- Email: [email protected]
Analysis of electron and hole trap states in novel storage phosphors: Undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Analysis of electron and hole trap states in novel storage phosphors: Undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics ⁎
Sayaka Nodaa, , Yutaka Fujimotoa, Masanori Koshimizua, Go Okadab, Takayuki Yanagidab, Keisuke Asaia a b
Tohoku University, 6-6-7, Aramaki Aoba Aoba-ku, Sendai 980-8579, Japan Nara Institute of Science and Technology, 8916-5, Takayama, Ikoma 630-0192, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermally stimulated luminescence TSL Storage phosphor Trap depth TSL glow curves Alkali halides
The electron and hole trap states of undoped, Eu-doped, and Ce-doped CsCaCl3 were analyzed using the absorption spectra and the TSL glow curves after the optical bleaching. The trap depths were estimated using the initial rise method. In CsCaCl3, F centers were the electron trap centers, and Vk centers stabilized by defects or impurities (referred to as Vk′ centers) were one type of the hole trap centers. The recombination of F–Vk′ centers occurred around 400–420 K. In addition to Vk′ centers, V2 centers, V3 centers, and V3 centers perturbed by dopant ions were also the hole trap centers. In Eu-doped CsCaCl3 ceramics, Eu-related hole centers were found, and their trap depths were estimated to be 1.55 and 1.63 eV. For the Ce-doped CsCaCl3 ceramics, Ce4+ ions were observed as hole trap centers. Only Ce-doped CsCaCl3 had charge-compensating defects, which contributed toward TSL glow curves. In all the samples, retrapping electrons or holes was observed and Ce-doped CsCaCl3 was considered to have the highest retrapping efficiency. In addition, the trapped electrons or holes in Eu-doped CsCaCl3 ceramics are more stabilized than those in undoped and Ce-doped CsCaCl3 ceramics.
1. Introduction Under irradiation, complementary defects, electron or hole trap centers, are generated in some insulator materials. Subsequently, the trapped electrons or holes can be stimulated thermally or optically to recombine with holes or electrons. In some cases, the recombination energy is released as luminescence [1–3]. Materials that exhibit such behavior are called storage phosphors, and the emitted light is called thermally stimulated luminescence (TSL) or optically stimulated luminescence (OSL). TSL and OSL in storage phosphors have been used for dosimeters [4], radiographies [5,6], and optical memories [7]. LiF: Mg, Ti and LiF: Mg, Cu, P are commercialized clinical dosimeters [4]. BaFBr: Eu2+ is one of the most famous storage phosphor materials used for imaging plates which are two-dimensional radiation detectors and used for X-ray diagnostics [5]. CsBr: Eu2+ is also used for imaging plates [6]. CaS: Ce, Sm and SrS: Eu, Sm are potential storage phosphors for optical memories [7]. The trap centers are usually defects or impurities that generate their own trapping levels in the bandgap of the host matrix [8]. The TSL and OSL properties of storage phosphors are greatly influenced by the trap depth, which is the energy required to release the trapped electrons or holes from the trapping level to the conduction or the valence band. The trap depth corresponds to the stability of the
⁎
trapped electrons or holes in a storage phosphor [9]. The stability of the trapped electrons or holes at room temperature (RT) is important for developing applications based on TSL or OSL in storage phosphors since the irradiation and storage in most applications proceed near RT. Deep traps prevent the recombination of electron–hole pairs for a long time around RT, whereas shallow traps easily release electrons or holes by thermal energy at RT. In this study, we focused on alkali halides and alkaline-earth halides doped with transition or rare-earth metal ions as a luminescence center, since these compounds contain or form a large number of electron and hole trap centers, which are efficiently occupied upon irradiation and can be thermally or optically stimulated [1]. In RbI crystals, doped ions influence the stability of trapped electrons and holes since different dopant ions lead to different trapping levels [1]. For example, In+doped RbI crystal has a deep trapping level assigned to a Vk center (an X2- molecule-like system consisting of two neighboring halogens X) agglomerated with an In+ ion and keeps trapped electrons or holes at RT with high stability. On the other hand, trapped electrons or holes of the Tl+-doped RbI crystal are not stable at RT. Thus, identification and characterization of the trap centers are essential for applications based on storage phosphors showing OSL or TSL. Most previous studies on TSL and OSL with alkali or alkaline-earth
Corresponding author. E-mail address: [email protected] (S. Noda).
https://doi.org/10.1016/j.nimb.2017.11.030 Received 20 July 2017; Received in revised form 9 November 2017; Accepted 29 November 2017 0168-583X/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Noda, S., Nuclear Inst, and Methods in Physics Research B (2017), https://doi.org/10.1016/j.nimb.2017.11.030
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
day, sealed in quartz tubes, and melted at 1273 K for 10 h. The synthesized transparent crystals were cut into thin pieces (0.50–0.68 mm).
halides are on binary compounds. There are only a few reports on storage phosphors based on multinary compounds. Martini et al. [10] reported on the TSL property of KMgF3 doped with Ce3+. They reported that two types of charge-compensating defects influence the trapping process of electrons or holes. Thus, ternary alkali and alkaline-earth halides are promising compounds for storage phosphors since they can contain many charge-compensating defects that capture electrons or holes. We focused on a ternary alkali and alkaline-earth halide, CsCaCl3, which is a novel scintillator material and has the perovskite structure [11–13]. CsCaCl3 is known to exhibit Auger-free luminescence and reported as a fast scintillator material [11]. Impurities doped CsCaCl3 crystals are also reported their scintillation properties. The Eu2+-doped CsCaCl3 has been reported to display an attractive scintillation light output based on the 5d–4f transition of Eu2+ and lower moisture sensitive than NaI: Tl and Cs2LiYCl6: Ce [12]. The Ce3+-doped CsCaCl3 has been reported as a promising scintillator with much higher light yield than undoped CsCaCl3 and faster decay time than Eu2+doped CsCaCl3 [13]. As other examples which we should focus on, the Yb2+-doped CsCaCl3 has been reported its luminescence properties to study the structure–luminescence relationship of divalent lanthanides in detail [14], and Tm2+-doped CsCaCl3 has also been reported its optical properties to understand the light-emission properties of Tm2+ [15]. We considered CsCaCl3 is a good base material for an impuritydoped storage phosphor and reported the TSL and OSL properties of undoped and impurities-doped CsCaCl3 ceramics [16,17]. For more detailed insight, the study of metastable states of CsCaCl3 is needed. The purpose of our study is to analyze the electron and hole trap centers formed in storage phosphors during irradiation and determine their trap depths in undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics. The TSL glow curves, which plots the TSL intensity versus temperature curve, is used for the analysis. Eu2+ and Ce3+ were chosen as activators for CsCaCl3 because their emission wavelengths (350–450 nm) [12,13] can be easily detected with a high sensitivity using generic photon detectors [18]. In addition, CsCaCl3 has two cation sites, Cs+ (0.167 nm) and Ca2+ (0.099 nm) [19], which are accessible to the Eu2+ or Ce3+ ions. Therefore, the formation of charge-compensating defects depends on the cation substituted by the dopant ion in the host lattice.
2.2. Optical and TSL measurements The samples were irradiated at RT with X-rays using an X-ray generator (RINT2200, Rigaku) with a Cu X-ray tube operating at 40 mA and 40 kV. The absorption spectrum of each crystal was measured, before and after irradiation, using a spectrophotometer (U-3500, HITACHI). For TSL measurements, the ceramic samples were irradiated with an X-ray at 45 Gy. After converging the phosphorescence of the ceramic samples for almost a day, the TSL glow curves of the ceramic samples were measured. The TSL glow curves were obtained with an original setup, which mainly consists of a heater (SAT0983a, Sakaguchi), a thermostat (SCR-SHQ-A, Sakaguchi), and a photomultiplier tube (PMT; H11890-210, Hamamatsu). The temperature used was 308–658 K, at a heating rate of 0.1 or 0.5 K/s. To eliminate the background signal that was not induced by the irradiation, the samples were measured once before the irradiation. The thermal radiation from the sample was cut with a radiation cut filter. The TSL glow curves were analyzed with initial rise method to determine the trap depths [8]. The heating rate of 0.5 K/s was used for the analysis of trap depths to suppress the influence of noise in the analysis [20]. To analyze the peaks at a higher temperature region, the peaks at lower temperatures were eliminated by heating the sample to the desired temperature. To identify the peaks in the TSL glow curves, the irradiated samples were optically bleached before measuring the TSL glow curves. After irradiating each sample with an X-ray and waiting for the convergence of the phosphorescence, each sample was illuminated with the monochromatized light corresponding to the absorption bands observed in undoped CsCaCl3 crystals (see Section 3.3). A fluorescence spectrophotometer (F-7000, Hitachi High-Tech) was used. For these measurements, the heating rate of 0.1 K/s was used to obtain precisely the glow peak temperatures. 3. Results and discussions 3.1. XRD analyses of CsCaCl3 ceramics
2. Experimental procedures Fig. 1 shows the XRD patterns of undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics. Most of the detected peaks were assigned to the
2.1. Syntheses of CsCaCl3 The undoped, Eu-doped, and Ce-doped CsCaCl3 ceramic samples (1 g) were prepared using the following starting materials: CsCl (99.999%, Kojundo Chemical Laboratory Co. Ltd., Japan) and CaCl2·2H2O (99.9%, Kojundo Chemical Laboratory Co. Ltd., Japan) powders. Then the dopant supplying materials were EuCl3·6H2O (99.9%, Sigma–Aldrich, Japan) and CeCl3·7H2O (99.9%, Sigma–Aldrich, Japan). The dopant concentration was 1 mol% in each sample. The starting and dopant supplying materials were then mixed stoichiometrically. The mixed powder was formed into a tablet (13 mm × 13 mm × ∼1 mm) and dried in a vacuum at 473 K to remove water. Subsequently, all the tablets were sintered at 673 K in a vacuum in sealed quartz tubes. The undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics were broken into shattered, reformed, and sintered again under the same conditions since these ceramics were very fragile after the first sintering process. All the obtained ceramics were not transparent. The undoped and Ce-doped CsCaCl3 ceramics were white, while the Eu-doped ceramics were pale gray in color. The phases of the synthesized CsCaCl3 ceramics were examined with powder X-ray diffraction (XRD) using an Ultima IV diffractometer (RIGAKU) in the 2θ range (5°–80°), operating with a Cu Kα radiation. For the absorption measurements, the undoped, Eu-doped, and Cedoped CsCaCl3 crystals were also synthesized. The starting materials and dopant supplying materials were the same as the synthesis of the ceramic samples. The dopant concentration in the melt was 0.5 mol%. The stoichiometric mixtures were dried in a vacuum at 673 K for one
Fig. 1. XRD patterns of undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics.
2
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
CsCaCl3 phase, but some were undefined. The largest diffraction peak of CsCaCl3, which is assigned to the (1 1 0) lattice plane, is 2θ = 23.27°. As shown at the bottom of Fig. 1, the (1 1 0) diffraction peak of Ce-doped CsCaCl3 is at a lower angle than that of the undoped or Eu-doped CsCaCl3. It is suggested that the lattice of Ce-doped CsCaCl3 expanded because of the incorporated Ce3+ ions. In CsCaCl3, two possible locations, Cs+ and Ca2+ sites are accessible to the dopant ions. The cations (Cs+ and Ca2+) in CsCaCl3 are of sixfold coordination [21]. The ionic radius of Cs+ and Ca2+, which are of the sixfold coordination, are 0.167 nm and 0.099 nm, respectively [19]. The ionic radius of Ca2+ in the sixfold coordination is much closer to that of Eu2+ (0.117 nm) [22] and Ce3+ (0.103 nm) [23] than that of Cs+. Therefore, Eu2+ and Ce3+ ions are considered as substitute for Ca2+. Unlike the Eu2+ doping, a charge-compensating interstitial or a cation vacancy must be taken into account in the case of Ce3+ doping [24]. According to a previous report of CaF2: Ce3+ crystals, Ce3+ ions substitute Ca2+ ions, thus, introducing charge-compensating F- interstitials and producing centers with different lattice symmetries around the Ce3+ ions [25]. According to another report on the KMgF3: Ce3+ crystal, Ce3+ ions substitute K+ sites and introduce cation vacancies [8].
Fig. 4. TSL glow curves of Ce-doped CsCaCl3 irradiated with an X-ray at 45 Gy.
temperatures, the glow curves were obtained after heating the sample at different temperatures. For “308 K” glow curves, the samples were kept at 308 K for a day before the measurements to wait for the converging the phosphorescence after the X-ray irradiation. For the other glow curves, the glow peaks at lower temperatures were eliminated by heating the sample to the temperature showed as legends to analyze the peaks at higher temperatures. The TSL glow curve of undoped CsCaCl3 has two peaks at 413 and 514 K with two shoulders around 341 and 446 K. The estimated trap depths corresponding to the peaks at 413 and 514 K were 0.852 and 1.10 eV, respectively. The calculated trap depths corresponding to the shoulders around 341 and 446 K were 0.586 and 0.996 eV, respectively. The TSL glow curve of the Eu-doped CsCaCl3 had two peaks at 413 and 514 K with some shoulders around 575 and 605 K. The estimated trap depths corresponding to the peaks at 413 and 514 K were 1.00 and 1.21 eV, respectively. The estimated trap depths corresponding to the shoulders around 575 and 605 K were 1.55 and 1.63 eV, respectively. The TSL glow curve of the Ce-doped CsCaCl3 had a peak at 376 K with some shoulders around 421, 460, 517, and 575 K. The estimated trap depth corresponding to the peaks and shoulders at 376, 421, 460, 517, and 575 K were 0.789, 0.842, 0.930, 1.02, and 1.26 eV, respectively.
3.2. Estimation of trap depths Figs. 2–4 show the TSL glow curves of undoped, Eu-doped, and Cedoped CsCaCl3 ceramics, respectively. The heating rate was 0.5 K/s. To derive the trap depths corresponding to the glow peaks at higher
3.3. Absorption properties of CsCaCl3 crystals Fig. 2. TSL glow curves of undopedCsCaCl3 irradiated with an X-ray at 45 Gy.
Fig. 5 shows the absorption spectra of undoped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. Two bands were observed at 370–450 nm and 480–550 nm in the spectra of the irradiated sample. After the irradiation, the undoped CsCaCl3 crystal was colored pale red. In alkali halides, the electrons generated by irradiation are generally trapped at anion vacancies in the lattice, and F centers are formed. The holes generated complementarily with electrons are trapped H centers at liquid helium temperature and Vk centers at liquid nitrogen temperature in the lattice. They were supposed to give rise to absorption bands assigned to H or Vk centers at shorter wavelengths than that of F centers [26,27]. At around room temperature, Vk centers become mobile and these are stabilized by defects or impurities which are contained unintentionally in the lattice [18,27]. We attributed the absorption band at 370–450 nm to Vk centers stabilized by defects or impurities (referred to as Vk′ centers) and the other band at 480–550 nm to F centers. The F centers and Vk′ centers are trap centers of electrons and holes in the undoped CsCaCl3. Fig. 6 shows the absorption spectra of the Eu-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. The band at 320–390 nm was assigned to the 4f–5d transition of Eu2+ since it coincided with the staircase structured band in photoluminescence excitation (PLE)
Fig. 3. TSL glow curves of Eu-doped CsCaCl3 irradiated with an X-ray at 45 Gy.
3
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
Fig. 5. Absorption spectra of undoped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy.
Fig. 7. (a) Absorption spectra of Ce-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. (b) The absorbance of Ce3+ versus X-ray dose plot.
center was an F center, and the hole trap center was a Vk′ center or a Ce3+ ion.
Fig. 6. Absorption spectra of Eu-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy.
3.4. TSL glow curves after optical bleaching spectrum of CsCaCl3: Eu2+ crystal around 330–400 nm that is attributed to the Eu2+ electronic transitions from the 4f7 state to the eg and t2g states of the 4f65d configuration [12]. The absorbance of this Eu2+ band hardly changed when the X-ray dose was increased. A weak band, associated with F centers, appearing after irradiation at 480–550 nm, indicates that the F centers are the electron trap centers for the Eudoped CsCaCl3. On the other hand, the band assigned to Vk′ centers is not observed. Based on these results, hole trap centers for Eu-doped CsCaCl3 are not defined. Fig. 7(a) shows the absorption spectra of the Ce-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. The band at 300–350 nm was assigned to the 4f–5d transition of Ce3+ since the photoluminescence excitation (PLE) spectrum of CsCaCl3: Ce3+ crystal has an intense band owing to the 4f–5d transition of Ce3+ at 255–350 nm [13]. Fig. 7(b) shows the absorbance in the Ce3+ band as a function of the Xray dose. The absorbance of the Ce3+ band reduced with the X-ray dose as shown in Fig. 7(b). Ce3+ ions are known to be hole trap centers and they form Ce4+ ions in some alkali earth halides [28–30]. The decrease in Ce3+ absorbance with the X-ray dose suggests that Ce3+ traps holes to form Ce4+. In addition to the Ce3+ band, two bands appear at 350–450 nm and 470–540 nm after irradiation, and these are associated with Vk′ centers and F centers, respectively. With the increasing X-ray dose, these bands are enhanced. For Ce-doped CsCaCl3 the electron trap
Fig. 8(a) and (b) show the TSL glow curves of undoped CsCaCl3 ceramics optically bleached by V-light and F-light after irradiation with an X-ray, where V- and F-light corresponds to the peak wavelengths of absorption bands of Vk′ centers (410 nm) and F centers (515 nm), respectively. The heating rate of TSL glow curves for optical bleaching was 0.1 K/s to obtain the temperatures of glow peaks more precisely. The peak at 396 K was effectively quenched by the V-light and F-light. The shoulder around 430 K was not quenched after 30 ss irradiation. These results indicate that the electron-hole pairs which recombine at 430 K have a different origin from those at 396 K. Besides the decay of F and Vk′ centers, the shoulder around 470 K was not quenched after V-light irradiation for 30 ss and it was enhanced after V-light irradiation for 5 min. It is suggested that some holes, which are released by the hole trap centers by V-light bleaching, are retrapped at hole trap centers corresponding to this shoulder around 470 K. Besides the shoulder around 470 K, the intensity around 550 K was also enhanced after V-light irradiation for 5 min, and it is attributed to hole retrapping centers. The assignments of these hole retrapping centers have not been defined yet. The peak at the highest temperature, 503 K, did not decrease after V and F-light irradiation for even 5 min. In alkali halides irradiated at RT, two kinds of V centers can be formed and they are observed as V2 and V3 bands in absorption spectra [31]. The V3 center was identified by 4
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
Fig. 9. TSL glow curves of Eu-doped CsCaCl3 (a) bleached by V-light (410 nm) for 30 s and 5 min, (b) bleached by F-light (515 nm) for 30 s and 5 min.
Fig. 8. TSL glow curves of undoped CsCaCl3 (a) bleached by V-light (410 nm) for 30 s and 5 min, (b) bleached by F-light (515 nm) for 30 s and 5 min.
irradiation and it was ascribed to the recombination of F–Vk′ centers like undoped CsCaCl3. Besides the peak at 398 K, a peak at 385 K was observed. The estimated trap depth corresponding to this peak at 385 K is 0.718 eV. It was quenched after F-light irradiation. On the other hand, it was not quenched after V-light irradiation. Therefore, it is suggested that the peak was not assigned to the recombination of F–Vk′ centers. We considered the peak at 385 K is ascribed to F centers perturbed by Eu2+ ions [33]. The intensity around 450 K slightly increased after V-light irradiation and after F-light irradiation for 30 ss. As the shoulder around 470 K was ascribed to retrapping centers in undoped CsCaCl3, there may be retrapping centers around 450 K in Eu-doped CsCaCl3. After V-light irradiation, the peak at 512 K and the shoulder around 560 K were reduced and shifted toward lower temperature region. On the other hand, these peak and shoulder were slightly enhanced by F-light irradiation for 30 s, and subsequently they significantly decreased without shifts of the peak or the shoulder after 5 min irradiation. The peak at 512 K is considered to have some relation with V3 centers since it is similar to the peak at 503 K assigned to V3 centers in undoped CsCaCl3. In contrast to the case of undoped CsCaCl3, the peak at 512 K bleached by V- and Flight irradiation. We considered that the peak at 512 K is assigned to V3 centers perturbed by Eu2+ ions. The shoulders around 560 K and 580 K were not observed in undoped CsCaCl3, and we assigned these shoulders to Eu-related centers. Hence, the hole trap centers for the Eu-doped CsCaCl3 are Vk′ centers, V3 centers perturbed by Eu2+ ions, and Eu-related centers. The
Seitz as being a complex center consisting of two positive vacancies and a neutral halogen molecule, and the V2 center as a pair of positive vacancies trapping a hole. [32] According to Seitz et al. [31] the V2 band bleaches relatively slowly when the alkali halides is irradiated with F-light, whereas the V3 band is difficult to bleach [31]. Therefore, the peak at 396 K is ascribed to the recombination of F–Vk′ centers since the peak was quenched efficiently by V- and F-light irradiation for 30 ss and for 5 min, respectively. The shoulder around 430 K is related to V2 centers since this shoulder was quenched more slowly by V- and F-light irradiation than the peak at 396 K. The peak at 503 K is related to V3 centers since this peak was not quenched by V- and F-light irradiation. In summary, F centers, Vk′ centers, V2 centers, V3 centers, and the hole retrapping centers were formed in undoped CsCaCl3 irradiated with Xray. Considering that peaks tend to shift to higher temperature at high heating rate [20], the peaks or shoulders at 396, 430, and 503 K, respectively, corresponds to the peaks at 413, 446, and 514 K in the TSL glow curves of undoped CsCaCl3 measured at 0.5 K/s (Fig. 2). The peak at 341 K in Fig. 2 did not correspond to the peaks or shoulders in Figure 7. We considered that this peak is due to phosphorescence because of its low temperature. The shoulder around 470 K was not observed in Fig. 2, and its trap depth is estimated to be 1.05 eV. Fig. 9(a) and (b) show the TSL glow curves of Eu-doped CsCaCl3 ceramics optically bleached by V- and F-light after X-ray irradiation, respectively. Although the absorption band assigned to Vk′ centers was not observed, the peak at 398 K decreased after V- and F-light 5
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
Table 1 Trap depths of undoped, Eu-doped, and Ce-doped CsCaCl3 calculated using the initial rise method and the assignment of each trap. Figures between brackets show the peaks and the shoulders observed in the TSL glow curves measured at 0.1 K/s. Tm indicates the temperature of a peak or a shoulder observed in the TSL glow curves measured at 0.5 K/s and ΔE indicates an estimated trap depth using initial rise method. Sample
Tm [K]
ΔE [eV]
Assignment
Undoped CsCaCl3
341 413 (3 9 6) 446 (4 3 0) (4 7 0) 514 (5 0 3) (5 5 0)
0.586 0.879 1.01 1.05 1.10 –
Phosphorescence F–Vk′ centers V2 centers Hole retrapping centers V3 centers Hole retrapping centers
Eu-doped CsCaCl3
(3 8 5) 413 (3 9 8) (4 5 0) 514 (5 1 2) 575 (5 6 0) 605 (5 8 0)
0.718 1.00 – 1.21 1.55 1.63
F centers perturbed by Eu2+ ions F–Vk′ centers Electron or hole retrapping centers V3 centers perturbed by Eu2+ ions Eu-related centers Eu-related centers
Ce-doped CsCaCl3
376 421 460 517 575
0.806 0.842 0.930 1.02 1.26
F–Ce4+ centers F–Vk′ centers Electron retrapping centers V3 centers perturbed by Ce3+ ions Charge-compensating defects
(3 7 5) (4 0 0) (4 4 0) (5 0 0) (5 4 5)
respectively, correspond to those at 376, 421, 460, 517, and 575 K in the TSL glow curves of Ce-doped CsCaCl3 measured at 0.5 K/s (Fig. 4). The estimated trap depths of all the samples are summarized in Table 1. The Eu-doped CsCaCl3 ceramics had the deepest traps associated with Eu ions, and it was the most stable storage phosphor of all the samples. In Ce-doped CsCaCl3, the shoulder assigned to retrapping centers around 440 K was observed, and they seem to have the highest retrapping efficiency in all the samples. In undoped CsCaCl3, a peak assigned to V3 centers was observed in the higher temperature region and it was difficult to be eliminated by light irradiation. The formation of V3 centers is considered to reduce the OSL efficiency. On the other hand, the V3 centers in Eu- and Ce-doped CsCaCl3 were perturbed by the dopant ions and they were able to be eliminated by light irradiation. It is suggested that the dopant ions improve the OSL efficiency.
Fig. 10. TSL glow curves of Ce-doped CsCaCl3 (a) bleached by V-light (410 nm) for 30 s and 5 min, (b) bleached by F-light (515 nm) for 30 s and 5 min.
4. Conclusion observed peaks and shoulders at 398, 512, 560, and 580 K correspond to those at 413, 514, 575, and 605 K in the TSL glow curves of Eu-doped CsCaCl3 measured at 0.5 K/s (Fig. 3), respectively. Fig. 10(a) and (b) show the TSL glow curves of Ce-doped CsCaCl3 ceramics optically bleached by V- and F-light after X-ray irradiation. After V- and F-light irradiation, the peak at 375 K and the shoulder around 400 K were quenched. The shoulder around 400 K is ascribed to the recombination of F–Vk′ centers since it is similar to the peak at 396 K observed in undoped CsCaCl3. The peak at 375 K was not observed in undoped CsCaCl3, and it is ascribed to the recombination of F–Ce4+ centers. The intensity of the peak at 375 K and the shoulder around 400 K hardly changed from 30 s to 5 min V-light irradiation. The remaining parts of the glow curve after V-light irradiation after 30 s and 5 min may be assigned to F centers. After F-light irradiation for 30 s, the shoulder around 440 K and the peak at 500 K were enhanced and a peak at 545 K appeared. It is suggested that these peaks and the shoulder are related to electron retrapping processes. Moreover, the peak at 500 K was quenched after Vlight irradiation for longer duration. Considering the similarity in temperature to the case in undoped CsCaCl3 and the optical bleaching properties, the peak at 500 K is assigned to V3 centers perturbed by Ce3+ ions. The peak at 545 K was not observed in the TSL glow curves of undoped and Eu-doped CsCaCl3. We attribute these peaks to chargecompensating defects. The observed peaks and shoulders at 375, 400, 440, 500, and 545 K,
The electron and hole trap states of undoped, Eu-doped, and Cedoped CsCaCl3 were analyzed using the absorption spectra and the TSL glow curves after the optical bleaching. The trap depths were estimated using the initial rise method. In CsCaCl3, F centers were the electron trap centers, and Vk centers stabilized by defects or impurities (referred to as Vk′ centers) were one of the hole trap centers. The recombination of F–Vk′ centers occurred around 400 K at the heating rate 0.1 K/s and 410–420 K at the heating rate 0.5 K/s. In addition to Vk′ centers, V2 centers, V3 centers, and V3 centers perturbed by dopant ions were also the hole trap centers. V3 centers are difficult to be eliminated by light irradiation, but V3 centers perturbed by dopant ions can be eliminated by light irradiation. In Ce-doped CsCaCl3, Ce4+ ions were observed as the hole trap centers. Only Cedoped CsCaCl3 had charge-compensating defects, which contributed toward TSL glow curves. In all the samples, retrapping electrons or holes was observed and Ce-doped CsCaCl3 was considered to have the highest retrapping efficiency. In addition, Eu-doped CsCaCl3 had the deepest trap centers related to Eu ions and it was the most stable storage phosphor for all the samples. References [1] M. Thoms, H. von Seggern, A. Winnacker, J. Appl. Phys. 76 (1994) 1800. [2] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, 1988.
6
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
[17] S. Noda, M. Koshimizu, Y. Fujimoto, G. Okada, K. Saeki, T. Yanagida, K. Asai, J. Ceram. Soc. Jpn. 125 (2017) 713. [18] S. Schweizer, Phys. Stat. Sol. (a) 187 (2001) 335. [19] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. Sect. B 25 (1969) 925. [20] A.J.J. Bos, Radiat. Meas. 41 (2007) S45. [21] K. Ephraim Babu, N. Murali, K. Vijaya Babu, Paulos Taddesse Shibeshi, V. Veeraiah, Acta. Phys. Pol. A 125 (2014) 1179. [22] A. Baran, S. Mahlik, M. Grinberg, P. Cai, S.I. Kim, H.J. Seo, J. Phys.: Condens. Matter 26 (2014) 385401. [23] D. Jia, W.M. Yen, J. Electrochem. Soc. 150 (3) (2003) H61. [24] Y. Tosaka, S. Adachi, ECS J. Solid State Sci. Technol. 3 (2) (2014) R14. [25] W.J. Manthey, Phys. Rev. B 8 (1973) 4086. [26] R.W. Christy, D.H. Phelps, Phys. Rev. 124 (1961) 1053. [27] Y. Farge, J. Phys. Colloques 34 (1973) C9–475. [28] D. Lapraz, H. Prévost, K. Idri, G. Angellier, L. Dusseau, Phys. Stat. Sol. (a) 15 (2006) 3793. [29] K. Chakrabarti, V.K. Mathur, L.A. Thomas, R.J. Abbundi, J. Appl. Phys. 65 (1989) 2021. [30] K. Chakrabarti, V.K. Mathur, Joanne F. Rhodes, R.J. Abbundi, J. Appl. Phys. 64 (1988) 1363. [31] F. Seitz, Phys. Rev. 79 (1950) 529. [32] T. Nagamiya, J. Phys. Soc. Jpn. 7 (1952) 358. [33] V. Chernov, R. Melendrez Ao, T.M. Piters, M. Barboza-Flores, Radiat. Meas. 33 (2001) 797.
[3] S.W.S. McKeever, Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing, 1995. [4] C.K. Harris, H.R. Elson, M.A.S. Lamba, A.E. Foster, Med. Phys. 24 (9) (1997) 1527. [5] H. von Seggern, T. Voigt, W. Knüpfer, G. Lange, J. Appl. Phys. 64 (1988) 1405. [6] H. Nanto, A. Nishimura, M. Kuroda, Y. Takei, Y. Nakano, T. Shoji, T. Yanagida, S. Kasai, Nucl. Instr. Methods Phys. Res. A 580 (2007) 278. [7] V.G. Kravets, Opt. Mater. 16 (2001) 369. [8] P. Kivits, H.J.L. Hagebeuk, J. Lumin. 15 (1977) 1. [9] P. Saadatkia, C. Varney, F. Selim, Luminescence- An Outlook on the Phenomena and their Applications, InTech, 2016 Chap. 10, p.225. [10] M. Martini, F. Meinardi, A. Scacco, Chem. Phys. Lett. 293 (1998) 43. [11] M. Koshimizu, N. Yahaba, R. Haruki, F. Nshikido, S. Kishimoto, K. Asai, Opt. Mater. 36 (2014) 1930. [12] M. Zhuravleva, B. Blalock, K. Yang, M. Koschan, C.L. Melcher, J. Cryst. Growth 352 (2012) 115. [13] Y. Fujimoto, K. Saeki, H. Tanaka, T. Yahaba, T. Yanagida, M. Koshimizu, K. Asai, Phys. Scr. 91 (2016) 094002. [14] M. Suta, W. Urland, C. Daul, C. Wickleder, Phys. Chem. Chem. Phys. 18 (2016) 13196. [15] J. Grimm, J. Freek Suyver, E. Beurer, G. Carver, Hans U. Güdel, Phys. Chem. B 110 (2006) 2093. [16] S. Noda, K. Saeki, M. Koshimizu, Y. Fujimoto, K. Asai, and T. Yanagida. In: Proc. 26th Symposium of Association for Condensed Matter Photophysics, Japan, 2015, 458, (in Japanese).
7
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Analysis of electron and hole trap states in novel storage phosphors: Undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics ⁎
Sayaka Nodaa, , Yutaka Fujimotoa, Masanori Koshimizua, Go Okadab, Takayuki Yanagidab, Keisuke Asaia a b
Tohoku University, 6-6-7, Aramaki Aoba Aoba-ku, Sendai 980-8579, Japan Nara Institute of Science and Technology, 8916-5, Takayama, Ikoma 630-0192, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermally stimulated luminescence TSL Storage phosphor Trap depth TSL glow curves Alkali halides
The electron and hole trap states of undoped, Eu-doped, and Ce-doped CsCaCl3 were analyzed using the absorption spectra and the TSL glow curves after the optical bleaching. The trap depths were estimated using the initial rise method. In CsCaCl3, F centers were the electron trap centers, and Vk centers stabilized by defects or impurities (referred to as Vk′ centers) were one type of the hole trap centers. The recombination of F–Vk′ centers occurred around 400–420 K. In addition to Vk′ centers, V2 centers, V3 centers, and V3 centers perturbed by dopant ions were also the hole trap centers. In Eu-doped CsCaCl3 ceramics, Eu-related hole centers were found, and their trap depths were estimated to be 1.55 and 1.63 eV. For the Ce-doped CsCaCl3 ceramics, Ce4+ ions were observed as hole trap centers. Only Ce-doped CsCaCl3 had charge-compensating defects, which contributed toward TSL glow curves. In all the samples, retrapping electrons or holes was observed and Ce-doped CsCaCl3 was considered to have the highest retrapping efficiency. In addition, the trapped electrons or holes in Eu-doped CsCaCl3 ceramics are more stabilized than those in undoped and Ce-doped CsCaCl3 ceramics.
1. Introduction Under irradiation, complementary defects, electron or hole trap centers, are generated in some insulator materials. Subsequently, the trapped electrons or holes can be stimulated thermally or optically to recombine with holes or electrons. In some cases, the recombination energy is released as luminescence [1–3]. Materials that exhibit such behavior are called storage phosphors, and the emitted light is called thermally stimulated luminescence (TSL) or optically stimulated luminescence (OSL). TSL and OSL in storage phosphors have been used for dosimeters [4], radiographies [5,6], and optical memories [7]. LiF: Mg, Ti and LiF: Mg, Cu, P are commercialized clinical dosimeters [4]. BaFBr: Eu2+ is one of the most famous storage phosphor materials used for imaging plates which are two-dimensional radiation detectors and used for X-ray diagnostics [5]. CsBr: Eu2+ is also used for imaging plates [6]. CaS: Ce, Sm and SrS: Eu, Sm are potential storage phosphors for optical memories [7]. The trap centers are usually defects or impurities that generate their own trapping levels in the bandgap of the host matrix [8]. The TSL and OSL properties of storage phosphors are greatly influenced by the trap depth, which is the energy required to release the trapped electrons or holes from the trapping level to the conduction or the valence band. The trap depth corresponds to the stability of the
⁎
trapped electrons or holes in a storage phosphor [9]. The stability of the trapped electrons or holes at room temperature (RT) is important for developing applications based on TSL or OSL in storage phosphors since the irradiation and storage in most applications proceed near RT. Deep traps prevent the recombination of electron–hole pairs for a long time around RT, whereas shallow traps easily release electrons or holes by thermal energy at RT. In this study, we focused on alkali halides and alkaline-earth halides doped with transition or rare-earth metal ions as a luminescence center, since these compounds contain or form a large number of electron and hole trap centers, which are efficiently occupied upon irradiation and can be thermally or optically stimulated [1]. In RbI crystals, doped ions influence the stability of trapped electrons and holes since different dopant ions lead to different trapping levels [1]. For example, In+doped RbI crystal has a deep trapping level assigned to a Vk center (an X2- molecule-like system consisting of two neighboring halogens X) agglomerated with an In+ ion and keeps trapped electrons or holes at RT with high stability. On the other hand, trapped electrons or holes of the Tl+-doped RbI crystal are not stable at RT. Thus, identification and characterization of the trap centers are essential for applications based on storage phosphors showing OSL or TSL. Most previous studies on TSL and OSL with alkali or alkaline-earth
Corresponding author. E-mail address: [email protected] (S. Noda).
https://doi.org/10.1016/j.nimb.2017.11.030 Received 20 July 2017; Received in revised form 9 November 2017; Accepted 29 November 2017 0168-583X/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Noda, S., Nuclear Inst, and Methods in Physics Research B (2017), https://doi.org/10.1016/j.nimb.2017.11.030
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
day, sealed in quartz tubes, and melted at 1273 K for 10 h. The synthesized transparent crystals were cut into thin pieces (0.50–0.68 mm).
halides are on binary compounds. There are only a few reports on storage phosphors based on multinary compounds. Martini et al. [10] reported on the TSL property of KMgF3 doped with Ce3+. They reported that two types of charge-compensating defects influence the trapping process of electrons or holes. Thus, ternary alkali and alkaline-earth halides are promising compounds for storage phosphors since they can contain many charge-compensating defects that capture electrons or holes. We focused on a ternary alkali and alkaline-earth halide, CsCaCl3, which is a novel scintillator material and has the perovskite structure [11–13]. CsCaCl3 is known to exhibit Auger-free luminescence and reported as a fast scintillator material [11]. Impurities doped CsCaCl3 crystals are also reported their scintillation properties. The Eu2+-doped CsCaCl3 has been reported to display an attractive scintillation light output based on the 5d–4f transition of Eu2+ and lower moisture sensitive than NaI: Tl and Cs2LiYCl6: Ce [12]. The Ce3+-doped CsCaCl3 has been reported as a promising scintillator with much higher light yield than undoped CsCaCl3 and faster decay time than Eu2+doped CsCaCl3 [13]. As other examples which we should focus on, the Yb2+-doped CsCaCl3 has been reported its luminescence properties to study the structure–luminescence relationship of divalent lanthanides in detail [14], and Tm2+-doped CsCaCl3 has also been reported its optical properties to understand the light-emission properties of Tm2+ [15]. We considered CsCaCl3 is a good base material for an impuritydoped storage phosphor and reported the TSL and OSL properties of undoped and impurities-doped CsCaCl3 ceramics [16,17]. For more detailed insight, the study of metastable states of CsCaCl3 is needed. The purpose of our study is to analyze the electron and hole trap centers formed in storage phosphors during irradiation and determine their trap depths in undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics. The TSL glow curves, which plots the TSL intensity versus temperature curve, is used for the analysis. Eu2+ and Ce3+ were chosen as activators for CsCaCl3 because their emission wavelengths (350–450 nm) [12,13] can be easily detected with a high sensitivity using generic photon detectors [18]. In addition, CsCaCl3 has two cation sites, Cs+ (0.167 nm) and Ca2+ (0.099 nm) [19], which are accessible to the Eu2+ or Ce3+ ions. Therefore, the formation of charge-compensating defects depends on the cation substituted by the dopant ion in the host lattice.
2.2. Optical and TSL measurements The samples were irradiated at RT with X-rays using an X-ray generator (RINT2200, Rigaku) with a Cu X-ray tube operating at 40 mA and 40 kV. The absorption spectrum of each crystal was measured, before and after irradiation, using a spectrophotometer (U-3500, HITACHI). For TSL measurements, the ceramic samples were irradiated with an X-ray at 45 Gy. After converging the phosphorescence of the ceramic samples for almost a day, the TSL glow curves of the ceramic samples were measured. The TSL glow curves were obtained with an original setup, which mainly consists of a heater (SAT0983a, Sakaguchi), a thermostat (SCR-SHQ-A, Sakaguchi), and a photomultiplier tube (PMT; H11890-210, Hamamatsu). The temperature used was 308–658 K, at a heating rate of 0.1 or 0.5 K/s. To eliminate the background signal that was not induced by the irradiation, the samples were measured once before the irradiation. The thermal radiation from the sample was cut with a radiation cut filter. The TSL glow curves were analyzed with initial rise method to determine the trap depths [8]. The heating rate of 0.5 K/s was used for the analysis of trap depths to suppress the influence of noise in the analysis [20]. To analyze the peaks at a higher temperature region, the peaks at lower temperatures were eliminated by heating the sample to the desired temperature. To identify the peaks in the TSL glow curves, the irradiated samples were optically bleached before measuring the TSL glow curves. After irradiating each sample with an X-ray and waiting for the convergence of the phosphorescence, each sample was illuminated with the monochromatized light corresponding to the absorption bands observed in undoped CsCaCl3 crystals (see Section 3.3). A fluorescence spectrophotometer (F-7000, Hitachi High-Tech) was used. For these measurements, the heating rate of 0.1 K/s was used to obtain precisely the glow peak temperatures. 3. Results and discussions 3.1. XRD analyses of CsCaCl3 ceramics
2. Experimental procedures Fig. 1 shows the XRD patterns of undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics. Most of the detected peaks were assigned to the
2.1. Syntheses of CsCaCl3 The undoped, Eu-doped, and Ce-doped CsCaCl3 ceramic samples (1 g) were prepared using the following starting materials: CsCl (99.999%, Kojundo Chemical Laboratory Co. Ltd., Japan) and CaCl2·2H2O (99.9%, Kojundo Chemical Laboratory Co. Ltd., Japan) powders. Then the dopant supplying materials were EuCl3·6H2O (99.9%, Sigma–Aldrich, Japan) and CeCl3·7H2O (99.9%, Sigma–Aldrich, Japan). The dopant concentration was 1 mol% in each sample. The starting and dopant supplying materials were then mixed stoichiometrically. The mixed powder was formed into a tablet (13 mm × 13 mm × ∼1 mm) and dried in a vacuum at 473 K to remove water. Subsequently, all the tablets were sintered at 673 K in a vacuum in sealed quartz tubes. The undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics were broken into shattered, reformed, and sintered again under the same conditions since these ceramics were very fragile after the first sintering process. All the obtained ceramics were not transparent. The undoped and Ce-doped CsCaCl3 ceramics were white, while the Eu-doped ceramics were pale gray in color. The phases of the synthesized CsCaCl3 ceramics were examined with powder X-ray diffraction (XRD) using an Ultima IV diffractometer (RIGAKU) in the 2θ range (5°–80°), operating with a Cu Kα radiation. For the absorption measurements, the undoped, Eu-doped, and Cedoped CsCaCl3 crystals were also synthesized. The starting materials and dopant supplying materials were the same as the synthesis of the ceramic samples. The dopant concentration in the melt was 0.5 mol%. The stoichiometric mixtures were dried in a vacuum at 673 K for one
Fig. 1. XRD patterns of undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics.
2
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
CsCaCl3 phase, but some were undefined. The largest diffraction peak of CsCaCl3, which is assigned to the (1 1 0) lattice plane, is 2θ = 23.27°. As shown at the bottom of Fig. 1, the (1 1 0) diffraction peak of Ce-doped CsCaCl3 is at a lower angle than that of the undoped or Eu-doped CsCaCl3. It is suggested that the lattice of Ce-doped CsCaCl3 expanded because of the incorporated Ce3+ ions. In CsCaCl3, two possible locations, Cs+ and Ca2+ sites are accessible to the dopant ions. The cations (Cs+ and Ca2+) in CsCaCl3 are of sixfold coordination [21]. The ionic radius of Cs+ and Ca2+, which are of the sixfold coordination, are 0.167 nm and 0.099 nm, respectively [19]. The ionic radius of Ca2+ in the sixfold coordination is much closer to that of Eu2+ (0.117 nm) [22] and Ce3+ (0.103 nm) [23] than that of Cs+. Therefore, Eu2+ and Ce3+ ions are considered as substitute for Ca2+. Unlike the Eu2+ doping, a charge-compensating interstitial or a cation vacancy must be taken into account in the case of Ce3+ doping [24]. According to a previous report of CaF2: Ce3+ crystals, Ce3+ ions substitute Ca2+ ions, thus, introducing charge-compensating F- interstitials and producing centers with different lattice symmetries around the Ce3+ ions [25]. According to another report on the KMgF3: Ce3+ crystal, Ce3+ ions substitute K+ sites and introduce cation vacancies [8].
Fig. 4. TSL glow curves of Ce-doped CsCaCl3 irradiated with an X-ray at 45 Gy.
temperatures, the glow curves were obtained after heating the sample at different temperatures. For “308 K” glow curves, the samples were kept at 308 K for a day before the measurements to wait for the converging the phosphorescence after the X-ray irradiation. For the other glow curves, the glow peaks at lower temperatures were eliminated by heating the sample to the temperature showed as legends to analyze the peaks at higher temperatures. The TSL glow curve of undoped CsCaCl3 has two peaks at 413 and 514 K with two shoulders around 341 and 446 K. The estimated trap depths corresponding to the peaks at 413 and 514 K were 0.852 and 1.10 eV, respectively. The calculated trap depths corresponding to the shoulders around 341 and 446 K were 0.586 and 0.996 eV, respectively. The TSL glow curve of the Eu-doped CsCaCl3 had two peaks at 413 and 514 K with some shoulders around 575 and 605 K. The estimated trap depths corresponding to the peaks at 413 and 514 K were 1.00 and 1.21 eV, respectively. The estimated trap depths corresponding to the shoulders around 575 and 605 K were 1.55 and 1.63 eV, respectively. The TSL glow curve of the Ce-doped CsCaCl3 had a peak at 376 K with some shoulders around 421, 460, 517, and 575 K. The estimated trap depth corresponding to the peaks and shoulders at 376, 421, 460, 517, and 575 K were 0.789, 0.842, 0.930, 1.02, and 1.26 eV, respectively.
3.2. Estimation of trap depths Figs. 2–4 show the TSL glow curves of undoped, Eu-doped, and Cedoped CsCaCl3 ceramics, respectively. The heating rate was 0.5 K/s. To derive the trap depths corresponding to the glow peaks at higher
3.3. Absorption properties of CsCaCl3 crystals Fig. 2. TSL glow curves of undopedCsCaCl3 irradiated with an X-ray at 45 Gy.
Fig. 5 shows the absorption spectra of undoped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. Two bands were observed at 370–450 nm and 480–550 nm in the spectra of the irradiated sample. After the irradiation, the undoped CsCaCl3 crystal was colored pale red. In alkali halides, the electrons generated by irradiation are generally trapped at anion vacancies in the lattice, and F centers are formed. The holes generated complementarily with electrons are trapped H centers at liquid helium temperature and Vk centers at liquid nitrogen temperature in the lattice. They were supposed to give rise to absorption bands assigned to H or Vk centers at shorter wavelengths than that of F centers [26,27]. At around room temperature, Vk centers become mobile and these are stabilized by defects or impurities which are contained unintentionally in the lattice [18,27]. We attributed the absorption band at 370–450 nm to Vk centers stabilized by defects or impurities (referred to as Vk′ centers) and the other band at 480–550 nm to F centers. The F centers and Vk′ centers are trap centers of electrons and holes in the undoped CsCaCl3. Fig. 6 shows the absorption spectra of the Eu-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. The band at 320–390 nm was assigned to the 4f–5d transition of Eu2+ since it coincided with the staircase structured band in photoluminescence excitation (PLE)
Fig. 3. TSL glow curves of Eu-doped CsCaCl3 irradiated with an X-ray at 45 Gy.
3
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
Fig. 5. Absorption spectra of undoped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy.
Fig. 7. (a) Absorption spectra of Ce-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. (b) The absorbance of Ce3+ versus X-ray dose plot.
center was an F center, and the hole trap center was a Vk′ center or a Ce3+ ion.
Fig. 6. Absorption spectra of Eu-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy.
3.4. TSL glow curves after optical bleaching spectrum of CsCaCl3: Eu2+ crystal around 330–400 nm that is attributed to the Eu2+ electronic transitions from the 4f7 state to the eg and t2g states of the 4f65d configuration [12]. The absorbance of this Eu2+ band hardly changed when the X-ray dose was increased. A weak band, associated with F centers, appearing after irradiation at 480–550 nm, indicates that the F centers are the electron trap centers for the Eudoped CsCaCl3. On the other hand, the band assigned to Vk′ centers is not observed. Based on these results, hole trap centers for Eu-doped CsCaCl3 are not defined. Fig. 7(a) shows the absorption spectra of the Ce-doped CsCaCl3 crystal irradiated with an X-ray at 0–20 kGy. The band at 300–350 nm was assigned to the 4f–5d transition of Ce3+ since the photoluminescence excitation (PLE) spectrum of CsCaCl3: Ce3+ crystal has an intense band owing to the 4f–5d transition of Ce3+ at 255–350 nm [13]. Fig. 7(b) shows the absorbance in the Ce3+ band as a function of the Xray dose. The absorbance of the Ce3+ band reduced with the X-ray dose as shown in Fig. 7(b). Ce3+ ions are known to be hole trap centers and they form Ce4+ ions in some alkali earth halides [28–30]. The decrease in Ce3+ absorbance with the X-ray dose suggests that Ce3+ traps holes to form Ce4+. In addition to the Ce3+ band, two bands appear at 350–450 nm and 470–540 nm after irradiation, and these are associated with Vk′ centers and F centers, respectively. With the increasing X-ray dose, these bands are enhanced. For Ce-doped CsCaCl3 the electron trap
Fig. 8(a) and (b) show the TSL glow curves of undoped CsCaCl3 ceramics optically bleached by V-light and F-light after irradiation with an X-ray, where V- and F-light corresponds to the peak wavelengths of absorption bands of Vk′ centers (410 nm) and F centers (515 nm), respectively. The heating rate of TSL glow curves for optical bleaching was 0.1 K/s to obtain the temperatures of glow peaks more precisely. The peak at 396 K was effectively quenched by the V-light and F-light. The shoulder around 430 K was not quenched after 30 ss irradiation. These results indicate that the electron-hole pairs which recombine at 430 K have a different origin from those at 396 K. Besides the decay of F and Vk′ centers, the shoulder around 470 K was not quenched after V-light irradiation for 30 ss and it was enhanced after V-light irradiation for 5 min. It is suggested that some holes, which are released by the hole trap centers by V-light bleaching, are retrapped at hole trap centers corresponding to this shoulder around 470 K. Besides the shoulder around 470 K, the intensity around 550 K was also enhanced after V-light irradiation for 5 min, and it is attributed to hole retrapping centers. The assignments of these hole retrapping centers have not been defined yet. The peak at the highest temperature, 503 K, did not decrease after V and F-light irradiation for even 5 min. In alkali halides irradiated at RT, two kinds of V centers can be formed and they are observed as V2 and V3 bands in absorption spectra [31]. The V3 center was identified by 4
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
Fig. 9. TSL glow curves of Eu-doped CsCaCl3 (a) bleached by V-light (410 nm) for 30 s and 5 min, (b) bleached by F-light (515 nm) for 30 s and 5 min.
Fig. 8. TSL glow curves of undoped CsCaCl3 (a) bleached by V-light (410 nm) for 30 s and 5 min, (b) bleached by F-light (515 nm) for 30 s and 5 min.
irradiation and it was ascribed to the recombination of F–Vk′ centers like undoped CsCaCl3. Besides the peak at 398 K, a peak at 385 K was observed. The estimated trap depth corresponding to this peak at 385 K is 0.718 eV. It was quenched after F-light irradiation. On the other hand, it was not quenched after V-light irradiation. Therefore, it is suggested that the peak was not assigned to the recombination of F–Vk′ centers. We considered the peak at 385 K is ascribed to F centers perturbed by Eu2+ ions [33]. The intensity around 450 K slightly increased after V-light irradiation and after F-light irradiation for 30 ss. As the shoulder around 470 K was ascribed to retrapping centers in undoped CsCaCl3, there may be retrapping centers around 450 K in Eu-doped CsCaCl3. After V-light irradiation, the peak at 512 K and the shoulder around 560 K were reduced and shifted toward lower temperature region. On the other hand, these peak and shoulder were slightly enhanced by F-light irradiation for 30 s, and subsequently they significantly decreased without shifts of the peak or the shoulder after 5 min irradiation. The peak at 512 K is considered to have some relation with V3 centers since it is similar to the peak at 503 K assigned to V3 centers in undoped CsCaCl3. In contrast to the case of undoped CsCaCl3, the peak at 512 K bleached by V- and Flight irradiation. We considered that the peak at 512 K is assigned to V3 centers perturbed by Eu2+ ions. The shoulders around 560 K and 580 K were not observed in undoped CsCaCl3, and we assigned these shoulders to Eu-related centers. Hence, the hole trap centers for the Eu-doped CsCaCl3 are Vk′ centers, V3 centers perturbed by Eu2+ ions, and Eu-related centers. The
Seitz as being a complex center consisting of two positive vacancies and a neutral halogen molecule, and the V2 center as a pair of positive vacancies trapping a hole. [32] According to Seitz et al. [31] the V2 band bleaches relatively slowly when the alkali halides is irradiated with F-light, whereas the V3 band is difficult to bleach [31]. Therefore, the peak at 396 K is ascribed to the recombination of F–Vk′ centers since the peak was quenched efficiently by V- and F-light irradiation for 30 ss and for 5 min, respectively. The shoulder around 430 K is related to V2 centers since this shoulder was quenched more slowly by V- and F-light irradiation than the peak at 396 K. The peak at 503 K is related to V3 centers since this peak was not quenched by V- and F-light irradiation. In summary, F centers, Vk′ centers, V2 centers, V3 centers, and the hole retrapping centers were formed in undoped CsCaCl3 irradiated with Xray. Considering that peaks tend to shift to higher temperature at high heating rate [20], the peaks or shoulders at 396, 430, and 503 K, respectively, corresponds to the peaks at 413, 446, and 514 K in the TSL glow curves of undoped CsCaCl3 measured at 0.5 K/s (Fig. 2). The peak at 341 K in Fig. 2 did not correspond to the peaks or shoulders in Figure 7. We considered that this peak is due to phosphorescence because of its low temperature. The shoulder around 470 K was not observed in Fig. 2, and its trap depth is estimated to be 1.05 eV. Fig. 9(a) and (b) show the TSL glow curves of Eu-doped CsCaCl3 ceramics optically bleached by V- and F-light after X-ray irradiation, respectively. Although the absorption band assigned to Vk′ centers was not observed, the peak at 398 K decreased after V- and F-light 5
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
Table 1 Trap depths of undoped, Eu-doped, and Ce-doped CsCaCl3 calculated using the initial rise method and the assignment of each trap. Figures between brackets show the peaks and the shoulders observed in the TSL glow curves measured at 0.1 K/s. Tm indicates the temperature of a peak or a shoulder observed in the TSL glow curves measured at 0.5 K/s and ΔE indicates an estimated trap depth using initial rise method. Sample
Tm [K]
ΔE [eV]
Assignment
Undoped CsCaCl3
341 413 (3 9 6) 446 (4 3 0) (4 7 0) 514 (5 0 3) (5 5 0)
0.586 0.879 1.01 1.05 1.10 –
Phosphorescence F–Vk′ centers V2 centers Hole retrapping centers V3 centers Hole retrapping centers
Eu-doped CsCaCl3
(3 8 5) 413 (3 9 8) (4 5 0) 514 (5 1 2) 575 (5 6 0) 605 (5 8 0)
0.718 1.00 – 1.21 1.55 1.63
F centers perturbed by Eu2+ ions F–Vk′ centers Electron or hole retrapping centers V3 centers perturbed by Eu2+ ions Eu-related centers Eu-related centers
Ce-doped CsCaCl3
376 421 460 517 575
0.806 0.842 0.930 1.02 1.26
F–Ce4+ centers F–Vk′ centers Electron retrapping centers V3 centers perturbed by Ce3+ ions Charge-compensating defects
(3 7 5) (4 0 0) (4 4 0) (5 0 0) (5 4 5)
respectively, correspond to those at 376, 421, 460, 517, and 575 K in the TSL glow curves of Ce-doped CsCaCl3 measured at 0.5 K/s (Fig. 4). The estimated trap depths of all the samples are summarized in Table 1. The Eu-doped CsCaCl3 ceramics had the deepest traps associated with Eu ions, and it was the most stable storage phosphor of all the samples. In Ce-doped CsCaCl3, the shoulder assigned to retrapping centers around 440 K was observed, and they seem to have the highest retrapping efficiency in all the samples. In undoped CsCaCl3, a peak assigned to V3 centers was observed in the higher temperature region and it was difficult to be eliminated by light irradiation. The formation of V3 centers is considered to reduce the OSL efficiency. On the other hand, the V3 centers in Eu- and Ce-doped CsCaCl3 were perturbed by the dopant ions and they were able to be eliminated by light irradiation. It is suggested that the dopant ions improve the OSL efficiency.
Fig. 10. TSL glow curves of Ce-doped CsCaCl3 (a) bleached by V-light (410 nm) for 30 s and 5 min, (b) bleached by F-light (515 nm) for 30 s and 5 min.
4. Conclusion observed peaks and shoulders at 398, 512, 560, and 580 K correspond to those at 413, 514, 575, and 605 K in the TSL glow curves of Eu-doped CsCaCl3 measured at 0.5 K/s (Fig. 3), respectively. Fig. 10(a) and (b) show the TSL glow curves of Ce-doped CsCaCl3 ceramics optically bleached by V- and F-light after X-ray irradiation. After V- and F-light irradiation, the peak at 375 K and the shoulder around 400 K were quenched. The shoulder around 400 K is ascribed to the recombination of F–Vk′ centers since it is similar to the peak at 396 K observed in undoped CsCaCl3. The peak at 375 K was not observed in undoped CsCaCl3, and it is ascribed to the recombination of F–Ce4+ centers. The intensity of the peak at 375 K and the shoulder around 400 K hardly changed from 30 s to 5 min V-light irradiation. The remaining parts of the glow curve after V-light irradiation after 30 s and 5 min may be assigned to F centers. After F-light irradiation for 30 s, the shoulder around 440 K and the peak at 500 K were enhanced and a peak at 545 K appeared. It is suggested that these peaks and the shoulder are related to electron retrapping processes. Moreover, the peak at 500 K was quenched after Vlight irradiation for longer duration. Considering the similarity in temperature to the case in undoped CsCaCl3 and the optical bleaching properties, the peak at 500 K is assigned to V3 centers perturbed by Ce3+ ions. The peak at 545 K was not observed in the TSL glow curves of undoped and Eu-doped CsCaCl3. We attribute these peaks to chargecompensating defects. The observed peaks and shoulders at 375, 400, 440, 500, and 545 K,
The electron and hole trap states of undoped, Eu-doped, and Cedoped CsCaCl3 were analyzed using the absorption spectra and the TSL glow curves after the optical bleaching. The trap depths were estimated using the initial rise method. In CsCaCl3, F centers were the electron trap centers, and Vk centers stabilized by defects or impurities (referred to as Vk′ centers) were one of the hole trap centers. The recombination of F–Vk′ centers occurred around 400 K at the heating rate 0.1 K/s and 410–420 K at the heating rate 0.5 K/s. In addition to Vk′ centers, V2 centers, V3 centers, and V3 centers perturbed by dopant ions were also the hole trap centers. V3 centers are difficult to be eliminated by light irradiation, but V3 centers perturbed by dopant ions can be eliminated by light irradiation. In Ce-doped CsCaCl3, Ce4+ ions were observed as the hole trap centers. Only Cedoped CsCaCl3 had charge-compensating defects, which contributed toward TSL glow curves. In all the samples, retrapping electrons or holes was observed and Ce-doped CsCaCl3 was considered to have the highest retrapping efficiency. In addition, Eu-doped CsCaCl3 had the deepest trap centers related to Eu ions and it was the most stable storage phosphor for all the samples. References [1] M. Thoms, H. von Seggern, A. Winnacker, J. Appl. Phys. 76 (1994) 1800. [2] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, 1988.
6
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
S. Noda et al.
[17] S. Noda, M. Koshimizu, Y. Fujimoto, G. Okada, K. Saeki, T. Yanagida, K. Asai, J. Ceram. Soc. Jpn. 125 (2017) 713. [18] S. Schweizer, Phys. Stat. Sol. (a) 187 (2001) 335. [19] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. Sect. B 25 (1969) 925. [20] A.J.J. Bos, Radiat. Meas. 41 (2007) S45. [21] K. Ephraim Babu, N. Murali, K. Vijaya Babu, Paulos Taddesse Shibeshi, V. Veeraiah, Acta. Phys. Pol. A 125 (2014) 1179. [22] A. Baran, S. Mahlik, M. Grinberg, P. Cai, S.I. Kim, H.J. Seo, J. Phys.: Condens. Matter 26 (2014) 385401. [23] D. Jia, W.M. Yen, J. Electrochem. Soc. 150 (3) (2003) H61. [24] Y. Tosaka, S. Adachi, ECS J. Solid State Sci. Technol. 3 (2) (2014) R14. [25] W.J. Manthey, Phys. Rev. B 8 (1973) 4086. [26] R.W. Christy, D.H. Phelps, Phys. Rev. 124 (1961) 1053. [27] Y. Farge, J. Phys. Colloques 34 (1973) C9–475. [28] D. Lapraz, H. Prévost, K. Idri, G. Angellier, L. Dusseau, Phys. Stat. Sol. (a) 15 (2006) 3793. [29] K. Chakrabarti, V.K. Mathur, L.A. Thomas, R.J. Abbundi, J. Appl. Phys. 65 (1989) 2021. [30] K. Chakrabarti, V.K. Mathur, Joanne F. Rhodes, R.J. Abbundi, J. Appl. Phys. 64 (1988) 1363. [31] F. Seitz, Phys. Rev. 79 (1950) 529. [32] T. Nagamiya, J. Phys. Soc. Jpn. 7 (1952) 358. [33] V. Chernov, R. Melendrez Ao, T.M. Piters, M. Barboza-Flores, Radiat. Meas. 33 (2001) 797.
[3] S.W.S. McKeever, Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing, 1995. [4] C.K. Harris, H.R. Elson, M.A.S. Lamba, A.E. Foster, Med. Phys. 24 (9) (1997) 1527. [5] H. von Seggern, T. Voigt, W. Knüpfer, G. Lange, J. Appl. Phys. 64 (1988) 1405. [6] H. Nanto, A. Nishimura, M. Kuroda, Y. Takei, Y. Nakano, T. Shoji, T. Yanagida, S. Kasai, Nucl. Instr. Methods Phys. Res. A 580 (2007) 278. [7] V.G. Kravets, Opt. Mater. 16 (2001) 369. [8] P. Kivits, H.J.L. Hagebeuk, J. Lumin. 15 (1977) 1. [9] P. Saadatkia, C. Varney, F. Selim, Luminescence- An Outlook on the Phenomena and their Applications, InTech, 2016 Chap. 10, p.225. [10] M. Martini, F. Meinardi, A. Scacco, Chem. Phys. Lett. 293 (1998) 43. [11] M. Koshimizu, N. Yahaba, R. Haruki, F. Nshikido, S. Kishimoto, K. Asai, Opt. Mater. 36 (2014) 1930. [12] M. Zhuravleva, B. Blalock, K. Yang, M. Koschan, C.L. Melcher, J. Cryst. Growth 352 (2012) 115. [13] Y. Fujimoto, K. Saeki, H. Tanaka, T. Yahaba, T. Yanagida, M. Koshimizu, K. Asai, Phys. Scr. 91 (2016) 094002. [14] M. Suta, W. Urland, C. Daul, C. Wickleder, Phys. Chem. Chem. Phys. 18 (2016) 13196. [15] J. Grimm, J. Freek Suyver, E. Beurer, G. Carver, Hans U. Güdel, Phys. Chem. B 110 (2006) 2093. [16] S. Noda, K. Saeki, M. Koshimizu, Y. Fujimoto, K. Asai, and T. Yanagida. In: Proc. 26th Symposium of Association for Condensed Matter Photophysics, Japan, 2015, 458, (in Japanese).
7