High energy-transfer rates from Sn2+ to Mn2+ centers are demonstrated in ZnO–P2O5 glass. Emission decay curves of Sn2+ suggest an energy exchange interaction between Sn2+ and Mn2+. It is notable that the high energy-transfer rates are attained for random phosphate glass and that the transfer rate becomes slower with increasing amounts of Mn2+. Because these glasses possess high internal quantum efficiencies independent of the Sn2+ or Mn2+ concentration, we emphasize that effective energy-transfer paths are generated in the transparent glass phosphor, which leads to the development of a transparent inorganic light-emitting material different from conventional rare-earth-containing powdered phosphors.
© 2015 Optical Society of America
Energy transfer between a donor and an acceptor has been demonstrated to enhance luminescence intensity via an antenna effect in many fields [1–16]. Energy transfer is important not only because it enhances emission intensity but also because it allows emission color tuning. Various kinds of inorganic phosphors employing energy transfer between RE cations have been examined, and while this is scientifically interesting, the net efficiency of emission is not sufficient if the donor ion possesses a forbidden excitation transition. Therefore, an emission center with high probability of transition to excited states is needed as a donor for practical applications. Considering an emission center exhibiting high transition probability, i.e., a parity-allowed excitation process, we can select two candidates for the donor: RE cations exhibiting 4fx–4f(x-1)5d transitions and ns2-type centers.
In the present study, we have focused on the ns2-type emission center as an activator. This emission is strongly affected by the coordination field, because the ns2-type centers (n ≧ 4) possess an electron in the outermost shell in both the ground state (ns2) and the excited state (ns1np1) [1, 17–21]. We have reported on the highest quantum efficiency (QE) in amorphous SnO–ZnO–P2O5 (SZP) low-melting glass [22–27]. It is notable that transparent glass without RE cations shows high UV-excited photoluminescence (PL) that is comparable to that of crystalline phosphor such as MgWO4. Glass possessing strong light emission will be considered as a novel emitting material capable of good formability that is quite important in the industrial manufacturing process. The emission is caused by Sn2+, which is the most conventional and harmless ns2-type center. We have also reported that white light emission can be obtained by co-doping of Mn2+ in SZP glasses owing to the energy transfer from Sn2+ to Mn2+ centers . White light emission was attained by the addition of Mn2+ cations, similar to the case of Mn2+-co-doped calcium halophosphate crystals containing Sb3+ centers .
Energy transfer from Sn2+ to Mn2+ in solid-state matter has been mainly reported for halide [10–12] and oxide crystal systems . In previous reports, it was suggested that the emission centers of Sn2+ and Mn2+ were not randomly distributed but rather formed small Sn–Mn clusters inside alkali halides, such as NaCl, KBr, and NaBr, even when a very small amount was added. In addition, the energy transfer due to exchange interactions enables high energy-transfer rates. However, because the number of studies on the energy-transfer mechanism of Sn2+–Mn2+ in oxide glasses is limited , the details have not been clarified yet. We expect that the main reason for the lack of such a report is the valence control of the Sn2+ cation in glass materials. It was reported that the Sn2+ center in the SZP glass is easily oxidized into Sn4+ [26, 28]. Therefore, its preparation in an inert atmosphere is required in order to reveal the precise mechanism. Recently, our group compared the emission property of SZP glasses prepared in air and Ar atmospheres and confirmed that most of the Sn2+ centers were oxidized during melting in air. On the other hand, only Sn2+ centers existed in the SZP glass prepared in an Ar atmosphere , thereby confirming that the discussion on the energy-transfer mechanism is dealing with real Sn2+ centers in oxide glasses. Therefore, this work reports on the energy-transfer mechanism from Sn2+ to Mn2+ centers in ZnO–P2O5 glasses containing different amounts of Sn2+ centers (donors). For the present study, the chemical compositions of the samples were selected to be xMnO–ySnO–(60-y)ZnO–40P2O5 (in molar ratio). Herein, this glass system is denoted as xMn-ySZP.
The xMn-ySZP glasses used in the present study were prepared according to the conventional melt-quenching method reported previously [26, 29]. The glass batch was set in a Ar-purged electric furnace at room temperature (r.t.). It took 2 h to heat up from r.t. to 1373 K; the temperature was then fixed at 1373 K for 30 min. After melting, the glass melt was quenched on a stainless steel plate at 473 K and then annealed for 1 h at the glass transition temperature (Tg) as measured by differential thermal analysis. Optical absorption spectra were measured using a U-3500 spectrophotometer (Hitachi). The PL and PL excitation (PLE) spectra were measured at r.t. by using an F-7000 fluorescence spectrophotometer (Hitachi). The emission decay at r.t. was measured using a Quantaurus-Tau instrument (Hamamatsu Photonics) with 280-nm LED. The absolute QE of the glass was measured using a Quantaurus-QY (Hamamatsu Photonics).
3. Results and discussion
Figure 1 shows the optical absorption spectra of the xMn-5.0SZP glasses. The observed absorption edges are due to the s–p transition of Sn2+ centers, and are slightly red-shifted because of the added MnO. Because such a shift of the absorption bands was not observed in xMn-1.0SZP and xMn-2.5SZP glasses, it is expected that the local coordination states of the Sn2+ centers in the xMn-5.0SZP glasses, whose Sn2+ concentration is far from the conventional concentration of crystalline phosphor, are affected by the Mn co-doping.
Figure 2 shows the PL spectra of the xMn-5.0SZP glasses excited by a photon energy of 4.5 eV. These emission spectra consist of two broad bands of the donor Sn2+ (2.86 eV) and acceptor Mn2+ centers (2.05 eV) . The emission intensity of Sn2+ decreases with increasing MnO amount, while that of Mn2+ increases in the concentration range less than 3.0 mol%. Such spectral changes due to energy transfer in glass were observed previously too . The decrease in emission intensity in the case of Mn2+ addition above 5.0 mol% suggests concentration quenching of Mn2+. It is notable that Sn2+ emission is hardly observed in the 10Mn-5.0SZP glass, indicating that most of the excitation energy of the Sn2+ centers is transferred to Mn2+ in the present system. In order to examine the energy-transfer rate, we measured the emission decay curves of MnO-doped SZP glasses.
Figure 3(a) shows the PL emission decay curves of Sn2+ centers of, exemplarily, the xMn-5.0SZP glasses. A decrease in the decay constants of the Sn2+ centers with increasing Mn amount is observed. Because the energy-transfer process from the ns2-cation to Mn2+ in calcium halophosphate is thought to be governed by the exchange interaction , we take this interaction into account for the experimental fitting. The emission decay curves of the donor for the dominating exchange interaction are represented by the following equation [1, 31–33], which uses the radiative lifetime of the donor (τD):1, 30] by the following relation:
The function g(z) in Eq. (1) can be expressed by
The coefficients related to derivatives of the gamma function of argument unity, i.e., h1, h2, and h3 are 1.73164699, 5.93433597, and 5.44487446, respectively . The fitting curves in Fig. 3(a) show good agreement with the experimental data. From these fittings, we can obtain the τ1/e values for the Sn2+ centers in the Mn-doped SZP glasses, shown in Fig. 3(b), for y = 1.0, 2.5, and 5.0. The emission decay constants of the glasses containing higher Sn concentrations exhibit slightly shorter lifetimes, suggesting concentration quenching between the Sn2+ centers.
If the decay constant of a Sn2+ center with a Mn2+ cation is denoted as τ, the energy-transfer efficiency ηET is written according to [33–35] as
Figure 4(a) shows ηET values for the xMn-ySZP glasses (y = 1.0, 2.5, and 5.0) as a function of the MnO amount. The values of ηET from Sn2+ to Mn2+ generally increase with increasing Mn2+ amount. It seems that the ηET values are independent of the SnO concentration. The present ηET values are comparable to that reported in a previous study on Sn2+–Mn2+ phosphate glass prepared in air , although the Sn2+/(Sn2+ + Sn4+) ratio may be different. That is because no clustering of Sn2+ and Mn2+ occurs independent of the Sn2+ amount, which is also suggested by the present ηET values. Figure 4(b) shows ηET as a function of the Mn2+/Sn2+ ratio. It suggests that the distance between the donor and acceptor becomes statistically shortened. Since ηET values depend on the Sn2+/Mn2+ ratio, the present data do not support the formation of Sn2+–Mn2+ clustering structures in the glasses prepared by the melt-quenching method. In other words, Sn2+ and Mn2+ centers are almost homogeneously dispersed in these glasses, like in a liquid state.
The QE values of these glasses are shown in Fig. 5.It is notable that these glasses show a high QE value of around 90% even at high MnO doping. It is expected that the QE values may be influenced by several factors such as the coordination state or aggregation state of Sn and emission efficiency of Mn. The efficiencies obtained in the present study are more than twice the previously reported QE values for Sn2+–RE3+ co-doped glasses . Therefore, these values clearly show the potential of the present MnO-doped SZP glasses for phosphor application with high QE. Although the generation of Sn2+–Mn2+ cluster-like structures has not been proven from the ηET data, the QE values strongly suggest that effective energy-transfer paths exist even in a random matrix.
We emphasize that the present Sn2+–Mn2+ co-doped glasses have high QE values that are comparable to that of crystalline phosphor, although the host matrix possesses a random network. The present glass system is attractive not only from the viewpoint of a RE-free inorganic amorphous material possessing effective emission properties but also from the aspect of representing a dispersion of different kinds of emission centers in a random matrix. Because monolithic materials can reduce optical loss due to scattering at the interface, transparent emission materials will be applicable in novel industrial applications in combination with high-power deep UV LEDs.
We have demonstrated energy transfer from Sn2+ to Mn2+ centers in ZnO–P2O5 glasses. The optical absorption edge of the glasses containing higher Sn concentrations is affected by the MnO amount. The emission decay curves of Sn2+ can be fitted using the exchange interaction, and the energy transfer efficiency suggests that Sn2+ and Mn2+ centers are almost homogeneously dispersed in these glasses, like in a liquid state. The internal quantum efficiency of Sn2+–Mn2+ co-doped glasses indicates that effective energy-transfer paths exist even in a random matrix. The high energy transfer rate is also a notable advantage of the present Sn2+-doped phosphate glass phosphor.
This work was partially supported by a JSPS KAKENHI Grant-in-Aid for Young Scientists (A) number 26709048, the Kyoto Technoscience Center, Collaborative Research Program of I.C.R., Kyoto University (grant #2014-31), and the SPRITS program, Kyoto University. H.M. gratefully acknowledges the support of Prof. Yasuhiro Yamada (I.C.R. Kyoto University) for his fruitful discussion.
References and links
1. W. M. Yen, S. Shionoya, and H. Yamamoto, Phosphor Handbook 2nd Edition (CRC Press, 2007).
2. A. Wachtel, “Sb3+ and Mn2+ activated calcium halophosphate phosphors from flux-grown apatites,” J. Electrochem. Soc. 103, 160–165 (1966).
3. K. H. Butler and C. W. Jerome, “Calcium halophosphate phosphors I. analysis of emission spectra,” J. Electrochem. Soc. 97(9), 265–270 (1950). [CrossRef]
4. M. Leskelä, T. Koskentalo, and G. Blasse, “Luminescence properties of Eu2+, Sn2+, and Pb2+ in SrB6O10 and Sr1-xMnxB6O10,” J. Solid State Chem. 59(3), 272–279 (1985). [CrossRef]
5. G. Blasse and A. Bril, “Study of energy transfer from Sb3+, Bi3+, Ce3+ to Sm3+, Eu3+, Tb3+, Dy3+,” J. Chem. Phys. 47(6), 1920–1926 (1967). [CrossRef]
6. G. Blasse and A. Bril, “Investigation of some Ce3+ Activated phosphors,” J. Chem. Phys. 47(12), 5139–5145 (1967). [CrossRef]
8. P. Dorenbos, “Systematic behavior in trivalent lanthanide charge transfer energies,” J. Phys. Condens. Matter 15(49), 8417–8434 (2003). [CrossRef]
9. J. A. Jiménez, “Emission properties of Sn2+ and Sm3+ Co-doped barium phosphate glass,” J. Electron. Mater. 43(9), 3588–3592 (2014). [CrossRef]
11. R. Aceves, G. U. Caldiño, O. J. Rubio, and E. Camarillo, “Nonradiative energy transfer Sn2+→Mn2+ in monocrystalline KBr,” J. Lumin. 65(3), 113–119 (1995). [CrossRef]
12. A. Méndez, F. Ramos, R. Guerrero, E. Camarillo, and U. C. Garcí, “Energy transfer mechanisms in the NaBr: Sn2+: Mn2+ phosphor,” J. Lumin. 79(4), 269–274 (1998). [CrossRef]
13. P. I. Paulose, G. Jose, V. Thomas, N. V. Unnikrishnan, and M. K. R. Warrier, “Sensitized fluorescence of Ce3+/Mn2+ system in phosphate glass,” J. Phys. Chem. Solids 64(5), 841–846 (2003). [CrossRef]
14. Y. Liu, Z. Zou, X. Liang, S. Wang, Z. Xing, and G. Chen, “Energy transfer and photoluminescence of zinc phosphate glasses co-doped with Tb3+ and Mn2+,” J. Am. Ceram. Soc. 93, 1891–1893 (2010).
15. U. Caldiño, J. L. Hernández-Pozos, C. Flores, A. Speghini, and M. Bettinelli, “Photoluminescence of Ce3+ and Mn2+ in zinc metaphosphate glasses,” J. Phys. Condens. Matter 17(46), 7297–7305 (2005). [CrossRef]
16. J. S. Kim, P. E. Jeon, J. C. Choi, H. L. Park, S. I. Mho, and G. C. Kim, “Warm-white-light emitting diode utilizing a single-phase full-color Ba3MgSi2O8:Eu2+, Mn2+ phosphor,” Appl. Phys. Lett. 84(15), 2931–2934 (2004). [CrossRef]
17. R. C. Ropp and R. W. Mooney, “Tin-activated alkaline earth pyrophosphate phosphors,” J. Electrochem. Soc. 107(1), 15–20 (1960). [CrossRef]
18. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non-Cryst. Solids 149(1-2), 77–95 (1992). [CrossRef]
20. T. S. Davis, E. R. Kreidler, J. A. Parodi, and T. F. Soules, “The luminescent properties of antimony in calcium halophosphates,” J. Lumin. 4(1), 48–62 (1971). [CrossRef]
21. R. Reisfeld, L. Boehm, and B. Barnett, “Luminescence and nonradiative relaxation of Pb2+, Sn2+, Sb3+, and Bi3+ in oxide glasses,” J. Solid State Chem. 15(2), 140–150 (1975). [CrossRef]
22. H. Masai, Y. Takahashi, T. Fujiwara, S. Matsumoto, and T. Yoko, “High photoluminescent property of low-melting Sn-doped phosphate glass,” Appl. Phys. Express 3(8), 082102 (2010). [CrossRef]
23. H. Masai, T. Fujiwara, S. Matsumoto, Y. Takahashi, K. Iwasaki, Y. Tokuda, and T. Yoko, “White light emission of Mn-doped SnO-ZnO-P2O5 glass containing no rare earth cation,” Opt. Lett. 36(15), 2868–2870 (2011). [CrossRef] [PubMed]
24. H. Masai, T. Tanimoto, T. Fujiwara, S. Matsumoto, Y. Tokuda, and T. Yoko, “Correlation between emission property and concentration of Sn2+ center in the SnO-ZnO-P2O5 glass,” Opt. Express 20(25), 27319–27326 (2012). [CrossRef] [PubMed]
25. H. Masai, T. Yanagida, Y. Fujimoto, M. Koshimizu, and T. Yoko, “Scintillation property of rare earth-free SnO-doped oxide glass,” Appl. Phys. Lett. 101(19), 191906 (2012). [CrossRef]
26. H. Masai, T. Tanimoto, S. Okumura, K. Teramura, S. Matsumoto, T. Yanagida, Y. Tokuda, and T. Yoko, “Correlation between preparation conditions and the photoluminescence properties of Sn2+ centers in ZnO-P2O5 glasses,” J. Mater. Chem. C 2(12), 2137–2143 (2014). [CrossRef]
27. H. Masai, Y. Hino, T. Yanagida, Y. Fujimoto, and T. Yoko, “Energy transfer from Sn2+ to RE3+ cations in ZnO-P2O5 glass,” Bull. Chem. Soc. Jpn. 87(4), 556–563 (2014). [CrossRef]
28. H. Segawa, S. Inoue, and K. Nomura, “Electronic states of SnO–ZnO–P2O5 glasses and photoluminescence properties,” J. Non-Cryst. Solids 358(11), 1333–1338 (2012). [CrossRef]
29. H. Masai, T. Tanimoto, T. Fujiwara, S. Matsumoto, Y. Takahashi, Y. Tokuda, and T. Yoko, “Fabrication of Sn-doped zinc phosphate glass using a platinum crucible,” J. Non-Crystal. Solids 358, 265–269 (2012).
30. D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21(5), 836–850 (1953). [CrossRef]
31. M. Inokuti and F. Hirayama, “Influence of energy transfer by the exchange mechanism on donor luminescence,” J. Chem. Phys. 43(6), 1978–1989 (1965). [CrossRef]
32. M. J. Weber, “Luminescence decay by energy migration and transfer: Observation of diffusion-limited relaxation,” Phys. Rev. B 4(9), 2932–2939 (1971). [CrossRef]
34. R. Reisfeld and N. Lieblich-Soffer, “Energy transfer from UO22+ to Sm3+ in phosphate glass,” J. Solid State Chem. 28(3), 391–395 (1979). [CrossRef]
35. P. I. Paulose, G. Jose, V. Thomas, N. V. Unnikrishnan, and M. K. R. Warrier, “Sensitized fluorescence of Ce3+/Mn2+ system in phosphate glass,” J. Phys. Chem. Solids 64(5), 841–846 (2003). [CrossRef]