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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 25 — Dec. 3, 2012
  • pp: 27319–27326
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Correlation between emission property and concentration of Sn2+ center in the SnO-ZnO-P2O5 glass

Hirokazu Masai, Toshiro Tanimoto, Takumi Fujiwara, Syuji Matsumoto, Yomei Tokuda, and Toshinobu Yoko  »View Author Affiliations


Optics Express, Vol. 20, Issue 25, pp. 27319-27326 (2012)
http://dx.doi.org/10.1364/OE.20.027319


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Abstract

The authors report on the correlation between the photoluminescence (PL) property and the SnO amount in SnO-ZnO-P2O5 (SZP) glass. In the PL excitation (PLE) spectra of the SZP glass containing Sn2+ emission center, two S1 states, one of which is strongly affected by SnO amount, are assumed to exist. The PLE band closely correlates with the optical band edge originating from Sn2+ species, and they both largely red-shifts with increasing amount of SnO. The emission decay time of the SZP glass decreased with increasing amount of SnO and the internal quantum efficiencies of the SZP glasses containing 1~5 mol% of SnO are comparable to that of MgWO4. It is expected that the composition-dependent S1 state (the lower energy excitation band) governs the quantum efficiency of the SZP glasses.

© 2012 OSA

1. Introduction

White light emitting device using light emitting diode (LED) is strongly desired as a next-generation light source because of the low electrical power consumption, the long lifespan and so on. Therefore, it will replace the conventional white fluorescent lamp in near future. Although various kinds of phosphors are actively developed all over the world, the host materials for these phosphors, such as silicone, still have a limitation against a high power or short wavelength excitation light source because of the degradation. Therefore, we would like to emphasize that low-melting inorganic glass can be applied as sealant for high intensity and shorter wavelength excitation white LED.

Based on such background, we have proposed a novel white LED device consisting of inorganic glass phosphor and LED source, and selected SnO-ZnO-P2O5 glass system as a potential candidate [1

1. 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]

5

5. H. Masai, S. Matsumoto, T. Fujiwara, Y. Tokuda, and T. Yoko, “Photoluminescent properties of Sb-doped phosphate glass,” J. Am. Ceram. Soc. 358, 265–269 (2012).

]. This glass is known as a low-melting glass that is used as a sealant of electronic devices [6

6. S. Matsumoto, N. Nakamura, and N. Wada, “Glass, coating material for light-emitting device, and light-emitting device,” WO 2009/088086 (2009).

, 7

7. R. Morena, “Phosphate glasses as alternatives to Pb-based sealing frits,” J. Non-Cryst. Solids 263–264, 382–387 (2000). [CrossRef]

]. If LED-sealing inorganic material itself shows light emission, i.e. spectrum conversion, it will be a novel light emitting device possessing both transparency and chemical durability. On the other hand, it has been reported that Sn-doped ZnO-Al2O3-P2O5 glass exhibits an emission property via the absorption of ultraviolet (UV) light [8

8. J. G. Hooley, “Fluorescent glass composition” US 2400147 (1946).

]. The origin of the emission by UV excitation is Sn2+ belonging to an ns2 type emission center. Since the ns2 type emission centers (n ≧ 4), such as Sn2+, Sb3+, Hg0, Tl+, Pb2+, and Bi3+, possess electrons in the outermost shell in both the ground state (ns2) and the excited state (ns1np1), the emission is strongly affected by the coordination field [9

9. W. M. Yen, S. Shionoya, and H. Yamamoto, Phosphor Handbook, 2nd edition (CRC Press, 2007).

]. Therefore, the broad emission of ns2 type emission center, which is quite different from sharp emission of conventional rare earth (RE)-containing phosphors, is suitable for broad-band white light emission.

Recently, our group has reported that SnO-ZnO-P2O5 low-melting glasses show light emission by UV excitation [2

2. 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]

]. The oxide glass prepared by conventional melt-quenching method showed white ~blue emission depending on the amount of SnO. It is notable that the transparent oxide glass containing no RE cation shows intense UV-excited emission comparable to crystal phosphor such as MgWO4; in other words, this shows the largest efficiency among glass materials without RE cation ever reported. In addition, we have also demonstrated the UV-excited white light emission property of MnO-codoped SnO-ZnO-P2O5 glasses [2

2. 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]

, 3

3. H. Masai, T. Fujiwara, S. Matsumoto, Y. Takahashi, K. Iwasaki, Y. Tokuda, and T. Yoko, “High efficient white light emission of rare earth-free MnO-SnO-ZnO-P2O5 glass,” J. Ceram. Soc. Jpn. 119(1394), 726–730 (2011). [CrossRef]

]. The white light emission consisting of broad bands can be tailored by addition of Mn2+ emission center instead of RE. It is notable that high efficiency comparable to the practical crystalline phosphor was also attained.

2. Experimental

The SnO-ZnO-P2O5 (SZP) glasses were prepared by a conventional melt-quenching method using a platinum crucible [4

4. 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-Cryst. Solids 358(2), 265–269 (2012). [CrossRef]

]. The chemical composition of the SZP glass was fixed at xSnO-60ZnO-40P2O5 (in mole%, x = 0 ~7.5). Batches consisting of ZnO and (NH4)2HPO4 were initially calcined at 800°C for 3 h in an ambient atmosphere. The calcined solid was mixed with SnO at room temperature (r. t.) and then melted at 1100°C for 30 min in an ambient atmosphere. The glass melt was quenched on a steel plate kept at 200°C and then annealed at the glass transition temperature Tg for 1 h. The samples were mechanically polished to obtain mirror surface.

The Tg was determined by differential thermal analysis (DTA) operated at a heating rate of 10 K/min using TG8120 (Rigaku). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at r.t. using 850 fluorescence spectrophotometer (Hitachi). The absorption spectra were measured at r.t. using U3500 spectrophotometer (Hitachi). The emission decay at r.t. was measured using a Quantaurus-Tau (Hamamatsu Photonics) whose excitation light source was 280 nm LED operated at a frequency of 10 kHz. The absolute quantum efficiency of the glass was measured using an integrating sphere C9920-02 (Hamamatsu Photonics) at r.t.

The quantum efficiency (QE) was normalized with respect to practical phosphor MgWO4 (3N) excited by a wavelength of 254 nm. The normalized quantum efficiency (NQE) that was calculated using Eq. (1):
NQE=PgΔEs/PsΔEg,
(1)
where Pg and Ps are the PL peak area of glass and the standard phosphor MgWO4, respectively; ΔEs and ΔEg are is the photon number absorbed by MgWO4 at 254 nm and the sample, respectively, and both are obtained by the difference between the peak area of blank and that of sample. The authors have confirmed that there was no difference in NQE between the bulk sample and the powdered sample.

3. Results and discussion

Figure 1
Fig. 1 Glass transition temperature Tg of the xSnO-60ZnO-40P2O5 (SZP) glasses as a function of SnO amount.
shows the variation of Tg with SnO amount in the xSnO-60ZnO-40P2O5 (SZP) glass. The Tg decreased with increasing amount of SnO. The observed change of the Tg values indicates that SnO is homogeneously dispersed in the glass network and plays a role of a network modifier to decrease the network dimension.

The obtained glasses were colorless and transparent. We have already reported that Sb2O3-added zinc phosphate glass, which also contained ns2 type Sb3+ emission center, was yellow-colored by addition of 1 mol% of Sb2O3 [5

5. H. Masai, S. Matsumoto, T. Fujiwara, Y. Tokuda, and T. Yoko, “Photoluminescent properties of Sb-doped phosphate glass,” J. Am. Ceram. Soc. 358, 265–269 (2012).

]. Since the present glasses were colorless even when 5 mol% SnO was added, SnO-addition to zinc phosphate glass has an advantage over Sb2O3-addition. First, we have examined the correlation between the PLE peak and the optical absorption band. Figure 2(a)
Fig. 2 (a) Optical absorption and PLE spectra of the SZP glasses containing different amounts of SnO (x = 0, 0.1, 1.0, and 5.0). (b) Relation between the peak energy of PLE band and the Egopt.
shows the optical absorption and PLE spectra of the SZP glasses containing different amounts of SnO (x = 0, 0.1, 1.0, and 5.0). The PLE spectra were measured at the peak photon energy of each PL spectrum. The optical band edge energy of 60ZnO-40P2O5 glass is over 6 eV, which is much larger than that of Sn-doped glasses. It was reported that binding energy of the Zn2p3/2 electrons in zinc phosphate glass is higher than that measured for ZnO powder [10

10. E. C. Onyiriuka, “Zinc phosphate glass surfaces studied by XPS,” J. Non-Cryst. Solids 163(3), 268–273 (1993). [CrossRef]

]. We speculate that the observed band edge is strongly affected by the coordination state of zinc in the phosphate glass, although the actual coordination state has not been confirmed yet. It is notable that addition of a small amount of SnO (0.1 mole%) brings about a large peak shift of PLE band, which clearly indicates that the PLE band derives from a Sn2+ species. Therefore, it can be said that a strong absorption band of the Sn2+ species exists within the band gap of the host zinc phosphate glass. Comparing the absorption spectra with the PLE spectra, we notice that the main excitation band position locates in the vicinity of the optical band edge. It is also found that both the PLE peak energy providing the maximum PL intensity and the optical absorption edge red-shift with increasing amount of SnO. Here, we introduce the optical band edge Egopt which is evaluated by the extrapolation of linear portion of the absorption coefficient within the region of 30 cm−1. Figure 2(b) shows a plot of the peak energy of PLE vs. the Egopt. There is a clear linear relation between them, indicating that a Sn2+ species is the origin of both the absorption edge and the PLE band, and gives strong emission under irradiation with a light whose photon energy corresponds to the optical band gap of Sn2+ center.

Figure 4
Fig. 4 PL-PLE contour plots of the SZP glasses (x = (a) 0, (b) 0.1, (c) 0.5, (d) 1.0, (e) 5.0 and (f) 6.0) whose emission intensity is shown on an identical linear scale using colors.
shows PL-PLE contour plots of the SZP glasses (x = 0, 0.1, 0.5, 1.0, 5.0 and 6.0) using an intensity axis on a linear scale. The 60ZnO-40P2O5 glass (x = 0) shows no emission, and the Sn-containing glasses shows broad emission in the range from 2 to 4 eV. In each figure, the photon energy of excitation is plotted as ordinate and that of emission as abscissa, and emission intensity axis is shown on an identical linear scale using colors. From these figures, we can easily understand the red-shift of PLE peak and the generation of S1’ state on the lower excitation energy side with increasing SnO amount. In addition, it is observed that either excitation to S1 state or S1’ state (at different excitation energy) gives similar emission spectra. Emission intensity of these glasses takes the maximum value in the range of from 1to 5 mole% SnO, and then, with further increasing amount of SnO, the intensity decreases, indicating that concentration quenching occurs by addition of more than 6 mole% SnO. It is notable that the concentration of Sn2+ emission center in the SZP glass where concentration quenching takes place is much higher than that of conventional emission centers such as RE cations. Although the mechanism has not been clarified yet, the ionic host phosphate glasses may prevent aggregation of Sn2+ cations to cause the emission quenching.

Here, the emission mechanism of the xSnO-60ZnO-40P2O5 glass is discussed based on the present results. Figure 6
Fig. 6 Plausible energy scheme for photoluminescence process of Sn2+ in the 0.1SnO-60ZnO-40P2O5 and 5SnO-60ZnO-40P2O5 glasses. Solid and dashed lines show radiative and non-radiative processes, respectively.
shows plausible energy schemes for photoluminescence process of Sn2+ in the 0.1SnO-60ZnO-40P2O5 and 5SnO-60ZnO-40P2O5 glasses. In the case of 0.1SnO-60ZnO-40P2O5 glass, the peak photon energies for S0–S1 excitation and T1–S0 relaxation are 5.3 eV and 2.9 eV, respectively. On the other hand, the peak photon energy for S0–S1 excitation in the 5SnO-60ZnO-40P2O5 glass is 4.4 eV, which is lower than that in the glass containing lower amount of SnO. In the glass phosphor, there are two excitation states: one is S1 state at a photon energy of 5.3 eV, and the other is S1’ state whose peak energy strongly depends on the SnO concentration. It is easily understood that the smaller the Stokes shift becomes, the more effective the emission process takes place. Therefore, it is expected that the high quantum efficiency of the present glass, which is comparable to crystalline phosphor, is attained by the unique energy scheme.

Table 1

Table 1. Normalized quantum efficiency of the SZP glasses containing different amounts of SnO. The quantum efficiency is normalized to the value of MgWO4 crystalline phosphor.

table-icon
View This Table
shows the normalized quantum efficiency of the SZP glass, which is calculated using the value of MgWO4 crystalline phosphor. The efficiency of the SZP glasses containing 1 ~5 mole% shows a very high value comparable to that of MgWO4 phosphor, suggesting that concentration quenching was not observed in the present phosphate glass of which the concentration of Sn2+ emission center is as high as 5 mole%, which is much higher compared to the case for usual crystalline phosphors. The color chromaticity coordinates of the glasses are shown in Fig. 7
Fig. 7 Color chromaticity coordinates of the SZP glasses.
. The emission color of the 0.1SnO-containing glass is white-blue, and becomes closer to blue with increasing amount of SnO. Therefore, we can control the emission properties of the glass phosphor to a certain extent by changing the amount of SnO.

The importance of the present study is to have elucidated the possibility of tuning the emission property by changing the local coordination filed of Sn2+ emission center. In addition, it is notable that the SZP glass containing high amount of SnO shows quite high quantum efficiency comparable to MgWO4 phosphor. Although the coordination states of two S1 states have not been clarified yet, the lower excitation band that closely correlates with the absorption edge has particularly a key for the broad emission. As we have demonstrated, the intrinsic emission properties of ns2-type emission centers are useful for designing white light emission. Further study for clarification of the emission centers will open a novel amorphous emission device containing no RE cation.

4. Conclusion

We have demonstrated deep-UV-induced PL properties in SnO-ZnO-P2O5 (SZP) glasses. The Tg of the SZP glass decreased with increasing amount of SnO, indicating that addition of SnO results in lower dimensional glass network. Both the peak position of PLE band and the optical absorption edge decreased in photon energy with increasing amount of SnO, indicating a strong correlation between them. The broad emission of the Sn2+ center is strongly affected by the coordination field, in other words, the chemical composition of the glass. We found that two excitation bands, S1 and S1’, exist and that the two bands depended on the SnO concentration. The emission decay indicates that emission quenching occurs in the SZP glass containing higher than 5mol% SnO. The internal quantum efficiency indicates that the concentration dependent PLE band (S1’) is also attributed to the allowed excitation band. The large shift of S1’ band is characteristic of the present RE-free SnO-ZnO-P2O5 glass.

Acknowledgments

The author (H.M.) thanks Dr. A. Wakamiya (Kyoto Univ.) for allowing measurement of emission decay. This work was partially supported by the Asahi Glass Foundation, the Inamori Foundation, and Kazuchika Okura Memorial Foundation.

References and links

1.

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]

2.

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]

3.

H. Masai, T. Fujiwara, S. Matsumoto, Y. Takahashi, K. Iwasaki, Y. Tokuda, and T. Yoko, “High efficient white light emission of rare earth-free MnO-SnO-ZnO-P2O5 glass,” J. Ceram. Soc. Jpn. 119(1394), 726–730 (2011). [CrossRef]

4.

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-Cryst. Solids 358(2), 265–269 (2012). [CrossRef]

5.

H. Masai, S. Matsumoto, T. Fujiwara, Y. Tokuda, and T. Yoko, “Photoluminescent properties of Sb-doped phosphate glass,” J. Am. Ceram. Soc. 358, 265–269 (2012).

6.

S. Matsumoto, N. Nakamura, and N. Wada, “Glass, coating material for light-emitting device, and light-emitting device,” WO 2009/088086 (2009).

7.

R. Morena, “Phosphate glasses as alternatives to Pb-based sealing frits,” J. Non-Cryst. Solids 263–264, 382–387 (2000). [CrossRef]

8.

J. G. Hooley, “Fluorescent glass composition” US 2400147 (1946).

9.

W. M. Yen, S. Shionoya, and H. Yamamoto, Phosphor Handbook, 2nd edition (CRC Press, 2007).

10.

E. C. Onyiriuka, “Zinc phosphate glass surfaces studied by XPS,” J. Non-Cryst. Solids 163(3), 268–273 (1993). [CrossRef]

11.

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]

OCIS Codes
(160.2750) Materials : Glass and other amorphous materials
(250.5230) Optoelectronics : Photoluminescence
(300.2140) Spectroscopy : Emission

ToC Category:
Materials

History
Original Manuscript: October 11, 2012
Revised Manuscript: November 6, 2012
Manuscript Accepted: November 13, 2012
Published: November 20, 2012

Citation
Hirokazu Masai, Toshiro Tanimoto, Takumi Fujiwara, Syuji Matsumoto, Yomei Tokuda, and Toshinobu Yoko, "Correlation between emission property and concentration of Sn2+ center in the SnO-ZnO-P2O5 glass," Opt. Express 20, 27319-27326 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-25-27319


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References

  1. H. Masai, Y. Takahashi, T. Fujiwara, S. Matsumoto, and T. Yoko, “High photoluminescent property of low-melting Sn-doped phosphate glass,” Appl. Phys. Express3(8), 082102 (2010). [CrossRef]
  2. 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]
  3. H. Masai, T. Fujiwara, S. Matsumoto, Y. Takahashi, K. Iwasaki, Y. Tokuda, and T. Yoko, “High efficient white light emission of rare earth-free MnO-SnO-ZnO-P2O5 glass,” J. Ceram. Soc. Jpn.119(1394), 726–730 (2011). [CrossRef]
  4. 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-Cryst. Solids358(2), 265–269 (2012). [CrossRef]
  5. H. Masai, S. Matsumoto, T. Fujiwara, Y. Tokuda, and T. Yoko, “Photoluminescent properties of Sb-doped phosphate glass,” J. Am. Ceram. Soc.358, 265–269 (2012).
  6. S. Matsumoto, N. Nakamura, and N. Wada, “Glass, coating material for light-emitting device, and light-emitting device,” WO 2009/088086 (2009).
  7. R. Morena, “Phosphate glasses as alternatives to Pb-based sealing frits,” J. Non-Cryst. Solids263–264, 382–387 (2000). [CrossRef]
  8. J. G. Hooley, “Fluorescent glass composition” US 2400147 (1946).
  9. W. M. Yen, S. Shionoya, and H. Yamamoto, Phosphor Handbook, 2nd edition (CRC Press, 2007).
  10. E. C. Onyiriuka, “Zinc phosphate glass surfaces studied by XPS,” J. Non-Cryst. Solids163(3), 268–273 (1993). [CrossRef]
  11. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non-Cryst. Solids149(1-2), 77–95 (1992). [CrossRef]

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