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Novel highly luminescent double-perovskite Ca2GdSbO6:Eu3+ red phosphors with high color purity for white LEDs: Synthesis, crystal structure, and photoluminescence properties
Journal of Luminescence 221 (2020) 117105
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Novel highly luminescent double-perovskite Ca2GdSbO6:Eu3þ red phosphors with high color purity for white LEDs: Synthesis, crystal structure, and photoluminescence properties Zongjie Zhang a, Liangling Sun a, Balaji Devakumar a, Jia Liang a, Shaoying Wang a, Qi Sun a, Sanjay J. Dhoble b, Xiaoyong Huang a, * a b
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China Department of Physics, R.T.M. Nagpur University, Nagpur, 440033, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Eu3þ ions Ca2GdSbO6 Red-emitting phosphors White LEDs Photoluminescence Double-perovskite
High-efficiency red-emitting phosphors are required to fabricate high-performance white light-emitting diodes (LEDs). Herein, the novel highly efficient Eu3þ-activated Ca2GdSbO6 double-perovskite red phosphors with good thermal stability toward warm-white LEDs were reported. A series of Ca2Gd(1-x)EuxSbO6 red phosphors with different Eu3þ doping concentrations (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were synthesized by using hightemperature solid-state reaction method. Under the excitation of 396 nm near-ultraviolet light, these Ca2Gd(15 7 3þ x)EuxSbO6 phosphors showed intense red emissions peaking at 612 nm due to the D0→ F2 transition of Eu 3þ ions. The strongest luminescence intensity reached when Eu doping concentration was x ¼ 0.5, and the critical distance between Eu3þ activators was calculated to be 7.97 Å. The concentration quenching mechanism was due to the dipole-dipole interaction of Eu3þ ions. The CIE color coordinates of the optimal Ca2Gd0.5Eu0.5SbO6 phosphors were determined to be (0.6629, 0.3367), and the corresponding color purity reached about 94.9%. Importantly, the Ca2Gd0.5Eu0.5SbO6 phosphors revealed outstanding internal quantum efficiency of 73% and good thermal stability. The emission intensity of Ca2Gd0.5Eu0.5SbO6 phosphors at 423 K still remained about 73% of its initial value at 303 K. Finally, a prototype white LED device was fabricated by coating the phosphor blend of commercial blue-emitting BaMgAl10O17:Eu2þ, green-emitting (Ba, Sr)2SiO4:Eu2þ and our as-prepared redemitting Ca2Gd0.5Eu0.5SbO6 on a 395 nm LED chip. Under 20 mA driven current, the device showed bright warmwhite light with CIE color coordinates of (0.3888, 0.3943), correlated color temperature of 3911 K, and color rendering index of 88.4. The results demonstrated that the developed novel red-emitting Ca2Gd0.5Eu0.5SbO6 phosphors could be used as potential color converters in white LEDs.
1. Introduction At present, environmental problems (resource exhaustion, water pollution and air pollution, etc.) are becoming more and more promi nent and attracting people’s attention [1]. Compared with traditional lighting sources such as incandescent lamps and fluorescent lamps, white light emitting diode (LEDs) have the advantages of low power consumption, high efficiency, long working life and environmental friendliness [2–6]. Currently, white LEDs are generally prepared by using YAG:Ce3þ yellow-emitting phosphors with blue InGaN chips, but they have high correlation color temperature (CCT) and low color rendering index (CRI) [7]. Therefore, scientific researchers are
committed to the developments of efficient white LEDs with low CCT and high CRI [8]. Among them, LED chips coated with tricolor (blue, green and red) phosphors can achieve good results. However, there is still a lack of red phosphors with good performance for white LEDs [9, 10]. Therefore, the research of red phosphors has become very urgent. Inorganic phosphors doped with rare earths with excellent optical properties have been widely used as color converters in white LEDs [11–13]. Particularly, Eu3þ doped phosphors could emit intense red light under UV excitation because of the 5D0→7FJ (J ¼ 1, 2, 3, 4) tran sition of Eu3þ ions [14–19]. Currently, double-perovskite compounds with A2BB’O6 structure have been extensively studied as hosts of lumi nescent materials. The thermal stability of these double-perovskite
* Corresponding author. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.jlumin.2020.117105 Received 26 October 2019; Received in revised form 5 February 2020; Accepted 7 February 2020 Available online 8 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. PXRD patterns of the Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors together with the standard card of Ca2GdTaO6 (JCPDS #73–0085). (b) The unit cell parameters of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors.
compounds is excellent due to abundant [B/B’O6] octahedron structures [20]. Moreover, recent researches proved that these kinds of double-perovskite compounds are outstanding host materials for Eu3þ ions, such as Ca2LuTaO6:Eu3þ [21], Ca2GdTaO6:Eu3þ [22], Ca2LuSbO6: Eu3þ [23], (Li, Na, K)LaMgWO6:Eu3þ [24], Ca3TeO6:Eu3þ [25], La2LiSbO6:Eu3þ [26], and BaLaMg(Sb, Nb)O6:Eu3þ [27]. In this paper, a series of the novel efficient red-emitting Eu3þ-acti vated Ca2GdSbO6 double-perovskite phosphors were demonstrated. The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra, luminescence decay lifetimes, internal quantum effi ciency (IQE), CIE color coordinates, color purity, and temperaturedependent emission spectra of Ca2GdSbO6:Eu3þ phosphors have been systematically studied. We observed that the as-prepared samples exhibited strong red luminescence at around 612 nm upon 396 nm excitation, and the IQE reached as high as 73%. These results indicated that Ca2Gd(1-x)EuxSbO6 phosphors could be considered as potential red phosphors for white LEDs.
reagent, Tianjin Kemiou Chemical Reagent Co., Ltd), Gd2O3 (99.99%, Jining Tianyi New Materials Co., Ltd), Sb2O5 (99%, Aladdin Industrial Corporation, Shanghai, China) and Eu2O3 (99.99%, Jining Tianyi New Materials Co., Ltd) were weighed and then mixed in an agate mortar. Afterwards, the obtained powders were transferred into crucibles and then calcined at 1500 � C for 6 h in a furnace in air. Once the reaction process was finished, the phosphor products were cooled down naturally to room temperature, and then were re-ground into to fine powders. 2.2. Characterization The phase purity of the as-prepared phosphors was identified via powder X-ray diffraction (PXRD) with Cu-Kα radiation (step size 0.02 and step time 0.2 s). The PXRD pattern Rietveld refinement of Ca2G d0.5Eu0.5SbO6 sample was performed with general structure analysis system (GSAS) [28,29], and the Vesta software was used for generating the crystal structure. An Edinburgh FS5 spectrometer, using a 150 W continuous-wave Xe lamp as a light source, was used to record the PLE and PL spectra of the samples. Decay curves and IQE of the samples were also obtained from the Edinburgh FS5 spectrometer equipped with a pulsed xenon lamp and an integrating sphere, respectively. All the above measurements were performed at room temperature. The temperature-dependent emission spectra were measured by the Edin burgh FS5 spectrometer equipped with a temperature controller.
2. Experimental section 2.1. Synthesis of the samples A series of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors with various Eu3þ doping concentrations were synthesized by high-temperature solid-state reaction. On the basis of the stoichiometric amounts of the formula, the raw materials including CaCO3 (analytical
Fig. 2. The Rietveld refinement of the PXRD pattern (a) and the crystal structure (b) of Ca2Gd0.5Eu0.5SbO6 phosphors. 2
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Journal of Luminescence 221 (2020) 117105
Table 1 Refined crystallographic parameters of Ca2Gd0.5Eu0.5SbO6 phosphors.
Table 3 Bond distances of Ca2Gd0.5Eu0.5SbO6 phosphors.
Ca2Gd0.5Eu0.5SbO6 Crystal system Space group Lattice parameters
Monoclinic P 21/n a ¼ 5.597(11) Å b ¼ 5.858(11) Å c ¼ 8.095(16) Å α ¼ γ ¼ 90� β ¼ 89.723(16) � V ¼ 265.439(6) Å3
Unit cell volume
Table 2 Atomic coordinates, occupancy and displacement parameters of Ca2G d0.5Eu0.5SbO6 phosphors. Atom
x
y
z
Wycoff position
Occupancy
Uiso(Ǻ2)
Ca
0.518 (3) 0 0 0 0.111 (14) 0.293 (3) 0.170 (8)
0.052 (6) 0 0 0 0.045 (2) 0.181 (4) 0.286 (3)
0.247 (6) 0.5 0.5 0 0.236 (3) 0.060 (4) 0.062 (4)
4e
1
2a 2a 2c 4e
0.5 0.5 1 1
4e
1
4e
1
0.034 (6) 0.044 0.044 0.036 0.055 (3) 0.056 (7) 0.047 (3)
Gd Eu Sb O1 O2 O3
Bond
Bond distance(Ǻ)
Ca–O1 Ca–O1 Ca–O2 Ca–O2 Ca–O3 Ca–O3 Ca–O3 (Gd–O1) � 2 (Gd–O2) � 2 (Gd–O3) � 2 (Sb–O1) � 2 (Sb–O2) � 2 (Sb–O3) � 2
2.356 2.476 2.707 2.919 2.916 2.703 2.363 2.251 2.263 2.30 2.044 2.023 2.005
2.3. White LED device fabrication The as-prepared Ca2Gd0.5Eu0.5SbO6 red phosphors, the commercial BaMgAl10O17:Eu2þ (BAM:Eu2þ) blue phosphors, commercial (Ba, Sr)2SiO4:Eu2þ green phosphors and silicone were mixed fully and then coated on a 395 nm near-UV LED chip to fabricate a prototype white LED device. An integrating sphere spectroradiometer (HAAS2000, Everfine) system was used to test the photoelectric properties of the LED device such as CRI, CCT, luminous efficacy (LE) and electroluminescence spectrum.
Fig. 3. The PLE (λem ¼ 612 nm) and PL (λex ¼ 396 nm) spectra of Ca2G d0.5Eu0.5SbO6 phosphors.
monoclinic structure with the space group of P21/n and the crystal pa rameters were found to be a ¼ 5.597(11) Å, b ¼ 5.858(11) Å, c ¼ 8.095 (16) Å, and V ¼ 265.439(6) Å3 (see Table 1). Atomic coordinates and displacement parameters obtained by the Rietveld refinement were lis ted in Table 2, while the bond distances of Ca2Gd0.5Eu0.5SbO6 phosphors were listed in Table 3. The crystal structure of the Ca2Gd0.5Eu0.5SbO6 phosphor was displayed in Fig. 2(b). In Ca2GdSbO6 host, the Sb5þ and Gd3þ ions were connected with six shared oxygen atoms into a [SbO6]/ [GdO6] octahedral.
3. Results and discussion 3.1. Phase purity and crystal structure The recorded PXRD patterns of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors together with the standard card of Ca2GdTaO6 (JCPDS #73–0085) were shown in Fig. 1(a). When x was higher than 0.4, the PXRD patterns of Ca2Gd(1-x)EuxSbO6 samples could match with the standard card except a faint difference peak around 2θ ¼ 53.9� . These powder samples with main phase of the Ca2Gd(1-x)EuxSbO6 single crystal combined with a small amount of CaO isostructural phase. The effect of impurity could be neglected because the intensity of dif ference peak was too low. The PXRD patterns of Ca2Gd(1-x)EuxSbO6 samples with other Eu3þ concentrations could match well with the standard card. The unit cell parameters of (including β, a, b, c, and V) of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors were shown in Fig. 1(b). With increasing Eu3þ concentration, a, b, c, and V presented an increasing trend. The ionic radii of Eu3þ (r ¼ 0.95 Å) is slightly larger than Gd3þ (r ¼ 0.94 Å) ions [22], and Eu3þ and Gd3þ ions have same valence. Thus, the Eu3þ ions were likely to replace Gd3þ ions in the Ca2GdSbO6 crystal lattice, and then formed Ca2Gd(1-x)EuxSbO6 solid solutions. Furthermore, the PXRD refinement results of Ca2Gd0.5Eu0.5SbO6 phosphors were displayed in Fig. 2. Through Rietveld refinement of PXRD pattern of Ca2Gd0.5Eu0.5SbO6 phosphors, we could fully under stand the crystal structure of the sample. The sample crystallized in
3.2. Photoluminescence properties Fig. 3 shows the PLE and PL spectra of Ca2Gd0.5Eu0.5SbO6 sample. The PLE spectrum monitored at 612 nm consisted of a broad excitation band in the range from 200 to 310 nm and a series of sharp excitation peaks ranging from 310 to 550 nm. The excitation band in the 250–310 nm wavelength range peaking at 294 nm was derived from the O2 →Eu3þ charge transfer band (CTB) [30–32]. The highest sharp excitation peak was obtained at 396 nm, which could be attributed to the 7F0→5L6 transition of Eu3þ ions. The others sharp excitation peaks at 322, 364, 383, 416, 466, and 535 nm were assigned to the transitions from 7F0 level to 5H6, 5D4, 5G2, 5D3, 5D2, and 5D1 levels of Eu3þ ions, respectively [33,34]. The observed results indicated that near-UV LED chip could efficiently excite the Ca2Gd0.5Eu0.5SbO6 phosphors. Under the excitation of 396 nm, the Ca2Gd0.5Eu0.5SbO6 phosphors showed bright red light. The corresponding PL spectrum included four main emission peaks at 591, 612, 656 and 702 nm, corresponding to the 5 D0→7F1, 5D0→7F2, 5D0→7F3, and 5D0→7F4 transitions of Eu3þ ions, 3
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Fig. 4. (a) PL spectra of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors under 396 nm excitation. (b) The PL intensity as a function of Eu3þ doping concentration in Ca2Gd(1-x)EuxSbO6 phosphors under 396 nm excitation. (c) The relationship of log(I/x) versus log(x) in Ca2Gd(1-x)EuxSbO6 phosphors. (d) Decay curves of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors excited at 396 nm when monitored at 612 nm.
respectively [35]. In the PL spectrum, the strongest emission peak was located at 612 nm. In order to find the optimal doping concentration of Eu3þ ions in Ca2GdSbO6:Eu3þ phosphors, a series of Ca2Gd(1-x)EuxSbO6 samples were prepared with various Eu3þ concentrations and measured their PL spectra. Fig. 4(a) displays the PL spectra of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors excited at 396 nm. All these samples exhibited intense red emissions. Clearly, all the PL spectra of the samples showed similar emission profiles with strong sharp red emission lines peaking at around 591, 612, 656 and 702 nm. Moreover, as shown in Fig. 4(b), the emission intensity obviously increased with increasing the Eu3þ doping concentration and reached a maximum when x ¼ 0.5, and then decreased when the Eu3þ concentration further increased owing to the concentration quenching effect. The optimal doping con centration of Eu3þ ions was x ¼ 0.5. The concentration quenching mechanism could be judged by the value of the critical distance (Rc) between the nearest Eu3þ ions. If Rc exceeds 5 Å, the electric multipolar interaction will be the dominant mechanism of the concentration quenching phenomenon; if not, the mechanism for concentration quenching will be an exchange interaction [30]. The following equation could be used to calculate the Rc [36–38]: � �1=3 3V (1) Rc ¼ 2 4π xc N
Table 4 The CIE chromaticity coordinates, color purity and asymmetry ratio of Ca2Gd(1x)EuxSbO6 phosphors. Eu3þ concentration (x)
Asymmetry ratio
CIE coordinates
Color purity
0.2 0.3 0.4 0.5 0.6 0.7 0.8
5.75 5.88 5.97 6.00 5.97 6.00 6.05
(0.6578, (0.6608, (0.6622, (0.6629, (0.6629, (0.6626, (0.6623,
93.4% 94.3% 94.7% 94.9% 94.8% 94.8% 94.7%
0.3414) 0.3386) 0.3373) 0.3367) 0.3366) 0.3369) 0.3371)
Eu3þ ions in Ca2Gd(1-x)EuxSbO6 phosphors, we used the following equation [39,40]: � � I ¼ k 1 þ βðxÞθ=3 x
1
(2)
where x and I are the doping concentration of Eu3þ ions and PL intensity of samples, respectively; k and β are constants; θ is the electric multipole index and the corresponding value of θ ¼ 6, 8 and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interac tion, respectively [41–43]. Fig. 4(c) shows the relationship between log (I/x) and log(x). The slope of the fitted line was obtained to be 2.16. Thus, the calculated value of θ was 6.48, which was close to 6. There fore, the dipole-dipole interaction could be attributed to be the main mechanism for concentration quenching effect in Ca2Gd(1-x)EuxSbO6 phosphors. As is well known, the asymmetry ratio (the (5D0→7F2)/(5D0→7F1) emission intensity ratio, namely, R/O ratio) can be used to represent the site symmetry of the Eu3þ ions in the lattice [44]. The magnetic dipole transition (5D0→7F1) and the electric dipole transition (5D0→7F2) are
where V stands for the volume of the unit cell; xc is the critical con centration of Eu3þ ions; N refers to the number of sites that can be available for activator ions in the unit cell. For Ca2Gd(1-x)EuxSbO6 phosphors, xc ¼ 0.5, N ¼ 2 and V ¼ 265.439 Å3, and then the Rc was calculated to be 7.97 Å. Therefore, the concentration quenching was caused by the electric multipolar interaction. In order to further research the energy transfer interaction between 4
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Journal of Luminescence 221 (2020) 117105
Fig. 6. The IQE of the Ca2Gd0.5Eu0.5SbO6 sample was measured by using the integrating sphere within the range of 376–750 nm (λex ¼ 396 nm). The inset shows the magnified view of the emission spectrum within the range of 575–725 nm.
0.863, 0.833, 0.773, and 0.682 corresponding x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. The observed mono-exponential decay curves meant that interaction between Eu3þ ions hardly took place [51]. However, when Eu3þ content x was higher than 0.5, the distance between Eu3þ activators largely decreased and the energy transfer became more frequent, resulting in great enhancement of non-radiative energy transfer and providing an extra decay channel which changed the decay curves [52]. Accordingly, the decay curves for the Ca2Gd(1- x)EuxSbO6 samples with x up to 0.6 were well fitted by double expo nential function [53,54]:
Fig. 5. The CIE chromaticity diagram of Ca2Gd0.5Eu0.5SbO6 phosphors upon 396 nm excitation. Inset displays the digital photograph of Ca2Gd0.5Eu0.5SbO6 sample under a 365 nm UV lamp.
related with Eu3þ ions centered at the inversion symmetry and non-inversion symmetry in the crystal structure, respectively [45]. All the values of asymmetry ratio for Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors were determined based on their PL spectra excited at 396 nm, and the results were listed in Table 4. The R/O values calculated for samples are between 5.75 and 6.05, which clearly illustrated that Eu3þ ions occupied low symmetry sites in the Ca2GdSbO6 host lattice without an inversion center [46]. The high R/O ratio is beneficial for achieving high-color-purity red emission. In this work, the R/O ratio value of Ca2Gd(1-x)EuxSbO6 phosphors was much higher than that of some previously reported red phosphors, such as Ca9Gd(PO4)7:Eu3þ (R/O ratio ¼ 4.43) [47] and Sr3La(PO4)3:Eu3þ (R/O ratio ¼ 2.445) [48]. Fig. 4(d) exhibits the decay curves (λem ¼ 612 nm; λex ¼ 396 nm) of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors. Mono-exponential decays were observed in the samples with Eu3þ content x below 0.6. By contrast, at higher concentrations (x ¼0.6, 0.7 and 0.8), the observed decay curves were nonexponential, and the nonexponential change became more prominent as Eu3þ content increased, indicating that more than one relaxation process existed. Generally, the distance between two neighboring activators reduced when their concentrations increased, and then the energy transfer pro cess between activators became more frequent, which will act as an extra luminescence decay channel. As a result, the decay time could be critically influenced by the energy migration process between activators in the host. Moreover, the relaxation rate of the extra energy-migration process may be different, yielding nonexponential decay curves. Since Eu3þ ions only occupied one site in Ca2GdSbO6 host, thus if there was no interaction between Eu3þ ions, the decay curve should be a monoexponential function. We found that when x was below 0.6, the decay curves could be fitted into single exponential function [49,50]: I ¼ I0 expð t = τÞ
IðtÞ ¼ A1 expð
t = τ1 Þ þ A2 expð
t = τ2 Þ
(4)
where I(t) represents the intensity of luminescence at time t; A1 and A2 are constants; τ1 is the decay time for the exponential components associated with the slow decay process; and τ2 is the decay time for the exponential components associated with the rapid one. The decay life times were calculated to be 0.536, 0.434 and 0.326 ms for x ¼ 0.6, 0.7 and 0.8, respectively. Similar phenomenon was also reported in Sr1.7Zn0.3CeO4:Eu3þ red phosphors [51]. 3.3. CIE color coordinates The CIE color coordinates (x, y) of the Ca2Gd(1-x)EuxSbO6 phosphors were measured according to their PL spectra under 396 nm excitation and shown in Table 4. Fig. 5 shows the CIE diagram of Ca2G d0.5Eu0.5SbO6 sample. Obviously, the Ca2Gd0.5Eu0.5SbO6 phosphors gave bright red emission under a 365 nm UV lamp. And the CIE color coordinates (0.6629, 0.3367) of the Ca2Gd0.5Eu0.5SbO6 phosphors were located at the red region, which were very close to that of the ideal red light of (0.670, 0.330) [55]. The values of color purity of Ca2Gd(1-x)EuxSbO6 phosphors were determined by using the following equation [56]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 (5) Color purity ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% ðxd xi Þ2 þ ðyd yi Þ2 Here, (x, y) present the CIE coordinates of the samples, (xi, yi) refers to the CIE coordinates of white illumination, and (xd, yd) corresponds to dominated wavelength. In this work, for the optimal Ca2Gd0.5Eu0.5SbO6 phosphors, (x, y) ¼ (0.6629, 0.3367), (xi, yi) ¼ (0.310, 0.316), and (xd, yd) ¼ (0.6829, 0.3169), so the calculated value of color purity was
(3)
where I and I0 are the intensities of luminescence at t and t ¼ 0, respectively; τ is the decay time. The values of τ were calculated to be 5
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phosphors. 3.4. IQE value To further investigate the luminescence properties, the IQE value of the optimal Ca2Gd0.5Eu0.5SbO6 sample was measured by using inte grating sphere method. Fig. 6 shows the IQE (η) value of Ca2G d0.5Eu0.5SbO6 phosphors, and the IQE value was estimated by the following expression [58]: R LS R ηIQE ¼ R (6) ER ES where LS represents the emission spectrum from the sample; ES is the integrated intensity of PLE spectrum of Ca2Gd0.5Eu0.5SbO6 phosphors; and ER indicates the excitation light spectrum of BaSO4 powder. The IQE (η) value of the Ca2Gd0.5Eu0.5SbO6 phosphors was measured to be about 73%, which was better than the commercial Y2O3:Eu3þ (12%) [59]. In addition, the value of external QE (EQE, ηEQE) could be obtained by following formula: R LS ηEQE ¼ R (7) ER Accordingly, the EQE of Ca2Gd0.5Eu0.5SbO6 phosphors was calcu lated to be 20%, which was higher than that of BaLu2Si3O10:Eu3þ phosphors (EQE ¼ 9%) [60]. This result showed that the Ca2G d0.5Eu0.5SbO6 phosphors could be used in solid-state lighting as a good red-emitting material. 3.5. Thermal stability In order to estimate the thermal stability of the Ca2Gd(1-x)EuxSbO6 phosphors, the PL spectra of the representative Ca2Gd0.5Eu0.5SbO6 phosphors were tested and analyzed with various temperatures ranging from 303 to 463 K. Temperature-dependent PL spectra of Ca2G d0.5Eu0.5SbO6 phosphors under 396 nm excitation were presented in Fig. 7(a). The emission spectra at various temperatures exhibited similar profiles, and the PL intensity reduced at higher temperatures. Fig. 7(b) shows the integrated PL intensity (550–750 nm range) of Ca2G d0.5Eu0.5SbO6 phosphors as a function of temperature. As can be seen, with the temperature increasing from 303 to 463 K, the intensity of all the peaks gradually decreased due to the thermal quenching, and the emission intensity at 423 K remained about 73% of its initial value at 303 K. This result demonstrated that the Ca2Gd0.5Eu0.5SbO6 phosphors could exhibit stable red emission light at high temperature. The activation energy (Ea) was studied by using the following equation [61,62]: � � I0 Ea ln 1 ¼ ln A (8) I kT where I0 represents the initial luminescence intensity, I is the intensity at different temperatures T; A, k and Ea are the constant, the Boltzmann coefficient and activation energy, respectively. As plotted in Fig. 7(c), the data could be well fitted, and the slope was obtained to be 0.171. So the value of the Ea of Ca2Gd0.5Eu0.5SbO6 compound was around 0.171 eV, which was better than that of Ca19Mg2(PO4)14:Eu3þ (Ea ¼ 0.14 eV), Na2Gd2B2O7:Eu3þ (Ea ¼ 0.148 eV), and Sr3Lu(PO4)3:Eu3þ (Ea ¼ 0.161 eV) phosphors [63–65]. Thus, the studies about temperature-dependent PL emphasized that the Ca2Gd0.5Eu0.5SbO6 phosphors have better thermal properties.
Fig. 7. (a) Temperature-dependent PL spectra of Ca2Gd0.5Eu0.5SbO6 phosphors under 396 nm excitation. (b) Normalized integrated PL intensity (550–750 nm range) of Ca2Gd0.5Eu0.5SbO6 phosphors at temperature from 303 to 463 K (λex ¼ 396 nm). (c) The curve of ln(I0/I-1) versus 1/kT.
94.9%, which were higher than that of the reported Eu3þ-doped red phosphors, such as Na2Gd(PO4)(MoO4):Eu3þ (92%) [31] and Ca2SiO4: Eu3þ (91.2%) [57]. The color purity values of all the Ca2Gd(1-x)EuxSbO6 samples (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were listed in Table 4, and all of these values exceeded 93%. Therefore, the as-prepared Ca2Gd(1-x)EuxSbO6 phosphors were high-color-purity red-emitting
3.6. Application in white LEDs Finally, the as-prepared Ca2Gd0.5Eu0.5SbO6 red phosphors, the commercial BAM:Eu2þ blue phosphors, and commercial (Ba,Sr)2SiO4: 6
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Fig. 8. (a) The electroluminescence spectrum of the fabricated warm-white LED device driven by 20 mA. Insets: (i) shows the fabricated LED device and (ii) demonstrates the corresponding luminescent image operated at a current of 20 mA. (b) The CIE chromaticity coordinates diagram of the fabricated warm-white LED device under 20 mA driven current.
Eu2þ green phosphors were coated on a 395 nm near-UV LED chip for manufacturing a white LED device. The potential application of Ca2G d0.5Eu0.5SbO6 red phosphors could be evaluated by testing the white LED device. Fig. 8(a) illustrates the electroluminescence spectrum of the as-fabricated white LED device under 20 mA driven current. The fabri cated LED lamp and the corresponding luminescent image were exhibited in insets (i) and (ii), respectively. Impressively, the LED device exhibited intense warm-white light with CIE color coordinates of (0.3888, 0.3943), high CRI ¼ 88.4, low CCT ¼ 3911 K, and good LE ¼ 23.2 lm/W. Fig. 8(b) shows the CIE chromaticity diagram of the asfabricated LED lamp under 20 mA driven current. The obtained CIE coordinates of (0.3888, 0.3943) were located in the warm-white light region. These results indicated that the Ca2Gd0.5Eu0.5SbO6 phosphors could be used as potential red-emitting phosphors in white LEDs.
CRediT authorship contribution statement
4. Conclusions
[1] G. Annadurai, B. Li, B. Devakumar, H. Guo, L. Sun, X. Huang, Synthesis, structural and photoluminescence properties of novel orange-red emitting Ba3Y2B6O15:Eu3þ phosphors, J. Lumin. 208 (2019) 75–81. [2] A.K. Vishwakarma, K. Jha, M. Jayasimhadri, A.S. Rao, K. Jang, B. Sivaiah, D. Haranath, Red light emitting BaNb2O6:Eu3þ phosphor for solid state lighting applications, J. Alloys Compd. 622 (2015) 97–101. [3] J. Liang, P. Du, H. Guo, L. Sun, B. Li, X. Huang, High-efficiency and thermal-stable Ca3La(GaO)3(BO3)4:Eu3þ red phosphors excited by near-UV light for white LEDs, Dyes Pigments 157 (2018) 40–46. [4] L. Sun, B. Devakumar, J. Liang, S. Wang, Q. Sun, X. Huang, Highly efficient Ce3þ → Tb3þ energy transfer induced bright narrowband green emissions from garnet-type Ca2YZr2(AlO4)3:Ce3þ,Tb3þ phosphors for white LEDs with high color rendering index, J. Mater. Chem. C 7 (2019) 10471–10480. [5] X. Huang, New red phosphors enable white LEDs to show both high luminous efficacy and color rendering index, Sci. Bull. 64 (2019) 879–880. [6] Q. Sun, S. Wang, B. Devakumar, B. Li, L. Sun, J. Liang, D. Chen, X. Huang, Novel far-red-emitting SrGdAlO4:Mn4þ phosphors with excellent responsiveness to phytochrome PFR for plant growth lighting, RSC Adv. 8 (2018) 39307–39313. [7] X. Yan, W. Li, K. Sun, Preparation and luminescent properties of a novel red emitting phosphor of Ca1 2xMxIn2O4:xEu3þ (M¼Li, Na, K) for white LED solid-state lighting, J. Alloys Compd. 508 (2010) 475–479. [8] M. Cui, J. Wang, M. Shang, J. Li, Q. Wei, P. Dang, H.S. Jang, J. Lin, Full visible light emission in Eu2þ,Mn2þ-doped Ca9LiY0.667(PO4)7 phosphors based on multiple crystal lattice substitution and energy transfer for warm white LEDs with high colour-rendering, J. Mater. Chem. C 7 (2019) 3644–3655. [9] Z. Sun, M. Wang, Z. Yang, Z. Jiang, K. Liu, Z. Ye, Enhanced red emission from Eu3 þ –Bi3þco-doped Ca2YSbO6phosphors for white light-emitting diode, J. Alloys Compd. 658 (2016) 453–458. [10] Y. Wei, L. Cao, L. Lv, G. Li, J. Hao, J. Gao, C. Su, C.C. Lin, H.S. Jang, P. Dang, J. Lin, Highly efficient blue emission and superior thermal stability of BaAl12O19:Eu2þ phosphors based on highly symmetric crystal structure, Chem. Mater. 30 (2018) 2389–2399. [11] Z. Lu, T. Huang, R. Deng, H. Wang, L. Wen, M. Huang, L. Zhou, C. Yao, Double perovskite Ca2GdNbO6:Mn4þ deep red phosphor: potential application for warm W-LEDs, Superlattice. Microst. 117 (2018) 476–487.
Zongjie Zhang: Investigation, Data curation, Writing - original draft. Liangling Sun: Investigation. Balaji Devakumar: Software. Jia Liang: Investigation. Shaoying Wang: Investigation. Qi Sun: Investigation. Sanjay J. Dhoble: Investigation. Xiaoyong Huang: Conceptualization, Supervision, Writing - review & editing. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190). References
In summary, a series of novel Ca2Gd(1-x)EuxSbO6 red-emitting phosphors were successfully manufactured, which could be efficiently excited by near-UV light and exhibited intense red emissions around 612 nm. The optimal doping concentration of Eu3þ ions was determined as x ¼ 0.5. The concentration quenching mechanism of Ca2Gd(1x)EuxSbO6 phosphors was attributed to the dipole-dipole interaction. The CIE color coordinates of the Ca2Gd0.5Eu0.5SbO6 phosphors were calculated to be (0.6629, 0.3367), which were very close to the co ordinates of ideal red light (0.670, 0.330), and its color purity was 94.9%. Meanwhile, the IQE and EQE of Ca2Gd0.5Eu0.5SbO6 phosphors reached as high as 73% and 20%, respectively. Furthermore, the PL intensity of Ca2Gd0.5Eu0.5SbO6 phosphors at 423 K remained approxi mately 73% of the value at 303 K, and Ea was found to be 0.171 eV, demonstrating that the Ca2Gd0.5Eu0.5SbO6 phosphors could retain the stable emission light at high temperature. Finally, a white LED lamp was fabricated by coating the as-prepared Ca2Gd0.5Eu0.5SbO6 red phosphors, the commercial BAM:Eu2þ blue phosphors, and commercial (Ba, Sr)2SiO4:Eu2þ green phosphors on a 395 nm near-UV LED chip. The LED lamp showed bright warm-white light with CIE color coordinates of (0.3888, 0.3943), correlated color temperature of 3911 K and color rendering index of 88.4 at a current of 20 mA. All the results made Ca2Gd0.5Eu0.5SbO6 phosphors attractive candidates as red-emitting color converters in solid-state lighting.
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Novel highly luminescent double-perovskite Ca2GdSbO6:Eu3þ red phosphors with high color purity for white LEDs: Synthesis, crystal structure, and photoluminescence properties Zongjie Zhang a, Liangling Sun a, Balaji Devakumar a, Jia Liang a, Shaoying Wang a, Qi Sun a, Sanjay J. Dhoble b, Xiaoyong Huang a, * a b
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China Department of Physics, R.T.M. Nagpur University, Nagpur, 440033, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Eu3þ ions Ca2GdSbO6 Red-emitting phosphors White LEDs Photoluminescence Double-perovskite
High-efficiency red-emitting phosphors are required to fabricate high-performance white light-emitting diodes (LEDs). Herein, the novel highly efficient Eu3þ-activated Ca2GdSbO6 double-perovskite red phosphors with good thermal stability toward warm-white LEDs were reported. A series of Ca2Gd(1-x)EuxSbO6 red phosphors with different Eu3þ doping concentrations (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were synthesized by using hightemperature solid-state reaction method. Under the excitation of 396 nm near-ultraviolet light, these Ca2Gd(15 7 3þ x)EuxSbO6 phosphors showed intense red emissions peaking at 612 nm due to the D0→ F2 transition of Eu 3þ ions. The strongest luminescence intensity reached when Eu doping concentration was x ¼ 0.5, and the critical distance between Eu3þ activators was calculated to be 7.97 Å. The concentration quenching mechanism was due to the dipole-dipole interaction of Eu3þ ions. The CIE color coordinates of the optimal Ca2Gd0.5Eu0.5SbO6 phosphors were determined to be (0.6629, 0.3367), and the corresponding color purity reached about 94.9%. Importantly, the Ca2Gd0.5Eu0.5SbO6 phosphors revealed outstanding internal quantum efficiency of 73% and good thermal stability. The emission intensity of Ca2Gd0.5Eu0.5SbO6 phosphors at 423 K still remained about 73% of its initial value at 303 K. Finally, a prototype white LED device was fabricated by coating the phosphor blend of commercial blue-emitting BaMgAl10O17:Eu2þ, green-emitting (Ba, Sr)2SiO4:Eu2þ and our as-prepared redemitting Ca2Gd0.5Eu0.5SbO6 on a 395 nm LED chip. Under 20 mA driven current, the device showed bright warmwhite light with CIE color coordinates of (0.3888, 0.3943), correlated color temperature of 3911 K, and color rendering index of 88.4. The results demonstrated that the developed novel red-emitting Ca2Gd0.5Eu0.5SbO6 phosphors could be used as potential color converters in white LEDs.
1. Introduction At present, environmental problems (resource exhaustion, water pollution and air pollution, etc.) are becoming more and more promi nent and attracting people’s attention [1]. Compared with traditional lighting sources such as incandescent lamps and fluorescent lamps, white light emitting diode (LEDs) have the advantages of low power consumption, high efficiency, long working life and environmental friendliness [2–6]. Currently, white LEDs are generally prepared by using YAG:Ce3þ yellow-emitting phosphors with blue InGaN chips, but they have high correlation color temperature (CCT) and low color rendering index (CRI) [7]. Therefore, scientific researchers are
committed to the developments of efficient white LEDs with low CCT and high CRI [8]. Among them, LED chips coated with tricolor (blue, green and red) phosphors can achieve good results. However, there is still a lack of red phosphors with good performance for white LEDs [9, 10]. Therefore, the research of red phosphors has become very urgent. Inorganic phosphors doped with rare earths with excellent optical properties have been widely used as color converters in white LEDs [11–13]. Particularly, Eu3þ doped phosphors could emit intense red light under UV excitation because of the 5D0→7FJ (J ¼ 1, 2, 3, 4) tran sition of Eu3þ ions [14–19]. Currently, double-perovskite compounds with A2BB’O6 structure have been extensively studied as hosts of lumi nescent materials. The thermal stability of these double-perovskite
* Corresponding author. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.jlumin.2020.117105 Received 26 October 2019; Received in revised form 5 February 2020; Accepted 7 February 2020 Available online 8 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. PXRD patterns of the Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors together with the standard card of Ca2GdTaO6 (JCPDS #73–0085). (b) The unit cell parameters of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors.
compounds is excellent due to abundant [B/B’O6] octahedron structures [20]. Moreover, recent researches proved that these kinds of double-perovskite compounds are outstanding host materials for Eu3þ ions, such as Ca2LuTaO6:Eu3þ [21], Ca2GdTaO6:Eu3þ [22], Ca2LuSbO6: Eu3þ [23], (Li, Na, K)LaMgWO6:Eu3þ [24], Ca3TeO6:Eu3þ [25], La2LiSbO6:Eu3þ [26], and BaLaMg(Sb, Nb)O6:Eu3þ [27]. In this paper, a series of the novel efficient red-emitting Eu3þ-acti vated Ca2GdSbO6 double-perovskite phosphors were demonstrated. The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra, luminescence decay lifetimes, internal quantum effi ciency (IQE), CIE color coordinates, color purity, and temperaturedependent emission spectra of Ca2GdSbO6:Eu3þ phosphors have been systematically studied. We observed that the as-prepared samples exhibited strong red luminescence at around 612 nm upon 396 nm excitation, and the IQE reached as high as 73%. These results indicated that Ca2Gd(1-x)EuxSbO6 phosphors could be considered as potential red phosphors for white LEDs.
reagent, Tianjin Kemiou Chemical Reagent Co., Ltd), Gd2O3 (99.99%, Jining Tianyi New Materials Co., Ltd), Sb2O5 (99%, Aladdin Industrial Corporation, Shanghai, China) and Eu2O3 (99.99%, Jining Tianyi New Materials Co., Ltd) were weighed and then mixed in an agate mortar. Afterwards, the obtained powders were transferred into crucibles and then calcined at 1500 � C for 6 h in a furnace in air. Once the reaction process was finished, the phosphor products were cooled down naturally to room temperature, and then were re-ground into to fine powders. 2.2. Characterization The phase purity of the as-prepared phosphors was identified via powder X-ray diffraction (PXRD) with Cu-Kα radiation (step size 0.02 and step time 0.2 s). The PXRD pattern Rietveld refinement of Ca2G d0.5Eu0.5SbO6 sample was performed with general structure analysis system (GSAS) [28,29], and the Vesta software was used for generating the crystal structure. An Edinburgh FS5 spectrometer, using a 150 W continuous-wave Xe lamp as a light source, was used to record the PLE and PL spectra of the samples. Decay curves and IQE of the samples were also obtained from the Edinburgh FS5 spectrometer equipped with a pulsed xenon lamp and an integrating sphere, respectively. All the above measurements were performed at room temperature. The temperature-dependent emission spectra were measured by the Edin burgh FS5 spectrometer equipped with a temperature controller.
2. Experimental section 2.1. Synthesis of the samples A series of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors with various Eu3þ doping concentrations were synthesized by high-temperature solid-state reaction. On the basis of the stoichiometric amounts of the formula, the raw materials including CaCO3 (analytical
Fig. 2. The Rietveld refinement of the PXRD pattern (a) and the crystal structure (b) of Ca2Gd0.5Eu0.5SbO6 phosphors. 2
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Table 1 Refined crystallographic parameters of Ca2Gd0.5Eu0.5SbO6 phosphors.
Table 3 Bond distances of Ca2Gd0.5Eu0.5SbO6 phosphors.
Ca2Gd0.5Eu0.5SbO6 Crystal system Space group Lattice parameters
Monoclinic P 21/n a ¼ 5.597(11) Å b ¼ 5.858(11) Å c ¼ 8.095(16) Å α ¼ γ ¼ 90� β ¼ 89.723(16) � V ¼ 265.439(6) Å3
Unit cell volume
Table 2 Atomic coordinates, occupancy and displacement parameters of Ca2G d0.5Eu0.5SbO6 phosphors. Atom
x
y
z
Wycoff position
Occupancy
Uiso(Ǻ2)
Ca
0.518 (3) 0 0 0 0.111 (14) 0.293 (3) 0.170 (8)
0.052 (6) 0 0 0 0.045 (2) 0.181 (4) 0.286 (3)
0.247 (6) 0.5 0.5 0 0.236 (3) 0.060 (4) 0.062 (4)
4e
1
2a 2a 2c 4e
0.5 0.5 1 1
4e
1
4e
1
0.034 (6) 0.044 0.044 0.036 0.055 (3) 0.056 (7) 0.047 (3)
Gd Eu Sb O1 O2 O3
Bond
Bond distance(Ǻ)
Ca–O1 Ca–O1 Ca–O2 Ca–O2 Ca–O3 Ca–O3 Ca–O3 (Gd–O1) � 2 (Gd–O2) � 2 (Gd–O3) � 2 (Sb–O1) � 2 (Sb–O2) � 2 (Sb–O3) � 2
2.356 2.476 2.707 2.919 2.916 2.703 2.363 2.251 2.263 2.30 2.044 2.023 2.005
2.3. White LED device fabrication The as-prepared Ca2Gd0.5Eu0.5SbO6 red phosphors, the commercial BaMgAl10O17:Eu2þ (BAM:Eu2þ) blue phosphors, commercial (Ba, Sr)2SiO4:Eu2þ green phosphors and silicone were mixed fully and then coated on a 395 nm near-UV LED chip to fabricate a prototype white LED device. An integrating sphere spectroradiometer (HAAS2000, Everfine) system was used to test the photoelectric properties of the LED device such as CRI, CCT, luminous efficacy (LE) and electroluminescence spectrum.
Fig. 3. The PLE (λem ¼ 612 nm) and PL (λex ¼ 396 nm) spectra of Ca2G d0.5Eu0.5SbO6 phosphors.
monoclinic structure with the space group of P21/n and the crystal pa rameters were found to be a ¼ 5.597(11) Å, b ¼ 5.858(11) Å, c ¼ 8.095 (16) Å, and V ¼ 265.439(6) Å3 (see Table 1). Atomic coordinates and displacement parameters obtained by the Rietveld refinement were lis ted in Table 2, while the bond distances of Ca2Gd0.5Eu0.5SbO6 phosphors were listed in Table 3. The crystal structure of the Ca2Gd0.5Eu0.5SbO6 phosphor was displayed in Fig. 2(b). In Ca2GdSbO6 host, the Sb5þ and Gd3þ ions were connected with six shared oxygen atoms into a [SbO6]/ [GdO6] octahedral.
3. Results and discussion 3.1. Phase purity and crystal structure The recorded PXRD patterns of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors together with the standard card of Ca2GdTaO6 (JCPDS #73–0085) were shown in Fig. 1(a). When x was higher than 0.4, the PXRD patterns of Ca2Gd(1-x)EuxSbO6 samples could match with the standard card except a faint difference peak around 2θ ¼ 53.9� . These powder samples with main phase of the Ca2Gd(1-x)EuxSbO6 single crystal combined with a small amount of CaO isostructural phase. The effect of impurity could be neglected because the intensity of dif ference peak was too low. The PXRD patterns of Ca2Gd(1-x)EuxSbO6 samples with other Eu3þ concentrations could match well with the standard card. The unit cell parameters of (including β, a, b, c, and V) of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors were shown in Fig. 1(b). With increasing Eu3þ concentration, a, b, c, and V presented an increasing trend. The ionic radii of Eu3þ (r ¼ 0.95 Å) is slightly larger than Gd3þ (r ¼ 0.94 Å) ions [22], and Eu3þ and Gd3þ ions have same valence. Thus, the Eu3þ ions were likely to replace Gd3þ ions in the Ca2GdSbO6 crystal lattice, and then formed Ca2Gd(1-x)EuxSbO6 solid solutions. Furthermore, the PXRD refinement results of Ca2Gd0.5Eu0.5SbO6 phosphors were displayed in Fig. 2. Through Rietveld refinement of PXRD pattern of Ca2Gd0.5Eu0.5SbO6 phosphors, we could fully under stand the crystal structure of the sample. The sample crystallized in
3.2. Photoluminescence properties Fig. 3 shows the PLE and PL spectra of Ca2Gd0.5Eu0.5SbO6 sample. The PLE spectrum monitored at 612 nm consisted of a broad excitation band in the range from 200 to 310 nm and a series of sharp excitation peaks ranging from 310 to 550 nm. The excitation band in the 250–310 nm wavelength range peaking at 294 nm was derived from the O2 →Eu3þ charge transfer band (CTB) [30–32]. The highest sharp excitation peak was obtained at 396 nm, which could be attributed to the 7F0→5L6 transition of Eu3þ ions. The others sharp excitation peaks at 322, 364, 383, 416, 466, and 535 nm were assigned to the transitions from 7F0 level to 5H6, 5D4, 5G2, 5D3, 5D2, and 5D1 levels of Eu3þ ions, respectively [33,34]. The observed results indicated that near-UV LED chip could efficiently excite the Ca2Gd0.5Eu0.5SbO6 phosphors. Under the excitation of 396 nm, the Ca2Gd0.5Eu0.5SbO6 phosphors showed bright red light. The corresponding PL spectrum included four main emission peaks at 591, 612, 656 and 702 nm, corresponding to the 5 D0→7F1, 5D0→7F2, 5D0→7F3, and 5D0→7F4 transitions of Eu3þ ions, 3
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Fig. 4. (a) PL spectra of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors under 396 nm excitation. (b) The PL intensity as a function of Eu3þ doping concentration in Ca2Gd(1-x)EuxSbO6 phosphors under 396 nm excitation. (c) The relationship of log(I/x) versus log(x) in Ca2Gd(1-x)EuxSbO6 phosphors. (d) Decay curves of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors excited at 396 nm when monitored at 612 nm.
respectively [35]. In the PL spectrum, the strongest emission peak was located at 612 nm. In order to find the optimal doping concentration of Eu3þ ions in Ca2GdSbO6:Eu3þ phosphors, a series of Ca2Gd(1-x)EuxSbO6 samples were prepared with various Eu3þ concentrations and measured their PL spectra. Fig. 4(a) displays the PL spectra of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors excited at 396 nm. All these samples exhibited intense red emissions. Clearly, all the PL spectra of the samples showed similar emission profiles with strong sharp red emission lines peaking at around 591, 612, 656 and 702 nm. Moreover, as shown in Fig. 4(b), the emission intensity obviously increased with increasing the Eu3þ doping concentration and reached a maximum when x ¼ 0.5, and then decreased when the Eu3þ concentration further increased owing to the concentration quenching effect. The optimal doping con centration of Eu3þ ions was x ¼ 0.5. The concentration quenching mechanism could be judged by the value of the critical distance (Rc) between the nearest Eu3þ ions. If Rc exceeds 5 Å, the electric multipolar interaction will be the dominant mechanism of the concentration quenching phenomenon; if not, the mechanism for concentration quenching will be an exchange interaction [30]. The following equation could be used to calculate the Rc [36–38]: � �1=3 3V (1) Rc ¼ 2 4π xc N
Table 4 The CIE chromaticity coordinates, color purity and asymmetry ratio of Ca2Gd(1x)EuxSbO6 phosphors. Eu3þ concentration (x)
Asymmetry ratio
CIE coordinates
Color purity
0.2 0.3 0.4 0.5 0.6 0.7 0.8
5.75 5.88 5.97 6.00 5.97 6.00 6.05
(0.6578, (0.6608, (0.6622, (0.6629, (0.6629, (0.6626, (0.6623,
93.4% 94.3% 94.7% 94.9% 94.8% 94.8% 94.7%
0.3414) 0.3386) 0.3373) 0.3367) 0.3366) 0.3369) 0.3371)
Eu3þ ions in Ca2Gd(1-x)EuxSbO6 phosphors, we used the following equation [39,40]: � � I ¼ k 1 þ βðxÞθ=3 x
1
(2)
where x and I are the doping concentration of Eu3þ ions and PL intensity of samples, respectively; k and β are constants; θ is the electric multipole index and the corresponding value of θ ¼ 6, 8 and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interac tion, respectively [41–43]. Fig. 4(c) shows the relationship between log (I/x) and log(x). The slope of the fitted line was obtained to be 2.16. Thus, the calculated value of θ was 6.48, which was close to 6. There fore, the dipole-dipole interaction could be attributed to be the main mechanism for concentration quenching effect in Ca2Gd(1-x)EuxSbO6 phosphors. As is well known, the asymmetry ratio (the (5D0→7F2)/(5D0→7F1) emission intensity ratio, namely, R/O ratio) can be used to represent the site symmetry of the Eu3þ ions in the lattice [44]. The magnetic dipole transition (5D0→7F1) and the electric dipole transition (5D0→7F2) are
where V stands for the volume of the unit cell; xc is the critical con centration of Eu3þ ions; N refers to the number of sites that can be available for activator ions in the unit cell. For Ca2Gd(1-x)EuxSbO6 phosphors, xc ¼ 0.5, N ¼ 2 and V ¼ 265.439 Å3, and then the Rc was calculated to be 7.97 Å. Therefore, the concentration quenching was caused by the electric multipolar interaction. In order to further research the energy transfer interaction between 4
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Fig. 6. The IQE of the Ca2Gd0.5Eu0.5SbO6 sample was measured by using the integrating sphere within the range of 376–750 nm (λex ¼ 396 nm). The inset shows the magnified view of the emission spectrum within the range of 575–725 nm.
0.863, 0.833, 0.773, and 0.682 corresponding x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. The observed mono-exponential decay curves meant that interaction between Eu3þ ions hardly took place [51]. However, when Eu3þ content x was higher than 0.5, the distance between Eu3þ activators largely decreased and the energy transfer became more frequent, resulting in great enhancement of non-radiative energy transfer and providing an extra decay channel which changed the decay curves [52]. Accordingly, the decay curves for the Ca2Gd(1- x)EuxSbO6 samples with x up to 0.6 were well fitted by double expo nential function [53,54]:
Fig. 5. The CIE chromaticity diagram of Ca2Gd0.5Eu0.5SbO6 phosphors upon 396 nm excitation. Inset displays the digital photograph of Ca2Gd0.5Eu0.5SbO6 sample under a 365 nm UV lamp.
related with Eu3þ ions centered at the inversion symmetry and non-inversion symmetry in the crystal structure, respectively [45]. All the values of asymmetry ratio for Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors were determined based on their PL spectra excited at 396 nm, and the results were listed in Table 4. The R/O values calculated for samples are between 5.75 and 6.05, which clearly illustrated that Eu3þ ions occupied low symmetry sites in the Ca2GdSbO6 host lattice without an inversion center [46]. The high R/O ratio is beneficial for achieving high-color-purity red emission. In this work, the R/O ratio value of Ca2Gd(1-x)EuxSbO6 phosphors was much higher than that of some previously reported red phosphors, such as Ca9Gd(PO4)7:Eu3þ (R/O ratio ¼ 4.43) [47] and Sr3La(PO4)3:Eu3þ (R/O ratio ¼ 2.445) [48]. Fig. 4(d) exhibits the decay curves (λem ¼ 612 nm; λex ¼ 396 nm) of Ca2Gd(1-x)EuxSbO6 (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) phosphors. Mono-exponential decays were observed in the samples with Eu3þ content x below 0.6. By contrast, at higher concentrations (x ¼0.6, 0.7 and 0.8), the observed decay curves were nonexponential, and the nonexponential change became more prominent as Eu3þ content increased, indicating that more than one relaxation process existed. Generally, the distance between two neighboring activators reduced when their concentrations increased, and then the energy transfer pro cess between activators became more frequent, which will act as an extra luminescence decay channel. As a result, the decay time could be critically influenced by the energy migration process between activators in the host. Moreover, the relaxation rate of the extra energy-migration process may be different, yielding nonexponential decay curves. Since Eu3þ ions only occupied one site in Ca2GdSbO6 host, thus if there was no interaction between Eu3þ ions, the decay curve should be a monoexponential function. We found that when x was below 0.6, the decay curves could be fitted into single exponential function [49,50]: I ¼ I0 expð t = τÞ
IðtÞ ¼ A1 expð
t = τ1 Þ þ A2 expð
t = τ2 Þ
(4)
where I(t) represents the intensity of luminescence at time t; A1 and A2 are constants; τ1 is the decay time for the exponential components associated with the slow decay process; and τ2 is the decay time for the exponential components associated with the rapid one. The decay life times were calculated to be 0.536, 0.434 and 0.326 ms for x ¼ 0.6, 0.7 and 0.8, respectively. Similar phenomenon was also reported in Sr1.7Zn0.3CeO4:Eu3þ red phosphors [51]. 3.3. CIE color coordinates The CIE color coordinates (x, y) of the Ca2Gd(1-x)EuxSbO6 phosphors were measured according to their PL spectra under 396 nm excitation and shown in Table 4. Fig. 5 shows the CIE diagram of Ca2G d0.5Eu0.5SbO6 sample. Obviously, the Ca2Gd0.5Eu0.5SbO6 phosphors gave bright red emission under a 365 nm UV lamp. And the CIE color coordinates (0.6629, 0.3367) of the Ca2Gd0.5Eu0.5SbO6 phosphors were located at the red region, which were very close to that of the ideal red light of (0.670, 0.330) [55]. The values of color purity of Ca2Gd(1-x)EuxSbO6 phosphors were determined by using the following equation [56]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 (5) Color purity ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% ðxd xi Þ2 þ ðyd yi Þ2 Here, (x, y) present the CIE coordinates of the samples, (xi, yi) refers to the CIE coordinates of white illumination, and (xd, yd) corresponds to dominated wavelength. In this work, for the optimal Ca2Gd0.5Eu0.5SbO6 phosphors, (x, y) ¼ (0.6629, 0.3367), (xi, yi) ¼ (0.310, 0.316), and (xd, yd) ¼ (0.6829, 0.3169), so the calculated value of color purity was
(3)
where I and I0 are the intensities of luminescence at t and t ¼ 0, respectively; τ is the decay time. The values of τ were calculated to be 5
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phosphors. 3.4. IQE value To further investigate the luminescence properties, the IQE value of the optimal Ca2Gd0.5Eu0.5SbO6 sample was measured by using inte grating sphere method. Fig. 6 shows the IQE (η) value of Ca2G d0.5Eu0.5SbO6 phosphors, and the IQE value was estimated by the following expression [58]: R LS R ηIQE ¼ R (6) ER ES where LS represents the emission spectrum from the sample; ES is the integrated intensity of PLE spectrum of Ca2Gd0.5Eu0.5SbO6 phosphors; and ER indicates the excitation light spectrum of BaSO4 powder. The IQE (η) value of the Ca2Gd0.5Eu0.5SbO6 phosphors was measured to be about 73%, which was better than the commercial Y2O3:Eu3þ (12%) [59]. In addition, the value of external QE (EQE, ηEQE) could be obtained by following formula: R LS ηEQE ¼ R (7) ER Accordingly, the EQE of Ca2Gd0.5Eu0.5SbO6 phosphors was calcu lated to be 20%, which was higher than that of BaLu2Si3O10:Eu3þ phosphors (EQE ¼ 9%) [60]. This result showed that the Ca2G d0.5Eu0.5SbO6 phosphors could be used in solid-state lighting as a good red-emitting material. 3.5. Thermal stability In order to estimate the thermal stability of the Ca2Gd(1-x)EuxSbO6 phosphors, the PL spectra of the representative Ca2Gd0.5Eu0.5SbO6 phosphors were tested and analyzed with various temperatures ranging from 303 to 463 K. Temperature-dependent PL spectra of Ca2G d0.5Eu0.5SbO6 phosphors under 396 nm excitation were presented in Fig. 7(a). The emission spectra at various temperatures exhibited similar profiles, and the PL intensity reduced at higher temperatures. Fig. 7(b) shows the integrated PL intensity (550–750 nm range) of Ca2G d0.5Eu0.5SbO6 phosphors as a function of temperature. As can be seen, with the temperature increasing from 303 to 463 K, the intensity of all the peaks gradually decreased due to the thermal quenching, and the emission intensity at 423 K remained about 73% of its initial value at 303 K. This result demonstrated that the Ca2Gd0.5Eu0.5SbO6 phosphors could exhibit stable red emission light at high temperature. The activation energy (Ea) was studied by using the following equation [61,62]: � � I0 Ea ln 1 ¼ ln A (8) I kT where I0 represents the initial luminescence intensity, I is the intensity at different temperatures T; A, k and Ea are the constant, the Boltzmann coefficient and activation energy, respectively. As plotted in Fig. 7(c), the data could be well fitted, and the slope was obtained to be 0.171. So the value of the Ea of Ca2Gd0.5Eu0.5SbO6 compound was around 0.171 eV, which was better than that of Ca19Mg2(PO4)14:Eu3þ (Ea ¼ 0.14 eV), Na2Gd2B2O7:Eu3þ (Ea ¼ 0.148 eV), and Sr3Lu(PO4)3:Eu3þ (Ea ¼ 0.161 eV) phosphors [63–65]. Thus, the studies about temperature-dependent PL emphasized that the Ca2Gd0.5Eu0.5SbO6 phosphors have better thermal properties.
Fig. 7. (a) Temperature-dependent PL spectra of Ca2Gd0.5Eu0.5SbO6 phosphors under 396 nm excitation. (b) Normalized integrated PL intensity (550–750 nm range) of Ca2Gd0.5Eu0.5SbO6 phosphors at temperature from 303 to 463 K (λex ¼ 396 nm). (c) The curve of ln(I0/I-1) versus 1/kT.
94.9%, which were higher than that of the reported Eu3þ-doped red phosphors, such as Na2Gd(PO4)(MoO4):Eu3þ (92%) [31] and Ca2SiO4: Eu3þ (91.2%) [57]. The color purity values of all the Ca2Gd(1-x)EuxSbO6 samples (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were listed in Table 4, and all of these values exceeded 93%. Therefore, the as-prepared Ca2Gd(1-x)EuxSbO6 phosphors were high-color-purity red-emitting
3.6. Application in white LEDs Finally, the as-prepared Ca2Gd0.5Eu0.5SbO6 red phosphors, the commercial BAM:Eu2þ blue phosphors, and commercial (Ba,Sr)2SiO4: 6
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Fig. 8. (a) The electroluminescence spectrum of the fabricated warm-white LED device driven by 20 mA. Insets: (i) shows the fabricated LED device and (ii) demonstrates the corresponding luminescent image operated at a current of 20 mA. (b) The CIE chromaticity coordinates diagram of the fabricated warm-white LED device under 20 mA driven current.
Eu2þ green phosphors were coated on a 395 nm near-UV LED chip for manufacturing a white LED device. The potential application of Ca2G d0.5Eu0.5SbO6 red phosphors could be evaluated by testing the white LED device. Fig. 8(a) illustrates the electroluminescence spectrum of the as-fabricated white LED device under 20 mA driven current. The fabri cated LED lamp and the corresponding luminescent image were exhibited in insets (i) and (ii), respectively. Impressively, the LED device exhibited intense warm-white light with CIE color coordinates of (0.3888, 0.3943), high CRI ¼ 88.4, low CCT ¼ 3911 K, and good LE ¼ 23.2 lm/W. Fig. 8(b) shows the CIE chromaticity diagram of the asfabricated LED lamp under 20 mA driven current. The obtained CIE coordinates of (0.3888, 0.3943) were located in the warm-white light region. These results indicated that the Ca2Gd0.5Eu0.5SbO6 phosphors could be used as potential red-emitting phosphors in white LEDs.
CRediT authorship contribution statement
4. Conclusions
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Zongjie Zhang: Investigation, Data curation, Writing - original draft. Liangling Sun: Investigation. Balaji Devakumar: Software. Jia Liang: Investigation. Shaoying Wang: Investigation. Qi Sun: Investigation. Sanjay J. Dhoble: Investigation. Xiaoyong Huang: Conceptualization, Supervision, Writing - review & editing. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190). References
In summary, a series of novel Ca2Gd(1-x)EuxSbO6 red-emitting phosphors were successfully manufactured, which could be efficiently excited by near-UV light and exhibited intense red emissions around 612 nm. The optimal doping concentration of Eu3þ ions was determined as x ¼ 0.5. The concentration quenching mechanism of Ca2Gd(1x)EuxSbO6 phosphors was attributed to the dipole-dipole interaction. The CIE color coordinates of the Ca2Gd0.5Eu0.5SbO6 phosphors were calculated to be (0.6629, 0.3367), which were very close to the co ordinates of ideal red light (0.670, 0.330), and its color purity was 94.9%. Meanwhile, the IQE and EQE of Ca2Gd0.5Eu0.5SbO6 phosphors reached as high as 73% and 20%, respectively. Furthermore, the PL intensity of Ca2Gd0.5Eu0.5SbO6 phosphors at 423 K remained approxi mately 73% of the value at 303 K, and Ea was found to be 0.171 eV, demonstrating that the Ca2Gd0.5Eu0.5SbO6 phosphors could retain the stable emission light at high temperature. Finally, a white LED lamp was fabricated by coating the as-prepared Ca2Gd0.5Eu0.5SbO6 red phosphors, the commercial BAM:Eu2þ blue phosphors, and commercial (Ba, Sr)2SiO4:Eu2þ green phosphors on a 395 nm near-UV LED chip. The LED lamp showed bright warm-white light with CIE color coordinates of (0.3888, 0.3943), correlated color temperature of 3911 K and color rendering index of 88.4 at a current of 20 mA. All the results made Ca2Gd0.5Eu0.5SbO6 phosphors attractive candidates as red-emitting color converters in solid-state lighting.
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