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Highly enhanced photoluminescence of SrTiO3:Pr by substitution of (Li0.5, La0.5) pair for Sr
PERGAMON
Solid State Communications 115 (2000) 99–104 www.elsevier.com/locate/ssc
Highly enhanced photoluminescence of SrTiO3:Pr by substitution of (Li0.5, La0.5) pair for Sr K.-A. Hyeon a, S.-H. Byeon a,*, J.-C. Park b, D.-K. Kim c, K.-S. Suh d a
College of Environment and Applied Chemistry, Kyung Hee University, Yong In, Kyung Ki 449-701, South Korea b Department of Chemistry of New Materials, Silla University, Pusan 617-736, South Korea c Department of Chemistry, College of Natural Sciences, Kyungpook National University, Taegu 702-701, South Korea d Semiconductor Division, ETRI, Yusong, P.O. Box 106, Taejon 305-600, South Korea Received 10 January 2000; accepted 7 March 2000 by C.N.R. Rao
Abstract Photoluminescence (PL) of SrTiO3 –(Li0.5La0.5)TiO3:Pr system was investigated. This system was characterized by a very intense red emission at 611 nm corresponding to the 1 D2 ! 3 H4 inner transition of Pr 3⫹ ions. A weak PL intensity of SrTiO3:Pr was remarkably increased when the Sr atom was replaced by the (Li, La) pair. Through the elemental analysis for the Li atom, the dependence of PL intensity on the composition and heating temperature is correlated with the amount of defect sites for the Li atoms. Based on the experimental data, it is proposed that the substitution of the (Li, La) pair for Sr of SrTiO3 likely produces the holes trapped near Li ⫹. The increased recombination probability of electrons and trapped holes would then cause the highly enhanced emission intensity of Pr3⫹ by the resultant energy transfer. 䉷 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: C. Point defects; D. Optical properties; E. Luminescence
1. Introduction Since the flat-panel display technology using plasma was developed in the late 1960s [1], extensive research in the area of luminescence has been focused on the phosphor development or improvement for low-voltage flat-panel displays. Field emission displays (FEDs), which were realized for the first time in 1991 [2], are currently being explored as a potential flat-panel display technology. One of the most serious obstacles to the commercialization of FEDs is the absence of efficient low-voltage phosphors that do not degrade under prolonged Coulomb loading. The phosphor compositions most widely considered for FEDs have been the conventional cathode ray tube (CRT) phosphors, vacuum fluorescent display (VFD) phosphors, and projection TV phosphors. Another important aspect of the FED phosphor system is the volatility of the phosphor. In particular, electron stimulated reactions have been shown to occur at the phosphor surface in several sulfide based * Corresponding author. Tel.: ⫹82-331-201-2457; fax: ⫹82-331202-7337. E-mail address: [email protected] (S.-H. Byeon).
materials. In the case of CRT-red phosphor Y2O2S:Eu, for instance, the sulfide gas released from the phosphor by electron irradiation not only causes degradation of the phosphor brightness but also increases the work function of the field emitters [3]. No evolution of gases in the assembly process and in operation is therefore required for the low-voltage FED phosphors. To avoid the volatile sulfur byproduct, more stable oxide phosphors are being examined. For this purpose, some oxides of the perovskite structure were investigated as one of the host matrices. CaZrO3:Pr phosphor gave the green emission [4]. The red emission was observed in CaTiO3:Pr and SrTiO3:Pr phosphors [5,6]. Particularly, the addition of Ga 3⫹ or Al 3⫹ to SrTiO3:Pr phosphor largely increased the luminous efficiency. The recombination of electrons and holes trapped at Ga 3⫹ and the resultant energy transfer was proposed to induce such an enhanced red emission [7]. Self-activated type phosphors such as LiGa5O8, ZnGa2O4, and a solid solution between Li0.5Ga2.5O4 and ZnGa2O4 were also intensively investigated [8–13]. A strong compositional dependence of luminescence was observed for lithium zinc gallate phosphor. In the present work, SrTiO3 –Li0.5La0.5TiO3 solid solution
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Table 1 Experimental host compositions determined by ICP Nominal host compositions
Li0.50La0.50TiO3 Li0.50La0.50TiO3 ⫹30% excess Li Li0.50La0.50TiO3 ⫹50% excess Li Li0.50La0.50TiO3 ⫹100% excess Li Li0.50La0.50TiO3 Li0.50La0.50TiO3 Li0.50La0.50TiO3 Li0.56La0.48TiO3 Li0.65La0.45TiO3 Li0.44La0.52TiO3 Li0.35La0.55TiO3
Heating times (h) at 1200⬚C
Experimental host compositions
4 4
Li0.39La0.50TiO2.95 Li0.50La0.50TiO3
4
Li0.50La0.50TiO3
4
Li0.50La0.50TiO3
6 8 12 4 4 4 4
Li0.33La0.50TiO2.92 Li0.32La0.50TiO2.91 Li0.21La0.50TiO2.86 Li0.39La0.48TiO2.92 Li0.39La0.45TiO2.87 Li0.31La0.52TiO2.94 Li0.16La0.55TiO2.91
doped by Pr was explored in an attempt to search a new oxide phosphor. It is known that the 3 P0 ! 3 H4 and/or 1 D2 ! 3 H4 transition can be induced for the Pr 3⫹ ion depending on the host lattice [14]. Each transition corresponds to the green and red emissions, respectively. For instance, La2O3:Pr phosphor shows the green emission but
cubic Y2O3:Pr the red emission [15]. Very intense red emission at 611 nm corresponding to the 1 D2 ! 3 H4 inner transition of Pr 3⫹ ions was observed in our solid solution. Interestingly, a weak photoluminescence (PL) intensity of SrTiO3:Pr was remarkably increased when the Sr atom was replaced by the (Li, La) pair. Through the elemental analysis for the Li atom of Lix La0:5 TiO2:75⫹x=2 :Pr, which shows the highest brightness, the dependence of PL intensity on the composition and heating temperature was correlated with the amount of defect sites for the Li atoms. The influence of different alkali metals such as Na and K were also systematically investigated.
2. Experimental The SrTiO3 –Li0.5La0.5TiO3:xPr (x mol%; 0.05–0.80) solid solutions were prepared by typical solid state reaction. Well ground stoichiometric mixtures of SrCO3 (4 N), Li2CO3 (4 N), La2O3 (4 N), TiO2 (4 N) and PrCl3·7H2O (3 N) dissolved in absolute ethanol were heated at 900⬚C for 5 h in air. The residues were reground and heated again at 1200⬚C for 4, 6, 8, and 12 h in air. Li0.5⫺3xLa0.5⫹xTiO3:Pr and A0.5La0.5TiO3:Pr (A Na and K) were also prepared by the same procedure for comparison. About 130, 150, and 200% of Li2CO3 were added in order to induce higher contents of Li in the matrix. Since the high temperature treatment should induce a loss of volatile Li component, the quantitative analysis using the inductively coupled plasma (ICP) method was carried out to determine the Li content in this series. About 10 mg of the sample and about 1 ml of conc. HCl (35 wt%) were sealed into a Pyrex glass tube and kept at about 150⬚C for ⬃3 h to dissolve. The powder X-ray diffraction patterns were recorded on a rotating anode installed diffractometer (18 kW). The Cu Ka radiation used was monochromated by a curved-crystal graphite. A spectrofluorometer (Hitachi, F-4500) was used for the PL measurement at room temperature. The sample loaded on a powder holder provided by Hitachi was mounted about 45⬚ to the excitation and source for PL measurement.
3. Results and discussion
Fig. 1. X-ray powder diffraction patterns of: (a) SrTiO3:0.2(mol%)Pr; (b) (Li0.39La0.50)TiO2.95:0.2(mol%)Pr; and (c) (Li0.50La0.50)TiO3:0.2(mol%)Pr heated at 1200⬚C for 4 h. Open circles and arrows in (b) represent the reflections resulted from the ordering of La 3⫹ and Li ⫹ or vacancies along the c-axis and the ordering of rock-salt type arrangement, respectively.
Elemental analysis using ICP revealed that a considerable loss of the lithium is induced at high temperature due to the evaporation, the amount of lithium loss being proportional to the heating time. For instance, heating the sample of nominal composition Li0.50La0.50TiO3:Pr at 1200⬚C for 12 h resulted in about 58% loss of Li content (Li0.21La0.50TiO2.86:Pr) and consequently the oxygen vacancy of ⬃4.7% per formula unit. The addition of higher than 30% excess Li2CO3 was needed to obtain stoichiometric Li0.50La0.50TiO3:Pr. The lithium contents of some prepared
K.-A. Hyeon et al. / Solid State Communications 115 (2000) 99–104
Fig. 2. Excitation spectra monitoring at the wavelength at 611 nm (top) and emission spectra exciting at the wavelength of 350 nm (bottom) of Sr1⫺xLix/2Lax/2TiO3:0.2(mol%)Pr (x 0:0; 0.2, 0.4, 0.6, 0.8, and 1.0) heated at 1200⬚C for 4 h. The nominal compositions were adopted for all samples.
samples, which were determined by ICP, are summarized in Table 1. Fig. 1 shows the X-ray powder diffraction patterns of SrTiO3:0.2(mol%)Pr 3⫹, (Li0.39La0.5)TiO2.95:0.2(mol%)Pr 3⫹, and (Li0.5La0.5)TiO3:0.2(mol%)Pr 3⫹, which were heated at 1200⬚C for 4 h. Substitution of the (Li, La) pair for Sr did not greatly affect the cubic perovskite structure of SrTiO3, a slight difference in unit cell parameter being only induced. The small diffraction lines marked in Fig. 1(b), which are not observed in the cubic SrTiO3, indicate that La 3⫹ and Li ⫹ or vacancy are ordered along the c-axis or with the rock-salt arrangement [16]. The relative intensities of these additional diffractions were dependent on the amount of Li and vacancy as expected. Based on the emission intensity, the activator concentration was optimized. The maximal intensity of PL was obtained at 0.2 mol% of Pr on heating the samples at
101
Fig. 3. Excitation spectra monitoring at the wavelength at 611 nm (top) and emission spectra exciting at the wavelength of 350 nm (bottom) of Lix La0:5 TiO2:75⫹x=2 :0.2(mol%)Pr.
1200⬚C for 4 h. Excitation and emission spectra of SrTiO3 –(Li0.5La0.5)TiO3:0.2(mol%)Pr solid solution are shown in Fig. 2. The excitation bands around 450, 470, 486 nm are attributed to 3 H4 ! 3 P2 , 3 P1 ; 3 P0 transition of the Pr 3⫹ ion, respectively [17–19]. The broad band around 350 nm is most probably related to the host lattice. Excitation of SrTiO3:Pr around this wavelength was associated with the electronic transition from valence band to conduction band of the host lattice with an energy band gap of ⬃3.4 eV [20,21]. The PL spectra of pure SrTiO3 showed a broad emission band around the 350–500 nm region [22]. Since the excitation bands of the Pr 3⫹ ion overlap this emission band (see Fig. 2), the energy transfer by recombination of an exciton could be responsible for the emission of Pr 3⫹. Both the broad excitation band around 350 nm and the sharp emission band at 611 nm are strongly influenced by the substitution of the (Li, La) pair for Sr. A dramatic increase in the intensity is observed for Li0.39La0.50TiO2.95:Pr containing no Sr. A shift of the excitation band toward shorter
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Fig. 4. Emission spectra exciting at the wavelength of 350 nm of Li0.5La0.5TiO3:0.2(mol%)Pr, x mol% Ga heated at 1200⬚C for 4 h. The nominal compositions were adopted for all samples.
wavelength is also observed with increasing the amount of the (Li, La) pair. It is generally accepted that the lattice constants and the bond lengths are the important factors to determine the band gap. On the contrary, the emission band at 611 nm shows no significant shift, indicating that this emission is associated with the inner transition of Pr 3⫹ ion. As shown in Fig. 1(b) and Table 1, the replacement of Sr by the (Li, La) pair gives rise to the formation of defective structures. The accompanied increase in the emission intensity might be accordingly attributed to the increased Li deficiency and therefore the oxygen vacancy in the host lattice. Since these oxygen vacancies are not statistically distributed but ordered along the c-axis or with the rock-salt arrangement, the sites offered for Pr 3⫹ would have uneven symmetry which is able to lift the parity selection rule. In order to
correlate the oxygen vacancy with the emission intensity, the PL spectra were investigated for the samples LixLa0.5TiO2.75⫹x/2:Pr containing different amounts of lithium ion. Fig. 3 shows the transition of their excitation and emission spectra profiles with x value. Although no strong change of excitation energy is observed, the band intensity is significantly dependent of the oxygen vacancy. The intensity of emission bands at 611 nm increases as x decreases from 0.50 to 0.39. When x is smaller than 0.33, the emission intensity is considerably reduced. Thus, the oxygen vacancies of about 1.6 mol% (i.e. x 0:39) give the maximal intensity of emission. This variation clearly indicates that the PL intensity is influenced by the amount of oxygen vacancy. Considering that these oxygen vacancies are ordered, the sufficient number of sites (⬃1.7% per formula unit) whose
Fig. 5. Emission spectra exciting at the wavelength of 350 nm of Li0:5⫺x La0:5⫹y TiO2:75⫹d=2 :0.2(mol%)Pr.
K.-A. Hyeon et al. / Solid State Communications 115 (2000) 99–104
103
Fig. 6. Emission spectra exciting at the wavelength of 350 nm of A0.5La0.5TiO3:0.2(mol%)Pr (A Li, Na, and K) heated at 1200⬚C for 4 h. The nominal compositions were adopted for all samples.
local symmetry is lower than cubic symmetry can be offered for the activators (Pr) of 0.2 mol%. Energy-band studies for ABO3 perovskite oxides showed that the energy bands originated by the A ions, which are far from the band gap, do not play an important role in determining electronic and optical properties [23]. That is, the energy-band structure is mainly determined by the BO6 octahedron. Similarly, the important effect of the (Li, La) 3⫹ pairs in (Lix⫹,La0.5 )TiO2.75⫹x/2:Pr would be their electrostatic contribution to the Madelung potentials and the crystal field. Indeed, the Li deficiency is not strongly correlated to the band gap energy as evidenced by the absorption band (Fig.3, top) which shows no large shift in spite of different x value. However, the accompanied oxygen vacancies in the TiO6 octahedral sublattice likely produce the holes trapped near the Li ⫹ sites. If the excitation of the host matrix is attributed to a band-to-band transition as proposed for SrTiO3:Pr, then, the higher concentration of trapped holes will increase the probability to recombine the excited electrons. The relatively narrow band gap, which is similar to that of SrTiO3 as compared in Fig. 2 (top), would cause the enhanced emission intensity of Pr 3⫹ by the resultant energy transfer. In an attempt to confirm the formation of trapped holes by the lithium deficiency, the effects of Ga addition on the PL spectra of Li0.5La0.5TiO3:0.2(mol%)Pr were explored. The addition of Ga 3⫹ or Al 3⫹ into SrTiO3:Pr phosphor resulted in the remarkable increase in the emission intensity of Pr 3⫹. As noted above, such an enhancement of emission was associated with the increased number of holes trapped at Ga 3⫹ which are able to recombine the electrons excited into the conduction band [7]. If there is already a sufficient amount of holes trapped near Li ⫹ before the addition of Ga 3⫹, however, it is expected that the additional acceptor levels will give no further influence on the recombination probability. The PL spectra of Li0.5La0.5TiO3:0.2(mol%)Pr,
x(mol%) Ga are compared as a function of x value in Fig. 4. In contrast to SrTiO3:Pr, Ga, the emission intensity weakens with increasing the amount of added Ga. Thus, it is found that the Li ⫹ deficiencies play a role similar to the addition of Ga 3⫹, both effects being related to the formation of oxygen vacancies. One interesting feature is that the PL intensities can be different in spite of similar oxygen vacancy as shown in Fig. 5. Compared with those of Li0.31La0.52TiO2.94:Pr and Li0.16La0.55TiO2.91:Pr, considerably higher emission intensities are observed for Li0.39La0.50TiO2.95:Pr and La0.16La0.48TiO2.92:Pr having the same oxygen vacancy within the experimental errors, respectively. This difference suggests that the Pr 3⫹ ions preferably occupy the sites for La 3⫹. Although the same oxygen vacancies give the same number of distorted sites for La 3⫹ and Pr 3⫹, more effective occupation of these sites by Pr 3⫹ could be induced in an La 3⫹poorer matrix. A much stronger emission intensity of Li0.39La0.45TiO2.87:Pr than that of Li0.21La0.50TiO2.86:Pr (Fig. 3, bottom; e)) with comparable oxygen vacancy also supports this picture. ⫹ 3⫹ ⫹ 3⫹ The substitutions of the (Na0.5 La0.5 ) and (K0.5 La0.5 ) pairs ⫹ 3⫹ 2⫹ instead of the (Li0.5La0.5 ) one for Sr were examined in order to change the size and distortion of the Pr 3⫹ site. ⫹ 3⫹ The PL spectra of (A0.5 La0.5 )TiO3:0.2(mol%)Pr where A Li, Na, and K are compared in Fig. 6. A strong decrease in the intensity and even a shift toward shorter wavelength of the emission band are observed with replacing A from Li ˚ ) and K ⫹ (1.64 A ˚) to K. Comparing the size of Li ⫹ (⬃1.2 A ˚ ) of La 3⫹ [24], it is indicated that a considwith that (1.36 A erable lattice expansion accompanied by an enlargement of the activator site will disrupt an efficient energy transfer and reduce the emission intensity. Finally, it could be noted that ⫹ 3⫹ the excitation band of (Na0.5 La0.5 )TiO3:Pr was observed at 365 nm wavelength, which can be easily obtained by a
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commercial UV lamp. The resulting emission intensity at 608 nm was comparable with that of YVO4:Eu when excited at 365 nm. An optimization of photoluminescent behavior ⫹ 3⫹ of (Na0.5 La0.5 )TiO3:Pr with potential application as a lamp phosphor is in progress. Acknowledgements This work was supported by the Ministry of Information and Communication (“Support project of university foundation research ’99” supervised by IITA). References [1] S.W. Depp, W.E. Howard, Sci. Am. March (1993) 90. [2] A. Ghis, R. Meyer, P. Rambaud, F. Levy, T. Leroux, IEEE Trans. Electron Devices 38 (1991) 2320. [3] S. Itoh, H. Toki, Y. Sato, K. Morimoto, T. Kishino, Jpn J. Appl. Phys. 32 (1993) 3955. [4] H.E. Hoefdraad, G. Blasse, Phys. Status Solidi A 89 (1975) K95. [5] S.H. Cho, J.S. Yoo, J.D. Lee, J. Electrochem. Soc. 143 (1996) L231. [6] H. Yamamoto, S. Okamoto, H. Toki, K. Tamura, S. Itoh, in: Third International Conference on the Science and Technology of Display Phosphors, California, November 1997, p. 3. [7] M.-A. Lee, S. Nahm, M.-H. Kim, K.-S. Suh, J.-D. Byun, J. Korean Ceram. Soc. 35 (1998) 757.
[8] W.L. Wanmaker, J.W. ter Vrugt, Philips Res. Rep. 24 (1969) 201. [9] G. Blasse, A. Bril, Mater. Res. Bull. 5 (1970) 231. [10] S. Itoh, H. Toki, Y. Sato, K. Morimoto, T. Kishino, J. Electrochem. Soc. 138 (1991) 1509. [11] L.E. Shea, R.K. Datta, J.J. Brown Jr., J. Electrochem. Soc. 141 (1994) 2198. [12] T. Omata, N. Ueda, K. Ueda, H. Kawazoe, Appl. Phys. Lett. 64 (1994) 1077. [13] K.-S. Suh, S.-J. Jang, K.-I. Cho, S. Nahm, J.-D. Byun, J. Vac. Sci. Technol., B 16 (1998) 858. [14] S.R. Rotman, in: H.L. Tuller (Ed.), Wide-gap luminescent materials: Theory and applications, Kluwer Academic, Dordrecht, MA, 1997, p. 41. [15] M. Okamura, M. Tamatani, A.K. Albessard, N. Matsuda, Jpn J. Appl. Phys. 36 (1997) 6411. [16] M. Itoh, Y. Inaguma, W.-H. Jung, L. Chen, T. Nakamura, Solid State Ionics 70/71 (1994) 203. [17] I.M. Batyaev, S.M. Shilov, C. Meiulebte, Optics and Spectroscopy 78 (1995) 705. [18] A.M. Srivastava, D.A. Doughty, W.W. Beers, J. Electrochem. Soc. 143 (1996) 4113. [19] G. Wittmann, R.M. Macfarlane, Optics Lett. 21 (1996) 426. [20] M. Cardona, Phys. Rev. A 140 (1965) 651. [21] S. Ahuja, T.R.N. Kutty, J. Photochem. Photobiol. A: Chem. 97 (1996) 99. [22] B.G. Yacobi, D.B. Holt, Cathodoluminescence Microscopy of Inorganic Solids, Plenum Press, New York, 1990, p. 247. [23] S. Ellialtioglu, T. Wolfram, Phys. Rev., B 18 (1978) 4509. [24] R.D. Shanon, Acta Crystallogr. A 32 (1976) 751.
Solid State Communications 115 (2000) 99–104 www.elsevier.com/locate/ssc
Highly enhanced photoluminescence of SrTiO3:Pr by substitution of (Li0.5, La0.5) pair for Sr K.-A. Hyeon a, S.-H. Byeon a,*, J.-C. Park b, D.-K. Kim c, K.-S. Suh d a
College of Environment and Applied Chemistry, Kyung Hee University, Yong In, Kyung Ki 449-701, South Korea b Department of Chemistry of New Materials, Silla University, Pusan 617-736, South Korea c Department of Chemistry, College of Natural Sciences, Kyungpook National University, Taegu 702-701, South Korea d Semiconductor Division, ETRI, Yusong, P.O. Box 106, Taejon 305-600, South Korea Received 10 January 2000; accepted 7 March 2000 by C.N.R. Rao
Abstract Photoluminescence (PL) of SrTiO3 –(Li0.5La0.5)TiO3:Pr system was investigated. This system was characterized by a very intense red emission at 611 nm corresponding to the 1 D2 ! 3 H4 inner transition of Pr 3⫹ ions. A weak PL intensity of SrTiO3:Pr was remarkably increased when the Sr atom was replaced by the (Li, La) pair. Through the elemental analysis for the Li atom, the dependence of PL intensity on the composition and heating temperature is correlated with the amount of defect sites for the Li atoms. Based on the experimental data, it is proposed that the substitution of the (Li, La) pair for Sr of SrTiO3 likely produces the holes trapped near Li ⫹. The increased recombination probability of electrons and trapped holes would then cause the highly enhanced emission intensity of Pr3⫹ by the resultant energy transfer. 䉷 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: C. Point defects; D. Optical properties; E. Luminescence
1. Introduction Since the flat-panel display technology using plasma was developed in the late 1960s [1], extensive research in the area of luminescence has been focused on the phosphor development or improvement for low-voltage flat-panel displays. Field emission displays (FEDs), which were realized for the first time in 1991 [2], are currently being explored as a potential flat-panel display technology. One of the most serious obstacles to the commercialization of FEDs is the absence of efficient low-voltage phosphors that do not degrade under prolonged Coulomb loading. The phosphor compositions most widely considered for FEDs have been the conventional cathode ray tube (CRT) phosphors, vacuum fluorescent display (VFD) phosphors, and projection TV phosphors. Another important aspect of the FED phosphor system is the volatility of the phosphor. In particular, electron stimulated reactions have been shown to occur at the phosphor surface in several sulfide based * Corresponding author. Tel.: ⫹82-331-201-2457; fax: ⫹82-331202-7337. E-mail address: [email protected] (S.-H. Byeon).
materials. In the case of CRT-red phosphor Y2O2S:Eu, for instance, the sulfide gas released from the phosphor by electron irradiation not only causes degradation of the phosphor brightness but also increases the work function of the field emitters [3]. No evolution of gases in the assembly process and in operation is therefore required for the low-voltage FED phosphors. To avoid the volatile sulfur byproduct, more stable oxide phosphors are being examined. For this purpose, some oxides of the perovskite structure were investigated as one of the host matrices. CaZrO3:Pr phosphor gave the green emission [4]. The red emission was observed in CaTiO3:Pr and SrTiO3:Pr phosphors [5,6]. Particularly, the addition of Ga 3⫹ or Al 3⫹ to SrTiO3:Pr phosphor largely increased the luminous efficiency. The recombination of electrons and holes trapped at Ga 3⫹ and the resultant energy transfer was proposed to induce such an enhanced red emission [7]. Self-activated type phosphors such as LiGa5O8, ZnGa2O4, and a solid solution between Li0.5Ga2.5O4 and ZnGa2O4 were also intensively investigated [8–13]. A strong compositional dependence of luminescence was observed for lithium zinc gallate phosphor. In the present work, SrTiO3 –Li0.5La0.5TiO3 solid solution
0038-1098/00/$ - see front matter 䉷 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00127-7
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Table 1 Experimental host compositions determined by ICP Nominal host compositions
Li0.50La0.50TiO3 Li0.50La0.50TiO3 ⫹30% excess Li Li0.50La0.50TiO3 ⫹50% excess Li Li0.50La0.50TiO3 ⫹100% excess Li Li0.50La0.50TiO3 Li0.50La0.50TiO3 Li0.50La0.50TiO3 Li0.56La0.48TiO3 Li0.65La0.45TiO3 Li0.44La0.52TiO3 Li0.35La0.55TiO3
Heating times (h) at 1200⬚C
Experimental host compositions
4 4
Li0.39La0.50TiO2.95 Li0.50La0.50TiO3
4
Li0.50La0.50TiO3
4
Li0.50La0.50TiO3
6 8 12 4 4 4 4
Li0.33La0.50TiO2.92 Li0.32La0.50TiO2.91 Li0.21La0.50TiO2.86 Li0.39La0.48TiO2.92 Li0.39La0.45TiO2.87 Li0.31La0.52TiO2.94 Li0.16La0.55TiO2.91
doped by Pr was explored in an attempt to search a new oxide phosphor. It is known that the 3 P0 ! 3 H4 and/or 1 D2 ! 3 H4 transition can be induced for the Pr 3⫹ ion depending on the host lattice [14]. Each transition corresponds to the green and red emissions, respectively. For instance, La2O3:Pr phosphor shows the green emission but
cubic Y2O3:Pr the red emission [15]. Very intense red emission at 611 nm corresponding to the 1 D2 ! 3 H4 inner transition of Pr 3⫹ ions was observed in our solid solution. Interestingly, a weak photoluminescence (PL) intensity of SrTiO3:Pr was remarkably increased when the Sr atom was replaced by the (Li, La) pair. Through the elemental analysis for the Li atom of Lix La0:5 TiO2:75⫹x=2 :Pr, which shows the highest brightness, the dependence of PL intensity on the composition and heating temperature was correlated with the amount of defect sites for the Li atoms. The influence of different alkali metals such as Na and K were also systematically investigated.
2. Experimental The SrTiO3 –Li0.5La0.5TiO3:xPr (x mol%; 0.05–0.80) solid solutions were prepared by typical solid state reaction. Well ground stoichiometric mixtures of SrCO3 (4 N), Li2CO3 (4 N), La2O3 (4 N), TiO2 (4 N) and PrCl3·7H2O (3 N) dissolved in absolute ethanol were heated at 900⬚C for 5 h in air. The residues were reground and heated again at 1200⬚C for 4, 6, 8, and 12 h in air. Li0.5⫺3xLa0.5⫹xTiO3:Pr and A0.5La0.5TiO3:Pr (A Na and K) were also prepared by the same procedure for comparison. About 130, 150, and 200% of Li2CO3 were added in order to induce higher contents of Li in the matrix. Since the high temperature treatment should induce a loss of volatile Li component, the quantitative analysis using the inductively coupled plasma (ICP) method was carried out to determine the Li content in this series. About 10 mg of the sample and about 1 ml of conc. HCl (35 wt%) were sealed into a Pyrex glass tube and kept at about 150⬚C for ⬃3 h to dissolve. The powder X-ray diffraction patterns were recorded on a rotating anode installed diffractometer (18 kW). The Cu Ka radiation used was monochromated by a curved-crystal graphite. A spectrofluorometer (Hitachi, F-4500) was used for the PL measurement at room temperature. The sample loaded on a powder holder provided by Hitachi was mounted about 45⬚ to the excitation and source for PL measurement.
3. Results and discussion
Fig. 1. X-ray powder diffraction patterns of: (a) SrTiO3:0.2(mol%)Pr; (b) (Li0.39La0.50)TiO2.95:0.2(mol%)Pr; and (c) (Li0.50La0.50)TiO3:0.2(mol%)Pr heated at 1200⬚C for 4 h. Open circles and arrows in (b) represent the reflections resulted from the ordering of La 3⫹ and Li ⫹ or vacancies along the c-axis and the ordering of rock-salt type arrangement, respectively.
Elemental analysis using ICP revealed that a considerable loss of the lithium is induced at high temperature due to the evaporation, the amount of lithium loss being proportional to the heating time. For instance, heating the sample of nominal composition Li0.50La0.50TiO3:Pr at 1200⬚C for 12 h resulted in about 58% loss of Li content (Li0.21La0.50TiO2.86:Pr) and consequently the oxygen vacancy of ⬃4.7% per formula unit. The addition of higher than 30% excess Li2CO3 was needed to obtain stoichiometric Li0.50La0.50TiO3:Pr. The lithium contents of some prepared
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Fig. 2. Excitation spectra monitoring at the wavelength at 611 nm (top) and emission spectra exciting at the wavelength of 350 nm (bottom) of Sr1⫺xLix/2Lax/2TiO3:0.2(mol%)Pr (x 0:0; 0.2, 0.4, 0.6, 0.8, and 1.0) heated at 1200⬚C for 4 h. The nominal compositions were adopted for all samples.
samples, which were determined by ICP, are summarized in Table 1. Fig. 1 shows the X-ray powder diffraction patterns of SrTiO3:0.2(mol%)Pr 3⫹, (Li0.39La0.5)TiO2.95:0.2(mol%)Pr 3⫹, and (Li0.5La0.5)TiO3:0.2(mol%)Pr 3⫹, which were heated at 1200⬚C for 4 h. Substitution of the (Li, La) pair for Sr did not greatly affect the cubic perovskite structure of SrTiO3, a slight difference in unit cell parameter being only induced. The small diffraction lines marked in Fig. 1(b), which are not observed in the cubic SrTiO3, indicate that La 3⫹ and Li ⫹ or vacancy are ordered along the c-axis or with the rock-salt arrangement [16]. The relative intensities of these additional diffractions were dependent on the amount of Li and vacancy as expected. Based on the emission intensity, the activator concentration was optimized. The maximal intensity of PL was obtained at 0.2 mol% of Pr on heating the samples at
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Fig. 3. Excitation spectra monitoring at the wavelength at 611 nm (top) and emission spectra exciting at the wavelength of 350 nm (bottom) of Lix La0:5 TiO2:75⫹x=2 :0.2(mol%)Pr.
1200⬚C for 4 h. Excitation and emission spectra of SrTiO3 –(Li0.5La0.5)TiO3:0.2(mol%)Pr solid solution are shown in Fig. 2. The excitation bands around 450, 470, 486 nm are attributed to 3 H4 ! 3 P2 , 3 P1 ; 3 P0 transition of the Pr 3⫹ ion, respectively [17–19]. The broad band around 350 nm is most probably related to the host lattice. Excitation of SrTiO3:Pr around this wavelength was associated with the electronic transition from valence band to conduction band of the host lattice with an energy band gap of ⬃3.4 eV [20,21]. The PL spectra of pure SrTiO3 showed a broad emission band around the 350–500 nm region [22]. Since the excitation bands of the Pr 3⫹ ion overlap this emission band (see Fig. 2), the energy transfer by recombination of an exciton could be responsible for the emission of Pr 3⫹. Both the broad excitation band around 350 nm and the sharp emission band at 611 nm are strongly influenced by the substitution of the (Li, La) pair for Sr. A dramatic increase in the intensity is observed for Li0.39La0.50TiO2.95:Pr containing no Sr. A shift of the excitation band toward shorter
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Fig. 4. Emission spectra exciting at the wavelength of 350 nm of Li0.5La0.5TiO3:0.2(mol%)Pr, x mol% Ga heated at 1200⬚C for 4 h. The nominal compositions were adopted for all samples.
wavelength is also observed with increasing the amount of the (Li, La) pair. It is generally accepted that the lattice constants and the bond lengths are the important factors to determine the band gap. On the contrary, the emission band at 611 nm shows no significant shift, indicating that this emission is associated with the inner transition of Pr 3⫹ ion. As shown in Fig. 1(b) and Table 1, the replacement of Sr by the (Li, La) pair gives rise to the formation of defective structures. The accompanied increase in the emission intensity might be accordingly attributed to the increased Li deficiency and therefore the oxygen vacancy in the host lattice. Since these oxygen vacancies are not statistically distributed but ordered along the c-axis or with the rock-salt arrangement, the sites offered for Pr 3⫹ would have uneven symmetry which is able to lift the parity selection rule. In order to
correlate the oxygen vacancy with the emission intensity, the PL spectra were investigated for the samples LixLa0.5TiO2.75⫹x/2:Pr containing different amounts of lithium ion. Fig. 3 shows the transition of their excitation and emission spectra profiles with x value. Although no strong change of excitation energy is observed, the band intensity is significantly dependent of the oxygen vacancy. The intensity of emission bands at 611 nm increases as x decreases from 0.50 to 0.39. When x is smaller than 0.33, the emission intensity is considerably reduced. Thus, the oxygen vacancies of about 1.6 mol% (i.e. x 0:39) give the maximal intensity of emission. This variation clearly indicates that the PL intensity is influenced by the amount of oxygen vacancy. Considering that these oxygen vacancies are ordered, the sufficient number of sites (⬃1.7% per formula unit) whose
Fig. 5. Emission spectra exciting at the wavelength of 350 nm of Li0:5⫺x La0:5⫹y TiO2:75⫹d=2 :0.2(mol%)Pr.
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Fig. 6. Emission spectra exciting at the wavelength of 350 nm of A0.5La0.5TiO3:0.2(mol%)Pr (A Li, Na, and K) heated at 1200⬚C for 4 h. The nominal compositions were adopted for all samples.
local symmetry is lower than cubic symmetry can be offered for the activators (Pr) of 0.2 mol%. Energy-band studies for ABO3 perovskite oxides showed that the energy bands originated by the A ions, which are far from the band gap, do not play an important role in determining electronic and optical properties [23]. That is, the energy-band structure is mainly determined by the BO6 octahedron. Similarly, the important effect of the (Li, La) 3⫹ pairs in (Lix⫹,La0.5 )TiO2.75⫹x/2:Pr would be their electrostatic contribution to the Madelung potentials and the crystal field. Indeed, the Li deficiency is not strongly correlated to the band gap energy as evidenced by the absorption band (Fig.3, top) which shows no large shift in spite of different x value. However, the accompanied oxygen vacancies in the TiO6 octahedral sublattice likely produce the holes trapped near the Li ⫹ sites. If the excitation of the host matrix is attributed to a band-to-band transition as proposed for SrTiO3:Pr, then, the higher concentration of trapped holes will increase the probability to recombine the excited electrons. The relatively narrow band gap, which is similar to that of SrTiO3 as compared in Fig. 2 (top), would cause the enhanced emission intensity of Pr 3⫹ by the resultant energy transfer. In an attempt to confirm the formation of trapped holes by the lithium deficiency, the effects of Ga addition on the PL spectra of Li0.5La0.5TiO3:0.2(mol%)Pr were explored. The addition of Ga 3⫹ or Al 3⫹ into SrTiO3:Pr phosphor resulted in the remarkable increase in the emission intensity of Pr 3⫹. As noted above, such an enhancement of emission was associated with the increased number of holes trapped at Ga 3⫹ which are able to recombine the electrons excited into the conduction band [7]. If there is already a sufficient amount of holes trapped near Li ⫹ before the addition of Ga 3⫹, however, it is expected that the additional acceptor levels will give no further influence on the recombination probability. The PL spectra of Li0.5La0.5TiO3:0.2(mol%)Pr,
x(mol%) Ga are compared as a function of x value in Fig. 4. In contrast to SrTiO3:Pr, Ga, the emission intensity weakens with increasing the amount of added Ga. Thus, it is found that the Li ⫹ deficiencies play a role similar to the addition of Ga 3⫹, both effects being related to the formation of oxygen vacancies. One interesting feature is that the PL intensities can be different in spite of similar oxygen vacancy as shown in Fig. 5. Compared with those of Li0.31La0.52TiO2.94:Pr and Li0.16La0.55TiO2.91:Pr, considerably higher emission intensities are observed for Li0.39La0.50TiO2.95:Pr and La0.16La0.48TiO2.92:Pr having the same oxygen vacancy within the experimental errors, respectively. This difference suggests that the Pr 3⫹ ions preferably occupy the sites for La 3⫹. Although the same oxygen vacancies give the same number of distorted sites for La 3⫹ and Pr 3⫹, more effective occupation of these sites by Pr 3⫹ could be induced in an La 3⫹poorer matrix. A much stronger emission intensity of Li0.39La0.45TiO2.87:Pr than that of Li0.21La0.50TiO2.86:Pr (Fig. 3, bottom; e)) with comparable oxygen vacancy also supports this picture. ⫹ 3⫹ ⫹ 3⫹ The substitutions of the (Na0.5 La0.5 ) and (K0.5 La0.5 ) pairs ⫹ 3⫹ 2⫹ instead of the (Li0.5La0.5 ) one for Sr were examined in order to change the size and distortion of the Pr 3⫹ site. ⫹ 3⫹ The PL spectra of (A0.5 La0.5 )TiO3:0.2(mol%)Pr where A Li, Na, and K are compared in Fig. 6. A strong decrease in the intensity and even a shift toward shorter wavelength of the emission band are observed with replacing A from Li ˚ ) and K ⫹ (1.64 A ˚) to K. Comparing the size of Li ⫹ (⬃1.2 A ˚ ) of La 3⫹ [24], it is indicated that a considwith that (1.36 A erable lattice expansion accompanied by an enlargement of the activator site will disrupt an efficient energy transfer and reduce the emission intensity. Finally, it could be noted that ⫹ 3⫹ the excitation band of (Na0.5 La0.5 )TiO3:Pr was observed at 365 nm wavelength, which can be easily obtained by a
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commercial UV lamp. The resulting emission intensity at 608 nm was comparable with that of YVO4:Eu when excited at 365 nm. An optimization of photoluminescent behavior ⫹ 3⫹ of (Na0.5 La0.5 )TiO3:Pr with potential application as a lamp phosphor is in progress. Acknowledgements This work was supported by the Ministry of Information and Communication (“Support project of university foundation research ’99” supervised by IITA). References [1] S.W. Depp, W.E. Howard, Sci. Am. March (1993) 90. [2] A. Ghis, R. Meyer, P. Rambaud, F. Levy, T. Leroux, IEEE Trans. Electron Devices 38 (1991) 2320. [3] S. Itoh, H. Toki, Y. Sato, K. Morimoto, T. Kishino, Jpn J. Appl. Phys. 32 (1993) 3955. [4] H.E. Hoefdraad, G. Blasse, Phys. Status Solidi A 89 (1975) K95. [5] S.H. Cho, J.S. Yoo, J.D. Lee, J. Electrochem. Soc. 143 (1996) L231. [6] H. Yamamoto, S. Okamoto, H. Toki, K. Tamura, S. Itoh, in: Third International Conference on the Science and Technology of Display Phosphors, California, November 1997, p. 3. [7] M.-A. Lee, S. Nahm, M.-H. Kim, K.-S. Suh, J.-D. Byun, J. Korean Ceram. Soc. 35 (1998) 757.
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