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A red-emitting long-afterglow phosphor of Eu3+, Ho3+ co-doped Y2O3
Journal of Alloys and Compounds 619 (2015) 244–247
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
A red-emitting long-afterglow phosphor of Eu3+, Ho3+ co-doped Y2O3 Wei Xie a,⇑, Yinhai Wang b, Changwei Zou a, Jun Quan a, Lexi Shao a a b
Research Center of Chemistry & Materials Science, Lingnan Normal University, Zhanjiang 524048, PR China School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 28 June 2014 Received in revised form 8 September 2014 Accepted 9 September 2014 Available online 16 September 2014 Keywords: Phosphors Luminescence Optical materials and properties Thermoluminescence
a b s t r a c t Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors were prepared by the solid-state reaction method. X-ray diffraction (XRD) patterns indicate that the Eu3+ and/or Ho3+ doping do not show obvious effect to the cubic Y2O3 crystal. Under UV 254 nm excitation, both samples present same photoluminescence (PL) properties, while Y2O3:Eu3+, Ho3+ phosphor shows a red-emitting long afterglow phenomenon, and the Eu3+ ion are the luminescent center during the decay process. The decay characteristic of Y2O3:Eu3+, Ho3+ phosphor is according with the double exponential equation. Thermoluminescence (TL) analysis indicates that the traps in the Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ host are both origin from Eu3+ ion. The co-doped Ho3+ ion shoaled the trap depth, which is benefit for the carriers to escape from the trap, and then the afterglow phenomenon happened. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Long afterglow phosphor is a kind of environmental friendly materials which could adsorb energy from the visible light, fluorescent lamp or UV light, and store the energy, and then release the energy gradually in form of afterglow [1–13]. Because of the advanced properties, long afterglow materials have been used in many fields, such as illumination in darkness, emergency sign, artwork, information storage, detection of radiation, display, and photocatalytic degradation [6–9]. Nowadays, the yellow-emitting and blue-emitting long afterglow phosphors have been used successfully, such as SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Nd3+ [1,10–13]. However, the duration and intensity of red-emitting long afterglow phosphor are inferior in comparison with the former two phosphors [5,14–16]. Therefore, the development of novel, high-intensity, long-duration, redemitting long afterglow phosphor is required. Recently, Xu and co-workers reported a new NIR long-persistent Cr3+:SrGa12O19 phosphor with an emission duration of more than 2 h and a broadband phosphorescence from 650 to 950 nm [17]. Duan et al. synthesized a novel long lasting phosphorescence phosphor La2Zr2O7:Sm3+, Ti4+, and the color of afterglow/photoluminescence can be adjusted [18]. Besides the novel host, Y2O3 is a conventional host for rare earth doping [19]. For example, Y2O3:Eu3+ was widely used as the phosphor in color kinescope. In this work, a red-emitting long afterglow material Y2O3:Eu3+, Ho3+ was synthesized, and ⇑ Corresponding author. Tel.: +86 759 3183424. E-mail address: [email protected] (W. Xie). http://dx.doi.org/10.1016/j.jallcom.2014.09.092 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
structure, morphology, photoluminescence and decay characteristics of the material were investigated, and the potential mechanism of the afterglow phenomenon was discussed by analysis of TL spectra.
2. Experimental The phosphor samples were prepared by the solid-state reac3+ 3+ tion method. According to Y1.99O3:Eu3+ 0.01 and Y1.98O3:Eu0.01, Ho0.01, stoichiometric composition of Y2O3 (4 N), Eu2O3 (4 N) and Ho2O3 (4 N) were mixed and milled thoroughly in an agate mortar for 2 h. 5 mol% of H3BO3 (3 N) was added as flux. The powders were sintered at 1500 °C for 4 h in air. Finally, the phosphors were obtained, respectively. All of the raw materials are purchased from Sinopharm Chemical Reagent Co., Ltd. The phase identification of the sample was carried out by PANalytical X’pet pro MRD X-ray powder diffraction (XRD) with Cu Ka irradiation (k = 1.5406 Å) at 40 kV tube voltage and 40 mA tube current at room temperature. The filter is Ni 0.5°, step size is 0.016°, time per step is 0.1 s. The morphology and grain size of phosphors were observed by a Philips XL-30 scanning electron microscope (SEM). The photoluminescence (PL) spectra were taken on a Hitachi F-7000 fluorescence spectrophotometer. The decay curves were registered by a GSZF-2A single-photon counter system (Tianjin Gangdong Sci.&Tech. Development Co. Ltd.). The thermoluminescence (TL) curves were measured using a FJ427A1 thermoluminescent dosimeter (Beijing Nuclear Instrument Factory). The heating rate was 1 K/s and the samples were excited by a UV
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254 nm light for 5 min and the measurement started with 10 min delay time. 3. Results and discussion As shown in Fig. 1(a), Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors show the same diffraction patterns, and the patterns are in perfect match with the standard powder peak positions of Y2O3 cubic phase (JCPDF No. 25-1011). Fig. 1(b) is the XRD patterns between 25° and 40°, it can be seen that there is no other phase and no apparent shift can be found, indicating that the doped Eu3+ or Ho3+ show no obvious influence to the cubic Y2O3 crystal, the reason is the content of rare earth ions is too small. Fig. 2(a) and (b) are the SEM micrographs of Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ powders, respectively. It can be clearly seen that the two samples have similar irregular morphology with round edge, and the grain sizes varied from 3 to 8 lm. It can be concluded that slightly rare earth ions doping show no significant influence on the grain sizes and morphology of phosphors. In order to determine the valence state of Ho3+, XPS was measured. As can be seen in Fig. 3, the Ho 4d5/2 peak centered at 161.1 eV, indicating the existence of Ho3+. Fig. 4 shows the photoluminescence spectra of the phosphors excited by UV light at the wavelength of 254 nm. Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors exhibited the same line position and strong red emission, and it can be found that the emission intensity of Y2O3:Eu3+, Ho3+ is a little higher than that of Y2O3:Eu3+. Sharp emissions peaking at 582, 588, 594, 600 and 612 nm can be assigned to the 5D0 ? 7FJ (where J is 0, 1 and 2) transitions of Eu3+ ion (marked in Fig. 4). No other emission can be observed in the Y2O3:Eu3+, Ho3+ sample, indicating that the doped Ho3+ ion do not show any significant emission under 254 nm excitation. In cubic Y2O3, there are two kinds of Y3+ sites, 75% of Y3+ sites are non centrosymmetric with C2 symmetry, and the other 25% are centrosymmetric with S6 symmetry [20–22]. The strongest red emission at 612 nm is due to the low symmetry position of the Eu3+ ion with an inversion center and this forced-electric dipole transition is often dominant in the emission spectrum. So the strongest emission at 612 nm is ascribed to Eu3+ ion which occupied on the C2 site, and these Eu3+ ions are dominant in the host. The samples were excited under UV 254 nm for 5 min, and then the excitation source was removed to examine whether the materials show long afterglow emission in the dark. Y2O3:Eu3+ phosphor reveals no persist phenomena while Y2O3:Eu3+, Ho3+ sample exhibits an obvious red afterglow lasting for about 10 min. Two minutes after the end of excitation, the phosphorescence spectrum of Y2O3:Eu3+, Ho3+ was recorded, and it is shown in the inset of Fig. 5. The emission peaks of the afterglow phosphorescence
remained the similar shape as the photoluminescence spectra, indicating that Eu3+ ion is the luminescent center during the decay process. As can be seen in the left inset of Fig. 5, the persistent luminescence of Y2O3:Eu, Ho material can be observed by the naked eyes and it can last for more than 5 min. The decay curve of the Y2O3:Eu3+, Ho3+ sample was recorded and shown in Fig. 5. It contains a rapid-decaying process and a slow-decaying process, the decay curve is similar to that of typical long afterglow materials, and it can be evaluated by fitting using double exponential equation which reflects the trend of the decay. The equation is as follows [10,23,24]:
I ¼ I1 exp
t
s1
þ I2 exp
t ;
s2
ð1Þ
where I represents the phosphorescent intensity; I1 and I2 are two constants; t represents the time; s1 and s2 are decay constants which decide the decay rate for the rapid and the slow exponentially decay components. The fitting results of the parameters of s1 and s2 and the fitting curves are shown in Fig. 5. The decay characteristic of Y2O3:Eu3+, Ho3+ phosphor is similar to the rare-earth doping alkaline-earth aluminates long afterglow materials, which have drawn much attention from 1990s. One of the typical aluminates is SrAl2O4:Eu2+, RE3+ (RE = Dy, Nd and Ho, etc.), which was successfully synthesized by Matsuzawa in 1996 [25]. Generally, the Eu2+ ion is the only luminescent center in SrAl2O4:Eu2+, RE3+ phosphors, and the doped RE3+ ions often generate the trap level in the host [10,23]. In order to exhibit long afterglow phenomena, the phosphor should have suitable trap level and trap density, and the trapped carriers does recombine in the luminescent centers occurred with the afterglow. The depth of the trap levels could be investigated by thermoluminescence curves (in Fig. 6) fitted by a general order kinetics formula [23,24,26]. The TL intensity as a function of temperature, T, is as follows:
Et IðTÞ ¼ sn0 exp kB T lðl1Þ Z T ðl 1Þs Et dT þ 1 exp ; b kB T T0
ð2Þ
where I(T) is the TL intensity, n0 is the concentration of trapped charges at T = 0, kB is the Boltzmann’s constant, b the heating rate (1 oC/s in this work), Et the activation energy (or depth of the trap), s the frequency factor and l the order of kinetics. Trap level depth Et is proportional to one corresponding glow peak temperature. As the effect of s on Tm is neglected and the frequency of an electron escaping the trap is assumed as 1/s, the thermal activation energy or the
Fig. 1. The XRD patterns of the phosphors.
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Fig. 2. SEM images of (a) Y2O3:Eu3+ and (b) Y2O3:Eu3+, Ho3+ phosphors.
Fig. 3. XPS spectrum of the Ho 4d5/2.
Fig. 5. The decay curve of the Y2O3:Eu3+, Ho3+ sample, the left inset is the afterglow photograph of Y2O3:Eu3+, Ho3+ phosphor, and right inset is the phosphorescence spectrum of Y2O3:Eu3+, Ho3+ which was examined at 2 min after the UV 254 nm light excitation for 5 min.
Fig. 4. The photoluminescence spectra of phosphors excited under UV 254 nm.
trap depth Et (eV) could be estimated by Et = Tm/500 [10,27,28], where Tm (K) is the glow peak temperature in the TL curve. In Fig. 6, the Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors present a broad band peaking at 363 K and 356 K, respectively. It can be seen that the traps exist in the phosphor, no matter the Ho3+ ion was doped or not. Hence, the trap in the phosphors is not caused by Ho3+ ion but Eu3+ ion. This conclusion is different from the mechanism of rare-earth ion doping aluminates and the distinction may be attributed to the difference of the different host matrix. The calculated trap depth of Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors is 0.726 eV and 0.712 eV, respectively. The two values reflect that the trap depth was shoaled by Ho3+ ion doping. Consequently, the carriers can escape from the trap more easily, and then the afterglow appeared. As shown in Fig. 6, the intensity of TL curves of
Fig. 6. The thermoluminescence curves of phosphors.
Y2O3:Eu3+, Ho3+ is lower than that of Y2O3:Eu3+, so we believe that the number of carriers in the Y2O3:Eu3+, Ho3+ is lower the than that of Y2O3:Eu3+ in the measurement process, because the carriers in the Y2O3:Eu3+, Ho3+ is released from the trap and caused the afterglow. When the TL curves are recorded, the decay process is almost finished, and the intensity of TL curves is proportional to the carriers which remained in the trap, so the intensity of TL curves of Y2O3:Eu3+, Ho3+ is lower than that of Y2O3:Eu3+. It can be concluded that the doped of Ho3+ ion is benefit for the carriers to escape from the traps caused by Eu3+ ion, and then generate afterglow phenomenon.
W. Xie et al. / Journal of Alloys and Compounds 619 (2015) 244–247
4. Conclusions Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors show the same photoluminescence spectra while Y2O3:Eu3+, Ho3+ exhibits a red long afterglow phenomenon. The decay characteristic of Y2O3:Eu3+, Ho3+ is described with the double exponential equation. The TL analysis indicates that the traps in the Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ were generated by Eu3+. The doped of Ho3+ shoaled the trap depth, which is beneficial for the carriers to escape from the trap, and then the afterglow appears sequentially. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 21271048, 61106124, 11304276), outstanding young teacher training program in Guangdong province (Yq2013113). References [1] [2] [3] [4] [5] [6] [7] [8]
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
A red-emitting long-afterglow phosphor of Eu3+, Ho3+ co-doped Y2O3 Wei Xie a,⇑, Yinhai Wang b, Changwei Zou a, Jun Quan a, Lexi Shao a a b
Research Center of Chemistry & Materials Science, Lingnan Normal University, Zhanjiang 524048, PR China School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 28 June 2014 Received in revised form 8 September 2014 Accepted 9 September 2014 Available online 16 September 2014 Keywords: Phosphors Luminescence Optical materials and properties Thermoluminescence
a b s t r a c t Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors were prepared by the solid-state reaction method. X-ray diffraction (XRD) patterns indicate that the Eu3+ and/or Ho3+ doping do not show obvious effect to the cubic Y2O3 crystal. Under UV 254 nm excitation, both samples present same photoluminescence (PL) properties, while Y2O3:Eu3+, Ho3+ phosphor shows a red-emitting long afterglow phenomenon, and the Eu3+ ion are the luminescent center during the decay process. The decay characteristic of Y2O3:Eu3+, Ho3+ phosphor is according with the double exponential equation. Thermoluminescence (TL) analysis indicates that the traps in the Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ host are both origin from Eu3+ ion. The co-doped Ho3+ ion shoaled the trap depth, which is benefit for the carriers to escape from the trap, and then the afterglow phenomenon happened. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Long afterglow phosphor is a kind of environmental friendly materials which could adsorb energy from the visible light, fluorescent lamp or UV light, and store the energy, and then release the energy gradually in form of afterglow [1–13]. Because of the advanced properties, long afterglow materials have been used in many fields, such as illumination in darkness, emergency sign, artwork, information storage, detection of radiation, display, and photocatalytic degradation [6–9]. Nowadays, the yellow-emitting and blue-emitting long afterglow phosphors have been used successfully, such as SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Nd3+ [1,10–13]. However, the duration and intensity of red-emitting long afterglow phosphor are inferior in comparison with the former two phosphors [5,14–16]. Therefore, the development of novel, high-intensity, long-duration, redemitting long afterglow phosphor is required. Recently, Xu and co-workers reported a new NIR long-persistent Cr3+:SrGa12O19 phosphor with an emission duration of more than 2 h and a broadband phosphorescence from 650 to 950 nm [17]. Duan et al. synthesized a novel long lasting phosphorescence phosphor La2Zr2O7:Sm3+, Ti4+, and the color of afterglow/photoluminescence can be adjusted [18]. Besides the novel host, Y2O3 is a conventional host for rare earth doping [19]. For example, Y2O3:Eu3+ was widely used as the phosphor in color kinescope. In this work, a red-emitting long afterglow material Y2O3:Eu3+, Ho3+ was synthesized, and ⇑ Corresponding author. Tel.: +86 759 3183424. E-mail address: [email protected] (W. Xie). http://dx.doi.org/10.1016/j.jallcom.2014.09.092 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
structure, morphology, photoluminescence and decay characteristics of the material were investigated, and the potential mechanism of the afterglow phenomenon was discussed by analysis of TL spectra.
2. Experimental The phosphor samples were prepared by the solid-state reac3+ 3+ tion method. According to Y1.99O3:Eu3+ 0.01 and Y1.98O3:Eu0.01, Ho0.01, stoichiometric composition of Y2O3 (4 N), Eu2O3 (4 N) and Ho2O3 (4 N) were mixed and milled thoroughly in an agate mortar for 2 h. 5 mol% of H3BO3 (3 N) was added as flux. The powders were sintered at 1500 °C for 4 h in air. Finally, the phosphors were obtained, respectively. All of the raw materials are purchased from Sinopharm Chemical Reagent Co., Ltd. The phase identification of the sample was carried out by PANalytical X’pet pro MRD X-ray powder diffraction (XRD) with Cu Ka irradiation (k = 1.5406 Å) at 40 kV tube voltage and 40 mA tube current at room temperature. The filter is Ni 0.5°, step size is 0.016°, time per step is 0.1 s. The morphology and grain size of phosphors were observed by a Philips XL-30 scanning electron microscope (SEM). The photoluminescence (PL) spectra were taken on a Hitachi F-7000 fluorescence spectrophotometer. The decay curves were registered by a GSZF-2A single-photon counter system (Tianjin Gangdong Sci.&Tech. Development Co. Ltd.). The thermoluminescence (TL) curves were measured using a FJ427A1 thermoluminescent dosimeter (Beijing Nuclear Instrument Factory). The heating rate was 1 K/s and the samples were excited by a UV
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254 nm light for 5 min and the measurement started with 10 min delay time. 3. Results and discussion As shown in Fig. 1(a), Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors show the same diffraction patterns, and the patterns are in perfect match with the standard powder peak positions of Y2O3 cubic phase (JCPDF No. 25-1011). Fig. 1(b) is the XRD patterns between 25° and 40°, it can be seen that there is no other phase and no apparent shift can be found, indicating that the doped Eu3+ or Ho3+ show no obvious influence to the cubic Y2O3 crystal, the reason is the content of rare earth ions is too small. Fig. 2(a) and (b) are the SEM micrographs of Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ powders, respectively. It can be clearly seen that the two samples have similar irregular morphology with round edge, and the grain sizes varied from 3 to 8 lm. It can be concluded that slightly rare earth ions doping show no significant influence on the grain sizes and morphology of phosphors. In order to determine the valence state of Ho3+, XPS was measured. As can be seen in Fig. 3, the Ho 4d5/2 peak centered at 161.1 eV, indicating the existence of Ho3+. Fig. 4 shows the photoluminescence spectra of the phosphors excited by UV light at the wavelength of 254 nm. Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors exhibited the same line position and strong red emission, and it can be found that the emission intensity of Y2O3:Eu3+, Ho3+ is a little higher than that of Y2O3:Eu3+. Sharp emissions peaking at 582, 588, 594, 600 and 612 nm can be assigned to the 5D0 ? 7FJ (where J is 0, 1 and 2) transitions of Eu3+ ion (marked in Fig. 4). No other emission can be observed in the Y2O3:Eu3+, Ho3+ sample, indicating that the doped Ho3+ ion do not show any significant emission under 254 nm excitation. In cubic Y2O3, there are two kinds of Y3+ sites, 75% of Y3+ sites are non centrosymmetric with C2 symmetry, and the other 25% are centrosymmetric with S6 symmetry [20–22]. The strongest red emission at 612 nm is due to the low symmetry position of the Eu3+ ion with an inversion center and this forced-electric dipole transition is often dominant in the emission spectrum. So the strongest emission at 612 nm is ascribed to Eu3+ ion which occupied on the C2 site, and these Eu3+ ions are dominant in the host. The samples were excited under UV 254 nm for 5 min, and then the excitation source was removed to examine whether the materials show long afterglow emission in the dark. Y2O3:Eu3+ phosphor reveals no persist phenomena while Y2O3:Eu3+, Ho3+ sample exhibits an obvious red afterglow lasting for about 10 min. Two minutes after the end of excitation, the phosphorescence spectrum of Y2O3:Eu3+, Ho3+ was recorded, and it is shown in the inset of Fig. 5. The emission peaks of the afterglow phosphorescence
remained the similar shape as the photoluminescence spectra, indicating that Eu3+ ion is the luminescent center during the decay process. As can be seen in the left inset of Fig. 5, the persistent luminescence of Y2O3:Eu, Ho material can be observed by the naked eyes and it can last for more than 5 min. The decay curve of the Y2O3:Eu3+, Ho3+ sample was recorded and shown in Fig. 5. It contains a rapid-decaying process and a slow-decaying process, the decay curve is similar to that of typical long afterglow materials, and it can be evaluated by fitting using double exponential equation which reflects the trend of the decay. The equation is as follows [10,23,24]:
I ¼ I1 exp
t
s1
þ I2 exp
t ;
s2
ð1Þ
where I represents the phosphorescent intensity; I1 and I2 are two constants; t represents the time; s1 and s2 are decay constants which decide the decay rate for the rapid and the slow exponentially decay components. The fitting results of the parameters of s1 and s2 and the fitting curves are shown in Fig. 5. The decay characteristic of Y2O3:Eu3+, Ho3+ phosphor is similar to the rare-earth doping alkaline-earth aluminates long afterglow materials, which have drawn much attention from 1990s. One of the typical aluminates is SrAl2O4:Eu2+, RE3+ (RE = Dy, Nd and Ho, etc.), which was successfully synthesized by Matsuzawa in 1996 [25]. Generally, the Eu2+ ion is the only luminescent center in SrAl2O4:Eu2+, RE3+ phosphors, and the doped RE3+ ions often generate the trap level in the host [10,23]. In order to exhibit long afterglow phenomena, the phosphor should have suitable trap level and trap density, and the trapped carriers does recombine in the luminescent centers occurred with the afterglow. The depth of the trap levels could be investigated by thermoluminescence curves (in Fig. 6) fitted by a general order kinetics formula [23,24,26]. The TL intensity as a function of temperature, T, is as follows:
Et IðTÞ ¼ sn0 exp kB T lðl1Þ Z T ðl 1Þs Et dT þ 1 exp ; b kB T T0
ð2Þ
where I(T) is the TL intensity, n0 is the concentration of trapped charges at T = 0, kB is the Boltzmann’s constant, b the heating rate (1 oC/s in this work), Et the activation energy (or depth of the trap), s the frequency factor and l the order of kinetics. Trap level depth Et is proportional to one corresponding glow peak temperature. As the effect of s on Tm is neglected and the frequency of an electron escaping the trap is assumed as 1/s, the thermal activation energy or the
Fig. 1. The XRD patterns of the phosphors.
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Fig. 2. SEM images of (a) Y2O3:Eu3+ and (b) Y2O3:Eu3+, Ho3+ phosphors.
Fig. 3. XPS spectrum of the Ho 4d5/2.
Fig. 5. The decay curve of the Y2O3:Eu3+, Ho3+ sample, the left inset is the afterglow photograph of Y2O3:Eu3+, Ho3+ phosphor, and right inset is the phosphorescence spectrum of Y2O3:Eu3+, Ho3+ which was examined at 2 min after the UV 254 nm light excitation for 5 min.
Fig. 4. The photoluminescence spectra of phosphors excited under UV 254 nm.
trap depth Et (eV) could be estimated by Et = Tm/500 [10,27,28], where Tm (K) is the glow peak temperature in the TL curve. In Fig. 6, the Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors present a broad band peaking at 363 K and 356 K, respectively. It can be seen that the traps exist in the phosphor, no matter the Ho3+ ion was doped or not. Hence, the trap in the phosphors is not caused by Ho3+ ion but Eu3+ ion. This conclusion is different from the mechanism of rare-earth ion doping aluminates and the distinction may be attributed to the difference of the different host matrix. The calculated trap depth of Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors is 0.726 eV and 0.712 eV, respectively. The two values reflect that the trap depth was shoaled by Ho3+ ion doping. Consequently, the carriers can escape from the trap more easily, and then the afterglow appeared. As shown in Fig. 6, the intensity of TL curves of
Fig. 6. The thermoluminescence curves of phosphors.
Y2O3:Eu3+, Ho3+ is lower than that of Y2O3:Eu3+, so we believe that the number of carriers in the Y2O3:Eu3+, Ho3+ is lower the than that of Y2O3:Eu3+ in the measurement process, because the carriers in the Y2O3:Eu3+, Ho3+ is released from the trap and caused the afterglow. When the TL curves are recorded, the decay process is almost finished, and the intensity of TL curves is proportional to the carriers which remained in the trap, so the intensity of TL curves of Y2O3:Eu3+, Ho3+ is lower than that of Y2O3:Eu3+. It can be concluded that the doped of Ho3+ ion is benefit for the carriers to escape from the traps caused by Eu3+ ion, and then generate afterglow phenomenon.
W. Xie et al. / Journal of Alloys and Compounds 619 (2015) 244–247
4. Conclusions Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ phosphors show the same photoluminescence spectra while Y2O3:Eu3+, Ho3+ exhibits a red long afterglow phenomenon. The decay characteristic of Y2O3:Eu3+, Ho3+ is described with the double exponential equation. The TL analysis indicates that the traps in the Y2O3:Eu3+ and Y2O3:Eu3+, Ho3+ were generated by Eu3+. The doped of Ho3+ shoaled the trap depth, which is beneficial for the carriers to escape from the trap, and then the afterglow appears sequentially. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 21271048, 61106124, 11304276), outstanding young teacher training program in Guangdong province (Yq2013113). References [1] [2] [3] [4] [5] [6] [7] [8]
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