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Optical properties of Ce3+-doped Li2O-B2O3-SiO2 glasses prepared in inert atmosphere have been examined. Clear splitting of 4f-5d excitation bands is observed in these glasses at room temperature. The emission decay constant depends on the excitation and emission energy, indicating the site distribution of Ce3+ in the random glass matrix. The absorption energies of Ce3+ are independent of the optical basicity, suggesting that the emission is affected by the local basicity of random network. The present findings provide the basic concepts for designing Ce-doped oxide glass for luminescent applications.
© 2015 Optical Society of America
1. Introduction
The Nobel Prize in Physics, 2014, for the development of the blue light emitting diode [1] is expected to focus attention on the development of phosphors containing appropriate activators. Although both the Eu2+ [2–4] and Ce3+ cations exhibit strong light emission arising from the allowed 4f-5d transition, the decay constant of Ce3+ is of the order of several tens of nanoseconds, which means that it provides a fast response [5–9]. There is currently a need for developing alternative thermal neutron detectors that can replace the conventional 3He-based systems [10]. A minimum requirement for these is that the host material should contain either 6Li or 10B, which have high interaction probabilities with thermal neutrons. Although conventional crystal phosphors are mostly powders [11], we feel that there is potential for monolithic bulk materials for thermal neutron detection. There are several reports on Li-containing solid-state materials exhibiting good scintillation properties towards a neutron source [12–15]. One of the preferred candidates is oxide glass [14, 15], since it possesses good chemical stability and a wide composition range. However, research on glass materials for detecting thermal neutrons has been limited, and only one material is commercially available: Li-glass [16]. Although some materials are commercially available [17], the chemical design of the host materials has not been fully clarified. We, therefore, believe that there is room for further study of glass-based detectors with high interaction probabilities with thermal neutrons.
Recently, we have reported the scintillation properties of Ce-doped lithium borosilicate [18], and Ce-doped aluminosilicate glasses [19], which have the same chemical composition as GS20 [20]. Although the compositions included lithium (and boron) species, the emission intensities were not very high, because melting in air led to the oxidation of Ce3+ to Ce4+. Because melting of oxide glasses in an air atmosphere often leads to such oxidations of the emission center [21, 22], Ce3+-containing oxide systems have mainly been reported for phosphate glasses exhibiting low optical basicity [7–9, 22]. In addition, the use of a metallic salt as Ce source rather than CeO2 is desirable for melting under inert conditions. It has often been reported that basicity of the glass influences the impurity cation valence [23–25], which can affect the Ce3+-Ce4+ redox reaction. We have, therefore, also examined the relationship between the basicity of the oxide and the optical properties by changing the chemical composition of the glass.
The aim of this present work is to study the optical absorption properties in the UV region of a Ce-containing lithium borosilicate glass prepared in an inert atmosphere. In addition, we discuss the photoluminescence (PL) properties of several lithium borosilicate glasses possessing different B2O3/SiO2 molar ratios.
2. Experimental
The chemical composition of present glass was xCe3+-40Li2O–yB2O3–(60-y)SiO2 (in mol%), with Ce3+ added in excess. A mixture of Li2CO3 (99.99%), B2O3 (99.9%), SiO2 (99.999%), and Ce(OCOCH3)3·H2O (99.9%) was melted in an electric furnace at 1000°C for 30 min in an Ar (99.999%) atmosphere. The glass melt was quenched on a stainless plate at 200°C and then annealed at glass transition temperature Tg for 1 h; this was measured by differential thermal analysis (DTA). The samples were mechanically polished to obtain a mirror surface.
Tg was determined on a TG8120 DTA system (Rigaku) operating at a heating rate of 10°C/min. The PL and PL excitation (PLE) spectra were recorded at room temperature (r.t.) using a F7000 fluorescence spectrophotometer (Hitachi High-Tech.). Band pass filters of 2.5 nm for the PL measurement were used for both excitation and emission. The absorption spectra at r.t. were recorded using a U3500 UV-vis-NIR spectrometer (Hitachi High-Tech.). The absolute quantum efficiency (QE) of emission of the glass was measured using an integrating sphere Quantaurus-QY(Hamamatsu Photonics). The errors of the measurement were ± 3%. The emission decay at r.t. was measured using Quantaurus-Tau (Hamamatsu Photonics) with several LED sources.
For X-ray induced scintillation spectra, samples were coupled to the monochromator equipped charge coupled device (CCD, Andor DU-420-BU2) via optical fiber. The emission spectra were measured by irradiation of X-ray supplied with bias voltage of 50 kV and tube current of 4 mA. The detailed description about the geometry can be shown the past work [26].
3. Results and discussion
3.1 Optical properties of Ce-doped 40Li2O-40B2O3-20SiO2 glasses
First, we examined Ce concentration-dependent optical property of xCe3+-40Li2O-40B2O3-20SiO2 (xCeLBS40) glasses. The glasses obtained were colorless and transparent, and did not show any precipitation of nano-crystallites. Figure 1 shows the optical absorption spectra of xCeLBS40 glasses at r.t. Absorption spectrum of the non-doped LBS40 glass is also shown. According to previous reports [7, 27, 28], absorption spectra of the Ce-containing LBS40 glasses can be deconvoluted to the intrinsic absorption of LBS40 glass and six absorptions due to five individual 4f-5d bands and a 4f-6s band. These are found by peak deconvolution using Gaussian functions, with the full width half maximum (FWHM) of 4500 cm−1, whose band width is broader than that in a phosphate glass [27]. The absorption coefficient of Ce3+ increases linearly with the Ce3+ concentration without the formation of any additional absorption band due to aggregation, suggesting that Ce3+ is dispersed homogenously in the glass matrix. The optical absorption edges red-shift with increasing amounts of Ce3+, which has also been observed in previous reports on Ce3+ [28] or Sn2+ [29], where excitation properties are seen to be affected by the coordination field.
Fig. 1 Optical absorption spectra of xCe3+-40Li2O-40B2O3-20SiO2 (xCeLBS40) glasses. Dashed lines show that PLE spectra can be deconvoluted into six excitation bands.
Figure 2(a) shows PL and PLE spectra of the xCeLBS40 glasses (x = 0.1, 0.5, 1.0, and 2.0). The asymmetric, broad excitation band occurs over a wide range of the UV region, whereas the PL peak indicates Gaussian-like spectra whose FWHM is approximately 5600 cm−1. In non-doped LBS40 glass possessing no excitation band in UV region (see Fig. 1), PL is hardly observed. Although it has been reported that the emission of Ce3+ is split in a crystal field into 2F5/2 and 2F7/2 bands, with a separation of approximately 2000 cm−1 [27], we were unable to fit these PL spectra using two components with a fixed photon energy. Therefore, we can conclude that both emission bands are broadened and affected by the coordination field, leading to a continuous energy shift. Figure 2(b) shows PL and PLE peak energies, as a function of the amount of Ce, together with the Stokes shift, which corresponds to the peak energy difference between the PLE and PL bands,. Since the PLE energy is almost constant, the increase in the Stokes shift upon increasing Ce concentration is mainly due to the red-shift of the PL band, i.e., narrowing of the f-d energy level for emission.
Fig. 2 (a) Normalized PL-PLE spectra of xCeLBS40 glasses containing different amounts of Ce. (b) PL-PLE peak energies as functions of the Ce content. The Stokes shift is also shown.
Figure 3(a) shows a PL-PLE 3D contour plot of the 0.5CeLBS glass. The vertical and horizontal axes show the photon energy of excitation and emission, respectively. The broad excitation band suggests an effective excitation continuum band, in agreement with Fig. 2(a). The peak energy of the emission red-shifts slightly with decreasing excitation photon energy, i.e., the lower the excitation energy, the smaller the emission energy. Both excitation and emission peak energy of Ce3+ of this glass are located at lower energies than those in the case of phosphate glass [7, 23], while they are higher than that of silicate glasses [23]. Figure 3(b) shows the emission decay curves of the 0.5CeLBS glass with different excitation energies. The inset shows the emission decay curves of the xCeLBS glasses with excitation at 35700 cm−1. The decay constants of the glasses in the concentration region (x = 0.1–2.0) are almost equal, as shown in the inset, suggesting that there is no concentration quenching. In contrast, the decay constant depends on the excitation energy, although all relaxation processes appear to be exponential; the lower the excitation energy is, the longer the lifetime becomes. The decay constants at different monitored energies by different excitation energies are summarized in Table 1. Differences in the rate constant for exponential decay mean that both excitation and emission energy levels possess site distribution due to random networks in the glass; the lifetime becomes longer as the emission decreases. Since the actual number of excited Ce3+ with lower photon energy excitation is much less than the total amount of Ce3+, the decay curve indicates that the amount of activated Ce3+ cation is very small, similar to what happened if excitation occurs in low-concentration material. It is noteworthy that the emission peak energy red-shifts with increasing Ce concentration (see Fig. 2(b)). Since similar decay constants are observed with these glasses, whose monitored peak energies were different, (inset of Fig. 3(b)), it can be expected that the peak shift can mainly be attributed to the local coordination change. Further, since the 0.1Ce-LBS40 glass exhibits the highest QE as shown in Table 2, it can be concluded that a strong absorption band does not directly connect to an effective light energy conversion.
Fig. 3 (a) PL-PLE 3D contour mapping of xCe-LBS40 glass. (b)Emission decay curves of the 0.5Ce-LBS40 glasses obtained at different excitation energies shown in Fig. 3(a). Inset shows the emission decay curves of the xCeLBS glasses with excitation of 35700 cm−1.
Table 1. PL decay constants of 0.5Ce-LBS40 glasses monitored at different energies.
Table 2. QE values of xCeLBS glasses.
Figure 4 shows the relationship between the optical absorption and the PLE spectrum of the 0.5CeLBS40 glass. The three vertical dotted lines designate different photon energies used for the emission decay measurement, as indicated in Fig. 3(b). As can be seen in Fig. 4, both 27400 and 29400 cm−1 excitations correlate with the lowest 2F-2D absorption band, while the excitation peak energy corresponds to the onset of the optical absorption edge. Deeper excitation gives a smaller emission intensity, which may be due to the energy loss by phonon vibration.
Fig. 4 Comparison between optical absorption and PLE spectrum of 0.5Ce3+-40Li2O-40B2O3-20SiO2 (0.5CeLBS40) glasses. Dotted lines indicate the excitation energies shown in Fig. 3. Dashed lines show that PLE spectra can be deconvoluted into five excitation bands.
3.2 Optical properties of 0.5Ce-doped 40Li2O-yB2O3-(60-y)SiO2 glasses
It has been reported that the valence state of Ce is affected by the local coordination state, i.e., local basicity, of the glass [27, 28]. In this study, we have tuned the basicity by changing the compositional ratio of the glass network oxides: B2O3 and SiO2. We can calculate the theoretical optical basicity Λth of Li2O-B2O3-SiO2 glass using the following Eq. (1):
where ΛMO and ΧMO are the optical basicity and metal oxide (MO) fraction in the glass composition, respectively. Because ΛLi2O, ΛB2O3, and ΛSiO2 are reported to be 1.00, 0.42, and 0.48, respectively [30], the substitution of B2O3 for SiO2 decreases the average optical basicity of the glass.Figure 5 shows the optical absorption spectra of the 0.5Ce3+-40Li2O-yB2O3-(60-y)SiO2 (0.5CeLBSy) glasses. Absorption coefficient at the absorption edge increases with increasing amounts of SiO2, suggesting that the optical basicity of the glass affects the dispersion and valence state of the Ce3+. If an increase in the optical absorption in the SiO2-rich glasses is brought about by a redox reaction transforming Ce3+ into Ce4+, the result corresponds to the conventional view that glass melts with lower basicity contain a dopant possessing lower valence state: i.e., Ce3+ is preferentially generated in the B2O3-rich glasses. Since the absorption tail of Ce4+ is observed at lower energy region [7], generation of Ce4+ brings the red-shift of the absorption tail. Indeed, the QE of the 0.5Ce-LBS40 glass is the highest (73%) among these 0.5Ce-containing glasses (see Table 3), which may be affected by the absorption due to Ce4+. It is noteworthy that these peak energies are the same, although relative intensity changes with the chemical composition. Therefore, the 2F-2D energy diagram is fixed although the macroscopic basicity of the glass has been changed. Similar results are also observed in Sn2+-center-containing phosphate glasses, in which the emission did not change the optical basicity [31]. Here, we have assumed that the local coordination state of Ce3+ affects each absorption (excitation) band, and that average Ce3+/Ce4+ ratio is affected by the macroscopic basicity of the glass. Although we have not obtained clear proof for this assumption, the results obtained will provide a guideline for Ce-containing oxide glass possessing good emission properties.
Table 3. QE values of 0.5CeLBSy glasses.
3.3 X-ray induced scintillation of Ce-doped 40Li2O-40B2O3-20SiO2 glasses
As a simulation of application for thermal neutron scintillation detectors, X-ray induced scintillation spectrum is a touchstone of the performance. Figure 6 shows X-ray induced scintillation spectra, whose irradiated dose was 1Gy, of the xCeLBS40 glasses containing different amounts of Ce. Emission peaks of these glasses are slightly red-shifted according to Ce concentration, which is also observed in PL spectra as shown in Fig. 2. We can estimate concentration quenching threshold using the correlation between Ce concentration and the scintillation intensity. From the emission intensity, it indicates that a concentration quenching slightly occurs around 2.0 mol% Ce in scintillation. It is notable that emission spectra shapes are non-symmetric, which might correlate with optical absorption (self-absorption) of the glass. Thus, it is suggested that large Stokes shift is preferable for improvement of efficient detection. The present data show the potential of the Li-containing glasses for neutron detection, because irradiation of neutron, whose energy is several orders of magnitude larger than that of X-ray, easier induces 10B or 6Li related nuclear reactions. In addition, the scintillation spectra shapes simultaneously show a strategy for design of glass-based materials exhibiting high emission efficiency.
4. Conclusion
We have examined the optical absorption and PL properties of lithium borosilicate glasses containing different amount of Ce3+. Clear f-d absorption bands are observed, whose relative intensities depend on the optical basicity of the glass. The lowest absorption band gives the strongest emission intensity, and the emission decay constant depends on the excitation and emission energy. The present study provides a guideline for obtaining Ce-containing non-phosphate glass phosphors.
Acknowledgment
This work was partially supported by the JSPS KAKENHI Grant-in-Aid for Young Scientists (A) Number 26709048. Collaborative Research Program of I.C.R., Kyoto University (grant #2014-31), and the SPRITS program, Kyoto University.
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