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Abstract
The construction of cerium-doped materials is of great technological importance for various applications, including smart lighting, biological shielding and high-energy ray and particle detection. A major challenge is the efficient prevention of the undesired colorization after cerium doping. Here we present the nanocrystallization method for constructing colorless cerium-doped glass with extremely high cerium concentration (15 mol%). The structure and optical characterizations confirm that the notable color change of glass is associated with the precipitation of CeF3 crystalline phase during heat-treatment. The chemical state investigation shows that most of cerium ions exist in the form of Ce3+ in both the glass and glass-ceramic samples. The chemical environment study indicates a dramatic change in the local structure unit from -Ce-O- to -Ce-F-, which is believed to dominate the decoloring phenomenon in cerium doped glass. As a result, a significant improvement in the ultraviolet excited luminescence (~35 times enhancement in intensity) and scintillating performance can be achieved, pointing to potential applications in X-ray detection.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Scintillators are luminescent materials that convert the incident particles or energetic photons into photons with lower energy, which can be detected via optoelectronic components or devices [1–3]. Compared with single crystal and ceramics, glass materials doped with rare earth ions offer superior advantages of high dopant solubility, large-volume and easy shaping of elements [4–9]. Especially, glass can be easily drawn into low-loss fibers and further constructed into compact fiber device which allows for simultaneous remote and real-time radiation monitoring where direct access is totally denied [10–13].
Cerium-doped glass provides an excellent model system for scintillators because of its attractive properties such as fast decay time (~50 ns), excellent chemical durability and compatible with commercial photomultiplier tubes (PMTs) [14–20]. Although the substantial progress in application of cerium-doped glass for radiation detection, the construction of cerium-activated scintillators with high dopant concentration remains a significant challenge. One underlying limitation is associated with the unexpected cerium clustering in the form of Ce-O-Ce bonds which gives rise to the yellow color and substantially quenches the radiation transition of the activator ions [21–23]. Here we present experimental observation and mechanistic investigation of crystallization mediated color bleaching and notable enhanced scintillation performance. We find that introduction of fluorine ion (F-) and the subsequent nanocrystallization results in the dramatic reduction in the absorption coefficient in the visible wavelength region (~60%) and a substantial increase in the ultraviolet excited luminescent intensity (~35 times).
2. Experimental
2.1. Material synthesis
Glass samples with nominal composition of 40SiO2-28Al2O3-17NaF-15CeF3 (in mol%) were prepared by melt-quenching method in air atmosphere. Raw materials (20 g) were melted in a covered alumina crucible at 1450 °C for 2 h. The homogenized melt was cast into a slab on a preheated brass plate to form the precursor glass (named as PG). Subsequently, PG was heat-treated for 2 h ranging from 600 to 800 °C to form transparent glass-ceramic (GC) samples, which were named as GC600, GC800, etc. All samples were cut and polished to plates of about 10 × 10 × 1 mm3 in size for further characterization.
2.2. Material characterization
The nanocrystal phase in PG and GC samples was examined by X-ray diffraction (XRD) using Cu/Kα radiation. The Raman spectra of PG and GC samples were recorded on a Renishaw InVia spectrometer with a 532 nm laser source. Microstructures were analyzed by transmission electron microscopy (TEM), which was performed using a JEOL 2010F (scanning) transmission electron microscope. The absorption spectra were measured by a JASCO FP-6500 double-beam spectrophotometer. The composition and oxidation state of cerium in these glasses were quantified using X-ray photoelectron spectroscopy (XPS). The X-ray absorption near-edge structure (XANES) spectra of the PG and GC samples at the Ce L3-edge were collected at room temperature in the transmission mode, using the 1W1B beam line (XAFS station) at Beijing Synchrotron Radiation Facility. The ring energy was operated at 1.85 GeV and 205 mA. The X-rays were monochromatized using an Si (111) double-crystal spectrometer, and the energy was calibrated by a Cr-foil reference. The Athena software, based on the IFEFFIT program, was used to analyze XANES data. The excitation and photoluminescence spectra were measured using a FLS920 fluorescence spectrophotometer (Edinburgh Instrument Ltd., U.K.). The X-ray excited luminescence spectra were recorded on a homemade spectrophotometer with an X-ray tube of W anode operating at 70 kV and 1.5 mA.
3. Results and discussion
A typical cerium-doped oxyfluoride glass system before and after crystallization was studied. Figure 1(a) shows the XRD patterns of PG and GC samples heat-treated at various temperatures. The broad diffraction band observed in PG sample can be attributed to the amorphous phase. After heat-treatment, the XRD patterns of GC samples present characteristic sharp diffraction peaks which can be indexed to the isostructural CeF3 crystalline phase (PDF 01-086-0967). The degree of crystallinity for GC800 can be calculated and it is estimated to be ~69.1%. The Raman scattering spectra of PG and GC samples are compared in Fig. 1(b). The Raman spectrum of PG is similar to that of the oxyfluoride glass, showing broad bands at ~320 and 380 cm−1 which indicate the amorphous nature of the sample. The Raman spectra of GC samples heat-treated at various temperatures exhibit sharp peaks which can be indexed to the symmetric stretching vibration of CeF3 crystalline phase (v2 vibration at 488 cm−1). With the enhancement of heat-treatment temperature, the Raman scattering intensity of fingerprint peaks gradually increases, indicating the improvement in crystallinity. To provide more information about the microstructure of the GC samples, TEM characterizations was performed and Fig. 1(c) displays the TEM images of GC680. Low-resolution TEM confirms that the nanocrystals are homogeneously dispersed in glassy matrix. High-resolution TEM exhibiting lattice fringes with the d-spacing of ~3.20 Å, which can be indexed to the (111) plane of crystalline CeF3. Above results provide direct evidence that the heat-treatment leads to the precipitation of CeF3 nanocrystals inside glass.
Fig. 1 XRD and TEM characterizations of glass and GC samples. (a) XRD patterns of the PG sample, GC samples and the reference data of PDF card No. 01-086-0967 for CeF3. (b) Raman spectra of the PG and GC samples. (c) Low-resolution and high-resolution TEM images of the GC680 sample.
Interestingly, the color of the samples was changed after nanocrystallization. As shown in Fig. 2(a), the PG sample is yellow while the GC samples presents light yellow and even colorless. There is a clear positive relationship between the color of the sample and the heat-treatment temperature. Absorption spectrum was employed to study the color change. As indicated in Fig. 2(b), a notable blue-shifting of the cut-off edges can be observed by increasing the heat-treatment temperature. Simultaneously, the absorption coefficient at 400 nm shows a sharp decrease during the occurrence of nanocrystallization. Figure 2(c) presents the typical absorption spectra of PG, GC640 and GC680 samples. It can be found that the absorption coefficient at 400 nm for GC680 is ~60% smaller compared with that of PG sample. This is crucial for cerium-doped luminescent materials because the radiative transition process of cerium dopant occurs around this waveband and the details will be discussed in the following section.
Fig. 2 Optical absorption change during nanocrystallization of glass. (a) The photographs of the PG and GC samples. (b) Absorbance at 400 nm and the bandgap of the PG and GC samples. (c) Absorption spectra of the PG, GC640 and GC680 samples.
The observed abnormal absorption change is mainly contributed by two dominant factors, the valence state and the local environment change of cerium dopant. XPS was employed to characterize the chemical state of cerium from glass powder. Figure 3(a) shows the fingerprint peaks of Ce 3d in PG and GC samples. Due to the occurrence of obvious overlapping of 3d photoelectron transitions for various cerium ions, curve-fitting was performed to distinguish them. Table 1 presents the binding energy values for identified peaks, six corresponding to Ce4+ state (v, u, v″, u″, v‴ and u‴) and four representing Ce3+ state (v0, u0, v′ and u′), as well as the relative position, full width at half maximum (FWHM), and area constrained determined from the Ce3+ and Ce4+ standards [24–26]. Figs. 3(b) and 3(c) show the curve-fitting results of PG and GC680 samples. The results clearly show that there are two oxidation states of cerium ions (Ce3+ and Ce4+) in the PG and GC680 samples. Notably, the characteristic bands of Ce4+ exhibit no obvious change in intensity. This fact rules out the contribution of valance state change to the colorization because it is well known that the appearance of Ce4+ ions will produce the yellow color of the sample.
Fig. 3 Characterization of the chemical state change during nanocrystallization of glass. (a) Ce 3d XPS spectra of the PG and various GC samples. (b) Curve-fitting in Ce 3d XPS spectra for a vacuum fracture surface from the PG sample. (c) Curve-fitting in Ce 3d XPS spectra for a vacuum fracture surface from the GC680 sample.
Table 1. Ce 3d XPS Peak-Fitting Constraints
†Calculated from the initial individual fit for u‴.
To further explore the valence state and the local environment change of cerium during nanocrystallization, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) characterizations were performed. The XANES spectra of cerium in PG, GC640 and GC680 were compared with that of the standard CeF3 and CeF4 samples. As shown in Fig. 4(a), the Ce L3-edge position of PG, GC640 and GC680 are similar to that of CeF3 (~5722 eV), while smaller than that of CeF4 (~5727 eV), indicating that cerium mainly exists in the form of Ce3+ in the glass and GC samples. The inconsistency between XANES and XPS results are supposed to be associated with the notable difference in test depth, 10 μm for XANES while less than 10 nm for XPS. Accordingly, XANES provides cerium signal in volume phase and XPS is dominated by the cerium signal closing to the sample surface which might be partially oxidized to Ce4+. Figure 4(b) shows the k2-weighted EXAFS spectra of PG, GC640, GC680 and CeF3 samples. It can be observed that the EXAFS signal from as-made glass is characterized by the single oscillation mode with relatively weak intensity. In contrast, the EXAFS signals from GC640 and GC680 samples are similar to CeF3 crystal and present multiple oscillation modes with notable increase in oscillation intensity. Above results suggest that the local structure in GC samples is featured in high-symmetry [CeF8]5- unit cells and that in PG is characterized by the existence of hybrid Ce-F and Ce-O bonding.
Fig. 4 Characterization of the chemical environment change during nanocrystallization of glass. (a) Ce L3-edge XANES spectra from PG, GC640, GC680 and the standard CeF3 and CeF4 samples. (b) k2-weighted χ(k) spectra of PG, GC640, GC680 and the standard CeF3 samples. (c) Absorption spectra of CeF3, CeO2, CePO4 and CeF4.
The above results collaboratively imply that the observed abnormal absorption change is mostly probably associated with the local chemical environment change during nanocrystallization. To further confirm this contribution, the absorption spectra of the standard samples composed of Ce-F and Ce-O frameworks were investigated. As shown in Fig. 4(c), the structure with Ce-F bonding (CeF3 and CeF4) exhibits a cut-off absorption less than 350 nm. In stark contrast, the structure with Ce-O bonding (CePO4 and CeO2) shows a notable red-shift and the cut-off absorption is estimated to be larger than 400 nm. In addition, the absorption coefficient of Ce3+ is much lower than Ce4+. Therefore it can be supposed that the existence of Ce3+ center and Ce-F bonding cooperatively contributes to the low absorption coefficient of CeF3. Taken together, above data suggest that the color change from PG to GC is originated from the local chemical environment modulation around Ce3+ from oxygen-rich to fluorine-rich structure during nanocrystallization.
The crystallization mediated color bleaching in cerium-doped glass leads to a significant improvement in the luminescent properties. Figure 5(a) shows the ultraviolet excited photoluminescence spectra of PG and GC samples. The typical broad emission band peaked at ~370 nm under excitation at 254 nm can be attributed to the allowed transitions from the 5d excited states of Ce3+ ions to its 4f ground state. Figure 5(b) shows the heat-treatment dependent luminescence intensity change of the PG and GC samples. Significantly, GC samples after nanocrystallization exhibit highly intense luminescence and the luminescence intensity of GC680 is ~35 times higher compared with the PG sample, which exhibits extremely weak luminescence. Figure 5(c) exhibits the corresponding decay curves of the luminescence at 370 nm for PG and GC samples. The results show that the nanocrystallization also leads to the extension of the fluorescence decay process and the lifetime of GC680 is estimated to be ~33 ns. The observed notable increase in decay lifetime with the enhancement of heat-treatment temperature should be associated with the change of the local chemistry around active centers [27,28]. Especially, the enrichment of F- ions may significantly decrease the phonon energy of the local structure around Ce3+, thus facilitating the radiative transition process of the active centers. The intense luminescence from Ce3+ with short lifetime observed in GC samples implies their potential applications as scintillating glass. To explore it, scintillating properties were characterized and Fig. 5(e) shows the X-ray excited luminescence spectra of PG and GC samples. It can be observed that the GC samples present obvious luminescence at 385 nm under irradiation with X-ray while PG sample does not show obvious signal. Figure 5(f) summarizes the heat-treatment temperature dependent X-ray excited luminescence signal and it is clear that the nanocrystallization leads to a dramatic enhancement in the signal intensity. The observed simultaneous improvement in the photoluminescence and X-ray induced luminescence can be ascribed to the success in effective suppression of the absorption around at near-ultraviolet region (Fig. 2(c)). The slight decrease in luminescence intensity from GC680 to GC800 might be associated with the optical scattering effect. Additionally, compared with photoluminescence emission spectra, X-ray excited luminescence spectra exhibit notable red-shift, which is resulted from the different interaction mechanism under ultraviolet light excitation and X-ray excitation in the PG and GC samples. Under ultraviolet excitation, Ce3+ ions are directly excited into excited states by absorbing high energy photons from excitation source, and then emit low energy photons. However, under X-ray excitation, scintillating process is much more complicated. Firstly, the initial energy is released via formation of “hot” electrons and holes. Then, the energy is transferred to luminescent centers through formation of excitonic states and groups of excited luminescent centers. Finally, the relaxation of excited luminescent centers results in the scintillating emission.
Fig. 5 Characterizations of the ultraviolet and X-ray excited luminescence. (a) Ultraviolet excited luminescence spectra of the PG and GC samples. The excitation wavelength is 254 nm. (b) Heat-treatment temperature dependent ultraviolet excited luminescence intensity. (c) Decay curves at 370 nm of various samples. (d) Heat-treatment temperature dependent ultraviolet excited luminescence lifetime at 370 nm. (e) X-ray excited luminescence of various samples. (f) Heat-treatment temperature dependent X-ray excited luminescence intensity.
4. Conclusion
In conclusion, we have presented a basic strategy for manipulating the colorization of cerium-doped glass. The principle, which involves the tuning the chemical environment around cerium through controllable nanocrystallization, provides convenient access to colorless cerium-doped glass. The protocol enables a significant increase of the radiative transition probability of the cerium ions. Thus it provides an effective way to construct efficient cerium-doped scintillating glass and simultaneously mitigates the stringent reducing atmosphere control issue associated with conventional methods. Furthermore, we note that the strategy should be general to other cerium-doped functional materials (e.g., phosphor and radiation absorber), point to promising applications in high-energy ray and particle detection, smart lighting and biological shielding.
Funding
National Key R&D Program of China (2018YFB1107200); National Natural Science Foundation (Grant 11474102); National Science Fund for Excellent Young Scholars of China (Grant 51622206); Local Innovative and Research Teams Project of Guangdong Peal River Talents Program (Grant 2017BT01X137); Tip-Top Scientific and Technological Innovative Youth Talents of Guangdong Special Support Program (Grant 2015TQ01C362); Fundamental Research Funds for the Central University; and Open Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), China (IPOC2016B003).
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