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Effect of synthesis conditions on Ce3 + luminescence in borate glasses
NOC-17374; No of Pages 4 Journal of Non-Crystalline Solids xxx (2015) xxx–xxx
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effect of synthesis conditions on Ce3 + luminescence in borate glasses Atul D. Sontakke a,⁎, Jumpei Ueda a,b, Setsuhisa Tanabe a a b
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan Delft University of Technology, Faculty of Applied Science, Department of Radiation Science and Technology (FAME-LMR), 2629 JB Delft, Netherlands
a r t i c l e
i n f o
Article history: Received 29 November 2014 Received in revised form 2 April 2015 Accepted 4 April 2015 Available online xxxx Keywords: Ce3 + luminescence; Quantum yield; Glass synthesis; Reducing conditions; Borate glass
a b s t r a c t Cerium exhibits two stable valence states, i.e. Ce3+ and Ce4+. In glassy hosts, Ce4+ strongly deactivates the Ce3+ luminescence and therefore it becomes essential to employ reducing conditions in the glass synthesis. In the present work, we report a systematic study on the effect of different synthesis conditions on Ce3+ luminescence properties in borate glasses. In the air atmosphere synthesized glasses, strong Ce4+ charge transfer (CT) transitions have been observed, which led to poor luminescence properties with quantum yield less than 1%. The Ce3+ luminescence significantly improved in glasses prepared under reducing synthesis atmosphere suggesting effective reduction of Ce4+ ions to the Ce3+ state. In both mild to strong reducing conditions, the decay profiles were single exponential with decay lifetime of about 46 ns. Among the studied synthesis conditions, the glass prepared under strong reducing condition using carbon enclosed double crucible plus 0.5 wt.% carbon doping showed best performance with luminescence quantum yield of about 42%, which is one of the highest value in the glassy hosts reported so far. © 2015 Published by Elsevier B.V.
1. Introduction Ce3 + doped inorganic materials such as glasses and crystals are widely used as phosphors for displays and lightings as well as the high energy radiation scintillators [1–3]. It is due to the superior light yield of Ce3+ ions arising from its parity allowed 5d–4f transitions. Moreover, the absence of cross-relaxation mechanisms and an insignificant multiphonon relaxation of excitation population due to the wide separation of 5d–4f energy levels have established Ce3+ as one of the most efficient luminescent ion. Y3Al5O12: Ce3 +, LaCl3: Ce3 +, LSO: Ce3 + and some glasses activated with Ce3+ ions are widely commercialized as efficient phosphors and radiation scintillators [1–5]. The redox reaction of cerium allows two stable valence states, i.e. Ce3 + and Ce4 +, whose equilibrium depends on the host material as well as on the synthesis conditions [6]. Ce4+, owing to its strong charge transfer (CT) absorption (Ce4+ + e− → Ce3+) in the UV–visible spectral region can act as a quenching center for the Ce3+ luminescence (Fig. 1). In the case of the glasses, the Ce4+ CT occurs at longer wavelength region and effectively overlaps the Ce3+ 4f–5d transitions [7]. Therefore special precautions such as the reducing synthesis atmosphere, selection of appropriate precursor chemicals, and doping with reducing agents are necessary in the synthesis of Ce3 + doped glasses. Reisfeld and Hormadaly used (NH4)2Ce(NO3)6 as a precursor chemical for Ce3+ and added mannitol as the reducing agent in the batch to maintain the reducing atmosphere during the synthesis of Ce3+ doped borate glass [8]. Chewpraditkul et al. used CeO2 as the precursor dopant ⁎ Corresponding author. E-mail address: [email protected] (A.D. Sontakke).
chemical and the melting was carried out in CO reducing atmosphere [7]. In other reports, Ce(NO3)3·6H2O was used as a precursor chemical and the melting was carried out under N2 plus graphite lumps; whereas, only N2 atmosphere was also used as the reducing atmosphere together with the oxide precursor chemicals in the glass synthesis [6,9]. From all these studies, it can be seen that the different reducing conditions were employed in the glass synthesis, which creates mild to strong reducing effects. There is no systematic study on the influence of such varied synthesis conditions on Ce3+ luminescence properties in the glass, which is highly important to precisely optimize the synthesis process. In this work, we present a systematic investigation on the effect of different synthesis conditions on the Ce3+ luminescence properties in a calcium–aluminum–lanthanum–borate glass system. We studied various synthesis conditions, such as reducing atmosphere (5% H2 gas flow, melting in carbon enclosed double crucibles, etc.), direct doping of carbon or aluminum metal as a reducing agent in the batch, and the use of CeO2 or CeF3 precursor dopant chemicals, and investigated its influence on the Ce3+ luminescence, decay lifetime and luminescence quantum yield.
2. Experimental High temperature melt-quenching method was used for the glass synthesis with base composition of 55 B2O3–20 CaO–10 Al2O3–15 La2O3 in mol%. The doped samples were obtained by substituting the 0.5 mol% of La2O3 with an equivalent amount of the dopant contents. CeO2 (99.99%) and CeF3 (99.99%) were used as precursor chemicals for the dopants. The melting was carried out in covered alumina
http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005 0022-3093/© 2015 Published by Elsevier B.V.
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
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A.D. Sontakke et al. / Journal of Non-Crystalline Solids xxx (2015) xxx–xxx
Fig. 1. Host referred electron binding energy (HRBE) diagram of Ce3+ ions in studied borate glass explaining the Ce3+ quenching mechanism by Ce4+ ions.
crucibles using different synthesis conditions and the details are presented in Table 1. The glass samples G-1 and G-2 were prepared under normal air atmosphere, G-3 was prepared in 5% H2–95% N2 reducing atmosphere, and the samples G-4 to G-6 were prepared in strongly reducing CO atmosphere provided by putting the covered batch crucible in a carbon filled enclosing crucible. For further reducing action, additional dopants were incorporated in the batch, such as the carbon powder (99.9%) or the Al metal (99.9%) as specified in Table 1. All the ingredients were mixed thoroughly using a mortar–pestle and melted at 1350 °C for 45 min. The melt was then quenched on a warm stainless steel mold. The cast glasses were annealed at 600 °C for stress removal and cooled slowly to room temperature. The well-annealed glasses were cut and polished in 10 × 10 × 2 mm3 dimensions for various measurements. Optical absorption spectra were recorded using a Shimadzu 3600 spectrophotometer in the wavelength range of 180–800 nm. To avoid the saturation effect due to the strong absorption in the UV region, absorption spectra were also measured using thin samples (~ 200 μm thickness). Photoluminescence (PL) and PL excitation (PLE) spectra of the studied glasses were recorded using a Shimadzu RF-5300 spectrophotometer in the 200–700 nm wavelength region. The PL quantum yield was measured using a 10 in. integrating sphere (Labsphere Inc., LMS-100) attached with a multi-channel CCD detector (Ocean Optics Inc., USB 2000) and a 372 nm LD (Nichia Co. Ltd., NDHU110APAE3) excitation source. Signals were calibrated using a standard halogen lamp (Labsphere, SCL-600) and an auxiliary halogen lamp for absolute spectral power distribution and absorption losses. The systematic error in quantum yield measurement is less than 5%. The PL decay curves were recorded using a PL lifetime measurement setup (Hamamatsu-Photonics, Quantarus Tau) equipped with a picosecond LED (λp: 340 nm; temporal resolution ~0.5 ns).
Fig. 2. Optical absorption spectra of the studied glasses (thickness ~ 2 mm). Inset: Absorption spectra recorded using thin samples (thickness ~ 200 μm).
figure for reference. In the cerium doped glasses, a strong absorption is observed in the UV region, thereby shifting the UV absorption band to the longer wavelength region. In the air atmosphere synthesized G-1 glass sample, this band shift is maximum, whereas it is least in the G-6 glass prepared under CO reducing atmosphere. Inset shows the absorption spectra of the G-1, G-6 and the base glass samples recorded using thin (~ 200 μm) specimens. In the case of air atmosphere synthesized G-1 glass, the absorption is very strong starting at about 400 nm and is attributed to the charge transfer (CT) transitions of the Ce4 + ions (Ce4 + + e− → Ce3 +) [10]. The presence of such intense CT absorption incorporates yellowish tint in the air atmosphere synthesized G-1 as well as the G-2 glasses. This suggests that the addition of carbon doping in the G-2 glass batch has very small reducing effect owing to its air atmosphere synthesis condition. Whereas, the G-3 to G-6 glasses prepared under reducing atmosphere conditions are all clear with no visible coloration. The absorption spectra of the thin specimen of these glasses showed well-resolved five absorption bands due to the Ce3 + 4f → 5dj transitions (j = 1–5) as represented in the absorption profile of the G-6 glass sample in the inset of Fig. 2. This clearly demonstrates that the reducing atmosphere is highly advantageous in the glass synthesis to effectively reduce the Ce4 + ions in to the Ce3 + state.
3. Results and discussion 3.1. Optical absorption spectra Fig. 2 shows the UV–visible optical absorption spectra of the studied glasses. The base glass absorption spectrum is also presented in the Table 1 Sample identification name and the respective reducing conditions used in glass synthesis. Sample name
G-1
G-2
G-3
G-4
G-5
G-6
CeO2 CeO2 CeF3 CeF3 Dopant precursors CeO2 CeO2 Additional – C (0.5 wt.%) – Ala (0.5 wt.%) – C (0.5 wt.%) dopants CO CO Synthesis Air Air 5% H2 CO atmosphere a In glass G-4, the additional Al metal doping was compensated with the Al2O3 contents in the batch to maintain the glass stoichiometry.
Fig. 3. PL spectra of the studied glasses (λEx: 345 nm). Inset shows the PL-PLE mapping of G-1 and G-6 glasses along with their PL glow images.
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
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3.2. Photoluminescence spectra and decay profiles The PL spectra of the glasses were recorded under 345 nm excitation and are presented in Fig. 3. All the glasses show broad PL profile peaking at around 385 nm due to the Ce3+ 5d1 → 4f transitions. The PL intensity is very weak in the air atmosphere synthesized G-1 and G-2 glasses compared to the other glasses synthesized in more reducing conditions. This is due to the abundance of Ce4 + ions in the G-1 and G-2 glasses which acts as strong deactivator sites for the Ce3 + luminescence (Fig. 1). Among the studied glasses, the G-6 glass exhibits the strongest PL intensity. This suggests that the CO synthesis atmosphere and use of fluoride (CeF3) dopant chemicals are advantageous to obtain superior Ce3+ luminescence properties. The inset of Fig. 3 presents the PL-PLE mapping of the G-1 and G-6 glasses along with their PL glow images under UV excitation. From the PL-PLE mapping, it can be seen that the Ce3 + PL is not symmetric, which indicates an inhomogeneous site distribution of dopant ions in the amorphous host [11]. Fig. 4 shows the PL decay curves of the studied glass samples. In the case of G-1 and G-2 glasses, the decay profile exhibits non-exponential behavior with the decay lifetime of 27.0 ns and 30.1 ns respectively. The non-exponential decay profiles suggest non-radiative energy transfer from the active ions to the quenching sites, i.e. Ce4 + in the present case. For G-3 to G-6 glasses, the decay profiles are almost single exponential with decay lifetime of about 46 ns. The 46 ns is one of the longest lifetime for the Ce3 + luminescence in glassy hosts [7–9]. The longer decay lifetime and exponential decay profiles indicate an effective control of non-radiative losses of the Ce3+ excited state population in G-3 to G-6 glasses. 3.3. Ce3+ luminescence quantum yield For more detail understanding, the luminescence quantum yield of Ce3+ PL in studied glasses was measured using the integrating sphere method. Fig. 5 represents the schematic of external quantum yield measurement using the integrating sphere method [12,13]. The difference between the sample spectrum (sample in the line of excitation beam) and the reference spectrum (sample off-line to the excitation beam) gives the spectral power absorbed and emitted by the sample. In figure, this is represented by the shaded part. On converting it to the absolute photon counts, the quantum yield can be derived as the ratio of the emitted photons with that of the absorbed photons [13]. Table 2 presents the luminescence quantum yield of Ce3 + in studied glasses along with the normalized integrated PL intensity and decay lifetimes. The quantum yield is less than 1% in G-1 and G-2 glasses, but it increases to 19% in G-3, 34% in G-4, 7% in G-5 and 42% in the G-6 glass. The PL intensity and the decay lifetime also showed significant enhancement
Fig. 5. Schematic representation of the luminescence quantum yield measurement using the integrating sphere method. The inset reveals the experimental setup used to record the sample spectrum and the reference spectrum. The shaded parts in the excitation and emission region represent the spectral absorption and the emission due to the sample, respectively.
from the G-1 and G-2 glasses to the other glasses prepared under reducing atmosphere. Note that the trend in the quantum yield is analogous with the PL intensity. From Table 2, it can be noticed that the decay lifetime is almost same for the G-3 to G-6 glass and the PL intensity shows a small variation. Despite this, the luminescence quantum yield exhibits significant change. In the case of G-3 and G-6 glass samples, the luminescence quantum yield exhibits about 120% enhancement; whereas, the PL intensity increases by only 7%. Same is the case for other glasses showing more change in quantum yield over a small change in PL intensity. Since the quantum yield is the ratio of emission with absorption, the observed huge increase in the quantum yield despite a small change in the PL intensity may be related to the difference in the absorption due to the samples. Fig. 6 shows the excitation and emission region spectra of reference and the sample signals of the quantum yield experiment in some of the studied glasses. In the case of the G-1 glass, the absorption is maximum, and it consistently decreases in the G-3 to G-4 and G-6 glass. The emission intensity follows the same trend as in the PL intensity in Fig. 3. Since the doping concentration is same in all the glasses, the observed difference in the excitation energy absorption by the glasses can be attributed to the absorption contribution due to the Ce4+ ions in the studied glasses. From the absorption spectra in Fig. 2, it is clear that the Ce4+ exhibits intense absorption in the UV region of the spectrum. The strong CT absorption due to the Ce4+ ions in the G-1 and G-2 glasses gives rise to a substantial loss of excitation signals, and therefore results in very low quantum yield. The reducing conditions in the G-3 to G-6 glasses significantly reduce the Ce4+ ions to the Ce3+ states which are evident from exponential behavior of the Ce3+ decay profiles and the almost saturated decay lifetime values. However, these glasses may still exhibit trace amount of Ce4+ impurities. These trace Ce4+ ions in the glasses do Table 2 Luminescence quantum yield (η), PL intensity (IPL), and the decay lifetime (τ) of studied glasses.
Fig. 4. PL decay curves of the studied borate glasses.
Glass
η (%)
IPL (±0.02)
τ (±0.5 ns)
G-1 G-2 G-3 G-4 G-5 G-6
0.5 ± 0.3 0.7 ± 0.3 19 ± 2 34 ± 2 7±2 42 ± 2
0.06 0.11 0.93 0.99 0.84 1
27.0 30.1 46.6 46.7 45.6 46.1
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
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the Ce3+ PL in glass hosts and strong reducing conditions are necessary to achieve high luminescence quantum yield. 4. Conclusion In summary, the effect of different synthesis conditions on Ce3 + photoluminescence, decay lifetime and the external quantum yield has been investigated in Ce3 + doped borate glasses. The airatmosphere synthesized glasses showed poor luminescence performance with luminescence quantum yield less than 1%. The results are attributed to the presence of Ce4 + ions due to the oxidizing atmosphere, which act as strong quenching centers for the Ce3 + luminescence. On the other hand, the glasses prepared under reducing atmosphere showed superior luminescence properties with exponential decay behavior. Though the mild reducing conditions such as 5% H2 gas flow could effectively reduce Ce4 + ions to the Ce 3 + state and reduced the non-radiative energy losses; a trace amount of Ce4 + still exists in the glass, which is responsible for its inferior luminescence quantum yield. Among the synthesis conditions studied, the glass prepared in strongly reducing carbon enclosed double crucible plus 0.5 wt.% carbon doping condition showed the best performance with luminescence quantum yield of about 42%, which is one of the highest value in glassy hosts reported so far. Acknowledgments This work is carried out under the JSPS Post-Doctoral Fellowship program (P 13372). References
Fig. 6. Sample spectrum and the reference spectrum of the quantum yield experiments of the studied glasses. The x-axis has been rescaled for clear view of the excitation region (368 nm–376 nm) and the emission region (378 nm–700 nm) of the measurement.
not sufficiently affect the Ce3 + decay kinetics, i.e. very weak nonradiative interactions with the Ce3+ ions, but it can still absorb the excitation signals owing to its strong absorption coefficient at the excitation region giving rise to an unwanted absorption loss by the glasses. In the case of the mild reducing conditions like G-3 glass, the absorption loss is more than that in the strong reducing condition in the G-6 glass. Therefore, it can be concluded that the reducing conditions play a vital role on
[1] D. Wisniewski, L.A. Boatner, J.O. Ramey, M. Wisniewska, J.S. Neal, G.E. Jellison, Explanatory research on the development of novel Ce3+ — activated phosphate glass scintillators, IEEE Trans. Nucl. Sci. 55 (2009) 3692. [2] V. Bachmann, C. Ronda, A. Meijerink, Temperature quenching of yellow Ce3+ luminescence in YAG Ce, Chem. Mater. 21 (2009) 2077. [3] J. Ueda, S. Tanabe, T. Nakanishi, Analysis of Ce3+ luminescence quenching in solid solutions between Y3Al5O12 and Y3Ga5O12 by temperature dependence of photoconductivity measurement, J. Appl. Phys. 110 (2011) 053102. [4] I.V. Khodyuk, P. Dorenbos, Nonproportional response of LaBr3: Ce and LaCl3: Ce scintillators to X-ray radiations, J. Phys. Condens. Matter 22 (2010) 485402. [5] M.S. Brown, S. Gundacker, A. Taylor, C. Tummeltshammer, E. Auffray, P. Lecoq, I. Papakonstantinou, Influence of depth of interaction upon the performance of scintillator detectors, PLoS One 9 (2014) e98177. [6] J. Bei, G. Qian, X. Liang, S. Yuan, Y. Yang, G. Chen, Optical properties of Ce3+ doped oxide glasses and correlation with optical basicity, Mater. Res. Bull. 42 (2007) 1195. [7] W. Chewpraditkul, X. He, D. Chen, Y. Shen, Q. Sheng, B. Yu, M. Nikl, R. Kucerkova, A. Beitlerova, C. Wanarak, A. Phunpueok, Luminescence and scintillation of Ce3+ — doped oxide glass with high Gd2O3 concentration, Phys. Status Solidi A 208 (2011) 2830. [8] R. Reisfeld, J. Hormadaly, Quantum yield of Ce3+ and energy transfer between Ce3+ and Tb3+ in borax glasses, J. Solid State Chem. 13 (1975) 283. [9] C. Jiang, Q. Zeng, F. Gan, New scintillator: cerium-doped dense oxide glass, Proc. SPIE 4134 (2000) 329. [10] J. Fu, J.M. Parker, R.M. Brown, P.S. Flower, Compositional dependence of scintillation yield of glasses with high Gd2O3 concentrations, J. Non-Cryst. Solids 326/327 (2003) 335. [11] E. Malchukova, B. Boizot, Tunable luminescence from Ce-doped aluminoborosilicate glasses, J. Rare Earths 32 (2014) 217. [12] S. Tanabe, S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, YAG glass–ceramic phosphor for white LED (II): luminescence characteristics, Proc. SPIE 5941 (2005) 594112. [13] J.C. de Mello, H.F. Wittmann, R.H. Friend, An improved experimental determination of external photoluminescence quantum efficiency, Adv. Mater. 9 (1997) 230.
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effect of synthesis conditions on Ce3 + luminescence in borate glasses Atul D. Sontakke a,⁎, Jumpei Ueda a,b, Setsuhisa Tanabe a a b
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan Delft University of Technology, Faculty of Applied Science, Department of Radiation Science and Technology (FAME-LMR), 2629 JB Delft, Netherlands
a r t i c l e
i n f o
Article history: Received 29 November 2014 Received in revised form 2 April 2015 Accepted 4 April 2015 Available online xxxx Keywords: Ce3 + luminescence; Quantum yield; Glass synthesis; Reducing conditions; Borate glass
a b s t r a c t Cerium exhibits two stable valence states, i.e. Ce3+ and Ce4+. In glassy hosts, Ce4+ strongly deactivates the Ce3+ luminescence and therefore it becomes essential to employ reducing conditions in the glass synthesis. In the present work, we report a systematic study on the effect of different synthesis conditions on Ce3+ luminescence properties in borate glasses. In the air atmosphere synthesized glasses, strong Ce4+ charge transfer (CT) transitions have been observed, which led to poor luminescence properties with quantum yield less than 1%. The Ce3+ luminescence significantly improved in glasses prepared under reducing synthesis atmosphere suggesting effective reduction of Ce4+ ions to the Ce3+ state. In both mild to strong reducing conditions, the decay profiles were single exponential with decay lifetime of about 46 ns. Among the studied synthesis conditions, the glass prepared under strong reducing condition using carbon enclosed double crucible plus 0.5 wt.% carbon doping showed best performance with luminescence quantum yield of about 42%, which is one of the highest value in the glassy hosts reported so far. © 2015 Published by Elsevier B.V.
1. Introduction Ce3 + doped inorganic materials such as glasses and crystals are widely used as phosphors for displays and lightings as well as the high energy radiation scintillators [1–3]. It is due to the superior light yield of Ce3+ ions arising from its parity allowed 5d–4f transitions. Moreover, the absence of cross-relaxation mechanisms and an insignificant multiphonon relaxation of excitation population due to the wide separation of 5d–4f energy levels have established Ce3+ as one of the most efficient luminescent ion. Y3Al5O12: Ce3 +, LaCl3: Ce3 +, LSO: Ce3 + and some glasses activated with Ce3+ ions are widely commercialized as efficient phosphors and radiation scintillators [1–5]. The redox reaction of cerium allows two stable valence states, i.e. Ce3 + and Ce4 +, whose equilibrium depends on the host material as well as on the synthesis conditions [6]. Ce4+, owing to its strong charge transfer (CT) absorption (Ce4+ + e− → Ce3+) in the UV–visible spectral region can act as a quenching center for the Ce3+ luminescence (Fig. 1). In the case of the glasses, the Ce4+ CT occurs at longer wavelength region and effectively overlaps the Ce3+ 4f–5d transitions [7]. Therefore special precautions such as the reducing synthesis atmosphere, selection of appropriate precursor chemicals, and doping with reducing agents are necessary in the synthesis of Ce3 + doped glasses. Reisfeld and Hormadaly used (NH4)2Ce(NO3)6 as a precursor chemical for Ce3+ and added mannitol as the reducing agent in the batch to maintain the reducing atmosphere during the synthesis of Ce3+ doped borate glass [8]. Chewpraditkul et al. used CeO2 as the precursor dopant ⁎ Corresponding author. E-mail address: [email protected] (A.D. Sontakke).
chemical and the melting was carried out in CO reducing atmosphere [7]. In other reports, Ce(NO3)3·6H2O was used as a precursor chemical and the melting was carried out under N2 plus graphite lumps; whereas, only N2 atmosphere was also used as the reducing atmosphere together with the oxide precursor chemicals in the glass synthesis [6,9]. From all these studies, it can be seen that the different reducing conditions were employed in the glass synthesis, which creates mild to strong reducing effects. There is no systematic study on the influence of such varied synthesis conditions on Ce3+ luminescence properties in the glass, which is highly important to precisely optimize the synthesis process. In this work, we present a systematic investigation on the effect of different synthesis conditions on the Ce3+ luminescence properties in a calcium–aluminum–lanthanum–borate glass system. We studied various synthesis conditions, such as reducing atmosphere (5% H2 gas flow, melting in carbon enclosed double crucibles, etc.), direct doping of carbon or aluminum metal as a reducing agent in the batch, and the use of CeO2 or CeF3 precursor dopant chemicals, and investigated its influence on the Ce3+ luminescence, decay lifetime and luminescence quantum yield.
2. Experimental High temperature melt-quenching method was used for the glass synthesis with base composition of 55 B2O3–20 CaO–10 Al2O3–15 La2O3 in mol%. The doped samples were obtained by substituting the 0.5 mol% of La2O3 with an equivalent amount of the dopant contents. CeO2 (99.99%) and CeF3 (99.99%) were used as precursor chemicals for the dopants. The melting was carried out in covered alumina
http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005 0022-3093/© 2015 Published by Elsevier B.V.
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
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Fig. 1. Host referred electron binding energy (HRBE) diagram of Ce3+ ions in studied borate glass explaining the Ce3+ quenching mechanism by Ce4+ ions.
crucibles using different synthesis conditions and the details are presented in Table 1. The glass samples G-1 and G-2 were prepared under normal air atmosphere, G-3 was prepared in 5% H2–95% N2 reducing atmosphere, and the samples G-4 to G-6 were prepared in strongly reducing CO atmosphere provided by putting the covered batch crucible in a carbon filled enclosing crucible. For further reducing action, additional dopants were incorporated in the batch, such as the carbon powder (99.9%) or the Al metal (99.9%) as specified in Table 1. All the ingredients were mixed thoroughly using a mortar–pestle and melted at 1350 °C for 45 min. The melt was then quenched on a warm stainless steel mold. The cast glasses were annealed at 600 °C for stress removal and cooled slowly to room temperature. The well-annealed glasses were cut and polished in 10 × 10 × 2 mm3 dimensions for various measurements. Optical absorption spectra were recorded using a Shimadzu 3600 spectrophotometer in the wavelength range of 180–800 nm. To avoid the saturation effect due to the strong absorption in the UV region, absorption spectra were also measured using thin samples (~ 200 μm thickness). Photoluminescence (PL) and PL excitation (PLE) spectra of the studied glasses were recorded using a Shimadzu RF-5300 spectrophotometer in the 200–700 nm wavelength region. The PL quantum yield was measured using a 10 in. integrating sphere (Labsphere Inc., LMS-100) attached with a multi-channel CCD detector (Ocean Optics Inc., USB 2000) and a 372 nm LD (Nichia Co. Ltd., NDHU110APAE3) excitation source. Signals were calibrated using a standard halogen lamp (Labsphere, SCL-600) and an auxiliary halogen lamp for absolute spectral power distribution and absorption losses. The systematic error in quantum yield measurement is less than 5%. The PL decay curves were recorded using a PL lifetime measurement setup (Hamamatsu-Photonics, Quantarus Tau) equipped with a picosecond LED (λp: 340 nm; temporal resolution ~0.5 ns).
Fig. 2. Optical absorption spectra of the studied glasses (thickness ~ 2 mm). Inset: Absorption spectra recorded using thin samples (thickness ~ 200 μm).
figure for reference. In the cerium doped glasses, a strong absorption is observed in the UV region, thereby shifting the UV absorption band to the longer wavelength region. In the air atmosphere synthesized G-1 glass sample, this band shift is maximum, whereas it is least in the G-6 glass prepared under CO reducing atmosphere. Inset shows the absorption spectra of the G-1, G-6 and the base glass samples recorded using thin (~ 200 μm) specimens. In the case of air atmosphere synthesized G-1 glass, the absorption is very strong starting at about 400 nm and is attributed to the charge transfer (CT) transitions of the Ce4 + ions (Ce4 + + e− → Ce3 +) [10]. The presence of such intense CT absorption incorporates yellowish tint in the air atmosphere synthesized G-1 as well as the G-2 glasses. This suggests that the addition of carbon doping in the G-2 glass batch has very small reducing effect owing to its air atmosphere synthesis condition. Whereas, the G-3 to G-6 glasses prepared under reducing atmosphere conditions are all clear with no visible coloration. The absorption spectra of the thin specimen of these glasses showed well-resolved five absorption bands due to the Ce3 + 4f → 5dj transitions (j = 1–5) as represented in the absorption profile of the G-6 glass sample in the inset of Fig. 2. This clearly demonstrates that the reducing atmosphere is highly advantageous in the glass synthesis to effectively reduce the Ce4 + ions in to the Ce3 + state.
3. Results and discussion 3.1. Optical absorption spectra Fig. 2 shows the UV–visible optical absorption spectra of the studied glasses. The base glass absorption spectrum is also presented in the Table 1 Sample identification name and the respective reducing conditions used in glass synthesis. Sample name
G-1
G-2
G-3
G-4
G-5
G-6
CeO2 CeO2 CeF3 CeF3 Dopant precursors CeO2 CeO2 Additional – C (0.5 wt.%) – Ala (0.5 wt.%) – C (0.5 wt.%) dopants CO CO Synthesis Air Air 5% H2 CO atmosphere a In glass G-4, the additional Al metal doping was compensated with the Al2O3 contents in the batch to maintain the glass stoichiometry.
Fig. 3. PL spectra of the studied glasses (λEx: 345 nm). Inset shows the PL-PLE mapping of G-1 and G-6 glasses along with their PL glow images.
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
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3.2. Photoluminescence spectra and decay profiles The PL spectra of the glasses were recorded under 345 nm excitation and are presented in Fig. 3. All the glasses show broad PL profile peaking at around 385 nm due to the Ce3+ 5d1 → 4f transitions. The PL intensity is very weak in the air atmosphere synthesized G-1 and G-2 glasses compared to the other glasses synthesized in more reducing conditions. This is due to the abundance of Ce4 + ions in the G-1 and G-2 glasses which acts as strong deactivator sites for the Ce3 + luminescence (Fig. 1). Among the studied glasses, the G-6 glass exhibits the strongest PL intensity. This suggests that the CO synthesis atmosphere and use of fluoride (CeF3) dopant chemicals are advantageous to obtain superior Ce3+ luminescence properties. The inset of Fig. 3 presents the PL-PLE mapping of the G-1 and G-6 glasses along with their PL glow images under UV excitation. From the PL-PLE mapping, it can be seen that the Ce3 + PL is not symmetric, which indicates an inhomogeneous site distribution of dopant ions in the amorphous host [11]. Fig. 4 shows the PL decay curves of the studied glass samples. In the case of G-1 and G-2 glasses, the decay profile exhibits non-exponential behavior with the decay lifetime of 27.0 ns and 30.1 ns respectively. The non-exponential decay profiles suggest non-radiative energy transfer from the active ions to the quenching sites, i.e. Ce4 + in the present case. For G-3 to G-6 glasses, the decay profiles are almost single exponential with decay lifetime of about 46 ns. The 46 ns is one of the longest lifetime for the Ce3 + luminescence in glassy hosts [7–9]. The longer decay lifetime and exponential decay profiles indicate an effective control of non-radiative losses of the Ce3+ excited state population in G-3 to G-6 glasses. 3.3. Ce3+ luminescence quantum yield For more detail understanding, the luminescence quantum yield of Ce3+ PL in studied glasses was measured using the integrating sphere method. Fig. 5 represents the schematic of external quantum yield measurement using the integrating sphere method [12,13]. The difference between the sample spectrum (sample in the line of excitation beam) and the reference spectrum (sample off-line to the excitation beam) gives the spectral power absorbed and emitted by the sample. In figure, this is represented by the shaded part. On converting it to the absolute photon counts, the quantum yield can be derived as the ratio of the emitted photons with that of the absorbed photons [13]. Table 2 presents the luminescence quantum yield of Ce3 + in studied glasses along with the normalized integrated PL intensity and decay lifetimes. The quantum yield is less than 1% in G-1 and G-2 glasses, but it increases to 19% in G-3, 34% in G-4, 7% in G-5 and 42% in the G-6 glass. The PL intensity and the decay lifetime also showed significant enhancement
Fig. 5. Schematic representation of the luminescence quantum yield measurement using the integrating sphere method. The inset reveals the experimental setup used to record the sample spectrum and the reference spectrum. The shaded parts in the excitation and emission region represent the spectral absorption and the emission due to the sample, respectively.
from the G-1 and G-2 glasses to the other glasses prepared under reducing atmosphere. Note that the trend in the quantum yield is analogous with the PL intensity. From Table 2, it can be noticed that the decay lifetime is almost same for the G-3 to G-6 glass and the PL intensity shows a small variation. Despite this, the luminescence quantum yield exhibits significant change. In the case of G-3 and G-6 glass samples, the luminescence quantum yield exhibits about 120% enhancement; whereas, the PL intensity increases by only 7%. Same is the case for other glasses showing more change in quantum yield over a small change in PL intensity. Since the quantum yield is the ratio of emission with absorption, the observed huge increase in the quantum yield despite a small change in the PL intensity may be related to the difference in the absorption due to the samples. Fig. 6 shows the excitation and emission region spectra of reference and the sample signals of the quantum yield experiment in some of the studied glasses. In the case of the G-1 glass, the absorption is maximum, and it consistently decreases in the G-3 to G-4 and G-6 glass. The emission intensity follows the same trend as in the PL intensity in Fig. 3. Since the doping concentration is same in all the glasses, the observed difference in the excitation energy absorption by the glasses can be attributed to the absorption contribution due to the Ce4+ ions in the studied glasses. From the absorption spectra in Fig. 2, it is clear that the Ce4+ exhibits intense absorption in the UV region of the spectrum. The strong CT absorption due to the Ce4+ ions in the G-1 and G-2 glasses gives rise to a substantial loss of excitation signals, and therefore results in very low quantum yield. The reducing conditions in the G-3 to G-6 glasses significantly reduce the Ce4+ ions to the Ce3+ states which are evident from exponential behavior of the Ce3+ decay profiles and the almost saturated decay lifetime values. However, these glasses may still exhibit trace amount of Ce4+ impurities. These trace Ce4+ ions in the glasses do Table 2 Luminescence quantum yield (η), PL intensity (IPL), and the decay lifetime (τ) of studied glasses.
Fig. 4. PL decay curves of the studied borate glasses.
Glass
η (%)
IPL (±0.02)
τ (±0.5 ns)
G-1 G-2 G-3 G-4 G-5 G-6
0.5 ± 0.3 0.7 ± 0.3 19 ± 2 34 ± 2 7±2 42 ± 2
0.06 0.11 0.93 0.99 0.84 1
27.0 30.1 46.6 46.7 45.6 46.1
Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005
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the Ce3+ PL in glass hosts and strong reducing conditions are necessary to achieve high luminescence quantum yield. 4. Conclusion In summary, the effect of different synthesis conditions on Ce3 + photoluminescence, decay lifetime and the external quantum yield has been investigated in Ce3 + doped borate glasses. The airatmosphere synthesized glasses showed poor luminescence performance with luminescence quantum yield less than 1%. The results are attributed to the presence of Ce4 + ions due to the oxidizing atmosphere, which act as strong quenching centers for the Ce3 + luminescence. On the other hand, the glasses prepared under reducing atmosphere showed superior luminescence properties with exponential decay behavior. Though the mild reducing conditions such as 5% H2 gas flow could effectively reduce Ce4 + ions to the Ce 3 + state and reduced the non-radiative energy losses; a trace amount of Ce4 + still exists in the glass, which is responsible for its inferior luminescence quantum yield. Among the synthesis conditions studied, the glass prepared in strongly reducing carbon enclosed double crucible plus 0.5 wt.% carbon doping condition showed the best performance with luminescence quantum yield of about 42%, which is one of the highest value in glassy hosts reported so far. Acknowledgments This work is carried out under the JSPS Post-Doctoral Fellowship program (P 13372). References
Fig. 6. Sample spectrum and the reference spectrum of the quantum yield experiments of the studied glasses. The x-axis has been rescaled for clear view of the excitation region (368 nm–376 nm) and the emission region (378 nm–700 nm) of the measurement.
not sufficiently affect the Ce3 + decay kinetics, i.e. very weak nonradiative interactions with the Ce3+ ions, but it can still absorb the excitation signals owing to its strong absorption coefficient at the excitation region giving rise to an unwanted absorption loss by the glasses. In the case of the mild reducing conditions like G-3 glass, the absorption loss is more than that in the strong reducing condition in the G-6 glass. Therefore, it can be concluded that the reducing conditions play a vital role on
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Please cite this article as: A.D. Sontakke, et al., Effect of synthesis conditions on Ce3 + luminescence in borate glasses, J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.005