Photoluminescence properties of Tb3+ and Ce3+ co-doped Sr2MgSi2O7 phosphors for solid-state lighting

JOURNAL OF RARE EARTHS, Vol. 33, No. 4, Apr. 2015, P. 366

Photoluminescence properties of Tb3+ and Ce3+ co-doped Sr2MgSi2O7 phosphors for solid-state lighting YU Hong (Ѣ⊧)1, CHEN Jinlei (䰜䞥⺞)2, PU Yong (㪆࢛)1,*, ZHANG Tiejun (ᓴ䪕‫)ݯ‬1, GAN Shucai (⫬ᷥᠡ)2,* (1. Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China; 2. College of Chemistry, Jilin University, Changchun 130026, China) Received 26 May 2014; revised 12 January 2015

Abstract: Sr2MgSi2O7:Tb3+,Ce3+ phosphors were synthesized by solid-state reaction and placed in a muffle furnace in a reducing atmosphere at 1300 ºC for 3 h. Photoluminescence properties and energy transfer were investigated. The Ce3+/Tb3+ energy transfer was thoroughly investigated by their emission/excitation spectra and photoluminescence lifetime, there was shortened lifetime of Ce3+ (from 51.31 to 50.06 ns) which could support evidence of energy transfer from Ce3+ to Tb3+ in the host. The varied emitted color of Sr1.97–yMgSi2O7:0.03Tb3+,yCe3+ phosphors could be achieved by altering the concentration of Ce3+, the chromaticity coordinates (x, y) varied from (0.225, 0.376) to (0.172, 0.231). In Sr1.96MgSi2O7:0.03Tb3+,0.01 Ce3+ phosphors, the results indicated that Sr2MgSi2O7:Tb3+,Ce3+ might be useful as tunable phosphors for ultraviolet white-light-emitting diodes. Keywords: Sr2MgSi2O7:Tb3+,Ce3+; photoluminescence; phosphor; energy transfer; rare earths  

White-light-emitting diodes (W-LEDs) have been paid more and more attention to due to the advantage of longer lifetime, higher rendering index, higher luminosity efficiency and lower energy consumption[1,2]. The most common method is realized by combining the blue light of GaN chips and the yellow emission of YAG:Ce3+ [3,4] , however, the type of white color varies with the input power and a poor color rendering index (Ra<80). Many efforts have been made to overcome these disadvantages mentioned above, novel phosphor materials which can be effectively excited by ultraviolet or blue light and emit strong blue, green, red light, gained importance[5]. Tricolor phosphors with high stability and intense absorption in UV spectral region are in demand to meet the optimum requirements of W-LEDs. It is well-known that Tb3+ ions as the significant luminescent activators in advanced lighting and display, shows the characteristic 4f-4f transitions[6], Ce3+ ions may act as highly efficient activators as the result of 4f-5d transition of Ce3+ ions that have been widely investigated. The 5d-4f emissions of Ce3+ ion can vary from long wavelength UV to red emission region dependent on the host composition, crystal structure, the lattice symmetry. The emission could easily shift, such strong host dependence of Ce3+ ions leads to its poor reproduction quality of optical properties of a phosphor[7]. Tb3+ ions have good optical properties in the green spectral region but have weak absorption in the UV region of

300–410 nm which does not match well with the UV chip, Ce3+ ions co-doped in the hosts could not only increase the intensity of Tb3+ ions but also can excited in the UV region of 300–410 nm, thus, it could overcome the drawbacks of solely doped Ce3+ ions or Tb3+ ions, generally, the Ce3+ ions can act as sensitizer in most hosts and lead to the energy transfer from Ce3+ to Tb3+. Tb3+ and Ce3+ co-doped phosphors have been investigated in some hosts[8, 9]. Silicate as the host of the phosphor material has the advantage of their outstanding thermal, chemical, and mechanical stability and structural diversity[10]. Recently, silicate compounds have been extensively studied as host lattices for phosphors when activated by Tb3+ and Ce3+ [11,12]. As far as we know, Sr2MgSi2O7 as a stable silicate host, their luminescence property and the energy transfer after being co-doped with Tb3+ and Ce3+ have not yet been investigated. In the present study, the Sr2MgSi2O7:Tb3+,Ce3+ phosphors were investigated by the emission/excitation spectra, photoluminescence decay behaviors. They could be effectively excited under the UV region and showed green to blue emission band achieved by altering the concentration ratio of Ce3+ to Tb3+. This suggests that Sr2MgSi2O7:Tb3+,Ce3+ phosphors may be a promising phosphor candidate in UV white LEDs.

1 Experimental

Foundation item: Project supported by Mineral and Ore Resources Comprehensive Utilization of Advanced Technology Popularization and Practical Research (MORCUATPPR), funded by China Geological Survey (12120113088300) * Corresponding authors: PU Yong, GAN Shucai (E-mail: [email protected], [email protected]; Tel.: +86-23-49891910, +86-432-88502259) DOI: 10.1016/S1002-0721(14)60428-2

YU Hong et al., Photoluminescence properties of Tb3+ and Ce3+ co-doped Sr2MgSi2O7 phosphors for solid-state lighting 367

1.1

Preparation of Sr2–x–yMgSi2O7:xTb3+,yCe3+ (x= 0.01–0.09, y=0; x=0, y=0.01; x=0.03, y=0.005– 0.05) samples

Sr2–x–yMgSi2O7:xTb3+,yCe3+ (x=0.01, 0.03, 0.05, 0.07, 0.09, y=0; x=0, y=0.01; x=0.03, y=0.005, 0.01, 0.03, 0.05) were prepared by a solid-state reaction technique. Analytical reagent grade (99.90%) SrCO3, (MgCO3)4·Mg(OH)2·5H2O, SiO2 and spectrographically pure (99.99%) CeO2, Tb4O7 were employed as reactants. Reactant samples were first quantified by the stoichiometric ratio and then thoroughly mixed by grinding them in an agate mortar for 2 h. The mixture was placed in a small corundum crucible, which was then transferred to a large corundum crucible, the space between the two corundum crucibles was filled with powdered graphite. Then the large corundum crucible was placed in a muffle furnace in a reducing atmosphere at 1300 ºC for 3 h. Finally, the large corundum crucible was cooled to room temperature and the phosphor samples were obtained. 1.2 Apparatus and measurements The powder X-ray diffraction (XRD) patterns were recorded with a Bruker D8-advance X-ray powder diffractometer with Cu KĮ radiation (Ȝ=0.15406 nm), the operation voltage and current were maintained at 40 kV and 40 mA, respectively. A scan rate of 2(°)/min was applied to record the patterns in the range of 2ș=10°–50°. The excitation and emission spectra were measured by a spectrofluorophotometer RF-5301PC series equipped with a 150 W xenon lamp. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz). All the experiments were performed at room temperature.

2 Results and discussion 2.1 Characteristic analysis of XRD The XRD patterns of Sr2–x–yMgSi2O7:xTb3+,yCe3+ phosphor samples with different Tb3+ and Ce3+ concentrations are shown in Fig. 1. All of the diffraction peaks are in accord with Sr2MgSi2O7 (JCPDS card No. 15-0016). All these samples are of single phase without any impurities. This indicates that doping of Tb3+ and Ce3+ in Sr2–x–yMgSi2O7 host with such a small concentration has no other phase specific changes. The crystal structure of Sr2MgSi2O7 has been refined to tetragonal, the corresponding lattice parameters are a=0.8025 nm, b=0.8025 nm, c=0.5181 nm, Z=2, V=0.33366 nm3. The approximate ionic radii of Tb3+ (0.092 nm) and Ce3+ (0.101 nm) are similar to that of Sr2+ (0.118 nm) which are much larger than those of Mg2+ (0.072 nm) and Si4+ (0.040 nm), possibly, the Tb3+ and Ce3+ ions both preferably occupy the Sr2+ ions[13].

Fig. 1 Typical XRD patterns of Sr2MgSi2O7:Tb3+,Ce3+ samples with different Ce3+ and Tb3+ concentrations

2.2 Photoluminescence of Sr2–xMgSi2O7:xTb3+ phosphors The PL excitation and emission spectra of Tb3+ doped Sr1.97MgSi2O7 phosphor samples with 3 mol.% concentration are presented in Fig. 2. Fig. 2(1) shows the excitation spectrum of the sample that monitored at the emission of 542 nm which consists of the band in the range of 220 to 350 nm with the maximum at 228 nm which is attributed to the 4f to 5d transition. Fig. 2(2) presents the emission spectrum of the sample excited by 228 nm which covers the region from 400 to 700 nm consisting of the characteristic transitions of Tb3+, namely, 5D4ĺ7F6 (488 nm), 5D4ĺ7F5 (542 nm), 5D4ĺ7F4 (584 nm) and 5 D4ĺ7F3 (620 nm), among these characteristic peaks of Tb3+, the emission of the 5D4ĺ7F5 transition at 542 nm is predominantly higher which can be illustrated by the large values of the reduced matrix elements at J=5 and the Judd-Ofelt theory[14]. Fig. 3 shows the emission intensity of Sr2–xMgSi2O7: xTb3+ (x=0.01–0.09) with different concentrations under the excitation of 228 nm. The emission intensities of all of the emission are enhanced significantly with the Tb3+ concentration increasing, then gradually decrease as the doping concentration becomes higher than 0.03. With

Fig. 2 Photoluminescence excitation (1) and emission (2) spectra of Sr1.97MgSi2O7:0.03Tb3+

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the addition of Tb3+ ions increasing, the distance of Tb3+ ion and Tb3+ ion becomes smaller, which leading to the high probability of energy transfer among the Tb3+ ions, thus, the loss of energy increases and lead the emission intensity reduced, namely, the concentration quenching occurred. Therefore, the optimum doping concentration of Tb3+ is fixed at 3 mol.%. 2.3 Photoluminescence of Sr2–x–yMgSi2O7:xTb3+,yCe3+ phosphors Fig. 4 presents the emission spectra of Sr2–x–yMgSi2O7: xTb3+,yCe3+ phosphors (x=0.03, y=0.005, 0.01, 0.03, 0.05; x=0, y=0.01). The inset figure in Fig. 4 shows the emission and excitation spectra of Sr1.99MgSi2O7:0.01Ce3+. Under the excitation of 332 nm, the emission spectra of Sr1.97–yMgSi2O7:0.03Tb3+,yCe3+ (y=0.005–0.05) show not only the band of Tb3+ ions but also the band of Ce3+ ions. With increasing of Ce3+ content, the emission intensity of the Tb3+ ions increases, then decreases as the doping concentration is higher than 0.01. It occurs concentration

JOURNAL OF RARE EARTHS, Vol. 33, No. 4, Apr. 2015

decreases as the doping concentration becomes higher quenching when the optimum doping concentration of Ce3+ is above 1 mol.%. The results demonstrate that the energy transfer from Ce3+ to Tb3+ occurred. Simultaneously, the luminescence intensity of Ce3+ in Sr1.99MgSi2O7:0.01Ce3+ phosphors is 583 a.u., while in the Sr1.96MgSi2O7:0.03Tb3+,0.01Ce3+ is 502 a.u.. It can be seen that the intensity of Ce3+ decreased after Tb3+ doped. We calculate the quantum efficiency of Sr2MgSi2O7:Ce3+ and the result is 62.3%. The calculate methods we used are calculating integral area of the emission peak and the classical theory of quantum conversion efficiency, and we made a comparison with the commercial blue phosphors (BaMgAl10O7:Eu2+) we purchased which quantum efficiency is 78.8%. 2.4 Energy transfer from Ce3+ to Tb3+ Fig. 5 shows the excitation spectra of Sr1.97–yMgSi2O7: 0.03Tb3+,yCe3+ (y=0.005–0.05) monitored at 542 nm. It can be seen clearly that all the excitation spectra consist of both the excitation band of Tb3+ and Ce3+, which also could indicate the presence of the energy transfer from Ce3+ to Tb3+ ions. The concentration quenching is due to energy transfer from one activator (donor) to another until the energy sink (acceptor) in the lattice is reached. Hence, the energy transfer will strongly depend on the distance (Rc) between the Tb3+ and Ce3+, which can be obtained using the following equation[15]

ª 3V º Rc | 2 « » ¬ 4S& c Z ¼

Fig. 3 Photoluminescence emission spectra of Sr2–xMgSi2O7: xTb3+ with different Tb3+ concentrations under the excitation of 228 nm (The inset shows the 488 and 542 nm emission intensities of Sr2–xMgSi2O7:xTb3+ (x=0.01– 0.09))

Fig. 4 Photoluminescence emission (Ȝex=332 nm) spectra of Sr2–x–yMgSi2O7:xTb3+,yCe3+ (The inset shows the excitation and emission spectra of Sr1.99MgSi2O7:0.01Ce3+)

1

3

(1)

where Xc is the total critical concentration of Tb3+ and Ce3+ (Xc=0.04), Z is the number of formula units in the Sr2MgSi2O7 unit cell (Z=2) and V is the volume of the unit cell (V=0.33366 nm3 in this case). The critical distance (Rc) between the donor and acceptor can be calculated from the critical concentration, for which the nonradiative transfer rate equals the internal decay rate (radiative rate). Blasse[16] assumed that, for the critical con-

Fig. 5 Photoluminescence excitation (Ȝem=542 nm) spectra of Sr1.97–yMgSi2O7:0.03Tb3+,yCe3+ (y=0.005–0.05)

YU Hong et al., Photoluminescence properties of Tb3+ and Ce3+ co-doped Sr2MgSi2O7 phosphors for solid-state lighting 369

centration, the average shortest distance between the nearest activator ions is equal to the critical distance. By taking the experimental and analytic values of V, N and Xc (0.33366 nm3, 2, 0.04, respectively), the critical distance Rc is estimated by Eq. (1) to be about 1.997 nm. 2.5 Photoluminescence lifetimes of Sr2–x–yMgSi2O7: xTb3+,yCe3+ phosphors To further investigate whether the energy transfer between Tb3+ and Ce3+ exist, the photoluminescence decay curves of Tb3+ and Ce3+ ions doped in Sr2–x–yMgSi2O7: xTb3+,yCe3+ phosphors are monitored at the emission peak wavelength 456, as shown in Fig. 6. For Ce3+, either it singly doped or co-doped with Tb3+ in the host are shown in Fig. 6(a) and (b), their decay curves can be well fitted by a double exponential function: I=A1exp(–t/t1)+A2exp(–t/t2) (2) where t1 and t2 are two components of the luminescence lifetime, A1 and A2 are the fitting parameters. The lifetime of Ce3+ could be evaluated by t=(A1t12+A2t22)/(A1t1+ A2t2)[17], where t is defined as the average lifetime. After calculating, the lifetime are 51.31 and 50.06 ns. There is shortened lifetime of Ce3+ (from 51.31 to 50.06 ns). The photoluminescence decay measurements again further support evidence of energy transfer from Ce3+ to Tb3+ in the host. Fig. 7 confirmes the process of the energy transfer from Ce3+ to Tb3+. Under the excitation of 332 nm, Ce3+ ions absorbed energy from the ground state 2FJ excited to the 5d level, and via nonradioactive resonance transfer,

the energy of excitation state directly to the 5D3 and 5D4 energy level of Tb3+ ions. The 5D3 energy level to the 5D4 energy level by nonradioactive relaxation, then energy transfer from 5D4 to 7FJ leading to emitting. The 5D4ĺ7F5 transfer emission of Tb3+ could obviously increased via the transfer from Ce3+ to Tb3+. 2.6 Chromaticity coordinates of Sr2–x–yMgSi2O7:xTb3+, yCe3+ phosphors The chromaticity coordinates of Sr2–x-yMgSi2O7:xTb3+, yCe3+ phosphors (x=0.03, y=0–0.05; x=0, y=0.01) are presented in Fig. 8. From Fig. 8, with the increasing of Ce3+ content, the chromaticity coordinates gradually move from green to blue areas. The chromaticity coordinates (x, y) varying systematically are (0.225, 0.376), (0.205, 0.318), (0.166, 0.194), (0.185, 0.275), (0.172, 0.231) and (0.148, 0.120), respectively. Corresponding color tone of the samples changes gradually from green to bluish green and eventually to blue. By altering the

Fig. 7 Partial energy level and visible emission transitions of Ce3+ and Tb3+

Fig. 6 Photoluminescence decay curves of Ce3+ (a, b) in Sr2–x–yMgSi2O7:xCe3+,yTb3+ phosphors

Fig. 8 Chromaticity coordinates of Sr1.97-yMgSi2O7:xTb3+,yCe3+ (y=0–0.09) under 332 nm excitation (1) x=0.03, y=0; (2) x=0.03, y=0.005; (3) x=0.03, y=0.01; (4) x= 0.03, y= 0.03; (5) x=0.03, y=0.05; (6) x=0, y=0.01

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concentration of Ce3+, the tunable emission properties of the Sr1.97–yMgSi2O7:0.03Tb3+,yCe3+ samples can be obtained. The Sr1.96MgSi2O7:0.03Tb3+,0.01Ce3+ phosphors emit blue light during excitation with an UV wavelength of 332 nm, which is accessible by UV LEDs. This result indicates that the Sr2MgSi2O7:Tb3+,Ce3+ phosphor may act as the blue light emitting phosphor for the development of W-LEDs under UV excitation.

3 Conclusions In summary, the photoluminescence properties and the Ce3+/Tb3+ energy transfer of the Sr2MgSi2O7:Tb3+,Ce3+ phosphors investigated. Energy transfer from Ce3+ to Tb3+ was confirmed by the evidences collected from the emission, excitation spectra, and photoluminescence decay curves. After Ce3+ co-doping, the emission intensity of Tb3+ ions increased gradually, then decreased as the doping concentration became higher than 0.01. The luminous color transferred from green to blue. The fluorescence lifetime of Ce3+ was decreased which provided the indirect evidence of the Ce3+/Tb3+ energy transfer. The results indicated that the Sr2MgSi2O7:Ce3+,Tb3+ phosphor might act as potential tunable phosphor for UV-LEDs. Acknowledgements: The work was also supported by Foundation of Chongqing University of Arts and Sciences (R2013CJ09, Y2013CJ28, Y2013CJ30, R2012CJ19).

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