- Email: [email protected]
Temperature-dependent luminescence of GdTaO4 single crystal
Journal of Luminescence 160 (2015) 90–94
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Temperature-dependent luminescence of GdTaO4 single crystal Huajun Yang a,b, Fang Peng a,b, Qingli Zhang a,n, Chaoshu Shi c, Changxin Guo c, Xiantao Wei c, Dunlu Sun a, Xiaofei Wang a, Renqin Dou a, Xue Xing a, Huili Zhang a a
The Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c Department of Physics, University of Science and Technology of China, Hefei 230026, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2014 Received in revised form 24 November 2014 Accepted 27 November 2014 Available online 8 December 2014
Recently, we proposed GdTaO4 as a promising high-density scintillator but the scintillation mechanism was still unknown. In this paper, we investigated the temperature-dependent luminescent properties of GdTaO4 crystal. The emission spectra are characterized by a broad band composed of two Gaussian components located at 2.2 eV and 2.7 eV of 8–300 K, which are both thermally quenched with the activation energy of 156 meV and 175 meV, respectively. These two bands are believed to be from different luminescent centers, judging from the excitation spectra, thermal activation energy and decay curves. Additionally, it was also found that the annealing atmosphere exerts little influence on the luminescence intensity. Based on the results, we tentatively assigned the high-energy band to selftrapped excitons (STE) localized in TaO34 groups and the low-energy band to relaxed excitons related to lattice imperfections. & 2014 Elsevier B.V. All rights reserved.
Keywords: GdTaO4 Temperature-dependent luminescence Scintillator Self-trapped excitons
1. Introduction The family of lanthanide orthotantalates has been widely investigated due to its high chemical stability, proton conductivity [1–3], luminescence [4–12], and so on. One characteristic of these materials is the very high density ranging from 7.8 g/cm3 of LaTaO4 to 9.75 g/cm3 of LuTaO4, which makes them very attractive as scintillator crystals [9] or X-ray phosphors [12]. Recently, crackfree GdTaO4 (hereafter abbreviated as GTO) crystal with large dimension was grown successfully by our group and it was found that GTO was a promising scintillator in terms of high density (8.94 g/cm3), and is the highest among current inorganic scintillators, relatively fast decay and more favorable light yield compared to PbWO4 [13]. However, there exists a slow component in either photoluminescence (PL) decay or scintillation decay, which is detrimental to the timing resolution in application that requires fast response such as high energy calorimetry. It should also be noted that the light yield of GTO is still too low to be used in medical imaging, although around three times as that of PbWO4 [14]. Apparently, to find its way in practical scintillating applications, the scintillation mechanism of GTO needs to be understood, thereby improving the scintillation efficiency and shortening the decay time.
n
Corresponding author. Tel./fax: þ 86 551 5591039. E-mail address: [email protected] (Q. Zhang).
http://dx.doi.org/10.1016/j.jlumin.2014.11.048 0022-2313/& 2014 Elsevier B.V. All rights reserved.
Rare earth orthotantalates have a band gap of around 5.3– 5.4 eV [15]. Unlike other orthotantalates, the charge transfer emission of TaO34 groups in GTO crystal is quenched by Gd3 þ [16]. Our previous studies have shown that GTO exhibits a broad band luminescence at room temperature which can be decomposed into two bands centered at 2.2 eV and 2.7 eV [13]. It is also likely to be intrinsic due to the widespread existence [13,17]. Nevertheless, the nature of the excited states is still unknown. Thus, more powerful optical experiments need to be performed. In this paper, we made temperature-dependent luminescent studies of GTO single crystal and detailed characteristics were presented.
2. Experimental High quality of GTO crystal was grown by the Czochralski method. The growth details can be found elsewhere [13]. 2 mm thick wafers with the same size were cut from the as-grown crystals and were then annealed at 1400 1C for 24 h under air atmosphere and flowing hydrogen atmosphere. The luminescent spectra and decay curves were recorded on the Jobin Yvon Fluorolog-3 spectrometer. 450 W Xeon lamp and a pulsed laser (266 nm) by the fourth harmonic of a Nd:YAG laser were used as the excitation sources for the steady-state and time-resolved luminescent measurements, respectively. For time-resolved luminescence, we used a photomultiplier with a variable load resistor and recorded the signal with a Tektronix TDS 2024 oscilloscope.
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
91
The resistance was set to be the minimum of 5 Ω all the time to obtain the best instrumental response of 97 ns.
3. Results To reveal detailed luminescent characteristics of GTO crystal, we performed temperature-dependent luminescent measurements including excitation spectra, emission spectra and decay curves. Fig. 1 shows the transmission, excitation and emission spectra of GTO at 8 K. In the transmission spectrum, the absorption peaks around 4.53 eV (274 nm) and 3.96 eV (313 nm) are corresponding to 8 S7/2-6IJ and 8S7/2-6PJ transitions of Gd3 þ [18,19], respectively. The absorption edge is situated at about 4.96 eV (250 nm). The emission spectrum can be well reproduced by two Gaussian components located at 2.72 eV and 2.23 eV. When monitored at 2.71 eV (457 nm) and 2.17 eV (571 nm), both the excitation spectra consist of a broad band above 4 eV and some parasitic peaks. The broad band above 4 eV can be assigned to charge transfer (CT) transition of TaO34 groups [18]. The peaks are corresponding to the characteristic 4f transitions of Gd3 þ , which are consistent with the transmission spectrum. Despite the Gd3 þ transitions, the excitation below the absorption edge is almost zero. Besides, the Stokes shift is very large for these two bands. The temperature-dependent PL spectra are shown in Fig. 2. The PL intensity increases slightly as the temperature rises from 8 K to 80 K and then it decreases with increasing temperature. It quenches rapidly above 150 K with the intensity decreasing by two orders of magnitude. All the spectra exhibit broad band spectra from 1.7 eV to 3.2 eV and there is no sign of zero-phonon line even at 8 K, which indicates a large electron–lattice coupling effect. Some impurityrelated peaks attached to the wide band appear when the intensity is weaker at higher temperature (see the spectra from 200 K to 300 K). These peaks can be ascribed to Eu3 þ and Tb3 þ impurities [13]. The spectra within the full measured temperature range are characterized by a highly asymmetric shape, which can all be decomposed into two Gaussian components centered at 2.2 eV and 2.7 eV (see Supplementary data). The temperature dependence of PL intensity I(T) of these two bands is plotted in Fig. 3. The lines become straight in Arrhenius plot, suggesting the existence of thermal activation process [20–22], which can be represented by the following equation: IðTÞ ¼
I0 1 þ Cexpð E=kTÞ
Fig. 2. PL spectra of GTO scintillator upon excitation with the photon energy of 4.53 eV within (a) 8–80 K, (b) 80–200 K, (c) 200–300 K.
ð1Þ
where I0 is the intensity at zero temperature (K), C is the fit constant, E represents the activation energy (eV) of the nonradiative process
Fig. 3. Temperature-dependent PL intensity of these two bands.
Fig. 1. Transmission, emission, and excitation spectra of GTO crystal at 8 K. The excitation energy is 4.53 eV (274 nm) and the emission is monitored at 2.71 eV (457 nm) and 2.17 eV (571 nm).
and k is the Boltzmann constant. The activation energy for 2.2 eV and 2.7 eV band is fitted to be 156 meV and 175 meV, respectively. It is yet to be proved by photoconductivity measurements whether the activation process is a photoionization process.
92
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
Temperature-dependent excitation spectra monitored at 2.71 eV and 2.17 eV are shown in Fig. 4a and b, respectively. As described above, the excitation consists of 4f transitions of Gd3 þ and CT transition of TaO34 group. For the high-energy band, the shape of the excitation spectra changes significantly with temperature. The excitation efficiency around 4 eV decreases faster than that of 4.53 eV. A broad band at about 3.5 eV also appears at higher temperature, which may be caused by some defects. While for the low-energy band, the excitation spectra are rather similar to each other. Some impurity-related peaks located at 3.53 eV and 3.39 eV
can be observed at higher temperature, which are corresponding to the 4f transitions of Eu3 þ impurities (emission spectra excited by photons around these peaks show the characteristics of Eu3 þ emission). The appearance of these peaks is consistent with the situation observed in the temperature-dependent emission spectra. Fig. 5a and b shows the luminescence decay curves monitored at 2.88 eV and 2.16 eV at some selected temperature points, respectively. The decays of both bands show a two-exponential manner in the full temperature range. It can be written as follows: IðtÞ ¼ a1 expð t=τ1 Þ þ a2 expð t=τ2 Þ þ C
ð2Þ
The fit results are summarized in Tables 1 and 2. The twoexponential behavior implies that the excited states experience additional nonradiative channel accompanying the radiative recombination [23]. For the high-energy band, both the fast component and slow component decrease with increase of the temperature. The decay of the high-energy band becomes very faster above 150 K, as shown in Fig. 5a, which agrees well with the temperature dependence of PL intensity. Abrupt increase of the lifetime at low temperature evidences the presence of a pair of closely spaced energy levels involved in the photoexcited dynamics [24,25]. For the low-energy band, the decays change gradually with increasing temperature, with the fast component changing slightly around 10 ns and the slow component decreasing from 661 ns to 124 ns. We calculate the average lifetime using the following equation [26]: R tIðtÞdt τave ¼ R ð2Þ IðtÞdt where I(t) denotes the PL intensity at time t corrected for the background. The average lifetime of 2.7 eV band at 20 K is calculated to be 14 μs, much longer than that of the 2.2 eV band with the average lifetime of 673 ns. Apparently, the decay rates of both bands are significantly different, which gives the further evidence that these two bands originate from different luminescent centers. The general trends of the decays can be outlined but the measured temperature-dependent lifetime may not be completely alike the real situation due to the slow response of our instrument (97 ns). Oxygen vacancy is the most obvious defect we can think of for the reasons that the crystal is grown in anoxic atmosphere and oxygen vacancies in many oxides emit broad luminescence [23,27]. To confirm whether the luminescence is related to oxygen vacancy, we investigated the annealing effect on the luminescence of GTO crystal. The air annealing effect had been reported in Ref. [13] and the effect of H2 annealing was further studied in this work, as shown in Fig. 6. The intensity of these samples can be classified in the following order: I air o I unannealed o I H2 However, the difference of these spectra looks very small. As mentioned above, the luminescence of GTO crystal is severely thermally quenched. Therefore, this cannot be treated as reliable evidence to confirm whether and how much oxygen vacancies contribute to the luminescence of GTO crystal. Further experiments need to be taken to clarify it.
4. Discussions
Fig. 4. Temperature-dependent excitation spectra monitored at (a) 457 nm (2.71 eV) and (b) 2.17 eV (571 nm). The intensity is normalized at 274 nm (4.53 eV).
Although with the same structure, the luminescence of GTO is totally different from that of YTaO4 and LuTaO4 which exhibits characteristic CT emission of TaO34 group (always located around 330 nm at 3.76 eV). This is ascribed to energy transfer from TaO34 to Gd3 þ in GTO, followed by concentration quenching [16]. For GTO, the luminescence is characterized by a super wide band composed of two Gaussian components from 8 K to 300 K. The Stokes shift is very large and the excitation efficiency is almost zero below the absorption edge, indicating that both emission bands of GTO come from exciton and interband excitation. It is also likely to be intrinsic
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
93
Fig. 5. Temperature-dependent luminescence decays of GTO monitored at (a) 2.88 eV and (b) 2.16 eV at some selected temperature points.
Table 1 Results of two-exponential fitting of luminescent decays monitored at 2.88 eV. Temperature (K)
20 50 100 120 150 180 220 280
Fast component
Slow component
τ1 (ns)
a1
τ2 (μs)
a2
886 629 371 342 280 33 11 15
36 30 80 94 165 13565 13520 168
14 11.6 9.2 8.5 6.4 1.45 1.29 –
11.5 6.7 8.5 8.5 10.2 21 8.7 –
Table 2 Results of two-exponential fitting of luminescent decays monitored at 2.16 eV. Temperature (K)
20 50 100 120 150 180 200 220 280
Fast component
Slow component
τ1 (ns)
a1
τ2 (ns)
a2
9.2 11.5 14.9 14.8 15.1 13 13 12 11.6
113.5 74.8 97.6 123 110 130 288 323 207
661 620 607 568 466 345 283 201 124
74 73.6 67 66 60 51 99 83 87
due to the widespread existence [13,17]. Besides, these two components exhibit different behaviors in terms of excitation spectra, thermal activation energy, and decays. One can conclude that they originate from different luminescent centers. Such characteristics of GTO crystal clearly remind us the situation in the ABO4 (A¼Pb, Ca, Ba, Sr, B¼W, Mo) [24,25,28] crystals, where the luminescence also
Fig. 6. Room-temperature PL spectra upon 4.53 eV excitation under different annealing atmospheres.
consists of two components. The Stokes shift of luminescence in ABO4 is very large and the temperature-dependent luminescence of high-energy band indicates a thermal activation of nonradiative process, too. Besides, the as-grown GTO crystal, which belongs to a distorted scheelite structure where Ta and four coordinated O atoms form an isolated and distorted tetrahedron [29], shares similar structure with ABO4 crystals. By analogy with them, we tentatively assigned the high-energy band to self-trapped excitons (STE) localized at TaO34 groups and low-energy band to relaxed excitons related to lattice imperfections [25]. The long lifetime of high-energy band of GTO at low temperature also evidences that this band originates from a forbidden transition, which is consistent with triplet–singlet transition of STE. It is worth mentioning that similar luminescence has also been observed in KTaO3 crystal, where the luminescence at low temperature consists of two components located around 2.61 eV and 2.28 eV [30,31] or around 2.1 eV and
94
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
2.5 eV in another paper [21]. The similarity is that the charge transfer emission from the O 2p to Ta 5d does not appear in both cases. It is quenched by Gd3 þ in GTO while in KTaO3 the charge transfer band is higher than the band gap (3.7 eV). Therefore, the luminescence of GTO is believed to be related to anion groups, namely TaO34 group, which also supports the above analysis. 5. Conclusions In summary, we revealed the detailed temperature-dependent luminescent characteristics of GTO crystal. The spectra consist of two bands, 2.2 eV band and 2.7 eV band from 8 K to 300 K. The temperature-dependent PL intensity of these two bands indicates the existence of thermal activation process. The activation energy of 2.2 eV band and 2.7 eV band is determined to be 156 meV and 175 meV, respectively. The lifetime of 2.7 eV band is found to be much longer than that of 2.2 eV band at low temperature. Based on the above, these two bands are believed to be from different luminescent centers. Besides, the annealing atmosphere exerts little influence on the PL intensity. We tentatively assign the high-energy band to self-trapped excitons (STE) localized at TaO34 groups and the low-energy band to relaxed excitons related to lattice imperfections. These results will be helpful to reveal the nature of scintillation mechanism of GTO crystal. However, there still exists many unknown aspects regarding the scintillation mechanism. Furthermore, more accurate temperature-dependent time-resolved luminescence measurements need to be performed to model the complicated radiation process. Our further study will focus on them. Acknowledgment Thanks are due to Shaoshuai Zhou for the time-resolved measurements. This work was financially supported by the National Natural Science Foundation of China (Grant nos. 51172236, 91122021, and 51272254). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2014.11.048.
References [1] R. Haugsrud, T. Norby, Nat. Mater. 5 (2006) 193. [2] R. Haugsrud, T. Norby, J. Am. Ceram. Soc. 90 (2007) 1116. [3] M. Nyman, M.A. Rodriguez, L.E.S. Rohwer, J.E. Martin, M. Waller, F.E. Osterloh, Chem. Mater. 21 (2009) 4731. [4] M. Gu, L.H. Xiao, X.L. Liu, R. Zhang, B.J. Liu, X. Xu, J. Alloy. Compd. 426 (2006) 390. [5] M.J.J. Lammers, G. Blasse, Mater. Res. Bull. 19 (1984) 759. [6] B. Li, Z.N. Gu, J.H. Lin, M.Z. Su, J. Mater. Sci. 35 (2000) 1139. [7] B. Liu, M. Gu, X.L. Liu, K. Han, S.M. Huang, C. Ni, G.B. Zhang, Z.M. Qi, Appl. Phys. Lett. 94 (2009) 061906. [8] B. Liu, K. Han, X.L. Liu, M. Gu, S.M. Huang, C. Ni, Z.M. Qi, G.B. Zhang, Solid State Commun. 144 (2007) 484. [9] W.P. Liu, Q.L. Zhang, W.L. Zhou, C.J. Gu, S.T. Yin, IEEE Trans. Nucl. Sci. 57 (2010) 1287. [10] H.J. Yang, Q.L. Zhang, P.Y. Zhou, K.J. Ning, J.Y. Gao, Y. He, J.Q. Luo, D.L. Sun, S.T. Yin, Proc. SPIE 8206 (2012) 820626. [11] H.J. Zhang, Y.H. Wang, L.C. Xie, J. Lumin. 130 (2010) 2089. [12] O. Voloshyna, S.V. Neicheva, N.G. Starzhinskiy, I.M. Zenya, S.S. Gridin, V.N. Baumer, O.T. Sidletskiy, Mater. Sci. Eng. B 178 (2013) 1491. [13] H. Yang, F. Peng, Q. Zhang, C. Guo, C. Shi, W. Liu, G. Sun, Y. Zhao, D. Zhang, D. Sun, S. Yin, M. Gu, R. Mao, CrystEngComm 16 (2014) 2480. [14] C.L. Melcher, Nucl. Instrum. Methods Phys. Res. A 537 (2005) 6. [15] M. Gu, X. Xu, X.L. Liu, L.Q. Qiu, R. Zhang, J. Sol–Gel Sci. Technol. 35 (2005) 193. [16] G. Blasse, A. Bril, J. Lumin. 3 (1970) 109. [17] L.I. Kazakova, I.S. Bykov, A.B. Dubovsky, J. Lumin. 72-4 (1997) 211. [18] L.L. Noto, S.S. Pitale, M.A. Gusowki, O.M. Ntwaeaborwa, J.J. Terblans, H.C. Swart, J. Lumin. 145 (2014) 907. [19] B. Liu, K. Han, X. Liu, M. Gu, S. Huang, C. Ni, Z. Qi, G. Zhang, Solid State Commun. 144 (2007) 484. [20] H. Feng, V. Jary, E. Mihokova, D.Z. Ding, M. Nikl, G.H. Ren, H.Y. Li, S.K. Pan, A. Beitlerova, R. Kucerkova, J. Appl. Phys. 108 (2010) 033519. [21] I. Katayama, K. Tanaka, J. Phys. Soc. Jpn. 75 (2006) 12064713. [22] M. Nikl, P. Bruza, D. Panek, M. Vrbova, E. Mihokova, J.A. Mares, A. Beitlerova, N. Kawaguchi, K. Fukuda, A. Yoshikawa, Appl. Phys. Lett. 102 (2013). [23] S. Sawai, T. Uchino, J. Appl. Phys. 112 (2012) 103523. [24] M. Nikl, P. Bohacek, E. Mihokova, M. Kobayashi, M. Ishii, Y. Usuki, V. Babin, A. Stolovich, S. Zazubovich, M. Bacci, J. Lumin. 87-89 (2000) 1136. [25] M. Itoh, T. Sakurai, Phys. Rev. B 73 (2006) 235106. [26] T. Passuello, F. Piccinelli, M. Trevisani, M. Giarola, G. Mariotto, L. Marciniak, D. Hreniak, M. Guzik, M. Fasoli, A. Vedda, V. Jary, M. Nikl, V. Causin, M. Bettinelli, A. Speghini, J. Mater. Chem. 22 (2012) 10639. [27] M. Itou, A. Fujiwara, T. Uchino, J. Phys. Chem. C 113 (2009) 20949. [28] M. Itoh, T. Kajitani, Phys. Rev. B 87 (2013) 085201. [29] L.H. Brixner, H.Y. Chen, J. Electrochem. Soc. 130 (1983) 2435. [30] E. Yamaichi, K. Watanabe, K. Imamiya, K. Ohi, J. Phys. Soc. Jpn. 56 (1987) 1890. [31] E. Yamaichi, K. Watanabe, K. Ohi, J. Phys. Soc. Jpn. 57 (1988) 2201.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Temperature-dependent luminescence of GdTaO4 single crystal Huajun Yang a,b, Fang Peng a,b, Qingli Zhang a,n, Chaoshu Shi c, Changxin Guo c, Xiantao Wei c, Dunlu Sun a, Xiaofei Wang a, Renqin Dou a, Xue Xing a, Huili Zhang a a
The Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c Department of Physics, University of Science and Technology of China, Hefei 230026, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2014 Received in revised form 24 November 2014 Accepted 27 November 2014 Available online 8 December 2014
Recently, we proposed GdTaO4 as a promising high-density scintillator but the scintillation mechanism was still unknown. In this paper, we investigated the temperature-dependent luminescent properties of GdTaO4 crystal. The emission spectra are characterized by a broad band composed of two Gaussian components located at 2.2 eV and 2.7 eV of 8–300 K, which are both thermally quenched with the activation energy of 156 meV and 175 meV, respectively. These two bands are believed to be from different luminescent centers, judging from the excitation spectra, thermal activation energy and decay curves. Additionally, it was also found that the annealing atmosphere exerts little influence on the luminescence intensity. Based on the results, we tentatively assigned the high-energy band to selftrapped excitons (STE) localized in TaO34 groups and the low-energy band to relaxed excitons related to lattice imperfections. & 2014 Elsevier B.V. All rights reserved.
Keywords: GdTaO4 Temperature-dependent luminescence Scintillator Self-trapped excitons
1. Introduction The family of lanthanide orthotantalates has been widely investigated due to its high chemical stability, proton conductivity [1–3], luminescence [4–12], and so on. One characteristic of these materials is the very high density ranging from 7.8 g/cm3 of LaTaO4 to 9.75 g/cm3 of LuTaO4, which makes them very attractive as scintillator crystals [9] or X-ray phosphors [12]. Recently, crackfree GdTaO4 (hereafter abbreviated as GTO) crystal with large dimension was grown successfully by our group and it was found that GTO was a promising scintillator in terms of high density (8.94 g/cm3), and is the highest among current inorganic scintillators, relatively fast decay and more favorable light yield compared to PbWO4 [13]. However, there exists a slow component in either photoluminescence (PL) decay or scintillation decay, which is detrimental to the timing resolution in application that requires fast response such as high energy calorimetry. It should also be noted that the light yield of GTO is still too low to be used in medical imaging, although around three times as that of PbWO4 [14]. Apparently, to find its way in practical scintillating applications, the scintillation mechanism of GTO needs to be understood, thereby improving the scintillation efficiency and shortening the decay time.
n
Corresponding author. Tel./fax: þ 86 551 5591039. E-mail address: [email protected] (Q. Zhang).
http://dx.doi.org/10.1016/j.jlumin.2014.11.048 0022-2313/& 2014 Elsevier B.V. All rights reserved.
Rare earth orthotantalates have a band gap of around 5.3– 5.4 eV [15]. Unlike other orthotantalates, the charge transfer emission of TaO34 groups in GTO crystal is quenched by Gd3 þ [16]. Our previous studies have shown that GTO exhibits a broad band luminescence at room temperature which can be decomposed into two bands centered at 2.2 eV and 2.7 eV [13]. It is also likely to be intrinsic due to the widespread existence [13,17]. Nevertheless, the nature of the excited states is still unknown. Thus, more powerful optical experiments need to be performed. In this paper, we made temperature-dependent luminescent studies of GTO single crystal and detailed characteristics were presented.
2. Experimental High quality of GTO crystal was grown by the Czochralski method. The growth details can be found elsewhere [13]. 2 mm thick wafers with the same size were cut from the as-grown crystals and were then annealed at 1400 1C for 24 h under air atmosphere and flowing hydrogen atmosphere. The luminescent spectra and decay curves were recorded on the Jobin Yvon Fluorolog-3 spectrometer. 450 W Xeon lamp and a pulsed laser (266 nm) by the fourth harmonic of a Nd:YAG laser were used as the excitation sources for the steady-state and time-resolved luminescent measurements, respectively. For time-resolved luminescence, we used a photomultiplier with a variable load resistor and recorded the signal with a Tektronix TDS 2024 oscilloscope.
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
91
The resistance was set to be the minimum of 5 Ω all the time to obtain the best instrumental response of 97 ns.
3. Results To reveal detailed luminescent characteristics of GTO crystal, we performed temperature-dependent luminescent measurements including excitation spectra, emission spectra and decay curves. Fig. 1 shows the transmission, excitation and emission spectra of GTO at 8 K. In the transmission spectrum, the absorption peaks around 4.53 eV (274 nm) and 3.96 eV (313 nm) are corresponding to 8 S7/2-6IJ and 8S7/2-6PJ transitions of Gd3 þ [18,19], respectively. The absorption edge is situated at about 4.96 eV (250 nm). The emission spectrum can be well reproduced by two Gaussian components located at 2.72 eV and 2.23 eV. When monitored at 2.71 eV (457 nm) and 2.17 eV (571 nm), both the excitation spectra consist of a broad band above 4 eV and some parasitic peaks. The broad band above 4 eV can be assigned to charge transfer (CT) transition of TaO34 groups [18]. The peaks are corresponding to the characteristic 4f transitions of Gd3 þ , which are consistent with the transmission spectrum. Despite the Gd3 þ transitions, the excitation below the absorption edge is almost zero. Besides, the Stokes shift is very large for these two bands. The temperature-dependent PL spectra are shown in Fig. 2. The PL intensity increases slightly as the temperature rises from 8 K to 80 K and then it decreases with increasing temperature. It quenches rapidly above 150 K with the intensity decreasing by two orders of magnitude. All the spectra exhibit broad band spectra from 1.7 eV to 3.2 eV and there is no sign of zero-phonon line even at 8 K, which indicates a large electron–lattice coupling effect. Some impurityrelated peaks attached to the wide band appear when the intensity is weaker at higher temperature (see the spectra from 200 K to 300 K). These peaks can be ascribed to Eu3 þ and Tb3 þ impurities [13]. The spectra within the full measured temperature range are characterized by a highly asymmetric shape, which can all be decomposed into two Gaussian components centered at 2.2 eV and 2.7 eV (see Supplementary data). The temperature dependence of PL intensity I(T) of these two bands is plotted in Fig. 3. The lines become straight in Arrhenius plot, suggesting the existence of thermal activation process [20–22], which can be represented by the following equation: IðTÞ ¼
I0 1 þ Cexpð E=kTÞ
Fig. 2. PL spectra of GTO scintillator upon excitation with the photon energy of 4.53 eV within (a) 8–80 K, (b) 80–200 K, (c) 200–300 K.
ð1Þ
where I0 is the intensity at zero temperature (K), C is the fit constant, E represents the activation energy (eV) of the nonradiative process
Fig. 3. Temperature-dependent PL intensity of these two bands.
Fig. 1. Transmission, emission, and excitation spectra of GTO crystal at 8 K. The excitation energy is 4.53 eV (274 nm) and the emission is monitored at 2.71 eV (457 nm) and 2.17 eV (571 nm).
and k is the Boltzmann constant. The activation energy for 2.2 eV and 2.7 eV band is fitted to be 156 meV and 175 meV, respectively. It is yet to be proved by photoconductivity measurements whether the activation process is a photoionization process.
92
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
Temperature-dependent excitation spectra monitored at 2.71 eV and 2.17 eV are shown in Fig. 4a and b, respectively. As described above, the excitation consists of 4f transitions of Gd3 þ and CT transition of TaO34 group. For the high-energy band, the shape of the excitation spectra changes significantly with temperature. The excitation efficiency around 4 eV decreases faster than that of 4.53 eV. A broad band at about 3.5 eV also appears at higher temperature, which may be caused by some defects. While for the low-energy band, the excitation spectra are rather similar to each other. Some impurity-related peaks located at 3.53 eV and 3.39 eV
can be observed at higher temperature, which are corresponding to the 4f transitions of Eu3 þ impurities (emission spectra excited by photons around these peaks show the characteristics of Eu3 þ emission). The appearance of these peaks is consistent with the situation observed in the temperature-dependent emission spectra. Fig. 5a and b shows the luminescence decay curves monitored at 2.88 eV and 2.16 eV at some selected temperature points, respectively. The decays of both bands show a two-exponential manner in the full temperature range. It can be written as follows: IðtÞ ¼ a1 expð t=τ1 Þ þ a2 expð t=τ2 Þ þ C
ð2Þ
The fit results are summarized in Tables 1 and 2. The twoexponential behavior implies that the excited states experience additional nonradiative channel accompanying the radiative recombination [23]. For the high-energy band, both the fast component and slow component decrease with increase of the temperature. The decay of the high-energy band becomes very faster above 150 K, as shown in Fig. 5a, which agrees well with the temperature dependence of PL intensity. Abrupt increase of the lifetime at low temperature evidences the presence of a pair of closely spaced energy levels involved in the photoexcited dynamics [24,25]. For the low-energy band, the decays change gradually with increasing temperature, with the fast component changing slightly around 10 ns and the slow component decreasing from 661 ns to 124 ns. We calculate the average lifetime using the following equation [26]: R tIðtÞdt τave ¼ R ð2Þ IðtÞdt where I(t) denotes the PL intensity at time t corrected for the background. The average lifetime of 2.7 eV band at 20 K is calculated to be 14 μs, much longer than that of the 2.2 eV band with the average lifetime of 673 ns. Apparently, the decay rates of both bands are significantly different, which gives the further evidence that these two bands originate from different luminescent centers. The general trends of the decays can be outlined but the measured temperature-dependent lifetime may not be completely alike the real situation due to the slow response of our instrument (97 ns). Oxygen vacancy is the most obvious defect we can think of for the reasons that the crystal is grown in anoxic atmosphere and oxygen vacancies in many oxides emit broad luminescence [23,27]. To confirm whether the luminescence is related to oxygen vacancy, we investigated the annealing effect on the luminescence of GTO crystal. The air annealing effect had been reported in Ref. [13] and the effect of H2 annealing was further studied in this work, as shown in Fig. 6. The intensity of these samples can be classified in the following order: I air o I unannealed o I H2 However, the difference of these spectra looks very small. As mentioned above, the luminescence of GTO crystal is severely thermally quenched. Therefore, this cannot be treated as reliable evidence to confirm whether and how much oxygen vacancies contribute to the luminescence of GTO crystal. Further experiments need to be taken to clarify it.
4. Discussions
Fig. 4. Temperature-dependent excitation spectra monitored at (a) 457 nm (2.71 eV) and (b) 2.17 eV (571 nm). The intensity is normalized at 274 nm (4.53 eV).
Although with the same structure, the luminescence of GTO is totally different from that of YTaO4 and LuTaO4 which exhibits characteristic CT emission of TaO34 group (always located around 330 nm at 3.76 eV). This is ascribed to energy transfer from TaO34 to Gd3 þ in GTO, followed by concentration quenching [16]. For GTO, the luminescence is characterized by a super wide band composed of two Gaussian components from 8 K to 300 K. The Stokes shift is very large and the excitation efficiency is almost zero below the absorption edge, indicating that both emission bands of GTO come from exciton and interband excitation. It is also likely to be intrinsic
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
93
Fig. 5. Temperature-dependent luminescence decays of GTO monitored at (a) 2.88 eV and (b) 2.16 eV at some selected temperature points.
Table 1 Results of two-exponential fitting of luminescent decays monitored at 2.88 eV. Temperature (K)
20 50 100 120 150 180 220 280
Fast component
Slow component
τ1 (ns)
a1
τ2 (μs)
a2
886 629 371 342 280 33 11 15
36 30 80 94 165 13565 13520 168
14 11.6 9.2 8.5 6.4 1.45 1.29 –
11.5 6.7 8.5 8.5 10.2 21 8.7 –
Table 2 Results of two-exponential fitting of luminescent decays monitored at 2.16 eV. Temperature (K)
20 50 100 120 150 180 200 220 280
Fast component
Slow component
τ1 (ns)
a1
τ2 (ns)
a2
9.2 11.5 14.9 14.8 15.1 13 13 12 11.6
113.5 74.8 97.6 123 110 130 288 323 207
661 620 607 568 466 345 283 201 124
74 73.6 67 66 60 51 99 83 87
due to the widespread existence [13,17]. Besides, these two components exhibit different behaviors in terms of excitation spectra, thermal activation energy, and decays. One can conclude that they originate from different luminescent centers. Such characteristics of GTO crystal clearly remind us the situation in the ABO4 (A¼Pb, Ca, Ba, Sr, B¼W, Mo) [24,25,28] crystals, where the luminescence also
Fig. 6. Room-temperature PL spectra upon 4.53 eV excitation under different annealing atmospheres.
consists of two components. The Stokes shift of luminescence in ABO4 is very large and the temperature-dependent luminescence of high-energy band indicates a thermal activation of nonradiative process, too. Besides, the as-grown GTO crystal, which belongs to a distorted scheelite structure where Ta and four coordinated O atoms form an isolated and distorted tetrahedron [29], shares similar structure with ABO4 crystals. By analogy with them, we tentatively assigned the high-energy band to self-trapped excitons (STE) localized at TaO34 groups and low-energy band to relaxed excitons related to lattice imperfections [25]. The long lifetime of high-energy band of GTO at low temperature also evidences that this band originates from a forbidden transition, which is consistent with triplet–singlet transition of STE. It is worth mentioning that similar luminescence has also been observed in KTaO3 crystal, where the luminescence at low temperature consists of two components located around 2.61 eV and 2.28 eV [30,31] or around 2.1 eV and
94
H. Yang et al. / Journal of Luminescence 160 (2015) 90–94
2.5 eV in another paper [21]. The similarity is that the charge transfer emission from the O 2p to Ta 5d does not appear in both cases. It is quenched by Gd3 þ in GTO while in KTaO3 the charge transfer band is higher than the band gap (3.7 eV). Therefore, the luminescence of GTO is believed to be related to anion groups, namely TaO34 group, which also supports the above analysis. 5. Conclusions In summary, we revealed the detailed temperature-dependent luminescent characteristics of GTO crystal. The spectra consist of two bands, 2.2 eV band and 2.7 eV band from 8 K to 300 K. The temperature-dependent PL intensity of these two bands indicates the existence of thermal activation process. The activation energy of 2.2 eV band and 2.7 eV band is determined to be 156 meV and 175 meV, respectively. The lifetime of 2.7 eV band is found to be much longer than that of 2.2 eV band at low temperature. Based on the above, these two bands are believed to be from different luminescent centers. Besides, the annealing atmosphere exerts little influence on the PL intensity. We tentatively assign the high-energy band to self-trapped excitons (STE) localized at TaO34 groups and the low-energy band to relaxed excitons related to lattice imperfections. These results will be helpful to reveal the nature of scintillation mechanism of GTO crystal. However, there still exists many unknown aspects regarding the scintillation mechanism. Furthermore, more accurate temperature-dependent time-resolved luminescence measurements need to be performed to model the complicated radiation process. Our further study will focus on them. Acknowledgment Thanks are due to Shaoshuai Zhou for the time-resolved measurements. This work was financially supported by the National Natural Science Foundation of China (Grant nos. 51172236, 91122021, and 51272254). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2014.11.048.
References [1] R. Haugsrud, T. Norby, Nat. Mater. 5 (2006) 193. [2] R. Haugsrud, T. Norby, J. Am. Ceram. Soc. 90 (2007) 1116. [3] M. Nyman, M.A. Rodriguez, L.E.S. Rohwer, J.E. Martin, M. Waller, F.E. Osterloh, Chem. Mater. 21 (2009) 4731. [4] M. Gu, L.H. Xiao, X.L. Liu, R. Zhang, B.J. Liu, X. Xu, J. Alloy. Compd. 426 (2006) 390. [5] M.J.J. Lammers, G. Blasse, Mater. Res. Bull. 19 (1984) 759. [6] B. Li, Z.N. Gu, J.H. Lin, M.Z. Su, J. Mater. Sci. 35 (2000) 1139. [7] B. Liu, M. Gu, X.L. Liu, K. Han, S.M. Huang, C. Ni, G.B. Zhang, Z.M. Qi, Appl. Phys. Lett. 94 (2009) 061906. [8] B. Liu, K. Han, X.L. Liu, M. Gu, S.M. Huang, C. Ni, Z.M. Qi, G.B. Zhang, Solid State Commun. 144 (2007) 484. [9] W.P. Liu, Q.L. Zhang, W.L. Zhou, C.J. Gu, S.T. Yin, IEEE Trans. Nucl. Sci. 57 (2010) 1287. [10] H.J. Yang, Q.L. Zhang, P.Y. Zhou, K.J. Ning, J.Y. Gao, Y. He, J.Q. Luo, D.L. Sun, S.T. Yin, Proc. SPIE 8206 (2012) 820626. [11] H.J. Zhang, Y.H. Wang, L.C. Xie, J. Lumin. 130 (2010) 2089. [12] O. Voloshyna, S.V. Neicheva, N.G. Starzhinskiy, I.M. Zenya, S.S. Gridin, V.N. Baumer, O.T. Sidletskiy, Mater. Sci. Eng. B 178 (2013) 1491. [13] H. Yang, F. Peng, Q. Zhang, C. Guo, C. Shi, W. Liu, G. Sun, Y. Zhao, D. Zhang, D. Sun, S. Yin, M. Gu, R. Mao, CrystEngComm 16 (2014) 2480. [14] C.L. Melcher, Nucl. Instrum. Methods Phys. Res. A 537 (2005) 6. [15] M. Gu, X. Xu, X.L. Liu, L.Q. Qiu, R. Zhang, J. Sol–Gel Sci. Technol. 35 (2005) 193. [16] G. Blasse, A. Bril, J. Lumin. 3 (1970) 109. [17] L.I. Kazakova, I.S. Bykov, A.B. Dubovsky, J. Lumin. 72-4 (1997) 211. [18] L.L. Noto, S.S. Pitale, M.A. Gusowki, O.M. Ntwaeaborwa, J.J. Terblans, H.C. Swart, J. Lumin. 145 (2014) 907. [19] B. Liu, K. Han, X. Liu, M. Gu, S. Huang, C. Ni, Z. Qi, G. Zhang, Solid State Commun. 144 (2007) 484. [20] H. Feng, V. Jary, E. Mihokova, D.Z. Ding, M. Nikl, G.H. Ren, H.Y. Li, S.K. Pan, A. Beitlerova, R. Kucerkova, J. Appl. Phys. 108 (2010) 033519. [21] I. Katayama, K. Tanaka, J. Phys. Soc. Jpn. 75 (2006) 12064713. [22] M. Nikl, P. Bruza, D. Panek, M. Vrbova, E. Mihokova, J.A. Mares, A. Beitlerova, N. Kawaguchi, K. Fukuda, A. Yoshikawa, Appl. Phys. Lett. 102 (2013). [23] S. Sawai, T. Uchino, J. Appl. Phys. 112 (2012) 103523. [24] M. Nikl, P. Bohacek, E. Mihokova, M. Kobayashi, M. Ishii, Y. Usuki, V. Babin, A. Stolovich, S. Zazubovich, M. Bacci, J. Lumin. 87-89 (2000) 1136. [25] M. Itoh, T. Sakurai, Phys. Rev. B 73 (2006) 235106. [26] T. Passuello, F. Piccinelli, M. Trevisani, M. Giarola, G. Mariotto, L. Marciniak, D. Hreniak, M. Guzik, M. Fasoli, A. Vedda, V. Jary, M. Nikl, V. Causin, M. Bettinelli, A. Speghini, J. Mater. Chem. 22 (2012) 10639. [27] M. Itou, A. Fujiwara, T. Uchino, J. Phys. Chem. C 113 (2009) 20949. [28] M. Itoh, T. Kajitani, Phys. Rev. B 87 (2013) 085201. [29] L.H. Brixner, H.Y. Chen, J. Electrochem. Soc. 130 (1983) 2435. [30] E. Yamaichi, K. Watanabe, K. Imamiya, K. Ohi, J. Phys. Soc. Jpn. 56 (1987) 1890. [31] E. Yamaichi, K. Watanabe, K. Ohi, J. Phys. Soc. Jpn. 57 (1988) 2201.