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Luminescence properties of a new red long-lasting phosphor: Mg2SiO4:Dy3+, Mn2+
Journal of Alloys and Compounds 455 (2008) 327–330
Luminescence properties of a new red long-lasting phosphor: Mg2SiO4:Dy3+, Mn2+ Lin Lin a,b , Min Yin a,b,∗ , Chaoshu Shi b , Weiping Zhang b a
b
Hefei National Laboratory for Physical Sciences at the Microscale, Hefei 230026, China Department of Physics, University of Science and Technology of China, Hefei 230026, China Received 28 July 2006; received in revised form 11 January 2007; accepted 13 January 2007 Available online 18 January 2007
Abstract The red long-lasting phosphor Mg2 SiO4 :Dy3+ , Mn2+ was prepared by solid state reaction, the luminescence and afterglow properties of which were investigated. The emission spectrum of Mg2 SiO4 :Mn2+ shows a broad band centered at about 650 nm, which is attributed to the 4 T1 (4 G) → 6 A1 (6 S) transitions of Mn2+ ions occupying two nonequivalent Mg2+ sites. The excitation bands of 650 nm emission in near ultraviolet and visible region were assigned. Mg2 SiO4 :Mn2+ samples co-doped with Dy3+ show higher initial phosphorescence intensity and longer afterglow time than Mn2+ doped ones. Both of them represent red afterglow. In addition, investigation of the effects of Dy3+ ions in co-doped samples was conducted, indicating that Dy3+ ions serve as trap centers. © 2007 Elsevier B.V. All rights reserved. Keywords: Luminescence; Dy; Mg2 SiO4 : Mn; Red afterglow
1. Introduction Great progress has been made in long-lasting phosphorescence since early 20th century. Long-lasting phosphors were widely used in emergency lighting, safety indication, road signs, and so on. Most of long-lasting phosphors are based on sulfide, aluminate, or silicate hosts. Recently, silicate phosphors have been paid considerable attention because of their multi-color phosphorescence and resistance to acid, alkali and oxygen. Comparing long-lasting phosphors of three primary colors, green and blue ones have much better afterglow performance than red ones. For example, the afterglow time of SrAl2 O4 :Eu2+ , Dy3+ (emitting at 520 nm, green) [1] and CaAl2 O4 : Eu2+ , Nd3+ (440 nm, blue) [2] is longer than 10 h, while that of Y2 O2 S:Eu3+ , Ti4+ , Mg2+ (617 nm, red) is only 5 h [3]. In 2003, Xiao-Jun Wang et al. [4] reported a red long-lasting phosphor: MgSiO3 :Eu2+ , Dy3+ , Mn2+ . Its red emission peaked at 660 nm can last over 4 h after UV irradiation.
∗ Corresponding author at: Department of Physics, University of Science and Technology of China, Hefei, China. Tel.: +86 551 3607412; fax: +86 551 3607417. E-mail address: [email protected] (M. Yin).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.01.059
In general, the Dy3+ dopants could act as electron trap centers in long-lasting phosphors based on oxysalt of alkaline earth ions [5]. Therefore, they should effectively postpone the afterglow of luminescence centers. In this work, a new red long-lasting silicate phosphor, Mg2 SiO4 :Dy3+ , Mn2+ was prepared by solidstate reaction. Red phosphorescence and afterglow properties of Mg2 SiO4 :Dy3+ , Mn2+ were investigated. 2. Experimental Mg2 SiO4 powder samples were synthesized by solid-state reaction. Mg(OH)2 ·4MgCO3 ·6H2 O (AR), SiO2 (AR), MnCO3 (CR), Dy2 O3 (4N) were used as starting materials. Their mixtures were sintered at 1300 ◦ C for 6 h in a reducing atmosphere. The samples’ atomic ratio of doped ions to Mg2+ is given as follows: (1) Mn2+ doped samples: Mn2+ (1–4%). (2) Dy3+ doped samples: Dy3+ (0–6%). (3) Samples co-doped with Dy3+ and Mn2+ :Dy3+ (1–8%), Mn2+ (3%). The excitation spectrum of Mg2 SiO4 :1% Mn2+ (Fig. 1a) was measured by a FLUOROLOG-TAU fluorescence spectrometer. Other excitation and emission spectra, phosphorescence decay curves, and phosphorescence spectra were recorded by a HITACHI-850 fluorescence spectrometer. A BR2000III thermo-luminescence (TL) meter was utilized to detect TL curves in the range of 25–300 ◦ C with a heating rate of 10 ◦ C/s, and samples used to record decay curve, phosphorescence spectra and TL curves were
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irradiated for 3 min with a low-pressure mercury lamp (254 nm) before measurement.
3. Results and discussion The XRD pattern of Mg2 SiO4 :Mn2+ sample (be omitted) can be indexed on Mg2 SiO4 (JCPDS data base No. 85-1364), excess SiO2 (JCPDS No. 77-1315) and a little MgSiO3 (JCPDS No. 11-0273). Unfortunately, formation of MgSiO3 could be hardly avoided in the synthesis of Mg2 SiO4 even by sol–gel method [6,7]. 3.1. Photo-luminescence properties 3.1.1. Excitation and emission spectra of Mn2+ -doped samples Fig. 1 represents the excitation and emission spectra of Mg2 SiO4 :1% Mn2+ . Excited by 215 nm UV light, which is the tail part of the VUV excitation band peaked at 180 nm, the emission spectrum (Fig. 1b) consists of a broad band at 650 nm. The emission band can be fitted by two Gaussian peaks at 645 and 723 nm, which should be attributed to the transitions of Mn2+ ions occupying two nonequivalent Mg2+ sites, mirror symmetric octahedron (M2 ) and inversion symmetric octahedron (M1 ), respectively [8]. The similar emission band was also observed and investigated in Mg2 SiO4 :Mn2+ samples prepared by sol–gel method [7]. Moreover, this band shifts to longer wavelength with an increase of Mn2+ concentration. For example, the emission band of Mg2 SiO4 :3% Mn2+ sample is shifted to about 660 nm under the same excitation. This phenomenon results from the relative intensity variety of the two peaks with the increasing Mn2+ concentration and the slight red shift of both Gaussian peaks, which may be the result of exchange interaction between Mn2+ ions [9]. The excitation spectrum (Fig. 1a) of 650 nm emission consists of a band peaked at 180 nm, corresponding to the absorption of
Fig. 1. Excitation and emission spectra of Mg2 SiO4 :1% Mn2+ Solid line (a) excitation spectrum (λem = 650 nm); (b) emission spectrum (λex = 215 nm). Dash line: fitted Gaussian peaks.
host lattice, and a band in the range of 225–300 nm, which can be attributed to the Mn2+ –O2− charge transfer transition [10,11], as well as several bands in near ultraviolet and visible region. The excitation spectrum in the range of 300–600 nm can be fitted by the sum of seven Gaussian peaks at 545, 461, 429, 412, 367, 349 and 316 nm (dash line). Since the octahedral crystal field of Mg2+ sites is slightly distorted, degeneration of the 4 E(4 G) and 4 A1 (4 G) levels is released. The fitting parameters B and /B of Tanabe–Sugano diagram were calculated as 735.0 cm−1 and 10.63, respectively. Accordingly, the seven peaks mentioned above can be attributed to the Mn2+ transitions from the ground state 6 A1 (6 S) to 4 T1 (4 G), 4 T2 (4 G), 4 E(4 G), 4 A1 (4 G), 4 T2 (4 D), 4 E(4 D) and 4 T (4 P), respectively. 1 In Fig. 1a, the strong excitation peaks in visible region can make the best use of energy of visible light from the sun, which is an advantage of this red long-lasting phosphor. 3.1.2. Excitation and emission spectra of Dy3+ -doped samples The excitation and emission spectra of Mg2 SiO4 :2% Dy3+ and the emission spectrum of un-doped Mg2 SiO4 are depicted in Fig. 2. Comparing the two emission spectra of Mg2 SiO4 :2% Dy3+ (Fig. 2b), and un-doped Mg2 SiO4 (Fig. 2c), one can see that the emission lines of Dy3+ ions (480, 487, 580 nm) in Mg2 SiO4 :Dy3+ are overlapped on the emission bands of un-doped Mg2 SiO4 , peaked at 460 and 640 nm. The emission bands of the un-doped sample may result from the Si Si oxygen vacancy [12] due to the reducing atmosphere during preparation. The three emission lines at 480, 487 and 580 nm in Fig. 2b can be assigned to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ , respectively. The excitation spectrum in the range of 270–500 nm consists of the f–f transitions of
Fig. 2. (a) Excitation spectrum of Mg2 SiO4 :2% Dy3+ (λem = 580 nm); (b) emission spectrum of Mg2 SiO4 :2% Dy3+ (λex = 215 nm); (c) emission spectrum of Mg2 SiO4 (λex = 215 nm).
L. Lin et al. / Journal of Alloys and Compounds 455 (2008) 327–330
:Dy3+ ,
Mn2+
Fig. 3. (a) Emission spectra of Mg2 SiO4 3% series with different concentration of Dy3+ (λex = 215 nm); (b) Intensity of fitted Gaussian peaks at 656 nm and 730 nm of Mg2 SiO4 :Dy3+ , 3% Mn2+ series (λex = 215 nm) vs. Dy3+ concentrations.
Dy3+ : 295 nm (6 H15/2 → 4 D7/2 ), 325 nm (6 H15/2 → 6 P3/2 ), 350 nm (6 H15/2 → 6 P7/2 ), 365 nm (6 H15/2 → 6 P5/2 ), 386 nm (6 H15/2 → 4 M21/2 ), 420 nm (6 H15/2 → 4 G11/2 ), 453 nm (6 H15/2 → 4 J15/2 ), 475 nm (6 H15/2 → 4 F9/2 ). The intensity of Dy3+ emission reaches its maximum while the concentration of Dy3+ is 2%. However, these intensities, as well as those of host’s emission, are only about 10% of that of Mn2+ when excited by 215 nm, which could be hardly detected in the emission spectrum of co-doped samples under UV excitation. 3.1.3. Emission spectra of samples co-doped with Dy3+ and Mn2+ ions A very interesting phenomenon is the sensitizing effect of Dy3+ to the emission of Mn2+ , which has not been reported in other long-lasting phosphors. Fig. 3a shows the emission spectra of Mg2 SiO4 co-doped with Dy3+ and Mn2+ , where the concentration of Mn2+ was fixed at 3% and that of Dy3+ varied from 0 to 6%. The intensities of red emission in all samples with Dy3+ are stronger than that in sample without Dy3+ . When Dy3+ concentration is 2%, the intensity of 656 nm band increases 1.5 times. The emission intensity of Mn2+ decreases for samples with higher concentration of Dy3+ , attributing to the concentration quenching of Dy3+ . Moreover, the shape of emission spectra of Mn2+ changes due to the adding of Dy3+ . Fig. 3b describes the dependence of emission intensity of Mn2+ on the concentration of Dy3+ . It is clear that the curves for 656 and 730 nm, which correspond to the emissions from Mn2+ ions located in M2 and M1 sites, respectively, are different: the Mn2+ ions in M2 sites are sensitized more effectively by Dy3+ . This result indicates that co-doped samples show higher initial intensity than Mn2+ -doped samples during the afterglow process. However, the mechanism of the sensitizing effect of Dy3+ is not clear now, and further investigation is needed.
329
Fig. 4. Experimental (symbols) and fitted (lines) decay curves of Mg2 SiO4 :3% Mn2+ (λem = 660 nm) and Mg2 SiO4 :8% Dy3+ , 3% Mn2+ (λem = 660 nm).
3.2. Afterglow properties Fig. 4 shows the decay curves of Mn2+ emission in Mn2+ doped and co-doped samples. Each can be fitted well by a hyperbolic curve rather than an exponential decay. The fitting curves and the parameters are represented in Fig. 4, which prove that co-doped Dy3+ ions increase the initial phosphorescence intensity of Mn2+ emission. In the darkness, the red afterglow of Mn2+ doped and codoped samples can be seen for 6 and 17 min, respectively after UV irradiation. Fig. 5 illustrates the phosphorescence spectra of co-doped sample at the recorded decay time of 1, 5 and 10 min after the removal of the excitation source, respectively. It is clear that only the emission of Mn2+ was detected in the co-doped sample in the decay process, and the shapes of the bands are similar at different decay times. Thus, the co-doped samples represent red afterglow, consisting with the observation of the naked eyes. It was reported that doping ions Dy3+ serve as trap centers in MgSiO3 :Eu2+ , Dy3+ , Mn2+ [4]. From the TL curves of Mg2 SiO4
Fig. 5. Phosphorescence spectra of Mg2 SiO4 :8% Dy3+ , 3% Mn2+ .
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the afterglow of samples [13]. Therefore, the doping of Dy3+ ions can postpone the afterglow of co-doped samples effectively. The peaks of the Mn2+ -doped sample at about 190 and 260 ◦ C are out of the proper range of TL peaks, so that defects of the Mn2+ -doped sample are hard to release trapped electrons at room temperature. This analysis is consistent with the results of the eye-observation. 4. Conclusions Red long-lasting phosphorescence was observed in Mg2 SiO4 :Dy3+ , Mn2+ powder samples prepared by solid-state reaction. In the excitation spectrum with 650 nm emission in Mg2 SiO4 :Mn2+ , there are several bands in the visible region, which can utilize the energy of visible light from the sun. Our experiments have shown that Mg2 SiO4 :Mn2+ samples co-doped with Dy3+ have higher initial phosphorescence intensity and longer decay time than Mn2+ doped ones. Afterglow time of Mn2+ doped and co-doped samples are 6 and 17 min, respectively. The experiment results indicate that the function of doped Dy3+ ions is trap centers. Moreover, the sensitizing effect of Dy3+ to the emission of Mn2+ was obtained in co-doped samples as a novel phenomenon. Acknowledgements Fig. 6. TL curves of Mg2 SiO4 samples: (a) 3% Mn2+ ; (b) 2% Dy3+ ; (c) 2% Dy3+ , 3% Mn2+ . Table 1 TL peaks of single doped and co-doped Mg2 SiO4 samples Concentration of Mn2+ and Dy3+
Peaks (◦ C)
3% Mn2+ 2% Dy3+ 2% Dy3+ , 3% Mn2+
194, 262 66, >300 74, 189, >300
doped with Mn2+ and/or Dy3+ (Fig. 6), we believe that Dy3+ ions act as the same role in co-doped Mg2 SiO4 samples. Fig. 6 depicts the TL curves of Mg2 SiO4 doped with (a) 3% Mn2+ ; (b) 2% Dy3+ ; and (c) 2% Dy3+ and 3% Mn2+ . Relative intensity of three TL curves in this figure is comparable, as the same irradiated area and irradiated time were used. Corresponding peaks of these TL curves are listed in Table 1. According to Fig. 6 and Table 1, it is obvious that the doped Dy3+ ions act as new trap centers, corresponding to TL peak at about 70 ◦ C. In general, the trap centers, corresponding to the TL peaks in the range of 50–110 ◦ C, are helpful to extend
This work is supported by National High Technology Research and Development Program of China (863 Program) (No. 2002 AA 324060) and National Nature Science Foundation of China (No. 10404028). References [1] T. Matsuzawa, Y. Aoki, N. Takeuchi, et al., J. Electrochem. Soc. 143 (1996) 2670–2673. [2] Y.H. Lin, Z.L. Tang, Z.T. Zhang, et al., J. Eur. Ceram. Soc. 23 (2003) 175–178. [3] Y. Murazaki, K. Arai, K. Ichinomiya, JPN. Rare Earths 35 (1999) 41– 45. [4] X.J. Wang, D.D. Jia, W.M. Yen, J. Lumin. 102–103 (2003) 34–37. [5] C.S. Shi, Z.M. Qi, Chin. J. Inorg. Mater. 19 (2004) 961–969. [6] A. Douy, J. Sol-Gel Sci. Technol. 24 (2002) 221–228. [7] L. Lin, M. Yin, C. Shi, et al., J. Rare Earths 24 (2006) 104–107 (Supp.). [8] W.Y. Jia, H.M. Liu, S. Jaffe, et al., Phys. Rev.: B43 (1991) 5234–5242. [9] C.R. Ronda, T. Amrein, J. Lumin. 69 (1996) 245–248. [10] J. Wang, S.B. Wang, Q. Su, J. Solid State Chem. 177 (2004) 895–900. [11] J. Lin, D.U. Sanger, M. Mennig, et al., Thin Solid Films 360 (2000) 39– 45. [12] R. Tohmon, H. Mizuno, Y. Ohki, et al., Phys. Rev. B 39 (1989) 1337–1345. [13] T.Z. Zhang, Q. Su, J. Soc. Inf. Disp. 8 (2000) 27–30.
Luminescence properties of a new red long-lasting phosphor: Mg2SiO4:Dy3+, Mn2+ Lin Lin a,b , Min Yin a,b,∗ , Chaoshu Shi b , Weiping Zhang b a
b
Hefei National Laboratory for Physical Sciences at the Microscale, Hefei 230026, China Department of Physics, University of Science and Technology of China, Hefei 230026, China Received 28 July 2006; received in revised form 11 January 2007; accepted 13 January 2007 Available online 18 January 2007
Abstract The red long-lasting phosphor Mg2 SiO4 :Dy3+ , Mn2+ was prepared by solid state reaction, the luminescence and afterglow properties of which were investigated. The emission spectrum of Mg2 SiO4 :Mn2+ shows a broad band centered at about 650 nm, which is attributed to the 4 T1 (4 G) → 6 A1 (6 S) transitions of Mn2+ ions occupying two nonequivalent Mg2+ sites. The excitation bands of 650 nm emission in near ultraviolet and visible region were assigned. Mg2 SiO4 :Mn2+ samples co-doped with Dy3+ show higher initial phosphorescence intensity and longer afterglow time than Mn2+ doped ones. Both of them represent red afterglow. In addition, investigation of the effects of Dy3+ ions in co-doped samples was conducted, indicating that Dy3+ ions serve as trap centers. © 2007 Elsevier B.V. All rights reserved. Keywords: Luminescence; Dy; Mg2 SiO4 : Mn; Red afterglow
1. Introduction Great progress has been made in long-lasting phosphorescence since early 20th century. Long-lasting phosphors were widely used in emergency lighting, safety indication, road signs, and so on. Most of long-lasting phosphors are based on sulfide, aluminate, or silicate hosts. Recently, silicate phosphors have been paid considerable attention because of their multi-color phosphorescence and resistance to acid, alkali and oxygen. Comparing long-lasting phosphors of three primary colors, green and blue ones have much better afterglow performance than red ones. For example, the afterglow time of SrAl2 O4 :Eu2+ , Dy3+ (emitting at 520 nm, green) [1] and CaAl2 O4 : Eu2+ , Nd3+ (440 nm, blue) [2] is longer than 10 h, while that of Y2 O2 S:Eu3+ , Ti4+ , Mg2+ (617 nm, red) is only 5 h [3]. In 2003, Xiao-Jun Wang et al. [4] reported a red long-lasting phosphor: MgSiO3 :Eu2+ , Dy3+ , Mn2+ . Its red emission peaked at 660 nm can last over 4 h after UV irradiation.
∗ Corresponding author at: Department of Physics, University of Science and Technology of China, Hefei, China. Tel.: +86 551 3607412; fax: +86 551 3607417. E-mail address: [email protected] (M. Yin).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.01.059
In general, the Dy3+ dopants could act as electron trap centers in long-lasting phosphors based on oxysalt of alkaline earth ions [5]. Therefore, they should effectively postpone the afterglow of luminescence centers. In this work, a new red long-lasting silicate phosphor, Mg2 SiO4 :Dy3+ , Mn2+ was prepared by solidstate reaction. Red phosphorescence and afterglow properties of Mg2 SiO4 :Dy3+ , Mn2+ were investigated. 2. Experimental Mg2 SiO4 powder samples were synthesized by solid-state reaction. Mg(OH)2 ·4MgCO3 ·6H2 O (AR), SiO2 (AR), MnCO3 (CR), Dy2 O3 (4N) were used as starting materials. Their mixtures were sintered at 1300 ◦ C for 6 h in a reducing atmosphere. The samples’ atomic ratio of doped ions to Mg2+ is given as follows: (1) Mn2+ doped samples: Mn2+ (1–4%). (2) Dy3+ doped samples: Dy3+ (0–6%). (3) Samples co-doped with Dy3+ and Mn2+ :Dy3+ (1–8%), Mn2+ (3%). The excitation spectrum of Mg2 SiO4 :1% Mn2+ (Fig. 1a) was measured by a FLUOROLOG-TAU fluorescence spectrometer. Other excitation and emission spectra, phosphorescence decay curves, and phosphorescence spectra were recorded by a HITACHI-850 fluorescence spectrometer. A BR2000III thermo-luminescence (TL) meter was utilized to detect TL curves in the range of 25–300 ◦ C with a heating rate of 10 ◦ C/s, and samples used to record decay curve, phosphorescence spectra and TL curves were
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irradiated for 3 min with a low-pressure mercury lamp (254 nm) before measurement.
3. Results and discussion The XRD pattern of Mg2 SiO4 :Mn2+ sample (be omitted) can be indexed on Mg2 SiO4 (JCPDS data base No. 85-1364), excess SiO2 (JCPDS No. 77-1315) and a little MgSiO3 (JCPDS No. 11-0273). Unfortunately, formation of MgSiO3 could be hardly avoided in the synthesis of Mg2 SiO4 even by sol–gel method [6,7]. 3.1. Photo-luminescence properties 3.1.1. Excitation and emission spectra of Mn2+ -doped samples Fig. 1 represents the excitation and emission spectra of Mg2 SiO4 :1% Mn2+ . Excited by 215 nm UV light, which is the tail part of the VUV excitation band peaked at 180 nm, the emission spectrum (Fig. 1b) consists of a broad band at 650 nm. The emission band can be fitted by two Gaussian peaks at 645 and 723 nm, which should be attributed to the transitions of Mn2+ ions occupying two nonequivalent Mg2+ sites, mirror symmetric octahedron (M2 ) and inversion symmetric octahedron (M1 ), respectively [8]. The similar emission band was also observed and investigated in Mg2 SiO4 :Mn2+ samples prepared by sol–gel method [7]. Moreover, this band shifts to longer wavelength with an increase of Mn2+ concentration. For example, the emission band of Mg2 SiO4 :3% Mn2+ sample is shifted to about 660 nm under the same excitation. This phenomenon results from the relative intensity variety of the two peaks with the increasing Mn2+ concentration and the slight red shift of both Gaussian peaks, which may be the result of exchange interaction between Mn2+ ions [9]. The excitation spectrum (Fig. 1a) of 650 nm emission consists of a band peaked at 180 nm, corresponding to the absorption of
Fig. 1. Excitation and emission spectra of Mg2 SiO4 :1% Mn2+ Solid line (a) excitation spectrum (λem = 650 nm); (b) emission spectrum (λex = 215 nm). Dash line: fitted Gaussian peaks.
host lattice, and a band in the range of 225–300 nm, which can be attributed to the Mn2+ –O2− charge transfer transition [10,11], as well as several bands in near ultraviolet and visible region. The excitation spectrum in the range of 300–600 nm can be fitted by the sum of seven Gaussian peaks at 545, 461, 429, 412, 367, 349 and 316 nm (dash line). Since the octahedral crystal field of Mg2+ sites is slightly distorted, degeneration of the 4 E(4 G) and 4 A1 (4 G) levels is released. The fitting parameters B and /B of Tanabe–Sugano diagram were calculated as 735.0 cm−1 and 10.63, respectively. Accordingly, the seven peaks mentioned above can be attributed to the Mn2+ transitions from the ground state 6 A1 (6 S) to 4 T1 (4 G), 4 T2 (4 G), 4 E(4 G), 4 A1 (4 G), 4 T2 (4 D), 4 E(4 D) and 4 T (4 P), respectively. 1 In Fig. 1a, the strong excitation peaks in visible region can make the best use of energy of visible light from the sun, which is an advantage of this red long-lasting phosphor. 3.1.2. Excitation and emission spectra of Dy3+ -doped samples The excitation and emission spectra of Mg2 SiO4 :2% Dy3+ and the emission spectrum of un-doped Mg2 SiO4 are depicted in Fig. 2. Comparing the two emission spectra of Mg2 SiO4 :2% Dy3+ (Fig. 2b), and un-doped Mg2 SiO4 (Fig. 2c), one can see that the emission lines of Dy3+ ions (480, 487, 580 nm) in Mg2 SiO4 :Dy3+ are overlapped on the emission bands of un-doped Mg2 SiO4 , peaked at 460 and 640 nm. The emission bands of the un-doped sample may result from the Si Si oxygen vacancy [12] due to the reducing atmosphere during preparation. The three emission lines at 480, 487 and 580 nm in Fig. 2b can be assigned to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ , respectively. The excitation spectrum in the range of 270–500 nm consists of the f–f transitions of
Fig. 2. (a) Excitation spectrum of Mg2 SiO4 :2% Dy3+ (λem = 580 nm); (b) emission spectrum of Mg2 SiO4 :2% Dy3+ (λex = 215 nm); (c) emission spectrum of Mg2 SiO4 (λex = 215 nm).
L. Lin et al. / Journal of Alloys and Compounds 455 (2008) 327–330
:Dy3+ ,
Mn2+
Fig. 3. (a) Emission spectra of Mg2 SiO4 3% series with different concentration of Dy3+ (λex = 215 nm); (b) Intensity of fitted Gaussian peaks at 656 nm and 730 nm of Mg2 SiO4 :Dy3+ , 3% Mn2+ series (λex = 215 nm) vs. Dy3+ concentrations.
Dy3+ : 295 nm (6 H15/2 → 4 D7/2 ), 325 nm (6 H15/2 → 6 P3/2 ), 350 nm (6 H15/2 → 6 P7/2 ), 365 nm (6 H15/2 → 6 P5/2 ), 386 nm (6 H15/2 → 4 M21/2 ), 420 nm (6 H15/2 → 4 G11/2 ), 453 nm (6 H15/2 → 4 J15/2 ), 475 nm (6 H15/2 → 4 F9/2 ). The intensity of Dy3+ emission reaches its maximum while the concentration of Dy3+ is 2%. However, these intensities, as well as those of host’s emission, are only about 10% of that of Mn2+ when excited by 215 nm, which could be hardly detected in the emission spectrum of co-doped samples under UV excitation. 3.1.3. Emission spectra of samples co-doped with Dy3+ and Mn2+ ions A very interesting phenomenon is the sensitizing effect of Dy3+ to the emission of Mn2+ , which has not been reported in other long-lasting phosphors. Fig. 3a shows the emission spectra of Mg2 SiO4 co-doped with Dy3+ and Mn2+ , where the concentration of Mn2+ was fixed at 3% and that of Dy3+ varied from 0 to 6%. The intensities of red emission in all samples with Dy3+ are stronger than that in sample without Dy3+ . When Dy3+ concentration is 2%, the intensity of 656 nm band increases 1.5 times. The emission intensity of Mn2+ decreases for samples with higher concentration of Dy3+ , attributing to the concentration quenching of Dy3+ . Moreover, the shape of emission spectra of Mn2+ changes due to the adding of Dy3+ . Fig. 3b describes the dependence of emission intensity of Mn2+ on the concentration of Dy3+ . It is clear that the curves for 656 and 730 nm, which correspond to the emissions from Mn2+ ions located in M2 and M1 sites, respectively, are different: the Mn2+ ions in M2 sites are sensitized more effectively by Dy3+ . This result indicates that co-doped samples show higher initial intensity than Mn2+ -doped samples during the afterglow process. However, the mechanism of the sensitizing effect of Dy3+ is not clear now, and further investigation is needed.
329
Fig. 4. Experimental (symbols) and fitted (lines) decay curves of Mg2 SiO4 :3% Mn2+ (λem = 660 nm) and Mg2 SiO4 :8% Dy3+ , 3% Mn2+ (λem = 660 nm).
3.2. Afterglow properties Fig. 4 shows the decay curves of Mn2+ emission in Mn2+ doped and co-doped samples. Each can be fitted well by a hyperbolic curve rather than an exponential decay. The fitting curves and the parameters are represented in Fig. 4, which prove that co-doped Dy3+ ions increase the initial phosphorescence intensity of Mn2+ emission. In the darkness, the red afterglow of Mn2+ doped and codoped samples can be seen for 6 and 17 min, respectively after UV irradiation. Fig. 5 illustrates the phosphorescence spectra of co-doped sample at the recorded decay time of 1, 5 and 10 min after the removal of the excitation source, respectively. It is clear that only the emission of Mn2+ was detected in the co-doped sample in the decay process, and the shapes of the bands are similar at different decay times. Thus, the co-doped samples represent red afterglow, consisting with the observation of the naked eyes. It was reported that doping ions Dy3+ serve as trap centers in MgSiO3 :Eu2+ , Dy3+ , Mn2+ [4]. From the TL curves of Mg2 SiO4
Fig. 5. Phosphorescence spectra of Mg2 SiO4 :8% Dy3+ , 3% Mn2+ .
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the afterglow of samples [13]. Therefore, the doping of Dy3+ ions can postpone the afterglow of co-doped samples effectively. The peaks of the Mn2+ -doped sample at about 190 and 260 ◦ C are out of the proper range of TL peaks, so that defects of the Mn2+ -doped sample are hard to release trapped electrons at room temperature. This analysis is consistent with the results of the eye-observation. 4. Conclusions Red long-lasting phosphorescence was observed in Mg2 SiO4 :Dy3+ , Mn2+ powder samples prepared by solid-state reaction. In the excitation spectrum with 650 nm emission in Mg2 SiO4 :Mn2+ , there are several bands in the visible region, which can utilize the energy of visible light from the sun. Our experiments have shown that Mg2 SiO4 :Mn2+ samples co-doped with Dy3+ have higher initial phosphorescence intensity and longer decay time than Mn2+ doped ones. Afterglow time of Mn2+ doped and co-doped samples are 6 and 17 min, respectively. The experiment results indicate that the function of doped Dy3+ ions is trap centers. Moreover, the sensitizing effect of Dy3+ to the emission of Mn2+ was obtained in co-doped samples as a novel phenomenon. Acknowledgements Fig. 6. TL curves of Mg2 SiO4 samples: (a) 3% Mn2+ ; (b) 2% Dy3+ ; (c) 2% Dy3+ , 3% Mn2+ . Table 1 TL peaks of single doped and co-doped Mg2 SiO4 samples Concentration of Mn2+ and Dy3+
Peaks (◦ C)
3% Mn2+ 2% Dy3+ 2% Dy3+ , 3% Mn2+
194, 262 66, >300 74, 189, >300
doped with Mn2+ and/or Dy3+ (Fig. 6), we believe that Dy3+ ions act as the same role in co-doped Mg2 SiO4 samples. Fig. 6 depicts the TL curves of Mg2 SiO4 doped with (a) 3% Mn2+ ; (b) 2% Dy3+ ; and (c) 2% Dy3+ and 3% Mn2+ . Relative intensity of three TL curves in this figure is comparable, as the same irradiated area and irradiated time were used. Corresponding peaks of these TL curves are listed in Table 1. According to Fig. 6 and Table 1, it is obvious that the doped Dy3+ ions act as new trap centers, corresponding to TL peak at about 70 ◦ C. In general, the trap centers, corresponding to the TL peaks in the range of 50–110 ◦ C, are helpful to extend
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