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Abstract
We examined the relationship between defect formation in the UV region by X-ray irradiation and its luminescence properties of binary zinc phosphate glasses. The emergence of absorption bands in the UV region linearly increased on increasing the irradiation dose. For up to 10 Gy irradiation of X-ray from a tungsten source, the generated absorption bands disappeared after annealing at 350 °C. Moreover, the thermally stimulated luminescence (TSL) intensity linearly increased on increasing the irradiation dose. There is a linear relationship between the peak area of TSL and that of the generated absorption bands. In contrast, the absorption, i.e., defect, generated by Cu-Ka 1000 Gy irradiation survived after annealing at 350 °C. The generated defects served as emission centers of photoluminescence (PL), which was confirmed by comparison between the optical absorption and PL excitation spectra.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Random network is a characteristic of glass that is different from the ordered crystal structure. As the random network is the origin of the liquidus phenomena of the matter, it strongly correlates with good formability. Glass exhibits a wide chemical composition range and it contains various local structures as well, i.e., the diversity of network connection [1]. The diversity of the glass network can allow the existence of defects with greater ease than in single crystals [2,3]. Although defects often act as energy loss sites for energy propagation, however, they can work as emission centers that are similar to activators in phosphors. Various kinds of defects exist in glass, and some of them act as emitting sites [4–8]. It may be said that the tailoring of defects in glass is a possible approach for preparing functional glasses.
Then again, various factors act as the origins of defects, e.g., the chemical composition, preparation process, and post treatment of glasses can affect the defects in glasses. It is reported that irradiation by ionizing particles, such as X-rays or thermal neutrons, to glass often induces the formation of defects in the matrix [9–12]. The formation of defects by ionizing-particle irradiation are affected by various kinds of radiation sources, which can react with electrons in the inner-shell owing to the large energy existing far beyond the band gap of the materials [9–12]. On the other hand, a thermal annealing process is effective in erasing the defects even after heating below the glass transition temperature, Tg, which is correlated to the thermal stability of the defects. For example, silver-doped phosphate glasses have been industrially used as a monitoring material and annealing is effective for its reuse [13].
Here, we have examined the relationship between X-ray-induced defect formation and thermally stimulated luminescence (TSL) of oxide glass. As it is expected that defect formation easily occurs in lower-melting glass, we focus on phosphate glasses whose optical changes after X-ray irradiation have been reported in several papers [14–27]. Among these phosphate glasses, we selected zinc phosphate (ZP) glass whose three dimensional structure was recently reported [1]. Although ZP glass without an activator exhibits weak emission owing to defects, the correlation between the defect formation and luminescence properties has not been fully clarified thus far. Since activator-doped ZP glasses exhibit good photoluminescence (PL) conversion efficiency [28–30], the examination of defects is important for further understanding. From the results of optical absorption and TSL, we have discussed the relationship between the X-ray-induced defect formation and relaxation process.
2. Experimental
The ZP glass was prepared by a conventional melt-quenching method. The nominal chemical composition of the ZP glass was 60ZnO–40P2O5 (in units of mol%). The preparation scheme in ambient atmosphere has been previously reported [1,31]. After quenching and annealing at the glass-transition temperature (Tg), the glass sample was cut into dimensions of 10 mm × 10 mm × 1 mm and mechanically polished using diamond slurry to obtain a mirror surface.
The Tg was measured by differential thermal analysis (DTA, Thermo Plus 8120, Rigaku) at a heating rate of 10 °C/min. Ultraviolet and visible (UV–vis) absorption spectra were measured by V670 spectrophotometer (JASCO, Japan). PL spectra and PL excitation (PLE) spectra were recorded with a fluorescence spectrophotometer (F-7000, Hitachi). Slits for achieving a spectral resolution of 2.5 nm were used for both excitation and emission measurements.
In the present study, we used two X-ray sources for irradiation. One was Spellman X-ray generator (XRBOP&N200X4550), which is equipped with a conventional X-ray tube with a tungsten (W) anode and Be window. The supplied bias voltage and tube current of W were 40 kV and 0.052 ~5.2 mA, respectively. Another X-ray source is a Cu-Kα anode whose supplied bias voltage was 40 kV in Miniflex 600 (RIGAKU) or RINT 2100 (RIGAKU). The tube currents of the RIGAKU Miniflex 600 and RINT 2100 were 15 mA and 40 mA, respectively. These X-ray doses were calibrated using an ionization chamber (PTWTN30013). The TSL glow curves of all glasses were measured by using a TSL reader (TL-2000, Nanogray Inc) after irradiation by X-rays at several different doses. The detection range is approximately 320-520 nm, which covers most of the observed emission region. The measurement temperature range and heating rate were 50–350 °C and 1 °C/s, respectively. Electron spinning resonance (ESR) measurements of ZP glasses were performed in the range of the X-band (~9.8 GHz) at room temperature (RT) using ESP300E (Bruker).
3. Results and discussion
The Tg of the transparent ZP glass was approximately 410 °C. Considering the Tg value, we set the annealing temperature at 350 °C as mentioned in the experimental section. Figure 1(a) shows the PL-PLE contour plot of the ZP glass. In the 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. As shown in the figure, there is a broad weak emission band. The upper figure shows the PL spectrum of the glass by 5.9 eV that is indicated in the figure using a dotted line. As there is no activator in the glass, it is expected that this emission observed in PL is caused by intrinsic defects in the ZP glass. Figure 1(b) shows the scintillation spectra of the glass with different irradiation doses using W X-ray source. Broad emission is observed whose intensities increase on increasing the irradiated dose. As there is a possibility that X-ray irradiation induces defect formation during irradiation, we checked the optical absorption spectra before and after X-ray irradiation.
Fig. 1 (a) PL-PLE contour plot of the ZP glass. Upper figure shows PL spectra of the ZP glass excited by 5.9 eV (210 nm) that is indicated by using a dotted line in the contour plot. (b) X-ray (tungsten source)-induced scintillation spectra of the ZP glass with different irradiation doses.
Figure 2(a) shows the optical absorption spectra of ZP glasses after X-ray (W) irradiation with different doses. Compared with the non-irradiated sample (0 Gy), absorption coefficient in the UV region increases. Since it is reported that the absorption edge of the ZP glasses is more than 6 eV [30], these generated bands exist between the valence band and the conduction band of the host ZP glass. It is notable that the absorption coefficient is continuously increased, suggesting that several defects exist in the ZP glass. Optical absorption spectra of the 10 Gy X-ray irradiated ZP glasses after annealing at 350 °C are shown in Fig. 2(b). Since the absorption measurements after X-ray irradiation followed by annealing were performed twice in order to check reproducibility, the average values are also shown in the inset. As shown in the inset, the difference of absorption coefficient is less than 0.05 cm−1. Therefore, we have found that the emerged absorption bands, i.e. generated defects, induced by this irradiation dose are erasable by thermal annealing. Figure 2(c) shows the difference optical absorption spectra, which are obtained by subtracting the spectrum of non-irradiated ZP glass as a standard. On increasing the irradiated dose, absorption coefficients of the emerged bands linearly increase. As mentioned above, the absorption coefficients continuously increase, although the spectra shapes are non-symmetric to the photon energy. It indicates that both effects (the asymmetric shape and the dose-dependent shape of the absorption peak) are included. We can, therefore, conclude that the generated defects consist of several species. The relationship between the emerged absorption area induced by X-ray irradiation and the irradiated dose is shown in Fig. 2(d). There is a linear relationship between the emerged absorption bands and irradiation dose.
Fig. 2 (a) Optical absorption spectra of ZP glass after different X-ray (tungsten source) irradiation doses. (b) Optical absorption spectra of the 10 Gy X-ray irradiated ZP glass after annealing. The X-ray irradiation and annealing processes were done twice in order to check reproducibility. Inset shows reproducibility of difference absorption spectra. (c) Difference absorption spectra of ZP glass after different irradiation doses, which are obtained by subtracting the spectrum of non-irradiated ZP glass. (d) Difference of absorption area induced by X-ray irradiation as a function of irradiation doses.
Figure 3(a) shows TSL glow curves of the ZP glass after different irradiation doses of W X-ray. It appears as though the glow curves exhibit similar shape. It is expected that the generated defects have similar structure, which is suggested by the optical absorption (Fig. 2(b)). In addition, there is no contradiction between the nonsymmetrical glow curves and emerged optical absorption bands, both of which suggest the formation of several defects in the glass after X-ray irradiation. Figure 3(b) shows the relationship between the TSL peak area and irradiation dose. There is an apparent linear relationship between them. Thus, it is expected that the number density of the emission center by X-ray irradiation increases linearly within this irradiation range.
Fig. 3 (a) TSL glow curves of ZP glass after different X-ray (tungsten source) irradiation doses. (b) TSL peak area as a function of irradiation doses.
Figure 4 shows the relationship between the TSL peak area and the difference of absorption area induced by X-ray irradiation. The figure suggests that there is a strong relationship between the two peak areas. As the concentrations of activators are not quantitatively calculated, we cannot conclude whether there is a direct relationship between them. However, the strong relationship between them suggests that the generated absorption band can act as a trap site for TSL.
Fig. 4 Relationship between the TSL peak area and the difference of absorption area induced by X-ray irradiation.
In order to examine the structural change by larger irradiation doses, other irradiation measurements were performed by using Cu-Kα source. After calibration, the lowest doses were estimated as ~100 Gy (using 3 min irradiation), ~330 Gy (using 10 min irradiation), and ~1000 Gy (using 30 min irradiation). Figure 5(a) shows the optical absorption spectra after Cu-Kα irradiation for 3 min, 10 min, and 30 min, respectively. Owing to the larger irradiation dose, larger increase in the absorption coefficient is observed. It is notable that the induced structural change using 30 min irradiation is not completely recovered after annealing at 350 °C as shown in Fig. 5(b). The absorption coefficient becomes approximately 1.1 times larger than of the ZP mother glass in the UV region (from 5.0 eV to 6.0 eV). As the spectrum shape is similar to that of the mother glass, it is expected that emerged absorption band surviving after annealing originated from structures similar to those in the mother glass.
Fig. 5 (a) Optical absorption spectra of ZP glass after X-ray (Cu-Kα) irradiation at different durations. (b) Optical absorption spectra of ZP glass after annealing of the ZP glass irradiated by Cu-K irradiation for 30 min and TSL measurement. The spectrum of non-irradiated glass is also shown for comparison.
On comparison of the optical absorption band with the PLE band, we observe that the intrinsic absorption of the ZP glass may be the origin of PL. If there was a relationship between them, it is expected that the intensity of PLE increases in accord with the increase of the optical absorption coefficient. Figure 6 shows the optical absorption spectra and PLE spectra of the ZP mother glass and ZP glass with 30 min Cu-Kα irradiation followed by annealing at 350°C. Since peak energy of PL is approximately 3.3 eV (see in Fig. 1(a)), excitation spectra were measured using the photon energy. The PLE intensity of the annealed glass is approximately 1.1 times higher than that of the ZP mother glass in the region from 5.5 eV to 6.0 eV, whose energy is lower than that of the band gap of the ZP glass [30]. As the degree of the intensity of PLE corresponds to that of optical absorption, it is suggested to be caused by the generated absorption band that function as emission centers to enhance the PL intensity. On the other hand, the increase in the absorption coefficient, i.e., generated defects, below 5.0 eV has little effect on PL intensity. The generated defects in this region are, therefore, suitable for non-radiative relaxation (generation of heat), or storage parts for luminescence stimulated by higher temperatures.
Fig. 6 Comparison of the optical absorption and PLE spectra of 3.76 eV-emission of two ZP glasses: ZP glass with 30 min Cu-Kα irradiation followed by annealing, and non-irradiated ZP mother glass. Dashed lines and solid lines indicate PLE spectra and optical absorption spectra, respectively.
It was previously reported that the absorption at 5.7 eV is correlated with the ESR active phosphate unit [32]. In order to check the relationship, we measured the ESR spectra before and after X-ray irradiation. Figure 7 shows ESR spectra of several ZP glasses measured at RT. In ZP glass after 40 kV and 40 mA Cu-Kα irradiation, phosphorus and oxygen related radiation-induced hole-trapped defect centers (POHC) and P1 species, which is assigned to PO32- [33], can be clearly observed. As the irradiation doses were more than several thousand Gy, a clear signal could be observed in RT. However, no clear signal is detected in the ZP glass with 30 min Cu-Kα (40 kV, 15 mA) irradiation followed by annealing, although a slight increase in the optical absorption coefficient is observed as shown in Fig. 6. It is expected that it is difficult to detect small amounts of defects by the present setup at RT, which is the reason for the origin of the generated absorption band at 5.7 eV. The correlation between the irradiation dose and formation of defects in ZP glasses will be discussed in a separate paper, with a precise quantification of irradiation doses measured at lower temperatures.
Fig. 7 ESR spectra of ZP glass with 30 min Cu-Kα (40 kV, 40 mA) irradiation, ZP glass with 30 min Cu-Kα (40 kV, 15 mA) irradiation followed by annealing, and ZP glass with no X-ray irradiation.
In the present study, we have demonstrated that X-ray irradiation induces absorption bands, i.e., defects that act as emission centers. If glasses contain activators that exhibit absorption band in the UV region, it is expected that X-ray irradiation will act as both the host matrix and activator. However, it is difficult to detect ESR-active species surviving after the annealing process by the measurements at RT. In order to understand the complex mechanism of ionizing-particle irradiation on glass matrix, it is necessary to check the basic measurements that can correlate both absorption and emission. Although non-radiative pathways are not discussed in the present study, the understanding of energy conversion will be necessary for further functionalization.
4. Summary
We have investigated X-ray-induced defect formation in ZnO–P2O5 glasses, from which clear TSL was observed. The X-ray-induced absorption coefficients in the UV region linearly increase on increasing the irradiation dose, which correlates with the TSL intensity as well. It is expected that several types of defects are generated by the irradiation that can be reversed by annealing when the irradiation dose is less than 10 Gy. A part of the generated defects induced by larger irradiation doses survive even after annealing at 350°C, and acts as emission centers to enhance the PLE intensity. From the relationship between the optical absorption and PLE spectra, it is suggested that certain generated traps can function as storage luminescent centers below the Tg.
Funding
Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (B) (18H01714).
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