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Cerium-, terbium- and europium-activated CaScAlSiO6 as a full-color emitting phosphor
Journal of Luminescence 147 (2014) 159–162
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Cerium-, terbium- and europium-activated CaScAlSiO6 as a full-color emitting phosphor Wei Lü a,n, Yongchao Jia a,b, Wenzhen Lv a,b, Qi Zhao a,b, Hongpeng You a,n a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 July 2013 Received in revised form 17 October 2013 Accepted 1 November 2013 Available online 13 November 2013
We reported a single-phased CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ as a potential full-color emitting phosphor for the application in fluorescent lamps. The CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ phosphor exhibits three bands under 254 nm excitation: one band situated at 380 nm is attributed to the 5d-4f transitions of Ce3 þ ions, the second band with sharp lines peaked at 542 nm is assigned to the 5D4-7FJ transitions of Tb3 þ ions, the third band in the orange–red region (580–700 nm) is originated from 5D0-7FJ transitions of Eu3 þ ions. The Commission Internationale de I’Eclairage (CIE) chromaticity coordinates (0.30, 0.30) and high color rendering index (CRI ¼88) can be achieved upon excitation of 254 nm light. It is suggested that CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ can serve as a potential single-phased full-color emitting phosphor for phosphor-converted fluorescent lamps. & 2013 Elsevier B.V. All rights reserved.
Keywords: Phosphor Full-color emitting Fluorescent lamps
1. Introduction Nowadays, rare earth activated phosphors have been widely utilized in illumination devices [1–3]. Especially, phosphors for fluorescent lamps (FLs) are frequently used and produced in the largest quantity for lighting because of their high efficiency and long lifetime. Although the basics of commercial FLs containing a mixture of triphosphors-the blue-emitting BaMgAl10O17:Eu2 þ [4], the red-emitting Y2O3:Eu3 þ [5], and the green-emitting LaPO4: Ce3 þ , Tb3 þ [6] under UV light at 254 nm were well-established, the development of new phosphors continues because of the importance of phosphor efficiency required for different applications [7–9]. As we known, trivalent Tb and Eu ions, as the promising species that provide optical emission in green and red regions, have been investigated by many groups [10–14]. Take the Eu3 þ ions for example, in the typical used for fluorescent lamps of Y2O3:Eu3 þ phosphor, it has strong optical absorption in the shortwave UV region of 254 nm and gives an efficient red emission [15]. Despite there were sweeping studies on the above fields including Tb3 þ and Eu3 þ single doped phosphor in the past years, single-phase full-color emitting phosphors for FLs application were rarely reported [16]. In this research, we report our recent investigation results on the synthesis and luminescence of a full-color emitting CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ phosphor. The CIE chromaticity coordinates
n
Corresponding authors. Tel.: þ 86 431 8526 2798; fax: þ 86 431 8569 8041. E-mail addresses: [email protected] (W. Lü), [email protected] (H. You).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.001
(0.30, 0.30) and high color rendering index (CRI ¼88) can be achieved upon excitation of 254 nm light. 2. Experimental section The Ca1 xSc1 y–zAlSiO6(CSAS):xCe3 þ , yTb3 þ , zEu3 þ phosphors were synthesized by a high-temperature solid state reaction. The constituent oxides or carbonates CaCO3 (99.9%), Sc2O3 (99.9%), SiO2 (99.9%), Al2O3 (99.9%), CeO2 (99.99%), Tb4O7 (99.99%) and Eu2O3 (99.99%) were employed as the raw materials. In the preparation, the complete burning process was achieved by a two-stage solid-state reaction. The starting materials were mixed together with required molar ratio. A small amount of high-purity CeO2 and Tb4O7 were added into the mixture. The reactants were mixed homogeneously by an agate mortar for 30 min, placed in a crucible with a lid, and then sintered in a tubular furnace at 1400 1C for 4 h in reductive atmosphere (10% H2 þ90% N2 mixed flowing gas). Then, a small amount of high-purity Eu2O3 was added into the obtained powder. The obtained powder were mixed homogeneously by an agate mortar for 15 min, placed in a crucible with a lid, and then sintered in a tubular furnace at 1400 1C in air. The structure of sintered samples was identified by powder X-ray diffraction (XRD) analysis (Bruker AXS D8), with graphite monochromatized Cu Kα radiation (λ¼ 0.15405 nm) operating at 40 kV and 40 mA. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed by using a Hitachi F4500 spectrometer equipped with a 150 W xenon lamp under a working voltage of 700 V. The size and
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morphology of the samples were inspected using a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). The CIE and CRI were calculated by SpectraWin software. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width ¼4 ns, gate¼50 ns) as the excitation source (Continuum Sunlite OPO). We determined the Ce3 þ concentrations at 4% which the PL intensities reach their maxima under 330 nm light excitation.
3. Results and discussion Fig. 1(a) shows the XRD patterns of CSAS:0.04Ce3 þ , CSAS: 0.04Tb3 þ , CSAS:0.04Eu3 þ and CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04 Eu3 þ samples. It is clearly observed that the samples are well coincident with the standard data of CSAS with JCPDS card no 770465 except for a small amount of impurities. These weak impurity peaks may be assigned to Sc2Si2O7 silicate phase. The intensity of impurity phases is so small that the effect on luminescent properties of rare-earth ions doped CSAS could be neglected. The schematic representation of the structural of CSAS host is illustrated in Fig. 1(b). As shown in Fig. 1(b), Ca2 þ and Sc3 þ ions are tightly surrounded with tetrahedral SiO4 and AlO4 units, and the oxygen atoms form bridges between tetrahedral SiO4 and AlO4. The CSAS host lattice contains two different cation sites:
Fig. 1. The XRD patterns of CSAS:0.04Ce3 þ , CSAS:0.04Tb3 þ , CSAS:0.04Eu3 þ and CSAS:0.04Ce3 þ . 0.04Tb3 þ , 0.04Eu3 þ and its schematic view of the structure and coordination environments.
Fig. 2. The typical SEM micrograph of CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ .
8-fold coordinated Ca2 þ site and 6-fold coordinated Sc2 þ site. For the consideration of ionic radii matching, it is demonstrated that Ce3 þ is expected to occupy Ca2 þ site because the ionic radius of Ce3 þ (1.14 Å) is close to that of Ca2 þ (1.12 Å); As for Tb3 þ (0.92 Å for CN ¼6, 1.04 Å for CN¼ 8) and Eu3 þ (0.95 Å for CN ¼6, 1.07 Å for CN¼8), accounting for ion valence, we presume that Tb3 þ and Eu3 þ may be favorable to occupy Sc3 þ (0.75 Å for CN¼ 6, 0.87 Å for CN¼8) site. Fig. 2 displays the representative SEM micrograph of CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ . The phosphor powders are uniform and the phosphor crystals show irregular shapes with the dimension of 1–10 mm. Fig. 3(a) shows the PL and PLE spectra of CSAS:0.04Ce3 þ . The PLE spectrum of CSAS:Ce3þ consists of three broad bands centered at 245 nm, 296 nm and 340 nm (the strongest), corresponding to the host absorption and 4f–5d transition of Ce3 þ . While the PL spectrum presents an intense violet light with a peak at 380 nm, which is originated from the 5d-4f transitions of Ce3þ ions. The PL and PLE spectra of CSAS:0.04Tb3 þ are presented in Fig. 3(b). A charge transfer band (CTB) centered at 233 nm can be found in the range of 200– 300 nm, which is attributed to the spin allowed 4f–5d transition of Tb3 þ ions. The other is composed of a series of weak narrow bands in the 300–500 nm regions, which correspond to absorption f–f transition of Tb3þ ions. The Tb3 þ emission lines are located at 485, 542 nm, 580 nm and 620 nm, which are assigned to the 5D4 to 7FJ ¼ (J¼6, 5, 4, 3) multiplet transitions, respectively [17]. In particular, the highest sharp line peaked at 542 nm is characteristic of 5D4–7F5 of Tb3 þ 4f–4f transitions. Comparing Fig. 3(a) and (b), it is clearly exhibited that there is a spectral overlap between the Ce3þ PL and Tb3þ PLE spectra, indicating the possibility of energy transfer from Ce3þ to Tb3þ in CSAS. Meanwhile, a shortening of the lifetimes of 5d–4f transition of Ce3þ with codoping Tb3þ ions can be observed, as shown in Fig. 4, which is another proof for the energy transfer from the Ce3þ to Tb3þ ions [18]. The corresponding energy levels scheme of Ce3 þ and Tb3þ and the possible optical transition involved in the energy transfer processes are schematically depicted in Fig. 5. When Ce3 þ ions absorb UV light, the excitation energy could be released not only by emitting blue light but also by transferring to Tb3 þ ions, which finally exhibits a green emission of Tb3 þ ions. Fig. 3(c) shows the PL and PLE spectra of CSAS:0.04Eu3þ . The PLE spectrum consists of two main features, the abroad band in the range of 200–300 nm is assigned to CTB from O2þ to Eu3þ [19], and the characteristic f–f transition lines of Eu3 þ are attributed to the transitions from 7F0 ground state to the different excited states of Eu3 þ , with weak peaks at 365 nm (5D4), 381 nm (5G3) and 391 nm (5L6), respectively. The PL spectrum yields multi-emission
W. Lü et al. / Journal of Luminescence 147 (2014) 159–162
Fig. 3. The PL and PLE spectra of CSAS:0.04Ce3 þ (a), CSAS:0.04Tb3 þ (b), CSAS:0.04Eu3 þ (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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and this transition is independent of the symmetry and the site occupied by Eu3þ ions in the host. While the transition of 5D0-7F2 belongs to a forced electric dipole transition and its intensity is very sensitive to the site symmetry of the Eu3þ ions [21–23]. As shown in Fig. 3, the 5D0-7F2 electric-dipole transition dominates the emission spectrum, which suggests that the Eu3þ is located in an asymmetric cation environment. This phenomenon is similar to the previous report [24,25]. Noteworthy, it is not found that there be a significant spectral overlap between the emission band of CSAS:Tb3þ and the excitation band of CSAS:Eu3þ . Meanwhile, the PLE spectrum monitoring the red lines of the Eu3þ is not consistent with that monitoring the Ce3þ emission. All these results demonstrated that, in our tri-doped CSAS:Ce3þ , Tb3 þ , Eu3 þ samples, the energy transfer from Tb3þ to Eu3þ and from Ce3 þ to Eu3þ is not considered to occur. These results show that the PLE spectra of the three samples are well suitable for the excitation wavelength of 254 nm, indicating that our sample can be used as a promising phosphor for FLs. The above experimental results exhibit that the emission of the Ce3 þ , Tb3 þ and Eu3 þ singly doped CSAS phosphors is located at around blue, green and red area, respectively, suggesting that tricolor (blue, green and red) emitting bands may be obtained
Fig. 4. The schematic energy levels of Ce3 þ , Tb3 þ and Eu3 þ , and the energy transfer processes in CSAS.
Fig. 6. The CIE chromaticity coordinates for these selected samples under excitation at 254 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Decay time curves of CSAS:0.04Ce3 þ and CSAS:0.04Ce3 þ , 0.08Tb3 þ excited at 340 nm and monitored at 380 nm.
lines from the 5D0 excited states to the 7FJ ground states of Eu3þ , including 5D0-7F1 (578 nm, 588 nm, 596 nm), 5D0-7F2 (609 nm, 620 nm), and 5D0-7F4 transitions (682 nm, 700 nm) [20]. Under the 254 nm excitation, the energy relaxes from the upper level to the 5D3 levels nonradiatively and then sequentially to the lower 5D levels, and gives the characteristic f–f transition of Eu3þ (Fig. 5). It is well-known that the 5D0-7F1 transition belongs to the magnetic-dipole transition which scarcely changes the crystal field strength around the Eu3 þ ions
Fig. 7. The PL spectra of white light emitting CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ phosphor in the visible range upon 254 nm excitation.
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when Ce3 þ , Tb3 þ and Eu3 þ are co-excited in the single-phase CSAS. Fig. 6 displays the CIE chromaticity coordinates for these selected samples under excitation at 254 nm. The CSAS:Ce3 þ , CSAS:Tb3 þ , and CSAS:Eu3 þ show blue, green and red luminescence with corresponding CIE chromaticity coordinates (0.17, 0.03), (0.31, 0.46), and (0.48, 0.35), respectively. Fig. 7 shows the PL spectra of white light emitting CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ phosphor in the visible range upon 254 nm excitation. The corresponding CIE color coordinates of the phosphor are (0.30, 0.30), which is located at white light area. The color rendering index (CRI) is as high as the present FLs based on triphosphors (CRI E85). Our results indicate that CSAS:Ce3 þ , Tb3 þ , Eu3 þ may have promising applications for present FLs. 4. Conclusion In summary, we have synthesized a novel full-color emitting phosphor CaScAlSiO6:Ce3þ , Tb3þ , Eu3 þ phosphors by solid state reaction. The obtained phosphor exhibits three emission colors: a blue band of 380 nm, a green band of 542 nm, and the red emission lines from 580 nm to 700 nm, which are assigned to the contribution from Ce3 þ , Tb3 þ and Eu3þ , respectively. Upon 254 nm light excitation, full-color emissions have been realized with the CIE chromaticity coordinates (0.30, 0.30) and high color rendering index (CRI¼ 88). All these results indicate that CSAS:Ce3þ , Tb3 þ , Eu3þ is a promising single-phase full-color emitting phosphor for FLs application. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 21271167) and the Fund for Creative Research Groups (Grant No. 21221061).
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] S. Nakamura, G. Fasol, The Blue Laser Diod, Springer, Berlin (1997) 216. [3] X.F. Li, J.D. Budai, F. Liu, J.Y. Howe, J.H. Zhang, X.J. Wang, Z.J. Gu, C.J. Sun, R.S. Meltzer, Z.W. Pan, Light: Sci. Appl. 2 (2013) e50. [4] Y. Shimomura, N. Kijima, J. Electrochem. Soc. 151 (8) (2004) H192. [5] Y. Shimomura, N. Kijima, J. Electrochem. Soc. 151 (4) (2004) H86. [6] W. Schaik, S. Lizzo, W. Smit, G. Blass, J. Electrochem. Soc. 140 (1) (1993) 216. [7] R.S. Yadav, V.K. Shukla, P. Mishra, S.K. Pandey, K. Kumar, V. Baranwal, M. Kumar, A.C. Pandey, J. Alloys Compd 547 (2013) 1. [8] D. Hou, W. Chen, X. Ding, H. Liang, L. Zheng, Jing Zhang, ECS J. Solid State Sci. Technol 2 (4) (2013) R79. [9] J. Liao, D. Zhou, H. You, H. Wen, Q. Zhou, B. Yang, Optik 124 (2013) 1362. [10] C.H. Huang, T.W. Kuo, T.M. Chen, Opt. Express 19 (2011) A1. [11] C.H. Huang, T.W. Kuo, T.M. Chen, ACS Appl. Mater. Interfaces 2 (5) (2010) 1395. [12] T.W. Kuo, T.M. Chen, J. Electrochem. Soc. 157 (6) (2010) J216. [13] T.W. Kuo, T.M. Chen, Opt. Mater. 32 (2010) 882. [14] G. Feng, W.H. Jiang, Y.B. Chen, R.J. Zeng, Mater. Lett. 65 (2011) 110. [15] S. Won Kang, J. Soo Yoo, J.Duk Lee, J. Electrochem. Soc. 145 (1998) 648. [16] W. Lü, Y.S. Luo, Z.D. Hao, X. Zhang, X.J. Wang, J.H. Zhang, J. Lumin. 132 (2012) 2439. [17] Y.C. Chiu, W.R. Liu, Y.T. Yeh, S.M. Jang, T.M. Chen, J. Electrochem. Soc. 156 (2009) J221. [18] W. Lü, N. Guo, Y.C. Jia, Q. Zhao, W.Z. Lv, M.M. Jiao, B.Q. Shao, H.P. You, Inorg. Chem. 52 (2013) 3007. [19] G. Blasse, B.C. Grabmeier, Luminescent MaterialsSpringer, Berlin, 1994. [20] S.F. Wang, K.K. Rao, Y.R. Wang, Y.F. Hsu, S.H. Chen, Y.C. Lu, J. Am. Ceram. Soc. 8 (2009) 1732. [21] R.S. Yadav, R.K. Dutta, M. Kumar, A.C. Pandey, J. Lumin. 129 (2009) 1078. [22] M.M. Haque, H.L. Lee, D.K. Kim, J. Alloys Compd 481 (2009) 792. [23] I. Omkaram, B. Vengala Rao, S. Buddhudu, J. Alloys Compd 474 (2009) 565. [24] S.A. Yan, Y.S. Chang, W.S. Hwang, Y.H. Chang, M. Yoshimura, C.S. Hwang, J. Alloys Compd 509 (2011) 5777. [25] X. He, M. Guan, C. Zhang, T. Shang, N. Lian, Y. Yao, J. Alloys Compd 509 (2011) L341.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Cerium-, terbium- and europium-activated CaScAlSiO6 as a full-color emitting phosphor Wei Lü a,n, Yongchao Jia a,b, Wenzhen Lv a,b, Qi Zhao a,b, Hongpeng You a,n a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 July 2013 Received in revised form 17 October 2013 Accepted 1 November 2013 Available online 13 November 2013
We reported a single-phased CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ as a potential full-color emitting phosphor for the application in fluorescent lamps. The CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ phosphor exhibits three bands under 254 nm excitation: one band situated at 380 nm is attributed to the 5d-4f transitions of Ce3 þ ions, the second band with sharp lines peaked at 542 nm is assigned to the 5D4-7FJ transitions of Tb3 þ ions, the third band in the orange–red region (580–700 nm) is originated from 5D0-7FJ transitions of Eu3 þ ions. The Commission Internationale de I’Eclairage (CIE) chromaticity coordinates (0.30, 0.30) and high color rendering index (CRI ¼88) can be achieved upon excitation of 254 nm light. It is suggested that CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ can serve as a potential single-phased full-color emitting phosphor for phosphor-converted fluorescent lamps. & 2013 Elsevier B.V. All rights reserved.
Keywords: Phosphor Full-color emitting Fluorescent lamps
1. Introduction Nowadays, rare earth activated phosphors have been widely utilized in illumination devices [1–3]. Especially, phosphors for fluorescent lamps (FLs) are frequently used and produced in the largest quantity for lighting because of their high efficiency and long lifetime. Although the basics of commercial FLs containing a mixture of triphosphors-the blue-emitting BaMgAl10O17:Eu2 þ [4], the red-emitting Y2O3:Eu3 þ [5], and the green-emitting LaPO4: Ce3 þ , Tb3 þ [6] under UV light at 254 nm were well-established, the development of new phosphors continues because of the importance of phosphor efficiency required for different applications [7–9]. As we known, trivalent Tb and Eu ions, as the promising species that provide optical emission in green and red regions, have been investigated by many groups [10–14]. Take the Eu3 þ ions for example, in the typical used for fluorescent lamps of Y2O3:Eu3 þ phosphor, it has strong optical absorption in the shortwave UV region of 254 nm and gives an efficient red emission [15]. Despite there were sweeping studies on the above fields including Tb3 þ and Eu3 þ single doped phosphor in the past years, single-phase full-color emitting phosphors for FLs application were rarely reported [16]. In this research, we report our recent investigation results on the synthesis and luminescence of a full-color emitting CaScAlSiO6:Ce3 þ , Tb3 þ , Eu3 þ phosphor. The CIE chromaticity coordinates
n
Corresponding authors. Tel.: þ 86 431 8526 2798; fax: þ 86 431 8569 8041. E-mail addresses: [email protected] (W. Lü), [email protected] (H. You).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.001
(0.30, 0.30) and high color rendering index (CRI ¼88) can be achieved upon excitation of 254 nm light. 2. Experimental section The Ca1 xSc1 y–zAlSiO6(CSAS):xCe3 þ , yTb3 þ , zEu3 þ phosphors were synthesized by a high-temperature solid state reaction. The constituent oxides or carbonates CaCO3 (99.9%), Sc2O3 (99.9%), SiO2 (99.9%), Al2O3 (99.9%), CeO2 (99.99%), Tb4O7 (99.99%) and Eu2O3 (99.99%) were employed as the raw materials. In the preparation, the complete burning process was achieved by a two-stage solid-state reaction. The starting materials were mixed together with required molar ratio. A small amount of high-purity CeO2 and Tb4O7 were added into the mixture. The reactants were mixed homogeneously by an agate mortar for 30 min, placed in a crucible with a lid, and then sintered in a tubular furnace at 1400 1C for 4 h in reductive atmosphere (10% H2 þ90% N2 mixed flowing gas). Then, a small amount of high-purity Eu2O3 was added into the obtained powder. The obtained powder were mixed homogeneously by an agate mortar for 15 min, placed in a crucible with a lid, and then sintered in a tubular furnace at 1400 1C in air. The structure of sintered samples was identified by powder X-ray diffraction (XRD) analysis (Bruker AXS D8), with graphite monochromatized Cu Kα radiation (λ¼ 0.15405 nm) operating at 40 kV and 40 mA. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed by using a Hitachi F4500 spectrometer equipped with a 150 W xenon lamp under a working voltage of 700 V. The size and
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morphology of the samples were inspected using a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). The CIE and CRI were calculated by SpectraWin software. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width ¼4 ns, gate¼50 ns) as the excitation source (Continuum Sunlite OPO). We determined the Ce3 þ concentrations at 4% which the PL intensities reach their maxima under 330 nm light excitation.
3. Results and discussion Fig. 1(a) shows the XRD patterns of CSAS:0.04Ce3 þ , CSAS: 0.04Tb3 þ , CSAS:0.04Eu3 þ and CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04 Eu3 þ samples. It is clearly observed that the samples are well coincident with the standard data of CSAS with JCPDS card no 770465 except for a small amount of impurities. These weak impurity peaks may be assigned to Sc2Si2O7 silicate phase. The intensity of impurity phases is so small that the effect on luminescent properties of rare-earth ions doped CSAS could be neglected. The schematic representation of the structural of CSAS host is illustrated in Fig. 1(b). As shown in Fig. 1(b), Ca2 þ and Sc3 þ ions are tightly surrounded with tetrahedral SiO4 and AlO4 units, and the oxygen atoms form bridges between tetrahedral SiO4 and AlO4. The CSAS host lattice contains two different cation sites:
Fig. 1. The XRD patterns of CSAS:0.04Ce3 þ , CSAS:0.04Tb3 þ , CSAS:0.04Eu3 þ and CSAS:0.04Ce3 þ . 0.04Tb3 þ , 0.04Eu3 þ and its schematic view of the structure and coordination environments.
Fig. 2. The typical SEM micrograph of CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ .
8-fold coordinated Ca2 þ site and 6-fold coordinated Sc2 þ site. For the consideration of ionic radii matching, it is demonstrated that Ce3 þ is expected to occupy Ca2 þ site because the ionic radius of Ce3 þ (1.14 Å) is close to that of Ca2 þ (1.12 Å); As for Tb3 þ (0.92 Å for CN ¼6, 1.04 Å for CN¼ 8) and Eu3 þ (0.95 Å for CN ¼6, 1.07 Å for CN¼8), accounting for ion valence, we presume that Tb3 þ and Eu3 þ may be favorable to occupy Sc3 þ (0.75 Å for CN¼ 6, 0.87 Å for CN¼8) site. Fig. 2 displays the representative SEM micrograph of CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ . The phosphor powders are uniform and the phosphor crystals show irregular shapes with the dimension of 1–10 mm. Fig. 3(a) shows the PL and PLE spectra of CSAS:0.04Ce3 þ . The PLE spectrum of CSAS:Ce3þ consists of three broad bands centered at 245 nm, 296 nm and 340 nm (the strongest), corresponding to the host absorption and 4f–5d transition of Ce3 þ . While the PL spectrum presents an intense violet light with a peak at 380 nm, which is originated from the 5d-4f transitions of Ce3þ ions. The PL and PLE spectra of CSAS:0.04Tb3 þ are presented in Fig. 3(b). A charge transfer band (CTB) centered at 233 nm can be found in the range of 200– 300 nm, which is attributed to the spin allowed 4f–5d transition of Tb3 þ ions. The other is composed of a series of weak narrow bands in the 300–500 nm regions, which correspond to absorption f–f transition of Tb3þ ions. The Tb3 þ emission lines are located at 485, 542 nm, 580 nm and 620 nm, which are assigned to the 5D4 to 7FJ ¼ (J¼6, 5, 4, 3) multiplet transitions, respectively [17]. In particular, the highest sharp line peaked at 542 nm is characteristic of 5D4–7F5 of Tb3 þ 4f–4f transitions. Comparing Fig. 3(a) and (b), it is clearly exhibited that there is a spectral overlap between the Ce3þ PL and Tb3þ PLE spectra, indicating the possibility of energy transfer from Ce3þ to Tb3þ in CSAS. Meanwhile, a shortening of the lifetimes of 5d–4f transition of Ce3þ with codoping Tb3þ ions can be observed, as shown in Fig. 4, which is another proof for the energy transfer from the Ce3þ to Tb3þ ions [18]. The corresponding energy levels scheme of Ce3 þ and Tb3þ and the possible optical transition involved in the energy transfer processes are schematically depicted in Fig. 5. When Ce3 þ ions absorb UV light, the excitation energy could be released not only by emitting blue light but also by transferring to Tb3 þ ions, which finally exhibits a green emission of Tb3 þ ions. Fig. 3(c) shows the PL and PLE spectra of CSAS:0.04Eu3þ . The PLE spectrum consists of two main features, the abroad band in the range of 200–300 nm is assigned to CTB from O2þ to Eu3þ [19], and the characteristic f–f transition lines of Eu3 þ are attributed to the transitions from 7F0 ground state to the different excited states of Eu3 þ , with weak peaks at 365 nm (5D4), 381 nm (5G3) and 391 nm (5L6), respectively. The PL spectrum yields multi-emission
W. Lü et al. / Journal of Luminescence 147 (2014) 159–162
Fig. 3. The PL and PLE spectra of CSAS:0.04Ce3 þ (a), CSAS:0.04Tb3 þ (b), CSAS:0.04Eu3 þ (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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and this transition is independent of the symmetry and the site occupied by Eu3þ ions in the host. While the transition of 5D0-7F2 belongs to a forced electric dipole transition and its intensity is very sensitive to the site symmetry of the Eu3þ ions [21–23]. As shown in Fig. 3, the 5D0-7F2 electric-dipole transition dominates the emission spectrum, which suggests that the Eu3þ is located in an asymmetric cation environment. This phenomenon is similar to the previous report [24,25]. Noteworthy, it is not found that there be a significant spectral overlap between the emission band of CSAS:Tb3þ and the excitation band of CSAS:Eu3þ . Meanwhile, the PLE spectrum monitoring the red lines of the Eu3þ is not consistent with that monitoring the Ce3þ emission. All these results demonstrated that, in our tri-doped CSAS:Ce3þ , Tb3 þ , Eu3 þ samples, the energy transfer from Tb3þ to Eu3þ and from Ce3 þ to Eu3þ is not considered to occur. These results show that the PLE spectra of the three samples are well suitable for the excitation wavelength of 254 nm, indicating that our sample can be used as a promising phosphor for FLs. The above experimental results exhibit that the emission of the Ce3 þ , Tb3 þ and Eu3 þ singly doped CSAS phosphors is located at around blue, green and red area, respectively, suggesting that tricolor (blue, green and red) emitting bands may be obtained
Fig. 4. The schematic energy levels of Ce3 þ , Tb3 þ and Eu3 þ , and the energy transfer processes in CSAS.
Fig. 6. The CIE chromaticity coordinates for these selected samples under excitation at 254 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Decay time curves of CSAS:0.04Ce3 þ and CSAS:0.04Ce3 þ , 0.08Tb3 þ excited at 340 nm and monitored at 380 nm.
lines from the 5D0 excited states to the 7FJ ground states of Eu3þ , including 5D0-7F1 (578 nm, 588 nm, 596 nm), 5D0-7F2 (609 nm, 620 nm), and 5D0-7F4 transitions (682 nm, 700 nm) [20]. Under the 254 nm excitation, the energy relaxes from the upper level to the 5D3 levels nonradiatively and then sequentially to the lower 5D levels, and gives the characteristic f–f transition of Eu3þ (Fig. 5). It is well-known that the 5D0-7F1 transition belongs to the magnetic-dipole transition which scarcely changes the crystal field strength around the Eu3 þ ions
Fig. 7. The PL spectra of white light emitting CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ phosphor in the visible range upon 254 nm excitation.
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when Ce3 þ , Tb3 þ and Eu3 þ are co-excited in the single-phase CSAS. Fig. 6 displays the CIE chromaticity coordinates for these selected samples under excitation at 254 nm. The CSAS:Ce3 þ , CSAS:Tb3 þ , and CSAS:Eu3 þ show blue, green and red luminescence with corresponding CIE chromaticity coordinates (0.17, 0.03), (0.31, 0.46), and (0.48, 0.35), respectively. Fig. 7 shows the PL spectra of white light emitting CSAS:0.04Ce3 þ , 0.04Tb3 þ , 0.04Eu3 þ phosphor in the visible range upon 254 nm excitation. The corresponding CIE color coordinates of the phosphor are (0.30, 0.30), which is located at white light area. The color rendering index (CRI) is as high as the present FLs based on triphosphors (CRI E85). Our results indicate that CSAS:Ce3 þ , Tb3 þ , Eu3 þ may have promising applications for present FLs. 4. Conclusion In summary, we have synthesized a novel full-color emitting phosphor CaScAlSiO6:Ce3þ , Tb3þ , Eu3 þ phosphors by solid state reaction. The obtained phosphor exhibits three emission colors: a blue band of 380 nm, a green band of 542 nm, and the red emission lines from 580 nm to 700 nm, which are assigned to the contribution from Ce3 þ , Tb3 þ and Eu3þ , respectively. Upon 254 nm light excitation, full-color emissions have been realized with the CIE chromaticity coordinates (0.30, 0.30) and high color rendering index (CRI¼ 88). All these results indicate that CSAS:Ce3þ , Tb3 þ , Eu3þ is a promising single-phase full-color emitting phosphor for FLs application. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 21271167) and the Fund for Creative Research Groups (Grant No. 21221061).
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