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Photoluminescence properties of silica monoliths codoped with terbium and germanium
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Materials Letters 62 (2008) 1945 – 1948 www.elsevier.com/locate/matlet
Photoluminescence properties of silica monoliths codoped with terbium and germanium I. Hernández a , G. Córdoba a , J. Padilla a , J. Méndez-Vivar a , R. Arroyo a,b,⁎ a
Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Apdo, Postal 55 534, México, D. F. 09820, México b Laboratorio Ligero, Facultad de Ciencias Básicas, Ingeniería y Tecnología, Universidad Autónoma de Tlaxcala, Calzada Apizaquito s/n, Apizaco, Tlaxcala, 90300, México Received 16 August 2007; accepted 25 October 2007 Available online 30 October 2007
Abstract We prepared SiO2 monoliths doped with 1 mol% of Tb3+ ions and 1, 3, 5, 10 and 15 mol% of GeO2 via the sol–gel process. The xerogels were sintered at 1100 °C in air during three hours. We also prepared materials in a H2/Ar reductive atmosphere at 550 °C. We studied the luminescent properties of these materials. Among the most important results, we found that the distribution of the Tb3+ ions in the materials network is homogeneous. In addition, the incorporation of GeO2 in the silica network leads to the quenching of the emission signals of Tb3+, due to an energy transference process from the dopant to the SiO2-GeO2 matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: Luminescence; Sol–gel preparation; Optical materials and properties
1. Introduction The preparation of materials with optical properties is currently of the outmost relevance due to their technological applications [1,2]. Recently the sol–gel process has been used extensively to obtain materials with luminescent properties [3–5]. Silica glasses doped with GeO2 have been extensively studied due to several interesting optical phenomena caused by defects related to the presence of germanium [6–8]. In SiO2-GeO2 films the photosensitivity and UV absorption are highly dependent from the GeO2 concentration, leading to the conclusion that the photosensitive processes related to germanium silicates depends to a great extent from the germanium content and the defects related to it [9]. GeO2, owes a high refractive index and tends to produce oxygen defects. This property of GeO2 has led to the synthesis of germanosilicates doped with Tb3+ ions [10,11]. The lumi⁎ Corresponding author. Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Apdo, Postal 55 534, México, D. F. 09820, México. Tel.: +55 5804 4641; fax: +55 5804 4666. E-mail address: [email protected] (R. Arroyo). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.047
nescent properties of silica xerogels codoped with Tb3+ and GeO2 have been recently studied, and the influence of the defects related to the latter when it is incorporated to the silica matrix [10]. Among the obtained results, it has been reported that the presence of germanium in the silica network increases the emission intensity of Tb3+ ions. A recent research [11] on the same system considers the possibility of using the defects related to GeO2 as sensitivity agents of the Tb3+ ions, due to the high absorptivity of those defects. In addition, the defects concentration can be controlled varying the amount of germanium and a reduction treatment. The goal of this research is to present the results derived from the study of the optical properties of Tb3+ and GeO2 codoped silica monoliths. According to our results, the increase of defects in these materials produces a quenching on the Tb3+ emission. In addition, the intensity of the emission increases when the monoliths are subjected to oxidation in air at high temperatures. 2. Experimental The materials were synthesized starting from tetraethyl orthosilicate (TEOS 98%, Aldrich), tetraethyl orthogermanate
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I. Hernández et al. / Materials Letters 62 (2008) 1945–1948
Fig. 1. Excitation spectra of samples calcined at 1100 °C.
(TEOG, 99.95+%, Aldrich) and terbium nitrate pentahydrate (99.9%, Aldrich). N,N-dimethylformamide (DMF) was used as drying control chemical additive. Samples with the nominal compositions (100-x-y)SiO2-xGeO2-yTbO1.5, where x = 0, 1,3,5,10,15 and y = 0 or 1, were prepared. The samples hereafter will be referred to as SGxTy. The molar ratio of TEOS, TEOG, terbium nitrate pentahydrate, DMF, ethanol and water were (100-x-y)/100:x/100:y/ 100:4:4:6. The experimental procedure consisted on the dissolution of terbium nitrate in deionized water, and then half of the required amount of ethanol was added. TEOS, DMF and the second half of ethanol was mixed separately, and finally TEOG was added. This mixture was stirred during five minutes at room temperature (25 °C). The aqueous solution was added, and later on, three drops of concentrated nitric acid were also added. The resulting sol was stirred during 20 min. and transferred to a Teflon™ vessel and sealed with aluminum foil. The samples were left in an oven at 60 °C during six days. The cover was then punctuated to allow the evaporation of a
Fig. 3. Emission spectra of monoliths containing various germanium amounts (td = 0.03 ms).
small amount of solvent and the temperature was gradually increased during five days up to 120 °C. The samples were kept at this temperature during ten days. The samples were then subjected to different thermal treatments in air and a mixture of hydrogen/argon (5 wt.%). The monoliths were transferred to a quartz vessel and thermally treated in air atmosphere in a furnace. The samples were heated at a rate of 5 °C/min. from room temperature (25 °C) up to 100 °C, and kept at that temperature for 1 h. Using the same heating rate the temperature was increased up to 1100 °C and kept to those conditions during three hours. Finally, the samples were left to cool down overnight to room temperature in the furnace. The treatment of the samples in hydrogen/argon atmosphere was performed in flux conditions. The sample was transferred to a quartz reactor at room temperature and subjected to a flux of hydrogen/argon (5 wt.%) at a rate of 10 mL/min. The temperature was then increased at a heating rate of 5 °C/min. up to 550 °C, and kept to this temperature during two hours. The sample was left to cool down overnight to room temperature in hydrogen/argon flux. These treatments were carried out in an equipment ISRI-RIG-100. The absorption spectra were carried out on a Varian model Cary 5 spectrophotometer. The fluorescence and phosphorescence spectra were obtained at room temperature using a luminescence Perkin Elmer LS-50B spectrophotometer. 3. Results and discussion
Fig. 2. Emission spectra of SG0T1 and SG5T1 samples.
The obtained monoliths were slightly opaque; however, when they were heated in air atmosphere at 1100 °C some fractures appeared and the appearance turned out to be clear glasses. The excitation spectra (λem = 546 nm) of samples containing different amounts of Tb3+ and GeO2 treated at 1100 °C appear in Fig. 1. The SG0T0 sample do not exhibit significant absorption bands between 200 and 400 nm; however,
I. Hernández et al. / Materials Letters 62 (2008) 1945–1948
the samples containing Tb3+ present absorption in the 225–400 nm range. The assignment can be done as it is described below. The three spectra present a very intense absorption band at 252 nm. The intensity of the band depends on the GeO2 concentration, decreasing when the amount of GeO2 increases. This band is assigned to the 4f8 → 4f75d terbium transition [12–14]. In addition, there can be observed low intensity absorption bands characteristic of Tb3+ that can be assigned to the 7 F6 → 5H6 (303 nm), 5H7 (318 nm), 5L7,8 (339 nm), 5D2 (352 nm), 5L10 (368 nm), and 5D3 (378 nm) transitions, respectively [15,16]. Fig. 2 presents the emission spectra (obtained after excitation at 254 nm) of samples SG0T1 and SG5T1. Two emission bands appear at 489 and 545 nm, characteristic of the 5D4 → 7F6 and 5D4 → 7F5 Tb3+ transitions, respectively [12–14]. An additional wide band appears at 400 nm, which can be assigned to the matrix emission [6]. The bands due to Tb3+ are better defined and more intense in the sample SG0T1; whereas the sample SG5T1 presents a lower intensity of the Tb3+ emission. This behavior suggests that the presence of germanium induces a quenching in the emission of Tb3+. This quenching cannot be due to a cross-relaxation process since the low terbium concentration and the method of synthesis lead to homogeneous materials. Since the incorporation of germanium produce the decaying of Tb3+ ion through the matrix, this process could occur with a diminishing in the intensity of the emission of the doping agent and simultaneously an increase in the intensity of the matrix signals. This statement is in agreement with the results in Fig. 3, where the emission spectra of all samples containing Tb3+ after excitation at 254 nm appear, the spectra were obtained with a td = 0.03 ms to eliminate the signals of the matrix and the dopant signals can be seen clearly. These results show clearly the existence of the 5D3 → 7F6 (378 nm), 7F5 (414 nm), 7F4 (437 nm) and 5D4 → 7F6 (489 nm), 7F5 (545 nm), 7F4 (588 nm), 7F3 (618 nm) transitions [15–18]. It is clearly observed in the spectra a gradual decreasing intensity of the 5D4 → 7FJ transitions when
1947
the germanium amount increases. Simultaneously, the intensity of the D3 → 7FJ transitions gradually increases. The decrease in the intensity of the signals that appear in Fig. 2 cannot be attributed to a cross-relaxation process, because it is not observed a decrease in the luminescence of 5D3; on the contrary, an increase in intensity is observed. In the crossrelaxation process the energy transference between two Tb3+ ions occurs as follows: a Tb3+ excited ion acts as donor in the 5D3 state relaxes nonradiatively to the 5D4 state, while the other Tb3+ ion, the acceptor in the base state is excited up to the 7F0 state [17]. The velocity of this crossrelaxation process increase with the diminishing of the donor–acceptor distance; in this way the emission of the 5D3 → 7FJ transitions should decrease with the closing in of the Tb3+ ions [17,18]; however, in the present case it is observed an increasing. This suggests that the distribution of the doping agent in the network occurs homogeneously, and at the same time the germanium addition favors the decaying through the network. This decaying affects both 5D3 → 7FJ and 5D4 → 7FJ emissions and therefore these emissions are more intense in the SG0T1 sample. In order to confirm this reasoning, the sample SG5T1 was treated to two oxidation-reduction cycles (see the Experimental section) in order to produce defects. Fig. 4 presents the emission spectra of the sample SG5T1 (treated in a reducting H2/Ar atmosphere) after exciting at 254 nm and using different delay times. The selected excitation wavelength was not only absorbed by the Tb3+ ions, but also by the matrix [6,7]. The spectrum obtained without delay time shows a wide band between 300 and 550 nm that is due to the emission of the SiO2-GeO2 matrix [6,7] and a weak signal at 545 nm, due to the 5D4 → 7F5 emission, of Tb3+. This indicates that the matrix emission predominates, without occurring the energy transference to the Tb3+ ions. When the delay time increases, the intensity of the matrix emission diminishes quickly and the Tb3+ signals gradually appear. Starting with a delay time of 0.04 ms, the signal due to the matrix emission exhibits a constant intensity and because its lifetime is very small, the most probable explanation is that the Tb3+ ions transfer energy to the matrix, and this one in turn keeps emitting. 5
4. Conclusions SiO2 monoliths doped with Tb3+ ions containing different amounts of GeO2 were successfully obtained via the sol–gel process. We found that the Tb3+ ions are homogeneously distributed inside the materials. In addition, the incorporation of GeO2 in the silica matrix leads to the quenching of the emission signals of the Tb3+ ions, due to an energy transference process from the dopant to the matrix. References
Fig. 4. Emission spectra of H2-reduced SG5T1 sample obtained with different delay times.
[1] M. Chiesa, M. Ferraris, E. Giamello, D. Milanese, J. Non-Cryst. Solids 328 (2003) 215. [2] V.K. Tikhomirov, A.B. Seddon, D. Furniss, M. Ferrari, J. Non-Cryst. Solids 326&327 (2003) 296. [3] A.A. Ismail, M. Abboudi, P. Holloway, H. El-Shall, Mater. Res. Bull. 42 (2007) 137. [4] S. Grandi, P. Mustarelli, A. Magistris, M. Gallorini, E. Rizzio, J. NonCryst. Solids 303 (2002) 208. [5] J. Qiu, N. Wada, F. Ogura, K. Kojima, K. Hirao, J. Phys., Condens. Matter 14 (2002) 2561. [6] S. Grandi, P. Mustarelli, S. Agnello, M. Cannas, A. Cannizzo, J. Sol-Gel Sci. Technol. 26 (2003) 915. [7] M. Takahashi, K. Ichii, Y. Tokuda, T. Uchino, T. Yoko, J. Nishii, T. Fujiwara, J. Appl. Phys. 92 (2002) 3442.
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[8] J.H. Jang, J. Koo, B.-S. Bae, J. Non-Cryst. Solids 259 (1999) 144. [9] K.D. Simmons, G.I. Stegeman, B.G. Potter Jr., J.H. Simmons, J. NonCryst. Solids 179 (1994) 254. [10] M. Bredol, S. Gutzov, Opt. Mater. 20 (2002) 233. [11] M. Bredol, S. Gutzov, Th. Jüstel, J. Non-Cryst. Solids 321 (2003) 225. [12] H.-M. Yang, J.-X. Shi, H.-B. Liang, M.-L. Gong, Mater. Res. Bull. 41 (2006) 867. [13] G. Wakefield, H.A. Keron, P.J. Dobson, J.L. Hutchison, J. Phys. Chem. Solids 60 (1999) 503.
[14] F. Zhao, P. Guo, G. Li, F. Liao, S. Tian, X. Jing, Mater. Res. Bull. 38 (2003) 931. [15] M. Flores, U. Caldiño, R. Arroyo, Opt. Mater. 28 (2006) 514. [16] M.E. Villafuerte-Castrejon, M.E. Álvarez R., U. Caldiño G., Ferroelectr. Lett. 28 (2001) 103. [17] C. Armellini, M. Ferrari, M. Montagna, G. Pucker, C. Bernard, A. Monteil, J. Non-Cryst. Solids 245 (1999) 115. [18] D. Jia, J. Zhu, B. Wu, S. E., J. Lumin. 93 (2001) 107.
Materials Letters 62 (2008) 1945 – 1948 www.elsevier.com/locate/matlet
Photoluminescence properties of silica monoliths codoped with terbium and germanium I. Hernández a , G. Córdoba a , J. Padilla a , J. Méndez-Vivar a , R. Arroyo a,b,⁎ a
Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Apdo, Postal 55 534, México, D. F. 09820, México b Laboratorio Ligero, Facultad de Ciencias Básicas, Ingeniería y Tecnología, Universidad Autónoma de Tlaxcala, Calzada Apizaquito s/n, Apizaco, Tlaxcala, 90300, México Received 16 August 2007; accepted 25 October 2007 Available online 30 October 2007
Abstract We prepared SiO2 monoliths doped with 1 mol% of Tb3+ ions and 1, 3, 5, 10 and 15 mol% of GeO2 via the sol–gel process. The xerogels were sintered at 1100 °C in air during three hours. We also prepared materials in a H2/Ar reductive atmosphere at 550 °C. We studied the luminescent properties of these materials. Among the most important results, we found that the distribution of the Tb3+ ions in the materials network is homogeneous. In addition, the incorporation of GeO2 in the silica network leads to the quenching of the emission signals of Tb3+, due to an energy transference process from the dopant to the SiO2-GeO2 matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: Luminescence; Sol–gel preparation; Optical materials and properties
1. Introduction The preparation of materials with optical properties is currently of the outmost relevance due to their technological applications [1,2]. Recently the sol–gel process has been used extensively to obtain materials with luminescent properties [3–5]. Silica glasses doped with GeO2 have been extensively studied due to several interesting optical phenomena caused by defects related to the presence of germanium [6–8]. In SiO2-GeO2 films the photosensitivity and UV absorption are highly dependent from the GeO2 concentration, leading to the conclusion that the photosensitive processes related to germanium silicates depends to a great extent from the germanium content and the defects related to it [9]. GeO2, owes a high refractive index and tends to produce oxygen defects. This property of GeO2 has led to the synthesis of germanosilicates doped with Tb3+ ions [10,11]. The lumi⁎ Corresponding author. Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Apdo, Postal 55 534, México, D. F. 09820, México. Tel.: +55 5804 4641; fax: +55 5804 4666. E-mail address: [email protected] (R. Arroyo). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.047
nescent properties of silica xerogels codoped with Tb3+ and GeO2 have been recently studied, and the influence of the defects related to the latter when it is incorporated to the silica matrix [10]. Among the obtained results, it has been reported that the presence of germanium in the silica network increases the emission intensity of Tb3+ ions. A recent research [11] on the same system considers the possibility of using the defects related to GeO2 as sensitivity agents of the Tb3+ ions, due to the high absorptivity of those defects. In addition, the defects concentration can be controlled varying the amount of germanium and a reduction treatment. The goal of this research is to present the results derived from the study of the optical properties of Tb3+ and GeO2 codoped silica monoliths. According to our results, the increase of defects in these materials produces a quenching on the Tb3+ emission. In addition, the intensity of the emission increases when the monoliths are subjected to oxidation in air at high temperatures. 2. Experimental The materials were synthesized starting from tetraethyl orthosilicate (TEOS 98%, Aldrich), tetraethyl orthogermanate
1946
I. Hernández et al. / Materials Letters 62 (2008) 1945–1948
Fig. 1. Excitation spectra of samples calcined at 1100 °C.
(TEOG, 99.95+%, Aldrich) and terbium nitrate pentahydrate (99.9%, Aldrich). N,N-dimethylformamide (DMF) was used as drying control chemical additive. Samples with the nominal compositions (100-x-y)SiO2-xGeO2-yTbO1.5, where x = 0, 1,3,5,10,15 and y = 0 or 1, were prepared. The samples hereafter will be referred to as SGxTy. The molar ratio of TEOS, TEOG, terbium nitrate pentahydrate, DMF, ethanol and water were (100-x-y)/100:x/100:y/ 100:4:4:6. The experimental procedure consisted on the dissolution of terbium nitrate in deionized water, and then half of the required amount of ethanol was added. TEOS, DMF and the second half of ethanol was mixed separately, and finally TEOG was added. This mixture was stirred during five minutes at room temperature (25 °C). The aqueous solution was added, and later on, three drops of concentrated nitric acid were also added. The resulting sol was stirred during 20 min. and transferred to a Teflon™ vessel and sealed with aluminum foil. The samples were left in an oven at 60 °C during six days. The cover was then punctuated to allow the evaporation of a
Fig. 3. Emission spectra of monoliths containing various germanium amounts (td = 0.03 ms).
small amount of solvent and the temperature was gradually increased during five days up to 120 °C. The samples were kept at this temperature during ten days. The samples were then subjected to different thermal treatments in air and a mixture of hydrogen/argon (5 wt.%). The monoliths were transferred to a quartz vessel and thermally treated in air atmosphere in a furnace. The samples were heated at a rate of 5 °C/min. from room temperature (25 °C) up to 100 °C, and kept at that temperature for 1 h. Using the same heating rate the temperature was increased up to 1100 °C and kept to those conditions during three hours. Finally, the samples were left to cool down overnight to room temperature in the furnace. The treatment of the samples in hydrogen/argon atmosphere was performed in flux conditions. The sample was transferred to a quartz reactor at room temperature and subjected to a flux of hydrogen/argon (5 wt.%) at a rate of 10 mL/min. The temperature was then increased at a heating rate of 5 °C/min. up to 550 °C, and kept to this temperature during two hours. The sample was left to cool down overnight to room temperature in hydrogen/argon flux. These treatments were carried out in an equipment ISRI-RIG-100. The absorption spectra were carried out on a Varian model Cary 5 spectrophotometer. The fluorescence and phosphorescence spectra were obtained at room temperature using a luminescence Perkin Elmer LS-50B spectrophotometer. 3. Results and discussion
Fig. 2. Emission spectra of SG0T1 and SG5T1 samples.
The obtained monoliths were slightly opaque; however, when they were heated in air atmosphere at 1100 °C some fractures appeared and the appearance turned out to be clear glasses. The excitation spectra (λem = 546 nm) of samples containing different amounts of Tb3+ and GeO2 treated at 1100 °C appear in Fig. 1. The SG0T0 sample do not exhibit significant absorption bands between 200 and 400 nm; however,
I. Hernández et al. / Materials Letters 62 (2008) 1945–1948
the samples containing Tb3+ present absorption in the 225–400 nm range. The assignment can be done as it is described below. The three spectra present a very intense absorption band at 252 nm. The intensity of the band depends on the GeO2 concentration, decreasing when the amount of GeO2 increases. This band is assigned to the 4f8 → 4f75d terbium transition [12–14]. In addition, there can be observed low intensity absorption bands characteristic of Tb3+ that can be assigned to the 7 F6 → 5H6 (303 nm), 5H7 (318 nm), 5L7,8 (339 nm), 5D2 (352 nm), 5L10 (368 nm), and 5D3 (378 nm) transitions, respectively [15,16]. Fig. 2 presents the emission spectra (obtained after excitation at 254 nm) of samples SG0T1 and SG5T1. Two emission bands appear at 489 and 545 nm, characteristic of the 5D4 → 7F6 and 5D4 → 7F5 Tb3+ transitions, respectively [12–14]. An additional wide band appears at 400 nm, which can be assigned to the matrix emission [6]. The bands due to Tb3+ are better defined and more intense in the sample SG0T1; whereas the sample SG5T1 presents a lower intensity of the Tb3+ emission. This behavior suggests that the presence of germanium induces a quenching in the emission of Tb3+. This quenching cannot be due to a cross-relaxation process since the low terbium concentration and the method of synthesis lead to homogeneous materials. Since the incorporation of germanium produce the decaying of Tb3+ ion through the matrix, this process could occur with a diminishing in the intensity of the emission of the doping agent and simultaneously an increase in the intensity of the matrix signals. This statement is in agreement with the results in Fig. 3, where the emission spectra of all samples containing Tb3+ after excitation at 254 nm appear, the spectra were obtained with a td = 0.03 ms to eliminate the signals of the matrix and the dopant signals can be seen clearly. These results show clearly the existence of the 5D3 → 7F6 (378 nm), 7F5 (414 nm), 7F4 (437 nm) and 5D4 → 7F6 (489 nm), 7F5 (545 nm), 7F4 (588 nm), 7F3 (618 nm) transitions [15–18]. It is clearly observed in the spectra a gradual decreasing intensity of the 5D4 → 7FJ transitions when
1947
the germanium amount increases. Simultaneously, the intensity of the D3 → 7FJ transitions gradually increases. The decrease in the intensity of the signals that appear in Fig. 2 cannot be attributed to a cross-relaxation process, because it is not observed a decrease in the luminescence of 5D3; on the contrary, an increase in intensity is observed. In the crossrelaxation process the energy transference between two Tb3+ ions occurs as follows: a Tb3+ excited ion acts as donor in the 5D3 state relaxes nonradiatively to the 5D4 state, while the other Tb3+ ion, the acceptor in the base state is excited up to the 7F0 state [17]. The velocity of this crossrelaxation process increase with the diminishing of the donor–acceptor distance; in this way the emission of the 5D3 → 7FJ transitions should decrease with the closing in of the Tb3+ ions [17,18]; however, in the present case it is observed an increasing. This suggests that the distribution of the doping agent in the network occurs homogeneously, and at the same time the germanium addition favors the decaying through the network. This decaying affects both 5D3 → 7FJ and 5D4 → 7FJ emissions and therefore these emissions are more intense in the SG0T1 sample. In order to confirm this reasoning, the sample SG5T1 was treated to two oxidation-reduction cycles (see the Experimental section) in order to produce defects. Fig. 4 presents the emission spectra of the sample SG5T1 (treated in a reducting H2/Ar atmosphere) after exciting at 254 nm and using different delay times. The selected excitation wavelength was not only absorbed by the Tb3+ ions, but also by the matrix [6,7]. The spectrum obtained without delay time shows a wide band between 300 and 550 nm that is due to the emission of the SiO2-GeO2 matrix [6,7] and a weak signal at 545 nm, due to the 5D4 → 7F5 emission, of Tb3+. This indicates that the matrix emission predominates, without occurring the energy transference to the Tb3+ ions. When the delay time increases, the intensity of the matrix emission diminishes quickly and the Tb3+ signals gradually appear. Starting with a delay time of 0.04 ms, the signal due to the matrix emission exhibits a constant intensity and because its lifetime is very small, the most probable explanation is that the Tb3+ ions transfer energy to the matrix, and this one in turn keeps emitting. 5
4. Conclusions SiO2 monoliths doped with Tb3+ ions containing different amounts of GeO2 were successfully obtained via the sol–gel process. We found that the Tb3+ ions are homogeneously distributed inside the materials. In addition, the incorporation of GeO2 in the silica matrix leads to the quenching of the emission signals of the Tb3+ ions, due to an energy transference process from the dopant to the matrix. References
Fig. 4. Emission spectra of H2-reduced SG5T1 sample obtained with different delay times.
[1] M. Chiesa, M. Ferraris, E. Giamello, D. Milanese, J. Non-Cryst. Solids 328 (2003) 215. [2] V.K. Tikhomirov, A.B. Seddon, D. Furniss, M. Ferrari, J. Non-Cryst. Solids 326&327 (2003) 296. [3] A.A. Ismail, M. Abboudi, P. Holloway, H. El-Shall, Mater. Res. Bull. 42 (2007) 137. [4] S. Grandi, P. Mustarelli, A. Magistris, M. Gallorini, E. Rizzio, J. NonCryst. Solids 303 (2002) 208. [5] J. Qiu, N. Wada, F. Ogura, K. Kojima, K. Hirao, J. Phys., Condens. Matter 14 (2002) 2561. [6] S. Grandi, P. Mustarelli, S. Agnello, M. Cannas, A. Cannizzo, J. Sol-Gel Sci. Technol. 26 (2003) 915. [7] M. Takahashi, K. Ichii, Y. Tokuda, T. Uchino, T. Yoko, J. Nishii, T. Fujiwara, J. Appl. Phys. 92 (2002) 3442.
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I. Hernández et al. / Materials Letters 62 (2008) 1945–1948
[8] J.H. Jang, J. Koo, B.-S. Bae, J. Non-Cryst. Solids 259 (1999) 144. [9] K.D. Simmons, G.I. Stegeman, B.G. Potter Jr., J.H. Simmons, J. NonCryst. Solids 179 (1994) 254. [10] M. Bredol, S. Gutzov, Opt. Mater. 20 (2002) 233. [11] M. Bredol, S. Gutzov, Th. Jüstel, J. Non-Cryst. Solids 321 (2003) 225. [12] H.-M. Yang, J.-X. Shi, H.-B. Liang, M.-L. Gong, Mater. Res. Bull. 41 (2006) 867. [13] G. Wakefield, H.A. Keron, P.J. Dobson, J.L. Hutchison, J. Phys. Chem. Solids 60 (1999) 503.
[14] F. Zhao, P. Guo, G. Li, F. Liao, S. Tian, X. Jing, Mater. Res. Bull. 38 (2003) 931. [15] M. Flores, U. Caldiño, R. Arroyo, Opt. Mater. 28 (2006) 514. [16] M.E. Villafuerte-Castrejon, M.E. Álvarez R., U. Caldiño G., Ferroelectr. Lett. 28 (2001) 103. [17] C. Armellini, M. Ferrari, M. Montagna, G. Pucker, C. Bernard, A. Monteil, J. Non-Cryst. Solids 245 (1999) 115. [18] D. Jia, J. Zhu, B. Wu, S. E., J. Lumin. 93 (2001) 107.