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Photoluminescence of ns2-type center-containing zinc borate glasses
NOC-17357; No of Pages 5 Journal of Non-Crystalline Solids xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
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Photoluminescence of ns2-type center-containing zinc borate glasses Hirokazu Masai a,⁎, Takayuki Yanagida b a b
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
a r t i c l e
i n f o
Article history: Received 19 December 2014 Received in revised form 9 March 2015 Accepted 27 March 2015 Available online xxxx Keywords: Zinc borate glass; Photoluminescence; Non-rare earth
a b s t r a c t Optical absorption and photoluminescence properties of Sn2+, Sb3+, and Te4+-doped zinc borate glasses are examined. The obtained ns2-type center-doped glasses prepared by non-quenching method using glassy carbon crucible show different absorption and photoluminescent (PL) properties compared with those prepared by a conventional rapid-quenched method. The optical absorption edges red-shift with increasing amount of ns2-type center. There is a linear relationship between the optical absorption edge and the PL excitation band among Sn2+, Sb3+, and Te4+ centers. Although these 5 s2 emission centers possess the same electron configuration, Sn2+ center possessing the longest decay constant exhibits the highest intensity and quantum efficiency. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Bulk oxide glass prepared via melting process generally exhibits high transparency and glass formability. Since the glass is prepared by a quenching method, i.e., solidification via super-cooled liquid state, the quenched liquid state greatly affects the physical property and structure of the resulting glass matrix. Although glass material is thought to be amorphous and inherently isotropic, it is reasonable that glass consists of many anisotropic structures randomly located to make non-periodic network. Therefore, the local coordination state should be considered if a dopant cation is incorporated into the glass matrix for phosphor applications. In response to an improvement of light emitting sources, such that the Nobel Prize in Physics 2014 was awarded for the invention of efficient blue light-emitting diode, various kinds of phosphors are actively developed all over the world [1]. The emission properties of atomic or ionic emission centers have attracted attention not only from scientific community, but also from industry. For practical applications, emission centers exhibiting allowed excitations are preferable. Therefore, as a candidate, rare earth cations, such as Ce3 + [2,3] and Eu2+ [4,5] possessing f–d transition, and ns2-type ions, such as Sb3+ [6,7], Tl+ [8,9], and Sn2+ [10–13] possessing s–p transition, can be listed [1]. It is notable that these emission centers possess electrons in the outermost shell, the emission properties are strongly affected by the coordination field. Our group has been focused on the ns2-type emission centers as an activator of oxide glasses, and reported the highest quantum efficiency (QE) for amorphous SnO–ZnO–P2O5 glass [14]. It is notable that ⁎ Corresponding author. E-mail address: [email protected] (H. Masai).
transparent glass with no RE cations has a high UV-excited emission comparable to a crystal phosphor such as MgWO4; further, this was the highest QE value ever reported for monolithic bulk glass. In addition, this glass also showed scintillation with excitation by ionizing radiation [15]. On the other hand, the UV-excited white light emission property of MnO-doped SnO–ZnO–P2O5 glasses has also been demonstrated [16]. The white light emission, consisting of broad bands, can be tailored by the addition of a Mn2 + emission center. In addition to Sn2+ [14–19] and Sb3+ [20] -doped zinc phosphate glasses, we have recently reported photoluminescent (PL) property of Te4+ cation in zinc borate glass [21]. Although Te4+ has so far not been considered as an emission center in conventional glass science, luminescence of Te4+ in an amorphous material was firstly reported in the paper [21]. These transparent emission materials are attractive not only from the scientific viewpoint quite different from conventional powdered phosphor, but also from an industrial application for strong LED light sources. However, it is difficult to discuss these PL properties of ns2-type center originating in the elements, because unified experiment has not yet been performed. The coordination states of ns2-cation containing glasses, in which amounts of ns2 cation were over 5 mol%, have been reported, and it is reported that Sb3+ and Te4+ cations take the pyramid and the trigonal pyramid structures [22–25], respectively, not the regular octahedron (Oh symmetry) observed in alkali halides. Therefore, it is worthwhile to examine optical and PL property of ns2-type centers in an oxide glass with a fixed material condition, and to discuss an elemental dependence. As mentioned above, PL property of the ns2-type center is affected by the local coordination field, depending not only on the chemical composition of host glass but also on the solidified cooling process. Here, we selected zinc borate glass as a host glass, because it possesses a relatively low melting temperature (~ 1100 °C), and photoluminescence of rare earth doped zinc borate
http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029 0022-3093/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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glass has been reported by many authors [21,26–28]. In addition to melting in an inert atmosphere [18], a glassy carbon (GC) crucible instead of a Pt crucible was used for preparation, which can prevent not only an oxidation reaction during the melting but also a diversity of glass network formation even by a conventional rapid melt-quenching method [29]. In the present study, we have prepared ns2-type centercontaining zinc borate glasses prepared by a conventional melting (without rapid quenching) method and examined the optical and PL properties. 2. Experimental The oxide glasses were prepared using a 30 cm3 glassy carbon (GC) crucible in Ar (99.999%) atmosphere. The nominal chemical composition of the base glass was 60ZnO–40B2O3 (mol%), and the ns2-type: SnO, SbO3/2, and TeO2 were added as an excess amount. The amounts of these additives were 0.1 mol% and 1.0 mol%. ZnO (99.99%) and B2O3 (99.99%), and oxides of ns2-type cation: SnO (99.5%), Sb2O3 (99.9%), TeO2 (99.999%) were mixed put into an Ar-purged electric furnace. The heating strategy was as follows: (1) the sample was heated to 1100 °C for 3 h, (2) temperature was maintained for 1 h, and then, (3) the sample was cooled to room temperature for 3 h. The obtained glasses without annealing were then cut and mechanically polished to obtain a mirror surface. The thicknesses of the samples were approximately 2 mm. The PL and PL excitation (PLE) spectra were measured at room temperature (r.t.) using an F7000 fluorescence spectrophotometer (Hitachi) with the band pass filters of 2.5 nm. The absorption spectra were measured at r.t. using a U3500 spectrophotometer (Hitachi). The photoluminescence dynamics were also evaluated using a QuantaurusTau (Hamamatsu Photonics) with a 280 nm LED. The absolute QE of the glass was measured using an integrating sphere (Quantaurus-QY, Hamamatsu Photonics) at r.t. The error of QE measurement was approximately ±0.02. 3. Results The obtained xSnO– and xSbO3/2–60ZnO–40B2O3 glasses were transparent, i.e., no absorption band in the visible region, with no precipitation of metal nanoparticles. On the other hand, formation of metal nanoparticles, whose average diameter was approximately 40 nm by XRD measurement using the Scherrer equation, was observed in the 1TeO2–60ZnO–40B2O3 glass whose apparent color was slightly yellowish. Fig. 1(a) shows the absorption spectra of the xMOy–60ZnO–40B2O3 glasses (x = 0.1 and 1.0, MOy = SnO, SbO3/2, and TeO2) along with the non-doped 60ZnO–40B2O3 glass measured at room temperature. In all glass systems, red-shifts of absorption edge with increasing MOy are observed, and the edge energy is in the following order of MO y: SbO3/2 N SnO N TeO2. Per a previous report [17,18], we introduced an optical band edge Egopt, which was determined by extrapolation of the absorption coefficient as shown in Fig. 1(a). Fig. 1(b) shows composition dependence of the Egopt of these glasses. The value of Egopt decreases with increasing amount of additive, which is also observed in other phosphate [17,18] or borate glasses [19]. Fig. 2 shows the PL and PLE spectra of three xMOy–60ZnO–40B2O3 glasses (x = 0.1 and 1.0): (a) Sn2+, (b) Sb3+, and (c) Te4+, respectively. All samples show non-symmetric PL and PLE shapes, which suggests a diversity of local coordination state of emission center in the glass. It seems that the PLE bands consist of at least two bands, and that two emission bands are so clearly observed in the case of a glass containing TeO2. In the TeO2-containing glass, the higher excitation band at ~5 eV can be assignable to a defect in the glass [30–33], because the emission is observed in the undoped zinc borate glass (see Fig. 3(d)). Although the actual structure of defect in UV region has not clarified yet, one plausible structure is an electron trap mainly consisting of borate unit [32, 33]. The lower emission band, which is attributable to Te4 + species
Fig. 1. (a) Optical absorption spectra of the xMOy–60ZnO–40B2O3 (x = 0.1, and 1.0, MOy = SnO, SbO3/2, and TeO2) glasses at room temperature along with the non-doped glass. (b) Optical absorption edge Eg opt as a function of amount of additive.
[21], exhibits a broad emission whose peak energy and half width at half maximum (HWHM) are approximately 2.8 eV and 1.3 eV, respectively. Although the electron configurations of Sn2+, Sb3 +, and Te4 + are the same, the HWHM of PL peak is in the following order of MOy: SbO3/2 b SnO b TeO2. The PLE peak energies red-shift with increasing MOy amount, which corresponds to the energy shift of the absorption edge (see Fig. 1). Fig. 3(a)–(c) shows the normalized PL–PLE contour plots of the 1MOy–60ZnO–40B2O3 glasses along with non-doped 60ZnO–40B2O3 glass (Fig. 3(d)) using an intensity axis on a linear scale. Internal quantum efficiency ϕ of these glasses is also shown. In each figure, the photon energy of excitation is plotted on the y-axis and that of emission on the x-axis, and the emission intensity axis is shown after normalization using the peak energy. Each mapping consists of several excitation bands, and the undoped 60ZnO–40B2O3 glass shows clear emission band whose excitation peak energy is around 4.8 eV. Compared with PL-PLE mapping of the non-doped sample, ns2-doped glasses exhibits different mapping owing to the doped emission centers. In the case of the Te-containing sample (Fig. 3(c)), the emissions of both Te4+ (solid circled region) and another emission band (dashed circled region) are observed, indicating weak PL intensity of Te4+ species. The ϕ values of the Sn2+-doped sample exhibits the highest among these glasses. The emission decay curves monitored at 2.95 eV for the 0.1SnO– and 0.1SbO3/2–60ZnO–40B2O3 glasses are shown in Fig. 4, where the samples were irradiated using UV light of 4.43 eV. The decay curve of
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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4. Discussion
Fig. 2. Normalized PL and PLE spectra of 0.1- and 1.0MOy–60ZnO–40B2O3 glasses at room temperature: (MOy = (a) SnO, (b) SbO3/2, and (c) TeO2). Black solid lines and colored marks indicate 0.1 mol% and 1.0 mol%-added glasses, respectively. In the TeO2-doped glass, an excitation peak attributable to Te4+ species is used for normalization.
0.1TeO2–60ZnO–40B2O3 glass is not shown in the figure, because it is not easy to decompose the decay curve into a Te4 +-originated component and host-related emission as shown in Fig. 2. These decay curves consist of two components: a faster component whose lifetime of nanoseconds and a slower component of microseconds. From the slower decay components, lifetime τ1/e was estimated as 5.5 μs (Sn2+) and 3.6 μs (Sb3+), respectively. The τ1/e value of 1.0SnO– and 1.0SbO3/2–60ZnO–40B2O3 glasses exhibited the same value, suggesting that no concentration quenching occurs in the present concentration region.
First, correlation between valence state of ns2-center and preparation condition of the glass is considered. Since metal nanoparticles were observed in the 1 mol% TeO2-containing glass, it suggests that metalation from TeO2 occurred, which is easily understood from the viewpoint of the Gibbs free energy at molten temperature. Oxidation reaction during melting was suitably prevented in the present inert preparation, because no precipitation of Te nanoparticles were observed in the 5TeO2–50ZnO–50B2O3 glass prepared in air atmosphere [21]. The boiling points of metal tin, antimony, and tellurium are 2602 °C, 1587 °C, 988 °C, respectively [34]. On the other hand, Sb2O3 and TeO2 can be evaporated at the boiling temperatures: 1425 °C (Sb2O3), and 1245 °C (TeO2), respectively [34]. Since no precipitation was observed in 0.5 mol%-doped TeO2 glass, the amount of precipitated Te amount is expected to be in the range of 0.1–0.5 mol%. However, we think that several amounts of Te species were evaporated during the melting. Generally, melting with a GC crucible in pursed pure Ar (99.999%) condition doesn't give at least oxidation condition of the melt. Therefore, we assume that valence states of the ns2-type cations in the present systems are Sn2 +, Sb3 +, and Te4 + (except for 1 mol% TeO2containing glass), respectively. Compared with Egopt value of the SnO-doped 60ZnO–40B2O3 glass prepared by a conventional rapid melt-quenching method [35], the Egopt value of the present glass prepared by non-rapid quenching using GC crucible locates at lower photon energy. On the other hand, emission decay curves were independent of concentration of ns2-center in the present region (below 1.0 mol%). It is reported that the higher the SnO concentration increases, the lower the Egopt value becomes due to local coordination change of Sn2 + center [17,18]. Therefore, it is expected that the coordination change of Sn2+ centers also occurs during the slow cooling to decrease the PLE peak energy. Comparing the optical absorption edge of 1.0 mol% Sb glass with that of 0.1 mol% Sn glass, we can obtain an assumption. That suggests a possibility that at least 50% of Sb3+ may be changed, although the melting temperature in inert atmosphere was 1100 °C, which was below the boiling temperatures of the Sb metal and Sb2O3. Considering the Ar condition, we propose a hypothesis that SbOx species exhibiting lower boiling temperature may be generated. As shown above, the PLE band located at the optical absorption edge, Egopt, is also characteristic of oxide glass containing Sn2 + emission centers [17,18]. Fig. 5 shows relationship between the peak energies of the PLE bands and the optical absorption edge Egopt of the xMOy– 60ZnO–40B2O3 glasses. Although 5 s2 cations are different, there is a linear relationship between the PLE peak and the Egopt value. The linear correlation among these 5 s2 cations indicates that the electron in the outermost shell affects absorption and luminescence although the PL efficiency depends on the element. It is also notable that PL spectra of Sn2 +- and Sb3 +-doped glasses independent of the concentration although PLE shaped are changed originating in the concentration. In addition, HWHM of PL band is also independent of the Egopt value, i.e., distribution of tail energy of impurity existing between valence and conduction bands. Not only the ns2-type cations but also transition metal cations exhibit the luminescence depending on the environmental state [13,36]. It suggests that distribution of local coordination state of ns2-center depends on the cation species possessing different symmetries. Table 1 shows optical parameters of the ns2-type center-doped– 60ZnO–40B2O3 glasses along with those of previous papers. In all glasses, both faster decay at nanoseconds and slower decay at microseconds are observed. From the decay constant, main radiative relaxation processes are triplet–singlet relaxation in ns2-type cations. Because the faster decay of Sn2 + center was not observed in the ZnO–P2O5 glass system [17], it is expected that relaxation at the nanosecond order in the borate glass originates from the different coordination states of the Sn2 + center in the phosphate glasses [18]. We have
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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Fig. 3. PL and PLE contour plots of the 1MOy–60ZnO–40B2O3 glasses at room temperature, where MOy are (a) SnO, (b) SbO3/2, and (c) TeO2, respectively. The non-doped glass (d) is also shown for comparison. PL quantum efficiency ϕ of each sample is depicted.
reported that the decay constant τ1/e of the Te4+ center in zinc borate glass is approximately 3 μs [21]. Therefore, the T1–S0 decay constants of these 5 s2 emission centers become shorter with increasing atomic number. On the other hand, internal quantum efficiency of Sn2+ is the highest among these glasses. Because it is generally recognized that the quantum efficiency is proportional to inverse of decay time, the observed results suggest that Sn2+ center exhibit more effective radiative relaxation process compared with other emission centers. Although we have now started examining the actual coordination state of ns2-center
We have examined the emission properties of ns2-type emission center-containing zinc borate glasses. In these glass prepared by nonquenched method, the optical absorption edge locates lower photon energy, indicating a local coordination change of emission center during
Fig. 4. Emission decay curves of the 1SnO– and 1SbO3/2–60ZnO–40B2O3 glasses at room temperature.
Fig. 5. Relationship between the peak energies of the PLE bands and the optical absorption edge Egopt of the xMOy–60ZnO–40B2O3 glasses. A dashed line is a guide for the eyes.
in oxide glass using XAFS analysis [18], further study is needed for understanding the basic science originating in each element. 5. Conclusion
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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Table 1 Optical parameters of the xMOy–60ZnO-40B2O3 glasses. Sn2+ Additive amount/mol% Excitation peak/eV Emission peak/eV Stokes shift/eV Decay constant 1/e/μs Internal quantum efficiency (QE) a
0.1 4.4 2.8 1.6 5.5 ~ 52%
Sb3+ 1.0 4.0 2.8 1.2 5.5 ~ 60%
1.0 (by melt- quenching[35]) 4.2 2.9 1.3 5.5 ~ 60%
0.1 4.7 3.0 1.7 3.6 ~ 32%
Te4+ 1.0 4.3 3.1 1.2 3.6 ~ 33%
0.1 4.1 3.0 1.1 (2.5)[21] ~ 8%
1.0a 4.1 3.0 1.1 – ~ 5%
means that metal nanoparticles were observed in the glass.
the cooling. There is a linear relationship between Egopt and the PLE peak of these ns2 center-doped zinc borate glasses. Although these decay constants are microsecond order, indicating a radiative relaxation of triplet–singlet transition, PL efficiency depends on the elements. Among Sn2 +, Sb3 +, and Te4 + centers, we have demonstrated that Sn2+ cations possess the most effective emission properties in oxide glasses. Acknowledgments This work was partially supported by the 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 Prof. Yasuhiro Yamada (I.C.R. Kyoto University) for his fruitful discussion. References [1] W.M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, 2nd edition CRC Press, Boca Raton, 2007. [2] G. Blasse, A. Bril, J. Chem. Phys. 47 (1967) 5139. [3] E.V.D. van Loef, P. Dorenbos, C.W.E. van Eijk, K. Kramer, H.U. Gudel, Appl. Phys. Lett. 79 (2001) 1573. [4] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [5] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [6] A. Wachtel, J. Electrochem. Soc. 113 (1966) 128. [7] S. Parke, R.S. Webb, J. Phys. D 4 (1971) 825. [8] Y. Toyozawa, M. Inoue, J. Phys. Soc. Jpn. 21 (1966) 1663. [9] R.H. Clapp, R.J. Ginther, J. Opt. Soc. Am. 37 (1947) 355. [10] R.C. Ropp, R.W. Mooney, J. Electrochem. Soc. 107 (1960) 15. [11] L. Skuja, J. Non-Cryst. Solids 149 (1992) 77. [12] R. Reisfeld, L. Boehm, B. Barnett, J. Solid State Chem. 15 (1975) 140. [13] D. Ehrt, J. Non-Cryst. Solids 348 (2004) 22.
[14] H. Masai, Y. Takahashi, T. Fujiwara, S. Matsumoto, T. Yoko, Appl. Phys. Express 3 (2010) 082102. [15] H. Masai, T. Yanagida, Y. Fujimoto, M. Koshimizu, T. Yoko, Appl. Phys. Lett. 101 (2012) 191906. [16] H. Masai, T. Fujiwara, S. Matsumoto, Y. Takahashi, K. Iwasaki, Y. Tokuda, T. Yoko, Opt. Lett. 36 (2011) 2868. [17] H. Masai, T. Tanimoto, T. Fujiwara, S. Matsumoto, Y. Tokuda, T. Yoko, Opt. Express 20 (2012) 27319. [18] H. Masai, T. Tanimoto, S. Okumura, K. Teramura, S. Matsumoto, T. Yanagida, Y. Tokuda, T. Yoko, J. Mater. Chem. C 2 (2014) 2137. [19] H. Masai, Y. Yamada, Y. Suzuki, K. Teramura, Y. Kanemitsu, T. Yoko, Sci. Rep. 3 (2013) 3541. [20] H. Masai, S. Matsumoto, T. Fujiwara, Y. Tokuda, T. Yoko, J. Am. Ceram. Soc. 95 (2012) 862. [21] H. Masai, Y. Yamada, S. Okumura, Y. Kanemitsu, T. Yoko, Opt. Lett. 38 (2013) 3780. [22] L. Koudelka, J. Šubčík, P. Mošner, L. Montagne, L. Delevoye, J. Non-Cryst. Solids 353 (2007) 1828. [23] D. Holland, A.C. Hannon, M.E. Smith, C.E. Johnson, M.F. Thomas, A.M. Beesley, Solid State Nucl. Magn. Reson. 26 (2004) 172. [24] K. Terashima, T. Hashimoto, T. Uchino, S.-H. Kim, T. Yoko, J. Ceram. Soc. Jpn. 104 (1996) 1008. [25] S. Sakida, S. Hayakawa, T. Yoko, J. Non-Cryst. Solids 243 (1999) 13. [26] A. Ivankov, J. Seekamp, W. Bauhofer, J. Lumin. 121 (2006) 123. [27] R.L. Dhiman, V.S. Kundu, A.S. Maan, D.R. Goyal, J. Optoelectron. Adv. Mater. 11 (2009) 1002. [28] A.D. Sontakke, A. Tarafder, K. Biswas, K. Annapurna, Physica B 404 (2009) 3525. [29] H. Masai, T. Tanimoto, T. Fujiwara, S. Matsumoto, Y. Takahashi, Y. Tokuda, T. Yoko, J. Non-Cryst. Solids 358 (2012) 265. [30] W.M. Pontuschka, S. Isotani, A. Piccini, J. Am. Ceram. Soc. 70 (1987) 59. [31] D.L. Griscom, P.C. Taylor, D.A. Ware, P.J. Bray, J. Chem. Phys. 48 (1968) 5158. [32] A. Bishay, J. Non-Cryst. Solids 3 (1970) 54. [33] F.H. El Batal, A.A. El Kheshen, M.A. Azooz, S.M. Abo-Naf, Opt. Mater. 30 (2008) 881. [34] D.R. Lide, CRC Handbook of Chemistry and Physics, 83rd edition CRC press, Boca Raton, 2002. 4–46. [35] Y. Suzuki, (The Master's Thesis) Department of Molecular Engineering, Kyoto University, Japan, 2012. (in Japanese). [36] G. Lakshminarayana, S. Buddhudu, Spectrochim. Acta A 63 (2006) 295.
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Photoluminescence of ns2-type center-containing zinc borate glasses Hirokazu Masai a,⁎, Takayuki Yanagida b a b
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
a r t i c l e
i n f o
Article history: Received 19 December 2014 Received in revised form 9 March 2015 Accepted 27 March 2015 Available online xxxx Keywords: Zinc borate glass; Photoluminescence; Non-rare earth
a b s t r a c t Optical absorption and photoluminescence properties of Sn2+, Sb3+, and Te4+-doped zinc borate glasses are examined. The obtained ns2-type center-doped glasses prepared by non-quenching method using glassy carbon crucible show different absorption and photoluminescent (PL) properties compared with those prepared by a conventional rapid-quenched method. The optical absorption edges red-shift with increasing amount of ns2-type center. There is a linear relationship between the optical absorption edge and the PL excitation band among Sn2+, Sb3+, and Te4+ centers. Although these 5 s2 emission centers possess the same electron configuration, Sn2+ center possessing the longest decay constant exhibits the highest intensity and quantum efficiency. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Bulk oxide glass prepared via melting process generally exhibits high transparency and glass formability. Since the glass is prepared by a quenching method, i.e., solidification via super-cooled liquid state, the quenched liquid state greatly affects the physical property and structure of the resulting glass matrix. Although glass material is thought to be amorphous and inherently isotropic, it is reasonable that glass consists of many anisotropic structures randomly located to make non-periodic network. Therefore, the local coordination state should be considered if a dopant cation is incorporated into the glass matrix for phosphor applications. In response to an improvement of light emitting sources, such that the Nobel Prize in Physics 2014 was awarded for the invention of efficient blue light-emitting diode, various kinds of phosphors are actively developed all over the world [1]. The emission properties of atomic or ionic emission centers have attracted attention not only from scientific community, but also from industry. For practical applications, emission centers exhibiting allowed excitations are preferable. Therefore, as a candidate, rare earth cations, such as Ce3 + [2,3] and Eu2+ [4,5] possessing f–d transition, and ns2-type ions, such as Sb3+ [6,7], Tl+ [8,9], and Sn2+ [10–13] possessing s–p transition, can be listed [1]. It is notable that these emission centers possess electrons in the outermost shell, the emission properties are strongly affected by the coordination field. Our group has been focused on the ns2-type emission centers as an activator of oxide glasses, and reported the highest quantum efficiency (QE) for amorphous SnO–ZnO–P2O5 glass [14]. It is notable that ⁎ Corresponding author. E-mail address: [email protected] (H. Masai).
transparent glass with no RE cations has a high UV-excited emission comparable to a crystal phosphor such as MgWO4; further, this was the highest QE value ever reported for monolithic bulk glass. In addition, this glass also showed scintillation with excitation by ionizing radiation [15]. On the other hand, the UV-excited white light emission property of MnO-doped SnO–ZnO–P2O5 glasses has also been demonstrated [16]. The white light emission, consisting of broad bands, can be tailored by the addition of a Mn2 + emission center. In addition to Sn2+ [14–19] and Sb3+ [20] -doped zinc phosphate glasses, we have recently reported photoluminescent (PL) property of Te4+ cation in zinc borate glass [21]. Although Te4+ has so far not been considered as an emission center in conventional glass science, luminescence of Te4+ in an amorphous material was firstly reported in the paper [21]. These transparent emission materials are attractive not only from the scientific viewpoint quite different from conventional powdered phosphor, but also from an industrial application for strong LED light sources. However, it is difficult to discuss these PL properties of ns2-type center originating in the elements, because unified experiment has not yet been performed. The coordination states of ns2-cation containing glasses, in which amounts of ns2 cation were over 5 mol%, have been reported, and it is reported that Sb3+ and Te4+ cations take the pyramid and the trigonal pyramid structures [22–25], respectively, not the regular octahedron (Oh symmetry) observed in alkali halides. Therefore, it is worthwhile to examine optical and PL property of ns2-type centers in an oxide glass with a fixed material condition, and to discuss an elemental dependence. As mentioned above, PL property of the ns2-type center is affected by the local coordination field, depending not only on the chemical composition of host glass but also on the solidified cooling process. Here, we selected zinc borate glass as a host glass, because it possesses a relatively low melting temperature (~ 1100 °C), and photoluminescence of rare earth doped zinc borate
http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029 0022-3093/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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glass has been reported by many authors [21,26–28]. In addition to melting in an inert atmosphere [18], a glassy carbon (GC) crucible instead of a Pt crucible was used for preparation, which can prevent not only an oxidation reaction during the melting but also a diversity of glass network formation even by a conventional rapid melt-quenching method [29]. In the present study, we have prepared ns2-type centercontaining zinc borate glasses prepared by a conventional melting (without rapid quenching) method and examined the optical and PL properties. 2. Experimental The oxide glasses were prepared using a 30 cm3 glassy carbon (GC) crucible in Ar (99.999%) atmosphere. The nominal chemical composition of the base glass was 60ZnO–40B2O3 (mol%), and the ns2-type: SnO, SbO3/2, and TeO2 were added as an excess amount. The amounts of these additives were 0.1 mol% and 1.0 mol%. ZnO (99.99%) and B2O3 (99.99%), and oxides of ns2-type cation: SnO (99.5%), Sb2O3 (99.9%), TeO2 (99.999%) were mixed put into an Ar-purged electric furnace. The heating strategy was as follows: (1) the sample was heated to 1100 °C for 3 h, (2) temperature was maintained for 1 h, and then, (3) the sample was cooled to room temperature for 3 h. The obtained glasses without annealing were then cut and mechanically polished to obtain a mirror surface. The thicknesses of the samples were approximately 2 mm. The PL and PL excitation (PLE) spectra were measured at room temperature (r.t.) using an F7000 fluorescence spectrophotometer (Hitachi) with the band pass filters of 2.5 nm. The absorption spectra were measured at r.t. using a U3500 spectrophotometer (Hitachi). The photoluminescence dynamics were also evaluated using a QuantaurusTau (Hamamatsu Photonics) with a 280 nm LED. The absolute QE of the glass was measured using an integrating sphere (Quantaurus-QY, Hamamatsu Photonics) at r.t. The error of QE measurement was approximately ±0.02. 3. Results The obtained xSnO– and xSbO3/2–60ZnO–40B2O3 glasses were transparent, i.e., no absorption band in the visible region, with no precipitation of metal nanoparticles. On the other hand, formation of metal nanoparticles, whose average diameter was approximately 40 nm by XRD measurement using the Scherrer equation, was observed in the 1TeO2–60ZnO–40B2O3 glass whose apparent color was slightly yellowish. Fig. 1(a) shows the absorption spectra of the xMOy–60ZnO–40B2O3 glasses (x = 0.1 and 1.0, MOy = SnO, SbO3/2, and TeO2) along with the non-doped 60ZnO–40B2O3 glass measured at room temperature. In all glass systems, red-shifts of absorption edge with increasing MOy are observed, and the edge energy is in the following order of MO y: SbO3/2 N SnO N TeO2. Per a previous report [17,18], we introduced an optical band edge Egopt, which was determined by extrapolation of the absorption coefficient as shown in Fig. 1(a). Fig. 1(b) shows composition dependence of the Egopt of these glasses. The value of Egopt decreases with increasing amount of additive, which is also observed in other phosphate [17,18] or borate glasses [19]. Fig. 2 shows the PL and PLE spectra of three xMOy–60ZnO–40B2O3 glasses (x = 0.1 and 1.0): (a) Sn2+, (b) Sb3+, and (c) Te4+, respectively. All samples show non-symmetric PL and PLE shapes, which suggests a diversity of local coordination state of emission center in the glass. It seems that the PLE bands consist of at least two bands, and that two emission bands are so clearly observed in the case of a glass containing TeO2. In the TeO2-containing glass, the higher excitation band at ~5 eV can be assignable to a defect in the glass [30–33], because the emission is observed in the undoped zinc borate glass (see Fig. 3(d)). Although the actual structure of defect in UV region has not clarified yet, one plausible structure is an electron trap mainly consisting of borate unit [32, 33]. The lower emission band, which is attributable to Te4 + species
Fig. 1. (a) Optical absorption spectra of the xMOy–60ZnO–40B2O3 (x = 0.1, and 1.0, MOy = SnO, SbO3/2, and TeO2) glasses at room temperature along with the non-doped glass. (b) Optical absorption edge Eg opt as a function of amount of additive.
[21], exhibits a broad emission whose peak energy and half width at half maximum (HWHM) are approximately 2.8 eV and 1.3 eV, respectively. Although the electron configurations of Sn2+, Sb3 +, and Te4 + are the same, the HWHM of PL peak is in the following order of MOy: SbO3/2 b SnO b TeO2. The PLE peak energies red-shift with increasing MOy amount, which corresponds to the energy shift of the absorption edge (see Fig. 1). Fig. 3(a)–(c) shows the normalized PL–PLE contour plots of the 1MOy–60ZnO–40B2O3 glasses along with non-doped 60ZnO–40B2O3 glass (Fig. 3(d)) using an intensity axis on a linear scale. Internal quantum efficiency ϕ of these glasses is also shown. In each figure, the photon energy of excitation is plotted on the y-axis and that of emission on the x-axis, and the emission intensity axis is shown after normalization using the peak energy. Each mapping consists of several excitation bands, and the undoped 60ZnO–40B2O3 glass shows clear emission band whose excitation peak energy is around 4.8 eV. Compared with PL-PLE mapping of the non-doped sample, ns2-doped glasses exhibits different mapping owing to the doped emission centers. In the case of the Te-containing sample (Fig. 3(c)), the emissions of both Te4+ (solid circled region) and another emission band (dashed circled region) are observed, indicating weak PL intensity of Te4+ species. The ϕ values of the Sn2+-doped sample exhibits the highest among these glasses. The emission decay curves monitored at 2.95 eV for the 0.1SnO– and 0.1SbO3/2–60ZnO–40B2O3 glasses are shown in Fig. 4, where the samples were irradiated using UV light of 4.43 eV. The decay curve of
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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4. Discussion
Fig. 2. Normalized PL and PLE spectra of 0.1- and 1.0MOy–60ZnO–40B2O3 glasses at room temperature: (MOy = (a) SnO, (b) SbO3/2, and (c) TeO2). Black solid lines and colored marks indicate 0.1 mol% and 1.0 mol%-added glasses, respectively. In the TeO2-doped glass, an excitation peak attributable to Te4+ species is used for normalization.
0.1TeO2–60ZnO–40B2O3 glass is not shown in the figure, because it is not easy to decompose the decay curve into a Te4 +-originated component and host-related emission as shown in Fig. 2. These decay curves consist of two components: a faster component whose lifetime of nanoseconds and a slower component of microseconds. From the slower decay components, lifetime τ1/e was estimated as 5.5 μs (Sn2+) and 3.6 μs (Sb3+), respectively. The τ1/e value of 1.0SnO– and 1.0SbO3/2–60ZnO–40B2O3 glasses exhibited the same value, suggesting that no concentration quenching occurs in the present concentration region.
First, correlation between valence state of ns2-center and preparation condition of the glass is considered. Since metal nanoparticles were observed in the 1 mol% TeO2-containing glass, it suggests that metalation from TeO2 occurred, which is easily understood from the viewpoint of the Gibbs free energy at molten temperature. Oxidation reaction during melting was suitably prevented in the present inert preparation, because no precipitation of Te nanoparticles were observed in the 5TeO2–50ZnO–50B2O3 glass prepared in air atmosphere [21]. The boiling points of metal tin, antimony, and tellurium are 2602 °C, 1587 °C, 988 °C, respectively [34]. On the other hand, Sb2O3 and TeO2 can be evaporated at the boiling temperatures: 1425 °C (Sb2O3), and 1245 °C (TeO2), respectively [34]. Since no precipitation was observed in 0.5 mol%-doped TeO2 glass, the amount of precipitated Te amount is expected to be in the range of 0.1–0.5 mol%. However, we think that several amounts of Te species were evaporated during the melting. Generally, melting with a GC crucible in pursed pure Ar (99.999%) condition doesn't give at least oxidation condition of the melt. Therefore, we assume that valence states of the ns2-type cations in the present systems are Sn2 +, Sb3 +, and Te4 + (except for 1 mol% TeO2containing glass), respectively. Compared with Egopt value of the SnO-doped 60ZnO–40B2O3 glass prepared by a conventional rapid melt-quenching method [35], the Egopt value of the present glass prepared by non-rapid quenching using GC crucible locates at lower photon energy. On the other hand, emission decay curves were independent of concentration of ns2-center in the present region (below 1.0 mol%). It is reported that the higher the SnO concentration increases, the lower the Egopt value becomes due to local coordination change of Sn2 + center [17,18]. Therefore, it is expected that the coordination change of Sn2+ centers also occurs during the slow cooling to decrease the PLE peak energy. Comparing the optical absorption edge of 1.0 mol% Sb glass with that of 0.1 mol% Sn glass, we can obtain an assumption. That suggests a possibility that at least 50% of Sb3+ may be changed, although the melting temperature in inert atmosphere was 1100 °C, which was below the boiling temperatures of the Sb metal and Sb2O3. Considering the Ar condition, we propose a hypothesis that SbOx species exhibiting lower boiling temperature may be generated. As shown above, the PLE band located at the optical absorption edge, Egopt, is also characteristic of oxide glass containing Sn2 + emission centers [17,18]. Fig. 5 shows relationship between the peak energies of the PLE bands and the optical absorption edge Egopt of the xMOy– 60ZnO–40B2O3 glasses. Although 5 s2 cations are different, there is a linear relationship between the PLE peak and the Egopt value. The linear correlation among these 5 s2 cations indicates that the electron in the outermost shell affects absorption and luminescence although the PL efficiency depends on the element. It is also notable that PL spectra of Sn2 +- and Sb3 +-doped glasses independent of the concentration although PLE shaped are changed originating in the concentration. In addition, HWHM of PL band is also independent of the Egopt value, i.e., distribution of tail energy of impurity existing between valence and conduction bands. Not only the ns2-type cations but also transition metal cations exhibit the luminescence depending on the environmental state [13,36]. It suggests that distribution of local coordination state of ns2-center depends on the cation species possessing different symmetries. Table 1 shows optical parameters of the ns2-type center-doped– 60ZnO–40B2O3 glasses along with those of previous papers. In all glasses, both faster decay at nanoseconds and slower decay at microseconds are observed. From the decay constant, main radiative relaxation processes are triplet–singlet relaxation in ns2-type cations. Because the faster decay of Sn2 + center was not observed in the ZnO–P2O5 glass system [17], it is expected that relaxation at the nanosecond order in the borate glass originates from the different coordination states of the Sn2 + center in the phosphate glasses [18]. We have
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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Fig. 3. PL and PLE contour plots of the 1MOy–60ZnO–40B2O3 glasses at room temperature, where MOy are (a) SnO, (b) SbO3/2, and (c) TeO2, respectively. The non-doped glass (d) is also shown for comparison. PL quantum efficiency ϕ of each sample is depicted.
reported that the decay constant τ1/e of the Te4+ center in zinc borate glass is approximately 3 μs [21]. Therefore, the T1–S0 decay constants of these 5 s2 emission centers become shorter with increasing atomic number. On the other hand, internal quantum efficiency of Sn2+ is the highest among these glasses. Because it is generally recognized that the quantum efficiency is proportional to inverse of decay time, the observed results suggest that Sn2+ center exhibit more effective radiative relaxation process compared with other emission centers. Although we have now started examining the actual coordination state of ns2-center
We have examined the emission properties of ns2-type emission center-containing zinc borate glasses. In these glass prepared by nonquenched method, the optical absorption edge locates lower photon energy, indicating a local coordination change of emission center during
Fig. 4. Emission decay curves of the 1SnO– and 1SbO3/2–60ZnO–40B2O3 glasses at room temperature.
Fig. 5. Relationship between the peak energies of the PLE bands and the optical absorption edge Egopt of the xMOy–60ZnO–40B2O3 glasses. A dashed line is a guide for the eyes.
in oxide glass using XAFS analysis [18], further study is needed for understanding the basic science originating in each element. 5. Conclusion
Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029
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Table 1 Optical parameters of the xMOy–60ZnO-40B2O3 glasses. Sn2+ Additive amount/mol% Excitation peak/eV Emission peak/eV Stokes shift/eV Decay constant 1/e/μs Internal quantum efficiency (QE) a
0.1 4.4 2.8 1.6 5.5 ~ 52%
Sb3+ 1.0 4.0 2.8 1.2 5.5 ~ 60%
1.0 (by melt- quenching[35]) 4.2 2.9 1.3 5.5 ~ 60%
0.1 4.7 3.0 1.7 3.6 ~ 32%
Te4+ 1.0 4.3 3.1 1.2 3.6 ~ 33%
0.1 4.1 3.0 1.1 (2.5)[21] ~ 8%
1.0a 4.1 3.0 1.1 – ~ 5%
means that metal nanoparticles were observed in the glass.
the cooling. There is a linear relationship between Egopt and the PLE peak of these ns2 center-doped zinc borate glasses. Although these decay constants are microsecond order, indicating a radiative relaxation of triplet–singlet transition, PL efficiency depends on the elements. Among Sn2 +, Sb3 +, and Te4 + centers, we have demonstrated that Sn2+ cations possess the most effective emission properties in oxide glasses. Acknowledgments This work was partially supported by the 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 Prof. Yasuhiro Yamada (I.C.R. Kyoto University) for his fruitful discussion. References [1] W.M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, 2nd edition CRC Press, Boca Raton, 2007. [2] G. Blasse, A. Bril, J. Chem. Phys. 47 (1967) 5139. [3] E.V.D. van Loef, P. Dorenbos, C.W.E. van Eijk, K. Kramer, H.U. Gudel, Appl. Phys. Lett. 79 (2001) 1573. [4] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [5] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [6] A. Wachtel, J. Electrochem. Soc. 113 (1966) 128. [7] S. Parke, R.S. Webb, J. Phys. D 4 (1971) 825. [8] Y. Toyozawa, M. Inoue, J. Phys. Soc. Jpn. 21 (1966) 1663. [9] R.H. Clapp, R.J. Ginther, J. Opt. Soc. Am. 37 (1947) 355. [10] R.C. Ropp, R.W. Mooney, J. Electrochem. Soc. 107 (1960) 15. [11] L. Skuja, J. Non-Cryst. Solids 149 (1992) 77. [12] R. Reisfeld, L. Boehm, B. Barnett, J. Solid State Chem. 15 (1975) 140. [13] D. Ehrt, J. Non-Cryst. Solids 348 (2004) 22.
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Please cite this article as: H. Masai, T. Yanagida, Photoluminescence of ns2-type center-containing zinc borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.03.029