- Email: [email protected]
Synthesis and photoluminescence properties of tunable emission phosphors Ca4Si2O7F2:Ce3+
Solid State Sciences 48 (2015) 193e196
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis and photoluminescence properties of tunable emission phosphors Ca4Si2O7F2:Ce3þ Mubiao Xie a, *, Guoxian Zhu a, Dongyu Li b, Rongkai Pan a, Xiaoping Zhou c, Wei Xie c a
Institute of Physical Chemistry, School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang, 524048, China School of Physics Science and Technology, Lingnan Normal University, Zhanjiang, 524048, China c Development Center for New Materials Engineering & Technology in Universities of Guangdong, Lingnan Normal University, Zhanjiang, 524048, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 7 August 2015 Accepted 11 August 2015 Available online 14 August 2015
The Ce3þ activated phosphors Ca4Si2O7F2:Ce3þ are prepared by a solid state reaction technique. The UV evis luminescence properties as well as fluorescence decay time spectra are investigated and discussed. The results revealed that there were two kinds of Ce3þ luminescence behavior with 408 and 470 nm emissions, respectively. Under 355 nm excitation, the Ce(1) emission (408 nm) is dominant at low doping concentration, and then the Ce(2) emission (470 nm) get more important with increasing of Ce3þ concentrations in the host. The phosphors Ca4Si2O7F2:xCe3þ show tunable emissions from blue area to green-blue area under near-ultraviolet light excitation, indicating a potential application in near-UV based w-LEDs. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Phosphor Photoluminescence Ca4Si2O7F2:Ce3þ w-LEDs
1. Introduction Nowadays, Much attention has been paid for white lightemitting diodes (w-LEDs), which can be realized through a combination of red, green and blue phosphors with UV (ultraviolet) or near-UV (350e410 nm) LEDs [1e3]. Therefore, it is necessary to develop new tri-color emitting phosphors with good stabilities and highly efficient under excitation wavelength in range of 350e410 nm. In consideration of the merits and drawbacks of different tri-color phosphors, it is better to develop color tunable phosphors for the fabrication of w-LEDs. Most color tunable phosphor can be obtained through co-doping two or more activators into the same host [4e8]. However, it will be more interesting that tunable emitting color would be realized by one kind of ions entering different crystallographic sites in a single host [9,10]. The trivalent rare-earth Ce3þ ions show broadband absorption and emission due to 4f7e4f65d1 transitions, which strongly dependent on the type of host lattices. Thus, the emission of Ce3þ can vary from the ultraviolet to the red spectral range. The luminescence of Ce3þ ions in Ca4Si2O7F2 have been studied by Wei Lü
* Corresponding author. E-mail address: [email protected] (M. Xie). http://dx.doi.org/10.1016/j.solidstatesciences.2015.08.009 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
and their studies revealed that the Ce3þ concentration has effect on the shape of the emission spectra [11]. In this work, we further report the preparation and luminescent properties of emissiontunable phosphors Ca4Si2O7F2:xCe3þ. The site occupation and energy transfer between different Ce3þ ions are discussed. 2. Experimental The Ca4Si2O7F2:xCe3þ phosphors were synthesized by a hightemperature solidestate reaction. The CaCO3 (AR), Li2CO3 (AR), SiO2 (AR), NH4F (AR) and CeO2 (99.99%) were used as the raw materials. Li ions are used as charge compensation ions here. The reactants were mixed homogeneously by an agate mortar, after mixing and thoroughly grinding, the mixtures were heated at 1000 C for 6 h in CO reducing atmosphere. The final products were cooled to room temperature (RT) by switching off the muffle furnace and ground again into white powder. The phase purity and structure of the final products were characterized by a powder X-ray diffraction (XRD) analysis using Cu Ka radiation (l ¼ 1.5405 Å, 40 kV, 30 mA) on a Rigaku D/max 2200 vpc X-Ray Diffractometer at room temperature (RT). The UV luminescence spectra and luminescence decay curves at RT were recorded on an Edinburgh FLS920 combined fluorescence lifetime and steady state spectrometer, which was equipped with a time-
194
M. Xie et al. / Solid State Sciences 48 (2015) 193e196
correlated single-photon counting (TCSPC) card. The quantum efficiency (QE) of the samples was measured by QY-2000 equipped with 355 nm LED lamp (Orient Koji, China). 3. Results and discussion The X-ray diffraction (XRD) patterns of samples Ca4Si2O7F2:xCe3þ were measured at RT. Fig. 1 presents the diffractograms of three typical samples Ca4Si2O7F2:0.01Ce3þ, (a), Ca4Si2O7F2:0.04Ce3þ (b) and Ca4Si2O7F2:0.10Ce3þ (c). The diffraction patterns of the samples are identical to each other, and all peaks agree well with the standard data Ca4Si2O7F2 (JCPDS 411474). The results suggest that the prepared samples belong to the pure phase and that Ce3þ ions have little effect on the phase structure. In Ca4Si2O7F2 structure, Ca2þ ions occupy four different crystal sites. Ca(1) is six coordinated surrounding by five oxygen and one fluorine atoms; Ca(2) is seven coordinated surrounding by six oxygen, one fluorine atoms; Ca(3) is seven coordinated surrounding by four oxygen and three fluorine atoms; Ca(4) is eight coordinated surrounding by five oxygen and three fluorine atoms [11]. Hence, when Ce3þ ions are induced into the Ca4Si2O7F2 host, Ce3þ ions may enter different Ca sites, and show different emissions. To classify this issue, the photoluminescence spectra of sample Ca4Si2O7F2:xCe3þ are measured at RT. The excitation spectra for sample Ca4Si2O7F2:0.01Ce3þ at RT are shown in Fig. 2(i). By motoring the emission at 408 nm, three excitation bands A (~355 nm), B (~320 nm) and C (~290 nm) which are attributed to the f / d transitions of Ce3þ are observed in curve a. To determine the peak positions, curve a is Gaussian fitted with a sum of three bands. By monitoring different emission wavelengths, obvious difference can be found in the excitation spectra. When the emission wavelength is fixed at 408 nm, as curve a in Fig. 2(i), the dominant excitation bands is band A. However, when the emission wavelength moves to 470 nm, as curve b in Fig. 2(i), the relative intensity of A, and C decreases. We surmise the relative intensities between these bands related to Ce3þ in different Ca2þ coordination sites in Ca4Si2O7F2 host, which is discussed as follow. To further study the site occupation phenomenon, two different excitation wavelengths 320 (band B) and 365 nm (band A) were chosen as excitation wavelength to measured the emission spectra,
Fig. 1. XRD patterns of samples Ca4Si2O7F2:0.01Ce3þ (a), Ca4Si2O7F2:0.04Ce3þ (b) and Ca4Si2O7F2:0.10Ce3þ (c).
Fig. 2. Excitation spectra of Ca4Si2O7F2:0.01Ce3þ (a: lem ¼ 408 nm; b: lem ¼ 470 nm) and emission spectra of Ca4Si2O7F2:0.08Ce3þ (c: lex ¼ 320 nm; d: lex ¼ 355 nm).
as shown in Fig. 2(ii). Upon 320 nm excitation, the emission spectrum (curve c) shows two broad emission bands with maximum peaks at 430 nm and 470 nm within the wavelength range 350e650 nm, which is due to 5d-4f transition of the Ce3þ. The energy separation of the two bands corresponds to the spineorbit splitting and amounts to about 2000 cm1. Obviously, curve c is unsymmetrical with a long tail at the long wavelength side and a weak shoulder at short wavelength side, indicating that another weak band may exist. To find the peak positions, curve c is Gaussian fitted with a sum of four peaks, which are marked D1 (~402 nm), D2 (~432 nm), E1 (~465 nm), and E2 (~508 nm). Generally, the emission of Ce3þ ions in a definite lattice site often occurs as two bands due to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 spineorbit split 4f ground states, especially for the case with small Stokes shift at lower temperature. The energy separation of the two bands corresponds to the spineorbit splitting and amounts to about 2000 cm1. The energy separation of bands D1D2 and E1-E2 are calculated to be 1726 and 1819 cm1, respectively. Obviously, the bands D1 and D2 belong to the same Ca site in the host lattice, while the bands E1 and E2 come from another Ca site. We mark bands D (D1, D2) and E (E1, E2) as Ce(1) and Ce(2) in order to expediently discuss following. When the excitation wavelength is moved to 355 nm, as shown in curve d in Fig. 2(ii), the relative intensity of band E gets stronger than that in curve c, indicating that different excitation wavelength have great effect on Ce(1) and Ce(2) emissions. Fig. 3(a) shows the excitation spectra of samples Ca4Si2O7F2:xCe3þ (x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10) at RT. The monitoring wavelength is fixed at 408 nm, which related to the corresponding emission band D in Fig. 2(ii). In comparison the excitation intensity of these excitation spectra, it is found that the relative intensities of all the three excitation bands decrease clearly with increasing of Ce3þ concentration. The decreasing rate of band A is more obvious than that of band B and C. When the monitoring wavelength is fixed at 470 nm (Fig. 3(b)), which related to the corresponding emission band E in Fig. 2(ii), the excitation band intensities of these three bands are difference with that in Fig. 3(a). Band B (~320 nm) is dominated in all the samples, except for the lowest concentration sample (x ¼ 0.01). For band B, the intensity increases at low Ce3þ doping concentration (x 0.02), and then decrease gradually with x values increasing. However, with Ce3þ
M. Xie et al. / Solid State Sciences 48 (2015) 193e196
195
Fig. 3. Excitation and emission spectra of Ca4Si2O7F2:xCe3þ (x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10).
doping concentration increasing, the intensities of band A decrease constantly. In addition, band C is very weak in all the excitation curves by monitoring 470 nm emission. The difference between Fig. 3(a) and (b) further indicates that two emission bands at 408 nm and 470 nm may come from different Ce3þ sites. To evaluate the influence of doping concentration (x value) and excitation wavelength on the emission intensity, the emission spectra of samples Ca4Si2O7F2:xCe3þ with different Ce3þ concentration under 355 nm and 320 excitation were measured, as shown in Fig. 3(c) and (d). It is found that the emission spectra are dependent strongly on the Ce3þ doping concentration and excitation wavelength. Under 355 nm excitation, as shown in Fig. 3(c), for low Ce3þ concentration examples (x 0.06), the Ce(1) emission is dominant. When x values increase up to 0.06, the Ce(2) emission become dominant. The inset in Fig. 3(c) exhibits the normalized emission spectra of samples Ca4Si2O7F2:xCe3þ under 355 nm excitation. It can be found more clearly that the emission band shifts to the long-wavelength side with increasing Ce3þ concentrations. The spectra shape under 320 nm excitation in Fig. 3(d) are different from that under 355 nm excitation. It is clearly to be seen that Ce(1) emission keeps dominant in all emission curves in Fig. 3(d). As we analyze the spectrum intensities of Fig. 3, we can find that the intensities of Ce(1) emission varies in accordance with PLE band A, while Ce(2) emission varies in accordance with PLE band B. So, can we get a conclusion that band A is related to Ce(1) site emission, and band B is related to Ce(2) site emission? If so, it would confuse us that way Ce(1) emission keeps dominant under 320 nm excitation. As a result, we can not attribute the excitation bands to two Ce site emissions based on the PL bands and PLE bands here. However, it is accepted that Ce3þ ions enter two kinds of Ca sites in Ca4Si2O7F2 host. The reasons for the difficulty in confirming the correlation between the PL bands and PLE bands may relate to two factors as follow: firstly, the excitation bands of two Ce site emissions is close to each other; Secondly, the intensities decay rate of two Ce site emissions is different. As we know that the Ce3þ ions may share the same PL spectra
bands when the coordination environments are similar or inapparent [12,13]. As discussed in the former crystallographic coordination information for Ca4Si2O7F2, Ca(1), Ca(2), Ca(3), Ca(4) and are both coordinated by oxygen and fluorine atoms with different coordination numbers 6, 7, 7, and 8. Their coordination environments are somewhat similar. Therefore, only two type of Ce3þ luminescence behavior can be found here. Upon excitation of band B at 320 nm and monitoring band D at 408 nm, the luminescence decay curves of samples Ca4Si2O7F2:xCe3þ are shown in Fig. 4(a). When Ce3þ concentration is low (x ¼ 0.01), the curve show single exponential decays. However, these curves slightly derivate single exponential decays with Ce3þ concentration increasing. The decay time as a function of the x value are depicted in the right inset. With x value increasing from
Fig. 4. Luminescence decay curves of Ca4Si2O7F2:xCe3þ (a:lex ¼ 320 nm, lex ¼ 408 nm; b:lex ¼ 355 nm,lem ¼ 470 nm).
196
M. Xie et al. / Solid State Sciences 48 (2015) 193e196
evaluate the phosphors. The external quantum efficiencies of Ca4Si2O7F2:0.01Ce3þ, Ca4Si2O7F2:0.04Ce3þ, and Ca4Si2O7F2:0.10Ce3þ are measured to be 41.62%, 23.36% and 14.49%, respectively. The quantum efficiency may be improved by optimizing the particle size, size distribution, morphology and crystalline defects of the phosphors. 4. Conclusions In summary, Ce3þ doped Ca4Si2O7F2 phosphors Ca4Si2O7F2:xCe3þ were prepared by a conventional solidestate reaction method. The Ce3þ ions occupy two different lattice sites with 408 and 470 nm emission bands in the host. Under 355 nm excitation, the dominant emission bands change from Ce(1) to Ce(2) with Ce3þ concentration increasing. As a consequence, the phosphors Ca4Si2O7F2:xCe3þ show a bright tunable color emission from blue to green-blue under the nUV light excitation, implying that the phosphors have a potential application in n-UV based LEDs. Acknowledgments
Fig. 5. CIE chromaticity diagram for Ca4Si2O7F2:xCe3þ excited at 355 nm. The inset shows the photos of samples Ca4Si2O7F2:xCe3þ taken under 365 nm excitation in a UV box.
0.01 to 0.10, the decay time decreases from 47 ns to 24 ns. It may relate to the energy transfer of Ce(1) / Ce(1) and Ce(1) / Ce(2) in the system. Upon excitation of band A at 355 nm and monitoring band E at 470 nm, the luminescence decay curves of samples Ca4Si2O7F2:xCe3þ are shown in Fig. 4(b). By comparing with decay curves in Fig. 4(a), all these curves can be well fitted using a single exponential equation, It ¼ I0 exp(t/t), where It and I0 are the luminescence intensities at time t and t ¼ 0, and t is the decay constant. The decay time as a function of the x value are depicted in the right inset, and the decay time remains nearly at about 40 ns. The chromaticity coordinates of samples Ca4Si2O7F2:xCe3þ under 355 nm excitation are presented in the CIE chromaticity diagram, as shown in Fig. 5. With increasing of Ce3þ concentrations, the chromaticity coordinate gradually move from blue area to green-blue area, suggesting that emitting color can be tunable by adjusting the doping concentration of Ce3þ ions. The results show that the phosphors Ca4Si2O7F2:xCe3þ have potential applications in n-UV based LEDs, especially for the high Ce3þ concentration samples. The external quantum efficiency of samples Ca4Si2O7F2:xCe3þ were measured by QY-2000 equipped with 355 nm LED lamp to
The work is financially supported by National Natural Science Foundation of China (Grant no. 21401165, 11404283), Natural Science Foundation of Guangdong Province (Grant no. 2014A030307040, 2014A030307028), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (2012LYM_0094, 2013LYM_0053) and Program of young creative talents in Universities of Guangdong (Nature Science, 2014KQNCX188). References [1] X.H. He, M.Y. Guan, N. Lian, J.H. Sun, T.M. Shang, J. Alloys. Compd. 492 (2010) 452e455. [2] A. Lakshmanan, R.S. Bhaskar, P.C. Thomas, R.S. Kumar, V.S. Kumar, M.T. Jose, Mater. Lett. 64 (2010) 1809e1812. [3] R. Wang, J. Xu, C. Chen, Mater. Lett. 68 (2012) 307e309. [4] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 156 (2009) J179eJ184. [5] X.M. Zhang, H.J. Seo, Phys. B 405 (2010) 2436e2439. [6] C.H. Huang, T.S. Chan, W.R. Liu, D.Y. Wang, Y.C. Chiu, Y.T. Yeh, T.M. Chen, J. Mater. Chem. 22 (2012) 20210e20216. [7] G.G. Li, Y. Zhang, D.L. Geng, M.M. Shang, C. Peng, Z.Y. Cheng, J. Lin, ACS Appl. Mater. Interf. 4 (2012) 296e305. [8] G.G. Li, D.L. Geng, M.M. Shang, C. Peng, Z.Y. Cheng, J. Lin, J. Mater. Chem. 21 (2011) 13334e13344. [9] Y.Y. Zhang, Z.G. Xia, H.K. Liu, Z.Y. Wang, M.L. Li, Chem. Phys. Lett. 593 (2014) 189e192. [10] W.P. Chen, H.B. Liang, H.Y. Ni, P. He, Q. Su, J. Electrochem. Soc. 157 (2010) J159eJ163. [11] W. Lü, Y.S. Luo, Z.D. Hao, X. Zhang, X.J. Wang, J.H. Zhang, Mater. Lett. 77 (2012) 45e47. [12] J. Zhou, Z.G. Xia, M.X. Yang, K. Shen, J. Mater. Chem. 22 (2012) 21935e21941. [13] Z.G. Xia, W.W. Wu, Dalton Trans. 42 (2013) 12989e12997.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis and photoluminescence properties of tunable emission phosphors Ca4Si2O7F2:Ce3þ Mubiao Xie a, *, Guoxian Zhu a, Dongyu Li b, Rongkai Pan a, Xiaoping Zhou c, Wei Xie c a
Institute of Physical Chemistry, School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang, 524048, China School of Physics Science and Technology, Lingnan Normal University, Zhanjiang, 524048, China c Development Center for New Materials Engineering & Technology in Universities of Guangdong, Lingnan Normal University, Zhanjiang, 524048, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 7 August 2015 Accepted 11 August 2015 Available online 14 August 2015
The Ce3þ activated phosphors Ca4Si2O7F2:Ce3þ are prepared by a solid state reaction technique. The UV evis luminescence properties as well as fluorescence decay time spectra are investigated and discussed. The results revealed that there were two kinds of Ce3þ luminescence behavior with 408 and 470 nm emissions, respectively. Under 355 nm excitation, the Ce(1) emission (408 nm) is dominant at low doping concentration, and then the Ce(2) emission (470 nm) get more important with increasing of Ce3þ concentrations in the host. The phosphors Ca4Si2O7F2:xCe3þ show tunable emissions from blue area to green-blue area under near-ultraviolet light excitation, indicating a potential application in near-UV based w-LEDs. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Phosphor Photoluminescence Ca4Si2O7F2:Ce3þ w-LEDs
1. Introduction Nowadays, Much attention has been paid for white lightemitting diodes (w-LEDs), which can be realized through a combination of red, green and blue phosphors with UV (ultraviolet) or near-UV (350e410 nm) LEDs [1e3]. Therefore, it is necessary to develop new tri-color emitting phosphors with good stabilities and highly efficient under excitation wavelength in range of 350e410 nm. In consideration of the merits and drawbacks of different tri-color phosphors, it is better to develop color tunable phosphors for the fabrication of w-LEDs. Most color tunable phosphor can be obtained through co-doping two or more activators into the same host [4e8]. However, it will be more interesting that tunable emitting color would be realized by one kind of ions entering different crystallographic sites in a single host [9,10]. The trivalent rare-earth Ce3þ ions show broadband absorption and emission due to 4f7e4f65d1 transitions, which strongly dependent on the type of host lattices. Thus, the emission of Ce3þ can vary from the ultraviolet to the red spectral range. The luminescence of Ce3þ ions in Ca4Si2O7F2 have been studied by Wei Lü
* Corresponding author. E-mail address: [email protected] (M. Xie). http://dx.doi.org/10.1016/j.solidstatesciences.2015.08.009 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
and their studies revealed that the Ce3þ concentration has effect on the shape of the emission spectra [11]. In this work, we further report the preparation and luminescent properties of emissiontunable phosphors Ca4Si2O7F2:xCe3þ. The site occupation and energy transfer between different Ce3þ ions are discussed. 2. Experimental The Ca4Si2O7F2:xCe3þ phosphors were synthesized by a hightemperature solidestate reaction. The CaCO3 (AR), Li2CO3 (AR), SiO2 (AR), NH4F (AR) and CeO2 (99.99%) were used as the raw materials. Li ions are used as charge compensation ions here. The reactants were mixed homogeneously by an agate mortar, after mixing and thoroughly grinding, the mixtures were heated at 1000 C for 6 h in CO reducing atmosphere. The final products were cooled to room temperature (RT) by switching off the muffle furnace and ground again into white powder. The phase purity and structure of the final products were characterized by a powder X-ray diffraction (XRD) analysis using Cu Ka radiation (l ¼ 1.5405 Å, 40 kV, 30 mA) on a Rigaku D/max 2200 vpc X-Ray Diffractometer at room temperature (RT). The UV luminescence spectra and luminescence decay curves at RT were recorded on an Edinburgh FLS920 combined fluorescence lifetime and steady state spectrometer, which was equipped with a time-
194
M. Xie et al. / Solid State Sciences 48 (2015) 193e196
correlated single-photon counting (TCSPC) card. The quantum efficiency (QE) of the samples was measured by QY-2000 equipped with 355 nm LED lamp (Orient Koji, China). 3. Results and discussion The X-ray diffraction (XRD) patterns of samples Ca4Si2O7F2:xCe3þ were measured at RT. Fig. 1 presents the diffractograms of three typical samples Ca4Si2O7F2:0.01Ce3þ, (a), Ca4Si2O7F2:0.04Ce3þ (b) and Ca4Si2O7F2:0.10Ce3þ (c). The diffraction patterns of the samples are identical to each other, and all peaks agree well with the standard data Ca4Si2O7F2 (JCPDS 411474). The results suggest that the prepared samples belong to the pure phase and that Ce3þ ions have little effect on the phase structure. In Ca4Si2O7F2 structure, Ca2þ ions occupy four different crystal sites. Ca(1) is six coordinated surrounding by five oxygen and one fluorine atoms; Ca(2) is seven coordinated surrounding by six oxygen, one fluorine atoms; Ca(3) is seven coordinated surrounding by four oxygen and three fluorine atoms; Ca(4) is eight coordinated surrounding by five oxygen and three fluorine atoms [11]. Hence, when Ce3þ ions are induced into the Ca4Si2O7F2 host, Ce3þ ions may enter different Ca sites, and show different emissions. To classify this issue, the photoluminescence spectra of sample Ca4Si2O7F2:xCe3þ are measured at RT. The excitation spectra for sample Ca4Si2O7F2:0.01Ce3þ at RT are shown in Fig. 2(i). By motoring the emission at 408 nm, three excitation bands A (~355 nm), B (~320 nm) and C (~290 nm) which are attributed to the f / d transitions of Ce3þ are observed in curve a. To determine the peak positions, curve a is Gaussian fitted with a sum of three bands. By monitoring different emission wavelengths, obvious difference can be found in the excitation spectra. When the emission wavelength is fixed at 408 nm, as curve a in Fig. 2(i), the dominant excitation bands is band A. However, when the emission wavelength moves to 470 nm, as curve b in Fig. 2(i), the relative intensity of A, and C decreases. We surmise the relative intensities between these bands related to Ce3þ in different Ca2þ coordination sites in Ca4Si2O7F2 host, which is discussed as follow. To further study the site occupation phenomenon, two different excitation wavelengths 320 (band B) and 365 nm (band A) were chosen as excitation wavelength to measured the emission spectra,
Fig. 1. XRD patterns of samples Ca4Si2O7F2:0.01Ce3þ (a), Ca4Si2O7F2:0.04Ce3þ (b) and Ca4Si2O7F2:0.10Ce3þ (c).
Fig. 2. Excitation spectra of Ca4Si2O7F2:0.01Ce3þ (a: lem ¼ 408 nm; b: lem ¼ 470 nm) and emission spectra of Ca4Si2O7F2:0.08Ce3þ (c: lex ¼ 320 nm; d: lex ¼ 355 nm).
as shown in Fig. 2(ii). Upon 320 nm excitation, the emission spectrum (curve c) shows two broad emission bands with maximum peaks at 430 nm and 470 nm within the wavelength range 350e650 nm, which is due to 5d-4f transition of the Ce3þ. The energy separation of the two bands corresponds to the spineorbit splitting and amounts to about 2000 cm1. Obviously, curve c is unsymmetrical with a long tail at the long wavelength side and a weak shoulder at short wavelength side, indicating that another weak band may exist. To find the peak positions, curve c is Gaussian fitted with a sum of four peaks, which are marked D1 (~402 nm), D2 (~432 nm), E1 (~465 nm), and E2 (~508 nm). Generally, the emission of Ce3þ ions in a definite lattice site often occurs as two bands due to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 spineorbit split 4f ground states, especially for the case with small Stokes shift at lower temperature. The energy separation of the two bands corresponds to the spineorbit splitting and amounts to about 2000 cm1. The energy separation of bands D1D2 and E1-E2 are calculated to be 1726 and 1819 cm1, respectively. Obviously, the bands D1 and D2 belong to the same Ca site in the host lattice, while the bands E1 and E2 come from another Ca site. We mark bands D (D1, D2) and E (E1, E2) as Ce(1) and Ce(2) in order to expediently discuss following. When the excitation wavelength is moved to 355 nm, as shown in curve d in Fig. 2(ii), the relative intensity of band E gets stronger than that in curve c, indicating that different excitation wavelength have great effect on Ce(1) and Ce(2) emissions. Fig. 3(a) shows the excitation spectra of samples Ca4Si2O7F2:xCe3þ (x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10) at RT. The monitoring wavelength is fixed at 408 nm, which related to the corresponding emission band D in Fig. 2(ii). In comparison the excitation intensity of these excitation spectra, it is found that the relative intensities of all the three excitation bands decrease clearly with increasing of Ce3þ concentration. The decreasing rate of band A is more obvious than that of band B and C. When the monitoring wavelength is fixed at 470 nm (Fig. 3(b)), which related to the corresponding emission band E in Fig. 2(ii), the excitation band intensities of these three bands are difference with that in Fig. 3(a). Band B (~320 nm) is dominated in all the samples, except for the lowest concentration sample (x ¼ 0.01). For band B, the intensity increases at low Ce3þ doping concentration (x 0.02), and then decrease gradually with x values increasing. However, with Ce3þ
M. Xie et al. / Solid State Sciences 48 (2015) 193e196
195
Fig. 3. Excitation and emission spectra of Ca4Si2O7F2:xCe3þ (x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10).
doping concentration increasing, the intensities of band A decrease constantly. In addition, band C is very weak in all the excitation curves by monitoring 470 nm emission. The difference between Fig. 3(a) and (b) further indicates that two emission bands at 408 nm and 470 nm may come from different Ce3þ sites. To evaluate the influence of doping concentration (x value) and excitation wavelength on the emission intensity, the emission spectra of samples Ca4Si2O7F2:xCe3þ with different Ce3þ concentration under 355 nm and 320 excitation were measured, as shown in Fig. 3(c) and (d). It is found that the emission spectra are dependent strongly on the Ce3þ doping concentration and excitation wavelength. Under 355 nm excitation, as shown in Fig. 3(c), for low Ce3þ concentration examples (x 0.06), the Ce(1) emission is dominant. When x values increase up to 0.06, the Ce(2) emission become dominant. The inset in Fig. 3(c) exhibits the normalized emission spectra of samples Ca4Si2O7F2:xCe3þ under 355 nm excitation. It can be found more clearly that the emission band shifts to the long-wavelength side with increasing Ce3þ concentrations. The spectra shape under 320 nm excitation in Fig. 3(d) are different from that under 355 nm excitation. It is clearly to be seen that Ce(1) emission keeps dominant in all emission curves in Fig. 3(d). As we analyze the spectrum intensities of Fig. 3, we can find that the intensities of Ce(1) emission varies in accordance with PLE band A, while Ce(2) emission varies in accordance with PLE band B. So, can we get a conclusion that band A is related to Ce(1) site emission, and band B is related to Ce(2) site emission? If so, it would confuse us that way Ce(1) emission keeps dominant under 320 nm excitation. As a result, we can not attribute the excitation bands to two Ce site emissions based on the PL bands and PLE bands here. However, it is accepted that Ce3þ ions enter two kinds of Ca sites in Ca4Si2O7F2 host. The reasons for the difficulty in confirming the correlation between the PL bands and PLE bands may relate to two factors as follow: firstly, the excitation bands of two Ce site emissions is close to each other; Secondly, the intensities decay rate of two Ce site emissions is different. As we know that the Ce3þ ions may share the same PL spectra
bands when the coordination environments are similar or inapparent [12,13]. As discussed in the former crystallographic coordination information for Ca4Si2O7F2, Ca(1), Ca(2), Ca(3), Ca(4) and are both coordinated by oxygen and fluorine atoms with different coordination numbers 6, 7, 7, and 8. Their coordination environments are somewhat similar. Therefore, only two type of Ce3þ luminescence behavior can be found here. Upon excitation of band B at 320 nm and monitoring band D at 408 nm, the luminescence decay curves of samples Ca4Si2O7F2:xCe3þ are shown in Fig. 4(a). When Ce3þ concentration is low (x ¼ 0.01), the curve show single exponential decays. However, these curves slightly derivate single exponential decays with Ce3þ concentration increasing. The decay time as a function of the x value are depicted in the right inset. With x value increasing from
Fig. 4. Luminescence decay curves of Ca4Si2O7F2:xCe3þ (a:lex ¼ 320 nm, lex ¼ 408 nm; b:lex ¼ 355 nm,lem ¼ 470 nm).
196
M. Xie et al. / Solid State Sciences 48 (2015) 193e196
evaluate the phosphors. The external quantum efficiencies of Ca4Si2O7F2:0.01Ce3þ, Ca4Si2O7F2:0.04Ce3þ, and Ca4Si2O7F2:0.10Ce3þ are measured to be 41.62%, 23.36% and 14.49%, respectively. The quantum efficiency may be improved by optimizing the particle size, size distribution, morphology and crystalline defects of the phosphors. 4. Conclusions In summary, Ce3þ doped Ca4Si2O7F2 phosphors Ca4Si2O7F2:xCe3þ were prepared by a conventional solidestate reaction method. The Ce3þ ions occupy two different lattice sites with 408 and 470 nm emission bands in the host. Under 355 nm excitation, the dominant emission bands change from Ce(1) to Ce(2) with Ce3þ concentration increasing. As a consequence, the phosphors Ca4Si2O7F2:xCe3þ show a bright tunable color emission from blue to green-blue under the nUV light excitation, implying that the phosphors have a potential application in n-UV based LEDs. Acknowledgments
Fig. 5. CIE chromaticity diagram for Ca4Si2O7F2:xCe3þ excited at 355 nm. The inset shows the photos of samples Ca4Si2O7F2:xCe3þ taken under 365 nm excitation in a UV box.
0.01 to 0.10, the decay time decreases from 47 ns to 24 ns. It may relate to the energy transfer of Ce(1) / Ce(1) and Ce(1) / Ce(2) in the system. Upon excitation of band A at 355 nm and monitoring band E at 470 nm, the luminescence decay curves of samples Ca4Si2O7F2:xCe3þ are shown in Fig. 4(b). By comparing with decay curves in Fig. 4(a), all these curves can be well fitted using a single exponential equation, It ¼ I0 exp(t/t), where It and I0 are the luminescence intensities at time t and t ¼ 0, and t is the decay constant. The decay time as a function of the x value are depicted in the right inset, and the decay time remains nearly at about 40 ns. The chromaticity coordinates of samples Ca4Si2O7F2:xCe3þ under 355 nm excitation are presented in the CIE chromaticity diagram, as shown in Fig. 5. With increasing of Ce3þ concentrations, the chromaticity coordinate gradually move from blue area to green-blue area, suggesting that emitting color can be tunable by adjusting the doping concentration of Ce3þ ions. The results show that the phosphors Ca4Si2O7F2:xCe3þ have potential applications in n-UV based LEDs, especially for the high Ce3þ concentration samples. The external quantum efficiency of samples Ca4Si2O7F2:xCe3þ were measured by QY-2000 equipped with 355 nm LED lamp to
The work is financially supported by National Natural Science Foundation of China (Grant no. 21401165, 11404283), Natural Science Foundation of Guangdong Province (Grant no. 2014A030307040, 2014A030307028), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (2012LYM_0094, 2013LYM_0053) and Program of young creative talents in Universities of Guangdong (Nature Science, 2014KQNCX188). References [1] X.H. He, M.Y. Guan, N. Lian, J.H. Sun, T.M. Shang, J. Alloys. Compd. 492 (2010) 452e455. [2] A. Lakshmanan, R.S. Bhaskar, P.C. Thomas, R.S. Kumar, V.S. Kumar, M.T. Jose, Mater. Lett. 64 (2010) 1809e1812. [3] R. Wang, J. Xu, C. Chen, Mater. Lett. 68 (2012) 307e309. [4] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 156 (2009) J179eJ184. [5] X.M. Zhang, H.J. Seo, Phys. B 405 (2010) 2436e2439. [6] C.H. Huang, T.S. Chan, W.R. Liu, D.Y. Wang, Y.C. Chiu, Y.T. Yeh, T.M. Chen, J. Mater. Chem. 22 (2012) 20210e20216. [7] G.G. Li, Y. Zhang, D.L. Geng, M.M. Shang, C. Peng, Z.Y. Cheng, J. Lin, ACS Appl. Mater. Interf. 4 (2012) 296e305. [8] G.G. Li, D.L. Geng, M.M. Shang, C. Peng, Z.Y. Cheng, J. Lin, J. Mater. Chem. 21 (2011) 13334e13344. [9] Y.Y. Zhang, Z.G. Xia, H.K. Liu, Z.Y. Wang, M.L. Li, Chem. Phys. Lett. 593 (2014) 189e192. [10] W.P. Chen, H.B. Liang, H.Y. Ni, P. He, Q. Su, J. Electrochem. Soc. 157 (2010) J159eJ163. [11] W. Lü, Y.S. Luo, Z.D. Hao, X. Zhang, X.J. Wang, J.H. Zhang, Mater. Lett. 77 (2012) 45e47. [12] J. Zhou, Z.G. Xia, M.X. Yang, K. Shen, J. Mater. Chem. 22 (2012) 21935e21941. [13] Z.G. Xia, W.W. Wu, Dalton Trans. 42 (2013) 12989e12997.