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Synthesis and luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs
Author’s Accepted Manuscript Synthesis and luminescence properties of Ce3+doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs Yaochun Qiang, Yuxi Yu, Guolong Chen, Jiyu Fang www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(15)30313-6 http://dx.doi.org/10.1016/j.jlumin.2015.11.041 LUMIN13740
To appear in: Journal of Luminescence Received date: 28 July 2015 Revised date: 25 October 2015 Accepted date: 25 November 2015 Cite this article as: Yaochun Qiang, Yuxi Yu, Guolong Chen and Jiyu Fang, Synthesis and luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.11.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs Yaochun Qianga, Yuxi Yua*, Guolong Chenb and Jiyu Fangc a
Fujian Key Laboratory of Advanced Materials, Department of Materials Science and
Engineering, College of Materials, Xiamen University, Xiamen 361005, China b
Fujian Engineering Research Center for Solid-state Lighting, Department of Electronic
Science, Xiamen University, Xiamen 361005, China c
Advanced Materials Processing and Analysis Center, Department of Materials Science and
Engineering, University of Central Florida, Orlando, Florida 32816, USA Abstract A series of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphors were successfully synthesized by a solid-state reaction method. The microstructure, morphology, luminescence spectra, luminescence quantum yield (QY) and thermal stability of the phosphor were investigated. The critical concentration of Ce3+ ions in Y3-mAl3.5Ga1.5O12:mCe3+ is m = 0.06. The QY of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is as high as 94 % under excitation at 450 nm and its luminescence intensity at 150 °C still maintains 90 % of that measured at 25 °C, which are just a little worse than those of commercial Lu3Al5O12:Ce3+ green phosphor but much better than those of commercial (Sr,Ba)2SiO4:Eu2+ green phosphor. A white LED lamp was fabricated by employing Y2.94Al3.5Ga1.5O12:0.06Ce3+ as a green phosphor and commercial (Ca,Sr)AlSiN3:Eu2+ as a red phosphor (628 nm), its Ra value, correlated color temperature *
Corresponding author. Tel.: +865923502958. E-mail addresses: [email protected] (Y. X. Yu) 1
(CCT), CIE1931 chromaticity coordinates and luminous efficiency is 84, 3081 K, (x = 0.4369, y = 0.4142) and 102 lm/W, respectively. The experimental results demonstrate that Y2.94Al3.5Ga1.5O12:0.06Ce3+ is a promising green phosphor not only can be used for high color rendering index white LEDs but also for high-power white LEDs. Keywords: Luminescence; Phosphors; YAGG:Ce; Thermal stability; White LEDs. 1. Introduction White light-emitting diodes (LEDs) have attracted extensive attention because they have much better properties than those of conventional incandescent and fluorescent lamps, such as longer lifetime, higher efficiency, better reliability, and so on [1,2]. White LEDs are generally manufactured by combining blue LED chips and yellow-emitting cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor. The main drawback of this type white LEDs is the low color rendering index (Ra), which is only about 75 [3,4]. In order to improve the color-rendering properties of white LEDs, YAG:Ce phosphor was replaced by green and red phosphors simultaneously [5,6], which made the white LEDs show excellent color-rendering properties, the Ra value was more than 90. So green phosphor is one of the key materials for making high Ra value white LEDs. Although a large amount of green phosphors have been investigated for use in solid state lighting, only a small number of green phosphors can be employed practically for blue LED-based white
LEDs. These green phosphors include (Sr,Ba)2SiO4:Eu2+ [6],
β-SiAlON:Eu2+ [7], Ca3Sc2Si3O12:Ce3+ [8], Lu3Al5O12:Ce3+ [9]. However, it was reported that (Sr,Ba)2SiO4:Eu2+ suffers from large thermal quenching of luminescence at high temperatures [10,11], which makes it unable to be used for high-power white LEDs. β-SiAlON:Eu2+ 2
exhibits excellent thermal stability and durability, but it is difficulty to synthesize (1800 2000 °C, 0.6 - 0.9 MPa gas pressure) and its quantum efficiency is low [12]. Ca3Sc2Si3O12:Ce3+ [8] and Lu3Al5O12:Ce3+ [9] shows excellent thermal stability, but their main raw materials (Sc2O3 and Lu2O3) are rare earth oxides, which are very limited and very expensive. Therefore, it is necessary to develop alternative green phosphors for white LEDs. It has been reported that Ce3+-doped yttrium aluminum gallium garnet (YAGG:Ce) is able to emit tunable green light when excited by blue light [13], which suggests that YAGG:Ce phosphor may serve as a green phosphor for white LEDs. However, to the best of our knowledge, there are no studies that identify the Ce3+ concentration-dependent and temperature-dependent luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs. From the viewpoint of its application, it is necessary to investigate the luminescence properties of the phosphor. In this study, a series of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphors were synthesized by a solid-state reaction method. The Ce3+ concentration-dependent luminescence properties was studied. The effect of NaF flux on microstructure and luminescence properties of the phosphors was investigated. The luminescence quantum yield and thermal stability (temperature-dependent luminescence intensity and color stability) of Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor were investigated and compared with those of commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors. In addition, the optical properties of the warm white LED lamp fabricated by combining blue LED lamp with the prepared Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor and commercial (Ca,Sr)AlSiN3:Eu2+ red phosphor (628 nm) were measured. 2. Experimental 3
2.1. Synthesis of Ce3+-doped Y3Al3.5Ga1.5O12 phosphors The samples of Y3-mAl3.5Ga1.5O12:mCe3+ (0.03 ≤ m ≤ 0.08) phosphors were synthesized from the stoichiometric mixture of Y2O3 (99.99%), Ga2O3 (99.99%), Al2O3 (99.99%) and CeO2 (99.99%), in which 0 - 5 wt% NaF (99%) was used as flux. The mixture was grounded with an agate mortar and pestle for 1 hour, and then placed in alumina crucibles, fired at 1480 °C for 4 h in a reducing atmosphere environment (5 v% H2 / 95 v% N2). After the cooling, the samples were slightly grounded with an agate mortar and pestle, washed with hot distilled water for 3 times, and then dried at 80 °C for 12 hours. 2.2. Preparation of white LED lamp A warm white LED lamp was fabricated by combining blue LED lamp (~ 5 W) with a phosphor containing silicone cap (Weight ≈ 2.8g, Thickness ≈ 0.45 mm) and a polycarbonate lampshade. The Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor (3.2 g) prepared with 3 wt% NaF and commercial (Ca,Sr)AlSiN3:Eu2+ red phosphor (0.22 g) were dispersed homogeneously into the silicone (20 g). The vacuum technique was used to remove the air bubbles in the mixture, and then the mixture was put into the molds and heated for a few seconds, several phosphor containing silicone caps were made. 2.3. Characterization The structure of Ce3+-doped Y3Al3.5Ga1.5O12 phosphors was analyzed by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) using Cu Kα1 radiation. The elemental composition of the sample was analyzed by X-ray energy dispersive spectroscopy (EDS, X-MaxN, Oxford, UK). The morphology was observed by scanning electron microscopy (SEM, TM3000, Hitachi, Japan). The luminescence spectra were measured by a fluorescence spectrometer (FLS920, 4
Edinburgh Instruments, UK) with Xe-lamp (450 W) as an excitation source. The luminescence quantum yield was measured by a fluorescence spectrometer (FLS980, Edinburgh Instruments, UK) with Xe-lamp (450 W) as an excitation source. The luminescence intensity and the Commission Internationale de L’Eclairage (CIE) 1931 chromaticity coordinates at various temperatures (25 - 200 °C) were recorded by an optical scanning spectrometer (Spectro320, Instrument Systems, Germany) with a narrow spectrum (FWHM = 10 nm) blue LED (emission peak at 458 nm) as an excitation source. A self-designed heating attachment was employed to heat samples, which mainly includes a resistive heater and a standard temperature controller (error range of 0.1 °C). The optical properties of the warm white LED lamp were measured by a LED lamp measurement system (YF1000, Everfine, China) under a forward-bias current of 60 mA at room temperature. 3. Results and discussion 3.1. Phase and composition analysis The XRD patterns of the samples (with different Ce3+ concentration) obtained without NaF are shown in Fig. 1a. The XRD patterns of the samples obtained with 0 ~ 5 wt% NaF as flux are shown in Fig. 1b. As shown in Fig. 1, the diffraction peaks of the samples obtained with no flux and with NaF as flux are matched well with those of the pure phase yttrium aluminum garnet (Y3Al5O12 JCPDS 33-0040) and no significant impurities or secondary phases are observed, which confirms that the obtained samples are single garnet phase. In order to confirm the percentage composition of the prepared microcrystal, the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample prepared at 1480 °C for 4 h with 3 wt% NaF as flux was chosen to do EDS analysis and the result is shown in Fig. 2. The result indicates that the 5
microcrystal of the sample is composed of 13.77 at% Yttrium (Y), 15.30 at% Aluminum (Al), 5.90 at% Gallium (Ga), 0.25 at% Cerium (Ce) and 64.78 at% Oxygen (O). As shown in Table 1, the experimental values and theoretical values (calculated from the chemical formula of Y2.94Ce0.06Ga1.5Al3.5O12) are very close. Combined with XRD result, EDS result demonstrates that the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample was successfully synthesized. As can be seen in Fig. 1, the diffraction peaks of the samples prepared with NaF are stronger (intensity) and sharper (shape) than those of the sample prepared without NaF, implying that NaF can promote the crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors. In addition, we note that the intensity of the diffraction peaks increases as the adding amount of NaF flux, and it reaches the maximum at 3 wt% NaF. With the further increase of NaF flux, the intensity decreases. The result reveals that too much NaF flux will have a negative effect on the crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor. We deduce that too much NaF flux may cause more crystal defects (interstitial defects, antisite defects and so on) which will worsen the luminescence properties of the phosphor. 3.2. Morphology of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors Fig. 3 shows the SEM images of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared with 0 5 wt% NaF. As seen from Fig. 3, the mean primary particle size of the sample prepared without flux is very small (less than 1μm) and the particle agglomeration is serious. It is obvious that the primary particle size of the samples was dramatically changed by adding NaF as flux. The mean primary particle size is about 2 μm, 4 μm, 5 μm, 6 μm and 7 μm for the sample prepared with 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%, respectively. In addition, we find out that the crystal faces of the samples prepared with NaF (≥ 2 wt%) are 6
distinguishable. Larger primary particle size and clear crystal faces demonstrate that the samples prepared with NaF are well-crystallized, indicating that NaF can greatly promote the growth and crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor. However, the significant over-sintering phenomenon was observed in Fig. 3e and Fig. 3f, especially in Fig. 3f, which was caused by too much NaF flux. The result indicates that too much NaF flux will worsen the dispersivity of the phosphor. 3.3. Luminescence spectra of Ce3+-doped Y3Al3.5Ga1.5O12 phosphors Fig. 4 shows the emission spectra of Y3-mAl3.5Ga1.5O12:mCe3+ (0.03 ≤ m ≤ 0.08) phosphors with various Ce3+ concentrations under the excitation wavelength of 450 nm. It is well known that the Ce3+ ion has a 4f1 configuration, the ground state consists of 2F5/2 level and 2F7/2 level. Thus, the broad emission bands of Y3-mAl3.5Ga1.5O12:mCe3+ phosphors should be ascribed to the electronic transitions of Ce3+ from the lowest 5d level (5d1) to the ground 4f levels (2F5/2 and 2F7/2) [14,15]. As can be seen, the luminescence intensity of Y3-mAl3.5Ga1.5O12:mCe3+ (0.03 ≤ m ≤ 0.08) phosphors increases with the increase of Ce3+ concentration, and it reaches the maximum at m = 0.06 (2.0 mol %). With the further increase of Ce3+ concentration (m > 0.06), the luminescence intensity decreases, which is the so-called concentration quenching of luminescence. In addition, with the increase of Ce3+ concentration, a continuous red shift of the emission spectra is observed. According to previous studies [16-18], the concentration quenching and the red shift should be mainly attributed to the energy transfer between Ce3+ ions. As suggested by Blasse [19], the critical distance Rc of energy transfer can be estimated by the equation: 7
1⁄3
3𝑉
𝑅𝑐 ≈ (4𝜋𝑥 𝑁)
(1)
𝑐
where Rc is the critical distance for energy transfer, V is the unit cell volume, and N is the number of sites per unit at which the activator atoms enter. xc is the critical concentration of Ce3+ ions. For Y3Al3.5Ga1.5O12 host, N = 24 [20]. On the basis of the XRD results showed in Fig.1a, V ≈ 1757.5 Å3. As shown in Fig. 3, the critical concentration of Ce3+ ions (xc) is 2.0 mol % (m = 0.06). Thus, according to the Eq. (2), Rc is calculated to be 19 Å. Fig. 5 shows the luminescence spectra of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared at 1480 °C for 4 h with 0 - 5 wt% NaF as flux. In Fig. 5a, two excitation bands of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors are observed, at around 345nm and 440nm, respectively. The result indicates that Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor can be effectively excited by blue LED (430 – 460 nm). The longer wavelength bands and the shorter ones should be ascribed to the electronic transitions of Ce3+ from the ground 4f level (2F5/2) to the lowest 5d level (5d1) and the second lowest 5d level (5d2) [14,15]. The broad emission bands of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor samples shown in Fig. 5b are attributed to the electronic transition of Ce3+ ions from the lowest 5d level to the ground 4f levels (5d1 → 2
F5/2,7/2)
[14,15].
As
can
be
seen
in
Fig.
5b,
the
emission
intensity
of
Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors significantly increases as the adding amount of NaF flux, and it reaches the maximum at 3 wt%. With the further increase of NaF flux, the emission intensity decreases. The variation trend is consistent with that of the XRD results in section 3.1. The result reveals that only appropriate amount of NaF flux can play a positive role in promoting the luminescence intensity. According to the XRD results in section 3.1, the decrease of the luminescence intensity should be mainly attributed to the increase of the 8
crystal defects caused by too much NaF flux. In order to evaluate the usefulness of this phosphor for white LED applications, the luminescence quantum yield of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor (prepared at 1480 °C for 4 h with 3 wt% NaF) was measured and compared with commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors for excitation at 450 nm. The luminescence quantum yield is 94 %, 82 % and 98 % for Y2.94Al3.5Ga1.5O12:0.06Ce3+, commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+, respectively. The result shows that Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor has a high luminescence quantum yield, indicating its great potential for use in white LEDs. 3.4. Thermal stability of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor It is well known that a lot of heat is generated when LED works, especially for high-power LED. Therefore, thermal stability (temperature-dependent luminescence intensity and color stability) is one of the important technological parameters for a phosphor used for white LEDs. In order to show the thermal stability of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor (prepared at 1480 °C for 4 h with 3 wt% NaF), its thermal stability was compared with that of commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors. The temperature-dependent luminescent intensity (excited at 458 nm) of the phosphors were measured at temperatures in the range of 25 - 200 °C, and the results are shown in Fig. 6. Obviously, the luminescence intensity of all samples falls as the temperature rises. The luminescence intensity of commercial (Sr,Ba)2SiO4:Eu2+ phosphor decreases much more rapidly.
At
150
°C,
the
luminescence
intensity
of
(Sr,Ba)2SiO4:Eu2+,
Y2.94Al3.5Ga1.5O12:0.06Ce3+ and Lu3Al5O12:Ce3+ maintains 79%, 90% and 93% of that 9
measured at 25 °C, respectively. It is easy to find out that the thermal quenching of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is only a little larger than that of Lu3Al5O12:Ce3+ phosphor but much smaller than that of (Sr,Ba)2SiO4:Eu2+ phosphor. Fig. 7 shows the CIE 1931 chromaticity coordinates of (Sr,Ba)2SiO4:Eu2+, Y2.94Al3.5Ga1.5O12:0.06Ce3+ and Lu3Al5O12:Ce3+ at various temperatures (25 - 200 °C). With the temperature increasing from 25 °C to 200 °C, the CIE1931 chromaticity coordinates of (Sr,Ba)2SiO4:Eu2+, Y2.94Al3.5Ga1.5O12:0.06Ce3+ and Lu3Al5O12:Ce3+ change from (0.2695, 0.6367) to (0.2692, 0.6062), (0.3854, 0.5671) to (0.3918, 0.5602) and (0.3592, 0.5766) to (0.3673, 0.5726), respectively. It is easy to find out that the change of commercial (Sr,Ba)2SiO4:Eu2+ phosphor is the largest and the change of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is as small as that of the commercial Lu3Al5O12:Ce3+ phosphor. The results reveal that the color stability Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is very good. All the thermal testing results demonstrate that Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor has excellent thermal stability, which indicates its great potential for use in high-power white LEDs as a green phosphor. 3.5. Application of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor in White LED lamp Fig. 8 presents the pictures, emission spectrum and optical properties of the warm white-LED lamp using Y2.94Al3.5Ga1.5O12:0.06Ce3+ as a green phosphor and commercial (Ca,Sr)AlSiN3:Eu2+ as a red phosphor. As can be seen, the Ra value of the white LED lamp is 84, its correlated color temperature (CCT) is 3081 K, its CIE1931 chromaticity coordinates is (x = 0.4369, y = 0.4142) and its luminous efficiency is 102 lm/W. The Ra value of the white LED lamp (84) is much higher than that of the traditional white LEDs (~ 75) prepared by combining
blue
LEDs
with
YAG:Ce 10
phosphor
[3,4],
which
demonstrates
Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor can be employed as a green phosphor for high Ra value white LEDs. 4. Conclusions A series of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphors were successfully synthesized by a
solid-state
reaction
method.
The
critical
concentration
of
Ce3+
ions
in
Y3-mAl3.5Ga1.5O12:mCe3+ is m = 0.06. NaF is a very good flux, which can dramatically promote the growth and crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor. The luminescence quantum yield and thermal stability of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor are just a little worse than those of commercial Lu3Al5O12:Ce3+ green phosphor but much better than those of commercial (Sr,Ba)2SiO4:Eu2+ green phosphor. The Ra value of the white LED lamp prepared by using Y2.94Al3.5Ga1.5O12:0.06Ce3+ as green phosphor is as high as 84. All the results demonstrate Y2.94Al3.5Ga1.5O12:0.06Ce3+ is a promising green phosphor not only can be used for high color rendering index white LEDs but also for high-power white LEDs. Acknowledgments This work was supported by the Natural Science Foundation of China (51175444), the Aviation Science Foundation of China (2013ZD68009), New Century Excellent Talents in Fujian Province University (2013), Natural Science Foundation of Fujian Province of China (2014J01206), Xiamen Municipal Bureau of Science and Technology (3502Z20143009) and Scientific and Technological Innovation Platform of Fujian Province of China (2014H2006). References [1] E. F. Schubert, J. K. Kim, Solid-State Light Sources Getting Smart, Science 308 (2005) 1274-1278. 11
[2] Y. Narukawa, M. Ichikawa, D. Sanga, M. Sano, T. Mukai, White light emitting diodes with super-high luminous efficacy, J. Phys. D: Appl. Phys. 43 (2010) 354002. [3] H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon, S. S. Kim, Enhancement of red spectral emission intensity of Y3Al5O12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs, J. Lumin. 126 (2007) 371-377. [4] H. S. Jang, Y. H. Won, D.Y. Jeon, Improvement of electroluminescent property of blue LED coated with highly luminescent yellow-emitting phosphors, Appl. Phys. B. 95 (2009) 715-720. [5] C. C. Yang, C. M. Lin, Y. J. Chen, Y. T. Wu, S. R. Chuang, et al, Highly stable three-band white light from an InGaN-based blue light emitting diode chip precoated with (oxy)nitride green/red phosphors, Appl. Phys. Lett. 90 (2007) 123503. [6] Y. H. Won, H. S. Jang, K. W. Cho, Y. S. Song, D. Y. Jeon, H. K. Kwon, Effect of phosphor geometry on the luminous efficiency of high-power white light-emitting diodes with excellent color rendering property, Opt. Lett. 34 (2009) 1-3. [7] N. Kimura, K. Sakuma, S. Hirafune, K. Asano, N. Hirosaki, R. J. Xie, Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode, Appl. Phys. Lett. 90 (2007) 051109. [8] Y. B. Chen, K. W. Cheah, M. L. Gong, Low thermal quenching and high-efficiency Ce3+, Tb3+-co-doped Ca3Sc2Si3O12 green phosphor for white light-emitting diodes. J. Lumin. 131 (2011) 1589-1593. [9] L. Chen, C. C. Lin, C. W. Yeh, R. S. Liu, Light Converting Inorganic Phosphors for White Light-Emitting Diodes, Mater. 3 (2010) 2172-2195. 12
[10] I. Baginskiy, R.S. Liu, C.L. Wang, R.T. Lin, Y.J. Yao, Temperature-dependent emission of strontium-barium orthosilicate (Sr2-xBax)SiO4:Eu2+ phosphors for high-power white light-emitting diodes, J. Electrochem. Soc. 158 (2011) 118-121. [11] Q.Y. Shao, H.Y. Lin, Y. Dong, J.Q. Jiang, Temperature-dependent photoluminescence properties of (Ba,Sr)2SiO4:Eu2+ phosphors for white LEDs applications, J. Lumin. 151 (2014) 165-169. [12] S. Yamada, H. Emotoa, M. Ibukiyama, N. Hirosaki, Properties of SiAlON powder phosphors for white LEDs, J. Eur. Ceram. Soc. 32 (2012) 1355-1358. [13] T.Y. Tien, E. F. Gibbons, R. G. Delosh, P. J. Zasmanidis, D. E. Smith, H. L. Stadler, Ce3+ activated Y3Al5O12 and some of its solid solutions, J. Electrochem. Soc. 120 (1973) 278-281. [14] R.R. Jacobs, W.F. Krupke, M.J. Weber, Measurement of excited state absorption loss for Ce3+ in Y3Al5O12 and implications for tunable 5d→4f rare earth lasers, Appl. Phys. Lett. 33 (1978) 410-412. [15] J. Ueda, S. Tanabe, T. Nakanishi, Analysis of Ce3+ luminescence quenching in solid solutions
between
Y3Al5O12
and
Y3Ga5O12
by
temperature
dependence
of
photoconductivity measurement, J. Appl. Phys. 110 (2011) 053102. [16] X. F. Song, R. L. Fu, S. Agathopoulos, H. He, X. R. Zhao, S. D. Zhang, Photoluminescence properties of Eu2+-activated CaSi2O2N2: Redshift and concentration quenching, J. Appl. Phys. 106 (2009) 033103. [17] A. A. Setlur, A. M. Srivastava, On the relationship between emission color and Ce3+ concentration in garnet phosphors, Opt. Mater. 29 (2007) 1647-1652. 13
[18] V. Bachmann, C. Ronda, O. Oeckler, W. Schnick, A. Meijerink, Color point tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+ for white light LEDs, Chem. Mater. 21 (2009) 316-325. [19] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24, 131 (1969) [20] A. Nakatsuka, A. Yoshiasa, T. Yamanaka, Acta Crystallogr. B55 (1999) 266-272.
Tables Table 1 The experimental and theoretical values of the composition of the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample prepared at 1480 °C for 4 h with 3 wt% NaF as flux. Element O Al Ga Y Ce Totals
Experimental atomic% 64.78 15.30 5.90 13.77 0.25 100
Theoretical atomic% 60.00 17.50 7.50 14.70 0.30 100
Figure Captions Fig. 1. XRD patterns of Y3-mAl3.5Ga1.5O12:mCe3+ (m = 0.03, 0.04, 0.05, 0.06, 0.07, and 0.08) phosphors obtained without flux (a) and Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors obtained with 0 ~ 5 wt% NaF as flux (b). Fig. 2. EDS result of the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample prepared at 1480 °C for 4 h with 14
3 wt% NaF as flux. Fig. 3. SEM images of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared with various amount of NaF (a) 0 wt%, (b) 1 wt%, (c) 2 wt%, (d) 3 wt%, (e) 4 wt% and (f) 5 wt%. Fig. 4. Emission spectra of Y3-mAl3.5Ga1.5O12:mCe3+ (m = 0.03, 0.04, 0.05, 0.06, 0.07, and 0.08) phosphors obtained without flux. Fig. 5. Excitation spectra (a), emission spectra (b) of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared with 0 - 5 wt% NaF. Fig. 6 Temperature-dependent luminescence intensity of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor and commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors. Fig. 7. CIE 1931 chromaticity coordinates of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor and commercial (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors at different temperatures (25 °C to 200 °C). Fig. 8. Pictures, emission spectrum and optical properties of the white LED lamp fabricated by using Y2.94Al3.5Ga1.5O12:0.06Ce3+ as a green phosphor and a commercial red phosphor.
15
Figures: Fig. 1
16
Fig. 2
17
Fig. 3
18
Fig. 4
19
Fig. 5
20
21
Fig. 6
22
Fig. 7
23
Fig. 8
24
PII: DOI: Reference:
S0022-2313(15)30313-6 http://dx.doi.org/10.1016/j.jlumin.2015.11.041 LUMIN13740
To appear in: Journal of Luminescence Received date: 28 July 2015 Revised date: 25 October 2015 Accepted date: 25 November 2015 Cite this article as: Yaochun Qiang, Yuxi Yu, Guolong Chen and Jiyu Fang, Synthesis and luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.11.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs Yaochun Qianga, Yuxi Yua*, Guolong Chenb and Jiyu Fangc a
Fujian Key Laboratory of Advanced Materials, Department of Materials Science and
Engineering, College of Materials, Xiamen University, Xiamen 361005, China b
Fujian Engineering Research Center for Solid-state Lighting, Department of Electronic
Science, Xiamen University, Xiamen 361005, China c
Advanced Materials Processing and Analysis Center, Department of Materials Science and
Engineering, University of Central Florida, Orlando, Florida 32816, USA Abstract A series of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphors were successfully synthesized by a solid-state reaction method. The microstructure, morphology, luminescence spectra, luminescence quantum yield (QY) and thermal stability of the phosphor were investigated. The critical concentration of Ce3+ ions in Y3-mAl3.5Ga1.5O12:mCe3+ is m = 0.06. The QY of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is as high as 94 % under excitation at 450 nm and its luminescence intensity at 150 °C still maintains 90 % of that measured at 25 °C, which are just a little worse than those of commercial Lu3Al5O12:Ce3+ green phosphor but much better than those of commercial (Sr,Ba)2SiO4:Eu2+ green phosphor. A white LED lamp was fabricated by employing Y2.94Al3.5Ga1.5O12:0.06Ce3+ as a green phosphor and commercial (Ca,Sr)AlSiN3:Eu2+ as a red phosphor (628 nm), its Ra value, correlated color temperature *
Corresponding author. Tel.: +865923502958. E-mail addresses: [email protected] (Y. X. Yu) 1
(CCT), CIE1931 chromaticity coordinates and luminous efficiency is 84, 3081 K, (x = 0.4369, y = 0.4142) and 102 lm/W, respectively. The experimental results demonstrate that Y2.94Al3.5Ga1.5O12:0.06Ce3+ is a promising green phosphor not only can be used for high color rendering index white LEDs but also for high-power white LEDs. Keywords: Luminescence; Phosphors; YAGG:Ce; Thermal stability; White LEDs. 1. Introduction White light-emitting diodes (LEDs) have attracted extensive attention because they have much better properties than those of conventional incandescent and fluorescent lamps, such as longer lifetime, higher efficiency, better reliability, and so on [1,2]. White LEDs are generally manufactured by combining blue LED chips and yellow-emitting cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor. The main drawback of this type white LEDs is the low color rendering index (Ra), which is only about 75 [3,4]. In order to improve the color-rendering properties of white LEDs, YAG:Ce phosphor was replaced by green and red phosphors simultaneously [5,6], which made the white LEDs show excellent color-rendering properties, the Ra value was more than 90. So green phosphor is one of the key materials for making high Ra value white LEDs. Although a large amount of green phosphors have been investigated for use in solid state lighting, only a small number of green phosphors can be employed practically for blue LED-based white
LEDs. These green phosphors include (Sr,Ba)2SiO4:Eu2+ [6],
β-SiAlON:Eu2+ [7], Ca3Sc2Si3O12:Ce3+ [8], Lu3Al5O12:Ce3+ [9]. However, it was reported that (Sr,Ba)2SiO4:Eu2+ suffers from large thermal quenching of luminescence at high temperatures [10,11], which makes it unable to be used for high-power white LEDs. β-SiAlON:Eu2+ 2
exhibits excellent thermal stability and durability, but it is difficulty to synthesize (1800 2000 °C, 0.6 - 0.9 MPa gas pressure) and its quantum efficiency is low [12]. Ca3Sc2Si3O12:Ce3+ [8] and Lu3Al5O12:Ce3+ [9] shows excellent thermal stability, but their main raw materials (Sc2O3 and Lu2O3) are rare earth oxides, which are very limited and very expensive. Therefore, it is necessary to develop alternative green phosphors for white LEDs. It has been reported that Ce3+-doped yttrium aluminum gallium garnet (YAGG:Ce) is able to emit tunable green light when excited by blue light [13], which suggests that YAGG:Ce phosphor may serve as a green phosphor for white LEDs. However, to the best of our knowledge, there are no studies that identify the Ce3+ concentration-dependent and temperature-dependent luminescence properties of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphor for white LEDs. From the viewpoint of its application, it is necessary to investigate the luminescence properties of the phosphor. In this study, a series of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphors were synthesized by a solid-state reaction method. The Ce3+ concentration-dependent luminescence properties was studied. The effect of NaF flux on microstructure and luminescence properties of the phosphors was investigated. The luminescence quantum yield and thermal stability (temperature-dependent luminescence intensity and color stability) of Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor were investigated and compared with those of commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors. In addition, the optical properties of the warm white LED lamp fabricated by combining blue LED lamp with the prepared Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor and commercial (Ca,Sr)AlSiN3:Eu2+ red phosphor (628 nm) were measured. 2. Experimental 3
2.1. Synthesis of Ce3+-doped Y3Al3.5Ga1.5O12 phosphors The samples of Y3-mAl3.5Ga1.5O12:mCe3+ (0.03 ≤ m ≤ 0.08) phosphors were synthesized from the stoichiometric mixture of Y2O3 (99.99%), Ga2O3 (99.99%), Al2O3 (99.99%) and CeO2 (99.99%), in which 0 - 5 wt% NaF (99%) was used as flux. The mixture was grounded with an agate mortar and pestle for 1 hour, and then placed in alumina crucibles, fired at 1480 °C for 4 h in a reducing atmosphere environment (5 v% H2 / 95 v% N2). After the cooling, the samples were slightly grounded with an agate mortar and pestle, washed with hot distilled water for 3 times, and then dried at 80 °C for 12 hours. 2.2. Preparation of white LED lamp A warm white LED lamp was fabricated by combining blue LED lamp (~ 5 W) with a phosphor containing silicone cap (Weight ≈ 2.8g, Thickness ≈ 0.45 mm) and a polycarbonate lampshade. The Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor (3.2 g) prepared with 3 wt% NaF and commercial (Ca,Sr)AlSiN3:Eu2+ red phosphor (0.22 g) were dispersed homogeneously into the silicone (20 g). The vacuum technique was used to remove the air bubbles in the mixture, and then the mixture was put into the molds and heated for a few seconds, several phosphor containing silicone caps were made. 2.3. Characterization The structure of Ce3+-doped Y3Al3.5Ga1.5O12 phosphors was analyzed by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) using Cu Kα1 radiation. The elemental composition of the sample was analyzed by X-ray energy dispersive spectroscopy (EDS, X-MaxN, Oxford, UK). The morphology was observed by scanning electron microscopy (SEM, TM3000, Hitachi, Japan). The luminescence spectra were measured by a fluorescence spectrometer (FLS920, 4
Edinburgh Instruments, UK) with Xe-lamp (450 W) as an excitation source. The luminescence quantum yield was measured by a fluorescence spectrometer (FLS980, Edinburgh Instruments, UK) with Xe-lamp (450 W) as an excitation source. The luminescence intensity and the Commission Internationale de L’Eclairage (CIE) 1931 chromaticity coordinates at various temperatures (25 - 200 °C) were recorded by an optical scanning spectrometer (Spectro320, Instrument Systems, Germany) with a narrow spectrum (FWHM = 10 nm) blue LED (emission peak at 458 nm) as an excitation source. A self-designed heating attachment was employed to heat samples, which mainly includes a resistive heater and a standard temperature controller (error range of 0.1 °C). The optical properties of the warm white LED lamp were measured by a LED lamp measurement system (YF1000, Everfine, China) under a forward-bias current of 60 mA at room temperature. 3. Results and discussion 3.1. Phase and composition analysis The XRD patterns of the samples (with different Ce3+ concentration) obtained without NaF are shown in Fig. 1a. The XRD patterns of the samples obtained with 0 ~ 5 wt% NaF as flux are shown in Fig. 1b. As shown in Fig. 1, the diffraction peaks of the samples obtained with no flux and with NaF as flux are matched well with those of the pure phase yttrium aluminum garnet (Y3Al5O12 JCPDS 33-0040) and no significant impurities or secondary phases are observed, which confirms that the obtained samples are single garnet phase. In order to confirm the percentage composition of the prepared microcrystal, the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample prepared at 1480 °C for 4 h with 3 wt% NaF as flux was chosen to do EDS analysis and the result is shown in Fig. 2. The result indicates that the 5
microcrystal of the sample is composed of 13.77 at% Yttrium (Y), 15.30 at% Aluminum (Al), 5.90 at% Gallium (Ga), 0.25 at% Cerium (Ce) and 64.78 at% Oxygen (O). As shown in Table 1, the experimental values and theoretical values (calculated from the chemical formula of Y2.94Ce0.06Ga1.5Al3.5O12) are very close. Combined with XRD result, EDS result demonstrates that the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample was successfully synthesized. As can be seen in Fig. 1, the diffraction peaks of the samples prepared with NaF are stronger (intensity) and sharper (shape) than those of the sample prepared without NaF, implying that NaF can promote the crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors. In addition, we note that the intensity of the diffraction peaks increases as the adding amount of NaF flux, and it reaches the maximum at 3 wt% NaF. With the further increase of NaF flux, the intensity decreases. The result reveals that too much NaF flux will have a negative effect on the crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor. We deduce that too much NaF flux may cause more crystal defects (interstitial defects, antisite defects and so on) which will worsen the luminescence properties of the phosphor. 3.2. Morphology of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors Fig. 3 shows the SEM images of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared with 0 5 wt% NaF. As seen from Fig. 3, the mean primary particle size of the sample prepared without flux is very small (less than 1μm) and the particle agglomeration is serious. It is obvious that the primary particle size of the samples was dramatically changed by adding NaF as flux. The mean primary particle size is about 2 μm, 4 μm, 5 μm, 6 μm and 7 μm for the sample prepared with 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%, respectively. In addition, we find out that the crystal faces of the samples prepared with NaF (≥ 2 wt%) are 6
distinguishable. Larger primary particle size and clear crystal faces demonstrate that the samples prepared with NaF are well-crystallized, indicating that NaF can greatly promote the growth and crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor. However, the significant over-sintering phenomenon was observed in Fig. 3e and Fig. 3f, especially in Fig. 3f, which was caused by too much NaF flux. The result indicates that too much NaF flux will worsen the dispersivity of the phosphor. 3.3. Luminescence spectra of Ce3+-doped Y3Al3.5Ga1.5O12 phosphors Fig. 4 shows the emission spectra of Y3-mAl3.5Ga1.5O12:mCe3+ (0.03 ≤ m ≤ 0.08) phosphors with various Ce3+ concentrations under the excitation wavelength of 450 nm. It is well known that the Ce3+ ion has a 4f1 configuration, the ground state consists of 2F5/2 level and 2F7/2 level. Thus, the broad emission bands of Y3-mAl3.5Ga1.5O12:mCe3+ phosphors should be ascribed to the electronic transitions of Ce3+ from the lowest 5d level (5d1) to the ground 4f levels (2F5/2 and 2F7/2) [14,15]. As can be seen, the luminescence intensity of Y3-mAl3.5Ga1.5O12:mCe3+ (0.03 ≤ m ≤ 0.08) phosphors increases with the increase of Ce3+ concentration, and it reaches the maximum at m = 0.06 (2.0 mol %). With the further increase of Ce3+ concentration (m > 0.06), the luminescence intensity decreases, which is the so-called concentration quenching of luminescence. In addition, with the increase of Ce3+ concentration, a continuous red shift of the emission spectra is observed. According to previous studies [16-18], the concentration quenching and the red shift should be mainly attributed to the energy transfer between Ce3+ ions. As suggested by Blasse [19], the critical distance Rc of energy transfer can be estimated by the equation: 7
1⁄3
3𝑉
𝑅𝑐 ≈ (4𝜋𝑥 𝑁)
(1)
𝑐
where Rc is the critical distance for energy transfer, V is the unit cell volume, and N is the number of sites per unit at which the activator atoms enter. xc is the critical concentration of Ce3+ ions. For Y3Al3.5Ga1.5O12 host, N = 24 [20]. On the basis of the XRD results showed in Fig.1a, V ≈ 1757.5 Å3. As shown in Fig. 3, the critical concentration of Ce3+ ions (xc) is 2.0 mol % (m = 0.06). Thus, according to the Eq. (2), Rc is calculated to be 19 Å. Fig. 5 shows the luminescence spectra of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared at 1480 °C for 4 h with 0 - 5 wt% NaF as flux. In Fig. 5a, two excitation bands of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors are observed, at around 345nm and 440nm, respectively. The result indicates that Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor can be effectively excited by blue LED (430 – 460 nm). The longer wavelength bands and the shorter ones should be ascribed to the electronic transitions of Ce3+ from the ground 4f level (2F5/2) to the lowest 5d level (5d1) and the second lowest 5d level (5d2) [14,15]. The broad emission bands of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor samples shown in Fig. 5b are attributed to the electronic transition of Ce3+ ions from the lowest 5d level to the ground 4f levels (5d1 → 2
F5/2,7/2)
[14,15].
As
can
be
seen
in
Fig.
5b,
the
emission
intensity
of
Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors significantly increases as the adding amount of NaF flux, and it reaches the maximum at 3 wt%. With the further increase of NaF flux, the emission intensity decreases. The variation trend is consistent with that of the XRD results in section 3.1. The result reveals that only appropriate amount of NaF flux can play a positive role in promoting the luminescence intensity. According to the XRD results in section 3.1, the decrease of the luminescence intensity should be mainly attributed to the increase of the 8
crystal defects caused by too much NaF flux. In order to evaluate the usefulness of this phosphor for white LED applications, the luminescence quantum yield of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor (prepared at 1480 °C for 4 h with 3 wt% NaF) was measured and compared with commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors for excitation at 450 nm. The luminescence quantum yield is 94 %, 82 % and 98 % for Y2.94Al3.5Ga1.5O12:0.06Ce3+, commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+, respectively. The result shows that Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor has a high luminescence quantum yield, indicating its great potential for use in white LEDs. 3.4. Thermal stability of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor It is well known that a lot of heat is generated when LED works, especially for high-power LED. Therefore, thermal stability (temperature-dependent luminescence intensity and color stability) is one of the important technological parameters for a phosphor used for white LEDs. In order to show the thermal stability of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor (prepared at 1480 °C for 4 h with 3 wt% NaF), its thermal stability was compared with that of commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors. The temperature-dependent luminescent intensity (excited at 458 nm) of the phosphors were measured at temperatures in the range of 25 - 200 °C, and the results are shown in Fig. 6. Obviously, the luminescence intensity of all samples falls as the temperature rises. The luminescence intensity of commercial (Sr,Ba)2SiO4:Eu2+ phosphor decreases much more rapidly.
At
150
°C,
the
luminescence
intensity
of
(Sr,Ba)2SiO4:Eu2+,
Y2.94Al3.5Ga1.5O12:0.06Ce3+ and Lu3Al5O12:Ce3+ maintains 79%, 90% and 93% of that 9
measured at 25 °C, respectively. It is easy to find out that the thermal quenching of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is only a little larger than that of Lu3Al5O12:Ce3+ phosphor but much smaller than that of (Sr,Ba)2SiO4:Eu2+ phosphor. Fig. 7 shows the CIE 1931 chromaticity coordinates of (Sr,Ba)2SiO4:Eu2+, Y2.94Al3.5Ga1.5O12:0.06Ce3+ and Lu3Al5O12:Ce3+ at various temperatures (25 - 200 °C). With the temperature increasing from 25 °C to 200 °C, the CIE1931 chromaticity coordinates of (Sr,Ba)2SiO4:Eu2+, Y2.94Al3.5Ga1.5O12:0.06Ce3+ and Lu3Al5O12:Ce3+ change from (0.2695, 0.6367) to (0.2692, 0.6062), (0.3854, 0.5671) to (0.3918, 0.5602) and (0.3592, 0.5766) to (0.3673, 0.5726), respectively. It is easy to find out that the change of commercial (Sr,Ba)2SiO4:Eu2+ phosphor is the largest and the change of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is as small as that of the commercial Lu3Al5O12:Ce3+ phosphor. The results reveal that the color stability Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor is very good. All the thermal testing results demonstrate that Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor has excellent thermal stability, which indicates its great potential for use in high-power white LEDs as a green phosphor. 3.5. Application of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor in White LED lamp Fig. 8 presents the pictures, emission spectrum and optical properties of the warm white-LED lamp using Y2.94Al3.5Ga1.5O12:0.06Ce3+ as a green phosphor and commercial (Ca,Sr)AlSiN3:Eu2+ as a red phosphor. As can be seen, the Ra value of the white LED lamp is 84, its correlated color temperature (CCT) is 3081 K, its CIE1931 chromaticity coordinates is (x = 0.4369, y = 0.4142) and its luminous efficiency is 102 lm/W. The Ra value of the white LED lamp (84) is much higher than that of the traditional white LEDs (~ 75) prepared by combining
blue
LEDs
with
YAG:Ce 10
phosphor
[3,4],
which
demonstrates
Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor can be employed as a green phosphor for high Ra value white LEDs. 4. Conclusions A series of Ce3+-doped Y3Al3.5Ga1.5O12 green phosphors were successfully synthesized by a
solid-state
reaction
method.
The
critical
concentration
of
Ce3+
ions
in
Y3-mAl3.5Ga1.5O12:mCe3+ is m = 0.06. NaF is a very good flux, which can dramatically promote the growth and crystallization of Y2.94Al3.5Ga1.5O12:0.06Ce3+ green phosphor. The luminescence quantum yield and thermal stability of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor are just a little worse than those of commercial Lu3Al5O12:Ce3+ green phosphor but much better than those of commercial (Sr,Ba)2SiO4:Eu2+ green phosphor. The Ra value of the white LED lamp prepared by using Y2.94Al3.5Ga1.5O12:0.06Ce3+ as green phosphor is as high as 84. All the results demonstrate Y2.94Al3.5Ga1.5O12:0.06Ce3+ is a promising green phosphor not only can be used for high color rendering index white LEDs but also for high-power white LEDs. Acknowledgments This work was supported by the Natural Science Foundation of China (51175444), the Aviation Science Foundation of China (2013ZD68009), New Century Excellent Talents in Fujian Province University (2013), Natural Science Foundation of Fujian Province of China (2014J01206), Xiamen Municipal Bureau of Science and Technology (3502Z20143009) and Scientific and Technological Innovation Platform of Fujian Province of China (2014H2006). References [1] E. F. Schubert, J. K. Kim, Solid-State Light Sources Getting Smart, Science 308 (2005) 1274-1278. 11
[2] Y. Narukawa, M. Ichikawa, D. Sanga, M. Sano, T. Mukai, White light emitting diodes with super-high luminous efficacy, J. Phys. D: Appl. Phys. 43 (2010) 354002. [3] H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon, S. S. Kim, Enhancement of red spectral emission intensity of Y3Al5O12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs, J. Lumin. 126 (2007) 371-377. [4] H. S. Jang, Y. H. Won, D.Y. Jeon, Improvement of electroluminescent property of blue LED coated with highly luminescent yellow-emitting phosphors, Appl. Phys. B. 95 (2009) 715-720. [5] C. C. Yang, C. M. Lin, Y. J. Chen, Y. T. Wu, S. R. Chuang, et al, Highly stable three-band white light from an InGaN-based blue light emitting diode chip precoated with (oxy)nitride green/red phosphors, Appl. Phys. Lett. 90 (2007) 123503. [6] Y. H. Won, H. S. Jang, K. W. Cho, Y. S. Song, D. Y. Jeon, H. K. Kwon, Effect of phosphor geometry on the luminous efficiency of high-power white light-emitting diodes with excellent color rendering property, Opt. Lett. 34 (2009) 1-3. [7] N. Kimura, K. Sakuma, S. Hirafune, K. Asano, N. Hirosaki, R. J. Xie, Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode, Appl. Phys. Lett. 90 (2007) 051109. [8] Y. B. Chen, K. W. Cheah, M. L. Gong, Low thermal quenching and high-efficiency Ce3+, Tb3+-co-doped Ca3Sc2Si3O12 green phosphor for white light-emitting diodes. J. Lumin. 131 (2011) 1589-1593. [9] L. Chen, C. C. Lin, C. W. Yeh, R. S. Liu, Light Converting Inorganic Phosphors for White Light-Emitting Diodes, Mater. 3 (2010) 2172-2195. 12
[10] I. Baginskiy, R.S. Liu, C.L. Wang, R.T. Lin, Y.J. Yao, Temperature-dependent emission of strontium-barium orthosilicate (Sr2-xBax)SiO4:Eu2+ phosphors for high-power white light-emitting diodes, J. Electrochem. Soc. 158 (2011) 118-121. [11] Q.Y. Shao, H.Y. Lin, Y. Dong, J.Q. Jiang, Temperature-dependent photoluminescence properties of (Ba,Sr)2SiO4:Eu2+ phosphors for white LEDs applications, J. Lumin. 151 (2014) 165-169. [12] S. Yamada, H. Emotoa, M. Ibukiyama, N. Hirosaki, Properties of SiAlON powder phosphors for white LEDs, J. Eur. Ceram. Soc. 32 (2012) 1355-1358. [13] T.Y. Tien, E. F. Gibbons, R. G. Delosh, P. J. Zasmanidis, D. E. Smith, H. L. Stadler, Ce3+ activated Y3Al5O12 and some of its solid solutions, J. Electrochem. Soc. 120 (1973) 278-281. [14] R.R. Jacobs, W.F. Krupke, M.J. Weber, Measurement of excited state absorption loss for Ce3+ in Y3Al5O12 and implications for tunable 5d→4f rare earth lasers, Appl. Phys. Lett. 33 (1978) 410-412. [15] J. Ueda, S. Tanabe, T. Nakanishi, Analysis of Ce3+ luminescence quenching in solid solutions
between
Y3Al5O12
and
Y3Ga5O12
by
temperature
dependence
of
photoconductivity measurement, J. Appl. Phys. 110 (2011) 053102. [16] X. F. Song, R. L. Fu, S. Agathopoulos, H. He, X. R. Zhao, S. D. Zhang, Photoluminescence properties of Eu2+-activated CaSi2O2N2: Redshift and concentration quenching, J. Appl. Phys. 106 (2009) 033103. [17] A. A. Setlur, A. M. Srivastava, On the relationship between emission color and Ce3+ concentration in garnet phosphors, Opt. Mater. 29 (2007) 1647-1652. 13
[18] V. Bachmann, C. Ronda, O. Oeckler, W. Schnick, A. Meijerink, Color point tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+ for white light LEDs, Chem. Mater. 21 (2009) 316-325. [19] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24, 131 (1969) [20] A. Nakatsuka, A. Yoshiasa, T. Yamanaka, Acta Crystallogr. B55 (1999) 266-272.
Tables Table 1 The experimental and theoretical values of the composition of the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample prepared at 1480 °C for 4 h with 3 wt% NaF as flux. Element O Al Ga Y Ce Totals
Experimental atomic% 64.78 15.30 5.90 13.77 0.25 100
Theoretical atomic% 60.00 17.50 7.50 14.70 0.30 100
Figure Captions Fig. 1. XRD patterns of Y3-mAl3.5Ga1.5O12:mCe3+ (m = 0.03, 0.04, 0.05, 0.06, 0.07, and 0.08) phosphors obtained without flux (a) and Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors obtained with 0 ~ 5 wt% NaF as flux (b). Fig. 2. EDS result of the Y2.94Al3.5Ga1.5O12:0.06Ce3+ sample prepared at 1480 °C for 4 h with 14
3 wt% NaF as flux. Fig. 3. SEM images of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared with various amount of NaF (a) 0 wt%, (b) 1 wt%, (c) 2 wt%, (d) 3 wt%, (e) 4 wt% and (f) 5 wt%. Fig. 4. Emission spectra of Y3-mAl3.5Ga1.5O12:mCe3+ (m = 0.03, 0.04, 0.05, 0.06, 0.07, and 0.08) phosphors obtained without flux. Fig. 5. Excitation spectra (a), emission spectra (b) of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphors prepared with 0 - 5 wt% NaF. Fig. 6 Temperature-dependent luminescence intensity of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor and commercially available (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors. Fig. 7. CIE 1931 chromaticity coordinates of Y2.94Al3.5Ga1.5O12:0.06Ce3+ phosphor and commercial (Sr,Ba)2SiO4:Eu2+ and Lu3Al5O12:Ce3+ green phosphors at different temperatures (25 °C to 200 °C). Fig. 8. Pictures, emission spectrum and optical properties of the white LED lamp fabricated by using Y2.94Al3.5Ga1.5O12:0.06Ce3+ as a green phosphor and a commercial red phosphor.
15
Figures: Fig. 1
16
Fig. 2
17
Fig. 3
18
Fig. 4
19
Fig. 5
20
21
Fig. 6
22
Fig. 7
23
Fig. 8
24