Enhanced luminescence and tunable color of Sr8CaSc(PO4)7:Eu2+, Ce3+, Mn2+ phosphor by energy transfer between Ce3+-Eu2+-Mn2+

Enhanced luminescence and tunable color of Sr8CaSc(PO4)7:Eu2+, Ce3+, Mn2+ phosphor by energy transfer between Ce3+-Eu2+-Mn2+

Journal of Alloys and Compounds 731 (2018) 796e804 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 731 (2018) 796e804

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Enhanced luminescence and tunable color of Sr8CaSc(PO4)7:Eu2þ, Ce3þ, Mn2þ phosphor by energy transfer between Ce3þ-Eu2þ-Mn2þ Jingyan Fan a, Jing Gou a, *, Yali Chen a, Binxun Yu a, **, Shengzhong Frank Liu a, b, *** a

Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China b Dalian National Laboratory for Clean Energy, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2017 Received in revised form 12 October 2017 Accepted 13 October 2017 Available online 16 October 2017

The crystal structure of Sr8CaSc(PO4)7:Eu2þ, Ce3þ, Mn2þ phosphor was refined and determined from Rietveld refinement method. The occupation for Eu2þ, Ce3þ at Ca2þ ion site and Mn2þ at Sc3þ site in Sr8CaSc(PO4)7 host material were proved by the XRDs profiles and Van Uitert equation. The designed trienergy transfer process of Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ occurred in Sr8CaSc(PO4)7 phosphor, and furtherly certified by their decay curves. The corresponding calculated energy transfer efficiencies indicated that the energy transfer Ce3þ/Mn2þ made the main contribution on the emission enhancement of Mn2þ ion. The chromaticity and color temperature of Sr8CaSc(PO4)7:xEu2þ, yCe3þ, zMn2þ phosphors were conveniently adjusted in a broad region by the tri-energy transfer and different excitation energy, which is from (0.2914, 0.3396), 7606 K to (0.4458, 0.3303), 2013 K under 311 nm excitation and (0.2248, 0.5466), 8000 K to (0.3978, 0.4932), 4231 K under 370 nm excitation, respectively. Meanwhile, the corresponding quantum efficiency was improved up to 36.3% in Sr8CaSc(PO4)7:1%Eu2þ, 2.5% Ce3þ, 2%Mn2þ phosphor by tri-energy transfer and high excitation energy 311 nm. The corresponding fabricated 310 SCSPO-LED and 370 SCSPO-LED showed the chromaticity and color temperature as (0.241, 0.345), 10818 K and (0.339, 0.254), 4673 K respectively, that prove Sr8CaSc(PO4)7:xEu2þ, yCe3þ, zMn2þ phosphors have potential application value on NUV WLEDs. © 2017 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence Energy transfer LED Luminescence Phosphor White light emission

1. Introduction White-light-emitting diodes (WLEDs) are replacing the traditional lighting systems because of their excellent features as high efficiency, high thermal stability, energy-saving, long lifetime, and environmental friendly [1e3]. The commercial methods for WLEDs manufacture are combining blue InGaN chip with yellow-emitting Y3Al5O12:Ce3þ (YAG:Ce) phosphor or coating three primary red, green and blue-emitting phosphors on the near-UV (NUV) chip.

* Corresponding author. ** Corresponding author. *** Corresponding author. Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China. E-mail addresses: [email protected] (J. Gou), [email protected] (B. Yu), [email protected] (S.F. Liu). https://doi.org/10.1016/j.jallcom.2017.10.099 0925-8388/© 2017 Elsevier B.V. All rights reserved.

However, in the first method the absence of red-emitting results in cool-white light emitting making this kind of WLEDs not suitable for indoor lighting, and in the second method the energy reabsorption between phosphors lead to the lower luminescence efficiency. Thus how to simply obtain high efficiency warm white light is still a challenge in the field of WLEDs. Energy transfer (ET) between different luminescent activators is considered as an effective approach to generate warm-white light from single-phase phosphor, that avoid the insufficiency of low correlated color temperature (CCT) and energy reabsorption. How to select appropriate ions and adjust the emission color by energy transfer is valuable to be explored. Because rare earth or transition metal ions have rich energy levels, a rare earth sensitizer and multiactivators (rare earth or transition metal ions) are designed doping into crystalline matrices, such as Eu2þ/Tb3þ/Mn2þ ions in (Sr3,Ca,Ba)(PO4)3Cl phosphor [4], Ce3þ/Tb3þ/Mn2þ ions in Ca3Gd7(PO4)(SiO4)5O2 phosphor [5], Tm3þ/Eu3þ/Tb3þ in LaF3 [6] and CaW1xMoxO4 phosphors [7]. The warm-white-emitting was reported to be directly obtained by efficient energy transfer of

J. Fan et al. / Journal of Alloys and Compounds 731 (2018) 796e804

Eu2þ-Mn2þ [8e10], however, how to further improve its transfer efficiency is also a challenge. Because of their overlapped PLE spectra, the introduction of Ce3þ as a sensitizer to enhance the luminescence efficiency of Eu2þ and Mn2þ is considered as a good choice. Whitlockite phosphate Sr8MA(PO4)7 (M ¼ Zn2þ, Mg2þ; A ¼ Gd3þ, Sc3þ, La3þ) derive from Sr9A(PO4)7, which have been developed as the key materials for NUV LEDs applications because of its rigid crystal structure, high chemical and thermal stability [9,11e15]. How to modify its constituents as a stable matrix for luminescent centres is very important. Due to the ionic radius of Ca2þ is closed to Sr2þ, the incorporation of Ca2þ can stabilize original crystal structure, thus Sr8CaSc(PO4)7 is chosen as aimed host material. The luminescence properties of the novel Sr8CaSc(PO4)7:Ce3þ, Eu2þ, Mn2þ phosphors is firstly explored and reported in this paper. In our research, the crystal structure and the luminescent properties of Sr8CaSc(PO4)7:Ce3þ, Eu2þ, Mn2þ phosphor were detailed investigated. The tunable Commission International de l’Eclairage (CIE) chromaticity coordinates and CCT can be obtained by the increasing incorporation of Mn2þ ions and different excitation energy. Furthermore, the mechanism for energy transfer from Ce3þ and Eu2þ to Mn2þ ions was certified by their photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra and fluorescence decay curves. The EL spectra of fabricated 310 SCSPO-LED and 370 SCSPO-LED proves that Sr8CaSc(PO4)7:Eu2þ, Ce3þ, Mn2þ phosphor is potential to be fabricated with NUV (300e420 nm) chip for the warm white-light LEDs application. 2. Experimental 2.1. Materials and synthesis The polycrystalline phosphors composed of Sr8CaSc(PO4)7:xEu2þ, yCe3þ, zMn2þ (SCSPO:xEu2þ, yCe3þ, zMn2þ) (0 < x  2.5%, 0  y  3%, 0  z  3%) were synthesized by a traditional solid-state reaction, in which the constituent raw materials SrCO3 (A. R., 99%); CaCO3 (A. R., 99.0%); Sc2O3 (A. R., 99.99%); (NH4)2HPO4 (98.5%); Ce(NO3)3$6H2O (A. R., 99.95%); Eu2O3 (A. R., 99.99%) and MnCO3 (A. R., 99.0%) were weighted in stoichiometric

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proportions. The mixed powders were sintered at 900  C for 6 h in air, then reground and calcined at 1450  C for 24 h under a reducing CO atmosphere. The SCSPO resin is constructed of an epoxy resin well-dispersed with SCSPO:1%Eu2þ, 2.5%Ce3þ, 1.0%Mn2þ and SCSPO:1%Eu2þ, 2.5%Ce3þ, 2.0%Mn2þ phosphors. The epoxy resin was made of Q-203 A and Q-203 B (VA:VB ¼ 2:1). The SCSPO-LED was fabricated by combining SCSPO resin on LED chips. 2.2. Materials characterization The crystal structure were refined and determined by SmartLab powder X-ray diffractometer. The X-ray diffraction (XRD) data were collected using Cu Ka radiation (DX-2700 powder X-ray diffractometer). The PLE, PL and EL spectra were all recorded by a Hitachi F-7000 fluorescence spectrophotometer with Xe lamp as the light source. All the fluorescence lifetime measurements were investigated by using an Edinburgh FLS980 fluorescence spectrophotometer. The Photoluminescence quantum efficiency (QE) was measured by an Absolute PL quantum yield measurement system Hamamatsu C9920-02G. All the measurements above were performed at room temperature. The thermal quenching measurements were investigated by using the Hitachi F-7000 fluorescence spectrophotometer equipped with a heating accessory (TCB1402C). 3. Results and discussion 3.1. Phase identification and crystal structure Fig. 1(a) shows the observed (black crosses), calculated (red solid line), and difference (bottom) XRD profiles for the Rietveld refinement of Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ and Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 2.5%Mn2þ phosphors at 298 K. The Rietveld refinement results indicate that the incorporation of Eu2þ, Ce3þ and Mn2þ ions in Sr8CaSc(PO4)7 host structure didn't cause any impurity. The Sr8CaSc(PO4)7:Eu2þ, Ce3þ, Mn2þ phosphors crystallize as a monoclinic structure with space group I2/a. For Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ, its lattice parameters were determined as a ¼ 18.0583(1) Å, b ¼ 10.6681(8) Å, c ¼ 18.4782(1) Å, a ¼ 90 , b ¼ 133.0128 , g ¼ 90 , and V ¼ 2598.05(31) Å3. Its refinement finally converged to RP ¼ 5.25%,

Fig. 1. (a) Observed (black crosses), calculated (red solid line), and difference (bottom) XRD profiles for the Rietveld refinement of Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ (down) and Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 2.5%Mn2þ (up) phosphors at 298 K. Bragg reflections are indicated by green tick marks; (b) the XRDs of Sr8CaSc(PO4)7, Sr8CaSc(PO4)7:1% Eu2þ, Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ and Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 1.5%Mn2þ phosphors. The magnified diffraction peaks at about 30 were inserted in the figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Rietveld Refinement and Crystal Data of Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ and Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 2.5%Mn2þ phosphors. Formula

Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ

Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 2.5%Mn2þ

2q range (deg.) T (K) symmetry space group a b c a ¼ g (deg.) b (deg.) V (Å3) Z Rp(%) Rwp(%)

10e90 298 monoclinic I2/a 18.0583(1) 10.6681(8) 18.4782(1) 90 133.0128 V ¼ 2598.05(31) 4 5.25 7.21 2.149

10e90 298 monoclinic I2/a 18.0365(9) 10.6519(9) 18.4424(6) 90 132.9982 2591.55(59) 4 5.24 6.95 1.95

c

RWP ¼ 7.21% and c2 ¼ 2.149. When the incorporation of Mn2þ increases to 2.5%, the lattice parameters changes as a ¼ 18.0365(9) Å, b ¼ 10.6519(9) Å, c ¼ 18.4424(6) Å, a ¼ 90 , b ¼ 132.9982 , g ¼ 90 , and V ¼ 2591.55(59) Å3, and its refinement finally converged to RP ¼ 5.24%, RWP ¼ 6.95% and c2 ¼ 1.95. The refinements with different Mn2þ incorporations both reveal the great fitting quality. With the incorporation of Mn2þ increasing to 2.5%, the shrank unit cell was observed. All the lattice parameters for Sr8CaSc(PO4)7:1% Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ and Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 2.5%Mn2þ were collected in Table 1, and the corresponding atom positions and occupancies factors were presented in Tables S1 and S2. Fig. 1 (b) presents the XRD patterns of Sr8CaSc(PO4)7, Sr8CaSc(PO4)7:1%Eu2þ, Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, and Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 2.5%Mn2þ for comparison, the corresponding magnified diffraction peak at about 30 were also inserted in Fig. 1(b). All the diffraction peaks can be indexed with the Powder Diffraction File (PDF) card No. 54e1186, that indicate all the samples well crystallized as Sr9Sc(PO4)7 structure, and the little doping amount of Eu2þ, Ce3þ and Mn2þ didn't cause any significant phase change. The TEM image and corresponding high-resolution TEM image of Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ phosphor are presented in Fig. 2 (a) and (b), respectively. The TEM image demonstrate the phosphor particle size as approximate 450 nm, the high-resolution TEM can further prove Sr8CaSc(PO4)7:1%Eu2þ, 2.5% Ce3þ, 0.5%Mn2þ phosphor was well crystallized as Sr9Sc(PO4)7 structure, and the measured interplanar distance d ¼ 0.2657 nm corresponds to (2, 2, 6) crystal face.

The simulated crystal structure of Sr8CaSc(PO4)7 is shown in Fig. 3, which is the isostructural b-Ca3(PO4)2 crystal structure. In the b-Ca3(PO4)2 crystal structure, the occupation sites of Ca2þ cations has five different positions and one vacancy; Ca1 and Ca2 are eightcoordinated with oxygen atoms, Ca3 is nine-coordinated, Ca4 site is half-occupied with nine-coordination, the calcium cations at Ca5 site form a distorted octahedron with their six coordinated oxygen atoms, and Ca6 is a vacancy. In the Sr8CaSc(PO4)7 structure, Sr and Ca occupy the same sites. The Sr/Ca2, (Sr/Ca1, Sr/Ca3, Sr/Ca4 and Sr/ Ca5), Sr/Ca6 sites correspond to the (Ca1, Ca2), Ca3 and (Ca4, Ca6) sites in the b-Ca3(PO4)2 crystal structure, respectively. Only Sc site corresponds to Ca5 site with six-coordination; and the fourcoordinated P present as tetrahedrons. The ionic radii of Sr2þ are 1.26 Å (CN ¼ 8) and 1.31 Å (CN ¼ 9), that of Ca2þ are 1.12 Å (CN ¼ 8) and 1.18 Å (CN ¼ 9), and six-coordinated Sc3þ is 0.745 Å, respectively. However, the ionic radii for six-, eight- and nine-coordinated Eu2þ are 1.17 (CN ¼ 6), 1.25 (CN ¼ 8), and 1.3 Å (CN ¼ 9), Ce3þ are 1.01 (CN ¼ 6), 1.143 (CN ¼ 8), and 1.196 Å (CN ¼ 9) and Mn2þ are 0.67 (CN ¼ 6) and 0.96 (CN ¼ 8). In the enlargement view of XRDs in Fig. 1 (b), the diffraction peaks both obviously move toward to the lower angle when Eu2þ and Ce3þ were separately incorporated into host crystal lattice, then move back toward higher angle with 1.5% Mn2þ introduction. The movement of diffraction peaks illustrates that the incorporation of Eu2þ and Ce3þ ions both expanded the unit cells; on the contrary, the introduction of Mn2þ ion shrank the unit cells. On the basis of ionic radii, the Eu2þ and Ce3þ ions are considered to occupy the smaller sites as Ca2þ and Sc3þ ions; and Mn2þ ions should also enter Ca2þ ion sites in the SCSPO matrix. 3.2. Photoluminescence properties Fig. 4 (a), (b) and (c) present the PLE and PL spectra of SCSPO:1% Eu2þ, SCSPO:1%Ce3þ and SCSPO:1%Mn2þ respectively for comparison. It is clearly observed that the PLE and PL spectra of Ce3þ both overlap with the PLE spectra of Eu2þ and Mn2þ ions very well, thus the energy transfers between the three ions are expected, eventually the tunable emission color can be obtained by adjusting the concentrations of Ce3þ, Eu2þ and Mn2þ ions in SCSPO matrix. For clarifying the energy transfer relationship between Ce3þ and 2þ Eu ions, the PLE and PL spectra of SCSPO:1%Eu2þ, yCe3þ (0  y  3%) are measured and presented in Fig. 5 (a) and (b). 1% is selected as the optimal doping amount of Eu2þ according to the PL spectra of SCSPO:xEu2þ in Fig. S1, and a broad absorption band between 260 and 450 nm can been observed in its PLE spectrum, which is attributed to the 4f7/4f65d1 transition of Eu2þ ions. This broad excitation band matches well with the emission of NUV LED

Fig. 2. (a) TEM image and (b) high-resolution TEM image of Sr8CaSc(PO4)7:1%Eu2þ, 2.5%Ce3þ, 0.5%Mn2þ phosphor.

J. Fan et al. / Journal of Alloys and Compounds 731 (2018) 796e804

Fig. 3. Crystal structure of Sr8CaSc(PO4)7.

Fig. 4. (a) PLE spectra (lem ¼ 505 nm, left) and PL spectra (lex ¼ 370 nm, right) of SCSPO:1%Eu2þ phosphor; (b) PLE spectra (lem ¼ 370 nm, left) and PL spectra (lex ¼ 311 nm, right) of SCSPO:1%Ce3þ phosphor; (c) PLE spectra (lem ¼ 606 nm, left) and PL spectra (lex ¼ 370 nm, right) of SCSPO:1%Mn2þ phosphor.

chip in the range of 300e400 nm. From the PL spectra of Eu2þ in Fig. S1, the emission bands locating at 505 nm belong to 4f65d1/4f7 transition of Eu2þ ions. The emission of Ce3þ ion was reported at about 368 nm, thus it can be considered as a well matched sensitizer to furtherly improve the emission of Eu2þ ion. In

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Fig. 5 (a), the intensive excitation band located at 311 nm and a weaker band with a maximum at 270 nm both are attributed to the electron transitions from Ce3þ 4f ground states to its excited 5d states split by the crystal field. All the Ce3þ emission bands centering at 370 nm are almost the same, which proves the crystal field environment around Ce3þ ions with different concentrations being similar. The broad asymmetric emission spectra of Ce3þ shows that Ce3þ has more than one emission centers in Sr8CaSc(PO4)7, which could be attributed to the typical Ce3þ 5d-4f transitions [11]. Under 311 nm excitation, with an increasing Ce3þ content, the emission intensity of Ce3þ reached the maximum at y ¼ 1.5%, and then decreased, meanwhile, the emission intensity of Eu2þ is enhanced until y>2.5% begin to decrease. With 370 nm excitation, the same increasing emission intensity of Eu2þ can be observed in Fig. 5 (b), and it also reached the maximum at y ¼ 2.5%. The enhancement on the emission intensity of Eu2þ could be attributed to the energy transfer between Ce3þ and Eu2þ. Therefore, the PLE spectra monitoring at 505 nm were normalized and presented in Fig. 5 (b), the intensive enhancement before 355 nm in the PLE spectra could due to the absorption of Ce3þ, which strongly validates the occurrence of the energy transfer from Ce3þ to Eu2þ ions. For obtaining white light emission, a series of Mn2þ ions with different concentrations were codoped in the phosphor as SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ. Fig. 6 illustrates the PL and PLE spectra of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphor. In Fig. 6 (a), the codoped phosphors show the violet emission bands due to the 5d-4f transition of Ce3þ ion, the green emission bands of Eu2þ ion and the red emission band attributing to the spinforbidden 4T1(4G)/6A1(6S) transition of Mn2þ ion. With the increasing of the Mn2þ content, the emission intensity of the Ce3þ and Eu2þ ions decrease monotonically excited by 311 nm, meanwhile the emission intensity of the Mn2þ ion firstly increases to the maximum at z ¼ 2.5% and then decreases because of the concentration quenching effect. The decreased emission intensities of Eu2þ also can be observed with increasing amount of Mn2þ under 370 nm excitation in Fig. 6 (b). The PLE spectra of SCSPO:1%Eu2þ, 2.5%Ce3þ, 2%Mn2þ phosphor monitoring at 606 nm is shown in Fig. 6 (c). The detected absorption bands of Eu2þ and Ce3þ ions at 370 nm, 311 nm and 270 nm furtherly confirm the energy transfers of Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ. Previous research results reveal that Mn2þ ion with eight-coordinated leads to a red emission.11 Thus, the emission of Mn2þ ion furtherly confirms the occupation site for Mn2þ at Ca2þ site, which is in

Fig. 5. (a) PLE spectra (lem ¼ 370 nm, left) and PL spectra (lex ¼ 311 nm, right); (b) PLE spectra (lem ¼ 505 nm, left) and PL spectra (lex ¼ 370 nm, right) of SCSPO:1%Eu2þ, yCe3þ (0  y  3%) phosphor.

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Fig. 6. PL spectra (a) (lex ¼ 311 nm); (b) (lex ¼ 370 nm) of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphor; (c) PLE spectra (lem ¼ 606 nm) of SCSPO:1%Eu2þ, 2.5%Ce3þ, 2% Mn2þ phosphor.

accordance with the conclusion from the XRDs in Fig. 1(b). The critical distance between the activator Mn2þ ions could be calculated using the relation proposed by Lammers et al. [16],

 RC z2

3V 4pxC Z

1=3 (1)

where V is the volume of the unit cell, xc is the sum of critical concentration of activator ion Mn2þ and sensitizer ions Eu2þ and Ce3þ, Z the number of formula units per unit cell. For SCSPO host, using Z ¼ 4, xc ¼ 0.06, and V ¼ 2591.56 Å3, the obtained Rc value is 27.42 Å. The larger value of calculated Rc means more stiffness of host matrix, that is due to phosphate tetrahedral network. According to the report of Van Uitert, the emission positions of the Eu2þ and Ce3þ ions are strongly dependent on their local environments; the possible crystallographic sites were investigated theoretically by the following equation [14,17]:

2

3  1 V nEa r V E ¼ Q 41   10 80 5 4

(2)

where E is the position of Eu2þ or Ce3þ ions' emission peaks, Q is the position in energy for the lower d-band edge for Eu2þ or Ce3þ ions (34000 cm1 and 50000 cm1 for Eu2þ and Ce3þ, respectively); V is the valence of Eu2þ (V ¼ 2) and Ce3þ (V ¼ 3), n is the number of anions in the immediate shell about the Eu2þ or Ce3þ ions, Ea is the electron affinity of the atoms that form anions (eV), and r is the

radius of host cation replaced by the Eu2þ or Ce3þ ions (Å). For phosphate phosphor, the value of Ea is approximately 2.19 eV. The experimental and calculated emission wavelengths of Eu2þ and Ce3þ ions in different coordination environments are all listed in Table 2. The result indicates that the emission band of Eu2þ with peak at 505 nm is attributed to eight-coordination Eu2þ luminescence centers, and the emission band centered at 370 nm can be assigned to six- and eight-coordinated Ce3þ ions. For further confirmation on the coordination environments of Ce3þ ions, the PL spectrum of SCSPO:1% Ce3þ was decomposed and shown in Fig. S2. From this figure, the emission band of Ce3þ consists of three decomposed emission bands at 356 nm, 380 nm and 411 nm. The 356 nm emission band is attributed to the eight-coordinated Ce3þ ion and the 380 nm and 411 nm emission bands belong to the Ce3þ ion at six-coordinated sites. In order to further validate the energy transfer process of Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ in SCSPO host lattice, the decay curves for Ce3þ and Eu2þ emissions in SCSPO:1% Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphors are respectively shown in Fig. 7 (a) and (b). The decay curves are successfully fitted using the following two exponential equation [18], I(t) ¼ I0þA1exp(t/t1) þA2exp(t/t2)

(3)

where I(t) and I0 are the luminescence intensities at times t; A1 and A2 are fitting constants; t1 and t2 are the decay times for corresponding exponential components. The average decay times (t*) can be calculated by the following formula,

J. Fan et al. / Journal of Alloys and Compounds 731 (2018) 796e804 Table 2 Experimental and calculated emission wavelengths of Eu2þ and Ce3þ ions in different coordination environments in the Sr8CaSc(PO4)7 host. Activators

n

r(Å)

Ecalcd (cm1)

lcalcd (nm)

lexp (nm)

Eu2þ

8 9 6 8 9

rCa/1.12 rCa/1.18 rSc/0.745 rCa/1.12 rCa/1.18

20333 21690 23844 30292 32250

492 461 419 330 310

505

Ce3þ

t* ¼

A1 t21 þ A2 t22 A1 t1 þ A2 t2

370

(4)

All the calculated average decay times are listed in Table 3. Obviously, the decay times of Ce3þ and Eu2þ emissions both gradually decrease with the increasing Mn2þ ion doping amount, which indicate the existence of the energy transfer process of Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ in SCSPO host lattice. Their energy transfer efficiencies hT can be calculated by the following equation by Paulose et al. and also listed in Table 3.

hT ¼ 1 

t t0

(5)

where t0 and t are the lifetimes of the sensitizer in the absence and presence of the activator, respectively. As a function of z (z ¼ 0.5%, 1%, 2%, 2.5% and 3%), the calculated hT for Ce3þ/Mn2þ energy transfer in SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ are 8.56%, 15.42%, 28.00%, 28.14% and 36.98%, respectively, and the hT for Eu2þ/Mn2þ energy transfer were also calculated as 0.54%, 10.22%, 13.10%, 13.34% and 14.52%. The larger hT for Ce3þ/Mn2þ energy transfer indicates that the energy transfer Ce3þ/Mn2þ

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presents dominant than Eu2þ/Mn2þ for Mn2þ ion's emission enhancement. To clearly understand the tri-energy transfer processes of Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ, the possible energy transfer routes are also presented in Fig. 8 as a reference. As illustrated in Fig. 8, the 5d band of Eu2þ partially overlaps the excited-state energy levels 4T2(4D) and 4T2(4G) of Mn2þ, therefore, we consider that the dual-process energy transfers of Eu2þ/Mn2þ may also take place in two ways of ① and ② except the occurrence of the emission from lower energy levels of 4f65d1 to 8S7/2. In energy transfer ①, the energy transfer occurs from the lowest 5d excited level of Eu2þ to the 4T2 (4G) emitting level of Mn2þ by multiphonon relaxation, then the electrons relax to the lower excited energy level 4T1(4G) and subsequently return to the ground state 6A1 by radiative transition. In the energy transfer ②, the electrons excited into higher 5d excited level of Eu2þ by direct excitation at 370 nm are transferred to the 4T2 (4D) emitting level of Mn2þ by multiphonon relaxation, then the electrons relax to 4T1 (4G) emitting level of Mn2þ by two steps, and eventually return to the ground state 6A1 by radiative transition. The energy transfers ③ and ④ both start from the 5d energy level of Ce3þ. Because of the overlaps between the 5d band of Ce3þ with Eu2þ 5d band and Mn2þ 4 T2 (4D) energy level, one part of the electrons in 5d energy level of Ce3þ jump back to the ground state 2T5/2 with the emission centering at 370 nm, another part electrons jump from Ce3þ 5d band to the energy level of Mn2þ 4T2 (4D) ④ or 5d energy level of Eu2þ ③. The followed energy transfer process ② from 5d energy level of Eu2þ to Mn2þ 4T2 (4D) energy level occur after energy transfer process ③. Then the electrons relax to 4T2 (4G) and following 4T1 (4G) emitting level of Mn2þ by two steps, and eventually return to the ground state 6A1 by radiative transition. The chromaticity, CCT and QE are important parameters for practical LED applications. The energy transfer between sensitizer

Fig. 7. (a) Decay curves of Ce3þ emission (lex ¼ 311 nm, lem ¼ 370 nm); (b) Decay curves of Eu2þ emission (lex ¼ 370 nm, lem ¼ 505 nm) in SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphors.

Table 3 The calculated average decay times and energy transfer efficiency of Eu2þ and Ce3þ.

lex and lem lex ¼ 311 nm, lem ¼ 370 nm lex ¼ 370 nm, lem ¼ 505 nm

z

t* (ns) hT t* (ns) hT

0

0.5%

1.0%

2.0%

2.5%

3.0%

21.0250

19.2250 8.56% 867.9604 0.54%

17.7839 15.42% 783.4416 10.22%

15.1379 28.00% 758.3446 13.10%

15.1080 28.14% 756.2563 13.34%

13.2498 36.98% 745.9474 14.52%

872.6581

802

J. Fan et al. / Journal of Alloys and Compounds 731 (2018) 796e804 Table 5 The QE of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphors with the excitation of 311 nm and 370 nm. SCSPO: 1%Eu2þ, 2.5%Ce3þ, zMn2þ

z z z z z

¼ ¼ ¼ ¼ ¼

QE

0 0.5% 1.0% 2.0% 2.5%

lex ¼ 311 nm

lex ¼ 311 nm

10.4% 23.5% 27.1% 36.3% 35.5%

10.4% 23.5% 27.1% 36.3% 35.5%

Fig. 8. The energy levels schemes of Ce3þ/Eu2þ/Mn2þ and the possible energy transfer modes for Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ in SCSPO host lattice.

and activator is an effective way for adjusting the CIE chromaticity coordinates and CCT, as well as improving the QE of phosphors. QE is defined as the ratio of the emitted photons to the absorbed photons, which can be calculated by following equation [19,20],

Z QE ¼

Iem ¼Z Iabs

LS Z ER  ES

(6)

where Iem and Iabs stand for the number of emitted and absorbed photons, and LS, ER and ES represent the spectra of the PL spectra (with sample), PLE spectra (without sample) and PLE spectra (with sample) in the integrating sphere, respectively. All the calculated chromaticity coordinates, CCT and QE of SCSPO: 1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphors are listed in Tables 4 and 5, respectively. Meanwhile, their chromaticity coordinates are shown in the CIE chromaticity diagram as Fig. 9 for more directly comparison. It is obviously observed that the chromaticity coordinates of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ phosphors locate at yellowish green region under 370 nm excitation, and with the increasing Mn2þ concentration the CCT decrease, meanwhile, the corresponding chromaticity coordinates move toward to yellow region, then move back when z ¼ 3.0%. When excited by 311 nm, the CCT of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ phosphors can also be decreased with the increasing Mn2þ amount, meanwhile, their chromaticity coordinates can be adjusted from cold white to yellowish pink region. The maximum QE of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ phosphors are observed as 36.3% (z ¼ 2.0%) and 14.8% (z ¼ 1.0%), respectively, under 311 nm and 370 nm excitation. Therefore, the

Fig. 9. CIE chromaticity diagram for the SCSPO: 1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphors excited at 311 nm (a-g) and 370 nm (1e6).

two phosphors are fabricated as SCSPO resins and subsequently combined with 310 nm and 370 nm LED chips as 310 SCSPO-LED and 370 SCSPO-LED. Generally, the thermal stability is also an important technological parameter for phosphors. Fig. 10 (a) shows the temperature dependence of the luminescence for SCSPO:1%Eu2þ, 2.5%Ce3þ, 1% Mn2þ phosphor. The relative intensity of Ce3þ ion decreased with temperature increasing. The thermal stability of Mn2þ emission present better than that of Eu2þ emission. As seen in Fig. 10 (a), the slightly blue-shifts of Eu2þ and Mn2þ emissions with increasing temperature both can be observed, which is caused by the thermally active phonon-assisted excitation from lower energy sublevels to higher energy sublevels in the excited states of Eu2þ and

Table 4 The CIE chromaticity coordinates (x, y) and CCT of SCSPO:1%Eu2þ, 2.5%Ce3þ, zMn2þ (0  z  3%) phosphors with the excitation of 311 nm and 370 nm. SCSPO: 1%Eu2þ, 2.5%Ce3þ, zMn2þ

z z z z z z

¼ ¼ ¼ ¼ ¼ ¼

0 0.5% 1.0% 2.0% 2.5% 3.0%

Chromaticity coordinates (x, y)

CCT

lex ¼ 311 nm

lex ¼ 370 nm

lex ¼ 311 nm

lex ¼ 370 nm

(0.2914, (0.3502, (0.3896, (0.4292, (0.4303, (0.4458,

(0.2248, 0.5466) (0.2490,0.5331) (0.2716,0.5229) (0.2819,0.5121) (0.3217,0.5294) (0.3978,0.4932)

7606 4678 3204 2110 2199 2013

8000 7472 6959 6746 5804 4231

0.3396) 0.3299) 0.3279) 0.3182) 0.3269) 0.3303)

K K K K K K

K K K K K K

J. Fan et al. / Journal of Alloys and Compounds 731 (2018) 796e804

803

Fig. 10. (a) PL spectra of SCSPO: 1%Eu2þ, 2.5%Ce3þ, 1%Mn2þ with various temperature ranging from 298 K to 423 K. The relationship between relative intensities of Eu2þ and Mn2þ emission bands and temperature (lex ¼ 370 nm) (inserted); (b) The relationship of ln(I0/I-1) versus 1/kT activation energy graph for thermal quenching of SCSPO: 1%Eu2þ, 2.5%Ce3þ, 1%Mn2þ phosphor.

Fig. 11. The EL spectra of (a) 310 SCSPO-LED (V ¼ 6 V, I ¼ 20 mA) and (b) 370 SCSPO-LED (V ¼ 10 V, I ¼ 200 mA); (c) The chromaticity coordinates (x,y) and color temperatures of 310 SCSPO-LED and 370 SCSPO-LED in the CIE 1931 chromaticity diagram; (d) The photographs of 310 SCSPO-LED and 370 SCSPO-LED.

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J. Fan et al. / Journal of Alloys and Compounds 731 (2018) 796e804

Mn2þ [21,22]. For further exploring the thermal quenching property of SCSPO: 1%Eu2þ, 2.5%Ce3þ, 1%Mn2þ phosphor, the Arrhenius equation was used to express the relationship between temperature and luminescence intensity. The activation energy (Ea) can be calculated by the following equation,

  I Ea ln 0 ¼ ln A  I kT

(7)

where I0 and I are the luminescence intensities of SCSPO: 1%Eu2þ, 2.5%Ce3þ, 1%Mn2þ at initial temperature and given temperature, respectively. A is a constant, k is Boltzmann's constant 1.38065  1023 J/K and Ea is the activation energy for thermal quenching. Fig. 10 (b) plots the relationship of ln(I0/I-1) versus 1/kT for thermal quenching of SCSPO:1%Eu2þ, 2.5%Ce3þ, 1%Mn2þ phosphor, which can be well fitted as linearity with a slope of 0.2619, thus the activation energy Ea for thermal quenching is estimated to be 0.2619 eV. The EL property of WLEDs can present its application potential. The EL spectra of 310 SCSPO-LED (V ¼ 6 V, I ¼ 20 mA) and 370 SCSPO-LED (V ¼ 10 V, I ¼ 200 mA) are illustrated in Fig. 11 (a) and (b), respectively. The corresponding chromaticity coordinates (x,y) and CCT of 310 SCSPO-LED and 370 SCSPO-LED were calculated and shown in the CIE 1931 chromaticity diagram as Fig. 11 (c), and their photographs are inserted in Fig. 11 (d). The calculated chromaticity coordinates (x,y) and color temperatures of 310 SCSPO-LED are (0.339, 0.254) and 4673 K, and those of 370 SCSPO-LED are (0.241, 0.345) and 10818 K. The homogeneous warm white and cold white emitting light can both are directly observed from the photographs of 310 SCSPO-LED and 370 SCSPO-LED. That illustrate the tunable chromaticity and CCT also can be obtained in SCSPO:Eu2þ, Ce3þ, Mn2þ phosphor by excited with the different LED emitting energy, which demonstrated that SCSPO:Eu2þ, Ce3þ, Mn2þ has potential application value on white NUV LEDs.

with higher excitation energy can lead the phosphors' chromaticity moving toward to warm color region, CCT greatly reduced and QE improved to the maximum. The EL properties of fabricated 310 SCSPO-LED and 370 SCSPO-LED both indicated that SCSPO:Eu2þ, Ce3þ, Mn2þ phosphor has potential application value on NUV WLEDs. Acknowledgements This research was financially supported by the National Key Research Program of China (2016YFA0202403), National Natural Science Foundation of China (21603140 and 21641001), the Fundamental Research Funds for the Central Universities (GK201603108 and GK201603048), and Natural Science Basic Research Plan in Shaanxi Province of China (2015JQ5140). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2017.10.099. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

4. Conclusions In summary, a novel white-light-emitting SCSPO:Eu2þ, Ce3þ, Mn2þ phosphor was synthesized and evaluated for use in NUV WLEDs. The tunable white-light-emission can be obtained by the design of Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ energy transfer in SCSPO host lattice, which furtherly can be certified by the PL, PLE spectra and decay curves of SCSPO:Eu2þ, Ce3þ, Mn2þ phosphor. The corresponding energy transfer efficiencies hT of Eu2þ/Mn2þ and Ce3þ/Mn2þ energy transfers with different Mn2þ concentration were all calculated, the higher energy transfer efficiency of Ce3þ/Mn2þ indicates that the energy transfer Ce3þ/Mn2þ presents dominant than Eu2þ/Mn2þ for the enhancement of Mn2þ emission. The Eu2þ/Mn2þ, Ce3þ/Mn2þ and Ce3þ/Eu2þ/Mn2þ tri-energy transfer processes cooperating

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