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High-temperature long persistent and photo-stimulated luminescence in Tb3+ doped gallate phosphor
Journal of Alloys and Compounds 701 (2017) 774e779
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High-temperature long persistent and photo-stimulated luminescence in Tb3þ doped gallate phosphor Xue Yu a, b, Shaobo Wang a, Yucheng Zhu a, Jiajing Liang a, Jianbei Qiu a, Xuhui Xu a, *, Wei Lu c a
College of Materials Science and Engineering, Kunming University of Science and Technology, Key Laboratory of Advanced Materials of Yunnan Province, Kunming, 650093, China Department of Chemical Engineering, New Mexico State University, Las Cruces, NM, 88003, United States c Department of Applied Physics and University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, 999077, Hong Kong, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2016 Received in revised form 4 January 2017 Accepted 19 January 2017 Available online 23 January 2017
The formation of suitable defects levels is crucial to the optical performance of the electron trapping materials which provide a wide range of applications from the initial civil uses to life sciences, energy and environmental engineering. SrGa2O4, possessing a one-dimensional chain structure of cations along a crystal direction, exhibits a self-activated blue long persistent luminescence (LPL) related to the intrinsic defects. Different trap depths are generated forming a successive defect structure in this threedimensional framework of SrGa2O4 containing channels occupied by Sr2þ when Tb3þ ions are introduced. The captured carriers in shallow traps are spontaneously released at room temperature and recombine in the luminescence center of Tb3þ, eventually causing characteristic emission of Tb3þ. The deeper traps are critically important to prevent the thermal fading of carriers at room temperature, therefore, LPL is achieved in SrGa2O4:Tb3þ phosphor at 328 K. Furthermore, a photo-stimulated luminescence (PSL) originated from Tb3þ ions is realized in SrGa2O4:Tb3þ phosphor induced by 980 nm laser diode with the releasing of the carriers trapped in the deeper traps at room temperature. Our results infer that the existence of a successive defect structure with multiple traps for the incorporation of the shallow and deep ones is conducive to the thermal stability of SrGa2O4:Tb3þ phosphor, which could provide a potential application in a rigorous environment with a higher thermal energy than the room temperature, such as solar energy utilization, and in vivo-imaging. © 2017 Elsevier B.V. All rights reserved.
Keywords: Long persistent luminescence Photo-stimulated luminescence Defects SrGa2O4:Tb3þ Thermal stability
1. Introduction Manipulating and tuning the charge carrier transport in photoactive materials has attracted much attention in the fields of photodetector, photovoltaics and photocatalysis, since it plays an important role in enhancing the photoconversion efficiency [1e5]. Considerable research efforts have been devoted to the amplify photon energy absorption as well as enhance charge separation and hinder the charge recombination to optimize the photo-related device performance by engineering the architecture of the corresponding counterparts [6e8]. Electron trapping materials (ETMs), possessing appropriate trap centers to capture charge carriers
* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.jallcom.2017.01.210 0925-8388/© 2017 Elsevier B.V. All rights reserved.
(electrons or holes) which can be subsequently released by thermal, optical, or physical stimulations, resulting in stimulated emissions from the emitting centers, has been also thoroughly investigated for their important applications, like biomedicine, clinical medicine, or energy and environmental engineering [9]. Therefore, the trapping and de-trapping processes of electrons and holes is of current interest due to the necessity to understand the mechanism of long persistent luminescence and optical storage phenomena, as well as to improve the performance of ETMs [10]. Via so-called “defect engineering”, such as type, distribution, depth of the defects could be managed by the introduction of lanthanum ions and size/morphology/homogeneity/bonding control [11e15]. Tremendous progresses have been achieved. Specifically, rather than employing doping to engineer defects, designed energy band structure [16e20] to modulate the charge carrier generation is an efficient approach which has been drawn
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considerable theoretical and experimental efforts nowadays. Now that the optical properties of ETMs are critically rely on the crystal structure of host matrixes and the properties of defects, simplify the study with a specific structure of the host to determine the nature and origin of the defects seems feasible and necessary. Recently, the host matrix of luminescence materials with a unique low-dimensional chain crystal structure has attracted great research interest [21e23]. The host possesses an unusual one dimensional chain structure of cations along a certain crystal direction, which could be beneficial to the formation of the trap level and excited state level, and especially could provide an effective transmission path in one direction between luminescent and trapping centers for efficient transportation of majority carriers [24]. Hence, in this work, the gallates of SrGa2O4 has been chosen as host lattice, the structure of which is characterized by layers of sixmembered rings of GaO4 tetrahedra perpendicular to the a axis, and the stacking of the layers parallel to the a axis results in a threedimensional framework containing channels that are occupied by the Sr cations [25]. Furthermore, low-dimensional structural materials are easy to use to implant rare earth ions into the host lattice and create traps located at suitable depths [24]. Tb3þ ions are introduced to SrGa2O4 due to its excellent green emission derived from the transition of 5D4-7F5. Traps with different depth are generated and a successive defect structure is formed in SrGa2O4:Tb3þ phosphors. The roles of these traps, especially the deeper ones which contributes to prevent the thermal fading of carries and stabilize them at room temperature, are investigated. The thermally stimulated LPL and photo-stimulated luminescence (PSL) process in SrGa2O4:Tb3þ phosphor is studied. The hightemperature-resistance LPL ability of SrGa2O4:Tb3þ phosphor provides a potential application in vivo bio-imaging or a harsh environment where the thermal energy is much higher than the room temperature. The aim of our work is to get a deeper understanding on the possibility to shape the spectroscopic properties of phosphors based on the unique low-dimensional host structure and the effective trapping centers. 2. Experimental procedure Sr1xGa2O4:xTb3þ (x ¼ 0, 0.005, 0.01, 0.03, 0.05 and 0.07) samples were synthesized by the conventional high-temperature solid state reaction. Stoichiometric amounts of Sr2CO3(A.R), Ga2O3(A.R), and Tb4O7 (99.99%) were mixed in an agate mortar with ethanol. After being fully ground, the mixtures were put into crucibles and calcined at 1200 C for 6 h in a reducing atmosphere (95:5 N2:H2). After cooling to room temperature naturally, the asobtained samples were ground into powder for the following measurements. The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) with Ni-filter Cu Ka radiation (l ¼ 0.154056 nm) at a scanning step of 0.02 . The XRD data were collected in the range of 10e40 by applying a D8ADVANCE/ Germany Bruker X ray diffractometer. The photoluminescence excitation (PLE), photoluminescence (PL), LPL, and PSL spectra were recorded by using a Hitachi F-7000 fluorescence spectrophotometer. The LPL lifetime curves were measured with a PR305 long afterglow instrument (Zhejiang University Sensing Instruments Co., Ltd., China) after the sample had been irradiated with UV light (254 nm and 365 nm) for 20min. When the sample vessel was heated to the corresponding temperature through the DC power, the decay curves of Sr0.995Ga2O4: 0.005 Tb3þ at different ambient temperature were recorded subsequently after ceasing the excitation of UV light for 20min. The thermo-luminescence (TL) curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory). The weight of the measured samples was constant
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(0.002 g). Prior to the TL measure, the samples were first exposed to the radiation from the UV light for 20min and then heated from room temperature to 550 K at a rate of 1 K/s. Before the PSL had been measured (lex ¼ 980 nm), the samples were pre-irradiated with UV light for 20 min and then placed in the dark for 20 h. A 980 nm laser diode is used as a stimulating source. 3. Results and discussion The purity of all the prepared samples was systematically checked by XRD measurements. Fig. 1 shows the typical XRD patterns of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.005, 0.01 and 0.03) samples. Clearly, all the diffraction peaks could be indexed to the phase of SrGa2O4 registered in JCPDF file No. 22-0905, which indicates that all the samples are identified as SrGa2O4 phase. No significant impure phases are observed when Tb3þ ions are doped, demonstrating the pure phase is synthesized in this work. Based on the effective ionic radius (r) of cations with different coordination number (CN) [26], the radii of Tb3þ (0.0995 nm) is closer to that of Sr2þ (0.1130 nm). Since both four-coordinated Ga3þ (0.0470 nm) sites are too small for Tb3þ ions to occupy, therefore, we conclude that Tb3þ ions tend to prefer the Sr2þ sites due to size consideration. As shown in the inset of Fig. 1, the structure of SrGa2O4 phase possesses a distinct open structure with tunnel-like cavities (a crystal direction) running throughout the host lattice [25,27]. Fig. 2 presents the PL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples under the excitation of 252 nm. As shown in Fig. 2, Ga3þ is the luminescence center of SrGa2O4 phosphor, and the emission wavelength is 407 nm, which is referred to the 4T1 / 4A2 transition of electrons in d orbits Ga3þ [27,28]. Whereas, the PL intensity of the transition of Ga3þ drastically decreased when Tb3þ ions are introduced. Under 252 nm excitation, the emission spectra yield from blue to green emissions in the region of 380e460 and 480e650 nm, which are due to the 5 D3 / 7FJ (J ¼ 6, 5, 4, 3) and 5D4/7FJ (J ¼ 6, 5, 4, 3) transitions of Tb3þ ions [29] in Sr1-xGa2O4:xTb3þ samples, respectively. In addition, the emission intensity of Tb3þ ions at 547 nm increases firstly, of which reaches the maximum when the doped concentration of Tb3þ is up to 0.005, and then decreases due to the concentration quenching. The inset of Fig. 2 exhibit the PLE spectra of SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ, respectively. The PLE spectrum of SrGa2O4
Fig. 1. XRD patterns of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.005, 0.01 and 0.03) samples and the JCPDF No.22-0905 of SrGa2O4, respectively. The inset is the crystal structure of the SrGa2O4 seen from a axis and c axis, respectively.
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Fig. 2. PL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples under 252 nm excitation. The inset is the PLE spectra of SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ sample and the decay time of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples, respectively.
exhibits an asymmetry band, which is consisted of two peaks located at 237 and 252 nm, respectively. The excitation band is attributed to the contribution of both host absorption and the O2-Ga3þ charge transfer band [30,31]. While monitored the emission of 547 nm originated from the characteristic transition of 5D4/7F5 of Tb3þ, it can be clearly seen that the excitation spectrum is made up of a broad peak in the region of 200e275 nm and a series of peaks from 275 to 400 nm, which is belonged to the host absorption and the 4f-4f transitions of Tb3þ, respectively. Besides, the emission band of SrGa2O4 spectral overlaps with the excitation band of Tb3þ, which indicates the energy transfer from SrGa2O4 host to Tb3þ according to Dexter's theory [32]. The energy transfer behavior from the host to the Tb3þ ions can be further recognized by the inserted decay curves of the Sr1-xGa2O4:xTb3þ samples in Fig. 2 intuitively. The average decay time t of SrGa2O4 host is determined to be 1.727, 1.608, 1.553, 1.460, 1.391 and 1.218ms for Sr13þ with x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03, xGa2O4:xTb respectively. The decay lifetime for Ga3þ is found to decrease with increasing Tb3þ dopant content, which is strong evidence for the energy transfer from the SrGa2O4 host to Tb3þ, as reported by Xia et al. [33] and Lü et al. [34], respectively. After the Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) series of samples had been pre-irradiated with 254 nm UV light for 20min, LPL spectra are recorded as shown in Fig. 3a. An obvious LPL phenomenon located at 407 nm is observed in the SrGa2O4 host matrix after the UV excitation sources had been switched off, which exhibits similar shape and position to the PL of samples as shown in Fig. 2. It is distinctly observed that a greenblue LPL is achieved in Sr1-xGa2O4:xTb3þ (x ¼ 0.001, 0.003, 0.005, 0.01, and 0.03) samples as depicted in Fig. 3a. Different from the LPL of the SrGa2O4 host, the characteristic emission peaks derived from the 5D3/7FJ (J ¼ 6, 5, 4, 3) and 5D4/7FJ (J ¼ 6, 5, 4, 3) transitions of Tb3þ is detected, while the LPL emission band of SrGa2O4 host is mostly insignificant. It indicates that Tb3þ ions act as the dominate emission centers in LPL process. The optimal LPL is observed in the Sr0.995Ga2O4:0.005 Tb3þ sample. With a further increasing concentration of Tb3þ, the LPL intensity of Tb3þ is drastically decreased. Compared with the PL energy transfer process of Fig. 2, the persistent energy transfer efficiency of LPL is more
effective than that of the PL process. It could be suggested that Tb3þ ions introduce foreign trapping centers that acted as the bridge, promoting the energy transfer efficiency from the SrGa2O4 host to Tb3þ of the LPL process. Fig. 3b is the LPL lifetime curves of Sr13þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples xGa2O4:xTb at room temperature, which shows that these samples exhibit different persistent times that consisted of a fast decay and a consequent slow decay with a long decay tail, implying the existence of various trap depths. Compared with the SrGa2O4 host, a remarkable longer persistent time is observed in the Tb3þ doped phosphors. Hence, it indicates that the incorporation of Tb3þ ions creates more defects (the intrinsic defects or foreign defects) in the SrGa2O4 host lattice, which act as trapping centers and have a significant influence on the LPL performance. The LPL lifetime curves show that the optimized persistent luminescence of Sr0.995Ga2O4:0.005 Tb3þ can be visually recognized (0.32mcd/m2) over 6 h at room temperature. Furthermore, the Sr0.995Ga2O4:0.005 Tb3þ sample exhibits a high-temperatureresistance LPL ability. The LPL lifetime curves of Sr0.995Ga2O4:0.005 Tb3þ is recorded with an increasing ambient temperature as shown in Fig. 3c. The LPL of Sr0.995Ga2O4:0.005 Tb3þ is detectable even at 328 K, which lasts for a period of time. It indicates that Sr0.995Ga2O4:0.005 Tb3þ phosphor provide a potential application in a rigorous environment of solar energy utilization, and in vivo-imaging where higher thermal energy than the room temperature is required. A suitable trap depth, slightly above the room temperature (320e393 K), is the key to excellent persistent luminescence [35]. Fig. 4 shows the TL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01, and 0.03) samples recorded immediately after UV lamp pre-radiation for 20min, respectively. As can be seen from Fig. 4, SrGa2O4 shows a relatively weak broadband between 300 and 450 K regardless of its weak intensity, which indicates that the host itself has proper native defects levels to generate LPL. This is consistent with the observation of blue LPL performance as depicted in Fig. 3. With respect to the intrinsic defects, because the high annealing temperatures may generate both O and Sr va00 cancies (V O and VSr , respectively) in SrGa2O4, the roles of these defects are expected to be crucial to the LPL phenomena. In the literature, V O is supposed to serve as an electron trap center, while 00 VSr possibly acts as a hole trap center [36]. As shown in Fig. 4, a remarkable growth of TL intensity with the introduction of Tb3þ is pronounced. Two bands centered at T1 (357 K) and T2 (410 K) are observed respectively, using Gaussian function for both the SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ sample as shown in the inset of Fig. 4. It is notable that the position of the TL peaks of Tb3þ doped SrGa2O4 samples is not significantly changed compared with SrGa2O4 host, which indicates the introduction of Tb3þ contributes to the increase number of the trapping centers without changing the depth of the traps. It is commonly considered that the lower and higher temperature of the TL bands is related to the shallower and deeper traps, respectively. Therefore, the increased shallower traps (357 K) are contributed to the significant improvement of the LPL properties of Tb3þ doped SrGa2O4 samples. Meanwhile, the increased deeper one is critically important to prevent the thermal fading of carriers at room temperature, which is consistent with the observation of the LPL phenomenon of Tb3þ doped SrGa2O4 samples at relative higher ambient temperature as depicted in Fig. 3c. In order to understand the trapping nature, the trap depth ET of the well fitted TL peaks, marked TA (357 K) and TB (410 K) by two Gaussian profiles of the asymmetric TL peak of SrGa2O4 and Sr0.995Ga2O4: 0.005 Tb3þ samples, in the inset of Fig. 4 is calculated by the following equation [36]:
X. Yu et al. / Journal of Alloys and Compounds 701 (2017) 774e779
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Fig. 3. (a) LPL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples, (b) LPL lifetime curves of Sr1-xGa2O4:xTb3þ (x ¼ 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples at room temperature, (c) LPL lifetime curves of Sr0.995Ga2O4: 0.005 Tb3þ samples at different ambient temperature.
2Tb
Fig. 4. TL curves of Sr1-xGa2O4:xTb3þ (x ¼ 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples recorded immediately after UV lamp pre-radiation for 20min. The inset is the TL curves of SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ, respectively.
h i k T2 B m E ¼ 2:52 þ 10:2 mg 0:42
u
! 2kB Tm
(1)
where, mg ¼ d/u is the symmetrical geometrical factor, and d ¼ T2Tm is the half width towards the fall-off of the glow peak, u ¼ T2T1 is the total half of width, and Tm, T1 and T2 are the peak temperature at the maximum and the temperatures on either side of the temperature at the maximum, corresponding to half intensity, respectively. k is the Boltzmann's constant. Then the calculated trap levels (E) of TA, TB are 0.7337 and 0.8719 eV, respectively. In the SrGa2O4 host, intrinsic defects, such as V O and 00 VSr , exist as mentioned above. With the doping of Tb3þ, considering the fact that Tb3þ replace the replace the Sr2þ site in the SrGa2O4 host matrix, two Tb3þ ions replace three Sr2þ ions to balance the charge of the phosphor, which creates two positive defects and one
3þ
negative defect (3Sr2þ !2TbSr þ VSr ). Therefore, more VSr and extra electron traps TbSr are formed. Although TL spectra can accurately demonstrate the range of a trap's depth, the situation of trap type could not be attributed through TL spectra [37]. In this respect, the broad TL spectrum covers from 300 to 525 K of Sr0.995Ga2O4:0.005 Tb3þ, which indicates that the traps distribute over a wide range of energies in the bandgap as traps of various depth are formed, is ascribed to the increased number of the intrinsic defects as well as the defect complexes. In addition, the successive trap structure of Sr0.995Ga2O4:0.005 Tb3þ as shown in the TL spectra suggest that the trapped carriers in deeper centers could be further released by photo-induction with near infrared excitation. To investigate the kinetic order of these samples, TL curves of Sr0.995Ga2O4:0.005 Tb3þ with different delay time are recorded in Fig. 5a. The TA band was greatly depleted after a delay of 20 h and then almost completely disappears. However, the intensities of TB band sustain the intensity without changing significantly after 10 h. Hence, it is safe to say that the relative shallow traps of TA could be ascribed to the LPL phenomenon at room temperature, while the presence of deep stable traps in SrGa2O4:Tb3þ can immobilize the energy permanently at room temperature. The relatively deep trap precludes the thermal release of the intercepted carriers at room temperature, which is an essential factor required for PSL phosphors. The detailed PSL properties for SrGa2O4:Tb3þ were investigated as follows. As shown in Fig. 5b, Sr0.995Ga2O4:0.005 Tb3þ phosphor exhibit strong PSL upon 980 nm stimulation after being pre-irradiated with UV light for 20min and then placed in the dark for 20 h to make sure the completely release of electrons trapped in the shallow trap (TA). The characteristic emission peaks derived from the 5D3 / 7FJ (J ¼ 6, 5, 4, 3) and 5D4 / 7FJ (J ¼ 6, 5, 4, 3) transitions of Tb3þ is detected under 980 nm excitation. The PSL intensities decline with an increased stimulation time, which confirms that it is not an up-conversion process but the release of the carriers in the traps of this phosphor induced by the excitation energy of 980 nm. Thus, it suggests that the deeper traps corresponding to the TB band could be responsible for the performance of PSL (lex ¼ 980 nm). Fig. 5c displays the TL map as a function of the delay time of the samples being placed in a dark room, and Fig. 5d is the PSL map as a function of the stimulation time of 00
00
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X. Yu et al. / Journal of Alloys and Compounds 701 (2017) 774e779
Fig. 5. (a) TL curves of Sr0.995Ga2O4:0.005 Tb3þ sample placed in a dark room for different periods of time (0, 5, 10, 15 and 20 h), (b) PSL emission spectra of Sr0.995Ga2O4: 0.005 Tb3þ under 980 nm excitation with different stimulation time (5, 15, 25, 35, and 45s), (c) TL map as a function of the delay time of the samples being placed in a dark room, (d) PSL map as a function of the stimulation time of 980 nm excitation, respectively.
980 nm excitation, respectively, from which the luminescence intensity distribution can be clearly observed. 4. Conclusion A visible LPL has been obtained in the unique structure of SrGa2O4 possessing a three-dimensional framework containing channels occupied by the cations of Sr2þ. It is found that both the energy transfer and the persistent energy transfer from SrGa2O4 to the emission centers of Tb3þ occurs effectively in this phosphor. The introduction of Tb3þ in SrGa2O4 contributes to the increase number of the trapping centers significantly. Actually, the LPL of SrGa2O4:Tb3þ can last for a period of time even at 328 K, ascribed to the stable deeper traps which prevent the thermal fading of carriers at room temperature. Besides, thanks to the successive trap structure, the carries captured in deeper defects could be released as a PSL induced by the excitation energy of 980 nm in SrGa2O4:Tb3þ phosphor. Our results indicate that SrGa2O4:Tb3þ phosphor could provide particular applications as optical probes for in vivo bioimaging or solar energy utilization, especially in an environment with a relative higher thermal energy. Acknowledgements This work was financially supported by the National Nature Science Foundation of China (61565009, 11664022), the Foundation of Natural Science of Yunnan Province (2016FB088), the Young Teachers Support Program of Faculty of Materials Science and
Engineering, Kunming University of Science and Technology (1096838507, 14078342), and the China Scholarship Council (201508535038). References [1] X. Wang, C. Liow, A. Bisht, X. Liu, T.C. Sum, X. Chen, S. Li, Adv. Mater. 27 (2015) 2207e2214. [2] M.Z. Ge, C.Y. Cao, S.H. Li, Y.X. Tang, L.N. Wang, N. Qi, J.Y. Huang, K.Q. Zhang, S.S.A. Deyab, Y.K. Lai, Nanoscale 8 (2016) 5226e5234. [3] J. Yao, J. Shao, Y. Wang, Z. Zhao, G. Yang, Nanoscale 7 (2015) 12535e12541. [4] Y. Li, X. Wei, X. Yan, J. Cai, A. Zhou, M. Yang, K. Liu, Phys. Chem. Chem. Phys. 18 (2016) 10255e10261. [5] B.J. Morgan, G.W. Watson, Phys. Rev. B 80 (2009) 233102. [6] Z. Zhang, Y. Huang, K. Liu, L. Guo, Q. Yuan, B. Dong, Adv. Mater. 27 (2015) 5906e5914. [7] R. Li, F. Zhang, D. Wang, J. Yang, M. Li, J. Zhu, X. Zhou, H. Han, C. Li, Nat. Commun. 4 (2013) 1432. [8] S. Sarkar, D. Basak, Appl. Phys. Lett. 103 (2013) 041112. [9] Y. Li, M. Geceviciusa, J. Qiu, Chem. Soc. Rev. 45 (2016) 2090e2136. [10] K.P. McKenna, A.L. Shluger, Nat. Mater. 7 (2008) 859e862. [11] H. Nan, Z. Wang, W. Wang, Z. Liang, Y. Lu, Q. Chen, D. He, P. Tan, F. Miao, X. Wang, J. Wang, Z. Ni, ACS Nano 8 (2014) 5738e5745. [12] D. Kulesza, E. Zych, J. Phys. Chem. C 117 (2013) 26921e26928. [13] Y. Li, S. Zhou, Yiyang Li, K. Sharafudeen, Z. Ma, G. Dong, M. Peng, J. Qiu, J. Mater. Chem. C 2 (2014) 2657e2663. [14] J. Xu, J. Ueda, S. Tanabe, J. Mater. Chem. C 4 (2016) 4380e4386. [15] M. Nikl, K. Kamada, V. Babin, J. Pejchal, K. Pilarova, E. Mihokova, A. Beitlerova, K. Bartosiewicz, S. Kurosawa, A. Yoshikawa, Cryst. Growth Des. 14 (2014) 4827e4833. [16] M. Fasoli, A. Vedda, M. Nikl, C. Jiang, B.P. Uberuaga, D.A. Andersson, K.J. McClellan, C.R. Stanek, Phys. Rev. B 84 (2011) 081102. [17] H. Luo, A.J.J. Bos, P. Dorenbos, J. Phys. Chem. C 120 (2016) 5916e5925. [18] H. Lin, B. Wang, Q. Huang, F. Huang, J. Xu, H. Chen, Z. Lin, J. Wang, T. Hua, Y. Wang, J. Mater. Chem. C 4 (2016) 10329e10338. [19] J. Ueda, P. Dorenbos, A. Bos, K. Kuroishi, S. Tanabe, J. Mater. Chem. C 3 (2015)
X. Yu et al. / Journal of Alloys and Compounds 701 (2017) 774e779 5642e5651. [20] J. Xu, J. Ueda, S. Tanabe, Opt. Mater. Express 5 (2015) 963e968. [21] E. Danielson, M. Devenney, D.M. Giaquinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojary, C.M. Reaves, W.H. Weinberg, D.X. Wu, Science 279 (1998) 837e839. [22] S.K. Gupta, M. Sahu, K. Krishnan, M.K. Saxena, V. Natarajan, S.V. Godbole, J. Mater. Chem. C 1 (2013) 7054e7063. [23] H. Li, R. Zhao, Y. Jia, W. Sun, J. Fu, L. Jiang, S. Zhang, R. Pang, C. Li, ACS Appl. Mater. Interfaces 6 (2014) 3163e3169. [24] Y. Liu, J. Kuang, B. Lei, C. Shi, J. Mater. Chem. 15 (2005) 4025e4031. [25] V. Kahlenberg, R.X. Fischer, C.S.J. Shaw, J. Solid State Chem. 153 (2000) 294e300. [26] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751e767. [27] T. Wang, X. Xu, D. Zhou, Y. Yang, J. Qiu, X. Yu, Inorg. Chem. 55 (2016) 894e901.
779
[28] S.H. Yang, H.F. Tu, J. Electrochem. Soc. 152 (2005) H1eH5. [29] B.V. Ratnam, M. Jayasimhadri, G.B. Kumar, K. Jang, S.S. Kim, Y.I. Lee, J.M. Lim, D.S. Shin, T.K. Song, J. Alloys Comp. 564 (2013) 100e104. [30] Q. Bin, T. Zilong, Z. Zhongtai, C. Lei, J. Rare Earths 25 (2007) 286e290. [31] M.E. Foley, R.W. Meulenberg, J.R. McBride, G.F. Strouse, Chem. Mater. 27 (2015) 8362e8374. [32] D.L. Dexter, J. Chem. Phys. 21 (1953) 836e850. [33] Z. Xia, R.S. Liu, J. Phys. Chem. C 116 (2012) 15604e15609. [34] W. Lü, W. Lv, Q. Zhao, M. Jiao, B. Shao, H. You, J. Mater. Chem. C 3 (2015) 2334e2340. [35] K.V. Eeckhout, A.J. Bos, D. Poelman, P.F. Smet, Phys. Rev. B 87 (2013) 045126. [36] B. Qu, B. Zhang, L. Wang, R. Zhou, X.C. Zeng, Chem. Mater. 27 (2015) 2195e2202. [37] P. Wang, X. Xu, D. Zhou, X. Yu, J. Qiu, Inorg. Chem. 54 (2015) 1690e1697.
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High-temperature long persistent and photo-stimulated luminescence in Tb3þ doped gallate phosphor Xue Yu a, b, Shaobo Wang a, Yucheng Zhu a, Jiajing Liang a, Jianbei Qiu a, Xuhui Xu a, *, Wei Lu c a
College of Materials Science and Engineering, Kunming University of Science and Technology, Key Laboratory of Advanced Materials of Yunnan Province, Kunming, 650093, China Department of Chemical Engineering, New Mexico State University, Las Cruces, NM, 88003, United States c Department of Applied Physics and University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, 999077, Hong Kong, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2016 Received in revised form 4 January 2017 Accepted 19 January 2017 Available online 23 January 2017
The formation of suitable defects levels is crucial to the optical performance of the electron trapping materials which provide a wide range of applications from the initial civil uses to life sciences, energy and environmental engineering. SrGa2O4, possessing a one-dimensional chain structure of cations along a crystal direction, exhibits a self-activated blue long persistent luminescence (LPL) related to the intrinsic defects. Different trap depths are generated forming a successive defect structure in this threedimensional framework of SrGa2O4 containing channels occupied by Sr2þ when Tb3þ ions are introduced. The captured carriers in shallow traps are spontaneously released at room temperature and recombine in the luminescence center of Tb3þ, eventually causing characteristic emission of Tb3þ. The deeper traps are critically important to prevent the thermal fading of carriers at room temperature, therefore, LPL is achieved in SrGa2O4:Tb3þ phosphor at 328 K. Furthermore, a photo-stimulated luminescence (PSL) originated from Tb3þ ions is realized in SrGa2O4:Tb3þ phosphor induced by 980 nm laser diode with the releasing of the carriers trapped in the deeper traps at room temperature. Our results infer that the existence of a successive defect structure with multiple traps for the incorporation of the shallow and deep ones is conducive to the thermal stability of SrGa2O4:Tb3þ phosphor, which could provide a potential application in a rigorous environment with a higher thermal energy than the room temperature, such as solar energy utilization, and in vivo-imaging. © 2017 Elsevier B.V. All rights reserved.
Keywords: Long persistent luminescence Photo-stimulated luminescence Defects SrGa2O4:Tb3þ Thermal stability
1. Introduction Manipulating and tuning the charge carrier transport in photoactive materials has attracted much attention in the fields of photodetector, photovoltaics and photocatalysis, since it plays an important role in enhancing the photoconversion efficiency [1e5]. Considerable research efforts have been devoted to the amplify photon energy absorption as well as enhance charge separation and hinder the charge recombination to optimize the photo-related device performance by engineering the architecture of the corresponding counterparts [6e8]. Electron trapping materials (ETMs), possessing appropriate trap centers to capture charge carriers
* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.jallcom.2017.01.210 0925-8388/© 2017 Elsevier B.V. All rights reserved.
(electrons or holes) which can be subsequently released by thermal, optical, or physical stimulations, resulting in stimulated emissions from the emitting centers, has been also thoroughly investigated for their important applications, like biomedicine, clinical medicine, or energy and environmental engineering [9]. Therefore, the trapping and de-trapping processes of electrons and holes is of current interest due to the necessity to understand the mechanism of long persistent luminescence and optical storage phenomena, as well as to improve the performance of ETMs [10]. Via so-called “defect engineering”, such as type, distribution, depth of the defects could be managed by the introduction of lanthanum ions and size/morphology/homogeneity/bonding control [11e15]. Tremendous progresses have been achieved. Specifically, rather than employing doping to engineer defects, designed energy band structure [16e20] to modulate the charge carrier generation is an efficient approach which has been drawn
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considerable theoretical and experimental efforts nowadays. Now that the optical properties of ETMs are critically rely on the crystal structure of host matrixes and the properties of defects, simplify the study with a specific structure of the host to determine the nature and origin of the defects seems feasible and necessary. Recently, the host matrix of luminescence materials with a unique low-dimensional chain crystal structure has attracted great research interest [21e23]. The host possesses an unusual one dimensional chain structure of cations along a certain crystal direction, which could be beneficial to the formation of the trap level and excited state level, and especially could provide an effective transmission path in one direction between luminescent and trapping centers for efficient transportation of majority carriers [24]. Hence, in this work, the gallates of SrGa2O4 has been chosen as host lattice, the structure of which is characterized by layers of sixmembered rings of GaO4 tetrahedra perpendicular to the a axis, and the stacking of the layers parallel to the a axis results in a threedimensional framework containing channels that are occupied by the Sr cations [25]. Furthermore, low-dimensional structural materials are easy to use to implant rare earth ions into the host lattice and create traps located at suitable depths [24]. Tb3þ ions are introduced to SrGa2O4 due to its excellent green emission derived from the transition of 5D4-7F5. Traps with different depth are generated and a successive defect structure is formed in SrGa2O4:Tb3þ phosphors. The roles of these traps, especially the deeper ones which contributes to prevent the thermal fading of carries and stabilize them at room temperature, are investigated. The thermally stimulated LPL and photo-stimulated luminescence (PSL) process in SrGa2O4:Tb3þ phosphor is studied. The hightemperature-resistance LPL ability of SrGa2O4:Tb3þ phosphor provides a potential application in vivo bio-imaging or a harsh environment where the thermal energy is much higher than the room temperature. The aim of our work is to get a deeper understanding on the possibility to shape the spectroscopic properties of phosphors based on the unique low-dimensional host structure and the effective trapping centers. 2. Experimental procedure Sr1xGa2O4:xTb3þ (x ¼ 0, 0.005, 0.01, 0.03, 0.05 and 0.07) samples were synthesized by the conventional high-temperature solid state reaction. Stoichiometric amounts of Sr2CO3(A.R), Ga2O3(A.R), and Tb4O7 (99.99%) were mixed in an agate mortar with ethanol. After being fully ground, the mixtures were put into crucibles and calcined at 1200 C for 6 h in a reducing atmosphere (95:5 N2:H2). After cooling to room temperature naturally, the asobtained samples were ground into powder for the following measurements. The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) with Ni-filter Cu Ka radiation (l ¼ 0.154056 nm) at a scanning step of 0.02 . The XRD data were collected in the range of 10e40 by applying a D8ADVANCE/ Germany Bruker X ray diffractometer. The photoluminescence excitation (PLE), photoluminescence (PL), LPL, and PSL spectra were recorded by using a Hitachi F-7000 fluorescence spectrophotometer. The LPL lifetime curves were measured with a PR305 long afterglow instrument (Zhejiang University Sensing Instruments Co., Ltd., China) after the sample had been irradiated with UV light (254 nm and 365 nm) for 20min. When the sample vessel was heated to the corresponding temperature through the DC power, the decay curves of Sr0.995Ga2O4: 0.005 Tb3þ at different ambient temperature were recorded subsequently after ceasing the excitation of UV light for 20min. The thermo-luminescence (TL) curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory). The weight of the measured samples was constant
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(0.002 g). Prior to the TL measure, the samples were first exposed to the radiation from the UV light for 20min and then heated from room temperature to 550 K at a rate of 1 K/s. Before the PSL had been measured (lex ¼ 980 nm), the samples were pre-irradiated with UV light for 20 min and then placed in the dark for 20 h. A 980 nm laser diode is used as a stimulating source. 3. Results and discussion The purity of all the prepared samples was systematically checked by XRD measurements. Fig. 1 shows the typical XRD patterns of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.005, 0.01 and 0.03) samples. Clearly, all the diffraction peaks could be indexed to the phase of SrGa2O4 registered in JCPDF file No. 22-0905, which indicates that all the samples are identified as SrGa2O4 phase. No significant impure phases are observed when Tb3þ ions are doped, demonstrating the pure phase is synthesized in this work. Based on the effective ionic radius (r) of cations with different coordination number (CN) [26], the radii of Tb3þ (0.0995 nm) is closer to that of Sr2þ (0.1130 nm). Since both four-coordinated Ga3þ (0.0470 nm) sites are too small for Tb3þ ions to occupy, therefore, we conclude that Tb3þ ions tend to prefer the Sr2þ sites due to size consideration. As shown in the inset of Fig. 1, the structure of SrGa2O4 phase possesses a distinct open structure with tunnel-like cavities (a crystal direction) running throughout the host lattice [25,27]. Fig. 2 presents the PL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples under the excitation of 252 nm. As shown in Fig. 2, Ga3þ is the luminescence center of SrGa2O4 phosphor, and the emission wavelength is 407 nm, which is referred to the 4T1 / 4A2 transition of electrons in d orbits Ga3þ [27,28]. Whereas, the PL intensity of the transition of Ga3þ drastically decreased when Tb3þ ions are introduced. Under 252 nm excitation, the emission spectra yield from blue to green emissions in the region of 380e460 and 480e650 nm, which are due to the 5 D3 / 7FJ (J ¼ 6, 5, 4, 3) and 5D4/7FJ (J ¼ 6, 5, 4, 3) transitions of Tb3þ ions [29] in Sr1-xGa2O4:xTb3þ samples, respectively. In addition, the emission intensity of Tb3þ ions at 547 nm increases firstly, of which reaches the maximum when the doped concentration of Tb3þ is up to 0.005, and then decreases due to the concentration quenching. The inset of Fig. 2 exhibit the PLE spectra of SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ, respectively. The PLE spectrum of SrGa2O4
Fig. 1. XRD patterns of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.005, 0.01 and 0.03) samples and the JCPDF No.22-0905 of SrGa2O4, respectively. The inset is the crystal structure of the SrGa2O4 seen from a axis and c axis, respectively.
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Fig. 2. PL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples under 252 nm excitation. The inset is the PLE spectra of SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ sample and the decay time of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples, respectively.
exhibits an asymmetry band, which is consisted of two peaks located at 237 and 252 nm, respectively. The excitation band is attributed to the contribution of both host absorption and the O2-Ga3þ charge transfer band [30,31]. While monitored the emission of 547 nm originated from the characteristic transition of 5D4/7F5 of Tb3þ, it can be clearly seen that the excitation spectrum is made up of a broad peak in the region of 200e275 nm and a series of peaks from 275 to 400 nm, which is belonged to the host absorption and the 4f-4f transitions of Tb3þ, respectively. Besides, the emission band of SrGa2O4 spectral overlaps with the excitation band of Tb3þ, which indicates the energy transfer from SrGa2O4 host to Tb3þ according to Dexter's theory [32]. The energy transfer behavior from the host to the Tb3þ ions can be further recognized by the inserted decay curves of the Sr1-xGa2O4:xTb3þ samples in Fig. 2 intuitively. The average decay time t of SrGa2O4 host is determined to be 1.727, 1.608, 1.553, 1.460, 1.391 and 1.218ms for Sr13þ with x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03, xGa2O4:xTb respectively. The decay lifetime for Ga3þ is found to decrease with increasing Tb3þ dopant content, which is strong evidence for the energy transfer from the SrGa2O4 host to Tb3þ, as reported by Xia et al. [33] and Lü et al. [34], respectively. After the Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) series of samples had been pre-irradiated with 254 nm UV light for 20min, LPL spectra are recorded as shown in Fig. 3a. An obvious LPL phenomenon located at 407 nm is observed in the SrGa2O4 host matrix after the UV excitation sources had been switched off, which exhibits similar shape and position to the PL of samples as shown in Fig. 2. It is distinctly observed that a greenblue LPL is achieved in Sr1-xGa2O4:xTb3þ (x ¼ 0.001, 0.003, 0.005, 0.01, and 0.03) samples as depicted in Fig. 3a. Different from the LPL of the SrGa2O4 host, the characteristic emission peaks derived from the 5D3/7FJ (J ¼ 6, 5, 4, 3) and 5D4/7FJ (J ¼ 6, 5, 4, 3) transitions of Tb3þ is detected, while the LPL emission band of SrGa2O4 host is mostly insignificant. It indicates that Tb3þ ions act as the dominate emission centers in LPL process. The optimal LPL is observed in the Sr0.995Ga2O4:0.005 Tb3þ sample. With a further increasing concentration of Tb3þ, the LPL intensity of Tb3þ is drastically decreased. Compared with the PL energy transfer process of Fig. 2, the persistent energy transfer efficiency of LPL is more
effective than that of the PL process. It could be suggested that Tb3þ ions introduce foreign trapping centers that acted as the bridge, promoting the energy transfer efficiency from the SrGa2O4 host to Tb3þ of the LPL process. Fig. 3b is the LPL lifetime curves of Sr13þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01 and 0.03) samples xGa2O4:xTb at room temperature, which shows that these samples exhibit different persistent times that consisted of a fast decay and a consequent slow decay with a long decay tail, implying the existence of various trap depths. Compared with the SrGa2O4 host, a remarkable longer persistent time is observed in the Tb3þ doped phosphors. Hence, it indicates that the incorporation of Tb3þ ions creates more defects (the intrinsic defects or foreign defects) in the SrGa2O4 host lattice, which act as trapping centers and have a significant influence on the LPL performance. The LPL lifetime curves show that the optimized persistent luminescence of Sr0.995Ga2O4:0.005 Tb3þ can be visually recognized (0.32mcd/m2) over 6 h at room temperature. Furthermore, the Sr0.995Ga2O4:0.005 Tb3þ sample exhibits a high-temperatureresistance LPL ability. The LPL lifetime curves of Sr0.995Ga2O4:0.005 Tb3þ is recorded with an increasing ambient temperature as shown in Fig. 3c. The LPL of Sr0.995Ga2O4:0.005 Tb3þ is detectable even at 328 K, which lasts for a period of time. It indicates that Sr0.995Ga2O4:0.005 Tb3þ phosphor provide a potential application in a rigorous environment of solar energy utilization, and in vivo-imaging where higher thermal energy than the room temperature is required. A suitable trap depth, slightly above the room temperature (320e393 K), is the key to excellent persistent luminescence [35]. Fig. 4 shows the TL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0, 0.001, 0.003, 0.005, 0.01, and 0.03) samples recorded immediately after UV lamp pre-radiation for 20min, respectively. As can be seen from Fig. 4, SrGa2O4 shows a relatively weak broadband between 300 and 450 K regardless of its weak intensity, which indicates that the host itself has proper native defects levels to generate LPL. This is consistent with the observation of blue LPL performance as depicted in Fig. 3. With respect to the intrinsic defects, because the high annealing temperatures may generate both O and Sr va00 cancies (V O and VSr , respectively) in SrGa2O4, the roles of these defects are expected to be crucial to the LPL phenomena. In the literature, V O is supposed to serve as an electron trap center, while 00 VSr possibly acts as a hole trap center [36]. As shown in Fig. 4, a remarkable growth of TL intensity with the introduction of Tb3þ is pronounced. Two bands centered at T1 (357 K) and T2 (410 K) are observed respectively, using Gaussian function for both the SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ sample as shown in the inset of Fig. 4. It is notable that the position of the TL peaks of Tb3þ doped SrGa2O4 samples is not significantly changed compared with SrGa2O4 host, which indicates the introduction of Tb3þ contributes to the increase number of the trapping centers without changing the depth of the traps. It is commonly considered that the lower and higher temperature of the TL bands is related to the shallower and deeper traps, respectively. Therefore, the increased shallower traps (357 K) are contributed to the significant improvement of the LPL properties of Tb3þ doped SrGa2O4 samples. Meanwhile, the increased deeper one is critically important to prevent the thermal fading of carriers at room temperature, which is consistent with the observation of the LPL phenomenon of Tb3þ doped SrGa2O4 samples at relative higher ambient temperature as depicted in Fig. 3c. In order to understand the trapping nature, the trap depth ET of the well fitted TL peaks, marked TA (357 K) and TB (410 K) by two Gaussian profiles of the asymmetric TL peak of SrGa2O4 and Sr0.995Ga2O4: 0.005 Tb3þ samples, in the inset of Fig. 4 is calculated by the following equation [36]:
X. Yu et al. / Journal of Alloys and Compounds 701 (2017) 774e779
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Fig. 3. (a) LPL spectra of Sr1-xGa2O4:xTb3þ (x ¼ 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples, (b) LPL lifetime curves of Sr1-xGa2O4:xTb3þ (x ¼ 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples at room temperature, (c) LPL lifetime curves of Sr0.995Ga2O4: 0.005 Tb3þ samples at different ambient temperature.
2Tb
Fig. 4. TL curves of Sr1-xGa2O4:xTb3þ (x ¼ 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples recorded immediately after UV lamp pre-radiation for 20min. The inset is the TL curves of SrGa2O4 and Sr0.995Ga2O4:0.005 Tb3þ, respectively.
h i k T2 B m E ¼ 2:52 þ 10:2 mg 0:42
u
! 2kB Tm
(1)
where, mg ¼ d/u is the symmetrical geometrical factor, and d ¼ T2Tm is the half width towards the fall-off of the glow peak, u ¼ T2T1 is the total half of width, and Tm, T1 and T2 are the peak temperature at the maximum and the temperatures on either side of the temperature at the maximum, corresponding to half intensity, respectively. k is the Boltzmann's constant. Then the calculated trap levels (E) of TA, TB are 0.7337 and 0.8719 eV, respectively. In the SrGa2O4 host, intrinsic defects, such as V O and 00 VSr , exist as mentioned above. With the doping of Tb3þ, considering the fact that Tb3þ replace the replace the Sr2þ site in the SrGa2O4 host matrix, two Tb3þ ions replace three Sr2þ ions to balance the charge of the phosphor, which creates two positive defects and one
3þ
negative defect (3Sr2þ !2TbSr þ VSr ). Therefore, more VSr and extra electron traps TbSr are formed. Although TL spectra can accurately demonstrate the range of a trap's depth, the situation of trap type could not be attributed through TL spectra [37]. In this respect, the broad TL spectrum covers from 300 to 525 K of Sr0.995Ga2O4:0.005 Tb3þ, which indicates that the traps distribute over a wide range of energies in the bandgap as traps of various depth are formed, is ascribed to the increased number of the intrinsic defects as well as the defect complexes. In addition, the successive trap structure of Sr0.995Ga2O4:0.005 Tb3þ as shown in the TL spectra suggest that the trapped carriers in deeper centers could be further released by photo-induction with near infrared excitation. To investigate the kinetic order of these samples, TL curves of Sr0.995Ga2O4:0.005 Tb3þ with different delay time are recorded in Fig. 5a. The TA band was greatly depleted after a delay of 20 h and then almost completely disappears. However, the intensities of TB band sustain the intensity without changing significantly after 10 h. Hence, it is safe to say that the relative shallow traps of TA could be ascribed to the LPL phenomenon at room temperature, while the presence of deep stable traps in SrGa2O4:Tb3þ can immobilize the energy permanently at room temperature. The relatively deep trap precludes the thermal release of the intercepted carriers at room temperature, which is an essential factor required for PSL phosphors. The detailed PSL properties for SrGa2O4:Tb3þ were investigated as follows. As shown in Fig. 5b, Sr0.995Ga2O4:0.005 Tb3þ phosphor exhibit strong PSL upon 980 nm stimulation after being pre-irradiated with UV light for 20min and then placed in the dark for 20 h to make sure the completely release of electrons trapped in the shallow trap (TA). The characteristic emission peaks derived from the 5D3 / 7FJ (J ¼ 6, 5, 4, 3) and 5D4 / 7FJ (J ¼ 6, 5, 4, 3) transitions of Tb3þ is detected under 980 nm excitation. The PSL intensities decline with an increased stimulation time, which confirms that it is not an up-conversion process but the release of the carriers in the traps of this phosphor induced by the excitation energy of 980 nm. Thus, it suggests that the deeper traps corresponding to the TB band could be responsible for the performance of PSL (lex ¼ 980 nm). Fig. 5c displays the TL map as a function of the delay time of the samples being placed in a dark room, and Fig. 5d is the PSL map as a function of the stimulation time of 00
00
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Fig. 5. (a) TL curves of Sr0.995Ga2O4:0.005 Tb3þ sample placed in a dark room for different periods of time (0, 5, 10, 15 and 20 h), (b) PSL emission spectra of Sr0.995Ga2O4: 0.005 Tb3þ under 980 nm excitation with different stimulation time (5, 15, 25, 35, and 45s), (c) TL map as a function of the delay time of the samples being placed in a dark room, (d) PSL map as a function of the stimulation time of 980 nm excitation, respectively.
980 nm excitation, respectively, from which the luminescence intensity distribution can be clearly observed. 4. Conclusion A visible LPL has been obtained in the unique structure of SrGa2O4 possessing a three-dimensional framework containing channels occupied by the cations of Sr2þ. It is found that both the energy transfer and the persistent energy transfer from SrGa2O4 to the emission centers of Tb3þ occurs effectively in this phosphor. The introduction of Tb3þ in SrGa2O4 contributes to the increase number of the trapping centers significantly. Actually, the LPL of SrGa2O4:Tb3þ can last for a period of time even at 328 K, ascribed to the stable deeper traps which prevent the thermal fading of carriers at room temperature. Besides, thanks to the successive trap structure, the carries captured in deeper defects could be released as a PSL induced by the excitation energy of 980 nm in SrGa2O4:Tb3þ phosphor. Our results indicate that SrGa2O4:Tb3þ phosphor could provide particular applications as optical probes for in vivo bioimaging or solar energy utilization, especially in an environment with a relative higher thermal energy. Acknowledgements This work was financially supported by the National Nature Science Foundation of China (61565009, 11664022), the Foundation of Natural Science of Yunnan Province (2016FB088), the Young Teachers Support Program of Faculty of Materials Science and
Engineering, Kunming University of Science and Technology (1096838507, 14078342), and the China Scholarship Council (201508535038). References [1] X. Wang, C. Liow, A. Bisht, X. Liu, T.C. Sum, X. Chen, S. Li, Adv. Mater. 27 (2015) 2207e2214. [2] M.Z. Ge, C.Y. Cao, S.H. Li, Y.X. Tang, L.N. Wang, N. Qi, J.Y. Huang, K.Q. Zhang, S.S.A. Deyab, Y.K. Lai, Nanoscale 8 (2016) 5226e5234. [3] J. Yao, J. Shao, Y. Wang, Z. Zhao, G. Yang, Nanoscale 7 (2015) 12535e12541. [4] Y. Li, X. Wei, X. Yan, J. Cai, A. Zhou, M. Yang, K. Liu, Phys. Chem. Chem. Phys. 18 (2016) 10255e10261. [5] B.J. Morgan, G.W. Watson, Phys. Rev. B 80 (2009) 233102. [6] Z. Zhang, Y. Huang, K. Liu, L. Guo, Q. Yuan, B. Dong, Adv. Mater. 27 (2015) 5906e5914. [7] R. Li, F. Zhang, D. Wang, J. Yang, M. Li, J. Zhu, X. Zhou, H. Han, C. Li, Nat. Commun. 4 (2013) 1432. [8] S. Sarkar, D. Basak, Appl. Phys. Lett. 103 (2013) 041112. [9] Y. Li, M. Geceviciusa, J. Qiu, Chem. Soc. Rev. 45 (2016) 2090e2136. [10] K.P. McKenna, A.L. Shluger, Nat. Mater. 7 (2008) 859e862. [11] H. Nan, Z. Wang, W. Wang, Z. Liang, Y. Lu, Q. Chen, D. He, P. Tan, F. Miao, X. Wang, J. Wang, Z. Ni, ACS Nano 8 (2014) 5738e5745. [12] D. Kulesza, E. Zych, J. Phys. Chem. C 117 (2013) 26921e26928. [13] Y. Li, S. Zhou, Yiyang Li, K. Sharafudeen, Z. Ma, G. Dong, M. Peng, J. Qiu, J. Mater. Chem. C 2 (2014) 2657e2663. [14] J. Xu, J. Ueda, S. Tanabe, J. Mater. Chem. C 4 (2016) 4380e4386. [15] M. Nikl, K. Kamada, V. Babin, J. Pejchal, K. Pilarova, E. Mihokova, A. Beitlerova, K. Bartosiewicz, S. Kurosawa, A. Yoshikawa, Cryst. Growth Des. 14 (2014) 4827e4833. [16] M. Fasoli, A. Vedda, M. Nikl, C. Jiang, B.P. Uberuaga, D.A. Andersson, K.J. McClellan, C.R. Stanek, Phys. Rev. B 84 (2011) 081102. [17] H. Luo, A.J.J. Bos, P. Dorenbos, J. Phys. Chem. C 120 (2016) 5916e5925. [18] H. Lin, B. Wang, Q. Huang, F. Huang, J. Xu, H. Chen, Z. Lin, J. Wang, T. Hua, Y. Wang, J. Mater. Chem. C 4 (2016) 10329e10338. [19] J. Ueda, P. Dorenbos, A. Bos, K. Kuroishi, S. Tanabe, J. Mater. Chem. C 3 (2015)
X. Yu et al. / Journal of Alloys and Compounds 701 (2017) 774e779 5642e5651. [20] J. Xu, J. Ueda, S. Tanabe, Opt. Mater. Express 5 (2015) 963e968. [21] E. Danielson, M. Devenney, D.M. Giaquinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojary, C.M. Reaves, W.H. Weinberg, D.X. Wu, Science 279 (1998) 837e839. [22] S.K. Gupta, M. Sahu, K. Krishnan, M.K. Saxena, V. Natarajan, S.V. Godbole, J. Mater. Chem. C 1 (2013) 7054e7063. [23] H. Li, R. Zhao, Y. Jia, W. Sun, J. Fu, L. Jiang, S. Zhang, R. Pang, C. Li, ACS Appl. Mater. Interfaces 6 (2014) 3163e3169. [24] Y. Liu, J. Kuang, B. Lei, C. Shi, J. Mater. Chem. 15 (2005) 4025e4031. [25] V. Kahlenberg, R.X. Fischer, C.S.J. Shaw, J. Solid State Chem. 153 (2000) 294e300. [26] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751e767. [27] T. Wang, X. Xu, D. Zhou, Y. Yang, J. Qiu, X. Yu, Inorg. Chem. 55 (2016) 894e901.
779
[28] S.H. Yang, H.F. Tu, J. Electrochem. Soc. 152 (2005) H1eH5. [29] B.V. Ratnam, M. Jayasimhadri, G.B. Kumar, K. Jang, S.S. Kim, Y.I. Lee, J.M. Lim, D.S. Shin, T.K. Song, J. Alloys Comp. 564 (2013) 100e104. [30] Q. Bin, T. Zilong, Z. Zhongtai, C. Lei, J. Rare Earths 25 (2007) 286e290. [31] M.E. Foley, R.W. Meulenberg, J.R. McBride, G.F. Strouse, Chem. Mater. 27 (2015) 8362e8374. [32] D.L. Dexter, J. Chem. Phys. 21 (1953) 836e850. [33] Z. Xia, R.S. Liu, J. Phys. Chem. C 116 (2012) 15604e15609. [34] W. Lü, W. Lv, Q. Zhao, M. Jiao, B. Shao, H. You, J. Mater. Chem. C 3 (2015) 2334e2340. [35] K.V. Eeckhout, A.J. Bos, D. Poelman, P.F. Smet, Phys. Rev. B 87 (2013) 045126. [36] B. Qu, B. Zhang, L. Wang, R. Zhou, X.C. Zeng, Chem. Mater. 27 (2015) 2195e2202. [37] P. Wang, X. Xu, D. Zhou, X. Yu, J. Qiu, Inorg. Chem. 54 (2015) 1690e1697.