Afterglow phenomenon in Erbium and Titanium codoped GD2O2S phosphors

Afterglow phenomenon in Erbium and Titanium codoped GD2O2S phosphors

ARTICLE IN PRESS Journal of Luminescence 117 (2006) 141–146 www.elsevier.com/locate/jlumin Afterglow phenomenon in Erbium and Titanium codoped GD2O2...

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ARTICLE IN PRESS

Journal of Luminescence 117 (2006) 141–146 www.elsevier.com/locate/jlumin

Afterglow phenomenon in Erbium and Titanium codoped GD2O2S phosphors Junwen Zhanga, Ying-Liang Liua,, Shi-qing Manb a

Department of Chemistry, Jinan University, Guangzhou 510632, PR China b Department of Physics, Jinan University, Guangzhou 510632, PR China Received 18 September 2004 Available online 1 July 2005

Abstract A series of new long-lasting phosphor Gd2O2S:xEr,Ti are prepared by the conventional high-temperature solid-state method and their luminescent properties are systematically investigated in this paper. The characteristic afterglow emission of Er, which is due to the transition of 4F9/2-4I15/2 and 4S3/2-4I15/2, is reported for the first time. XRD, photoluminescence, long-lasting phosphorescence and decay curves are used to characterize the synthesized phosphors. The possible energy transfer mechanism of Gd2O2S:xEr,Ti is also investigated. r 2005 Elsevier B.V. All rights reserved. Keywords: Afterglow; Energy transfer; Erbium ion; Titanium ion

1. Introduction Long-lasting phosphor, in which energy could be reserved under the (ultraviolet) UV lamp or infrared femtosecond laser and finally thermal release as luminescence, have been widely studied in various rare earth-doped crystals and glasses [1–3]. These materials can be widely used in areas such as safety indication, lighting in emergency, instruments in automobiles, luminous paint and optical data storage, etc. Till now, the active Corresponding author. Tel.: +86 0431 85221813;

fax: +86 020 85221697. E-mail address: [email protected] (Y.-L. Liu).

centers in persistent luminescence phosphors doped with rare earth ions are almost Ce3+, Pr3+, Tb3+, Eu2+ and Eu3+, Sm3+ [4–9], and the most efficient long-lasting afterglow phosphors are still based on alkaline-earth aluminates, e.g., SrAl2O4:Dy,Eu (green), CaAl2O4:Nd,Eu (blue). In this paper, we report another persistent phosphor of Gd2O2S:Er,Ti. To our best knowledge, there have been no reports about the persistent phosphorescence from Er. In our work, the obvious long-lasting phosphorescence of Er3+ in Gd2O2S phosphor are observed for the first time. The afterglow emission peaks located at 555 and 675 nm are attributed to the characteristic transition of 4F9/2-4I15/2 and 4S3/2-4I15/2 from

0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.05.001

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Er. The phosphorescence of Gd2O2S:Ti, Gd2O2S:Er and the double-doped phosphor are compared and studied. It is apparent that the emission of Ti becomes very weak and is barely detectable with the increase of Er concentration, meanwhile the characteristic emissions of Er are strongly enhanced in the double-doped phosphor. The decay time of the Gd2O2S:Er,Ti coincided with that of Gd2O2S:Ti. And the single-doped Er3+ materials ion just has a very short lifetime. Based on these facts, energy transfer process is introduced into explaining our observation. The corresponding mechanism is given.

3. Results and discussions 3.1. XRD of Gd2O2S:Er,Ti The X-ray diffraction (XRD) pattern of 8% Er3+-doped Gd2O2S sample is shown in Fig. 1, and by comparing the pattern with JCPD card no. 26-1422, we believe that our samples are chemically and structurally Gd2O2S. The phase analysis demonstrates that Gd2O2S:Er,Ti phosphor is hexagonal with the cell dimensions: a ¼ 0:3852 nm and c ¼ 0:6667 nm. According to the XRD analysis, we conclude that the Ti ions and Er ions substitute Gd3+ without disturbing the crystal lattice.

2. Experiment 3.2. The fluorescence properties of Gd2O2S:Er,Ti phosphor The excitation spectra monitored by 555 and 675 nm are shown in parts (a) and (b), respectively, of Fig. 2. As shown in the figures, it is found that the excitation spectrum monitored by 555 nm is mostly composed by the band absorption near 254 nm and two excitation peaks at 375 nm. The former can be attributed to the host absorption [10], and the later belong to the characteristic absorption of Er3+ [11]. When the excitation spectrum monitored at 675 nm is checked, not only the strong band absorption at 254 nm can be

10000 8000 Intensity(a.u.)

Er3+-doped gadolinium oxysulfide phosphor was prepared by flux fusion method with stoichiometric amount of Gd2O3, S, Er2O3, and TiO2 as raw materials, using binary flux composition (S and Na2CO3 at a ratio of 1:1 at 30 wt% of the total weight of raw material). The TiO2 and MgCO3 were added into the system at 2% and 1% molar ratio, respectively. The starting materials were Gd2O3(5N), Er2O3(4N), S (99.99%), and Na2CO3 (99.99%). The Er3+ dopant concetration was 2%, 4%, 8% molar ratio to Gd3+. The raw materials Gd2O3, Er2O3, S, Na2CO3, TiO2, and MgCO3 were homogeneously mixed and first preheated at 400 1C for 2 h then fired at 1050 1C for 6 h using alumina crucibles with alumina lids in weak reducing atmosphere (CO gas produced by kryptol in high temperature) with subsequent air-quenching to get the raw product. After the firing process, the air-cooled raw products were washed by 5% hydrochloric acid to remove the residual sulfur and flux byproduct, then were washed by ultrapure water for several times till the pH value was 7, then washed by alcohol and dried at 80 1C. The structures of all the synthesized powder samples were checked by MSAL-XRD2 X-ray diffractometer at 40 kV of 20 mA and 4.01(2y)/min scanning rate. The photoluminescence spectra and afterglow decay curve were recorded on an American VARIAN fluorescence spectrophotometer using 150 W xenon arc lamp as the excitation source.

6000 4000 2000 0 10

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2θ (deg) Fig. 1. The XRD patterns of Gd2O2S:Er,Ti.

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200 300 400 500 600 700 Wavelenghth (nm)

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c

a

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20 10 0 -10 200

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Wavelenghth/nm Fig. 2. Photoluminescence spectra of Gd2O2S:Er,Ti phosphor: (a) excitation spectrum of Gd2O2S:Er,Ti (l ¼ 555 nm), (b) excitation spectrum of Gd2O2S:Er,Ti (l ¼ 675 nm), (c) emission spectrum of Gd2O2S:Er,Ti (l ¼ 375 nm), (d) emission spectrum of Gd2O2S:Er,Ti (l ¼ 254 nm).

seen, but also some peak absorptions in the range from 400 and 600 nm are observed when this area is multiplied by 50 (inset). The strongest line is located at 550 nm which induce the transition of 4 I15/2-4F9/2. The emission spectra of Gd2O2S:Er,Ti are shown in parts (c) and (d), respectively, of Fig. 2. There are some stronger emission peaks at 540, 555 and 675 nm, which are due to the Er3+ transition of 4H11/2-4I15/2, 4S3/2-4I15/2 and 4F9/24 I15/2 [12]. Besides, an interesting feature observed in our experiment is that the luminescence of Er3+ is different as the phosphor is excited by UV radiation of different wavelength. When the phosphor is excited at 375 nm, there is a stronger emission peak located at 555 nm and a weaker emission at 615 nm. When excited by 254 nm, the emission at 675 nm increases, meanwhile the emission located at 555 nm decreases. Based on the emission spectra, we can speculate that the excited spectrum at 375 nm can result in the transitions 4I15/2-4H11/2, 4I15/2-4S3/2 , and the band absorption located at 254 nm which attributed to the host absorption [10] may induce the emission at 675 nm through energy transfer.

The long-lasting phosphorescence of these phosphors have been checked and studied. Fig. 3 shows the afterglow spectra of gadolinium oxysulfide phosphor with different Er3+ concentration. It is found that all the afterglow spectra are composed by two kinds of emissions: two emission peaks located at 555 and 675 nm, and band emission at about 590 nm. However, the former is observed in the luminescence spectra of Fig. 2, and they are ascribed to the transitions of 4S3/24 I15/2 and 4F9/2-4I15/2 from Er3+. The band afterglow emission belongs to Ti ion [12,13]. Moreover, the band emission of afterglow is similar to the fluorescence spectra of the Ti3+doped CaGdAlO4 crystal and which is due to the transition of 2E-2T2 [14]. The shape and bandwidth of these two band emissions are found identical. So we speculate that the band afterglow emission of Gd2O2S:Er,Ti phosphor may be attributed to the transition of 2E-2T2 of Ti3+ ions. That some portion of the Ti ions exist in the host in their trivalent state mostly is based on these reasons: (a) fraction of Ti4+ ions undergo the reduction in valence because the synthesis of phosphors are done in weak reducing atmosphere

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3.3. The persistent phosphorescence characteristic of Gd2O2S:Er,Ti phosphor

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Fig. 3. Afterglow spectra of Gd2O2S:xEr,Ti (x ¼ 0:02, 0.04, 0.08). The inset pattern: (a) is from the phosphor single-doped Er and (b) is from the phosphor single-doped Ti.

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(carbon monoxide); (b) once part of Ti ions enter into the host as Ti4+, they will substitute the Y3+ and form an electron affinity TiGd. When irradiated under the UV light, electrons are generated and they are trapped by the electron affinity TiGd, which may induce the formation of trivalent Ti ions. This process can be demonstrated by the band absorption from the excitation spectra. Further work is still being carried on. Meanwhile, it is observed that there are some changes in afterglow spectra with the increase of Er3+ concentration: (i) the band emission due to Ti3+ becomes weaker and weaker, when the concentration is 8%, only trace emission of Ti3+ is observed. (ii) the emission located at 555 nm due to 4S3/2-4I15/2 becomes weaker and at 675 nm due to 4F9/2-4I15/2 becomes stronger, respectively. In addition, we have checked the single-doped Ti sample and the single-doped Er3+sample, whose luminescence are shown at (a) and (b), respectively, in Fig. 3. It is found that the phosphor single-doped with Ti just has a strong band emission at about 590 nm, however the singledoped Er3+ sample has rarely afterglow emission. The lifetimes of single-doped Er, single-doped Ti and codoped samples are measured for 2 min, 1.5 and 1.2 h. It is found that the afterglow lifetime of doubly doped sample coincides with the lifetime of Gd2O2S:Ti. The afterglow decay curves of these samples are shown in Fig. 4. The decay curves of Gd2O2S:Ti and Gd2O2S:Er,Ti are shown in parts (a) and (b), respectively. According to them, the similarity of afterglow decay curve of the two phosphors is obviously observed. And the singledoped Er3+ material just has a weak afterglow emission shown in part (c) and a very short lifetime. These phosphorescence characteristics are evaluated based on the decay times that have been calculated using a curve-fitting technique. The decay curves are fitted using the equation [15]: I ¼ A1 expðt=T 1 Þ þ A2 expðt=T 2 Þ,

(1)

where I is the phosphorescence intensity at any time ‘t’ after switching off the excitation illumination, A1 and A2 are constants, and T 1 and T 2 are decay times for the exponential components, respectively. The results are shown in Table 1. It is obvious that Gd2O2S:Ti and Gd2O2S:Er,Ti can

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t/min Fig. 4. Long-lasting phosphorescence decay curve of singledoped Ti or Er and double-doped Ti and Er.

Table 1 Decay times of the phosphorescence of Gd2O2S:Ti, Gd2O2S:Er,Ti and Gd2O2S:Er Compounds

Gd2O2S:Ti Gd2O2S:Er,Ti Gd2O2S:Er

Decay lifetimes (min) T1

T2

1.07 1.01 0.66

14.08 13.09 —

be considered as two decay process: fast decay and slow decay, and Gd2O2S:Er is just considered a fast decay. The T values of Gd2O2S:Er,Ti coincide with that of Gd2O2S:Ti. 3.4. Persistent energy transfer from Ti to Er According to the theory of energy transfer developed by Dexter [17], the energy transfer process through multipolar interaction depends on the extent of overlap of the emission spectrum of sensitizer with the absorption spectrum of the acceptor, the relative orientation of interacting dipoles, and the distance between the sensitizer and the acceptor. It is apparent that the Ti3+emission in the doubly doped sample disappears by contributing its energy to a different afterglow,

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which can be attributed to Er3+. So we may consider that Ti3+acts as sensitizer (S) and Er3+ as acceptor (A). It is observed that the long-lasting orange afterglow of Ti ions is converted to the characteristic luminescence of Er3+; since the Ti excitation lasts for an extended period, the converted afterglow also persists for the similar duration. From Table 1, the T values of Gd2O2S:Er,Ti coincide with that of Gd2O2S:Ti. From part (b) of Fig. 2 and part (b) of Fig. 3, an obvious overlap between the emission spectrum of S and the absorption spectrum of A can be seen. Thus an efficient energy transfer may be speculated. Persistent energy tranfer process is followed by the emission from Er3+ and the quenching of the Ti3+ emission. However, the corresponding work is still carried on.

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I 13/2

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Fig. 5. The energy level of Er3+ and the process of energy transfer from Ti3+ to Er3+.

3.5. Mechanism of afterglow Much research has been done on the principle of the afterglow phenomenon and the mechanism for afterglow phenomenon has not been well established. Recently, Jia [5,18] has reported a new mechanism of afterglow due to energy transfer in CaAlO2:Ce,Tb. Here, energy transfer process from Ti to Er3+ is exploited to explain the persistent phosphorescence of Gd2O2S:Er,Ti. The energy level of Er3+ and the probable process of energy transfer from Ti3+ to Er3+ is shown in Fig. 5. The possible mechanism of Gd2O2S:Er,Ti phosphor is as following: initially, the electrons are excited by UV light exposure involving the transition from the ground state (2T2) of the Ti3+ to an excited state (2E), and then one part of the excited electrons transit back to lower energy levels of Ti3+ and the luminescence is observed. Meanwhile, some excited electrons can be stored in the traps. The orange long-lasting phosphorescence occurs when the electrons located in the trap are thermally released and then cascade to the ground state. When excited electrons in the 2E come back to 2T2, the released energy would transfer to Er3+, and electron in 4 I15/2 would be excited to 4F9/2 (see Fig. 5). Er3+ in 4 F9/2 energy level may have two transition approaches: (i) it directly releases nonradiatively

to the 4I15/2 with red emission; (ii) since the 4F9/2 has a very short lifetime which is about 1 ms and 4I11/2 has a much longer lifetime of 1.3 ms [15], some Er3+ will rapidly release to 4I11/2. This permits the energy transfer up-conversion of 4I11/2-4F7/2 [11]. Then Er ion will decay nonradiatively to 2H11/2 and 4I3/2, thus resulting in 4S3/2-4I15/2 and blue luminescence.

4. Conclusion In conclusion, we have firstly observed the persistent phosphorescence from Er3+ in Gd2O2S:Er,Ti. The phosphor has been synthesized and its luminescence property has been systematically checked and studied. The orange afterglow from Ti ions decreases and the characteristic persistent phosphorescence from Er3+ becomes strong with an increase of doped Er3+ concentration. Based on the analysis of the excitation spectra, emission spectra and decay curves of these phosphors, energy transfer process, which brings the persistent phosphorescence of Er3+ is speculated. The corresponding afterglow mechanism is given, too.

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Acknowledgements This present work was supported by the National Natural Science Foundations of China (Grant no. 50472077 and 20171018), the Natural Science Foundations of Guangdong Province (Grant no. 013201 and 36706), and Foundation from Key Laboratory of Rare Earth Chemistry and Physics of Changchun Institute of Applied Chemistry (R020202K). Reference [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [2] T. Kinoshita, M. Yamazaki, H. Kawazoe, H. Hosono, J. Appl. Phys. 86 (1999) 3729. [3] E. Danielson, M. Devenney, D.M. Giaguinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojanary, X.D. Wu, Science 1279 (1998) 837.

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