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Photoluminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method
Materials Science and Engineering B 146 (2008) 99–102
Photoluminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method Jee Hee Lee, Young Jin Kim ∗ Department of Materials Science & Engineering, Kyonggi University, Suwon 443-760, Kyonggi-Do, Republic of Korea
Abstract Sr2 SiO4 :Eu2+ phosphors were prepared by a flux method. Two emission bands at 495 nm and 560 nm were observed, which originated from Eu(I) and Eu(II) that were substituted for Sr(I) and Sr(II), respectively. The preference of Eu2+ ions for Sr(I) and Sr(II) strongly depended on the amounts of flux and firing temperatures. The increase of Eu2+ concentration led to the energy transfer from Eu(I) to Eu(II) emitting center, resulting in the red-shift, and the phase transformation from - to ␣’-Sr2 SiO4 were observed. © 2007 Elsevier B.V. All rights reserved. Keywords: Europium oxide; Optical properties; Strontium silicate; Energy transfer
1. Introduction White light emitting diodes (LEDs) composed of blue LEDs and yellow emitting phosphors have been developed and are widely applied to lighting systems in these days. Y3 Al5 O12 :Ce3+ (YAG:Ce3+ ) and strontium silicate materials are used as yellow phosphors [1–3], while YAG:Ce3+ powders are commercially most prevailing ones at present. Since Sr2 SiO4 :Eu2+ phosphors have some merits of the stability under high irradiation powers and temperature and the durability in the packaging resin, they are also commercially used in white LEDs instead of YAG:Ce3+ [2,4]. 4f–5d transition of Eu2+ ions causes various emission colors depending on the excitation wavelengths, and so various color temperatures for white LEDs can be realized [5–7]. Strontium orthosilicate (Sr2 SiO4 ) has two crystallographic phases, -Sr2 SiO4 (monoclinic) and ␣’-Sr2 SiO4 (orthorhombic), of which the transition temperature is about 358 K. The structures of - and ␣’-Sr2 SiO4 are same with those of Ca2 SiO4 and -K2 SO4 , respectively. There are two cation sites of Sr2+ in Sr2 SiO4 . Sr(I) is ten-coordinated and Sr(II) is ninecoordinated by oxygen atoms, resulting in two emission bands at 460–490 nm and around 560 nm by doping Eu2+ ions [2,8,9]. Eu2+ ions are readily substituted for Sr2+ sites, because the ionic ˚ and 10-coordinated radius of nine-coordinated Eu2+ (1.30 A)
∗
Corresponding author. Tel.: +82 31 249 9766; fax: +82 31 244 6300. E-mail address: [email protected] (Y.J. Kim).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.052
˚ are almost same with those of nine-coordinated Eu2+ (1.35 A) 2+ ˚ ˚ respectively Sr (1.31 A) and 10-coordinated Sr2+ (1.36 A), [10]. The short (460–490 nm) and long (around 560 nm) emission wavelength originate from Eu2+ ions substituted for Sr(I) and Sr(II), respectively [2,6]. So the emission spectra rely on the ratio of Eu2+ ions at Sr(I)–Sr(II) sites. However, it has not been definitely verified which sites Eu2+ ions prefer, Sr(I) or Sr(II) under various synthesizing circumstances. Also, the relative intensity of two emission bands strongly depends both on the excitation wavelength and on the Eu2+ concentrations. In this work, to investigate the preference of Eu2+ for Sr(I) and Sr(II) and the effects of firing conditions on the photoluminescence (PL), we synthesized Sr2 SiO4 :Eu2+ powders by solid-state reaction with a flux NH4 Cl at various circumstances such as firing temperatures, the amount of a flux, and dopant concentrations. 2. Experiment SrCO3 (Aldrich, 99.9+ %), SiO2 (high purity chemical, quartz, 99.9%), Eu2 O3 (Aldrich, 99.99%), NH4 Cl (Jin Chem., 99%) were used as starting materials. SrCO3 and SiO2 were mixed together by 2:1 mole ratio and various amounts of NH4 Cl were added as a flux. The mixtures were ball-milled for 24 h and fired at 800–1300 ◦ C for 3 h under 5% H2 atmosphere (50 sccm) in electric tube furnace. The crystalline phases of prepared powders were determined by XRD (X-ray diffractome˚ ter, SIEMENS D5005) using Cu K␣ radiation (λ = 1.5406 A). PL (Photoluminescence) properties were measured by PL (PSI
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Darsa 5000) system of which an excitation source was a Xe lamp. 3. Results and discussion XRD patterns of 1.95SrCO3 –SiO2 –0.025Eu2 O3 fired at 800 ◦ C for 3 h under 5% H2 gas with various amounts of a flux, NH4 Cl. Without a flux, besides Sr2 SiO4 phase, SrCO3 and Eu2 O3 remained as un-reacted due to the low temperature for the reactions. By adding 1 wt% NH4 Cl, the reaction was more activated by a flux, resulting in a single ␣’-Sr2 SiO4 phase, but even a little Eu2 O3 was still observed, which partly remained not to be substituted for Sr2+ sites. Finally, a single ␣’-Sr2 SiO4 phase was synthesized at 2 wt% NH4 Cl, and it was maintained up to 10 wt%. However, at 20 wt% Sr2 SiO4 peaks remarkably weakened and weak SrSiO3 peaks newly appeared. This indicated that the excess flux led to the formation of an unintended phase SrSiO3 during the reaction process. NH4 Cl enhanced not only the reaction as shown in Fig. 1, but also the luminescence characteristics of silicate phosphors [11]. A flux method is well-known to accelerate the kinetics of the formation of the desired compounds by enhancing diffusion coefficients. This method is very simple and easy to synthesize compound powders with desirable characteristics, including very fine size, narrow size distribution, single-crystal particles, high purity, and good chemical homogeneity [7,12,13]. Excitation and emission spectra of Sr2 SiO4 :Eu2+ (2.5 mol%) synthesized at 800 ◦ C with 2 wt% NH4 Cl are shown in Fig. 2. Eu2+ ions could be substituted for two cation sites of Sr2 SiO4 , 10-coordinated Sr(I) and nine-coordinated Sr(II), respectively, leading to two emission bands around 495 nm and 560 nm. The crystal fields surrounding the Eu(II) are stronger than that surrounding of Eu(I), and so the emission wavelength due to Eu(II) is longer than that due to Eu(I). Consequently, 495 nm and 560 nm emissions were attributed to Eu(I) and Eu(II), respectively [2,6,14]. The excitation spectrum for 495 nm emission
Fig. 1. XRD pattern of 1.95SrCO3 –SiO2 –0.025Eu2 O3 powders fired at 800 ◦ C with various NH4 Cl contents: (a) 0 wt%, (b) 1 wt%, (c) 2 wt%, (d) 5 wt%, (e) 10 wt%, and (f) 20 wt%.
Fig. 2. Excitation and emission spectra of Eu2+ (2.5 mol%) doped Sr2 SiO4 fired at 800 ◦ C for 3 h with 2 wt% NH4 Cl.
showed a single band peaked at 320 nm, while that for 560 nm emission exhibited a broad band from 280 nm to 450 nm, peaking at 320 nm and 370 nm. In Sr2 SiO4 , the crystallographic symmetry of Eu2+ ions is extremely low due to the coexistence of Eu(I) and Eu(II), which causes the large crystal field splitting of the 5d level, resulting in the extension of the absorption bands into the visible region (∼450 nm) [15]. 560 emission could be effectively excited both by 320 nm and by 370 nm, but 495 nm emission only by 320 nm. As shown in Fig. 2, between two emission bands, 495 nm emission is relatively strong under 320 nm excitation; but 560 nm emission under 370 nm excitation. However, under 410 nm excitation, luminescent intensity at 495 nm rapidly dropped and PL spectra exhibited a nearly single band at 560 nm. PL spectra excited by 320 nm and 370 nm of 1.95SrCO3 – SiO2 –0.025Eu2 O3 fired at 800 ◦ C as a function of NH4 Cl is shown in Fig. 3(a) and (b). Depending on the excitation wavelengths of 320 nm and 370 nm as well as on the amount of a flux, the relative intensities of 495 nm to 560 nm were changed. Under 320 nm excitation (Fig. 3 (a)), relative intensity of 495 nm to 560 nm, (I495nm /I560nm ), increased with increasing the flux amount, and emission spectra exhibited a nearly single band peaking at 320 nm with 10 wt% NH4 Cl and more. This implied that the concentration of Eu(I) increased comparing with Eu(II) by increasing the flux amount. It could be verified with 370 nm excitation as shown in Fig. 3(b), in which I495nm /I560nm also increased with increasing the flux amount, maintaining two emission bands. As shown in Fig. 2, 370 nm excitation was effective for 560 nm emission, but much less for 495 nm emission. Nevertheless, under 370 nm excitation, I495nm /I560nm increased with increasing the flux amounts. This demonstrated that Sr(I) sites were preferred by Eu2+ ions to Sr(II) sites with increasing the flux amounts. The intensity of PL spectra corresponded to the crystallinity of Sr2 SiO4 as shown in Fig. 1, where Sr2 SiO4 single phase with high quality crystallinity could be obtained at 1–5 wt% NH4 Cl. Long wavelength tail was observed in the emission spectra, which was ascribed to the reduction of the energy of Eu2+ ions due to positive charges in Sr2 SiO4 crystal [16].
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Fig. 3. PL spectra of 1.95SrCO3 –SiO2 –0.025Eu2 O3 fired at 800 ◦ C with various NH4 Cl contents: (a) λex = 320 nm and (b) λex = 370 nm.
Fig. 4 shows PL spectra of Sr2 SiO4 :Eu2+ (2.5 mol%) powders fired at different firing temperatures. Both I495nm /I560nm ratios of PL spectra excited with 320 nm and 370 nm decreased with increasing the firing temperature. Moreover, even with 320 nm excitation that was effective for 495 nm emission, PL spectra showed a nearly single band around 560 nm rather than at 495 nm at 1000 ◦ C and more. Until now, it is not verified which site Eu2+ ions prefer, Sr(I) or Sr(II), but this result apparently indicated that Eu2+ ions preferred Sr(II) to Sr(I) with increasing the firing temperature. The intensities of PL increased with increasing the firing temperature, and also the blue-shift was observed. The increase of firing temperatures contributed to the enhancement of the crystallinity as well as to the preference of substitutional Eu2+ ions. As a result, it could be speculated that the blue-shift in PL was ascribed to the changes of crystal field surrounding Eu2+ ions due to the micro-structural variations. XRD of Sr2−x Eux SiO4 prepared at 1300 ◦ C with 2 wt% NH4 Cl is shown Fig. 5. The transition temperature for ␣’- ↔ Sr2 SiO4 is 358K. XRD patterns of ␣’- and -Sr2 SiO4 are similar to each other and both the phases can coexist, because this phase transformation is completed by the rearrangement in short-range order without the disconnection of coordination bonds [17]. A single -Sr2 SiO4 phase was formed at x = 0.005. With increasing Eu2+ concentration up to x = 0.05, ␣’-Sr2 SiO4 phase gradually increased, coexisting with -Sr2 SiO4 . At x = 0.07, a single phase ␣’-Sr2 SiO4 finally could be synthesized.
Fig. 4. PL spectra of 2.5 mol% Eu2+ doped Sr2 SiO4 as a function of firing temperature with 2 wt% NH4 Cl: (a) λex = 320 nm and (b) λex = 370 nm.
Fig. 6 shows PL spectra of Sr2−x Eux SiO4 powders. The interesting points were that with increasing Eu2+ concentration the relative PL intensity around 560 nm to 495 nm (I560 nm /I495 nm ) increased and the red-shift was observed in PL under the excitation wavelengths, 320 nm and 370 nm. I560 nm /I495 nm changes
Fig. 5. XRD patterns of Sr2−x Eux SiO4 fired at 1300 ◦ C with 2 wt% NH4 Cl: (a) x = 0.005, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05, and (e) x = 0.07.
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4. Conclusion Sr2 SiO4 :Eu2+ phosphors were prepared by a flux method. The effects of the amounts of a flux, firing temperature, and Eu2+ concentrations on the structural transformations and luminescent properties were investigated. Two emission bands at 495 nm and 560 nm were observed, which originated from Eu(I) and Eu(II), respectively. The concentration of Eu(I) increased comparing with Eu(II) by increasing the flux amount, while it decreased with increasing the firing temperature. With increasing Eu2+ concentration, the energy transfer from Eu(I) of high energy emitting center to Eu(II) of low energy emitting center increased, leading to a long wavelength emission, and also the red-shift and the phase transformation from - to ␣’-Sr2 SiO4 were observed. Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R01-2005-000-10530-0). References
Fig. 6. PL spectra of Sr2−x Eux SiO4 fired at 1300 ◦ C with 2 wt% NH4 Cl as a function of Eu concentrations (x = 0.005–0.07): (a) λex = 320 nm and (b) λex = 370 nm.
can be explained by Rc (critical distance of energy transfer) and Rav (average distance of Eu2+ –Eu2+ ions). Blass explained the change of relative intensities of two emission bands that originated from two Eu2+ sites in Rb2 ZnBr4 by Rc and Rav [18]. If Rav Rc with increasing Eu2+ concentration, the energy transfer is generated from Eu(I) of high energy emitting center to Eu(II) of low energy emitting center, resulting in the increase of I560 nm /I495 nm . This mechanism can be applied to ␣’-Sr2 SiO4 and -Sr2 SiO4 , because both Rb2 ZnBr4 :Eu2+ and ␣’-Sr2 SiO4 have a same crystal structure with -K2 SO4 , and also exhibit similar luminescent properties [9,18]. In this experiment, the energy transfer phenomena could be apparently observed in Fig. 6(a). Under 320 nm excitation, I560nm /I495nm drastically reversed with increasing Eu2+ concentration from x = 0.005 to x = 0.01. Energy transfer probability of among Eu2+ ions themselves increased due to the increase of Eu2+ concentration. And so, in 5d orbit of Eu2+ ion, the energy transfer is possible from high energy level to low energy level, causing the red-shift [19].
[1] K. Toda, Y. kawakami, S. Kousaka, Y. Ito, A. Komeno, K. Uematsu, M. Sato, IEICE Trans. Electron. E89-C (2006) 1406–1412. [2] N. Lakshminarasimhan, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H152–H156. [3] J.K. Park, K.J. Choi, C.H. Kim, H.D. Park, S.Y. Choi, Electrochem. SolidState Lett. 7 (2004) H15–H17. [4] G. Roth, Phosphor Global Summit, 2007. [5] G. Blasse, J. Chem. Phys. 51 (1969) 3529–3530. [6] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, Solid-State Commun. 133 (2005) 187–190. [7] A. Nag, T.R.N. Kutty, J. Alloys Compd. 354 (2003) 221–231. [8] A. Nag, T.R.N. Kutty, J. Mater. Chem. 14 (2004) 1598–1604. [9] S.H.M. Poort, W. Janssen, G. Blasse, J. Alloys Compd. 260 (1997) 93–97. [10] R.D. Shannon, Acta Cryst. A 32 (1976) 751–767. [11] H.S. Kang, Y.C. Kang, K.Y. Jung, S.B. Park, Mater. Sci. Eng. B 121 (2005) 81–85. [12] F. Bondioli, A.B. Corradi, T. Manfredini, Chem. Mater. 12 (2002) 324–330. [13] S. Shionoya, W.M. Yen, Phosphor Handbook, Phosphor Research Society, CRC Press, 1999. [14] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, Heidelberg, 1994. [15] G. Blasse, W.L. Wanmaker, J.W. ter Vrugt, A. Bril, Philips Res. Repts. 23 (1968) 189–200. [16] S.H.M. Poort, H.M. Reijnhoudt, H.O.T. van der Kuip, G. Blasse, J. Alloys Compd. 241 (1996) 75–81. [17] M. Catti, G. Gazzoni, Acta Cryst. B 39 (1983) 679–684. [18] G. Blasse, J. Solid State Chem. 62 (1986) 207–211. [19] J. Qiu, K. Miura, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 213/214 (1997) 266–270.
Photoluminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method Jee Hee Lee, Young Jin Kim ∗ Department of Materials Science & Engineering, Kyonggi University, Suwon 443-760, Kyonggi-Do, Republic of Korea
Abstract Sr2 SiO4 :Eu2+ phosphors were prepared by a flux method. Two emission bands at 495 nm and 560 nm were observed, which originated from Eu(I) and Eu(II) that were substituted for Sr(I) and Sr(II), respectively. The preference of Eu2+ ions for Sr(I) and Sr(II) strongly depended on the amounts of flux and firing temperatures. The increase of Eu2+ concentration led to the energy transfer from Eu(I) to Eu(II) emitting center, resulting in the red-shift, and the phase transformation from - to ␣’-Sr2 SiO4 were observed. © 2007 Elsevier B.V. All rights reserved. Keywords: Europium oxide; Optical properties; Strontium silicate; Energy transfer
1. Introduction White light emitting diodes (LEDs) composed of blue LEDs and yellow emitting phosphors have been developed and are widely applied to lighting systems in these days. Y3 Al5 O12 :Ce3+ (YAG:Ce3+ ) and strontium silicate materials are used as yellow phosphors [1–3], while YAG:Ce3+ powders are commercially most prevailing ones at present. Since Sr2 SiO4 :Eu2+ phosphors have some merits of the stability under high irradiation powers and temperature and the durability in the packaging resin, they are also commercially used in white LEDs instead of YAG:Ce3+ [2,4]. 4f–5d transition of Eu2+ ions causes various emission colors depending on the excitation wavelengths, and so various color temperatures for white LEDs can be realized [5–7]. Strontium orthosilicate (Sr2 SiO4 ) has two crystallographic phases, -Sr2 SiO4 (monoclinic) and ␣’-Sr2 SiO4 (orthorhombic), of which the transition temperature is about 358 K. The structures of - and ␣’-Sr2 SiO4 are same with those of Ca2 SiO4 and -K2 SO4 , respectively. There are two cation sites of Sr2+ in Sr2 SiO4 . Sr(I) is ten-coordinated and Sr(II) is ninecoordinated by oxygen atoms, resulting in two emission bands at 460–490 nm and around 560 nm by doping Eu2+ ions [2,8,9]. Eu2+ ions are readily substituted for Sr2+ sites, because the ionic ˚ and 10-coordinated radius of nine-coordinated Eu2+ (1.30 A)
∗
Corresponding author. Tel.: +82 31 249 9766; fax: +82 31 244 6300. E-mail address: [email protected] (Y.J. Kim).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.052
˚ are almost same with those of nine-coordinated Eu2+ (1.35 A) 2+ ˚ ˚ respectively Sr (1.31 A) and 10-coordinated Sr2+ (1.36 A), [10]. The short (460–490 nm) and long (around 560 nm) emission wavelength originate from Eu2+ ions substituted for Sr(I) and Sr(II), respectively [2,6]. So the emission spectra rely on the ratio of Eu2+ ions at Sr(I)–Sr(II) sites. However, it has not been definitely verified which sites Eu2+ ions prefer, Sr(I) or Sr(II) under various synthesizing circumstances. Also, the relative intensity of two emission bands strongly depends both on the excitation wavelength and on the Eu2+ concentrations. In this work, to investigate the preference of Eu2+ for Sr(I) and Sr(II) and the effects of firing conditions on the photoluminescence (PL), we synthesized Sr2 SiO4 :Eu2+ powders by solid-state reaction with a flux NH4 Cl at various circumstances such as firing temperatures, the amount of a flux, and dopant concentrations. 2. Experiment SrCO3 (Aldrich, 99.9+ %), SiO2 (high purity chemical, quartz, 99.9%), Eu2 O3 (Aldrich, 99.99%), NH4 Cl (Jin Chem., 99%) were used as starting materials. SrCO3 and SiO2 were mixed together by 2:1 mole ratio and various amounts of NH4 Cl were added as a flux. The mixtures were ball-milled for 24 h and fired at 800–1300 ◦ C for 3 h under 5% H2 atmosphere (50 sccm) in electric tube furnace. The crystalline phases of prepared powders were determined by XRD (X-ray diffractome˚ ter, SIEMENS D5005) using Cu K␣ radiation (λ = 1.5406 A). PL (Photoluminescence) properties were measured by PL (PSI
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Darsa 5000) system of which an excitation source was a Xe lamp. 3. Results and discussion XRD patterns of 1.95SrCO3 –SiO2 –0.025Eu2 O3 fired at 800 ◦ C for 3 h under 5% H2 gas with various amounts of a flux, NH4 Cl. Without a flux, besides Sr2 SiO4 phase, SrCO3 and Eu2 O3 remained as un-reacted due to the low temperature for the reactions. By adding 1 wt% NH4 Cl, the reaction was more activated by a flux, resulting in a single ␣’-Sr2 SiO4 phase, but even a little Eu2 O3 was still observed, which partly remained not to be substituted for Sr2+ sites. Finally, a single ␣’-Sr2 SiO4 phase was synthesized at 2 wt% NH4 Cl, and it was maintained up to 10 wt%. However, at 20 wt% Sr2 SiO4 peaks remarkably weakened and weak SrSiO3 peaks newly appeared. This indicated that the excess flux led to the formation of an unintended phase SrSiO3 during the reaction process. NH4 Cl enhanced not only the reaction as shown in Fig. 1, but also the luminescence characteristics of silicate phosphors [11]. A flux method is well-known to accelerate the kinetics of the formation of the desired compounds by enhancing diffusion coefficients. This method is very simple and easy to synthesize compound powders with desirable characteristics, including very fine size, narrow size distribution, single-crystal particles, high purity, and good chemical homogeneity [7,12,13]. Excitation and emission spectra of Sr2 SiO4 :Eu2+ (2.5 mol%) synthesized at 800 ◦ C with 2 wt% NH4 Cl are shown in Fig. 2. Eu2+ ions could be substituted for two cation sites of Sr2 SiO4 , 10-coordinated Sr(I) and nine-coordinated Sr(II), respectively, leading to two emission bands around 495 nm and 560 nm. The crystal fields surrounding the Eu(II) are stronger than that surrounding of Eu(I), and so the emission wavelength due to Eu(II) is longer than that due to Eu(I). Consequently, 495 nm and 560 nm emissions were attributed to Eu(I) and Eu(II), respectively [2,6,14]. The excitation spectrum for 495 nm emission
Fig. 1. XRD pattern of 1.95SrCO3 –SiO2 –0.025Eu2 O3 powders fired at 800 ◦ C with various NH4 Cl contents: (a) 0 wt%, (b) 1 wt%, (c) 2 wt%, (d) 5 wt%, (e) 10 wt%, and (f) 20 wt%.
Fig. 2. Excitation and emission spectra of Eu2+ (2.5 mol%) doped Sr2 SiO4 fired at 800 ◦ C for 3 h with 2 wt% NH4 Cl.
showed a single band peaked at 320 nm, while that for 560 nm emission exhibited a broad band from 280 nm to 450 nm, peaking at 320 nm and 370 nm. In Sr2 SiO4 , the crystallographic symmetry of Eu2+ ions is extremely low due to the coexistence of Eu(I) and Eu(II), which causes the large crystal field splitting of the 5d level, resulting in the extension of the absorption bands into the visible region (∼450 nm) [15]. 560 emission could be effectively excited both by 320 nm and by 370 nm, but 495 nm emission only by 320 nm. As shown in Fig. 2, between two emission bands, 495 nm emission is relatively strong under 320 nm excitation; but 560 nm emission under 370 nm excitation. However, under 410 nm excitation, luminescent intensity at 495 nm rapidly dropped and PL spectra exhibited a nearly single band at 560 nm. PL spectra excited by 320 nm and 370 nm of 1.95SrCO3 – SiO2 –0.025Eu2 O3 fired at 800 ◦ C as a function of NH4 Cl is shown in Fig. 3(a) and (b). Depending on the excitation wavelengths of 320 nm and 370 nm as well as on the amount of a flux, the relative intensities of 495 nm to 560 nm were changed. Under 320 nm excitation (Fig. 3 (a)), relative intensity of 495 nm to 560 nm, (I495nm /I560nm ), increased with increasing the flux amount, and emission spectra exhibited a nearly single band peaking at 320 nm with 10 wt% NH4 Cl and more. This implied that the concentration of Eu(I) increased comparing with Eu(II) by increasing the flux amount. It could be verified with 370 nm excitation as shown in Fig. 3(b), in which I495nm /I560nm also increased with increasing the flux amount, maintaining two emission bands. As shown in Fig. 2, 370 nm excitation was effective for 560 nm emission, but much less for 495 nm emission. Nevertheless, under 370 nm excitation, I495nm /I560nm increased with increasing the flux amounts. This demonstrated that Sr(I) sites were preferred by Eu2+ ions to Sr(II) sites with increasing the flux amounts. The intensity of PL spectra corresponded to the crystallinity of Sr2 SiO4 as shown in Fig. 1, where Sr2 SiO4 single phase with high quality crystallinity could be obtained at 1–5 wt% NH4 Cl. Long wavelength tail was observed in the emission spectra, which was ascribed to the reduction of the energy of Eu2+ ions due to positive charges in Sr2 SiO4 crystal [16].
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Fig. 3. PL spectra of 1.95SrCO3 –SiO2 –0.025Eu2 O3 fired at 800 ◦ C with various NH4 Cl contents: (a) λex = 320 nm and (b) λex = 370 nm.
Fig. 4 shows PL spectra of Sr2 SiO4 :Eu2+ (2.5 mol%) powders fired at different firing temperatures. Both I495nm /I560nm ratios of PL spectra excited with 320 nm and 370 nm decreased with increasing the firing temperature. Moreover, even with 320 nm excitation that was effective for 495 nm emission, PL spectra showed a nearly single band around 560 nm rather than at 495 nm at 1000 ◦ C and more. Until now, it is not verified which site Eu2+ ions prefer, Sr(I) or Sr(II), but this result apparently indicated that Eu2+ ions preferred Sr(II) to Sr(I) with increasing the firing temperature. The intensities of PL increased with increasing the firing temperature, and also the blue-shift was observed. The increase of firing temperatures contributed to the enhancement of the crystallinity as well as to the preference of substitutional Eu2+ ions. As a result, it could be speculated that the blue-shift in PL was ascribed to the changes of crystal field surrounding Eu2+ ions due to the micro-structural variations. XRD of Sr2−x Eux SiO4 prepared at 1300 ◦ C with 2 wt% NH4 Cl is shown Fig. 5. The transition temperature for ␣’- ↔ Sr2 SiO4 is 358K. XRD patterns of ␣’- and -Sr2 SiO4 are similar to each other and both the phases can coexist, because this phase transformation is completed by the rearrangement in short-range order without the disconnection of coordination bonds [17]. A single -Sr2 SiO4 phase was formed at x = 0.005. With increasing Eu2+ concentration up to x = 0.05, ␣’-Sr2 SiO4 phase gradually increased, coexisting with -Sr2 SiO4 . At x = 0.07, a single phase ␣’-Sr2 SiO4 finally could be synthesized.
Fig. 4. PL spectra of 2.5 mol% Eu2+ doped Sr2 SiO4 as a function of firing temperature with 2 wt% NH4 Cl: (a) λex = 320 nm and (b) λex = 370 nm.
Fig. 6 shows PL spectra of Sr2−x Eux SiO4 powders. The interesting points were that with increasing Eu2+ concentration the relative PL intensity around 560 nm to 495 nm (I560 nm /I495 nm ) increased and the red-shift was observed in PL under the excitation wavelengths, 320 nm and 370 nm. I560 nm /I495 nm changes
Fig. 5. XRD patterns of Sr2−x Eux SiO4 fired at 1300 ◦ C with 2 wt% NH4 Cl: (a) x = 0.005, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05, and (e) x = 0.07.
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4. Conclusion Sr2 SiO4 :Eu2+ phosphors were prepared by a flux method. The effects of the amounts of a flux, firing temperature, and Eu2+ concentrations on the structural transformations and luminescent properties were investigated. Two emission bands at 495 nm and 560 nm were observed, which originated from Eu(I) and Eu(II), respectively. The concentration of Eu(I) increased comparing with Eu(II) by increasing the flux amount, while it decreased with increasing the firing temperature. With increasing Eu2+ concentration, the energy transfer from Eu(I) of high energy emitting center to Eu(II) of low energy emitting center increased, leading to a long wavelength emission, and also the red-shift and the phase transformation from - to ␣’-Sr2 SiO4 were observed. Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R01-2005-000-10530-0). References
Fig. 6. PL spectra of Sr2−x Eux SiO4 fired at 1300 ◦ C with 2 wt% NH4 Cl as a function of Eu concentrations (x = 0.005–0.07): (a) λex = 320 nm and (b) λex = 370 nm.
can be explained by Rc (critical distance of energy transfer) and Rav (average distance of Eu2+ –Eu2+ ions). Blass explained the change of relative intensities of two emission bands that originated from two Eu2+ sites in Rb2 ZnBr4 by Rc and Rav [18]. If Rav Rc with increasing Eu2+ concentration, the energy transfer is generated from Eu(I) of high energy emitting center to Eu(II) of low energy emitting center, resulting in the increase of I560 nm /I495 nm . This mechanism can be applied to ␣’-Sr2 SiO4 and -Sr2 SiO4 , because both Rb2 ZnBr4 :Eu2+ and ␣’-Sr2 SiO4 have a same crystal structure with -K2 SO4 , and also exhibit similar luminescent properties [9,18]. In this experiment, the energy transfer phenomena could be apparently observed in Fig. 6(a). Under 320 nm excitation, I560nm /I495nm drastically reversed with increasing Eu2+ concentration from x = 0.005 to x = 0.01. Energy transfer probability of among Eu2+ ions themselves increased due to the increase of Eu2+ concentration. And so, in 5d orbit of Eu2+ ion, the energy transfer is possible from high energy level to low energy level, causing the red-shift [19].
[1] K. Toda, Y. kawakami, S. Kousaka, Y. Ito, A. Komeno, K. Uematsu, M. Sato, IEICE Trans. Electron. E89-C (2006) 1406–1412. [2] N. Lakshminarasimhan, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H152–H156. [3] J.K. Park, K.J. Choi, C.H. Kim, H.D. Park, S.Y. Choi, Electrochem. SolidState Lett. 7 (2004) H15–H17. [4] G. Roth, Phosphor Global Summit, 2007. [5] G. Blasse, J. Chem. Phys. 51 (1969) 3529–3530. [6] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, Solid-State Commun. 133 (2005) 187–190. [7] A. Nag, T.R.N. Kutty, J. Alloys Compd. 354 (2003) 221–231. [8] A. Nag, T.R.N. Kutty, J. Mater. Chem. 14 (2004) 1598–1604. [9] S.H.M. Poort, W. Janssen, G. Blasse, J. Alloys Compd. 260 (1997) 93–97. [10] R.D. Shannon, Acta Cryst. A 32 (1976) 751–767. [11] H.S. Kang, Y.C. Kang, K.Y. Jung, S.B. Park, Mater. Sci. Eng. B 121 (2005) 81–85. [12] F. Bondioli, A.B. Corradi, T. Manfredini, Chem. Mater. 12 (2002) 324–330. [13] S. Shionoya, W.M. Yen, Phosphor Handbook, Phosphor Research Society, CRC Press, 1999. [14] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, Heidelberg, 1994. [15] G. Blasse, W.L. Wanmaker, J.W. ter Vrugt, A. Bril, Philips Res. Repts. 23 (1968) 189–200. [16] S.H.M. Poort, H.M. Reijnhoudt, H.O.T. van der Kuip, G. Blasse, J. Alloys Compd. 241 (1996) 75–81. [17] M. Catti, G. Gazzoni, Acta Cryst. B 39 (1983) 679–684. [18] G. Blasse, J. Solid State Chem. 62 (1986) 207–211. [19] J. Qiu, K. Miura, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 213/214 (1997) 266–270.