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Luminescent Properties of BaSi2O5:Eu2+ Phosphor Film Fabricated by Spin-Coating of Ba-Eu Precursor on SiO2 Glass
Luminescent Properties of BaSi2O5:Eu2+ Phosphor Film Fabricated by Spin-Coating of Ba-Eu Precursor on SiO2 Glass
Journal of the Optical Society of Korea. 2014. Feb, 18(1): 45-49
Copyright © 2014, Journal of the Optical Society of Korea
  • Received : January 01, 2014
  • Accepted : January 01, 2014
  • Published : February 25, 2014
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About the Authors
Je Hong Park
AIDEN, 134, Gongdan-ro, Heungdeok-gu, Cheongju 361-290, Korea
Jong Su Kim
Department of Image Science and Engineering, Pukyong National University, 599-1, Daeyeon 3-dong, Nam-gu, Busan 608-739, Korea
Jong Tae Kim
Department of Image Science and Engineering, Pukyong National University, 599-1, Daeyeon 3-dong, Nam-gu, Busan 608-739, Korea
jtkim@pknu.ac.kr
Abstract
Well-crystallized BaSi 2 O 5 :Eu 2+ phosphor films were synthesized by heat treatment of spin-coated BaO:Eu on SiO 2 glass. We investigated luminescence-structure properties of these phosphor films as a function of heat-treatment temperature. From x-ray diffraction patterns, our BaSi 2 O 5 :Eu 2+ phosphor films revealed that (111)- and (204)-crystal planes of BaSi2O5 crystal were dominantly increased with an increase of heat-treatment temperature. Photoluminescence intensities of BaSi 2 O 5 :Eu 2+ phosphor films were increased with amount of these crystal planes. It can be explained that Eu 2+ ions were stably occupied at specific crystal orientation of BaSi 2 O 5 crystal, enhancing the luminescent intensities of BaSi 2 O 5 :Eu 2+ phosphor films. In addition, our BaSi 2 O 5 :Eu 2+ phosphor films had transmittance of 70% at 510 nm,.due to the dense morphology and specific crystallinity of BaSi 2 O 5 :Eu 2+ phosphor films.
Keywords
I. INTRODUCTION
silicate system is useful luminescent host, the luminescence process of Eu 2+ ions activated in the silicate system is generated by the 4f 6 5d-4f 7 transition of Eu 2+ ions acting as a color center. Blasse et al. have systemically reported the luminescent properties of Eu 2+ ions activated in binary and ternary silicates [1] . Tanabe et al. reported the preparation of Eu-doped BaSi 2 O 5 glass ceramic phosphors and investigated the luminescent properties with various heat-treatment temperatures [2] . However, the optical and luminescent properties related to crystal properties of Eu-doped silicate phosphors have not been reported in film-type phosphors.
Film-type phosphors offer noteworthy advantages over conventional powder-type phosphors for LED [3] and display applications, such as dense uniformity, long-term stability, thermal and chemical stability, and superior adhesion to the substrate [4] . Film-type phosphors can be fabricated using various techniques, including sputtering deposition [5 , 6] , pulsed laser deposition [7] , atomic layer deposition [8] and chemical vapor deposition [9] . However, these methods are expensive and require complicated equipment for high vacuum techniques and other rigorous conditions. One of the methods for synthesizing film-type phosphors is the solution based spin-coating technique. This method is simple, inexpensive and does not need vacuum techniques as compared to other methods for synthesizing film-type phosphors. In addition, this method has the possibility of large-area coatings with high uniformity on various substrates including glass [10 , 11] . The properties of the film-type phosphors are generally influenced by crystallinity and morphology of film dependent on growth condition and post-heat treatment [5 , 12] . Especially, pores and grain boundaries for a polycrystalline film have a decisive effect on optical properties of the films [13] .
In our experiment, we used a simple method for filmtype phosphors. We exploited the reaction at the interface between substrate material and spin-coated film material on the substrate for making film-type phosphors. In previous research, we had obtained phosphor film with high luminescence and dense morphology through this method [14 , 15] . In this paper, we report luminescence-structure properties of BaSi 2 O 5 :Eu 2+ phosphor films formed on SiO 2 glass through heat treatment of spin-coated BaO:Eu film on SiO 2 glass.
II. EXPERIMENTAL
Sol solution of BaO:Eu was prepared for spin coating using the following procedure. Barium acetate [Ba(CH 3 COO) 2 ] was dissolved in 2-methoxyethanol and acetic acid solution at 60℃. The solution was stirred at 60℃ to obtain a clear and homogeneous mixture. Subsequently, europium acetate hydrate [Eu(CH 3 COO)H 2 O] was added to the solution and stirred at 60℃ for 2 hrs. Eu concentration was fixed to 11 mol% with respect to Ba. The BaO:Eu films were deposited by spin coating on 2 glass substrates (3000 rpm for 20 sec.). The films were dried at 300℃ for 10 min in air ambient to eliminate the solvent and organic residue. The same spin coating procedures were repeated 10 times. Finally, the films were heat treated at 1000, 1050, 1100, 1150℃ for 5 hrs in reduction atmosphere [H 2 (5%)/N 2 (95%)]. The obtained films were identified by X-ray diffraction (XRD) analysis using a Rigaku D/MAX 2500 with Cu Kα radiation. The cross-section images were obtained by conventional scanning electron microscopy (FESEM, JSM-6700F). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained using PR-650 and Hitachi F4500 fluorescence spectrometer. Optical transmittance was measured using a UV-visible spectrometer (Hitachi U3000). The vacuum ultraviolet (VUV) photoluminescence spectrum obtained under vacuum ultraviolet (146 nm) by deuterium lamp.
III. RESULTS AND DISCUSSION
Figure 1(a) shows the formation procedure of BaSi 2 O 5 :Eu 2+ phosphor film and Fig. 1(b) , (c) shows cross-section SEM images of spin-coated BaO:Eu film and formed BaSi 2 O 5 :Eu 2+ phosphor film on SiO 2 glass after heat treatment at 1100℃ for 5hrs in reduction atmosphere (5% H 2 / 95% N 2 ). Figure 1(b) reveals that spin-coated BaO:Eu film is stacked up to 300 nm with small grains from SiO 2 glass. And the grains are agglomerated because of the dry process for elimination of residual organic material during the spin-coating process. After heat treatment, BaSi 2 O 5 :Eu 2+ phosphor film formed on SiO 2 glass with thickness is about 800 nm as seen in Fig. 1(c) . It can be explain that thermal diffusion and reaction is generated at the interface of BaO:Eu film and SiO 2 glass during heat treatment. Consequently dense BaSi 2 O 5 :Eu 2+ phosphor film is formed on SiO 2 glass. Figure 1(c) shows that the interface of formed BaSi 2 O 5 :Eu 2+ phosphor film and SiO 2 glass is not clear and it shows a corrugated shape. This means that the diffusion rate is not uniform at all points of the interface, therefore, it naturally induces a corrugated interface.
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Schematic of formation procedure of BaSi2O5:Eu2+ phosphor film (a). Cross-section SEM images of (b) spin-coated BaO:Eu film and (c) BaSi2O5:Eu2+ phosphor film after heat treatment at 1100℃ for 5hrs in reduction atmosphere.
Figure 2 shows XRD patterns of BaSi 2 O 5 :Eu 2+ phosphor films with increasing heat-treatment temperature. Broad signals appear at around 2θ = 21° in all samples, indicating amorphous SiO 2 glass. Spin-coated BaO:Eu film shows sharp signal peaking at 2θ = 40° , this signal is monoclinic Eu 2 O 3 phase (JCPDS file 71-0589). Usually monoclinic Eu 2 O 3 crystal is stable at high temperatures and pressure [16] . Chen et. al reported on appearance of pure monoclinic Eu 2 O 3 formed at low temperatures and pressure in air [17] . BaO phases are not found in spin-coated BaO:Eu film. In this stage, Eu ions are not completely occupied at Ba sites and monoclinic Eu 2 O 3 phase and amorphous BaO coexist in the form of a cluster. After heat treatment, Eu 2 O 3 and BaO crystals are not detected in all heat treated samples as seen in Fig. 2 . Heat treated samples are well matched with BaSi 2 O 5 crystal (JCPDS file 71-1441), which means that spin-coated BaO:Eu film and SiO 2 glass are completely reacted at the interface during heat treatment and Eu ions completely occupied in Ba sites due to the high temperature soaking. During the heat treatment, monoclinic Eu 2 O 3 crystal is broken down due to the high temperature soaking, and Eu ions are migrated into Ba sites concurrent with synthesizing BaSi 2 O 5 crystal at the interface. Consequently dense BaSi 2 O 5 :Eu 2+ phosphor film formed on SiO 2 glass as seen in SEM images of Fig. 1(c) . And they have some preferred crystalline which is revealed (110)-, (111)-, (204)-crystal planes as seen in XRD patterns of Fig. 2 . Increasing the heat-treatment temperature peak intensity of (111)-, (204)-crystal planes is drastically increased. In addition a cristobalite phase appears having (101)-crystal plane at 1150℃ heat treated sample. It is due to the transition from amorphous SiO 2 glass to crystallized SiO 2 with increasing heat-treatment temperature.
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XRD patterns of spin-coated BaO:Eu film (a) and BaSi2O5:Eu2+ phosphor films heat treated at 1000, 1050, 1100 and 1150℃ for 5hrs in reduction atmosphere (b).
Figure 3(a) shows PL spectra of BaSi 2 O 5: Eu 2+ phosphor films excited by 365 nm UV-lamp as a function of heattreatment temperature. All samples show broad-green emission peaking at 510 nm which originates from the 5d-4f transition of Eu 2+ ions due to the strong coupling of 5d electrons with the host lattice [1] . With increasing heat-treatment temperature, PL intensity is significantly increased and then maximized at 1100℃. From XRD patterns from Fig. 2(b) , peak intensity of (111)-, (204)-crystal planes of BaSi 2 O 5 :Eu 2+ phosphor films is significantly increased with increasing heat-treatment temperature. This means that PL properties of BaSi 2 O 5 :Eu 2+ phosphor films are deeply related to these crystal planes. Up to 1050℃, crystallization of (111)-, (204)-crystal planes is poor and begin to drastically appear these crystal planes at 1100 and 1150℃ heat treated samples as seen in XRD patterns of Fig. 2(b) . PL intensity of BaSi 2 O 5 :Eu 2+ phosphor films increases with increasing the signal of these crystal planes as seen in Fig. 3(b) . It can be explained that Eu 2+ ions are selectively migrated into specific crystal planes and (111)-, (204)-crystal plans of BaSi 2 O 5 crystal offer stable site for activation of Eu 2+ ions. PL intensity of the sample treated at 1150℃ slightly decreased. It is due to the phase impurity (cristobalite) acting as luminescence killer. PL-excitation spectrum monitored at 510 nm of BaSi 2 O 5 :Eu 2+ phosphor films shows reasonable excitation in near the UV region as seen in the inset of Fig. 3(a) . Thus our phosphor films can be applied as a green phosphor of a green LED or a white LED excited by a UV chip.
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PL spectra of samples annealed at 1000, 1050, 1100, and 1150℃ for 5hrs (a). PL intensities and XRD peak intensities of (204) crystal plane with increasing the heattreatment temperatures (b).
Figure 4 shows vacuum ultraviolet (VUV)- PL and PLexcitation spectra of BaSi 2 O 5 :Eu 2+ phosphor film heat treated at 1100℃. BaSi 2 O 5 :Eu 2+ phosphor film has high excitation at 167 nm wavelength as seen in VUV-PLE of Fig. 4 . Our phosphor films also can be used in the green component of plasma display panels (PDP).
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VUV-PL(E) spectra of sample annealed at 1100℃ for 5hrs.
Figure 5 shows transmittance spectrum and photograph of BaSi 2 O 5 :Eu 2+ phosphor films on SiO 2 glass excited by 365 nm UV lamp under daylight lamp. Transmittances of our sample revealed about 70% at 510 nm wavelength range. It is due to reduction of scattering by reducing grain boundaries and pores. Generally, the transparency of a ceramic is limited by light scattering from grain boundaries or pores [13] . Our BaSi 2 O 5 :Eu 2+ phosphor film shows dense morphology as seen in SEM images of Fig. 1 . In addition, the phosphor film has some preferred crystal planes as seen in XRD patterns of Fig. 2 , which is different from general poly-crystalline ceramic powder. These factors affect optical properties of our film. It can reduce pores and grain boundaries.
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Transmittance spectrum and photograph of BaSi2O5:Eu2+ phosphor films excited by 365 nm UV lamp under daylight lamp.
IV. CONCLUSIONS
Green-emission BaSi 2 O 5 :Eu 2+ phosphor films were successfully formed on SiO 2 glass using a simple method through thermal-diffusion reaction at the interface between spin-coated BaO:Eu film and SiO 2 glass. We investigated luminescence-structure properties of these phosphor films with increasing the heat-treatment temperatures. The BaSi 2 O 5 : Eu 2+ phosphor films revealed specific crystal structure and this crystal property was profoundly linked to the luminescent property of BaSi 2 O 5 :Eu 2+ phosphor films. In addition, our BaSi 2 O 5 :Eu 2+ phosphor films had excellent optical transmittance; it is due to the crystallinity and dense morphology of the BaSi 2 O 5 :Eu 2+ phosphor films. Our phosphors films formed through a simple synthesis method can be applied in lighting and display devices such as W-LED and PDP.
Acknowledgements
This work was supported by the Pukyong National University Research Abroad Fund in 2009 (PS-2009-022).
References
Blasse G. , Wanmaker W. L. , Vrugt J. W. ter , Bril A. 1968 “Fluorescence of Eu2+activated alkaline-earth aluminates,” Philips Res. Rep. 23 189 - 200
Nakanishi T. , Tanabe S. 2008 “Preparation of BaSi2O5:Eu2+glass ceramic phosphors and luminescent properties,” J. Light & Vis. Env. http://dx.doi.org/10.2150/jlve.32.93 32 93 - 96
Ryu Han-Youl 2013 “Analysis on the luminous efficiency of phosphor-conversion white light-emitting diode,” Journal of the Optical Society of Korea http://dx.doi.org/10.3807/JOSK.2013.17.1.022 17 (1) 22 - 26
Cho K. G. , Kumar D. , Lee D. G. , Jones S. L. , Holloway P. H. , Singh R. K. 1997 “Improved luminescence properties of pulsed laser deposited thin films on diamond coated silicon substrates,” Appl. Phys. Lett. 3335 - 3337
Zhang D. S. , Ko K. Y. , Park H. K. , Yoon D. H. , Do Y. R. 2008 “A study of the factors influencing the brightness of the photoluminescence of sputter-deposited Y2O3:Eu3+film phosphors,” J. Electrochem. Soc. 155 J111 - J116
Seo K. I. , Park J. H. , Kim J. S. , Na Y. H. , Choi J. C. , Bae J. S. 2009 “Green-emissive transparent BaSi2O5:Eu2+film phosphor on quartz glass created by a sputtering thermal diffusion process,” Solid State Commun http://dx.doi.org/10.1016/j.ssc.2009.05.038 149 1578 - 1581
Kim Kang Min , Chung Jun Ho , Ryu Jeong Ho 2012 “Thin film deposition of Tb3Al5O12:Ce by pulsed laser ablation and effects of low-temperature post-annealing,“ Journal of the Optical Society of Korea http://dx.doi.org/10.3807/JOSK.2012.16.1.076 16 (1) 76 - 79
Marinsek M. , Zupan K. , Maeek J. 2002 “Ni-YSZ cermet anodes prepared by citrate/nitrate combustion synthesis,” J. Power Sources http://dx.doi.org/10.1016/S0378-7753(01)01056-4 106 178 - 188
Minami T. , Kuroi Y. , Takata S. 1996 “Preparation of ZnGa2O4: Mn phosphor thin films as emitting layers for electroluminescent devices,” J. Vac. Sci. Technol. A 14 1736 - 1740
Liu Z. , Li J. , Ya J. , Xin Y. , Jin Z. 2008 “Mechanism and characteristics of porous ZnO films by sol-gel method with PEG template,” Mater. Lett. http://dx.doi.org/10.1016/j.matlet.2007.08.010 62 1190 - 1193
Kumar N. , Kaur R. , Mehra R. M. 2007 “Photoluminescence studies in sol-gel derived ZnO films,” J. Lumin. http://dx.doi.org/10.1016/j.jlumin.2006.11.012 126 784 - 788
Yi S. S. , Bae J. S. , Shim K. S. , Jeong J. H. , Park J. C. , Holloway P. H. 2004 “Enhanced luminescence of Gd2O3:Eu3+thin-film phosphors by Li doping,” Appl. Phys. Lett. http://dx.doi.org/10.1063/1.1641526 84 353 - 355
Apetz R. , Bruggen M. P. B. 2003 “Transparent alumina: A light-scattering model,” J. Am. Ceram. Soc. http://dx.doi.org/10.1111/j.1151-2916.2003.tb03325.x 86 480 - 486
Park J. , Park K. , Lee J. , Kim J. , Kim S. M. , Kung P. 2010 “Excellent brightness with shortening lifetime of textured Zn2SiO4:Mn2+phosphor films on quartz glass,” Jpn. J. Appl. Phys http://dx.doi.org/10.1143/JJAP.49.042603 49 042603 - 042607
Park J. , Park K. , Lee J. , Lee S. , Kim J. , Kim G. , Yoo J. 2013 “A simple synthesis method for Zn2SiO4:Mn2+phosphor films and their optical and luminescence properties,” J. Lumin. http://dx.doi.org/10.1016/j.jlumin.2012.09.011 134 71 - 74
Chen G. , Stump N. A. , Haire R. G. , Peterson J. R. 1992 “Study of the phase behavior of Eu2O3under pressure via luminescence of Eu3+,” J. Alloys Compounds http://dx.doi.org/10.1016/0925-8388(92)90347-C 181 503 - 509
Chen W. , Joly A. G. , Kowalchuk C. M. , Malm J. O. , Huang Y. , Bovin J. O. 2002 “Structure, luminescence, and dynamics of Eu2O3nanoparticles in MCM-41,” J. Phys. Chem. B 106 7034 - 7041