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Effect of temperature on the luminescence processes of SrAl2O4:Eu2+
Radiation Measurements 38 (2004) 727 – 730 www.elsevier.com/locate/radmeas
E ect of temperature on the luminescence processes of SrAl2O4: Eu2+ Tuomas Aitasaloa; b , Jorma H,ols,aa , H,ogne Jungnerc , Jean-Claude Krupad , Mika Lastusaaria , Janina Legendziewicze , Janne Niittykoskia; b;∗ a University
of Turku, Department of Chemistry, Turku FIN-20014, Finland School of Materials Research, Turku, Finland c University of Helsinki, Dating Laboratory, P.O. Box 64, Helsinki FIN-00014, Finland d Institut de Physique Nucl0 eaire, CNRS-IN2P3, Orsay Cedex F-91406, France e University of Wroclaw, Faculty of Chemistry, 14 F. Joliot-Curie, Wroclaw PL-50-383, Poland b Graduate
Received 16 January 2004; received in revised form 16 January 2004; accepted 25 January 2004
Abstract The UV excited and persistent luminescence properties as well as thermoluminescence (TL) of Eu 2+ doped strontium aluminates, SrAl2 O4 :Eu2+ were studied at di erent temperatures. Two luminescence bands peaking at 445 and 520 nm were observed at 20 K but only the latter at 295 K. Both Sr-sites in the lattice are thus occupied by Eu2+ but at room temperature e?cient energy transfer occurs between the two sites. The UV excited and persistent luminescence spectra were similar at 295 K but the excitation spectra were di erent. Thus the luminescent centre is the same in both phenomena but excitation processes are di erent. Two TL peaks were observed between 50 and 250 ◦ C in the glow curve. Multiple traps were, however, observed by preheating and initial rise methods. With longer delay times only the high temperature TL peak was observed. The persistent luminescence is mainly due to slow fading of the low temperature TL peak but the step-wise feeding process from high temperature traps is also probable. c 2004 Elsevier Ltd. All rights reserved. Keywords: Persistent luminescence; Thermoluminescence; Europium; Strontium aluminate
1. Introduction The alkaline earth aluminates doped with the Eu2+ and R ions, MAl2 O4 :Eu2+ ; R 3+ (M=Ca and Sr, R =e:g. Nd or Dy), are novel and e?cient persistent luminescence (afterglow) materials used, e.g. in luminous paints (Murayama, 1999). The Eu2+ ion acts as a luminescent centre with luminescence at the blue (max = 440 nm) and green (520 nm) regions for CaAl2 O4 :Eu2+ and SrAl2 O4 :Eu2+ , respectively. More recently, new applications for these materials, 3+
∗ Corresponding author. University of Turku, Department of Chemistry, Turku FIN-20014, Finland. Tel.: +358-2-3336734; fax: +358-2-3336700. E-mail address: [email protected] (J. Niittykoski).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.031
e.g. radiation detection (Kowatari et al., 2002) and sensing of the structural damage and fracture of materials (Xu et al., 1999, 2000; Akiyama et al., 2002) have been presented. The development of new materials would be greatly facilitated if the exact luminescence mechanisms and the trap identity were known. Although several studies of the persistent luminescence phenomena have already been published (e.g. Matsuzawa et al., 1996; Nakazawa and Mochida, 1997; Jia et al., 1998; H,ols,a et al., 2001) the mechanisms of the persistent luminescence of the MAl2 O4 :Eu2+ ; R 3+ materials have not been elucidated. The theories presented so far are even contradictory. In the present work, UV excited luminescence and persistent luminescence as well as excitation and luminescence dynamics of SrAl2 O4 :Eu2+ were studied at di erent temperatures. Thermoluminescence (TL) measurements were also carried out.
T. Aitasalo et al. / Radiation Measurements 38 (2004) 727 – 730
2. Experimental
400
where k is the Boltzmann’s constant. The trap depth E can now be found from the slope of a straight line obtained by plotting ln(I ) against 1=T . The temperature range used for the analysis was restricted in such a way that the TL intensity did not exceed 10–15% of the peak maximum temperature (McKeever, 1985).
Intensity / Arb. Unit
The polycrystalline SrAl2 O4 :Eu materials were prepared by a solid state reaction between strontium carbonate (SrCO3 ), aluminium oxide (Al2 O3 ) and europium oxide (Eu2 O3 ; 0:5 mol%), with boron oxide (B2 O3 ; 1 mol%) as a Lux in a reducing (N2 + 12% H2 ) atmosphere at 1300 ◦ C for 4 h. The luminescence spectra at 20 and 295 K were measured using an Ocean Optics PC2000 spectrometer equipped with an optical Hber. The luminescence decay curves were measured with a Lecroy 9350M oscilloscope. The third harmonic (355 nm) of a Nd:YAG laser was used as the UV excitation source. The excitation spectra were measured with a SLM Aminco SPF-500 spectrometer equipped with a 300 W xenon lamp. The persistent luminescence at 295 K was measured with a Perkin Elmer LS-5 spectrometer. The TL glow curves were measured with a RisH TL/OSL-DA-12 system using the heating rate of 5 ◦ C s−1 . Prior to the TL measurements, the samples were irradiated by a 20 W incandescent lamp for 10–120 s. The TL delay times from 3 min up to 10 h were used. The preheating method (McKeever, 1985) was used to study the number of the trapping levels in detail. The samples were heated to a temperature Tstop and then rapidly cooled down to the room temperature. Then the TL experiment was performed. This procedure was repeated for the Tstop values between 50 and 250 ◦ C in steps of 5◦ C. The initial rise method (McKeever, 1985) was used to determine the trap depths from the preheated glow curves. At the start of the glow curve the thermoluminescence intensity (I ) is, irrespective of the kinetic order, exponentially dependent on temperature E I = constant × exp − ; kT
2+
295 K
2+
SrAl2O4:Eu
300
20
λexc= 355 nm
20 K
15
200
10 AG, 295 K
100
5
0
0 450
500 550 Wavelength / nm
AG Intensity / Arb. Unit
728
600
Fig. 1. UV excited and persistent (AG) luminescence spectra of SrAl2 O4 : Eu2+ at 20 and 295 K.
nescence (Poort et al., 1995). The persistent luminescence spectrum at 295 K was identical to the UV excited one but was not at all observed at 20 K due to lack of su?cient thermal energy to release the trapped excitation energy. The emitting centre can be concluded to be the same Eu2+ ion in both luminescence phenomena. The luminescence decay curve of the 520 nm band was found nearly exponential ( = 1:23 s) at 20 K but multi-exponential ( 1 = 0:74 s) at 295 K (Fig. 2). The short rise observed at the beginning of the 520 nm curve could be due to energy transfer from the site corresponding to the 445 nm emission. On the other hand, the rise in the 295 K curve at longer times (ca. 1 s) was connected to the persistent luminescence phenomenon. The decay curve of the 445 nm band was very fast ( 1 = 0:42 s) at 20 K. The long decay tail in the 445 nm band decay curve is, with high probability, due to the slightly overlapping intense emission from the 520 nm band.
2+
100
SrAl2O4:Eu
520 nm
100
λexc= 355 nm
3.1. Luminescence The UV excited luminescence of SrAl2 O4 : Eu2+ was observed at 295 K as one broad but symmetric band centred at 520 nm, but at 20 K as two bands centred at 445 and 520 nm (Fig. 1). The luminescence is due to the 4f 6 5d 1 → 4f 7 transitions of the Eu2+ ion. According to structural studies (Schulze and M,uller-Buschbaum, 1981), there are two Sr sites in the SrAl2 O4 lattice. At low temperatures, the Eu2+ luminescence from both sites can be observed, while at higher temperatures an e?cient energy transfer between the sites causes the quenching of the higher energy lumi-
445 nm
Intensity / Arb. Unit
3. Results and discussion
20 K 0.0
0.1
0.2
10 λem= 520 nm, 20 K λem= 520 nm, 295 K λem= 445 nm, 20 K
1 0
1
2 Lifetime / µs
3
4
Fig. 2. Luminescence decay curves of SrAl2 O4 : Eu2+ at 20 and 295 K, with UV excitation.
T. Aitasalo et al. / Radiation Measurements 38 (2004) 727 – 730 2+
SrAl2O4:Eu
λem= 520 nm, 77 K
150
λem= 445 nm, 77 K
120 90 AG, λem= 520 nm, 295 K
60
2+
irradiation 120 s SrAl2O4:Eu delay 3 min
200 TL Intesity / Arb. Unit
Intensity / Arb. Unit
180
150 60 s 3 min
100
400
10 s 10 h
0
450
Fig. 3. Excitation spectra of the UV excited and persistent (AG) luminescence of SrAl2 O4 : Eu2+ at 77 and 295 K.
The excitation spectra of SrAl2 O4 : Eu2+ were dependent on the luminescence band. The excitation spectra at 77 K consisted of two main bands at 320 and 350 nm for the 520 nm emission but at 270 and 320 nm for the 445 nm emission (Fig. 3). No clear Hne structure due to the 7 F0 – 6 levels was observed. The excitation spectrum of persistent luminescence was observed at the same wavelength region but no clear band structure was observed. A strong increase in intensity below 280 nm is possibly due to an exciton. The excitation processes for the UV excited and persistent luminescence are signiHcantly di erent.
50
100
150 200 Temperature / °C
250
Fig. 4. TL curves of SrAl2 O4 : Eu2+ with di erent irradiation and delay times.
0.8
0.7 Etrap / eV
300 350 Wavelength / nm
10 s 3 min
50
30 250
729
0.6 2+
SrAl2O4:Eu 0.5
3.2. Thermoluminescence Two TL peaks at 80 and 160 ◦ C were observed when the delay time was 3 min (Fig. 4). These peaks correspond to a shallow and a deep traps, respectively. The kinetics of the TL peaks was studied by measuring the peak positions as a function of the irradiation dose. The Hrst TL peak position was observed to move slightly to higher and the second one to lower temperatures with increasing irradiation times. The TL peaks seemed to merge with long exposing times. The dose behaviour of the Hrst peak, which cannot be explained easily with simple the Hrst or second order kinetics, could be due to the di erent Hlling rates of individual overlapping or continuously distributed traps (Lakshmanan, 1999; Sakurai and Gartia, 1997). On the other hand, the second TL peak does not follow the behaviour of the Hrst one but obviously the second order kinetics. Only the high temperature peak was observed when the delay time was 10 h. Because the persistent luminescence was hardly observed when the delay time was 10 h, it can be concluded that the persistent luminescence at 295 K is mainly due to the slow fading of the low temperature TL peaks of SrAl2 O4 : Eu2+ . The intensity of the high temperature TL peak was also decreased and thus the very slow emptying of deeper traps or a feeding process to shallow traps is also possible.
60
90
120 150 Tstop / °C
180
Fig. 5. Trap depths of SrAl2 O4 : Eu2+ estimated by initial rise method from preheated glow curves.
Several traps were observed by the preheating and initial rise methods. The Hrst TL peak is due to at least three traps at ca. 0.55, 0.60 and 0:65 eV (Fig. 5). The energies of the traps are, however, very close to each other and thus the real amount of traps is uncertain. Continuous distribution of the traps may also be possible. The second TL peak corresponds probably to only a one trap at ca. 0:75 eV. 4. Conclusions Two luminescence bands for SrAl2 O4 : Eu2+ centred at 445 and 520 nm were observed at 20 K but only one band at 520 nm at 295 K. The change is probably due to energy transfer between the two Eu2+ sites at higher temperatures. The UV excited and persistent luminescence spectra were similar but the excitation spectra di ered. There is thus the same emitting Eu2+ centre but di erent excitation process
730
T. Aitasalo et al. / Radiation Measurements 38 (2004) 727 – 730
for the UV excited and persistent luminescence. Two TL peaks were observed at 80 and 160 ◦ C but mainly the low temperature one had e ect on the persistent luminescence at 295 K. Acknowledgements The authors thank the Academy of Finland (project #5066/2000), the Graduate School of Materials Research (Turku, Finland), the University of Turku and the Marie Curie Fellowship program of the European Union for Hnancial support. References Akiyama, M., Xu, C.-N., Liu, Y., Nonaka, K., Watanabe, T., 2002. InLuence of Eu, Dy co-doped strontium aluminate composition on mechanoluminescence intensity. J. Lumin. 97, 13–18. H,ols,a, J., Jungner, H., Lastusaari, M., Niittykoski, J., 2001. Persistent luminescence of Eu2+ doped alkaline earth aluminates, MAl2 O4 : Eu2+ . J. Alloys Compd. 94–95, 159–163. Jia, W., Yuan, H., Lu, L., Liu, H., Yen, W.M., 1998. Phosphorescent dynamics in SrAl2 O4 : Eu2+ ; Dy3+ single crystal Hbers. J. Lumin. 76&77, 424–428. Kowatari, M., Koyama, D., Satoh, Y., Iinuma, K., Uchida, S., 2002. The temperature dependence of luminescence from a
long-lasting phosphor exposed to ionizing radiation. Nucl. Instrum. Methods Phys. Res. A 480, 431–439. Lakshmanan, A.R., 1999. Photoluminescence and thermostimulated luminescence processes in rare-earth-doped CaSO4 phosphors. Prog. Mater. Sci. 44, 1–187. Matsuzawa, T., Aoki, Y., Takeuchi, N., Murayama, Y., 1996. A new long phosphorescence phosphor with high brightness, SrAl2 O4 : Eu2+ ; Dy3+ . J. Electrochem. Soc. 143, 2670–2673. McKeever, S.W.S., 1985. Thermoluminescence of Solids, Cambridge University Press, New York, pp. 76–80. Murayama, Y., 1999. Luminous paints. In: Shionoya, S., Yen, W.M. (Eds.), Phosphor Handbook. CRC Press, Boca Raton, FL, USA, pp. 651–658. Nakazawa, E., Mochida, T., 1997. Traps in SrAl2 O4 : Eu2+ phosphor with rare-earth doping. J. Lumin. 72–74, 236–237. Poort, S.H.M., Blockpoel, W.P., Blasse, G., 1995. Luminescence of Eu2+ in barium and strontium aluminate and gallate. Chem. Mater. 7, 1547–1551. Sakurai, T., Gartia, R.K., 1997. Evidence of continuous trap distribution in the glow curve of a Brown microcline. J. Appl. Phys. 82, 5722–5727. Schulze, A.-R., M,uller-Buschbaum, H.K., 1981. Zur Struktur von monoklinem SrAl2 O4 . Z. Anorg. Allg. Chem. 475, 205–210. Xu, C.-N., Watanabe, T., Akiyama, M., Zheng, X.-G., 1999. Direct view of stress distribution in solid by mechanoluminescence. Appl. Phys. Lett. 74, 2414–2416. Xu, C.-N., Zheng, X.-G., Akiyama, M., Nonaka, K., Watanabe, T., 2000. Dynamic visualization of stress distribution by mechanoluminescence. Appl. Phys. Lett. 76, 179–181.
E ect of temperature on the luminescence processes of SrAl2O4: Eu2+ Tuomas Aitasaloa; b , Jorma H,ols,aa , H,ogne Jungnerc , Jean-Claude Krupad , Mika Lastusaaria , Janina Legendziewicze , Janne Niittykoskia; b;∗ a University
of Turku, Department of Chemistry, Turku FIN-20014, Finland School of Materials Research, Turku, Finland c University of Helsinki, Dating Laboratory, P.O. Box 64, Helsinki FIN-00014, Finland d Institut de Physique Nucl0 eaire, CNRS-IN2P3, Orsay Cedex F-91406, France e University of Wroclaw, Faculty of Chemistry, 14 F. Joliot-Curie, Wroclaw PL-50-383, Poland b Graduate
Received 16 January 2004; received in revised form 16 January 2004; accepted 25 January 2004
Abstract The UV excited and persistent luminescence properties as well as thermoluminescence (TL) of Eu 2+ doped strontium aluminates, SrAl2 O4 :Eu2+ were studied at di erent temperatures. Two luminescence bands peaking at 445 and 520 nm were observed at 20 K but only the latter at 295 K. Both Sr-sites in the lattice are thus occupied by Eu2+ but at room temperature e?cient energy transfer occurs between the two sites. The UV excited and persistent luminescence spectra were similar at 295 K but the excitation spectra were di erent. Thus the luminescent centre is the same in both phenomena but excitation processes are di erent. Two TL peaks were observed between 50 and 250 ◦ C in the glow curve. Multiple traps were, however, observed by preheating and initial rise methods. With longer delay times only the high temperature TL peak was observed. The persistent luminescence is mainly due to slow fading of the low temperature TL peak but the step-wise feeding process from high temperature traps is also probable. c 2004 Elsevier Ltd. All rights reserved. Keywords: Persistent luminescence; Thermoluminescence; Europium; Strontium aluminate
1. Introduction The alkaline earth aluminates doped with the Eu2+ and R ions, MAl2 O4 :Eu2+ ; R 3+ (M=Ca and Sr, R =e:g. Nd or Dy), are novel and e?cient persistent luminescence (afterglow) materials used, e.g. in luminous paints (Murayama, 1999). The Eu2+ ion acts as a luminescent centre with luminescence at the blue (max = 440 nm) and green (520 nm) regions for CaAl2 O4 :Eu2+ and SrAl2 O4 :Eu2+ , respectively. More recently, new applications for these materials, 3+
∗ Corresponding author. University of Turku, Department of Chemistry, Turku FIN-20014, Finland. Tel.: +358-2-3336734; fax: +358-2-3336700. E-mail address: [email protected] (J. Niittykoski).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.031
e.g. radiation detection (Kowatari et al., 2002) and sensing of the structural damage and fracture of materials (Xu et al., 1999, 2000; Akiyama et al., 2002) have been presented. The development of new materials would be greatly facilitated if the exact luminescence mechanisms and the trap identity were known. Although several studies of the persistent luminescence phenomena have already been published (e.g. Matsuzawa et al., 1996; Nakazawa and Mochida, 1997; Jia et al., 1998; H,ols,a et al., 2001) the mechanisms of the persistent luminescence of the MAl2 O4 :Eu2+ ; R 3+ materials have not been elucidated. The theories presented so far are even contradictory. In the present work, UV excited luminescence and persistent luminescence as well as excitation and luminescence dynamics of SrAl2 O4 :Eu2+ were studied at di erent temperatures. Thermoluminescence (TL) measurements were also carried out.
T. Aitasalo et al. / Radiation Measurements 38 (2004) 727 – 730
2. Experimental
400
where k is the Boltzmann’s constant. The trap depth E can now be found from the slope of a straight line obtained by plotting ln(I ) against 1=T . The temperature range used for the analysis was restricted in such a way that the TL intensity did not exceed 10–15% of the peak maximum temperature (McKeever, 1985).
Intensity / Arb. Unit
The polycrystalline SrAl2 O4 :Eu materials were prepared by a solid state reaction between strontium carbonate (SrCO3 ), aluminium oxide (Al2 O3 ) and europium oxide (Eu2 O3 ; 0:5 mol%), with boron oxide (B2 O3 ; 1 mol%) as a Lux in a reducing (N2 + 12% H2 ) atmosphere at 1300 ◦ C for 4 h. The luminescence spectra at 20 and 295 K were measured using an Ocean Optics PC2000 spectrometer equipped with an optical Hber. The luminescence decay curves were measured with a Lecroy 9350M oscilloscope. The third harmonic (355 nm) of a Nd:YAG laser was used as the UV excitation source. The excitation spectra were measured with a SLM Aminco SPF-500 spectrometer equipped with a 300 W xenon lamp. The persistent luminescence at 295 K was measured with a Perkin Elmer LS-5 spectrometer. The TL glow curves were measured with a RisH TL/OSL-DA-12 system using the heating rate of 5 ◦ C s−1 . Prior to the TL measurements, the samples were irradiated by a 20 W incandescent lamp for 10–120 s. The TL delay times from 3 min up to 10 h were used. The preheating method (McKeever, 1985) was used to study the number of the trapping levels in detail. The samples were heated to a temperature Tstop and then rapidly cooled down to the room temperature. Then the TL experiment was performed. This procedure was repeated for the Tstop values between 50 and 250 ◦ C in steps of 5◦ C. The initial rise method (McKeever, 1985) was used to determine the trap depths from the preheated glow curves. At the start of the glow curve the thermoluminescence intensity (I ) is, irrespective of the kinetic order, exponentially dependent on temperature E I = constant × exp − ; kT
2+
295 K
2+
SrAl2O4:Eu
300
20
λexc= 355 nm
20 K
15
200
10 AG, 295 K
100
5
0
0 450
500 550 Wavelength / nm
AG Intensity / Arb. Unit
728
600
Fig. 1. UV excited and persistent (AG) luminescence spectra of SrAl2 O4 : Eu2+ at 20 and 295 K.
nescence (Poort et al., 1995). The persistent luminescence spectrum at 295 K was identical to the UV excited one but was not at all observed at 20 K due to lack of su?cient thermal energy to release the trapped excitation energy. The emitting centre can be concluded to be the same Eu2+ ion in both luminescence phenomena. The luminescence decay curve of the 520 nm band was found nearly exponential ( = 1:23 s) at 20 K but multi-exponential ( 1 = 0:74 s) at 295 K (Fig. 2). The short rise observed at the beginning of the 520 nm curve could be due to energy transfer from the site corresponding to the 445 nm emission. On the other hand, the rise in the 295 K curve at longer times (ca. 1 s) was connected to the persistent luminescence phenomenon. The decay curve of the 445 nm band was very fast ( 1 = 0:42 s) at 20 K. The long decay tail in the 445 nm band decay curve is, with high probability, due to the slightly overlapping intense emission from the 520 nm band.
2+
100
SrAl2O4:Eu
520 nm
100
λexc= 355 nm
3.1. Luminescence The UV excited luminescence of SrAl2 O4 : Eu2+ was observed at 295 K as one broad but symmetric band centred at 520 nm, but at 20 K as two bands centred at 445 and 520 nm (Fig. 1). The luminescence is due to the 4f 6 5d 1 → 4f 7 transitions of the Eu2+ ion. According to structural studies (Schulze and M,uller-Buschbaum, 1981), there are two Sr sites in the SrAl2 O4 lattice. At low temperatures, the Eu2+ luminescence from both sites can be observed, while at higher temperatures an e?cient energy transfer between the sites causes the quenching of the higher energy lumi-
445 nm
Intensity / Arb. Unit
3. Results and discussion
20 K 0.0
0.1
0.2
10 λem= 520 nm, 20 K λem= 520 nm, 295 K λem= 445 nm, 20 K
1 0
1
2 Lifetime / µs
3
4
Fig. 2. Luminescence decay curves of SrAl2 O4 : Eu2+ at 20 and 295 K, with UV excitation.
T. Aitasalo et al. / Radiation Measurements 38 (2004) 727 – 730 2+
SrAl2O4:Eu
λem= 520 nm, 77 K
150
λem= 445 nm, 77 K
120 90 AG, λem= 520 nm, 295 K
60
2+
irradiation 120 s SrAl2O4:Eu delay 3 min
200 TL Intesity / Arb. Unit
Intensity / Arb. Unit
180
150 60 s 3 min
100
400
10 s 10 h
0
450
Fig. 3. Excitation spectra of the UV excited and persistent (AG) luminescence of SrAl2 O4 : Eu2+ at 77 and 295 K.
The excitation spectra of SrAl2 O4 : Eu2+ were dependent on the luminescence band. The excitation spectra at 77 K consisted of two main bands at 320 and 350 nm for the 520 nm emission but at 270 and 320 nm for the 445 nm emission (Fig. 3). No clear Hne structure due to the 7 F0 – 6 levels was observed. The excitation spectrum of persistent luminescence was observed at the same wavelength region but no clear band structure was observed. A strong increase in intensity below 280 nm is possibly due to an exciton. The excitation processes for the UV excited and persistent luminescence are signiHcantly di erent.
50
100
150 200 Temperature / °C
250
Fig. 4. TL curves of SrAl2 O4 : Eu2+ with di erent irradiation and delay times.
0.8
0.7 Etrap / eV
300 350 Wavelength / nm
10 s 3 min
50
30 250
729
0.6 2+
SrAl2O4:Eu 0.5
3.2. Thermoluminescence Two TL peaks at 80 and 160 ◦ C were observed when the delay time was 3 min (Fig. 4). These peaks correspond to a shallow and a deep traps, respectively. The kinetics of the TL peaks was studied by measuring the peak positions as a function of the irradiation dose. The Hrst TL peak position was observed to move slightly to higher and the second one to lower temperatures with increasing irradiation times. The TL peaks seemed to merge with long exposing times. The dose behaviour of the Hrst peak, which cannot be explained easily with simple the Hrst or second order kinetics, could be due to the di erent Hlling rates of individual overlapping or continuously distributed traps (Lakshmanan, 1999; Sakurai and Gartia, 1997). On the other hand, the second TL peak does not follow the behaviour of the Hrst one but obviously the second order kinetics. Only the high temperature peak was observed when the delay time was 10 h. Because the persistent luminescence was hardly observed when the delay time was 10 h, it can be concluded that the persistent luminescence at 295 K is mainly due to the slow fading of the low temperature TL peaks of SrAl2 O4 : Eu2+ . The intensity of the high temperature TL peak was also decreased and thus the very slow emptying of deeper traps or a feeding process to shallow traps is also possible.
60
90
120 150 Tstop / °C
180
Fig. 5. Trap depths of SrAl2 O4 : Eu2+ estimated by initial rise method from preheated glow curves.
Several traps were observed by the preheating and initial rise methods. The Hrst TL peak is due to at least three traps at ca. 0.55, 0.60 and 0:65 eV (Fig. 5). The energies of the traps are, however, very close to each other and thus the real amount of traps is uncertain. Continuous distribution of the traps may also be possible. The second TL peak corresponds probably to only a one trap at ca. 0:75 eV. 4. Conclusions Two luminescence bands for SrAl2 O4 : Eu2+ centred at 445 and 520 nm were observed at 20 K but only one band at 520 nm at 295 K. The change is probably due to energy transfer between the two Eu2+ sites at higher temperatures. The UV excited and persistent luminescence spectra were similar but the excitation spectra di ered. There is thus the same emitting Eu2+ centre but di erent excitation process
730
T. Aitasalo et al. / Radiation Measurements 38 (2004) 727 – 730
for the UV excited and persistent luminescence. Two TL peaks were observed at 80 and 160 ◦ C but mainly the low temperature one had e ect on the persistent luminescence at 295 K. Acknowledgements The authors thank the Academy of Finland (project #5066/2000), the Graduate School of Materials Research (Turku, Finland), the University of Turku and the Marie Curie Fellowship program of the European Union for Hnancial support. References Akiyama, M., Xu, C.-N., Liu, Y., Nonaka, K., Watanabe, T., 2002. InLuence of Eu, Dy co-doped strontium aluminate composition on mechanoluminescence intensity. J. Lumin. 97, 13–18. H,ols,a, J., Jungner, H., Lastusaari, M., Niittykoski, J., 2001. Persistent luminescence of Eu2+ doped alkaline earth aluminates, MAl2 O4 : Eu2+ . J. Alloys Compd. 94–95, 159–163. Jia, W., Yuan, H., Lu, L., Liu, H., Yen, W.M., 1998. Phosphorescent dynamics in SrAl2 O4 : Eu2+ ; Dy3+ single crystal Hbers. J. Lumin. 76&77, 424–428. Kowatari, M., Koyama, D., Satoh, Y., Iinuma, K., Uchida, S., 2002. The temperature dependence of luminescence from a
long-lasting phosphor exposed to ionizing radiation. Nucl. Instrum. Methods Phys. Res. A 480, 431–439. Lakshmanan, A.R., 1999. Photoluminescence and thermostimulated luminescence processes in rare-earth-doped CaSO4 phosphors. Prog. Mater. Sci. 44, 1–187. Matsuzawa, T., Aoki, Y., Takeuchi, N., Murayama, Y., 1996. A new long phosphorescence phosphor with high brightness, SrAl2 O4 : Eu2+ ; Dy3+ . J. Electrochem. Soc. 143, 2670–2673. McKeever, S.W.S., 1985. Thermoluminescence of Solids, Cambridge University Press, New York, pp. 76–80. Murayama, Y., 1999. Luminous paints. In: Shionoya, S., Yen, W.M. (Eds.), Phosphor Handbook. CRC Press, Boca Raton, FL, USA, pp. 651–658. Nakazawa, E., Mochida, T., 1997. Traps in SrAl2 O4 : Eu2+ phosphor with rare-earth doping. J. Lumin. 72–74, 236–237. Poort, S.H.M., Blockpoel, W.P., Blasse, G., 1995. Luminescence of Eu2+ in barium and strontium aluminate and gallate. Chem. Mater. 7, 1547–1551. Sakurai, T., Gartia, R.K., 1997. Evidence of continuous trap distribution in the glow curve of a Brown microcline. J. Appl. Phys. 82, 5722–5727. Schulze, A.-R., M,uller-Buschbaum, H.K., 1981. Zur Struktur von monoklinem SrAl2 O4 . Z. Anorg. Allg. Chem. 475, 205–210. Xu, C.-N., Watanabe, T., Akiyama, M., Zheng, X.-G., 1999. Direct view of stress distribution in solid by mechanoluminescence. Appl. Phys. Lett. 74, 2414–2416. Xu, C.-N., Zheng, X.-G., Akiyama, M., Nonaka, K., Watanabe, T., 2000. Dynamic visualization of stress distribution by mechanoluminescence. Appl. Phys. Lett. 76, 179–181.