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Influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Nd3+
RARE METALS, Vol. 27, No. 4, Aug 2008, p. 335
Influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Nd3+ TENG Xiaoming, ZHUANG Weidong, and HE Huaqiang National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem Advanced Materials Co. Ltd., Beijing 100088, China Received 5 September 2007; received in revised form 27 October 2007; accepted 8 November 2007
Abstract The long afterglow phosphor CaAl2O4:Eu2+,Nd3+ was prepared by the high temperature solid-state reaction method, and the influence of La3+ and Dy3+ on the properties of the long afterglow phosphor was studied by X-ray diffraction (XRD), photoluminescence (PL), and thermoluminescence (TL). The XRD pattern shows the host phase of CaAl2O4 is produced and no impurity phase appears. The peak wavelength of the phosphor does not vary with La3+ and Dy3+ doping. It implies that the crystal field, which affects the 5d electron states of Eu2+, is not changed dramatically after doping of La3+ and Dy3+. The TL spectra indicate that the phosphor doped with La3+ or Dy3+ produces different depths of trap energy level. In the mechanism of long afterglow luminescence, it is considered that La3+ or Dy3+ works as trap energy level. The decay time lies on the number of electrons in the trap energy level and the rate of the electrons returning to the excitation level. Keywords: rare earth; aluminate; long afterglow; phosphor; electron traps
1. Introduction The aluminate long afterglow phosphor can store energy after short time irradiation by sunlight or fluorescent lamp, and then emit visible light in the darkness. Furthermore, the aluminate long afterglow phosphor can overcome the disadvantages of sulfide phosphors, and has characteristics of high luminescent brightness, long afterglow time, good chemical stability and without pollution and so on. Thus the aluminate long afterglow phosphor is widely applied to luminescent paint, night illumination, instrumental display etc. [1-4]. Studies on the aluminate system have been mainly focused on the doping with the second activator except Eu since 1990s, for example, Dy and Nd. It is expected that the phosphor can form the proper trap energy level by importing microelements. So the luminescent brightness and afterglow time are enhanced [5-10]. Tang et al. studied the luminescent characteristic of SrAl2O4: Eu2+ in 1995 [3]. Yamamoto et al. reported the long afterglow characteristic of SrAl2O4: Eu2+, Dy3+ and CaAl2O4: Eu2+, Dy3+ in 1997 [6]. In this article, the influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Corresponding author: ZHUANG Weidong
E-mail: [email protected]
Nd3+ was studied. It is proved that the La3+ and Dy3+ works as trap energy level. The afterglow time lies on the number of electrons in the trap energy level and the rate that the electrons return to the excitation level.
2. Experimental 2.1. Preparation The long afterglow phosphor CaAl2O4: Eu2+, Nd3+, RE3+ (RE = La or Dy) was prepared by the high temperature solid-state reactions. The raw materials were CaCO3 (3N), Al2O3(4N), Eu2O3(4N), Nd2O3(4N), La2O3(4N), Dy2O3(4N) and H3BO3 is added as flux. The mixture of raw materials was ground and then sintered in reducing atmosphere at 1300qC for about 3 h. After cooling and grinding, the sample of the aluminate long afterglow phosphor CaAl2O4: Eu2+, Nd3+, RE3+ was obtained. 2.2. Measurements The luminescent brightness and the afterglow time of the sample were measured using a ST-900PM photometer. The excitation and emission spectra of the sample were measured using FluoroMax-II spectrofluorometer made in the
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USA. The crystal structure of the sample was checked using a MXP2NAHF X-ray diffractometer. The TL spectra were measured by the Lab-made 1100AUTOMATED TL SYSTEM.
3. Results and discussion 3.1. Excitation and emission spectra of CaAl2O4: Eu2+, Nd3+, RE3+ phosphor Fig. 1 shows the emission spectra of CaAl2O4: Eu2+, Nd3+, RE3+ phosphor. The emission spectra (Oex = 365 nm) show a broad band from Eu2+ about at 440 nm. This emission is due to the transition of Eu2+ that originates from 4f65d excitation state to 4f7 ground state. The peak wavelength of the phosphor does not vary with the doped RE3+. It implies that the crystal field, which affects the 5d electron states of Eu2+, is not changed dramatically by the variations of the doped RE3+.
Fig. 2. Excitation spectra of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
Fig. 1. Emission spectra of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
Fig. 2 shows the excitation spectra of the phosphor. There are two excitation peaks at 275 nm and 338 nm in the excitation spectra (Oem = 440 nm), which is consistent with the absorption of the host CaAl2O4 and the transition of Eu2+ that is from 4f7 to 4f65d1, respectively [11]. 3.2. X-ray diffraction analysis Fig. 3 shows the XRD patterns of CaAl2O4: Eu2+, Nd3+, RE3+ samples. It has a monoclinic crystal structure. Its lattice parameters are a = 0.8698 nm, b = 0.8092 nm and c = 1.5208 nm, (JCPDS: 23-1036). The XRD patterns show that the host phase of CaAl2O4 is formed for the samples with different doped RE3+. It is consistent with the XRD pattern of the phosphor without RE3+ doping. There is only a single phase that belongs to CaAl2O4 and no impurity phase appears. It is argued that this single phase has been developed fully through the preparation procedure.
Fig. 3. XRD patterns of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
Table 1 shows the crystal lattice parameters of the long afterglow phosphor doped with different RE3+. The electric charge and radius of RE3+ are different from those of Ca2+. When La3+ replaces Ca2+, the crystal lattice will expand because the radius of La3+ is larger than that of Ca2+. On the contrary, when Dy3+ replaces Ca2+, the crystal lattice will shrink because the radius of Dy3+ is smaller than that of Ca2+. When La3+ and Dy3+ replace Ca2+ simultaneously, the parameter changes are not much.
Teng X.M. et al., Influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Nd3+ Table 1. Crystal lattice parameters of the phosphor Phosphor
a / nm
b / nm
c / nm
CaAl2O4:Eu2+,Nd3+
0.8708
0.8103
1.5219
0.8715
0.8107
1.5225
0.8704
0.8099
1.5216
0.8710
0.8104
1.5221
CaAl2O4:Eu2+,Nd3+,La3+ 2+
3+
CaAl2O4:Eu ,Nd ,Dy
3+
CaAl2O4:Eu2+,Nd3+,La3+,Dy3+
3.3. TL spectra analysis In general, the initial brightness and afterglow time are related to the density and depth of the trap energy level besides the density of luminescent center. The TL spectra are measured to know the density and depth of the trap energy level. Fig. 4 shows the TL spectra of CaAl2O4: Eu2+, Nd3+, RE3+. The spectra are all unitary. The intensity of the thermal spectrum peak stands for the number of electrons captured by the trap energy level or the number of trap energy level. The CaAl2O4: Eu2+, Nd3+, RE3+ phosphors all have different depths of trap energy level (Table 2). The depth of CaAl2O4: Eu2+, Nd3+ phosphor is the lowest, and the CaAl2O4: Eu2+, Nd3+ doped with La3+ and Dy3+ is the highest. The deeper the trap energy level, the slower the rate of the electrons releasing from the traps, so the afterglow time of CaAl2O4: Eu2+, Nd3+, RE3+ become longer. It is consistent with theoretical analysis.
Fig. 4. TL spectra of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+. Table 2. Trap energy levels of CaAl2O4: Eu2+, Nd3+, RE3+ Phosphor 2+
Tm 3+
CaAl2O4:Eu ,Nd
CaAl2O4:Eu2+,Nd3+,La3+
T1
T2
E / eV
139.2
110.9
166.4
0.76
149.9
121.1
179.0
0.80
CaAl2O4:Eu2+,Nd3+,Dy3+
145.3
116.4
175.2
0.79
CaAl2O4:Eu2+,Nd3+,La3+,Dy3+
153.6
125.1
182.5
0.83
The calculation of the trap energy adopts the empirical formula, which was found in 1969 by Chen [12].
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E = CIJ (kTm2)/IJ BIJ (2kTm), where, IJ = Tm T1, į = T2 Tm, Ȧ = T2 T1, µg = į/Ȧ, k = Tm + 273.15; Tm is the temperature of the TL spectra peak; T1 and T2 is the temperature of the half high peak, furthermore T1 < T2; CIJ and BIJ are the constant. 3.4. Influence of La3+ and Dy3+ doping on the properties of long afterglow phosphor Table 3 and Fig. 5 show the initial brightness and the decay curve of CaAl2O4: Eu2+, Nd3+, RE3+, respectively. Phosphor doping La3+ and Dy3+ not only enhances its luminescent brightness but also greatly prolongs its afterglow time. Table 3. Comparison of the brightness of CaAl2O4: Eu2+, Nd3+, RE3+ Phosphor
Relative brightness
2+
3+
CaAl2O4: Eu , Nd 2+
3+
71.1 3+
CaAl2O4: Eu , Nd , La
89.0
CaAl2O4: Eu2+, Nd3+, Dy3+ 2+
3+
3+
CaAl2O4: Eu , Nd , La , Dy
83.2 3+
100.0
Fig. 5. Long decay curves of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
In general, the aluminate long afterglow phosphor activated by Eu2+ can produce trap energy level with some quantity and depth due to doping with other rare earth ions, and then prolong the luminescent time [13-14]. When the phosphor doped with La3+ or Dy3+, unequal replacement is produced between La3+, Dy3+ and Ca2+. Thus more trap energy level is produced and the depth of the energy level is deeper. Otherwise, La3+ or Dy3+ possesses proper affinity toward to the electrons in the energy level, which can release electrons slowly and produce long afterglow luminescence. The more the trap energy level, the more electrons are stored in the energy level. The deeper the energy level, the slower the rate of electron release. Therefore the La3+ or Dy3+ addi-
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tion enhances the initial brightness and prolongs the luminescent time. 3.5. Mechanism on long afterglow luminescence of CaAl2O4: Eu2+, Nd3+, RE3+ Many researches have been conducted on the long afterglow luminescence about the aluminates phosphor. The typical mechanism is due to doping with co-activator, and a new energy level named trap energy level is produced in the system [6]. Fig. 6 shows the mechanism of long afterglow luminescence of CaAl2O4: Eu2+, Nd3+, RE3+. The luminescence is produced by the 4f o 5d transition of Eu. The Eu2+ ions have influences on the structure of host crystal since the radius of Eu2+ is larger than that of Ca2+. The phosphor produces distortion of the host crystal and changes the structure of the crystal. When the phosphor is doped with La3+ or Dy3+, the new trap energy level is produced due to the difference radii between RE3+ and Ca2+. The energy level lies between the excitation level and ground level of Eu2+. When the phosphor is excited, a part of the electrons return to the lower energy level and produce luminescence. The other electrons are stored in the trap energy level via relaxation process. When the electrons in the trap energy level absorb energy, they return to the excitation state again, and then transfer to the ground state and produce luminescence. The escape of captured electrons is a continuous process; thus the phosphor exhibits the character of long afterglow luminescence. The depth of the trap energy level is important for the long afterglow phosphor; if the energy level is too shallow, the electrons easily escape from the traps at room temperature, and the afterglow time is too short or the luminescence cannot be observed at all; if the energy level is too deep, the number of electrons escaping from the trap is too less, sometimes it does not exist. Both are not beneficial to long afterglow luminescence.
RARE METALS, Vol. 27, No. 4, Aug 2008
afterglow time is; the more the absorbed energy is, the easier the electrons conquer the energy distance between the trap energy level and excitation level. Thus the phosphor produces the phenomenon of long afterglow luminescence, but it does not mean to prolong the luminescent time when the absorbed energies continuously increase. If the adequate energies cause the electrons to return to the excitation level once, it does not help to prolong the luminescent time. Contrarily, if the energies are too low, it does not cause the electrons return to the excitation level, and the afterglow luminescence is not observed. Thus the afterglow time lies on the number of electrons in the trap energy level and the rate at which the electrons return to the excitation level. The intensity of the afterglow lies in the rate at which the electrons in trap energy level return to the excitation level in unit time.
4. Conclusions (1) The peak wavelength of the phosphor does not vary with the RE3+ doping. It implies that the crystal field, which affects the 5d electron states of Eu2+, is not changed dramatically by the variations of RE3+ (2) The result of XRD analysis indicates that the host phase of CaAl2O4 forms with the La3+ or Dy3+ doping. No impurity phase appears. (3) The TL spectra indicate that the phosphor doped with La3+ or Dy3+ produce a deeper trap energy level than the other samples. (4) The phosphor doped with La3+ or Dy3+ will not only enhance its luminescent brightness but also greatly prolong its afterglow time. The La3+ or Dy3+ works as trap energy level. The afterglow time lies on the depth of the trap energy level and the rate at which the electrons return to the excitation level.
Acknowledgements This study is financially supported by the National Natural Science Foundation of China (No. 50204002) and the National High-Tech Research and Development Program of China (No. 2001AA324080).
References Fig. 6. Mechanism of long afterglow luminescence in CaAl2O4: Eu2+, Nd3+, RE3+.
The afterglow time is also concerned with the absorbed energies and the electrons stored in the trap energy level: the more the electrons in the trap energy level, the longer the
[1] Abbruscato V., Optical and electrical properties of SrAl2O4: Eu2+, J. Electrochem. Soc., 1971, 118 (6): 930. [2] Murayam Y., Long phosphorescent materials, Ceram. Jpn., 1997, 32 (1), 40. [3] Tang M.D., Li C.K., and Gao Z.W., Research of phosphorescence characteristics on SrAl2O4: Eu2+ phosphor, Chin. J. Lumin. (in Chinese), 1995, 16 (1): 51.
Teng X.M. et al., Influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Nd3+ [4] Li X.D., A new long persistence phosphor, J. Rare Earth Inf. (in Chinese), 1997, (3): 20. [5] Matsuzawa T., Aoki Y., Takeuchi N., and Murayama Y., A new long phosphorescent phosphor with high brightness, SrAl2O4: Eu2+, Dy3+, J. Electrochem. Soc., 1996, 143: 2670. [6] Yamamoto H. and Matsuzawa T., Mechanism of long phosphorescence of SrAl2O4: Eu2+, Dy3+ and CaAl2O4: Eu2+, Dy3+ , J. Lumin., 1997, 72-74: 287. [7] Kamada M., Murakami J., and Ohno N., Excitation spectra of a long-persistent phosphor SrAl2O4: Eu, Dy in vacuum ultraviolet region, J. Lumin., 2000, 87-89: 1042. [8] Yu J.J., Xiao Z.G., and Ning G.L., Modification of luminescence properties of SrAl2O4 afterglow phosphors by doping with (NH4)2HPO4, J. Rare Earths, 2005, 23 (S2): 39. [9] Geng X.J., Chen Y.J., Qiu G.M., Xiao L.J., Yan C.H., and Sun Y.B., Synthesis of long afterglow photoluminescent materials Sr2MgSi2O7: Eu2+, Dy3+ by Sol-Gel Method, J. Rare
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Earths, 2005, 23 (3): 292. [10] Hu J., Sun J.L., Liu J.L., Shi A., Xu M., and Gao Q.Q., Defect chemistry of long afterglow material SrAl2O4: Eu, Dy, J. Rare Earths, 2005, 23 (S2): 35. [11] Meng X.G., Wang Y.S., Sun L., He Z.Y., Deng C.Y., and Li H.L., Characteristics and mechanism of (Eu2+, Nd3+): CaAl2O4 long afterglow, Chin. J. Lumin. (in Chinese), 2003, 23 (3): 356. [12] Chen R., J. Glow curves with general order kinetics, Electrochem. Soc. Solid State Sci., 1969, 116: 1254. [13] Jia W.Y., Yuan H.B., and Yen W.M., Phosphorescent dynamics in SrAl2O4 single crystal fibers, J. Lumin., 1998, 76: 424. [14] Matsuzawa T., Nabae T., Sasajima K., Kumuro S., and Morikawa T., Effects of composition on the long phosphorescence SrAl2O4: Eu, Dy phosphor crystals, J. Electrochem. Soc., 1997, 144 (9): L243.
Influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Nd3+ TENG Xiaoming, ZHUANG Weidong, and HE Huaqiang National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem Advanced Materials Co. Ltd., Beijing 100088, China Received 5 September 2007; received in revised form 27 October 2007; accepted 8 November 2007
Abstract The long afterglow phosphor CaAl2O4:Eu2+,Nd3+ was prepared by the high temperature solid-state reaction method, and the influence of La3+ and Dy3+ on the properties of the long afterglow phosphor was studied by X-ray diffraction (XRD), photoluminescence (PL), and thermoluminescence (TL). The XRD pattern shows the host phase of CaAl2O4 is produced and no impurity phase appears. The peak wavelength of the phosphor does not vary with La3+ and Dy3+ doping. It implies that the crystal field, which affects the 5d electron states of Eu2+, is not changed dramatically after doping of La3+ and Dy3+. The TL spectra indicate that the phosphor doped with La3+ or Dy3+ produces different depths of trap energy level. In the mechanism of long afterglow luminescence, it is considered that La3+ or Dy3+ works as trap energy level. The decay time lies on the number of electrons in the trap energy level and the rate of the electrons returning to the excitation level. Keywords: rare earth; aluminate; long afterglow; phosphor; electron traps
1. Introduction The aluminate long afterglow phosphor can store energy after short time irradiation by sunlight or fluorescent lamp, and then emit visible light in the darkness. Furthermore, the aluminate long afterglow phosphor can overcome the disadvantages of sulfide phosphors, and has characteristics of high luminescent brightness, long afterglow time, good chemical stability and without pollution and so on. Thus the aluminate long afterglow phosphor is widely applied to luminescent paint, night illumination, instrumental display etc. [1-4]. Studies on the aluminate system have been mainly focused on the doping with the second activator except Eu since 1990s, for example, Dy and Nd. It is expected that the phosphor can form the proper trap energy level by importing microelements. So the luminescent brightness and afterglow time are enhanced [5-10]. Tang et al. studied the luminescent characteristic of SrAl2O4: Eu2+ in 1995 [3]. Yamamoto et al. reported the long afterglow characteristic of SrAl2O4: Eu2+, Dy3+ and CaAl2O4: Eu2+, Dy3+ in 1997 [6]. In this article, the influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Corresponding author: ZHUANG Weidong
E-mail: [email protected]
Nd3+ was studied. It is proved that the La3+ and Dy3+ works as trap energy level. The afterglow time lies on the number of electrons in the trap energy level and the rate that the electrons return to the excitation level.
2. Experimental 2.1. Preparation The long afterglow phosphor CaAl2O4: Eu2+, Nd3+, RE3+ (RE = La or Dy) was prepared by the high temperature solid-state reactions. The raw materials were CaCO3 (3N), Al2O3(4N), Eu2O3(4N), Nd2O3(4N), La2O3(4N), Dy2O3(4N) and H3BO3 is added as flux. The mixture of raw materials was ground and then sintered in reducing atmosphere at 1300qC for about 3 h. After cooling and grinding, the sample of the aluminate long afterglow phosphor CaAl2O4: Eu2+, Nd3+, RE3+ was obtained. 2.2. Measurements The luminescent brightness and the afterglow time of the sample were measured using a ST-900PM photometer. The excitation and emission spectra of the sample were measured using FluoroMax-II spectrofluorometer made in the
336
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USA. The crystal structure of the sample was checked using a MXP2NAHF X-ray diffractometer. The TL spectra were measured by the Lab-made 1100AUTOMATED TL SYSTEM.
3. Results and discussion 3.1. Excitation and emission spectra of CaAl2O4: Eu2+, Nd3+, RE3+ phosphor Fig. 1 shows the emission spectra of CaAl2O4: Eu2+, Nd3+, RE3+ phosphor. The emission spectra (Oex = 365 nm) show a broad band from Eu2+ about at 440 nm. This emission is due to the transition of Eu2+ that originates from 4f65d excitation state to 4f7 ground state. The peak wavelength of the phosphor does not vary with the doped RE3+. It implies that the crystal field, which affects the 5d electron states of Eu2+, is not changed dramatically by the variations of the doped RE3+.
Fig. 2. Excitation spectra of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
Fig. 1. Emission spectra of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
Fig. 2 shows the excitation spectra of the phosphor. There are two excitation peaks at 275 nm and 338 nm in the excitation spectra (Oem = 440 nm), which is consistent with the absorption of the host CaAl2O4 and the transition of Eu2+ that is from 4f7 to 4f65d1, respectively [11]. 3.2. X-ray diffraction analysis Fig. 3 shows the XRD patterns of CaAl2O4: Eu2+, Nd3+, RE3+ samples. It has a monoclinic crystal structure. Its lattice parameters are a = 0.8698 nm, b = 0.8092 nm and c = 1.5208 nm, (JCPDS: 23-1036). The XRD patterns show that the host phase of CaAl2O4 is formed for the samples with different doped RE3+. It is consistent with the XRD pattern of the phosphor without RE3+ doping. There is only a single phase that belongs to CaAl2O4 and no impurity phase appears. It is argued that this single phase has been developed fully through the preparation procedure.
Fig. 3. XRD patterns of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
Table 1 shows the crystal lattice parameters of the long afterglow phosphor doped with different RE3+. The electric charge and radius of RE3+ are different from those of Ca2+. When La3+ replaces Ca2+, the crystal lattice will expand because the radius of La3+ is larger than that of Ca2+. On the contrary, when Dy3+ replaces Ca2+, the crystal lattice will shrink because the radius of Dy3+ is smaller than that of Ca2+. When La3+ and Dy3+ replace Ca2+ simultaneously, the parameter changes are not much.
Teng X.M. et al., Influence of La3+ and Dy3+ on the properties of the long afterglow phosphor CaAl2O4: Eu2+, Nd3+ Table 1. Crystal lattice parameters of the phosphor Phosphor
a / nm
b / nm
c / nm
CaAl2O4:Eu2+,Nd3+
0.8708
0.8103
1.5219
0.8715
0.8107
1.5225
0.8704
0.8099
1.5216
0.8710
0.8104
1.5221
CaAl2O4:Eu2+,Nd3+,La3+ 2+
3+
CaAl2O4:Eu ,Nd ,Dy
3+
CaAl2O4:Eu2+,Nd3+,La3+,Dy3+
3.3. TL spectra analysis In general, the initial brightness and afterglow time are related to the density and depth of the trap energy level besides the density of luminescent center. The TL spectra are measured to know the density and depth of the trap energy level. Fig. 4 shows the TL spectra of CaAl2O4: Eu2+, Nd3+, RE3+. The spectra are all unitary. The intensity of the thermal spectrum peak stands for the number of electrons captured by the trap energy level or the number of trap energy level. The CaAl2O4: Eu2+, Nd3+, RE3+ phosphors all have different depths of trap energy level (Table 2). The depth of CaAl2O4: Eu2+, Nd3+ phosphor is the lowest, and the CaAl2O4: Eu2+, Nd3+ doped with La3+ and Dy3+ is the highest. The deeper the trap energy level, the slower the rate of the electrons releasing from the traps, so the afterglow time of CaAl2O4: Eu2+, Nd3+, RE3+ become longer. It is consistent with theoretical analysis.
Fig. 4. TL spectra of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+. Table 2. Trap energy levels of CaAl2O4: Eu2+, Nd3+, RE3+ Phosphor 2+
Tm 3+
CaAl2O4:Eu ,Nd
CaAl2O4:Eu2+,Nd3+,La3+
T1
T2
E / eV
139.2
110.9
166.4
0.76
149.9
121.1
179.0
0.80
CaAl2O4:Eu2+,Nd3+,Dy3+
145.3
116.4
175.2
0.79
CaAl2O4:Eu2+,Nd3+,La3+,Dy3+
153.6
125.1
182.5
0.83
The calculation of the trap energy adopts the empirical formula, which was found in 1969 by Chen [12].
337
E = CIJ (kTm2)/IJ BIJ (2kTm), where, IJ = Tm T1, į = T2 Tm, Ȧ = T2 T1, µg = į/Ȧ, k = Tm + 273.15; Tm is the temperature of the TL spectra peak; T1 and T2 is the temperature of the half high peak, furthermore T1 < T2; CIJ and BIJ are the constant. 3.4. Influence of La3+ and Dy3+ doping on the properties of long afterglow phosphor Table 3 and Fig. 5 show the initial brightness and the decay curve of CaAl2O4: Eu2+, Nd3+, RE3+, respectively. Phosphor doping La3+ and Dy3+ not only enhances its luminescent brightness but also greatly prolongs its afterglow time. Table 3. Comparison of the brightness of CaAl2O4: Eu2+, Nd3+, RE3+ Phosphor
Relative brightness
2+
3+
CaAl2O4: Eu , Nd 2+
3+
71.1 3+
CaAl2O4: Eu , Nd , La
89.0
CaAl2O4: Eu2+, Nd3+, Dy3+ 2+
3+
3+
CaAl2O4: Eu , Nd , La , Dy
83.2 3+
100.0
Fig. 5. Long decay curves of CaAl2O4: Eu2+, Nd3+, RE3+: (a) CaAl2O4: Eu2+, Nd3+; (b) CaAl2O4: Eu2+, Nd3+, La3+; (c) CaAl2O4: Eu2+, Nd3+, Dy3+; (d) CaAl2O4: Eu2+, Nd3+, La3+, Dy3+.
In general, the aluminate long afterglow phosphor activated by Eu2+ can produce trap energy level with some quantity and depth due to doping with other rare earth ions, and then prolong the luminescent time [13-14]. When the phosphor doped with La3+ or Dy3+, unequal replacement is produced between La3+, Dy3+ and Ca2+. Thus more trap energy level is produced and the depth of the energy level is deeper. Otherwise, La3+ or Dy3+ possesses proper affinity toward to the electrons in the energy level, which can release electrons slowly and produce long afterglow luminescence. The more the trap energy level, the more electrons are stored in the energy level. The deeper the energy level, the slower the rate of electron release. Therefore the La3+ or Dy3+ addi-
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tion enhances the initial brightness and prolongs the luminescent time. 3.5. Mechanism on long afterglow luminescence of CaAl2O4: Eu2+, Nd3+, RE3+ Many researches have been conducted on the long afterglow luminescence about the aluminates phosphor. The typical mechanism is due to doping with co-activator, and a new energy level named trap energy level is produced in the system [6]. Fig. 6 shows the mechanism of long afterglow luminescence of CaAl2O4: Eu2+, Nd3+, RE3+. The luminescence is produced by the 4f o 5d transition of Eu. The Eu2+ ions have influences on the structure of host crystal since the radius of Eu2+ is larger than that of Ca2+. The phosphor produces distortion of the host crystal and changes the structure of the crystal. When the phosphor is doped with La3+ or Dy3+, the new trap energy level is produced due to the difference radii between RE3+ and Ca2+. The energy level lies between the excitation level and ground level of Eu2+. When the phosphor is excited, a part of the electrons return to the lower energy level and produce luminescence. The other electrons are stored in the trap energy level via relaxation process. When the electrons in the trap energy level absorb energy, they return to the excitation state again, and then transfer to the ground state and produce luminescence. The escape of captured electrons is a continuous process; thus the phosphor exhibits the character of long afterglow luminescence. The depth of the trap energy level is important for the long afterglow phosphor; if the energy level is too shallow, the electrons easily escape from the traps at room temperature, and the afterglow time is too short or the luminescence cannot be observed at all; if the energy level is too deep, the number of electrons escaping from the trap is too less, sometimes it does not exist. Both are not beneficial to long afterglow luminescence.
RARE METALS, Vol. 27, No. 4, Aug 2008
afterglow time is; the more the absorbed energy is, the easier the electrons conquer the energy distance between the trap energy level and excitation level. Thus the phosphor produces the phenomenon of long afterglow luminescence, but it does not mean to prolong the luminescent time when the absorbed energies continuously increase. If the adequate energies cause the electrons to return to the excitation level once, it does not help to prolong the luminescent time. Contrarily, if the energies are too low, it does not cause the electrons return to the excitation level, and the afterglow luminescence is not observed. Thus the afterglow time lies on the number of electrons in the trap energy level and the rate at which the electrons return to the excitation level. The intensity of the afterglow lies in the rate at which the electrons in trap energy level return to the excitation level in unit time.
4. Conclusions (1) The peak wavelength of the phosphor does not vary with the RE3+ doping. It implies that the crystal field, which affects the 5d electron states of Eu2+, is not changed dramatically by the variations of RE3+ (2) The result of XRD analysis indicates that the host phase of CaAl2O4 forms with the La3+ or Dy3+ doping. No impurity phase appears. (3) The TL spectra indicate that the phosphor doped with La3+ or Dy3+ produce a deeper trap energy level than the other samples. (4) The phosphor doped with La3+ or Dy3+ will not only enhance its luminescent brightness but also greatly prolong its afterglow time. The La3+ or Dy3+ works as trap energy level. The afterglow time lies on the depth of the trap energy level and the rate at which the electrons return to the excitation level.
Acknowledgements This study is financially supported by the National Natural Science Foundation of China (No. 50204002) and the National High-Tech Research and Development Program of China (No. 2001AA324080).
References Fig. 6. Mechanism of long afterglow luminescence in CaAl2O4: Eu2+, Nd3+, RE3+.
The afterglow time is also concerned with the absorbed energies and the electrons stored in the trap energy level: the more the electrons in the trap energy level, the longer the
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