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Sm3+) phosphors with double-perovskite structures
Optical Materials 98 (2019) 109428
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Luminescence properties of high thermal stability Sr2LaNbO6: xLn3þ(Ln3þ¼Eu3þ/Sm3þ) phosphors with double-perovskite structures GengQiao Hu, Shuangping Yi *, ZhiXiong Fang, Zhengfa Hu, Weiren Zhao School of Physics and Optoelectronic Engineering, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou, Guangdong, 510006, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Niobate W-LED Thermal stability Double-perovskite
A series of novel single-phased niobate phosphors Sr2LaNbO6:xLn3þ(Ln3þ ¼ Eu3þ/Sm3þ) (SLNO:Ln3þ) with red light emission was successfully synthesized by a conventional solid-state method and characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and photoluminescence. The phosphors showed high thermal stabilities, and at 423 K they still retained 60.7% and 80.7% emission efficiencies for the Eu3þ and Sm3þ variants, respectively. There is a difference in decay time between Eu3þ and Sm3þ due to the energy transit in different energy levels. The crystalline structure was described as a double-perovskite structure with several hexa-coordinated sites. The synthesized Eu3þ phosphor had more favorable characteristics for LEDs and a pro totype device was fabricated with blue-green commercial LED phosphors, Sr2LaNbO6:0.13Eu3þ, and a 395 nmemitting InGaN chip, which exhibited warm white light, with a color temperature of 4833 K. The current study demonstrates that Sr2LaNbO6:0.13Eu3þ can be a potent red phosphor in LED applications.
1. Introduction In recent decades, luminescent materials based on rare earth ions have been widely used for applications including lighting [1–3], dis plays [4,5], solar cells [6,7], sensors [8–10], and bioimaging [11,12]. White-light-emitting diodes (W-LEDs) are especially popular as light sources due to their long service life and environmental friendliness. Currently, commercial white LEDs are generally fabricated by combining Y3Al5O12:Ce3þ yellow-emitting phosphors excited by blue chips. Thus, high correlated color temperature (Tc > 4500 K), which is not ideal for indoor lighting applications. Niobate [13–15] is a matrix raw material that has emerged in recent years, and it has received attention because of its luminescent, electro-optical, piezoelectric, photo-elastic, and nonlinear properties together with chemical stability. Hydrothermal processing has been shown to be one of the most important fabrication methods for niobate. The rare earth ions Eu3þ and Sm3þ are usually used as the red lumi nescence centers in phosphors [16,17] owing to their abundant elec tronic energy levels with 4G65/2 → H7/2(Sm3þ) and 5D70→F2(Eu3þ). In this work, the Sr2La1-xNbO6:xLn3þ(Ln3þ ¼ Eu3þ/Sm3þ) red-emitting phos phors were prepared by a traditional solid-state reaction method heated at 1200 � C for 3 h, and the crystal structure, decay time, and lumines cence spectrum were measured. By comparing the intensities of several
(Ln3þ ¼ Eu3þ/Sm3þ) concentrations of doped SLNO, we show that SLNO:xEu3þ and SLNO:xSm3þ can achieve a maximum performance at Eu3þ(x ¼ 0.13) and Sm3þ(x ¼ 0.07), respectively. We compared the thermal stabilities and internal quantum efficiencies of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ and determined that SLNO:0.13Eu3þ is more suit able for W-LED applications [14]. By combining the Sr2LaNbO6:0.13Eu3þ phosphor and commercial green phosphors with a 395 nm LED chip, a warm white LED with a color temperature of 4833 K and CRI(Color rendering index) is 72.9 was produced, which demon strated that the niobate had potential in LED applications. 2. Experimental procedure The Eu3þ/Sm3þ ion doped Sr2LaNbO6 was synthesized via conven tional high temperature solid-state reaction method. Original materials were Sm2O3(99.9%), Eu2O3(99.9%), La2O3(99.9%), Nb2O5(99.9%), and Sr2O3(99.9%), which were weighed in a proper stoichiometric ratio. After thoroughly mixing and milling, the samples were heated to 1200 � C and preserved for 5 h. Finally, all the as-obtained samples were furnace-cooled tond retained as fine powders for further measurements.
* Corresponding author. E-mail address: [email protected] (S. Yi). https://doi.org/10.1016/j.optmat.2019.109428 Received 11 August 2019; Received in revised form 23 September 2019; Accepted 27 September 2019 Available online 14 October 2019 0925-3467/© 2019 Published by Elsevier B.V.
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Optical Materials 98 (2019) 109428
around the host and the dopant ions should not be greater than 30%. When Eu3þ and Sm3þ ions replace La3þ ions, the ratio can be calculated by the following formula [20]: Dr ¼
RmðCNÞ RdðCNÞ RmðCNÞ
(1)
where Rm(CN) and Rd(CN) represent the radius of the host ions and doped ions, respectively; and CN is the coordination number. The per centage of Eu3þ ions is 13.8%, and percentage of Sm3þ ions is 7.2%. Both results are less than 30%, thus, the La3þ sites are substituted by the Eu3þ and Sm3þ in the lattice. 3.2. XRD characterization and X-ray photoelectron spectroscopy (XPS) The synthesized phosphor powder was analyzed by x-ray diffraction (XRD), using Cu kα radiation, which affirmed the phases of Sr2La0.87NbO6:0.13Eu3þ and Sr2La0.93NbO6:0.07Sm3þ. Fig. 2a and b shows that the results agree well with those of the Joint Committee on Powder Diffraction Standards (JCPDS) for Card No. 47–0461, which is consistent with the double-perovskite structure Sr2LaNbO6 with a space group of 225 and no diffraction peak corresponding to any impurity phase. These results indicate that phases of Sr2LaNbO6 were obtained. The results of X-ray photoelectron spectroscopy on SLNO:0.07Sm3þ and SLNO 0.13Eu3þ are shown in Fig. 2c and d, respectively. The signals of Eu3d and Sm3d were only weakly detected, because of their lower concentrations relative to those of other elements. Peaks at (3d, 1201 eV) and (3d, 1040 eV) represent the binding energies of Eu3d and Sm3d, respectively. Furthermore, the binding energy data (calibrated using C (1 s, 284.05 eV) as the reference) from SLNO:Ln3þ (x ¼ Eu/Sm) were consistent with the previous reported data for niobates. The peaks at approximately 530 eV, 132.6 eV, 205.85 eV, and 854.7 eV repre sented the binding energies of O(1s), Sr(3d), Nb(3d), and La(3d), respectively.
Fig. 1. Projection view of crystal structure of Sr2LaNbO6 unit cell and the co ordination environment of each ion.
3. Results and discussion 3.1. Crystal structure The double-perovskite crystal structure [18,19] of Sr2LaNbO6 is shown in Fig. 1 with lattice constants (a ¼ b ¼ c ¼ 8.324 Å, V ¼ 576.76 Å3, and Z ¼ 4.). The unit cell structure allowed conservation of a number of octahedral sites (NbO6). The different element effective ion sizes are as follows: Sr2þ (CN ¼ 12, 1.44 Å), La3þ (CN ¼ 6, 1.032 Å), Nb5þ (CN ¼ 6, 0.64 Å), Eu3þ (CN ¼ 6, 0.89 Å), and Sm3þ (CN ¼ 6, 0.958 Å). By observation and comparison we see that because the ionic radius of La3þ is close to those of Eu3þ and Sm3þ, which tend to occupy the La3þ site. The acceptable difference percentage of the ionic radius
Fig. 2. (a) XRD patterns of Sr2La0.87NbO6:0.13Eu3þ; (b) XRD patterns of Sr2La0.93NbO6:0.07Sm3þ; (c) XPS spectrum of SLNO:0.13Eu3þ phosphor; (d)XPS spectrum of SLNO:0.07Sm3þ phosphor. 2
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Fig. 3. (a) Excitation and emission spectrum of SLNO:0.13Eu3þ and SLNO: 0.07Sm3þ; (b) Excitation spectrum of SLNO:xEu3þ (x ¼ 0.01, 0.04, 0.07, 0.10, 0.13, 0.16) monitored at 612 nm; (c) Emission spectrum of SLNO:xEu3þ excited at 394 nm; (d) Excitation spectrum of SLNO:xSm3þ (x ¼ 0.01, 0.04, 0.07, 0.10, 0.13); (e) Emission spectrum of SLNO:xSm3þ excited at 405 nm.
Fig. 4. (a) Decay curves of SLNO:0.13Eu3þ at room temperature (λex ¼ 394 nm, λem ¼ 612 nm); (b) Temperature-dependent emission spectra of SLNO:0.13Eu3þ phosphor in the range of 300–510 K (λex ¼ 394 nm); (c) Decay curves of SLNO:0.07Sm3þ at room temperature (λex ¼ 405 nm, λem ¼ 605 nm); (d) Temperaturedependent emission spectra of SLNO:0.07Sm3þ phosphor in the range of 300–510 K (λex ¼ 405 nm); (e) CIE for SLNO:0.13Eu3þ and SLNO:0.07Sm3þ phosphors.
4. Photoluminescence properties
photoluminescence reaches optimal concentration at 0.13 mol. Continuing to add Eu3þ, the photoluminescence soon declines caused by concentration quenching. The critical transfer distance (Rc) can be calculated using [4]:
4.1. Photoluminescence properties of Sr2La1-xNbO6:xEu3þ The luminescence process of the Sr2LaxNbO6:Eu3þ spectrum showed sharp peaks between 330 nm and 700 nm located at 345, 363, 380, 394, 405, and 473 nm seen in excitation spectrum of SLNO:xEu3þ by excita tion at 612 nm. The peaks are assigned to the ground states 7F0 5→D4, 7F0 5 7 5 7 5 →G4, F0→L8, and F0→D3 shown in Fig. 3b and the emission spectrum of Sr2LaxNbO6:Eu3þ illustrated in Fig. 3c. Additional sharp peaks at 595, 612, 655, and 710 nm correspond to 5D70→F1, 5D70→F2, 5D70→F3, and 5 7 D0→F4 excited by 394 nm, respectively. The figure depicts some sharp peaks at 347, 362, 380, 408, and 473 nm attributed to 6H5/2 → 4K17/2, 6 H5/2 → 4H7/2, 6H5/2 → 4P7/2, 6H5/2 → 4K11/2,and 6H5/2 → 4I11/2 at 612 nm excitation. Fig. 3b and c shows the photoluminescence intensity. It is significant that, along with the increase in concentration of Eu3þ, the intensity of
� Rc ¼ 2
3V 4πCN
�13
(2)
where V is the volume of the host lattice (V ¼ 576.76 Å), N is the number of sites available for dopants in the unit cell (N ¼ 6), and C represents the critical doped concentration (C ¼ 0.13). Thus, RC is ~11.21934 Å. This indicates that the formation of energy transfer is a multipolar interaction. 4.2. Photoluminescence properties of Sr2La1-XNbO6:xSm3þ As presented in Fig. 3d and e, with the concentration of Sm3þ doped 3
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Fig. 5. (a) Normalized emission intensity of SLNO:0.13Eu3þ phosphors as a function of temperature; (b) Normalized emission intensity of SLNO:0.07Sm3þ phosphors as a function of temperature; (c) Plot of In(I0/I-1) versus 1/kT and the calculated activation energy(Ea) for the SLNO:0.13Eu3þ phosphor; (d) Plot of In(I0/I-1) versus 1/kT and the calculated activation energy (Ea) for the SLNO:0.07Sm3þ phosphor; (e) Temperature-dependent internal quantum efficiency of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ phosphor range from 323 K to 523 K.
0.6207 and 0.3769 for Sr2La0.87NbO6:0.13Eu3þ, and 0.6109 and 0.387 for Sr2La0.93NbO6:0.07Sm3þ.
in phosphor, Fig. 4e shows an increasing trend in luminescence in tensity, and reaches an optimal concentration of 7%. The SLNO:Sm3þ phosphors express several excitation peaks around 324 nm–475 nm. The sharp peaks located at 348, 363, 380, 405, 424, 451, and 472 nm are assigned to a transition to the ground state 6H5/2 and to the excited states 4 K17/2, 4H7/2, 4P7/2,4K11/2,4P5/2, 4G9/2, and 4I11/2, respectively. The strongest peak corresponding to the (6H45/2 → K11/2) transition state is at 405 nm. The emission spectra of Sr2LaxNbO6:xSm3þ phosphors recorded at 405 nm excitation are shown in Fig. 4d, and they consist of three emission peaks located at 567 nm (yellow), 608 nm (orange), and 656 (red), attributed to the 4G65/2 → H5/2, 4G65/2 → H7/2, and 4G65/2 → H9/2 transitions, respectively. The peak at 608 nm is much stronger than the ones at 567 nm or 656 nm, due to the Sm3þ substitution in the Sr2LaNbO6 lattice for La3þ, creating a hexahedron with high-symmetry. The RC is ~13.79058 Å for this structure.
4.4. Thermal quenching properties and quantum efficiency Fig. 5a and (b) show the temperature-dependent emission spectra of SLNO:0.07Sm3þ and SLNO:0.13Eu3þ over the temperature range 300 K–510 K under excitation by 394 nm and 405 nm, respectively. With the increase in temperature, the intensity of the phosphor luminosity is also weakened to some extent due to thermal quenching. Fig. 5(c) and (d) present the integrated emission intensity of the SLNO:0.07Sm3þ and SLNO:0.13Eu3þ samples as a function of temperature. SLNO:0.07Sm3þ and SLNO:0.13Eu3þ can maintain emission intensities of 80.7% and 60.7% at normal temperature and 430 K, respectively. To further confirm which SLNO: 0.13Eu3þ and SLNO:0.07Sm3þ phosphor is more suitable for w-LED application, we experimented with the variable temperature quantum efficiency of the sample. Fig. 5 (e) shows a com parison of the internal quantum efficiencies of SLNO:0.07Sm3þ and SLNO:0.13Eu3þ at 430 K. SLNO:0.13Eu3þ has an internal quantum ef ficiency of 21.7%, which is significantly better than SLNO:0.07Sm3þ with a strength of 2.0%, indicating that the SLNO:0.13Eu3þ phosphor is more suitable for application of W-LED. The thermal characteristics of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ phosphors are related to Nb5þ energy states [23]. Two quenching mechanisms exist for the phosphors, one attributed to displacement between the excited states and ground state [24], and the other related to the thermal stimulated ionization process from the 5d level to the conduction band (CB) [25]. Electrons excited by light of a certain wavelength can transition to the CB and subsequently be seized by the trap, after which the electrons can be liberated to the CB and recaptured by Eu3þ. Therefore, thermal stability is related to the ionization process, which may be affected by the band structure. The band gap is narrowed by doping with Nb5þ, which is beneficial to thermal ionization. When the temperature increases, electrons are more easily activated into the CB, and the band gap decreases. Therefore, more excited state energy is lost and thermal quenching weakens. The activation energy for the aforementioned thermal quenching can be determined by using the following equation [21,26,27]:
4.3. The lifetime and CIE color coordinate Fig. 4(a) and (c) show the decay curves of the 395 nm excitation of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ, respectively. The corresponding decay lifetimes could be obtained by the following expression [2,21,22]: � � � � t t I ¼ A1 exp þ A2 exp (3)
τ1
τ2
where IðtÞ is the intensity of SLNO:Ln3þ(Ln ¼ Eu/Sm) at time t, A1 and A2 are constants, t represents the lifetime, and τ1 and τ2 express the short and long lifetimes for exponential components, respectively. The average (τs) can be calculated by Ref. [20]: �� τs ¼ A1 τ1 2 þ A2 τ2 2 ðA1 τ1 þ A2 τ2 Þ (4) The decay time decreased clearly as Sm3þ increased, Fig. 4d shows the decay time of SLNO:0.07Sm3þ phosphor calculated as 0.280 ms. In Fig. 4b, the decay time of SLNO:0.13Eu3þ phosphor is shown as 0.846 ms, which is much longer than the decay time for SLNO:0.07Sm3þ. Fig. 5e depicts the CIE coordinate positions of SLNO: xLn3þ(Ln3þ ¼ Eu3þ/Sm3þ), where the CIE chromaticity coordinates (x, y) of samples with different compositions are labeled. The CIE co ordinates of SLNO:0.07Sm3þ and SLNO:0.13Eu3þ were clearly depicted, and both phosphors emit red light. The CIE coordinate positions were
IðTÞ ¼
4
I0 1 þ c expð ΔE=kTÞ
(5)
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Optical Materials 98 (2019) 109428
Fig. 6. EL spectrum of the fabricated W-LED lamp with 395 nm NUV chip, SLNO:0.13Eu3þ and commercial green phosphors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Acknowledgments
where I0 and I(T) are the emission intensity at the initial temperature and measured temperature T, respectively; k is Boltzmann’s constant; c is a constant, and △E is the activation energy. To validate the potential application of the synthesized SLNO:0.13Eu3þ red phosphor for w-LEDs, SLNO:0.13Eu3þ and indus trial green powder were dispersed into organic silicon resin and coated on an nvled chip by using 395 nm nvled excitation packaging technology [28,29]. The CIE coordinates of the 395 nm LED chip combined with SLNO:0.13Eu3þ phosphor and commercial green phosphor are shown in Fig. 6 with (x ¼ 0.3626, y ¼ 0.4623) and TC ¼ 4833 K. This device ex hibits standard neutral white light, and the CRI (color rendering index) is 72.9, which can be improved by adding blue phosphors.
This research was supported by the Science and Technology Special Development in Guangdong Province of China (Grant Nos. 2016A010103029), the Science and Technology Project of Guangzhou of China (Grant Nos. 201607010179), and by the Special Funds for Scientific and Technological Innovation Strategy in Guangdong Prov ince (Grant Nos. 2018B090902003). References [1] K. Anilkumar, S. Damodaraiah, S. Babu, V.R. Prasad, Y.C. Ratnakaram, Emission spectra and energy transfer studies in Dy3þ and Dy3þ/Eu3þ co-doped potassium fluorophosphate glasses for white light applications, J. Lumin. 205 (2019) 190–196. [2] W. Chen, Z. Liu, L. Shen, C. Shen, L. Ding, Z. Zhang, H. Zhang, W. Xiang, X. Liang, Design and energy transfer mechanism for single-phased Gd2MgTiO6: Bi3þ, Eu3þ tunable white light-emitting phosphors, J. Mater. Sci. 54 (2018) 4056–4072. [3] Z. Sun, Z. Zhu, Z. Guo, Z.-c. Wu, Z. Yang, T. Zhang, X. Zhang, Electronic structure and luminescent properties of Ce3þ-doped Ba3Lu2B6O15, a high-efficient blueemitting phosphor, Ceram. Int. 45 (2019) 7143–7150. [4] Y. Jin, Y. Hu, H. Wu, H. Duan, L. Chen, Y. Fu, G. Ju, Z. Mu, M. He, A deep red phosphor Li 2 MgTiO 4 :Mn 4þ exhibiting abnormal emission: potential application as color converter for warm w-LEDs, Chem. Eng. J. 288 (2016) 596–607. [5] D. Rajesh, K. Brahmachary, Y.C. Ratnakaram, N. Kiran, A.P. Baker, G.-G. Wang, Energy transfer based emission analysis of Dy 3þ/Eu 3þ co-doped ZANP glasses for white LED applications, J. Alloy. Comp. 646 (2015) 1096–1103. [6] Y.Y. Cheng, B. Fückel, R.W. MacQueen, T. Khoury, R.G.C.R. Clady, T.F. Schulze, N. J. Ekins-Daukes, M.J. Crossley, B. Stannowski, K. Lips, T.W. Schmidt, Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion, Energy Environ. Sci. 5 (2012) 6953. [7] J. de Wild, A. Meijerink, J.K. Rath, W.G.J.H.M. van Sark, R.E.I. Schropp, Towards upconversion for amorphous silicon solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 1919–1922. � c, S. Stojadinovi�c, M.D. Drami�canin, Custom-built thermometry apparatus [8] A. Ciri� and luminescence intensity ratio thermometry of ZrO2:Eu3þ and Nb2O5:Eu3þ, Meas. Sci. Technol. 30 (2019). [9] Z. Sun, Z. Fu, G. Liu, Designing down- and up-conversion dual-mode luminescence of lanthanide-doped phosphors for temperature sensing, J. Lumin. 206 (2019) 176–184. [10] X. Tian, S. Lian, C. Ji, Z. Huang, J. Wen, Z. Chen, H. Peng, S. Wang, J. Li, J. Hu, Y. Peng, Enhanced photoluminescence and ultrahigh temperature sensitivity from NaF flux assisted CaTiO3: Pr3þ red emitting phosphor, J. Alloy. Comp. 784 (2019) 628–640. [11] L.L. Li, R. Zhang, L. Yin, K. Zheng, W. Qin, P.R. Selvin, Y. Lu, Biomimetic surface engineering of lanthanide-doped upconversion nanoparticles as versatile bioprobes, Angew. Chem. 51 (2012) 6121–6125. [12] F. Wang, D. Banerjee, Y. Liu, X. Chen, X. Liu, Upconversion nanoparticles in biological labeling, imaging, and therapy, Analyst 135 (2010) 1839–1854. [13] I. Mackevic, J. Grigorjevaite, M. Janulevicius, A. Linkeviciute, S. Sakirzanovas, A. Katelnikovas, Synthesis and optical properties of highly efficient red-emitting K2LaNb5O15:Eu3þ phosphors, Opt. Mater. 89 (2019) 25–33. [14] E. Pavitra, G.S.R. Raju, S.M. Ghoreishian, C.H. Kwak, J.Y. Park, Y.-K. Han, Y. S. Huh, Novel orange-emitting Ba2LaNbO6:Eu3þ nanophosphors for NUV-based WLEDs and photocatalytic water purification, Ceram. Int. 45 (2019) 4781–4789.
5. Conclusions In summary, two new double-perovskite crystal structure phosphors SLNO:xLn3þ(Ln ¼ Sm, Eu) were successfully synthesized by a traditional high temperature solid-state method. The luminescent properties and decay lifetimes were clearly demonstrated, thermal stability was investigated, and excellent thermal stability character of the Sr2La13þ phosphor was shown. The XPS spectra confirmed that Eu3þ xNbO6:xEu and Sm3þ occupy the La3þ sites, which are coordinated by six oxides with a broad excitation band ranging from 300 to 550 nm, which are consistent with the result of the UV–vis spectrum and thus can be effi ciently excited by near-UV or blue light. The SLNO:xSm3þ and SLNO: xEu3þ phosphors exhibit red emission maximizing at 613 nm and 608 nm, respectively. The experiment proves that the phosphor has good thermal stability, and at 423 K, thermal stabilities of 60.7% and 80.7% were demonstrated. We further confirmed by measuring the temperature-varying quantum efficiency that Sr2La0.87NbO6:0.13Eu3þ is more suitable for LED applications than Sr2La0.93NbO6:0.07Sm3þ. Finally, by mixing A and B, and then using a 395 nm chip, a neutral white light with a color temperature of 4833 K was obtained. In conclusion, this work provides a potential new red niobate phosphor with a double-perovskite crystal structure that can be used for W-LEDs. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Luminescence properties of high thermal stability Sr2LaNbO6: xLn3þ(Ln3þ¼Eu3þ/Sm3þ) phosphors with double-perovskite structures GengQiao Hu, Shuangping Yi *, ZhiXiong Fang, Zhengfa Hu, Weiren Zhao School of Physics and Optoelectronic Engineering, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou, Guangdong, 510006, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Niobate W-LED Thermal stability Double-perovskite
A series of novel single-phased niobate phosphors Sr2LaNbO6:xLn3þ(Ln3þ ¼ Eu3þ/Sm3þ) (SLNO:Ln3þ) with red light emission was successfully synthesized by a conventional solid-state method and characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and photoluminescence. The phosphors showed high thermal stabilities, and at 423 K they still retained 60.7% and 80.7% emission efficiencies for the Eu3þ and Sm3þ variants, respectively. There is a difference in decay time between Eu3þ and Sm3þ due to the energy transit in different energy levels. The crystalline structure was described as a double-perovskite structure with several hexa-coordinated sites. The synthesized Eu3þ phosphor had more favorable characteristics for LEDs and a pro totype device was fabricated with blue-green commercial LED phosphors, Sr2LaNbO6:0.13Eu3þ, and a 395 nmemitting InGaN chip, which exhibited warm white light, with a color temperature of 4833 K. The current study demonstrates that Sr2LaNbO6:0.13Eu3þ can be a potent red phosphor in LED applications.
1. Introduction In recent decades, luminescent materials based on rare earth ions have been widely used for applications including lighting [1–3], dis plays [4,5], solar cells [6,7], sensors [8–10], and bioimaging [11,12]. White-light-emitting diodes (W-LEDs) are especially popular as light sources due to their long service life and environmental friendliness. Currently, commercial white LEDs are generally fabricated by combining Y3Al5O12:Ce3þ yellow-emitting phosphors excited by blue chips. Thus, high correlated color temperature (Tc > 4500 K), which is not ideal for indoor lighting applications. Niobate [13–15] is a matrix raw material that has emerged in recent years, and it has received attention because of its luminescent, electro-optical, piezoelectric, photo-elastic, and nonlinear properties together with chemical stability. Hydrothermal processing has been shown to be one of the most important fabrication methods for niobate. The rare earth ions Eu3þ and Sm3þ are usually used as the red lumi nescence centers in phosphors [16,17] owing to their abundant elec tronic energy levels with 4G65/2 → H7/2(Sm3þ) and 5D70→F2(Eu3þ). In this work, the Sr2La1-xNbO6:xLn3þ(Ln3þ ¼ Eu3þ/Sm3þ) red-emitting phos phors were prepared by a traditional solid-state reaction method heated at 1200 � C for 3 h, and the crystal structure, decay time, and lumines cence spectrum were measured. By comparing the intensities of several
(Ln3þ ¼ Eu3þ/Sm3þ) concentrations of doped SLNO, we show that SLNO:xEu3þ and SLNO:xSm3þ can achieve a maximum performance at Eu3þ(x ¼ 0.13) and Sm3þ(x ¼ 0.07), respectively. We compared the thermal stabilities and internal quantum efficiencies of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ and determined that SLNO:0.13Eu3þ is more suit able for W-LED applications [14]. By combining the Sr2LaNbO6:0.13Eu3þ phosphor and commercial green phosphors with a 395 nm LED chip, a warm white LED with a color temperature of 4833 K and CRI(Color rendering index) is 72.9 was produced, which demon strated that the niobate had potential in LED applications. 2. Experimental procedure The Eu3þ/Sm3þ ion doped Sr2LaNbO6 was synthesized via conven tional high temperature solid-state reaction method. Original materials were Sm2O3(99.9%), Eu2O3(99.9%), La2O3(99.9%), Nb2O5(99.9%), and Sr2O3(99.9%), which were weighed in a proper stoichiometric ratio. After thoroughly mixing and milling, the samples were heated to 1200 � C and preserved for 5 h. Finally, all the as-obtained samples were furnace-cooled tond retained as fine powders for further measurements.
* Corresponding author. E-mail address: [email protected] (S. Yi). https://doi.org/10.1016/j.optmat.2019.109428 Received 11 August 2019; Received in revised form 23 September 2019; Accepted 27 September 2019 Available online 14 October 2019 0925-3467/© 2019 Published by Elsevier B.V.
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around the host and the dopant ions should not be greater than 30%. When Eu3þ and Sm3þ ions replace La3þ ions, the ratio can be calculated by the following formula [20]: Dr ¼
RmðCNÞ RdðCNÞ RmðCNÞ
(1)
where Rm(CN) and Rd(CN) represent the radius of the host ions and doped ions, respectively; and CN is the coordination number. The per centage of Eu3þ ions is 13.8%, and percentage of Sm3þ ions is 7.2%. Both results are less than 30%, thus, the La3þ sites are substituted by the Eu3þ and Sm3þ in the lattice. 3.2. XRD characterization and X-ray photoelectron spectroscopy (XPS) The synthesized phosphor powder was analyzed by x-ray diffraction (XRD), using Cu kα radiation, which affirmed the phases of Sr2La0.87NbO6:0.13Eu3þ and Sr2La0.93NbO6:0.07Sm3þ. Fig. 2a and b shows that the results agree well with those of the Joint Committee on Powder Diffraction Standards (JCPDS) for Card No. 47–0461, which is consistent with the double-perovskite structure Sr2LaNbO6 with a space group of 225 and no diffraction peak corresponding to any impurity phase. These results indicate that phases of Sr2LaNbO6 were obtained. The results of X-ray photoelectron spectroscopy on SLNO:0.07Sm3þ and SLNO 0.13Eu3þ are shown in Fig. 2c and d, respectively. The signals of Eu3d and Sm3d were only weakly detected, because of their lower concentrations relative to those of other elements. Peaks at (3d, 1201 eV) and (3d, 1040 eV) represent the binding energies of Eu3d and Sm3d, respectively. Furthermore, the binding energy data (calibrated using C (1 s, 284.05 eV) as the reference) from SLNO:Ln3þ (x ¼ Eu/Sm) were consistent with the previous reported data for niobates. The peaks at approximately 530 eV, 132.6 eV, 205.85 eV, and 854.7 eV repre sented the binding energies of O(1s), Sr(3d), Nb(3d), and La(3d), respectively.
Fig. 1. Projection view of crystal structure of Sr2LaNbO6 unit cell and the co ordination environment of each ion.
3. Results and discussion 3.1. Crystal structure The double-perovskite crystal structure [18,19] of Sr2LaNbO6 is shown in Fig. 1 with lattice constants (a ¼ b ¼ c ¼ 8.324 Å, V ¼ 576.76 Å3, and Z ¼ 4.). The unit cell structure allowed conservation of a number of octahedral sites (NbO6). The different element effective ion sizes are as follows: Sr2þ (CN ¼ 12, 1.44 Å), La3þ (CN ¼ 6, 1.032 Å), Nb5þ (CN ¼ 6, 0.64 Å), Eu3þ (CN ¼ 6, 0.89 Å), and Sm3þ (CN ¼ 6, 0.958 Å). By observation and comparison we see that because the ionic radius of La3þ is close to those of Eu3þ and Sm3þ, which tend to occupy the La3þ site. The acceptable difference percentage of the ionic radius
Fig. 2. (a) XRD patterns of Sr2La0.87NbO6:0.13Eu3þ; (b) XRD patterns of Sr2La0.93NbO6:0.07Sm3þ; (c) XPS spectrum of SLNO:0.13Eu3þ phosphor; (d)XPS spectrum of SLNO:0.07Sm3þ phosphor. 2
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Fig. 3. (a) Excitation and emission spectrum of SLNO:0.13Eu3þ and SLNO: 0.07Sm3þ; (b) Excitation spectrum of SLNO:xEu3þ (x ¼ 0.01, 0.04, 0.07, 0.10, 0.13, 0.16) monitored at 612 nm; (c) Emission spectrum of SLNO:xEu3þ excited at 394 nm; (d) Excitation spectrum of SLNO:xSm3þ (x ¼ 0.01, 0.04, 0.07, 0.10, 0.13); (e) Emission spectrum of SLNO:xSm3þ excited at 405 nm.
Fig. 4. (a) Decay curves of SLNO:0.13Eu3þ at room temperature (λex ¼ 394 nm, λem ¼ 612 nm); (b) Temperature-dependent emission spectra of SLNO:0.13Eu3þ phosphor in the range of 300–510 K (λex ¼ 394 nm); (c) Decay curves of SLNO:0.07Sm3þ at room temperature (λex ¼ 405 nm, λem ¼ 605 nm); (d) Temperaturedependent emission spectra of SLNO:0.07Sm3þ phosphor in the range of 300–510 K (λex ¼ 405 nm); (e) CIE for SLNO:0.13Eu3þ and SLNO:0.07Sm3þ phosphors.
4. Photoluminescence properties
photoluminescence reaches optimal concentration at 0.13 mol. Continuing to add Eu3þ, the photoluminescence soon declines caused by concentration quenching. The critical transfer distance (Rc) can be calculated using [4]:
4.1. Photoluminescence properties of Sr2La1-xNbO6:xEu3þ The luminescence process of the Sr2LaxNbO6:Eu3þ spectrum showed sharp peaks between 330 nm and 700 nm located at 345, 363, 380, 394, 405, and 473 nm seen in excitation spectrum of SLNO:xEu3þ by excita tion at 612 nm. The peaks are assigned to the ground states 7F0 5→D4, 7F0 5 7 5 7 5 →G4, F0→L8, and F0→D3 shown in Fig. 3b and the emission spectrum of Sr2LaxNbO6:Eu3þ illustrated in Fig. 3c. Additional sharp peaks at 595, 612, 655, and 710 nm correspond to 5D70→F1, 5D70→F2, 5D70→F3, and 5 7 D0→F4 excited by 394 nm, respectively. The figure depicts some sharp peaks at 347, 362, 380, 408, and 473 nm attributed to 6H5/2 → 4K17/2, 6 H5/2 → 4H7/2, 6H5/2 → 4P7/2, 6H5/2 → 4K11/2,and 6H5/2 → 4I11/2 at 612 nm excitation. Fig. 3b and c shows the photoluminescence intensity. It is significant that, along with the increase in concentration of Eu3þ, the intensity of
� Rc ¼ 2
3V 4πCN
�13
(2)
where V is the volume of the host lattice (V ¼ 576.76 Å), N is the number of sites available for dopants in the unit cell (N ¼ 6), and C represents the critical doped concentration (C ¼ 0.13). Thus, RC is ~11.21934 Å. This indicates that the formation of energy transfer is a multipolar interaction. 4.2. Photoluminescence properties of Sr2La1-XNbO6:xSm3þ As presented in Fig. 3d and e, with the concentration of Sm3þ doped 3
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Fig. 5. (a) Normalized emission intensity of SLNO:0.13Eu3þ phosphors as a function of temperature; (b) Normalized emission intensity of SLNO:0.07Sm3þ phosphors as a function of temperature; (c) Plot of In(I0/I-1) versus 1/kT and the calculated activation energy(Ea) for the SLNO:0.13Eu3þ phosphor; (d) Plot of In(I0/I-1) versus 1/kT and the calculated activation energy (Ea) for the SLNO:0.07Sm3þ phosphor; (e) Temperature-dependent internal quantum efficiency of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ phosphor range from 323 K to 523 K.
0.6207 and 0.3769 for Sr2La0.87NbO6:0.13Eu3þ, and 0.6109 and 0.387 for Sr2La0.93NbO6:0.07Sm3þ.
in phosphor, Fig. 4e shows an increasing trend in luminescence in tensity, and reaches an optimal concentration of 7%. The SLNO:Sm3þ phosphors express several excitation peaks around 324 nm–475 nm. The sharp peaks located at 348, 363, 380, 405, 424, 451, and 472 nm are assigned to a transition to the ground state 6H5/2 and to the excited states 4 K17/2, 4H7/2, 4P7/2,4K11/2,4P5/2, 4G9/2, and 4I11/2, respectively. The strongest peak corresponding to the (6H45/2 → K11/2) transition state is at 405 nm. The emission spectra of Sr2LaxNbO6:xSm3þ phosphors recorded at 405 nm excitation are shown in Fig. 4d, and they consist of three emission peaks located at 567 nm (yellow), 608 nm (orange), and 656 (red), attributed to the 4G65/2 → H5/2, 4G65/2 → H7/2, and 4G65/2 → H9/2 transitions, respectively. The peak at 608 nm is much stronger than the ones at 567 nm or 656 nm, due to the Sm3þ substitution in the Sr2LaNbO6 lattice for La3þ, creating a hexahedron with high-symmetry. The RC is ~13.79058 Å for this structure.
4.4. Thermal quenching properties and quantum efficiency Fig. 5a and (b) show the temperature-dependent emission spectra of SLNO:0.07Sm3þ and SLNO:0.13Eu3þ over the temperature range 300 K–510 K under excitation by 394 nm and 405 nm, respectively. With the increase in temperature, the intensity of the phosphor luminosity is also weakened to some extent due to thermal quenching. Fig. 5(c) and (d) present the integrated emission intensity of the SLNO:0.07Sm3þ and SLNO:0.13Eu3þ samples as a function of temperature. SLNO:0.07Sm3þ and SLNO:0.13Eu3þ can maintain emission intensities of 80.7% and 60.7% at normal temperature and 430 K, respectively. To further confirm which SLNO: 0.13Eu3þ and SLNO:0.07Sm3þ phosphor is more suitable for w-LED application, we experimented with the variable temperature quantum efficiency of the sample. Fig. 5 (e) shows a com parison of the internal quantum efficiencies of SLNO:0.07Sm3þ and SLNO:0.13Eu3þ at 430 K. SLNO:0.13Eu3þ has an internal quantum ef ficiency of 21.7%, which is significantly better than SLNO:0.07Sm3þ with a strength of 2.0%, indicating that the SLNO:0.13Eu3þ phosphor is more suitable for application of W-LED. The thermal characteristics of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ phosphors are related to Nb5þ energy states [23]. Two quenching mechanisms exist for the phosphors, one attributed to displacement between the excited states and ground state [24], and the other related to the thermal stimulated ionization process from the 5d level to the conduction band (CB) [25]. Electrons excited by light of a certain wavelength can transition to the CB and subsequently be seized by the trap, after which the electrons can be liberated to the CB and recaptured by Eu3þ. Therefore, thermal stability is related to the ionization process, which may be affected by the band structure. The band gap is narrowed by doping with Nb5þ, which is beneficial to thermal ionization. When the temperature increases, electrons are more easily activated into the CB, and the band gap decreases. Therefore, more excited state energy is lost and thermal quenching weakens. The activation energy for the aforementioned thermal quenching can be determined by using the following equation [21,26,27]:
4.3. The lifetime and CIE color coordinate Fig. 4(a) and (c) show the decay curves of the 395 nm excitation of SLNO:0.13Eu3þ and SLNO:0.07Sm3þ, respectively. The corresponding decay lifetimes could be obtained by the following expression [2,21,22]: � � � � t t I ¼ A1 exp þ A2 exp (3)
τ1
τ2
where IðtÞ is the intensity of SLNO:Ln3þ(Ln ¼ Eu/Sm) at time t, A1 and A2 are constants, t represents the lifetime, and τ1 and τ2 express the short and long lifetimes for exponential components, respectively. The average (τs) can be calculated by Ref. [20]: �� τs ¼ A1 τ1 2 þ A2 τ2 2 ðA1 τ1 þ A2 τ2 Þ (4) The decay time decreased clearly as Sm3þ increased, Fig. 4d shows the decay time of SLNO:0.07Sm3þ phosphor calculated as 0.280 ms. In Fig. 4b, the decay time of SLNO:0.13Eu3þ phosphor is shown as 0.846 ms, which is much longer than the decay time for SLNO:0.07Sm3þ. Fig. 5e depicts the CIE coordinate positions of SLNO: xLn3þ(Ln3þ ¼ Eu3þ/Sm3þ), where the CIE chromaticity coordinates (x, y) of samples with different compositions are labeled. The CIE co ordinates of SLNO:0.07Sm3þ and SLNO:0.13Eu3þ were clearly depicted, and both phosphors emit red light. The CIE coordinate positions were
IðTÞ ¼
4
I0 1 þ c expð ΔE=kTÞ
(5)
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Fig. 6. EL spectrum of the fabricated W-LED lamp with 395 nm NUV chip, SLNO:0.13Eu3þ and commercial green phosphors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Acknowledgments
where I0 and I(T) are the emission intensity at the initial temperature and measured temperature T, respectively; k is Boltzmann’s constant; c is a constant, and △E is the activation energy. To validate the potential application of the synthesized SLNO:0.13Eu3þ red phosphor for w-LEDs, SLNO:0.13Eu3þ and indus trial green powder were dispersed into organic silicon resin and coated on an nvled chip by using 395 nm nvled excitation packaging technology [28,29]. The CIE coordinates of the 395 nm LED chip combined with SLNO:0.13Eu3þ phosphor and commercial green phosphor are shown in Fig. 6 with (x ¼ 0.3626, y ¼ 0.4623) and TC ¼ 4833 K. This device ex hibits standard neutral white light, and the CRI (color rendering index) is 72.9, which can be improved by adding blue phosphors.
This research was supported by the Science and Technology Special Development in Guangdong Province of China (Grant Nos. 2016A010103029), the Science and Technology Project of Guangzhou of China (Grant Nos. 201607010179), and by the Special Funds for Scientific and Technological Innovation Strategy in Guangdong Prov ince (Grant Nos. 2018B090902003). References [1] K. Anilkumar, S. Damodaraiah, S. Babu, V.R. Prasad, Y.C. Ratnakaram, Emission spectra and energy transfer studies in Dy3þ and Dy3þ/Eu3þ co-doped potassium fluorophosphate glasses for white light applications, J. Lumin. 205 (2019) 190–196. [2] W. Chen, Z. Liu, L. Shen, C. Shen, L. Ding, Z. Zhang, H. Zhang, W. Xiang, X. Liang, Design and energy transfer mechanism for single-phased Gd2MgTiO6: Bi3þ, Eu3þ tunable white light-emitting phosphors, J. Mater. Sci. 54 (2018) 4056–4072. [3] Z. Sun, Z. Zhu, Z. Guo, Z.-c. Wu, Z. Yang, T. Zhang, X. Zhang, Electronic structure and luminescent properties of Ce3þ-doped Ba3Lu2B6O15, a high-efficient blueemitting phosphor, Ceram. Int. 45 (2019) 7143–7150. [4] Y. Jin, Y. Hu, H. Wu, H. Duan, L. Chen, Y. Fu, G. Ju, Z. Mu, M. He, A deep red phosphor Li 2 MgTiO 4 :Mn 4þ exhibiting abnormal emission: potential application as color converter for warm w-LEDs, Chem. Eng. J. 288 (2016) 596–607. [5] D. Rajesh, K. Brahmachary, Y.C. Ratnakaram, N. Kiran, A.P. Baker, G.-G. Wang, Energy transfer based emission analysis of Dy 3þ/Eu 3þ co-doped ZANP glasses for white LED applications, J. Alloy. Comp. 646 (2015) 1096–1103. [6] Y.Y. Cheng, B. Fückel, R.W. MacQueen, T. Khoury, R.G.C.R. Clady, T.F. Schulze, N. J. Ekins-Daukes, M.J. Crossley, B. Stannowski, K. Lips, T.W. Schmidt, Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion, Energy Environ. Sci. 5 (2012) 6953. [7] J. de Wild, A. Meijerink, J.K. Rath, W.G.J.H.M. van Sark, R.E.I. Schropp, Towards upconversion for amorphous silicon solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 1919–1922. � c, S. Stojadinovi�c, M.D. Drami�canin, Custom-built thermometry apparatus [8] A. Ciri� and luminescence intensity ratio thermometry of ZrO2:Eu3þ and Nb2O5:Eu3þ, Meas. Sci. Technol. 30 (2019). [9] Z. Sun, Z. Fu, G. Liu, Designing down- and up-conversion dual-mode luminescence of lanthanide-doped phosphors for temperature sensing, J. Lumin. 206 (2019) 176–184. [10] X. Tian, S. Lian, C. Ji, Z. Huang, J. Wen, Z. Chen, H. Peng, S. Wang, J. Li, J. Hu, Y. Peng, Enhanced photoluminescence and ultrahigh temperature sensitivity from NaF flux assisted CaTiO3: Pr3þ red emitting phosphor, J. Alloy. Comp. 784 (2019) 628–640. [11] L.L. Li, R. Zhang, L. Yin, K. Zheng, W. Qin, P.R. Selvin, Y. Lu, Biomimetic surface engineering of lanthanide-doped upconversion nanoparticles as versatile bioprobes, Angew. Chem. 51 (2012) 6121–6125. [12] F. Wang, D. Banerjee, Y. Liu, X. Chen, X. Liu, Upconversion nanoparticles in biological labeling, imaging, and therapy, Analyst 135 (2010) 1839–1854. [13] I. Mackevic, J. Grigorjevaite, M. Janulevicius, A. Linkeviciute, S. Sakirzanovas, A. Katelnikovas, Synthesis and optical properties of highly efficient red-emitting K2LaNb5O15:Eu3þ phosphors, Opt. Mater. 89 (2019) 25–33. [14] E. Pavitra, G.S.R. Raju, S.M. Ghoreishian, C.H. Kwak, J.Y. Park, Y.-K. Han, Y. S. Huh, Novel orange-emitting Ba2LaNbO6:Eu3þ nanophosphors for NUV-based WLEDs and photocatalytic water purification, Ceram. Int. 45 (2019) 4781–4789.
5. Conclusions In summary, two new double-perovskite crystal structure phosphors SLNO:xLn3þ(Ln ¼ Sm, Eu) were successfully synthesized by a traditional high temperature solid-state method. The luminescent properties and decay lifetimes were clearly demonstrated, thermal stability was investigated, and excellent thermal stability character of the Sr2La13þ phosphor was shown. The XPS spectra confirmed that Eu3þ xNbO6:xEu and Sm3þ occupy the La3þ sites, which are coordinated by six oxides with a broad excitation band ranging from 300 to 550 nm, which are consistent with the result of the UV–vis spectrum and thus can be effi ciently excited by near-UV or blue light. The SLNO:xSm3þ and SLNO: xEu3þ phosphors exhibit red emission maximizing at 613 nm and 608 nm, respectively. The experiment proves that the phosphor has good thermal stability, and at 423 K, thermal stabilities of 60.7% and 80.7% were demonstrated. We further confirmed by measuring the temperature-varying quantum efficiency that Sr2La0.87NbO6:0.13Eu3þ is more suitable for LED applications than Sr2La0.93NbO6:0.07Sm3þ. Finally, by mixing A and B, and then using a 395 nm chip, a neutral white light with a color temperature of 4833 K was obtained. In conclusion, this work provides a potential new red niobate phosphor with a double-perovskite crystal structure that can be used for W-LEDs. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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