Synthesis and luminescence properties of KCaPO4:Eu2+,Tb3+,Mn2+ for white-light-emitting diodes (WLED)

JOURNAL OF RARE EARTHS, Vol. 33, No. 8, Aug. 2015, P. 825

Synthesis and luminescence properties of KCaPO4:Eu2+,Tb3+,Mn2+ for white-light-emitting diodes (WLED) FANG Hongwei (ᮍᅣ࿕), HUANG Shan (咘ቅ), WEI Xiantao (䶺‫ܜ‬⍯), DUAN Changkui (↉ᯠ༢), YIN Min (ል⇥), CHEN Yonghu (䰜∌㰢)* (Department of Physics, University of Science and Technology of China, Hefei 230026, China) Received 8 December 2014; revised 2 March 2015

Abstract: In order to obtain a single-host white-light phosphor, a series of KCaPO4 powder samples tri-doped with Eu2+, Tb3+ and Mn2+ were synthesized via high-temperature solid-state reaction method. Their structural and luminescence properties were investigated. Under proper ultraviolet excitation (255–405 nm), white light was obtained, consisting of blue, green and red emissions stemming from Eu2+, Tb3+, Mn2+ ions respectively. The temperature stability of our sample was analyzed by studying the variation tendency of CIE chromaticity coordinates at different temperatures. The results indicated that this phosphor could yield good color stability when utilized in WLED. Keywords: single-phase white-light phosphor; photoluminescence; energy transfer; color stability; CIE chromaticity coordinates; rare earths

In recent years, white light emitting diode (WLED) has drawn much attention due to its good performance characteristics, such as high efficiency, long lifespan, energy saving and environmentally-friendly aspects[1,2]. Among different kinds of WLED lamps, it is most promising for phosphors converted WLEDs to be applied in a wide variety of fields such as industrial illumination and solid light-emission devices[3]. There are two main important approaches based on the combination of a single LED chip with phosphors. One is to partially convert the emission of the blue LED chip into appropriate visible light. The most typical example for this is the combination of YAG:Ce3+ yellow phosphor with a blue emitting InGaN LED, which has been widely utilized in lighting sources, automobile lamps and backlighting, etc.[4–6] However, YAG:Ce3+ phosphor suffers from some drawbacks such as poor color rendering index and low stability of color temperature[4,7,8]. The other approach is usually based on the mixing of red (R), green (G) and blue (B) lights converted from the excitation of UV (or near-UV) LED by different color phosphors. A common path to help us achieve this goal involves mixing different color phosphors, for example, Y2MoO6:Eu3+ (red), 12CaO-7Al2O3:Ce3+,Tb3+ (green) and BaMgAl10O17:Eu2+ (blue)[9–13]. However, the mixture of multiple phosphors may be subjected to color shift due to the host aging, resulting from different hosts involved usually sustaining different aging conditions. In order to enhance the color

stability, a single-phase white phosphors is proposed to be a potential solution, since different activators in only one kind of host would experience similar aging condition because of similar atomic surroundings the activators would face. In recent years, phosphate was widely used in WLED, and it has been reported that KCaPO4 can be used as a potential candidate of UV-LED phosphor host for its high temperature stability[14–16]. To our best knowledge, there has been some reports about the luminescence properties of singly-doped and co-doped KCaPO4 phosphors[15–19], but no report on the tunable color emission of the tri-doped KCaPO4:Eu2+,Tb3+,Mn2+ phosphor. In this paper, powder samples were successfully synthesized and their luminescence properties were investigated under UV excitation. By studying the variation tendency of CIE chromaticity coordinates with different temperatures, we concluded that KCaPO4:Eu2+,Tb3+,Mn2+ phosphor could be a candidate for WLED phosphors due to its good temperature stability.

1 Experimental The samples of KCaPO4:Eu2+,Tb3+,Mn2+ with different doping concentrations including KCaPO4:0.9%Eu2+, x%Tb3+,1.5%Mn2+ (x=5, 6, 7, 8), KCaPO4:y%Eu2+, 7%Tb3+,1.5%Mn2+ (y=0.9, 3.5–7) and KCaPO4:7%Eu2+, 7%Tb3+,2%Mn2+ were synthesized by high-temperature

Foundation item: Project supported by National Key Basic Research Program of China (2013CB921800), the National Natural Science Foundation of China (11374291, 11204292, 11274299, 11311120047), the Fundamental Research Funds for the Central Universities (WK2030020021) and Anhui Provincial Natural Science Foundation (1308085QE75) * Corresponding author: CHEN Yonghu (E-mail: [email protected]; Tel.: +86-551-63606024) DOI: 10.1016/S1002-0721(14)60491-9

826

solid-state reaction method. The constituent oxides and carbonates consisted of K2CO3 (A.R.), CaCO3 (A.R.), (NH4)H2PO4 (A.R.), MnCO3 (A.R.), Eu2O3 (99.99%) and Tb4O7 (99.99%). Stoichiometric molar ratio of the raw materials were thoroughly mixed, with fully ground and pre-fired at 650 ºC for 6 h under a weak reducing atmosphere (5% H2 and 95% N2). Subsequently after being ground again, the obtained sample were re-calcined at 1000 ºC for 10 h. The crystal structures were analyzed by an X-ray diffractometer (Rigaku-TTR-III) with Cu K radiation (=0.15418 nm) in the 2 range from 10° to 70°. Excitation and emission spectra were recorded with a spectrometer (HITACHI 850), which utilizes a 150 W Xe lamp as its excitation source.

2 Results and discussion 2.1 Structural properties The XRD patterns of some samples with the standard data of KCaPO4 are shown in Fig. 1. At first, to ascertain how Ca2+ ions are substituted by the doping ions, we compared two possible charge balanced the molar ratio: (1) a pair of Tb3+ ion and K+ ion substitute for two Ca2+ ions, one Mn2+ ion replaces one K+ ion and leaves one K+ vacancy, and one Eu2+ ion occupies one Ca2+ site; (2) one Tb3+ ion simultaneously replaces one Ca2+-K+ pair, while the replacement of Eu2+ and Mn2+ ions are not changed. The doping concentration of the samples we choose to ascertain proper substitution is KCaPO4:7% Eu2+,7%Tb3+, 2%Mn2+. The nominal chemical formula for our two possible situation is K1.03Ca0.79Eu0.07Tb0.07- Mn0.02PO4 and K0.89Ca0.84Eu0.07Tb0.07Mn0.02PO4 respectively, and the XRD patterns of two samples are exhibited in Fig. 1(1)

Fig. 1 (1) and (2) XRD patterns of the sample KCaPO4:7%Eu2+, 7%Tb3+,2%Mn2+ with different nominal chemical and formulus (K0.89Ca0.84Eu0.07Tb0.07Mn0.02PO4 K1.03Ca0.79Eu0.07Tb0.07Mn0.02PO4 ); (3), (4), (5) XRD patterns of samples KCaPO4:0.9%Eu2+,5%Tb3+, 1.5%Mn2+,KCaPO4:0.9%Eu2+,7%Tb3+,1.5%Mn2+ and KCaPO4:7%Eu2+,7%Tb3+,1.5%Mn2+ respectively; (6) The standard KCaPO4 data No. 33-1002

JOURNAL OF RARE EARTHS, Vol. 33, No. 8, Aug. 2015

and (2). It can be seen that Fig. 1(2) shows no obvious impurity phases, while noticeable deviations occur in Fig. 1(1). This result suggests that the second assumption of doping is more reasonable. Then the XRD patterns of three other samples are checked, including KCaPO4:0.9%Eu2+,5%Tb3+,1.5%Mn2+,KCaPO4:0.9%Eu2+, 7%Tb3+,1.5%Mn2+ and KCaPO4:7%Eu2+,7%Tb3+, 2+ 1.5%Mn . All results show that different doping centers would not lead to unintended phases. Their XRD patterns are shown in Fig. 1(3), (4) and (5) respectively. 2.2 Luminescence properties The PLE and PL spectra of the sample KCaPO4:0.9% Eu2+,7%Tb3+,1.5%Mn2+ are shown in Fig. 2. The PL spectrum consists of blue, green and orange-reddish emissions. Under UV excitation at 355 nm, a blue broadband emission with the maximum at 463 nm is generated, which can be attributed to the 5d-4f transition of Eu2+. Besides, some narrow emissions stemming from the transitions between 4f energy levels of Tb3+ can be observed, consisting of a green emission centered at around 544 nm which is related to the 5D47F5 transition; a blue emission whose peak locates at near 487 nm which can

Fig. 2 PLE and PL spectra of the sample KCaPO4:7%Eu2+,7% Tb3+,2%Mn2+ respectively (a) PLE spectra monitoring the emission wavelength 516 nm (solid line), 544 nm (dashed line) and 670 nm (dotted line) respectively; (b) PL spectrum of tri-doped sample under 355 nm excitation

FANG Hongwei et al., Synthesis and luminescence properties of KCaPO4:Eu2+,Tb3+,Mn2+ for white-light-…

be ascribed to the 5D47F6 transition; and the emission peaks situated at the red and orange-reddish region which correspond to 5D47F3 and 5D47F4 transition respectively. Additionally, a broad red emission band also arises from the 3d-3d transitions of Mn2+, when Mn2+ ions are doped into octahedron crystal field (owing to Oh symmetry)[18]. There are a broad-band excitation ranging from 250–450 nm and a narrow-band excitation centered at 225 nm in our PLE spectrum for our tri-doped sample. The weak narrow-band excitation can be ascribed to 4f8-4f75d1 transition of Tb3+, and as for the broad-band excitation, apart from the 4f-4f transition absorption of Eu2+˄250–400 nm), the 4f-4f transition absorption of Tb3+ (350–400 nm) and the contribution of Mn2+ (300– 450 nm) including the ground state 6A1g to the excited state (4T1g, 4T2g , etc.) should be considered as well[18,19]. By observing the PLE and PL, there exists an overlap between the absorption and the emission of Eu2+, so we make 516 nm as a substitution for the wavelength at maximum emission intensity to monitor the absorption. And for Mn2+, as the overlap between the emission spectra of Tb3+ and Mn2+, we choose 670 nm to measure the excitation spectrum of Mn2+. Meanwhile, we monitor 544 nm to acquire the PLE spectrum of Tb3+. It is noticeable that no characteristic absorption at 225 nm can be observed in Mn2+ singly-doped KCaPO4 samples, but a narrow-band absorption at 225 nm can be seen obviously in Fig. 2(a) for our tri-doped samples[19]. Therefore, we can conclude that there exists a energy transfer (ET) process from Tb3+ to Mn2+ because of the similar spectral shape at 225 nm for PLEs of Tb3+ and Mn2+ in our tri- doped samples. What’s more, it has been reported that the ET process occurs between Eu2+ and Mn2+ in Eu2+, Mn2+ co-doped KCaPO4 samples[19]. Compared with the excitation intensity of Mn2+ in Eu2+, Mn2+ co-doped KCaPO4 samples, the one in Mn2+ singly-doped sample is much weaker because of the spin-forbidden properties in absorption transition process. By further observation, the excitation intensity of 225 nm which results from the ET process between Tb3+ and Mn2+ is weak. So we can speculate that the broad-band excitation ranging from 250 to 430 nm in the PLE of Mn2+ is mainly contributed by the excitation of Eu2+, which means ET process also occurs between Eu2+ and Mn2+ in our tri-doped samples. Fig. 3 shows the emission spectra of samples KCaPO4: x%Tb3+ (x=5, 6, 7, 8˅under 225 nm excitation for the 5d band of Tb3+. The variation tendency of integrated intensity for the emission spectrum under different doping concentrations of Tb3+ is presented in inner illustration. It illustrates that, when doping concentration of Tb3+ is around 7%, the highest emission intensity can be obtained. The curve experiences a slow increase and then decline, resulting from the concentration quenching of

827

Fig. 3 PL spectra of sample KCaPO4:x%Tb3+ (x=5, 6, 7, 8) (The inset shows the tendency between the integrated emission intensity and the doping concentration of Tb3+. The integral region covers from 400 to 675 nm)

Tb3+. Therefore, we choose 7% as our optimized doping concentration to conduct further study. Fig. 4 shows the emission spectra of sample KCaPO4: x%Eu2+,7%Tb3+,1.5%Mn2+ (x=0.9, 3.5–7.0) under 355 nm excitation. Here we normalize the emission spectra of the samples with different doping concentrations of Eu2+ at 544 nm (Tb3+) to research the relative emission intensity between Eu2+ and Mn2+. It can be observed that, as the doping concentration of Eu2+ increases, the relative red emission intensity shows a gradual increase. The reasons can be summarized into these as follows. When the doping concentration of Eu2+ is low (0.9%, grey curve), there is almost no emission of Mn2+, for sufficiently short distance between Mn2+ and the sensitizer (Eu2+) is needed to achieve the ET process. With doping more and more Eu2+, it is much easier for each Mn2+ ion to have an Eu2+ ion in its near neighborhood. Therefore, the raised Eu2+ can be a contributing factor to promote the ET process from Eu2+ to Mn2+, enhancing the red emission of Mn2+. The white light emission is observed in the sample KCaPO4:7%Eu2+,7%Tb3+ and 2% Mn2+ upon UV excitation. In order to check the dependence of the emission color on excitation wavelength, we measure the emission

Fig. 4 PL spectra of sample KCaPO4:x%Eu2+,7%Tb3+,1.5% Mn2+ (x=0.9, 3.5–7.0) under 355 nm excitation

828

JOURNAL OF RARE EARTHS, Vol. 33, No. 8, Aug. 2015

spectra under the excitation from 255 to 405 nm. The chromaticity coordinates were calculated from the recorded emission spectra and then is listed in Table 1. As we see, when the excitation wavelength varies from 255 to 405 nm, the chromaticity coordinate x changes from 0.322 to 0.324, while y sees a variation between 0.300 and 0.347. It is intuitive to see that the white light can be obtained on a wide range of excitation from 255 to 405 nm, with the CIE chromaticity coordinates only witness a slight shift, which suggests that there exists a wide range of excitation wavelength to obtain white light emission. To further study how CIE coordinates change with different doping concentrations, we calculate the values from the PL spectra in Fig. 5. When doping concentration of Eu2+ varies from y=3.5 to y=7, the luminescence colors of the samples change from blue color region to white color region. And the CIE coordinates for the samples of KCaPO4:y%Eu2+,7%Tb3+,1.5%Mn2+ (y=5–7) are quite close to white light region, which shows that whitelight emission can be obtained if the doping concentrations are proper in our tri-doped KCaPO4:Eu2+,Tb3+,Mn2+ samples. Finally, the temperature stability is also a crucial factor for WLED phosphor. When the temperature is lower than 470 K, the sample KCaPO4:7%Eu2+,7%Tb3+,2% Mn2+ displays as a white light source, while it tends to be a little blue above 520 K. The relation between integrated intensity and temperature for our sample are shown

in Fig. 6. As all known, 340–360 K tends to be a proper working temperature interval that should be kept in daily LED processing technique. From Fig. 5, we can obtain that the integrated intensity does not witness an obvious change under 330 K, which indicates our sample will almost not be affected by the temperature quenching in the practical operating temperature region. Additionally, we can see from Fig. 7 that, although the CIE points shift as the variation of temperature, they still situates in the region of white light, which indicates that the color of our sample can keep stable on a wide range of temperature.

Fig. 6 Variation tendency between integrated intensity of the emission and the temperature for sample KCaPO4: 7%Eu2+,7%Tb3+,2%Mn2+ under 355 nm excitation. The PL spectrum is integrated ranging from 400 to 725 nm

Table 1 CIE chromaticity coordinate points (x, y) of sample KCaPO4:7%Eu2+,7%Tb3+,2%Mn2+ under different excitation wavelengths varying from 255 to 405 nm ex/nm

255

275

305

315

335

355

365

395

405

x

0.322 0.312 0.320 0.322 0.319 0.313 0.319 0.333 0.324

y

0.300 0.302 0.312 0.312 0.306 0.306 0.329 0.344 0.347

Fig. 7 Variation tendency between CIE chromaticity coordinate points and the temperature for sample KCaPO4:7%Eu2+, 7%Tb3+,2%Mn2+ under 355 nm excitation, with temperature changing from room temperature to 570 K

3 Conclusions Fig. 5 Variation tendency between CIE chromaticity coordinate points and the doping concentration of Eu2+ for sample KCaPO4:y%Eu2+,7%Tb3+,2%Mn2+ under 355 nm excitation, with y changing from 3.5% to 7%

A series of KCaPO4:Eu2+,Tb3+,Mn2+ phosphors were synthesized by a high-temperature solid-state reaction method. The structural and luminescence properties of these samples were investigated and the possible energy

FANG Hongwei et al., Synthesis and luminescence properties of KCaPO4:Eu2+,Tb3+,Mn2+ for white-light-…

transfer processes were studied by analyzing the PLE and PL spectra. White-light-emission was obtained under UV excitation and the range of excitation wavelength was very wide. When changing the excitation wavelength from 255 to 405 nm, the shift of CIE chromaticity coordinate points was not very large. Further studies are needed to achieve a better chromaticity by doping some other ions such as Dy3+ to enhance the yellow light emitting.

References: [1] Pimputkar S, Speck J S, DenBaars S P, Nakamura S. Prospects for LED lighting. Nat. Photonics, 2009, 3: 181. [2] Schubert E F, Kim J K. Solid-state light sources getting smart. Science, 2005, 308: 1274. [3] Phillips J M, Coltrin M E, Crawford M H, Fischer A J, Krames M R, Mueller M R, Mueller G O, Ohno Y, Rohwer L E, Simmons J A, Tsao J Y. Research challenges to ultra-efficient inorganic solid-state lighting. Laser Photonics Rev., 2007, 1: 307. [4] Uchida Y, Taguchi T. Lighting theory and luminous characteristics of white light-emitting diodes. Opt. Eng., 2005, 44: 124003. [5] Liu T Y, Lei X M, Xu L, Wang X, Luo W, Wang Y Z, Zhao Z Y, Jiao H. Effect of post-processing on luminescence properties of YAG:Ce3+ phosphors. J. Rare Earths, 2014, 32: 156. [6] Yam F K, Hassan Z. InGaN: An overview of the growth kinetics, physical properties and emission mechanisms. Superlattices Microstruct., 2008, 43: 1. [7] Smet P F, Parmentier A B, Poelman D. Selecting conversion phosphors for white light-emitting diodes. J. Electrochem. Soc., 2011, 158: R37. [8] Lee S, Seo S. Optimization of yttrium aluminum garnet: Ce3+ phosphors for white light-emitting diodes by combinatorial chemistry method. J. Electrochem. Soc., 2002, 149: J85. [9] Yasuo S, Naoto K. Effect of ammonium chloride addition

829

on spray pyrolysis synthesis of BaMgAl10O17:Eu2+ phosphor without post-heating. J. Electrochem. Soc., 2004, 151: H192. [10] Jin H J, Yang P, Tian L H. Improved luminescence of Y2MoO6:Eu3+ by doping Li+ ions for light-emitting diode applications. J. Lumin., 2012, 132: 1188. [11] Yu J L, Ming Y L, Fan Y, X Y W, Wen L Y, Xue J L. Preparation and characteristics of Ca2NaMg2V3O12:Sm3+ single-phased white-emitting phosphors. J. Inorg. Organomet. Polym., 2013, 23: 684. [12] Deng K M, Gong T, Chen Y H, Duan C K, Yin M. Efficient red-emitting phosphor for near-ultraviolet-based solid-state lighting. Opt. Lett., 2011, 36: 4470. [13] Liu X L, Liu Y X, Yan D T, Zhu H C, Liu C G, Xu C S. Luminescence and energy transfer characteristics of Ce3+ and Tb3+ co-doped nanoporous 12CaO-7Al2O3 phosphors. J. Nanosci. Nanotech., 2011, 10: 1166. [14] Poort S H M, Janssen W, Blasse G. Optical properties of Eu2+-activated orthosilicates and orthophosphates. J. Alloys Compd., 1997, 260: 93. [15] Zhang S Y, Huang Y L, Seo H J. The spectroscopy and structural sites of Eu2+ ions doped KCaPO4 phosphor. J. Electrochem. Soc., 2010, 157: J261. [16] Guo N, Song Y H, You H P, Jia G, Yang M, Lu Y J. Optical properties and energy transfer of NaCaPO4:Ce3+,Tb3+ phosphors for potential application in light-emitting diodes. Eur. J. Inorg. Chem., 2010, 29: 4636. [17] Li P L, Wang Z J, Guo Q L, Yang Z P. Preparation and luminescent characteristics of KCaPO4:Eu3+ red phosphor for white LED. J. Chin. Soc. Rare Earths (in Chin.), 2010, 28: 755. [18] Liu Z R, Zhong R X. Energy transfer and luminescent properties in y-Zn3(PO4)2:Mn2+, Ga3+. J. Chin. Soc. Rare Earths, 2013, 31: 197. [19] Wang Z J, Li Y L, Yang Z P, Guo Q L. Luminescence characteristics of KCaPO4: Tb3+ green phosphor. J. Funct. Mater., 2011, 42: 884. [20] Hu J, Xie H D, Huang Y L, Wei D L, Seo H. Luminescence spectroscopy and erengy transfer in Eu2+/Mn2+doped KCaPO4 phosphors. Appl. Phys. B, 2014, 114: 461.