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Enhancement of cathodoluminescence intensities of Y2O3:Eu and Gd2O3:Eu phosphors by incorporation of Li ions
ARTICLE IN PRESS
Journal of Luminescence 114 (2005) 275–280 www.elsevier.com/locate/jlumin
Enhancement of cathodoluminescence intensities of Y2O3:Eu and Gd2O3:Eu phosphors by incorporation of Li ions Sang Hoon Shina,, Jong Hyuk Kangb, Duk Young Jeonb, Dong Sik Zanga a
Electronic Materials Lab., Corporate R&D Center, Samsung SDI Co., Ltd., 428-5 Gongse-ri, Giheung-eup, Yongin, Gyeonggi 449-577, Korea b Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Received 4 October 2004 Available online 14 March 2005
Abstract Li-doped Y2O3:Eu and Gd2O3:Eu phosphors have been studied as potential red phosphors for application to field emission displays. These phosphors showed cathodoluminescence increases when Li ions are doped. It is found that the lattice of Gd2O3:Eu phosphors transforms from monoclinic to cubic as the Li ions are doped. However, Y2O3:Eu phosphors keep their cubic phases upon Li doping. Also, this investigation has determined that the differences in intensity ratios originate from two different luminescent sites in each phosphor. From this study, it is understood that Li ions affect the luminescence of the Gd2O3:Eu phosphor by influencing its lattice symmetry. On the other hand, the luminescence of Y2O3:Eu phosphors is increased by a kind of charge compensation caused by Li doping. r 2005 Elsevier B.V. All rights reserved. PACS: 78.60.Hk; 74.62.Bf; 42.79.Kr Keywords: Cathodoluminescence; Y2O3:Eu; Gd2O3:Eu
1. Introduction Sesquioxide phosphors, such as Y2O3:Eu and Gd2O3:Eu are useful phosphors in many display Corresponding
author. Tel.: +82 31 288 4637; fax: +82 31 288 4646. E-mail addresses: [email protected], [email protected] (S.H. Shin), [email protected] (J.H. Kang), [email protected] (D.Y. Jeon).
devices, particularly Y2O3:Eu, which has long been used as a lamp phosphor [1]. Recently, Li-doped Gd2O3:Eu has shown good luminescence in lowvoltage-operating field emission display (FED) conditions [2]. Since a small prototype FED (o 400 diagonal) was made, many kinds of phosphors have been tested as potential phosphors for FEDs [3]. Even though the tip material for FEDs has been changed to carbon nanotube (CNT), the low accelerating voltage condition
0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.02.002
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does not produce sufficient brightness [4]. Therefore, the operation of FEDs now uses a higher voltage (43 kV) than has been used in the past [5]. Because of the increased operating voltage and current conditions, oxide phosphors appear suitable for the new FED’s operation conditions. Many researches have studied the stability and aging characteristics of oxide phosphors [6–8]. Also, it has been found that an increased luminescence of the Y2O3:Eu phosphor is achieved when some flux materials and doping of group I or II elements are added at firing stages [9,10]. In other recent studies, the manufacturing process or miniaturization to nano-size of, mainly, Y2O3:Eu phosphors have been studied [11,12]. This research considers Li-doped Y2O3:Eu and Gd2O3:Eu. Some studies have shown that adding Li to Y2O3:Eu and/or Gd2O3:Eu causes site change in the crystal structure or morphology of the phosphors [13,14]. In this paper, we have tried to separate the effects of Li addition on site symmetry from those on morphological change (or flux action). From this study, it is found that Li ions have little effect on the site symmetry of Y2O3:Eu phosphor compared with Gd2O3:Eu phosphor but, on the contrary, Li ions seem to affect the surface valence of Y2O3:Eu phosphor.
by using a demountable ultra-high vacuum chamber equipped with a laboratory-built CL spectrometer. Measurement of CL was carried out at an excitation energy of 5 keV and a beam current density of 60 mA/cm2 because, in the near future, higher excitation energy will be used to increase the brightness of FED. The luminescence measurements were conducted on powder samples and an X-ray powder diffraction (XRD) measurement was carried out on a Philips X-Pert MPD1 X-ray diffractometer.
3. Results and discussion CL intensities of Y2O3:Eu and Gd2O3:Eu showed their maxima when Eu ions were doped at 5 mol% of both Y and Gd ions. Fig. 1 shows the
2. Experimental To synthesize Y2O3:Eu and Gd2O3:Eu, a conventional solid state firing method was used. Y2O3 and/or Gd2O3 were used as the host lattice and Eu2O3 was used as the activator. The doping of Li ions was achieved by using LiNO3 or Li2CO3. The raw materials were mixed and sintered at 1300 1C or higher for 3 h in air. After sintering, the phosphor was ball-milled with glass beads. The ball-mill process was performed, such that the mass of the phosphor, glass beads, and distilled water were set to be equal. The milling speed was fixed at 80 revolutions per minute (rpm). After being ball-milled, the phosphor was sieved with a 400 dots/in mesh. In order to investigate optical properties, the cathodoluminescence (CL) of phosphors was measured at room temperature. CL was measured
Fig. 1. CL spectra of (a) Y2O3:Eu and (b) Gd2O3:Eu. CL measurements were conducted at an excitation energy of 5 keV and beam current density of 60 mA/cm2.
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CL spectra of Y2O3:Eu and Gd2O3:Eu. Fig. 1(a) shows that the main emission band of Y2O3:Eu at about 611 nm is assigned to the 5D0-7F2 transition of the Eu3+ ion and another transition originating from the 5D0 level could also be observed. The 5D0-7F2 transition could be seen in Gd2O3:Eu phosphor. However, the transition band of the Gd2O3:Eu phosphor is broader than that of Y2O3:Eu, as shown in Fig. 1(b). Fig. 2 shows the CL spectra of Y1.95xLixO3: Eu0.05 and Gd1.95xLixO3:Eu0.05 phosphors. To dope Li ions, Li2CO3 were used at each phosphor. In the case of Y1.95xLixO3:Eu0.05 phosphors, it seems that there are no shape changes of spectra as Li ions are doped. However, in the case of Gd1.95xLixO3:Eu0.05 phosphors, the spectra of Gd1.95xLixO3:Eu0.05 become similar to those of Y1.95xLixO3:Eu0.05 phosphors as the amount of Li ions is increased.
Fig. 2. CL spectra of (a) Y1.95xLixO3:Eu0.05 and (b) Gd1.95xLixO3:Eu0.05. In these phosphors, Li2CO3 was used as a doping agent of Li ions.
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In order to distinguish the doping effect of Li ions out of Li compounds from additional flux action of the same compounds, LiNO3 was used as another doping agent of Li ions in Y2O3:Eu and Gd2O3:Eu phosphors. As Li ions were doped, the CL intensities of the phosphors were increased, irrespective of Li compounds. Figs. 3 and 4 show the relative intensities of Y1.95xLixO3:Eu0.05 and Gd1.95xLixO3:Eu0.05, respectively, scaled by undoped phosphors as a reference. It is known that the emission of Eu3+ ion is affected by the crystal symmetry of Y2O3 and Gd2O3. When its crystal symmetry is cubic, this type of structure offers crystallographically two different sites to the impurities, one with C2 and the other with S6 symmetry [15]. In principle, the transition of the
Fig. 3. Relative intensities of Y1.95xLixO3:Eu0.05 phosphors as a function of the doping amount of Li ions. The doping agents of Li ions are (a) Li2CO3 and (b) LiNO3. The Li un-doped sample was scaled as unit. The occupancy ratio of the two different sites (ORS) is defined as the intensity ratio of the 5 D0-7F2 (611–630 nm) to the 5D0-7F1 (at about 590 nm).
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Fig. 4. Relative intensities of Gd1.95xLixO3:Eu0.05 phosphors as a function of the doping amount of Li ions. The doping agents of Li ions are (a) Li2CO3 and (b) LiNO3. The Li undoped sample was scaled as unit. The occupancy ratio of the two different sites (ORS) is defined as the intensity ratio of the 5 D0-7F2 (611–630 nm) to the 5D0-7F1 (at about 590 nm).
Eu3+ ion is mainly affected by the symmetry of the crystal field. In the case of Eu3+ ions with S6 symmetry, the 5D0-7F1 magnetic-dipole transition is dominant. If we define the degree of site asymmetry as the intensity ratio of the 5D0-7F2 (611–630 nm) to the 5D0-7F1 (at about 590 nm), the local symmetry of surrounding impurity ions, i.e., the occupancy ratio of the two different sites (ORS) could easily be obtained [2,16]. In the figures, changes of ORS were represented as a function of the doping amount of the Li ions. As can be seen, there is no ORS change in Y2O3:Eu when the doping amount of Li ions was varied even though two different Li doping agents were used. However, CL intensities did change, when the amounts of Li ions were varied, as shown in Fig. 3(a) and (b).
However, in the case of the Gd2O3:Eu phosphor, the solubility of the Li ions is dependent on the phosphor’s doping agent, and the trends of CL intensities and ORS appear quite different from those in the case of the Y2O3:Eu phosphor when Li ions are added, as shown in Fig. 4. As indicated in Ref. [2], it seems that the Eu3+ and Li+ ions are not assumed to occupy C2 and S6 sites in a statistical way. If the two ions are statistically distributed in the lattice, it is expected that there would be no change in ORS with increased Li doping. It shows that the Li+ ions are introduced into the C2 site with the higher population of lattice. When Li2CO3 is used as a doping agent of Li ions, the sum of the reduced symmetry and the flux action of the agent contributes to the CL increase of Gd1.95xLixO3:Eu0.05. The ORS change in Gd1.95xLixO3:Eu0.05 is similar to the trend of CL intensities, as shown in Fig. 4(a) and (b). As indicated earlier, there is little change in ORS in the case of Y1.95xLixO3:Eu0.05 phosphors. Therefore, it is thought that the Li ions affect the CL increase of Gd1.95xLixO3:Eu0.05 phosphors differently than in Y1.95xLixO3:Eu0.05 phosphors. To check the crystal lattice of each phosphor, XRD was measured. From Table 1 and Fig. 5, in the case of Y1.95xLixO3:Eu0.05 phosphors, the host lattices are assigned to be cubic phases [17]. As shown, the crystal structure is nearly unchanged even though the doping amount of Li ions is increased. However, in the case of Gd1.95xLixO3:Eu0.05 phosphors, the host lattice transformed from monoclinic to cubic as the doping amount of Li ions was increased, as shown in Table 1 and Fig. 6 [18,19]. Despite having the same cubic structures [Fig. 5(d) and (e) and Fig. 6(d) and (e)], the trends of CL intensities and ORS of Y1.95xLixO3:Eu0.05 were different from those of Gd1.95xLixO3:Eu0.05, as shown in Figs. 3 and 4. From Table 1, in the case of Y1.95xLixO3:Eu0.05 phosphors, the lattice parameters were nearly unchanged as the doping amount of Li was increased. As indicated above, however, in the case of Gd1.95xLixO3:Eu0.05 phosphors, the lattice transformed from monoclinic to cubic as the doping amount of Li was increased. As one can see, the lattice parameters of the cubic phases of
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Table 1 Crystal information of Y1.95xLixO3:Eu0.05 and Gd1.95xLixO3:Eu0.05 phosphors Amount of Li doping (x) (mol)
a 0 (reference) 0.01 0.03 0.05 0.10 a
Gd1.95xLixO3:Eu0.05 Lattice parameter (A˚)
Y1.95xLixO3:Eu0.05 Lattice parameter (A˚) b
10.602 10.600 10.603 10.603 10.540
c
a
b
c
b (1)
14.078 14.077 14.095 10.813a 10.800a
3.571 3.571 3.571
8.758 8.756 8.752
100.047 100.036 100.047
Small amount of monoclinic phases are mixed.
Fig. 5. XRD patterns of Y1.95xLixO3:Eu0.05 phosphors.
Y1.95xLixO3:Eu0.05 and Gd1.95xLixO3:Eu0.05 are similar. The ionic radius of Y3+ is 0.89 A˚ and that of Gd3+ is 0.94 A˚ [20]. If dopant ions, such as Eu3+ or Li+, are introduced in each phosphor lattice, the Gd2O3 lattice would be more deformed by the dopant ions than Y2O3 lattice. Therefore, it
Fig. 6. XRD patterns of Gd1.95xLixO3:Eu0.05 phosphors. The monoclinic phase of the Gd1.95xLixO3:Eu0.05 phosphor is changed to a cubic phase as the doping amount of Li ions is increased.
is thought that the statistical distribution of Eu3+ or Li+ ions over the C2 and S6 sites is more difficult in the Gd2O3 lattice than in the Y2O3 lattice. As demonstrated in Fig. 4, the change of
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ORS shows that the Li+ ions are introduced into the C2 site with a higher population of Gd1.95xLixO3:Eu0.05 lattice. Thus, the increase of CL intensities of Gd1.95xLixO3:Eu0.05 phosphors would be from the reduced symmetry of this site. On the other hand, a CL increase of the Y1.95xLixO3:Eu0.05 phosphor lattices is thought to be similar to that discussed in Ref. [11]. The substitution of Y3+ ions by Li+ ions might relieve the oxygen vacancies on the surface of the phosphor if a suitable amount of Li ions were doped. Therefore, the luminescence of Y1.95xLixO3: Eu0.05 phosphors would increase. 4. Summary and conclusion CL properties of Li-doped Y2O3:Eu and Gd2O3:Eu phosphors have been studied. When Li ions were doped, the CL intensities of both Y2O3:Eu and Gd2O3:Eu phosphors increased. The increased CL intensity of Y1.95xLixO3: Eu0.05 phosphors is likely due to the charge compensation resulting from the substitution effect of Li+ ions to Y3+ ions. In the case of Gd1.95xLixO3:Eu0.05, however, the CL intensity increased due to the lowering of local symmetry that was induced by the Li ions. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Heidelberg, 1994, pp. 116–118. [2] J.C. Park, H.K. Moon, D.K. Kim, S.H. Byeon, B.C. Kim, K.S. Suh, Appl. Phys. Lett. 77 (2000) 2162.
[3] J.M. Kim, N.S. Park, J.H. Choi, J.W. Lim, S.K. Hong, B.I. Gorfinkel, E.V. Roussina, V.V. Mishin, N.P. Abanshin, A.R. Zoulkameev, SID ’96 Digest, San Diego, CA, 1996, pp. 57–60. [4] W.B. Choi, D.S. Chung, S.H. Park, J.M. Kim, SID ’99 Digest, San Jose, CA, 1999, pp. 1134–1137. [5] J.E. Jang, S.J. Lee, W.Y. Jin, Y.J. Moon, J.E. Jung, C.G. Lee, T.S. Oh, J.S. Choi, S.K. Kang, J.M. Kim, Proceedings of the Tenth International Display Workshop, Hiroshima, Japan, 2002, pp. 947–950. [6] H. Bechtel, W. Czarnojan, M. Hasse, D. Wadow, J. SID 4 (3) (1996) 219. [7] S.M. Jacobsen, J. SID 4 (4) (1996) 331. [8] B.K. Wagner, J. Penczek, S. Yang, F.L. Zhang, C. Stoffers, C.J. Summers, Proceedings of the International Display Research Conference, Toronto, Canada, 1997, pp. 330–333. [9] G.Y. Hong, K. Yoo, S.J. Moon, J.S. Yoo, J. Electrochem. Soc. 150 (4) (2003) H67. [10] S.M. Park, C.H. Kim, J.K. Park, H.D. Park, H.G. Jang, J. Kor. Chem. Soc. 45 (3) (2001) 236. [11] J. Zhang, Z. Tang, Z. Zhang, W. Fu, J. Wang, Y. Lin, Mater. Sci. Eng. A 334 (2002) 246. [12] G.Y. Hong, K. Yoo, S.J. Moon, J.S. Yoo, J. Electrochem. Soc. 150 (2003) H67. [13] L. Sun, C. Qian, C. Liao, X. Wang, C. Yan, Solid State Commun. 119 (2001) 393. [14] Y.C. Kang, H.S. Roh, S.B. Park, H.D. Park, J. Eur. Ceram. Soc. 22 (2002) 1661. [15] M. Buijs, A. Meyerink, G. Blasse, J. Lumin. 37 (1987) 9. [16] N. Rakov, F.E. Ramos, G. Hirata, M. Xiao, Appl. Phys. Lett. 83 (2003) 272. [17] JCPDS card number: 25-1011, JCPDS-International Center for Diffraction Data, 1996. [18] JCPDS card number: 43-1015, JCPDS-International Center for Diffraction Data, 1996. [19] JCPDS card number: 76-0155, JCPDS-International Center for Diffraction Data, 1996. [20] P.M.B. Walker (Ed.), Materials Science and Technology Dictionary, Chambers Harrap Publishers Ltd., Edinburgh, 1993, pp. 342–344.
Journal of Luminescence 114 (2005) 275–280 www.elsevier.com/locate/jlumin
Enhancement of cathodoluminescence intensities of Y2O3:Eu and Gd2O3:Eu phosphors by incorporation of Li ions Sang Hoon Shina,, Jong Hyuk Kangb, Duk Young Jeonb, Dong Sik Zanga a
Electronic Materials Lab., Corporate R&D Center, Samsung SDI Co., Ltd., 428-5 Gongse-ri, Giheung-eup, Yongin, Gyeonggi 449-577, Korea b Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Received 4 October 2004 Available online 14 March 2005
Abstract Li-doped Y2O3:Eu and Gd2O3:Eu phosphors have been studied as potential red phosphors for application to field emission displays. These phosphors showed cathodoluminescence increases when Li ions are doped. It is found that the lattice of Gd2O3:Eu phosphors transforms from monoclinic to cubic as the Li ions are doped. However, Y2O3:Eu phosphors keep their cubic phases upon Li doping. Also, this investigation has determined that the differences in intensity ratios originate from two different luminescent sites in each phosphor. From this study, it is understood that Li ions affect the luminescence of the Gd2O3:Eu phosphor by influencing its lattice symmetry. On the other hand, the luminescence of Y2O3:Eu phosphors is increased by a kind of charge compensation caused by Li doping. r 2005 Elsevier B.V. All rights reserved. PACS: 78.60.Hk; 74.62.Bf; 42.79.Kr Keywords: Cathodoluminescence; Y2O3:Eu; Gd2O3:Eu
1. Introduction Sesquioxide phosphors, such as Y2O3:Eu and Gd2O3:Eu are useful phosphors in many display Corresponding
author. Tel.: +82 31 288 4637; fax: +82 31 288 4646. E-mail addresses: [email protected], [email protected] (S.H. Shin), [email protected] (J.H. Kang), [email protected] (D.Y. Jeon).
devices, particularly Y2O3:Eu, which has long been used as a lamp phosphor [1]. Recently, Li-doped Gd2O3:Eu has shown good luminescence in lowvoltage-operating field emission display (FED) conditions [2]. Since a small prototype FED (o 400 diagonal) was made, many kinds of phosphors have been tested as potential phosphors for FEDs [3]. Even though the tip material for FEDs has been changed to carbon nanotube (CNT), the low accelerating voltage condition
0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.02.002
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does not produce sufficient brightness [4]. Therefore, the operation of FEDs now uses a higher voltage (43 kV) than has been used in the past [5]. Because of the increased operating voltage and current conditions, oxide phosphors appear suitable for the new FED’s operation conditions. Many researches have studied the stability and aging characteristics of oxide phosphors [6–8]. Also, it has been found that an increased luminescence of the Y2O3:Eu phosphor is achieved when some flux materials and doping of group I or II elements are added at firing stages [9,10]. In other recent studies, the manufacturing process or miniaturization to nano-size of, mainly, Y2O3:Eu phosphors have been studied [11,12]. This research considers Li-doped Y2O3:Eu and Gd2O3:Eu. Some studies have shown that adding Li to Y2O3:Eu and/or Gd2O3:Eu causes site change in the crystal structure or morphology of the phosphors [13,14]. In this paper, we have tried to separate the effects of Li addition on site symmetry from those on morphological change (or flux action). From this study, it is found that Li ions have little effect on the site symmetry of Y2O3:Eu phosphor compared with Gd2O3:Eu phosphor but, on the contrary, Li ions seem to affect the surface valence of Y2O3:Eu phosphor.
by using a demountable ultra-high vacuum chamber equipped with a laboratory-built CL spectrometer. Measurement of CL was carried out at an excitation energy of 5 keV and a beam current density of 60 mA/cm2 because, in the near future, higher excitation energy will be used to increase the brightness of FED. The luminescence measurements were conducted on powder samples and an X-ray powder diffraction (XRD) measurement was carried out on a Philips X-Pert MPD1 X-ray diffractometer.
3. Results and discussion CL intensities of Y2O3:Eu and Gd2O3:Eu showed their maxima when Eu ions were doped at 5 mol% of both Y and Gd ions. Fig. 1 shows the
2. Experimental To synthesize Y2O3:Eu and Gd2O3:Eu, a conventional solid state firing method was used. Y2O3 and/or Gd2O3 were used as the host lattice and Eu2O3 was used as the activator. The doping of Li ions was achieved by using LiNO3 or Li2CO3. The raw materials were mixed and sintered at 1300 1C or higher for 3 h in air. After sintering, the phosphor was ball-milled with glass beads. The ball-mill process was performed, such that the mass of the phosphor, glass beads, and distilled water were set to be equal. The milling speed was fixed at 80 revolutions per minute (rpm). After being ball-milled, the phosphor was sieved with a 400 dots/in mesh. In order to investigate optical properties, the cathodoluminescence (CL) of phosphors was measured at room temperature. CL was measured
Fig. 1. CL spectra of (a) Y2O3:Eu and (b) Gd2O3:Eu. CL measurements were conducted at an excitation energy of 5 keV and beam current density of 60 mA/cm2.
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CL spectra of Y2O3:Eu and Gd2O3:Eu. Fig. 1(a) shows that the main emission band of Y2O3:Eu at about 611 nm is assigned to the 5D0-7F2 transition of the Eu3+ ion and another transition originating from the 5D0 level could also be observed. The 5D0-7F2 transition could be seen in Gd2O3:Eu phosphor. However, the transition band of the Gd2O3:Eu phosphor is broader than that of Y2O3:Eu, as shown in Fig. 1(b). Fig. 2 shows the CL spectra of Y1.95xLixO3: Eu0.05 and Gd1.95xLixO3:Eu0.05 phosphors. To dope Li ions, Li2CO3 were used at each phosphor. In the case of Y1.95xLixO3:Eu0.05 phosphors, it seems that there are no shape changes of spectra as Li ions are doped. However, in the case of Gd1.95xLixO3:Eu0.05 phosphors, the spectra of Gd1.95xLixO3:Eu0.05 become similar to those of Y1.95xLixO3:Eu0.05 phosphors as the amount of Li ions is increased.
Fig. 2. CL spectra of (a) Y1.95xLixO3:Eu0.05 and (b) Gd1.95xLixO3:Eu0.05. In these phosphors, Li2CO3 was used as a doping agent of Li ions.
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In order to distinguish the doping effect of Li ions out of Li compounds from additional flux action of the same compounds, LiNO3 was used as another doping agent of Li ions in Y2O3:Eu and Gd2O3:Eu phosphors. As Li ions were doped, the CL intensities of the phosphors were increased, irrespective of Li compounds. Figs. 3 and 4 show the relative intensities of Y1.95xLixO3:Eu0.05 and Gd1.95xLixO3:Eu0.05, respectively, scaled by undoped phosphors as a reference. It is known that the emission of Eu3+ ion is affected by the crystal symmetry of Y2O3 and Gd2O3. When its crystal symmetry is cubic, this type of structure offers crystallographically two different sites to the impurities, one with C2 and the other with S6 symmetry [15]. In principle, the transition of the
Fig. 3. Relative intensities of Y1.95xLixO3:Eu0.05 phosphors as a function of the doping amount of Li ions. The doping agents of Li ions are (a) Li2CO3 and (b) LiNO3. The Li un-doped sample was scaled as unit. The occupancy ratio of the two different sites (ORS) is defined as the intensity ratio of the 5 D0-7F2 (611–630 nm) to the 5D0-7F1 (at about 590 nm).
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Fig. 4. Relative intensities of Gd1.95xLixO3:Eu0.05 phosphors as a function of the doping amount of Li ions. The doping agents of Li ions are (a) Li2CO3 and (b) LiNO3. The Li undoped sample was scaled as unit. The occupancy ratio of the two different sites (ORS) is defined as the intensity ratio of the 5 D0-7F2 (611–630 nm) to the 5D0-7F1 (at about 590 nm).
Eu3+ ion is mainly affected by the symmetry of the crystal field. In the case of Eu3+ ions with S6 symmetry, the 5D0-7F1 magnetic-dipole transition is dominant. If we define the degree of site asymmetry as the intensity ratio of the 5D0-7F2 (611–630 nm) to the 5D0-7F1 (at about 590 nm), the local symmetry of surrounding impurity ions, i.e., the occupancy ratio of the two different sites (ORS) could easily be obtained [2,16]. In the figures, changes of ORS were represented as a function of the doping amount of the Li ions. As can be seen, there is no ORS change in Y2O3:Eu when the doping amount of Li ions was varied even though two different Li doping agents were used. However, CL intensities did change, when the amounts of Li ions were varied, as shown in Fig. 3(a) and (b).
However, in the case of the Gd2O3:Eu phosphor, the solubility of the Li ions is dependent on the phosphor’s doping agent, and the trends of CL intensities and ORS appear quite different from those in the case of the Y2O3:Eu phosphor when Li ions are added, as shown in Fig. 4. As indicated in Ref. [2], it seems that the Eu3+ and Li+ ions are not assumed to occupy C2 and S6 sites in a statistical way. If the two ions are statistically distributed in the lattice, it is expected that there would be no change in ORS with increased Li doping. It shows that the Li+ ions are introduced into the C2 site with the higher population of lattice. When Li2CO3 is used as a doping agent of Li ions, the sum of the reduced symmetry and the flux action of the agent contributes to the CL increase of Gd1.95xLixO3:Eu0.05. The ORS change in Gd1.95xLixO3:Eu0.05 is similar to the trend of CL intensities, as shown in Fig. 4(a) and (b). As indicated earlier, there is little change in ORS in the case of Y1.95xLixO3:Eu0.05 phosphors. Therefore, it is thought that the Li ions affect the CL increase of Gd1.95xLixO3:Eu0.05 phosphors differently than in Y1.95xLixO3:Eu0.05 phosphors. To check the crystal lattice of each phosphor, XRD was measured. From Table 1 and Fig. 5, in the case of Y1.95xLixO3:Eu0.05 phosphors, the host lattices are assigned to be cubic phases [17]. As shown, the crystal structure is nearly unchanged even though the doping amount of Li ions is increased. However, in the case of Gd1.95xLixO3:Eu0.05 phosphors, the host lattice transformed from monoclinic to cubic as the doping amount of Li ions was increased, as shown in Table 1 and Fig. 6 [18,19]. Despite having the same cubic structures [Fig. 5(d) and (e) and Fig. 6(d) and (e)], the trends of CL intensities and ORS of Y1.95xLixO3:Eu0.05 were different from those of Gd1.95xLixO3:Eu0.05, as shown in Figs. 3 and 4. From Table 1, in the case of Y1.95xLixO3:Eu0.05 phosphors, the lattice parameters were nearly unchanged as the doping amount of Li was increased. As indicated above, however, in the case of Gd1.95xLixO3:Eu0.05 phosphors, the lattice transformed from monoclinic to cubic as the doping amount of Li was increased. As one can see, the lattice parameters of the cubic phases of
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Table 1 Crystal information of Y1.95xLixO3:Eu0.05 and Gd1.95xLixO3:Eu0.05 phosphors Amount of Li doping (x) (mol)
a 0 (reference) 0.01 0.03 0.05 0.10 a
Gd1.95xLixO3:Eu0.05 Lattice parameter (A˚)
Y1.95xLixO3:Eu0.05 Lattice parameter (A˚) b
10.602 10.600 10.603 10.603 10.540
c
a
b
c
b (1)
14.078 14.077 14.095 10.813a 10.800a
3.571 3.571 3.571
8.758 8.756 8.752
100.047 100.036 100.047
Small amount of monoclinic phases are mixed.
Fig. 5. XRD patterns of Y1.95xLixO3:Eu0.05 phosphors.
Y1.95xLixO3:Eu0.05 and Gd1.95xLixO3:Eu0.05 are similar. The ionic radius of Y3+ is 0.89 A˚ and that of Gd3+ is 0.94 A˚ [20]. If dopant ions, such as Eu3+ or Li+, are introduced in each phosphor lattice, the Gd2O3 lattice would be more deformed by the dopant ions than Y2O3 lattice. Therefore, it
Fig. 6. XRD patterns of Gd1.95xLixO3:Eu0.05 phosphors. The monoclinic phase of the Gd1.95xLixO3:Eu0.05 phosphor is changed to a cubic phase as the doping amount of Li ions is increased.
is thought that the statistical distribution of Eu3+ or Li+ ions over the C2 and S6 sites is more difficult in the Gd2O3 lattice than in the Y2O3 lattice. As demonstrated in Fig. 4, the change of
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ORS shows that the Li+ ions are introduced into the C2 site with a higher population of Gd1.95xLixO3:Eu0.05 lattice. Thus, the increase of CL intensities of Gd1.95xLixO3:Eu0.05 phosphors would be from the reduced symmetry of this site. On the other hand, a CL increase of the Y1.95xLixO3:Eu0.05 phosphor lattices is thought to be similar to that discussed in Ref. [11]. The substitution of Y3+ ions by Li+ ions might relieve the oxygen vacancies on the surface of the phosphor if a suitable amount of Li ions were doped. Therefore, the luminescence of Y1.95xLixO3: Eu0.05 phosphors would increase. 4. Summary and conclusion CL properties of Li-doped Y2O3:Eu and Gd2O3:Eu phosphors have been studied. When Li ions were doped, the CL intensities of both Y2O3:Eu and Gd2O3:Eu phosphors increased. The increased CL intensity of Y1.95xLixO3: Eu0.05 phosphors is likely due to the charge compensation resulting from the substitution effect of Li+ ions to Y3+ ions. In the case of Gd1.95xLixO3:Eu0.05, however, the CL intensity increased due to the lowering of local symmetry that was induced by the Li ions. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Heidelberg, 1994, pp. 116–118. [2] J.C. Park, H.K. Moon, D.K. Kim, S.H. Byeon, B.C. Kim, K.S. Suh, Appl. Phys. Lett. 77 (2000) 2162.
[3] J.M. Kim, N.S. Park, J.H. Choi, J.W. Lim, S.K. Hong, B.I. Gorfinkel, E.V. Roussina, V.V. Mishin, N.P. Abanshin, A.R. Zoulkameev, SID ’96 Digest, San Diego, CA, 1996, pp. 57–60. [4] W.B. Choi, D.S. Chung, S.H. Park, J.M. Kim, SID ’99 Digest, San Jose, CA, 1999, pp. 1134–1137. [5] J.E. Jang, S.J. Lee, W.Y. Jin, Y.J. Moon, J.E. Jung, C.G. Lee, T.S. Oh, J.S. Choi, S.K. Kang, J.M. Kim, Proceedings of the Tenth International Display Workshop, Hiroshima, Japan, 2002, pp. 947–950. [6] H. Bechtel, W. Czarnojan, M. Hasse, D. Wadow, J. SID 4 (3) (1996) 219. [7] S.M. Jacobsen, J. SID 4 (4) (1996) 331. [8] B.K. Wagner, J. Penczek, S. Yang, F.L. Zhang, C. Stoffers, C.J. Summers, Proceedings of the International Display Research Conference, Toronto, Canada, 1997, pp. 330–333. [9] G.Y. Hong, K. Yoo, S.J. Moon, J.S. Yoo, J. Electrochem. Soc. 150 (4) (2003) H67. [10] S.M. Park, C.H. Kim, J.K. Park, H.D. Park, H.G. Jang, J. Kor. Chem. Soc. 45 (3) (2001) 236. [11] J. Zhang, Z. Tang, Z. Zhang, W. Fu, J. Wang, Y. Lin, Mater. Sci. Eng. A 334 (2002) 246. [12] G.Y. Hong, K. Yoo, S.J. Moon, J.S. Yoo, J. Electrochem. Soc. 150 (2003) H67. [13] L. Sun, C. Qian, C. Liao, X. Wang, C. Yan, Solid State Commun. 119 (2001) 393. [14] Y.C. Kang, H.S. Roh, S.B. Park, H.D. Park, J. Eur. Ceram. Soc. 22 (2002) 1661. [15] M. Buijs, A. Meyerink, G. Blasse, J. Lumin. 37 (1987) 9. [16] N. Rakov, F.E. Ramos, G. Hirata, M. Xiao, Appl. Phys. Lett. 83 (2003) 272. [17] JCPDS card number: 25-1011, JCPDS-International Center for Diffraction Data, 1996. [18] JCPDS card number: 43-1015, JCPDS-International Center for Diffraction Data, 1996. [19] JCPDS card number: 76-0155, JCPDS-International Center for Diffraction Data, 1996. [20] P.M.B. Walker (Ed.), Materials Science and Technology Dictionary, Chambers Harrap Publishers Ltd., Edinburgh, 1993, pp. 342–344.