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Synthesis and luminescent properties of Dy3+:GAG nanophosphors
Journal of Alloys and Compounds 481 (2009) 730–734
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and luminescent properties of Dy3+ :GAG nanophosphors G. Seeta Rama Raju a , Hong Chae Jung a , Jin Young Park a , C.M. Kanamadi a , Byung Kee Moon a,∗ , Jung Hyun Jeong a , Se-Mo Son b , Jung Hwan Kim c a
Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea Division of Image Science and Information Engineering, Pukyong National University, Busan 608-739, Republic of Korea c Department of Physics, Dong Eui University, Busan 614-714, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 4 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Dy3+ :GAG nanophosphor Solvothermal synthesis White light emission XRD Concentrations effect on PLE and PL
a b s t r a c t Dy3+ :GAG nanophosphors have been newly synthesized by solvothermal process. Spherically shaped particles in the nanometer range prepared by this method were evidenced by X-ray diffraction and SEM measurements. The synthesis, structural and luminescent properties along with chromaticity coordinates of the nanophosphor samples with the compositions xDy3+ :Gd (1−x) Al5 O12 (x = 0.5–5 mol%) are featured in this paper. Furthermore, the yellow to blue intensity ratio of Dy3+ :GAG nanophosphors varies from 0.63 to 0.55. Such luminescent powders are expected to find potential applications such as fluorescent lamps and optical display systems. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Currently, materials with reduced domain size are of interest in ceramics or phosphors for structural and optical applications, and new insights into material properties in the nanodimension are of fundamental interest [1,2]. Thus, size effects of materials with characteristic functional behavior such as magnetic [3], electric [4] and optical properties [5] are addressed. It is well known that phosphors are essential materials for the development of high resolution optical display systems, imaging devices and lamps. Rare earth (RE) ions activated phosphors, have found to be excellent luminescent materials because of their marked improvements in lumen output, color rendering index, energy efficiency and greater radiation stability [6–9]. In recent years, interest in the rare earth doped nanocrystalline host has increased due to their unique optical behavior [10]. Luminescent nanophosphors are also attractive in the field of nanobiotechnology as they have dimensions (diameters) in good match with those of biological structures, such as DNA, proteins, and antibodies. They can be used for analytical, diagnostic, and therapeutic purposes [10–12]. The RE ions are characterized by a partially filled 4f shell that is shielded by 5s2 and 5p6 electrons. The combination of Gd2 O3 –Al2 O3 systems, such as Gd3 Al5 O12 (GAG), GdAlO3 (GAP) and Gd4 Al2 O9 (GAM) are known as chemically stable and suitable as hosts for replacement of RE3+ (Ce3+ ,
∗ Corresponding author. Tel.: +82 51 629 5569; fax: +82 51 629 5549. E-mail address: [email protected] (B.K. Moon). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.095
Nd3+ , Er3+ , Eu3+ , Tb3+ , Sm3+ and Dy3+ ) activators [13–16]. Among these, Gadolinium aluminum garnet (GAG) is one of the important phosphor host materials with cubic structure. In previous studies, GAG powder was prepared by quenching the glass-crystallized product from a PbO or PbF2 flux at a high temperature; however, the purity of the product was not described [15]. Matyusheuko et al. [17] indicated that it is impossible to prepare GAG powder by the solid-state reaction method. However, in recent years, several wet chemical techniques such as co-precipitation method, sol–gel, combustion, hydrothermal synthesis, solvothermal synthesis, glycothermal method, and spray-pyrolysis synthesis were used to prepare the phosphor precursor at lower temperatures [18–24]. Although, phosphor materials synthesized by chemical methods have many advantages, i.e. high purity, homogenous composition and fine grains, they require additional heat treatment at higher temperatures to get well-crystallized products with efficient luminescent properties. Furthermore Dy3+ ions can show strong luminescence in a variety of lattices and exhibits both blue (4 F9/2 → 6 H15/2 ) and yellow (4 F9/2 → 6 H13/2 ) emissions, which are necessary for the development of white light emission and are very useful in high resolution optical display systems [25,26]. In this work, we have synthesized the Dy3+ :GAG nanophosphors by means of solvothermal process. This process is one of the most prominent processes to control the particle size in nanometers, morphology and distribution of phosphor particles with efficient luminescent properties. Upon going through the literature, it has become quite clear that Dy3+ activated GAG nanophosphors by
G. Seeta Rama Raju et al. / Journal of Alloys and Compounds 481 (2009) 730–734
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means of solvothermal method have not been reported so far. In this paper, the first time, we report on the luminescent properties of different concentrations of Dy3+ :GAG nanophosphors by means of solvothermal process. 2. Experimental 2.1. Synthesis Different concentrations (0.5, 1, 2, 3 and 5 mol%) of Dy3+ doped Gd3 Al5 O12 (GAG) nanophosphor samples were prepared by means of solvothermal process. The stoichiometric amounts of high purity grade gadolinium nitrate hexahydrate (Gd(NO3 )3 ·6H2 O), dysprosium nitrate pentahydrate (Dy(NO3 )3 ·5H2 O), and aluminum isopropoxide {[(CH3 )2 CHO]3 Al} were dissolved in 40 ml of 2-propanol. All reagents were used without any further purification and stirred vigorously by using magnetic stirrer until the homogeneous solution was formed and transferred into stainless steel autoclave with a Teflon liner (80 ml capacity and 50% filling). It was then heated to 230 ◦ C at a rate of 2 ◦ C/min and maintained for 5 h with magnetic stirring (at 180 rpm) to make stable networks of Gd–O–Al and Dy–O–Al. After cooling gradually down to room temperature, the precipitate was separated by a centrifugal separator with 3000 rpm for 3 min and then dried at 60 ◦ C for a day in ambient atmosphere. The dried powder was sintered at 800, 900, 1100, 1350 and 1550 ◦ C for 5 h. 2.2. Characterization Thermogravimetric/differential thermal analysis (TG/DTA) of the Dy3+ :GAG nanophosphors were carried out with a Material Analysis and Characterization TGDTA 2000. This experiment was carried out at a heating rate of 5 ◦ C/min and the samples were heated from room temperature to 1000 ◦ C. X-ray diffraction patterns of Dy3+ :GAG nanophosphors were recorded on X’ PERT PRO X-ray diffractometer with CuK␣ = 1.5406 Å. The morphology and size of the Dy3+ :GAG sintered particles were examined by means of scanning electron microscopy (SEM) model HITACHI S-4200 FESEM. Osmium coating was sprayed on the sample surfaces by using Hitachi fine coat ion sputter E-1010 unit to avoid possible charging of specimens before SEM observation was made on each time. The room temperature photoluminescent spectra of Dy3+ :GAG nanophosphors were recorded on a PTI (Photon Technology International) fluorimeter with a Xe-arc lamp of power 60 W and the lifetimes were measured with a phosphorimeter attachment to the main system with a Xe-flash lamp (25 W power).
3. Results and discussion Fig. 1 illustrates the TG/DTA curves of the powder precursor of Dy3+ :GAG obtained by the solvothermal method (range 25–1000 ◦ C). The TG curve shows two distinct weight loss steps up to 465 ◦ C; no further weight loss was registered up to 1000 ◦ C. The weight loss is related to the decomposition of organic matrix. On the DTA curve, three main exothermic effects were observed with maxima at 275 , 378 and 920 ◦ C. The first two exothermic peaks indicating that the thermal events can be associated with
Fig. 1. TG/DTA curves of Dy3+ :GAG precipitate powder.
Fig. 2. XRD patterns of Dy3+ :GAG nanophosphors at different sintering temperatures (‘’ indicates orthorhombic phase).
the burnout of organic species involved in the precursor powders of the residual nitrogen and the third exothermic peak is due to crystallization of GAG powder from the amorphous component. These results were well agreement with the XRD analysis. Fig. 2 compares the XRD patterns of GAG nanophosphors at different sintering temperatures (800, 900, 1100, 1350, and 1550 ◦ C) for 5 h. The phosphor was found to be amorphous until the precipitate powder was sintered at 800 ◦ C. From 900 ◦ C, different phase development was observed such as orthorhombic GAP (O-GAP) and cubic GAG. At 900 ◦ C the main phase is O-GAP with the presence of GAG peaks. By increasing the temperature the intensity of the OGAP peaks were decreased and GAG peaks were increased. Finally at 1350 ◦ C all the orthorhombic peaks were disappeared and pure GAG phase was observed. At higher temperatures also the GAG phase was stable. It is well known that the pure phase is favorable for luminescent properties of phosphors. The diffraction peaks are in well agreement with the standard JCPDS card [PDF (73-1371)] with space group Ia3d. In general, the crystallite size can be estimated by using the Scherrer’s equation, Dh k l = k/ˇcos , where D is the average grain size, k = 0.9 is shape factor, is the X-ray wave´˚ ˇ is the full width at half maximum (FWHM) length (1.5406 A), and is the diffraction angle of an observed peak, respectively. The strongest diffraction peaks are used to calculate the crystallite size of Dy3+ :GAG nanophosphor, sintered at 1350 ◦ C, which yields an average value of about 87 nm. Fig. 3 shows the SEM image of Dy3+ :GAG nanophosphor and from which spherical shaped particles are noticed, those might be due to the occurrence of agglomerations amongst the GAG particles during the period of sample sintering at 1350 ◦ C for 5 h. It is well known that spherical-shaped particles (≤2 m) are of greater importance because of their high packing density, lower scattering of light and brighter luminescence performance. Fig. 4 compares the PLE spectra (within the range of 200–500 nm) of Dy3+ :GAG nanophosphors sintered at 1350 ◦ C, which were measured with the emission wavelength fixed at 482 nm corresponding to the electronic transition (4 F9/2 → 6 H15/2 ). The spectra consists of broad excitation band at 250–295 nm with a maximum at 273 nm, which might be overlapping of charge transfer states (CTS) due to Dy3+ and O2− interactions and the host absorption band (HAB). Some sharp lines in the longer wavelength region due to the f–f transitions of Dy3+ are observed, which are assigned to the electronic transitions (6 H15/2 → 6 P3/2 )
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Fig. 5. Concentrations effect on the emission spectra of Dy3+ :GAG nanophosphors. Fig. 3. SEM image of Dy3+ :GAG nanophosphor.
at 326 nm, (6 H15/2 → 6 P7/2 ) at 351 nm, (6 H15/2 → 6 P5/2 ) at 366 nm, (6 H15/2 → 4 K17/2 ) at 379 nm, (6 H15/2 → 4 I13/2 ) at 387 nm, (6 H15/2 → 4 G11/2 ) at 427 nm and (6 H15/2 → 4 I15/2 ) at 451 nm. From the excitation spectra, it is clear that by increasing the Dy3+ concentration the HAB decreases due to the change of Dy3+ polarization of the surrounding O2− ions in the host lattice [27,28]. Fig. 5 shows the concentrations effect on the emission of Dy3+ :GAG nanophosphors sintered at 1350 ◦ C, which were excited at 351 nm. The PL spectra shows the two main groups of lines in the blue region (460–500 nm) and yellow region (555–610 nm) and also some weak lines observed in red region. These blue, yellow, and red emissions are assigned to the electronic transitions (4 F9/2 → 6 H15/2 ), (4 F9/2 → 6 H13/2 ) and (4 F9/2 → 6 H11/2 ), respectively. The blue (4 F9/2 → 6 H15/2 ) emission corresponding to the magnetic dipole transition and the yellow (4 F9/2 → 6 H13/2 ) emission belongs to the hypersensitive (forced electric dipole) transition with the selection rule, J = 2. The crystal field splitting components of Dy3+ can be observed and is well correlated with the Kramer’s doublets (2J + 1)/2, where J is the total angular momentum of the electrons [29,30]. It indicates that Dy3+ ions are well substituted into Gd3+ sites. In this Dy3+ :GAG nanophosphor, the integrated intensity of blue emission is greater than that of the yellow emission, this can be explained according to the following reason: It is well known
Fig. 4. Concentrations effect on the excitation spectra of Dy3+ :GAG nanophosphors.
that the yellow (4 F9/2 → 6 H13/2 ) emission of Dy3+ belongs to the hypersensitive (forced electric dipole) transition with the selection rule, J = 2, which is strongly influenced by the outside surrounding environment. The blue (4 F9/2 → 6 H15/2 ) emission corresponding to the magnetic dipole transition hardly varies with the crystal field symmetry around the Dy3+ ion. When Dy3+ is located at a low symmetry local site (without inversion symmetry), the yellow emission is often dominant in the emission spectrum and when Dy3+ is at a high symmetry local site (with inversion symmetry center), the blue emission is stronger than the yellow emission and is dominant in the emission spectrum [27]. The latter case occurs for Dy3+ doped GAG nanophosphors, this can also be explained according to the crystal structure of Gd3 Al5 O12 . The cubic unit cell of a general garnet compound C3 A2 D3 O12 contains eight formula units, where C, A, D are metal ions occupying different symmetry sites. It has a bcc structure (space group Ia3d) with 160 atoms in the cubic conventional cell. The Gd ions (C atom) occupy the 24(c) sites and each is dodecahedroally coordinated to eight O with (D2 point symmetry, with inversion center). The O atoms occupy the 96(h) sites whose exact locations depend on three structural parameters x, y, and z and are different for different garnet oxides. There are two different sites for Al, Aloct (A atom) occupy the 16(a) site with octahedral point symmetry (C3i ) and Altet (D atoms) occupy the 24(d) sites with tetrahedral point symmetry (S4 ) [31]. These structural units may show the different behavior to internal stresses, the dodecahedral unit being the less rigid one [32]. Considering the ionic radii, r(Dy3+ ) = 0.912 Å (for octahedral site) and 1.027 Å (for dodecahedron site), r(Gd3+ ) = 1.053 Å for dodecahedral site and r(Al3+ ) = 0.39 Å (for tetrahedral site) and 0.535 Å (for octahedral site). It is easier for dodecahedral site of Dy3+ ions to replace the dodecahedral site of Gd3+ ions than the other octahedral and tetrahedral sites of Al3+ ions in the GAG host lattice. The spectral property of the Dy3+ confirms that the Dy3+ ions occupy the dodecahedral Gd3+ sites in the Gd3 Al5 O12 host lattice. Note that, when the concentration of Dy3+ increases from 0.5 to 1 mol%, the emission intensity increases, and exceeds the concentration more than 1 mol%, the emission intensity decreases due to concentration quenching. The concentration quenching might be elucidated by two factors, (i) the excitation migration due to resonance between the activators is enhanced when the doping concentration is increased, and thus the excitation energy reaches quenching centers, and (ii) the activators are paired or coagulated and are changed to quenching center. Furthermore, the yellow to blue (Y/B) ratio also depends upon the concentration, and when the concentration increases from 0.5 to 1 mol%, the Y/B ratio also increases from 0.61 to 0.63 and when the concentration increases above 1 mol% the Y/B ratio decreases from 0.63 at 1 mol% to 0.55 at 5 mol%. This can be understood if one considers
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Fig. 7. Effect of concentration quenching on Y/B ratio and lifetimes.
Fig. 6. Decay curves of the different concentrations of Dy3+ :GAG nanophosphors.
Table 1 Chromaticity coordinates and corresponding color temperatures with excitation wavelength.
that the J = 2 transition probability changes with a polarity of the neighboring ions [27]. The decay of the luminescence of the 4 F9/2 level of Dy3+ :GAG nanophosphors has been recorded under excitation at 351 nm (6 H15/2 → 5 P7/2 ) and emission at 482 nm (4 F9/2 → 6 H15/2 ) are shown in Fig. 6. At lower concentrations the decay curves were well fitted by a single exponential function and at 5 mol%, however, the observed decay curve was non-exponential. With the increase of Dy3+ concentration, the distance between the Dy3+ ions decreases; subsequently, the energy transfer between Dy3+ ions is more frequent. Therefore the energy transfer process between the Dy3+ ions provides an extra decay channel to change the decay curves, resulting in a non-exponential decay curve. For comparison purpose the average lifetime of this bi-exponential curve is calculated by the following equation [33]:
Sample
Excitation (nm)
Chromaticity coordinates
Color temperature (K)
GAG:0.5Dy3+ GAG:1Dy3+ GAG:2Dy3+ GAG:3Dy3+ GAG:5Dy3+ CIE white light point ProPhoto/ColorMatch PAL/SECAM/HDTV NTSC
351 351 351 351 351
(0.310, 0.340) (0.325, 0.350) (0.300, 0.320) (0.290, 0.300) (0.270, 0.280) (0.33, 0.33) (0.3457, 0.3585) (0.3127, 0.329) (0.3101, 0.3162)
7100 6950 7800 8000 14500
∞ I(t)tdt exp t = avg =
0 ∞
I(t)dt 0
where I(t) represents the luminescence intensity at a time t. The lifetimes of 0.5, 1, 2, 3 and 5 mol% Dy3+ doped GAG nanophosphors are 649, 738, 552, 484 and 378 s, respectively. The concentration effects on the Y/B ratio and lifetime process are shown in Fig. 7 The Commission International de l’Eclairage (CIE) chromaticity coordinates for Dy3+ :GAG nanophosphors were calculated and listed in Table 1 along with their color temperatures. The CIE chromaticity coordinates are also represented in Fig. 8. We have observed that the Dy3+ :GAG exhibits excellent CIE coordinates of (0.310, 0.340), (0.325, 0.350), (0.300, 0.320), (0.290, 0.300) and (0.270, 0.280) for 0.5, 1, 2, 3 and 5 mol%, respectively, which are quite close to that of the CIE white light point (0.33, 0.33). Some other color systems chromaticity coordinates are presented in table for comparison purpose. From the table it is clear that at very low concentrations of the Dy3+ :GAG nanophosphors gives cool white light and by increasing the concentration it shifts to very cool white light (bluish white). This makes it possible to make a UV-white LED capable of emitting cool white desired for outdoor illumina-
Fig. 8. CIE diagram represented with our obtained chromaticity coordinates.
tion applications. Also, at higher concentrations these phosphors are useful for optical display systems, because the chromaticity coordinates of Dy3+ :GAG nanophosphors approaches the National Television System Committee (NTSC). The above observations hint at the promising application of Dy3+ :GAG nanophosphor to produce white-light for UV-LEDs as well as optical display systems. 4. Conclusions In summary, it could be concluded that we have successfully synthesized the different concentrations of Dy3+ :GAG nanophosphors by means of solvothermal process and sintering at 1350 ◦ C. The structural and the luminescent properties of Dy3+ :GAG
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nanophosphors have been studied by the measurement of their XRD, SEM, PLE and PL spectra along with their lifetimes. The emission spectra show two strong bands in blue (4 F9/2 → 6 H15/2 ) and yellow (4 F9/2 → 6 H13/2 ) regions. Based on the emission spectral intensities, we could calculate the chromaticity coordinates of Dy3+ :GAG nanophosphors to demonstrate the color stability of these phosphors and which approaches the CIE ideal white light condition. The Dy3+ :GAG nanophosphors clearly exhibit excellent chromaticity coordinates with high color stability when Dy3+ is doped with GAG. Acknowledgements This work was supported by Korea Research Foundation Grant funded by the Korean Government (KRF-2007-412-J00902 and KRF-2008-314-C00098), and also partially supported by a Grantin-Aid for the National Core Research Center Program from MEST and KOSEF (No. R15-2006-022-03001). References [1] T. Fukui, K. Toda, S. Kirihara, in: Masuo Hosokawa, Kiyoshi Nogi, Makio Naito, Toyokazu Yokoyama (Eds.), Nanoparticle Technology Handbook, Elsevier, 2007, pp. 319–374 (Chapter 6). [2] S. Wohlrab, M. Weiss, H. Du, S. Kaskel, Chem. Mater. 18 (2006) 4227. [3] M. Drofenik, D. Lisjak, D. Makovec, Mater. Sci. Forum 494 (2005) 129. [4] A. Ru¨diger, T. Schneller, A. Roelofs, S. Tiedke, T. Schmitz, R. Waser, Appl. Phys. A 80 (2005) 1247. [5] C. Noguez, Opt. Mater. 27 (2005) 1204. [6] J. W-, T.-M. Yang, Chen, Appl. Phys. Lett. 88 (2006) 101903.
[7] R.A. Rodriguez, E.D. Rosa, P. Salas, R. Melendrez, M. Barboza-Flores, J. Phys. D: Appl. Phys. 38 (2005) 3854. [8] G. Seeta Rama Raju, S. Buddhudu, Mater. Lett. 62 (2008) 1259. [9] Y. Pan, M. Wu, Q. Su, J. Phys. Chem. Solids 65 (2004) 845. [10] S.K. Singh, K. Kumar, S.B. Rai, Appl. Phys. B 94 (2009) 165. [11] P.L.A.M. Corstjens, M. Zuiderwijk, M. Nilsson, H. Feindt, R.S. Niedbala, H.J. Tanke, Anal. Biochem. 312 (2003) 191. [12] O.V. Salata, J. Nanobiotecnol. 2 (2004) 3. [13] C.-C. Chiang, M.-S. Tsai, M.-H. Hon, J. Electrochem. Soc. 155 (6) (2008) B517. [14] S. Do Han, S.P. Khatkar, V.B. Taxak, D. Kumar, J.Y. Park, Mater. Sci. Eng. B 127 (2006) 272. [15] C.-C. Chiang, M.-S. Tsai, M.-H. Hon, J. Electrochem. Soc. 154 (10) (2007) J326. [16] H. Kimura, T. Numazawa, M. Sato, J. Mater. Sci. Lett. 13 (1994) 1164. [17] N.N. Matyushenko, E.P. Shevyakova, E.V. Lifshits, N.V. Lapina, Russ. J. Inorg. Chem. 30 (1985) 942. [18] D. Jia, Y. Wang, X. Guo, K. Li, Y.K. Zou, W. Jia, J. Electrochem. Soc. 154 (1) (2007) J1. [19] F.L. Yuan, H.J. Ryu, Mater. Sci. Eng. B 107 (2004) 14. [20] J.Y. Choe, D. Ravichandran, S.M. Blomquist, K.W. Kirchner, E.W. Forsythe, D.C. Morton, J. Lumin. 93 (2001) 119. [21] S.K. Shi, J.Y. Wang, Chin. J. Inorg. Chem. 18 (2002) 431. [22] G. Zou, H. Li, Y. Zhang, K. Xiong, Y. Qian, Nanotechnology 17 (2006) S313. [23] X. Li, H. Liu, J.Y. Wang, H.M. Cui, F. Han, Mater. Res. Bull. 39 (2004) 1923. [24] Y.C. Kang, Y.S. Chung, S.B. Park, J. Am. Chem. Soc. 82 (1999) 2056. [25] J. Kuang, Y. Liu, J. Zhang, J. Solid State Chem. 179 (2006) 266. [26] B. Liu, L. Kong, C. Shi, J. Lumin. 123 (2007) 121–122. [27] T. Kano, in: W.M. Yen, S. Shionoya, H. Yamamoto (Eds.), Phosphor Handbook, Second edition, CRC Press, 2006, pp. 205–210, Chapter 3. [28] X. Xiao, B. Yan, J. Non-Cryst. Solids 352 (2006) 3047. [29] J. Mulak, M. Mulak, J. Phys. A: Math. Theor. 40 (2007) 2063. [30] J.B. Gruber, B. Zandi, U.V. Valiev, Sh.A. Rakhimov, J. Appl. Phys. 94 (2003) 1030. [31] H. Xu, H. Yang, Phys. Stat. Sol. (A). 204 (2007) 1203. [32] E. Antic-Fidancev, J. Holsa, M. Lastusaari, A. Lupeil, Phys. Rev. B. 64 (2001) 195108. [33] F. Lahoz, I.R. Martin, J. Mendz- Ramos, P. Nunez, J. Chem. Phys. 120 (2004) 6180.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and luminescent properties of Dy3+ :GAG nanophosphors G. Seeta Rama Raju a , Hong Chae Jung a , Jin Young Park a , C.M. Kanamadi a , Byung Kee Moon a,∗ , Jung Hyun Jeong a , Se-Mo Son b , Jung Hwan Kim c a
Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea Division of Image Science and Information Engineering, Pukyong National University, Busan 608-739, Republic of Korea c Department of Physics, Dong Eui University, Busan 614-714, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 4 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Dy3+ :GAG nanophosphor Solvothermal synthesis White light emission XRD Concentrations effect on PLE and PL
a b s t r a c t Dy3+ :GAG nanophosphors have been newly synthesized by solvothermal process. Spherically shaped particles in the nanometer range prepared by this method were evidenced by X-ray diffraction and SEM measurements. The synthesis, structural and luminescent properties along with chromaticity coordinates of the nanophosphor samples with the compositions xDy3+ :Gd (1−x) Al5 O12 (x = 0.5–5 mol%) are featured in this paper. Furthermore, the yellow to blue intensity ratio of Dy3+ :GAG nanophosphors varies from 0.63 to 0.55. Such luminescent powders are expected to find potential applications such as fluorescent lamps and optical display systems. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Currently, materials with reduced domain size are of interest in ceramics or phosphors for structural and optical applications, and new insights into material properties in the nanodimension are of fundamental interest [1,2]. Thus, size effects of materials with characteristic functional behavior such as magnetic [3], electric [4] and optical properties [5] are addressed. It is well known that phosphors are essential materials for the development of high resolution optical display systems, imaging devices and lamps. Rare earth (RE) ions activated phosphors, have found to be excellent luminescent materials because of their marked improvements in lumen output, color rendering index, energy efficiency and greater radiation stability [6–9]. In recent years, interest in the rare earth doped nanocrystalline host has increased due to their unique optical behavior [10]. Luminescent nanophosphors are also attractive in the field of nanobiotechnology as they have dimensions (diameters) in good match with those of biological structures, such as DNA, proteins, and antibodies. They can be used for analytical, diagnostic, and therapeutic purposes [10–12]. The RE ions are characterized by a partially filled 4f shell that is shielded by 5s2 and 5p6 electrons. The combination of Gd2 O3 –Al2 O3 systems, such as Gd3 Al5 O12 (GAG), GdAlO3 (GAP) and Gd4 Al2 O9 (GAM) are known as chemically stable and suitable as hosts for replacement of RE3+ (Ce3+ ,
∗ Corresponding author. Tel.: +82 51 629 5569; fax: +82 51 629 5549. E-mail address: [email protected] (B.K. Moon). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.095
Nd3+ , Er3+ , Eu3+ , Tb3+ , Sm3+ and Dy3+ ) activators [13–16]. Among these, Gadolinium aluminum garnet (GAG) is one of the important phosphor host materials with cubic structure. In previous studies, GAG powder was prepared by quenching the glass-crystallized product from a PbO or PbF2 flux at a high temperature; however, the purity of the product was not described [15]. Matyusheuko et al. [17] indicated that it is impossible to prepare GAG powder by the solid-state reaction method. However, in recent years, several wet chemical techniques such as co-precipitation method, sol–gel, combustion, hydrothermal synthesis, solvothermal synthesis, glycothermal method, and spray-pyrolysis synthesis were used to prepare the phosphor precursor at lower temperatures [18–24]. Although, phosphor materials synthesized by chemical methods have many advantages, i.e. high purity, homogenous composition and fine grains, they require additional heat treatment at higher temperatures to get well-crystallized products with efficient luminescent properties. Furthermore Dy3+ ions can show strong luminescence in a variety of lattices and exhibits both blue (4 F9/2 → 6 H15/2 ) and yellow (4 F9/2 → 6 H13/2 ) emissions, which are necessary for the development of white light emission and are very useful in high resolution optical display systems [25,26]. In this work, we have synthesized the Dy3+ :GAG nanophosphors by means of solvothermal process. This process is one of the most prominent processes to control the particle size in nanometers, morphology and distribution of phosphor particles with efficient luminescent properties. Upon going through the literature, it has become quite clear that Dy3+ activated GAG nanophosphors by
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means of solvothermal method have not been reported so far. In this paper, the first time, we report on the luminescent properties of different concentrations of Dy3+ :GAG nanophosphors by means of solvothermal process. 2. Experimental 2.1. Synthesis Different concentrations (0.5, 1, 2, 3 and 5 mol%) of Dy3+ doped Gd3 Al5 O12 (GAG) nanophosphor samples were prepared by means of solvothermal process. The stoichiometric amounts of high purity grade gadolinium nitrate hexahydrate (Gd(NO3 )3 ·6H2 O), dysprosium nitrate pentahydrate (Dy(NO3 )3 ·5H2 O), and aluminum isopropoxide {[(CH3 )2 CHO]3 Al} were dissolved in 40 ml of 2-propanol. All reagents were used without any further purification and stirred vigorously by using magnetic stirrer until the homogeneous solution was formed and transferred into stainless steel autoclave with a Teflon liner (80 ml capacity and 50% filling). It was then heated to 230 ◦ C at a rate of 2 ◦ C/min and maintained for 5 h with magnetic stirring (at 180 rpm) to make stable networks of Gd–O–Al and Dy–O–Al. After cooling gradually down to room temperature, the precipitate was separated by a centrifugal separator with 3000 rpm for 3 min and then dried at 60 ◦ C for a day in ambient atmosphere. The dried powder was sintered at 800, 900, 1100, 1350 and 1550 ◦ C for 5 h. 2.2. Characterization Thermogravimetric/differential thermal analysis (TG/DTA) of the Dy3+ :GAG nanophosphors were carried out with a Material Analysis and Characterization TGDTA 2000. This experiment was carried out at a heating rate of 5 ◦ C/min and the samples were heated from room temperature to 1000 ◦ C. X-ray diffraction patterns of Dy3+ :GAG nanophosphors were recorded on X’ PERT PRO X-ray diffractometer with CuK␣ = 1.5406 Å. The morphology and size of the Dy3+ :GAG sintered particles were examined by means of scanning electron microscopy (SEM) model HITACHI S-4200 FESEM. Osmium coating was sprayed on the sample surfaces by using Hitachi fine coat ion sputter E-1010 unit to avoid possible charging of specimens before SEM observation was made on each time. The room temperature photoluminescent spectra of Dy3+ :GAG nanophosphors were recorded on a PTI (Photon Technology International) fluorimeter with a Xe-arc lamp of power 60 W and the lifetimes were measured with a phosphorimeter attachment to the main system with a Xe-flash lamp (25 W power).
3. Results and discussion Fig. 1 illustrates the TG/DTA curves of the powder precursor of Dy3+ :GAG obtained by the solvothermal method (range 25–1000 ◦ C). The TG curve shows two distinct weight loss steps up to 465 ◦ C; no further weight loss was registered up to 1000 ◦ C. The weight loss is related to the decomposition of organic matrix. On the DTA curve, three main exothermic effects were observed with maxima at 275 , 378 and 920 ◦ C. The first two exothermic peaks indicating that the thermal events can be associated with
Fig. 1. TG/DTA curves of Dy3+ :GAG precipitate powder.
Fig. 2. XRD patterns of Dy3+ :GAG nanophosphors at different sintering temperatures (‘’ indicates orthorhombic phase).
the burnout of organic species involved in the precursor powders of the residual nitrogen and the third exothermic peak is due to crystallization of GAG powder from the amorphous component. These results were well agreement with the XRD analysis. Fig. 2 compares the XRD patterns of GAG nanophosphors at different sintering temperatures (800, 900, 1100, 1350, and 1550 ◦ C) for 5 h. The phosphor was found to be amorphous until the precipitate powder was sintered at 800 ◦ C. From 900 ◦ C, different phase development was observed such as orthorhombic GAP (O-GAP) and cubic GAG. At 900 ◦ C the main phase is O-GAP with the presence of GAG peaks. By increasing the temperature the intensity of the OGAP peaks were decreased and GAG peaks were increased. Finally at 1350 ◦ C all the orthorhombic peaks were disappeared and pure GAG phase was observed. At higher temperatures also the GAG phase was stable. It is well known that the pure phase is favorable for luminescent properties of phosphors. The diffraction peaks are in well agreement with the standard JCPDS card [PDF (73-1371)] with space group Ia3d. In general, the crystallite size can be estimated by using the Scherrer’s equation, Dh k l = k/ˇcos , where D is the average grain size, k = 0.9 is shape factor, is the X-ray wave´˚ ˇ is the full width at half maximum (FWHM) length (1.5406 A), and is the diffraction angle of an observed peak, respectively. The strongest diffraction peaks are used to calculate the crystallite size of Dy3+ :GAG nanophosphor, sintered at 1350 ◦ C, which yields an average value of about 87 nm. Fig. 3 shows the SEM image of Dy3+ :GAG nanophosphor and from which spherical shaped particles are noticed, those might be due to the occurrence of agglomerations amongst the GAG particles during the period of sample sintering at 1350 ◦ C for 5 h. It is well known that spherical-shaped particles (≤2 m) are of greater importance because of their high packing density, lower scattering of light and brighter luminescence performance. Fig. 4 compares the PLE spectra (within the range of 200–500 nm) of Dy3+ :GAG nanophosphors sintered at 1350 ◦ C, which were measured with the emission wavelength fixed at 482 nm corresponding to the electronic transition (4 F9/2 → 6 H15/2 ). The spectra consists of broad excitation band at 250–295 nm with a maximum at 273 nm, which might be overlapping of charge transfer states (CTS) due to Dy3+ and O2− interactions and the host absorption band (HAB). Some sharp lines in the longer wavelength region due to the f–f transitions of Dy3+ are observed, which are assigned to the electronic transitions (6 H15/2 → 6 P3/2 )
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Fig. 5. Concentrations effect on the emission spectra of Dy3+ :GAG nanophosphors. Fig. 3. SEM image of Dy3+ :GAG nanophosphor.
at 326 nm, (6 H15/2 → 6 P7/2 ) at 351 nm, (6 H15/2 → 6 P5/2 ) at 366 nm, (6 H15/2 → 4 K17/2 ) at 379 nm, (6 H15/2 → 4 I13/2 ) at 387 nm, (6 H15/2 → 4 G11/2 ) at 427 nm and (6 H15/2 → 4 I15/2 ) at 451 nm. From the excitation spectra, it is clear that by increasing the Dy3+ concentration the HAB decreases due to the change of Dy3+ polarization of the surrounding O2− ions in the host lattice [27,28]. Fig. 5 shows the concentrations effect on the emission of Dy3+ :GAG nanophosphors sintered at 1350 ◦ C, which were excited at 351 nm. The PL spectra shows the two main groups of lines in the blue region (460–500 nm) and yellow region (555–610 nm) and also some weak lines observed in red region. These blue, yellow, and red emissions are assigned to the electronic transitions (4 F9/2 → 6 H15/2 ), (4 F9/2 → 6 H13/2 ) and (4 F9/2 → 6 H11/2 ), respectively. The blue (4 F9/2 → 6 H15/2 ) emission corresponding to the magnetic dipole transition and the yellow (4 F9/2 → 6 H13/2 ) emission belongs to the hypersensitive (forced electric dipole) transition with the selection rule, J = 2. The crystal field splitting components of Dy3+ can be observed and is well correlated with the Kramer’s doublets (2J + 1)/2, where J is the total angular momentum of the electrons [29,30]. It indicates that Dy3+ ions are well substituted into Gd3+ sites. In this Dy3+ :GAG nanophosphor, the integrated intensity of blue emission is greater than that of the yellow emission, this can be explained according to the following reason: It is well known
Fig. 4. Concentrations effect on the excitation spectra of Dy3+ :GAG nanophosphors.
that the yellow (4 F9/2 → 6 H13/2 ) emission of Dy3+ belongs to the hypersensitive (forced electric dipole) transition with the selection rule, J = 2, which is strongly influenced by the outside surrounding environment. The blue (4 F9/2 → 6 H15/2 ) emission corresponding to the magnetic dipole transition hardly varies with the crystal field symmetry around the Dy3+ ion. When Dy3+ is located at a low symmetry local site (without inversion symmetry), the yellow emission is often dominant in the emission spectrum and when Dy3+ is at a high symmetry local site (with inversion symmetry center), the blue emission is stronger than the yellow emission and is dominant in the emission spectrum [27]. The latter case occurs for Dy3+ doped GAG nanophosphors, this can also be explained according to the crystal structure of Gd3 Al5 O12 . The cubic unit cell of a general garnet compound C3 A2 D3 O12 contains eight formula units, where C, A, D are metal ions occupying different symmetry sites. It has a bcc structure (space group Ia3d) with 160 atoms in the cubic conventional cell. The Gd ions (C atom) occupy the 24(c) sites and each is dodecahedroally coordinated to eight O with (D2 point symmetry, with inversion center). The O atoms occupy the 96(h) sites whose exact locations depend on three structural parameters x, y, and z and are different for different garnet oxides. There are two different sites for Al, Aloct (A atom) occupy the 16(a) site with octahedral point symmetry (C3i ) and Altet (D atoms) occupy the 24(d) sites with tetrahedral point symmetry (S4 ) [31]. These structural units may show the different behavior to internal stresses, the dodecahedral unit being the less rigid one [32]. Considering the ionic radii, r(Dy3+ ) = 0.912 Å (for octahedral site) and 1.027 Å (for dodecahedron site), r(Gd3+ ) = 1.053 Å for dodecahedral site and r(Al3+ ) = 0.39 Å (for tetrahedral site) and 0.535 Å (for octahedral site). It is easier for dodecahedral site of Dy3+ ions to replace the dodecahedral site of Gd3+ ions than the other octahedral and tetrahedral sites of Al3+ ions in the GAG host lattice. The spectral property of the Dy3+ confirms that the Dy3+ ions occupy the dodecahedral Gd3+ sites in the Gd3 Al5 O12 host lattice. Note that, when the concentration of Dy3+ increases from 0.5 to 1 mol%, the emission intensity increases, and exceeds the concentration more than 1 mol%, the emission intensity decreases due to concentration quenching. The concentration quenching might be elucidated by two factors, (i) the excitation migration due to resonance between the activators is enhanced when the doping concentration is increased, and thus the excitation energy reaches quenching centers, and (ii) the activators are paired or coagulated and are changed to quenching center. Furthermore, the yellow to blue (Y/B) ratio also depends upon the concentration, and when the concentration increases from 0.5 to 1 mol%, the Y/B ratio also increases from 0.61 to 0.63 and when the concentration increases above 1 mol% the Y/B ratio decreases from 0.63 at 1 mol% to 0.55 at 5 mol%. This can be understood if one considers
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Fig. 7. Effect of concentration quenching on Y/B ratio and lifetimes.
Fig. 6. Decay curves of the different concentrations of Dy3+ :GAG nanophosphors.
Table 1 Chromaticity coordinates and corresponding color temperatures with excitation wavelength.
that the J = 2 transition probability changes with a polarity of the neighboring ions [27]. The decay of the luminescence of the 4 F9/2 level of Dy3+ :GAG nanophosphors has been recorded under excitation at 351 nm (6 H15/2 → 5 P7/2 ) and emission at 482 nm (4 F9/2 → 6 H15/2 ) are shown in Fig. 6. At lower concentrations the decay curves were well fitted by a single exponential function and at 5 mol%, however, the observed decay curve was non-exponential. With the increase of Dy3+ concentration, the distance between the Dy3+ ions decreases; subsequently, the energy transfer between Dy3+ ions is more frequent. Therefore the energy transfer process between the Dy3+ ions provides an extra decay channel to change the decay curves, resulting in a non-exponential decay curve. For comparison purpose the average lifetime of this bi-exponential curve is calculated by the following equation [33]:
Sample
Excitation (nm)
Chromaticity coordinates
Color temperature (K)
GAG:0.5Dy3+ GAG:1Dy3+ GAG:2Dy3+ GAG:3Dy3+ GAG:5Dy3+ CIE white light point ProPhoto/ColorMatch PAL/SECAM/HDTV NTSC
351 351 351 351 351
(0.310, 0.340) (0.325, 0.350) (0.300, 0.320) (0.290, 0.300) (0.270, 0.280) (0.33, 0.33) (0.3457, 0.3585) (0.3127, 0.329) (0.3101, 0.3162)
7100 6950 7800 8000 14500
∞ I(t)tdt exp t = avg =
0 ∞
I(t)dt 0
where I(t) represents the luminescence intensity at a time t. The lifetimes of 0.5, 1, 2, 3 and 5 mol% Dy3+ doped GAG nanophosphors are 649, 738, 552, 484 and 378 s, respectively. The concentration effects on the Y/B ratio and lifetime process are shown in Fig. 7 The Commission International de l’Eclairage (CIE) chromaticity coordinates for Dy3+ :GAG nanophosphors were calculated and listed in Table 1 along with their color temperatures. The CIE chromaticity coordinates are also represented in Fig. 8. We have observed that the Dy3+ :GAG exhibits excellent CIE coordinates of (0.310, 0.340), (0.325, 0.350), (0.300, 0.320), (0.290, 0.300) and (0.270, 0.280) for 0.5, 1, 2, 3 and 5 mol%, respectively, which are quite close to that of the CIE white light point (0.33, 0.33). Some other color systems chromaticity coordinates are presented in table for comparison purpose. From the table it is clear that at very low concentrations of the Dy3+ :GAG nanophosphors gives cool white light and by increasing the concentration it shifts to very cool white light (bluish white). This makes it possible to make a UV-white LED capable of emitting cool white desired for outdoor illumina-
Fig. 8. CIE diagram represented with our obtained chromaticity coordinates.
tion applications. Also, at higher concentrations these phosphors are useful for optical display systems, because the chromaticity coordinates of Dy3+ :GAG nanophosphors approaches the National Television System Committee (NTSC). The above observations hint at the promising application of Dy3+ :GAG nanophosphor to produce white-light for UV-LEDs as well as optical display systems. 4. Conclusions In summary, it could be concluded that we have successfully synthesized the different concentrations of Dy3+ :GAG nanophosphors by means of solvothermal process and sintering at 1350 ◦ C. The structural and the luminescent properties of Dy3+ :GAG
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nanophosphors have been studied by the measurement of their XRD, SEM, PLE and PL spectra along with their lifetimes. The emission spectra show two strong bands in blue (4 F9/2 → 6 H15/2 ) and yellow (4 F9/2 → 6 H13/2 ) regions. Based on the emission spectral intensities, we could calculate the chromaticity coordinates of Dy3+ :GAG nanophosphors to demonstrate the color stability of these phosphors and which approaches the CIE ideal white light condition. The Dy3+ :GAG nanophosphors clearly exhibit excellent chromaticity coordinates with high color stability when Dy3+ is doped with GAG. Acknowledgements This work was supported by Korea Research Foundation Grant funded by the Korean Government (KRF-2007-412-J00902 and KRF-2008-314-C00098), and also partially supported by a Grantin-Aid for the National Core Research Center Program from MEST and KOSEF (No. R15-2006-022-03001). References [1] T. Fukui, K. Toda, S. Kirihara, in: Masuo Hosokawa, Kiyoshi Nogi, Makio Naito, Toyokazu Yokoyama (Eds.), Nanoparticle Technology Handbook, Elsevier, 2007, pp. 319–374 (Chapter 6). [2] S. Wohlrab, M. Weiss, H. Du, S. Kaskel, Chem. Mater. 18 (2006) 4227. [3] M. Drofenik, D. Lisjak, D. Makovec, Mater. Sci. Forum 494 (2005) 129. [4] A. Ru¨diger, T. Schneller, A. Roelofs, S. Tiedke, T. Schmitz, R. Waser, Appl. Phys. A 80 (2005) 1247. [5] C. Noguez, Opt. Mater. 27 (2005) 1204. [6] J. W-, T.-M. Yang, Chen, Appl. Phys. Lett. 88 (2006) 101903.
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