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Studies on mechanoluminescence from SrAl2O4:Eu, Dy phosphor
Materials Chemistry and Physics 80 (2003) 20–22
Material science communication
Studies on mechanoluminescence from SrAl2 O4 :Eu, Dy phosphor Yuan-Hua Lin∗ , Zhimin Dang, Yuan Deng, Ce Wen Nan State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsing Hua University, Beijing 100 084, PR China Received 22 April 2002; received in revised form 11 September 2002; accepted 24 September 2002
Abstract Luminescence induced by mechanical stress has been observed in the SrAl2 O4 :Eu, Dy phosphors. The Dy3+ doping concentration and the loading pressure show strong influence on the intensity but the Eu2+ doped samples show little effect of mechanoluminescence (ML). The origin of the ML may be ascribed to the holes released due to the stress on the phosphors and then recombine with metastable Eu+1 . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanoluminescence (ML); Stress; Phosphor; Hole
1. Introduction Mechanoluminescence (ML) is an interesting luminescent phenomenon, which is a light emission caused by mechanical stimuli such as grinding, cutting, collision, striking, friction, and so on [1–3]. According to a rough estimate, about 50% of inorganic salts and organic molecular solids can exhibit ML. The effective use of this phenomenon can be employed to detect mechanical stress remotely by converting mechanical energy into visible lights [4,5]. Usually, piezoelectric materials were employed to detect mechanical stress, but the technique required electrodes in physical contact with the test materials to detect the electrical signals. The ML materials can be used to detect mechanical stress remotely if the emission produced by ML is strong enough, but the intensity in such cases is generally so low as to make practical application difficult. Up to now, nondestructive ML has been observed only in a limited number of materials. SrAl2 O4 -based phosphor co-doped with rare earth ions with excellent photoresistance, great brightness, long-lasting time, no radiation and environmental capability was developed recently, which resulted in their wide applications in many fields. The thermoluminescence (TL) and photoluminescence (PL) have been observed in this phosphor [6,7]. In this paper, ML was observed for the first time in the SrAl2 O4 :Eu, Dy phosphor. The effects of the concentration of Eu and Dy on the properties of ML have been investigated. The results indicate that both the load applied to ∗ Corresponding author. Tel.: +86-10-62773741; fax: +86-10-62773587. E-mail address: [email protected] (Y.-H. Lin).
the SrAl2 O4 :Eu, Dy phosphor and the Dy3+ concentration doped have great influence on the intensity of the ML.
2. Experimental SrCO3 , ␣-Al2 O3 , Eu(NO3 )3 , Dy(NO3 )3 and H3 BO3 were used as the raw materials (all reagents are of analytical purity). Six kinds of phosphors with various doping concentrations of Eu and Dy shown in Table 1 were successfully prepared by a traditional ceramic synthesis method as reported previously [8]. The intensity of the ML was measured by a home-designed apparatus, while the samples were irradiated by 365 nm lights for 5 min. All measurements were carried out at room temperature.
3. Results and discussion The sample to be studied was placed on a flat plate (all phosphor samples decayed about 30 h after excitation by 365 nm light for 5 min in order to eliminate the effect of afterglow). Pressure was applied to the samples as the Reynolds’ method [9]. A weight of 500 g was dropped through a distance of 20 cm, and the optical detector was placed and inclined near the phosphor sample about 3 cm at the dark place. Correspondingly, the ML can be observed while a sudden load was applied to the phosphor samples, and the intensity of the ML varied with the concentration of Dy3+ doped phosphors (S-1, S-2, S-3, and S-4) shown in Table 1. In addition, the ML results in Table 1 indicate that the changes of the concentration of Eu2+ doped phos-
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 7 8 - 9
Y.-H. Lin et al. / Materials Chemistry and Physics 80 (2003) 20–22
21
Table 1 Influence of Eu2+ and Dy3+ concentration on the ML intensitya Samples
Composition
ML intensity (mcd m−2 )
S-1 S-2 S-3 S-4 S-5 S-6
Sr0.99 Al2 O4 :Eu0.01 Sr0.98 Al2 O4 :Eu0.01 , Dy0.01 Sr0.97 Al2 O4 :Eu0.01 , Dy0.02 Sr0.96 Al2 O4 :Eu0.01 , Dy0.03 Sr0.975 Al2 O4 :Eu0.005 , Dy0.02 Sr0.965 Al2 O4 :Eu0.015 , Dy0.02
8.5 18.3 37.4 24.5 37.2 37.7
a
A weight of 500 g was employed.
phors (S-3, S-5, and S-6) have very little effect on the ML intensity. The color of ML of these samples is very similar to that of photoluminescence after excited by the UV lights, which can be observed obviously in the dark place. In fact, the radius of Sr2+ ions (1.13 Å) is nearly equivalent to that of Eu2+ (1.12 Å), and so there are very little crystallographic distortions when Eu2+ enters Sr2+ sites. But when the Dy3+ ions are co-doped in the SrAl2 O4 host, some distortion occurs due to the different radius and valency of Dy3+ (0.99 Å) and Sr2+ , the defect chemistry equation can be expressed as follows: Dy2 O3 → 2DySr • + VSr + 3OxO
(1)
Therefore, Sr vacancies will be produced for maintaining the charge balance. In fact, some trap levels can be produced due to these vacancies. As previously reported [10,11], the thermoluminescence of aluminates-based phosphors (e.g. SrAl2 O4 , CaAl2 O4 ) indicated the thermoluminescent properties were affected by the defects formed by codoping with rare earth ions (e.g. Dy3+ , Nd3+ ). Thus, these Sr vacancies may play as an important role to produce the ML. Table 2 shows the effects of different loading pressure on the ML properties of Sr0.97 Al2 O4 : Eu0.01 , Dy0.02 phosphor (S-3), and indicates that the intensity of the ML increased with loading pressure. Fig. 1 can then be proposed to understand the mechanism of ML for SrAl2 O4 :Eu, Dy phosphors, which accompany the following process: Stress + Filled traps(holes) → Filled traps + Released holes
(2)
Released holes + (Eu+ )∗ → (Eu2+ )∗
(3)
(Eu2+ )∗ → Eu2+ + hν
(4)
As the previous report on the long afterglow mechanism of aluminates-based phosphors [12], the pairs of hole and Table 2 Effects of the loading weight on the intensity of the ML for Sr0.97 Al2 O4 : Eu0.01 , Dy0.02 phosphor (sample S-3) Different weight (g)
ML intensity (mcd m−2 )
50 200 500
12.4 20.8 37.4
Fig. 1. Schematic graph of the mechanism of ML for SrAl2 O4 :Eu, Dy phosphors.
electron will be produced when phosphors are excited by lights, and some of free holes are captured by trap levels. When the excitation source is removed, the trapped holes are released thermally to the valence band and migrate to recombine with the metastable state Eu and lead to the long afterglow. Therefore, if the stress are loaded on the phosphor, some of the filled traps can release the captured holes, and then the released holes can recombine with metastable (Eu+ )∗ and transfer as to another metastable (Eu2+ )∗ form, finally relax and return to the ground state of Eu2+ accompanying the luminescence. Thus, the ML intensity should be influenced by the amount of filled traps and the depth of trap levels. As in the previous report, Dy played an important role on prolonging the afterglow time and acted as the trap level in the SrAl2 O4 :Eu, Dy phosphor [13]. The results of Table 1 indicate that a stronger ML intensity will be observed while increasing the Dy3+ doping concentration. But when the doping concentration reached 0.03 mol per aluminate, the ML intensity decreased. It is possible that too much traps recapture the free holes released by the loaded stress and resulted in the ML quenching.
4. Conclusions Six samples of SrAl2 O4 :Eu, Dy phosphors with different doping concentrations have been prepared via a ceramic synthesis method, and all phosphor samples show the ML. The results indicate that the Dy3+ doping concentration and the loading pressure show strong influence on the ML intensity but the Eu2+ doped sample show little effect. The origin of the ML may be caused by the holes released by the stress on the phosphors.
Acknowledgements This work was financially supported by NSF of China, under grant No. 59872016.
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Y.-H. Lin et al. / Materials Chemistry and Physics 80 (2003) 20–22
References [1] A.J. Walton, Adv. Phys. 26 (1977) 887. [2] G. Alzetta, I. Chudacek, R. Scarmozzino, Phys. Status Solidi A 1 (1970) 775. [3] Y. Enomoto, H. Hashimoto, Nature 346 (1990) 641. [4] C.N. Xu, M. Akiyama, P. Sun, T. Watanabe, Appl. Phys. Lett. 70 (1997) 1639. [5] N. Takada, S. Hieda, J. Sugiyama, et al., Synth. Met. 111 (2000) 587. [6] Y.H. Lin, Z.T. Zhang, F. Zhang, et al., Mater. Chem. Phys. 65 (2000) 103.
[7] W. Jia, H. Yuan, L. Lu, et al., J. Cryst. Growth 200 (1999) 179. [8] Y.H. Lin, Z.T. Zhang, Z.L. Tang, et al., Mater. Chem. Phys. 76 (2001) 142. [9] G.T. Reynolds, J. Lumin. 75 (2000) 295. [10] M. Ohta, M. Maruyama, T. Hayakawa, T. Nishijo, J. Ceram. Soc. Jpn. 108 (2000) 284. [11] Y.H. Lin, Z.L. Tang, Z.T. Zhang, et al., Mater. Eng. 8 (2001) 29. [12] T. Matsuzawa, T. Nabae, T. Katsumata, et al., J. Electrochem. Soc. 144 (1997) L243. [13] H. Yamamoto, T. Matsuzawa, J. Lumin. 72 (1997) 287.
Material science communication
Studies on mechanoluminescence from SrAl2 O4 :Eu, Dy phosphor Yuan-Hua Lin∗ , Zhimin Dang, Yuan Deng, Ce Wen Nan State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsing Hua University, Beijing 100 084, PR China Received 22 April 2002; received in revised form 11 September 2002; accepted 24 September 2002
Abstract Luminescence induced by mechanical stress has been observed in the SrAl2 O4 :Eu, Dy phosphors. The Dy3+ doping concentration and the loading pressure show strong influence on the intensity but the Eu2+ doped samples show little effect of mechanoluminescence (ML). The origin of the ML may be ascribed to the holes released due to the stress on the phosphors and then recombine with metastable Eu+1 . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanoluminescence (ML); Stress; Phosphor; Hole
1. Introduction Mechanoluminescence (ML) is an interesting luminescent phenomenon, which is a light emission caused by mechanical stimuli such as grinding, cutting, collision, striking, friction, and so on [1–3]. According to a rough estimate, about 50% of inorganic salts and organic molecular solids can exhibit ML. The effective use of this phenomenon can be employed to detect mechanical stress remotely by converting mechanical energy into visible lights [4,5]. Usually, piezoelectric materials were employed to detect mechanical stress, but the technique required electrodes in physical contact with the test materials to detect the electrical signals. The ML materials can be used to detect mechanical stress remotely if the emission produced by ML is strong enough, but the intensity in such cases is generally so low as to make practical application difficult. Up to now, nondestructive ML has been observed only in a limited number of materials. SrAl2 O4 -based phosphor co-doped with rare earth ions with excellent photoresistance, great brightness, long-lasting time, no radiation and environmental capability was developed recently, which resulted in their wide applications in many fields. The thermoluminescence (TL) and photoluminescence (PL) have been observed in this phosphor [6,7]. In this paper, ML was observed for the first time in the SrAl2 O4 :Eu, Dy phosphor. The effects of the concentration of Eu and Dy on the properties of ML have been investigated. The results indicate that both the load applied to ∗ Corresponding author. Tel.: +86-10-62773741; fax: +86-10-62773587. E-mail address: [email protected] (Y.-H. Lin).
the SrAl2 O4 :Eu, Dy phosphor and the Dy3+ concentration doped have great influence on the intensity of the ML.
2. Experimental SrCO3 , ␣-Al2 O3 , Eu(NO3 )3 , Dy(NO3 )3 and H3 BO3 were used as the raw materials (all reagents are of analytical purity). Six kinds of phosphors with various doping concentrations of Eu and Dy shown in Table 1 were successfully prepared by a traditional ceramic synthesis method as reported previously [8]. The intensity of the ML was measured by a home-designed apparatus, while the samples were irradiated by 365 nm lights for 5 min. All measurements were carried out at room temperature.
3. Results and discussion The sample to be studied was placed on a flat plate (all phosphor samples decayed about 30 h after excitation by 365 nm light for 5 min in order to eliminate the effect of afterglow). Pressure was applied to the samples as the Reynolds’ method [9]. A weight of 500 g was dropped through a distance of 20 cm, and the optical detector was placed and inclined near the phosphor sample about 3 cm at the dark place. Correspondingly, the ML can be observed while a sudden load was applied to the phosphor samples, and the intensity of the ML varied with the concentration of Dy3+ doped phosphors (S-1, S-2, S-3, and S-4) shown in Table 1. In addition, the ML results in Table 1 indicate that the changes of the concentration of Eu2+ doped phos-
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 7 8 - 9
Y.-H. Lin et al. / Materials Chemistry and Physics 80 (2003) 20–22
21
Table 1 Influence of Eu2+ and Dy3+ concentration on the ML intensitya Samples
Composition
ML intensity (mcd m−2 )
S-1 S-2 S-3 S-4 S-5 S-6
Sr0.99 Al2 O4 :Eu0.01 Sr0.98 Al2 O4 :Eu0.01 , Dy0.01 Sr0.97 Al2 O4 :Eu0.01 , Dy0.02 Sr0.96 Al2 O4 :Eu0.01 , Dy0.03 Sr0.975 Al2 O4 :Eu0.005 , Dy0.02 Sr0.965 Al2 O4 :Eu0.015 , Dy0.02
8.5 18.3 37.4 24.5 37.2 37.7
a
A weight of 500 g was employed.
phors (S-3, S-5, and S-6) have very little effect on the ML intensity. The color of ML of these samples is very similar to that of photoluminescence after excited by the UV lights, which can be observed obviously in the dark place. In fact, the radius of Sr2+ ions (1.13 Å) is nearly equivalent to that of Eu2+ (1.12 Å), and so there are very little crystallographic distortions when Eu2+ enters Sr2+ sites. But when the Dy3+ ions are co-doped in the SrAl2 O4 host, some distortion occurs due to the different radius and valency of Dy3+ (0.99 Å) and Sr2+ , the defect chemistry equation can be expressed as follows: Dy2 O3 → 2DySr • + VSr + 3OxO
(1)
Therefore, Sr vacancies will be produced for maintaining the charge balance. In fact, some trap levels can be produced due to these vacancies. As previously reported [10,11], the thermoluminescence of aluminates-based phosphors (e.g. SrAl2 O4 , CaAl2 O4 ) indicated the thermoluminescent properties were affected by the defects formed by codoping with rare earth ions (e.g. Dy3+ , Nd3+ ). Thus, these Sr vacancies may play as an important role to produce the ML. Table 2 shows the effects of different loading pressure on the ML properties of Sr0.97 Al2 O4 : Eu0.01 , Dy0.02 phosphor (S-3), and indicates that the intensity of the ML increased with loading pressure. Fig. 1 can then be proposed to understand the mechanism of ML for SrAl2 O4 :Eu, Dy phosphors, which accompany the following process: Stress + Filled traps(holes) → Filled traps + Released holes
(2)
Released holes + (Eu+ )∗ → (Eu2+ )∗
(3)
(Eu2+ )∗ → Eu2+ + hν
(4)
As the previous report on the long afterglow mechanism of aluminates-based phosphors [12], the pairs of hole and Table 2 Effects of the loading weight on the intensity of the ML for Sr0.97 Al2 O4 : Eu0.01 , Dy0.02 phosphor (sample S-3) Different weight (g)
ML intensity (mcd m−2 )
50 200 500
12.4 20.8 37.4
Fig. 1. Schematic graph of the mechanism of ML for SrAl2 O4 :Eu, Dy phosphors.
electron will be produced when phosphors are excited by lights, and some of free holes are captured by trap levels. When the excitation source is removed, the trapped holes are released thermally to the valence band and migrate to recombine with the metastable state Eu and lead to the long afterglow. Therefore, if the stress are loaded on the phosphor, some of the filled traps can release the captured holes, and then the released holes can recombine with metastable (Eu+ )∗ and transfer as to another metastable (Eu2+ )∗ form, finally relax and return to the ground state of Eu2+ accompanying the luminescence. Thus, the ML intensity should be influenced by the amount of filled traps and the depth of trap levels. As in the previous report, Dy played an important role on prolonging the afterglow time and acted as the trap level in the SrAl2 O4 :Eu, Dy phosphor [13]. The results of Table 1 indicate that a stronger ML intensity will be observed while increasing the Dy3+ doping concentration. But when the doping concentration reached 0.03 mol per aluminate, the ML intensity decreased. It is possible that too much traps recapture the free holes released by the loaded stress and resulted in the ML quenching.
4. Conclusions Six samples of SrAl2 O4 :Eu, Dy phosphors with different doping concentrations have been prepared via a ceramic synthesis method, and all phosphor samples show the ML. The results indicate that the Dy3+ doping concentration and the loading pressure show strong influence on the ML intensity but the Eu2+ doped sample show little effect. The origin of the ML may be caused by the holes released by the stress on the phosphors.
Acknowledgements This work was financially supported by NSF of China, under grant No. 59872016.
22
Y.-H. Lin et al. / Materials Chemistry and Physics 80 (2003) 20–22
References [1] A.J. Walton, Adv. Phys. 26 (1977) 887. [2] G. Alzetta, I. Chudacek, R. Scarmozzino, Phys. Status Solidi A 1 (1970) 775. [3] Y. Enomoto, H. Hashimoto, Nature 346 (1990) 641. [4] C.N. Xu, M. Akiyama, P. Sun, T. Watanabe, Appl. Phys. Lett. 70 (1997) 1639. [5] N. Takada, S. Hieda, J. Sugiyama, et al., Synth. Met. 111 (2000) 587. [6] Y.H. Lin, Z.T. Zhang, F. Zhang, et al., Mater. Chem. Phys. 65 (2000) 103.
[7] W. Jia, H. Yuan, L. Lu, et al., J. Cryst. Growth 200 (1999) 179. [8] Y.H. Lin, Z.T. Zhang, Z.L. Tang, et al., Mater. Chem. Phys. 76 (2001) 142. [9] G.T. Reynolds, J. Lumin. 75 (2000) 295. [10] M. Ohta, M. Maruyama, T. Hayakawa, T. Nishijo, J. Ceram. Soc. Jpn. 108 (2000) 284. [11] Y.H. Lin, Z.L. Tang, Z.T. Zhang, et al., Mater. Eng. 8 (2001) 29. [12] T. Matsuzawa, T. Nabae, T. Katsumata, et al., J. Electrochem. Soc. 144 (1997) L243. [13] H. Yamamoto, T. Matsuzawa, J. Lumin. 72 (1997) 287.