High-brightness and high-color purity red-emitting ... - OSA Publishing

Letter

Vol. 43, No. 6 / 15 March 2018 / Optics Letters

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High-brightness and high-color purity red-emitting Ca3LuAlO3BO34:Eu3 phosphors with internal quantum efficiency close to unity for near-ultraviolet-based white-light-emitting diodes XIAOYONG HUANG,*

SHAOYING WANG, BIN LI, QI SUN,

AND

HENG GUO

Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China *Corresponding author: [email protected] Received 11 January 2018; revised 30 January 2018; accepted 4 February 2018; posted 13 February 2018 (Doc. ID 319439); published 12 March 2018

In this work, we reported on high-brightness Eu3 -activated Ca 3 LuAlO3 BO3 4 (CLAB) red-emitting phosphors. Under 397 nm excitation, the CLAB:Eu3 phosphors showed intense red emissions at around 621 nm with CIE coordinates of (0.657, 0.343). The optimal doping concentration of Eu3 ions was found to be 30 mol. %, and the CLAB:0.3Eu3 sample possessed high-color purity of 93% and ultra-high internal quantum efficiency as great as 98.5%. Importantly, the CLAB:0.3Eu3 also had good thermal stability. Finally, a white-light-emitting diode (WLED) lamp with good colorrendering index was fabricated by using a 365 nm ultraviolet chip and the phosphor blends of CLAB:0.3Eu3 red-emitting phosphors, Ba;Sr2 SiO4 ∶Eu2 green-emitting phosphors, and BaMgAl10 O7 ∶Eu2 blue-emitting phosphors. © 2018 Optical Society of America OCIS codes: (160.4670) Optical materials; (160.5690) Rare-earthdoped materials; (250.5230) Photoluminescence. https://doi.org/10.1364/OL.43.001307

Recently, solid-state lighting-based phosphors-converted whitelight-emitting diodes (WLEDs) have been considered as the promising next-generation light source to replace conventional incandescent and fluorescent lamps due to their outstanding energy efficiency, long operation lifetime, and environment friendliness [1,2]. At present, the widely used approach to realize WLEDs is the combination of blue LED chips and yellowemitting YAG:Ce3 phosphors. However, the WLEDs obtained show a high correlated color temperature (CCT ≈ 7750 K) and a poor color-rendering index (CRI ≈ 70–80), which results from the lack of red spectral contribution [3,4]. Just recently, an alternative approach for fabricating WLEDs, which couples nearultraviolet (near-ultraviolet [UV], 350–420 nm) chips with green, blue, and red tricolor phosphors, has gained considerable 0146-9592/18/061307-04 Journal © 2018 Optical Society of America

attention [5]. The widely used red phosphors for near-UV-based WLEDs are Y 2 O2 S:Eu3 and some nitrides and oxynitridesbased compounds [such as CaAlSiN3 :Eu2 , M2 Si5 N8 :Eu2 (M Ca, Sr, Ba)] [6,7]. However, Y 2 O2 S:Eu3 are chemically unstable and have relatively low luminescence efficiency [8]. Nitrides and oxynitrides with good thermal stability, however, are very expensive because they require the fastidious synthesis procedures including high nitrogen pressure and ultra-high temperature. Hence, it is urgent to exploit near-UV-excitable oxidebased red-emitting phosphors with high brightness and good stability, as well as low cost. Eu3 ions have been well known for their efficient red emissions due to the 5 D2 − 7 F J (J 0 − 4) transitions [9,10]. On the other hand, selecting a suitable host material is very important to get high-efficiency inorganic phosphors. Borates proved to be an outstanding class of host materials for optical materials. Recently, several groups reported the luminescent properties of rare-earth-ions doped Ca3 YAlO3 BO3 4 (CYAB) phosphors [11,12]. Huang explored Eu3 -activated CYAB phosphors as a red component for the white-light fluorescent lamps under 254 nm excitation [12]. In this Letter, we reported novel high-brightness red-emitting Eu3 -activated Ca3 LuAlO3 BO3 4 (CLAB) phosphors with color purity as high as 93%. Under 397 nm excitation, CLAB:Eu3 exhibited intense red emissions peaking at 621 nm with CIE coordinates of (x 0.657, y 0.343). The optimal doping concentration of Eu3 ions was 30 mol. %, and the internal quantum efficiency (IQE) of the CLAB:0.3Eu3 sample reached up to 98.5%. The phosphors also had good thermal stability. All the results demonstrated that the CLAB:0.3Eu3 phosphor could be an eligible red-emitting phosphor candidate for WLEDs. CLAB:Eu3 phosphors were fabricated via a high-temperature solid-state reaction technique. Raw materials of H3 BO3 (analytical reagent), CaCO3 (analytical reagent), Lu2 O3 (99.99%), H3 BO3

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(analytical reagent), Al2 O3 (analytical reagent), and Eu2 O3 (99.99%) were weighted and ground in an agate mortar, and then the obtained mixtures were put in the alumina crucibles and sintered at 1100°C for 4 h. The final products were ground and collected for further characterization. The x-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 x-ray diffractometer using Cu Kα radiation. The room-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra and luminescence decay lifetimes of phosphors were measured by an Edinburgh FS5 spectrometer equipped with a 150 W continued-wavelength Xenon lamp and a pulsed Xenon lamp, respectively. Temperature-dependent PL spectra were recorded by using an Edinburgh FS5 spectrometer with a temperature controller. The IQE of the phosphor sample was measured on an Edinburgh FS5 spectrometer equipped with an integrating sphere coated with BaSO4 . Figure 1 shows the XRD patterns of CLAB:0.3Eu3 and CLAB:1.0Eu3 phosphors. All the patterns are in good agreement with the standard data of CYAB (ICSD-172154), indicating that the as-prepared CLAB:Eu3 compounds were pure single phase. Since the radii of Lu3 (0.848 Å) and Eu3 ions (0.950 Å) are very close [13], Lu3 ions can be readily replaced by Eu3 ions and the crystal structure just slightly changes. Figure 2 presents the PLE and PL spectra of the CLAB:0.3Eu3 phosphors. When monitored at 621 nm, the PLE spectrum consisted of a broad PLE band in the 230– 310 nm wavelength range with a maximum at 262 nm, which was attributed to O2− -Eu3 charge transfer band (CTB) and a series of sharp peaks at 319 nm (7 F 0 → 5 H 5 transition), 362 nm (7 F 0 → 5 D4 transition), 377 nm (7 F 0 → 5 G 4 transition), 384 nm (7 F 0 → 5 L7 transition), 397 nm (7 F 0 → 5 L6 transition), 413 nm (7 F 0 → 5 D3 transition), and 464 nm (7 F 0 → 5 D2 transition) [5,10]. Among the characteristic 4f − 4f transitions of Eu3 , the strongest PLE peak at 397 nm matched well with the emission wavelength of near-UV LED chips, indicating that the CLAB:0.3Eu3 phosphors would be efficiently excited by near-UV light. Under 397 nm excitation, the CLAB:0.3Eu3 sample exhibited bright-red emissions, as shown in the inset of Fig. 2. The PL spectrum of CLAB:0.3Eu3 mainly contained the characteristic emission peaks of Eu3 ions due to the 5 D0 → 7 F J (J 0, 1, 2, 3, 4) transitions, namely 588 nm (5 D0 → 7 F 0 transition), 598 nm (5 D0 → 7 F 1 transition), 621 nm (transition), 648 nm

(5 D0 → 7 F 3 transition), and 700 nm (5 D0 → 7 F 4 transition) [5,10]. The most intense emission peak was located at 621 nm, corresponding to the electric dipole transition 5 D0 → 7 F 2 , which indicated that the local symmetry of Eu3 site belongs to non-inversion symmetry in CLAB host. The existence of a non-inversion center around the Eu3 ions is beneficial to obtain red phosphors with high-color purity [10,14]. Figure 3(a) shows the PL spectra of CLAB:xEu3 (x 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) phosphors under 397 nm excitation. All samples exhibited intense red emissions corresponding to 5 D0 → 7 F J (J 0, 1, 2, 3, 4) transitions, and their emission intensities were strongly dependent on the Eu3 concentrations. The optimal doping concentration was 30 mol. %, whereas the red emission intensity decreased rapidly once x was larger than 0.3, owing to the concentration quenching effect. The high concentration quenching threshold in CLAB:xEu3 phosphors is beneficial for achieving high luminescence efficiency. Besides, the critical average distance between Eu3 ions in the CLAB host lattice can be roughly evaluated by using the following equation [15]: 1∕3 3V ; (1) Rc 2 4πx c Z

Fig. 1. XRD patterns of CLAB:xEu3 (x 0.1 and 1.0) phosphors. The standard data of CYAB (ICSD-172154) were also shown as references.

Fig. 2. PLE and PL spectra of CLAB:0.3Eu3 phosphors. Inset shows the photograph of CLAB:0.3Eu3 sample under a 365 nm UV lamp.

where R c is the critical distance, V is the volume of the unit cell, x c refers to the critical doping concentration, and Z is the number of formula units per unit cell. In this Letter, the x c 0.3; V 556.94 Å 3 ; Z 2. Therefore, the critical average distance for Eu3 ions in CLAB:xEu3 was determined to be about 12.1 Å. Generally, the nonradiative energy transfer among the activators can take place via either exchange, interaction, or electric multipole interaction. The exchange interaction requires the critical distance to be smaller than 5 Å, and thus electric multipole interaction was responsible for the nonradiative energy transfer in CLAB:xEu3 phosphors [5]. In order to figure out the mechanism of aforementioned nonradiative energy transfer, the relation between logI ∕x and logx can be calculated by the following equation [16]: logI ∕x A − θ∕3 log x;

(2)

Letter

Fig. 3. (a) PL spectra of CLAB:xEu3 (x 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) phosphors under 397 nm excitation. The inset shows the integrated PL intensity of CLAB:xEu3 as a function of Eu3 doping concentration. (b) Plot of logI ∕x versus logx for the 621 nm emission of Eu3 ions in CLAB:xEu3 phosphors pumped at 397 nm.


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to be 2.022, 1.919, 1.765, 1.520, 1.261, and 1.092 ms with increasing contents of Eu3 ions from x 0.1 to x 1. The decreased lifetimes of CLAB:xEu3 with increasing Eu3 concentrations were attributed to the rise of the nonradiative energy transfer among Eu3 ions. Based on the PL spectrum in Fig. 2, the CIE chromaticity coordinate of CLAB:0.3Eu3 was determined to be (0.657, 0.343), which was very close to the ideal red light (0.670, 0.330). Importantly, the CIE chromaticity coordinate of CLAB:0.3Eu3 phosphors was better than that of the commercial phosphors Y 2 O2 S:Eu3 (0.622, 0.351) [10]. Furthermore, the color purity of CLAB:0.3Eu3 phosphors was calculated to be 93%. In order to further investigate the luminescence performance of the CLAB:0.3Eu3 sample, we measured its IQE. Under 397 nm excitation, the IQE value of CLAB:0.3Eu3 reached as great as 98.5%, which was much higher than the commercial phosphors Y 2 O2 S:Eu3 (IQE: 35%) [17]. The external QE (EQE) of the CLAB:0.3Eu3 sample was determined to be about 29.1%, which could be further improved by optimizing the synthesis procedure. Figure 5(a) presents the temperature-dependent PL spectra of the CLAB:0.3Eu3 sample under 397 nm excitation. It can be seen that with increasing temperature from 303 to 523 K, the PL spectra almost maintained the same profiles, while the PL intensity of CLAB:0.3Eu3 decreased due to thermal quenching. The PL intensity at 423 K was about 67% of that at 303 K, and thus CLAB:0.3Eu3 phosphors possessed

where I is the emission intensity, x is the dopant concentration, A is concentration, and θ 6, 8, and 10 correspond to electric dipole–dipole, dipole–quadrupole, and quadrupole– quadrupole interaction, respectively. As demonstrated in Fig. 3(b), the relation between the logI ∕x and logx was linear and the slope was fitted to be approximately −1.293. Hence, the θ value was determined to be 3.879, which was close to 6. Consequently, the mechanism of nonradiative energy transfer among the Eu3 ions in the CLAB host was dominated by dipole–dipole interaction. Figure 4 shows the decay curves of all the CLAB:xEu3 samples. The obtained average decay lifetimes are determined

Fig. 4. Decay curves of the Eu3 621 nm emission in CLAB:xEu3 phosphors under 397 nm excitation.

Fig. 5. (a) Temperature-dependent PL spectra of CLAB:0.3Eu3 phosphor excited at 397 nm. The inset shows normalized PL emission intensity of CLAB:0.3Eu3 sample as a function of temperature. (b) Plot of lnI 0 ∕I − 1 versus 1∕kT of the CLAB:0.3Eu3 sample.

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Letter In summary, we reported a high-brightness red-emitting CLAB:Eu3 phosphor. Under 397 nm excitation, CLAB:Eu3 exhibited intense red emissions with optimal doping concentration of 30 mol. %. The CIE chromaticity coordinates of CLAB:0.3Eu3 were (0.657, 0.343), and the color purity was calculated as high as 93%. Importantly, the IQE of CLAB:0.3Eu3 phosphors reached up to 98.5%. Moreover, this phosphor had good thermal stability with the activation energy of 0.315 eV. Finally, a prototype WLED lamp was fabricated by using 395 nm near-UV-emitting LED chips and tricolor phosphor blends (blue-emitting BAM:Eu2 phosphors, green-emitting Ba; Sr2 SiO4 :Eu2 phosphors, red-emitting CLAB:0.3Eu3 phosphors). These results demonstrated the promising prospect of CLAB:Eu3 as red phosphor for application in WLEDs.

Fig. 6. EL spectrum of the fabricated WLED lamp with 395 nm near-UV LED chip and BAM:Eu2 (BaMgAl10 O7 :Eu2 ) blueemitting phosphors, Ba; Sr2 SiO4 :Eu2 green-emitting phosphors, and CLAB:0.3Eu3 red-emitting phosphors driven by 20 mA current.

good thermal stability and they are suitable for application in WLEDs. The modified Arrhenius equation was then used to fit the thermal quenching data for activation energy calculation [10]: −1 IT E 1 C exp a ; (3) I0 kT where I 0 is the initial emission intensity, I T is the intensity at temperature, T , E a is the activation energy, C is a constant for a certain host, and k is the Boltzmann constant, respectively. Figure 5(b) shows the plot of lnI 0 ∕I − 1 versus 1∕kT, and the experimental data can be linear-fitted with a slope of −0.315. Thus, the activation energy of thermal quenching was 0.315 eV, which is higher than some previous Eu3 -activated red phosphors, such as BaZrGe3 O9 :Eu3 (0.175 eV) [5], CaW 0.4 Mo0.6 O4 :Eu3 (0.239 eV) [10], and K 2 Tb0.5 Eu0.5 PO4 WO4 (0.19 eV) [17]. In order to evaluate the potential application of CLAB:Eu3 phosphors, a WLED lamp was fabricated by utilizing a nearUV-emitting LED chip (395 nm) and BaMgAl10 O7 :Eu2 blue-emitting phosphor, Ba; Sr2 SiO4 :Eu2 green-emitting phosphor, and CLAB:0.3Eu3 red-emitting phosphor. Figure 6 shows the electroluminescence (EL) spectrum of the fabricated WLED device under the driven current of 20 mA. Bright white light was observed, and the CIE chromaticity coordinates CCT, CRI, and luminous efficiency were measured to be (0.304, 0.339), 6898 K, 81, and 30.89 lm/W, respectively.

Funding. National Natural Science Foundation of China (NSFC) (51502190); Open Fund of the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology (SCUT) (2017-skllmd-01). REFERENCES 1. X. Huang, Nat. Photonics 8, 748 (2014). 2. W. Dai, Y. Lei, M. Xu, P. Zhao, Z. Zhang, and J. Zhou, Sci. Rep. 7, 12872 (2017). 3. C.-W. Yeh, W.-T. Chen, R.-S. Liu, S.-F. Hu, H.-S. Sheu, J.-M. Chen, and H. T. Hintzen, J. Am. Chem. Soc. 134, 14108 (2012). 4. W. Dai, Y. Lei, J. Zhou, Y. Zhao, Y. Zheng, M. Xu, S. Wang, and F. Shen, J. Am. Ceram. Soc. 100, 5174 (2017). 5. Q. Zhang, X. Wang, X. Ding, and Y. Wang, Inorg. Chem. 56, 6990 (2017). 6. R.-J. Xie, N. Hirosaki, Y. Li, and T. Takeda, Materials 3, 3777 (2010). 7. P. Pust, V. Weiler, C. Hecht, A. Tücks, A. S. Wochnik, A.-K. Henß, D. Wiechert, C. Scheu, P. J. Schmidt, and W. Schnick, Nat. Mater. 13, 891 (2014). 8. Y. Huang, Y. Nakai, T. Tsuboi, and H. J. Seo, Opt. Express 19, 6303 (2011). 9. P. Du, X. Huang, and J. S. Yu, Chem. Eng. J. 337, 91 (2018). 10. X. Huang, B. Li, H. Guo, and D. Chen, Dyes Pigm. 143, 86 (2017). 11. C.-H. Huang and T.-M. Chen, J. Phys. Chem. C 115, 2349 (2011). 12. C.-H. Huang, T.-W. Kuo, and T.-M. Chen, Opt. Express 19, A1 (2011). 13. F. Xie, Z. Dong, D. Wen, J. Yan, J. Shi, J. Shi, and M. Wu, Ceram. Int. 41, 9610 (2015). 14. J. Zhong, D. Chen, W. Zhao, Y. Zhou, H. Yu, L. Chen, and Z. Ji, J. Mater. Chem. C 3, 4500 (2015). 15. G. Blasse, Phys. Lett. A 28, 444 (1968). 16. D. L. Dexter, J. Chem. Phys. 21, 836 (1953). 17. D. Wen, J. Feng, J. Li, J. Shi, M. Wu, and Q. Su, J. Mater. Chem. C 3, 2107 (2015).