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Novel Yellowish-green Emitting Ca10(PO4)6O:Ce3+ Phosphor: Structural Refinement, Preferential Site Occupancy and Color Tuning Guogang Li,*,a Yun Zhao,a Yi Wei,a Ying Tian,a Zewei Quanc and Jun Lin*,b
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Novel apatite structure phosphate phosphor Ca10(PO4)6O:Ce were firstly reported and identified, whose emission peaks were widely shifted from 410 nm (blue light) to 510 nm (yellowishgreen light that is closer to yellow light in eyes) with increasing 3+ Ce doping level under n-UV excitation due to a preferential occupancy at different crystallographic sites.
Developing rare earth ions activated phosphor materials with excellent luminescence performances can promote their application in the wide range of technology such as solid-state lighting, flat 1-3 panel displays, biological diagnostic and biomedical areas etc. Many studies have devoted to explore strategies for designing novel phosphor materials and optimizing their luminescence 4 properties including single crystal growth method, cationic/anionic 5-7 heuristics optimization based solid state substitutions, 8 combinatorial chemistry method, single-particle-diagnosis 9 10 approach, design of energy transfers at different sites, and so on. These approaches basically refer the change of coordination environment surrounding activators (especially Ce3+ and Eu2+ ions) because the 5d-4f transitions are sensitive to structural variation. Among various cationic/anionic substitutions, crystal–site engineering approach has been well designed for finding new solidstate luminescence materials.11 This methodology frequently forms solid solutions, and just control the preferential occupancy of activators at different crystallographic sites with different coordination environment, resulting in the adjustment of the optical properties of the existing materials.12-15 Therefore, the selection of host materials that commonly requires several cationic sites for accommodating activator ions, and the controllable variation of local crystal lattice are important. Apatite compounds are one of the most attractive host materials due to facile synthesis,
a.
Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China. E-mail: [email protected] b. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail: [email protected]; Fax: (+86) 431-85698041 c. Department of Chemistry, South University of Science and Technology of China, Shenzhen, Guangdong 518055, P. R. China Electronic Supplementary Information (ESI) available: [Refined structure parameters, XRD patterns, Absorption spectra, Lifetime decay curves, CIE color coordinates, emission peaks, QEs, FWHM, and Normalized Gaussian fitting PLE and PL spectra of Ca10(1-x)Ce10x(PO4)6O (x = 0–0.30)]. See DOI: 10.1039/x0xx00000x
16, 17
low cost, and highly physical-chemical stability. This structure commonly contains two kinds of cation sites, the nine-coordinated 4f sites with C3 point symmetry and the seven- or eight-coordinated 6h sites with Cs point symmetry, and both sites are easily occupied 18-22 by a variety of rare earth ions and transition metal ions. Moreover, the coordination environment of 4f and 6h sites could be effected by compositional substitution, generating color-tunable emission. Herein, we constructured a novel apatite-structured phosphate 3+ phosphor [Ca10(PO4)6O:Ce , CPO:Ce] by high temperature solid state reaction, and its crystal structure was firstly identified by the 3+ Rietveld refinement method. By changing the Ce doping concentration, the emission colors could be tuned from blue light to yellowish-green light that more closer to yellow light in eyes, and the corresponding luminescence mechanism was explored. In addition, the thermal stability of Ce3+ emission at the substituted sites in CPO:Ce phosphors were also revealed. The experimental section and a series of characterizations are shown in supporting information [SI]. Figure 1a shows the Rietveld fitting of the XRD pattern of representative Ca8Ce2(PO4)6O sample according to the starting 23 material of Ca10(PO4)6S from the literature. The Refined structure parameters and values of cell parameters of Ca10(PO4)6O host and 3+ the representative Ca8Ce2(PO4)6O and other Ce -doped Ca10(1x)Ce10x(PO4)6O (x = 0–0.30) samples are listed in Table S1-S3 [SI]. According to the refinement data, the Ca8Ce2(PO4)6O sample crystallizes in hexagonal phase with space group P63/m (173), a = b = 3 9.4367 Å, c = 6.9114 Å, α = β = 90°, γ = 120°, V = 533.02 Å and Z = 1. All atom positions, fraction factors, and thermal vibration parameters were refined by convergence and satisfied well the 2 reflection conditions, Rwp = 3.93%, Rp = 2.70%, and χ = 1.864, as shown in Table S1 [SI]. This result indicates the formation of singlephase and the crystal structure of CPO host are unchanged with the 3+ introduction of Ce ions. Although there is a charge-unbalance in 3+ 2+ Ca10(1-x)Ce10x(PO4)6O by substituting Ce ions for Ca ions, the asprepared Ca10(1-x)Ce10x(PO4)6O (x = 0–0.30) samples can hold singlephase at x ≤ 0.30 and some impurity phase appears after x ≥ 0.35, as shown in Figure S1 [SI]. The ICP analysis of the Ca, Ce and P contents (Table S4, SI) indicate that the atom ratios of (Ca+Ce)/P in the representative Ca10(1-x)Ce10x(PO4)6O (x = 0–0.15) samples are all
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Average bond length (A)
proposed as follow: When doping Ce ions into CPO:xCe system, they will randomly enter into Ca1−Ca3 sites. It is generally known (a) 2.65 that the bond length of Ce−O has significant effect on the crystal 5 d(Ca1-O) 2.60 field strength (Dq), i.e., Dq is proportional to 1/r , and the loose site d(Ca2-O) 3+ accommodating Ce activators corresponds to a high-energy d(Ca3-O) 2.55 emission, and on the contrary a low-energy emission will 26,27 2.50 appear. Because the d(Ca2-O) and d(Ca3-O) are smaller than d(Ca1-O) in CPO:xCe (Figure 3a), the emission of Ce2 and Ce3 are 2.45 3+ lower energy emission relative to Ce1. Therefore, at a low Ce 3+ 2.40 doping level, Ce ions mainly substitutes at Ca1 sites. With 3+ enhancing x values in CPO:xCe, more Ce ions gradually locate at 0.00 0.05 0.10 0.15 0.20 0.25 0.30 3+ Ce concentration (x, mol%) the Ca2 and Ca3 sites and ultimately occupy the Ca3 sites. A linear increase in d(Ca3-O) make the Ca3 polyhedron expand and distort Figure 3. (a) The average bond length (Å) [d(Ca1-O), d(Ca2-O) and the crystal lattice. To release lattice strain, the Ca1−O and Ca2−O 3+ d(Ca3-O)] as a function of Ce doping contents (x) in Ca10(1- bond lengths at neighboring Ca1 and Ca2 polyhedrons decrease (Figure 3b), resulting the decrease in d(Ca1-O) and d(Ca2-O). This x)Ce10x(PO4)6O (x = 0–0.30). (e) A schematic explanation of the site3+ phenomenon increases 5d-orbital crystal field splitting of Ce1 and preferential occupancy of Ce ions with its doping contents. Ce2, causing a continuous redshift in the emission spectra, which with the different emission wavelengths. The results also verify that could be confirmed by the normalized Gaussian-fitting PL spectra of the emission spectrum of Ca10(1-x)Ce10x(PO4)6O (x = 0.05, 0.10, 0.20) Ce1 and Ce2 emission centers in Figure S1 [SI]. The redshift of Ce3+ consists of three different emission peaks, indicating the existence ions at Ca3 sites is mainly attributed to a gradual increase of Ce3 of three emission sites. Interestingly, the emission peaks gradually emission with x. Finally, the increasing FWHM should originate from 3+ shift from 410 nm to 510 nm with the increase of Ce content, the increase of relative intensity of Ce3+ at low-energy sites than which is accompanied by an increase in FWHM from 58 nm to 142 that of high-energy sites. In addition, although the luminescence nm, as shown in Figure 2b. Thus, a wide range of tunable emission efficiency of the studied samples are not high (Table S5, SI), it offers from blue light to yellowish-green light are observed, which could a new insight in the wide-range-tunable photoluminescence be confirmed by their CIE color coordinates in Table S5 and Figure properties apatite-structured phosphors. Moreover, the S5 [SI] and luminescent photos in Figure 2c. Therefore, novel luminescence performances of the studied samples could be further yellowish-green emission was firstly observed in CPO:Ce phosphor. improved by process optimization. 3+ The optimal Ce -doping concentration is optimized to be 15 mol% The thermal stability is extremely important in ensuring a high 2+ of Ca in CPO host by comparing their emission intensity (integral efficiency and stability of phosphor-converted devices.24, 28-30 area). Seen from Figure 2b, at x < 0.15, the blue emission dominates, Figures 4 show thermal quenching behaviour of CPO:xCe from room while x > 0.15 the yellowish-green emission are main. The temperature to 573 K. The emission intensities of all samples redshifted emission and increasing FWHM mean that the 3+ coordination environments around Ce ions are continuously changed with x. An obvious red shift in the emission spectra of 1% 1.0 EM 340 nm Ca8Ce2(PO4)6O sample under different UV wavelengths (the insert in 5% 3+ 2+ 10% Figure 2b) further demonstrate that Ce ions enter multi Ca sites 15% 3+ 0.8 and a possible distortion of crystal lattice distortion around Ce 20% 25% ions. In addition, two obvious absorption bands 240-320 nm and 30% 320-400 nm of CPO:xCe were observed as shown in Figure S2 [SI]. 0.6 The first absorption band dominates at x < 0.15, while the second absorption band dominates after x ≥ 0.15. This result also 0.34 3+ 0.4 demonstrate the existence of different Ce luminescence centers. 0.32 To reveal the real red shift mechanism, the average Ca-O bond 0.30 3+ length [d(Ca-O)] with the doping of Ce ions were investigated. 0.2 0.28 2+ Figure 3a show the dependence of d(Ca-O) at three Ca sites and 0.26 3+ 0 5 10 15 20 25 30 3+ the Ce doping concentration. Obviously, linear decrease in d(Ca1Ce content (mol%) 3+ 0.0 O) is observed when enhancing the Ce doping concentration in 300 350 400 450 500 T (K) CPO, while the d(Ca3-O) gradually increase with x from 0 to 0.30. It is noted that although the d(Ca2-O) undergos a first increasing and then decreasing process and the turning point is x = 0.15, it also shows a downward trend. The above phenomena imply that with Figure 4. Thermal quenching behaviour of photoluminescence for 3+ 3+ the increase of Ce concentration more Ce ions gradually occupy Ca10(1-x)Ce10x(PO4)6O (x = 0–0.30) samples (λex = 340 nm). The insert 3+ the Ca2 and Ca3 sites at x < 0.15 and ultimately enter the Ca3 sites shows the dependence of activation energy Ea and Ce doping 12,13 contents. at 0.15 < x < 0.30, which is consistent with the previous reports. Consequently, a possible redshifted luminescence mechanism is
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decrease with increasing environmental temperature due to the non-radiative transitions under high work temperature. The overall trend of thermal quenching behaviour progressively deteriorate 3+ across the overall samples with increasing Ce concentration, especially, an obvious decrease appear at x = 0.20. This is mainly 3+ because that the Ce emission at Ca2 and Ca3 dominate at a high doping level (x ≥ 0.20), resulting a red shift emission. While this red shift cause the increase in Stokes shift, which should mean an 31 decrease in thermal energy (Ea). To verify this result, the Ea of CPO:xCe (x = 0.01-0.30) samples were calculated by IT/I0 = [1 + D −1 exp(−Ea/kT)] , where It (intensity at T), I0 (intensity at T = 0), D, and activation energy Ea are refined variables. Clearly, the Ea of CPO:xCe samples gradually decrease with x, suggesting that the probability of nonradiative transition can be weakened by thermal activation. Therefore, the thermal stability of CPO:xCe samples gradually 3+ become bad with the increase of Ce concentration. However, it still can remain 76% for x = 0.15 and 51% for x = 0.20 at 423 K of the room temperature emission intensity. Moreover, it could be further improved by optimizing doping level and synthesis condition. In summary, a novel yellowish-green emitting phosphor with apatite structure were firstly prepared through solid state reaction process. Three available cation sites that coordinated with nine (Ca1), nine (Ca2) and seven (Ca3) oxygen atoms, respectively, for activator ions were identified, which offered a probability for multiple emission. Under n-UV excitation, the luminescent colors of 3+ Ca10(PO4)6O:Ce phosphors could be continuously tuned from blue 3+ light (410 nm) to yellowish-green light (510 nm) with increasing Ce doping level, which could be explained by a preferential occupancy 3+ at different crystallographic sites. The thermal stability of Ce emission at the substituted sites in CPO:Ce phosphors were also revealed, which could obtain 76% for x = 0.15 and 51% for x = 0.20 3+ at 423 K of the original RT intensity at 423 K in Ca10(1-x)(PO4)6O:xCe system. In view of the above results, the reported emission-tunable 3+ Ca10(PO4)6O:Ce phosphors are potential phosphors for applying in n-UV based WLEDs devices. We acknowledge the financial support from National Basic Research Program of China (Grants No. 2010CB327704), the National Natural Science Foundation of China (Grants No. NSFC 21301162, 60977013, 91433110, U1301242, 21221061) and the Fundamental Research Founds for National University, China University of Geosciences (Wuhan) (Grant No. CUG130402, CUG130614, CUG130624, GBL31510). Zewei Quan acknowledges the funding support (FRG-SUSTC1501A-17) from South University of Science and Technology of China.
Notes and references 1 N. C. George, K. A. Denault and R. Seshadri, Annu. Rev. Mater. Res., 2013. 43, 481–501. 2 Z. G. Yi, S. J. Zeng, W. Lu, H. B. Wang, L. Rao, H. R. Liu and J. H. Hao, ACS Appl. Mater. Interfaces, 2014, 6, 3839–3846. 3 Y. S. Liu, D. T. Tu, H. M. Zhu and X. Y. Chen, Chem. Soc. Rev., 2013, 42, 6924–6958. 4 P. Pust, V. Weiler, C. Hecht, A. Tücks, A.S. Wochnik, A.-K. Henß, D. Wiechert, C. Scheu, P. J. Schmidt and W. Schnick, Nature Mater., 2014, 13, 891–896.
5 S. S. Wang, W. T. Che, Y. Li, J. Wang, H. S. Sheu and R. S. Liu, J. Am. Chem. Soc., 2013, 135, 12504−12507. 6 Z. Y. Zhao, Z. G. Yang, Y. R. Shi, C. Wang, B. T. Liu, G. Zhu and Y. H. Wang, J. Mater. Chem. C, 2013, 1, 1407–1412. 7 Z. G. Xia, C. G. Ma, M. S. Molokeev, Q. L. Liu, K. Rickert and K. R. Poeppelmeier, J. Am. Chem. Soc., 2015, 137, 12494–12497. 8 W. B. Park, S. P. Singh and K. S. Sohn, J. Am. Chem. Soc., 2014, 136, 2363−2373. 9 N. Hirosaki, T. Takeda, S. Funahashi and R. J. Xie, Chem. Mater., 2014, 26, 4280–4288. 10 C. H. Huang, T. S. Chan, W. R. Liu, D. Y. Wang, Y. C. Chiu, Y. T. Yeh and T. M. Chen, J. Mater. Chem., 2012, 22, 20210–20216. 11 G. G. Li, Y. Tian, Y. Zhao and J. Lin, Chem. Soc. Rev., 2015, 44, 8688–8713. 12 W. P. Chen, H. B. Liang, H. Y. Ni, P. He and Q. Su, J. Electrochem. Soc., 2010, 157, J159–J163. 13 C. M. Liu, H. B. Liang, X. J. Kuang, J. P. Zhong, S. S. Sun and Y. Tao, Inorg. Chem., 2012, 51, 8802−8809. 14 X. J. Zhang, J. Wang, L. Huang, F. J. Pan, Y. Chen, B. F. Lei, M. Y. Peng and M. M. Wu, ACS Appl. Mater. Interfaces, 2015, 7, 10044– 10054. 15 Y. F. Xia, J. Chen, Y. G. Liu, M. S. Molokeev, M. Guan, Z. H. Huang and M. H. Fang, Dalton Trans., 2016, DOI: 10.1039/C5DT03786G. 16 D. Y. Zhai, L. X. Ning, Y. C. Huang and G. K. Liu, J. Phys. Chem. C, 2014, 118, 16051–16059. 17 Z. X. Tao, Y. L. Huang and H. J. Seo, Dalton Trans., 2013, 42, 2121–2129. 18 G. G. Li, D. L. Geng, M. M. Shang, C. Peng, Z. Y. Cheng and J. Lin, J. Mater. Chem., 2011, 21, 13334–13344 19 W. Lv, Y. C. Jia, W. Z. Lv, Q. Zhao and H. P. You, New J. Chem., 2013, 37, 3701–3705. 20 S. Y. Qi, Y. L. Huang, Y. D. Li, P. Q. Cai, S. II. Kim and H. J. Seo, J. Mater. Chem. B, 2014, 2, 6387–6396. 21 Z. G. Xia, M. S. Molokeev, W. B. Im, S. Unithrattil and Q. L. Liu, J. Phys. Chem. C, 2015, 119, 9488–9495. 22 Z. F. Pan, Y. Xu, Q. S. Hu, W. Q. Li, H. Zhou and Y. F. Zheng, RSC Adv., 2015, 5, 9489–9496. 23 P. R. Suitch, A. Taitai, J. L. Lacout, R. A. Young, J. Solid State Chem., 1986, 63, 267–277. 24 C. W. Yeh, W. T. Chen, R. S. Liu, S. F. Hu, H. S. Sheu, J. M. Chen and H. T. Hintzen, J. Am. Chem. Soc., 2012, 134, 14108–14117. 25 W. R. Liu, C. H. Huang, C. W. Yeh, Y. C. Chiu, Y. T. Yeh and R. S. Liu, RSC Adv. 2013, 3, 9023–9028. 26 P. Dorenbos, ECS Solid State Letters, 2013, 2, R3001–R3011. 27 C. K. Jørgensen, Modern Aspects of Ligand Field Theory, Amsterdam: North-Holland, 1971. 28 X. Piao, K. Machida, T. Horikawa, H. Hanzawa, Y. Shimomura and N. Kijima, Chem. Mater., 2007, 19, 4592−4599. 29 Y. Q. Li, N. Hirosaki, R. J. Xie, T. Takeda and M. Mitomo, Chem. Mater., 2008, 20, 6704–6714. 30 K.-S. Sohn, B. Lee, R.-J. Xie and N. Hirosaki, Opt. Lett., 2009, 34, 3427–3429. 31 G. G. Li, C.-C. Lin, W.-T. Chen, M. S. Molokeev, V. V. Atuchin, C.-Y. Chiang, W. Z. Zhou, C.-W. Wang, W.-H. Li, H.-S. Sheu, T.-S. Chan, C. G. Ma, and R.-S. Liu, Chem. Mater., 2014, 26, 2991−3001.
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Novel Yellowish-green Emitting Ca10(PO4)6O:Ce3+ Phosphor: Structural Refinement, Preferential Site Occupancy and Color Tuning Guogang Li,*,a Yun Zhao,a Yi Wei,a Ying Tian,a Zewei Quanc and Jun Lin*,b
www.rsc.org/ 3+
Novel apatite structure phosphate phosphor Ca10(PO4)6O:Ce were firstly reported and identified, whose emission peaks were widely shifted from 410 nm (blue light) to 510 nm (yellowishgreen light that is closer to yellow light in eyes) with increasing 3+ Ce doping level under n-UV excitation due to a preferential occupancy at different crystallographic sites.
Developing rare earth ions activated phosphor materials with excellent luminescence performances can promote their application in the wide range of technology such as solid-state lighting, flat 1-3 panel displays, biological diagnostic and biomedical areas etc. Many studies have devoted to explore strategies for designing novel phosphor materials and optimizing their luminescence 4 properties including single crystal growth method, cationic/anionic 5-7 heuristics optimization based solid state substitutions, 8 combinatorial chemistry method, single-particle-diagnosis 9 10 approach, design of energy transfers at different sites, and so on. These approaches basically refer the change of coordination environment surrounding activators (especially Ce3+ and Eu2+ ions) because the 5d-4f transitions are sensitive to structural variation. Among various cationic/anionic substitutions, crystal–site engineering approach has been well designed for finding new solidstate luminescence materials.11 This methodology frequently forms solid solutions, and just control the preferential occupancy of activators at different crystallographic sites with different coordination environment, resulting in the adjustment of the optical properties of the existing materials.12-15 Therefore, the selection of host materials that commonly requires several cationic sites for accommodating activator ions, and the controllable variation of local crystal lattice are important. Apatite compounds are one of the most attractive host materials due to facile synthesis,
a.
Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China. E-mail: [email protected] b. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail: [email protected]; Fax: (+86) 431-85698041 c. Department of Chemistry, South University of Science and Technology of China, Shenzhen, Guangdong 518055, P. R. China Electronic Supplementary Information (ESI) available: [Refined structure parameters, XRD patterns, Absorption spectra, Lifetime decay curves, CIE color coordinates, emission peaks, QEs, FWHM, and Normalized Gaussian fitting PLE and PL spectra of Ca10(1-x)Ce10x(PO4)6O (x = 0–0.30)]. See DOI: 10.1039/x0xx00000x
16, 17
low cost, and highly physical-chemical stability. This structure commonly contains two kinds of cation sites, the nine-coordinated 4f sites with C3 point symmetry and the seven- or eight-coordinated 6h sites with Cs point symmetry, and both sites are easily occupied 18-22 by a variety of rare earth ions and transition metal ions. Moreover, the coordination environment of 4f and 6h sites could be effected by compositional substitution, generating color-tunable emission. Herein, we constructured a novel apatite-structured phosphate 3+ phosphor [Ca10(PO4)6O:Ce , CPO:Ce] by high temperature solid state reaction, and its crystal structure was firstly identified by the 3+ Rietveld refinement method. By changing the Ce doping concentration, the emission colors could be tuned from blue light to yellowish-green light that more closer to yellow light in eyes, and the corresponding luminescence mechanism was explored. In addition, the thermal stability of Ce3+ emission at the substituted sites in CPO:Ce phosphors were also revealed. The experimental section and a series of characterizations are shown in supporting information [SI]. Figure 1a shows the Rietveld fitting of the XRD pattern of representative Ca8Ce2(PO4)6O sample according to the starting 23 material of Ca10(PO4)6S from the literature. The Refined structure parameters and values of cell parameters of Ca10(PO4)6O host and 3+ the representative Ca8Ce2(PO4)6O and other Ce -doped Ca10(1x)Ce10x(PO4)6O (x = 0–0.30) samples are listed in Table S1-S3 [SI]. According to the refinement data, the Ca8Ce2(PO4)6O sample crystallizes in hexagonal phase with space group P63/m (173), a = b = 3 9.4367 Å, c = 6.9114 Å, α = β = 90°, γ = 120°, V = 533.02 Å and Z = 1. All atom positions, fraction factors, and thermal vibration parameters were refined by convergence and satisfied well the 2 reflection conditions, Rwp = 3.93%, Rp = 2.70%, and χ = 1.864, as shown in Table S1 [SI]. This result indicates the formation of singlephase and the crystal structure of CPO host are unchanged with the 3+ introduction of Ce ions. Although there is a charge-unbalance in 3+ 2+ Ca10(1-x)Ce10x(PO4)6O by substituting Ce ions for Ca ions, the asprepared Ca10(1-x)Ce10x(PO4)6O (x = 0–0.30) samples can hold singlephase at x ≤ 0.30 and some impurity phase appears after x ≥ 0.35, as shown in Figure S1 [SI]. The ICP analysis of the Ca, Ce and P contents (Table S4, SI) indicate that the atom ratios of (Ca+Ce)/P in the representative Ca10(1-x)Ce10x(PO4)6O (x = 0–0.15) samples are all
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Average bond length (A)
proposed as follow: When doping Ce ions into CPO:xCe system, they will randomly enter into Ca1−Ca3 sites. It is generally known (a) 2.65 that the bond length of Ce−O has significant effect on the crystal 5 d(Ca1-O) 2.60 field strength (Dq), i.e., Dq is proportional to 1/r , and the loose site d(Ca2-O) 3+ accommodating Ce activators corresponds to a high-energy d(Ca3-O) 2.55 emission, and on the contrary a low-energy emission will 26,27 2.50 appear. Because the d(Ca2-O) and d(Ca3-O) are smaller than d(Ca1-O) in CPO:xCe (Figure 3a), the emission of Ce2 and Ce3 are 2.45 3+ lower energy emission relative to Ce1. Therefore, at a low Ce 3+ 2.40 doping level, Ce ions mainly substitutes at Ca1 sites. With 3+ enhancing x values in CPO:xCe, more Ce ions gradually locate at 0.00 0.05 0.10 0.15 0.20 0.25 0.30 3+ Ce concentration (x, mol%) the Ca2 and Ca3 sites and ultimately occupy the Ca3 sites. A linear increase in d(Ca3-O) make the Ca3 polyhedron expand and distort Figure 3. (a) The average bond length (Å) [d(Ca1-O), d(Ca2-O) and the crystal lattice. To release lattice strain, the Ca1−O and Ca2−O 3+ d(Ca3-O)] as a function of Ce doping contents (x) in Ca10(1- bond lengths at neighboring Ca1 and Ca2 polyhedrons decrease (Figure 3b), resulting the decrease in d(Ca1-O) and d(Ca2-O). This x)Ce10x(PO4)6O (x = 0–0.30). (e) A schematic explanation of the site3+ phenomenon increases 5d-orbital crystal field splitting of Ce1 and preferential occupancy of Ce ions with its doping contents. Ce2, causing a continuous redshift in the emission spectra, which with the different emission wavelengths. The results also verify that could be confirmed by the normalized Gaussian-fitting PL spectra of the emission spectrum of Ca10(1-x)Ce10x(PO4)6O (x = 0.05, 0.10, 0.20) Ce1 and Ce2 emission centers in Figure S1 [SI]. The redshift of Ce3+ consists of three different emission peaks, indicating the existence ions at Ca3 sites is mainly attributed to a gradual increase of Ce3 of three emission sites. Interestingly, the emission peaks gradually emission with x. Finally, the increasing FWHM should originate from 3+ shift from 410 nm to 510 nm with the increase of Ce content, the increase of relative intensity of Ce3+ at low-energy sites than which is accompanied by an increase in FWHM from 58 nm to 142 that of high-energy sites. In addition, although the luminescence nm, as shown in Figure 2b. Thus, a wide range of tunable emission efficiency of the studied samples are not high (Table S5, SI), it offers from blue light to yellowish-green light are observed, which could a new insight in the wide-range-tunable photoluminescence be confirmed by their CIE color coordinates in Table S5 and Figure properties apatite-structured phosphors. Moreover, the S5 [SI] and luminescent photos in Figure 2c. Therefore, novel luminescence performances of the studied samples could be further yellowish-green emission was firstly observed in CPO:Ce phosphor. improved by process optimization. 3+ The optimal Ce -doping concentration is optimized to be 15 mol% The thermal stability is extremely important in ensuring a high 2+ of Ca in CPO host by comparing their emission intensity (integral efficiency and stability of phosphor-converted devices.24, 28-30 area). Seen from Figure 2b, at x < 0.15, the blue emission dominates, Figures 4 show thermal quenching behaviour of CPO:xCe from room while x > 0.15 the yellowish-green emission are main. The temperature to 573 K. The emission intensities of all samples redshifted emission and increasing FWHM mean that the 3+ coordination environments around Ce ions are continuously changed with x. An obvious red shift in the emission spectra of 1% 1.0 EM 340 nm Ca8Ce2(PO4)6O sample under different UV wavelengths (the insert in 5% 3+ 2+ 10% Figure 2b) further demonstrate that Ce ions enter multi Ca sites 15% 3+ 0.8 and a possible distortion of crystal lattice distortion around Ce 20% 25% ions. In addition, two obvious absorption bands 240-320 nm and 30% 320-400 nm of CPO:xCe were observed as shown in Figure S2 [SI]. 0.6 The first absorption band dominates at x < 0.15, while the second absorption band dominates after x ≥ 0.15. This result also 0.34 3+ 0.4 demonstrate the existence of different Ce luminescence centers. 0.32 To reveal the real red shift mechanism, the average Ca-O bond 0.30 3+ length [d(Ca-O)] with the doping of Ce ions were investigated. 0.2 0.28 2+ Figure 3a show the dependence of d(Ca-O) at three Ca sites and 0.26 3+ 0 5 10 15 20 25 30 3+ the Ce doping concentration. Obviously, linear decrease in d(Ca1Ce content (mol%) 3+ 0.0 O) is observed when enhancing the Ce doping concentration in 300 350 400 450 500 T (K) CPO, while the d(Ca3-O) gradually increase with x from 0 to 0.30. It is noted that although the d(Ca2-O) undergos a first increasing and then decreasing process and the turning point is x = 0.15, it also shows a downward trend. The above phenomena imply that with Figure 4. Thermal quenching behaviour of photoluminescence for 3+ 3+ the increase of Ce concentration more Ce ions gradually occupy Ca10(1-x)Ce10x(PO4)6O (x = 0–0.30) samples (λex = 340 nm). The insert 3+ the Ca2 and Ca3 sites at x < 0.15 and ultimately enter the Ca3 sites shows the dependence of activation energy Ea and Ce doping 12,13 contents. at 0.15 < x < 0.30, which is consistent with the previous reports. Consequently, a possible redshifted luminescence mechanism is
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DOI: 10.1039/C5CC09782G
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decrease with increasing environmental temperature due to the non-radiative transitions under high work temperature. The overall trend of thermal quenching behaviour progressively deteriorate 3+ across the overall samples with increasing Ce concentration, especially, an obvious decrease appear at x = 0.20. This is mainly 3+ because that the Ce emission at Ca2 and Ca3 dominate at a high doping level (x ≥ 0.20), resulting a red shift emission. While this red shift cause the increase in Stokes shift, which should mean an 31 decrease in thermal energy (Ea). To verify this result, the Ea of CPO:xCe (x = 0.01-0.30) samples were calculated by IT/I0 = [1 + D −1 exp(−Ea/kT)] , where It (intensity at T), I0 (intensity at T = 0), D, and activation energy Ea are refined variables. Clearly, the Ea of CPO:xCe samples gradually decrease with x, suggesting that the probability of nonradiative transition can be weakened by thermal activation. Therefore, the thermal stability of CPO:xCe samples gradually 3+ become bad with the increase of Ce concentration. However, it still can remain 76% for x = 0.15 and 51% for x = 0.20 at 423 K of the room temperature emission intensity. Moreover, it could be further improved by optimizing doping level and synthesis condition. In summary, a novel yellowish-green emitting phosphor with apatite structure were firstly prepared through solid state reaction process. Three available cation sites that coordinated with nine (Ca1), nine (Ca2) and seven (Ca3) oxygen atoms, respectively, for activator ions were identified, which offered a probability for multiple emission. Under n-UV excitation, the luminescent colors of 3+ Ca10(PO4)6O:Ce phosphors could be continuously tuned from blue 3+ light (410 nm) to yellowish-green light (510 nm) with increasing Ce doping level, which could be explained by a preferential occupancy 3+ at different crystallographic sites. The thermal stability of Ce emission at the substituted sites in CPO:Ce phosphors were also revealed, which could obtain 76% for x = 0.15 and 51% for x = 0.20 3+ at 423 K of the original RT intensity at 423 K in Ca10(1-x)(PO4)6O:xCe system. In view of the above results, the reported emission-tunable 3+ Ca10(PO4)6O:Ce phosphors are potential phosphors for applying in n-UV based WLEDs devices. We acknowledge the financial support from National Basic Research Program of China (Grants No. 2010CB327704), the National Natural Science Foundation of China (Grants No. NSFC 21301162, 60977013, 91433110, U1301242, 21221061) and the Fundamental Research Founds for National University, China University of Geosciences (Wuhan) (Grant No. CUG130402, CUG130614, CUG130624, GBL31510). Zewei Quan acknowledges the funding support (FRG-SUSTC1501A-17) from South University of Science and Technology of China.
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