Effect of NH 4 F Flux on Structural and Luminescent Properties of Sr 2 SiO 4 :Eu 2+ Phosphors Prepared by Solid-State Reaction Method

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1 J /1/157 8 /J31/5/$28. The Electrochemical Society Effect of F Flux on Structural and Luminescent Properties of :Eu 2+ Phosphors Prepared by Solid-State Reaction Method Hai Guo, a,z XiaoFeng Wang, a,b XinBo Zhang, a YueFeng Tang, a LingXia Chen, a and ChongGen Ma c a Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang 3214, People s Republic of China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 3127, People s Republic of China c College of Mathematics and Physics, Chongqing University of Post and Telecommunications, Chongqing 65, People s Republic of China :Eu 2+ phosphors were prepared by a conventional solid-state reaction method at a relatively low temperature 11 C by using F as a flux. The effect of F flux and Eu 2+ doping concentrations on the structural and luminescent properties of :Eu 2+ was investigated in detail. The structural properties were studied by X-ray diffraction and a field-emission-scanning electron microscope. The blueshift of emission peaks with different Eu 2+ concentrations was observed and analyzed. The F flux increased the content of monoclinic - phase and enhanced the luminescent performance of :Eu 2+ phosphor significantly, and the F flux is more superior to the Cl and Li 2 CO 3 fluxes. Our investigation indicates that :Eu 2+ is a good light-conversion phosphor candidate for near-uv chip and that F is a good flux for - :Eu The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted December 18, 9; revised manuscript received May 24, 1. Published June 3, 1. White light-emitting diodes LEDs, the next generation of solidstate lighting, have attracted significant attention recently due to their potential application in many fields, such as device indicators, backlights, automobile headlights, general illumination, etc They show many advantages over the existing incandescent and halogen lamps in terms of power efficiency, reliability, and long lifetime Moreover, they present promising features such as compactness, environment friendliness, and low maintenance. There are two strategies of producing a high intensity white light using LEDs. 8,12,17,18 One is to use individual LEDs that emit three primary colors, red, green, and blue, and then to mix all the colors to produce a white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to a broadspectrum white light, much in the same way a fluorescent light bulb works. At present, the common way to generate white light is through a combination of blue LED with yellow-emitting phosphor materials. It is very important to design new light-conversion phosphors for solid-state lighting which can be excited under near-uv and blue regions. Consequently, many light-conversion phosphors have been reported in recent years, such as Eu 2+ -activated binary and ternary silicates, 7-1,19-21 Eu 2+ -doped nitridosilicates and oxotridosilicates, 11-13,22,23 Sr,Ca 2 P 2 O 7 :Eu 2+,Mn 2+, 24 Sr 3 Al 2 O 5 Cl 2 :Ce 3+,Eu 2+, 25 Ca 2 GeO 4 :Ce 3+,Li +, 26 etc. Among all these phosphors, :Eu 2+ has attracted great interest because it absorbs UV radiation and emits a white light. It has been reported that :Eu 2+ phosphors show a stronger yellow emission than the conventional yttrium aluminum garnet:ce phosphors. 9 Besides, due to its special structural features and excellent physical and chemical stability, it is expected that the :Eu 2+ phosphors may be applied in white LED phosphors. A solid-state reaction method is usually used to prepare :Eu 2+ phosphors. In this method, the reactants are mixed thoroughly and the mixture is then heated or fired under an appropriate atmosphere. To ensure the proper reactivity between the constituents of the phosphor, the reactants are generally granular with radii in the micrometer range. To further facilitate the reaction and to improve the crystallinity of the luminescent materials, flux agents or molten salts are often added to provide a more interactive medium for the reaction. 21,27,28 The use of an interaction medium often results in lower reaction temperatures and allows for the optimization of the grain size of the luminophors being synthesized. z ghh@zjnu.cn A suitable flux increases the reactivity of the constituents by dissolving at least one of the reactants and providing a medium to incubate the crystallization of the phosphor. The amounts used are typically small, on the order of a few mole percent or a few weight percent. In previous literatures, boric acid 18,29,3 and lithium carbonate 12,31 were important fluxes in the preparation of :Eu 2+ phosphors. Subsequently, Cl was found to be an excellent flux for :Eu 2+, 2 but using F as a flux for :Eu 2+ has scarcely been reported. In this paper, we synthesized :Eu 2+ powders by a solidstate reaction method using F as a flux. The influence of the F flux on the structural and luminescent properties of :Eu 2+ was studied in detail and the results show that the F flux is superior to the Cl and Li 2 CO 3 fluxes for elaborate :Eu 2+ phosphors. Furthermore, an energy level scheme of Eu I and Eu II in :Eu 2+ was developed to briefly explain the luminescent properties of :Eu 2+. Experimental The powder samples of :Eu 2+ were prepared using a conventional high temperature solid-state method. Eu 2 O 3 with purity of 99.99% was purchased from Shanghai Yuelong Nonferrous Metals Co., Ltd., China. SrCO 3, SiO 2,HNO 3, F, Cl, Li 2 CO 3, and H 3 BO 3 all with purity of analytical reagent were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Briefly, a1mol/l Eu NO 3 3 solution was first prepared by dissolving Eu 2 O 3 in hot nitric acid.2 mol/l. Stoichiometric amounts of the SrCO 3, SiO 2, and Eu NO 3 3 solution and different kinds of fluxes F, Cl, Li 2 CO 3, and H 3 BO 3 were uniformly ground in an agate mortar and then dried at 1 C in air for about 12 h to evaporate the solvent from the Eu NO 3 3 solution. Then the mixtures were heat-treated at 11 C for 4hinareducing atmosphere a mixture of 5% H 2 and 95% N 2 by a Nabertherm R8/75/12- P3 tube furnace. The crystalline structures of the prepared powders were investigated by X-ray diffraction XRD on a Philips X Pert PRO SUPER XRD apparatus with Cu K radiation =.1556 nm as the incident radiation. A morphology study of :Eu 2+ powders was performed on a Hitachi S-48 field-emission-scanning electron microscope FESEM. An electron beam with 5 kv acceleration voltage and 1 A current was used for imaging. The sample was gold-sputtered before the FESEM analysis. The excitation and emission spectra were measured by using an FS9 spectrofluorometer

2 J311 (31) FXwt% (h) (g) (f ) (e) (d) (c) (b) 8% 7% 6% 5% 4% 3% 2% (a) 1% No flux α ' - β Figure 1. XRD pattern of :Eu 2+.6 mol % powders fired at 11 C with various F contents: a 1, b 2, c 3, d 4, e 5, f 6, g 7, and h 8wt%. Edinburgh Instruments with a continuous wave Xe lamp 45 W as the excitation light source and an RR928P photomultiplier for signal detection. All the measurements were carried out at room temperature. Results and Discussion Orthorhombic - and monoclinic - are two different crystallographic modifications of. In the structure of - and -,+ ions are located at two kinds of inequivalent lattice sites, and their coordination numbers are 9 and 1, respectively. When the Eu 2+ ions are introduced into the matrix, they take the place of +. Besides, the Eu 2+ ions in the - and or - matrix show almost the same luminescent spectra in terms of shape, but - :Eu 2+ has a higher luminescent efficiency. 21,32 Therefore, - is very crucial for practical applications and is an effective method to improve luminescent efficiency. Figure 1 shows the XRD patterns of the :Eu 2+.6 mol % powders fired at 11 C with various amounts of F flux as well as the standard JCPDS card no for - and no for - as reference. Different amounts of flux have different effects on the structure of :Eu 2+. Without an F flux, the sample s crystal structure is a mixed phase composed of - and - phases, and the phase is dominant. When the amount of Fis less than 3 wt % Fig. 1a-c, mixed-phase phosphors are also obtained, and the intensities of the diffraction peaks of the - phase gradually decrease with increasing F flux. When the amount of F is from 4 to 6 wt % Fig. 1d-f, almost pure - phase phosphors without an impurity phase are prepared. With more F more than 7 wt %, Fig. 1g and h, weak - peaks appear again. The purity of the phase can be calculated qualitatively according to the formula 21 = I / I + I 1 Table I. The = I Õ I + I,, and as a function of various F contents in :Eu 2+ (.6 mol %) powders. F content wt % range of less than 6 wt % and then decreases with a further increase in flux amount. XRD patterns indicate that the optimal dosage of the F flux is about 6 wt % for the preparation of monoclinic - :Eu 2+ phosphors and excess flux leads to the formation of an unintended phase during the reaction process. The photoluminescent PL properties of phosphors included in white LEDs are importantly correlated with the morphology and grain size of the phosphor powder. Figure 2 gives the FESEM image of the :Eu 2+.6 mol % powders fired at 11 C with 6 wt % F flux. It indicates that the :Eu 2+ phosphors have a good dispersion and a relatively regular shape and that the average particle size of the phosphors is about 8 m, which favors its practical applications. Figure 3 presents the PL spectra of :Eu 2+ fired at 11 C for4hwithdifferent amounts of F flux excited by 41 nm. The mass content of F varies from to 8 wt %. The broad emission bands are ascribed to a strong coupling interaction between the Eu 2+ ions and the host matrix, which can be attributed to the lowest 5d level to 4f 8 S 7/2 ground state transition of the Eu 2+ ions. The F flux can greatly enhance the luminescent intensities of :Eu 2+ phosphors. The luminescent intensities increase with the content of F and reach a maximum when the content of F is 6 wt %; such phenomenon is consistent with the XRD patterns. The luminescent intensities improve about five times by adding 6 wt % F flux. The effect of fluorescence enhancement is very significant and proves that the F flux is helpful in enhancing the crystallization degree and decreasing surface defect. where I and I are integral peak intensities of 112 for the phase and 31 for the phase in the XRD patterns, respectively. These two peaks are most suitable to discriminate the and phases from each other. The results are listed in Table I, which clearly shows that increases with an increase in flux content in the Figure 2. FESEM images of :Eu 2+ powders fired at 11 C with 6 wt% F flux.

3 J % 5% 4% 3% 2% 1% FXwt% Table I gives the relationship of,, and as a function of various F contents in :Eu 2+.6 mol % powders. It indicates that the PL intensities are strictly dependent on the value of. We can conclude that the enhancement of the luminescence properties is due to the increase in the phase. To investigate the contribution of Eu 2+ ions toward the structural and luminescent properties of the :Eu phosphors, we synthesized powders with 6 wt % F flux by varying the concentration of Eu 2+. Figure 4 shows the XRD patterns of doped with different Eu 2+ contents. Table II presents the relationship of = I / I + I,, and as a function of Eu 2+ concentrations. The value of increases gradually when the concentration of Eu 2+ changes from.1 to.6 mol %. When the concentration of Eu 2+ is.6 mol %, almost reaches 1, which means that the pure - is prepared Fig. 4e. The weak phase appears again when the Eu 2+ ion content is further increased. The corresponding PL spectra also drastically depend on the value of, as given in Fig. 6% 7% 8% :Eu 2+.6mol% Figure 3. Emission spectra of :Eu 2+ with various F contents and ex = 41 nm. :Eu 2+ Xmol% (h) 1. % (g).8 % Table II. The = I Õ I + I,, and as a function of various Eu 2+ concentrations in powders with 6wt% F flux. Eu 2+ concentration mol % and Table II. The PL intensities initially increase with Eu 2+ content, reaching a maximum at.6 mol %, and then decrease as the Eu 2+ concentration is further increased. The decrease in PL intensities may be caused by concentration quenching and or the decrease in value of. The change in Eu 2+ content from.6 to.7 mol % is very small and the Eu 2+ concentration is quite low. As a result, the concentration quenching is almost impossible to occur. We can say that the decrease in PL intensities with the Eu 2+ concentration mostly depends on the value of with Eu 2+ content from.6 to.7 mol %. Another interesting phenomenon is that the in the yellow region continuously move to shorter wavelengths blueshift with an increase in Eu 2+ content from.1 to.6 mol % and then move to longer wavelengths redshift with a further increase in Eu 2+ content. That is to say, the blueshift with the high purity of the phase. The blueshift of Eu 2+ emissions is determined by the strength of the crystal field in different crystal environments. 33 There are differences in the distance of ligands Sr O, the angle of the a and c axes, and the crystallographic symmetry between - and -. It was reported that in the phase orthorhombic, the tetrahedron was abnormally small and Sr O bond strength was weak due to the mirror symmetry, and in - monoclinic, the tetrahedron had a normal size and Sr O bond strength was enhanced due to the loss of the 1 symmetry plane. Eu 2+ ions in the phase sense a stronger crystal field than in the phase. As a result, the optical transitions of Eu 2+ ions in the phase are at a higher energy than in the phase. 21,34 Figures 4 and 5 and Table II further prove that the phase contributes to the increase in the PL intensities and the blueshift. In (31) (f).7 % (e).6 % (d).5 % (c).4 % (b).3 % (a).1 % α ' - β %.8 % 1. % :Eu 2+ Xmol%.6 %.5 %.4 %.3 %.1 % Figure 4. XRD pattern of powders fired at 11 C with various Eu 2+ concentrations: a.1, b.3, c.4, d.5, e.6, f.7, g.8, and h 1. mol % Figure 5. Emission spectra of :Eu 2+ with various Eu 2+ concentrations and ex = 41 nm.

4 J313 :Eu 2+.6 mol% F flux Table III. The = I Õ I + I,, and as a function of different kinds of fluxes (H 3 BO 3,Li 2 CO 3, Cl, and F) and without flux. Cl flux Samples with varied flux Li 2 CO 3 flux H 3 BO 3 flux No flux No flux H 3 BO Li 2 CO Cl F α ' - (31) β Figure 6. XRD pattern of the :Eu 2+.6 mol % powders with different kinds of fluxes H 3 BO 3,Li 2 CO 3, Cl, and F and without flux. fact, the blueshift of the with the amount of F flux is also observed in Fig. 3 and Table I, even though it is not obvious. In the solid-state reaction method, the fluxes have a great influence on the crystallite size distribution and luminescent efficiency. However, no attention has been paid on the effect of different kinds of fluxes on the luminescent properties of :Eu 2+ phosphors. Figure 6 displays the XRD of the :Eu 2+.6 mol % powders with different kinds of fluxes H 3 BO 3,Li 2 CO 3, Cl, and F and without flux. The dosage of each flux is optimized by the PL intensities with other conditions unchanged. A single - was prepared when using a H 3 BO 3 flux. A mixed phase composed of the - and - phases was prepared with Li 2 CO 3 and Cl fluxes. Pure - phase phosphors can only be prepared by using an F flux. Figure 7 shows the PL spectra of :Eu 2+.6 mol % powders with different kinds of fluxes and without flux. The emission spectra of phosphors synthesized with different fluxes are similar in shape but different in intensity. Most of the fluxes can enhance the PL intensities of :Eu 2+ except H 3 BO 3. F can greatly enhance the luminous efficiency, and the enhancement effect of F is far superior to that of Cl and Li 2 CO 3 fluxes. The H 3 BO 3 flux is harmful to luminescence, which must be caused by the low luminescence of the single phase. These results indicate that the luminous efficiency of - is stronger than that of mixed-phase and -. Table III lists the = I / I + I,, and as a function of different kinds of fluxes H 3 BO 3,Li 2 CO 3, Cl, and F and without flux. This is another proof that the form is responsible for the enhancement of the luminous efficiency and the blueshift. In summary, - is clearly superior to - in PL intensities for application in white LEDs. The F flux can evidently contribute to the single - phase formation, resulting in an increase in PL intensities and blueshift. The luminescent enhancement effect of the F flux is much better than that of the other fluxes. All these results prove that the F flux is an excellent flux for :Eu 2+ phosphors. The luminescent properties of :Eu 2+ phosphors are very complicated but few literatures detail the energy level scheme of Eu 2+ in. Therefore, the luminescent mechanisms of Eu 2+ in matrix were briefly investigated. Figure 8 presents the excitation and emission spectra of :Eu 2+ fired at 11 C for 4hwith6wt% F. The broadness of the excitation spectrum means that the phosphors can be well excited by near-uv light from 25 to 4 nm, matching well with LED GaN-based emission, which is essential for improving the efficiency of white LEDs. The emission spectra excited by different sources show two intense broad emission bands centered at 474 and 5 nm with different intensity ratios. There are two cation sites of + in. Sr I is ten-coordinated and Sr II is nine-coordinated by oxygen atoms. When Eu 2+ ions are doped into the host, Eu 2+ ions are readily substituted for + sites. As a result, there are two types 1 :Eu 2+.6 mol% excitation emission 8 F =3nm 6 Li 2 CO 3 without flux Cl λ em =5nm λ em =474nm =37nm H 3 BO Figure 7. Emission spectra of :Eu 2+ with different kinds of fluxes H 3 BO 3,Li 2 CO 3, Cl, and F and ex = 41 nm Figure 8. Excitation and emission spectra of :Eu 2+.6 mol % phosphors with 6 wt % F flux.

5 J314 Acknowledgments This work was partly supported by the National Natural Science Foundation of China no , open fund of State Key Laboratory of Silicon Materials, Zhejiang University no. SKL9-17, and the Foundation of Jinhua Science and Technology Bureau no The authors are grateful to Guanqun Xie and Yunlong Xie for the measurement of FESEM images and XRD patterns of the samples. Zhejiang Normal University assisted in meeting the publication costs of this article. Figure 9. The energy level scheme, excitation, and emission mechanism of Eu I and Eu II ion in. of Eu 2+ emission centers when Eu 2+ is incorporated into Sr sites of,eu I and Eu II, leading to two emission bands around 474 and 5 nm, respectively. 19,35,36 The luminescent properties of :Eu 2+ can be further explained by the energy level scheme of Eu I and Eu II in :Eu 2+ with optical transitions, as displayed in Fig. 9. When the excitation source is 3 nm, the excitation efficiencies for Eu I and Eu II ions are almost equal; therefore, the emission spectrum for the 3 nm excitation exhibits two emission bands, centering at 474 and 5 nm. When the excitation source is 37 nm, the excitation efficiency of Eu I ion is smaller than that of Eu II ion, so the relative emission intensity of the 5 nm emission is stronger than that of the 474 nm emission. There is only one 5 nm emission peak excited by 41 nm. The reason is that only 41 nm light can excite the Eu II ion effectively. Conclusions A yellow-emitting phosphor, - :Eu 2+, was successfully prepared by using a solid-state reaction method at a relatively low temperature 11 C. F flux increased the content of monoclinic - phase and enhanced the luminescent performance of the :Eu 2+ phosphor significantly. The luminescent intensities can be improved about five times by adding 6 wt % F flux, and it was found that F flux is far superior to Cl and Li 2 CO 3 fluxes. Results show that - was evidently superior to - in luminescent applications. :Eu 2+ phosphors give intense broad emissions ranging from 425 to 65 nm when excited by near-uv light from 25 to 4 nm. The excellent luminescent properties make :Eu 2+ suitable for white LED phosphors pumped by near-uv chips, and F is an excellent flux for :Eu 2+. References 1. E. F. Schubert and J. K. Kim, Science, 38, J. H. Lee and Y. J. Kim, Mater. Sci. Eng., B, 146, S. Nakamura, T. Mukai, and M. Senoh, Appl. Phys. Lett., 64, S. Yao and D. Chen, Opt. Laser Technol.,, V. Haerle, B. Hahn, S. Kaiser, A. Weimar, S. Bader, F. Eberhard, A. Plössl, and D. Eisert, Phys. Status Solidi A, 1, G. Dong, X. Xiao, X. Liu, B. Qian, Q. Zhang, G. Lin, Z. Ma, D. Chen, and J. Qiu, J. Electrochem. Soc., 156, J J. K. Park, K. J. Choi, S. H. Park, C. H. Kim, and H. K. Kim, J. Electrochem. Soc., 152, H J. K. Park, K. J. Choi, J. H. Yeon, S. J. Lee, and C. H. Kim, Appl. Phys. Lett., 88, J. K. Park, C. H. Kim, S. H. Park, H. D. Park, and S. Y. Choi, Appl. Phys. Lett., 84, J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, Appl. Phys. Lett., 82, R. J. Xie, N. Hirosaki, M. Mitomo, K. Sakuma, and N. Kimura, Appl. Phys. Lett., 89, R. J. Xie, N. Hirosaki, M. Mitomo, K. Takahashi, and K. Sakuma, Appl. Phys. Lett., 88, R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, and M. Mitomo, Appl. Phys. Lett., 84, V. Bachmann, C. Ronda, O. Oeckler, W. Schnick, and A. Meijerink, Chem. Mater., 21, C. C. Yang, C. M. Lin, Y. J. Chen, Y. T. Wu, S. R. Chuang, R. S. Liu, and S. F. Hu, Appl. Phys. Lett., 9, N. Narendran, Y. Gu, J. P. Freyssinier, H. Yu, and L. Deng, J. Cryst. Growth, 268, T. Jüstel, H. Nikol, and C. Ronda, Angew. Chem., Int. Ed., 37, N. Lakshminarasimhan and U. V. Varadaraju, J. Electrochem. Soc., 152, H J. S. Kim, P. E. Jeon, J. C. Choi, and H. L. Park, Solid State Commun., 133, J. S. Kim, Y. H. Park, S. M. Kim, J. C. Choi, and H. L. Park, Solid State Commun., 133, J. H. Lee and Y. J. Kim, J. Ceram. Proc. Res., 1, K. S. Sohn, B. Lee, R. J. Xie, and N. Hirosaki, Opt. Lett., 34, K. S. Sohn, S. Lee, R. J. Xie, and N. Hirosaki, Appl. Phys. Lett., 95, T. G. Kim, Y. S. Kim, and S. J. Im, J. Electrochem. Soc., 156, J Y. Song, G. Jia, M. Yang, Y. Huang, H. You, and H. Zhang, Appl. Phys. Lett., 94, Z. Jiang, Y. Wang, Z. Ci, and H. Jiao, J. Electrochem. Soc., 156, J S. Xu, L. Sun, Y. Zhang, H. Ju, S. Zhao, D. Deng, H. Wang, and B. Wang, J. Rare Earths, 27, L. H. Cheng, H. Y. Zhong, J. S. Sun, X. Q. Zhang, Y. Peng, T. Yu, and X. X. Zhao, J. Lumin., 26, N. Lakshminarasimhan and U. V. Varadaraju, Mater. Res. Bull., 43, L. Zhang, X. Zhou, H. Zeng, H. Chen, and X. Dong, Mater. Lett., 62, H. S. Yoo, W. B. Im, S. Vaidyanathan, B. J. Park, and D. Y. Jeon, J. Electrochem. Soc., 155, J X. Sun, J. Zhang, X. Zhang, Y. Luo, and X. Wang, J. Rare Earths, 26, G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, New York M. Catti, G. Gazzoni, and G. Ivaldi, Acta Crystallogr., C39, H. He, R. Fu, X. Song, D. Wang, and J. Chen, J. Lumin., 128, A. Nag and T. R. N. Kutty, J. Mater. Chem., 14,