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ScienceDirect Materials Today: Proceedings 2 (2015) 4111 – 4117
7th International Symposium On Macro- and Supramolecular Architectures and Materials
Luminescence of alternating SiO2:Tb and SiO2:Ce thin films produced by sol-gel spin coating H.A.A. Seed Ahmeda,b, A. Yousifa,b, H.C. Swarta, R.E. Kroona,* a
Department of Physics, University of the Free State, Bloemfontein ZA9300, South Africa b Department of Physics, University of Khartoum, Khartoum, Sudan
Abstract Thin films of different numbers of SiO2:Tb layers as well as alternating layers of SiO2:Tb and SiO2:Ce were fabricated by the solgel spin coating method in order to study the effect of Ce3+ on the Tb3+ emission. The prepared films were annealed at 1000°C in a H2/Ar reducing atmosphere to prevent the occurrence of the tetravalent state of the Ce ions. The topography of the films was investigated by atomic force microscopy and small empty areas were detected which were confirmed by scanning electron microscopy. The luminescence intensity from Tb3+ was found to increase monotonically with the number of deposited layers up to sixteen layers. In the films consisting of alternating layers the Tb3+ ions could be excited through Ce3+ absorption as a result of energy transfer from Ce3+ to Tb3+ ions. When exciting the Tb3+ ions via the Ce3+ ion absorption wavelength of 320 nm, the green emission at 545 nm reached a maximum intensity for four double-layers and then decreased if more layers were added. 3D images obtained by time-of-flight secondary ion mass spectroscopy showed the influence of the heating process on the layers of the thin films. © AllAll rights reserved. © 2015 2014Elsevier ElsevierLtd. Ltd. rights reserved. Selection under responsibility of theofConference Committee Members of 7th International Symposium on Selectionand andpeer-review peer-review under responsibility the Conference Committee Members of 7th International Symposium on MacroArchitectures and and Materials. Macro-and andSupramolecular Supramolecular Architectures Materials. Keywords: Luminescence; Thin films; Sol-gel; Spin coating; SiO2; Tb; Ce
* Corresponding author. Tel.: +27-51-4012884; fax: +27-51-4013507. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of 7th International Symposium on Macro- and Supramolecular Architectures and Materials. doi:10.1016/j.matpr.2015.08.041
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H.A.A. Seed Ahmed et al. / Materials Today: Proceedings 2 (2015) 4111 – 4117
1. Introduction Luminescence materials or phosphors have potential applications in modern technologies such as lighting and displays [1,2]. Phosphors are usually in the form of powders or thin films. However, thin films are widely used due to their several advantages and unique properties [3]. Thin films are formed by using various techniques, among which the sol-gel method combined with spin coating deposition is considered to be one of the simplest and most economical [4]. Sol-gel silica (SiO2) has been widely used as a host material for lanthanide ions because it has good physical properties and chemical stability [5]. Energy transfer can play an important role in phosphor materials as it can be used to enhance the luminescence efficiency [6]. Ce 3+ ions are an efficient sensitizer for Tb3+ ions and their co-doping has been studied in a variety of hosts [7-13]. Tb3+ ions are used as the activator because their bright green emission is suitable for many applications. Although Tb3+ ions can be excited efficiently through their allowed 4f-5d transition, it can be advantageous to use Ce3+ ions as a sensitizer because their allowed 4f-5d transition occurs at a longer (and therefore generally more accessible and convenient) wavelength. Recently, we reported on the high efficiency energy transfer from Ce3+ to Tb3+ as a result of incorporating the two ions in silica [14]. In the present work, we achieved the same result by depositing alternating thin film layers of SiO 2:Tb and SiO2:Ce instead of incorporating the two ions directly in the same host. 2. Experimental Thin films of different numbers of SiO2:Tb layers as well as alternating layers of SiO2:Tb and SiO2:Ce were fabricated by the sol-gel spin coating method. Solutions of SiO2:Tb and SiO2:Ce were produced by allowing tetraethylorthosilicate (TEOS) to react with water. Ethanol was used as a solvent and nitric acid was added to catalyze the reaction. Tb or Ce nitrate was added for doping. The molar ratio of TEOS:water:ethanol was 1:5:10, and the HNO3 concentration was 0.15 M in water. The dopant concentration of Tb or Ce was 2 mol% of the Si content. TEOS was mixed with ethanol and stirred for 30 min, after which the acidified water was added and stirring continued for another 30 min. After that Tb or Ce nitrate dissolved in a little ethanol was added to the mixture which was then stirred for a further 4 h. To fabricate thin films, each ml of the SiO2:Tb and SiO2:Ce sols to be used was first diluted by 20 ml of ethanol having 0.5 mmol of CTAB dissolved in it, to control the viscosity. Thin films were deposited layer by layer on Si(111) substrates. The substrates were cleaned ultrasonically using acetone, methanol and distilled water. An SPS SPIN 150 spin coater from Semiconductor Production Systems was used to produce the thin films. For each layer, a small amount of the solution was dropped onto the substrate which was then spun at 3000 rpm for 40 s, followed by drying on a hot plate. The prepared films were annealed at 1000°C in a H2/Ar reducing atmosphere to prevent the occurrence of the tetravalent state of the Ce ions. The surface topography was examined using a Shimadzu SPM-9600 atomic force microscopy (AFM). The scanning electron microscopy (SEM) of the thin films was performed using a JSM-7800F microscope. Photoluminescence (PL) measurements were made at room temperature using a Cary Eclipse fluorescence spectrophotometer equipped with a xenon lamp. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements were performed with a TOF-SIMS5 instrument from ION-TOF using a 1 kV oxygen sputter gun for depth profiling. 3. Results and Discussion The surface topography of the thin films was investigated by the AFM technique. The images for all the films showed almost the same topography, having smooth grains with an rms surface roughness between 0.4-1.0 nm. Fig. 1(a) and (b) show the AFM image of four double-layers of SiO2:Tb/SiO2:Ce film before and after annealing, respectively. The surface texture of the film after annealing was smoother compared to the film before annealing which can be attributed to stress which the film may be experienced during heating. AFM images of the films show some uncovered circular holes distributed randomly on the surface, which were confirmed by SEM images as well (see Fig. 1(c) and (d)). These holes can be ascribed due to empty uncovered areas, possibly as a result of tiny bubbles in the diluted sol material used for spin coating.
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Fig. 1. (a) AFM image of a thin film comprising four double-layers of SiO2:Tb/SiO2:Ce before annealing. (b) AFM image of the same thin film after annealing. (c) and (d) show the SEM images of the thin films in (a) and (b) respectively.
Fig. 1 (c) and (d) show SEM images of four double-layer thin films of SiO2:Tb/SiO2:Ce before and after annealing, respectively. These show that the holes increased in size during annealing. In addition the SEM image of the annealed film shows small white dots which are not present in the unannealed films. These white dots are silica clusters which are known to form at high temperatures [15].
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Fig. 2. (a) The excitation and emission spectra of sixteen layers of SiO2:Tb; (b) PL intensity of Tb emission at 545 nm versus the number of layers for the SiO2:Tb films.
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Fig. 2 (a) shows the excitation and emission spectra of a sample having sixteen layers of SiO2:Tb spin coated on the substrate. The optimum excitation wavelength of 226 nm is associated with the 4f-5d transition of Tb3+ ions [16]. The emission spectra showed the characteristic emission bands attributed to the 5D4-7FJ transitions (J = 6, 5, 4, 3) with the dominant green band of the 5D4-7F5 transition at 545 nm. There was also blue emission from 5D3-7FJ transitions in the wavelength region below 450 nm. During previous measurements using sol-gel powder samples [17] rather than thin films investigated here, the blue luminescence from 5D3-7FJ transitions of Tb3+ ions was not observed in samples having a relatively high Tb concentration of 1 mol%, which was suggested to be high enough to allow cross-relaxation energy transfer between Tb3+ ions which quenches emission from the upper 5D3 level (whereas for powder samples having a lower Tb concentration of 0.1 mol%, the Tb3+ ions were considered sufficiently separated to inhibit cross-relaxation and hence emission corresponding to 5D3-7FJ transitions was observed). In this study the initial concentration of Tb in the origin solution used for depositing the films was 2 mol%, but surprisingly the blue 5D3-7FJ emissions were still observed rather than being quenched due to crossrelaxation as expected. This interesting result suggests that cross-relaxation between Tb ions occurs less in the thin film samples than powder samples. This might be because in the thin film layers the Tb 3+ ions are distributed in one rather than three dimensions and hence have less close neighbours for cross-relaxation, or it may be that the dilution of the sol with ethanol and CTAB before spin coating produces a more homogeneous distribution of Tb 3+ ions compared to powder samples in which Tb 3+ ion clustering (a well-known issue for lanthanide ions doped in silica) might have occurred and thereby increased the cross-relaxation effect. Fig. 2 (b) illustrates the PL intensity of emission from Tb3+ ions versus the number of layers for the different SiO2:Tb thin films. The luminescence intensity from Tb3+ ions was found to increase monotonically with the number of deposited layers up to sixteen layers. It can be seen that the intensity has slightly increased from one to two layers, and then significantly increased from four to eight, while from eight to sixteen layers the increment rate was less. This suggests that the intensity will not be increased greatly more when the number of layers has increased above sixteen, which is expected because after a certain number of layers the excitation photons will not reach the bottom layers, therefore increasing the number of layers will not have a further effect on the intensity which would reach a saturated maximum level.
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The excitation and emission spectra of seven double-layers of alternating SiO2:Tb/SiO2:Ce are shown in Fig. 3 (a). The excitation spectrum monitored for Tb3+ emission at 545 nm has two peaks, one at 226 nm associated with the 4f-5d transition of Tb3+ ions (as in Fig. 2 (a)), and the other one is around 320 nm which is attributed to the 4f-5d transition of Ce3+ ions [18]. In such films consisting of alternating layers the Tb 3+ ions can be excited through Ce 3+ absorption as a result of energy transfer from Ce 3+ to Tb3+ ions. When exciting the Tb3+ via the Ce3+ absorption wavelength of 320 nm, as shown in Fig. 3 (a), the blue emission from the 5D3-7FJ transitions does not occur,
H.A.A. Seed Ahmed et al. / Materials Today: Proceedings 2 (2015) 4111 – 4117
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although it is still present when one excites the Tb3+ directly at 226 nm. Ntwaeaborwa et al. [10] reported that the energy transfer from Ce3+ to Tb3+ in silica involves interaction between the 5D3/2 state of Ce3+ and the 5D4 state of Tb3+ as suggested in the energy level scheme of Fig. 3 (b). Our results are in agreement with this proposal since when the Tb3+ ions are excited by energy transfer from the Ce 3+ ions no blue emission from the 5D3-7FJ transitions are observed, suggesting the 5D3 state of Tb3+ ions is not populated.
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The intensity of the green emission of Tb3+ ions at 545 nm after excitation via the Ce3+ ions at the more accessible wavelength of 320 nm as a function of the number of alternating SiO2:Tb/SiO2:Ce double-layers is shown in Fig. 4. The maximum intensity was reached for four double-layers and then decreased if more layers were added. To investigate this, depth profiles of the Tb and Ce distributions were made using TOF-SIMS. Fig. 5. (a) shows the ToF-SIMS 3D images for the overlay of measured TbO+ ions (brown) corresponding to Tb3+ and the measured CeO+ ions (green) corresponding to Ce3+ of a sample with four double-layers of SiO2:Tb/SiO2:Ce before annealing. The same sample after annealing in shown in Fig. 5 (b) while Fig. 5 (c) correspond to a thin film with seven double-layers of SiO2:Tb/SiO2:Ce after annealing. From the images of the four double-layers of SiO2:Tb/SiO2:Ce before and after annealing it is clear that the heat treatment caused the layers to diffuse into one another. A comparison of Fig. 5 (b) and (c) shows that the layers diffused much better in the case of four doublelayers compared to that of seven double-layers film. This explains why the thin film with four double-layers gave the maximum emission as seen in Fig. 4. The diffusion dynamics of these alternately layered thin films still requires further investigation. 4. Conclusion Thin films of different numbers of SiO2:Tb layers as well as alternating layers of SiO2:Tb and SiO2:Ce were successfully fabricated by the sol-gel spin coating method. The topography of the thin films was investigated by AFM showing smooth grains with an rms surface roughness between 0.4-1.0 nm. The luminescence intensity from Tb3+ was found to increase monotonically with the number of deposited layers up to sixteen layers. In those films consisting of alternating layers, the Tb3+ ions can be excited through Ce3+ absorption as a result of energy transfer from Ce3+ to Tb3+ ions. The green emission of Tb3+ ions reached a maximum intensity for four double-layers when exciting via the Ce3+ absorption and then decreased if more layers were added. ToF-SIMS 3D images showed that the layers were diffused in each other after annealing. The best diffusion of layers was detected for the four doublelayers film having the maximum emission.
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Fig. 5. ToF-SIMS 3D images for the overlay of measured TbO+ ions corresponding to Tb3+ (brown) and measured CeO+ ions corresponding to Ce3+ (green) for (a) four double-layers of SiO2:Tb/SiO2:Ce, unannealed; (b) four double-layers of SiO2:Tb/Ce, annealed; (c) seven double-layers of SiO2:Tb/SiO2:Ce film, annealed.
Acknowledgements This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation of South Africa. SEM images were obtained at the Centre of Microscopy at the University of the Free State for which the authors are grateful.
H.A.A. Seed Ahmed et al. / Materials Today: Proceedings 2 (2015) 4111 – 4117
References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. [2] D.S. Zang, J.H. Song, D.H. Park, Y.C. Kim, D.H. Yoon, J. Lumin. 129 (2009) 1088–1093. [3] M. Gedvilas, B. Voisiat, K. Regelskis, G. Račiukaitis, Thin Solid Films 571 (2014) 102–107. [4] H. Sakamoto, J. Qiu, A. Makishima, Sci. Technol. Adv. Mater. 4 (2003) 69–76. [5] Q. Guodong, W. Minquan, W. Mang, F. Xianping, H. Zhanglian, J. Lumin.75 (1997) 63–69. [6] E. Nakazawa, S. Shionoya, J. Chem. Phys. 47 (1967) 3211–3219. [7] K.G.Tshabalala, S.H. Cho, J.K. Park, S.S. Pitale, I.M. Nagpure, R.E. Kroon, H.C. Swart, O.M. Ntwaeaborwa J. Alloys Compd. 509 (2011) 10115–10120. [8] L. Xiou, Y. Xie, M. He, Y. Chen, W. Li, W. Yu, J. Rare Earths 28 (2010) 225–228. [9] F. Zhenxiao, B. Wenbo Solid State Sci. 10 (2008) 1062–1067. [10] O.M. Ntwaeaborwa, H.C. Swart, R.E. Kroon, P.H. Holloway, J.R. Botha, J.Phys. Chem. Solids 67 (2006) 1749–1753. [11] G.E. Malashkevich, G.I. Semkova, A.P. Stupak, A.V. Sukhodolov, Phys. Solid State 46 (2004) 1425–1431. [12] U. Caldiño, A. Speghini, F. Álvarez, S. Berneschi, M. Bettinelli, M. Brenci, G.C. Righini, Opt. Mat. 33 (2011) 1892–1897. [13] U. Caldiño, A. Speghini, M. Bettinelli J. Phys. Condens. Matter 18 (2006) 3499–3508. [14] H.A.A. Seed Ahmed, O.M. Ntwaeaborwa, R.E. Kroon, J. Lumin. 135 (2013) 15–19. [15] I.V. Schweigert, K.E.J. Lehtinen, M J. Carrier, M.R. Zachariah, Phys. Rev. B 65 (2002) 235410. [16] R.E. Kroon, H.A.A Seed Ahmed, M.A. Gusowski, 2011 in Proceedings of SAIP2011, the 56th Annual Conference of the South African Institute of Physics, edited by I. Basson and A.E. Botha (University of South Africa, Pretoria),162. [17] H.A.A. Seed Ahmed, O.M. Ntwaeaborwa, M.A. Gusowski, J.R. Botha, , R.E. Kroon, Physica B 407 (2012) 1653–1655. [18] M. Fasoli, A. Vedda, A. Lauria, F. Moretti, E. Rizzelli, N. Chiodini, F. Meinardi, M. Nikl J. Non-Cryst. Solids 355 (2009) 1140–1144.
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ScienceDirect Materials Today: Proceedings 2 (2015) 4111 – 4117
7th International Symposium On Macro- and Supramolecular Architectures and Materials
Luminescence of alternating SiO2:Tb and SiO2:Ce thin films produced by sol-gel spin coating H.A.A. Seed Ahmeda,b, A. Yousifa,b, H.C. Swarta, R.E. Kroona,* a
Department of Physics, University of the Free State, Bloemfontein ZA9300, South Africa b Department of Physics, University of Khartoum, Khartoum, Sudan
Abstract Thin films of different numbers of SiO2:Tb layers as well as alternating layers of SiO2:Tb and SiO2:Ce were fabricated by the solgel spin coating method in order to study the effect of Ce3+ on the Tb3+ emission. The prepared films were annealed at 1000°C in a H2/Ar reducing atmosphere to prevent the occurrence of the tetravalent state of the Ce ions. The topography of the films was investigated by atomic force microscopy and small empty areas were detected which were confirmed by scanning electron microscopy. The luminescence intensity from Tb3+ was found to increase monotonically with the number of deposited layers up to sixteen layers. In the films consisting of alternating layers the Tb3+ ions could be excited through Ce3+ absorption as a result of energy transfer from Ce3+ to Tb3+ ions. When exciting the Tb3+ ions via the Ce3+ ion absorption wavelength of 320 nm, the green emission at 545 nm reached a maximum intensity for four double-layers and then decreased if more layers were added. 3D images obtained by time-of-flight secondary ion mass spectroscopy showed the influence of the heating process on the layers of the thin films. © AllAll rights reserved. © 2015 2014Elsevier ElsevierLtd. Ltd. rights reserved. Selection under responsibility of theofConference Committee Members of 7th International Symposium on Selectionand andpeer-review peer-review under responsibility the Conference Committee Members of 7th International Symposium on MacroArchitectures and and Materials. Macro-and andSupramolecular Supramolecular Architectures Materials. Keywords: Luminescence; Thin films; Sol-gel; Spin coating; SiO2; Tb; Ce
* Corresponding author. Tel.: +27-51-4012884; fax: +27-51-4013507. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of 7th International Symposium on Macro- and Supramolecular Architectures and Materials. doi:10.1016/j.matpr.2015.08.041
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1. Introduction Luminescence materials or phosphors have potential applications in modern technologies such as lighting and displays [1,2]. Phosphors are usually in the form of powders or thin films. However, thin films are widely used due to their several advantages and unique properties [3]. Thin films are formed by using various techniques, among which the sol-gel method combined with spin coating deposition is considered to be one of the simplest and most economical [4]. Sol-gel silica (SiO2) has been widely used as a host material for lanthanide ions because it has good physical properties and chemical stability [5]. Energy transfer can play an important role in phosphor materials as it can be used to enhance the luminescence efficiency [6]. Ce 3+ ions are an efficient sensitizer for Tb3+ ions and their co-doping has been studied in a variety of hosts [7-13]. Tb3+ ions are used as the activator because their bright green emission is suitable for many applications. Although Tb3+ ions can be excited efficiently through their allowed 4f-5d transition, it can be advantageous to use Ce3+ ions as a sensitizer because their allowed 4f-5d transition occurs at a longer (and therefore generally more accessible and convenient) wavelength. Recently, we reported on the high efficiency energy transfer from Ce3+ to Tb3+ as a result of incorporating the two ions in silica [14]. In the present work, we achieved the same result by depositing alternating thin film layers of SiO 2:Tb and SiO2:Ce instead of incorporating the two ions directly in the same host. 2. Experimental Thin films of different numbers of SiO2:Tb layers as well as alternating layers of SiO2:Tb and SiO2:Ce were fabricated by the sol-gel spin coating method. Solutions of SiO2:Tb and SiO2:Ce were produced by allowing tetraethylorthosilicate (TEOS) to react with water. Ethanol was used as a solvent and nitric acid was added to catalyze the reaction. Tb or Ce nitrate was added for doping. The molar ratio of TEOS:water:ethanol was 1:5:10, and the HNO3 concentration was 0.15 M in water. The dopant concentration of Tb or Ce was 2 mol% of the Si content. TEOS was mixed with ethanol and stirred for 30 min, after which the acidified water was added and stirring continued for another 30 min. After that Tb or Ce nitrate dissolved in a little ethanol was added to the mixture which was then stirred for a further 4 h. To fabricate thin films, each ml of the SiO2:Tb and SiO2:Ce sols to be used was first diluted by 20 ml of ethanol having 0.5 mmol of CTAB dissolved in it, to control the viscosity. Thin films were deposited layer by layer on Si(111) substrates. The substrates were cleaned ultrasonically using acetone, methanol and distilled water. An SPS SPIN 150 spin coater from Semiconductor Production Systems was used to produce the thin films. For each layer, a small amount of the solution was dropped onto the substrate which was then spun at 3000 rpm for 40 s, followed by drying on a hot plate. The prepared films were annealed at 1000°C in a H2/Ar reducing atmosphere to prevent the occurrence of the tetravalent state of the Ce ions. The surface topography was examined using a Shimadzu SPM-9600 atomic force microscopy (AFM). The scanning electron microscopy (SEM) of the thin films was performed using a JSM-7800F microscope. Photoluminescence (PL) measurements were made at room temperature using a Cary Eclipse fluorescence spectrophotometer equipped with a xenon lamp. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements were performed with a TOF-SIMS5 instrument from ION-TOF using a 1 kV oxygen sputter gun for depth profiling. 3. Results and Discussion The surface topography of the thin films was investigated by the AFM technique. The images for all the films showed almost the same topography, having smooth grains with an rms surface roughness between 0.4-1.0 nm. Fig. 1(a) and (b) show the AFM image of four double-layers of SiO2:Tb/SiO2:Ce film before and after annealing, respectively. The surface texture of the film after annealing was smoother compared to the film before annealing which can be attributed to stress which the film may be experienced during heating. AFM images of the films show some uncovered circular holes distributed randomly on the surface, which were confirmed by SEM images as well (see Fig. 1(c) and (d)). These holes can be ascribed due to empty uncovered areas, possibly as a result of tiny bubbles in the diluted sol material used for spin coating.
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Fig. 1. (a) AFM image of a thin film comprising four double-layers of SiO2:Tb/SiO2:Ce before annealing. (b) AFM image of the same thin film after annealing. (c) and (d) show the SEM images of the thin films in (a) and (b) respectively.
Fig. 1 (c) and (d) show SEM images of four double-layer thin films of SiO2:Tb/SiO2:Ce before and after annealing, respectively. These show that the holes increased in size during annealing. In addition the SEM image of the annealed film shows small white dots which are not present in the unannealed films. These white dots are silica clusters which are known to form at high temperatures [15].
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Fig. 2 (a) shows the excitation and emission spectra of a sample having sixteen layers of SiO2:Tb spin coated on the substrate. The optimum excitation wavelength of 226 nm is associated with the 4f-5d transition of Tb3+ ions [16]. The emission spectra showed the characteristic emission bands attributed to the 5D4-7FJ transitions (J = 6, 5, 4, 3) with the dominant green band of the 5D4-7F5 transition at 545 nm. There was also blue emission from 5D3-7FJ transitions in the wavelength region below 450 nm. During previous measurements using sol-gel powder samples [17] rather than thin films investigated here, the blue luminescence from 5D3-7FJ transitions of Tb3+ ions was not observed in samples having a relatively high Tb concentration of 1 mol%, which was suggested to be high enough to allow cross-relaxation energy transfer between Tb3+ ions which quenches emission from the upper 5D3 level (whereas for powder samples having a lower Tb concentration of 0.1 mol%, the Tb3+ ions were considered sufficiently separated to inhibit cross-relaxation and hence emission corresponding to 5D3-7FJ transitions was observed). In this study the initial concentration of Tb in the origin solution used for depositing the films was 2 mol%, but surprisingly the blue 5D3-7FJ emissions were still observed rather than being quenched due to crossrelaxation as expected. This interesting result suggests that cross-relaxation between Tb ions occurs less in the thin film samples than powder samples. This might be because in the thin film layers the Tb 3+ ions are distributed in one rather than three dimensions and hence have less close neighbours for cross-relaxation, or it may be that the dilution of the sol with ethanol and CTAB before spin coating produces a more homogeneous distribution of Tb 3+ ions compared to powder samples in which Tb 3+ ion clustering (a well-known issue for lanthanide ions doped in silica) might have occurred and thereby increased the cross-relaxation effect. Fig. 2 (b) illustrates the PL intensity of emission from Tb3+ ions versus the number of layers for the different SiO2:Tb thin films. The luminescence intensity from Tb3+ ions was found to increase monotonically with the number of deposited layers up to sixteen layers. It can be seen that the intensity has slightly increased from one to two layers, and then significantly increased from four to eight, while from eight to sixteen layers the increment rate was less. This suggests that the intensity will not be increased greatly more when the number of layers has increased above sixteen, which is expected because after a certain number of layers the excitation photons will not reach the bottom layers, therefore increasing the number of layers will not have a further effect on the intensity which would reach a saturated maximum level.
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Wavelength (nm) Fig. 3. (a) The excitation and emission spectra of seven double-layers of alternating SiO2:Tb and SiO2:Ce; (b) Schematic energy levels of the Ce3+ and Tb3+ ions showing the energy transfer mechanism [10].
The excitation and emission spectra of seven double-layers of alternating SiO2:Tb/SiO2:Ce are shown in Fig. 3 (a). The excitation spectrum monitored for Tb3+ emission at 545 nm has two peaks, one at 226 nm associated with the 4f-5d transition of Tb3+ ions (as in Fig. 2 (a)), and the other one is around 320 nm which is attributed to the 4f-5d transition of Ce3+ ions [18]. In such films consisting of alternating layers the Tb 3+ ions can be excited through Ce 3+ absorption as a result of energy transfer from Ce 3+ to Tb3+ ions. When exciting the Tb3+ via the Ce3+ absorption wavelength of 320 nm, as shown in Fig. 3 (a), the blue emission from the 5D3-7FJ transitions does not occur,
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although it is still present when one excites the Tb3+ directly at 226 nm. Ntwaeaborwa et al. [10] reported that the energy transfer from Ce3+ to Tb3+ in silica involves interaction between the 5D3/2 state of Ce3+ and the 5D4 state of Tb3+ as suggested in the energy level scheme of Fig. 3 (b). Our results are in agreement with this proposal since when the Tb3+ ions are excited by energy transfer from the Ce 3+ ions no blue emission from the 5D3-7FJ transitions are observed, suggesting the 5D3 state of Tb3+ ions is not populated.
Luminescence intensity (a. u.)
900 800 700 600 500 400 300 200 100 0
0
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5
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No of alternating layers Fig. 4. PL intensity of Tb3+ emission at 545 nm versus the number of alternating layers for the different SiO2:Tb and SiO2:Ce, for excitation via the Ce3+ ions at the more accessible wavelength of 320 nm.
The intensity of the green emission of Tb3+ ions at 545 nm after excitation via the Ce3+ ions at the more accessible wavelength of 320 nm as a function of the number of alternating SiO2:Tb/SiO2:Ce double-layers is shown in Fig. 4. The maximum intensity was reached for four double-layers and then decreased if more layers were added. To investigate this, depth profiles of the Tb and Ce distributions were made using TOF-SIMS. Fig. 5. (a) shows the ToF-SIMS 3D images for the overlay of measured TbO+ ions (brown) corresponding to Tb3+ and the measured CeO+ ions (green) corresponding to Ce3+ of a sample with four double-layers of SiO2:Tb/SiO2:Ce before annealing. The same sample after annealing in shown in Fig. 5 (b) while Fig. 5 (c) correspond to a thin film with seven double-layers of SiO2:Tb/SiO2:Ce after annealing. From the images of the four double-layers of SiO2:Tb/SiO2:Ce before and after annealing it is clear that the heat treatment caused the layers to diffuse into one another. A comparison of Fig. 5 (b) and (c) shows that the layers diffused much better in the case of four doublelayers compared to that of seven double-layers film. This explains why the thin film with four double-layers gave the maximum emission as seen in Fig. 4. The diffusion dynamics of these alternately layered thin films still requires further investigation. 4. Conclusion Thin films of different numbers of SiO2:Tb layers as well as alternating layers of SiO2:Tb and SiO2:Ce were successfully fabricated by the sol-gel spin coating method. The topography of the thin films was investigated by AFM showing smooth grains with an rms surface roughness between 0.4-1.0 nm. The luminescence intensity from Tb3+ was found to increase monotonically with the number of deposited layers up to sixteen layers. In those films consisting of alternating layers, the Tb3+ ions can be excited through Ce3+ absorption as a result of energy transfer from Ce3+ to Tb3+ ions. The green emission of Tb3+ ions reached a maximum intensity for four double-layers when exciting via the Ce3+ absorption and then decreased if more layers were added. ToF-SIMS 3D images showed that the layers were diffused in each other after annealing. The best diffusion of layers was detected for the four doublelayers film having the maximum emission.
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a
b
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Fig. 5. ToF-SIMS 3D images for the overlay of measured TbO+ ions corresponding to Tb3+ (brown) and measured CeO+ ions corresponding to Ce3+ (green) for (a) four double-layers of SiO2:Tb/SiO2:Ce, unannealed; (b) four double-layers of SiO2:Tb/Ce, annealed; (c) seven double-layers of SiO2:Tb/SiO2:Ce film, annealed.
Acknowledgements This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation of South Africa. SEM images were obtained at the Centre of Microscopy at the University of the Free State for which the authors are grateful.
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