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Photoluminescence properties of M2MgSi2O7: Re2+(M=Ba, Sr, Ca)
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 158–161 www.elsevier.com/locate/jlumin
Photoluminescence properties of M2MgSi2O7: Re2+(M ¼ Ba, Sr, Ca) Dawei He, Yanning Shi, Dan Zhou, Tao Hou Key Laboratory of Luminescence and Optical Information, Ministry of Education & Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China Available online 10 March 2006
Abstract Excitation spectrum of M2MgSi2O7: Eu2+ was composed of two broad bands centered around 310 and 395 nm, respectively, in which the latter belonged to 4f-5d transitions of Eu2+. It was found that the introduction of Zn2+ into Ba2MgSi2O7: Eu2+ effectively increased its emission intensity without changing the position of emission peak. A further Eu2+ and Ce3+ co-doped pyrosilicate phosphor was found to be an efficient phosphor excited by long UV light and emitted in blue around 438 nm, which was originated from the 5d to 4f transition of Ce3+ ion in M2MgSi2O7: Eu2+, Ce3+. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare earths; Luminescence; Long UV excitation
1. Introduction
3. Results and discussion
As reported by G. Blasse, the luminescence properties of alkaline earth silicates activated by Eu2+ displayed a good emission under 254 nm excitation [1]. However, photoluminescence properties of M2MgSi2O7: Eu2+ excited by long UV radiation (380–420 nm) have not yet been investigated. In this work, Eu2+-activated alkaline earth silicate phosphors were investigated [2,3].
3.1. Phase composition of the synthesized phosphors
2. Experimental procedure The phosphors were synthesized by solid-state reaction method. The raw materials were weighed as the nominal composition and mixed thoroughly in an agate mortar. Subsequently, the mixture was sintered in a weak reducing atmosphere at 1150 1C for 2 h. Phase identification of the synthesized phosphors was carried out via X-ray diffraction (XRD). Emission and excitation spectra of the phosphors were recorded using a fluorescent spectrofluorometer equipped with a 450 W xenon lamp as excitation source. Corresponding author. Tel.: +86 10 51688018; fax: +86 10 51683933.
E-mail address: [email protected] (D. He). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.159
X-ray (Fig. 1) powder diffraction data of M2MgSi2O7 sample were in agreement with JEPDS standard card (100049(1), 75-1736(2), 88-0777(3)), which indicated that the co-doped Eu2+ had little influence on the structure of luminescent host. A single-phased M2MgSi2O7 phosphors were obtained in this synthesis process. 3.2. Luminescence characteristics of M2MgSi2O7: Eu2+ Excitation spectrum of M2MgSi2O7: Eu2+ is presented in Fig. 2 (l1em ¼ 504 nm, l2em ¼ 465 nm, l3em ¼ 475 nm). Two broad excitation bands, centered at 310 and 395 nm, can be observed in Fig. 2. The main excitation band at 395 nm belongs to 4f-5d transitions of Eu2+ and its broad band extends to 430 nm, which indicates a strong absorption in long UV band (370–420 nm) from M2MgSi2O7: Eu2+ phosphor. No contribution from Eu3+ in emission peaks were observed, which proved that Eu3+ in the crystallized matrix had been reduced to Eu2+ completely. A fine structure was observed on the low energetic excitation bands, and this fine structure was
ARTICLE IN PRESS D. He et al. / Journal of Luminescence 122–123 (2007) 158–161
159
1.0
3
(Ba) Relative intensity (a,u)
Intensity (a.u.)
0.8
(Sr)
(Ca)
0.6 2 0.4
1
0.2
0.0 0
20
40
60 2θ (degree)
80
100
120 300
Fig. 1. The XRD patterns of M2 Mg Si2O7: Eu2+ (M ¼ Ba, Sr, Ca).
6x105
350 400 Wavelength (nm)
450
Fig. 3. Excitation spectra of Ba2(Mg1x, Znx)Si2O7:Eu2+(lem ¼ 507 nm), where 1. x ¼ 0, 2. x ¼ 0.8, 3. x ¼ 1.0.
M=Ba 3 1.0 M=Sr
Relative intensity (a,u)
Intensity (a.u.)
5x105 4x105 M=Ca
3x105 2x105 1x105
0.8
0.6 2 0.4 1 0.2
0 250
300
350 Wavelength (nm)
400
450
Fig. 2. Excitation spectra of M2MgSi2O7: Eu2+. (M ¼ Ba(lem ¼ 504 nm), M ¼ Sr(lem ¼ 465 nm), M ¼ Ca(lem ¼ 475 nm)). 6
assigned to the splitting of the 4f configuration in the 4f65d1 excited state into seven 7FJ levels [4]. 3.3. Luminescence characteristics of Ba2 (Mg1x, Znx)Si2O7: Eu2+ We prepared series solid solutions Ba2 (Mg1x, Znx) Si2O7: Eu2+. Ba2MgSi2O7 (akermannite) and Ba2ZnSi2O7 (hardystonite) are isomorphous and belong to the members of the melilite group of silicates. The placement of Zn for Mg could form solid solutions at random proportion and had little influence on the crystal structure. Figs. 3 and 4 showed the excitation and emission spectra of Ba2 (Mgx, Zn1x)Si2O7: Eu2+ with different Zn2+ content (x ¼ 0, 0.8,1.0). The main excitation band was located at about 395 nm, and the main emission band at around 504 nm. There was an increase in both excitation intensity and emission intensity with the increase of Zn2+ content,
0.0 450
500
550 Wavelength (nm)
600
650
Fig. 4. Emission spectra of Ba2(Mg1x, Znx)Si2O7:Eu2+(lex ¼ 380 nm), where 1. x ¼ 0, 2. x ¼ 0.8, 3. x ¼ 1.0.
and when x ¼ 0.8, it reaches maximum. Then, with the increase of the Zn2+ content continuously, the spectra intensity slowly went down. Besides, as Zn2+was introduced into Ba2MgSi2O7: Eu2+, the excitation spectra became broader, which meant long UV could be more efficiently absorbed. There was no obvious wavelength shift of the Eu2+ emission band with the introduction of Zn into Ba2MgSi2O7: Eu2+. This suggested that the Eu2+ ions substituted neither Mg2+ nor Zn2+ site in Ba2(Mg1x, Znx) Si2O7: Eu2+. The other possible sites for Eu2+ incorporation into Ba2MgSi2O7 lattice were the Ba2+ sites or Si4+ sites. Si (0.026 nm) was small, so Eu2+ (0.112 nm) ions could hardly incorporate into a tetrahedral [SiO4]. From the above observations, Eu2+ could only occupy Ba2+ sites in the Ba2MgSi2O7 lattice and form the
ARTICLE IN PRESS D. He et al. / Journal of Luminescence 122–123 (2007) 158–161
160
corresponding emission center, which peaked at about 507 nm.
Table 1 The calculated values of position in energy of the lower d-band edge for Eu2+occupying different crystallographic sites
3.4. Luminescence characteristics of Ba2MgSi2O7: Eu2+, yCe3+
n
r (nm)
Energy (cm1)
lem (nm)
6 8
0.135 0.142
20 577 23 383
487 428
Fig. 5 shows the excitation and emission spectra of Ba2MgSi2O7: Eu2+, yCe3+ (y ¼ 0,0.006). Curve 1 is the emission spectrum of Ba2MgSi2O7: Eu2+ and curve 2 is the emission spectrum of Ba2MgSi2O7: Eu2+, Ce3+ at the same Eu2+ content. In addition to a green emission band at 507 nm, Ba2MgSi2O7: Eu2+, Ce3+ exhibited a strong purplish-blue band at 438 nm, which was even higher than its green peak. Owing to Ce3+ and Eu2+ excited by 365 nm UV light, this blue emission band centered at 438 nm originated from the 5d to 4f transition of Ce3+ ion in M2MgSi2O7: Eu2+, Ce3+. Thus, co-doping with a small amount of Ce3+ not only enhanced the emission intensity of Ba2MgSi2O7: Eu2+ but also changed the luminescence color of Ba2MgSi2O7: Eu2+ from green to bluish-green. This might be ascribed to the fact that the co-doping of Eu2+ and Ce3+ could greatly enhance the absorption of phosphor in UV band, which led to the increase of emission intensity. Curves 3 and 4 are the excitation spectra of Eu2+(lem ¼ 507 nm) in both Ba2MgSi2O7: Eu2+and Ba2MgSi2O7: Eu2+, Ce3+. The two curves were similar in profile, but the excitation intensity in Ba2MgSi2O7: Eu2+, Ce3+ was higher than in Ba2MgSi2O7: Eu2+(curve 4). Besides, curve 4 showed two other excitation peaks at around 410 and 418 nm, which would definitely enhance the efficient absorption in long UV band. The excitation spectrum of Ba2MgSi2O7:Ce3+ was composed of a broad band centered at 330 nm, which belonged to 4f-5d transitions of Ce3+, and its broad band extended to 380 nm. The emission spectrum showed a purplish-blue band at 396 nm with a half-width of 66 nm.
1.0
Relative intensity
0.8
2 0.6
3
1
0.2
0.0 300
350
E ¼ Q½1 ðV =4Þ1=V 10ðn ea rÞ=8
(1)
Q is the position in energy for the lower d-band edge for the free ion; here Q is 34,000 cm1 for Eu2+. V is the valence of the active cation; here V is 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. r is the radius of the host cation replaced by the active cation in the host crystal. The value of ea is different when Eu2+ is introduced into different anion complexes with various coordination numbers. Here, ea is 2.5. Therefore, we used Eq. (1), and the calculated values are given in Table 1. As can be seen from Table 1, when Ba2+ is situated in different environments, the calculated values are different. When these Ba2+ sites are replaced by Eu2+, the positions of the emission peaks are different, which demonstrates that Eu2+ occupies various emission centers in the phosphor. 4. Conclusions
4
0.4
As the emission intensity of Ba2MgSi2O7:Ce3+ under 380 nm was very low, the emission band of 438 nm in Ba2MgSi2O7:Eu2+, Ce3+ under 380 nm excitation might be the emission of Eu2+ in pyrosilicate host, either. That is to say, we believed there were two different lattice sites Ba2+ occupying in Ba2MgSi2O7 host. According to an early report [5], the position in energy of the lower d-band edge for Eu2+ or Ce3+ in various compounds can be calculated using the following empirical relation which provides a good fit to the emission peak for Eu2+:
400 450 500 Wavelength (nm)
550
600
650
Fig. 5. Excitation and emission spectrum of Ba2 Mg Si2O7: Eu2+, yCe3+, where 1. y ¼ 0, 2. y ¼ 0.006, 3. y ¼ 0, 4. y ¼ 0.006.
A series of phosphors of M2MgSi2O7: Eu2+ (M ¼ Ba, Sr, Ca) have been synthesized and studied. M2MgSi2O7: Eu2+ phosphor exhibited strong absorption in long UV band (370–420 nm), the fine structure was observed on the low energetic excitation bands. Eu2+ occupied Ba2+ sites in the Ba2MgSi2O7 lattice and formed the corresponding emission center at around 507 nm. It was found that the introduction of Zn2+ into Ba2MgSi2O7: Eu2+ effectively increased its emission intensity without changing the position of emission peak. Eu2+ and Ce3+ co-doped pyrosilicate phosphor was found to be an efficient bluish-green phosphor under the excitation of long UV light.
ARTICLE IN PRESS D. He et al. / Journal of Luminescence 122–123 (2007) 158–161
Acknowledgment This work was financially supported by the National Science Foundation of Beijing (no. 2052019). References [1] G. Blasse, W.L. Wanmaker, J.W. Ter Vrugt, Philips Res. Rep. 23 (1968) 189.
161
[2] J.K. Sheu, S.J. Chang, C.H. Kuo, IEEE Photonics Technol. Lett. 15 (1) (2003) 150. [3] L.G. Van Uitert, J. Lumin. 29 (1) (1984) 274. [4] Z. Xiao, L. Xingren, J. Electrochem. Soc. 139 (2) (1992) 622. [5] )J.V. Smith, Am. Minerol. 38 (1953) 643.
Journal of Luminescence 122–123 (2007) 158–161 www.elsevier.com/locate/jlumin
Photoluminescence properties of M2MgSi2O7: Re2+(M ¼ Ba, Sr, Ca) Dawei He, Yanning Shi, Dan Zhou, Tao Hou Key Laboratory of Luminescence and Optical Information, Ministry of Education & Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China Available online 10 March 2006
Abstract Excitation spectrum of M2MgSi2O7: Eu2+ was composed of two broad bands centered around 310 and 395 nm, respectively, in which the latter belonged to 4f-5d transitions of Eu2+. It was found that the introduction of Zn2+ into Ba2MgSi2O7: Eu2+ effectively increased its emission intensity without changing the position of emission peak. A further Eu2+ and Ce3+ co-doped pyrosilicate phosphor was found to be an efficient phosphor excited by long UV light and emitted in blue around 438 nm, which was originated from the 5d to 4f transition of Ce3+ ion in M2MgSi2O7: Eu2+, Ce3+. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare earths; Luminescence; Long UV excitation
1. Introduction
3. Results and discussion
As reported by G. Blasse, the luminescence properties of alkaline earth silicates activated by Eu2+ displayed a good emission under 254 nm excitation [1]. However, photoluminescence properties of M2MgSi2O7: Eu2+ excited by long UV radiation (380–420 nm) have not yet been investigated. In this work, Eu2+-activated alkaline earth silicate phosphors were investigated [2,3].
3.1. Phase composition of the synthesized phosphors
2. Experimental procedure The phosphors were synthesized by solid-state reaction method. The raw materials were weighed as the nominal composition and mixed thoroughly in an agate mortar. Subsequently, the mixture was sintered in a weak reducing atmosphere at 1150 1C for 2 h. Phase identification of the synthesized phosphors was carried out via X-ray diffraction (XRD). Emission and excitation spectra of the phosphors were recorded using a fluorescent spectrofluorometer equipped with a 450 W xenon lamp as excitation source. Corresponding author. Tel.: +86 10 51688018; fax: +86 10 51683933.
E-mail address: [email protected] (D. He). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.159
X-ray (Fig. 1) powder diffraction data of M2MgSi2O7 sample were in agreement with JEPDS standard card (100049(1), 75-1736(2), 88-0777(3)), which indicated that the co-doped Eu2+ had little influence on the structure of luminescent host. A single-phased M2MgSi2O7 phosphors were obtained in this synthesis process. 3.2. Luminescence characteristics of M2MgSi2O7: Eu2+ Excitation spectrum of M2MgSi2O7: Eu2+ is presented in Fig. 2 (l1em ¼ 504 nm, l2em ¼ 465 nm, l3em ¼ 475 nm). Two broad excitation bands, centered at 310 and 395 nm, can be observed in Fig. 2. The main excitation band at 395 nm belongs to 4f-5d transitions of Eu2+ and its broad band extends to 430 nm, which indicates a strong absorption in long UV band (370–420 nm) from M2MgSi2O7: Eu2+ phosphor. No contribution from Eu3+ in emission peaks were observed, which proved that Eu3+ in the crystallized matrix had been reduced to Eu2+ completely. A fine structure was observed on the low energetic excitation bands, and this fine structure was
ARTICLE IN PRESS D. He et al. / Journal of Luminescence 122–123 (2007) 158–161
159
1.0
3
(Ba) Relative intensity (a,u)
Intensity (a.u.)
0.8
(Sr)
(Ca)
0.6 2 0.4
1
0.2
0.0 0
20
40
60 2θ (degree)
80
100
120 300
Fig. 1. The XRD patterns of M2 Mg Si2O7: Eu2+ (M ¼ Ba, Sr, Ca).
6x105
350 400 Wavelength (nm)
450
Fig. 3. Excitation spectra of Ba2(Mg1x, Znx)Si2O7:Eu2+(lem ¼ 507 nm), where 1. x ¼ 0, 2. x ¼ 0.8, 3. x ¼ 1.0.
M=Ba 3 1.0 M=Sr
Relative intensity (a,u)
Intensity (a.u.)
5x105 4x105 M=Ca
3x105 2x105 1x105
0.8
0.6 2 0.4 1 0.2
0 250
300
350 Wavelength (nm)
400
450
Fig. 2. Excitation spectra of M2MgSi2O7: Eu2+. (M ¼ Ba(lem ¼ 504 nm), M ¼ Sr(lem ¼ 465 nm), M ¼ Ca(lem ¼ 475 nm)). 6
assigned to the splitting of the 4f configuration in the 4f65d1 excited state into seven 7FJ levels [4]. 3.3. Luminescence characteristics of Ba2 (Mg1x, Znx)Si2O7: Eu2+ We prepared series solid solutions Ba2 (Mg1x, Znx) Si2O7: Eu2+. Ba2MgSi2O7 (akermannite) and Ba2ZnSi2O7 (hardystonite) are isomorphous and belong to the members of the melilite group of silicates. The placement of Zn for Mg could form solid solutions at random proportion and had little influence on the crystal structure. Figs. 3 and 4 showed the excitation and emission spectra of Ba2 (Mgx, Zn1x)Si2O7: Eu2+ with different Zn2+ content (x ¼ 0, 0.8,1.0). The main excitation band was located at about 395 nm, and the main emission band at around 504 nm. There was an increase in both excitation intensity and emission intensity with the increase of Zn2+ content,
0.0 450
500
550 Wavelength (nm)
600
650
Fig. 4. Emission spectra of Ba2(Mg1x, Znx)Si2O7:Eu2+(lex ¼ 380 nm), where 1. x ¼ 0, 2. x ¼ 0.8, 3. x ¼ 1.0.
and when x ¼ 0.8, it reaches maximum. Then, with the increase of the Zn2+ content continuously, the spectra intensity slowly went down. Besides, as Zn2+was introduced into Ba2MgSi2O7: Eu2+, the excitation spectra became broader, which meant long UV could be more efficiently absorbed. There was no obvious wavelength shift of the Eu2+ emission band with the introduction of Zn into Ba2MgSi2O7: Eu2+. This suggested that the Eu2+ ions substituted neither Mg2+ nor Zn2+ site in Ba2(Mg1x, Znx) Si2O7: Eu2+. The other possible sites for Eu2+ incorporation into Ba2MgSi2O7 lattice were the Ba2+ sites or Si4+ sites. Si (0.026 nm) was small, so Eu2+ (0.112 nm) ions could hardly incorporate into a tetrahedral [SiO4]. From the above observations, Eu2+ could only occupy Ba2+ sites in the Ba2MgSi2O7 lattice and form the
ARTICLE IN PRESS D. He et al. / Journal of Luminescence 122–123 (2007) 158–161
160
corresponding emission center, which peaked at about 507 nm.
Table 1 The calculated values of position in energy of the lower d-band edge for Eu2+occupying different crystallographic sites
3.4. Luminescence characteristics of Ba2MgSi2O7: Eu2+, yCe3+
n
r (nm)
Energy (cm1)
lem (nm)
6 8
0.135 0.142
20 577 23 383
487 428
Fig. 5 shows the excitation and emission spectra of Ba2MgSi2O7: Eu2+, yCe3+ (y ¼ 0,0.006). Curve 1 is the emission spectrum of Ba2MgSi2O7: Eu2+ and curve 2 is the emission spectrum of Ba2MgSi2O7: Eu2+, Ce3+ at the same Eu2+ content. In addition to a green emission band at 507 nm, Ba2MgSi2O7: Eu2+, Ce3+ exhibited a strong purplish-blue band at 438 nm, which was even higher than its green peak. Owing to Ce3+ and Eu2+ excited by 365 nm UV light, this blue emission band centered at 438 nm originated from the 5d to 4f transition of Ce3+ ion in M2MgSi2O7: Eu2+, Ce3+. Thus, co-doping with a small amount of Ce3+ not only enhanced the emission intensity of Ba2MgSi2O7: Eu2+ but also changed the luminescence color of Ba2MgSi2O7: Eu2+ from green to bluish-green. This might be ascribed to the fact that the co-doping of Eu2+ and Ce3+ could greatly enhance the absorption of phosphor in UV band, which led to the increase of emission intensity. Curves 3 and 4 are the excitation spectra of Eu2+(lem ¼ 507 nm) in both Ba2MgSi2O7: Eu2+and Ba2MgSi2O7: Eu2+, Ce3+. The two curves were similar in profile, but the excitation intensity in Ba2MgSi2O7: Eu2+, Ce3+ was higher than in Ba2MgSi2O7: Eu2+(curve 4). Besides, curve 4 showed two other excitation peaks at around 410 and 418 nm, which would definitely enhance the efficient absorption in long UV band. The excitation spectrum of Ba2MgSi2O7:Ce3+ was composed of a broad band centered at 330 nm, which belonged to 4f-5d transitions of Ce3+, and its broad band extended to 380 nm. The emission spectrum showed a purplish-blue band at 396 nm with a half-width of 66 nm.
1.0
Relative intensity
0.8
2 0.6
3
1
0.2
0.0 300
350
E ¼ Q½1 ðV =4Þ1=V 10ðn ea rÞ=8
(1)
Q is the position in energy for the lower d-band edge for the free ion; here Q is 34,000 cm1 for Eu2+. V is the valence of the active cation; here V is 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. r is the radius of the host cation replaced by the active cation in the host crystal. The value of ea is different when Eu2+ is introduced into different anion complexes with various coordination numbers. Here, ea is 2.5. Therefore, we used Eq. (1), and the calculated values are given in Table 1. As can be seen from Table 1, when Ba2+ is situated in different environments, the calculated values are different. When these Ba2+ sites are replaced by Eu2+, the positions of the emission peaks are different, which demonstrates that Eu2+ occupies various emission centers in the phosphor. 4. Conclusions
4
0.4
As the emission intensity of Ba2MgSi2O7:Ce3+ under 380 nm was very low, the emission band of 438 nm in Ba2MgSi2O7:Eu2+, Ce3+ under 380 nm excitation might be the emission of Eu2+ in pyrosilicate host, either. That is to say, we believed there were two different lattice sites Ba2+ occupying in Ba2MgSi2O7 host. According to an early report [5], the position in energy of the lower d-band edge for Eu2+ or Ce3+ in various compounds can be calculated using the following empirical relation which provides a good fit to the emission peak for Eu2+:
400 450 500 Wavelength (nm)
550
600
650
Fig. 5. Excitation and emission spectrum of Ba2 Mg Si2O7: Eu2+, yCe3+, where 1. y ¼ 0, 2. y ¼ 0.006, 3. y ¼ 0, 4. y ¼ 0.006.
A series of phosphors of M2MgSi2O7: Eu2+ (M ¼ Ba, Sr, Ca) have been synthesized and studied. M2MgSi2O7: Eu2+ phosphor exhibited strong absorption in long UV band (370–420 nm), the fine structure was observed on the low energetic excitation bands. Eu2+ occupied Ba2+ sites in the Ba2MgSi2O7 lattice and formed the corresponding emission center at around 507 nm. It was found that the introduction of Zn2+ into Ba2MgSi2O7: Eu2+ effectively increased its emission intensity without changing the position of emission peak. Eu2+ and Ce3+ co-doped pyrosilicate phosphor was found to be an efficient bluish-green phosphor under the excitation of long UV light.
ARTICLE IN PRESS D. He et al. / Journal of Luminescence 122–123 (2007) 158–161
Acknowledgment This work was financially supported by the National Science Foundation of Beijing (no. 2052019). References [1] G. Blasse, W.L. Wanmaker, J.W. Ter Vrugt, Philips Res. Rep. 23 (1968) 189.
161
[2] J.K. Sheu, S.J. Chang, C.H. Kuo, IEEE Photonics Technol. Lett. 15 (1) (2003) 150. [3] L.G. Van Uitert, J. Lumin. 29 (1) (1984) 274. [4] Z. Xiao, L. Xingren, J. Electrochem. Soc. 139 (2) (1992) 622. [5] )J.V. Smith, Am. Minerol. 38 (1953) 643.