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Investigation of the luminescence properties of Dy3+-doped α-Gd2(MoO4)3 phosphors
Physica B 405 (2010) 4457–4461
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Physica B journal homepage: www.elsevier.com/locate/physb
Investigation of the luminescence properties of Dy3 + -doped a-Gd2(MoO4)3 phosphors Lihong Cheng a, Xiangping Li a,n, Jiashi Sun a, Haiyang Zhong a, Yue Tian a, Jing Wan a, Weili Lu a, Yanfeng Zheng a, Tingting Yu a, Libo Huang a, Hongquan Yu b, Baojiu Chen a,n a b
Department of Physics, Dalian Maritime University, Dalian, Liaoning 116026, PR China College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian, Liaoning 116028, PR China
a r t i c l e in fo
abstract
Article history: Received 14 March 2010 Received in revised form 3 August 2010 Accepted 6 August 2010
A series of a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations was synthesized by solid state reaction method. The crystal structure and morphology of the phosphors were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The luminescence properties of Dy3 + in a-Gd2(MoO4)3 were systematically studied. The electric dipole–dipole interaction between Dy3 + ions was identified as the main mechanism for luminescence quenching, according to the analysis of concentration quenching and the fluorescent dynamics. The chromatic nature of the phosphors was also analyzed in detail. & 2010 Elsevier B.V. All rights reserved.
Keywords: Phosphors Solid state reaction Luminescence properties Concentration quenching
1. Introduction Since the ever-increasing demands for novel lighting sources with high luminous efficiency and high environment safety, a tremendous emphasis is being placed on white light-emitting diodes (LEDs) as a more preferable replacement for conventional lighting sources [1]. In addition to the approach of combining blue LED with yellow phosphor, white light can also be produced by mixing the red, green and blue lights emitted from three different LEDs, or by pumping tricolor phosphors with a deep blue/ultraviolet (UV) LED or laser diode (LD) [2,3]. The latter has received much attention due to its high tolerance to UV chip’s color variation and excellent color rendering index [4]. In order to achieve practically applicable white LEDs, novel tricolor phosphors, which can be effectively excited by near UV light, are required. Recently, trivalent dysprosium ions (Dy3 + ) doped phosphors have been extensively studied due to the potential applications in white light emission, because of its intense blue (484 nm, 4F9/2-6H15/2) and yellow (575 nm, 4F9/2-6H13/2) emissions [5,6]. White light emissions from Dy3 + -doped phosphors have been reported in different hosts, such as borates [4], vanadates [7,8], molybdates [9], silicates [6], etc. Among these hosts, molybdates have attracted great interest due to their
n
Corresponding authors. Tel./fax: + 86 41184728909. E-mail addresses: [email protected] (X. Li), [email protected] (B. Chen). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.015
excellent spectral properties, chemical durability, as well as potential applications in catalysis, laser and phosphor [2,10–12]. To our best knowledge, there has been no report on the luminescence properties of Dy3 + -doped monoclinic a-Gd2(MoO4)3 phosphors. In this study, Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations were synthesized by solid state reaction method. Effective energy transfer from molybdate matrix to activator Dy3 + was observed, which yielded a high efficiency emission coming from Dy3 + under UV light excitation. The reaction mechanism between Dy3 + was studied based on the analysis of concentration quenching and fluorescent dynamics. The chromatic properties of the phosphors were also discussed.
2. Experimental A series of phosphors Gd2(MoO4)3 with various concentrations of Dy3 + (0.1%, 0.2%, 1.0%, 2.0%, 3.0%, 5.0%, 7.0% and 10% in molar) was synthesized by solid state reaction method in air atmosphere. According to the desired stoichiometric ratio, the starting materials gadolinium oxide (Gd2O3, 99.99%), dysprosium oxide (Dy2O3, 99.99%) and molybdenum oxide (MoO3, 99.99%) were weighed and well mixed in an agate mortar. NH4HF2 (analysis grade) was used as flux to improve the chemical reaction. Each batch of the mixtures was put into an alumina crucible and calcined in a muffle furnace at 800 1C for 4 h, and then the resultant powder with white body color was obtained.
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The crystal structure of the phosphors was identified by a Shimadzu X-ray diffractometer (XRD)-6000 operating at 100 kV and 40 mA. The Cu Ka radiation (l ¼0.15406 nm) was used as X-ray source. The morphologies were characterized by a Hitachi S-4800 field emission scanning electron microscopy (FE-SEM). The excitation and emission spectra of the phosphors were recorded using a Hitachi F-4600 fluorospectrometer equipped with a 150 W Xe lamp as an excitation source. This fluorospectrometer was well intensity-calibrated for the excitation and emission spectra. All the measurements were performed at room temperature.
3. Results and discussion Fig. 1 presents the XRD pattern of the as-synthesized Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + . It can be seen that all of the diffraction peaks observed can be identified as Gd2(MoO4)3 phase with a monoclinic structure (JCPDS 25-0338). No extraneous phase emerged in the pattern. The corresponding SEM image is shown in Fig. 2. As can be seen from the image, the sample exhibits flower-like structure, which is built by some thin sheets. It is well known that the phosphor with regular morphology and fine size is benefit to the improvement of luminescent performance in the devices [13]. Moreover, the morphology of the resultant phosphors usually depends on the morphology of the starting materials, the sintering temperature, reaction duration and the amount of flux. At this point, the morphology of the studied phosphors must be improved in order to meet the practical applications. The other samples with different Dy3 + concentrations have similar crystalline quality and morphology as that in the case of 5 mol% Dy3 + -doped phosphor. Fig. 3 shows the excitation spectra of a-Gd2(MoO4)3 phosphors with different Dy3 + concentrations by monitoring 577 nm emission. We have observed two kinds of excitation bands: one is a broad band centered at 274 nm; the other is composed of several sharp lines range from 305 to 500 nm. The dominated broad excitation band is responsible for the charge-transfer band (CTB) of Mo–O, implying an energy transfer from MoO24 to Dy3 + . The sharp lines located at 325, 355, 370, 390, 425, 455 and 475 nm belong to the intrinsic f–f transitions of Dy3 + from the ground state 6H15/2 to the excited state 4L19/2, 6P7/2, 6P5/2, 4I13/2, 4G11/2, 4 I15/2 and 4F9/2, respectively. It should be noted that the intensity of CTB is much more intense than that of f–f transitions, indicating
Fig. 1. XRD pattern of a-Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + (top part) and the standard pattern for monoclinic a-Gd2(MoO4)3 (bottom part).
Fig. 2. SEM image of a-Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + .
Fig. 3. Excitation spectra of Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations by monitoring the emission at 577 nm.
a very efficient energy transfer from hosts to Dy3 + . Then, Dy3 + doped a-Gd2(MoO4)3 phosphors can be effectively excited by near UV light. Therefore, molybdates could be a good matrix material for incorporating and sensitizing Dy3 + to achieve intense emissions. However, this result is different from that reported by Xue et al. [9] in orthorhombic Dy3 + -doped Gd2(MoO4)3, in which no obvious UV excitation band was observed. This spectral difference is caused by the different crystal fields in monoclinic and orthorhombic Gd2(MoO4)3, since the spectral properties of rare earth doped materials are decided by crystal field environments surrounding rare earth ions. Similar phenomenon has also been observed from rare earth doped tungstate phosphors [2,14]. Some researchers have given qualitative analysis on the nature of MoO24 and WO24 [15]. However, it is a pity that there is still a lack of theoretical expression for the relationship between crystal structure and the charge transfer nature. Fig. 4 shows the emission spectra of Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations excited by 274 nm UV light. It can be seen from all the emission spectra that there are two dominating emissions at 485 and 577 nm and as well one weak emission at 668 nm, corresponding to 4F9/2-6H15/2, 4F9/2-6H13/2
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interaction is the main mechanism for the concentration quenching of Dy3 + -doped a-Gd2(MoO4)3 phosphors. Considering the energy match rule, three possible cross-relaxation channels among Dy3 + are shown in Fig. 6, denoted as CR1, CR2 and CR3. The Dy3 + ions at 4F9/2 level can be de-excited to (6F9/2/6H7/2), (6H9/2/6F11/2) or 6F1/2 level via these three cross-relaxation processes, in the meanwhile the ground state Dy3 + ions accepting the energies from the Dy3 + at 4F9/2 level will arrive at 6F3/2, 6F5/2 and (6H9/2/6F11/2) level. Finally, all the Dy3 + ions involved in CR1, CR2 and CR3 processes will get in their ground states, thus the luminescence related to 4F9/2 level are quenched. In order to further study the energy transfer behavior, the fluorescent decays for 1 and 5 mol% Dy3 + -doped a-Gd2(MoO4)3 phosphors were measured monitoring 577 nm emission corresponding to 4F9/2-6H13/2 transition, while excited with 274 nm pulsed light. The measured fluorescent decay curves are plotted in a single-log coordinates system (as shown in Fig. 7), where the open-circled and open-squared dots are for the data obtained Fig. 4. Fluorescent emission spectra of Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations upon 274 nm excitation.
and 4F9/2-6H11/2 transitions, respectively. The 4F9/2-6H13/2 transition belongs to a forced electric dipole transition, which is allowed only in the case that the Dy3 + ions locate at the local sites with noninversion center symmetry. Therefore, the more intense 577 nm emission demonstrates that Dy3 + ions lie in this host lattice without inversion center [16]. The integrated emission intensity ratios of 4 F9/2-6H13/2 to 4F9/2-6H15/2 for all the samples were calculated to be about a constant value 1.8, which demonstrates that the Dy3 + introduction does not result in an obvious change of the crystal structure and crystal field environment surrounding the Dy3 + ions even at such a high doping level as 10 mol%. From Fig. 4, it can be seen that the profile of Dy3 + emission spectra is independent of the Dy3 + concentration, but the emission intensity changes with an increase of Dy3 + concentration. The concentration dependences of integrated intensities for both the transitions of 4F9/2-6H15/2 and 4F9/2-6H13/2 are shown in Fig. 5. It is found that both luminescence intensities for 4 F9/2-6H15/2 and 4F9/2-6H13/2 increase with the increase of Dy3 + concentration in the concentration range less than 6 mol%, and then decrease slightly with the further increase in Dy3 + concentration. The optimal doping concentration of Dy3 + for obtaining maximum emission intensity is estimated to be 6 mol%. Indubitably, the distances between the Dy3 + ions become smaller with the increase of Dy3 + concentration, the non-radiative relaxation would occur among Dy3 + ions, thus yielding luminescence quenching. Van Uitert [17] has developed a phenomenological expression for the relationship between the luminescence intensity and the doping concentration of luminescent center, which is mathematically represented through the following form: IðCÞ ¼
C K½1 þ bC Q =3
Fig. 5. The dependences of integrated emission intensities of 4F9/2-6H15/2 and 4 F9/2-6H13/2 transitions on Dy3 + concentration (lex ¼274 nm). The solid dots are the experimental data, the solid curves are the fitting results.
ð1Þ
where C is the activator concentration; K and b are the constants for a certain system; Q represents the interaction mechanism between rare earth ions, Q¼6, 8 or 10 for electric dipole–dipole (D–D), electric dipole–quadrupole (D–Q) or electric quadrupole– quadrupole (Q–Q) interactions, respectively. Then we can use this equation to fit the experimental results of the relationship between integrated emission intensity and Dy3 + concentration. The fitting results are shown in Fig. 5, denoted as the solid curves. We can see that the experimental data are fitted well to Eq. (1). According to this fitting, the Q values can be obtained. It equals to 6.0 and 6.1 for 485 and 577 nm emissions, respectively, which are very close to the theoretical value 6 for the electric dipole– dipole interaction, thus indicating that the electric dipole–dipole
Fig. 6. Energy level scheme of Dy3 + represents the mechanisms for different observed emissions and possible cross-relaxation processes.
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Fig. 7. Fluorescent decay curves for a-Gd2(MoO4)3 phosphors doped with 1 mol% (open square) and 5 mol% (open circle) Dy3 + . Straight lines represent the monoexponential fitting; solid lines show the fitted results based on IH model.
from the samples doped with 1 and 5 mol% Dy3 + , respectively. It can be seen that both the decay curves show non-exponential dependence of emission intensity on the decay time. The monoexponential fittings (see the solid straight lines in Fig. 7) exhibit large deviation from the experimental fluorescent decay data. The non-exponential decays are resulted from the nonradiative energy transfer processes between Dy3 + ions as mentioned above, which can provide extra decay channel to change the decay curves. In order to determine the nature of the possible interaction mechanism between Dy3 + ions, Inokuti– Hirayama (IH) model is applied to the analysis of the decay curves. From the IH model, the transient luminescence intensity I(t) can be expressed as follow [18]: " 3=s # t t IðtÞ ¼ Ið0Þexp A ð2Þ
t
t
3+
where t is the radiative transition lifetime of Dy in Gd2(MoO4)3 matrix, I(0)is the intensity at time t ¼0 and A is the nonradiative energy transfer factor. s has the same meaning as the aforementioned Q in Eq. (1). Here, s¼6, 8 or 10 for D–D, D–Q or Q–Q interactions, respectively. Based on the IH model, the experimental data were fitted to Eq. (2). It was found that the best fits were obtained when s¼ 6. This fact means that in both the cases of doping 1 and 5 mol% Dy3 + , the electric D–D interaction between Dy3 + is responsible for the fluorescence quenching. It is therefore unquestionable that the main nonradiative relaxation process between Dy3 + ions is electric D–D interaction, which is in accordance with the conclusion derived from the above analysis on the relationship between the luminescence intensity and Dy3 + doping concentration. Color coordinates are one of the important factors for evaluating phosphors’ performance. It is a well known fact that the color coordinates are the same if the spectra profiles are identical. In such case, the color coordinates for the sample doped with 5 mol% Dy3 + were calculated using the intensity-calibrated emission spectra data and the chromatic standard issued by the Commission International de I’Eclairage in 1931 (CIE 1931). Three calculated color coordinates as solid circle dots for the blue (485 nm), yellow (577 nm) and the full emissions (blue +yellow) are shown in Fig. 8. It can be seen that the straight line between the points of blue and yellow emissions in the chromatic space crosses the white light region, which means that the white light can be formed mixing these two emissions. However, the
Fig. 8. CIE color coordinates for a-Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + , the solid circle dots for blue, yellow and full emissions.
calculated color coordinates for full emissions are x¼0.3822, y¼0.4236, which are a little far from the equal energy point (x ¼0.3333, y¼0.3333), indicating that the relative intensity of blue emission is weak for forming high quality white light. In order to further improve the CIE color coordinates and achieve good quality white light emission, some other dopants, for instance Tm3 + , emitting blue may need to be introduced into this phosphor system.
4. Conclusions Dy3 + -doped Gd2(MoO4)3 phosphors were successfully synthesized via a solid state reaction. The effect of Dy3 + concentration on the luminescence properties was investigated using fluorescent spectra. The electric dipole–dipole interaction between Dy3 + ions was confirmed through the analysis of concentration quenching and the fluorescent dynamics. The color coordinates for Dy3 + doped a-Gd2(MoO4)3 phosphors were calculated to be x¼0.3822, y¼0.4236. In order to achieve high quality white light emission, some other luminescent centers emitting blue are suggested to be doped.
Acknowledgements This work was partially supported by NSFC (National Natural Science Foundation of China, Grant no. 50972021 and 50802010), Fundamental Research Funds for the Central Universities (Grant no. 2009QN066), Natural Science Foundation of Liaoning Province (Grant nos. 20082139, 20092146 and 20092147) and Foundation of Education Department of Liaoning Province (Grant no. 2009A117). References [1] S.Y. Gao, H.J. Zhang, R.P. Deng, X.M. Wang, D.H. Sun, G.L. Zheng, Appl. Phys. Lett. 89 (2006) 123125. [2] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [3] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, Appl. Phys. Lett. 85 (2004) 3696. [4] P.L. Li, Z.P. Yang, Z.J. Wang, Q.L. Guo, Mater. Lett. 62 (2008) 1455.
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J. Kuang, Y. Liu, J. Zhang, J. Solid State Chem. 179 (2006) 266. B. Liu, L.J. Kong, C.S. Shi, J. Lumin. 122–123 (2007) 121. A. Bao, H. Yang, C.Y. Tao, Y. Zhang, L.L. Han, J. Lumin. 128 (2008) 60. S.D. Han, S.P. Khatkar, V.B. Taxak, G. Sharma, D. Kumar, Mater. Sci. Eng. B 129 (2006) 126. Y.N. Xue, F. Xiao, Q.Y. Zhang, Z.H. Jiang, J. Rare Earths 27 (2009) 753. W. Kuang, Y. Fan, K. Yao, Y. Chen., J. Solid State Chem. 140 (1998) 354. R.A. Rocha, E.N. Muccillo, J. Alloy Comp. 400 (2005) 83. E. Sani, A. Toncelli, M. Tonelli, D.A. Lis, E.V. Zharikov, K.A. Subbotin, V.A. Smirnov, J. Appl. Phys. 97 (2005) 123531.
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[13] Y.C. Kang, M.A. Lim, H.D. Park, M. Han, J. Electrochem. Soc. 150 (2003) H7. [14] Y.S. Hu, W.D. Zhuang, H.Q. Ye, D.H. Wang, S.S. Zhang, X.W. Huang, J. Alloy Comp. 390 (2005) 226. [15] X.X. Zhao, X.J. Wang, B.J. Chen, Q.Y. Meng, B. Yan, W.H. Di, Opt. Mater. 29 (2007) 1680. [16] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Hang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [17] L.G.Van Uitert, J. Electrochem. Soc. 114 (1967) 1048. [18] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Investigation of the luminescence properties of Dy3 + -doped a-Gd2(MoO4)3 phosphors Lihong Cheng a, Xiangping Li a,n, Jiashi Sun a, Haiyang Zhong a, Yue Tian a, Jing Wan a, Weili Lu a, Yanfeng Zheng a, Tingting Yu a, Libo Huang a, Hongquan Yu b, Baojiu Chen a,n a b
Department of Physics, Dalian Maritime University, Dalian, Liaoning 116026, PR China College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian, Liaoning 116028, PR China
a r t i c l e in fo
abstract
Article history: Received 14 March 2010 Received in revised form 3 August 2010 Accepted 6 August 2010
A series of a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations was synthesized by solid state reaction method. The crystal structure and morphology of the phosphors were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The luminescence properties of Dy3 + in a-Gd2(MoO4)3 were systematically studied. The electric dipole–dipole interaction between Dy3 + ions was identified as the main mechanism for luminescence quenching, according to the analysis of concentration quenching and the fluorescent dynamics. The chromatic nature of the phosphors was also analyzed in detail. & 2010 Elsevier B.V. All rights reserved.
Keywords: Phosphors Solid state reaction Luminescence properties Concentration quenching
1. Introduction Since the ever-increasing demands for novel lighting sources with high luminous efficiency and high environment safety, a tremendous emphasis is being placed on white light-emitting diodes (LEDs) as a more preferable replacement for conventional lighting sources [1]. In addition to the approach of combining blue LED with yellow phosphor, white light can also be produced by mixing the red, green and blue lights emitted from three different LEDs, or by pumping tricolor phosphors with a deep blue/ultraviolet (UV) LED or laser diode (LD) [2,3]. The latter has received much attention due to its high tolerance to UV chip’s color variation and excellent color rendering index [4]. In order to achieve practically applicable white LEDs, novel tricolor phosphors, which can be effectively excited by near UV light, are required. Recently, trivalent dysprosium ions (Dy3 + ) doped phosphors have been extensively studied due to the potential applications in white light emission, because of its intense blue (484 nm, 4F9/2-6H15/2) and yellow (575 nm, 4F9/2-6H13/2) emissions [5,6]. White light emissions from Dy3 + -doped phosphors have been reported in different hosts, such as borates [4], vanadates [7,8], molybdates [9], silicates [6], etc. Among these hosts, molybdates have attracted great interest due to their
n
Corresponding authors. Tel./fax: + 86 41184728909. E-mail addresses: [email protected] (X. Li), [email protected] (B. Chen). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.015
excellent spectral properties, chemical durability, as well as potential applications in catalysis, laser and phosphor [2,10–12]. To our best knowledge, there has been no report on the luminescence properties of Dy3 + -doped monoclinic a-Gd2(MoO4)3 phosphors. In this study, Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations were synthesized by solid state reaction method. Effective energy transfer from molybdate matrix to activator Dy3 + was observed, which yielded a high efficiency emission coming from Dy3 + under UV light excitation. The reaction mechanism between Dy3 + was studied based on the analysis of concentration quenching and fluorescent dynamics. The chromatic properties of the phosphors were also discussed.
2. Experimental A series of phosphors Gd2(MoO4)3 with various concentrations of Dy3 + (0.1%, 0.2%, 1.0%, 2.0%, 3.0%, 5.0%, 7.0% and 10% in molar) was synthesized by solid state reaction method in air atmosphere. According to the desired stoichiometric ratio, the starting materials gadolinium oxide (Gd2O3, 99.99%), dysprosium oxide (Dy2O3, 99.99%) and molybdenum oxide (MoO3, 99.99%) were weighed and well mixed in an agate mortar. NH4HF2 (analysis grade) was used as flux to improve the chemical reaction. Each batch of the mixtures was put into an alumina crucible and calcined in a muffle furnace at 800 1C for 4 h, and then the resultant powder with white body color was obtained.
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The crystal structure of the phosphors was identified by a Shimadzu X-ray diffractometer (XRD)-6000 operating at 100 kV and 40 mA. The Cu Ka radiation (l ¼0.15406 nm) was used as X-ray source. The morphologies were characterized by a Hitachi S-4800 field emission scanning electron microscopy (FE-SEM). The excitation and emission spectra of the phosphors were recorded using a Hitachi F-4600 fluorospectrometer equipped with a 150 W Xe lamp as an excitation source. This fluorospectrometer was well intensity-calibrated for the excitation and emission spectra. All the measurements were performed at room temperature.
3. Results and discussion Fig. 1 presents the XRD pattern of the as-synthesized Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + . It can be seen that all of the diffraction peaks observed can be identified as Gd2(MoO4)3 phase with a monoclinic structure (JCPDS 25-0338). No extraneous phase emerged in the pattern. The corresponding SEM image is shown in Fig. 2. As can be seen from the image, the sample exhibits flower-like structure, which is built by some thin sheets. It is well known that the phosphor with regular morphology and fine size is benefit to the improvement of luminescent performance in the devices [13]. Moreover, the morphology of the resultant phosphors usually depends on the morphology of the starting materials, the sintering temperature, reaction duration and the amount of flux. At this point, the morphology of the studied phosphors must be improved in order to meet the practical applications. The other samples with different Dy3 + concentrations have similar crystalline quality and morphology as that in the case of 5 mol% Dy3 + -doped phosphor. Fig. 3 shows the excitation spectra of a-Gd2(MoO4)3 phosphors with different Dy3 + concentrations by monitoring 577 nm emission. We have observed two kinds of excitation bands: one is a broad band centered at 274 nm; the other is composed of several sharp lines range from 305 to 500 nm. The dominated broad excitation band is responsible for the charge-transfer band (CTB) of Mo–O, implying an energy transfer from MoO24 to Dy3 + . The sharp lines located at 325, 355, 370, 390, 425, 455 and 475 nm belong to the intrinsic f–f transitions of Dy3 + from the ground state 6H15/2 to the excited state 4L19/2, 6P7/2, 6P5/2, 4I13/2, 4G11/2, 4 I15/2 and 4F9/2, respectively. It should be noted that the intensity of CTB is much more intense than that of f–f transitions, indicating
Fig. 1. XRD pattern of a-Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + (top part) and the standard pattern for monoclinic a-Gd2(MoO4)3 (bottom part).
Fig. 2. SEM image of a-Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + .
Fig. 3. Excitation spectra of Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations by monitoring the emission at 577 nm.
a very efficient energy transfer from hosts to Dy3 + . Then, Dy3 + doped a-Gd2(MoO4)3 phosphors can be effectively excited by near UV light. Therefore, molybdates could be a good matrix material for incorporating and sensitizing Dy3 + to achieve intense emissions. However, this result is different from that reported by Xue et al. [9] in orthorhombic Dy3 + -doped Gd2(MoO4)3, in which no obvious UV excitation band was observed. This spectral difference is caused by the different crystal fields in monoclinic and orthorhombic Gd2(MoO4)3, since the spectral properties of rare earth doped materials are decided by crystal field environments surrounding rare earth ions. Similar phenomenon has also been observed from rare earth doped tungstate phosphors [2,14]. Some researchers have given qualitative analysis on the nature of MoO24 and WO24 [15]. However, it is a pity that there is still a lack of theoretical expression for the relationship between crystal structure and the charge transfer nature. Fig. 4 shows the emission spectra of Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations excited by 274 nm UV light. It can be seen from all the emission spectra that there are two dominating emissions at 485 and 577 nm and as well one weak emission at 668 nm, corresponding to 4F9/2-6H15/2, 4F9/2-6H13/2
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interaction is the main mechanism for the concentration quenching of Dy3 + -doped a-Gd2(MoO4)3 phosphors. Considering the energy match rule, three possible cross-relaxation channels among Dy3 + are shown in Fig. 6, denoted as CR1, CR2 and CR3. The Dy3 + ions at 4F9/2 level can be de-excited to (6F9/2/6H7/2), (6H9/2/6F11/2) or 6F1/2 level via these three cross-relaxation processes, in the meanwhile the ground state Dy3 + ions accepting the energies from the Dy3 + at 4F9/2 level will arrive at 6F3/2, 6F5/2 and (6H9/2/6F11/2) level. Finally, all the Dy3 + ions involved in CR1, CR2 and CR3 processes will get in their ground states, thus the luminescence related to 4F9/2 level are quenched. In order to further study the energy transfer behavior, the fluorescent decays for 1 and 5 mol% Dy3 + -doped a-Gd2(MoO4)3 phosphors were measured monitoring 577 nm emission corresponding to 4F9/2-6H13/2 transition, while excited with 274 nm pulsed light. The measured fluorescent decay curves are plotted in a single-log coordinates system (as shown in Fig. 7), where the open-circled and open-squared dots are for the data obtained Fig. 4. Fluorescent emission spectra of Dy3 + -doped a-Gd2(MoO4)3 phosphors with various Dy3 + concentrations upon 274 nm excitation.
and 4F9/2-6H11/2 transitions, respectively. The 4F9/2-6H13/2 transition belongs to a forced electric dipole transition, which is allowed only in the case that the Dy3 + ions locate at the local sites with noninversion center symmetry. Therefore, the more intense 577 nm emission demonstrates that Dy3 + ions lie in this host lattice without inversion center [16]. The integrated emission intensity ratios of 4 F9/2-6H13/2 to 4F9/2-6H15/2 for all the samples were calculated to be about a constant value 1.8, which demonstrates that the Dy3 + introduction does not result in an obvious change of the crystal structure and crystal field environment surrounding the Dy3 + ions even at such a high doping level as 10 mol%. From Fig. 4, it can be seen that the profile of Dy3 + emission spectra is independent of the Dy3 + concentration, but the emission intensity changes with an increase of Dy3 + concentration. The concentration dependences of integrated intensities for both the transitions of 4F9/2-6H15/2 and 4F9/2-6H13/2 are shown in Fig. 5. It is found that both luminescence intensities for 4 F9/2-6H15/2 and 4F9/2-6H13/2 increase with the increase of Dy3 + concentration in the concentration range less than 6 mol%, and then decrease slightly with the further increase in Dy3 + concentration. The optimal doping concentration of Dy3 + for obtaining maximum emission intensity is estimated to be 6 mol%. Indubitably, the distances between the Dy3 + ions become smaller with the increase of Dy3 + concentration, the non-radiative relaxation would occur among Dy3 + ions, thus yielding luminescence quenching. Van Uitert [17] has developed a phenomenological expression for the relationship between the luminescence intensity and the doping concentration of luminescent center, which is mathematically represented through the following form: IðCÞ ¼
C K½1 þ bC Q =3
Fig. 5. The dependences of integrated emission intensities of 4F9/2-6H15/2 and 4 F9/2-6H13/2 transitions on Dy3 + concentration (lex ¼274 nm). The solid dots are the experimental data, the solid curves are the fitting results.
ð1Þ
where C is the activator concentration; K and b are the constants for a certain system; Q represents the interaction mechanism between rare earth ions, Q¼6, 8 or 10 for electric dipole–dipole (D–D), electric dipole–quadrupole (D–Q) or electric quadrupole– quadrupole (Q–Q) interactions, respectively. Then we can use this equation to fit the experimental results of the relationship between integrated emission intensity and Dy3 + concentration. The fitting results are shown in Fig. 5, denoted as the solid curves. We can see that the experimental data are fitted well to Eq. (1). According to this fitting, the Q values can be obtained. It equals to 6.0 and 6.1 for 485 and 577 nm emissions, respectively, which are very close to the theoretical value 6 for the electric dipole– dipole interaction, thus indicating that the electric dipole–dipole
Fig. 6. Energy level scheme of Dy3 + represents the mechanisms for different observed emissions and possible cross-relaxation processes.
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Fig. 7. Fluorescent decay curves for a-Gd2(MoO4)3 phosphors doped with 1 mol% (open square) and 5 mol% (open circle) Dy3 + . Straight lines represent the monoexponential fitting; solid lines show the fitted results based on IH model.
from the samples doped with 1 and 5 mol% Dy3 + , respectively. It can be seen that both the decay curves show non-exponential dependence of emission intensity on the decay time. The monoexponential fittings (see the solid straight lines in Fig. 7) exhibit large deviation from the experimental fluorescent decay data. The non-exponential decays are resulted from the nonradiative energy transfer processes between Dy3 + ions as mentioned above, which can provide extra decay channel to change the decay curves. In order to determine the nature of the possible interaction mechanism between Dy3 + ions, Inokuti– Hirayama (IH) model is applied to the analysis of the decay curves. From the IH model, the transient luminescence intensity I(t) can be expressed as follow [18]: " 3=s # t t IðtÞ ¼ Ið0Þexp A ð2Þ
t
t
3+
where t is the radiative transition lifetime of Dy in Gd2(MoO4)3 matrix, I(0)is the intensity at time t ¼0 and A is the nonradiative energy transfer factor. s has the same meaning as the aforementioned Q in Eq. (1). Here, s¼6, 8 or 10 for D–D, D–Q or Q–Q interactions, respectively. Based on the IH model, the experimental data were fitted to Eq. (2). It was found that the best fits were obtained when s¼ 6. This fact means that in both the cases of doping 1 and 5 mol% Dy3 + , the electric D–D interaction between Dy3 + is responsible for the fluorescence quenching. It is therefore unquestionable that the main nonradiative relaxation process between Dy3 + ions is electric D–D interaction, which is in accordance with the conclusion derived from the above analysis on the relationship between the luminescence intensity and Dy3 + doping concentration. Color coordinates are one of the important factors for evaluating phosphors’ performance. It is a well known fact that the color coordinates are the same if the spectra profiles are identical. In such case, the color coordinates for the sample doped with 5 mol% Dy3 + were calculated using the intensity-calibrated emission spectra data and the chromatic standard issued by the Commission International de I’Eclairage in 1931 (CIE 1931). Three calculated color coordinates as solid circle dots for the blue (485 nm), yellow (577 nm) and the full emissions (blue +yellow) are shown in Fig. 8. It can be seen that the straight line between the points of blue and yellow emissions in the chromatic space crosses the white light region, which means that the white light can be formed mixing these two emissions. However, the
Fig. 8. CIE color coordinates for a-Gd2(MoO4)3 phosphor doped with 5 mol% Dy3 + , the solid circle dots for blue, yellow and full emissions.
calculated color coordinates for full emissions are x¼0.3822, y¼0.4236, which are a little far from the equal energy point (x ¼0.3333, y¼0.3333), indicating that the relative intensity of blue emission is weak for forming high quality white light. In order to further improve the CIE color coordinates and achieve good quality white light emission, some other dopants, for instance Tm3 + , emitting blue may need to be introduced into this phosphor system.
4. Conclusions Dy3 + -doped Gd2(MoO4)3 phosphors were successfully synthesized via a solid state reaction. The effect of Dy3 + concentration on the luminescence properties was investigated using fluorescent spectra. The electric dipole–dipole interaction between Dy3 + ions was confirmed through the analysis of concentration quenching and the fluorescent dynamics. The color coordinates for Dy3 + doped a-Gd2(MoO4)3 phosphors were calculated to be x¼0.3822, y¼0.4236. In order to achieve high quality white light emission, some other luminescent centers emitting blue are suggested to be doped.
Acknowledgements This work was partially supported by NSFC (National Natural Science Foundation of China, Grant no. 50972021 and 50802010), Fundamental Research Funds for the Central Universities (Grant no. 2009QN066), Natural Science Foundation of Liaoning Province (Grant nos. 20082139, 20092146 and 20092147) and Foundation of Education Department of Liaoning Province (Grant no. 2009A117). References [1] S.Y. Gao, H.J. Zhang, R.P. Deng, X.M. Wang, D.H. Sun, G.L. Zheng, Appl. Phys. Lett. 89 (2006) 123125. [2] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [3] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, Appl. Phys. Lett. 85 (2004) 3696. [4] P.L. Li, Z.P. Yang, Z.J. Wang, Q.L. Guo, Mater. Lett. 62 (2008) 1455.
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