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Sr2LaSbO6:Mn4+ far-red phosphor for plant cultivation: Synthesis, luminescence properties and emission enhancement by Al3+ ions
Journal of Luminescence 221 (2020) 117091
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Sr2LaSbO6:Mn4þ far-red phosphor for plant cultivation: Synthesis, luminescence properties and emission enhancement by Al3þ ions Lei Shi a, Shuang Wang a, Ya-jie Han a, Zhi-xin Ji a, Di Ma b, Zhong-fei Mu c, Zhi-yong Mao d, Da-jian Wang d, Zhi-wei Zhang a, *, Lu Liu a, ** a
Chemical Engineering College, Hebei Normal University of Science and Technology, Qinhuangdao, 066600, China College of Biomass Science and Engineering, SiChuan University, Chengdu, 610065, China School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou, 510006, PR China d Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of Technology, Tianjin, 300384, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Sr2LaSbO6:Mn4þ Al3þ Photoluminescence Far-red emitting phosphors Plant cultivation and growth LEDs
A series of novel Mn4þ-activated double perovskite antimonate Sr2LaSbO6 (SLSO) far-red emitting phosphors were successfully synthesized by a high-temperature solid-state method. X-ray diffraction (XRD), photo luminescence excitation (PLE) and emission (PL) spectra, temperature-dependent photoluminescence spectra, CIE chromaticity coordinates, as well as internal quantum efficiency (IQE) were used to analyze these phosphor samples. Under 334 nm excitation, the SLSO:Mn4þ phosphors emit far red emission peaking around 694 nm due to the Mn4þ: 2Eg→4A2g transition. The optimal doping concentration of Mn4þ was 0.8 mol%, moreover, the critical distance (Rc) was 40.89 Å. The dipole-quadrupole interaction may be responsible for the concentration quenching mechanism of Mn4þ ions in the SLSO:Mn4þ phosphors. Furthermore, various Al3þ improved SLSO:0.008Mn4þ phosphors are investigated. When Al3þ is doped with 0.8%, the PL intensity increased by 2.92 times. In addition, the IQE of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ were found to be as high as 35.2% and 38.1%, respectively. Impressively, all chromaticity coordinates of samples located in far-red region. Therefore, these superior luminescence properties of SLSO:Mn4þ sample demonstrated its potential applications in plant cultivation and growth light-emitting diodes (LEDs).
1. Introduction Nowadays, with the frequent occurrence of natural disasters, tradi tional agriculture is facing more and more risks and challenges, such as climate change, drought, flooding and so on. In order to make plants grow safely and effectively, as well as meet human needs, people are committed to developing greenhouse industry[1–5]. There are many factors affecting plant growth, among light accounts for a large pro portion. Previous studies have shown that different wavelength spectra of light exhibit different effects on plant growth. For instance, blue light (400–500 nm) mainly participates in photosynthesis, red light (620–690 nm) mainly affects phototaxis of plants. Importantly, the ef fects of far red light (700–740 nm) absorbed by photosensitive pigment PFR on plant morphology, including seed germination, depolarization, stem elongation, leaf expansion, shade avoidance and flowering induc tion [6–10]. Therefore, it is urgent to find a reliable light source to
regulate the growth of plants. The LEDs have become the main artificial light source for plant cultivation because of its controllable light-color, low power consumption, low radiation heat output and environmental friendliness[11–15]. The preparation technology of blue and red LEDs have become mature, while the far-red emission LED manufacturing technology needs to be improved, in especial, the far-red emission phosphor. Therefore, the purpose of this paper is to develop a novel far-red phosphor for plant growth LED to meet the requirements of plant growth. Generally speaking, Mn4þ ions with 3d3 electronic configuration can exist stably in octahedral coordination environment. In addition, as a non-rare earth element, Mn is cheaper than most rare earth elements, such as Eu, Dy, Ce and Yb. At the same time, Mn4þ ions embody strong absorption ability from near-ultraviolet (UV) to blue region in different crystal field environments, showing red or deep red emission in the range of 600–750 nm [16–22]. Therefore, the strong emission
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.-w. Zhang), [email protected] (L. Liu). https://doi.org/10.1016/j.jlumin.2020.117091 Received 1 December 2019; Received in revised form 15 January 2020; Accepted 2 February 2020 Available online 5 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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characteristics of Mn4þ-activated phosphors in the far-red region match well with the spectral range required for plant growth and can be applied to indoor plant lighting. In addition, Mn4þ-activated double perovskite structure phosphors have been widely reported for their excellent physical, chemical properties, thermal stability and optical properties, such as Ba2LaSbO6:Mn4þ [23], Ba2LaNbO6:Mn4þ [24], CaYMgSbO6:Mn4þ [25]. Meanwhile, Sr2LaSbO6 (SLSO) with [SbO6] octahedron belongs to A2BB’O6 type double perovskite, which makes it capably activated by Mn4þ to become a good luminescent materials. The luminescent properties of the phosphor can be improved by doping Al3þ. For example, Al3þ-doped can improve the PL intensity of LaBSiO5:Eu3þ phosphor[26]. Under the excitation of 464 nm, the PL intensity of Ca3Sr3(VO4)4: 0.05Eu3þ, 0.06Al3þ is about 1.47 times that of Ca3Sr3(VO4)4: 0.05Eu3þ [27]. The PL intensity of LiGaTiO4:Mn4þ phosphor can be increased by 2.2 times by co-doping Al3þ ions[28]. The incorporation of Al3þ into Ca3Sc2Si3O12:Ce3þ can inhibit the formation of the impurity phases Sc2O3 and CeO2, and enhance the photo luminescence intensity as well as quantum efficiency[29]. The substi tution of Sc3þ with Al3þ increased the crystal field splitting of Ce3þ and resulted in the red shift of photoluminescence. SLSO:Mn4þ phosphors were synthesized by high temperature solid state reaction in air atmosphere. The crystal structure and morphology of the samples were characterized, and the concentration quenching mechanism and luminescence mechanism were studied in detail. SLSO: Mn4þ phosphors possess excellent luminescent properties. Furthermore, the PL intensity and IQE of the SLSO:Mn4þ is further improved by codoping Al3þ ions, because Al3þ ions can hinder the energy transfer be tween Mn4þ-Mn4þ ion pairs. All the results show that the synthesized phosphors have excellent luminescent properties and are suitable for plant growth lighting.
Fig. 1. Schematic diagram of SLSO crystal structure. Table 1 Radius deviation between host ions and doped ions. Ions La3þ Sb5þ Sr2þ
2. Experimental section 2.1. Materials synthesis SLSO:xMn4þ (x ¼ 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.013 and 0.015) and Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors were synthesized by high temperature solid-state method and SrCO3, La2O3, Al2O3, Sb2O3 and MnCO3 are the main raw materials. The chemical reaction equation is as follows:
8SrCO3 þ2La2 O3 þ 2ð1
xÞSb2 O3 þ xO2 þ 4xMnCO3 ¼ 4Sr2 LaSbð1
CN 6 6 6
Radius (Å) 1.03 0.60 1.18
Rr (%) Mn4þ (CN ¼ 6, 0.53 Å)
Al3þ (CN ¼ 6, 0.535 Å)
48.5 11.67 55.08
48.06 10.83 54.66
The thermal stability of samples was measured by a fluorescence spec trometer (Hitachi F-7000) equipped with a temperature controller. The internal quantum efficiencies (IQE) of phosphors were measured with barium sulfate coated integrating sphere on spectrophotometer.
xÞO6 :xMn4þ þ ð4x þ 8ÞCO2 ↑
3. Results and discussion 3.1. SLSO:xMn4þphosphors
The raw materials were weighed according to the stoichiometric ratio, and then ground. The ground powders were sintered at 1300 � C for 10 h in a muffle furnace. The samples after sintering were ground again.
3.1.1. Crystal structure Fig. 1 displays a schematic diagram of SLSO crystal structure. SLSO crystal belongs to double perovskite type structure. It has monoclinic structure with space group P21/n, and the lattice parameters are as follows: a ¼ 5.816 Å, b ¼ 5.880 Å, c ¼ 8.304 Å, α ¼ γ ¼ 90� , and β ¼ 90.19� , V ¼ 573.32 Å3[30, 31]. In this lattice, La3þ and Sb5þ ions occupy octahedral sites by sharing an arrangement of oxygen atoms. In addition, Sr2þ ions are located in the center of the gap between [SbO6] and [LaO6] octahedrons. The premise of ion substitution is the radius deviation (Rr) between doped and substituted ions is reasonable within 30%[32, 33]. In order to analyze which sites Mn4þ or/and Al3þ ions occupy in the host lattice, the radius deviation of the related ions is calculated by equation (1), and the
2.2. Sample characterization The as-prepared samples were analyzed by X-ray powder diffrac tometer (D/MAX 2500 TC, 40 KV and 200 mA). The morphology and particle size of the samples were observed by field emission scanning electron microscopy (FESEM) (Hitachi-8010). The diffuse reflectance (DR) spectra of powder samples were collected by ultraviolet-2450 spectrophotometer (Ultraviolet-2450 of Shimadzu, Japan) with BaSO4 as reference material. The PL and PLE spectra of phosphors were measured by using Hitachi F-7000 fluorescence spectrophotometer under 400 V voltage and using xenon lamp (150W) as excitation source. 2
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Fig. 2. (a)XRD patterns of Sr2LaSb(1-x)O6:xMn4þ (x ¼ 0.005–0.015) phosphors; (b) Local enlargement of (a).
Fig. 3. FESEM images of SLSO:0.008Mn4þ:(a) 5000 � ; (b) 25,000 � .
inferred from Table 1 that when Mn4þ or/and Al3þ ions are introduced into SLSO crystals, they tend to occupy the octahedral position of [SbO6], rather than [LaO6] or [SrO6] octahedral.
results are listed in Table 1. Rr ¼ 100 �
Rm ðCNÞ Rd ðCNÞ Rm ðCNÞ
(1)
3.1.2. phase analysis Fig. 2(a) shows the XRD patterns of as-synthetized SLSO: xMn4þ phosphor. Here, compare the XRD pattern of SLSO: Mn4þ with the
where Rm(CN) and Rd(CN) are radii of matrix ions with different coor dination numbers (CN) and doped ions, respectively. Therefore, it can be
Fig. 4. (a) PLE spectra of SLSO:xMn4þ measured at room temperature (λem ¼ 694 nm); (b) The PLE spectrum of SLSO:0.008 Mn4þ using Gaussian decomposi tion method. 3
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Journal of Luminescence 221 (2020) 117091
Fig. 6. Log(I/x) dependent on log(x).
those of the standard card 47–0461 (SLNO), indicating that SLSO: xMn4þ phosphors have been successfully synthesized and Mn4þ-doping will not change the crystal structure of SLSO. Fig. 2(b) illustrates a partially enlarged view of Fig. 2(a) in the range from 29.5 to 31� . Apparently, the diffraction peak shifts to a large angle with the increase of Mn4þ doping concentration. This phenomenon can be explained by Bragg’s law: 2dsinθ ¼ nλ, in this formula, symbol λ, d, and θ represent the wavelength of the X-ray, interplanar spacing and diffraction angle, respectively. When Sb5þ (CN ¼ 6, 0.6 Å) is replaced by smaller Mn4þ (CN ¼ 6, 0.53 Å), the plane spacing of the crystal cells decreases, leading to the contraction of the lattice cell, which causes the diffraction peak to move to a large angle. 3.1.3. FESEM Fig. 3 presents the SEM images of the SLSO:0.008Mn4þ phosphors. In the sintering process, most of the small particles will gather together. SLSO:0.008Mn4þ phosphor exhibits irregular morphology and the par ticle size is smaller than 4 μm. It can be clearly seen from the figure that the surface of the particles is smooth and no voids and stomata, indi cating that there are almost no defects in the particles. 3.1.4. Luminescence properties Fig. 4 shows the PLE spectra of SLSO: xMn4þ (x ¼ 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.013 and 0.015) phosphors and the Gauss decomposition spectrum of SLSO: 0.008Mn4þ monitored at 694 nm, respectively. The PLE spectrum of SLSO:0.008Mn4þ can be decomposed into two excitation bands using Gauss decomposition method, which can be assigned to charge transfer band (CTB) of O2 →Mn4þ (4.51 eV), 4A2g→4T1g (4.04 eV), 4A2g→2T2g (3.57 eV) and 4 A2g→4T2g (2.59 eV) transitions of Mn4þ ions [34]. The PLE spectra indicate that SLSO: xMn4þ can be effectively excited by AlInGaN chip [35]. Fig. 5(a) depicts the PL spectra of SLSO: xMn4þ phosphors (x ¼ 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.013 and 0.015) under 334 nm excitation. The PL spectra exhibit red emission peaking at 694 nm, which is imputed to the Mn4þ: 2Eg→4A2g transition[36,37]. Fig. 5(b) shows the normalized PL spectrum of SLSO:0.008Mn4þ and the ab sorption spectrum of phytochrome PFR and PR, the absorption peak of PFR is near 713 nm [6]. Distinctly, the emission spectrum of SLSO:0.008Mn4þ is completely covered by the absorption spectrum of PFR. Therefore, the far-red emission wavelength of SLSO:Mn4þ phosphor can be well matched with the absorption wavelength of phytochrome PFR, indicating that SLSO: Mn4þ phosphor has potential application for indoor plant growth under artificial light. Fig. 5(c) illustrates the relationship between the integrated PL
Fig. 5. (a) PL spectra of SLSO:xMn4þ measured at room temperature (λex ¼ 334 nm); (b) Normalized PL spectrum of SLSO:0.008Mn4þ, the absorption spectrum of phytochrome PFR and PR; (c) The integrated PL intensity versus the concentration of Mn4þ.
standard card of Sr2LaNbO6 (SLNO). Theoretically, SLSO: Mn4þ and SLNO have the same crystal structure, so the positions of the diffraction peaks are similar. However, due to the small difference in the radii of Sb5þ/Mn4þ and Nb5þ, there is a slight difference in the positions of diffraction peaks between the two substances. Therefore, the standard card (SLNO) can be used as a criterion to judge whether SLSO: Mn4þ is synthesized. The diffraction patterns of all the samples coincide with 4
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Fig. 7. XRD pattern of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors; (b) Local enlargement of (a).
Fig. 8. UV-VIS DR spectra of SLSO, SLSO:0.005Mn4þ, SLSO:0.008Mn4þ, SLSO:0.015Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ.
intensity and doping concentration of Mn4þ. With the increase of Mn4þ doping concentration, the PL intensity increases, reaching the maximum when x ¼ 0.008. After that, the PL intensity decreases with the increase of concentration, that is, concentration quenching occurs. The energy transfer mechanism between Mn4þ ions may be exchange interaction or multi-pole interaction. The critical distance (Rc) of concentration quenching of Mn4þ can be calculated from Formula (2) [38]: � �1 3V 3 Rc ¼ 2 4π xc N
(2)
where N is the number of ions that can be substituted in SLSO lattice, xc is the critical content of Mn4þ and V is the cell volume of SLSO. For SLSO: xMn4þ phosphors, N ¼ 2, xc ¼ 0.008, V ¼ 573.32 Å3. By substituting the values of N, xc and V into formula (2), the value of Rc is calculated to be 40.89 Å, which is much longer than 5 Å. Only when the distance between the two luminescent centers is less than 5 Å, can the ion exchange interaction occur. Therefore, multi-polar interaction is the main energy transfer mechanism for the concentration quenching. In order to further determine the type of interaction between Mn4þ ions, the relationship between log (x) and log (I/x) is analyzed, as dis played in Fig. 6. The linear equation between log (x) and log (I/x) is as follows [39]:
Fig. 9. (a) PLE and (b) PL spectra of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors.
5
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electrical multi-polarity. From the data in Fig. 6, the equation of fitting curve is followed as y ¼ 1.24169–2.01569 x. Combining with formula (3), we can see that - (θ/3) ¼ - 2.46, so θ ¼ 7.38, close to 8. Because θ ¼ 8 denotes dipole-quadrupole interaction, the type of interaction between Mn4þ ions in SLSO: xMn4þ phosphors is attributed to the dipolequadrupole interaction. 3.2. Sr2La2Sb0.996-yO6:0.008Mn4þ, yAl3þ phosphors 3.2.1. Phase analysis Fig. 7 (a) plots XRD pattern of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors, for which it is expected that Al3þ substitutes Sb5þ. Similarly, PDF#47–0461 (SLNO) is the standard card (The specific description is in section 3.1.2). Obviously, the peak shape is sharp and no new impurity peaks were detected, indicating that Al3þ-doped did not change the crystal structure of the host. As shown in Fig. 7 (b), the diffraction peak shifts to a large angle with the increase of Al3þ doping concentration, the reason for this phenomenon is consistent with the description in section 3.1.2.
Fig. 10. PL intensity of 2Eg→4A2g transition as a function of Al3þ concentration for Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ (y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors.
logðI = xÞ ¼ c
ðθ = 3ÞlogðβxÞ
3.2.2. DR spectra analysis Fig. 8 records the DR spectra of SLSO, SLSO:0.005Mn4þ, SLSO:0.008Mn4þ, SLSO:0.015Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ. The DR curve of SLSO host presents strong absorption in ultraviolet region due to the host absorption and the absorption becomes weaker platform with the increase of wavelength. Compared with SLSO host, the DR spectra of SLSO: Mn4þ and SLSO: Mn4þ, Al3þ phosphors exhibit some redundant valleys, indicating that Mn4þ-doped can lead to local energy
(3)
where I denotes the PL intensity at a certain concentration, x stands for the concentration of Mn4þ ions, and c and β are constant under the same excitation conditions and matrix lattice. In addition, θ represents
Fig. 11. Temperature-dependent normalized integrated intensity of (a) SLSO:0.008Mn4þ and (b) SLSO:0.008Mn4þ, 0.008Al3þ phosphors; Inset: the integrated PL intensity at various temperatures. (c) Electronic transition diagram of fluorescence thermal quenching phenomenon of Mn4þ. 6
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Fig. 12. The relationship between Ln(I0/I-1) and 1/KBT. (a) SLSO:0.008Mn4þ, (b) SLSO:0.008Mn4þ, 0.008Al3þ phosphors.
levels in the bandgap of SLSO. Among them, the valley before 300 nm is attributed to the CTB of O2 →Mn4þ and host absorption, while the other valleys located at 320 and 505 nm are attributed to the transition of Mn4þ ions, namely, spin-allowed Mn4þ: 4A2g→4T1g and 4A2g→4T2g transitions. Among them, Mn4þ-doped can enhance the absorption ability of phosphors, and the doping of Al3þ deepens this phenomenon. Meanwhile, the results of DR spectra are similar to those of PLE, which further proves that SLSO: Mn4þ and SLSO: Mn4þ, Al3þ have strong ab sorption ability in UV-NUV and visible region.
Table 2 The I423K/I298K and Ea values of the related phosphors. Phosphor 4þ
Cs2NbOF5:Mn CaLaMgSbO6:Mn4þ Mg3Ga2GeO8:Mn4þ Ca3La2W2O12:Mn4þ NaMgGdTeO6:Mn4þ Sr2MgGe2O7:Mn4þ LiLaMgWO6:Mn4þ NaMgLaTeO6:Mn4þ Ca14Ga10Zn6O35:Mn4þ SrMg2La2W2O12:Mn4þ Ca2LaSbO6:Mn4þ CaYMgSbO6:Mn4þ Sr3NaSbO6:Mn4þ Sr3LiSbO6:Mn4þ
3.2.3. PLE and PL spectra of Sr2LaSb0.996-yO6:0.008Mn4þ, yAl3þ phosphors Fig. 9(a) illustrates the PLE spectra of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ (y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) monitored at 694 nm, which are consistent with that of PLE spectra of SLSO:Mn4þ. In order to improve the PL intensity of SLSO:0.008Mn4þ phosphors, Al3þ ions were introduced into the phosphors. Fig. 9(b) describes the PL spectra of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ (y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors under 334 nm excitation. It can be seen that the profile of PL spectra of SLSO:0.008Mn4þ, yAl3þ phosphors have no significant distinction with increasing Mn4þ con centrations under 334 nm excitation, but the PL intensity have changed obviously. The correlation between the PL intensity of SLSO:0.008Mn4þ, yAl3þ and the doping concentration of Al3þ as plotted in Fig. 10. With the increase of doping concentration of Al3þ, the PL intensity of the SLSO:0.008Mn4þ, yAl3þ phosphors also increases. When the doping concentration of Al3þ is found to be x ¼ 0.008, the PL intensity of the material reaches its maximum, and continues to increase the doping concentration of Al3þ, the PL intensity begins to decrease. The PL in tensity of SLSO:0.008Mn4þ, 0.008Al3þ is 2.92 times than that of SLSO:0.008Mn4þ, indicating that doping a small amount of Al3þ can greatly improve the PL intensity of the SLSO:0.008Mn4þ phosphors. It is necessary to study the mechanism of enhanced PL intensity. Generally, due to the charge difference between Mn4þ and Sb5þ, when Mn4þ re places Sb5þ, the structural defect Mn;Sb will form Mn4þ-Mn4þ pair and oxygen vacancy, which can be expressed as the following equation: ; � �� 2Mn� Mn þ 2SbSb →2MnMn þ VO
I423K/I298K
Ea (eV)
Reference
61 54 72 29 70 51.5 49 75 41.2 57.5 17.1 53.2 39.8 66.2
0.261 0.34 0.2177 0.38 0.2532 0.24 0.39 0.255 0.29 0.295 0.35 0.381 0.35 0.25
[47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [33] [25] [57] [58]
transfer among neighboring Mn4þ luminescent centers can be efficiently hindered by Al3þ-Mn4þ structure [40]. The hindrance of non radiative energy transfer of adjacent Mn4þ ions means that more energy of Mn4þ itself is released in the form of light, thus the PL intensity and lumi nescence performance of SLSO:Mn4þ phosphor are effectively improved [41,42]. Nevertheless, excessive Al3þ entrance the lattice becomes quite difficult due to the limitation of charge conservation. Residual Al3þ not only causes impurities in phosphors, but also provides conditions for the formation of Al3þ-Mn2þ pairs, which causing the reduction of luminous centers [43]. 3.2.4. Thermal stability For LED, especially for high-power LED devices, the thermal quenching characteristic of phosphor is a key issue to be considered. The temperature-dependent PL spectra of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ phosphors are presented in Fig. 11 (a) and (b). The illustrations in Fig. 11 (a) and (b) illustrated the relationship between temperature and integrated PL intensity. The PL intensity of the samples decreases significantly with the increase of temperature, lead ing to thermal quenching. When the temperature is 423 K, SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ phosphors, the in tensity remains at 35.1% and 33.9% of the initial temperature (298 K), respectively. The thermal quenching of the samples is due to the nonradiative transition in Mn4þ. As shown in Fig. 11 (c), the ground state electrons at 4A2g can transit to the excited states 4T1g and 4T2g. Then, the electrons relax to the lowest excited state 2Eg through non-radiative transition, and then the 2Eg→4A2g radiative transition occurs. Whereas, with the temperature gradually increases, more electrons in the 2Eg state are likely to be thermally excited through the intersection of 4 A2g and 2Eg states, and the ground state is non radiative relaxation,
(4)
In general, this negative vacancy is harmful to the luminescence intensity of Mn4þ ions, because the energy transfer from the lumines cence center to the vacancy defect becomes more efficient. For SLSO:Mn4þ, Al3þ phosphor, an Al3þ ion (CN ¼ 6, Rc ¼ 0.535 Å) is extremely inclined to replace a Sb5þ ion (CN ¼ 6, Rc ¼ 0.60 Å) in the lattice, one of Mn4þ ions in the Mn4þ-Mn4þ pair will be substituted by Al3þ, which induces the formation of Al3þ-Mn4þ pair. Al3þ as a non luminescent center without energy transfer, the nonradiative energy 7
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Fig. 13. Reference BaSO4 absorption, sample absorption and sample emission of (a) SLSO:0.008Mn4þ; (b) SLSO:0.008Mn4þ, 0.008Al3þ phosphors collected using an integrating sphere.
resulting in the decrease of the emission intensity of Mn4þ [44]. Therefore, the phenomenon of thermal quenching is continuous rather than sudden. The activation energy (ΔEa) is defined as the energy deference from the bottom of the excited state to the crossing point, which is an important parameter[45]. In order to explore the effect of thermal quenching, the ΔEa was obtained using the Arenius equation[46]: IT ¼
I0 1 þ A⋅expð ΔEa =KTÞ
The equation can be converted into the following forms: � � I0 ΔEa Ln 1 ¼ LnA IT KB T
to 750 nm, whose peak is attributed to the Mn4þ: 2Eg→4A2g transition. When the doping concentration of Mn4þ is x ¼ 0.008, the PL intensity of phosphor reaches its maximum, and then concentration quenching oc curs, due to the dipole-dipole interaction. In addition, the effect of codoping Al3þ on the luminescent properties of SLSO: Mn4þ was also studied. Al3þ-doped can hinder the energy transfer between Mn4þ-Mn4þ ion pairs and significantly improve the luminescence of Mn4þ. When the doping concentration of Al3þ is 0.8 mol%, the PL intensity is the highest, which is 2.92 times that of SLSO:0.008Mn4þ. Furthermore, the CIE color coordinates of phosphors are located in the far-red region, which matches well with PFR of plant growth. Therefore, these phosphors have potential applications in the field of LED for plant growth.
(5)
(6)
Acknowledgments
where I0 is the PL intensity at the initial temperature, IT corresponds to the luminous intensity at different temperatures, A is a constant and KB is a Boltzmann constant (KB ¼ 8.629 � l0 5 eV/K). The relationship between 1/KBT and Ln (I0/I-l) is plotted in Fig. 12. The ΔEa value of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ are 0.353 eV and 0.338 eV, respectively. Table 2 present the I423K/I298K and Ea values of the related phosphors. After comparison, the thermal stability of SLSO: Mn4þ is regular, which is not better than that of related phosphors, such as CaLaMgSbO6:Mn4þ and LiLaMgWO6:Mn4þ. Therefore, it is particu larly important to improve the thermal stability of SLSO:Mn4þ, for example, by changing the sintering temperature, doping flux and charge compensation ions.
This work was supported by the Doctoral Research Foundation of Hebei Normal University of Science and Technology (Grant No. 2016YB003). 2018 National College Students’ innovation and entre preneurship training project (Grant No. 201810798006). Hebei Master’s Innovation Subsidy Project in 2019 (Project No. CXZSS2019118). Na tional Natural Science Foundation of China (No. 51777138). Natural Science Foundation of Tianjin City (No. 18JCZDJC99700 and 18JCYBJC87400). We also gratefully acknowledge the instrumental analysis center of Hebei Normal University of Science and Technology. References [1] Y. Zhao, L. Shi, Y. Han, et al., Luminescent properties of Zn2þ-doped CaAl12O19: Mn4þ deep-red phosphor for indoor plant cultivation, Ceram. Int. 45 (2019) 8265–8270. [2] J. Liang, L. Sun, B. Devakumar, et al., Far-red-emitting double-perovskite CaLaMgSbO6:Mn4þ phosphors with high photoluminescence efficiency and thermal stability for indoor plant cultivation LEDs, RSC Adv. 8 (2018) 31666. [3] Q. Sun, S.Y. Wang, B. Devakumar, Synthesis and photoluminescence properties of novel far-red-emitting BaLaMgNbO6:Mn4þ phosphors for plant growth LEDs, RSC Adv. 8 (2018) 28538–28545. [4] R.P. Cao, Z.H. Shi, G.J. Quan, et al., Preparation and luminescence properties of Li2MgZrO4:Mn4þ red phosphor for plant growth, J. Lumin. 188 (2017) 577–581. [5] J. Xiang, J. Chen, N. Zhang, et al., Far red and near infrared double-wavelength emitting phosphor Gd2ZnTiO6:Mn4þ, Yb3þ for plant cultivation LEDs, Dyes Pigments 154 (2019) 257–262. [6] X. Huang, H. Guo, Finding a novel highly efficient Mn4þ -activated Ca3La2W2O12 far-red emitting phosphor with excellent responsiveness to phytochrome PFR : towards indoor plant cultivation application, Dyes Pigments 152 (2018) 36–42. [7] X. Huang, J. Liang, B. Li, et al., High-efficiency and thermally stable far-redemitting NaLaMgWO6:Mn4þ phosphorsfor indoor plant growth light-emitting diodes, Opt. Lett. 43 (2018) 3305–3308. [8] J. Long, X. Yuan, C. Ma, et al., Strongly enhanced luminescence of Sr4Al14O25:Mn4þ phosphor by co-doping B3þ and Naþ ions with red emission for plant growth LEDs, RSC Adv. 8 (2018) 1469–1476. [9] Q. Sun, S. Wang, B. Li, et al., Synthesis and photoluminescence properties of deep red-emitting CaGdAlO4:Mn4þ phosphors for plant growth LEDs, J. Lumin. 203 (2018) 371–375.
3.2.5. IQE Internal quantum efficiency is also an important parameter in prac tical applications. Internal quantum efficiency is usually calculated by the ratio of the number of photons emitted to the number of photons absorbed[41], which can be calculated by equation (6) [59]: R Ls I R ηint ¼ em ¼ R (7) Iabs ER Es where LS is the emission spectrum of the phosphor, and ES and ER are the spectra of the absorption of phosphor and BaSO4 reference material in the integral sphere, respectively. Fig. 13 depicts the internal quantum efficiency of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ (λex ¼ 310 nm). 4. Conclusion In summary, a novel type of red phosphor SLSO:Mn4þ with double perovskite structure was prepared by traditional solid state reaction method. Under 334 nm excitation, the emission spectra ranges from 600 8
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[10] Z. Zhou, M. Xia, Y. Zhong, et al., Dy3þ@Mn4þ co-doped Ca14Ga10 mAlmZn6O35 farred emitting phosphors with high brightness and improved luminescence and energy transfer properties for plant growth LED lights, J. Mater. Chem. C 5 (2017) 8201–8210. [11] A. Reinhardt, A. Zych, Ingo K€ ohler, et al., Disordered langasites La3Ga5MO14 : Eu3þ (M ¼ Si, Ge, Ti) as red-emitting LED phosphors, Dalton Trans. 47 (2018) 5703–5713. [12] K. Park, H. Kim, D. Hakeem, Photoluminescence properties of Eu3þ-and Tb3þdoped YAlO3 phosphors for white LED applications, Ceram. Int. 42 (2016) 10526–10530. [13] G. Annadurai, S. Kennedy, Synthesis and photoluminescence properties of Ba2CaZn2Si6O17:Eu3þ red phosphors for white LED applications, J. Lumin. 169 (2016) 690–694. [14] Q. Yang, G. Li, Y. Wei, et al., Synthesis and photoluminescence properties of redemitting NaLaMgWO6:Sm3þ, Eu3þ phosphors for white LED applications, J. Lumin. 199 (2018) 323–330. [15] N. Khaidukov, T. Zorenko, A. Iskaliyeva, et al., Synthesis and luminescent properties of prospective Ce3þ doped silicate garnet phosphors for white LED converters, J. Lumin. 192 (2017) 328–336. [16] D. Chen, Y. Zhou, W. Xu, et al., Enhanced luminescence of Mn4þ:Y3Al5O12 red phosphor via impurity doping, J. Mater. Chem. C 4 (2016) 1704. [17] S.P. Singh, M. Kim, W.B. Park, et al., Discovery of a red-emitting Li3RbGe8O18:Mn4 þ phosphor in the alkali-germanate system: structural determination and electronic calculations, Inorg. Chem. 55 (2016) 10310–10319. [18] D. Chen, Y. Zhou, J. Zhong, A review on Mn4þ activator in solids for warm white light-emitting diodes, RSC Adv. 6 (2016) 86285–86296. [19] C. Stoll, J. Bandemehr, F. Kraus, HF-free synthesis of Li2SiF6:Mn4þ: a red-emitting phosphor, Inorg. Chem. 58 (2019) 5518–5523. [20] D. Chen, Y. Liu, J. Chen, et al., Yb3þ/Ln3þ/Mn4þ (Ln ¼ Er, Ho, and Tm) doped Na3ZrF7 phosphors: oil–water interface cation exchange synthesis, dual-modal luminescence and anti-counterfeiting, J. Mater. Chem. C 7 (2019) 1321–1329. [21] D.K. Amarasinghe, F.A. Rabuffetti, Bandshift luminescence thermometry using Mn4 þ :Na4Mg(WO4)3 phosphors, Chem. Mater. 31 (2019) 10197–10204. [22] J. Zhong, D. Chen, S. Yuan, et al., Tunable optical properties and enhanced thermal quenching of non-rare-earth double-perovskite (Ba1-xSrx)2YSbO6:Mn4þ red phosphors based on composition modulation, Inorg. Chem. 57 (2018) 8978–8987. [23] Y. Ren, R. Cao, T. Chen, et al., Photoluminescence properties of Ba2LaSbO6:Mn4þ deep-red-emitting phosphor for plant growth LEDs, J. Lumin. 209 (2019) 1–7. [24] Z. Lu, Y. Meng, L. Wen, et al., Double perovskite Ba2LaNbO6:Mn4þ,Yb3þphosphors: potential application to plant-cultivation LEDs, Dyes Pigments 160 (2019) 395–402. [25] L. Shi, Y. Han, Z. Ji, et al., Highly efcient and thermally stable CaYMgSbO6:Mn4þ double perovskite red phosphor for indoor plant growth, J. Mater. Sci. Mater. Electron. 30 (2019) 3107. [26] Z. Wang, C. Ping, L. Yong, et al., Novel red phosphors LaBSiO5 co-doped with Eu3þ, Al3þ for near-UV light-emitting diodes, Opt. Mater. 37 (2014) 277–280. [27] Y. Wu, K. Qiu, Q. Tang, et al., Luminescence enhancement of Al3þ co-doped Ca3Sr3(VO4)4:Eu3þ red-emitting phosphors for white LEDs, Ceram. Int. 44 (2018) 8190–8195. [28] R. Cao, J. Huang, X. Ceng, et al., LiGaTiO4:Mn4þ red phosphor: synthesis, luminescence properties and emission enhancement by Mg2þ and Al3þ ions, Ceram. Int. 42 (2016) 13296–13300. [29] Y. Wu, Y. Chan, Y. Nien, et al., Crystal structure and optical performance of Al3þ and Ce3þ codoped Ca3Sc2Si3O12 green phosphors for white LEDs, J. Am. Ceram. Soc. 96 (2013) 234–240. [30] A. Dutta, P. Kumari, T. Sinha, Electrical properties of double perovskite oxide Sr2LaSbO6: an impedance spectroscopic study, El Mater. Lett. 11 (2015) 775–781. [31] A. Faik, E. Iturbe-Zabalo, I. Urcelay, et al., Crystal structures and high-temperature phase transitions of the new ordered double perovskites Sr2SmSbO6 and Sr2LaSbO6, J. Solid State Chem. 182 (2009) 2656–2663. [32] K. Li, H. Lian, R.V. Deun, Site occupancy and photoluminescence properties of a novel deep-red-emitting phosphor NaMgGdTeO6:Mn4þ with perovskite structure for w-LEDs, J. Lumin. 198 (2018) 155–162. [33] L. Shi, Y. Han, Z. Zhang, et al., Synthesis and photoluminescence properties of novel Ca2LaSbO6: Mn4þ double perovskite phosphor for plant growth LEDs, Ceram. Int. 45 (2019) 4739–4746. [34] J. Du, O. Clercq, D. Poelman, Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4þ, R (R¼ Naþ, Ca2þ, Sr2þ, Ba2þ) ceramics, Ceram. Int. 44 (2018) 21613–21620.
[35] W. Shu, L. Jiang, S. Xiao, et al., GeO2 dopant induced enhancement of red emission in CaAl12O19:Mn4þ phosphor, Mater. Sci. Eng. B 177 (2) (2012) 274–277. [36] E. Song, Y. Zhou, X. Yang, et al., Highly efficient and stable narrow-band red phosphor Cs2SiF6:Mn4þ for high-power warm white LED applications, ACS Photonics 4 (10) (2017) 2556–2565. [37] E. Song, Y. Zhou, Y. Wei, et al., A thermally stable narrow-band green-emitting phosphor MgAl2O4:Mn2þ for wide color gamut backlight display application, J. Mater. Chem. C 7 (2019) 8192–8198. [38] Z. Zhang, D. Ma, Y. Yue, et al., Wide-band excited LaBMoO6:Eu3þ red phosphor for white-light-emitting diode, J. Commer. 636 (2015) 113–116. [39] J. Liang, L. Sun, B. Devakumar, et al., Novel Mn4þ-activated LiLaMgWO6 far-red emitting phosphors: high photoluminescence efficiency, good thermal stability, and potential applications in plant cultivation LEDs, RSC Adv. 8 (2018) 27144–27151. [40] D. Chen, Y. Zhou, W. Xu, et al., Enhanced luminescence of Mn4þ: Y3Al5O12 red phosphor via impurity doping, J. Mater. Chem. C 4 (2016) 1704–1712. [41] J. Zhong, D. Chen, X. Chen, et al., Efficient rare-earth free red-emitting Ca2YSbO6: Mn4þ,M(M ¼ Liþ, Naþ, Kþ, Mg2þ) phosphors for white light-emitting diodes, Dalton Trans. 47 (2018) 6528–6537. [42] L. Wang, Y. Xu, D. Wang, et al., Deep red phosphors SrAl12O19:Mn4þ,M (M ¼ Liþ, Naþ, Kþ, Mg2þ) for high colour rendering white LEDs, Phys. Status Solidi 210 (2013) 1433–1437. [43] J. Li, W. Liu, Y. Li, et al., Effect of Si4þ substitution on luminescence of Y3Al5O12: Mn4þ, Ceram. Int. 44 (2018) 16099–16102. [44] C. Yang, Z. Zhang, G. Hu, et al., A novel deep red phosphor Ca14Zn6Ga10O35:Mn4þ as color converter for warm W-LEDs: structure and luminescence properties, J Com. 694 (2017) 1201–1208. [45] Y. Wu, Y. Zhuang, Y. Lv, et al., A high-performance non-rare-earth deep-redemitting Ca14-xSrxZn6Al10O35:Mn4þ phosphor for high-power plant growth LEDs, J Com. 781 (2019) 702–709. [46] H. Chen, H. Lin, Q. Huang, et al., A novel double-perovskite Gd2ZnTiO6:Mn4þ red phosphor for UV-based w-LEDs: structure and luminescence properties, J. Mater. Chem. C 4 (2016) 2374–2381. [47] H. Ming, J. Zhang, L. Liu, et al., A novel Cs2NbOF5:Mn4þ oxyfluoride red phosphor for light-emitting diode devices, Dalton Trans 47 (2018) 16048–16056. [48] J. Liang, L. Sun, B. Devakumar, et al., Far-red-emitting double-perovskite CaLaMgSbO6:Mn4þ phosphors with high photoluminescence efficiency and thermal stability for indoor plant cultivation LEDs, RSC Adv. 8 (2018) 31666. [49] X. Ding, G. Zhu, W. Geng, et al., Rare-earth-Free high-efficiency narrow-band redemitting Mg3Ga2GeO8:Mn4þ phosphor excited by near-UV light for WhiteLightemitting diodes, Inorg. Chem. 55 (2016) 154–162. [50] X. Huang, H. Guo, Finding a novel highly efficient Mn4þ-activated Ca3La2W2O12 far-red emitting phosphor with excellent responsiveness to phytochrome PFR: towards indoor plant cultivation application, Dyes Pigments 152 (2018) 36–42. [51] K. Li, H. Lian, R. Deun, et al., Site occupancy and photoluminescence properties of a novel deep-red-emitting phosphor NaMgGdTeO6:Mn4þ with perovskite structure for w-LEDs, J. Lumin. 198 (2018) 155–162. [52] W. Chen, Y. Cheng, L. Shen, et al., Red-emitting Sr2MgGe2O7:Mn4þ phosphors: structure, luminescence properties, and application in warm white light emitting diodes, JAll Com 762 (2018) 688–696. [53] J. Liang, L. Sun, B. Devakumar, et al., Novel Mn4þ-activated LiLaMgWO6 far-red emitting phosphors: high photoluminescence efficiency, good thermal stability, and potential applications in plant cultivation LEDs, RSC Adv. 8 (2018) 27144. [54] K. Li, H. Lian, R. Deun, et al., A far-red-emitting NaMgLaTeO6:Mn4þ phosphor with perovskite structure for indoor plant growth, Dyes Pigments 162 (2019) 214–221. [55] Y. Zhong, S. Gai, M. Xia, et al., Enhancing quantum efficiency and tuning photoluminescence properties in far-red-emitting phosphor Ca14Ga10Zn6O35:Mn4þ based on chemical unit engineering, Chem. Eng. J. 374 (2019) 381–391. [56] S. Wang, Q. Sun, B. Devakumar, et al., Novel SrMg2La2W2O12:Mn4þ far-red phosphors with high quantum efficiency and thermal stability towards applications in indoor plant cultivation LEDs, RSC Adv. 8 (2018) 30191. [57] L. Shi, J. Li, Y. Han, et al., Highly efficient and thermally stable of a novel red phosphor Sr3NaSbO6:Mn4þ for indoor plant growth, J. Lumin. 208 (2019) 201–207. [58] L. Shi, Y. Han, Y. Zhao, et al., Synthesis and photoluminescence properties of novel Sr3LiSbO6:Mn4þ red phosphor for indoor plant growth, Opt. Mater. 89 (2019) 609–614. [59] B. Wang, H. Lin, J. Xu, et al., CaMg2Al16O27:Mn4þ-based red phosphor: a potential color converter for high-powered warm W-LED, ACS App. Mater. Inter. 6 (2014) 22905–22913.
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Sr2LaSbO6:Mn4þ far-red phosphor for plant cultivation: Synthesis, luminescence properties and emission enhancement by Al3þ ions Lei Shi a, Shuang Wang a, Ya-jie Han a, Zhi-xin Ji a, Di Ma b, Zhong-fei Mu c, Zhi-yong Mao d, Da-jian Wang d, Zhi-wei Zhang a, *, Lu Liu a, ** a
Chemical Engineering College, Hebei Normal University of Science and Technology, Qinhuangdao, 066600, China College of Biomass Science and Engineering, SiChuan University, Chengdu, 610065, China School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou, 510006, PR China d Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of Technology, Tianjin, 300384, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Sr2LaSbO6:Mn4þ Al3þ Photoluminescence Far-red emitting phosphors Plant cultivation and growth LEDs
A series of novel Mn4þ-activated double perovskite antimonate Sr2LaSbO6 (SLSO) far-red emitting phosphors were successfully synthesized by a high-temperature solid-state method. X-ray diffraction (XRD), photo luminescence excitation (PLE) and emission (PL) spectra, temperature-dependent photoluminescence spectra, CIE chromaticity coordinates, as well as internal quantum efficiency (IQE) were used to analyze these phosphor samples. Under 334 nm excitation, the SLSO:Mn4þ phosphors emit far red emission peaking around 694 nm due to the Mn4þ: 2Eg→4A2g transition. The optimal doping concentration of Mn4þ was 0.8 mol%, moreover, the critical distance (Rc) was 40.89 Å. The dipole-quadrupole interaction may be responsible for the concentration quenching mechanism of Mn4þ ions in the SLSO:Mn4þ phosphors. Furthermore, various Al3þ improved SLSO:0.008Mn4þ phosphors are investigated. When Al3þ is doped with 0.8%, the PL intensity increased by 2.92 times. In addition, the IQE of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ were found to be as high as 35.2% and 38.1%, respectively. Impressively, all chromaticity coordinates of samples located in far-red region. Therefore, these superior luminescence properties of SLSO:Mn4þ sample demonstrated its potential applications in plant cultivation and growth light-emitting diodes (LEDs).
1. Introduction Nowadays, with the frequent occurrence of natural disasters, tradi tional agriculture is facing more and more risks and challenges, such as climate change, drought, flooding and so on. In order to make plants grow safely and effectively, as well as meet human needs, people are committed to developing greenhouse industry[1–5]. There are many factors affecting plant growth, among light accounts for a large pro portion. Previous studies have shown that different wavelength spectra of light exhibit different effects on plant growth. For instance, blue light (400–500 nm) mainly participates in photosynthesis, red light (620–690 nm) mainly affects phototaxis of plants. Importantly, the ef fects of far red light (700–740 nm) absorbed by photosensitive pigment PFR on plant morphology, including seed germination, depolarization, stem elongation, leaf expansion, shade avoidance and flowering induc tion [6–10]. Therefore, it is urgent to find a reliable light source to
regulate the growth of plants. The LEDs have become the main artificial light source for plant cultivation because of its controllable light-color, low power consumption, low radiation heat output and environmental friendliness[11–15]. The preparation technology of blue and red LEDs have become mature, while the far-red emission LED manufacturing technology needs to be improved, in especial, the far-red emission phosphor. Therefore, the purpose of this paper is to develop a novel far-red phosphor for plant growth LED to meet the requirements of plant growth. Generally speaking, Mn4þ ions with 3d3 electronic configuration can exist stably in octahedral coordination environment. In addition, as a non-rare earth element, Mn is cheaper than most rare earth elements, such as Eu, Dy, Ce and Yb. At the same time, Mn4þ ions embody strong absorption ability from near-ultraviolet (UV) to blue region in different crystal field environments, showing red or deep red emission in the range of 600–750 nm [16–22]. Therefore, the strong emission
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.-w. Zhang), [email protected] (L. Liu). https://doi.org/10.1016/j.jlumin.2020.117091 Received 1 December 2019; Received in revised form 15 January 2020; Accepted 2 February 2020 Available online 5 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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characteristics of Mn4þ-activated phosphors in the far-red region match well with the spectral range required for plant growth and can be applied to indoor plant lighting. In addition, Mn4þ-activated double perovskite structure phosphors have been widely reported for their excellent physical, chemical properties, thermal stability and optical properties, such as Ba2LaSbO6:Mn4þ [23], Ba2LaNbO6:Mn4þ [24], CaYMgSbO6:Mn4þ [25]. Meanwhile, Sr2LaSbO6 (SLSO) with [SbO6] octahedron belongs to A2BB’O6 type double perovskite, which makes it capably activated by Mn4þ to become a good luminescent materials. The luminescent properties of the phosphor can be improved by doping Al3þ. For example, Al3þ-doped can improve the PL intensity of LaBSiO5:Eu3þ phosphor[26]. Under the excitation of 464 nm, the PL intensity of Ca3Sr3(VO4)4: 0.05Eu3þ, 0.06Al3þ is about 1.47 times that of Ca3Sr3(VO4)4: 0.05Eu3þ [27]. The PL intensity of LiGaTiO4:Mn4þ phosphor can be increased by 2.2 times by co-doping Al3þ ions[28]. The incorporation of Al3þ into Ca3Sc2Si3O12:Ce3þ can inhibit the formation of the impurity phases Sc2O3 and CeO2, and enhance the photo luminescence intensity as well as quantum efficiency[29]. The substi tution of Sc3þ with Al3þ increased the crystal field splitting of Ce3þ and resulted in the red shift of photoluminescence. SLSO:Mn4þ phosphors were synthesized by high temperature solid state reaction in air atmosphere. The crystal structure and morphology of the samples were characterized, and the concentration quenching mechanism and luminescence mechanism were studied in detail. SLSO: Mn4þ phosphors possess excellent luminescent properties. Furthermore, the PL intensity and IQE of the SLSO:Mn4þ is further improved by codoping Al3þ ions, because Al3þ ions can hinder the energy transfer be tween Mn4þ-Mn4þ ion pairs. All the results show that the synthesized phosphors have excellent luminescent properties and are suitable for plant growth lighting.
Fig. 1. Schematic diagram of SLSO crystal structure. Table 1 Radius deviation between host ions and doped ions. Ions La3þ Sb5þ Sr2þ
2. Experimental section 2.1. Materials synthesis SLSO:xMn4þ (x ¼ 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.013 and 0.015) and Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors were synthesized by high temperature solid-state method and SrCO3, La2O3, Al2O3, Sb2O3 and MnCO3 are the main raw materials. The chemical reaction equation is as follows:
8SrCO3 þ2La2 O3 þ 2ð1
xÞSb2 O3 þ xO2 þ 4xMnCO3 ¼ 4Sr2 LaSbð1
CN 6 6 6
Radius (Å) 1.03 0.60 1.18
Rr (%) Mn4þ (CN ¼ 6, 0.53 Å)
Al3þ (CN ¼ 6, 0.535 Å)
48.5 11.67 55.08
48.06 10.83 54.66
The thermal stability of samples was measured by a fluorescence spec trometer (Hitachi F-7000) equipped with a temperature controller. The internal quantum efficiencies (IQE) of phosphors were measured with barium sulfate coated integrating sphere on spectrophotometer.
xÞO6 :xMn4þ þ ð4x þ 8ÞCO2 ↑
3. Results and discussion 3.1. SLSO:xMn4þphosphors
The raw materials were weighed according to the stoichiometric ratio, and then ground. The ground powders were sintered at 1300 � C for 10 h in a muffle furnace. The samples after sintering were ground again.
3.1.1. Crystal structure Fig. 1 displays a schematic diagram of SLSO crystal structure. SLSO crystal belongs to double perovskite type structure. It has monoclinic structure with space group P21/n, and the lattice parameters are as follows: a ¼ 5.816 Å, b ¼ 5.880 Å, c ¼ 8.304 Å, α ¼ γ ¼ 90� , and β ¼ 90.19� , V ¼ 573.32 Å3[30, 31]. In this lattice, La3þ and Sb5þ ions occupy octahedral sites by sharing an arrangement of oxygen atoms. In addition, Sr2þ ions are located in the center of the gap between [SbO6] and [LaO6] octahedrons. The premise of ion substitution is the radius deviation (Rr) between doped and substituted ions is reasonable within 30%[32, 33]. In order to analyze which sites Mn4þ or/and Al3þ ions occupy in the host lattice, the radius deviation of the related ions is calculated by equation (1), and the
2.2. Sample characterization The as-prepared samples were analyzed by X-ray powder diffrac tometer (D/MAX 2500 TC, 40 KV and 200 mA). The morphology and particle size of the samples were observed by field emission scanning electron microscopy (FESEM) (Hitachi-8010). The diffuse reflectance (DR) spectra of powder samples were collected by ultraviolet-2450 spectrophotometer (Ultraviolet-2450 of Shimadzu, Japan) with BaSO4 as reference material. The PL and PLE spectra of phosphors were measured by using Hitachi F-7000 fluorescence spectrophotometer under 400 V voltage and using xenon lamp (150W) as excitation source. 2
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Fig. 2. (a)XRD patterns of Sr2LaSb(1-x)O6:xMn4þ (x ¼ 0.005–0.015) phosphors; (b) Local enlargement of (a).
Fig. 3. FESEM images of SLSO:0.008Mn4þ:(a) 5000 � ; (b) 25,000 � .
inferred from Table 1 that when Mn4þ or/and Al3þ ions are introduced into SLSO crystals, they tend to occupy the octahedral position of [SbO6], rather than [LaO6] or [SrO6] octahedral.
results are listed in Table 1. Rr ¼ 100 �
Rm ðCNÞ Rd ðCNÞ Rm ðCNÞ
(1)
3.1.2. phase analysis Fig. 2(a) shows the XRD patterns of as-synthetized SLSO: xMn4þ phosphor. Here, compare the XRD pattern of SLSO: Mn4þ with the
where Rm(CN) and Rd(CN) are radii of matrix ions with different coor dination numbers (CN) and doped ions, respectively. Therefore, it can be
Fig. 4. (a) PLE spectra of SLSO:xMn4þ measured at room temperature (λem ¼ 694 nm); (b) The PLE spectrum of SLSO:0.008 Mn4þ using Gaussian decomposi tion method. 3
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Fig. 6. Log(I/x) dependent on log(x).
those of the standard card 47–0461 (SLNO), indicating that SLSO: xMn4þ phosphors have been successfully synthesized and Mn4þ-doping will not change the crystal structure of SLSO. Fig. 2(b) illustrates a partially enlarged view of Fig. 2(a) in the range from 29.5 to 31� . Apparently, the diffraction peak shifts to a large angle with the increase of Mn4þ doping concentration. This phenomenon can be explained by Bragg’s law: 2dsinθ ¼ nλ, in this formula, symbol λ, d, and θ represent the wavelength of the X-ray, interplanar spacing and diffraction angle, respectively. When Sb5þ (CN ¼ 6, 0.6 Å) is replaced by smaller Mn4þ (CN ¼ 6, 0.53 Å), the plane spacing of the crystal cells decreases, leading to the contraction of the lattice cell, which causes the diffraction peak to move to a large angle. 3.1.3. FESEM Fig. 3 presents the SEM images of the SLSO:0.008Mn4þ phosphors. In the sintering process, most of the small particles will gather together. SLSO:0.008Mn4þ phosphor exhibits irregular morphology and the par ticle size is smaller than 4 μm. It can be clearly seen from the figure that the surface of the particles is smooth and no voids and stomata, indi cating that there are almost no defects in the particles. 3.1.4. Luminescence properties Fig. 4 shows the PLE spectra of SLSO: xMn4þ (x ¼ 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.013 and 0.015) phosphors and the Gauss decomposition spectrum of SLSO: 0.008Mn4þ monitored at 694 nm, respectively. The PLE spectrum of SLSO:0.008Mn4þ can be decomposed into two excitation bands using Gauss decomposition method, which can be assigned to charge transfer band (CTB) of O2 →Mn4þ (4.51 eV), 4A2g→4T1g (4.04 eV), 4A2g→2T2g (3.57 eV) and 4 A2g→4T2g (2.59 eV) transitions of Mn4þ ions [34]. The PLE spectra indicate that SLSO: xMn4þ can be effectively excited by AlInGaN chip [35]. Fig. 5(a) depicts the PL spectra of SLSO: xMn4þ phosphors (x ¼ 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.013 and 0.015) under 334 nm excitation. The PL spectra exhibit red emission peaking at 694 nm, which is imputed to the Mn4þ: 2Eg→4A2g transition[36,37]. Fig. 5(b) shows the normalized PL spectrum of SLSO:0.008Mn4þ and the ab sorption spectrum of phytochrome PFR and PR, the absorption peak of PFR is near 713 nm [6]. Distinctly, the emission spectrum of SLSO:0.008Mn4þ is completely covered by the absorption spectrum of PFR. Therefore, the far-red emission wavelength of SLSO:Mn4þ phosphor can be well matched with the absorption wavelength of phytochrome PFR, indicating that SLSO: Mn4þ phosphor has potential application for indoor plant growth under artificial light. Fig. 5(c) illustrates the relationship between the integrated PL
Fig. 5. (a) PL spectra of SLSO:xMn4þ measured at room temperature (λex ¼ 334 nm); (b) Normalized PL spectrum of SLSO:0.008Mn4þ, the absorption spectrum of phytochrome PFR and PR; (c) The integrated PL intensity versus the concentration of Mn4þ.
standard card of Sr2LaNbO6 (SLNO). Theoretically, SLSO: Mn4þ and SLNO have the same crystal structure, so the positions of the diffraction peaks are similar. However, due to the small difference in the radii of Sb5þ/Mn4þ and Nb5þ, there is a slight difference in the positions of diffraction peaks between the two substances. Therefore, the standard card (SLNO) can be used as a criterion to judge whether SLSO: Mn4þ is synthesized. The diffraction patterns of all the samples coincide with 4
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Fig. 7. XRD pattern of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors; (b) Local enlargement of (a).
Fig. 8. UV-VIS DR spectra of SLSO, SLSO:0.005Mn4þ, SLSO:0.008Mn4þ, SLSO:0.015Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ.
intensity and doping concentration of Mn4þ. With the increase of Mn4þ doping concentration, the PL intensity increases, reaching the maximum when x ¼ 0.008. After that, the PL intensity decreases with the increase of concentration, that is, concentration quenching occurs. The energy transfer mechanism between Mn4þ ions may be exchange interaction or multi-pole interaction. The critical distance (Rc) of concentration quenching of Mn4þ can be calculated from Formula (2) [38]: � �1 3V 3 Rc ¼ 2 4π xc N
(2)
where N is the number of ions that can be substituted in SLSO lattice, xc is the critical content of Mn4þ and V is the cell volume of SLSO. For SLSO: xMn4þ phosphors, N ¼ 2, xc ¼ 0.008, V ¼ 573.32 Å3. By substituting the values of N, xc and V into formula (2), the value of Rc is calculated to be 40.89 Å, which is much longer than 5 Å. Only when the distance between the two luminescent centers is less than 5 Å, can the ion exchange interaction occur. Therefore, multi-polar interaction is the main energy transfer mechanism for the concentration quenching. In order to further determine the type of interaction between Mn4þ ions, the relationship between log (x) and log (I/x) is analyzed, as dis played in Fig. 6. The linear equation between log (x) and log (I/x) is as follows [39]:
Fig. 9. (a) PLE and (b) PL spectra of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors.
5
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electrical multi-polarity. From the data in Fig. 6, the equation of fitting curve is followed as y ¼ 1.24169–2.01569 x. Combining with formula (3), we can see that - (θ/3) ¼ - 2.46, so θ ¼ 7.38, close to 8. Because θ ¼ 8 denotes dipole-quadrupole interaction, the type of interaction between Mn4þ ions in SLSO: xMn4þ phosphors is attributed to the dipolequadrupole interaction. 3.2. Sr2La2Sb0.996-yO6:0.008Mn4þ, yAl3þ phosphors 3.2.1. Phase analysis Fig. 7 (a) plots XRD pattern of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ(y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors, for which it is expected that Al3þ substitutes Sb5þ. Similarly, PDF#47–0461 (SLNO) is the standard card (The specific description is in section 3.1.2). Obviously, the peak shape is sharp and no new impurity peaks were detected, indicating that Al3þ-doped did not change the crystal structure of the host. As shown in Fig. 7 (b), the diffraction peak shifts to a large angle with the increase of Al3þ doping concentration, the reason for this phenomenon is consistent with the description in section 3.1.2.
Fig. 10. PL intensity of 2Eg→4A2g transition as a function of Al3þ concentration for Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ (y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors.
logðI = xÞ ¼ c
ðθ = 3ÞlogðβxÞ
3.2.2. DR spectra analysis Fig. 8 records the DR spectra of SLSO, SLSO:0.005Mn4þ, SLSO:0.008Mn4þ, SLSO:0.015Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ. The DR curve of SLSO host presents strong absorption in ultraviolet region due to the host absorption and the absorption becomes weaker platform with the increase of wavelength. Compared with SLSO host, the DR spectra of SLSO: Mn4þ and SLSO: Mn4þ, Al3þ phosphors exhibit some redundant valleys, indicating that Mn4þ-doped can lead to local energy
(3)
where I denotes the PL intensity at a certain concentration, x stands for the concentration of Mn4þ ions, and c and β are constant under the same excitation conditions and matrix lattice. In addition, θ represents
Fig. 11. Temperature-dependent normalized integrated intensity of (a) SLSO:0.008Mn4þ and (b) SLSO:0.008Mn4þ, 0.008Al3þ phosphors; Inset: the integrated PL intensity at various temperatures. (c) Electronic transition diagram of fluorescence thermal quenching phenomenon of Mn4þ. 6
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Fig. 12. The relationship between Ln(I0/I-1) and 1/KBT. (a) SLSO:0.008Mn4þ, (b) SLSO:0.008Mn4þ, 0.008Al3þ phosphors.
levels in the bandgap of SLSO. Among them, the valley before 300 nm is attributed to the CTB of O2 →Mn4þ and host absorption, while the other valleys located at 320 and 505 nm are attributed to the transition of Mn4þ ions, namely, spin-allowed Mn4þ: 4A2g→4T1g and 4A2g→4T2g transitions. Among them, Mn4þ-doped can enhance the absorption ability of phosphors, and the doping of Al3þ deepens this phenomenon. Meanwhile, the results of DR spectra are similar to those of PLE, which further proves that SLSO: Mn4þ and SLSO: Mn4þ, Al3þ have strong ab sorption ability in UV-NUV and visible region.
Table 2 The I423K/I298K and Ea values of the related phosphors. Phosphor 4þ
Cs2NbOF5:Mn CaLaMgSbO6:Mn4þ Mg3Ga2GeO8:Mn4þ Ca3La2W2O12:Mn4þ NaMgGdTeO6:Mn4þ Sr2MgGe2O7:Mn4þ LiLaMgWO6:Mn4þ NaMgLaTeO6:Mn4þ Ca14Ga10Zn6O35:Mn4þ SrMg2La2W2O12:Mn4þ Ca2LaSbO6:Mn4þ CaYMgSbO6:Mn4þ Sr3NaSbO6:Mn4þ Sr3LiSbO6:Mn4þ
3.2.3. PLE and PL spectra of Sr2LaSb0.996-yO6:0.008Mn4þ, yAl3þ phosphors Fig. 9(a) illustrates the PLE spectra of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ (y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) monitored at 694 nm, which are consistent with that of PLE spectra of SLSO:Mn4þ. In order to improve the PL intensity of SLSO:0.008Mn4þ phosphors, Al3þ ions were introduced into the phosphors. Fig. 9(b) describes the PL spectra of Sr2LaSb0.992-yO6:0.008Mn4þ, yAl3þ (y ¼ 0, 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012) phosphors under 334 nm excitation. It can be seen that the profile of PL spectra of SLSO:0.008Mn4þ, yAl3þ phosphors have no significant distinction with increasing Mn4þ con centrations under 334 nm excitation, but the PL intensity have changed obviously. The correlation between the PL intensity of SLSO:0.008Mn4þ, yAl3þ and the doping concentration of Al3þ as plotted in Fig. 10. With the increase of doping concentration of Al3þ, the PL intensity of the SLSO:0.008Mn4þ, yAl3þ phosphors also increases. When the doping concentration of Al3þ is found to be x ¼ 0.008, the PL intensity of the material reaches its maximum, and continues to increase the doping concentration of Al3þ, the PL intensity begins to decrease. The PL in tensity of SLSO:0.008Mn4þ, 0.008Al3þ is 2.92 times than that of SLSO:0.008Mn4þ, indicating that doping a small amount of Al3þ can greatly improve the PL intensity of the SLSO:0.008Mn4þ phosphors. It is necessary to study the mechanism of enhanced PL intensity. Generally, due to the charge difference between Mn4þ and Sb5þ, when Mn4þ re places Sb5þ, the structural defect Mn;Sb will form Mn4þ-Mn4þ pair and oxygen vacancy, which can be expressed as the following equation: ; � �� 2Mn� Mn þ 2SbSb →2MnMn þ VO
I423K/I298K
Ea (eV)
Reference
61 54 72 29 70 51.5 49 75 41.2 57.5 17.1 53.2 39.8 66.2
0.261 0.34 0.2177 0.38 0.2532 0.24 0.39 0.255 0.29 0.295 0.35 0.381 0.35 0.25
[47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [33] [25] [57] [58]
transfer among neighboring Mn4þ luminescent centers can be efficiently hindered by Al3þ-Mn4þ structure [40]. The hindrance of non radiative energy transfer of adjacent Mn4þ ions means that more energy of Mn4þ itself is released in the form of light, thus the PL intensity and lumi nescence performance of SLSO:Mn4þ phosphor are effectively improved [41,42]. Nevertheless, excessive Al3þ entrance the lattice becomes quite difficult due to the limitation of charge conservation. Residual Al3þ not only causes impurities in phosphors, but also provides conditions for the formation of Al3þ-Mn2þ pairs, which causing the reduction of luminous centers [43]. 3.2.4. Thermal stability For LED, especially for high-power LED devices, the thermal quenching characteristic of phosphor is a key issue to be considered. The temperature-dependent PL spectra of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ phosphors are presented in Fig. 11 (a) and (b). The illustrations in Fig. 11 (a) and (b) illustrated the relationship between temperature and integrated PL intensity. The PL intensity of the samples decreases significantly with the increase of temperature, lead ing to thermal quenching. When the temperature is 423 K, SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ phosphors, the in tensity remains at 35.1% and 33.9% of the initial temperature (298 K), respectively. The thermal quenching of the samples is due to the nonradiative transition in Mn4þ. As shown in Fig. 11 (c), the ground state electrons at 4A2g can transit to the excited states 4T1g and 4T2g. Then, the electrons relax to the lowest excited state 2Eg through non-radiative transition, and then the 2Eg→4A2g radiative transition occurs. Whereas, with the temperature gradually increases, more electrons in the 2Eg state are likely to be thermally excited through the intersection of 4 A2g and 2Eg states, and the ground state is non radiative relaxation,
(4)
In general, this negative vacancy is harmful to the luminescence intensity of Mn4þ ions, because the energy transfer from the lumines cence center to the vacancy defect becomes more efficient. For SLSO:Mn4þ, Al3þ phosphor, an Al3þ ion (CN ¼ 6, Rc ¼ 0.535 Å) is extremely inclined to replace a Sb5þ ion (CN ¼ 6, Rc ¼ 0.60 Å) in the lattice, one of Mn4þ ions in the Mn4þ-Mn4þ pair will be substituted by Al3þ, which induces the formation of Al3þ-Mn4þ pair. Al3þ as a non luminescent center without energy transfer, the nonradiative energy 7
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Fig. 13. Reference BaSO4 absorption, sample absorption and sample emission of (a) SLSO:0.008Mn4þ; (b) SLSO:0.008Mn4þ, 0.008Al3þ phosphors collected using an integrating sphere.
resulting in the decrease of the emission intensity of Mn4þ [44]. Therefore, the phenomenon of thermal quenching is continuous rather than sudden. The activation energy (ΔEa) is defined as the energy deference from the bottom of the excited state to the crossing point, which is an important parameter[45]. In order to explore the effect of thermal quenching, the ΔEa was obtained using the Arenius equation[46]: IT ¼
I0 1 þ A⋅expð ΔEa =KTÞ
The equation can be converted into the following forms: � � I0 ΔEa Ln 1 ¼ LnA IT KB T
to 750 nm, whose peak is attributed to the Mn4þ: 2Eg→4A2g transition. When the doping concentration of Mn4þ is x ¼ 0.008, the PL intensity of phosphor reaches its maximum, and then concentration quenching oc curs, due to the dipole-dipole interaction. In addition, the effect of codoping Al3þ on the luminescent properties of SLSO: Mn4þ was also studied. Al3þ-doped can hinder the energy transfer between Mn4þ-Mn4þ ion pairs and significantly improve the luminescence of Mn4þ. When the doping concentration of Al3þ is 0.8 mol%, the PL intensity is the highest, which is 2.92 times that of SLSO:0.008Mn4þ. Furthermore, the CIE color coordinates of phosphors are located in the far-red region, which matches well with PFR of plant growth. Therefore, these phosphors have potential applications in the field of LED for plant growth.
(5)
(6)
Acknowledgments
where I0 is the PL intensity at the initial temperature, IT corresponds to the luminous intensity at different temperatures, A is a constant and KB is a Boltzmann constant (KB ¼ 8.629 � l0 5 eV/K). The relationship between 1/KBT and Ln (I0/I-l) is plotted in Fig. 12. The ΔEa value of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ are 0.353 eV and 0.338 eV, respectively. Table 2 present the I423K/I298K and Ea values of the related phosphors. After comparison, the thermal stability of SLSO: Mn4þ is regular, which is not better than that of related phosphors, such as CaLaMgSbO6:Mn4þ and LiLaMgWO6:Mn4þ. Therefore, it is particu larly important to improve the thermal stability of SLSO:Mn4þ, for example, by changing the sintering temperature, doping flux and charge compensation ions.
This work was supported by the Doctoral Research Foundation of Hebei Normal University of Science and Technology (Grant No. 2016YB003). 2018 National College Students’ innovation and entre preneurship training project (Grant No. 201810798006). Hebei Master’s Innovation Subsidy Project in 2019 (Project No. CXZSS2019118). Na tional Natural Science Foundation of China (No. 51777138). Natural Science Foundation of Tianjin City (No. 18JCZDJC99700 and 18JCYBJC87400). We also gratefully acknowledge the instrumental analysis center of Hebei Normal University of Science and Technology. References [1] Y. Zhao, L. Shi, Y. Han, et al., Luminescent properties of Zn2þ-doped CaAl12O19: Mn4þ deep-red phosphor for indoor plant cultivation, Ceram. Int. 45 (2019) 8265–8270. [2] J. Liang, L. Sun, B. Devakumar, et al., Far-red-emitting double-perovskite CaLaMgSbO6:Mn4þ phosphors with high photoluminescence efficiency and thermal stability for indoor plant cultivation LEDs, RSC Adv. 8 (2018) 31666. [3] Q. Sun, S.Y. Wang, B. Devakumar, Synthesis and photoluminescence properties of novel far-red-emitting BaLaMgNbO6:Mn4þ phosphors for plant growth LEDs, RSC Adv. 8 (2018) 28538–28545. [4] R.P. Cao, Z.H. Shi, G.J. Quan, et al., Preparation and luminescence properties of Li2MgZrO4:Mn4þ red phosphor for plant growth, J. Lumin. 188 (2017) 577–581. [5] J. Xiang, J. Chen, N. Zhang, et al., Far red and near infrared double-wavelength emitting phosphor Gd2ZnTiO6:Mn4þ, Yb3þ for plant cultivation LEDs, Dyes Pigments 154 (2019) 257–262. [6] X. Huang, H. Guo, Finding a novel highly efficient Mn4þ -activated Ca3La2W2O12 far-red emitting phosphor with excellent responsiveness to phytochrome PFR : towards indoor plant cultivation application, Dyes Pigments 152 (2018) 36–42. [7] X. Huang, J. Liang, B. Li, et al., High-efficiency and thermally stable far-redemitting NaLaMgWO6:Mn4þ phosphorsfor indoor plant growth light-emitting diodes, Opt. Lett. 43 (2018) 3305–3308. [8] J. Long, X. Yuan, C. Ma, et al., Strongly enhanced luminescence of Sr4Al14O25:Mn4þ phosphor by co-doping B3þ and Naþ ions with red emission for plant growth LEDs, RSC Adv. 8 (2018) 1469–1476. [9] Q. Sun, S. Wang, B. Li, et al., Synthesis and photoluminescence properties of deep red-emitting CaGdAlO4:Mn4þ phosphors for plant growth LEDs, J. Lumin. 203 (2018) 371–375.
3.2.5. IQE Internal quantum efficiency is also an important parameter in prac tical applications. Internal quantum efficiency is usually calculated by the ratio of the number of photons emitted to the number of photons absorbed[41], which can be calculated by equation (6) [59]: R Ls I R ηint ¼ em ¼ R (7) Iabs ER Es where LS is the emission spectrum of the phosphor, and ES and ER are the spectra of the absorption of phosphor and BaSO4 reference material in the integral sphere, respectively. Fig. 13 depicts the internal quantum efficiency of SLSO:0.008Mn4þ and SLSO:0.008Mn4þ, 0.008Al3þ (λex ¼ 310 nm). 4. Conclusion In summary, a novel type of red phosphor SLSO:Mn4þ with double perovskite structure was prepared by traditional solid state reaction method. Under 334 nm excitation, the emission spectra ranges from 600 8
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