- Email: [email protected]
Sol–gel synthesis, structural and optical properties of rare earth ions (Sm3+ or Dy3+) activated Ca3Ga2Si3O12 powder phosphors
Journal of Luminescence 131 (2011) 2503–2508
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Sol–gel synthesis, structural and optical properties of rare earth ions (Sm3 þ or Dy3 þ ) activated Ca3Ga2Si3O12 powder phosphors M. Bhushana Reddy a, C. Nageswara Raju a, S. Sailaja a, B. Vengala Rao b, B. Sudhakar Reddy a,n a b
Department of Physics (Research Centre), S.V. Degree College, Kadapa, Y.S.R. District 516003, India Department of Physics, Changwon National University, Changwon 641773, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 12 February 2011 Received in revised form 5 June 2011 Accepted 9 June 2011 Available online 28 June 2011
Structural, morphological and optical properties of rare earth ions (RE3 þ ¼ Sm3 þ or Dy3 þ ) activated Ca3Ga2Si3O12 (CaGaSi) phosphors synthesized by the sol–gel method are reported. XRD results confirmed the cubic phase structure of RE3 þ :CaGaSi phosphors. From the SEM images of RE3 þ :CaGaSi phosphors, it is observed that the particles are agglomerated. Photoluminescence spectra of Sm3 þ : CaGaSi phosphors have shown bright orange red emission at 598 nm (4G5/2-6H7/2) with an excitation wavelength of lexci ¼ 401 nm. In the case of Dy3 þ :CaGaSi phosphors bright yellow emission has been observed at 574 nm (4F9/2- 6H13/2) with lexci ¼ 451 nm. From the PL spectral results, the rare earth ion concentration of CaGaSi phosphors is optimized. & 2011 Elsevier B.V. All rights reserved.
Keywords: Sol–gel method Emission Excitation Sm3 þ and Dy3 þ :CaGaSi phosphors
1. Introduction In recent years, an extensive investigation has been carried out on phosphors including aluminates, silicates, aluminoborates, aluminosilicates, nitrides, borates, sulfides, etc. Among these, silicate based phosphors have great importance because of their properties such as good thermal and chemical stability to high irradiation powers, stable crystal structure, temperature and durability in the packaging resin, good luminescence output, excellent host materials for rare earth ions, high color purity and are also used in the preparation of white LEDs [1–3]. Calcium gallate phosphors are the promising host materials for rare earth ions and has a wide band gap because the materials activated by rare earth ions result in a large variety in luminescence [4]. By considering the above importance and significance, we have motivated to select Ca3Ga2Si3O12 host phosphor for the present work. Rare earth ions have good luminescent properties such as high color purity because of their electronic transitions within the 4fn energy levels. The rare earth ions doped phosphors have also attracted much attention because of their use in the field of fluorescent lamps, display applications, cathode-ray tubes, solid-state lasers, field-emission displays, plasma displays and in the development of white light emitting diodes [5]. The rare earth ions are considered as the important activators for luminescent devices. Among different rare earth ions, Sm3 þ is significant due
n
Corresponding author. Tel.: þ91 8562 244367; fax: þ 91 8562 259763. E-mail address: [email protected] (B. Sudhakar Reddy).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.06.015
to its applications in high density optical storage, temperature sensors, under sea communication, various fluorescent devices, color display and visible solid-state lasers due to its bright emission in orange or red region and consists of transitions from the excited state 4G5/2 to the ground state 6H5/2 and also to the higher levels 6Hj (j¼ 7/2, 9/2 and 11/2) [6]. Samarium is an active ion for different inorganic host lattices and usually acts as a powerful emitting center because of its energy level structure and high luminescence efficiency. Recently, investigations on novel red emitting phosphors have enhanced the importance of samarium doped crystalline materials, glasses and thin films. In the rare earth family, another interesting ion is Dy3 þ (4f9) because the analysis of luminescence from 4F9/2 level of Dy3 þ ions is very interesting as it ranges in the visible and NIR regions and also Dy3 þ ions could be considered as an appropriate luminescence center for the white light emitting phosphors. Trivalent dysprosium ion exhibits an intense transition from 4F9/2 to 6H13/2 level with the yellow light emission at 575 nm, which is hypersensitive, and the electric dipole transition. On the other hand Dy3 þ ion also shows the blue emission due to the transition from 4F9/2 to 6 H15/2, which is magnetic dipole allowed and its intensity is not altered by the local environment of Dy3 þ in the crystal lattice [7]. In order to explain the significance of RE3 þ (Sm3 þ , Dy3 þ ): CaGaSi luminescent phosphors, the sol–gel method is most suitable. As most of the silicates have high melting points, solid-state diffusion method has been used for the synthesis of silicates. Sol–gel process is an efficient technique for the preparation of phosphors due to its good mixing of starting materials and relatively low reaction temperature, which results in more homogeneous
2504
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
products than those obtained by solid-state reactions [8]. We have reported earlier on the synthesis and the luminescence properties of Eu3 þ or Tb3 þ :Ca3Ga2Si3O12 phosphors [9]. In the present paper, we have undertaken another couple of rare earth ions (Sm3 þ and Dy3 þ ) doped Ca3Ga2Si3O12 phosphors in order to study the structural, morphological and optical properties for novel applications.
2. Experimental procedure Ca3Ga2(1 x)Si3O12:RE32xþ (RE ¼Sm, Dy and x ¼1, 5, 10 and 15 mol %) phosphors were prepared by sol–gel method. Stoichiometric amounts of CaCO3, Ga2O3 (99.99%, Sigma Aldrich), Sm2O3 (99.99%, Sigma Aldrich), Dy2O3 (99.99%, Sigma Aldrich) and tetraethyl-ortho-silicate (TEOS, Sigma Aldrich) were taken as starting materials. CaCO3 and RE2O3 were first dissolved in nitric acid and then mixed with double distilled water and ethanol (volume ratio¼1:4) solution containing citric acid as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1:2; subsequently TEOS was added to this solution and stirred well. Ammonia was added to the above solution, which neutralizes the excess nitrate; this results in the formation of gel. The gel thus formed was dried in ambient atmosphere at 100 1C. The powder thus obtained after grinding the dried gel was kept in a furnace for precalcination at 400 1C for 4 h and then calcined at the required temperature 1000 1C for 6 h to obtain the phosphor samples. We also prepared the samples at different temperatures but it was found that the emission intensity is good at 1000 1C; hence this temperature is preferred. The chemical reaction for the synthesis of Ca3Ga2Si3O12 and the equation when the rare earth ions are introduced in the host powder phosphors are as follows:
Fig. 1. XRD pattern of Sm3 þ (5 mol %):CaGaSi phosphors.
3CaCO3 þ Ga2O3 þ3TEOS-Ca3Ga2Si3O12 þresidue 3CaCO3 þ (1 x)Ga2O3 þ3TEOSþ xRE2O3Ca3Ga2(1 x)Si3O12:RE32xþ þresidue Structural characterization of these samples has been carried out from the X-ray powder diffraction measurements on a XRD ˚ 3003 TT Seifert diffractometer with CuKa radiation (l ¼1.5406 A) at 40 kV and 20 mA and the 2y range was varied between 201 and 701. Morphology of the rare earth ions activated CaGaSi phosphor was examined on a ZEISS-EVO-MA15 ESEM. The scanning electron microscopy (SEM) image was obtained for phosphors using a 35 m camera attached to a high resolution recording system. The FTIR spectrum (4000–450 cm 1) was recorded on a Bruker IFS 66VFT-IR spectrometer with KBr pellets. Both the excitation and emission spectra of CaGaSi:RE3 þ (RE ¼Sm or Dy) phosphors were recorded on a SPEX Fluorolog-3 fluorimeter attached with a Xe arc lamp (450 W) with the data max software for acquiring the spectral data. A Xe flash lamp with a phosphorimeter attachment was used to measure the lifetimes of the emission transitions of these phosphors.
3. Results and discussion 3.1. Sm3 þ :CaGaSi phosphors The crystal structure of (5 mol %) Sm3 þ :CaGaSi powder phosphor is known by means of single crystal X-ray diffraction method and the pattern is presented in Fig. 1. From the XRD profile, it is observed that the rare earth ions (Sm3 þ ) do not influence the crystal phase of Ca3Ga2Si3O12 host phosphors, which have cubic structure and are well consistent with the standard JCPDS-74-1576. The SEM observation of the (5 mol %) Sm3 þ :CaGaSi powders indicates that the particles have the diameter of
Fig. 2. SEM image of Sm3 þ (5 mol %):CaGaSi phosphors.
around 200 nm and are agglomerated as shown in Fig. 2. The FTIR spectrum of (5 mol %) Sm3 þ :CaGaSi phosphor is shown in Fig. 3 and the assignment of peak positions is presented in Table 1. Assignments have been made by the earlier reported literature [10–12]. Fig. 4 shows the excitation spectrum of 5 mol % of Sm3 þ :CaGaSi powder phosphors. The excitation spectrum has shown five excitation bands at 401 nm (6H5/2-4F7/2), 415 nm (6H5/2-(6P, 4P)5/2),
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
2505
439 nm (6H5/2-4G9/2), 471 nm (6H5/2-4I11/2) and 525 nm (6H5/24 F3/2) and these peaks are due to the 4f–4f inner shell transitions of Sm3 þ . Among the five excitation bands, the prominent excitation band at 401 nm (6H5/2-4F7/2) has been chosen for the measurement of emission spectra of CaGaSi:Sm3 þ phosphors. Fig. 5 shows the emission spectra of CaGaSi:Sm3 þ phosphors. From this figure four emission bands at 562, 598, 645 and 706 nm are observed and are assigned to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2 -6H11/2 transitions [13–15]. Among these, the prominent luminescent band is located at 598 nm, which emit reddish orange light. The emission band at 645 nm corresponding to the transition 4 G5/2-6H9/2 is electric dipole allowed and is sensitive to the crystal field environment. The most intense band at 598 nm is partially magnetic and partially electric dipole transition and the third band located at 562 nm is magnetic dipole transition and is hard to the variations in the crystal field [16]. From the emission spectra, it is also observed that the emission intensities are observed to be increasing gradually from 1 to 5 mol % and beyond this concentration the emission intensities decrease due to concentration quenching; hence 5 mol% of Sm3 þ is the optimum concentration. Inset of Fig. 5 shows the emission intensity of Sm3 þ as a function of its doping concentration (mol%) in CaGaSi:Sm3 þ phosphors. Fig. 6 Fig. 4. Excitation spectrum of Sm3 þ (5 mol %):CaGaSi phosphors.
Fig. 3. FTIR spectrum of Sm3 þ (5 mol %):CaGaSi phosphors.
Fig. 5. Emission spectra and inset shows emission intensity of Sm3þ as a function of its doping concentration in Ca3Ga2(1 x)Si3O12:Smx (x¼ 1, 5, 10 and 15 mol %) phosphors.
Table1 FTIR assignments of (0.5 mol %) RE3 þ (RE ¼Sm or Dy):CaGaSi phosphors. Sm3 þ :CaGaSi phosphors
Dy3 þ :CaGaSi phosphors
Wavenumber (cm 1)
Assignments
Wavenumber (cm 1)
Assignments
3745 3444 2920 2850 1631
The first region 2900–3800 cm 1 with broad band centered at 3444 cm 1 is assigned to the stretching mode of OH groups The second region 1500–2900 cm 1 originates from the absorption of H2O and (NO3) groups
3743 3445 2918 2848 1634
The first region 2900–3800 cm 1 with broad band centered at 3445 cm 1 is assigned to the stretching mode of OH groups The second region 1500–2900 cm 1 originates from the absorption of H2O and (NO3) groups
Tetrahedral SiO4 units can be assigned based on symmetric stretching (n1), symmetric bending (n2) anti-symmetric stretching (n3), anti-symmetric bending (n4) 1471 The third region 500–1500 cm 1 shows the 1469 The third region 500–1500 cm 1 shows the stretching (721 and 1469 cm 1) and bending stretching (715 and 1471 cm 1) and bending 999 996 vibrations (520 cm 1) of silicate bonds vibrations (521 cm 1) of silicate bonds 929 934 721 715 520 521
2506
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
phase and there is no effect of Dy3 þ ions on the crystal structure. Fig. 8 shows the SEM image of (5 mol %) Dy3 þ :CaGaSi phosphor. This image displays that the phosphor particles are distributed randomly that indicates the phenomenon of agglomeration and the grain size is around 220 nm. The FTIR spectrum of (5 mol %) Dy3 þ :CaGaSi phosphor is presented in Fig. 9, and from the measurement of FTIR spectrum, the assignment of wavenumbers is reported in Table 1. Assignment of peak positions has been made by the earlier reported literature [10–12]. Excitation spectrum of (5 mol %) Dy3 þ :CaGaSi phosphor is shown in Fig. 10, with the excitation bands located at 348 nm (6H15/2-6P7/2), 363 nm (6H15/2-6P3/2), 386 nm (6H15/2-4I13/2, 4F7/2), 424 nm
Fig. 6. Decay curve of the emission transition and inset shows energy level diagram of Sm3 þ (5 mol %):CaGaSi phosphors.
Fig. 8. SEM image of Dy3 þ (5 mol %):CaGaSi phosphors.
Fig. 7. XRD profile of Dy3 þ (5 mol %):CaGaSi phosphors.
presents the decay curve of the emission transition of (5 mol %) Sm3 þ :CaGaSi phosphors along with its lifetime in the same figure. Inset of Fig. 6 shows the energy level diagram of (5 mol %) Sm3 þ :CaGaSi phosphors. 3.2. Dy3 þ :CaGaSi phosphors The XRD profile of 5 mol% Dy3 þ :CaGaSi phosphor is shown in Fig. 7 and indicates that these phosphors also have the cubic
Fig. 9. FTIR spectrum of Dy3 þ (5 mol %):CaGaSi phosphors.
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
2507
Fig. 10. Excitation spectrum of Dy3 þ (5 mol %):CaGaSi phosphors.
Fig. 12. Decay curve of the emission transition and inset shows energy level diagram of Dy3 þ (5 mol %):CaGaSi phosphors.
5 mol % and after reaching the critical concentration (x¼5 mol%) it decreases; this indicates that concentration quenching occurs. As the concentration of Dy3 þ ions increases the distance between the activator ions becomes smaller; this leads to the non-radiative energy transfer from one activator to another. The non-radiative energy transfer occurs as a result of an exchange interaction [17]. Thus, from the emission spectra, we have been observed that 5 mol % is the optimized concentration. Inset of Fig. 11 shows the emission intensity of Dy3 þ as a function of its doping concentration (mol%) in Ca3GaSi:Dy3 þ phosphors. The decay curve of the emission transition of (5 mol%) Dy3 þ :CaGaSi phosphor is presented in Fig. 12. The energy level diagram of (5 mol %) Dy3 þ :CaGaSi phosphor is shown in the inset of Fig. 12.
4. Conclusions
Fig. 11. Emission spectra and inset shows emission intensity of Dy3 þ as a function of its doping concentration in Ca3Ga2(1 x)Si3O12:Smx (x¼ 1, 5, 10 and 15 mol %) phosphors.
(6H15/2-4G11/2), 451 nm (6H15/2-4I15/2) and 470 nm (6H15/2-4F9/2). Fig. 11 presents the emission spectra of Dy3þ :CaGaSi phosphors and from this, three emission transitions are observed and are assigned to 4 F9/2-6H15/2 (482 nm), 4F9/2-6H13/2 (574 nm) and 4F9/2-6H11/2 (664 nm). The transition 4F9/2-6H13/2 at 574 nm with bright yellow emission is a forced electric dipole transition, which is allowed only when Dy3 þ is not at an inversion center and is familiar as the hypersensitive transition. In the present work, the emission at 574 nm (4F9/2-6H13/2) is the prominent one, indicating that the site occupied by Dy3 þ is not at an inversion center. In the case of emission spectra of Ca3GaSi:Dy3þ phosphors, the emission intensity also increases with the increase in Dy3 þ concentration from 1 to
We concluded that Sm3 þ and Dy3 þ ions activated CaGaSi phosphors have been synthesized by sol–gel method. From the XRD and SEM spectral studies, Sm3 þ and Dy3 þ :CaGaSi phosphors have shown cubic structure and the particles are agglomerated. Sm3 þ and Dy3 þ :CaGaSi phosphors have shown bright orange red and yellow emissions, respectively. The effect of the concentration on luminescence of Sm3 þ and Dy3 þ :CaGaSi phosphors has been investigated systematically. From the photoluminescence spectra of both Sm3 þ and Dy3 þ :CaGaSi phosphors, it has been observed that 5 mol% is the optimized concentration. Based on the spectral results, 5 mol% Sm3 þ and Dy3 þ :CaGaSi phosphors are brightly luminescent and are used as novel optical materials.
Acknowledgment This work was supported by the UGC-SERO, Hyderabad, in the form of Minor Research Project No. F MRP-3200/09(MRP/UGCSERO) sanctioned to the author (MBR), who would like to thank
2508
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
the Joint Secretary and Head, UGC-SERO, Hyderabad, and the authors also acknowledge the Sophisticated Analytical Instrument Facility (SAIF), IIT, Chennai, for extending instrumental facilities. References [1] Xinguo Zhang, Menglian Gong, J. Alloys Compd. 509 (2011) 2850. [2] Cheng Guang, Liu Quansheng, Cheng Liqun, Lu Liping, Sun Haiying, Wu Yiqing, Bai Zhaohui, Zhang Xiyan, Qiu Guanming, J. Rare Earth 28 (4) (2010) 526. [3] W.J. Park, Y.H. Song, D.H. Yoon, Mater. Sci. Eng. B 173 (2010) 76. [4] Yong-Kyu Kim, Dong-Hee Cho, Yong-Kwang Jeong, Min-Kook Nah, Kwang-Bok Kim, Jun-Gill Kang, Mater. Res. Bull. 45 (2010) 905. [5] Zhiguo Xia, Daimei Chen, Min Yang, Ting Ying, J. Phys. Chem. Solids 71 (2010) 175. [6] Xinmin Zhang, Hyo Jin Seo, J. Alloys Compd. 509 (2011) 2007. [7] Jian-Xin Meng, Chuang-Tao Yang, Qing-Qing Chen, J. Lumin. 130 (2010) 1320.
[8] Wang Yi, Zeng Qingguang, Shejun Hu, Kunbin Wu, Junhong Jiang, Genting Chen, J. Rare Earth 28 (2) (2010) 176. [9] M. Bhushana Reddy, S. Sailaja, K. Vemasevana Raju, C. Nageswara Raju, P. Giridhar, B. Vengala Rao, B. Sudhakar Reddy, J. Biol. Chem. Lumin. (2011). [10] B. Vengal Rao, N. Yung-Tang, L. Tzung-Heng, C. In-Gann, J. Am. Ceram. Soc. 92 (12) (2009) 2953. [11] Y. Liangzhu, F. Min, L. Yuejiao, L. Chao, W. Xiuying, Y. Xibin, Front. Chem. Chin. 2 (4) (2007) 442. [12] M. Yu, J. Lin, Y.H. Zhou, S.B. Wang, H.J. Zhang, J. Mater. Chem. 12 (2002) 86. [13] Li Changmin, Yang Dianlai, Zhang Yingying, Wang Zhiqiang, Lin Hai, J. Rare Earth 25 (2007) 143. [14] Xianghong He, Jian Zhou, Ning Lian, Jianhua Sun, Mingyun Guan, J. Lumin. 130 (2010) 743. [15] Bingful Lei, Yingliang Liu, Gongben Tang, Zeren Ye, Chunshan Shi, Mater. Chem. Phys. 87 (2004) 227. [16] Vijay Singh, S. Watanabe, T.K. Gundu Rao, J.F.D. Chubaci, Ho-Young Kwak, J. Non-Cryst. Solids 356 (2010) 1185. [17] Xue Yanna, Xiao Fen, Zhang Qinyuan, Jiang Zhonghong, J. Rare Earth 27 (5) (2009) 753.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Sol–gel synthesis, structural and optical properties of rare earth ions (Sm3 þ or Dy3 þ ) activated Ca3Ga2Si3O12 powder phosphors M. Bhushana Reddy a, C. Nageswara Raju a, S. Sailaja a, B. Vengala Rao b, B. Sudhakar Reddy a,n a b
Department of Physics (Research Centre), S.V. Degree College, Kadapa, Y.S.R. District 516003, India Department of Physics, Changwon National University, Changwon 641773, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 12 February 2011 Received in revised form 5 June 2011 Accepted 9 June 2011 Available online 28 June 2011
Structural, morphological and optical properties of rare earth ions (RE3 þ ¼ Sm3 þ or Dy3 þ ) activated Ca3Ga2Si3O12 (CaGaSi) phosphors synthesized by the sol–gel method are reported. XRD results confirmed the cubic phase structure of RE3 þ :CaGaSi phosphors. From the SEM images of RE3 þ :CaGaSi phosphors, it is observed that the particles are agglomerated. Photoluminescence spectra of Sm3 þ : CaGaSi phosphors have shown bright orange red emission at 598 nm (4G5/2-6H7/2) with an excitation wavelength of lexci ¼ 401 nm. In the case of Dy3 þ :CaGaSi phosphors bright yellow emission has been observed at 574 nm (4F9/2- 6H13/2) with lexci ¼ 451 nm. From the PL spectral results, the rare earth ion concentration of CaGaSi phosphors is optimized. & 2011 Elsevier B.V. All rights reserved.
Keywords: Sol–gel method Emission Excitation Sm3 þ and Dy3 þ :CaGaSi phosphors
1. Introduction In recent years, an extensive investigation has been carried out on phosphors including aluminates, silicates, aluminoborates, aluminosilicates, nitrides, borates, sulfides, etc. Among these, silicate based phosphors have great importance because of their properties such as good thermal and chemical stability to high irradiation powers, stable crystal structure, temperature and durability in the packaging resin, good luminescence output, excellent host materials for rare earth ions, high color purity and are also used in the preparation of white LEDs [1–3]. Calcium gallate phosphors are the promising host materials for rare earth ions and has a wide band gap because the materials activated by rare earth ions result in a large variety in luminescence [4]. By considering the above importance and significance, we have motivated to select Ca3Ga2Si3O12 host phosphor for the present work. Rare earth ions have good luminescent properties such as high color purity because of their electronic transitions within the 4fn energy levels. The rare earth ions doped phosphors have also attracted much attention because of their use in the field of fluorescent lamps, display applications, cathode-ray tubes, solid-state lasers, field-emission displays, plasma displays and in the development of white light emitting diodes [5]. The rare earth ions are considered as the important activators for luminescent devices. Among different rare earth ions, Sm3 þ is significant due
n
Corresponding author. Tel.: þ91 8562 244367; fax: þ 91 8562 259763. E-mail address: [email protected] (B. Sudhakar Reddy).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.06.015
to its applications in high density optical storage, temperature sensors, under sea communication, various fluorescent devices, color display and visible solid-state lasers due to its bright emission in orange or red region and consists of transitions from the excited state 4G5/2 to the ground state 6H5/2 and also to the higher levels 6Hj (j¼ 7/2, 9/2 and 11/2) [6]. Samarium is an active ion for different inorganic host lattices and usually acts as a powerful emitting center because of its energy level structure and high luminescence efficiency. Recently, investigations on novel red emitting phosphors have enhanced the importance of samarium doped crystalline materials, glasses and thin films. In the rare earth family, another interesting ion is Dy3 þ (4f9) because the analysis of luminescence from 4F9/2 level of Dy3 þ ions is very interesting as it ranges in the visible and NIR regions and also Dy3 þ ions could be considered as an appropriate luminescence center for the white light emitting phosphors. Trivalent dysprosium ion exhibits an intense transition from 4F9/2 to 6H13/2 level with the yellow light emission at 575 nm, which is hypersensitive, and the electric dipole transition. On the other hand Dy3 þ ion also shows the blue emission due to the transition from 4F9/2 to 6 H15/2, which is magnetic dipole allowed and its intensity is not altered by the local environment of Dy3 þ in the crystal lattice [7]. In order to explain the significance of RE3 þ (Sm3 þ , Dy3 þ ): CaGaSi luminescent phosphors, the sol–gel method is most suitable. As most of the silicates have high melting points, solid-state diffusion method has been used for the synthesis of silicates. Sol–gel process is an efficient technique for the preparation of phosphors due to its good mixing of starting materials and relatively low reaction temperature, which results in more homogeneous
2504
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
products than those obtained by solid-state reactions [8]. We have reported earlier on the synthesis and the luminescence properties of Eu3 þ or Tb3 þ :Ca3Ga2Si3O12 phosphors [9]. In the present paper, we have undertaken another couple of rare earth ions (Sm3 þ and Dy3 þ ) doped Ca3Ga2Si3O12 phosphors in order to study the structural, morphological and optical properties for novel applications.
2. Experimental procedure Ca3Ga2(1 x)Si3O12:RE32xþ (RE ¼Sm, Dy and x ¼1, 5, 10 and 15 mol %) phosphors were prepared by sol–gel method. Stoichiometric amounts of CaCO3, Ga2O3 (99.99%, Sigma Aldrich), Sm2O3 (99.99%, Sigma Aldrich), Dy2O3 (99.99%, Sigma Aldrich) and tetraethyl-ortho-silicate (TEOS, Sigma Aldrich) were taken as starting materials. CaCO3 and RE2O3 were first dissolved in nitric acid and then mixed with double distilled water and ethanol (volume ratio¼1:4) solution containing citric acid as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1:2; subsequently TEOS was added to this solution and stirred well. Ammonia was added to the above solution, which neutralizes the excess nitrate; this results in the formation of gel. The gel thus formed was dried in ambient atmosphere at 100 1C. The powder thus obtained after grinding the dried gel was kept in a furnace for precalcination at 400 1C for 4 h and then calcined at the required temperature 1000 1C for 6 h to obtain the phosphor samples. We also prepared the samples at different temperatures but it was found that the emission intensity is good at 1000 1C; hence this temperature is preferred. The chemical reaction for the synthesis of Ca3Ga2Si3O12 and the equation when the rare earth ions are introduced in the host powder phosphors are as follows:
Fig. 1. XRD pattern of Sm3 þ (5 mol %):CaGaSi phosphors.
3CaCO3 þ Ga2O3 þ3TEOS-Ca3Ga2Si3O12 þresidue 3CaCO3 þ (1 x)Ga2O3 þ3TEOSþ xRE2O3Ca3Ga2(1 x)Si3O12:RE32xþ þresidue Structural characterization of these samples has been carried out from the X-ray powder diffraction measurements on a XRD ˚ 3003 TT Seifert diffractometer with CuKa radiation (l ¼1.5406 A) at 40 kV and 20 mA and the 2y range was varied between 201 and 701. Morphology of the rare earth ions activated CaGaSi phosphor was examined on a ZEISS-EVO-MA15 ESEM. The scanning electron microscopy (SEM) image was obtained for phosphors using a 35 m camera attached to a high resolution recording system. The FTIR spectrum (4000–450 cm 1) was recorded on a Bruker IFS 66VFT-IR spectrometer with KBr pellets. Both the excitation and emission spectra of CaGaSi:RE3 þ (RE ¼Sm or Dy) phosphors were recorded on a SPEX Fluorolog-3 fluorimeter attached with a Xe arc lamp (450 W) with the data max software for acquiring the spectral data. A Xe flash lamp with a phosphorimeter attachment was used to measure the lifetimes of the emission transitions of these phosphors.
3. Results and discussion 3.1. Sm3 þ :CaGaSi phosphors The crystal structure of (5 mol %) Sm3 þ :CaGaSi powder phosphor is known by means of single crystal X-ray diffraction method and the pattern is presented in Fig. 1. From the XRD profile, it is observed that the rare earth ions (Sm3 þ ) do not influence the crystal phase of Ca3Ga2Si3O12 host phosphors, which have cubic structure and are well consistent with the standard JCPDS-74-1576. The SEM observation of the (5 mol %) Sm3 þ :CaGaSi powders indicates that the particles have the diameter of
Fig. 2. SEM image of Sm3 þ (5 mol %):CaGaSi phosphors.
around 200 nm and are agglomerated as shown in Fig. 2. The FTIR spectrum of (5 mol %) Sm3 þ :CaGaSi phosphor is shown in Fig. 3 and the assignment of peak positions is presented in Table 1. Assignments have been made by the earlier reported literature [10–12]. Fig. 4 shows the excitation spectrum of 5 mol % of Sm3 þ :CaGaSi powder phosphors. The excitation spectrum has shown five excitation bands at 401 nm (6H5/2-4F7/2), 415 nm (6H5/2-(6P, 4P)5/2),
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
2505
439 nm (6H5/2-4G9/2), 471 nm (6H5/2-4I11/2) and 525 nm (6H5/24 F3/2) and these peaks are due to the 4f–4f inner shell transitions of Sm3 þ . Among the five excitation bands, the prominent excitation band at 401 nm (6H5/2-4F7/2) has been chosen for the measurement of emission spectra of CaGaSi:Sm3 þ phosphors. Fig. 5 shows the emission spectra of CaGaSi:Sm3 þ phosphors. From this figure four emission bands at 562, 598, 645 and 706 nm are observed and are assigned to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2 -6H11/2 transitions [13–15]. Among these, the prominent luminescent band is located at 598 nm, which emit reddish orange light. The emission band at 645 nm corresponding to the transition 4 G5/2-6H9/2 is electric dipole allowed and is sensitive to the crystal field environment. The most intense band at 598 nm is partially magnetic and partially electric dipole transition and the third band located at 562 nm is magnetic dipole transition and is hard to the variations in the crystal field [16]. From the emission spectra, it is also observed that the emission intensities are observed to be increasing gradually from 1 to 5 mol % and beyond this concentration the emission intensities decrease due to concentration quenching; hence 5 mol% of Sm3 þ is the optimum concentration. Inset of Fig. 5 shows the emission intensity of Sm3 þ as a function of its doping concentration (mol%) in CaGaSi:Sm3 þ phosphors. Fig. 6 Fig. 4. Excitation spectrum of Sm3 þ (5 mol %):CaGaSi phosphors.
Fig. 3. FTIR spectrum of Sm3 þ (5 mol %):CaGaSi phosphors.
Fig. 5. Emission spectra and inset shows emission intensity of Sm3þ as a function of its doping concentration in Ca3Ga2(1 x)Si3O12:Smx (x¼ 1, 5, 10 and 15 mol %) phosphors.
Table1 FTIR assignments of (0.5 mol %) RE3 þ (RE ¼Sm or Dy):CaGaSi phosphors. Sm3 þ :CaGaSi phosphors
Dy3 þ :CaGaSi phosphors
Wavenumber (cm 1)
Assignments
Wavenumber (cm 1)
Assignments
3745 3444 2920 2850 1631
The first region 2900–3800 cm 1 with broad band centered at 3444 cm 1 is assigned to the stretching mode of OH groups The second region 1500–2900 cm 1 originates from the absorption of H2O and (NO3) groups
3743 3445 2918 2848 1634
The first region 2900–3800 cm 1 with broad band centered at 3445 cm 1 is assigned to the stretching mode of OH groups The second region 1500–2900 cm 1 originates from the absorption of H2O and (NO3) groups
Tetrahedral SiO4 units can be assigned based on symmetric stretching (n1), symmetric bending (n2) anti-symmetric stretching (n3), anti-symmetric bending (n4) 1471 The third region 500–1500 cm 1 shows the 1469 The third region 500–1500 cm 1 shows the stretching (721 and 1469 cm 1) and bending stretching (715 and 1471 cm 1) and bending 999 996 vibrations (520 cm 1) of silicate bonds vibrations (521 cm 1) of silicate bonds 929 934 721 715 520 521
2506
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
phase and there is no effect of Dy3 þ ions on the crystal structure. Fig. 8 shows the SEM image of (5 mol %) Dy3 þ :CaGaSi phosphor. This image displays that the phosphor particles are distributed randomly that indicates the phenomenon of agglomeration and the grain size is around 220 nm. The FTIR spectrum of (5 mol %) Dy3 þ :CaGaSi phosphor is presented in Fig. 9, and from the measurement of FTIR spectrum, the assignment of wavenumbers is reported in Table 1. Assignment of peak positions has been made by the earlier reported literature [10–12]. Excitation spectrum of (5 mol %) Dy3 þ :CaGaSi phosphor is shown in Fig. 10, with the excitation bands located at 348 nm (6H15/2-6P7/2), 363 nm (6H15/2-6P3/2), 386 nm (6H15/2-4I13/2, 4F7/2), 424 nm
Fig. 6. Decay curve of the emission transition and inset shows energy level diagram of Sm3 þ (5 mol %):CaGaSi phosphors.
Fig. 8. SEM image of Dy3 þ (5 mol %):CaGaSi phosphors.
Fig. 7. XRD profile of Dy3 þ (5 mol %):CaGaSi phosphors.
presents the decay curve of the emission transition of (5 mol %) Sm3 þ :CaGaSi phosphors along with its lifetime in the same figure. Inset of Fig. 6 shows the energy level diagram of (5 mol %) Sm3 þ :CaGaSi phosphors. 3.2. Dy3 þ :CaGaSi phosphors The XRD profile of 5 mol% Dy3 þ :CaGaSi phosphor is shown in Fig. 7 and indicates that these phosphors also have the cubic
Fig. 9. FTIR spectrum of Dy3 þ (5 mol %):CaGaSi phosphors.
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
2507
Fig. 10. Excitation spectrum of Dy3 þ (5 mol %):CaGaSi phosphors.
Fig. 12. Decay curve of the emission transition and inset shows energy level diagram of Dy3 þ (5 mol %):CaGaSi phosphors.
5 mol % and after reaching the critical concentration (x¼5 mol%) it decreases; this indicates that concentration quenching occurs. As the concentration of Dy3 þ ions increases the distance between the activator ions becomes smaller; this leads to the non-radiative energy transfer from one activator to another. The non-radiative energy transfer occurs as a result of an exchange interaction [17]. Thus, from the emission spectra, we have been observed that 5 mol % is the optimized concentration. Inset of Fig. 11 shows the emission intensity of Dy3 þ as a function of its doping concentration (mol%) in Ca3GaSi:Dy3 þ phosphors. The decay curve of the emission transition of (5 mol%) Dy3 þ :CaGaSi phosphor is presented in Fig. 12. The energy level diagram of (5 mol %) Dy3 þ :CaGaSi phosphor is shown in the inset of Fig. 12.
4. Conclusions
Fig. 11. Emission spectra and inset shows emission intensity of Dy3 þ as a function of its doping concentration in Ca3Ga2(1 x)Si3O12:Smx (x¼ 1, 5, 10 and 15 mol %) phosphors.
(6H15/2-4G11/2), 451 nm (6H15/2-4I15/2) and 470 nm (6H15/2-4F9/2). Fig. 11 presents the emission spectra of Dy3þ :CaGaSi phosphors and from this, three emission transitions are observed and are assigned to 4 F9/2-6H15/2 (482 nm), 4F9/2-6H13/2 (574 nm) and 4F9/2-6H11/2 (664 nm). The transition 4F9/2-6H13/2 at 574 nm with bright yellow emission is a forced electric dipole transition, which is allowed only when Dy3 þ is not at an inversion center and is familiar as the hypersensitive transition. In the present work, the emission at 574 nm (4F9/2-6H13/2) is the prominent one, indicating that the site occupied by Dy3 þ is not at an inversion center. In the case of emission spectra of Ca3GaSi:Dy3þ phosphors, the emission intensity also increases with the increase in Dy3 þ concentration from 1 to
We concluded that Sm3 þ and Dy3 þ ions activated CaGaSi phosphors have been synthesized by sol–gel method. From the XRD and SEM spectral studies, Sm3 þ and Dy3 þ :CaGaSi phosphors have shown cubic structure and the particles are agglomerated. Sm3 þ and Dy3 þ :CaGaSi phosphors have shown bright orange red and yellow emissions, respectively. The effect of the concentration on luminescence of Sm3 þ and Dy3 þ :CaGaSi phosphors has been investigated systematically. From the photoluminescence spectra of both Sm3 þ and Dy3 þ :CaGaSi phosphors, it has been observed that 5 mol% is the optimized concentration. Based on the spectral results, 5 mol% Sm3 þ and Dy3 þ :CaGaSi phosphors are brightly luminescent and are used as novel optical materials.
Acknowledgment This work was supported by the UGC-SERO, Hyderabad, in the form of Minor Research Project No. F MRP-3200/09(MRP/UGCSERO) sanctioned to the author (MBR), who would like to thank
2508
M. Bhushana Reddy et al. / Journal of Luminescence 131 (2011) 2503–2508
the Joint Secretary and Head, UGC-SERO, Hyderabad, and the authors also acknowledge the Sophisticated Analytical Instrument Facility (SAIF), IIT, Chennai, for extending instrumental facilities. References [1] Xinguo Zhang, Menglian Gong, J. Alloys Compd. 509 (2011) 2850. [2] Cheng Guang, Liu Quansheng, Cheng Liqun, Lu Liping, Sun Haiying, Wu Yiqing, Bai Zhaohui, Zhang Xiyan, Qiu Guanming, J. Rare Earth 28 (4) (2010) 526. [3] W.J. Park, Y.H. Song, D.H. Yoon, Mater. Sci. Eng. B 173 (2010) 76. [4] Yong-Kyu Kim, Dong-Hee Cho, Yong-Kwang Jeong, Min-Kook Nah, Kwang-Bok Kim, Jun-Gill Kang, Mater. Res. Bull. 45 (2010) 905. [5] Zhiguo Xia, Daimei Chen, Min Yang, Ting Ying, J. Phys. Chem. Solids 71 (2010) 175. [6] Xinmin Zhang, Hyo Jin Seo, J. Alloys Compd. 509 (2011) 2007. [7] Jian-Xin Meng, Chuang-Tao Yang, Qing-Qing Chen, J. Lumin. 130 (2010) 1320.
[8] Wang Yi, Zeng Qingguang, Shejun Hu, Kunbin Wu, Junhong Jiang, Genting Chen, J. Rare Earth 28 (2) (2010) 176. [9] M. Bhushana Reddy, S. Sailaja, K. Vemasevana Raju, C. Nageswara Raju, P. Giridhar, B. Vengala Rao, B. Sudhakar Reddy, J. Biol. Chem. Lumin. (2011). [10] B. Vengal Rao, N. Yung-Tang, L. Tzung-Heng, C. In-Gann, J. Am. Ceram. Soc. 92 (12) (2009) 2953. [11] Y. Liangzhu, F. Min, L. Yuejiao, L. Chao, W. Xiuying, Y. Xibin, Front. Chem. Chin. 2 (4) (2007) 442. [12] M. Yu, J. Lin, Y.H. Zhou, S.B. Wang, H.J. Zhang, J. Mater. Chem. 12 (2002) 86. [13] Li Changmin, Yang Dianlai, Zhang Yingying, Wang Zhiqiang, Lin Hai, J. Rare Earth 25 (2007) 143. [14] Xianghong He, Jian Zhou, Ning Lian, Jianhua Sun, Mingyun Guan, J. Lumin. 130 (2010) 743. [15] Bingful Lei, Yingliang Liu, Gongben Tang, Zeren Ye, Chunshan Shi, Mater. Chem. Phys. 87 (2004) 227. [16] Vijay Singh, S. Watanabe, T.K. Gundu Rao, J.F.D. Chubaci, Ho-Young Kwak, J. Non-Cryst. Solids 356 (2010) 1185. [17] Xue Yanna, Xiao Fen, Zhang Qinyuan, Jiang Zhonghong, J. Rare Earth 27 (5) (2009) 753.