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Synthesis and optical studies of novel Eu2+ and Ce3+ doped BaMg8Al18Si18O72 phosphors
Solid State Sciences 14 (2012) 607e610
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis and optical studies of novel Eu2þ and Ce3þ doped BaMg8Al18Si18O72 phosphors V.B. Pawade a, *, N.S. Dhoble b, S.J. Dhoble c a
Department of Applied Physics, VNIET, Nagpur 440023, India Department of Chemistry, Sevadal Mahila Mahavidyalaya, Nagpur 440009, India c Department of Physics, RTM Nagpur University, Nagpur 440033, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2011 Received in revised form 8 January 2012 Accepted 9 February 2012 Available online 21 February 2012
Novel Eu2þ and Ce3þ activated BaMg8Al18Si18O72 phosphors was prepared by combustion method and their PL characteristics were investigated. The result shows that all samples can be excited efficiently by near UV excitation under 334 nm and 316 nm. The emission was observed for BaMg8Al18Si18O72:Eu2þ phosphor at 437 nm corresponding to d / f transition, under 334 nm broad-band excitation, whereas BaMg8Al18Si18O72:Ce3þ phosphor shows emission band at 376 nm under 316 nm excitation. Phase purity of the phosphor was checked with the help of XRD pattern. SEM analysis shows the external morphology of the combustion synthesized phosphor. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Combustion synthesis Phosphors Optical properties W-leds Scintillators
1. Introduction Alkaline earth silicates have attracted much attention and become an interesting topic in the field of luminescent materials because of their high chemical stability and water-resistant property by comparing with sulfide phosphorescent phosphors and strontium aluminates phosphors [1e4]. However, the research of silicate-based long afterglow materials has been developed several years ago and the study mainly concentrates on the alkaline earth silicate host [5e7]. Single-host tricolor phosphor and NUV LED combination is a promising host to obtain white LEDs. Eu2þ is the most well-known and extensively used activator in many silicates, borates and other inorganic compounds. Its emission comes from 5d / 4f transition, and the emission wavelength can vary in very broad ranges. Among the oxide materials, if a silicate is used, the firing temperature cannot be very high. Eu2þ (4f7) ions which shows a 5de4f emission which can vary from long wavelength ultraviolet to visible in some oxides hosts. The first white LED had been commercialized by Nichia Co., in which a blue LED was coated with yellow phosphor (YAG:Ce3þ). However, the white LEDs based on a blue LED and yellow phosphor combination has several
* Corresponding author. Tel.: þ91 9823232068. E-mail address: [email protected] (V.B. Pawade). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.02.004
disadvantages including the halo effect of blue/yellow color separation and poor color rendering property caused by a lack of red component in the spectrum. Recently, Ce3þ activated scintillation materials are widely used in high-energy physics, nuclear medical imaging and security inspection. High density, high scintillation light yield and fast decay time are the most desired properties [8]. However, even for commercial scintillation crystals, such as NaI:Tlþ, Bi4Ge3O12, LaBr3:Ce3þ, Lu2SiO5:Ce3þ, they also have their own shortcomings. For example, NaI:Tlþ has slow decay time and highly hygroscopic characteristic; Bi4Ge3O12 exhibits a relatively long decay time; LaBr3:Ce3þ is more hygroscopic than NaI:Tlþ and Lu2SiO5:Ce3þ has a long afterglow [9e11]. Therefore, besides optimizing the performance of these crystals, it’s necessary to research and develop novel scintillators with excellent properties. Based on excellent luminescence properties, Ce3þ doped inorganic materials are applied in lightings, displays and detectors for ionizing radiation [12,13]. Ce3þ can also be used as a reference ion to predict the 5d energies of other rare earth ions in same host lattices [14]. Recently, we have reported Eu2þ and Ce3þ luminescence in ternary hexa aluminates based blue phosphors for solid-state lighting purpose, also Eu2þ luminescence in halo silicates based phosphors was studied for the near UV-white leds [15e18]. The reported work has examined the emission properties of Eu2þ and Ce3þ activated in novel BaMg8Al18Si18O72 phosphors first time successfully and such broad emission in divalent and trivalent impurities ions are very
608
V.B. Pawade et al. / Solid State Sciences 14 (2012) 607e610
important to study energy transfer mechanism. Therefore, the investigation on the spectroscopic properties of Ce3þ and Eu2þ activated in novel host lattices is important for the basic research to develop white LEDs, it will be the new illuminating source to replace traditional incandescent lamp and fluorescent lamps for the service lifetimes, energy and environmental considerations. 2. Experimental The reported BaMg8Al18Si18O72 phosphors were prepared by combustion methods at 550 C. The starting materials were used as follows, Ba(NO3)2, Mg(NO3)2$6H2O, Al(NO3)3$9H2O Merck (99.99% purity), SiO2 (99.99% purity), Eu2O3, (NH4)2Ce(NO3)6 Merck (99.99% purity), urea (NH2eCOeNH2, Merck). Ingredients of the above compound were mixed properly in agate mortar and a pasty, a gel solution was formed, it was then transfer to silica crucible, and kept inside a muffle furnace at 550 C. The flame with the foamy powder formed, that powder was collected and analyzed by XRD and photoluminescence measurement. Photoluminescence (PL) emission and photoluminescence excitation (PLE) were measured using a Shimadzu RF5301PC spectrofluorophotometer at room temperature, same amount of sample was used to measure emission and excitation keeping slitwidth at 1.5 nm, PL sensitivity at high for both measurements. 3. Results and discussion 3.1. X-ray diffraction pattern of BaMg8Al18Si18O72 phosphor The phase purities of the phosphor was checked by powder X-ray diffraction (XRD) using a PAN-analytical diffractometer with CuKa radiation (1.5405 Å) operating voltage at 40 kV, 30 mA and scan step time at 10.3377 s. Fig. 1 shows the XRD patterns of BaMg8Al18Si18O72 phosphor that have standard reference file No. 00-039-0273. It has hexagonal structure, having lattice parameter a ¼ b ¼ 9.807, c ¼ 9.400 with space group P6/mcc [19]. Therefore crystal structure of the Ba-substitute indialite is shown in Fig. 2, the actual structure for this compound does not exists we just shows the coordination of Ba, Mg ions in BaMg8Al18Si18O72 host lattices. Average crystallite size calculated by Debye Scherrer formula is approximately 17 nm for the sample as prepared by combustion method.
Fig. 2. Crystal structure and coordination of Ba and Mg ions in crystal lattice of BaMg8Al18Si18O72 phosphor.
A detailed study at high magnification showed that they had rod shaped particles, composing of different sizes in diameter as indicated in SEM micrographs. The particles has sizes of 359 nm, 411 nm, 533 nm, 847 nm, 533 nm agglomerates are measured under 1 mm resolution. Therefore, in general, the methodology presented in this work provides a simple method for preparing rod shaped structured BaMg8Al18Si18O72 phosphor powders via a simple combustion based route. 3.3. Photoluminescence characterization 3.3.1. Eu2þ luminescence in BaMg8Al18Si18O72 phosphor The excitation spectra of Eu2þ activated BaMg8Al18Si18O72 phosphor shows the strong broad peak observed at 334 nm
3.2. SEM characterization SEM micrographs of the combustion synthesized as prepared BaMg8Al18Si18O72 phosphor powders (550 C) is given in Fig. 3. In general, the particles are agglomerated and basically irregular in shape, with a substantial variation in particle size and morphology.
Fig. 1. X-ray diffraction pattern of BaMg8Al18Si18O72 phosphor.
Fig. 3. SEM morphology of the BaMg8Al18Si18O72 phosphor.
V.B. Pawade et al. / Solid State Sciences 14 (2012) 607e610
609
Fig. 6. PL excitation and emission spectra of BaMg8Al18Si18O72:Ce3þ phosphor lex ¼ 316 nm, and lem ¼ 376 nm.
Fig. 4. Excitation spectra of BaMg8Al18Si18O72:Eu2þ phosphor lem ¼ 437 nm.
(monitor at 437 nm) near UV region and is favorable to solid-state lighting excitation (Fig. 4). From emission spectra of Eu2þ activated BaMg8Al18Si18O72 phosphor as depicted in Fig. 5, shows broad emission band centered at 437 nm keeping excitation at 334 nm near UV region. The luminescence of Eu2þ ion consists of the 4f 6 5d1e4f 7(8S7/2) broad-band emission, which belongs to electricdipole allowed transition and has the properties of large absorption of UV light and broad emission ranging from ultraviolet to visible light depending on different crystal-lattice environment. The Eu2þ can stay in the sites of Ba2þ without a large change of lattice parameters. This means that Eu2þ can substitute for Ba2þ to form solid-state solution in BaMg8Al18Si18O72 host to some extent. This indicates that one of the Ba2þ sites is preferentially occupied by the Eu2þ ions and that the second site is filled only with higher
Fig. 5. Emission spectra of BaMg8Al18Si18O72:Eu2þ phosphor lex ¼ 334 nm.
dopant concentrations. According to the crystal structure, the first Ba2þ site (2a) has the multiplicity of two and a site symmetry of C3 while the second one (6c) has six and C1. Both Ba2þ sites have ninecoordination and the sites are similar in average size (d(BaeO) Ave ¼ 2.86 and 2.87 Å). However, the lower symmetry site has also shorter BaeO distances (2.69 Å) corresponding to those typical of Eu2þeO (2.68 Å). It would therefore seem reasonable that this 6c site is filled preferentially. As Al3þ ion was added and substituted for Si4þ ion, the local negative charge was produced .Al3þ is closer to Ba (II) in space, the local negative charge made the electron cloud density around Ba (II) denser than that of Ba (I), which means that the blue peak increases with the increase in the concentration of Al3þ ion. The overall emission extends on the whole visible spectrum which explains the blue fluorescence perceived with naked eyes under UV illumination. The strong excitation band at 260e380 nm range indicates that phosphor can be excited only by the UV-LED chip, which is essential for improving the efficiency and quality of white light-emitting diodes. 3.3.2. Ce3þ luminescence in BaMg8Al18Si18O72 phosphor Fig. 6 shows the emission spectrum of BaMg8Al18Si18O72:Ce3þ phosphor. The emission band extends from 350 to 500 nm and have a maxim at about 376 nm, keeping broad-excitation band centered
Fig. 7. Gaussian fitted curve of BaMg8Al18Si18O72:Ce3þ (5 mol %) phosphor lex ¼ 316 nm.
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4. Conclusion
Fig. 8. Energy level diagram and 5de4f transition in Eu2þ and Ce3þ ions [14,21].
at 316 nm. This emission band is assigned due to transitions from the lowest Ce3þ 5d excited state to the 4f ground state levels 2F5/2 and 2F7/2. Usually for Ce3þ ions, the energy gap between the maxima coincides with the spin-orbit splitting of Ce3þ ground state, which amounts to about 2000 cm1. The excitation band shows in Fig. 6 have two bands which was appears at 257 nm and 316 nm, out of this two band the phosphor was excited at 316 nm which has broad nature, there is no more excitation bands which is due to direct excitation of the Ce3þ ions via transitions to the components of the Ce3þ 5d configuration. Thus significant change of the Ce3þ emission in BaMg8Al18Si18O72 is observed by varying the Ce3þ concentration in the range from x ¼ 1e10 mol %. However, for higher x, the formation of Ce3þ centers, locally compensated by point defects, can be expected. The stokes-shift for BaMg8Al18Si18O72:Ce3þ phosphor observed at 5550 cm1 which is higher than observed values in many aluminates based phosphors. The emission spectra gives isolated broad-band under uv-region, such phosphor may be a dense scintillating materials. As explained above the doublet band of Ce3þ is not clearly observed, but it is resolved by using curve fitting [20]. The fitted band observed at 398 nm (25,125 cm1) and 371 nm (26,954 cm1) as shown in Fig. 8, the theoretically observed energy difference in these two bands comes out to be 1829 cm1 which is consistent with the energy difference between 2F7/2 and 2F5/2 level (2000 cm1) [14] as shown in Fig. 7. These bands occurs due to the transition from 5d level to the ground state of the Ce3þ ion. Fig. 7 shows the energy levels diagram for Eu2þ and Ce3þ ions. From this it is observed that, the broad-band emission in Eu2þ and Ce3þ ions, usually occurred due to 5d / 4f transition. And it is very interesting to study the mechanism of energy transfer in such ions, which is our future work for the reported novel BaMg8Al18Si18O72 phosphors.
Eu2þ and Ce3þ activated BaMg8Al18Si18O72 host phosphors, was prepared by the combustion synthesis at the constant temperature 550 C. The excitation spectrum was a wide broad-band, for BaMg8Al18Si18O72:Eu2þ phosphor under near uv region at 334 nm, it gives isolated broad blue emission due to 4f65D0 / 4f7 transition of Eu2þ ion, covers from 400 to 650 nm and peaking at 437 nm in the blue region of the spectrum. Whereas BaMg8Al18Si18O72:Ce3þ shows the broad emission band at 376 nm under excitation at 316 nm, such phosphor may have potential application for whiteleds also it is to be a good scintillating materials. The Average crystallite size of the prepared phosphor is comes out to be approximately 17 nm. From XRD and SEM it revealed that the resulting single-phase BaMg8Al18Si18O72 phosphor consists of a variety of agglomerated particle sizes. From the luminescence studies our future plan is to study energy transfer mechanism and phosphorescence behavior of the reported materials by other synthesis routs. Acknowledgments One of the authors NSD is thankful to UGC, New Delhi for financial assistance. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
S.H.M. Poort, W.P. Blokpoel, G. Blasse, Chem. Mater. 7 (1995) 1547. Z.Y. He, X.J. Wang, W.M. Yen, J. Lumin. 309 (2006) 119. K.Y. Jung, H.W. Lee, H.K. Jung, Chem. Mater. 18 (2006) 2249. X.J. Li, Y.D. Qu, X.H. Xie, Z.L. Wang, R.Y. Li, Mater. Lett. 60 (2006) 3673. B. Liu, C.S. Shi, M. Yin, L. Dong, Z.G. Xiao, J. Alloys Compd. 387 (2005) 65. Y. Chen, B. Liu, M. Kirm, Z. Qi, C. Shi, M. True, S. Vielhauer, G. Zimmerer, J. Lumin. 118 (2006) 70. J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696. C.W.E. van Eijk, Nucl. Instrum. Methods A 460 (2001) 1. C.W.E. van Eijk, Nucl. Instrum. Methods A 509 (2003) 17. A. Iltis, M.R. Mayhugh, P. Menge, C.M. Rozsa, O. Selles, V. Solovyev, Nucl. Instrum. Methods A 563 (2006) 359. P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys. Condens. Matter 6 (1994) 4167. C. Ronda, Luminescence: From Theory to Applications, Wiley-Vch Verlag GmbH & Co. KGaA, Weinheim, 2008. P.H. Holloway, T.A. Trottier, B. Abrams, C. Kondoleon, S.L. Jones, J.S. Sebastian, W.J. Thomes, J. Vac. Sci. Technol. B 17 (1999) 758. P. Dorenbos, J. Lumin. 91 (2000) 155. V.B. Pawade, S.J. Dhoble, Luminescence 26 (2011) 722. V.B. Pawade, S.J. Dhoble, Opt. Comm. 284 (2011) 4185. V.B. Pawade, N.S. Dhoble, S.J. Dhoble, J. Opto. Adv. Mat. Rapid Comm. 5 (2011) 208. Subrata Das, A. Amarnath Reddy, Shahzad Ahmad, R. Nagarajan, G. Vijaya Prakash, Chem. Phys. Lett. 508 (2011) 117. G. Fisscher, J. Geiger, Corning Glass Works Private comm., Corning, New York, USA, 1988 X. Zhang, B. Park, N. Choi, J. Kim, G.C. Kim, J.H. Yoo, Mater. Lett. 63 (2009) 700. A. Ellens., et al., J. Lumin. 59 (1994) 293.
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis and optical studies of novel Eu2þ and Ce3þ doped BaMg8Al18Si18O72 phosphors V.B. Pawade a, *, N.S. Dhoble b, S.J. Dhoble c a
Department of Applied Physics, VNIET, Nagpur 440023, India Department of Chemistry, Sevadal Mahila Mahavidyalaya, Nagpur 440009, India c Department of Physics, RTM Nagpur University, Nagpur 440033, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2011 Received in revised form 8 January 2012 Accepted 9 February 2012 Available online 21 February 2012
Novel Eu2þ and Ce3þ activated BaMg8Al18Si18O72 phosphors was prepared by combustion method and their PL characteristics were investigated. The result shows that all samples can be excited efficiently by near UV excitation under 334 nm and 316 nm. The emission was observed for BaMg8Al18Si18O72:Eu2þ phosphor at 437 nm corresponding to d / f transition, under 334 nm broad-band excitation, whereas BaMg8Al18Si18O72:Ce3þ phosphor shows emission band at 376 nm under 316 nm excitation. Phase purity of the phosphor was checked with the help of XRD pattern. SEM analysis shows the external morphology of the combustion synthesized phosphor. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Combustion synthesis Phosphors Optical properties W-leds Scintillators
1. Introduction Alkaline earth silicates have attracted much attention and become an interesting topic in the field of luminescent materials because of their high chemical stability and water-resistant property by comparing with sulfide phosphorescent phosphors and strontium aluminates phosphors [1e4]. However, the research of silicate-based long afterglow materials has been developed several years ago and the study mainly concentrates on the alkaline earth silicate host [5e7]. Single-host tricolor phosphor and NUV LED combination is a promising host to obtain white LEDs. Eu2þ is the most well-known and extensively used activator in many silicates, borates and other inorganic compounds. Its emission comes from 5d / 4f transition, and the emission wavelength can vary in very broad ranges. Among the oxide materials, if a silicate is used, the firing temperature cannot be very high. Eu2þ (4f7) ions which shows a 5de4f emission which can vary from long wavelength ultraviolet to visible in some oxides hosts. The first white LED had been commercialized by Nichia Co., in which a blue LED was coated with yellow phosphor (YAG:Ce3þ). However, the white LEDs based on a blue LED and yellow phosphor combination has several
* Corresponding author. Tel.: þ91 9823232068. E-mail address: [email protected] (V.B. Pawade). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.02.004
disadvantages including the halo effect of blue/yellow color separation and poor color rendering property caused by a lack of red component in the spectrum. Recently, Ce3þ activated scintillation materials are widely used in high-energy physics, nuclear medical imaging and security inspection. High density, high scintillation light yield and fast decay time are the most desired properties [8]. However, even for commercial scintillation crystals, such as NaI:Tlþ, Bi4Ge3O12, LaBr3:Ce3þ, Lu2SiO5:Ce3þ, they also have their own shortcomings. For example, NaI:Tlþ has slow decay time and highly hygroscopic characteristic; Bi4Ge3O12 exhibits a relatively long decay time; LaBr3:Ce3þ is more hygroscopic than NaI:Tlþ and Lu2SiO5:Ce3þ has a long afterglow [9e11]. Therefore, besides optimizing the performance of these crystals, it’s necessary to research and develop novel scintillators with excellent properties. Based on excellent luminescence properties, Ce3þ doped inorganic materials are applied in lightings, displays and detectors for ionizing radiation [12,13]. Ce3þ can also be used as a reference ion to predict the 5d energies of other rare earth ions in same host lattices [14]. Recently, we have reported Eu2þ and Ce3þ luminescence in ternary hexa aluminates based blue phosphors for solid-state lighting purpose, also Eu2þ luminescence in halo silicates based phosphors was studied for the near UV-white leds [15e18]. The reported work has examined the emission properties of Eu2þ and Ce3þ activated in novel BaMg8Al18Si18O72 phosphors first time successfully and such broad emission in divalent and trivalent impurities ions are very
608
V.B. Pawade et al. / Solid State Sciences 14 (2012) 607e610
important to study energy transfer mechanism. Therefore, the investigation on the spectroscopic properties of Ce3þ and Eu2þ activated in novel host lattices is important for the basic research to develop white LEDs, it will be the new illuminating source to replace traditional incandescent lamp and fluorescent lamps for the service lifetimes, energy and environmental considerations. 2. Experimental The reported BaMg8Al18Si18O72 phosphors were prepared by combustion methods at 550 C. The starting materials were used as follows, Ba(NO3)2, Mg(NO3)2$6H2O, Al(NO3)3$9H2O Merck (99.99% purity), SiO2 (99.99% purity), Eu2O3, (NH4)2Ce(NO3)6 Merck (99.99% purity), urea (NH2eCOeNH2, Merck). Ingredients of the above compound were mixed properly in agate mortar and a pasty, a gel solution was formed, it was then transfer to silica crucible, and kept inside a muffle furnace at 550 C. The flame with the foamy powder formed, that powder was collected and analyzed by XRD and photoluminescence measurement. Photoluminescence (PL) emission and photoluminescence excitation (PLE) were measured using a Shimadzu RF5301PC spectrofluorophotometer at room temperature, same amount of sample was used to measure emission and excitation keeping slitwidth at 1.5 nm, PL sensitivity at high for both measurements. 3. Results and discussion 3.1. X-ray diffraction pattern of BaMg8Al18Si18O72 phosphor The phase purities of the phosphor was checked by powder X-ray diffraction (XRD) using a PAN-analytical diffractometer with CuKa radiation (1.5405 Å) operating voltage at 40 kV, 30 mA and scan step time at 10.3377 s. Fig. 1 shows the XRD patterns of BaMg8Al18Si18O72 phosphor that have standard reference file No. 00-039-0273. It has hexagonal structure, having lattice parameter a ¼ b ¼ 9.807, c ¼ 9.400 with space group P6/mcc [19]. Therefore crystal structure of the Ba-substitute indialite is shown in Fig. 2, the actual structure for this compound does not exists we just shows the coordination of Ba, Mg ions in BaMg8Al18Si18O72 host lattices. Average crystallite size calculated by Debye Scherrer formula is approximately 17 nm for the sample as prepared by combustion method.
Fig. 2. Crystal structure and coordination of Ba and Mg ions in crystal lattice of BaMg8Al18Si18O72 phosphor.
A detailed study at high magnification showed that they had rod shaped particles, composing of different sizes in diameter as indicated in SEM micrographs. The particles has sizes of 359 nm, 411 nm, 533 nm, 847 nm, 533 nm agglomerates are measured under 1 mm resolution. Therefore, in general, the methodology presented in this work provides a simple method for preparing rod shaped structured BaMg8Al18Si18O72 phosphor powders via a simple combustion based route. 3.3. Photoluminescence characterization 3.3.1. Eu2þ luminescence in BaMg8Al18Si18O72 phosphor The excitation spectra of Eu2þ activated BaMg8Al18Si18O72 phosphor shows the strong broad peak observed at 334 nm
3.2. SEM characterization SEM micrographs of the combustion synthesized as prepared BaMg8Al18Si18O72 phosphor powders (550 C) is given in Fig. 3. In general, the particles are agglomerated and basically irregular in shape, with a substantial variation in particle size and morphology.
Fig. 1. X-ray diffraction pattern of BaMg8Al18Si18O72 phosphor.
Fig. 3. SEM morphology of the BaMg8Al18Si18O72 phosphor.
V.B. Pawade et al. / Solid State Sciences 14 (2012) 607e610
609
Fig. 6. PL excitation and emission spectra of BaMg8Al18Si18O72:Ce3þ phosphor lex ¼ 316 nm, and lem ¼ 376 nm.
Fig. 4. Excitation spectra of BaMg8Al18Si18O72:Eu2þ phosphor lem ¼ 437 nm.
(monitor at 437 nm) near UV region and is favorable to solid-state lighting excitation (Fig. 4). From emission spectra of Eu2þ activated BaMg8Al18Si18O72 phosphor as depicted in Fig. 5, shows broad emission band centered at 437 nm keeping excitation at 334 nm near UV region. The luminescence of Eu2þ ion consists of the 4f 6 5d1e4f 7(8S7/2) broad-band emission, which belongs to electricdipole allowed transition and has the properties of large absorption of UV light and broad emission ranging from ultraviolet to visible light depending on different crystal-lattice environment. The Eu2þ can stay in the sites of Ba2þ without a large change of lattice parameters. This means that Eu2þ can substitute for Ba2þ to form solid-state solution in BaMg8Al18Si18O72 host to some extent. This indicates that one of the Ba2þ sites is preferentially occupied by the Eu2þ ions and that the second site is filled only with higher
Fig. 5. Emission spectra of BaMg8Al18Si18O72:Eu2þ phosphor lex ¼ 334 nm.
dopant concentrations. According to the crystal structure, the first Ba2þ site (2a) has the multiplicity of two and a site symmetry of C3 while the second one (6c) has six and C1. Both Ba2þ sites have ninecoordination and the sites are similar in average size (d(BaeO) Ave ¼ 2.86 and 2.87 Å). However, the lower symmetry site has also shorter BaeO distances (2.69 Å) corresponding to those typical of Eu2þeO (2.68 Å). It would therefore seem reasonable that this 6c site is filled preferentially. As Al3þ ion was added and substituted for Si4þ ion, the local negative charge was produced .Al3þ is closer to Ba (II) in space, the local negative charge made the electron cloud density around Ba (II) denser than that of Ba (I), which means that the blue peak increases with the increase in the concentration of Al3þ ion. The overall emission extends on the whole visible spectrum which explains the blue fluorescence perceived with naked eyes under UV illumination. The strong excitation band at 260e380 nm range indicates that phosphor can be excited only by the UV-LED chip, which is essential for improving the efficiency and quality of white light-emitting diodes. 3.3.2. Ce3þ luminescence in BaMg8Al18Si18O72 phosphor Fig. 6 shows the emission spectrum of BaMg8Al18Si18O72:Ce3þ phosphor. The emission band extends from 350 to 500 nm and have a maxim at about 376 nm, keeping broad-excitation band centered
Fig. 7. Gaussian fitted curve of BaMg8Al18Si18O72:Ce3þ (5 mol %) phosphor lex ¼ 316 nm.
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V.B. Pawade et al. / Solid State Sciences 14 (2012) 607e610
4. Conclusion
Fig. 8. Energy level diagram and 5de4f transition in Eu2þ and Ce3þ ions [14,21].
at 316 nm. This emission band is assigned due to transitions from the lowest Ce3þ 5d excited state to the 4f ground state levels 2F5/2 and 2F7/2. Usually for Ce3þ ions, the energy gap between the maxima coincides with the spin-orbit splitting of Ce3þ ground state, which amounts to about 2000 cm1. The excitation band shows in Fig. 6 have two bands which was appears at 257 nm and 316 nm, out of this two band the phosphor was excited at 316 nm which has broad nature, there is no more excitation bands which is due to direct excitation of the Ce3þ ions via transitions to the components of the Ce3þ 5d configuration. Thus significant change of the Ce3þ emission in BaMg8Al18Si18O72 is observed by varying the Ce3þ concentration in the range from x ¼ 1e10 mol %. However, for higher x, the formation of Ce3þ centers, locally compensated by point defects, can be expected. The stokes-shift for BaMg8Al18Si18O72:Ce3þ phosphor observed at 5550 cm1 which is higher than observed values in many aluminates based phosphors. The emission spectra gives isolated broad-band under uv-region, such phosphor may be a dense scintillating materials. As explained above the doublet band of Ce3þ is not clearly observed, but it is resolved by using curve fitting [20]. The fitted band observed at 398 nm (25,125 cm1) and 371 nm (26,954 cm1) as shown in Fig. 8, the theoretically observed energy difference in these two bands comes out to be 1829 cm1 which is consistent with the energy difference between 2F7/2 and 2F5/2 level (2000 cm1) [14] as shown in Fig. 7. These bands occurs due to the transition from 5d level to the ground state of the Ce3þ ion. Fig. 7 shows the energy levels diagram for Eu2þ and Ce3þ ions. From this it is observed that, the broad-band emission in Eu2þ and Ce3þ ions, usually occurred due to 5d / 4f transition. And it is very interesting to study the mechanism of energy transfer in such ions, which is our future work for the reported novel BaMg8Al18Si18O72 phosphors.
Eu2þ and Ce3þ activated BaMg8Al18Si18O72 host phosphors, was prepared by the combustion synthesis at the constant temperature 550 C. The excitation spectrum was a wide broad-band, for BaMg8Al18Si18O72:Eu2þ phosphor under near uv region at 334 nm, it gives isolated broad blue emission due to 4f65D0 / 4f7 transition of Eu2þ ion, covers from 400 to 650 nm and peaking at 437 nm in the blue region of the spectrum. Whereas BaMg8Al18Si18O72:Ce3þ shows the broad emission band at 376 nm under excitation at 316 nm, such phosphor may have potential application for whiteleds also it is to be a good scintillating materials. The Average crystallite size of the prepared phosphor is comes out to be approximately 17 nm. From XRD and SEM it revealed that the resulting single-phase BaMg8Al18Si18O72 phosphor consists of a variety of agglomerated particle sizes. From the luminescence studies our future plan is to study energy transfer mechanism and phosphorescence behavior of the reported materials by other synthesis routs. Acknowledgments One of the authors NSD is thankful to UGC, New Delhi for financial assistance. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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