Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A long persistent phosphor based on recombination centers originating from Zn imperfections Yang Li, Xi Du, Kaniyarakkal Sharafudeen, Chenxing Liao, Jianrong Qiu ⇑ State Key Laboratory of Luminescent Materials and Devices, Institute of Optical Communication Materials, School of Materials Science and Technology, 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
A blue-white long persistent
phosphor. Zn imperfections as emitting centers
without any rare-earth doping. An energy transfer and remission
process is proposed.
a r t i c l e
i n f o
Article history: Received 28 September 2013 Received in revised form 2 December 2013 Accepted 5 December 2013 Available online 17 December 2013 Keywords: Recombination luminescence Zn imperfections Long persistent luminescence
a b s t r a c t The recombination luminescence from Zn imperfections has been extensively investigated; however, there have been few reports on the long persistent luminescence of Zn imperfections as emitting centers. Here, we observed a long persistent luminescence in blue-white visible region from 6 ZnO:3 GeO2:Al2O3 phosphor with Zn imperfections as emitting centers. Persistent luminescence could be observed beyond 2 h with naked eyes. The properties of traps were also elaborated by the measurements of thermo-luminescence spectra and photo-stimulated luminescence decay curves. Furthermore, a long persistent phosphor with warm white color was developed by doping Cr3+ into 6 ZnO:3 GeO2:Al2O3 phosphor. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Long persistent phosphors (LPPs) have been widely used in various areas, including safety indications and emergency lighting etc. [1]. Generally, long persistent phosphors require materials have abilities to form electrons reservoir, as well as accommodate suitable emitting centers. The emission wavelength of the long persistent phosphors is mainly determined by the emitters, while the persistence luminescence intensity and time are restricted by the defects [2]. Up to now, emitting centers in long persistent phos⇑ Corresponding author. Tel.: +86 20 87113646; fax: +86 20 87114204. E-mail address: [email protected] (J. Qiu). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.046
phors have been focused on the discrete luminescent centers, e.g. rare earth ions [3]. Except the discrete luminescent centers, it is well known that recombination centers are also a kind of important emitting centers, and defects usually act not only as the traps centers, but also as the emitting centers. However, there have been few reports on the long persistent luminescence from defects as emitting centers. The recombination photo-luminescence of Zn imperfections has been extensively investigated [4,5], but long persistent luminescence from Zn imperfections as emitting centers was rare. Herein, we observed a long persistent luminescence in blue-white visible region from 6 ZnO:3 GeO2:Al2O3 phosphor with Zn imperfections as emitting centers. The persistent luminescence could be
8
Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
observed beyond 2 h with naked eyes. The properties of the related traps were also elaborated by the thermo-luminescence (TL) spectra and photo-stimulated luminescence (PSL) decay curves. In addition, a long persistent phosphor with warm white color was obtained by doping Cr3+ ion into 6 ZnO:3 GeO2:Al2O3 phosphor. Experimental Phosphors with the compositions of 6 ZnO:3 GeO2:Al2O3 (GZA), 6 ZnO: 3 GeO2: Al2O3: 0.02 mol% Cr3+ (GZA1) were prepared with solid state reaction method. The 4 N pure GeO2, ZnO, Al2O3, and Cr2O3, were used as the raw materials, and mixed homogeneously in an agate mortar. The mixed powders were pre-fired at 1000 °C in air for 4 h, and were then ground to fine powders. The prepared powders were pressed into discs with diameters of 10 mm using a hydraulic press with the pressure of 10 MPa for 10 min. The discs were sintered again at 1400 °C in air for 4 h. X-ray diffraction (XRD) measurements were performed on a D8 advance Bruker diffract meter (Cu Ka radiation) equipped with a Vantec-1 linear detector. The data were collected between 10° and 130° (2h) with a 0.0164° step size at room temperature, confirming the presence of ZnGe2O4 (JCPDS no 11–0687) as the sole crystalline phase (Fig. s1). A FEI Quanta 400, field emission environmental SEM fitted with an oxford energy dispersive spectrometry (EDS) analyzer was employed for surface investigation and features detection, further confirming the presence of the sole crystalline phase (Fig. s2). All of the luminescence spectra were measured with Edinburgh instrument FLS 920 using a R928P photomultiplier as the signal collector and equipped with a 500 W xenon lamp. Thermo-luminescence (TL) glow curves were measured with a FJ-427A TL meter. Photographs were taken by a Canon digital camera un-irradiated or irradiated by the 4 W 254 nm UV lamp. Results and discussion Photoluminescence properties Fig. 1 shows the excitation and emission spectra of GZA phosphor. Two excitation bands were observed around 270 and 310 nm when monitored the emission at 488 nm. Under the excitation at 270 nm, GZA phosphor exhibits a bluish–white broad
emission band at 488 nm (Inset. b), which can be readily Gaussian-resolved into three bands located at 452, 512, 601 nm. The same photoluminescence has been observed in ZnO powders, ZnO nano-particles, and other compounds [5–7]. The broad emission band should be assigned to the transitions associated with Zn imperfections [8]. Atomic and electronic structures of native Zn imperfections have also been extensively investigated both theoretically and experimentally [9,10]. The emitting centers of blue, green, and orange bands are considered to be the intrinsic defects [10], including O vacancy (VO), Zn vacancy (VZn), Zn interstitial (Zni), O interstitial (Oi), and so on. [11]. Fan et al. suggested that radiative recombination transition between the shallow traps and deep acceptors (VO or VZn) gives rise to emission bands at 2.51, and 2.36 eV, respectively [12]. Wang et al. assigned the emissions at 2.9 and 2.38 eV to the transitions from Zn interstitial (Zni) to valence band, and from conduction band to OZn defect in ZnO, respectively [13]. Liu et al. also observed the similar luminescence ZnGe2O4 excited at 270 nm, and the emission is assigned to the donor–acceptor recombination, VO and Zni were donors, whereas ionized VGe and VZn were the acceptors [14]. Therefore, the blue, green, and orange emissions observed from GZA phosphor should be ascribed to the transitions from Ge interstitial (Zni), Zn interstitial (Zni) and O vacancy (VO) to Zn vacancy (VZn), respectively [15].
Long persistent luminescence properties After irradiation by a 254 nm UV lamp for 20 min, long persistent luminescence was also observed from the GZA sample (Fig. 2a). Obviously, the emission intensity of persistent luminescence decreased gradually (Fig. 2a) and the emission peak showed a red shift from 488 to 550 nm (Fig. 2b), indicating that the traps distributes over a wide range of energies. The persistent luminescence was sufficiently bright to be visually confirmed for about 2 h in the dark (Fig. 2c). The decay curve of persistent luminescence follows a hyperbolic function, indicating that emitting centers of persistent luminescence are recombination centers [16]. The decay curves of persistent luminescence after irradiation by the lights from 280 to 360 nm were shown in Fig. 3. Effectiveness for the acquisition of persistent luminescence decreases with the increment of excitation wavelength. It is also observed that persistent luminescence can be effectively achieved under ultraviolet illumination, but less effectively under visible light illumination (The inset of Fig. 3), and the maximum locates at 270 nm, which is similar to the excitation maximum shown in Fig. 1, indicating that shallow traps near the conduction band may be responsible for persistent luminescence [17].
Defects properties Thermo-luminescence spectra
Fig. 1. Emission spectrum (excited at 270 nm) and excitation spectrum by monitoring emission at 488 nm of GZA phosphor. The emission is Gaussianresolved into three bands located at 452 (green solid line curve), 512 (red solid line curve), and 601 nm (black solid line curve), respectively. The inset shows the photographs un-irradiated (a) or irradiated (b) by a 254 nm UV lamp. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Thermo-luminescence (TL) spectra are a useful tool for revealing the nature of traps [18–20]. TL curves acquired at 1, 10, 30 min, 1, 3, and 12 h after the stoppage of irradiation were shown in Fig. 4. All curves consist of a broad band, but with different maximum at 77, 105, 119, 139, 173, and 177 °C, respectively. The shift of TL peak to higher temperature confirms the continuous distribution of shallow traps and deep traps. Significantly, the position of TL peak acquired at 3 h and 12 h after the stoppage of irradiation is similar, which means that shallow traps near the conduction band are responsible for persistent luminescence, and electrons in deep traps cannot be released completely, and plenty of electrons are still detained in deep traps at room temperature.
Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
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Fig. 2. Afterglow spectra recorded at 10 s, 1, 2, 10, 30 and 90 min after the stoppage of irradiation (a), afterglow peak as a function of time (b) and afterglow intensity monitored at 500 nm as a function of time and the hyperbolic curve fitting (c). The sample GZA was pre-irradiated by a 4 W 254 nm UV lamp for 20 min.
Fig. 3. Afterglow decay curves irradiated by 280, 300, 320, 340, and 360 nm light for 20 min, the monitoring wavelength is 500 nm. The inset shows the afterglow intensity I10s monitored at 500 nm as a function of the excitation wavelengths 250– 450 nm. The sample GZA was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min.
Photo-stimulated luminescence decay curves To confirm this conjecture, photo-stimulated luminescence (PSL) decay curves monitored at 500 nm were measured and shown in Fig. 5. Before the measurements, GZA phosphor was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min. From Fig. 2c, the emission intensity of persistent luminescence became
Fig. 4. Thermo-luminescence curves over 30–280 °C acquired at 1, 10, 30 min, 1, 3, and 12 h after the stoppage of irradiation. The sample GZA was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min.
very faint at 3 h after the stoppage of irradiation. The sample was then annealed at 250 °C for 5 min, and irradiated with a Xenon lamp. Apparent photo-stimulated luminescence at 700 nm was observed in a flash excited at 390 nm. The emission decays when the Xenon lamp continuously irradiates the phosphor. Similar luminescence was also observed excited at 420, 450 and 480 nm. Recently, Liu et al. have observed a ‘‘downconversion’’ photo-stimulated luminescence from LiGa5O8:Cr3+ phosphor [21]. The luminescence monitored at 716 nm could be excited by the light of 380–1000 nm, indicating a large number of electrons were
10
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Fig. 5. Photo-stimulated luminescence decay curves monitored at 500 nm, the excitation wavelength is 390, 420, 450 and 480 nm, respectively. Before the measurements, the phosphor first pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min, the phosphor was annealed at 250 °C for 5 min at 3 h after the stoppage of irradiation. ‘‘On’’ means the excitation source (Xenon lamp, 500 W) turns on. Fig. 7. Afterglow decay curves of 500 nm (red curve) and 688 nm (black curve) emissions. The upper inset shows the CIE chromaticity. The under inset shows the photoluminescence spectrum during 270 nm excitation (orange curve) and the afterglow spectrum measured at 30 s after stopping the excitation (violet curve). The sample GZA1 was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Long persistent phosphor with warm white color
Fig. 6. Energy transfer and remission process involving the Zn-defects.
reserved into the deeper traps. In our work, similar ‘‘downconversion’’ photo-stimulated luminescence was observed, indicating GZA phosphor also had the deeper traps and plenty of electrons were captured into these deep traps. To simplify the description, we assign the shallow traps and the filled deep traps in the PSL process as TRAP-2 and TRAP-1, respectively in Fig. 6. Under excitation at 270 nm, electrons from the valence band of phosphor are excited to the conduction band. Subsequently, the excited free electrons are trapped by Gei, Zni, and Vo in TRAP-2. Recombination of the native defects results in the intrinsic broad band emission with color of greenish white. After a sufficient illumination time, all of the continuous traps are filled, with the deeper traps being filled mainly through nonradiative relaxation from the shallow traps. After the stoppage of the irradiation, the direct recombination between the electron traps (Gei, Zni, and Vo) and hole traps (VZn) in TRAP-2 dominates the persistent luminescence process. Several minutes later, owing to the depletion of the shallower traps, the red shift of emission wavelength appears at the room temperature. Three hours later, the direct recombination maybe disappear due to the exhaustion of all shallow traps, but actually, many electrons are still detained in the deep traps. Under the stimulation of a visible light for a short time at room temperature, some of the electrons in TRAP-1 can be photo-released, resulting in the regenerative luminescence [22].
Based on the CIE chromaticity diagram, it is expected that GZA phosphor has a potential application as a long persistent phosphor with the color of warm white (WLPP), when doping an activation ion with red or near-infrared (NIR) persistent luminescence (Fig. 7). Since phosphors doping with Cr3+ usually show the photoluminescence or persistent luminescence with the color of red and NIR, Cr3+ is chosen as a suitable candidate to prepare the WLPP (sample GZA1) [17]. Fig. 7 showed the photoluminescence spectrum during the excitation (kEx = 270 nm), the afterglow spectrum measured at 30 s after stopping the excitation, and the afterglow decay curves of 500 and 688 nm emissions. Emissions at 688, 698, and 713 nm observed from the photoluminescence spectrum should be deservedly assigned to the transition of Cr3+. It is confirmed that GZA1 phosphor can be used as a persistent phosphor with warm white color from the afterglow spectrum. The further verification is shown in the afterglow decay curves and CIE chromaticity diagram, indicating the rationality of the design for the WLPP.
Conclusion In summary, we have observed a long persistent luminescence in blue-white visible region from 6 ZnO:3 GeO2:Al2O3 phosphor with Zn imperfections as recombination centers. Long persistent luminescence could be observed beyond 2 h with the naked eyes. The properties of traps have also been elaborated by the thermoluminescence spectra and photo-stimulated luminescence decay curves. In addition, by doping Cr3+ into 6 ZnO:3 GeO2:Al2O3 phosphor, a long persistent phosphor with warm white color has been obtained. 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
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composition, defect equilibrium, modulation of the phosphor with warm white color are in progress. Acknowledgements We are grateful for the financial support from the National Natural Science Foundation of China (Grants Nos. 51072054, 51072060, 51132004), Guangdong Natural Science Foundation (Grant No. S2011030001349). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.046. References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670–2673. [2] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931–2933. [3] K. Kumar, A.K. Singh, S.B. Rai, Spectrochim. Acta A 102 (2013) 212–218. [4] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983–7990.
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[5] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42–46. [6] L. Vayssieres, Adv. Mater. 15 (2003) 464–466. [7] P.D. Yang, H.Q. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R.R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323–331. [8] S. Pearton, Prog. Mater. Sci. 50 (2005) 293–340. [9] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev. B 61 (2000) 15019–15027. [10] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Prog. Mater. Sci. 54 (2009) 89–136. [11] S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205. [12] J.C. Fan, K.M. Sreekanth, Z. Xie, S.L. Chang, K.V. Rao, Prog. Mater. Sci. 58 (2013) 874–985. [13] M. Wang, Y. Zhou, Y. Zhang, E. Jung Kim, S. Hong Hahn, S. Gie Seong, Appl. Phys. Lett. 100 (2012) 101906. [14] Z.S. Liu, X.P. Jing, L.X. Wang, J. Electronchem. Soc. 154 (2007) H500–H506. [15] D. Moncke, S. Reibstein, D. Schumacher, L. Wondraczek, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.029. [16] B. Guo, Z.R. Qiu, K.S. Wong, Appl. Phys. Lett. 82 (2003) 2290–2292. [17] Z. Pan, Y.Y. Lu, F. Liu, Nat. Mater. 11 (2012) 58–63. [18] B. Umesh, B. Eraiah, H. Nagabhushana, S.C. Sharma, D.V. Sunitha, B.M. Nagabhushana, C. Shivakumara, J.L. Rao, R.P.S. Chakradhar, Spectrochim. Acta A 93 (2012) 228–234. [19] D. Zwingel, J. Lumin. 5 (1972) 385–405. [20] T. Aitasalo, J. Hölsä, H. Jungner, J.C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, Radiat. Measur. 38 (2004) 727–730. [21] F. Liu, W. Yan, Y.J. Chuang, Z. Zhen, J. Xie, Z. Pan, Sci. Rep. 3 (2013) 1554. [22] G.J. Gao, L. Wondraczek, J. Mater. Chem. C 1 (2013) 1952–1958.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A long persistent phosphor based on recombination centers originating from Zn imperfections Yang Li, Xi Du, Kaniyarakkal Sharafudeen, Chenxing Liao, Jianrong Qiu ⇑ State Key Laboratory of Luminescent Materials and Devices, Institute of Optical Communication Materials, School of Materials Science and Technology, 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
A blue-white long persistent
phosphor. Zn imperfections as emitting centers
without any rare-earth doping. An energy transfer and remission
process is proposed.
a r t i c l e
i n f o
Article history: Received 28 September 2013 Received in revised form 2 December 2013 Accepted 5 December 2013 Available online 17 December 2013 Keywords: Recombination luminescence Zn imperfections Long persistent luminescence
a b s t r a c t The recombination luminescence from Zn imperfections has been extensively investigated; however, there have been few reports on the long persistent luminescence of Zn imperfections as emitting centers. Here, we observed a long persistent luminescence in blue-white visible region from 6 ZnO:3 GeO2:Al2O3 phosphor with Zn imperfections as emitting centers. Persistent luminescence could be observed beyond 2 h with naked eyes. The properties of traps were also elaborated by the measurements of thermo-luminescence spectra and photo-stimulated luminescence decay curves. Furthermore, a long persistent phosphor with warm white color was developed by doping Cr3+ into 6 ZnO:3 GeO2:Al2O3 phosphor. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Long persistent phosphors (LPPs) have been widely used in various areas, including safety indications and emergency lighting etc. [1]. Generally, long persistent phosphors require materials have abilities to form electrons reservoir, as well as accommodate suitable emitting centers. The emission wavelength of the long persistent phosphors is mainly determined by the emitters, while the persistence luminescence intensity and time are restricted by the defects [2]. Up to now, emitting centers in long persistent phos⇑ Corresponding author. Tel.: +86 20 87113646; fax: +86 20 87114204. E-mail address: [email protected] (J. Qiu). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.046
phors have been focused on the discrete luminescent centers, e.g. rare earth ions [3]. Except the discrete luminescent centers, it is well known that recombination centers are also a kind of important emitting centers, and defects usually act not only as the traps centers, but also as the emitting centers. However, there have been few reports on the long persistent luminescence from defects as emitting centers. The recombination photo-luminescence of Zn imperfections has been extensively investigated [4,5], but long persistent luminescence from Zn imperfections as emitting centers was rare. Herein, we observed a long persistent luminescence in blue-white visible region from 6 ZnO:3 GeO2:Al2O3 phosphor with Zn imperfections as emitting centers. The persistent luminescence could be
8
Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
observed beyond 2 h with naked eyes. The properties of the related traps were also elaborated by the thermo-luminescence (TL) spectra and photo-stimulated luminescence (PSL) decay curves. In addition, a long persistent phosphor with warm white color was obtained by doping Cr3+ ion into 6 ZnO:3 GeO2:Al2O3 phosphor. Experimental Phosphors with the compositions of 6 ZnO:3 GeO2:Al2O3 (GZA), 6 ZnO: 3 GeO2: Al2O3: 0.02 mol% Cr3+ (GZA1) were prepared with solid state reaction method. The 4 N pure GeO2, ZnO, Al2O3, and Cr2O3, were used as the raw materials, and mixed homogeneously in an agate mortar. The mixed powders were pre-fired at 1000 °C in air for 4 h, and were then ground to fine powders. The prepared powders were pressed into discs with diameters of 10 mm using a hydraulic press with the pressure of 10 MPa for 10 min. The discs were sintered again at 1400 °C in air for 4 h. X-ray diffraction (XRD) measurements were performed on a D8 advance Bruker diffract meter (Cu Ka radiation) equipped with a Vantec-1 linear detector. The data were collected between 10° and 130° (2h) with a 0.0164° step size at room temperature, confirming the presence of ZnGe2O4 (JCPDS no 11–0687) as the sole crystalline phase (Fig. s1). A FEI Quanta 400, field emission environmental SEM fitted with an oxford energy dispersive spectrometry (EDS) analyzer was employed for surface investigation and features detection, further confirming the presence of the sole crystalline phase (Fig. s2). All of the luminescence spectra were measured with Edinburgh instrument FLS 920 using a R928P photomultiplier as the signal collector and equipped with a 500 W xenon lamp. Thermo-luminescence (TL) glow curves were measured with a FJ-427A TL meter. Photographs were taken by a Canon digital camera un-irradiated or irradiated by the 4 W 254 nm UV lamp. Results and discussion Photoluminescence properties Fig. 1 shows the excitation and emission spectra of GZA phosphor. Two excitation bands were observed around 270 and 310 nm when monitored the emission at 488 nm. Under the excitation at 270 nm, GZA phosphor exhibits a bluish–white broad
emission band at 488 nm (Inset. b), which can be readily Gaussian-resolved into three bands located at 452, 512, 601 nm. The same photoluminescence has been observed in ZnO powders, ZnO nano-particles, and other compounds [5–7]. The broad emission band should be assigned to the transitions associated with Zn imperfections [8]. Atomic and electronic structures of native Zn imperfections have also been extensively investigated both theoretically and experimentally [9,10]. The emitting centers of blue, green, and orange bands are considered to be the intrinsic defects [10], including O vacancy (VO), Zn vacancy (VZn), Zn interstitial (Zni), O interstitial (Oi), and so on. [11]. Fan et al. suggested that radiative recombination transition between the shallow traps and deep acceptors (VO or VZn) gives rise to emission bands at 2.51, and 2.36 eV, respectively [12]. Wang et al. assigned the emissions at 2.9 and 2.38 eV to the transitions from Zn interstitial (Zni) to valence band, and from conduction band to OZn defect in ZnO, respectively [13]. Liu et al. also observed the similar luminescence ZnGe2O4 excited at 270 nm, and the emission is assigned to the donor–acceptor recombination, VO and Zni were donors, whereas ionized VGe and VZn were the acceptors [14]. Therefore, the blue, green, and orange emissions observed from GZA phosphor should be ascribed to the transitions from Ge interstitial (Zni), Zn interstitial (Zni) and O vacancy (VO) to Zn vacancy (VZn), respectively [15].
Long persistent luminescence properties After irradiation by a 254 nm UV lamp for 20 min, long persistent luminescence was also observed from the GZA sample (Fig. 2a). Obviously, the emission intensity of persistent luminescence decreased gradually (Fig. 2a) and the emission peak showed a red shift from 488 to 550 nm (Fig. 2b), indicating that the traps distributes over a wide range of energies. The persistent luminescence was sufficiently bright to be visually confirmed for about 2 h in the dark (Fig. 2c). The decay curve of persistent luminescence follows a hyperbolic function, indicating that emitting centers of persistent luminescence are recombination centers [16]. The decay curves of persistent luminescence after irradiation by the lights from 280 to 360 nm were shown in Fig. 3. Effectiveness for the acquisition of persistent luminescence decreases with the increment of excitation wavelength. It is also observed that persistent luminescence can be effectively achieved under ultraviolet illumination, but less effectively under visible light illumination (The inset of Fig. 3), and the maximum locates at 270 nm, which is similar to the excitation maximum shown in Fig. 1, indicating that shallow traps near the conduction band may be responsible for persistent luminescence [17].
Defects properties Thermo-luminescence spectra
Fig. 1. Emission spectrum (excited at 270 nm) and excitation spectrum by monitoring emission at 488 nm of GZA phosphor. The emission is Gaussianresolved into three bands located at 452 (green solid line curve), 512 (red solid line curve), and 601 nm (black solid line curve), respectively. The inset shows the photographs un-irradiated (a) or irradiated (b) by a 254 nm UV lamp. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Thermo-luminescence (TL) spectra are a useful tool for revealing the nature of traps [18–20]. TL curves acquired at 1, 10, 30 min, 1, 3, and 12 h after the stoppage of irradiation were shown in Fig. 4. All curves consist of a broad band, but with different maximum at 77, 105, 119, 139, 173, and 177 °C, respectively. The shift of TL peak to higher temperature confirms the continuous distribution of shallow traps and deep traps. Significantly, the position of TL peak acquired at 3 h and 12 h after the stoppage of irradiation is similar, which means that shallow traps near the conduction band are responsible for persistent luminescence, and electrons in deep traps cannot be released completely, and plenty of electrons are still detained in deep traps at room temperature.
Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
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Fig. 2. Afterglow spectra recorded at 10 s, 1, 2, 10, 30 and 90 min after the stoppage of irradiation (a), afterglow peak as a function of time (b) and afterglow intensity monitored at 500 nm as a function of time and the hyperbolic curve fitting (c). The sample GZA was pre-irradiated by a 4 W 254 nm UV lamp for 20 min.
Fig. 3. Afterglow decay curves irradiated by 280, 300, 320, 340, and 360 nm light for 20 min, the monitoring wavelength is 500 nm. The inset shows the afterglow intensity I10s monitored at 500 nm as a function of the excitation wavelengths 250– 450 nm. The sample GZA was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min.
Photo-stimulated luminescence decay curves To confirm this conjecture, photo-stimulated luminescence (PSL) decay curves monitored at 500 nm were measured and shown in Fig. 5. Before the measurements, GZA phosphor was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min. From Fig. 2c, the emission intensity of persistent luminescence became
Fig. 4. Thermo-luminescence curves over 30–280 °C acquired at 1, 10, 30 min, 1, 3, and 12 h after the stoppage of irradiation. The sample GZA was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min.
very faint at 3 h after the stoppage of irradiation. The sample was then annealed at 250 °C for 5 min, and irradiated with a Xenon lamp. Apparent photo-stimulated luminescence at 700 nm was observed in a flash excited at 390 nm. The emission decays when the Xenon lamp continuously irradiates the phosphor. Similar luminescence was also observed excited at 420, 450 and 480 nm. Recently, Liu et al. have observed a ‘‘downconversion’’ photo-stimulated luminescence from LiGa5O8:Cr3+ phosphor [21]. The luminescence monitored at 716 nm could be excited by the light of 380–1000 nm, indicating a large number of electrons were
10
Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
Fig. 5. Photo-stimulated luminescence decay curves monitored at 500 nm, the excitation wavelength is 390, 420, 450 and 480 nm, respectively. Before the measurements, the phosphor first pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min, the phosphor was annealed at 250 °C for 5 min at 3 h after the stoppage of irradiation. ‘‘On’’ means the excitation source (Xenon lamp, 500 W) turns on. Fig. 7. Afterglow decay curves of 500 nm (red curve) and 688 nm (black curve) emissions. The upper inset shows the CIE chromaticity. The under inset shows the photoluminescence spectrum during 270 nm excitation (orange curve) and the afterglow spectrum measured at 30 s after stopping the excitation (violet curve). The sample GZA1 was pre-irradiated by a 4 W 254 nm ultraviolet lamp for 20 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Long persistent phosphor with warm white color
Fig. 6. Energy transfer and remission process involving the Zn-defects.
reserved into the deeper traps. In our work, similar ‘‘downconversion’’ photo-stimulated luminescence was observed, indicating GZA phosphor also had the deeper traps and plenty of electrons were captured into these deep traps. To simplify the description, we assign the shallow traps and the filled deep traps in the PSL process as TRAP-2 and TRAP-1, respectively in Fig. 6. Under excitation at 270 nm, electrons from the valence band of phosphor are excited to the conduction band. Subsequently, the excited free electrons are trapped by Gei, Zni, and Vo in TRAP-2. Recombination of the native defects results in the intrinsic broad band emission with color of greenish white. After a sufficient illumination time, all of the continuous traps are filled, with the deeper traps being filled mainly through nonradiative relaxation from the shallow traps. After the stoppage of the irradiation, the direct recombination between the electron traps (Gei, Zni, and Vo) and hole traps (VZn) in TRAP-2 dominates the persistent luminescence process. Several minutes later, owing to the depletion of the shallower traps, the red shift of emission wavelength appears at the room temperature. Three hours later, the direct recombination maybe disappear due to the exhaustion of all shallow traps, but actually, many electrons are still detained in the deep traps. Under the stimulation of a visible light for a short time at room temperature, some of the electrons in TRAP-1 can be photo-released, resulting in the regenerative luminescence [22].
Based on the CIE chromaticity diagram, it is expected that GZA phosphor has a potential application as a long persistent phosphor with the color of warm white (WLPP), when doping an activation ion with red or near-infrared (NIR) persistent luminescence (Fig. 7). Since phosphors doping with Cr3+ usually show the photoluminescence or persistent luminescence with the color of red and NIR, Cr3+ is chosen as a suitable candidate to prepare the WLPP (sample GZA1) [17]. Fig. 7 showed the photoluminescence spectrum during the excitation (kEx = 270 nm), the afterglow spectrum measured at 30 s after stopping the excitation, and the afterglow decay curves of 500 and 688 nm emissions. Emissions at 688, 698, and 713 nm observed from the photoluminescence spectrum should be deservedly assigned to the transition of Cr3+. It is confirmed that GZA1 phosphor can be used as a persistent phosphor with warm white color from the afterglow spectrum. The further verification is shown in the afterglow decay curves and CIE chromaticity diagram, indicating the rationality of the design for the WLPP.
Conclusion In summary, we have observed a long persistent luminescence in blue-white visible region from 6 ZnO:3 GeO2:Al2O3 phosphor with Zn imperfections as recombination centers. Long persistent luminescence could be observed beyond 2 h with the naked eyes. The properties of traps have also been elaborated by the thermoluminescence spectra and photo-stimulated luminescence decay curves. In addition, by doping Cr3+ into 6 ZnO:3 GeO2:Al2O3 phosphor, a long persistent phosphor with warm white color has been obtained. 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
Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 7–11
composition, defect equilibrium, modulation of the phosphor with warm white color are in progress. Acknowledgements We are grateful for the financial support from the National Natural Science Foundation of China (Grants Nos. 51072054, 51072060, 51132004), Guangdong Natural Science Foundation (Grant No. S2011030001349). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.046. References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670–2673. [2] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931–2933. [3] K. Kumar, A.K. Singh, S.B. Rai, Spectrochim. Acta A 102 (2013) 212–218. [4] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983–7990.
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