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Greatly improved photoluminescence properties of the electrodeposited Y2O3:Eu3+ thin film phosphors by the addition of Na+ and K+ ions
Thin Solid Films 520 (2011) 174–178
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Greatly improved photoluminescence properties of the electrodeposited Y2O3:Eu 3+ thin film phosphors by the addition of Na + and K + ions L. Wang, N. Liao, H. Zeng, L. Shi, H. Jia, N. Wang, S. Guo, D. Jin ⁎ Materials Engineering Center, Zhejiang Sci-Tech University, Hangzhou 310018, P.R. China
a r t i c l e
i n f o
Article history: Received 25 October 2010 Received in revised form 1 July 2011 Accepted 1 July 2011 Available online 8 July 2011 Keywords: Y2O3:Eu3+ Thin film phosphors Electro-deposition Sodium Potassium Photoluminescence Microstructure
a b s t r a c t In this work, Y2O3:Eu3+ thin film phosphors were prepared by electro-deposition method. The effect of Na+ and K+ ions on the photoluminescence properties of Y2O3:Eu3+ thin film phosphor was studied in details. It was found that the addition of Na+ and K+ ions could improve the photoluminescence intensity by 3 to 4 times. The highly improved photoluminescence intensity may be caused by different factors. The improved crystallinity and the increased optical volume caused by the flux effect of Na+ and K+ ions could be the major reasons for the enhanced photoluminescence intensity. It was also found that the average lifetime of Y2O3:Eu3+ thin film phosphors could be adjusted by the molar amount of Na+ and K+ ions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Due to the special electronic structure, rare earth (RE) elements, such as europium [1–3], terbium [4–6], and cerium [7,8] have been used as activators or sensitizers for the luminescent materials. Among the RE elements, Eu3+ is one of the mostly used activators for the red phosphors, such as Y2O3:Eu 3+[9,10], Gd2O3:Eu 3+[11,12], YVO4: Eu 3+[13,14], and so on. As the development of the field emission displays devices, thin film phosphor (TFP) has drawn enormous interests. Different methods, such as pulsed laser deposition [15–17], chemical vapor deposition [18,19], and spray-pyrolysis [20–22] were employed to prepare the RE doped TFP. Up to now, there are very few reports on the preparation of TFP by the electro-deposition method [23]. However, the photoluminescence intensity of the electrodeposited Y2O3:Eu 3+ is relatively low. Similar problems were also found in the Y2O3:Eu3+ TFP prepared by other methods. Great efforts were dedicated to improve the photoluminescence efficiency and intensity of Eu3+ activated TFP. Lee et al. reported the improvement of the extraction efficiency by patterning the Y2O3: Eu 3+ TFP surface with polystyrene nanospheres [24]. Yi et al. reported Li+ ions have great effect on the photoluminescence properties of Gd2O3:Eu3+ phosphors [25]. Cho et al. analyzed the factors governing the enhanced photoluminescence of Li+ doped Y2O3:Eu 3+ phosphors [26]. Liu et al. studied the effect of Li+, K +, Na +, Al3+and other metal
⁎ Corresponding author. Tel.: + 86 571 86843265; fax: + 86 571 86843266. E-mail address: [email protected] (D. Jin). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.07.002
ions on the photoluminescence properties of GdTaO4:Eu 3+ phosphors [27]. In this work, Y2O3:Eu 3+ TFP was prepared by the electro-deposition method. Na + and K + ions were added individually into the Y2O3:Eu3+ TFP. After the addition of Na+ and K + ions, the crystal structure of the Y2O3:Eu 3+ TFP remained cubic while the crystallinity and the optical volume of the Y2O3:Eu3+ TFP were greatly increased. Photoluminescence characterization showed that the addition of Na + and K + ions improved the photoluminescence intensity of Y2O3:Eu 3+ TFP by 3 to 4 times, which is different from the results achieved by Liu et al. [27]. As in Liu's work, the addition of Na+ and K + ions showed very limited improvement of the photoluminescence intensity. 2. Experimental details Y(NO3)3 and Eu(NO3)3 solutions with concentration of 0.1 M were used as the precursors for the synthesis of Y2O3:Eu 3+ TFP. According to the previous work [23], the volume ratio of Y(NO3)3 solution to Eu (NO3)3 solution was fixed as 10:1. NaNO3 and KNO3 solutions with a concentration of 0.1 M were used for the doping. All the chemicals were of analytical grade and used without further purification. Electro-deposition of Y2O3:Eu3+ TFP was performed in a 3-electrode cell. The working electrode was indium tin oxide (ITO) coated glass (sheet resistance: 20 Ω/□), the counter electrode was platinum foil and the reference electrode was Ag/AgCl/saturated KCl. Before the deposition, both ITO glass and Pt foil were degreased in acetone and washed by deionized water. The applied deposition potential was set as −1.2 V. The bath temperature was 60 °C and the deposition time for each
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sample was 30 min. After the deposition, all the films were cleaned and dried in air at 60 °C for 2 h. Doping of Na+ ion was performed by dripping certain molar amount of NaNO3 solution on the surface of the as-prepared films. Then, Y2O3:Eu3+ TFP doped with Na+ was annealed in air at 600 °C for 2 h. K + ions were doped in the Y2O3:Eu 3+ TFP by the same procedure as that of Na + ions. The structural properties of all the samples were characterized by XRD (X-ray diffraction) and SEM (scanning electron microscopy). XRD was carried out by a Thermo ARL X'TRA X-ray powder diffractometer with Cu Kα radiation (1.54 Å). The voltage and current applied were 40 kV and 30 mA, respectively. The scanning speed was 2°/min. Surface morphology of Y2O3:Eu 3+ film was characterized by SEM with incident electron energy of 5 kV–15 kV. Photoluminescence spectra were taken at room temperature on a Hitachi F-4600 fluorescence-spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The wavelength of the excitation radiation was 235 nm. 3. Results and discussions Y2O3:Eu 3+ TFPs were synthesized by a two-step process: formations of Y(OH)3 and Eu(OH)3 followed by an annealing process, which was discussed in the previous work [23]. 3.1. Structural properties XRD was used to characterize the structural properties of the electrodeposited Y2O3:Eu 3+ TFP. The typical XRD patterns of Y2O3:Eu 3+ TFP with and without Na + and K+ were shown in Fig. 1(a). All the diffraction peaks could be indexed to the cubic phase of yttrium oxide (JCPDS No. 43-1036). There is no trace of other phases in the diffraction patterns, which indicates that Na + and K+ ions were incorporated into the Y2O3 lattice. The diffraction intensity and the FWHM (full width at half maximum) of the diffraction peaks can be obtained from Fig. 1(a). The strongest (222) diffraction peak was taken as an example, and the diffraction intensity and the FWHM for those samples were calculated and plotted in Fig. 1(b). Apparently, Y2O3:Eu3+ TFP with Na+ and K + ions have much higher diffraction intensity and smaller FWHM, which indicates the improvement of the crystallinity of Y2O3:Eu3+ TFPs. As believed, the highly improved crystallinity was caused by the flux effect of the Na+ and K + ions. SEM images of the Y2O3:Eu3+ TFPs: (a) pure Y2O3:Eu 3+ film, (b) with 0.1 mmol Na+, and (c) with 0.1 mmol K + are shown in Fig. 2. Apparently, all the TFPs have a similar plate-like surface morphology. However, the plate thicknesses are different for the given samples. The plate thickness of the pure Y2O3:Eu 3+ film is around 11 nm, while the plate thicknesses of the Y2O3:Eu 3+ film with Na + and K + ions are 23 nm and 19 nm, respectively. Meanwhile, the thickness of the Y2O3:Eu 3+ film also changes as Na+ and K + were added. The thicknesses of the three given samples are around 6.86 μm, 11.25 μm, and 10.6 μm, respectively. Apparently, as Na + and K+ ions were added into the Y2O3:Eu3+ film, the film thickness increased, i.e., the optical volume increased, which could be important for the photoluminescence properties of the Y2O3:Eu 3+ TFP. 3.2. Photoluminescence properties The effect of Na + and K + ions on the photoluminescence properties of Y2O3:Eu3+ TFP was studied in details. The emission spectra of the Y2O3:Eu3+ samples: (a) pure Y2O3:Eu 3+ film, (b) with 0.1 mmol Na+, and (c) with 0.1 mmol K+ are shown in Fig. 3. There are totally three emission bands corresponding to the 5D0– 7F1, 5D0– 7F2 and 5D0– 7F3 transitions of Eu 3+ ions, respectively. Emission from the parity forbidden electric dipole 5D0– 7F2 transition has stronger intensity than that from the magnetic dipole transition 5D0–7F1 indicates that Eu 3+ is located at the site with no inversion symmetry in Y2O3 lattice (C2 site) [28]. The emission bands do not shift as Na + and K + ions were
Fig. 1. (a) XRD patterns of the Y2O3:Eu3+ TFPs without doping, with 0.1 mmol Na+, and with 0.1 mmol K+ ions, and (b) the calculated FWHM and diffraction intensity of the (222) diffraction peak of the Y2O3:Eu3+ TFP without doping, with 0.1 mmol Na+, and with 0.1 mmol K+ ions.
added, which may be due to the shield effect of outer shell electrons. However, the photoluminescence intensity of the Y2O3:Eu 3+ TFP was improved by 3 to 4 times due to the addition of Na + and K + ions. The effect of the amount of Na+ and K + ions on the photoluminescence properties of the Y2O3:Eu3+ TFP was also studied and results are shown in Fig. 4. Apparently, the photoluminescence intensity of Y2O3:Eu3+ TFP varies with the molar amount of Na+ and K+ ions and the optimum molar amount for both Na+ and K + ions is 0.1 mmol. The origin of the greatly improved photoluminescence intensity of Y2O3:Eu 3+ TFP with Na + and K + is complicated. There are several factors may contribute to the improvement of the photoluminescence intensity. However, it is difficult to distinguish one from the other. The first and the most possible factor is the crystallinity of the film. As indicated by the XRD results, the crystallinity of the Y2O3:Eu 3+ TFP was greatly improved by the addition of Na + and K + ions. The flux effect of Na + and K + ions gives larger grain size, better crystallinity and lower concentration of defects, which results in the longer oscillation length for the optical transition and the reduction of luminescence quenching caused by the surface states [26,29,30]. The second factor could be the increase in the optical volume. It is well known that the optical volume is an important factor in the photoluminescence intensity increase. As reported, the photoluminescence intensity of TFP increased with increases in the thickness of the film to a certain thickness, beyond which the emission intensity is a constant [31]. To verify the effect of the optical volume on the photoluminescence properties, the Y2O3:Eu 3+ TFPs: (a) pure Y2O3: Eu 3+ film, (b) with 0.1 mmol Na +, and (c) with 0.1 mmol K + with
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Fig. 3. The photoluminescence emission spectra of the Y2O3:Eu3+ TFP: pure Y2O3:Eu3+, with 0.1 mmol Na+, and with 0.1 mmol K+.
The decay curves can not be fit by a single exponential function, but can be well fit by a second-order exponential function in a form: t t I = A1 exp − + A2 exp − : τ1 τ2
ð1Þ
Fig. 2. SEM images of the Y2O3:Eu3+ TFPs: (a) pure Y2O3:Eu3+, (b) with 0.1 mmol Na+, and (c) with 0.1 mmol K+.
similar thickness were prepared and studied. As shown in Fig. 5, with similar thickness, the photoluminescence intensity of Y2O3:Eu 3+:Na + (0.1 mmol) and Y2O3:Eu 3+:K + (0.1 mmol) was increased by 2 times. The result indicates that the film thickness, i.e., the optical volume, is also an important factor for the highly improved photoluminescence. There are also some other factors may cause the improvement of photoluminescence properties. Such as the oxygen vacancies created by the addition of Na + and K + ions [32,33] and the change of lattice symmetry caused by the addition of Na + and K + ions. The photoluminescence decay curves of the 5D0–7F2 transition of 3+ Eu ions in the Y2O3:Eu3+ TFP with and without Na + and K + ions were characterized and results are shown in Figs. 6 and 7, respectively.
Fig. 4. Photoluminescence Intensity of the 5D0–7F2 transition as a function of molar amount of (a) Na+ and (b) K+ in the Y2O3:Eu3+ TFP.
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Fig. 5. The photoluminescence emission spectra of the Y2O3:Eu3+ TFP with similar thickness (around 6.86 μm): pure Y2O3:Eu3+, with 0.1 mmol Na+, and with 0.1 mmol K+.
Fig. 7. (a) Decay curves of the 5D0–7F2 transition of the Y2O3:Eu3+ TFP with and without K+, and (b) The average lifetime of the 5D0–7F2 transition as a function of molar amount of K+.
The average lifetime τ is defined according to the following equation [34]: 2 2 τ = A1 τ1 + A2 τ2 = A1 τ1 + A2 τ2 :
ð2Þ
In Figs. 6(b) and 7(b), the average lifetime is plotted as a function of the molar amount of Na+ and K + ions. As shown, the average lifetime of Y2O3:Eu 3+ TFP varies with the molar amount of Na+ and K + ions. The concentration of surface states, which act as quenching centers, may decrease and give rise to the average lifetime for the Y2O3:Eu3+ TFP with Na+ and K + ions. On the other hand, the average lifetime varies with the molar amount of Na+ and K + ions. It can be ascribed to the variation of radiative transition rate caused by the addition of Na+ and K + ions [35]. 4. Conclusions
Fig. 6. (a) Decay curves of the 5D0–7F2 transition of the Y2O3:Eu3+ TFP with and without Na+, and (b) the average lifetime of the 5D0–7F2 transition as a function of the molar amount of Na+.
Y2O3:Eu3+ thin film phosphors were achieved by an electrodeposition process followed by an annealing process. The effect of Na+ and K+ ions on the photoluminescence properties was studied. It is found that the photoluminescence intensity of the Y2O3:Eu 3+ TFP was improved by 3 to 4 times as Na + and K + ions were added. The highly enhanced photoluminescence intensity of the Y2O3:Eu 3+ TFP may be mainly caused by a combined effect of the improved crystallinity and the increase in optical volume. Factors, such as the concentration of oxygen
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vacancies and the change of lattice symmetry, may also improve the photoluminescence intensity. It is also found that the average lifetime of the 5D0– 7F2 transition increased as Na + and K+ ions were added and varied with the molar amount of Na + and K + ions, which may be caused by the variation of the surface states concentration and the radiative transition rate. Acknowledgments This work is supported by the National Science Foundation of China under Grant 60906031, the Zhejiang Provincial Natural Science Foundation of China under Grant Y1090742, the Qianjiang Talent Programme of Zhejiang Province under grant 2009R10018, the Scientific Research Fund of Zhejiang Provincial Education Department under Grant 20070361, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry) under Grant 111383A4C08690. References [1] [2] [3] [4] [5]
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[9] S.H. Shin, J.H. Kang, D.Y. Jeon, D.S. Zang, J. Lumin. 114 (2005) 275. [10] A. Konrad, J. Almanstotter, J. Reichardt, A. Gahn, R. Tidecks, K. Samwer, J. Appl. Phys. 85 (1999) 1796. [11] J. Park, H. Moon, D. Kim, S. Byeon, B. Kim, K. Suh, Appl. Phys. Lett. 77 (2000) 2162. [12] J.K. Park, A.M. Park, C.H. Kim, H.D. Park, S.Y. Choi, J. Electrochem. Soc. 150 (2003) H27. [13] S.Z. Toma, F.F. Mikus, J.E. Mathers, J. Electrochem. Soc. 114 (1967) 953. [14] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118. [15] K.S. Sohn, D.H. Park, J.G. Yoo, J.S. Kim, Jpn. J. Appl. Phys. 44 (2005) 1787. [16] Y. Fu, G. Zhang, Z. Qi, H. Zhou, C. Shi, J. Lumin. 119–120 (2006) 581. [17] G.A. Hirata, O.A. Lopez, L.E. Shea, J.Y. Yi, T. Cheeks, J. McKittrick, J. Siqueiros, M. Avalos-Borja, A. Esparza, C. Falcony, J. Vac. Sci. Technol. A 14 (1996) 1694. [18] A. Mikami, K. Terada, M. Yoshida, S. Nakajima, J. Cryst. Growth 117 (1992) 991. [19] K. Hirabayashi, H. Kozawaguchi, B. Tsujiyama, Jpn. J. Appl. Phys. 27 (1988) 587. [20] J. Hao, S.A. Studenikin, M. Cocivera, J. Lumin. 93 (2001) 313. [21] M. Garcia-Hipolito, R. Martinez, O. Alvarez-Fregoso, E. Martineza, C. Falcony, J. Lumin. 93 (2001) 9. [22] A.E. Esparza-Garcia, M. Garcia-Hipolito, M.A. Aguilar-Frutis, C. Falcony, J. Electrochem. Soc. 150 (2003) H53. [23] L. Wang, H. Jia, X. Yu, Y. Zhang, P. Du, Z. Xi, D. Jin, Electrochem. Solid-State Lett. 12 (2009) E20. [24] Y.K. Lee, J.R. Oh, Y.R. Do, Appl. Phys. Lett. 91 (2007) 041907. [25] S. Yi, J.S. Bae, K.S. Shim, J.H. Jeong, J. Park, P.H. Holloway, Appl. Phys. Lett. 84 (2004) 353. [26] J.Y. Cho, Y.R. Do, Y. Huh, Appl. Phys. Lett. 89 (2006) 131915. [27] X. Liu, K. Han, M. Gu, L. Xiao, C. Ni, S. Huang, B. Liu, Solid State Commun. 142 (2007) 680. [28] G. Blasse, A. Bril, W.C. Nieuwpoort, J. Phys. Chem. Solids 27 (1966) 1587. [29] J.S. Bae, J.H. Jeong, K.S. Shim, B.K. Moon, S. Yi, J.H. Kim, Y.S. Kim, J.S. Lee, Appl. Surf. Sci. 252 (2006) 4564. [30] K.C. Misbra, J.K. Berkowitz, K.H. Johnson, P.C. Schmidt, Phys. Rev. B 45 (1992) 10902. [31] P. Rao, Solid State Commun. 99 (1996) 439. [32] J.K. Park, J.M. Kim, K.N. Kim, C.H. Kim, H.D. Park, Electrochem. Solid-State Lett. 7 (2005) H39. [33] O.A. Lopez, J. McKittrick, L.E. Shea, J. Lumin. 71 (1997) 1. [34] C.K. Lin, M.L. Pang, M. Yu, J. Lin, J. Lumin. 114 (2005) 299. [35] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Greatly improved photoluminescence properties of the electrodeposited Y2O3:Eu 3+ thin film phosphors by the addition of Na + and K + ions L. Wang, N. Liao, H. Zeng, L. Shi, H. Jia, N. Wang, S. Guo, D. Jin ⁎ Materials Engineering Center, Zhejiang Sci-Tech University, Hangzhou 310018, P.R. China
a r t i c l e
i n f o
Article history: Received 25 October 2010 Received in revised form 1 July 2011 Accepted 1 July 2011 Available online 8 July 2011 Keywords: Y2O3:Eu3+ Thin film phosphors Electro-deposition Sodium Potassium Photoluminescence Microstructure
a b s t r a c t In this work, Y2O3:Eu3+ thin film phosphors were prepared by electro-deposition method. The effect of Na+ and K+ ions on the photoluminescence properties of Y2O3:Eu3+ thin film phosphor was studied in details. It was found that the addition of Na+ and K+ ions could improve the photoluminescence intensity by 3 to 4 times. The highly improved photoluminescence intensity may be caused by different factors. The improved crystallinity and the increased optical volume caused by the flux effect of Na+ and K+ ions could be the major reasons for the enhanced photoluminescence intensity. It was also found that the average lifetime of Y2O3:Eu3+ thin film phosphors could be adjusted by the molar amount of Na+ and K+ ions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Due to the special electronic structure, rare earth (RE) elements, such as europium [1–3], terbium [4–6], and cerium [7,8] have been used as activators or sensitizers for the luminescent materials. Among the RE elements, Eu3+ is one of the mostly used activators for the red phosphors, such as Y2O3:Eu 3+[9,10], Gd2O3:Eu 3+[11,12], YVO4: Eu 3+[13,14], and so on. As the development of the field emission displays devices, thin film phosphor (TFP) has drawn enormous interests. Different methods, such as pulsed laser deposition [15–17], chemical vapor deposition [18,19], and spray-pyrolysis [20–22] were employed to prepare the RE doped TFP. Up to now, there are very few reports on the preparation of TFP by the electro-deposition method [23]. However, the photoluminescence intensity of the electrodeposited Y2O3:Eu 3+ is relatively low. Similar problems were also found in the Y2O3:Eu3+ TFP prepared by other methods. Great efforts were dedicated to improve the photoluminescence efficiency and intensity of Eu3+ activated TFP. Lee et al. reported the improvement of the extraction efficiency by patterning the Y2O3: Eu 3+ TFP surface with polystyrene nanospheres [24]. Yi et al. reported Li+ ions have great effect on the photoluminescence properties of Gd2O3:Eu3+ phosphors [25]. Cho et al. analyzed the factors governing the enhanced photoluminescence of Li+ doped Y2O3:Eu 3+ phosphors [26]. Liu et al. studied the effect of Li+, K +, Na +, Al3+and other metal
⁎ Corresponding author. Tel.: + 86 571 86843265; fax: + 86 571 86843266. E-mail address: [email protected] (D. Jin). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.07.002
ions on the photoluminescence properties of GdTaO4:Eu 3+ phosphors [27]. In this work, Y2O3:Eu 3+ TFP was prepared by the electro-deposition method. Na + and K + ions were added individually into the Y2O3:Eu3+ TFP. After the addition of Na+ and K + ions, the crystal structure of the Y2O3:Eu 3+ TFP remained cubic while the crystallinity and the optical volume of the Y2O3:Eu3+ TFP were greatly increased. Photoluminescence characterization showed that the addition of Na + and K + ions improved the photoluminescence intensity of Y2O3:Eu 3+ TFP by 3 to 4 times, which is different from the results achieved by Liu et al. [27]. As in Liu's work, the addition of Na+ and K + ions showed very limited improvement of the photoluminescence intensity. 2. Experimental details Y(NO3)3 and Eu(NO3)3 solutions with concentration of 0.1 M were used as the precursors for the synthesis of Y2O3:Eu 3+ TFP. According to the previous work [23], the volume ratio of Y(NO3)3 solution to Eu (NO3)3 solution was fixed as 10:1. NaNO3 and KNO3 solutions with a concentration of 0.1 M were used for the doping. All the chemicals were of analytical grade and used without further purification. Electro-deposition of Y2O3:Eu3+ TFP was performed in a 3-electrode cell. The working electrode was indium tin oxide (ITO) coated glass (sheet resistance: 20 Ω/□), the counter electrode was platinum foil and the reference electrode was Ag/AgCl/saturated KCl. Before the deposition, both ITO glass and Pt foil were degreased in acetone and washed by deionized water. The applied deposition potential was set as −1.2 V. The bath temperature was 60 °C and the deposition time for each
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sample was 30 min. After the deposition, all the films were cleaned and dried in air at 60 °C for 2 h. Doping of Na+ ion was performed by dripping certain molar amount of NaNO3 solution on the surface of the as-prepared films. Then, Y2O3:Eu3+ TFP doped with Na+ was annealed in air at 600 °C for 2 h. K + ions were doped in the Y2O3:Eu 3+ TFP by the same procedure as that of Na + ions. The structural properties of all the samples were characterized by XRD (X-ray diffraction) and SEM (scanning electron microscopy). XRD was carried out by a Thermo ARL X'TRA X-ray powder diffractometer with Cu Kα radiation (1.54 Å). The voltage and current applied were 40 kV and 30 mA, respectively. The scanning speed was 2°/min. Surface morphology of Y2O3:Eu 3+ film was characterized by SEM with incident electron energy of 5 kV–15 kV. Photoluminescence spectra were taken at room temperature on a Hitachi F-4600 fluorescence-spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The wavelength of the excitation radiation was 235 nm. 3. Results and discussions Y2O3:Eu 3+ TFPs were synthesized by a two-step process: formations of Y(OH)3 and Eu(OH)3 followed by an annealing process, which was discussed in the previous work [23]. 3.1. Structural properties XRD was used to characterize the structural properties of the electrodeposited Y2O3:Eu 3+ TFP. The typical XRD patterns of Y2O3:Eu 3+ TFP with and without Na + and K+ were shown in Fig. 1(a). All the diffraction peaks could be indexed to the cubic phase of yttrium oxide (JCPDS No. 43-1036). There is no trace of other phases in the diffraction patterns, which indicates that Na + and K+ ions were incorporated into the Y2O3 lattice. The diffraction intensity and the FWHM (full width at half maximum) of the diffraction peaks can be obtained from Fig. 1(a). The strongest (222) diffraction peak was taken as an example, and the diffraction intensity and the FWHM for those samples were calculated and plotted in Fig. 1(b). Apparently, Y2O3:Eu3+ TFP with Na+ and K + ions have much higher diffraction intensity and smaller FWHM, which indicates the improvement of the crystallinity of Y2O3:Eu3+ TFPs. As believed, the highly improved crystallinity was caused by the flux effect of the Na+ and K + ions. SEM images of the Y2O3:Eu3+ TFPs: (a) pure Y2O3:Eu 3+ film, (b) with 0.1 mmol Na+, and (c) with 0.1 mmol K + are shown in Fig. 2. Apparently, all the TFPs have a similar plate-like surface morphology. However, the plate thicknesses are different for the given samples. The plate thickness of the pure Y2O3:Eu 3+ film is around 11 nm, while the plate thicknesses of the Y2O3:Eu 3+ film with Na + and K + ions are 23 nm and 19 nm, respectively. Meanwhile, the thickness of the Y2O3:Eu 3+ film also changes as Na+ and K + were added. The thicknesses of the three given samples are around 6.86 μm, 11.25 μm, and 10.6 μm, respectively. Apparently, as Na + and K+ ions were added into the Y2O3:Eu3+ film, the film thickness increased, i.e., the optical volume increased, which could be important for the photoluminescence properties of the Y2O3:Eu 3+ TFP. 3.2. Photoluminescence properties The effect of Na + and K + ions on the photoluminescence properties of Y2O3:Eu3+ TFP was studied in details. The emission spectra of the Y2O3:Eu3+ samples: (a) pure Y2O3:Eu 3+ film, (b) with 0.1 mmol Na+, and (c) with 0.1 mmol K+ are shown in Fig. 3. There are totally three emission bands corresponding to the 5D0– 7F1, 5D0– 7F2 and 5D0– 7F3 transitions of Eu 3+ ions, respectively. Emission from the parity forbidden electric dipole 5D0– 7F2 transition has stronger intensity than that from the magnetic dipole transition 5D0–7F1 indicates that Eu 3+ is located at the site with no inversion symmetry in Y2O3 lattice (C2 site) [28]. The emission bands do not shift as Na + and K + ions were
Fig. 1. (a) XRD patterns of the Y2O3:Eu3+ TFPs without doping, with 0.1 mmol Na+, and with 0.1 mmol K+ ions, and (b) the calculated FWHM and diffraction intensity of the (222) diffraction peak of the Y2O3:Eu3+ TFP without doping, with 0.1 mmol Na+, and with 0.1 mmol K+ ions.
added, which may be due to the shield effect of outer shell electrons. However, the photoluminescence intensity of the Y2O3:Eu 3+ TFP was improved by 3 to 4 times due to the addition of Na + and K + ions. The effect of the amount of Na+ and K + ions on the photoluminescence properties of the Y2O3:Eu3+ TFP was also studied and results are shown in Fig. 4. Apparently, the photoluminescence intensity of Y2O3:Eu3+ TFP varies with the molar amount of Na+ and K+ ions and the optimum molar amount for both Na+ and K + ions is 0.1 mmol. The origin of the greatly improved photoluminescence intensity of Y2O3:Eu 3+ TFP with Na + and K + is complicated. There are several factors may contribute to the improvement of the photoluminescence intensity. However, it is difficult to distinguish one from the other. The first and the most possible factor is the crystallinity of the film. As indicated by the XRD results, the crystallinity of the Y2O3:Eu 3+ TFP was greatly improved by the addition of Na + and K + ions. The flux effect of Na + and K + ions gives larger grain size, better crystallinity and lower concentration of defects, which results in the longer oscillation length for the optical transition and the reduction of luminescence quenching caused by the surface states [26,29,30]. The second factor could be the increase in the optical volume. It is well known that the optical volume is an important factor in the photoluminescence intensity increase. As reported, the photoluminescence intensity of TFP increased with increases in the thickness of the film to a certain thickness, beyond which the emission intensity is a constant [31]. To verify the effect of the optical volume on the photoluminescence properties, the Y2O3:Eu 3+ TFPs: (a) pure Y2O3: Eu 3+ film, (b) with 0.1 mmol Na +, and (c) with 0.1 mmol K + with
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Fig. 3. The photoluminescence emission spectra of the Y2O3:Eu3+ TFP: pure Y2O3:Eu3+, with 0.1 mmol Na+, and with 0.1 mmol K+.
The decay curves can not be fit by a single exponential function, but can be well fit by a second-order exponential function in a form: t t I = A1 exp − + A2 exp − : τ1 τ2
ð1Þ
Fig. 2. SEM images of the Y2O3:Eu3+ TFPs: (a) pure Y2O3:Eu3+, (b) with 0.1 mmol Na+, and (c) with 0.1 mmol K+.
similar thickness were prepared and studied. As shown in Fig. 5, with similar thickness, the photoluminescence intensity of Y2O3:Eu 3+:Na + (0.1 mmol) and Y2O3:Eu 3+:K + (0.1 mmol) was increased by 2 times. The result indicates that the film thickness, i.e., the optical volume, is also an important factor for the highly improved photoluminescence. There are also some other factors may cause the improvement of photoluminescence properties. Such as the oxygen vacancies created by the addition of Na + and K + ions [32,33] and the change of lattice symmetry caused by the addition of Na + and K + ions. The photoluminescence decay curves of the 5D0–7F2 transition of 3+ Eu ions in the Y2O3:Eu3+ TFP with and without Na + and K + ions were characterized and results are shown in Figs. 6 and 7, respectively.
Fig. 4. Photoluminescence Intensity of the 5D0–7F2 transition as a function of molar amount of (a) Na+ and (b) K+ in the Y2O3:Eu3+ TFP.
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Fig. 5. The photoluminescence emission spectra of the Y2O3:Eu3+ TFP with similar thickness (around 6.86 μm): pure Y2O3:Eu3+, with 0.1 mmol Na+, and with 0.1 mmol K+.
Fig. 7. (a) Decay curves of the 5D0–7F2 transition of the Y2O3:Eu3+ TFP with and without K+, and (b) The average lifetime of the 5D0–7F2 transition as a function of molar amount of K+.
The average lifetime τ is defined according to the following equation [34]: 2 2 τ = A1 τ1 + A2 τ2 = A1 τ1 + A2 τ2 :
ð2Þ
In Figs. 6(b) and 7(b), the average lifetime is plotted as a function of the molar amount of Na+ and K + ions. As shown, the average lifetime of Y2O3:Eu 3+ TFP varies with the molar amount of Na+ and K + ions. The concentration of surface states, which act as quenching centers, may decrease and give rise to the average lifetime for the Y2O3:Eu3+ TFP with Na+ and K + ions. On the other hand, the average lifetime varies with the molar amount of Na+ and K + ions. It can be ascribed to the variation of radiative transition rate caused by the addition of Na+ and K + ions [35]. 4. Conclusions
Fig. 6. (a) Decay curves of the 5D0–7F2 transition of the Y2O3:Eu3+ TFP with and without Na+, and (b) the average lifetime of the 5D0–7F2 transition as a function of the molar amount of Na+.
Y2O3:Eu3+ thin film phosphors were achieved by an electrodeposition process followed by an annealing process. The effect of Na+ and K+ ions on the photoluminescence properties was studied. It is found that the photoluminescence intensity of the Y2O3:Eu 3+ TFP was improved by 3 to 4 times as Na + and K + ions were added. The highly enhanced photoluminescence intensity of the Y2O3:Eu 3+ TFP may be mainly caused by a combined effect of the improved crystallinity and the increase in optical volume. Factors, such as the concentration of oxygen
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vacancies and the change of lattice symmetry, may also improve the photoluminescence intensity. It is also found that the average lifetime of the 5D0– 7F2 transition increased as Na + and K+ ions were added and varied with the molar amount of Na + and K + ions, which may be caused by the variation of the surface states concentration and the radiative transition rate. Acknowledgments This work is supported by the National Science Foundation of China under Grant 60906031, the Zhejiang Provincial Natural Science Foundation of China under Grant Y1090742, the Qianjiang Talent Programme of Zhejiang Province under grant 2009R10018, the Scientific Research Fund of Zhejiang Provincial Education Department under Grant 20070361, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry) under Grant 111383A4C08690. References [1] [2] [3] [4] [5]
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