor increases slightly in the range 30–100 1C, and then decreases rapidly with the increasing temperature. The relative luminance of sulfide falls by 10% at 150 1C, and by 34% at 200 1C. In contrast, the NaEu(WO4)2 phosphor exhibits an excellent thermal stability. By exciting of 460 nm LED, its relative luminance is quite constant up to 160 1C, and falls only by 5% at 200 1C. As a 400 nm LED was used as an excitation source, at 150 1C the NaEu(WO4)2 phosphor still can retain 98% of its luminance at room temperature. The relative luminance of the Y2O2S:Eu3+ increases at elevated temperatures because its excitation maximum moves to a longer wavelength [16]. This is disadvantageous for practical application in the field of white LEDs, since it will alter the chromatic parameters of the device as a function of temperature [16].
Acknowledgements This work was financially supported by a research grant from the Ministry of Education of China (Grant no. 707029) and the Natural Science Foundation of Jiangsu Province (Grant no. BK2008317). References [1] M.S. Shur, A. Zukauskas, Proc. IEEE 93 (2005) 1691. [2] S. Nakamura, S. Perton, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Laser, Springer, Berlin, Germany, 1997. [3] C.F. Guo, D.X. Huang, Q. Su, Mater. Sci. Eng. B 130 (2006) 189. [4] Y.S. Hu, W.D. Zhuang, H.Q. Ye, D.H. Wang, S.S. Zhang, X.W. Huang, J. Alloy. Compd. 390 (2005) 226. [5] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [6] Z. Wang, H. Liang, M. Gong, Q. Su, Electrochem. Solid-State Lett. 8 (2005) H33. [7] Z. Wang, H. Liang, L. Zhou, H. Wu, M. Gong, Q. Su, Chem. Phys. Lett. 412 (2005) 313. [8] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H168. [9] T. Kim, S. Kang, J. Lumin. 122–123 (2007) 964. [10] C.H. Chiu, M.F. Wang, C.S. Lee, T.M. Chen, J. Solid State Chem. 180 (2007) 619. [11] Z.L. Wang, H.B. Liang, M.L. Gong, Q. Su, Opt. Mater. 29 (2007) 896. [12] J.P. Feist, A.L. Heyes, Meas. Sci. Technol. 11 (2000) 942. [13] K. Yao, M.W. Wang, S.X. Liu, L.D. Zhang, W.J. Li, J. Rare Earths 24 (2006) 524. [14] J.H. Ryu, Y.G. Park, H.S. Won, S.H. Kim, H. Suzuki, J.M. Lee, C. Yoon, M. Nazarov, D.Y. Noh, B. Tsukerblatc, J. Electrochem. Soc. 155 (2008) J99. [15] R.J. Xie, N. Hirosaki, Sci. Technol. Adv. Mater. 8 (2007) 588. [16] G. Gundiah, Y. Shimomura, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 455 (2008) 279.