- Email: [email protected]
Preparation and luminescence of rare-earth-activated Y2SiO5 thin films by metallorganic decomposition
Journal of Luminescence 87}89 (2000) 1297}1299
Preparation and luminescence of rare-earth-activated Y SiO thin "lms by metallorganic decomposition Yun Liu *, Chao-Nan Xu , Hiroaki Matsui , Takeshi Imamura , Tadahiko Watanabe Department of Inorganic Composite Materials, Kyushu National Industrial Research Institute, 807-1, Shuku, Saga 841-0052, Japan Department of Physics, Saga University, Saga 840-8502, Japan
Abstract The clear precursors of MV Y\V SiO (M"Eu, Sm, and Ce) were successfully synthesized by a metallorganic decomposition (MOD) technique for the "rst time. Dense and crack-free yttrium silicate (Y SiO )-based thin "lms appear to have the X }Y SiO structure on quartz substrates after annealing above 9003C. The photoluminescence (PL) and triboluminescence (TL) of these "lms were investigated. Results show that the europium-activated Y SiO thin "lms possess higher PL and TL properties than the other Y SiO thin "lms activated with Ce and Sm ions. The PL intensity is so strong that the red light emission at 611 nm is visible to the naked eye. The high TL intensity appears in the pure Y SiO thin "lm but it does not exist in the bulk material. This indicates that preparation of luminescent "lms is not only bene"cial for the fabrication of a planar display but also for improvement in the luminescent property. 2000 Elsevier Science B.V. All rights reserved. Keywords: Metallorganic decomposition; Y SiO thin "lm; Rare-earth activator
1. Introduction Yttrium silicate (Y SiO ) is a well-known luminescent host material for various rare-earth activators. Its high chemical stability makes it a promising candidate for integrated planar displays; therefore, formation of a luminescent yttrium silicate (Y SiO ) thin "lm is very important in this application. Ouyang and his coworkers have studied the preparation and electroluminescence of the Ce : Y SiO thin "lm by RF magnetron sputtering and used it in a multilayer display [1]. The metallorganic decomposition (MOD) is a solution deposition method. In comparison with the sputtering method, MOD has the advantages of a simple process, low cost, easily adjusted composition and easily changed activators. This process is compatible with
* Corresponding author. Tel.: #81-942-82-5161; fax: #81942-83-9858. E-mail addresses: [email protected] (Y. Liu), [email protected] (C.-N. Xu)
optical integrated circuits, and it is easy to realize the fabrication of planar displays. The present study was done to reveal the e!ect of di!erent activators on the luminescence of the thin "lms. Moreover, it is possible to improve the luminescent property of the materials. This work will promote the development of planar luminescent displays.
2. Experimental Precursors of the rare-earth-activated Y SiO were synthesized by the metallorganic decomposition (MOD) technique. The starting materials are yttrium nitrates, tetraethyl orthosilicate (TEOS) and diethylene glycol monomethyl ether. Rare-earth activators Eu>, Ce> and Sm> are obtained from europium nitrate, cerium nitrate and samarium oxide, respectively. Among them, the samarium oxide must react with acetic acid to form samarium acetate in order to dissolve it with the other starting materials in the same solution. Precursors with various compositions, such as Y SiO (YSO),
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 9 7 - 9
1298
Y. Liu et al. / Journal of Luminescence 87}89 (2000) 1297}1299
Eu Y SiO (EYSO), Ce Y SiO (C5YSO), Ce Y SiO (C10YSO), Sm Y SiO (SYSO) and Ce Eu Y SiO (CEYSO), were formed. After mixing the yttrium nitrate and europium nitrate in diethylene glycol monomethyl ether solvent, stirring and heating at 953C for 20 min, a slight #occulence phenomenon exists in the solution, and at the same time, the solution shows very high viscosity. In order to remove this #occulence and decrease the viscosity, acetylacetone was added to this solution. Moreover, small amounts of 2-propanol and polyvinyl alcohol (PVA) were mixed as additives to promote the formation of thin "lms. The typical concentration of precursors is 0.5 M. These precursors possess a high stability, in which the transparency of the precursor basically remains constant for a half-year, as measured by a UV}visible spectrophotometer (UV2100, Shimadzu). The thin "lms were deposited on quartz and silicon substrates by spin coating at 2500 rpm for 10 min followed by rapid thermal treatment at 6003C for 5 min. This process was repeated to achieve the desired thickness (about 150 nm), then the samples were heat-treated at temperatures between 8503C and 10503C. Dense and crack-free thin "lms were obtained on both substrates. The XRD analysis shows that these thin "lms possess the X }Y SiO structure after annealing above 9003C.
3. Results and discussions 3.1. Photoluminescence The photoluminescence spectra of the thin "lms were measured by a #uorescence spectrophotometer (F-3010, Hitachi). The typical PL spectra of YSO-based thin "lms are shown in Fig. 1. No PL phenomenon has been found in the pure YSO thin "lm. Various activators generate di!erent emission peaks: the EYSO and SYSO emit the red light, and the CYSO shows blue emission. As shown in Fig. 1, The EYSO thin "lm gives strong emission with peaks around 587, 611 and 630 nm. The emission peaks are due to the energy transitions from D to F , F , F . The emitted photon energy is in agreement with that of the bulk EYSO materials [2]. The highest peak at 611 nm corresponds to the transition from D to F . The red-light emission in the EYSO thin "lm is so strong that it is visible to the naked eye. Fig. 2 shows that the emission intensity increases with increasing thermal annealing temperature due to improvement of the crystallinity of the thin "lms. A similar PL spectrum of the EYSO thin "lm is observed on the silicon substrate. In addition, the emission intensity of the thin "lms is enhanced with increasing "lm thickness. No PL is observed in the SYSO thin "lm using an excitation below 295 nm, but a orange}red light emission peak appears under excitation at 402 nm as shown
Fig. 1. PL spectra of the YSO-based thin "lms on quartz substrates. (Ex"228 nm for YSO and EYSO, 402 nm for SYSO, 271 nm for CYSO and CEYSO).
Fig. 2. E!ect of thermal treatment temperatures on the PL intensities of the EYSO thin "lms on quartz substrates.
in Fig. 1. It is ascribed to the energy transfer from G to H . This intensity is obviously weaker than the EYSO thin "lm; therefore, the SYSO thin "lm shows a low e$cient luminescent material in this study. This
Y. Liu et al. / Journal of Luminescence 87}89 (2000) 1297}1299
1299
Table 1 TL intensities of the YSO-based thin "lms
Fig. 3. TL curve of the EYSO thin "lm on a quartz.
result is consistent with the bulk SYSO materials in the literature [2,3]. The C5YSO thin "lm shows a di!erent PL behavior in comparison with the EYSO and SYSO thin "lms as shown in Fig. 1. It has a maximum at about 410 nm due to the 5d}4f emission in Ce> ions. A broad emission band including several peaks is observed because there are several transitions from 4f ground state to di!erent crystal-"eld components of the 5d excited state of Ce> ions. For the co-activated CEYSO thin "lm, we could not detect light emission at 611 nm activated by Eu>, but the emission peak of the Ce> activator still appeared. There may be two reasons for this result: Eu> is quenched by Ce>; or the amount of the Eu> activator is too small in this case, because the optimum concentration for Eu> was reported to be 13.8 mol% for Eu> [2]. This is still under investigation. 3.2. Triboluminescence The YSO is not only a luminescent host material but also a high-temperature-resistance material. We hope the YSO "lm can detect the mechanical behavior at high temperature by means of the TL, which is induced during applied mechanical treatment [4,5]. It is characterized by a photon counter system reported previously [4], which generates the friction using a rotating rod with the speed of 150 rpm. Fig. 3 shows a typical TL response to friction. The maximum TL intensities of the YSO-based thin "lms are shown in Table 1. TL is observed in the pure YSO thin "lm, but it does not exist in the corresponding bulk materials. In addition, the EYSO thin "lm with 150 nm thickness also exhibits highest TL intensity. This indicates that the luminescent "lm is not only bene"cial for fabrication of planar devices but also for improve-
Sample
TL intensity (a.u.)
YSO/quartz EYSO/quartz EYSO/silicon C5YSO/quartz C10YSO/quartz CEYSO/quartz SYSO/quartz
120 537 14 87 73 86 109
ment of the luminescent property. The strongest TL performance appears in the EYSO "lm on a quartz substrate, but it is weaker on a silicon substrate. The other rare-earth-activated YSO thin "lms also show the TL performances although their PL intensities are weak as shown in Section 3.1. It is seen that the TL performance has a relationship not only with activators but also with its structure because the mechanism of TL greatly depend on the defect structure and mechanical properties [4,5]. The mechanism of TL in YSO "lms still remains to be investigated.
4. Conclusions The YSO-based thin "lms were successfully formed by the MOD method for the "rst time. Among Eu-, Sm- and Ce-activated YSO thin "lms, the europium-activated Y SiO thin "lms possess higher PL and TL properties than the other Y SiO thin "lms. The PL intensity of the EYSO thin "lm with 150 nm thickness is so strong that the red light emission at 611 nm is visible to the naked eye. In addition, the higher TL of the pure Y SiO thin "lm indicates that preparation of luminescent thin "lms is not only bene"cial for the fabrication of a planar display but also for improvement in the luminescent property.
References [1] X. Ouyang, A.H. Kitai, T. Xiao, J. Appl. Phys. 79 (6) (1996) 3229. [2] J. Shmulovich, G.W. Berkstresser, C.D. Brandle, A. Valentino, J. Electrochem. Soc. 135 (12) (1988) 3141. [3] J. Lin, Q. Su, H.J. Zhang, S.B. Wang, Mater. Res. Bull. 31 (2) (1996) 189. [4] C.N. Xu, T. Wataanabe, M. Akiyama, X.G. Zheng, Appl. Phys. Lett. 74 (9) (1999) 1236. [5] C.N. Xu, T. Wataanabe, M. Akiyama, X.G. Zheng, Appl. Phys. Lett. 74 (17) (1999) 2414.
Preparation and luminescence of rare-earth-activated Y SiO thin "lms by metallorganic decomposition Yun Liu *, Chao-Nan Xu , Hiroaki Matsui , Takeshi Imamura , Tadahiko Watanabe Department of Inorganic Composite Materials, Kyushu National Industrial Research Institute, 807-1, Shuku, Saga 841-0052, Japan Department of Physics, Saga University, Saga 840-8502, Japan
Abstract The clear precursors of MV Y\V SiO (M"Eu, Sm, and Ce) were successfully synthesized by a metallorganic decomposition (MOD) technique for the "rst time. Dense and crack-free yttrium silicate (Y SiO )-based thin "lms appear to have the X }Y SiO structure on quartz substrates after annealing above 9003C. The photoluminescence (PL) and triboluminescence (TL) of these "lms were investigated. Results show that the europium-activated Y SiO thin "lms possess higher PL and TL properties than the other Y SiO thin "lms activated with Ce and Sm ions. The PL intensity is so strong that the red light emission at 611 nm is visible to the naked eye. The high TL intensity appears in the pure Y SiO thin "lm but it does not exist in the bulk material. This indicates that preparation of luminescent "lms is not only bene"cial for the fabrication of a planar display but also for improvement in the luminescent property. 2000 Elsevier Science B.V. All rights reserved. Keywords: Metallorganic decomposition; Y SiO thin "lm; Rare-earth activator
1. Introduction Yttrium silicate (Y SiO ) is a well-known luminescent host material for various rare-earth activators. Its high chemical stability makes it a promising candidate for integrated planar displays; therefore, formation of a luminescent yttrium silicate (Y SiO ) thin "lm is very important in this application. Ouyang and his coworkers have studied the preparation and electroluminescence of the Ce : Y SiO thin "lm by RF magnetron sputtering and used it in a multilayer display [1]. The metallorganic decomposition (MOD) is a solution deposition method. In comparison with the sputtering method, MOD has the advantages of a simple process, low cost, easily adjusted composition and easily changed activators. This process is compatible with
* Corresponding author. Tel.: #81-942-82-5161; fax: #81942-83-9858. E-mail addresses: [email protected] (Y. Liu), [email protected] (C.-N. Xu)
optical integrated circuits, and it is easy to realize the fabrication of planar displays. The present study was done to reveal the e!ect of di!erent activators on the luminescence of the thin "lms. Moreover, it is possible to improve the luminescent property of the materials. This work will promote the development of planar luminescent displays.
2. Experimental Precursors of the rare-earth-activated Y SiO were synthesized by the metallorganic decomposition (MOD) technique. The starting materials are yttrium nitrates, tetraethyl orthosilicate (TEOS) and diethylene glycol monomethyl ether. Rare-earth activators Eu>, Ce> and Sm> are obtained from europium nitrate, cerium nitrate and samarium oxide, respectively. Among them, the samarium oxide must react with acetic acid to form samarium acetate in order to dissolve it with the other starting materials in the same solution. Precursors with various compositions, such as Y SiO (YSO),
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 9 7 - 9
1298
Y. Liu et al. / Journal of Luminescence 87}89 (2000) 1297}1299
Eu Y SiO (EYSO), Ce Y SiO (C5YSO), Ce Y SiO (C10YSO), Sm Y SiO (SYSO) and Ce Eu Y SiO (CEYSO), were formed. After mixing the yttrium nitrate and europium nitrate in diethylene glycol monomethyl ether solvent, stirring and heating at 953C for 20 min, a slight #occulence phenomenon exists in the solution, and at the same time, the solution shows very high viscosity. In order to remove this #occulence and decrease the viscosity, acetylacetone was added to this solution. Moreover, small amounts of 2-propanol and polyvinyl alcohol (PVA) were mixed as additives to promote the formation of thin "lms. The typical concentration of precursors is 0.5 M. These precursors possess a high stability, in which the transparency of the precursor basically remains constant for a half-year, as measured by a UV}visible spectrophotometer (UV2100, Shimadzu). The thin "lms were deposited on quartz and silicon substrates by spin coating at 2500 rpm for 10 min followed by rapid thermal treatment at 6003C for 5 min. This process was repeated to achieve the desired thickness (about 150 nm), then the samples were heat-treated at temperatures between 8503C and 10503C. Dense and crack-free thin "lms were obtained on both substrates. The XRD analysis shows that these thin "lms possess the X }Y SiO structure after annealing above 9003C.
3. Results and discussions 3.1. Photoluminescence The photoluminescence spectra of the thin "lms were measured by a #uorescence spectrophotometer (F-3010, Hitachi). The typical PL spectra of YSO-based thin "lms are shown in Fig. 1. No PL phenomenon has been found in the pure YSO thin "lm. Various activators generate di!erent emission peaks: the EYSO and SYSO emit the red light, and the CYSO shows blue emission. As shown in Fig. 1, The EYSO thin "lm gives strong emission with peaks around 587, 611 and 630 nm. The emission peaks are due to the energy transitions from D to F , F , F . The emitted photon energy is in agreement with that of the bulk EYSO materials [2]. The highest peak at 611 nm corresponds to the transition from D to F . The red-light emission in the EYSO thin "lm is so strong that it is visible to the naked eye. Fig. 2 shows that the emission intensity increases with increasing thermal annealing temperature due to improvement of the crystallinity of the thin "lms. A similar PL spectrum of the EYSO thin "lm is observed on the silicon substrate. In addition, the emission intensity of the thin "lms is enhanced with increasing "lm thickness. No PL is observed in the SYSO thin "lm using an excitation below 295 nm, but a orange}red light emission peak appears under excitation at 402 nm as shown
Fig. 1. PL spectra of the YSO-based thin "lms on quartz substrates. (Ex"228 nm for YSO and EYSO, 402 nm for SYSO, 271 nm for CYSO and CEYSO).
Fig. 2. E!ect of thermal treatment temperatures on the PL intensities of the EYSO thin "lms on quartz substrates.
in Fig. 1. It is ascribed to the energy transfer from G to H . This intensity is obviously weaker than the EYSO thin "lm; therefore, the SYSO thin "lm shows a low e$cient luminescent material in this study. This
Y. Liu et al. / Journal of Luminescence 87}89 (2000) 1297}1299
1299
Table 1 TL intensities of the YSO-based thin "lms
Fig. 3. TL curve of the EYSO thin "lm on a quartz.
result is consistent with the bulk SYSO materials in the literature [2,3]. The C5YSO thin "lm shows a di!erent PL behavior in comparison with the EYSO and SYSO thin "lms as shown in Fig. 1. It has a maximum at about 410 nm due to the 5d}4f emission in Ce> ions. A broad emission band including several peaks is observed because there are several transitions from 4f ground state to di!erent crystal-"eld components of the 5d excited state of Ce> ions. For the co-activated CEYSO thin "lm, we could not detect light emission at 611 nm activated by Eu>, but the emission peak of the Ce> activator still appeared. There may be two reasons for this result: Eu> is quenched by Ce>; or the amount of the Eu> activator is too small in this case, because the optimum concentration for Eu> was reported to be 13.8 mol% for Eu> [2]. This is still under investigation. 3.2. Triboluminescence The YSO is not only a luminescent host material but also a high-temperature-resistance material. We hope the YSO "lm can detect the mechanical behavior at high temperature by means of the TL, which is induced during applied mechanical treatment [4,5]. It is characterized by a photon counter system reported previously [4], which generates the friction using a rotating rod with the speed of 150 rpm. Fig. 3 shows a typical TL response to friction. The maximum TL intensities of the YSO-based thin "lms are shown in Table 1. TL is observed in the pure YSO thin "lm, but it does not exist in the corresponding bulk materials. In addition, the EYSO thin "lm with 150 nm thickness also exhibits highest TL intensity. This indicates that the luminescent "lm is not only bene"cial for fabrication of planar devices but also for improve-
Sample
TL intensity (a.u.)
YSO/quartz EYSO/quartz EYSO/silicon C5YSO/quartz C10YSO/quartz CEYSO/quartz SYSO/quartz
120 537 14 87 73 86 109
ment of the luminescent property. The strongest TL performance appears in the EYSO "lm on a quartz substrate, but it is weaker on a silicon substrate. The other rare-earth-activated YSO thin "lms also show the TL performances although their PL intensities are weak as shown in Section 3.1. It is seen that the TL performance has a relationship not only with activators but also with its structure because the mechanism of TL greatly depend on the defect structure and mechanical properties [4,5]. The mechanism of TL in YSO "lms still remains to be investigated.
4. Conclusions The YSO-based thin "lms were successfully formed by the MOD method for the "rst time. Among Eu-, Sm- and Ce-activated YSO thin "lms, the europium-activated Y SiO thin "lms possess higher PL and TL properties than the other Y SiO thin "lms. The PL intensity of the EYSO thin "lm with 150 nm thickness is so strong that the red light emission at 611 nm is visible to the naked eye. In addition, the higher TL of the pure Y SiO thin "lm indicates that preparation of luminescent thin "lms is not only bene"cial for the fabrication of a planar display but also for improvement in the luminescent property.
References [1] X. Ouyang, A.H. Kitai, T. Xiao, J. Appl. Phys. 79 (6) (1996) 3229. [2] J. Shmulovich, G.W. Berkstresser, C.D. Brandle, A. Valentino, J. Electrochem. Soc. 135 (12) (1988) 3141. [3] J. Lin, Q. Su, H.J. Zhang, S.B. Wang, Mater. Res. Bull. 31 (2) (1996) 189. [4] C.N. Xu, T. Wataanabe, M. Akiyama, X.G. Zheng, Appl. Phys. Lett. 74 (9) (1999) 1236. [5] C.N. Xu, T. Wataanabe, M. Akiyama, X.G. Zheng, Appl. Phys. Lett. 74 (17) (1999) 2414.