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On the photoluminescence of multi-sites Ce3+ in T-phase orthosilicate and energy transfer from Ce3+ to Tb3+
Journal of Alloys and Compounds 748 (2018) 871e875
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
On the photoluminescence of multi-sites Ce3þ in T-phase orthosilicate and energy transfer from Ce3þ to Tb3þ Xubo Tong a, Xinmin Zhang a, *, Luyi Wu a, Hongzhi Zhang a, Hyo Jin Seo b a
School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, 410004, China Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan, 608-737, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2018 Received in revised form 8 March 2018 Accepted 20 March 2018 Available online 21 March 2018
Ce3þ, Tb3þ co-doped T-phase orthosilicate Ba1.2Ca0.8SiO4 phosphors were prepared by means of solid state reactions. The synthesized samples were investigated using XRD and PL emission and excitation spectra. The emission spectra for Ce3þ doped Ba1.2Ca0.8SiO4 phosphors show broad bands in the 320 e550 nm spectral region. The excitation spectra exhibit several absorption peaks in the range of 250 e400 nm. Both the emission and excitation spectra are dependent on the excitation or monitoring wavelengths. We attribute these results to Ce3þ ions occupying two different crystallographic sites (i.e., Ce I and Ce II). The Ce3þ, Tb3þ co-doped Ba1.2Ca0.8SiO4 phosphors show Tb3þ-related line emissions in the 475e600 nm spectral region and Ce3þ-related band emissions in the 350e525 nm spectral region when the 4f / 5d transition of Ce3þ is excited (lex ¼ 345 nm), indicating that energy transfer from Ce3þ to Tb3þ takes place. Moreover, the emission intensity of 5D4 / 7F5 transition for Ba1.00Ce0.02Tb0.08Li0.10Ca0.8SiO4 is 30 times stronger than that of Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample under 345-nm UV excitation. © 2018 Elsevier B.V. All rights reserved.
Keywords: T-phase orthosilicate Site occupation Energy transfer Photoluminescence
1. Introduction Recently, white light-emitting-diodes (WLEDs) have received considerable attention due to their energy saving, long lifetime, and other advantages. As we know, the phosphors play an essential role for the phosphor-converted white light-emitting-diodes (pcWLEDs) [1e4]. It can be achieved by combination of a near-UV (nUV) LED chip and tricolor phosphors, each emitting a relatively narrow spectrum of red, green or blue light upon receiving ultraviolet radiation from the LED chip [5,6]. It is well known that rareearth ions play an important role in luminescent materials based on their 4f / 4f or 5d / 4f transitions. The energy levels of the Tb3þ ion arise from the 4f8 configuration. The emission of it is due to 5 D4/ 7FJ transitions which are mainly in the green and shows sharp lines in the spectrum. However, its optical absorption transitions are strongly forbidden by the parity selection rule, resulting in a weak absorption in the n-UV region. The Ce3þ emission occurs from the lowest crystal field component of the 5 d1 configuration to the two levels of the ground state and can vary from long wavelength UV to yellow depending strongly on the host lattice [7e9].
* Corresponding author. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.jallcom.2018.03.252 0925-8388/© 2018 Elsevier B.V. All rights reserved.
The energy transfer between different ion species can take place when they have closed matched energy levels. The energy transfer results either in the enhancement or in the quenching of emission. The Tb3þ emission can be remarkably sensitized by Ce3þ ion, which has been confirmed in LaPO4:Ce, Tb system for a long time. Similar results about energy transfer from Ce3þ to Tb3þ are reported in K2MgSi3O8:Ce3þ,Tb3þ [10], K2Ba3(P2O7)2:Ce3þ,Tb3þ [11], and Ba2Y5B5O17:Ce3þ, Tb3þ [12]. In this study, we try to synthesize and investigate the Tb3þ luminescence and Ce-Tb energy transfer in one of the silicates hosts. Silicates act as the important inorganic mineral materials. The basis for silicate structures is the SieO bond and the [SiO4] tetrahedron. Among them, alkaline earth orthosilicates which can be represented with the general formula M2SiO4 (M ¼ Mg, Ca, Sr, and Ba) including the end members or intermediate compositions. Solid solution is a very common phenomenon in minerals. However, M2SiO4 orthosilicate compounds show diversity of the phases, despite the fact that they often have very similar chemical formulas. The most important factor which controls solid solution formation is the relative size of the cations. For example, Ca2-xSrxSiO4 compounds have a0 H-, a0 L-, b-, and g-phase at room temperature [13]; while five different modulated phases, am-, Y-, X-, a0 L- and T-phase, were found in the Ba2SiO4-Ca2SiO4 system [14]. It is well known
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that the luminescent behaviors of silicates doped with rare earth or transition metal ions have attracted much attention recently years, and the luminescence properties are closely related to the crystal structure of host lattice. Xia reported the structural phase transformation of Ca2-xSrxSiO4:Ce3þ phosphor by Sr2þ substituting for Ca2þ within 0 x < 2 and studied the relationship between structure and property [13]. Recently, orthosilicate-based phosphors doped with Eu2þ and Ce3þ have attracted much attention due to their applications in the field of WLEDs [15]. The modulated Tphase occurs for samples of Ba2-xCaxSiO4 (0.45 x 0.80) and it is one of the most chemically stable phase [16]. The T-phase is made of five Ba/Ca sites. The photoluminescence (PL) properties of Eu2þ, Ce3þ single doped and Eu-Mn, Ce-Mn co-doped T-phase Ba2xCaxSiO4 were reported by Park and Choi [17,18]. However, there are no reports about the detailed PL properties of Ce3þ, Tb3þ, and the energy transfer from Ce3þ to Tb3þ in Ba1.2Ca0.8SiO4. In addition, purity Ba1.2Ca0.8SiO4 is very easy to synthesize. Accordingly, in this paper, Ba1.2Ca0.8SiO4:Ce3þ,Tb3þ phosphors were prepared via the solid state reaction method, and their PL properties, especially for the Ce3þ site occupation and energy transfer from Ce3þ to Tb3þ, were investigated in detail. 2. Experimental
the corresponding standard data for the hexagonal phase of Ba1.2Ca0.8SiO4, indicating that all samples under investigation show to be single phase. The emission spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4 under UV light excitation are given in Fig. 2a. As can be seen in Fig. 2a, the emission spectra show complicated luminescence behavior and are strongly dependent on the excitation wavelengths. The emission spectra under 254, 260, 275, 285, 325 and 330 nm excitation are similar to each other and exhibit broad band extending from 320 to 550 nm. The luminescence is ascribed to the 5d / 4f transition of Ce3þ [19]. A main peak at ~383 nm and two shoulder peaks at ~405 and ~445 nm are observed in the spectra. The emission spectra under 345, 365 and 385 nm excitation exhibit huge difference compared to other emission spectra. The host lattice has a great influence on the spectral position of the Ce3þ emission band [20,21]. So, the complicated emission spectra could result from more than one Ce3þ center [22]. The PL emission spectrum for the sample under 275 nm excitation can be fitted into four Gaussian peaks and the results are shown in the inset of Fig. 2a. The energy difference of two subbands peaking at about 350 and 380 nm is 2000 cm1, corresponding to the 5d excited states to the spin-orbit splitting of the ground states (2F5/2 and 2F7/2). The spectral position of the Ce3þ emission band depends on three factors. Among them, crystal field splitting of the 5 d1 configuration has a great influence on the
Ba1.2Ca0.8SiO4:Ce3þ, Tb3þ phosphors were prepared via a traditional high-temperature solid-state reaction method. BaCO3 (99.9%), CaCO3 (99.9%), Li2CO3 (99.9%), SiO2 (99.9%), CeO2 (99.99%) and Tb4O7 (99.99%) were used as the starting materials. The raw materials Li2CO3 was added as a charge compensation reagent. They were mixed and ground according to the given stoichiometric ratio. The mixtures were prefired at 500 C for 2 h and finally heated at 1100 C for 2 h in a CO reducing atmosphere. The crystal structures of the samples were examined by X-ray diffraction (XRD) analysis using a Beijing Puxi XD-2 diffractometer with monochromatized Cu Ka radiation (l ¼ 0.15418 nm). Excitation and emission spectra were taken on a RF-5301 PC spectrofluorometer equipped with a 150 W Xenon Lamp as the excitation source. 3. Results and discussion XRD patterns of the as-prepared phosphors were collected to verify the phase purity. Typical diffraction patterns are shown in Fig. 1. All the diffraction peaks of the samples are consistent with
Fig. 1. XRD patterns of (a) Ba0.96Ce0.02Tb0.10Li0.12Ca0.8SiO4, (b) Ba1.04Tb0.08Li0.08Ca0.8SiO4, (c) Ba1.0Ce0.10Li0.10Ca0.8SiO4 and (d) the simulated XRD pattern calculated from the structure of Ba1.2Ca0.8SiO4.
Fig. 2. (a) Emission spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4. The inset shows the Gaussian fit of emission spectrum excited by 275-nm UV light; (b) Excitation spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4. The excitation and emission wavelengths are listed in the figures.
X. Tong et al. / Journal of Alloys and Compounds 748 (2018) 871e875
emission wavelength. The bond length (R) of Ba/Ca-O affects the crystal field strength (Dq) significantly, i.e., Dq is proportional to 1/ R5 [23]. The Ba/Ca sites with big coordination numbers will feel weak crystal field strength, whereas those with small coordination numbers will feel strong crystal field strength. When the doped Ce3þ incorporated into different Ba/Ca sites, the crystal field splitting D is different, and then Ce3þ will show different emissions. There are five Ba/Ca sites in the crystal structure of Ba1.2Ca0.8SiO4 (i.e., one site having 6-fold coordination, two sites having 10-fold coordination and two sites having 12-fold coordination) [18]. We conclude that the doped Ce3þ could substitute for all the Ba/Ca sites, and mark as Ce I site for those at 10/12-coordination Ba/Ca sites and Ce II site for those at 6-coordination Ba/Ca sites. To be specific, the two subbands peaking at 350 and 380 nm originate from d-f transition of Ce I site, and the other two subbands peaking at about 400 and 430 nm are ascribed to d-f transition of Ce II site. In short, the doped Ce3þ ions can occupy two different sites at the same time. The excitation spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4 for different emission wavelengths are given in Fig. 2b. The excitation spectra are also dependent on monitoring wavelengths. When different emission wavelengths are monitored, two groups of excitation spectra are observed. One is located in the wavelength range 250e350 nm (Group I, lem ¼ 340, 360 and 380 nm) and another is in the wavelength range 250e400 nm (Group II, lem ¼ 430, 435, 445 and 480 nm). The excitation spectra of Group I can be ascribed to the 4f / 5d transitions in Ce I site, while that of Group II can be ascribed to the 4f / 5d transitions in Ce II site. The others are the overlap of the excitation spectra of Ce I and Ce II (lem ¼ 400, 410 and 420 nm). Moreover, all the excitation spectra show several subbands. The sub-bands of Group I peak at 260 (38460), 275 (36360) and 325 nm (30770 cm1), while those of Group II peak at 255 (39215), 285 (35090), 330 (30300) and 365 nm (27400 cm1). Based on the fact that two groups excitation spectra are obtained when different emission wavelengths are monitored, it further confirms that two different Ba/Ca sites are available for the doped Ce3þ ions to occupy. The lowest crystal field component of the 5d1 configuration of Ce I and Ce II sites situate at about 30770 cm1 (325 nm) and 27400 cm1 (365 nm), respectively. The emission spectra for Ba1.2-2xCexLixCa0.8SiO4 at different Ce3þ doping concentrations under 325 nm UV excitation are shown in Fig. 3. No obvious difference is observed for the emission spectra except for the relative intensity. The optimum concentration is to be 0.02.
3þ
Fig. 3. Ce concentration dependence of the emission spectra for Ba1.2-2xCexLixCa0.8SiO4 (lex ¼ 325 nm).
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The excitation and emission spectra of Ba1.04Tb0.08Li0.08Ca0.8SiO4 are shown in Fig. 4. The emission spectrum consists of a series of sharp emission lines in the 350e650 nm spectral region. These emission lines are attributed to the 4f8 / 4f8 transitions of Tb3þ [24]. The excitation spectra for the 367 and 540 nm emissions are similar to each other. The weak absorption peaks, corresponding to the parity-forbidden 4f8 / 4f8 transitions, can be observed in the 300e400 nm spectral region. The absorption peaks for the 4f8 / 4f75d transitions are also observed in the wavelength region below ~300 nm. This region can be divided into two regions: spin-allowed transition region (<275 nm) and spin-forbidden transition region (275e300 nm). The transition probability of spin-allowed transitions is higher than that of spin-forbidden ones and thus provide relatively strong intensities. The excitation and emission spectra of Ba1.12Ce0.02Tb0.02Li0.04Ca0.8SiO4 sample are presented in Fig. 5. When excitation is carried out with 345 nm UV light lying within the 4f / 5d absorption band of Ce3þ ion, the emission spectrum consists of broad band emission, corresponding to the 5d / 4f transitions of Ce3þ ions, and line emissions corresponding to the 5D4 / 7FJ of Tb3þ ions simultaneously. The 5D3 / 7FJ emissions overlap with the 5d / 4f transitions of Ce3þ ions. The result indicates that energy transfer from Ce3þ to Tb3þ takes place [25e27]. The excitation spectra for different emissions are different. The excitation spectra of the Tb3þ 5 D4 emissions (lem ¼ 545 and 485 nm) are nearly consistent with that of the Ce II emission (lem ¼ 450 nm) in the 260e400 nm spectral region. Identity of the excitation spectra for these two kinds of emissions further confirms that the energy transfer could occur from Ce II to Tb3þ in Ce3þ, Tb3þ co-doped T-phase Ba1.2Ca0.8SiO4 crystal. In order to further investigate the energy transfer process between the Ce3þ and Tb3þ ions in the host lattice, a series of Ce3þ, Tb3þ co-doped Ba1.16-2xCe0.02TbxLi0.02þxCa0.8SiO4 samples were prepared. The concentration of Ce3þ was fixed at the optimal value of 0.02 and the content of Tb3þ was varied in the range of 0.01e0.10. The emission spectra for the samples under 345 nm-UV excitation are shown in Fig. 6. With increasing Tb3þ concentration, the relative emission intensities of the Ce3þ ions decreased remarkably which is ascribed to the enhancement of energy transfer from Ce3þ to Tb3þ ions [10,28]; whereas the relative emission intensities of Tb3þ increase initially and reach a maximum value at x ¼ 0.08, beyond
Fig. 4. Excitation and emission spectra of Ba1.04Tb0.08Li0.08Ca0.8SiO4. The excitation and monitoring wavelengths were: (a) lex ¼ 250 nm; (b) lem ¼ 367 nm and (c) lem ¼ 540 nm. The inset shows the enlarged zone of excitation spectrum range from 320 to 400 nm.
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4. Conclusions We prepared Ce3þ, Tb3þ co-doped Ba1.2Ca0.8SiO4 phosphors by the solid state reactions. The synthesized phosphors were investigated using XRD and PL emission and excitation spectra. For the Ce3þ singly doped Ba1.2Ca0.8SiO4 phosphors, both the emission spectra and the excitation spectra are dependent on the excitation or monitoring wavelength, indicating that the Ce3þ ions occupy two different crystallographic sites (i.e., Ce I and Ce II). Under 345nm UV excitation, the emission intensity of 5D4 / 7F5 transition of Tb3þ in the Ce3þ, Tb3þ co-doped phosphor is 30 times stronger than that in Tb3þ singly doped sample, which indicates the occurrence of an efficient energy transfer from Ce3þ to Tb3þ. Whether Ce I or CeII is involved in this energy transfer needs further study. Acknowledgements
Fig. 5. Excitation and emission spectra of Ce3þ, Tb3þ co-doped Ba1.12Ce0.02Tb0.02Li0.04Ca0.8SiO4 sample. The excitation and monitoring wavelengths are listed in the figure.
Fig. 6. Tb3þ concentration dependence of emission spectra for Ce3þ, Tb3þ co-doped Ba1.16-2xCe0.02TbxLi0.02þxCa0.8SiO4 samples and Tb3þ-doped Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample (lex ¼ 345 nm).
which it decreases. As discussed above, the line emissions from 5D3 overlap obviously with the d-f transition emission of Ce3þ. The Ce3þ emission changes its shape with increasing Tb3þ concentrations could also be attributed to the concentration quenching of the 5D3 emission of Tb3þ except for energy transfer. In addition, as can be seen from Fig. 6 that the emissions from Ce I and Ce II sites decrease simultaneously in intensity. The energy transfer form Ce I to Tb3þ cannot be excluded because the absorption bands of Ce I and Ce II sites in the range of 250e350 nm are similar to each other. The emission spectrum of Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample is also presented in Fig. 6. Under 345-nm UV excitation, the emission intensity of 5D4 / 7F5 transition of Tb3þ for Ba1.00Ce0.02Tb0.08Li0.10Ca0.8SiO4 (curve e in Fig. 6) is 30 times stronger than that of Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample (curve g in Fig. 6), which further confirm the energy transfer takes place from Ce3þ to Tb3þ. In short, there is no doubt that the energy transfer from Ce3þ to Tb3þ happens. Whether Ce I or CeII is involved in this energy transfer needs further study.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
On the photoluminescence of multi-sites Ce3þ in T-phase orthosilicate and energy transfer from Ce3þ to Tb3þ Xubo Tong a, Xinmin Zhang a, *, Luyi Wu a, Hongzhi Zhang a, Hyo Jin Seo b a
School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, 410004, China Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan, 608-737, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2018 Received in revised form 8 March 2018 Accepted 20 March 2018 Available online 21 March 2018
Ce3þ, Tb3þ co-doped T-phase orthosilicate Ba1.2Ca0.8SiO4 phosphors were prepared by means of solid state reactions. The synthesized samples were investigated using XRD and PL emission and excitation spectra. The emission spectra for Ce3þ doped Ba1.2Ca0.8SiO4 phosphors show broad bands in the 320 e550 nm spectral region. The excitation spectra exhibit several absorption peaks in the range of 250 e400 nm. Both the emission and excitation spectra are dependent on the excitation or monitoring wavelengths. We attribute these results to Ce3þ ions occupying two different crystallographic sites (i.e., Ce I and Ce II). The Ce3þ, Tb3þ co-doped Ba1.2Ca0.8SiO4 phosphors show Tb3þ-related line emissions in the 475e600 nm spectral region and Ce3þ-related band emissions in the 350e525 nm spectral region when the 4f / 5d transition of Ce3þ is excited (lex ¼ 345 nm), indicating that energy transfer from Ce3þ to Tb3þ takes place. Moreover, the emission intensity of 5D4 / 7F5 transition for Ba1.00Ce0.02Tb0.08Li0.10Ca0.8SiO4 is 30 times stronger than that of Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample under 345-nm UV excitation. © 2018 Elsevier B.V. All rights reserved.
Keywords: T-phase orthosilicate Site occupation Energy transfer Photoluminescence
1. Introduction Recently, white light-emitting-diodes (WLEDs) have received considerable attention due to their energy saving, long lifetime, and other advantages. As we know, the phosphors play an essential role for the phosphor-converted white light-emitting-diodes (pcWLEDs) [1e4]. It can be achieved by combination of a near-UV (nUV) LED chip and tricolor phosphors, each emitting a relatively narrow spectrum of red, green or blue light upon receiving ultraviolet radiation from the LED chip [5,6]. It is well known that rareearth ions play an important role in luminescent materials based on their 4f / 4f or 5d / 4f transitions. The energy levels of the Tb3þ ion arise from the 4f8 configuration. The emission of it is due to 5 D4/ 7FJ transitions which are mainly in the green and shows sharp lines in the spectrum. However, its optical absorption transitions are strongly forbidden by the parity selection rule, resulting in a weak absorption in the n-UV region. The Ce3þ emission occurs from the lowest crystal field component of the 5 d1 configuration to the two levels of the ground state and can vary from long wavelength UV to yellow depending strongly on the host lattice [7e9].
* Corresponding author. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.jallcom.2018.03.252 0925-8388/© 2018 Elsevier B.V. All rights reserved.
The energy transfer between different ion species can take place when they have closed matched energy levels. The energy transfer results either in the enhancement or in the quenching of emission. The Tb3þ emission can be remarkably sensitized by Ce3þ ion, which has been confirmed in LaPO4:Ce, Tb system for a long time. Similar results about energy transfer from Ce3þ to Tb3þ are reported in K2MgSi3O8:Ce3þ,Tb3þ [10], K2Ba3(P2O7)2:Ce3þ,Tb3þ [11], and Ba2Y5B5O17:Ce3þ, Tb3þ [12]. In this study, we try to synthesize and investigate the Tb3þ luminescence and Ce-Tb energy transfer in one of the silicates hosts. Silicates act as the important inorganic mineral materials. The basis for silicate structures is the SieO bond and the [SiO4] tetrahedron. Among them, alkaline earth orthosilicates which can be represented with the general formula M2SiO4 (M ¼ Mg, Ca, Sr, and Ba) including the end members or intermediate compositions. Solid solution is a very common phenomenon in minerals. However, M2SiO4 orthosilicate compounds show diversity of the phases, despite the fact that they often have very similar chemical formulas. The most important factor which controls solid solution formation is the relative size of the cations. For example, Ca2-xSrxSiO4 compounds have a0 H-, a0 L-, b-, and g-phase at room temperature [13]; while five different modulated phases, am-, Y-, X-, a0 L- and T-phase, were found in the Ba2SiO4-Ca2SiO4 system [14]. It is well known
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that the luminescent behaviors of silicates doped with rare earth or transition metal ions have attracted much attention recently years, and the luminescence properties are closely related to the crystal structure of host lattice. Xia reported the structural phase transformation of Ca2-xSrxSiO4:Ce3þ phosphor by Sr2þ substituting for Ca2þ within 0 x < 2 and studied the relationship between structure and property [13]. Recently, orthosilicate-based phosphors doped with Eu2þ and Ce3þ have attracted much attention due to their applications in the field of WLEDs [15]. The modulated Tphase occurs for samples of Ba2-xCaxSiO4 (0.45 x 0.80) and it is one of the most chemically stable phase [16]. The T-phase is made of five Ba/Ca sites. The photoluminescence (PL) properties of Eu2þ, Ce3þ single doped and Eu-Mn, Ce-Mn co-doped T-phase Ba2xCaxSiO4 were reported by Park and Choi [17,18]. However, there are no reports about the detailed PL properties of Ce3þ, Tb3þ, and the energy transfer from Ce3þ to Tb3þ in Ba1.2Ca0.8SiO4. In addition, purity Ba1.2Ca0.8SiO4 is very easy to synthesize. Accordingly, in this paper, Ba1.2Ca0.8SiO4:Ce3þ,Tb3þ phosphors were prepared via the solid state reaction method, and their PL properties, especially for the Ce3þ site occupation and energy transfer from Ce3þ to Tb3þ, were investigated in detail. 2. Experimental
the corresponding standard data for the hexagonal phase of Ba1.2Ca0.8SiO4, indicating that all samples under investigation show to be single phase. The emission spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4 under UV light excitation are given in Fig. 2a. As can be seen in Fig. 2a, the emission spectra show complicated luminescence behavior and are strongly dependent on the excitation wavelengths. The emission spectra under 254, 260, 275, 285, 325 and 330 nm excitation are similar to each other and exhibit broad band extending from 320 to 550 nm. The luminescence is ascribed to the 5d / 4f transition of Ce3þ [19]. A main peak at ~383 nm and two shoulder peaks at ~405 and ~445 nm are observed in the spectra. The emission spectra under 345, 365 and 385 nm excitation exhibit huge difference compared to other emission spectra. The host lattice has a great influence on the spectral position of the Ce3þ emission band [20,21]. So, the complicated emission spectra could result from more than one Ce3þ center [22]. The PL emission spectrum for the sample under 275 nm excitation can be fitted into four Gaussian peaks and the results are shown in the inset of Fig. 2a. The energy difference of two subbands peaking at about 350 and 380 nm is 2000 cm1, corresponding to the 5d excited states to the spin-orbit splitting of the ground states (2F5/2 and 2F7/2). The spectral position of the Ce3þ emission band depends on three factors. Among them, crystal field splitting of the 5 d1 configuration has a great influence on the
Ba1.2Ca0.8SiO4:Ce3þ, Tb3þ phosphors were prepared via a traditional high-temperature solid-state reaction method. BaCO3 (99.9%), CaCO3 (99.9%), Li2CO3 (99.9%), SiO2 (99.9%), CeO2 (99.99%) and Tb4O7 (99.99%) were used as the starting materials. The raw materials Li2CO3 was added as a charge compensation reagent. They were mixed and ground according to the given stoichiometric ratio. The mixtures were prefired at 500 C for 2 h and finally heated at 1100 C for 2 h in a CO reducing atmosphere. The crystal structures of the samples were examined by X-ray diffraction (XRD) analysis using a Beijing Puxi XD-2 diffractometer with monochromatized Cu Ka radiation (l ¼ 0.15418 nm). Excitation and emission spectra were taken on a RF-5301 PC spectrofluorometer equipped with a 150 W Xenon Lamp as the excitation source. 3. Results and discussion XRD patterns of the as-prepared phosphors were collected to verify the phase purity. Typical diffraction patterns are shown in Fig. 1. All the diffraction peaks of the samples are consistent with
Fig. 1. XRD patterns of (a) Ba0.96Ce0.02Tb0.10Li0.12Ca0.8SiO4, (b) Ba1.04Tb0.08Li0.08Ca0.8SiO4, (c) Ba1.0Ce0.10Li0.10Ca0.8SiO4 and (d) the simulated XRD pattern calculated from the structure of Ba1.2Ca0.8SiO4.
Fig. 2. (a) Emission spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4. The inset shows the Gaussian fit of emission spectrum excited by 275-nm UV light; (b) Excitation spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4. The excitation and emission wavelengths are listed in the figures.
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emission wavelength. The bond length (R) of Ba/Ca-O affects the crystal field strength (Dq) significantly, i.e., Dq is proportional to 1/ R5 [23]. The Ba/Ca sites with big coordination numbers will feel weak crystal field strength, whereas those with small coordination numbers will feel strong crystal field strength. When the doped Ce3þ incorporated into different Ba/Ca sites, the crystal field splitting D is different, and then Ce3þ will show different emissions. There are five Ba/Ca sites in the crystal structure of Ba1.2Ca0.8SiO4 (i.e., one site having 6-fold coordination, two sites having 10-fold coordination and two sites having 12-fold coordination) [18]. We conclude that the doped Ce3þ could substitute for all the Ba/Ca sites, and mark as Ce I site for those at 10/12-coordination Ba/Ca sites and Ce II site for those at 6-coordination Ba/Ca sites. To be specific, the two subbands peaking at 350 and 380 nm originate from d-f transition of Ce I site, and the other two subbands peaking at about 400 and 430 nm are ascribed to d-f transition of Ce II site. In short, the doped Ce3þ ions can occupy two different sites at the same time. The excitation spectra of Ba1.18Ce0.01Li0.01Ca0.8SiO4 for different emission wavelengths are given in Fig. 2b. The excitation spectra are also dependent on monitoring wavelengths. When different emission wavelengths are monitored, two groups of excitation spectra are observed. One is located in the wavelength range 250e350 nm (Group I, lem ¼ 340, 360 and 380 nm) and another is in the wavelength range 250e400 nm (Group II, lem ¼ 430, 435, 445 and 480 nm). The excitation spectra of Group I can be ascribed to the 4f / 5d transitions in Ce I site, while that of Group II can be ascribed to the 4f / 5d transitions in Ce II site. The others are the overlap of the excitation spectra of Ce I and Ce II (lem ¼ 400, 410 and 420 nm). Moreover, all the excitation spectra show several subbands. The sub-bands of Group I peak at 260 (38460), 275 (36360) and 325 nm (30770 cm1), while those of Group II peak at 255 (39215), 285 (35090), 330 (30300) and 365 nm (27400 cm1). Based on the fact that two groups excitation spectra are obtained when different emission wavelengths are monitored, it further confirms that two different Ba/Ca sites are available for the doped Ce3þ ions to occupy. The lowest crystal field component of the 5d1 configuration of Ce I and Ce II sites situate at about 30770 cm1 (325 nm) and 27400 cm1 (365 nm), respectively. The emission spectra for Ba1.2-2xCexLixCa0.8SiO4 at different Ce3þ doping concentrations under 325 nm UV excitation are shown in Fig. 3. No obvious difference is observed for the emission spectra except for the relative intensity. The optimum concentration is to be 0.02.
3þ
Fig. 3. Ce concentration dependence of the emission spectra for Ba1.2-2xCexLixCa0.8SiO4 (lex ¼ 325 nm).
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The excitation and emission spectra of Ba1.04Tb0.08Li0.08Ca0.8SiO4 are shown in Fig. 4. The emission spectrum consists of a series of sharp emission lines in the 350e650 nm spectral region. These emission lines are attributed to the 4f8 / 4f8 transitions of Tb3þ [24]. The excitation spectra for the 367 and 540 nm emissions are similar to each other. The weak absorption peaks, corresponding to the parity-forbidden 4f8 / 4f8 transitions, can be observed in the 300e400 nm spectral region. The absorption peaks for the 4f8 / 4f75d transitions are also observed in the wavelength region below ~300 nm. This region can be divided into two regions: spin-allowed transition region (<275 nm) and spin-forbidden transition region (275e300 nm). The transition probability of spin-allowed transitions is higher than that of spin-forbidden ones and thus provide relatively strong intensities. The excitation and emission spectra of Ba1.12Ce0.02Tb0.02Li0.04Ca0.8SiO4 sample are presented in Fig. 5. When excitation is carried out with 345 nm UV light lying within the 4f / 5d absorption band of Ce3þ ion, the emission spectrum consists of broad band emission, corresponding to the 5d / 4f transitions of Ce3þ ions, and line emissions corresponding to the 5D4 / 7FJ of Tb3þ ions simultaneously. The 5D3 / 7FJ emissions overlap with the 5d / 4f transitions of Ce3þ ions. The result indicates that energy transfer from Ce3þ to Tb3þ takes place [25e27]. The excitation spectra for different emissions are different. The excitation spectra of the Tb3þ 5 D4 emissions (lem ¼ 545 and 485 nm) are nearly consistent with that of the Ce II emission (lem ¼ 450 nm) in the 260e400 nm spectral region. Identity of the excitation spectra for these two kinds of emissions further confirms that the energy transfer could occur from Ce II to Tb3þ in Ce3þ, Tb3þ co-doped T-phase Ba1.2Ca0.8SiO4 crystal. In order to further investigate the energy transfer process between the Ce3þ and Tb3þ ions in the host lattice, a series of Ce3þ, Tb3þ co-doped Ba1.16-2xCe0.02TbxLi0.02þxCa0.8SiO4 samples were prepared. The concentration of Ce3þ was fixed at the optimal value of 0.02 and the content of Tb3þ was varied in the range of 0.01e0.10. The emission spectra for the samples under 345 nm-UV excitation are shown in Fig. 6. With increasing Tb3þ concentration, the relative emission intensities of the Ce3þ ions decreased remarkably which is ascribed to the enhancement of energy transfer from Ce3þ to Tb3þ ions [10,28]; whereas the relative emission intensities of Tb3þ increase initially and reach a maximum value at x ¼ 0.08, beyond
Fig. 4. Excitation and emission spectra of Ba1.04Tb0.08Li0.08Ca0.8SiO4. The excitation and monitoring wavelengths were: (a) lex ¼ 250 nm; (b) lem ¼ 367 nm and (c) lem ¼ 540 nm. The inset shows the enlarged zone of excitation spectrum range from 320 to 400 nm.
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4. Conclusions We prepared Ce3þ, Tb3þ co-doped Ba1.2Ca0.8SiO4 phosphors by the solid state reactions. The synthesized phosphors were investigated using XRD and PL emission and excitation spectra. For the Ce3þ singly doped Ba1.2Ca0.8SiO4 phosphors, both the emission spectra and the excitation spectra are dependent on the excitation or monitoring wavelength, indicating that the Ce3þ ions occupy two different crystallographic sites (i.e., Ce I and Ce II). Under 345nm UV excitation, the emission intensity of 5D4 / 7F5 transition of Tb3þ in the Ce3þ, Tb3þ co-doped phosphor is 30 times stronger than that in Tb3þ singly doped sample, which indicates the occurrence of an efficient energy transfer from Ce3þ to Tb3þ. Whether Ce I or CeII is involved in this energy transfer needs further study. Acknowledgements
Fig. 5. Excitation and emission spectra of Ce3þ, Tb3þ co-doped Ba1.12Ce0.02Tb0.02Li0.04Ca0.8SiO4 sample. The excitation and monitoring wavelengths are listed in the figure.
Fig. 6. Tb3þ concentration dependence of emission spectra for Ce3þ, Tb3þ co-doped Ba1.16-2xCe0.02TbxLi0.02þxCa0.8SiO4 samples and Tb3þ-doped Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample (lex ¼ 345 nm).
which it decreases. As discussed above, the line emissions from 5D3 overlap obviously with the d-f transition emission of Ce3þ. The Ce3þ emission changes its shape with increasing Tb3þ concentrations could also be attributed to the concentration quenching of the 5D3 emission of Tb3þ except for energy transfer. In addition, as can be seen from Fig. 6 that the emissions from Ce I and Ce II sites decrease simultaneously in intensity. The energy transfer form Ce I to Tb3þ cannot be excluded because the absorption bands of Ce I and Ce II sites in the range of 250e350 nm are similar to each other. The emission spectrum of Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample is also presented in Fig. 6. Under 345-nm UV excitation, the emission intensity of 5D4 / 7F5 transition of Tb3þ for Ba1.00Ce0.02Tb0.08Li0.10Ca0.8SiO4 (curve e in Fig. 6) is 30 times stronger than that of Ba1.04Tb0.08Li0.08Ca0.8SiO4 sample (curve g in Fig. 6), which further confirm the energy transfer takes place from Ce3þ to Tb3þ. In short, there is no doubt that the energy transfer from Ce3þ to Tb3þ happens. Whether Ce I or CeII is involved in this energy transfer needs further study.
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