- Email: [email protected]
Dy-doped Sr3MgSi2O8
ARTICLE IN PRESS
Journal of Luminescence 114 (2005) 131–136 www.elsevier.com/locate/jlumin
Effects of dopant concentrations on phosphorescence properties of Eu/Dy-doped Sr3MgSi2O8 A.A. Sabbagh Alvania,, F. Moztarzadehb, A.A. Sarabia a
Department of Polymer Engineering, Amirkabir University of Technology, P.O.Box 15875-4413, Tehran, Iran b Department of Biomedical Engineering, Amirkabir University of Technology, 424, Hafez Ave., Tehran, Iran Received 17 July 2004 Available online 2 February 2005
Abstract Long afterglow Sr3MgSi2O8: Eu, Dy phosphor with high brightness was prepared by sintering at high temperature and weak reductive atmosphere. The luminescent properties of this photoluminescent pigment were studied systematically by investigating concentration effects. The analytical results indicated that the main emission peaks appear at 482 nm. The excitation and emission spectra of this phosphor show that both of them are broadband. This is ascribed to the 4f 7 ! 4f 6 5d1 transition of Eu2+ in the pigment matrix, which is in good agreement with the calculated value of 470 nm, and implies that luminescent centers Eu2+ occupy the deca-coordinated Sr2+ sites with the host of Sr3MgSi2O8. r 2005 Elsevier B.V. All rights reserved. Keywords: Afterglow; Photoluminescent pigment; Emission; Luminescence
1. Introduction It is generally believed that the phosphorescence of Eu2+ is due to 4f 7 ! 4f 6 5d1 transition [1,2]. Although, 4f electrons of Eu2+ are not sensitive to the changes of the crystal field strength due to the shielding function of outer shell, the peak positions in the emission spectra strongly, depend on the nature of Eu2+ surroundings in the host lattices.
Corresponding author.
E-mail address: [email protected] (A.A. Sabbagh Alvani).
Therefore, Eu2+ ions can emit variety of visible lights in various crystal fields [3]. Alkaline earth silicates are useful luminescent hosts since they have stable crystal structure, high physical and chemical stability. They have also excellent weather resistance and exhibit blue to green emission under excitation with ultraviolet rays and/or visible light rays [1,2]. There are many investigations on rare earth ion activation of these hosts. Barry prepared Ca3MgSi2O8:Eu, Ba3MgSi2O8: Eu, phosphor by prefiring at 600 1C in air using NH4Cl as a flux, and then firing at 1200 1C for 4 h in atmosphere of 96% N2/4% H2 [4]. He found the location of the main emission peaks at
0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.12.012
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136
room temperature to be at 475 nm. Huang and coworkers [5] prepared Ca3MgSi2O8:Ce phosphor and found that two kinds of emission centers exist in the host. However, the long afterglow was not observed in these phosphors. Lin and co-workers [6] prepared Ca3MgSi2O8: Eu2+ , Dy3+ phosphor with blue-emitting long afterglow. In further investigations, they examined luminescence of a series of host materials Viz. R3MgSi2O8 (R ¼ Ca, Sr, Ba) in comparative study [7]. In spite of excellent phosphorescence properties of Sr3MgSi2O8 host material, less attention has been paid to it in the literature. In this paper, we aim to reveal phosphorescence behavior of this excellent host material via a systematic research, particularly concentration effects.
Intensity (counts)
132
14900 13900 12900 11900 10900 8900 9900 7900 6900 5900 4900 3900 2900 1900 900 -100
Sr3Mgsi208 : Eu,dy
Eu/Dy 2.0 1.5 1.0 0.5
5
15
25
35
45
55
65
Two theta (°) Fig. 1. The XRD patterns of SMS-ED phosphor with ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
180 160
2. Experimental procedure
3. Results and discussion XRD pattern of SMS-ED sample is shown in Fig. 1. The sample was sintered at 1200 1C and
Relative Intensity
140
SrCO3, 4MgCO3 Mg(OH)2 5H2O, SiO2, Eu2O3, Dy2O3, and NH4Cl were used as the raw materials. All of the applied materials are highly purified. Small quantities of NH4Cl (0.5 m/o per mol of one silicate) were used as the flux material. The raw materials were mixed homogeneously by the fast mill for 5 h. A new long afterglow blueemitting SMS-ED photoluminescent phosphor was prepared at 1200 1C for 3 h and weak reductive atmosphere of flowing 4%H2–96%N2 gas. Then optical properties and phase composition of the product were fully characterized. The phosphor powders were analyzed using a Siemens D5 X-Ray Diffractometer (CuKa and l: 1.540510 A˚). ‘‘Luminescence spectrometers LS5/ 5B’’ fluorescence spectrophotometer was used to detect the excitation and emission spectra of the products. The decay curve of afterglow was measured by a brightness meter, and the powder samples were irradiated by a standard lamp for 15 min. All measurements were carried out at room temperature.
120 100 80 60 40 20 0 -20 300 325 350 375 400 425 450 475 500 525 550 575 600
Excitation Wavelength (nm) Fig. 2. The excitation spectra of SMS-ED phosphor with ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
reductive atmosphere for 3 h. The ratios of Eu/ Dy ¼ 0.5, 1.0, 1.5, 2.0 were used. It is evident from Fig. 1 that the co-doped Eu and Dy ions have little influence on the structure of the luminescent materials. All peaks are assigned to the phase of Sr3MgSi2O8 as well [8]. The doped Eu2+ ions in SMS host give rise to a blue emission under UV excitation. Figs. 2 and 3 show the excitation and emission spectra of SMSED at room temperature. As shown in Figs. 2 and 3, two broad excitation bands centered at 356 and 395 nm were observed. The main emission peak at 482 nm is seen as the
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136 180 160
Sr3Mgsi2O8 : Eu,Dy a37 : SMS-ED 37 Emission (Eu/Dy = 0.5)
140
a38 : SMS-ED 38 Emission (Eu/Dy =1.0)
120
a39 : SMS-ED 39 Emission (Eu/Dy =1.5)
Table 1 The calculated value of position in energy of the lower d-band edge for Eu2+ occupying different crystallographic sites
With exctitation 356 nm
Relative Intensity
With exctitation 356 nm
Ions
n
r (nm)
Energy (cm 1)
Emitting wavelength (nm)
Sr2+
6 8 10 12
0.118 0.126 0.136 0.144
16647 18886 21148 23151
600 529 470 431
Mg2+
6 8
0.072 0.089
14295 16679
699 599
With exctitation 356 nm
100
a40 : SMS-ED 40 Emission (Eu/Dy =2.0) With exctitation 356 nm
80 a40
60 40
a38 a37
20
133
a39
0 -20 300 325 350 375 400 425 450 475 500 525 550 575 600
Emission Wavelength (nm) Fig. 3. The emission spectra of SMS-ED phosphor with ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
typical emission of Eu2+ ascribed to the 4f 7 ! 4f 6 5d1 transition. However, no special emission peaks of Eu3+ were observed in the spectra. This means that Eu3+ in the crystal matrix has been completely reduced to Eu2+. The special Dy3+ emission peak is not present, which may be ascribed to the function of the hole or electron traps and energy transporting for Dy3+. It is not the cause of the luminescent centers in the SMS-ED host crystal lattice. The doped materials are rare earth elements such as Dy3+ and Eu3+. Therefore, for obtaining Eu2+, a one-step-reduction process may be performed. Using thermodynamic relations and assumption of reduction process, oxide forms of rare earth elements react with H2 and change to a reduced form. Thus, the following relations can be considered: Eu2 O3 þ H2 ! 2EuO þ H2 O;
(1)
Dy2 O3 þ H2 ! 2DyO þ H2 O:
(2)
Using Gibbs free energy, it could be shown: DG ¼ DG þ RT Ln K; K Eu1200 C ¼ 2:16 10 8 ;
(3) K Dy1200 C ffi 0:
Based on Eq. (3), it is not possible to reduce Dy3+ to Dy2+. Thus it could be considered that, this element is only existed in the form of tri-valant
(Dy3+). Considering non-zero equilibrium constant of Eu2+ production, there is a possibility of having Eu2+ in the reaction. Lin et al., have applied the rare earth ions as codoped elements in their investigations. They found that these elements have no strong effect on the emission wavelength. Based on this result they concluded that rare earth elements have little effect on luminescent center in Eu environment as well [9]. As previously reported [10], the energy position for the lower d-band edge in Eu2+ or Ce3+, for various compounds can be determined by E ¼ Q½1 ðV =4Þ1=v 10 ðn ea rÞ=8 :
(4)
Eq. (4) has a good agreement with experimental results, where Q is the position of energy for the lower d-band edge for the free ion, (Q ¼ 34; 000 cm 1 for Eu2+); V is the valence of the active cation, (V ¼ 2); n is the number of anions in the immediate shell about this ion, and ea is the electron affinity of the atoms that form the anions (ea ¼ 2.5 eV); r is the radius of the host cation replaced by the active cation in the host crystal. The values of position in energy of the lower d-band edge for Eu2+ occupying different crystallographic sites are shown in Table 1. Ionic radius of the elements incorporated in the phosphorescent material are shown in Table 2. According to Table 2, ionic radius of Eu2+ is equal to 1.35 (n ¼ 10), at the same coordination number the ionic radius of Sr2+ is equal to 1.36. As it is illustrated in Table 2, these two radius are close to each other and as a result Eu2+ could be
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136
Table 2 Ionic radius element in phosphor 2+
2+
8
2+
2+
n
Eu
Sr
Mg
Dy
6 8 10 12
1.17 1.25 1.35 —
1.18 1.26 1.36 1.44
0.57 0.72 0.89 —
0.91 1.03 — —
Luminescence intensity (mcd.m-2)
134
7 6 5 4 3 2 1 0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Time (hour)
Fig. 5. The typical decay curve of Sr3MgSi2O8:Eu,Dy phosphor.
Fig. 4. The decay curves of phosphor With ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
replaced in Sr2+ site. This is not the case for Mg2+. It is in good agreement with the work of Lin et al. [7]. The luminescent decay curves of different phosphors co-doped with various amount of Eu2+ under excitation by standard lamp for 15 min at room temperature are shown in Fig. 4. According to Fig. 4, all of the mentioned ratios (e.g. Eu/Dy ¼ 0.5, 1.0, 1.5, 2.0) have a rapid decay and then a long-lasting phosphorescence. At constant Dy3+ content (0.01 mol) in SMS-ED phosphor, the intensity of emission at 482 nm increases by increasing the Eu2+ content. When the ratio is greater than 1.5, the luminescence intensity is increased accordingly. This is straightforward to the result of increasing Eu concentration. The typical decay curve of Sr3MgSi2O8: Eu, Dy phosphor is shown in Fig. 5. The initial afterglow intensity of phosphorescent material is 58.5 mcd/ m2. After removing excitation source, it is obvious
that, its afterglow drops on 5.3 mcd/m2 and lasts for more than 10 h. The duration can reach over 10 h in the lack of the light perception by the darkadapted human eye. Lin et al. have reported 5 h afterglow duration in their works [6]. They believe that this effect is due to the presence of strontium in the structure of the phosphorescent product. Like the mechanism of the long afterglow of SMSED phosphor, the co-doped Dy3+ may play an important role in prolonging the duration. In the previous reports regarding strontium aluminates phosphors (e.g. SrAl2O4: Eu, Dy [6,11,12]) various researchers believed that the co-doped Dy3+ works as the hole trap levels and prolonged the afterglow. Thus, in the SMS-ED host, it is proposed that the Dy3+ may also act as trap levels, which capture the free holes. Then, it will be released from the trapped holes, and recombine with electrons accompanying the luminescence. The schematic luminescent mechanism can be found in Fig. 5 of Ref. [13]. For the sake of clarity, a typical scheme is illustrated in Fig. 6. As the electrons located at ground level are excited, they move to the excited state, and a hole is created in their place. In the case of excited electron, due to its instability, they tend to return to hole location immediately (states 1 and 2), which is called fluorescence phenomenon. On the other hand, due to the existence of Dy3+, a trap level is made in the structure. Because of this trap level, energy level of the hole decreases and reaches the valance band (state 3), and immigrates in this level
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136
135
Fig. 6. The schematic graph of luminescent mechanism of SMS-ED phosphor.
toward trap levels (state 4) [13]. Thus the hole would be trapped in trap level (state 5). On the other hand, the electron in excited state tends to return to ground level or stable energy level, also due to the instability of trap level, the hole tends to return to the ground level, as well. Thus, excited electron and the hole will reach together in ground level during the time (long afterglow). This is called phosphorescence phenomenon. According to the comparative study of Lin et al., the most suitable depth among R3MgSi2O8 (R ¼ Ca, Sr, Ba) host materials is related to Sr3MgSi2O8. Since Dy trap level is different in various host materials, it is of great importance to find a host material providing a suitable depth. In the class of R3MgSi2O8, Sr has a superior position among alkaline earth metals. Therefore, it is the reason for our particular attention to this new host material, Sr3MgSi2O8.
the long afterglow properties can be achieved in our research applied conditions. The main excitation peaks are at 356 and 395 nm, respectively. The main emission peak is measured to be at 482 nm. These data could be the sign of 4f7-4f65d1 transition of Eu2+ in pigment matrix. Like luminescence mechanism of SMS-ED pigment, the co-doped Dy3+ ions played an important role as the hole trap levels and captured the free holes, and thus prolonged the duration.
4. Conclusions
References
The work was devoted to study Sr3MgSi2O8 photoluminescent pigment co-doped with Eu, Dy ion. Based on this investigation, it is proposed that
Acknowledgements The authors acknowledge Material and Energy Research Center for their help in carrying out characterization tests. In addition, helpful discussions and suggestions of the referees are gratefully acknowledged.
[1] G. Blasse, W.L. Wanamaker, et al., Philips Res. Rep. 23 (1968) 189. [2] K. Yamazaki, H. Nakabayashi, Y. Kotera, A. Ueno, J. Electrochem. Soc. 133 (1986) 657.
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[3] G. Blasse, A. Bril, Philips-Res. Rep. 23 (1968) 201. [4] T.L. Barry, J. Electrochem. Soc. 115 (1968) 733. [5] L. Huang, X. Zhang, X. Liu, J. Alloys Compounds 305 (2000) 14. [6] Y. Lin, Z. Zhang, Z. Tang, X. Wang, J. Zhang, Z. Zheng, J. Eur. Ceram. Soc. 21 (2001) 683. [7] Y. Lin, Z. Tang, Z. Zhang, C.W. Nan, J. Alloys Compounds 348 (2003) 76. [8] Klasens, et al., J. Electrochem. Soc. 104 (1957) 93.
[9] Y. Lin, Z. Tang, et al., J. Eur. Ceram. Soc. 23 (2003) 175. [10] L.G. Van Uitert, J. Lumin. 29 (1984) 1. [11] M. Ohta, M. Maruyama, T. Hayakawa, J. Ceram. Soc. Japan 108 (2000) 284. [12] Y. Lin, Z. Zhang, Z. Tang, J. Zhang, Z. Zheng, X. Lu, Mater. Chem. Phys. 70 (2001) 156. [13] Y. Lin, Z. Tang, Z. Zhang, J. Zhang, Q. Chen, Mater. Sci. Eng. B 86 (2001) 79.
Journal of Luminescence 114 (2005) 131–136 www.elsevier.com/locate/jlumin
Effects of dopant concentrations on phosphorescence properties of Eu/Dy-doped Sr3MgSi2O8 A.A. Sabbagh Alvania,, F. Moztarzadehb, A.A. Sarabia a
Department of Polymer Engineering, Amirkabir University of Technology, P.O.Box 15875-4413, Tehran, Iran b Department of Biomedical Engineering, Amirkabir University of Technology, 424, Hafez Ave., Tehran, Iran Received 17 July 2004 Available online 2 February 2005
Abstract Long afterglow Sr3MgSi2O8: Eu, Dy phosphor with high brightness was prepared by sintering at high temperature and weak reductive atmosphere. The luminescent properties of this photoluminescent pigment were studied systematically by investigating concentration effects. The analytical results indicated that the main emission peaks appear at 482 nm. The excitation and emission spectra of this phosphor show that both of them are broadband. This is ascribed to the 4f 7 ! 4f 6 5d1 transition of Eu2+ in the pigment matrix, which is in good agreement with the calculated value of 470 nm, and implies that luminescent centers Eu2+ occupy the deca-coordinated Sr2+ sites with the host of Sr3MgSi2O8. r 2005 Elsevier B.V. All rights reserved. Keywords: Afterglow; Photoluminescent pigment; Emission; Luminescence
1. Introduction It is generally believed that the phosphorescence of Eu2+ is due to 4f 7 ! 4f 6 5d1 transition [1,2]. Although, 4f electrons of Eu2+ are not sensitive to the changes of the crystal field strength due to the shielding function of outer shell, the peak positions in the emission spectra strongly, depend on the nature of Eu2+ surroundings in the host lattices.
Corresponding author.
E-mail address: [email protected] (A.A. Sabbagh Alvani).
Therefore, Eu2+ ions can emit variety of visible lights in various crystal fields [3]. Alkaline earth silicates are useful luminescent hosts since they have stable crystal structure, high physical and chemical stability. They have also excellent weather resistance and exhibit blue to green emission under excitation with ultraviolet rays and/or visible light rays [1,2]. There are many investigations on rare earth ion activation of these hosts. Barry prepared Ca3MgSi2O8:Eu, Ba3MgSi2O8: Eu, phosphor by prefiring at 600 1C in air using NH4Cl as a flux, and then firing at 1200 1C for 4 h in atmosphere of 96% N2/4% H2 [4]. He found the location of the main emission peaks at
0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.12.012
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136
room temperature to be at 475 nm. Huang and coworkers [5] prepared Ca3MgSi2O8:Ce phosphor and found that two kinds of emission centers exist in the host. However, the long afterglow was not observed in these phosphors. Lin and co-workers [6] prepared Ca3MgSi2O8: Eu2+ , Dy3+ phosphor with blue-emitting long afterglow. In further investigations, they examined luminescence of a series of host materials Viz. R3MgSi2O8 (R ¼ Ca, Sr, Ba) in comparative study [7]. In spite of excellent phosphorescence properties of Sr3MgSi2O8 host material, less attention has been paid to it in the literature. In this paper, we aim to reveal phosphorescence behavior of this excellent host material via a systematic research, particularly concentration effects.
Intensity (counts)
132
14900 13900 12900 11900 10900 8900 9900 7900 6900 5900 4900 3900 2900 1900 900 -100
Sr3Mgsi208 : Eu,dy
Eu/Dy 2.0 1.5 1.0 0.5
5
15
25
35
45
55
65
Two theta (°) Fig. 1. The XRD patterns of SMS-ED phosphor with ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
180 160
2. Experimental procedure
3. Results and discussion XRD pattern of SMS-ED sample is shown in Fig. 1. The sample was sintered at 1200 1C and
Relative Intensity
140
SrCO3, 4MgCO3 Mg(OH)2 5H2O, SiO2, Eu2O3, Dy2O3, and NH4Cl were used as the raw materials. All of the applied materials are highly purified. Small quantities of NH4Cl (0.5 m/o per mol of one silicate) were used as the flux material. The raw materials were mixed homogeneously by the fast mill for 5 h. A new long afterglow blueemitting SMS-ED photoluminescent phosphor was prepared at 1200 1C for 3 h and weak reductive atmosphere of flowing 4%H2–96%N2 gas. Then optical properties and phase composition of the product were fully characterized. The phosphor powders were analyzed using a Siemens D5 X-Ray Diffractometer (CuKa and l: 1.540510 A˚). ‘‘Luminescence spectrometers LS5/ 5B’’ fluorescence spectrophotometer was used to detect the excitation and emission spectra of the products. The decay curve of afterglow was measured by a brightness meter, and the powder samples were irradiated by a standard lamp for 15 min. All measurements were carried out at room temperature.
120 100 80 60 40 20 0 -20 300 325 350 375 400 425 450 475 500 525 550 575 600
Excitation Wavelength (nm) Fig. 2. The excitation spectra of SMS-ED phosphor with ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
reductive atmosphere for 3 h. The ratios of Eu/ Dy ¼ 0.5, 1.0, 1.5, 2.0 were used. It is evident from Fig. 1 that the co-doped Eu and Dy ions have little influence on the structure of the luminescent materials. All peaks are assigned to the phase of Sr3MgSi2O8 as well [8]. The doped Eu2+ ions in SMS host give rise to a blue emission under UV excitation. Figs. 2 and 3 show the excitation and emission spectra of SMSED at room temperature. As shown in Figs. 2 and 3, two broad excitation bands centered at 356 and 395 nm were observed. The main emission peak at 482 nm is seen as the
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136 180 160
Sr3Mgsi2O8 : Eu,Dy a37 : SMS-ED 37 Emission (Eu/Dy = 0.5)
140
a38 : SMS-ED 38 Emission (Eu/Dy =1.0)
120
a39 : SMS-ED 39 Emission (Eu/Dy =1.5)
Table 1 The calculated value of position in energy of the lower d-band edge for Eu2+ occupying different crystallographic sites
With exctitation 356 nm
Relative Intensity
With exctitation 356 nm
Ions
n
r (nm)
Energy (cm 1)
Emitting wavelength (nm)
Sr2+
6 8 10 12
0.118 0.126 0.136 0.144
16647 18886 21148 23151
600 529 470 431
Mg2+
6 8
0.072 0.089
14295 16679
699 599
With exctitation 356 nm
100
a40 : SMS-ED 40 Emission (Eu/Dy =2.0) With exctitation 356 nm
80 a40
60 40
a38 a37
20
133
a39
0 -20 300 325 350 375 400 425 450 475 500 525 550 575 600
Emission Wavelength (nm) Fig. 3. The emission spectra of SMS-ED phosphor with ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
typical emission of Eu2+ ascribed to the 4f 7 ! 4f 6 5d1 transition. However, no special emission peaks of Eu3+ were observed in the spectra. This means that Eu3+ in the crystal matrix has been completely reduced to Eu2+. The special Dy3+ emission peak is not present, which may be ascribed to the function of the hole or electron traps and energy transporting for Dy3+. It is not the cause of the luminescent centers in the SMS-ED host crystal lattice. The doped materials are rare earth elements such as Dy3+ and Eu3+. Therefore, for obtaining Eu2+, a one-step-reduction process may be performed. Using thermodynamic relations and assumption of reduction process, oxide forms of rare earth elements react with H2 and change to a reduced form. Thus, the following relations can be considered: Eu2 O3 þ H2 ! 2EuO þ H2 O;
(1)
Dy2 O3 þ H2 ! 2DyO þ H2 O:
(2)
Using Gibbs free energy, it could be shown: DG ¼ DG þ RT Ln K; K Eu1200 C ¼ 2:16 10 8 ;
(3) K Dy1200 C ffi 0:
Based on Eq. (3), it is not possible to reduce Dy3+ to Dy2+. Thus it could be considered that, this element is only existed in the form of tri-valant
(Dy3+). Considering non-zero equilibrium constant of Eu2+ production, there is a possibility of having Eu2+ in the reaction. Lin et al., have applied the rare earth ions as codoped elements in their investigations. They found that these elements have no strong effect on the emission wavelength. Based on this result they concluded that rare earth elements have little effect on luminescent center in Eu environment as well [9]. As previously reported [10], the energy position for the lower d-band edge in Eu2+ or Ce3+, for various compounds can be determined by E ¼ Q½1 ðV =4Þ1=v 10 ðn ea rÞ=8 :
(4)
Eq. (4) has a good agreement with experimental results, where Q is the position of energy for the lower d-band edge for the free ion, (Q ¼ 34; 000 cm 1 for Eu2+); V is the valence of the active cation, (V ¼ 2); n is the number of anions in the immediate shell about this ion, and ea is the electron affinity of the atoms that form the anions (ea ¼ 2.5 eV); r is the radius of the host cation replaced by the active cation in the host crystal. The values of position in energy of the lower d-band edge for Eu2+ occupying different crystallographic sites are shown in Table 1. Ionic radius of the elements incorporated in the phosphorescent material are shown in Table 2. According to Table 2, ionic radius of Eu2+ is equal to 1.35 (n ¼ 10), at the same coordination number the ionic radius of Sr2+ is equal to 1.36. As it is illustrated in Table 2, these two radius are close to each other and as a result Eu2+ could be
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136
Table 2 Ionic radius element in phosphor 2+
2+
8
2+
2+
n
Eu
Sr
Mg
Dy
6 8 10 12
1.17 1.25 1.35 —
1.18 1.26 1.36 1.44
0.57 0.72 0.89 —
0.91 1.03 — —
Luminescence intensity (mcd.m-2)
134
7 6 5 4 3 2 1 0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Time (hour)
Fig. 5. The typical decay curve of Sr3MgSi2O8:Eu,Dy phosphor.
Fig. 4. The decay curves of phosphor With ratio ðEu=Dy ¼ 0:5; 1:0; 1:5; 2:0Þ:
replaced in Sr2+ site. This is not the case for Mg2+. It is in good agreement with the work of Lin et al. [7]. The luminescent decay curves of different phosphors co-doped with various amount of Eu2+ under excitation by standard lamp for 15 min at room temperature are shown in Fig. 4. According to Fig. 4, all of the mentioned ratios (e.g. Eu/Dy ¼ 0.5, 1.0, 1.5, 2.0) have a rapid decay and then a long-lasting phosphorescence. At constant Dy3+ content (0.01 mol) in SMS-ED phosphor, the intensity of emission at 482 nm increases by increasing the Eu2+ content. When the ratio is greater than 1.5, the luminescence intensity is increased accordingly. This is straightforward to the result of increasing Eu concentration. The typical decay curve of Sr3MgSi2O8: Eu, Dy phosphor is shown in Fig. 5. The initial afterglow intensity of phosphorescent material is 58.5 mcd/ m2. After removing excitation source, it is obvious
that, its afterglow drops on 5.3 mcd/m2 and lasts for more than 10 h. The duration can reach over 10 h in the lack of the light perception by the darkadapted human eye. Lin et al. have reported 5 h afterglow duration in their works [6]. They believe that this effect is due to the presence of strontium in the structure of the phosphorescent product. Like the mechanism of the long afterglow of SMSED phosphor, the co-doped Dy3+ may play an important role in prolonging the duration. In the previous reports regarding strontium aluminates phosphors (e.g. SrAl2O4: Eu, Dy [6,11,12]) various researchers believed that the co-doped Dy3+ works as the hole trap levels and prolonged the afterglow. Thus, in the SMS-ED host, it is proposed that the Dy3+ may also act as trap levels, which capture the free holes. Then, it will be released from the trapped holes, and recombine with electrons accompanying the luminescence. The schematic luminescent mechanism can be found in Fig. 5 of Ref. [13]. For the sake of clarity, a typical scheme is illustrated in Fig. 6. As the electrons located at ground level are excited, they move to the excited state, and a hole is created in their place. In the case of excited electron, due to its instability, they tend to return to hole location immediately (states 1 and 2), which is called fluorescence phenomenon. On the other hand, due to the existence of Dy3+, a trap level is made in the structure. Because of this trap level, energy level of the hole decreases and reaches the valance band (state 3), and immigrates in this level
ARTICLE IN PRESS A.A. Sabbagh Alvani et al. / Journal of Luminescence 114 (2005) 131–136
135
Fig. 6. The schematic graph of luminescent mechanism of SMS-ED phosphor.
toward trap levels (state 4) [13]. Thus the hole would be trapped in trap level (state 5). On the other hand, the electron in excited state tends to return to ground level or stable energy level, also due to the instability of trap level, the hole tends to return to the ground level, as well. Thus, excited electron and the hole will reach together in ground level during the time (long afterglow). This is called phosphorescence phenomenon. According to the comparative study of Lin et al., the most suitable depth among R3MgSi2O8 (R ¼ Ca, Sr, Ba) host materials is related to Sr3MgSi2O8. Since Dy trap level is different in various host materials, it is of great importance to find a host material providing a suitable depth. In the class of R3MgSi2O8, Sr has a superior position among alkaline earth metals. Therefore, it is the reason for our particular attention to this new host material, Sr3MgSi2O8.
the long afterglow properties can be achieved in our research applied conditions. The main excitation peaks are at 356 and 395 nm, respectively. The main emission peak is measured to be at 482 nm. These data could be the sign of 4f7-4f65d1 transition of Eu2+ in pigment matrix. Like luminescence mechanism of SMS-ED pigment, the co-doped Dy3+ ions played an important role as the hole trap levels and captured the free holes, and thus prolonged the duration.
4. Conclusions
References
The work was devoted to study Sr3MgSi2O8 photoluminescent pigment co-doped with Eu, Dy ion. Based on this investigation, it is proposed that
Acknowledgements The authors acknowledge Material and Energy Research Center for their help in carrying out characterization tests. In addition, helpful discussions and suggestions of the referees are gratefully acknowledged.
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