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Near-infrared long-persistent phosphor of Zn3Ga2Ge2O10: Cr3+ sintered in different atmosphere
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 385–389
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Near-infrared long-persistent phosphor of Zn3Ga2Ge2O10: Cr3+ sintered in different atmosphere Yiling Wu, Yang Li ⇑, Xixi Qin, Ruchun Chen, Dakun Wu, Shijian Liu, Jianrong Qiu ⇑ State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NIR and visible afterglow emission. Phosphorescent properties varied
with the sintering atmosphere. The optimized environment is
confirmed as the Argon or Air.
a r t i c l e
i n f o
Article history: Received 30 March 2015 Received in revised form 25 May 2015 Accepted 28 June 2015 Available online 29 June 2015 Keywords: NIR long-persistent phosphorescence Defect Atmosphere
a b s t r a c t A variety of materials sintered in different atmosphere have been well investigated, but there are few reports on the long-persistent phosphorescent materials, especially the near-infrared long-persistent phosphorescent materials sintered in various atmosphere. Changing the surrounding atmosphere is an effective method to improve the afterglow properties of the materials. In this work, we fabricate a typical kind of near-infrared long-persistent phosphorescent materials of Zn3Ga2Ge2O10: 0.5% Cr3+ in neutral, oxidizing, and reducing atmosphere. By analyzing the XRD patterns, afterglow spectra, decay and thermo-luminescence curves, we discuss the great effects on the structure, long persistent properties and trap properties of the phosphor. This work of obtaining the Zn3Ga2Ge2O10: 0.5% Cr3+ is of great potential in the applications in night-vision surveillance and in vivo bio-imaging. Ó 2015 Published by Elsevier B.V.
1. Introduction Long persistent phosphorescence (LPP) is the occurrence of luminescence which can last for several minutes or hours after the stoppage of excitation [1–3]. Phosphors with persistent luminescence have been rapidly developed in the past decade, largely stimulated by the green persistent phosphors, SrAl2O4: Eu2+, Dy3+, discovered in 1996 [4]. Up to now, there are persistent phosphors for each of the primary colours, with the representative ones ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Li), [email protected] (J. Qiu). http://dx.doi.org/10.1016/j.saa.2015.06.117 1386-1425/Ó 2015 Published by Elsevier B.V.
including CaAl2O4: Eu2+, Nd3+ (blue, >10 h) [5], SrAl2O4: Eu2+, Nd3+ (green, >30 h) [4] and Y2O2S: Eu3+, Mg2+, Ti4+ (red, >5 h) [6]. They have been widely applied in various important fields as security signs, emergency route signs, safety indication, indicators of control panels in dark environments or in the night [7–9]. In recent years, there are increasing demands for persistent phosphors in the near-infrared (NIR) region (700–2500 nm) in applications as identification markers in security and optical probes in vivo bio-imaging [10,11]. Since their emissions are in the region of the biologically transparent window (650–1100 nm) [12,13], moreover, since the emission lifetime is sufficiently long to permit late time-gated imaging, long persistent phosphors
386
Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 385–389
(LPPs) are unique and superior to other labels [14–18]. In such a case, auto-fluorescence from tissue organic components during imaging can be completely avoided, which gives rise to an extraordinary high signal-to-noise ratio [19,20]. Thus, long persistent phosphor (LPP) probes are expected to open up the possibility of advanced optical imaging with high resolution and weak light disturbance for factually assessing the structural and functional processes in cells, tissues and other complex systems. The past few years have witnessed great progress in the development of the LPP, for example, Chermont [11] and colleagues have demonstrated in vivo imaging by using a LPP probe with the duration of around 1 h. Fluorescence resonance energy transfer inhibition assay for a-fetoprotein excreted during cancer cell growth has also been realized with the help of a LPP probe. However, there are still challenges to broaden the application of LPPs [21,22]. Especially, the nature of traps in LPPs is still a subject of discussion and needs to be revealed. Until now the analysis of trap properties is still conducted independently, giving rise to an incomplete description for the LPPs [10,23]. Generally, in LPPs, two sorts of active centres are involved: traps and emitters [8]. Traps play a crucial role in all the suggested persistent luminescence mechanisms. Three of their main properties are their ‘type’, ‘concentration’, and ‘depth’ [24]. Many strategies are proposed to regulate these properties, such as doping or co-doping, raising the preparation temperature of phosphors and changing the surrounding atmosphere and pressure during the calcination processes [25–28]. Therein, changing the surrounding atmosphere is an effective method to improve the afterglow properties of the phosphors. Under different sintering atmosphere, the kinds and the numbers of defects, including cation vacancy, anion vacancy, interstitial vacancy will be diverse [24,29]. On the basis of this idea, we chose a typical kind of near-infrared long-persistent phosphorescent materials of Zn3Ga2Ge2O10: 0.5% Cr3+ [8] as the research object, which is of great potential in the applications in night-vision surveillance and in vivo bio-imaging. We sinter Zn3Ga2Ge2O10: 0.5% Cr3+ in neutral, oxidizing, and reducing atmosphere, to come up with the best sintering atmosphere for this LPP, and to learn more about the properties of the afterglow in Zn3Ga2Ge2O10: 0.5% Cr3+. We hope to figure out the role of sintering atmosphere in the growth process of afterglow, as well as to learn more about the mechanism governing the afterglow phenomenon in the Zn3Ga2Ge2O10: 0.5% Cr3+ phosphor.
respectively. The chemical compositions and preparation conditions of the phosphors are shown in Table 1. X-ray diffraction (XRD) analysis was studied with a powder diffractometer (Bruker D8) using Cu Ka radiation (k = 0.154 nm) at a scanning rate of 0.2 °/min. Afterglow spectra and decay curves were measured by a high-resolution spectrofluorometer (Edinburgh Instruments FLS920) equipped with a 500 W xenon lamp as excitation sources at room-temperature. Thermoluminescence glow curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory) by heating the irradiated samples from room temperature to 600 K at a linear rate of 2 K/s. The samples for measuring afterglow spectra, decay curves, and TL were pre-irradiated by using a 4 W 254 nm UV lamp for 10 min. 3. Results and discussion 3.1. Structural characterization Fig. 1(a) shows the diffraction patterns of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in Argon, Air, Nitrogen, (b) shows the two samples sintered in C powder, as well as the mixture of Nitrogen and Hydrogen. Unfortunately, there is not any data of Zn3Ga2Ge2O10 in the crystal structure database. In fact, the Zn3Ga2Ge2O10 is a solid solution of ZnGa2O4 and Zn2GeO4. The crystal structure of Zn3Ga2Ge2O10 is the same with that of ZnGa2O4 (PDF#38-1240), as is shown in Fig. 1(a). Hence, in the structure of Zn3Ga2Ge2O10, Ge play the role of substitution of Ga, conducive to the formation of traps, while ZnGa2O4 is the dominant crystal structure. As for ZnGa2O4, it is a typical AB2O4 type spinel crystal, its space group is Fd3m. In this kind of crystal structure [30], there are two kinds of cations in one unit-cell, A and B are surrounded by four and six oxygen anions forming a tetrahedron and an octahedron, respectively. In ZnGa2O4, A sites are occupied by Zn2+, B by Ga3+. It is reported that the existence of anti-site defects ([ZnGa]’, [GaZn]°) [24] in the structure of ZnGa2O4 makes great contribution to the performance of long afterglow. In Fig. 1(b), the testing samples are sintered in reducing atmosphere. In these conditions, the XRD patterns show that the crystal structure of the samples is the same with Ga2O3 (PDF#43-1012), because of the sublimation of other raw materials. Before sintering, the weight of the raw materials of each sample is 1.6000 g, including 0.5613 g of Ga2O3. However, after being sintered in reducing atmosphere, there is only about 0.4500 g left. Thus, we can infer that the reducing atmosphere is the adverse condition for fabricating the NIR long persistence phosphor Zn3Ga2Ge2O10: 0.5% Cr3+.
2. Materials and experiments The sample Zn3Ga2Ge2O10: 0.5% Cr3+ was synthesized by a solid-state reaction method. Stoichiometric amounts of the 4 N pure Zn2O3, Ga2O3, GeO2 and Cr2O3 were firstly mixed and ground homogeneously in an agate mortar. Five sets of Zn3Ga2Ge2O10: 0.5% Cr3+ were prepared by sintering at normal atmospheric pressure at 1300 °C for 4 h, followed by the samples cooled to room temperature in the furnace. The series differed only in the atmosphere at which the sintering was performed: ambient air (oxidizing), Argon (neutral), Nitrogen (neutral), C powder (reducing), the mixture of Nitrogen and Hydrogen (19:1 by volume) (reducing),
3.2. Long persistent phosphorescence properties Fig. 2 shows the afterglow spectra of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere. Five samples are sintered in Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen, respectively. The spectra of the sample sintered in Argon show a narrow-band emission peak at 713 nm, and a much weaker emission band at 520 nm. When the atmosphere changes into Air or Nitrogen, there is no certain change in
Table 1 Chemical compositions of the phosphors in different preparation conditions. No
Composition
Sintering temperature (°C)
Sintering time (h)
Atmosphere
Doping content
1 2 3 4 5
Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+
1300 1300 1300 1300 1300
4 4 4 4 4
Air Nitrogen Argon 95% Nitrogen + 5% Hydrogen C powders
0.5% 0.5% 0.5% 0.5% 0.5%
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Fig. 1. (a) XRD patterns of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in Argon, Air, Nitrogen, (b) XRD patterns of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in C powder, as well as the mixture of Nitrogen and Hydrogen.
Fig. 2. Afterglow spectra of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. Before the measurements, the phosphors are pre-irradiated using a UV lamp for 10 min. The peak on the left is corresponding to the left coordinates, and the right to the right side.
emission wavelength but the emission intensity decreases. While sintered in C power or the mixture of Nitrogen and Hydrogen, the emission band at 520 nm is undetectable, simultaneously, the narrow-band emission becomes broad-band emission, and the intensity is much lower. It is well known that the NIR emission peaking at 713 nm is characteristic of Cr3+ ions and can be attributed to the spin-forbidden 2E ? 4A2 transition in the octahedron site in the strong crystalline field [31]. Besides, the associated broad background emission ranging from 650 nm to >800 nm originated mostly from the phonon sidebands of the 2E ? 4A2 transition. The coexistence of the narrow-band emission peak and the broad-band emission indicate that the crystalline field strength of Zn3Ga2Ge2O10 is medium. As for the emission band at 520 nm, we consider it as recombination centers originating from intrinsic defects [32]. We also detect the afterglow curves monitored at 700 nm after irradiation, which are shown in Fig. 3. The data is recorded as a function of persistent luminescence intensity versus time, and
Fig. 3. The afterglow decay curves of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. Before the measurements, the phosphors are pre-irradiated using a UV lamp for 10 min.
the recordings last for 1200 s. (In fact, the afterglow of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in Air can last for more than 100 h [8].) The initial intensities decrease with the atmosphere changing, which is consistent with the afterglow spectra in Fig. 1. The afterglow intensity decreases quickly at the beginning and then decays much more slowly, following a hyperbolic function, indicating that emitting centers of persistent luminescence are recombination centers [33,34]. In fact, unlike the luminescence decay curve, there is not a common exponential decay function of the afterglow decay curve. There are several versions of the decay functions of the afterglow. If we want to compare the decay rate of different phosphors directly, we can normalize the curves. However, afterglow decay time is defined as the detectable duration time observed by the naked eyes (>0.32 mcd/m2) in the dark after stopping excitation. Not only the decay rate but also the initial intensity and detectable time are important for comparison of different phosphors. For instance, in Fig. 3, the afterglow of the sample sintered in Nitrogen and Hydrogen decays very slowly, but its duration time is much shorter than others. Thus, here, we show the comprehensive properties in Fig. 3, and find that the afterglow of the samples sintered in protective or oxidizing
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atmosphere is stronger and can last much longer than in reducing atmosphere. 3.3. Trap properties As an important factor determining the persistent duration, the trap depths need to be tested and analyzed using the thermo-luminescence (TL) curves [35,36]. Usually, shallow traps are easily emptied at room temperature, deep traps, on the other hand, are hardly emptied at room temperature, and parts of captured electrons are still stored in deep traps. For the application in bio-imaging, especially for repeatedly rejuvenated bio-imaging, the trap depth is an important indicator [37]. As shown in Fig. 4, the TL curves of the five samples are similar. All curves consist of a broad band. However, the peaks of the TL curves shift to lower temperature regularly. In details, the peaks of the TL curves of the samples sintered in Argon, Air, Nitrogen, the mixture of Nitrogen and Hydrogen, and C powder are 400, 390, 380, 370, 360 K, respectively. The level depths of traps observed from TL curves can be estimated by using the following equations [7]:
E¼
Tm 500
ð1Þ
k¼
1240 E
ð2Þ
4. Conclusion In summary, we fabricate a typical kind of Near-infrared long-persistent phosphorescent materials of Zn3Ga2Ge2O10: 0.5% Cr3+ in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. We demonstrate the structure, spectroscopic property and trap properties by taking the experiments of XRD, afterglow spectra and TL curves. In the structure of Zn3Ga2Ge2O10, Ge play the role of substitution of Ga, conducive to the formation of traps, while ZnGa2O4 is the dominant crystal structure. Besides, when sintered in reducing atmosphere, the crystal structure is the same with Ga2O3. The long persistent phosphorescence properties of samples sintered in Argon, Air, and Nitrogen are better than those in reducing atmosphere. Noteworthily, the peaks of the TL curves of the five samples sintered in Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen shift to lower temperature regularly. We confirm Argon and Air as the optimized environment. Nevertheless, the exact defect centers for persistent luminescence, and the electron transfer mode between Cr3+ and defect centers are still not clear now. Further investigations on the effect of phosphor composition and defect equilibrium, are in progress. Acknowledgements
where, Tm is the temperature for which the glow curve reaches a maximum. Therefore, the trap level depths corresponding to the TL peaks at 400, 390, 380, 370, and 360 K of the five samples are roughly estimated to be 0.8, 0.78, 0.76, 0.74, and 0.72 eV, respectively. We can infer that the shallow traps are easily emptied at room temperature leading to the poor properties of afterglow of the samples sintered in C powder and the mixture of Nitrogen and Hydrogen. Furthermore, according to Eq. (2), deep defects play important role in the green emission. In addition, Argon and Air are of benefit to forming deep defects.
Fig. 4. Thermo-luminescence (TL) curve of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. Before the measurements, the phosphors are pre-irradiated using a UV lamp for 10 min.
This work was financially supported by National Natural Science Foundation of China (Grant Nos. 51132004, 51072754, 51472091), Guangdong Natural Science Foundation (Grant Nos. S2011030001349, 2014A030310444), National Basic Research Program of China (Grant Nos. 2011CB808100), China Postdoctoral Science Foundation (Grant Nos. 2015M570707) and Fundamental Research Funds for the Central Universities (Grant Nos. 2013ZM0001). This work was also supported by the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics.). References [1] H. Wu, Y. Hu, X. Wang, Radiat. Meas. 46 (2011) 591–594. [2] W. Yan, F. Liu, Y. Lu, X. Wang, M. Yin, Z. Pan, Opt. Express 18 (2010) 20215– 20221. [3] Y. Li, Y.Y. Li, R. Chen, K. Sharafudeen, S. Zhou, M. Gecevicius, H. Wang, G. Dong, Y. Wu, X. Qin, J. Qiu, NPG Asia Mater. 7 (2015) 1–11. [4] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670–2673. [5] H. Yamamoto, T. Matsuzawa, J. Lumin. 72–74 (1997) 287–289. [6] X.X. Wang, Z.T. Zhang, Z.L. Tang, Y.H. Lin, Mater. Chem. Phys. 80 (2003) 1–5. [7] K. Van den Eeckhout, P.F. Smet, D. Poelman, Materials 3 (2010) 2536–2566. [8] Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 11 (2012) 58–63. [9] M. Shivram, S.C. Prashantha, H. Nagabhushana, S.C. Sharma, K. Thyagarajan, R. Harikrishna, B.M. Nagabhushana, Spectroc. Acta A 120 (2014) 395–400. [10] A. Abdukayum, J.-T. Chen, Q. Zhao, X.-P. Yan, J. Am. Chem. Soc. 135 (2013) 14125–14133. [11] Q.L.M. de Chermont, C. Chaneac, J. Seguin, F. Pelle, S. Maitrejean, J.-P. Jolivet, D. Gourier, M. Bessodes, D. Scherman, Proc. Natl. Acad. Sci. USA 104 (2007) 9266– 9271. [12] G. Hong, J.C. Lee, J.T. Robinson, U. Raaz, L. Xie, N.F. Huang, J.P. Cooke, H. Dai, Nat. Med. 18 (2012) 1841–1846. [13] D.J. Naczynski, M.C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C.M. Roth, R.E. Riman, P.V. Moghe, Nat. Commun. 4 (2013) 1–10. [14] T. Maldiney, G. Sraiki, B. Viana, D. Gourier, C. Richard, D. Scherman, M. Bessodes, K. Van den Eeckhout, D. Poelman, P.F. Smet, Opt. Mater. Express 2 (2012) 261–268. [15] T. Maldiney, A. Bessiere, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, Nat. Mater. 13 (2014) 418–426. [16] L. Zhan-Jun, Z. Hong-Wu, S. Meng, S. Jiang-Shan, F. Hai-Xia, J. Mater. Chem. 22 (2012) 24713–24720. [17] Y. Li, R.C. Chen, Y.Y. Li, K. Sharafudeen, S.J. Liu, D.K. Wu, Y.L. Wu, X.X. Qin, J.R. Qiu, Microchim. Acta. (2015), http://dx.doi.org/10.1007/s00604-015-1486-8. [18] Y. Li, S. Zhou, G. Dong, M. Peng, L. Wondraczek, J. Qiu, Sci. Rep. 4 (2014) 1–6. [19] A.M. Smith, M.C. Mancini, S. Nie, Nat. Nanotechnol. 4 (2009) 710–711. [20] C.-H. Quek, K.W. Leong, Nanomaterial 2 (2012) 92–112.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Near-infrared long-persistent phosphor of Zn3Ga2Ge2O10: Cr3+ sintered in different atmosphere Yiling Wu, Yang Li ⇑, Xixi Qin, Ruchun Chen, Dakun Wu, Shijian Liu, Jianrong Qiu ⇑ State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NIR and visible afterglow emission. Phosphorescent properties varied
with the sintering atmosphere. The optimized environment is
confirmed as the Argon or Air.
a r t i c l e
i n f o
Article history: Received 30 March 2015 Received in revised form 25 May 2015 Accepted 28 June 2015 Available online 29 June 2015 Keywords: NIR long-persistent phosphorescence Defect Atmosphere
a b s t r a c t A variety of materials sintered in different atmosphere have been well investigated, but there are few reports on the long-persistent phosphorescent materials, especially the near-infrared long-persistent phosphorescent materials sintered in various atmosphere. Changing the surrounding atmosphere is an effective method to improve the afterglow properties of the materials. In this work, we fabricate a typical kind of near-infrared long-persistent phosphorescent materials of Zn3Ga2Ge2O10: 0.5% Cr3+ in neutral, oxidizing, and reducing atmosphere. By analyzing the XRD patterns, afterglow spectra, decay and thermo-luminescence curves, we discuss the great effects on the structure, long persistent properties and trap properties of the phosphor. This work of obtaining the Zn3Ga2Ge2O10: 0.5% Cr3+ is of great potential in the applications in night-vision surveillance and in vivo bio-imaging. Ó 2015 Published by Elsevier B.V.
1. Introduction Long persistent phosphorescence (LPP) is the occurrence of luminescence which can last for several minutes or hours after the stoppage of excitation [1–3]. Phosphors with persistent luminescence have been rapidly developed in the past decade, largely stimulated by the green persistent phosphors, SrAl2O4: Eu2+, Dy3+, discovered in 1996 [4]. Up to now, there are persistent phosphors for each of the primary colours, with the representative ones ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Li), [email protected] (J. Qiu). http://dx.doi.org/10.1016/j.saa.2015.06.117 1386-1425/Ó 2015 Published by Elsevier B.V.
including CaAl2O4: Eu2+, Nd3+ (blue, >10 h) [5], SrAl2O4: Eu2+, Nd3+ (green, >30 h) [4] and Y2O2S: Eu3+, Mg2+, Ti4+ (red, >5 h) [6]. They have been widely applied in various important fields as security signs, emergency route signs, safety indication, indicators of control panels in dark environments or in the night [7–9]. In recent years, there are increasing demands for persistent phosphors in the near-infrared (NIR) region (700–2500 nm) in applications as identification markers in security and optical probes in vivo bio-imaging [10,11]. Since their emissions are in the region of the biologically transparent window (650–1100 nm) [12,13], moreover, since the emission lifetime is sufficiently long to permit late time-gated imaging, long persistent phosphors
386
Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 385–389
(LPPs) are unique and superior to other labels [14–18]. In such a case, auto-fluorescence from tissue organic components during imaging can be completely avoided, which gives rise to an extraordinary high signal-to-noise ratio [19,20]. Thus, long persistent phosphor (LPP) probes are expected to open up the possibility of advanced optical imaging with high resolution and weak light disturbance for factually assessing the structural and functional processes in cells, tissues and other complex systems. The past few years have witnessed great progress in the development of the LPP, for example, Chermont [11] and colleagues have demonstrated in vivo imaging by using a LPP probe with the duration of around 1 h. Fluorescence resonance energy transfer inhibition assay for a-fetoprotein excreted during cancer cell growth has also been realized with the help of a LPP probe. However, there are still challenges to broaden the application of LPPs [21,22]. Especially, the nature of traps in LPPs is still a subject of discussion and needs to be revealed. Until now the analysis of trap properties is still conducted independently, giving rise to an incomplete description for the LPPs [10,23]. Generally, in LPPs, two sorts of active centres are involved: traps and emitters [8]. Traps play a crucial role in all the suggested persistent luminescence mechanisms. Three of their main properties are their ‘type’, ‘concentration’, and ‘depth’ [24]. Many strategies are proposed to regulate these properties, such as doping or co-doping, raising the preparation temperature of phosphors and changing the surrounding atmosphere and pressure during the calcination processes [25–28]. Therein, changing the surrounding atmosphere is an effective method to improve the afterglow properties of the phosphors. Under different sintering atmosphere, the kinds and the numbers of defects, including cation vacancy, anion vacancy, interstitial vacancy will be diverse [24,29]. On the basis of this idea, we chose a typical kind of near-infrared long-persistent phosphorescent materials of Zn3Ga2Ge2O10: 0.5% Cr3+ [8] as the research object, which is of great potential in the applications in night-vision surveillance and in vivo bio-imaging. We sinter Zn3Ga2Ge2O10: 0.5% Cr3+ in neutral, oxidizing, and reducing atmosphere, to come up with the best sintering atmosphere for this LPP, and to learn more about the properties of the afterglow in Zn3Ga2Ge2O10: 0.5% Cr3+. We hope to figure out the role of sintering atmosphere in the growth process of afterglow, as well as to learn more about the mechanism governing the afterglow phenomenon in the Zn3Ga2Ge2O10: 0.5% Cr3+ phosphor.
respectively. The chemical compositions and preparation conditions of the phosphors are shown in Table 1. X-ray diffraction (XRD) analysis was studied with a powder diffractometer (Bruker D8) using Cu Ka radiation (k = 0.154 nm) at a scanning rate of 0.2 °/min. Afterglow spectra and decay curves were measured by a high-resolution spectrofluorometer (Edinburgh Instruments FLS920) equipped with a 500 W xenon lamp as excitation sources at room-temperature. Thermoluminescence glow curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory) by heating the irradiated samples from room temperature to 600 K at a linear rate of 2 K/s. The samples for measuring afterglow spectra, decay curves, and TL were pre-irradiated by using a 4 W 254 nm UV lamp for 10 min. 3. Results and discussion 3.1. Structural characterization Fig. 1(a) shows the diffraction patterns of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in Argon, Air, Nitrogen, (b) shows the two samples sintered in C powder, as well as the mixture of Nitrogen and Hydrogen. Unfortunately, there is not any data of Zn3Ga2Ge2O10 in the crystal structure database. In fact, the Zn3Ga2Ge2O10 is a solid solution of ZnGa2O4 and Zn2GeO4. The crystal structure of Zn3Ga2Ge2O10 is the same with that of ZnGa2O4 (PDF#38-1240), as is shown in Fig. 1(a). Hence, in the structure of Zn3Ga2Ge2O10, Ge play the role of substitution of Ga, conducive to the formation of traps, while ZnGa2O4 is the dominant crystal structure. As for ZnGa2O4, it is a typical AB2O4 type spinel crystal, its space group is Fd3m. In this kind of crystal structure [30], there are two kinds of cations in one unit-cell, A and B are surrounded by four and six oxygen anions forming a tetrahedron and an octahedron, respectively. In ZnGa2O4, A sites are occupied by Zn2+, B by Ga3+. It is reported that the existence of anti-site defects ([ZnGa]’, [GaZn]°) [24] in the structure of ZnGa2O4 makes great contribution to the performance of long afterglow. In Fig. 1(b), the testing samples are sintered in reducing atmosphere. In these conditions, the XRD patterns show that the crystal structure of the samples is the same with Ga2O3 (PDF#43-1012), because of the sublimation of other raw materials. Before sintering, the weight of the raw materials of each sample is 1.6000 g, including 0.5613 g of Ga2O3. However, after being sintered in reducing atmosphere, there is only about 0.4500 g left. Thus, we can infer that the reducing atmosphere is the adverse condition for fabricating the NIR long persistence phosphor Zn3Ga2Ge2O10: 0.5% Cr3+.
2. Materials and experiments The sample Zn3Ga2Ge2O10: 0.5% Cr3+ was synthesized by a solid-state reaction method. Stoichiometric amounts of the 4 N pure Zn2O3, Ga2O3, GeO2 and Cr2O3 were firstly mixed and ground homogeneously in an agate mortar. Five sets of Zn3Ga2Ge2O10: 0.5% Cr3+ were prepared by sintering at normal atmospheric pressure at 1300 °C for 4 h, followed by the samples cooled to room temperature in the furnace. The series differed only in the atmosphere at which the sintering was performed: ambient air (oxidizing), Argon (neutral), Nitrogen (neutral), C powder (reducing), the mixture of Nitrogen and Hydrogen (19:1 by volume) (reducing),
3.2. Long persistent phosphorescence properties Fig. 2 shows the afterglow spectra of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere. Five samples are sintered in Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen, respectively. The spectra of the sample sintered in Argon show a narrow-band emission peak at 713 nm, and a much weaker emission band at 520 nm. When the atmosphere changes into Air or Nitrogen, there is no certain change in
Table 1 Chemical compositions of the phosphors in different preparation conditions. No
Composition
Sintering temperature (°C)
Sintering time (h)
Atmosphere
Doping content
1 2 3 4 5
Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+ Zn3Ga2Ge2O10:Cr3+
1300 1300 1300 1300 1300
4 4 4 4 4
Air Nitrogen Argon 95% Nitrogen + 5% Hydrogen C powders
0.5% 0.5% 0.5% 0.5% 0.5%
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Fig. 1. (a) XRD patterns of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in Argon, Air, Nitrogen, (b) XRD patterns of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in C powder, as well as the mixture of Nitrogen and Hydrogen.
Fig. 2. Afterglow spectra of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. Before the measurements, the phosphors are pre-irradiated using a UV lamp for 10 min. The peak on the left is corresponding to the left coordinates, and the right to the right side.
emission wavelength but the emission intensity decreases. While sintered in C power or the mixture of Nitrogen and Hydrogen, the emission band at 520 nm is undetectable, simultaneously, the narrow-band emission becomes broad-band emission, and the intensity is much lower. It is well known that the NIR emission peaking at 713 nm is characteristic of Cr3+ ions and can be attributed to the spin-forbidden 2E ? 4A2 transition in the octahedron site in the strong crystalline field [31]. Besides, the associated broad background emission ranging from 650 nm to >800 nm originated mostly from the phonon sidebands of the 2E ? 4A2 transition. The coexistence of the narrow-band emission peak and the broad-band emission indicate that the crystalline field strength of Zn3Ga2Ge2O10 is medium. As for the emission band at 520 nm, we consider it as recombination centers originating from intrinsic defects [32]. We also detect the afterglow curves monitored at 700 nm after irradiation, which are shown in Fig. 3. The data is recorded as a function of persistent luminescence intensity versus time, and
Fig. 3. The afterglow decay curves of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. Before the measurements, the phosphors are pre-irradiated using a UV lamp for 10 min.
the recordings last for 1200 s. (In fact, the afterglow of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in Air can last for more than 100 h [8].) The initial intensities decrease with the atmosphere changing, which is consistent with the afterglow spectra in Fig. 1. The afterglow intensity decreases quickly at the beginning and then decays much more slowly, following a hyperbolic function, indicating that emitting centers of persistent luminescence are recombination centers [33,34]. In fact, unlike the luminescence decay curve, there is not a common exponential decay function of the afterglow decay curve. There are several versions of the decay functions of the afterglow. If we want to compare the decay rate of different phosphors directly, we can normalize the curves. However, afterglow decay time is defined as the detectable duration time observed by the naked eyes (>0.32 mcd/m2) in the dark after stopping excitation. Not only the decay rate but also the initial intensity and detectable time are important for comparison of different phosphors. For instance, in Fig. 3, the afterglow of the sample sintered in Nitrogen and Hydrogen decays very slowly, but its duration time is much shorter than others. Thus, here, we show the comprehensive properties in Fig. 3, and find that the afterglow of the samples sintered in protective or oxidizing
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atmosphere is stronger and can last much longer than in reducing atmosphere. 3.3. Trap properties As an important factor determining the persistent duration, the trap depths need to be tested and analyzed using the thermo-luminescence (TL) curves [35,36]. Usually, shallow traps are easily emptied at room temperature, deep traps, on the other hand, are hardly emptied at room temperature, and parts of captured electrons are still stored in deep traps. For the application in bio-imaging, especially for repeatedly rejuvenated bio-imaging, the trap depth is an important indicator [37]. As shown in Fig. 4, the TL curves of the five samples are similar. All curves consist of a broad band. However, the peaks of the TL curves shift to lower temperature regularly. In details, the peaks of the TL curves of the samples sintered in Argon, Air, Nitrogen, the mixture of Nitrogen and Hydrogen, and C powder are 400, 390, 380, 370, 360 K, respectively. The level depths of traps observed from TL curves can be estimated by using the following equations [7]:
E¼
Tm 500
ð1Þ
k¼
1240 E
ð2Þ
4. Conclusion In summary, we fabricate a typical kind of Near-infrared long-persistent phosphorescent materials of Zn3Ga2Ge2O10: 0.5% Cr3+ in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. We demonstrate the structure, spectroscopic property and trap properties by taking the experiments of XRD, afterglow spectra and TL curves. In the structure of Zn3Ga2Ge2O10, Ge play the role of substitution of Ga, conducive to the formation of traps, while ZnGa2O4 is the dominant crystal structure. Besides, when sintered in reducing atmosphere, the crystal structure is the same with Ga2O3. The long persistent phosphorescence properties of samples sintered in Argon, Air, and Nitrogen are better than those in reducing atmosphere. Noteworthily, the peaks of the TL curves of the five samples sintered in Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen shift to lower temperature regularly. We confirm Argon and Air as the optimized environment. Nevertheless, the exact defect centers for persistent luminescence, and the electron transfer mode between Cr3+ and defect centers are still not clear now. Further investigations on the effect of phosphor composition and defect equilibrium, are in progress. Acknowledgements
where, Tm is the temperature for which the glow curve reaches a maximum. Therefore, the trap level depths corresponding to the TL peaks at 400, 390, 380, 370, and 360 K of the five samples are roughly estimated to be 0.8, 0.78, 0.76, 0.74, and 0.72 eV, respectively. We can infer that the shallow traps are easily emptied at room temperature leading to the poor properties of afterglow of the samples sintered in C powder and the mixture of Nitrogen and Hydrogen. Furthermore, according to Eq. (2), deep defects play important role in the green emission. In addition, Argon and Air are of benefit to forming deep defects.
Fig. 4. Thermo-luminescence (TL) curve of Zn3Ga2Ge2O10: 0.5% Cr3+ sintered in different atmosphere: Argon, Air, Nitrogen, C powder, as well as the mixture of Nitrogen and Hydrogen. Before the measurements, the phosphors are pre-irradiated using a UV lamp for 10 min.
This work was financially supported by National Natural Science Foundation of China (Grant Nos. 51132004, 51072754, 51472091), Guangdong Natural Science Foundation (Grant Nos. S2011030001349, 2014A030310444), National Basic Research Program of China (Grant Nos. 2011CB808100), China Postdoctoral Science Foundation (Grant Nos. 2015M570707) and Fundamental Research Funds for the Central Universities (Grant Nos. 2013ZM0001). This work was also supported by the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics.). References [1] H. Wu, Y. Hu, X. Wang, Radiat. Meas. 46 (2011) 591–594. [2] W. Yan, F. Liu, Y. Lu, X. Wang, M. Yin, Z. Pan, Opt. Express 18 (2010) 20215– 20221. [3] Y. Li, Y.Y. Li, R. Chen, K. Sharafudeen, S. Zhou, M. Gecevicius, H. Wang, G. Dong, Y. Wu, X. Qin, J. Qiu, NPG Asia Mater. 7 (2015) 1–11. [4] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670–2673. [5] H. Yamamoto, T. Matsuzawa, J. Lumin. 72–74 (1997) 287–289. [6] X.X. Wang, Z.T. Zhang, Z.L. Tang, Y.H. Lin, Mater. Chem. Phys. 80 (2003) 1–5. [7] K. Van den Eeckhout, P.F. Smet, D. Poelman, Materials 3 (2010) 2536–2566. [8] Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 11 (2012) 58–63. [9] M. Shivram, S.C. Prashantha, H. Nagabhushana, S.C. Sharma, K. Thyagarajan, R. Harikrishna, B.M. Nagabhushana, Spectroc. Acta A 120 (2014) 395–400. [10] A. Abdukayum, J.-T. Chen, Q. Zhao, X.-P. Yan, J. Am. Chem. Soc. 135 (2013) 14125–14133. [11] Q.L.M. de Chermont, C. Chaneac, J. Seguin, F. Pelle, S. Maitrejean, J.-P. Jolivet, D. Gourier, M. Bessodes, D. Scherman, Proc. Natl. Acad. Sci. USA 104 (2007) 9266– 9271. [12] G. Hong, J.C. Lee, J.T. Robinson, U. Raaz, L. Xie, N.F. Huang, J.P. Cooke, H. Dai, Nat. Med. 18 (2012) 1841–1846. [13] D.J. Naczynski, M.C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C.M. Roth, R.E. Riman, P.V. Moghe, Nat. Commun. 4 (2013) 1–10. [14] T. Maldiney, G. Sraiki, B. Viana, D. Gourier, C. Richard, D. Scherman, M. Bessodes, K. Van den Eeckhout, D. Poelman, P.F. Smet, Opt. Mater. Express 2 (2012) 261–268. [15] T. Maldiney, A. Bessiere, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, Nat. Mater. 13 (2014) 418–426. [16] L. Zhan-Jun, Z. Hong-Wu, S. Meng, S. Jiang-Shan, F. Hai-Xia, J. Mater. Chem. 22 (2012) 24713–24720. [17] Y. Li, R.C. Chen, Y.Y. Li, K. Sharafudeen, S.J. Liu, D.K. Wu, Y.L. Wu, X.X. Qin, J.R. Qiu, Microchim. Acta. (2015), http://dx.doi.org/10.1007/s00604-015-1486-8. [18] Y. Li, S. Zhou, G. Dong, M. Peng, L. Wondraczek, J. Qiu, Sci. Rep. 4 (2014) 1–6. [19] A.M. Smith, M.C. Mancini, S. Nie, Nat. Nanotechnol. 4 (2009) 710–711. [20] C.-H. Quek, K.W. Leong, Nanomaterial 2 (2012) 92–112.
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