Intense red-emitting Y_4Al_2O_9:Eu^3+phosphor with short decay time and high color purity for advanced plasma display panel

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Intense red-emitting Y_4Al_2O_9:Eu^3+phosphor with short decay time and high color purity for advanced plasma display panel
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  Intense red-emitting Y 4 Al 2 O 9 :Eu 3+ phosphor with short decay time and high color purity for advanced plasma display panel Ravishanker Yadav, Atif F. Khan, Ashish Yadav, Harish Chander, Divi Haranath, Bipin Kr. Gupta, Virendra Shanker and Santa Chawla*  National Physical Laboratory, Council of Scientific and Industrial Research, Dr K S Krishnan Road,  New Delhi – 110 012, India * santa@nplindia.org Abstract:  A new phosphor Y 4 Al 2 O 9 :Eu 3+  (YAM:Eu 3+ ) emitting intense monochromatic red at 612 nm under vacuum ultraviolet (VUV) and ultraviolet (UV) excitations has been developed for application in next generation plasma display panels (PDPs). The developed phosphor has better luminescence efficiency, colour purity and shorter decay time than commercial (Y,Gd)BO 3 :Eu 3+  red emitting PDP phosphor. High color purity (x = 0.67, y = 0.32) under VUV excitation with short decay time (1.03 msec) and excellent stability against degradation during PDP panel preparation suggest that YAM:Eu 3+  is a potential candidate for present and future PDPs. Surface coating by SiO 2  further improved phosphor characteristics. © 2009 Optical Society of America OCIS codes:  (160.2540) Fluorescent and luminescent materials; (250.5230) Photoluminescence; (300.6280) Spectroscopy, fluorescence and luminescence. References and links 1. Z. Tian, H. Liang, H. Lin, Q. Su, B. Guo, G. Zhang, and Y. Fu, “Luminescence of NaGdFPO 4 :Ln 3+  after VUV excitation: A comparison with GdPO 4 :Ln 3+  (Ln=Ce, Tb),” J. Solid State Chem. 179 (5), 1356–1362 (2006). 2. R. P. Rao, and D. J. Devine, “RE-activated lanthanide phosphate phosphors for PDP applications,” J. Lumin. 87-89 , 1260–1263 (2000). 3. H. M. Yang, J. X. Shi, H. B. Liang, and M. L. Gong, “Novel red phosphor Mg 2 GeO 4  doped with Eu 3+  for PDP applications,” J. Mater. Sci. Eng. B 127 (2-3), 276–279 (2006). 4. S. Zhang, Y. Hou, H. Fujii, T. Onishi, M. Kokubu, M. Obata, H. Tanno, T. Kono, and H. Uchiike, “ Effect of Nonstoichiometry on the Deterioration of Eu 2+  -Doped Hexagonal Aluminate Phosphor for Plasma Display Applications, ” Jpn. J. Appl. Phys. 42 (Part 1, No. 2A), 477–480 (2003). 5. Q. Zeng, H. Tanno, K. Egoshi, N. Tanamachi, and S. Zhang, “Ba 5 SiO 4 :Eu 2+ : An intense blue emission phosphor under vacumm ultraviolet and near-ultraviolet excitation,” Appl. Phys. Lett. 88 (5), 051906 (2006). 6. G. Bizarri, and B. Moine, “On BaMgAl 10 O 17 : Eu 2+  phosphor degradation mechanism: thermal treatment effects,” J. Lumin. 113 (3-4), 199–213 (2005). 7. C. Okazaki, M. Shiiki, T. Suzuki, and K. Suzuki, “Luminescence saturation properties of PDP phosphors,” J. Lumin. 87-89 , 1280–1282 (2000). 8. C. R. Ronda, “Recent achievements in research on phosphors for lamps and displays,” J. Lumin. 49 , 72 (1997). 9. K. S. Sohn, S. S. Kim, and H. D. Park, “Lumnescence quenching in thermally-treated barium magnesium aluminate phosphor,” Appl. Phys. Lett. 81 (10), 1759 (2002). 10. S. Shionoya, and W. M. Yen, eds., Phosphor Handbook  , (CRC Press, NY, 1999), p. 629. 11. B. Han, H. Liang, H. Ni, Q. Su, G. Yang, J. Shi, and G. Zhang, “Intense red light emission of Eu 3+ -doped LiGd(PO 3 ) 4  for mercury-free lamps and plasma display panels application,” Opt. Express 17 (9), 7138–7144 (2009). 12. Y. C. Kang, Y. S. Chung, and S. B. Park, “Preparation of YAG:Europium Red Phosphors by Spray Pyrolysis Using a Filter-Expansion Aerosol Generator,” J. Am. Ceram. Soc. 82 , 2056 (1999). 13. W. Y. Ching, and Y. N. Xu, “Nonscalability and nontransferbility in the eletronic properties of the Y-Al-O systems,” Phys. Rev. B 59 (20), 12815 (1999). 14. V. Lupei, N. Pavel, and T. Taira, “Highly efficient continuous-wave 946-nm Nd:YAG laser emission under direct 885-nm pumping,” Appl. Phys. Lett. 81 (15), 2677 (2002). #116258 - $15.00 USDReceived 27 Aug 2009; revised 30 Oct 2009; accepted 1 Nov 2009; published 17 Nov 2009 (C) 2009 OSA23 November 2009 / Vol. 17, No. 24 / OPTICS EXPRESS 22023  15. C. H. Lu, and R. Jagannathan, “Cerium-ion-doped yttrium aluminium garnet nanophosphors prepared through sol-gel pyrolysis for luminescent lighting,” Appl. Phys. Lett. 80 (19), 3608 (2002). 16. J. Y. Choe, D. Ravichandran, S. M. Blomquist, D. C. Morton, K. W. Kirchner, M. H. Ervin, and U. Lee, “Alkoxy sol-gel derived Y 3–  x  Al 5 O 12 :Tb  x   thin films as efficient cathodoluminescent phosphors,” Appl. Phys. Lett. 78 (24), 3800 (2001). 17. G. Xia, S. Zhou, J. Zhang, S. Wang, H. Wang, and J. Xu, “Sol–gel combustion synthesis and luminescence of Y 4 Al 2 O 9 :Eu 3+  nanocrystal,” J. Non-Cryst. Solids 351 (37-39), 2979–2982 (2005). 18. D. Y. Wang, and Y. H. Wang, “Photoluminescence of Y 4 Al 2 O 9 :Re (Re= Tb 3+ , Eu 3+ ) under VUV excitation,” J. Alloy. Comp. 425 (1-2), L5–L7 (2006). 19. H. Yamane, M. Shimada, and B. A. Hunter, “High-Temperature Neutron Diffraction Study of Y 4 Al 2 O 9 ,” J. Solid State Chem. 141 (2), 466–474 (1998). 20. P. B. Wagh, A. V. Rao, and D. Haranath, “Influence of molar ratios of precursor, solvent and water on physical properties of citric acid catalyzed TEOS silica aerogels,” Mater. Chem. Phys. 53 (1), 41–47 (1998). 21. JCPDS card no. 22–0987. 22. J. P. Boeuf, “plasma display panels:physics, recent developments and key issues,” J. Phys. D Appl. Phys. 36 (6), R53–R79 (2003). 23. K. Saito, and A. J. Ikushima, ““ Absorption edge in silica glass,” Phys. Rev. B 62 (13), 8584–8587 (2000). 24. I. Y. Jung, Soc. Inform. Display Digest, 1325 (2007). 25. I. Y. Jung, Y. Cho, S. G. Lee, S. H. Sohn, D. K. Kim, D. K. Lee, and Y. M. Kweon, “Optical properties of BaMgAl 10 O 17 :Eu 2+  phosphor coated with SiO 2  for a plasma display panel,” Appl. Phys. Lett. 87 (19), 191908 (2005). 26. C. W. Cho, U. Paik, D. H. Park, Y. C. Kim, and D. S. Zang, “Design of fine phosphor system for thr improvement in the luminescent properties of the phosphor layer in the plasma display panel: Theoritical and experimental analysis,” Appl. Phys. Lett. 93 (3), 031505 (2008). 27. L. S. Wang, X. M. Liu, Z. W. Quan, D. Y. Kong, J. Yang, and J. Lin, “Luminescence properties of Y 0.9 − x Gd x Eu 0.1 Al 3 (BO 3 ) 4  (0 ≤ x ≤ 0.9) phosphors prepared by spray pyrolysis process,” J. Lumin. 122-123 , 36–39 (2007). 28. O. L. Malta, H. F. Vrito, J. F. S. Menezes, F. R. G. Silve, S. Alves, Jr., F. S. Farias, Jr., and A. V. M. Andrade, “Spectroscopic properties of a new light-converting device Eu(thenoyltrifluoroacetonate) 3  2(dibenzyl sulfoxide). A theoretical analysis based on structural data obtained from a sparkle model,” J. Lumin. 75 (3), 255–268 (1997). 29. M. Yin, W. Zhang, L. Lou, S. Xia, and J. C. Krupa, “Spectroscopic properties of Eu 3+  ions in X 1 –Y 2 SiO 5  at nanometric scale,” Physica B 254 (1-2), 141–147 (1998). 30. D. S. Zang, J. H. Song, D. H. Park, Y. C. Kim, and D. H. Yoon, “New fast-decaying green and red phosphor for 3D application of plasma display panels,” J. Lumin. 129 (9), 1088–1093 (2009). 31. J. P. Rainho, L. D. Carlos, and J. Rocha, “New phosphors based on Eu 3+ -doped microporous titanosilicates,” J. Lumin. 87-89 , 1083–1086 (2000). 1. Introduction Plasma display panels (PDPs) dominate the segment of the next generation flat panel displays which uses VUV excitation of Red, Green and Blue (RGB) phosphors for image display. For full coloured large area flat display, PDP is a very promising technique due to the progress made in technology by way of improvements in cost, resolution, lifetime, power consumption, high performance and luminescence efficiency which results in the reductions of thickness and weight [1,2] Motivated by the advances in PDP technique, the demand of highly efficient vacuum ultraviolet (VUV) phosphors has increased tremendously in the past decade [3]. The demand became crucial as phosphors play a very important role in PDPs in terms of augmented performance such as higher efficiency for lower power consumption and higher reliability for longer lifetime. Luminescence characteristics of phosphors and their behaviour under panel making process, energetic discharge ions, electrons and solarization from VUV generated by the Xe-Ne gas plasma are important factors for PDP. In PDP’s plasma resonance, vacuum ultraviolet (VUV) radiation lines of Xe atoms at 147 nm and a molecular excimer Xe 2  at 172 nm are used for excitation of phosphors to emit visible luminescence in red, green and blue. All the three red, blue and green phosphors have shown degradation in luminescence intensity due to the thermal treatment of PDP manufacturing process and this remains a major problem for PDP phosphors [4–7]. Another challenge in PDP technology today is to improve the purity of red phosphor so as to obtain monochromatic red emission with good color coordinates [8,9]. Commercially used (Y,Gd)BO 3 :Eu 3+  (YGB:Eu 3+ ) red phosphor has poor color purity with a dominant orange component and the CIE chromaticity x value (0.65) [10] falls short of declared standard value of 0.67 by National Television #116258 - $15.00 USDReceived 27 Aug 2009; revised 30 Oct 2009; accepted 1 Nov 2009; published 17 Nov 2009 (C) 2009 OSA23 November 2009 / Vol. 17, No. 24 / OPTICS EXPRESS 22024  Standard Committee (NTSC). For viewing high speed motor sports, video gaming and other high definition programs, decay time of the phosphor should be as low as possible to avoid image overlap. Decay time of YGB:Eu 3+  phosphor is long (12 msec) which makes this phosphor unsuitable for next generation PDP sets. It is, therefore, necessary to develop highly efficient, baking resistant, high color purity red-emitting phosphor with shorter decay time. As an alternative, the search for new efficient red-emitting phosphor other than YGB:Eu 3+  is being conducted and reviewed by many researchers [3,11] Europium-doped Yttrium Aluminium Garnet (YAG) has attracted substantial attention in recent years because of its high resistance to electron irradiation which makes Yttria – Alumina system promising candidates in Cathode Ray Tubes, Field Emission Display, Vacuum Fluorescent Displays [12] etc. The Yttria – Alumina system has several phases, including YAlO 3  (YAP: Yttrium Aluminium Polymorphs), Y 4 Al 2 O 9  (YAM: Yttrium Aluminium Monoclinic), and Y 3 Al 5 O 12  (YAG: Yttrium Aluminium Garnet). Commonly, YAP and YAM form as an intermediate product of YAG phase in solid state reactions. Even if YAG is synthesized with a stochiometric mixture of Y 2 O 3  and Al 2 O 3 , two detrimental phases, YAP and YAM, often co-exist as by-products [12,13]. We have deliberately synthesised single phase YAM:Eu 3+  by conventional solid state reaction technique and investigated the luminescence properties in detail to see the effect of changed crystal field in Eu 3+  emission characteristics. The luminescence properties of YAM:Eu 3+  under UV excitation was reported earlier by many researchers for different applications [14–16]. However, very few reports [17,18] are available on the VUV excitation properties of YAM:Eu 3+  for PDP application so far. To the best of our knowledge, we report for the first time Eu 3+  doped YAM that is highly efficient, degradation controlled, pure red-emitting phosphor with additional feature of short decay time suitable for advanced PDPs. 2. Experimental To synthesize YAM:Eu 3+  phosphor by solid state reaction technique [19], stoichiometric amounts of starting material Y 2 O 3  (99.99%), α -Al 2 O 3  (99.9%), and Eu 2 O 3  (99.99%) were weighed according to the molar ratio of (Y 1-x Eu x ) 4 Al 2 O 9  (x = 0.01 ≤  x ≤ 0.50). The powders were mechanically ground with 10 mole% H 3 BO 3  (99.9%) as flux in a mortar pestle, packed in an alumina crucible and subjected to two stage calcinations and sintering process between 700 and 1300°C in air atmosphere for 2-10 hours. Initial calcination was done at 700°C for 2 hours, at this temperature melting of boric acid (H 3 BO 3 ) starts which enables proper mixing of the precursor materials. Addition of H 3 BO 3  drastically reduces the synthesis temperature from 1500°C to ~1300°C and also promotes the crystallization by acting as a high temperature solvent. Therefore second stage sintering was done at 1300°C for 10 hrs to improve the crystallinity.The sintered mass was ground thoroughly to obtain fine powder phosphor. Mechanical grinding and rigorous ultrasonication in alcohol leads to de-agglomeration of particles. The YAM:Eu 3+  powder grains were then subjected to surface treatment for silica coating from a precursor solution of tetraethylorthosilicate (TEOS) by Stöber process [20] followed by high temperature annealing for uniform coating of the grains. Surface coating of phosphor particles was basically done for the purpose of preventing the degradation of the phosphors due to panel baking process in air at temperature ~500°C. For simulation of thermal baking process in PDP panel, the YAM:Eu 3+  powder was mixed with organic vehicle used for screen printing process and subsequently fired in air atmosphere at 500°C for 30 min to remove the organic volatiles from the phosphor. 3. Results and discussion X-Ray diffraction (XRD) was employed to identify the phase by Rigaku Miniflex II X-ray powder diffractometer using Cu K α  radiation. #116258 - $15.00 USDReceived 27 Aug 2009; revised 30 Oct 2009; accepted 1 Nov 2009; published 17 Nov 2009 (C) 2009 OSA23 November 2009 / Vol. 17, No. 24 / OPTICS EXPRESS 22025   Fig. 1. XRD pattern of prepared YAM:Eu 3+  PDP phosphor The XRD (Fig. 1) revealed monoclinic monophasic YAM [21] without any precipitated phase implying effectiveness of the synthesis process in single phase crystallization. The dopant Eu 3+  ions effectively substitutes Y 3+  ions. Figure 2 shows the scanning electron microscope image composed of near rounded particles of ~1 µ m size of as synthesized YAM:Eu 3+ . Fig. 2. SEM micrograph of as synthesized YAM:Eu 3+  phosphor sample. Low sintering temperature controls the growth of individual phosphor particles. Silica coating process provided a near uniform distribution of separated grains of approximate 1 µ m size. TEM image (Fig. 3) clearly shows the individual silica coated phosphor grains of size below 1 µ m with nearly spherical morphology. As theoretical considerations based on Mie scattering theory have shown that best phosphor efficiency can be achieved in 1-2 µ m grain size [22], the particle size of developed phosphor seems ideally suitable. The prepared YAM:Eu 3+  phosphor possesses an intense monochromatic red-emission at 612 nm under UV excitation (225-275 nm) (Fig. 4) and under VUV excitation at 147 nm, 172 nm (Fig. 5). The photoluminescence (PL) spectra of Eu 3+  in synthesized powder YAM and commercial YGB under UV excitation are shown in Fig. 4. #116258 - $15.00 USDReceived 27 Aug 2009; revised 30 Oct 2009; accepted 1 Nov 2009; published 17 Nov 2009 (C) 2009 OSA23 November 2009 / Vol. 17, No. 24 / OPTICS EXPRESS 22026   Fig. 3. TEM image of silica coated YAM:Eu 3+  particles. The PL emission from the YAM:Eu 3+  phosphor exhibits a strong emission peak at 612 nm which can be compared to the three emission peaks of commercial YGB: Eu 3+  phosphor under identical conditions. The strongest emission peak observed at 612 nm is due to the electric dipole transition of 5 D 0 - 7 F 2 . It is clearly seen from the figure that emission intensity of YGB:Eu 3+  phosphor is much lower with an undesirable 590 nm ( 5 D 0 - 7 F 1 ) peak that impedes the colour purity. The integrated PL emission intensity (inset in Fig. 4) of Y 3.8 Eu 0.2 Al 2 O 9  is 23% more than that of standard red emitting YGB:Eu 3+  red phosphor which is represented as blue line. PL emission spectra of YAM:Eu 3+  phosphor under 147 and 172 nm VUV excitation (Fig. 5) show a strong peak at 612 nm showing the monochromaticity of the developed phosphor in both the VUV excitation wavelengths employed in a conventional PDP panel. The spectrum under 147 nm excitation [Fig. 5(a)], however, was measured under vacuum in a different setup which may be the reason for slight change in the emission line width. Fig. 4. (color online) Room-temperature emission of YAM: 0.2Eu 3+  (Red line) phosphor and commercial YGB:Eu 3+  (Blue line) phosphor ( λ  ex  = 250 nm). The inset shows integrated emission intensity of the same phosphors. In order to optimize the Eu 3+  doping concentration for maximum luminescence intensity, Eu 3+  concentration was varied according to (Y 1-x Eu x ) 4 Al 2 O 9  with x = 0.01, 0.03, 0.07, 0.10, 0.20, 0.30 and 0.50. It was observed that the integrated emission intensities from Eu 3+  initially increased with an increase of the concentration x, reaching a maximum value at x = 0.20. The PL intensity decreased with further increase of Eu 3+  concentration in the phosphor sample due the concentration quenching effect. Thus, the optimum concentration for Eu 3+  is 0.20 moles of Y 3+  in Y 4 Al 2 O 9  host. In general, the concentration quenching of luminescence at higher #116258 - $15.00 USDReceived 27 Aug 2009; revised 30 Oct 2009; accepted 1 Nov 2009; published 17 Nov 2009 (C) 2009 OSA23 November 2009 / Vol. 17, No. 24 / OPTICS EXPRESS 22027
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