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Luminescence measurements on Sm2+-doped sol–gel glasses
Journal of Non-Crystalline Solids 304 (2002) 238–243 www.elsevier.com/locate/jnoncrysol
Luminescence measurements on Sm2þ -doped sol–gel glasses Vilma C. Costa b
a,*
, Yongrong Shen a, Ana M.M. Santos b, Kevin L. Bray
a
a Department of Chemistry, Washington State University, Pullman, WA 99164, USA Center of Nuclear Technology Development/CNEN, CP 941, Pampulha, Belo Horizonte, MG 30123-970, Brazil
Abstract Sol–gel glass samples containing nominally 1, 2, 5, and 10 wt% Sm2 O3 doped into the molar compositions 5Na2 O– 10Al2 O3 –85SiO2 and 10Al2 O3 –90SiO2 were prepared from metal alkoxide solution. After heat treatment in air at 500 °C, the samples were heated at 750 °C under flowing H2 or at 800 °C under a flowing H2 /N2 to reduce Sm3þ to Sm2þ . Samarium ions in the divalent and trivalent states were identified by luminescence and lifetime measurements. Samples incorporated with Sm2þ showed emission with peaks at 683, 700, 725 and 763 nm due to transitions 5 D0 ! 7 F0;1;2;3 , respectively, of Sm2þ ions. The luminescence properties of Sm2þ ions are discussed in relation to the concentration of Sm2 O3 and the glass matrix composition. The relative proportion of Sm2þ decreases with increasing Sm2 O3 doping concentration. Preliminary high pressure luminescence results for Sm2þ in the glasses show a complete quenching of Sm2þ luminescence and an irreversible local modification in the glass network. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Hx; 42.70.Ce; 62.50.þp
1. Introduction Divalent samarium (Sm2þ ) doped crystals and glasses have attracted attention for potential applications in lasers [1] and memory devices [2]. Interest in materials containing Sm2þ has increased because of their non-linear optical hole burning properties [3,4] and applications as optical datastorage materials [3–5]. Macfarlane and Shelby [6] first observed persistent spectral hole burning spectra in Sm2þ activated halide crystals at 2 K. Recently, persistent spectral hole burning has been
*
Corresponding author. Tel.: +1-509 335 2424; fax: +1-509 335 8867. E-mail address: [email protected] (V.C. Costa).
observed at room temperature in fluoride crystals [3], borate glasses [4], and fluorohafnate glasses [7]. A new approach for the preparation of Sm2þ -doped aluminosilicate glasses using the sol–gel process has been reported [8] and room temperature persistent spectral hole burning has been observed in these systems [5,9]. Glass systems are expected to have larger memory density in devices than crystals because of their inhomogeneous line widths and convenient sample preparation [4,5,7–9]. The sol– gel technique is a glass preparation method for which lower processing temperatures are required relative to conventional melting techniques and the porosities of sol–gel glasses increases reduction of Sm3þ to Sm2þ in presence of H2 atmosphere [8–11]. Sm2þ has a ground 4f6 configuration and an excited 4f5 5d configuration. The close proximity in
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 0 2 9 - 3
V.C. Costa et al. / Journal of Non-Crystalline Solids 304 (2002) 238–243
energy of the two configurations leads to an electronic mixing between the two configurations and to an alteration of the optical properties of Sm2þ . With increasing hydrostatic pressure in a sample, we can tune the extent of mixing of the two configurations by varying their energy separation. As a result, we can systematically investigate how the extent of mixing varies the static and dynamic emission properties of Sm2þ and correlate the emission properties with crystal field strength and coordination environment. There has been considerable progress in understanding the spectral properties of rare earth ions under pressure. The effects of pressure on spectral properties of Pr3þ [12], Nd3þ [13], Sm2þ [14] and Eu3þ [15] in crystals and Eu3þ [16] and Sm3þ [17] in glasses have been reported. In this paper, we present spectroscopic characterization of samarium doped aluminosilicate derived sol–gel glasses and preliminary high pressure Sm2þ luminescence results.
2. Experimental 2.1. Sample preparation Sol–gel glasses containing nominally 1, 2, 5, and 10 wt% Sm2 O3 doped into the molar compositions 5Na2 O–10Al2 O3 –85SiO2 and 10Al2 O3 –90SiO2 were prepared from metal alkoxide solution following the procedure presented by Nogami et al. [10]. Tetraethoxysilane (TEOS, Si(OC2 H9 )4 ), Al(OC4 Hsec 4 )3 , SmCl3 6H2 O or Sm(NO3 )3 6H2 O and CH3 CO2 Na were used as starting reagents. TEOS was hydrolyzed for 1 h with a solution of H2 O, C2 H5 OH in the molar ratio of 1:1 per mol of TEOS. In the first hydrolysis, HCl was added to achieve an initial solution pH of 1.5–2. Aluminum precursor, Al(OC4 Hsec 9 )3 , was added to the previously prepared solution. After stirring for 15 min, an ethanolic solution of either SmCl3 6H2 O or Sm(NO3 )3 6H2 O and CH3 CO2 Na was added and the solution was further stirred for 15 min. The resultant solution was hydrolyzed by adding the mixed solution of H2 O, C2 H5 OH and HCl in a 4:1:0.011 mol per mol of Al alkoxide.
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After stirring 30 min, the sols were cast into plastic vials where they were allowed to form gels at room temperature. After gelation, the samples were aged at 60 °C for two days and then dried at 90 °C for two days. After 2 h heat treatment in air at 500 °C, the glasses were heated up to 800 °C for 24 h under a flowing mixed gas of 20%H2 – 80%N2 or at 750 °C for 10 h under a flowing H2 . For sample in the composition 5Na2 O–10Al2 O3 – 85SiO2 containing 2 wt% Sm2 O3 , a similar preparation approach described above was used and the glasses were, also, preheated in air at 700 °C for 2 h. 2.2. Spectroscopic measurements Luminescence spectra were obtained upon an excitation at 488.0 nm by an argon laser. Samarium luminescence was dispersed on a 1-m monochromator (Spex 1704) and detected by a photomultiplier tube (Hamamatsu R928). All reported luminescence spectra have been corrected for the spectral response of instrument. For luminescence decay measurements, a pulsed Nd:YAG laser (a second harmonic laser beam of 532 nm) was used to excite Sm2þ from the 7 F0 (4f6 ) ground state to the excited 4f5 5d state. The spectra were detected by the 1-m monochromator and decay data were recorded by a digital storage oscilloscope (LeCroy 140). Lifetimes were measured at room temperature. Luminescence decays were monitored at the peak wavelength of the 5 D0 ! 7 F0 emission line of Sm2þ . For high-pressure luminescence experiments, pressure was generated by a diamond anvil cell (DAC) [18]. The sample chamber consisted of a 250 lm hole drilled in a indented inconel gasket. A piece of glass sample was loaded in the sample chamber along with a ruby pressure calibrant. A spectroscopic oil (poly-dimethylsiloxane) served as the pressure transmitting medium.
3. Results 3.1. Ambient pressure luminescence When the gels were heated in air at 500 °C for 2 h before heating in H2 atmosphere, we observed
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Fig. 1. Luminescence spectra of a 5Na2 O–10Al2 O3 –85SiO2 glass composition containing 2 wt% Sm2 O3 : (a) glass heated at 700 °C for 2 h in air and (b) glass heated at 750 °C in H2 gas for 10 h.
no luminescence of Sm2þ and Sm3þ . We attributed the absence of a luminescent signal to a nonradiative quenching by residual water and organic fragments remaining in the sol–gel matrix after heating at 500 °C [19]. Treatment at temperatures higher than 500 °C is needed to remove them and therefore expected to promote the luminescence of Sm3þ and/or Sm2þ . As an example, Fig. 1(a) shows the luminescence spectrum of 5Na2 O–10Al2 O3 –85SiO2 sample containing 2 wt% Sm2 O3 . The sample was heated in air at 700 °C for 2 h. The observed luminescence spectrum consisted of several peaks around 565, 600, 650 and 710 nm, which are attributed to 4 G5=2 ! 6 H5=2 , 6 H7=2 , 6 H9=2 , and 6 H11=2 transitions of Sm3þ [5,14], respectively. Fig. 1(b) shows the luminescence of the same composition after heat treatment at 750 °C for 10 h in a flowing H2 . The spectral changes are consistent with a partial reduction of Sm3þ to Sm2þ in the glass. Upon excitation into 4f5 5d state of Sm2þ (kexc ¼ 488 nm), 5 D0 luminescence of Sm2þ is yielded through a non-radiative 4f5 5d ! 5 D0 relaxation and subsequent radiative transitions from the 5 D0 level into lower 7 FJ levels [12,14]. The peaks at 683, 700, 725 and 763 nm are assigned to the 5 D0 ! 7 F0 , 7 F1 , 7 F2 , and 7 F3 transitions of Sm2þ , respectively [5, 14]. A similar trend in the luminescence spectrum was also observed for the 10Al2 O3 –90SiO2 glass
Fig. 2. Luminescence spectra of 10Al2 O3 –90SiO2 glasses containing: (a) 10 wt% Sm2 O3 , (b) 2 wt% Sm2 O3 , and (c) 1 wt% Sm2 O3 . The glasses were heated in a mixed H2 /N2 gas at 800 °C for 24 h. The spectra were normalized by the area of the spectrum over 550–800 nm.
composition under the two heat treatments above used for 5Na2 O–10Al2 O3 –85SiO2 glasses. The emission spectra of 10Al2 O3 –90SiO2 glasses containing 1, 2, and 10 wt% Sm2 O3 after heating at 800 °C in a mixed H2 /N2 gas for 24 h are shown in Fig. 2. The spectra indicated that the relative proportion of Sm2þ decreased with increasing Sm2 O3 doping concentration. As the Sm2 O3 concentration increases the relative intensity of the peaks assigned 5 D0 ! 7 F0 , 7 F1 , 7 F2 , and 7 F3 transitions of Sm2þ decreases. A comparison of the luminescence spectra of 5Na2 O–10Al2 O3 –85SiO2 and 10Al2 O3 –90SiO2 glasses containing 2 wt% Sm2 O3 treated at 750 °C in H2 for 10 h is shown in Fig. 3. The spectra (Fig. 3) clearly show that the greater relative proportion of Sm2þ luminescence is in the sample containing Na2 O. This indicates that inclusion of Na2 O in the glass composition increased the ratio of Sm2þ to Sm3þ upon heating in H2 . 3.2. Ambient pressure decay Luminescence decay curves at room temperature for the 5 D0 level of Sm2þ were obtained at excitation wavelength of 532 nm and measured at emission wavelength of 683 nm. The decay curves were non-exponential and appeared to consist of a fast component and a slow component. Since the
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Fig. 3. Luminescence spectra of glasses containing 2 wt% Sm2 O3 : (a) 10Al2 O3 –90SiO2 and (b) 5Na2 O–10Al2 O3 –85SiO2 . The glasses were heated at 750 °C in H2 gas for 10 h. The spectra were normalized by the area of the spectrum over 550– 800 nm.
decay curves were non-exponential, average 5 D0 lifetimes of Sm2þ were obtained by a common approach that is used to determine the lifetime for luminescent materials with a non-exponential decay [20,21]. The first step is to normalize the decay curve by its initial intensity (t ¼ 0) and the second step is to calculate the area of the normalized decay curve. The 5 D0 lifetimes of Sm2þ ranged from 0.60 to 0.86 ms in Na2 O–Al2 O3 –SiO2 and Al2 O3 – SiO2 matrices.
Fig. 4. Representative high pressure emission spectra of 10Al2 O3 –90SiO2 glass containing 2 wt% Sm2 O3 . The sample was heated at 750 °C in H2 gas for 10 h. The release spectrum is the result obtained upon returning the pressure to ambient pressure at the conclusion of the experiment. The sharp lines near 700 nm originate from the ruby pressure calibrant used in the experiment.
4. Discussion 4.1. Ambient pressure luminescence
3.3. High pressure luminescence Fig. 4 presents representative luminescence spectra for the 10Al2 O3 –90SiO2 glass composition containing 2 wt% Sm2 O3 under pressures up to 190 kbar. The sample used for our high-pressure study was heated to 750 °C in a H2 atmosphere for 10 h. The ambient pressure spectrum is included in Fig. 3(b). A redshift of 0.03 nm/kbar and an intensity decrease were observed for the 5 D0 ! 7 F0 transition of Sm2þ as pressure was increased. Above 74 kbar, Sm2þ emission disappeared. Upon release of pressure, the emission spectrum did not revert back to the original ambient pressure spectrum. An overall decrease in emission intensity and no Sm2þ emission was observed upon release of pressure at ambient pressure (the top spectrum of Fig. 4).
The experimental results for the decrease in the Sm2þ emission intensity relative to the Sm3þ emission intensity with increasing Sm2 O3 doping concentration (Fig. 2) indicate that the ratio Sm2þ / Sm3þ decreases upon increasing the Sm2 O3 doping concentration at a given Al/Si ratio. This trend is consistent with a recent nuclear magnetic resonance (NMR) and X-ray diffraction study of Sm2 O3 -doped Al2 O3 –SiO2 glasses reported by Jin et al. [11]. Jin et al. proposed that the presence of Al3þ increases the change of Sm3þ to Sm2þ . They argued that glasses doped with Sm3þ contain Al3þ almost exclusively in the form of network forming AlO4 tetrahedra. They further showed that heat treatment in a reducing (H2 ) atmosphere converted AlO4 tetrahedra into AlO6 octahedra and that this conversion coincides with the conversion of Sm3þ
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to Sm2þ . According to their AlO4 $ AlO6 model, we expect that the increasing non-bridging oxygen concentration resulting from the formation of AlO6 octahedra satisfies the increased site coordination of the larger Sm2þ ions. The data presented by Jin et al. showed for a given Al/Si ratio that the relative amount of Sm2þ decreased with increasing Sm2 O3 doping concentration. In other words, the fractional conversion of Sm3þ to Sm2þ decreased as the number of non-bridging oxygens available for Sm3þ decreased. Our results for the 5Na2 O– 10Al2 O3 –85SiO2 glass composition are also consistent with this interpretation. Addition of Na2 O to Al2 O3 –SiO2 glasses at fixed Al2 O3 content increased in the non-bridging oxygen concentration. A comparison of the results for 2 wt% Sm2 O3 : 5Na2 O–10Al2 O3 –85SiO2 (Fig. 3(b)) with those for 2 wt% Sm2 O3 :10Al2 O3 –90SiO2 (Fig. 3(a)) shows that the relative proportion of Sm2þ is greater in the presence of Na2 O. 4.2. High pressure luminescence The loss of Sm2þ emission intensity with pressure could be due to an enhancement of the nonradiative decay of Sm2þ or a pressure-induced oxidation of Sm2þ to Sm3þ . Previous high pressure studies of Sm2þ -doped crystals [14,22] showed that high pressure induces a larger redshift of the excited 4f5 5d state of Sm2þ than the 5 D0 level. Since the 4f5 5d state of Sm2þ is close in energy to the emitting 5 D0 sate of the ground 4f6 configuration of Sm2þ , it is conceivable that pressure induces a 4f5 5d–5 D0 (4f6 ) electronic crossover [14,22]. When the 4f5 5d state interacts more strongly with the glass matrix than with crystal host lattices, a pressure-induced electronic crossover could increase the non-radiative decay rate of Sm2þ and lead to a quenching of the 5 D0 luminescence. Alternatively, pressure may convert Sm2þ to Sm3þ . Based on the work of Jin et al. [11], such conversion would be likely accompanied by a local structural transformation of the glass involving back-conversion from AlO6 octahedra to AlO4 tetrahedra. In other words, pressure could induce the reverse of the reduction reaction. This process is plausible because, thermodynamically, pressure stabilizes the smallest volume state of a system
[18]. Jin et al. [11] reported a decrease in sample density upon reduction of Sm3þ to Sm2þ . Thus, oxidation of Sm2þ to Sm3þ is consistent with stabilization of the lowest volume state of the glass. Our results upon release of pressure indicate that an irreversible transformation occurs in the glass with increasing pressure. The irreversible transformation may involve a permanent structure change in the glass and is consistent with the proposed pressure-induced Sm2þ –Sm3þ oxidation.
5. Conclusion We have prepared and characterized Sm2þ doped Al2 O3 –SiO2 and Na2 O–Al2 O3 –SiO2 sol–gel glasses. The high pressure luminescence results demonstrated a pressure-induced irreversible modification in local sol–gel glass networks that governs the Sm2þ luminescence. Extension of our high pressure luminescence study to other divalent rare earth ions in sol–gel glasses is necessary to generalize structure-property relations.
Acknowledgements V.C.C. acknowledges financial support from Brazilian National Council of Research, CNPq and Dr Wilmar B. Ferraz (CDTN-Brazil) for his kind help with the heat treatment of glasses. Further acknowledgement is made to the donors of the Petroleum Research Fund, administered by the ACS, and the National Science Foundation for support of this research.
References [1] [2] [3] [4] [5] [6] [7] [8]
S. Kuck, Appl. Phys. B 72 (2001) 515. K. Hirao, J. Non-Cryst. Solids 196 (1996) 16. R. Jaaniso, H. Bill, Europhys. Lett. 16 (1991) 569. D.H. Cho, K. Hirao, N. Soga, J. Non-Cryst. Solids 189 (1995) 181. K. Fujita, K. Nouchi, K. Hirao, J. Lumin. 86 (2000) 305. R.M. Macfarlane, R.M. Shelby, Opt. Lett. 9 (1984) 533. K. Hirao, S. Todoroki, N. Soga, J. Lumin. 55 (1993) 217. M. Nogami, Y. Abe, K. Hirao, D.H. Cho, Appl. Phys. Lett. 66 (1995) 2952.
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[16] M.J. Lochhead, K.L. Bray, Phys. Rev. B 52 (1995) 15763. [17] C.K. Jayasankar, P. Babu, Th. Tr€ oster, W.B. Holzapfel, J. Lumin. 91 (2000) 33. [18] K.L. Bray (Ed.), Topics in Current Chemistry, vol. 213, Springer, Berlin, 1990, p. 1. [19] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, San Diego, CA, 1990. [20] G. Armagan, B. Di Bartolo, A.M. Buoncristiani, J. Lumin. 44 (1989) 129. [21] P.R. Wamsley, K.L. Bray, J. Lumin. 63 (1995) 31. [22] C.S. Yoo, H.B. Radousky, N.C. Holmes, N.M. Edelstein, Phys. Rev. B 44 (1991) 830.
Luminescence measurements on Sm2þ -doped sol–gel glasses Vilma C. Costa b
a,*
, Yongrong Shen a, Ana M.M. Santos b, Kevin L. Bray
a
a Department of Chemistry, Washington State University, Pullman, WA 99164, USA Center of Nuclear Technology Development/CNEN, CP 941, Pampulha, Belo Horizonte, MG 30123-970, Brazil
Abstract Sol–gel glass samples containing nominally 1, 2, 5, and 10 wt% Sm2 O3 doped into the molar compositions 5Na2 O– 10Al2 O3 –85SiO2 and 10Al2 O3 –90SiO2 were prepared from metal alkoxide solution. After heat treatment in air at 500 °C, the samples were heated at 750 °C under flowing H2 or at 800 °C under a flowing H2 /N2 to reduce Sm3þ to Sm2þ . Samarium ions in the divalent and trivalent states were identified by luminescence and lifetime measurements. Samples incorporated with Sm2þ showed emission with peaks at 683, 700, 725 and 763 nm due to transitions 5 D0 ! 7 F0;1;2;3 , respectively, of Sm2þ ions. The luminescence properties of Sm2þ ions are discussed in relation to the concentration of Sm2 O3 and the glass matrix composition. The relative proportion of Sm2þ decreases with increasing Sm2 O3 doping concentration. Preliminary high pressure luminescence results for Sm2þ in the glasses show a complete quenching of Sm2þ luminescence and an irreversible local modification in the glass network. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Hx; 42.70.Ce; 62.50.þp
1. Introduction Divalent samarium (Sm2þ ) doped crystals and glasses have attracted attention for potential applications in lasers [1] and memory devices [2]. Interest in materials containing Sm2þ has increased because of their non-linear optical hole burning properties [3,4] and applications as optical datastorage materials [3–5]. Macfarlane and Shelby [6] first observed persistent spectral hole burning spectra in Sm2þ activated halide crystals at 2 K. Recently, persistent spectral hole burning has been
*
Corresponding author. Tel.: +1-509 335 2424; fax: +1-509 335 8867. E-mail address: [email protected] (V.C. Costa).
observed at room temperature in fluoride crystals [3], borate glasses [4], and fluorohafnate glasses [7]. A new approach for the preparation of Sm2þ -doped aluminosilicate glasses using the sol–gel process has been reported [8] and room temperature persistent spectral hole burning has been observed in these systems [5,9]. Glass systems are expected to have larger memory density in devices than crystals because of their inhomogeneous line widths and convenient sample preparation [4,5,7–9]. The sol– gel technique is a glass preparation method for which lower processing temperatures are required relative to conventional melting techniques and the porosities of sol–gel glasses increases reduction of Sm3þ to Sm2þ in presence of H2 atmosphere [8–11]. Sm2þ has a ground 4f6 configuration and an excited 4f5 5d configuration. The close proximity in
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 0 2 9 - 3
V.C. Costa et al. / Journal of Non-Crystalline Solids 304 (2002) 238–243
energy of the two configurations leads to an electronic mixing between the two configurations and to an alteration of the optical properties of Sm2þ . With increasing hydrostatic pressure in a sample, we can tune the extent of mixing of the two configurations by varying their energy separation. As a result, we can systematically investigate how the extent of mixing varies the static and dynamic emission properties of Sm2þ and correlate the emission properties with crystal field strength and coordination environment. There has been considerable progress in understanding the spectral properties of rare earth ions under pressure. The effects of pressure on spectral properties of Pr3þ [12], Nd3þ [13], Sm2þ [14] and Eu3þ [15] in crystals and Eu3þ [16] and Sm3þ [17] in glasses have been reported. In this paper, we present spectroscopic characterization of samarium doped aluminosilicate derived sol–gel glasses and preliminary high pressure Sm2þ luminescence results.
2. Experimental 2.1. Sample preparation Sol–gel glasses containing nominally 1, 2, 5, and 10 wt% Sm2 O3 doped into the molar compositions 5Na2 O–10Al2 O3 –85SiO2 and 10Al2 O3 –90SiO2 were prepared from metal alkoxide solution following the procedure presented by Nogami et al. [10]. Tetraethoxysilane (TEOS, Si(OC2 H9 )4 ), Al(OC4 Hsec 4 )3 , SmCl3 6H2 O or Sm(NO3 )3 6H2 O and CH3 CO2 Na were used as starting reagents. TEOS was hydrolyzed for 1 h with a solution of H2 O, C2 H5 OH in the molar ratio of 1:1 per mol of TEOS. In the first hydrolysis, HCl was added to achieve an initial solution pH of 1.5–2. Aluminum precursor, Al(OC4 Hsec 9 )3 , was added to the previously prepared solution. After stirring for 15 min, an ethanolic solution of either SmCl3 6H2 O or Sm(NO3 )3 6H2 O and CH3 CO2 Na was added and the solution was further stirred for 15 min. The resultant solution was hydrolyzed by adding the mixed solution of H2 O, C2 H5 OH and HCl in a 4:1:0.011 mol per mol of Al alkoxide.
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After stirring 30 min, the sols were cast into plastic vials where they were allowed to form gels at room temperature. After gelation, the samples were aged at 60 °C for two days and then dried at 90 °C for two days. After 2 h heat treatment in air at 500 °C, the glasses were heated up to 800 °C for 24 h under a flowing mixed gas of 20%H2 – 80%N2 or at 750 °C for 10 h under a flowing H2 . For sample in the composition 5Na2 O–10Al2 O3 – 85SiO2 containing 2 wt% Sm2 O3 , a similar preparation approach described above was used and the glasses were, also, preheated in air at 700 °C for 2 h. 2.2. Spectroscopic measurements Luminescence spectra were obtained upon an excitation at 488.0 nm by an argon laser. Samarium luminescence was dispersed on a 1-m monochromator (Spex 1704) and detected by a photomultiplier tube (Hamamatsu R928). All reported luminescence spectra have been corrected for the spectral response of instrument. For luminescence decay measurements, a pulsed Nd:YAG laser (a second harmonic laser beam of 532 nm) was used to excite Sm2þ from the 7 F0 (4f6 ) ground state to the excited 4f5 5d state. The spectra were detected by the 1-m monochromator and decay data were recorded by a digital storage oscilloscope (LeCroy 140). Lifetimes were measured at room temperature. Luminescence decays were monitored at the peak wavelength of the 5 D0 ! 7 F0 emission line of Sm2þ . For high-pressure luminescence experiments, pressure was generated by a diamond anvil cell (DAC) [18]. The sample chamber consisted of a 250 lm hole drilled in a indented inconel gasket. A piece of glass sample was loaded in the sample chamber along with a ruby pressure calibrant. A spectroscopic oil (poly-dimethylsiloxane) served as the pressure transmitting medium.
3. Results 3.1. Ambient pressure luminescence When the gels were heated in air at 500 °C for 2 h before heating in H2 atmosphere, we observed
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Fig. 1. Luminescence spectra of a 5Na2 O–10Al2 O3 –85SiO2 glass composition containing 2 wt% Sm2 O3 : (a) glass heated at 700 °C for 2 h in air and (b) glass heated at 750 °C in H2 gas for 10 h.
no luminescence of Sm2þ and Sm3þ . We attributed the absence of a luminescent signal to a nonradiative quenching by residual water and organic fragments remaining in the sol–gel matrix after heating at 500 °C [19]. Treatment at temperatures higher than 500 °C is needed to remove them and therefore expected to promote the luminescence of Sm3þ and/or Sm2þ . As an example, Fig. 1(a) shows the luminescence spectrum of 5Na2 O–10Al2 O3 –85SiO2 sample containing 2 wt% Sm2 O3 . The sample was heated in air at 700 °C for 2 h. The observed luminescence spectrum consisted of several peaks around 565, 600, 650 and 710 nm, which are attributed to 4 G5=2 ! 6 H5=2 , 6 H7=2 , 6 H9=2 , and 6 H11=2 transitions of Sm3þ [5,14], respectively. Fig. 1(b) shows the luminescence of the same composition after heat treatment at 750 °C for 10 h in a flowing H2 . The spectral changes are consistent with a partial reduction of Sm3þ to Sm2þ in the glass. Upon excitation into 4f5 5d state of Sm2þ (kexc ¼ 488 nm), 5 D0 luminescence of Sm2þ is yielded through a non-radiative 4f5 5d ! 5 D0 relaxation and subsequent radiative transitions from the 5 D0 level into lower 7 FJ levels [12,14]. The peaks at 683, 700, 725 and 763 nm are assigned to the 5 D0 ! 7 F0 , 7 F1 , 7 F2 , and 7 F3 transitions of Sm2þ , respectively [5, 14]. A similar trend in the luminescence spectrum was also observed for the 10Al2 O3 –90SiO2 glass
Fig. 2. Luminescence spectra of 10Al2 O3 –90SiO2 glasses containing: (a) 10 wt% Sm2 O3 , (b) 2 wt% Sm2 O3 , and (c) 1 wt% Sm2 O3 . The glasses were heated in a mixed H2 /N2 gas at 800 °C for 24 h. The spectra were normalized by the area of the spectrum over 550–800 nm.
composition under the two heat treatments above used for 5Na2 O–10Al2 O3 –85SiO2 glasses. The emission spectra of 10Al2 O3 –90SiO2 glasses containing 1, 2, and 10 wt% Sm2 O3 after heating at 800 °C in a mixed H2 /N2 gas for 24 h are shown in Fig. 2. The spectra indicated that the relative proportion of Sm2þ decreased with increasing Sm2 O3 doping concentration. As the Sm2 O3 concentration increases the relative intensity of the peaks assigned 5 D0 ! 7 F0 , 7 F1 , 7 F2 , and 7 F3 transitions of Sm2þ decreases. A comparison of the luminescence spectra of 5Na2 O–10Al2 O3 –85SiO2 and 10Al2 O3 –90SiO2 glasses containing 2 wt% Sm2 O3 treated at 750 °C in H2 for 10 h is shown in Fig. 3. The spectra (Fig. 3) clearly show that the greater relative proportion of Sm2þ luminescence is in the sample containing Na2 O. This indicates that inclusion of Na2 O in the glass composition increased the ratio of Sm2þ to Sm3þ upon heating in H2 . 3.2. Ambient pressure decay Luminescence decay curves at room temperature for the 5 D0 level of Sm2þ were obtained at excitation wavelength of 532 nm and measured at emission wavelength of 683 nm. The decay curves were non-exponential and appeared to consist of a fast component and a slow component. Since the
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Fig. 3. Luminescence spectra of glasses containing 2 wt% Sm2 O3 : (a) 10Al2 O3 –90SiO2 and (b) 5Na2 O–10Al2 O3 –85SiO2 . The glasses were heated at 750 °C in H2 gas for 10 h. The spectra were normalized by the area of the spectrum over 550– 800 nm.
decay curves were non-exponential, average 5 D0 lifetimes of Sm2þ were obtained by a common approach that is used to determine the lifetime for luminescent materials with a non-exponential decay [20,21]. The first step is to normalize the decay curve by its initial intensity (t ¼ 0) and the second step is to calculate the area of the normalized decay curve. The 5 D0 lifetimes of Sm2þ ranged from 0.60 to 0.86 ms in Na2 O–Al2 O3 –SiO2 and Al2 O3 – SiO2 matrices.
Fig. 4. Representative high pressure emission spectra of 10Al2 O3 –90SiO2 glass containing 2 wt% Sm2 O3 . The sample was heated at 750 °C in H2 gas for 10 h. The release spectrum is the result obtained upon returning the pressure to ambient pressure at the conclusion of the experiment. The sharp lines near 700 nm originate from the ruby pressure calibrant used in the experiment.
4. Discussion 4.1. Ambient pressure luminescence
3.3. High pressure luminescence Fig. 4 presents representative luminescence spectra for the 10Al2 O3 –90SiO2 glass composition containing 2 wt% Sm2 O3 under pressures up to 190 kbar. The sample used for our high-pressure study was heated to 750 °C in a H2 atmosphere for 10 h. The ambient pressure spectrum is included in Fig. 3(b). A redshift of 0.03 nm/kbar and an intensity decrease were observed for the 5 D0 ! 7 F0 transition of Sm2þ as pressure was increased. Above 74 kbar, Sm2þ emission disappeared. Upon release of pressure, the emission spectrum did not revert back to the original ambient pressure spectrum. An overall decrease in emission intensity and no Sm2þ emission was observed upon release of pressure at ambient pressure (the top spectrum of Fig. 4).
The experimental results for the decrease in the Sm2þ emission intensity relative to the Sm3þ emission intensity with increasing Sm2 O3 doping concentration (Fig. 2) indicate that the ratio Sm2þ / Sm3þ decreases upon increasing the Sm2 O3 doping concentration at a given Al/Si ratio. This trend is consistent with a recent nuclear magnetic resonance (NMR) and X-ray diffraction study of Sm2 O3 -doped Al2 O3 –SiO2 glasses reported by Jin et al. [11]. Jin et al. proposed that the presence of Al3þ increases the change of Sm3þ to Sm2þ . They argued that glasses doped with Sm3þ contain Al3þ almost exclusively in the form of network forming AlO4 tetrahedra. They further showed that heat treatment in a reducing (H2 ) atmosphere converted AlO4 tetrahedra into AlO6 octahedra and that this conversion coincides with the conversion of Sm3þ
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to Sm2þ . According to their AlO4 $ AlO6 model, we expect that the increasing non-bridging oxygen concentration resulting from the formation of AlO6 octahedra satisfies the increased site coordination of the larger Sm2þ ions. The data presented by Jin et al. showed for a given Al/Si ratio that the relative amount of Sm2þ decreased with increasing Sm2 O3 doping concentration. In other words, the fractional conversion of Sm3þ to Sm2þ decreased as the number of non-bridging oxygens available for Sm3þ decreased. Our results for the 5Na2 O– 10Al2 O3 –85SiO2 glass composition are also consistent with this interpretation. Addition of Na2 O to Al2 O3 –SiO2 glasses at fixed Al2 O3 content increased in the non-bridging oxygen concentration. A comparison of the results for 2 wt% Sm2 O3 : 5Na2 O–10Al2 O3 –85SiO2 (Fig. 3(b)) with those for 2 wt% Sm2 O3 :10Al2 O3 –90SiO2 (Fig. 3(a)) shows that the relative proportion of Sm2þ is greater in the presence of Na2 O. 4.2. High pressure luminescence The loss of Sm2þ emission intensity with pressure could be due to an enhancement of the nonradiative decay of Sm2þ or a pressure-induced oxidation of Sm2þ to Sm3þ . Previous high pressure studies of Sm2þ -doped crystals [14,22] showed that high pressure induces a larger redshift of the excited 4f5 5d state of Sm2þ than the 5 D0 level. Since the 4f5 5d state of Sm2þ is close in energy to the emitting 5 D0 sate of the ground 4f6 configuration of Sm2þ , it is conceivable that pressure induces a 4f5 5d–5 D0 (4f6 ) electronic crossover [14,22]. When the 4f5 5d state interacts more strongly with the glass matrix than with crystal host lattices, a pressure-induced electronic crossover could increase the non-radiative decay rate of Sm2þ and lead to a quenching of the 5 D0 luminescence. Alternatively, pressure may convert Sm2þ to Sm3þ . Based on the work of Jin et al. [11], such conversion would be likely accompanied by a local structural transformation of the glass involving back-conversion from AlO6 octahedra to AlO4 tetrahedra. In other words, pressure could induce the reverse of the reduction reaction. This process is plausible because, thermodynamically, pressure stabilizes the smallest volume state of a system
[18]. Jin et al. [11] reported a decrease in sample density upon reduction of Sm3þ to Sm2þ . Thus, oxidation of Sm2þ to Sm3þ is consistent with stabilization of the lowest volume state of the glass. Our results upon release of pressure indicate that an irreversible transformation occurs in the glass with increasing pressure. The irreversible transformation may involve a permanent structure change in the glass and is consistent with the proposed pressure-induced Sm2þ –Sm3þ oxidation.
5. Conclusion We have prepared and characterized Sm2þ doped Al2 O3 –SiO2 and Na2 O–Al2 O3 –SiO2 sol–gel glasses. The high pressure luminescence results demonstrated a pressure-induced irreversible modification in local sol–gel glass networks that governs the Sm2þ luminescence. Extension of our high pressure luminescence study to other divalent rare earth ions in sol–gel glasses is necessary to generalize structure-property relations.
Acknowledgements V.C.C. acknowledges financial support from Brazilian National Council of Research, CNPq and Dr Wilmar B. Ferraz (CDTN-Brazil) for his kind help with the heat treatment of glasses. Further acknowledgement is made to the donors of the Petroleum Research Fund, administered by the ACS, and the National Science Foundation for support of this research.
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