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Synthesis and photoluminescence properties of Sm3+ and Dy3+ ions activated Ca2Gd2W3O14 phosphors
Journal of Molecular Structure 1003 (2011) 115–120
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and photoluminescence properties of Sm3+ and Dy3+ ions activated Ca2Gd2W3O14 phosphors S. Sailaja a, S.J. Dhoble b, B. Sudhakar Reddy a,⇑ a b
Department of Physics (Research Centre), SV Degree College, Kadapa 516 003, India Department of Physics, RTM Nagpur University, Nagpur 440 033, India
a r t i c l e
i n f o
Article history: Received 26 June 2011 Received in revised form 26 July 2011 Accepted 26 July 2011 Available online 6 August 2011 Keywords: XRD SEM EDAX FTIR Emission
a b s t r a c t Rare earth ions (Sm3+ and Dy3+) activated calcium gadolinium tungstate [Ca2Gd2W3O14] powder phosphors were synthesized by solid state reaction method at 1000 °C. The obtained phosphors were characterized by XRD, SEM, EDAX, FTIR and PL spectroscopy. The results of XRD patterns indicate the tetragonal phase structure of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors. From the SEM images of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors, it is observed that the average grain size is in the range 100–250 nm. The functional groups are identified by using the FTIR spectra and the elements present in the composition are confirmed by the EDAX profiles. Emission spectra of Sm3+: Ca2Gd2W3O14 powder phosphors have shown strong red emission at 646 nm (4G5/2 ? 6H9/2) with an excitation wavelength kexci = 402 nm (6H5/2 ? 4F7/2) and Dy3+: Ca2Gd2W3O14 phosphors have shown yellow emission at 574 nm (4F9/2 ? 6H13/2) with an excitation wavelength kexci = 385 nm (6H15/2 ? 4I13/2). Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, inorganic phosphors have attracted much attention of researchers due to their applications in white light emitting diodes (WLEDs), solid state lasers, cathode ray tubes, X-ray detectors, medical diagnosis, fluorescent lamps, electroluminescence, optical markers, laser materials, phosphors, fluorescent tubes, in the field of artificial production of light, field emission displays, scintillators, amplifiers, plasma display panels, high definition projection televisions, radiation dosimetry, X-ray imaging, color display and in fiber optic communication, etc. [1–3]. It is well known that, over the recent past the investigation on white light emitting diodes is of great importance due to their excellent properties such as high brightness, long lifetime, low power consumption, environment-friendly, good reliability, fast response and applications as the replacement of incandescent and fluorescent lamps, solid state lighting sources, indicator lights, display backlighting and flash lights [4]. Among different methods of preparation of white light emitting diodes, the easy and important one is the combination of UV-LED with cyan- and orange-emitting phosphor materials [5]. Thus the preparation of orange light emitting phosphor materials is necessary. Moreover rare earth ions activated phosphors have attracted much attention of scientists because the research related to the development of phosphors is giving more information to understand the physical processes of energy absorption and relaxation.
⇑ Corresponding author. Tel./fax: +91 8562 259059 (O). E-mail address: [email protected] (B. Sudhakar Reddy). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.07.048
Further, the f–f transition absorption and emission of the crystalline hosts activated by rare earth ions are of great importance because of their applications as luminescent optical materials emitting in the visible and near-IR regions. Among the rare earth ions, one of the important activator ion is Sm3+ for producing intense orange light. Sm3+ ions in various hosts show bright emission in orange or red regions because of the transitions from the excited state 4G5/2 to the ground state 6H5/2 and also to the higher levels 6Hj (j = 7/2, 9/2 and 11/2) and found the applications in high density optical storage, temperature sensors, under sea communication, various fluorescent devices, color display and visible solid-state lasers [6]. Another important ion is Dy3+ having two important emission bands, one in the yellow region (575 nm) corresponding to the hypersensitive transition 4F9/2 ? 6H13/2 (DL = 2, DJ = 2) and it is electric dipole allowed one; another is in the blue region (480 nm) corresponds to the 4F9/2 ? 6H15/2 transition is magnetic dipole allowed and its intensity is not considerably altered by the local environment of Dy3+ in the crystal lattice. Moreover the intensity of hypersensitive transition 4F9/2 ? 6H13/2 depends on the local symmetry around Dy3+ ions. The relative intensities of these two emission bands depends on the host composition, doping concentration and the excitation wavelength. By suitably adjusting yellow to blue emission intensity ratio, these Dy3+ ions doped phosphors are also used in the fabrication of WLEDs. The photoluminescent analysis of Dy3+ (4f9) ions is very interesting because the emission from 4F9/2 level of Dy3+ ions ranges in the visible and NIR regions and also [7,8]. Selection of suitable host material is also an important factor for the preparation of luminescent materials for different applications.
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Fig. 3. SEM image of 5 mol% of Sm3+: Ca2 Gd2W3O14 phosphors.
Fig. 1. XRD profiles of 5 mol% of Sm3+: Ca2Gd2W3O14 phosphors at different temperatures.
Fig. 4. SEM image of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
tungsten exhibits interesting properties such as ferroelectricity and catalysis [12]. We have earlier reported on the synthesis, photoluminescence and mechanoluminescence properties of Eu3+ ions activated Ca2Gd2W3O14 phosphors [13]. In the present work, we have reported on the synthesis, structural and photoluminescence properties of another couple of rare earth ions such as Sm3+ and Dy3+ activated Ca2Gd2W3O14 phosphors. 2. Experimental studies Fig. 2. XRD profiles of 5 mol% of Dy3+: Ca2Gd2W3O14 phosphors at different temperatures.
Generally the phosphor host materials are based on borates, phosphates, aluminates, silicates, tungstates, molybdates, vanadates, etc., Among the several inorganic phosphors, tungstates are the best host materials for rare earth ions because of their strong covalent interaction, high stability, strong visible luminescence and found the potential applications for lasers, optical fibers, white light emitting diodes, plasma display panels, quantum electronics, solid-state optoelectronic devices, scintillators and phosphors [9– 11]. On the other hand, the phosphors containing gadolinium and
Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors were synthesized by solid state reaction method. Starting materials CaCO3, Gd2O3, WO3, Sm2O3 and Dy2O3 are purchased from Sigma Aldrich and are used as received without any further purification. Appropriate amount of CaCO3, Gd2O3, WO3, Sm2O3 and Dy2O3 were thoroughly mixed in an agate mortar. Afterwards, the mixtures were put into alumina crucibles and calcined in a muffle furnace at 800 °C, 1000 °C, 1200 °C and 1400 °C for 2 h in air and then cooled in air to ambient temperature. The final samples were white powders and taken to characterization. The chemical reaction used in the synthesis of Ca2Gd2W3O14 phosphors is given as follows:
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Fig. 5. EDAX profile of 5 mol% of Sm3+: Ca2 Gd2W3O14 phosphors.
Fig. 6. EDAX profile of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
2CaCO3 þ Gd2 O3 þ 3WO3 ! Ca2 Gd2 W3 O14 þ 2CO2 Structural characterization of these samples has been carried out from the X-ray powder diffraction measurements on a XRD 3003TT Seifert diffractometer with Cu Ka radiation (k = 1.5406 Å) at 40 kV and 20 mA and the 2h range was varied between 20° and 55°. Morphology of the powder phosphors were examined on a ZEISS-EVO-MA15 ESEM. The scanning electron microscopy (SEM) image was obtained for samples by using a 35 m camera at-
tached to a high resolution recording system. The elemental analysis has been carried out by energy dispersive X-ray analysis (EDAX) using an X-ray detector attached to the SEM instrument. The FT-IR spectrum (4000–450 cm1) was recorded on a Perkin Elmer Spectrum1 spectrometer with KBr pellets. Both the excitation and emission spectra were obtained on a SPEX Fluorolog-2 Fluorimeter (Model II) with Data max software to acquire the data with a Xe-arc lamp (150 W) as the excitation source.
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Fig. 9. Emission spectra of Sm3+: Ca2 Gd2W3O14 phosphors. Fig. 7. FTIR spectra of 5 mol% of Sm3+ and Dy3+: Ca2 Gd2W3O14 phosphors.
Fig. 10. Emission intensity of the transition at 646 nm of Sm3+ as a function of its doping concentration in Sm3+: Ca2 Gd2W3O14 phosphors.
Fig. 8. Excitation spectrum of 5 mol% of Sm3+: Ca2 Gd2W3O14 phosphors.
3. Results and discussion 3.1. Structural, morphological, elemental and FTIR studies of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors In order to know the crystal phase structure, the X-ray diffraction profiles of (5 mol%) Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors
prepared by solid state reaction method at different temperatures starting from 800 °C to 1400 °C were measured are shown in Figs. 1 and 2 respectively. From both the figures, it was observed that the intensity was influenced by the synthesis temperature and it was good enough at 1000 °C, thus we have used the 1000 °C as the optimized temperature for all the remaining measurements. The diffraction peaks in the measured patterns are consistent with the standard JCPDS card No. 41-0186. This indicates that the prepared phosphors exist in a tetragonal phase structure and the doping of rare earth (Sm3+ and Dy3+) ions does not influence the intensity
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sizes, and the average grain size is expected to be around 100– 250 nm. Energy dispersive X-ray analysis technique was used to confirm the presence of elements in the prepared phosphors. EDAX profiles of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors are shown in Figs. 5 and 6, which confirm the presence of Ca, Gd, W, O and both of the rare earth ions (Sm3+ and Dy3+) in their respective profiles. Fourier Transform Infrared spectroscopy was used to identify the functional groups present in the prepared phosphors. FTIR spectra of (5 mol%) Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors are shown in Fig. 7. In FTIR spectra, the band at 809 cm1 can be assigned to the stretching mode of W–O bonds and the band at 607 cm1 in the range from 800 to 600 cm1 corresponds to the W–O vibrations respectively [12]. The bending vibration of H2O molecule and O– H stretching vibrations are present at about 1480 cm1and in the range from 3800 to 2700 cm1 [14]. 3.2. Photoluminescence studies
Fig. 11. Excitation spectrum of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
Fig. 12. Emission spectrum of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
as well as the crystal structure of the phosphors. The size and the shape of the prepared phosphors were measured by SEM images. Figs. 3 and 4 show the SEM images of (5 mol%) Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors. It is observed that, both of them shows, the particles are randomly distributed having various shapes and
3.2.1. Sm3+: Ca2 Gd2W3O14 phosphors The excitation spectrum of (5 mol%) Sm3+: Ca2 Gd2W3O14 phosphor is shown in Fig. 8. In the wavelength region 380– 450 nm, three excitation peaks are observed and are located at 402 nm (6H5/2 ? 4F7/2), 418 nm [6H5/2 ? (6P, 4P)5/2] and 436 nm (6H5/2 ? 4G9/2) which are attributed to f–f transitions of Sm3+. Charge transfer band of Sm3+–O2 interaction or host absorption band is not observed because the interaction of Sm3+ ions with the host lattice is very weak, therefore energy transfer does not occurs between Sm3+ and host [15]. From the excitation spectrum it was found that, the intensity of f–f transition at 402 nm is high compared with the other transitions and has been chosen for the measurement of emission spectra of Sm3+: Ca2Gd2W3O14 phosphors. The most intense peak at 402 nm clearly indicates that, these phosphors are effectively excited by near ultraviolet light emitting diodes. Fig. 9 shows the emission spectra of Sm3+: Ca2Gd2W3O14 phosphors. From the emission spectra four emission peaks are observed at 481 nm, 565 nm, 603 nm, 646 nm and which are assigned to 4M15/2 ? 6H5/2, 4 G5/2 ? 6H5/2, 4G5/2 ? 6H7/2 and 4G5/2 ? 6H9/2 transitions respectively. Among these, the transition at 646 nm (4G5/2 ? 6H9/2) is having the maximum intensity which corresponds to the red emission of Sm3+: Ca2Gd2W3O14 phosphors. In general, most of the compounds activated by Sm3+ ions show intense reddish-orange emission at 601 nm (4G5/2 ? 6H7/2) because these transitions are predominant when compared with the other transitions of Sm3+. In the present investigation, the transition at 646 nm in the red region is having maximum intensity and is in agreement with the previously reported literature [16]. From the emission spectra, it is also observed that the emission band at 646 nm (4G5/2 ? 6H9/2), split in two Stark components, this depends on the local environment of Sm3+ ions in the crystalline structure. It is a known fact that, the luminescence performance of the powder phosphors depends mainly on the concentration of activator ions, thus the identification of optimum dopant concentration is necessary. From the emission spectra, it is observed that the emission intensities are increasing gradually from 0.5 to 5 mol% and beyond this concentration the emission intensities are decreasing due to concentration quenching effect. This is because as the concentration of the luminescent ions increases, the distance between them decreases and causes the non-radiative energy transfer from one activator to another activator ion leading to the lowering of fluorescence intensity [17]. Hence 5 mol% of Sm3+ is the optimum dopant concentration. Fig. 10 shows the emission intensity of Sm3+ as a function of its doping concentration (mol%) in Sm3+: Ca2Gd2W3O14 phosphors. 3.2.2. Dy3+: Ca2 Gd2W3O14 phosphors Excitation spectrum of (5 mol%) Dy3+: Ca2Gd2W3O14 phosphor is shown in Fig. 11. From the excitation spectrum, three excitation
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bands are observed at 363 nm (6H15/2 ? 6P5/2), 385 nm (6H15/2 ? 4I13/2) and 419 nm (6H15/2 ? 4G11/2). The most intense excitation peak at 385 nm has been selected for the measurement of emission spectrum of Dy3+: Ca2Gd2W3O14 phosphors. Fig. 12 presents the emission spectrum of Dy3+: Ca2Gd2W3O14 phosphors and from this five emission bands have been observed and are located at 492 nm (4F9/2 ? 6H13/2), 526 nm (4F9/2 ? 6H13/2), 574 nm (4F9/2 ? 6H13/2), 611 nm (6F1/2 ? 6H13/2) and 635 nm (6F1/2 ? 6 H13/2) [18]. From the emission spectrum, it is clear that the emission was observed in the three regions namely blue, yellow and red regions. Among these three regions, the yellow region was most intense with the band located at 574 nm (4F9/2 ? 6H13/2). It is well known that, the transition 4F9/2 ? 6H13/2 at 574 nm with bright yellow emission is a forced electric dipole transition following the selection rule DJ = 2, which is allowed only when Dy3+ is not at an inversion center and it is also called as the hypersensitive transition because its intensity is very sensitive to the outside crystal field environment surrounding Dy3+ [8]. Thus 5 mol% of Dy3+: Ca2Gd2W3O14 phosphors with bright yellow emission can be used as novel optical luminescent materials.
4. Conclusions It could be concluded that, we have observed an intense red and yellow emission from Sm3+ and Dy3+ ions activated Ca2Gd2W3O14 powder phosphors synthesized by the solid state reaction method. The emission intensity has reached the maximum value at 5 mol% in the case of Sm3+: Ca2Gd2W3O14 phosphors. Thus 5 mol% is the optimized concentration. The prepared phosphors are also characterized by the XRD, SEM, EDAX and FTIR spectra. From these spectral results, we have studied the structural, morphological, elemental properties and also identified the functional groups present in the composition of the powder phosphors. By considering the luminescent performance (5 mol%), Sm3+ and Dy3+: Ca2Gd2W3O14
phosphors are the potential candidates for the applications as novel luminescent materials in optical systems. Acknowledgements The authors acknowledge the Sophisticated Analytical Instrument Facility (SAIF), IIT, Chennai for extending instrumental facilities. References [1] Xiang Ying Chen, ShiPing Bao, YuCheng Wu, J. Solid State Chem. 183 (2010) 2004. [2] Jiapeng Fu, Qinghong Zhang, Yaogang Li, Hongzhi Wang, J. Lumin. 130 (2010) 231. [3] Jinsheng Liao, Shaoan Zhang, Hangying You, He-Rui Wen, Jing-Lin Chen, Weixiong You, Opt. Mater. 33 (2011) 953. [4] Xihua Dou, Weiren Zhao, Enhai Song, Guoxiong Zhou, Chunyu Yi, Mingkang Zhou, Spectrochim. Acta A 78 (2011) 821. [5] Zheng-Hua Ju, Rui-Ping Wei, Jing-Xin Ma, Chao-Ran Pang, Wei-Sheng. Liu, J. Alloy. Compd. 507 (2010) 133. [6] Xinmin Zhang, Hyo Jin Seo, J. Alloy. Compd. 509 (2011) 2007. [7] Jian-Xin Meng, Chuang-Tao Yang, Qing-Qing Chen, J. Lumin. 130 (2010) 1320. [8] Yue Tiana, Baojiu Chen, Bining Tian, Ruinian Hu, Jiashi Sun, Lihong Cheng, Haiyang Zhong, Xiangping Li, Jinsu Zhang, Yanfeng Zheng, Tingting Yu, Libo Huanga, Qingyu Meng, J. Alloy. Compd. 509 (2011) 6096. [9] M. Guzik, E. Tomaszewicz, S.M. Kaczmarek, J. Cybinsca, H. Fuks, J. Non-Cryst. Solids 356 (2010) 902. [10] Jinsheng Liao, Bao Qiu, Huasheng Lai, J. Lumin. 129 (2009) 668. [11] Shao-An Yan, Yee-Shin Chang, Jian-Wen Wang, Weng-Sing Hwang, Yen-Hwei Chang, Mater. Res. Bull. 46 (2011) 1231. [12] Fang Lei, Bing Yan, Hao-Hong Chen, J. Solid State Chem. 181 (2008) 2845. [13] S. Sailaja, S.J. Dhoble, Nameetha Brahme, B. Sudhakar Reddy, J. Mater. Sci. (2011), doi:10.1007/s10853-011-5759-2. [14] M.K. Chong, K. Pita, C.H. Kam, J. Phys. Chem. Solids 66 (2005) 213. [15] Dong Tu, Yujun Liang, Rong Liu, Zheng Cheng, Fan Yang, Wenlong Yang, J. Alloy. Compd. 509 (2011) 5596. [16] Xianghong He, Jian Zhou, Ning Lian, Jianhua Sun, Mingyun Guan, J. Lumin. 130 (2010) 743–747. [17] Bingfu Lei, Shi-Qing Man, Yingliang Liu, Song Yue, Mater. Chem. Phys. 124 (2010) 912. [18] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4450.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and photoluminescence properties of Sm3+ and Dy3+ ions activated Ca2Gd2W3O14 phosphors S. Sailaja a, S.J. Dhoble b, B. Sudhakar Reddy a,⇑ a b
Department of Physics (Research Centre), SV Degree College, Kadapa 516 003, India Department of Physics, RTM Nagpur University, Nagpur 440 033, India
a r t i c l e
i n f o
Article history: Received 26 June 2011 Received in revised form 26 July 2011 Accepted 26 July 2011 Available online 6 August 2011 Keywords: XRD SEM EDAX FTIR Emission
a b s t r a c t Rare earth ions (Sm3+ and Dy3+) activated calcium gadolinium tungstate [Ca2Gd2W3O14] powder phosphors were synthesized by solid state reaction method at 1000 °C. The obtained phosphors were characterized by XRD, SEM, EDAX, FTIR and PL spectroscopy. The results of XRD patterns indicate the tetragonal phase structure of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors. From the SEM images of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors, it is observed that the average grain size is in the range 100–250 nm. The functional groups are identified by using the FTIR spectra and the elements present in the composition are confirmed by the EDAX profiles. Emission spectra of Sm3+: Ca2Gd2W3O14 powder phosphors have shown strong red emission at 646 nm (4G5/2 ? 6H9/2) with an excitation wavelength kexci = 402 nm (6H5/2 ? 4F7/2) and Dy3+: Ca2Gd2W3O14 phosphors have shown yellow emission at 574 nm (4F9/2 ? 6H13/2) with an excitation wavelength kexci = 385 nm (6H15/2 ? 4I13/2). Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, inorganic phosphors have attracted much attention of researchers due to their applications in white light emitting diodes (WLEDs), solid state lasers, cathode ray tubes, X-ray detectors, medical diagnosis, fluorescent lamps, electroluminescence, optical markers, laser materials, phosphors, fluorescent tubes, in the field of artificial production of light, field emission displays, scintillators, amplifiers, plasma display panels, high definition projection televisions, radiation dosimetry, X-ray imaging, color display and in fiber optic communication, etc. [1–3]. It is well known that, over the recent past the investigation on white light emitting diodes is of great importance due to their excellent properties such as high brightness, long lifetime, low power consumption, environment-friendly, good reliability, fast response and applications as the replacement of incandescent and fluorescent lamps, solid state lighting sources, indicator lights, display backlighting and flash lights [4]. Among different methods of preparation of white light emitting diodes, the easy and important one is the combination of UV-LED with cyan- and orange-emitting phosphor materials [5]. Thus the preparation of orange light emitting phosphor materials is necessary. Moreover rare earth ions activated phosphors have attracted much attention of scientists because the research related to the development of phosphors is giving more information to understand the physical processes of energy absorption and relaxation.
⇑ Corresponding author. Tel./fax: +91 8562 259059 (O). E-mail address: [email protected] (B. Sudhakar Reddy). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.07.048
Further, the f–f transition absorption and emission of the crystalline hosts activated by rare earth ions are of great importance because of their applications as luminescent optical materials emitting in the visible and near-IR regions. Among the rare earth ions, one of the important activator ion is Sm3+ for producing intense orange light. Sm3+ ions in various hosts show bright emission in orange or red regions because of the transitions from the excited state 4G5/2 to the ground state 6H5/2 and also to the higher levels 6Hj (j = 7/2, 9/2 and 11/2) and found the applications in high density optical storage, temperature sensors, under sea communication, various fluorescent devices, color display and visible solid-state lasers [6]. Another important ion is Dy3+ having two important emission bands, one in the yellow region (575 nm) corresponding to the hypersensitive transition 4F9/2 ? 6H13/2 (DL = 2, DJ = 2) and it is electric dipole allowed one; another is in the blue region (480 nm) corresponds to the 4F9/2 ? 6H15/2 transition is magnetic dipole allowed and its intensity is not considerably altered by the local environment of Dy3+ in the crystal lattice. Moreover the intensity of hypersensitive transition 4F9/2 ? 6H13/2 depends on the local symmetry around Dy3+ ions. The relative intensities of these two emission bands depends on the host composition, doping concentration and the excitation wavelength. By suitably adjusting yellow to blue emission intensity ratio, these Dy3+ ions doped phosphors are also used in the fabrication of WLEDs. The photoluminescent analysis of Dy3+ (4f9) ions is very interesting because the emission from 4F9/2 level of Dy3+ ions ranges in the visible and NIR regions and also [7,8]. Selection of suitable host material is also an important factor for the preparation of luminescent materials for different applications.
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Fig. 3. SEM image of 5 mol% of Sm3+: Ca2 Gd2W3O14 phosphors.
Fig. 1. XRD profiles of 5 mol% of Sm3+: Ca2Gd2W3O14 phosphors at different temperatures.
Fig. 4. SEM image of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
tungsten exhibits interesting properties such as ferroelectricity and catalysis [12]. We have earlier reported on the synthesis, photoluminescence and mechanoluminescence properties of Eu3+ ions activated Ca2Gd2W3O14 phosphors [13]. In the present work, we have reported on the synthesis, structural and photoluminescence properties of another couple of rare earth ions such as Sm3+ and Dy3+ activated Ca2Gd2W3O14 phosphors. 2. Experimental studies Fig. 2. XRD profiles of 5 mol% of Dy3+: Ca2Gd2W3O14 phosphors at different temperatures.
Generally the phosphor host materials are based on borates, phosphates, aluminates, silicates, tungstates, molybdates, vanadates, etc., Among the several inorganic phosphors, tungstates are the best host materials for rare earth ions because of their strong covalent interaction, high stability, strong visible luminescence and found the potential applications for lasers, optical fibers, white light emitting diodes, plasma display panels, quantum electronics, solid-state optoelectronic devices, scintillators and phosphors [9– 11]. On the other hand, the phosphors containing gadolinium and
Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors were synthesized by solid state reaction method. Starting materials CaCO3, Gd2O3, WO3, Sm2O3 and Dy2O3 are purchased from Sigma Aldrich and are used as received without any further purification. Appropriate amount of CaCO3, Gd2O3, WO3, Sm2O3 and Dy2O3 were thoroughly mixed in an agate mortar. Afterwards, the mixtures were put into alumina crucibles and calcined in a muffle furnace at 800 °C, 1000 °C, 1200 °C and 1400 °C for 2 h in air and then cooled in air to ambient temperature. The final samples were white powders and taken to characterization. The chemical reaction used in the synthesis of Ca2Gd2W3O14 phosphors is given as follows:
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Fig. 5. EDAX profile of 5 mol% of Sm3+: Ca2 Gd2W3O14 phosphors.
Fig. 6. EDAX profile of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
2CaCO3 þ Gd2 O3 þ 3WO3 ! Ca2 Gd2 W3 O14 þ 2CO2 Structural characterization of these samples has been carried out from the X-ray powder diffraction measurements on a XRD 3003TT Seifert diffractometer with Cu Ka radiation (k = 1.5406 Å) at 40 kV and 20 mA and the 2h range was varied between 20° and 55°. Morphology of the powder phosphors were examined on a ZEISS-EVO-MA15 ESEM. The scanning electron microscopy (SEM) image was obtained for samples by using a 35 m camera at-
tached to a high resolution recording system. The elemental analysis has been carried out by energy dispersive X-ray analysis (EDAX) using an X-ray detector attached to the SEM instrument. The FT-IR spectrum (4000–450 cm1) was recorded on a Perkin Elmer Spectrum1 spectrometer with KBr pellets. Both the excitation and emission spectra were obtained on a SPEX Fluorolog-2 Fluorimeter (Model II) with Data max software to acquire the data with a Xe-arc lamp (150 W) as the excitation source.
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Fig. 9. Emission spectra of Sm3+: Ca2 Gd2W3O14 phosphors. Fig. 7. FTIR spectra of 5 mol% of Sm3+ and Dy3+: Ca2 Gd2W3O14 phosphors.
Fig. 10. Emission intensity of the transition at 646 nm of Sm3+ as a function of its doping concentration in Sm3+: Ca2 Gd2W3O14 phosphors.
Fig. 8. Excitation spectrum of 5 mol% of Sm3+: Ca2 Gd2W3O14 phosphors.
3. Results and discussion 3.1. Structural, morphological, elemental and FTIR studies of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors In order to know the crystal phase structure, the X-ray diffraction profiles of (5 mol%) Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors
prepared by solid state reaction method at different temperatures starting from 800 °C to 1400 °C were measured are shown in Figs. 1 and 2 respectively. From both the figures, it was observed that the intensity was influenced by the synthesis temperature and it was good enough at 1000 °C, thus we have used the 1000 °C as the optimized temperature for all the remaining measurements. The diffraction peaks in the measured patterns are consistent with the standard JCPDS card No. 41-0186. This indicates that the prepared phosphors exist in a tetragonal phase structure and the doping of rare earth (Sm3+ and Dy3+) ions does not influence the intensity
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sizes, and the average grain size is expected to be around 100– 250 nm. Energy dispersive X-ray analysis technique was used to confirm the presence of elements in the prepared phosphors. EDAX profiles of Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors are shown in Figs. 5 and 6, which confirm the presence of Ca, Gd, W, O and both of the rare earth ions (Sm3+ and Dy3+) in their respective profiles. Fourier Transform Infrared spectroscopy was used to identify the functional groups present in the prepared phosphors. FTIR spectra of (5 mol%) Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors are shown in Fig. 7. In FTIR spectra, the band at 809 cm1 can be assigned to the stretching mode of W–O bonds and the band at 607 cm1 in the range from 800 to 600 cm1 corresponds to the W–O vibrations respectively [12]. The bending vibration of H2O molecule and O– H stretching vibrations are present at about 1480 cm1and in the range from 3800 to 2700 cm1 [14]. 3.2. Photoluminescence studies
Fig. 11. Excitation spectrum of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
Fig. 12. Emission spectrum of 5 mol% of Dy3+: Ca2 Gd2W3O14 phosphors.
as well as the crystal structure of the phosphors. The size and the shape of the prepared phosphors were measured by SEM images. Figs. 3 and 4 show the SEM images of (5 mol%) Sm3+ and Dy3+: Ca2Gd2W3O14 phosphors. It is observed that, both of them shows, the particles are randomly distributed having various shapes and
3.2.1. Sm3+: Ca2 Gd2W3O14 phosphors The excitation spectrum of (5 mol%) Sm3+: Ca2 Gd2W3O14 phosphor is shown in Fig. 8. In the wavelength region 380– 450 nm, three excitation peaks are observed and are located at 402 nm (6H5/2 ? 4F7/2), 418 nm [6H5/2 ? (6P, 4P)5/2] and 436 nm (6H5/2 ? 4G9/2) which are attributed to f–f transitions of Sm3+. Charge transfer band of Sm3+–O2 interaction or host absorption band is not observed because the interaction of Sm3+ ions with the host lattice is very weak, therefore energy transfer does not occurs between Sm3+ and host [15]. From the excitation spectrum it was found that, the intensity of f–f transition at 402 nm is high compared with the other transitions and has been chosen for the measurement of emission spectra of Sm3+: Ca2Gd2W3O14 phosphors. The most intense peak at 402 nm clearly indicates that, these phosphors are effectively excited by near ultraviolet light emitting diodes. Fig. 9 shows the emission spectra of Sm3+: Ca2Gd2W3O14 phosphors. From the emission spectra four emission peaks are observed at 481 nm, 565 nm, 603 nm, 646 nm and which are assigned to 4M15/2 ? 6H5/2, 4 G5/2 ? 6H5/2, 4G5/2 ? 6H7/2 and 4G5/2 ? 6H9/2 transitions respectively. Among these, the transition at 646 nm (4G5/2 ? 6H9/2) is having the maximum intensity which corresponds to the red emission of Sm3+: Ca2Gd2W3O14 phosphors. In general, most of the compounds activated by Sm3+ ions show intense reddish-orange emission at 601 nm (4G5/2 ? 6H7/2) because these transitions are predominant when compared with the other transitions of Sm3+. In the present investigation, the transition at 646 nm in the red region is having maximum intensity and is in agreement with the previously reported literature [16]. From the emission spectra, it is also observed that the emission band at 646 nm (4G5/2 ? 6H9/2), split in two Stark components, this depends on the local environment of Sm3+ ions in the crystalline structure. It is a known fact that, the luminescence performance of the powder phosphors depends mainly on the concentration of activator ions, thus the identification of optimum dopant concentration is necessary. From the emission spectra, it is observed that the emission intensities are increasing gradually from 0.5 to 5 mol% and beyond this concentration the emission intensities are decreasing due to concentration quenching effect. This is because as the concentration of the luminescent ions increases, the distance between them decreases and causes the non-radiative energy transfer from one activator to another activator ion leading to the lowering of fluorescence intensity [17]. Hence 5 mol% of Sm3+ is the optimum dopant concentration. Fig. 10 shows the emission intensity of Sm3+ as a function of its doping concentration (mol%) in Sm3+: Ca2Gd2W3O14 phosphors. 3.2.2. Dy3+: Ca2 Gd2W3O14 phosphors Excitation spectrum of (5 mol%) Dy3+: Ca2Gd2W3O14 phosphor is shown in Fig. 11. From the excitation spectrum, three excitation
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bands are observed at 363 nm (6H15/2 ? 6P5/2), 385 nm (6H15/2 ? 4I13/2) and 419 nm (6H15/2 ? 4G11/2). The most intense excitation peak at 385 nm has been selected for the measurement of emission spectrum of Dy3+: Ca2Gd2W3O14 phosphors. Fig. 12 presents the emission spectrum of Dy3+: Ca2Gd2W3O14 phosphors and from this five emission bands have been observed and are located at 492 nm (4F9/2 ? 6H13/2), 526 nm (4F9/2 ? 6H13/2), 574 nm (4F9/2 ? 6H13/2), 611 nm (6F1/2 ? 6H13/2) and 635 nm (6F1/2 ? 6 H13/2) [18]. From the emission spectrum, it is clear that the emission was observed in the three regions namely blue, yellow and red regions. Among these three regions, the yellow region was most intense with the band located at 574 nm (4F9/2 ? 6H13/2). It is well known that, the transition 4F9/2 ? 6H13/2 at 574 nm with bright yellow emission is a forced electric dipole transition following the selection rule DJ = 2, which is allowed only when Dy3+ is not at an inversion center and it is also called as the hypersensitive transition because its intensity is very sensitive to the outside crystal field environment surrounding Dy3+ [8]. Thus 5 mol% of Dy3+: Ca2Gd2W3O14 phosphors with bright yellow emission can be used as novel optical luminescent materials.
4. Conclusions It could be concluded that, we have observed an intense red and yellow emission from Sm3+ and Dy3+ ions activated Ca2Gd2W3O14 powder phosphors synthesized by the solid state reaction method. The emission intensity has reached the maximum value at 5 mol% in the case of Sm3+: Ca2Gd2W3O14 phosphors. Thus 5 mol% is the optimized concentration. The prepared phosphors are also characterized by the XRD, SEM, EDAX and FTIR spectra. From these spectral results, we have studied the structural, morphological, elemental properties and also identified the functional groups present in the composition of the powder phosphors. By considering the luminescent performance (5 mol%), Sm3+ and Dy3+: Ca2Gd2W3O14
phosphors are the potential candidates for the applications as novel luminescent materials in optical systems. Acknowledgements The authors acknowledge the Sophisticated Analytical Instrument Facility (SAIF), IIT, Chennai for extending instrumental facilities. References [1] Xiang Ying Chen, ShiPing Bao, YuCheng Wu, J. Solid State Chem. 183 (2010) 2004. [2] Jiapeng Fu, Qinghong Zhang, Yaogang Li, Hongzhi Wang, J. Lumin. 130 (2010) 231. [3] Jinsheng Liao, Shaoan Zhang, Hangying You, He-Rui Wen, Jing-Lin Chen, Weixiong You, Opt. Mater. 33 (2011) 953. [4] Xihua Dou, Weiren Zhao, Enhai Song, Guoxiong Zhou, Chunyu Yi, Mingkang Zhou, Spectrochim. Acta A 78 (2011) 821. [5] Zheng-Hua Ju, Rui-Ping Wei, Jing-Xin Ma, Chao-Ran Pang, Wei-Sheng. Liu, J. Alloy. Compd. 507 (2010) 133. [6] Xinmin Zhang, Hyo Jin Seo, J. Alloy. Compd. 509 (2011) 2007. [7] Jian-Xin Meng, Chuang-Tao Yang, Qing-Qing Chen, J. Lumin. 130 (2010) 1320. [8] Yue Tiana, Baojiu Chen, Bining Tian, Ruinian Hu, Jiashi Sun, Lihong Cheng, Haiyang Zhong, Xiangping Li, Jinsu Zhang, Yanfeng Zheng, Tingting Yu, Libo Huanga, Qingyu Meng, J. Alloy. Compd. 509 (2011) 6096. [9] M. Guzik, E. Tomaszewicz, S.M. Kaczmarek, J. Cybinsca, H. Fuks, J. Non-Cryst. Solids 356 (2010) 902. [10] Jinsheng Liao, Bao Qiu, Huasheng Lai, J. Lumin. 129 (2009) 668. [11] Shao-An Yan, Yee-Shin Chang, Jian-Wen Wang, Weng-Sing Hwang, Yen-Hwei Chang, Mater. Res. Bull. 46 (2011) 1231. [12] Fang Lei, Bing Yan, Hao-Hong Chen, J. Solid State Chem. 181 (2008) 2845. [13] S. Sailaja, S.J. Dhoble, Nameetha Brahme, B. Sudhakar Reddy, J. Mater. Sci. (2011), doi:10.1007/s10853-011-5759-2. [14] M.K. Chong, K. Pita, C.H. Kam, J. Phys. Chem. Solids 66 (2005) 213. [15] Dong Tu, Yujun Liang, Rong Liu, Zheng Cheng, Fan Yang, Wenlong Yang, J. Alloy. Compd. 509 (2011) 5596. [16] Xianghong He, Jian Zhou, Ning Lian, Jianhua Sun, Mingyun Guan, J. Lumin. 130 (2010) 743–747. [17] Bingfu Lei, Shi-Qing Man, Yingliang Liu, Song Yue, Mater. Chem. Phys. 124 (2010) 912. [18] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4450.