- Email: [email protected]
Synthesis and luminescence properties of Eu2+-activated phosphor Ba3LaK(PO4)3F for n-UV white-LEDs
Accepted Manuscript Synthesis and luminescence properties of Eu2+ -activated phosphor Ba3LaK(PO4)3F for n-UV white-LEDs Xiaoxue Ma, Lefu Mei, Haikun Liu, Libing Liao, Kun Nie, Yuqin Liu, Zhaohui Li PII: DOI: Reference:
S0277-5387(16)30420-X http://dx.doi.org/10.1016/j.poly.2016.09.001 POLY 12188
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
28 February 2016 15 August 2016 1 September 2016
Please cite this article as: X. Ma, L. Mei, H. Liu, L. Liao, K. Nie, Y. Liu, Z. Li, Synthesis and luminescence properties of Eu2+ -activated phosphor Ba3LaK(PO4)3F for n-UV white-LEDs, Polyhedron (2016), doi: http://dx.doi.org/ 10.1016/j.poly.2016.09.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and luminescence properties of Eu2+ -activated phosphor Ba3LaK(PO4)3F for n-UV white-LEDs Xiaoxue Ma1, Lefu Mei1,*, Haikun Liu1, Libing Liao1,*, Kun Nie1,Yuqin Liu1 , Zhaohui Li1,2 1
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes,
National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing,100083, China 2
Geosciences Department, University of Wisconsin Parkside, Kenosha, WI 53141-2000, USA
*
Author to whom correspondence should be addressed: Lefu Mei, Libing Liao
e-mail: [email protected];[email protected] Tel.: +86-10-82322039;+86-10-8233-1701 Fax.: +86-10-82322974;+86-10-8232-1701
Abstract The simple high-temperature solid-state route was adopted to synthesize a series of related
Ba3LaK(PO4)3F:xEu2+
(BLKPF:xEu2+)
blue
phosphors.
The
photoluminescence (PL) and diffuse reflection spectra have been investigated in detail. These phosphors which can be well excited by the 388 nm near ultraviolet light, produce the emitted blue light peaks located at about 460 nm. The critical Eu 2+ concentration quenching mechanism was ascribed to the dipole–dipole interaction, and the optimum concentration was eventually proved to be 0.1mol. All above results indicate that Ba3LaK(PO4)3F:xEu2+ phosphor can act as an appropriate candidate of the blue phosphor in the future.
1． ．Introduction Under the background of global energy shortage，white light emitting diodes (w-LEDs) was called the fourth generation lighting source and has been widely applied owing to its excellent properties, such as environmentally-friendly, energy saving, low-consumption, highly-efficiency and so on[1-4]. One method to generate the practical white light emission was to combine tri-color phosphors with a n-UV-InGaN chips (350–420 nm)[5-7]. Among tri-color phosphors, the blue component occupies a large proportion, so it is meaningful to explore blue-emitting
phosphors for the development of w-LEDs. Since light-emitting wavelength of the chips available in the market are most at 350–420 nm which locates at n-UV region, it is important to explore phosphors that can be excited by the n-UV optical source[8-11]. Phosphor consists of activator and matrix; as a super activator, Eu2+ ions can generate blue spectra with 4f-5d transition and have been intensely studied among the rare earth family [12-14]. Beyond this, good phosphors also need an appropriate substrate. Apatite is an excellent substrate because of its good chemical stability and it can be formulated as A10[PO4]6Z2; A often represents divalent cations such as Mg2+, Ca2+, Sr2+, Ba2+, Mn2+ and Pb2+; Z is denoted by F-, Cl-, OH-, or O2-[15,16]. Apatite compounds have been widely investigated in many fields for various applications, such as Sr5(PO4)2SiO4, Ca5(PO4)2SiO4[17-20] and so on. To enrich its applications, apatite needs more forms and there are many opportunities to build up new inorganic framework to obtain new compounds with apatite structure since it possesses wide accommodation ability in A sites [21-25]. The more the frameworks are, the more selections do the engineers have. Up to now, there is no report about Eu 2+-doped Ba3LaK(PO4)3F phosphor with an apatite structure[26-29]. In this study, a series of new Ba3 LaK(PO4)3F:Eu2+ phosphors with apatite structure have been reported, and their photoluminescence properties have been discussed in detail. The synthesis technique with a relative low temperature and reductive atmosphere provided by CO requires low energy consumption and is easy to achieve compared to the phosphors which must be prepared at high temperature and under high pressure. According to all these results, the phosphor could have potential application in the n-UV- wLEDs as blue components.
2. Experimental A sequence of phosphors Ba3LaK(PO4)3F:xEu 2+ were setted by the conservative high-temperature solid-state method. The fresh materials were (NH4)2HPO4 (A.R.), BaCO3 (A.R.), K2CO3 (A.R.), NH4HF2 (A.R.), La2O3 (99.99%), and Eu2O3 (99.99%), which were used immediately without any handling. X-ray diffraction patterns
(Cu-Kα line, 40 kV, 30 mA) with radiation (λ=0.15406 nm) were obtained using an X-ray powder diffractometer (XD-3, PGENERAL, China). Diffuse flection spectra on as-setted phosphors were recorded on a UV–vis–NIR spectrophotometer (UV-3600) connected with an integrating sphere, reference standard of which was BaSO4 powder. Photoluminescence excitation spectra and photoluminescence emission spectra at room temperature were recorded on a fluorescence spectrophotometer (Hitachi F-4600) and the xenon lamp (400 V, 150 W) was excitation source as the matching device. A 400 nm cut-off filter was applied in the measurement so that the second-order emission of source radiation can be accurately eliminated. Doping concentrations of the europium ions in Ba3LaK(PO4)3F:xEu 2+ were x=0.01,0.03,0.05,0.10,0.15,0.20 and 0.30. Amounts of the fresh materials were prepared according to this formula, then the selected fresh materials were blended and ground drastically in an agate mortar until the mixture was homogeneous. After that the stoichiometric combination was parked into an alumina crucible and annealed at 1080℃ for 3h. At last, the combinations were cooled to room temperature in the furnace naturally, and ground into powder again for the measurements mentioned above.
3. Results and discussions We adopted XRD analysis to verify structures of the as-setted samples. As shown in Fig.1, the representative XRD patterns include the Ba3LaK(PO4)3F and the Ba3LaK(PO4)3F:0.1Eu2+phosphor. It is obvious that the XRD patterns of them can be matched perfectly to the phase of Ba3LaNa(PO4)3F(JCPDS No.71-1337 ), which belongs to apatite structure with the hexagonal system and the group space P63/m. In view of the positions and intensities of diffraction peak matched well with the reference JCPDS file, we can see that the doping caused by Eu ions did not bring about variation in the structure ， which indicates the successful isomorphic replacement of Eu atoms to Ba atoms and K atoms to Na atoms in the present structure[30]. We measured the important spectra of different samples to evaluate the application
value of the phosphor which has an apatite structure. Fig.2 depicts the photoluminescence excitation (PLE), photoluminescence emission (PL) and the diffuse reflection spectra of Ba2.9LaK(PO4)3F:0.1Eu2+. The PLE spectrum of Ba2.9LaK(PO4)3F:0.1Eu 2+ ranges from 200 to 450 nm and the diffuse reflection spectrum of the Ba2.9LaK(PO4)3F:0.1Eu2+ phosphor shows a broad absorption in the 225–425 nm (n-UV) range, which results from the 4f–5d absorption of the Eu2+ ions, and they match well with each other. The characteristic excitation spectrum centers at 388 nm, which should be ascribed to transition 4f7–4f65d 1, suggesting that the excitation wavelength of as-grown phosphors goes well with the near-UV-LED chips. Upon 388 nm excitation, the phosphor shows a broad blue band centered at 460 nm, which is attributed to the electric-dipole-allowed transition from the lowest level of the 5d excited state (4f65d1) to the 4f ground state (4f7(8S7/2) of doped Eu2+ ions[31]. Obviously, the shape of the PL spectrum is not Gaussian peak in the long wavelength region, and the unsymmetrical emission spectrum can be deconvoluted into two Gaussian peaks centering at 455 and 479 nm which are shown as two dotted curves in Fig. 2 respectively. Since 455nm is close to 460nm, we choose 460 nm and 479 nm as the monitored spectra. The spectral profiles of excitation spectra monitored via 460nm and 479 nm are different, which indicates there may be two different Eu2+ emission centers and is in accordance with the apatite crystal structure. Then we measured PL spectra of samples with different doping contents. The PL spectra of Ba3LaK(PO4)3F:xEu2+ (x=0.01,0.03,0.05,0.10,0.15,0.20 and 0.30) excited by 388 nm are demonstrated in Fig.3 (a) . As is shown in Fig.3(a),the emission intensity of Eu 2+ ions firstly increases and peaks at x=0.10, then the emission intensity decreases as a result of concentration quenching effect. The following equation can be used to calculate the interaction type between sensitizers or between sensitizer and activator [32]: I/x= K [1+β(x)θ/3]-1 Here x represents the activator concentration and it is not less than the optimal concentration, I/x is the emission intensity (I) of unit activator concentration (x), K and β are constants once the excitation condition is fixed, θ indicates the electric
multipolar character and θ=6, 8, 10 respectively represents dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q). Fig.3(b) plots the lg(I/x) dependence on lg(x) and shows a linear dependence between them. The slope of the straight line can be calculated to be -1.6992 and the value of θ can be calculated to be 5.0976 as a result, which is close to 6 and means the quenching interactions of Ba3LaK(PO4)3F:xEu 2+ phosphors is dipole–dipole. The CIE chromaticity diagram for the Ba2.9LaK(PO4)3F:0.1Eu2+ phosphor under 365 nm UV excitation is shown in Fig. 4. The color coordinate is calculated to be (0.147, 0.092), which means that the phosphor can be used as a blue-emitting phosphor for w-LEDs application. Meanwhile, the typical digital photo of the Ba2.9LaK(PO4)3F:0.1Eu 2+ phosphor under 365 nm UV lamp is shown in the inset of Fig. 4, and the blue light emission can be seen by naked eyes. All the results above prove that Ba3LaK(PO4)3F:xEu2+ phosphors with 4f–5d absorption efficiency in n-UV region can play an important role in blue emitting phosphor and are potentially useful for ~370 nm GaN-based w-LEDs.
4. Conclusions In summary, blue phosphor Ba3LaK(PO4)3F:xEu2+ was acquired via conservative high-temperature solid-state method. The phosphor showed the emitted blue light peaks locate at about 460 nm ascribed to the 4f–5d transition of Eu2+. It was further proved that concentration quenching of Eu2+ ions in Ba3LaK(PO4)3F:0.1Eu2+ phosphors resulted from the dipole–dipole interactions. All the spectrum features indicate that this blue phosphor might be a component for w-LEDs.
Acknowledgments This present work was supported by the National Natural Science Foundation of China (Grant nos. 41172053), the Fundamental Research Funds for the Central Universities (Grant nos. 2652015306), and Science and Technology Innovation Fund of the China University of Geosciences (Beijing).
References [1] Z.P. Ci, M.D. Que, Y.R. Shi, G. Zhu, Y.H. Wang, Inorg. Chem. 53(2014) 2195. [2] H. Chen, J.Y. Gao, S.J. Su, X. Zhang, Z.H. Wang, IEEE T. Biomed. Circ. S. 9(2015) 227. [3] M.M. Jiao, N. Guo, W. Lu, Y.C. Jia, W.Z. Lv, Q. Zhao, B. Q. Shao, H.P. You, Inorg. Chem. 52(2013) 10340. [4] X. Zhang, M. Liu, B. Wang, H. Chen, Z.H. Wang, IEEE T. Circuits-I. 61 (2014) 2. [5] Q. Peng, C. Zhang, X.J. Zhao, X.G. Sun. F.L. Li, H. Chen, Z.H. Wang, IEEE T. Ind. Electron. (2015) 1. [6] H.S. Roh, S. Hur, H.J. Song, I.J. Park, D.K.Yim, D.W. Kim, K.S. Hong. Mater. Lett. 70(2012) 37. [7] R.Y. Mi, C.L. Zhao, Z.G. Xia, J. Am. Ceram. Soc. 97(2014) 1802. [8] K. Nie, Q. An, Y.H. Zhang, Nanoscale 8(2016) 8791. [9] S.J. Su, J.Y. Gao, Z. Cao, H. Chen, Z.H. Wang, in: 2015 IEEE, IEEE, 2015, p. 1. [10] X.X Ma, L.F. Mei, H.K Liu, L.B Liao, Y.Q. Liu, K. Nie, Z.H. Li, Chem. Phys. Lett. 653(2016) 212. [11] C. Zeng, H.W. Huang, Y.M. Hu, S.H. Miao, j. Zhou, Mater. Res. Bull.76(2016) 62. [12] P.F. Smet, K. Korthout, J.E. Van Haecke, D. Poelman, Mat. Sci. Eng. B-Solid 146(2008) 264. [13] J.H. Gan, Y.L. Huang, L. Shi, X.B. Qiao, H.J. Seo, Mater. Lett. 63(2009) 2160. [14] R.F. Wang, D.C. Zhou, J.B. Qiu, Y. Yang, C. Wang, J. Alloy Compd. 629(2015) 310. [15] A.K. Mishra, S.K. Mishra, B.D. Gupta, Opt. Commun. 344(2015) 86. [16] L.F. Chen, J.Y. Liu, J.S. Wen, H.D. Mao, F. Ge, D.M. Zhao, Opt. Commun. 338(2015) 110. [17] X.Y. Huang, J. Alloy Compd. 628(2015) 240. [18] F.W. Mo, P.C. Chen, A.X. Guan, W. Zhang, L.Y. Zhou, J. Rare Earth 33(2015) 1064. [19] J.S. Huo, J.D. Zhu, S.H. Wu, H.J. Zhang, Q.M. Wang, J. Lumin. 158(2015) 417. [20] W.J. Yang, L.Y. Luo, T.M. Chen, N.S. Wang, Chem. Mater. 17(2005) 3883. [21] R.H. Krishna, B.M. Nagabhushana, B.N. Sherikar, N.S. Murthy, C. Shivakumara, T. Thomas, Chem. Eng. J. 267(2015) 317. [22] K.W. Chae, T.R. Park, C.I. Cheon, N.I. Cho, J.S. Kim, J. Lumin. 132(2012) 2293. [23] J.P. Leonard, T. Gunnlaugsson, J. Fluoresc. 15(2005) 585. [24] A.C. Ferrand, D. Imbert, A.S. Chauvin, C.D.B. Vandevyver, J.C.G. Bunzli, Chem-Eur. J. 13(2007) 8678. [25] B.Y. Qu, B. Zhang, L. Wang, R.L. Zhou, X.C. Zeng, Chem. Mater. 27(2015) 2195. [26] S. Rodriguez, P. Elizondo, S. Bernes, N. Perez, R. Bustos, E. Garcia-Espana, Polyhedron 85(2015) 10. [27] R.Q. Li, G.H. Yu, Y.M. Liang, N.N. Zhang, Y.L. Liu, S.C. Gan, J. Colloid Interf. Sci. 460(2015) 273.
[28] Z. Chen, J.H. Zhang, S. Chen, M.Y. Lin, C.Q. He, G.D. Xu, M.M. Wang, X.F. Yu, J.Q. Zou, K. Guo, J. Alloy Compd. 632(2015) 756. [29] Z.C. Wu, S. Wang, J. Liu, J.H. Yin, S.P. Kuang, J. Alloy Compd. 644(2015) 274. [30] W.B. Im, S. Brinkley, J. Hu, A. Mikhailovsky, S.P. DenBaars, R. Seshadri, Chem. Mater. 22(2010) 2842. [31] Y.Y. Zhang, Z.G. Xia, W.W. Wu, J. Am. Ceram. Soc. 96(2013) 1043. [32] H.K. Liu, Y.Y. Zhang, L.B. Liao, Q.F. Guo, L.F. Mei, Ceram. Int. 40(2014) 13709.
Figures
Fig. 1 XRD patterns of Ba3LaK(PO4)3F and Ba2.9LaK(PO4)3F:0.1Eu2+ samples. The standard data for Ba3LaNa(PO4)3F( JCPDS No.71-1317) is also shown as a reference.
Fig. 2 Photoluminescence excitation (PLE, left) spectra, photoluminescence emission (PL, right) and diffuse reflection spectra (blue) of Ba2.9LaK(PO4)3F:0.1Eu 2+ phosphor.
Fig. 3(a) PL spectra of Ba3-xLaK(PO4)3F:xEu2+ phosphors and the dependence of PL intensity on the Eu 2+ doping concentration. The optimum doping Eu2+ concentration is x=0.10. Insert shows the relationship between Eu2+ contents and the peaks of emission intensity. (b) The fitting curves of lg(I/x) vs. lg(x) in Ba3-xLaK(PO4)3F:xEu 2+ phosphors.
Fig. 4. CIE chromaticity diagram for Ba2.9LaK(PO4)3F:0.1Eu2+ phosphor excited at 365nm. Digital photo of blue light emission is also shown as the evidence.
Highlights: NO.1 An inorganic framework of Ba3LaK(PO4)3F:Eu2+ phosphors with apatite structure has been synthesized. NO.2 The synthesis technique uses a relative low temperature. NO.3 Broad-band absorption originating from the f-d transition of Eu2+ can be found.
S0277-5387(16)30420-X http://dx.doi.org/10.1016/j.poly.2016.09.001 POLY 12188
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
28 February 2016 15 August 2016 1 September 2016
Please cite this article as: X. Ma, L. Mei, H. Liu, L. Liao, K. Nie, Y. Liu, Z. Li, Synthesis and luminescence properties of Eu2+ -activated phosphor Ba3LaK(PO4)3F for n-UV white-LEDs, Polyhedron (2016), doi: http://dx.doi.org/ 10.1016/j.poly.2016.09.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and luminescence properties of Eu2+ -activated phosphor Ba3LaK(PO4)3F for n-UV white-LEDs Xiaoxue Ma1, Lefu Mei1,*, Haikun Liu1, Libing Liao1,*, Kun Nie1,Yuqin Liu1 , Zhaohui Li1,2 1
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes,
National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing,100083, China 2
Geosciences Department, University of Wisconsin Parkside, Kenosha, WI 53141-2000, USA
*
Author to whom correspondence should be addressed: Lefu Mei, Libing Liao
e-mail: [email protected];[email protected] Tel.: +86-10-82322039;+86-10-8233-1701 Fax.: +86-10-82322974;+86-10-8232-1701
Abstract The simple high-temperature solid-state route was adopted to synthesize a series of related
Ba3LaK(PO4)3F:xEu2+
(BLKPF:xEu2+)
blue
phosphors.
The
photoluminescence (PL) and diffuse reflection spectra have been investigated in detail. These phosphors which can be well excited by the 388 nm near ultraviolet light, produce the emitted blue light peaks located at about 460 nm. The critical Eu 2+ concentration quenching mechanism was ascribed to the dipole–dipole interaction, and the optimum concentration was eventually proved to be 0.1mol. All above results indicate that Ba3LaK(PO4)3F:xEu2+ phosphor can act as an appropriate candidate of the blue phosphor in the future.
1． ．Introduction Under the background of global energy shortage，white light emitting diodes (w-LEDs) was called the fourth generation lighting source and has been widely applied owing to its excellent properties, such as environmentally-friendly, energy saving, low-consumption, highly-efficiency and so on[1-4]. One method to generate the practical white light emission was to combine tri-color phosphors with a n-UV-InGaN chips (350–420 nm)[5-7]. Among tri-color phosphors, the blue component occupies a large proportion, so it is meaningful to explore blue-emitting
phosphors for the development of w-LEDs. Since light-emitting wavelength of the chips available in the market are most at 350–420 nm which locates at n-UV region, it is important to explore phosphors that can be excited by the n-UV optical source[8-11]. Phosphor consists of activator and matrix; as a super activator, Eu2+ ions can generate blue spectra with 4f-5d transition and have been intensely studied among the rare earth family [12-14]. Beyond this, good phosphors also need an appropriate substrate. Apatite is an excellent substrate because of its good chemical stability and it can be formulated as A10[PO4]6Z2; A often represents divalent cations such as Mg2+, Ca2+, Sr2+, Ba2+, Mn2+ and Pb2+; Z is denoted by F-, Cl-, OH-, or O2-[15,16]. Apatite compounds have been widely investigated in many fields for various applications, such as Sr5(PO4)2SiO4, Ca5(PO4)2SiO4[17-20] and so on. To enrich its applications, apatite needs more forms and there are many opportunities to build up new inorganic framework to obtain new compounds with apatite structure since it possesses wide accommodation ability in A sites [21-25]. The more the frameworks are, the more selections do the engineers have. Up to now, there is no report about Eu 2+-doped Ba3LaK(PO4)3F phosphor with an apatite structure[26-29]. In this study, a series of new Ba3 LaK(PO4)3F:Eu2+ phosphors with apatite structure have been reported, and their photoluminescence properties have been discussed in detail. The synthesis technique with a relative low temperature and reductive atmosphere provided by CO requires low energy consumption and is easy to achieve compared to the phosphors which must be prepared at high temperature and under high pressure. According to all these results, the phosphor could have potential application in the n-UV- wLEDs as blue components.
2. Experimental A sequence of phosphors Ba3LaK(PO4)3F:xEu 2+ were setted by the conservative high-temperature solid-state method. The fresh materials were (NH4)2HPO4 (A.R.), BaCO3 (A.R.), K2CO3 (A.R.), NH4HF2 (A.R.), La2O3 (99.99%), and Eu2O3 (99.99%), which were used immediately without any handling. X-ray diffraction patterns
(Cu-Kα line, 40 kV, 30 mA) with radiation (λ=0.15406 nm) were obtained using an X-ray powder diffractometer (XD-3, PGENERAL, China). Diffuse flection spectra on as-setted phosphors were recorded on a UV–vis–NIR spectrophotometer (UV-3600) connected with an integrating sphere, reference standard of which was BaSO4 powder. Photoluminescence excitation spectra and photoluminescence emission spectra at room temperature were recorded on a fluorescence spectrophotometer (Hitachi F-4600) and the xenon lamp (400 V, 150 W) was excitation source as the matching device. A 400 nm cut-off filter was applied in the measurement so that the second-order emission of source radiation can be accurately eliminated. Doping concentrations of the europium ions in Ba3LaK(PO4)3F:xEu 2+ were x=0.01,0.03,0.05,0.10,0.15,0.20 and 0.30. Amounts of the fresh materials were prepared according to this formula, then the selected fresh materials were blended and ground drastically in an agate mortar until the mixture was homogeneous. After that the stoichiometric combination was parked into an alumina crucible and annealed at 1080℃ for 3h. At last, the combinations were cooled to room temperature in the furnace naturally, and ground into powder again for the measurements mentioned above.
3. Results and discussions We adopted XRD analysis to verify structures of the as-setted samples. As shown in Fig.1, the representative XRD patterns include the Ba3LaK(PO4)3F and the Ba3LaK(PO4)3F:0.1Eu2+phosphor. It is obvious that the XRD patterns of them can be matched perfectly to the phase of Ba3LaNa(PO4)3F(JCPDS No.71-1337 ), which belongs to apatite structure with the hexagonal system and the group space P63/m. In view of the positions and intensities of diffraction peak matched well with the reference JCPDS file, we can see that the doping caused by Eu ions did not bring about variation in the structure ， which indicates the successful isomorphic replacement of Eu atoms to Ba atoms and K atoms to Na atoms in the present structure[30]. We measured the important spectra of different samples to evaluate the application
value of the phosphor which has an apatite structure. Fig.2 depicts the photoluminescence excitation (PLE), photoluminescence emission (PL) and the diffuse reflection spectra of Ba2.9LaK(PO4)3F:0.1Eu2+. The PLE spectrum of Ba2.9LaK(PO4)3F:0.1Eu 2+ ranges from 200 to 450 nm and the diffuse reflection spectrum of the Ba2.9LaK(PO4)3F:0.1Eu2+ phosphor shows a broad absorption in the 225–425 nm (n-UV) range, which results from the 4f–5d absorption of the Eu2+ ions, and they match well with each other. The characteristic excitation spectrum centers at 388 nm, which should be ascribed to transition 4f7–4f65d 1, suggesting that the excitation wavelength of as-grown phosphors goes well with the near-UV-LED chips. Upon 388 nm excitation, the phosphor shows a broad blue band centered at 460 nm, which is attributed to the electric-dipole-allowed transition from the lowest level of the 5d excited state (4f65d1) to the 4f ground state (4f7(8S7/2) of doped Eu2+ ions[31]. Obviously, the shape of the PL spectrum is not Gaussian peak in the long wavelength region, and the unsymmetrical emission spectrum can be deconvoluted into two Gaussian peaks centering at 455 and 479 nm which are shown as two dotted curves in Fig. 2 respectively. Since 455nm is close to 460nm, we choose 460 nm and 479 nm as the monitored spectra. The spectral profiles of excitation spectra monitored via 460nm and 479 nm are different, which indicates there may be two different Eu2+ emission centers and is in accordance with the apatite crystal structure. Then we measured PL spectra of samples with different doping contents. The PL spectra of Ba3LaK(PO4)3F:xEu2+ (x=0.01,0.03,0.05,0.10,0.15,0.20 and 0.30) excited by 388 nm are demonstrated in Fig.3 (a) . As is shown in Fig.3(a),the emission intensity of Eu 2+ ions firstly increases and peaks at x=0.10, then the emission intensity decreases as a result of concentration quenching effect. The following equation can be used to calculate the interaction type between sensitizers or between sensitizer and activator [32]: I/x= K [1+β(x)θ/3]-1 Here x represents the activator concentration and it is not less than the optimal concentration, I/x is the emission intensity (I) of unit activator concentration (x), K and β are constants once the excitation condition is fixed, θ indicates the electric
multipolar character and θ=6, 8, 10 respectively represents dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q). Fig.3(b) plots the lg(I/x) dependence on lg(x) and shows a linear dependence between them. The slope of the straight line can be calculated to be -1.6992 and the value of θ can be calculated to be 5.0976 as a result, which is close to 6 and means the quenching interactions of Ba3LaK(PO4)3F:xEu 2+ phosphors is dipole–dipole. The CIE chromaticity diagram for the Ba2.9LaK(PO4)3F:0.1Eu2+ phosphor under 365 nm UV excitation is shown in Fig. 4. The color coordinate is calculated to be (0.147, 0.092), which means that the phosphor can be used as a blue-emitting phosphor for w-LEDs application. Meanwhile, the typical digital photo of the Ba2.9LaK(PO4)3F:0.1Eu 2+ phosphor under 365 nm UV lamp is shown in the inset of Fig. 4, and the blue light emission can be seen by naked eyes. All the results above prove that Ba3LaK(PO4)3F:xEu2+ phosphors with 4f–5d absorption efficiency in n-UV region can play an important role in blue emitting phosphor and are potentially useful for ~370 nm GaN-based w-LEDs.
4. Conclusions In summary, blue phosphor Ba3LaK(PO4)3F:xEu2+ was acquired via conservative high-temperature solid-state method. The phosphor showed the emitted blue light peaks locate at about 460 nm ascribed to the 4f–5d transition of Eu2+. It was further proved that concentration quenching of Eu2+ ions in Ba3LaK(PO4)3F:0.1Eu2+ phosphors resulted from the dipole–dipole interactions. All the spectrum features indicate that this blue phosphor might be a component for w-LEDs.
Acknowledgments This present work was supported by the National Natural Science Foundation of China (Grant nos. 41172053), the Fundamental Research Funds for the Central Universities (Grant nos. 2652015306), and Science and Technology Innovation Fund of the China University of Geosciences (Beijing).
References [1] Z.P. Ci, M.D. Que, Y.R. Shi, G. Zhu, Y.H. Wang, Inorg. Chem. 53(2014) 2195. [2] H. Chen, J.Y. Gao, S.J. Su, X. Zhang, Z.H. Wang, IEEE T. Biomed. Circ. S. 9(2015) 227. [3] M.M. Jiao, N. Guo, W. Lu, Y.C. Jia, W.Z. Lv, Q. Zhao, B. Q. Shao, H.P. You, Inorg. Chem. 52(2013) 10340. [4] X. Zhang, M. Liu, B. Wang, H. Chen, Z.H. Wang, IEEE T. Circuits-I. 61 (2014) 2. [5] Q. Peng, C. Zhang, X.J. Zhao, X.G. Sun. F.L. Li, H. Chen, Z.H. Wang, IEEE T. Ind. Electron. (2015) 1. [6] H.S. Roh, S. Hur, H.J. Song, I.J. Park, D.K.Yim, D.W. Kim, K.S. Hong. Mater. Lett. 70(2012) 37. [7] R.Y. Mi, C.L. Zhao, Z.G. Xia, J. Am. Ceram. Soc. 97(2014) 1802. [8] K. Nie, Q. An, Y.H. Zhang, Nanoscale 8(2016) 8791. [9] S.J. Su, J.Y. Gao, Z. Cao, H. Chen, Z.H. Wang, in: 2015 IEEE, IEEE, 2015, p. 1. [10] X.X Ma, L.F. Mei, H.K Liu, L.B Liao, Y.Q. Liu, K. Nie, Z.H. Li, Chem. Phys. Lett. 653(2016) 212. [11] C. Zeng, H.W. Huang, Y.M. Hu, S.H. Miao, j. Zhou, Mater. Res. Bull.76(2016) 62. [12] P.F. Smet, K. Korthout, J.E. Van Haecke, D. Poelman, Mat. Sci. Eng. B-Solid 146(2008) 264. [13] J.H. Gan, Y.L. Huang, L. Shi, X.B. Qiao, H.J. Seo, Mater. Lett. 63(2009) 2160. [14] R.F. Wang, D.C. Zhou, J.B. Qiu, Y. Yang, C. Wang, J. Alloy Compd. 629(2015) 310. [15] A.K. Mishra, S.K. Mishra, B.D. Gupta, Opt. Commun. 344(2015) 86. [16] L.F. Chen, J.Y. Liu, J.S. Wen, H.D. Mao, F. Ge, D.M. Zhao, Opt. Commun. 338(2015) 110. [17] X.Y. Huang, J. Alloy Compd. 628(2015) 240. [18] F.W. Mo, P.C. Chen, A.X. Guan, W. Zhang, L.Y. Zhou, J. Rare Earth 33(2015) 1064. [19] J.S. Huo, J.D. Zhu, S.H. Wu, H.J. Zhang, Q.M. Wang, J. Lumin. 158(2015) 417. [20] W.J. Yang, L.Y. Luo, T.M. Chen, N.S. Wang, Chem. Mater. 17(2005) 3883. [21] R.H. Krishna, B.M. Nagabhushana, B.N. Sherikar, N.S. Murthy, C. Shivakumara, T. Thomas, Chem. Eng. J. 267(2015) 317. [22] K.W. Chae, T.R. Park, C.I. Cheon, N.I. Cho, J.S. Kim, J. Lumin. 132(2012) 2293. [23] J.P. Leonard, T. Gunnlaugsson, J. Fluoresc. 15(2005) 585. [24] A.C. Ferrand, D. Imbert, A.S. Chauvin, C.D.B. Vandevyver, J.C.G. Bunzli, Chem-Eur. J. 13(2007) 8678. [25] B.Y. Qu, B. Zhang, L. Wang, R.L. Zhou, X.C. Zeng, Chem. Mater. 27(2015) 2195. [26] S. Rodriguez, P. Elizondo, S. Bernes, N. Perez, R. Bustos, E. Garcia-Espana, Polyhedron 85(2015) 10. [27] R.Q. Li, G.H. Yu, Y.M. Liang, N.N. Zhang, Y.L. Liu, S.C. Gan, J. Colloid Interf. Sci. 460(2015) 273.
[28] Z. Chen, J.H. Zhang, S. Chen, M.Y. Lin, C.Q. He, G.D. Xu, M.M. Wang, X.F. Yu, J.Q. Zou, K. Guo, J. Alloy Compd. 632(2015) 756. [29] Z.C. Wu, S. Wang, J. Liu, J.H. Yin, S.P. Kuang, J. Alloy Compd. 644(2015) 274. [30] W.B. Im, S. Brinkley, J. Hu, A. Mikhailovsky, S.P. DenBaars, R. Seshadri, Chem. Mater. 22(2010) 2842. [31] Y.Y. Zhang, Z.G. Xia, W.W. Wu, J. Am. Ceram. Soc. 96(2013) 1043. [32] H.K. Liu, Y.Y. Zhang, L.B. Liao, Q.F. Guo, L.F. Mei, Ceram. Int. 40(2014) 13709.
Figures
Fig. 1 XRD patterns of Ba3LaK(PO4)3F and Ba2.9LaK(PO4)3F:0.1Eu2+ samples. The standard data for Ba3LaNa(PO4)3F( JCPDS No.71-1317) is also shown as a reference.
Fig. 2 Photoluminescence excitation (PLE, left) spectra, photoluminescence emission (PL, right) and diffuse reflection spectra (blue) of Ba2.9LaK(PO4)3F:0.1Eu 2+ phosphor.
Fig. 3(a) PL spectra of Ba3-xLaK(PO4)3F:xEu2+ phosphors and the dependence of PL intensity on the Eu 2+ doping concentration. The optimum doping Eu2+ concentration is x=0.10. Insert shows the relationship between Eu2+ contents and the peaks of emission intensity. (b) The fitting curves of lg(I/x) vs. lg(x) in Ba3-xLaK(PO4)3F:xEu 2+ phosphors.
Fig. 4. CIE chromaticity diagram for Ba2.9LaK(PO4)3F:0.1Eu2+ phosphor excited at 365nm. Digital photo of blue light emission is also shown as the evidence.
Highlights: NO.1 An inorganic framework of Ba3LaK(PO4)3F:Eu2+ phosphors with apatite structure has been synthesized. NO.2 The synthesis technique uses a relative low temperature. NO.3 Broad-band absorption originating from the f-d transition of Eu2+ can be found.