- Email: [email protected]
Spectral changes of Tb3+ fluorescence in borosilicate glasses
Journal of Luminescence 87}89 (2000) 679}681
Spectral changes of Tb> #uorescence in borosilicate glasses Kazuhiko Tonooka*, Okio Nishimura Hokkaido National Industrial Research Institute, Toyohira-ku, 2-17 Tsukisamu-higashi, Sapporo 062-8517, Japan
Abstract A remarkable spectral change in the Tb> #uorescence has been found for borosilicate glasses prepared by a sol}gel method. The Tb> #uorescence color changed from green to yellow or red after calcination, suggesting that the crystal "eld acting on Tb> become very strong. XPS analysis of the samples revealed this spectral change was related to boron loss during the heat treatment. 2000 Elsevier Science B.V. All rights reserved. Keywords: Spectral change; Tb> #uorescence; Sol}gel borosilicate glass
1. Introduction
2. Sample preparation
Terbium-activated phosphors are well known as excellent emitters of green light. The #uorescence of Tb> under UV excitation is mainly due to the D PF and H D PFH ( j"0,2, 6) transitions [1,2], as shown in Fig. 1. The Tb> #uorescence in most oxide glasses consists of four main emission lines around 490 (blue), 545 (green), 580 (yellow) and 620 (red) nm, which correspond to the D PFH ( j"6, 2, 3) transitions. Since the emission due to the D PF transition (+545 nm) mostly dominates over all other emissions, the Tb> #uorescence usually appears green to the human eye. The #uorescence intensity is considered to be dependent on the host lattice through the crystal "eld. It is known that the sol}gel derived glasses can easily change in their structures during "ring. Therefore, the #uorescence properties of Tb> in sol}gel glasses were expected to be a!ected by heat treatment. In this work, spectral changes in the Tb> #uorescence were examined for the sol}gel derived borosilicate glasses. Sol}gel technology is suitable for preparation of amorphous bulk, "bers and "lms [3,4]. Multicomponent sol}gel processing allows the preparation of a large number of materials doped with organic molecules.
The initial stage of a sol}gel processing for a simple one-component silica glass is hydrolysis and polymerization of a silicon alkoxide. A microporous gel-glass is formed by a complex sequence of polymerization, sol formation, gelation, and gel drying. This gel body will sinter into a dense non-porous glass during "ring at about 10003C. Preparation of multicomponent glasses from a mixture of alkoxides containing di!erent metals is complicated by the di!erent hydrolysis rates of di!erent alkoxides. Terbium nitrate was chosen because of its solvability in water as a source of Tb>. Silicon tetraethoxide and triethyl borate were used as sources of SiO and B O , respectively. Our "rst composition 83SiO ) 17B O ) 0.4Tb O (mol%) was prepared according to the following scheme: (1) mixing of 0.028 mol of silicon tetraethoxide with 0.03 mol of ethanol, Tb nitrate in a minimal amount of water and dimethylformamide (DMF); the total amount of water was 0.6 mol; (2) holding the solution at room temperature for +24 h; (3) adding 0.012 mol of triethyl borate to the solution; and (4) holding the solution at +603C in an oven to form wet gel. DMF was added to the solution to avoid the gel from cracking by modifying the surface tension of the interstitial liquid and the pore size of the gel [5]. After 2}3 days a transparent wet gel was formed. The gel body shrank into about one-tenth of its volume by itself during drying and "ring in air. Firing at 6003C resulted in the formation of an transparent amorphous bulk.
* Corresponding author. Tel.: #81-11-857-8954; fax: #8111-857-8900. E-mail address: [email protected] (K. Tonooka)
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 3 5 5 - 5
680
K. Tonooka, O. Nishimura / Journal of Luminescence 87}89 (2000) 679}681
3. Fluorescence measurements The #uorescence spectra were measured using a #uorospectrometer at room temperature. Fig. 2 shows the #uorescence spectra of Tb> in 83SiO ) 17B O and SiO glass samples calcined at 6003C for 5 h. Emissions corresponding to the D PFH ( j"6, 5, 4, 3) transition were observed for these samples doped with 0.4 mol% Tb O . The emissions from the D level were too weak to observe in our samples prepared by the sol}gel method. The Tb> luminescence in SiO }B O samples was twice as strong as that in SiO samples. After calcination at 8003C, the Tb> #uorescence color of the borosilicate sample changed to yellow, while that of the silicate sample remained green. The color of the radiation is determined by the spectral energy distribution of the emitted light. This #uorescence color change to yellow was found due to an increased intensity of the emissions corresponding to the D PF and D PF transitions. To look into this spectral change further, we examined the Tb> #uorescence for SiO }B O samples calcined at higher temperatures. Fig. 3 shows the #uorescence spectra of borosilicate samples doped with 0.4 mol% Tb O calcined at 6003C, 8003C and 10003C for 5 h. The emission due to the D PF transition reached its maximum for the sample calcined at 8003C. The relative intensity of the emissions due to the D PF and D PF transitions increased with increasing calcina-
Fig. 1. Energy diagram and main emission process of Tb>.
Fig. 2. Fluorescence spectra of Tb> in borosilicate and SiO samples calcined at 6003C.
tion temperature. The color of the Tb> #uorescence was found to be red for the sample calcined at 10003C, whereas the #uorescence intensity was reduced to about onetenth of that of the sample calcined at 6003C.
4. Discussion Since there was no considerable change in the energy distribution of the Tb> #uorescence for the sol}gel silicate samples after "rings, boron was suggested to be a key element a!ecting the spectral change. Then the atomic fractions of Si, B, O, and Tb in the samples were investigated by using X-ray photoelectron spectroscopy (XPS). The B/Si ratio of the samples prepared from B/Si"0.42 in precursor solutions decreased from 0.35 for the sample calcined at 6003C, to 0.13 for that calcined at 8003C, then to 0.08 for that calcined at 10003C, as shown in Fig. 4. The reduction of boron concentration with increasing calcination temperature is considered due to the volatilization of boron species. Researchers [6}8] reported that the boron loss during processing increases as the concentration of boron in the precursor solution increases. The formation of boric acid and its eventual volatilization were reported for borosilicate glass "lms "red to low temperatures (+4003C). The porous characteristics of the glass samples and the slow heating (+103C/h) in air would have enhanced the boron loss.
Fig. 3. Fluorescence spectra of Tb> in borosilicate glasses calcined at various temperatures.
Fig. 4. Dependence of the B/Si atomic ratio in the sol}gel SiO }B O : Tb glasses on the calcination temperature.
K. Tonooka, O. Nishimura / Journal of Luminescence 87}89 (2000) 679}681
Fluorescence spectra and intensity of RE ions are dependent on the symmetry and strength of crystal "eld, because the optical radiation of RE ion is mainly due to the forced electric dipole transition. Kuboniwa and Hoshina [9] reported a good agreement between observed #uorescence and calculation for Tb> in some oxides such as YPO , YBO and ScBO . Their theoretical calculation along the Judd}Ofelt [10,11] approximation showed that the D PF transition has the largest probability in the D PFH ( j"3, 4, 5, 6) transitions for all these matrices. It was also pointed out that the intensity of D PF transition can become comparable to that of D PF when the crystal "eld is very strong, where the D state of the 4f(S)5d con"guration was assumed to be admixed into the 4f con"guration. Our rough estimation based on the calculations in Ref. [9] led to the ratios of the electric-dipole transition probabilities as 2 : 1 : 2 for the D PF H ( j"3, 4, 5) transitions. This prediction agrees well with our "nding in the Tb>-doped borosilicate sample calcined at 10003C. Accordingly, the strengthened crystal "eld acting on Tb> due to the volatilization of boron species from the host glass was suggested to be a probable mechanism for the change in the #uorescence properties of Tb>-doped borosilicate glasses. The volatilization of boron oxides was expected to modify the crystal "eld on Tb> in the borosilicate glasses through the oxygen defects. In silicate glasses also, oxygen defects leading to changes of optical absorption and #uorescence were reported to be caused by thermal annealing, UV exposure, or c irradiation [12,13].
5. Conclusions The strong dependence of the #uorescence properties of Tb> on heat treatment was found in borosilicate
681
glasses prepared by the sol}gel method. The intensity of the Tb> #uorescence increased with the calcination temperature up to 8003C, but at 10003C a sharp decrease was observed. The dominant factor determining the #uorescence color is the spectral energy distribution, which generally consists of four major emission lines around 490, 545, 580 and 620 nm for the Tb> #uorescence. The spectral changes due to the heat treatments were found to be large enough to show either green, yellow, or red #uorescence for the 83SiO ) 17B O ) 0.4Tb O glass samples. These spectral changes in the Tb> #uorescence were suggested to be due to the boron loss and the generation of oxygen defects in borosilicate glasses. The sol}gel processing and the borosilicate network seemed essential for such large spectral changes in the Tb> #uorescence. The unusual Tb> #uorescence observed in this study was recognized as a case of strong crystal "eld.
References [1] L.G. Van Uitert, L.F. Johnson, J. Chem. Phys. 44 (1966) 3514. [2] E. Nakazawa, S. Shionoya, J. Chem. Phys. 47 (1967) 3211. [3] H. Dislich, J. Non-Cryst. Solids 63 (1984) 237. [4] P. Hintz, H. Dislich, J. Non-Cryst. Solids 82 (1986) 411. [5] L.L. Hench, G. Orcel, J.L. Nogues, in: C.J. Brinker, D.R. Ulrich, D.E. Clark (Eds.), Better Ceramics Through Chemistry II, Materials Research Society, Pittsburgh, 1986, p. 35. [6] E. Arai, Y. Terunuma, J. Electrochem. Soc. 121 (1974) 676. [7] S. Yamanaka, J. Akagi, M. Hattori, J. Non-Cryst. Solids 70 (1985) 279. [8] E.A. Taft, J. Electrochem. Soc. 118 (1971) 1985. [9] S. Kuboniwa, T. Hoshina, Japan J. Appl. Phys. 32 (1972) 1059. [10] B.R. Judd, Phys. Rev. 127 (1962) 750. [11] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [12] E.M. Dianov et al., Opt. Lett. 22 (1997) 1089. [13] Y. Sakurai et al., J. Appl. Phys. 86 (1999) 370.
Spectral changes of Tb> #uorescence in borosilicate glasses Kazuhiko Tonooka*, Okio Nishimura Hokkaido National Industrial Research Institute, Toyohira-ku, 2-17 Tsukisamu-higashi, Sapporo 062-8517, Japan
Abstract A remarkable spectral change in the Tb> #uorescence has been found for borosilicate glasses prepared by a sol}gel method. The Tb> #uorescence color changed from green to yellow or red after calcination, suggesting that the crystal "eld acting on Tb> become very strong. XPS analysis of the samples revealed this spectral change was related to boron loss during the heat treatment. 2000 Elsevier Science B.V. All rights reserved. Keywords: Spectral change; Tb> #uorescence; Sol}gel borosilicate glass
1. Introduction
2. Sample preparation
Terbium-activated phosphors are well known as excellent emitters of green light. The #uorescence of Tb> under UV excitation is mainly due to the D PF and H D PFH ( j"0,2, 6) transitions [1,2], as shown in Fig. 1. The Tb> #uorescence in most oxide glasses consists of four main emission lines around 490 (blue), 545 (green), 580 (yellow) and 620 (red) nm, which correspond to the D PFH ( j"6, 2, 3) transitions. Since the emission due to the D PF transition (+545 nm) mostly dominates over all other emissions, the Tb> #uorescence usually appears green to the human eye. The #uorescence intensity is considered to be dependent on the host lattice through the crystal "eld. It is known that the sol}gel derived glasses can easily change in their structures during "ring. Therefore, the #uorescence properties of Tb> in sol}gel glasses were expected to be a!ected by heat treatment. In this work, spectral changes in the Tb> #uorescence were examined for the sol}gel derived borosilicate glasses. Sol}gel technology is suitable for preparation of amorphous bulk, "bers and "lms [3,4]. Multicomponent sol}gel processing allows the preparation of a large number of materials doped with organic molecules.
The initial stage of a sol}gel processing for a simple one-component silica glass is hydrolysis and polymerization of a silicon alkoxide. A microporous gel-glass is formed by a complex sequence of polymerization, sol formation, gelation, and gel drying. This gel body will sinter into a dense non-porous glass during "ring at about 10003C. Preparation of multicomponent glasses from a mixture of alkoxides containing di!erent metals is complicated by the di!erent hydrolysis rates of di!erent alkoxides. Terbium nitrate was chosen because of its solvability in water as a source of Tb>. Silicon tetraethoxide and triethyl borate were used as sources of SiO and B O , respectively. Our "rst composition 83SiO ) 17B O ) 0.4Tb O (mol%) was prepared according to the following scheme: (1) mixing of 0.028 mol of silicon tetraethoxide with 0.03 mol of ethanol, Tb nitrate in a minimal amount of water and dimethylformamide (DMF); the total amount of water was 0.6 mol; (2) holding the solution at room temperature for +24 h; (3) adding 0.012 mol of triethyl borate to the solution; and (4) holding the solution at +603C in an oven to form wet gel. DMF was added to the solution to avoid the gel from cracking by modifying the surface tension of the interstitial liquid and the pore size of the gel [5]. After 2}3 days a transparent wet gel was formed. The gel body shrank into about one-tenth of its volume by itself during drying and "ring in air. Firing at 6003C resulted in the formation of an transparent amorphous bulk.
* Corresponding author. Tel.: #81-11-857-8954; fax: #8111-857-8900. E-mail address: [email protected] (K. Tonooka)
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 3 5 5 - 5
680
K. Tonooka, O. Nishimura / Journal of Luminescence 87}89 (2000) 679}681
3. Fluorescence measurements The #uorescence spectra were measured using a #uorospectrometer at room temperature. Fig. 2 shows the #uorescence spectra of Tb> in 83SiO ) 17B O and SiO glass samples calcined at 6003C for 5 h. Emissions corresponding to the D PFH ( j"6, 5, 4, 3) transition were observed for these samples doped with 0.4 mol% Tb O . The emissions from the D level were too weak to observe in our samples prepared by the sol}gel method. The Tb> luminescence in SiO }B O samples was twice as strong as that in SiO samples. After calcination at 8003C, the Tb> #uorescence color of the borosilicate sample changed to yellow, while that of the silicate sample remained green. The color of the radiation is determined by the spectral energy distribution of the emitted light. This #uorescence color change to yellow was found due to an increased intensity of the emissions corresponding to the D PF and D PF transitions. To look into this spectral change further, we examined the Tb> #uorescence for SiO }B O samples calcined at higher temperatures. Fig. 3 shows the #uorescence spectra of borosilicate samples doped with 0.4 mol% Tb O calcined at 6003C, 8003C and 10003C for 5 h. The emission due to the D PF transition reached its maximum for the sample calcined at 8003C. The relative intensity of the emissions due to the D PF and D PF transitions increased with increasing calcina-
Fig. 1. Energy diagram and main emission process of Tb>.
Fig. 2. Fluorescence spectra of Tb> in borosilicate and SiO samples calcined at 6003C.
tion temperature. The color of the Tb> #uorescence was found to be red for the sample calcined at 10003C, whereas the #uorescence intensity was reduced to about onetenth of that of the sample calcined at 6003C.
4. Discussion Since there was no considerable change in the energy distribution of the Tb> #uorescence for the sol}gel silicate samples after "rings, boron was suggested to be a key element a!ecting the spectral change. Then the atomic fractions of Si, B, O, and Tb in the samples were investigated by using X-ray photoelectron spectroscopy (XPS). The B/Si ratio of the samples prepared from B/Si"0.42 in precursor solutions decreased from 0.35 for the sample calcined at 6003C, to 0.13 for that calcined at 8003C, then to 0.08 for that calcined at 10003C, as shown in Fig. 4. The reduction of boron concentration with increasing calcination temperature is considered due to the volatilization of boron species. Researchers [6}8] reported that the boron loss during processing increases as the concentration of boron in the precursor solution increases. The formation of boric acid and its eventual volatilization were reported for borosilicate glass "lms "red to low temperatures (+4003C). The porous characteristics of the glass samples and the slow heating (+103C/h) in air would have enhanced the boron loss.
Fig. 3. Fluorescence spectra of Tb> in borosilicate glasses calcined at various temperatures.
Fig. 4. Dependence of the B/Si atomic ratio in the sol}gel SiO }B O : Tb glasses on the calcination temperature.
K. Tonooka, O. Nishimura / Journal of Luminescence 87}89 (2000) 679}681
Fluorescence spectra and intensity of RE ions are dependent on the symmetry and strength of crystal "eld, because the optical radiation of RE ion is mainly due to the forced electric dipole transition. Kuboniwa and Hoshina [9] reported a good agreement between observed #uorescence and calculation for Tb> in some oxides such as YPO , YBO and ScBO . Their theoretical calculation along the Judd}Ofelt [10,11] approximation showed that the D PF transition has the largest probability in the D PFH ( j"3, 4, 5, 6) transitions for all these matrices. It was also pointed out that the intensity of D PF transition can become comparable to that of D PF when the crystal "eld is very strong, where the D state of the 4f(S)5d con"guration was assumed to be admixed into the 4f con"guration. Our rough estimation based on the calculations in Ref. [9] led to the ratios of the electric-dipole transition probabilities as 2 : 1 : 2 for the D PF H ( j"3, 4, 5) transitions. This prediction agrees well with our "nding in the Tb>-doped borosilicate sample calcined at 10003C. Accordingly, the strengthened crystal "eld acting on Tb> due to the volatilization of boron species from the host glass was suggested to be a probable mechanism for the change in the #uorescence properties of Tb>-doped borosilicate glasses. The volatilization of boron oxides was expected to modify the crystal "eld on Tb> in the borosilicate glasses through the oxygen defects. In silicate glasses also, oxygen defects leading to changes of optical absorption and #uorescence were reported to be caused by thermal annealing, UV exposure, or c irradiation [12,13].
5. Conclusions The strong dependence of the #uorescence properties of Tb> on heat treatment was found in borosilicate
681
glasses prepared by the sol}gel method. The intensity of the Tb> #uorescence increased with the calcination temperature up to 8003C, but at 10003C a sharp decrease was observed. The dominant factor determining the #uorescence color is the spectral energy distribution, which generally consists of four major emission lines around 490, 545, 580 and 620 nm for the Tb> #uorescence. The spectral changes due to the heat treatments were found to be large enough to show either green, yellow, or red #uorescence for the 83SiO ) 17B O ) 0.4Tb O glass samples. These spectral changes in the Tb> #uorescence were suggested to be due to the boron loss and the generation of oxygen defects in borosilicate glasses. The sol}gel processing and the borosilicate network seemed essential for such large spectral changes in the Tb> #uorescence. The unusual Tb> #uorescence observed in this study was recognized as a case of strong crystal "eld.
References [1] L.G. Van Uitert, L.F. Johnson, J. Chem. Phys. 44 (1966) 3514. [2] E. Nakazawa, S. Shionoya, J. Chem. Phys. 47 (1967) 3211. [3] H. Dislich, J. Non-Cryst. Solids 63 (1984) 237. [4] P. Hintz, H. Dislich, J. Non-Cryst. Solids 82 (1986) 411. [5] L.L. Hench, G. Orcel, J.L. Nogues, in: C.J. Brinker, D.R. Ulrich, D.E. Clark (Eds.), Better Ceramics Through Chemistry II, Materials Research Society, Pittsburgh, 1986, p. 35. [6] E. Arai, Y. Terunuma, J. Electrochem. Soc. 121 (1974) 676. [7] S. Yamanaka, J. Akagi, M. Hattori, J. Non-Cryst. Solids 70 (1985) 279. [8] E.A. Taft, J. Electrochem. Soc. 118 (1971) 1985. [9] S. Kuboniwa, T. Hoshina, Japan J. Appl. Phys. 32 (1972) 1059. [10] B.R. Judd, Phys. Rev. 127 (1962) 750. [11] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [12] E.M. Dianov et al., Opt. Lett. 22 (1997) 1089. [13] Y. Sakurai et al., J. Appl. Phys. 86 (1999) 370.