- Email: [email protected]
Irradiation effects on the absorption edge of silica glass
Journal of Non-Crystalline Solids 353 (2007) 559–563 www.elsevier.com/locate/jnoncrysol
Irradiation effects on the absorption edge of silica glass E. Vella b
a,*
, R. Boscaino a, G. Navarra a, B. Boizot
b
a University of Palermo, Department of Physical and Astronomical Sciences, Via Archirafi 36, 90123 Palermo, Italy Laboratoire des Solides Irradie´s, UMR 7642 CEA-CNRS-Ecole Polytechnique, Ecole Polytechnique, 91128 Palaiseau cedex, France
Available online 7 February 2007
Abstract Vacuum ultraviolet absorption experiments were carried out on a variety of specimens of amorphous silica b-irradiated at different doses from 103 to 5 · 106 kGy. Changes in the width of the absorption (Urbach) edge were investigated. These changes strongly depend on the kind of silica considered: in particular the Urbach energy of silica of industrial manufacture increases in the irradiated samples, whereas in sol–gel silica it is poorly influenced by the irradiation. The fictive temperature of the different materials before and after irradiation was also monitored. The changes of the Urbach energy and of the fictive temperature are tentatively discussed considering the disorder degree induced by irradiation. 2007 Elsevier B.V. All rights reserved. PACS: 78.55.Qr; 78.40.Pg; 61.80.Fe Keywords: Radiation effects; Absorption; FTIR measurements; Silica; Radiation; Long range order; Medium-range order
1. Introduction The absorption coefficient a(E) of amorphous silica (aSiO2) in the range from 10 cm1 to 103 cm1 is well described by the exponential law [1] E Eg aðEÞ ¼ a0 exp ð1Þ Eu known as Urbach law. In Eq. (1) a0 is a parameter distinctive of the material (in a-SiO2a0 5 · 103 cm1), Eg is the optical energy gap (8.5 eV in a-SiO2 at T = 300 K) and Eu is the so called Urbach energy. The inverse of Eu is the logarithmic slope of a(E) and so its value determines the width of the absorption tail. Low values of Eu correspond to a steep absorption edge and to a high transparency in the vacuum-UV (VUV) spectral region, while materials with a lower transmittance in the Urbach region are characterized by high values of Eu. The Urbach tail of the absorption edge is usually ascribed to the optical elec*
Corresponding author. E-mail address: eleonora.vella@fisica.unipa.it (E. Vella).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.11.011
tronic transitions between extended states and near-edge localized states. The formation of localized states with energies at the boundaries of the energy gap is one of the effects of the structural disorder on the electronic structure of amorphous materials. This is the reason why the Urbach energy is frequently used as a measure of the degree of structural disorder [2,3]. Another parameter useful to characterize the structural disorder of amorphous silica is the fictive temperature (Tf). The fictive temperature is the high temperature at which a-SiO2 is allowed to reach thermal equilibrium before the rapid quench to room temperature. In other words, Tf is the temperature at which the actual structural disorder would be at thermal equilibrium. So the variations of the disorder, induced by after-growth external treatments, can be quantified as variations of Tf [4]. It has been observed [5] that the value of the fictive temperature is correlated to the positions of the 1100 cm1 (m1) IR fundamental absorption band and of its overtone at 2260 cm1 (m2) through the following relations: m1 ¼ 1114:51 þ
11603:51 ; Tf
ð2Þ
43809:21 : Tf
ð3Þ
The correlations given in Eqs. (2) and (3) are relevant, since the frequency of this vibrational mode depends on the average value of the Si–O–Si bond angle [6]. This paper is concerned with the structural disorder induced by irradiation, studied through the analysis of the changes of Eu and of Tf in irradiated materials in comparison with the not irradiated ones. In order to examine the effects of high irradiation doses, samples were b-irradiated in the range from 103 to 5 · 106 kGy. We measured the optical absorption spectra of a variety of silica glasses before and after irradiation and we extracted the value of the Urbach energy. At the same time, we monitored the position of the band at 2260 cm1 and using Eq. (3) we determined their fictive temperatures. The induced variations of Eu and Tf have been studied as a function of the accumulated dose. 2. Experimental procedure We report here the results obtained in five different types of silica glass [7]: (a) Type I (natural dry): Infrasil 301 (I301), Puropsil A (QPA); (b) Type II (natural wet): Homosil (HM), Herasil 1 (H1); (c) Type III (synthetic wet): Suprasil 311 (S311), Corning 7940-5F (CNG5F), Suprasil 1 (S1); (d) Type IV: Suprasil 300 (S300), Suprasil F300 (F300); (e) sol–gel. In the following we will refer to the different materials with the nicknames put in brackets. The materials (a)–(d) are of industrial manufacture: I301, H1, HM, S311, S1, S300 and F300 are trademarks by Heareus Co. [8]; QPA has been supplied by Quartz and Silice [9] while CNG5F by Corning. The sol–gel samples were custom-made using the aerogel technique. For each silica type we used a set of samples with thickness d = 0.2 mm or 0.5 mm. Samples were first characterized for their VUV and IR properties and subjected to birradiation. This was accomplished using a Van de Graaff accelerator (electrons energy = 2.5 MeV; doses from 1.2 · 103 kGy to 5 · 106 kGy). The IR absorption measurements were carried out using a spectrophotometer BIORAD (Mod. FTS-40A) with a spectral resolution of 1 cm1. To avoid the effect of water in air, the absorption spectrum of the empty beam line was subtracted from the spectrum of each sample, after suitable normalization. The error associated with the determination of the position of the IR band of our interest is ±0.5 cm1. VUV absorption spectra were measured at room temperature using an ACTON spectrophotometer (Mod.SP150) working in N2 flux (typically 80 l/min). The measurements were carried out with a bandwidth (FWHM) of 0.4 nm. We have preliminarily verified that the effect of the finite bandwidth on the measured value of Eu is always less than 5% in our experimental conditions. Experimental spectra were corrected for the PMT dark current and for the reflec-
tions from sample surfaces. These were estimated using literature data of the dispersion of the refractive index in silica [10]. Using averaging techniques optical densities lower than 2.7 could be measured with an accuracy of ±0.03. The sample thickness of 0.2 mm allows to measure values of the absorption coefficient as high as a(E) 300 cm1; this condition sets the upper limit of the spectral range investigated from 6 eV to 8.3 eV (from k 207 nm to k 149 nm). This range is wide enough to evidence the exponential region of the absorption edge and to measure Eu. In the spectral range considered the absorption spectra of silica glass, and of irradiated materials in particular, show several intense bands. So, in the analysis of the exponential spectra we included not only the exponential term of Eq. (1) but also a suitable number of Gaussian bands. The experimental setup just described and the analysis procedure allow a reliable estimate of Eu within 5% for values of the Urbach energy as low as 45 meV. This was also verified by measuring the absorption edge in a sample of crystalline quartz: the obtained value of the Urbach energy (48 ± 4 meV) is in fair agreement with literature data [3]. 3. Results We measured first the Urbach energy and the fictive temperature of the different as-grown materials. Fig. 1 shows a typical measured spectrum and the Urbach edge as determined by our fitting procedure. In particular the spectrum reported was taken in a sample of QPA. Similar spectra were observed for the other as-grown materials examined. In addition to the Urbach law two Gaussian bands were generally included in the fitting procedure: one centered at 7.54 eV with FWHM of 0.6 eV (E band) and the other one centered at 7.9 eV with a FWHM of 0.4 eV. In some materials two minor bands 150
-1
m2 ¼ 2228:64 þ
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
Absorption coefficient (cm )
560
100
50
0 6
7
8
Energy (eV) Fig. 1. (d) VUV absorption spectrum of a sample of QPA. The full line plots the Urbach edge as determined by the fitting procedure. The Urbach energy is (72 ± 5) meV.
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
Materials
I
I301 QPA
62 72
1503 1468
II
HM H1
77 75
1438 1416
III
S311 S1 CNG5F
73 116 155
1251 1295 1244
F300 S300 SG
91 96 229
1481 1339 1222
IV Sol–gel
Urbach energy (meV)
Fictive temperature (K)
2
Urbach energy (meV)
150
100
50 1400
100
10
Type I Type II Type III Type IV Sol-gel
1300
QPA
120
60
250
1200
F300
80
at 6.25 eV and at 6.85 eV were required to get satisfying fitting. The measured values of Eu and of Tf of the not-irradiated materials are listed in Table 1 [11]. In Fig. 2, the Urbach energy of the different not irradiated materials is plotted versus their fictive temperature. With the relevant exception of the S311 material, as a general trend the fictive temperature is lower in the as-grown materials characterized by higher values of the Urbach energy. Different samples of each material were then subjected to different irradiation doses. Figs. 3(a) and (b) show typical results of the Urbach energy of the irradiated materials versus the irradiation dose. In Fig. 3(a) we report the results obtained in dry materials (QPA natural and F300 synthetic). The Urbach energies of the two as-grown materials are significantly different (Eu = 72 meV in QPA, Eu = 91 meV in F300). The Eu values grow with the irradiation dose in both materials, but the induced variation at the highest explored dose is different (DEu = 55 meV in QPA, DEu = 84 meV in F300). The results reported in Fig. 3(b) regard wet materials (HM natural and S1 syn-
200
180 160 140 120 100 80
1500
1600
Fictive temperature (K) Fig. 2. Urbach energy versus fictive temperature of (d) type I, (j) type II, (h) type III, (s) type IV and (m) sol–gel materials, respectively. The values of Eu and Tf of each material are listed in Table 1.
3
4
10
5
10 10 Irradiation dose (kGy)
6
10
6
10
10
7
b 250 Urbach energy (meV)
Type
a Urbach energy (meV)
Table 1 Urbach energies and fictive temperatures of the as-grown materials determined in this study
561
200
150
HM S1 SG
100
50 2 10
3
10
4
10
5
10
10
7
Irradiation dose (kGy) Fig. 3. Urbach energy versus the irradiation dose of (a) two dry materials (QPA, and F300) and of (b) two wet materials (HM and S1) and a sol–gel material (SG). The dashed lines indicate the value of the Urbach energy measured in each as-grown material.
thetic). The Urbach energies of the as-grown samples of the two materials are different (Eu = 77 meV in HM, Eu = 116 meV in S1), while in this particular case the amounts of the variation of Eu after irradiation are similar (DEu = 33 meV in HM, DEu = 32 meV in S1). Similar trends of the Urbach energy as a function of the irradiation dose were observed for the other industrial materials (wet or dry) examined. In Fig. 3(b) we also report the Urbach energy as a function of the irradiation dose in SG. The value of Eu in the sol–gel as-grown samples (Eu = 228 meV) is higher than that measured in any of the materials of industrial manufacture. Moreover, within the experimental error the Urbach energy does not change significantly after irradiation. We have also measured the fictive temperature of each sample examined. Fig. 4 shows the change of Tf with the irradiation dose for a wet material (S1), a dry one (F300) and the sol–gel one (SG). As far as the two materials of industrial manufacture are concerned, the value of Tf increases with the irradiation dose with a similar growth, while the amount of its variation depend on the material
562
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
F300 S1 SG
Fictive temperature (K)
2200 2000 1800 1600 1400 1200 2
10
3
10
4
10
5
10
6
10
7
10
Irradiation dose (kGy) Fig. 4. Fictive temperature versus irradiation dose for a dry material (F300), a wet material (S1) and the sol–gel one (SG). The dashed lines indicate the value of Tf in each material before irradiation.
considered (DTf = 570 K in F300; DTf = 360 K in S1). Similar trends were observed for the other industrial materials examined. The fictive temperature of the sol–gel material, instead, is almost not affected by irradiation within the experimental uncertainties. 4. Discussion The experimental values reported in Table 1 indicate that the Urbach energy and the fictive temperature show a high variability in different types of silica. In particular Eu and Tf varies respectively from 60 meV to 160 meV and from 1250 K to 1500 K in traditionally prepared materials, while in the sol–gel material Eu reaches the value of 230 meV and Tf is 1220 K. The fictive temperature of the sol–gel materials is lower than that of any of the industrial materials examined: this is the consequence of the fact that the highest temperature reached during the process of synthesis of these materials (1200 K) is lower than the others one (1600 K). Our results suggest, also, that the values of the Urbach energy and of the fictive temperature in glassy silica are not determined by being natural or synthetic nor by the SiOH content (wet or dry). Furthermore, one of the most interesting evidences is that, excepting the S311 material, in as-grown materials low values of the Urbach energy go with high values of the fictive temperature and vice versa. This result is somewhat unexpected since a higher disorder degree should correspond to higher values both of Tf and of Eu, an expectation not confirmed by our results. As far as the effect of irradiation is concerned, our results indicate a clear difference between the traditionally prepared materials and the sol–gel ones. In the first ones the Urbach energy increases monotonically with the irradiation dose in the range of doses from 104 to 5 · 106 kGy and in particular in the samples irradiated at a dose of 5 · 106 kGy Eu can reach a value even twice than that
of the pristine material. The effect of b-irradiation on the Urbach energy is similar to that previously observed in the literature as a consequence of UV laser irradiation [12], even if generally stronger. In the sol–gel material, instead, Eu does not change after irradiation within the experimental error. This result, together with the fact that the not irradiated sol–gel silica is characterized by a high value of Eu, suggests that in these materials, in spite of the low temperature reached during synthesis, the structural disorder is so marked to be almost not affected by irradiation. Apart from the sol–gel materials, the trends of growth of Eu and of Tf are similar to each other in all the examined materials. However, a remarkable difference in their growth is the dose range. In fact Eu starts increasing after a dose of 104 kGy, while Tf remain constant up to a dose of 105 kGy. A tentative explanation of this different behavior of Eu and Tf may be the following. The Urbach energy is influenced not only from the structural disorder peculiar to the amorphous structure, but also by the presence of point defects, which yield a relevant contribution to the density of the near-edge electronic localized states [13,14]. On the other hand, the fictive temperature, being related to the properties of the whole vitreous matrix, can vary only if a damage extended to the network as a whole is produced and this may occur only at higher irradiation doses. Finally it is worth noting that we found two different behaviors of the Urbach energy versus the fictive temperature in the as-grown materials and in the irradiated ones. Before irradiation the fictive temperature is higher for materials with a lower Urbach energy, while after irradiation both Eu and Tf increase. In order to understand this result it is important to consider that even if the Urbach energy and the fictive temperature both characterize the structural disorder of the system, at the same time they may be correlated to different structural aspects. From this point of view the fact that the correlation between Tf and Eu has different features in the as-grown materials and in the irradiated ones is an evidence of the complexity of the problem. In our opinion the study of the effect of irradiation on other structural parameters can be useful in order to get a deeper comprehension of this issue. One of these structural parameter is, for example, the distribution of n-membered rings, and in particular of the three-membered ones [15]. The investigation of the relationship of the variations of the rings distribution with Tf and Eu could bring useful information on this question. Work is in progress in this direction. Acknowledgements The authors thank S. Grandi (University of Pavia, Italy) for preparing sol–gel materials and G. Napoli and G. Tricomi for technical assistance. S. Esnouf and S. Guillous are acknowledged for taking care of b ray irradiation. We wish to thank M. Leone, S. Agnello and G. Buscarino for helpful discussions and suggestions.
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
References [1] S.R. Elliott, Physics of Amorphous Materials, 1st Ed., Longman Scientific & Technical, Harlow (GB), 1989, p. 234. [2] K. Saito, A.J. Ikushima, Phys. Rev. B 62 (2000) 8584. [3] I.T. Godmanis, A.N. Trukhin, K. Hubner, Phys. Status Solidi B 116 (1983) 279. [4] H. Hosono, Y. Ikuta, T. Kinoshita, K. Kajihara, M. Hirano, Phys. Rev. Lett. 87 (2001) 175501. [5] A. Agarwal, K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 185 (1995) 191. [6] F.L. Galeener, Phys. Rev. B 19 (1979) 4292. [7] R. Bruckner, J. Non-Cryst. Solids 5 (1970) 123.
[8] [9] [10] [11]
[12] [13] [14] [15]
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Heraeus, Quartzglas, Germany, catalog POL-0/102/E. Quartz and Silice, Nemours, France, catalog OPT-91-3. H.R. Philipp, J. Phys. Chem. Solids 32 (1971) 1935. R. Boscaino, E. Vella, G. Navarra, in: Proceedings of WFOPC2005 Fourth IEEE/LEOS Workshop on Fibres and Optical Passive Components, Mondello (Palermo), IT, 2005. F. Ku¨hnlenz, S. Bark-Zollmann, H. Stafast, W. Triebel, J. Non-Cryst. Solids 278 (2000) 115. R. Atta-Fynn, P. Biswas, P. Ordejo´n, D.A. Drabold, Phys. Rev. B 69 (2004) 85207. T. Koslowski, W. Kob, K. Wollmayr, Phys. Rev. B 56 (1997) 9469. B. Boizot, S. Agnello, B. Reynard, R. Boscaino, G. Petite, J. NonCryst. Solids 325 (2003) 22.
Irradiation effects on the absorption edge of silica glass E. Vella b
a,*
, R. Boscaino a, G. Navarra a, B. Boizot
b
a University of Palermo, Department of Physical and Astronomical Sciences, Via Archirafi 36, 90123 Palermo, Italy Laboratoire des Solides Irradie´s, UMR 7642 CEA-CNRS-Ecole Polytechnique, Ecole Polytechnique, 91128 Palaiseau cedex, France
Available online 7 February 2007
Abstract Vacuum ultraviolet absorption experiments were carried out on a variety of specimens of amorphous silica b-irradiated at different doses from 103 to 5 · 106 kGy. Changes in the width of the absorption (Urbach) edge were investigated. These changes strongly depend on the kind of silica considered: in particular the Urbach energy of silica of industrial manufacture increases in the irradiated samples, whereas in sol–gel silica it is poorly influenced by the irradiation. The fictive temperature of the different materials before and after irradiation was also monitored. The changes of the Urbach energy and of the fictive temperature are tentatively discussed considering the disorder degree induced by irradiation. 2007 Elsevier B.V. All rights reserved. PACS: 78.55.Qr; 78.40.Pg; 61.80.Fe Keywords: Radiation effects; Absorption; FTIR measurements; Silica; Radiation; Long range order; Medium-range order
1. Introduction The absorption coefficient a(E) of amorphous silica (aSiO2) in the range from 10 cm1 to 103 cm1 is well described by the exponential law [1] E Eg aðEÞ ¼ a0 exp ð1Þ Eu known as Urbach law. In Eq. (1) a0 is a parameter distinctive of the material (in a-SiO2a0 5 · 103 cm1), Eg is the optical energy gap (8.5 eV in a-SiO2 at T = 300 K) and Eu is the so called Urbach energy. The inverse of Eu is the logarithmic slope of a(E) and so its value determines the width of the absorption tail. Low values of Eu correspond to a steep absorption edge and to a high transparency in the vacuum-UV (VUV) spectral region, while materials with a lower transmittance in the Urbach region are characterized by high values of Eu. The Urbach tail of the absorption edge is usually ascribed to the optical elec*
Corresponding author. E-mail address: eleonora.vella@fisica.unipa.it (E. Vella).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.11.011
tronic transitions between extended states and near-edge localized states. The formation of localized states with energies at the boundaries of the energy gap is one of the effects of the structural disorder on the electronic structure of amorphous materials. This is the reason why the Urbach energy is frequently used as a measure of the degree of structural disorder [2,3]. Another parameter useful to characterize the structural disorder of amorphous silica is the fictive temperature (Tf). The fictive temperature is the high temperature at which a-SiO2 is allowed to reach thermal equilibrium before the rapid quench to room temperature. In other words, Tf is the temperature at which the actual structural disorder would be at thermal equilibrium. So the variations of the disorder, induced by after-growth external treatments, can be quantified as variations of Tf [4]. It has been observed [5] that the value of the fictive temperature is correlated to the positions of the 1100 cm1 (m1) IR fundamental absorption band and of its overtone at 2260 cm1 (m2) through the following relations: m1 ¼ 1114:51 þ
11603:51 ; Tf
ð2Þ
43809:21 : Tf
ð3Þ
The correlations given in Eqs. (2) and (3) are relevant, since the frequency of this vibrational mode depends on the average value of the Si–O–Si bond angle [6]. This paper is concerned with the structural disorder induced by irradiation, studied through the analysis of the changes of Eu and of Tf in irradiated materials in comparison with the not irradiated ones. In order to examine the effects of high irradiation doses, samples were b-irradiated in the range from 103 to 5 · 106 kGy. We measured the optical absorption spectra of a variety of silica glasses before and after irradiation and we extracted the value of the Urbach energy. At the same time, we monitored the position of the band at 2260 cm1 and using Eq. (3) we determined their fictive temperatures. The induced variations of Eu and Tf have been studied as a function of the accumulated dose. 2. Experimental procedure We report here the results obtained in five different types of silica glass [7]: (a) Type I (natural dry): Infrasil 301 (I301), Puropsil A (QPA); (b) Type II (natural wet): Homosil (HM), Herasil 1 (H1); (c) Type III (synthetic wet): Suprasil 311 (S311), Corning 7940-5F (CNG5F), Suprasil 1 (S1); (d) Type IV: Suprasil 300 (S300), Suprasil F300 (F300); (e) sol–gel. In the following we will refer to the different materials with the nicknames put in brackets. The materials (a)–(d) are of industrial manufacture: I301, H1, HM, S311, S1, S300 and F300 are trademarks by Heareus Co. [8]; QPA has been supplied by Quartz and Silice [9] while CNG5F by Corning. The sol–gel samples were custom-made using the aerogel technique. For each silica type we used a set of samples with thickness d = 0.2 mm or 0.5 mm. Samples were first characterized for their VUV and IR properties and subjected to birradiation. This was accomplished using a Van de Graaff accelerator (electrons energy = 2.5 MeV; doses from 1.2 · 103 kGy to 5 · 106 kGy). The IR absorption measurements were carried out using a spectrophotometer BIORAD (Mod. FTS-40A) with a spectral resolution of 1 cm1. To avoid the effect of water in air, the absorption spectrum of the empty beam line was subtracted from the spectrum of each sample, after suitable normalization. The error associated with the determination of the position of the IR band of our interest is ±0.5 cm1. VUV absorption spectra were measured at room temperature using an ACTON spectrophotometer (Mod.SP150) working in N2 flux (typically 80 l/min). The measurements were carried out with a bandwidth (FWHM) of 0.4 nm. We have preliminarily verified that the effect of the finite bandwidth on the measured value of Eu is always less than 5% in our experimental conditions. Experimental spectra were corrected for the PMT dark current and for the reflec-
tions from sample surfaces. These were estimated using literature data of the dispersion of the refractive index in silica [10]. Using averaging techniques optical densities lower than 2.7 could be measured with an accuracy of ±0.03. The sample thickness of 0.2 mm allows to measure values of the absorption coefficient as high as a(E) 300 cm1; this condition sets the upper limit of the spectral range investigated from 6 eV to 8.3 eV (from k 207 nm to k 149 nm). This range is wide enough to evidence the exponential region of the absorption edge and to measure Eu. In the spectral range considered the absorption spectra of silica glass, and of irradiated materials in particular, show several intense bands. So, in the analysis of the exponential spectra we included not only the exponential term of Eq. (1) but also a suitable number of Gaussian bands. The experimental setup just described and the analysis procedure allow a reliable estimate of Eu within 5% for values of the Urbach energy as low as 45 meV. This was also verified by measuring the absorption edge in a sample of crystalline quartz: the obtained value of the Urbach energy (48 ± 4 meV) is in fair agreement with literature data [3]. 3. Results We measured first the Urbach energy and the fictive temperature of the different as-grown materials. Fig. 1 shows a typical measured spectrum and the Urbach edge as determined by our fitting procedure. In particular the spectrum reported was taken in a sample of QPA. Similar spectra were observed for the other as-grown materials examined. In addition to the Urbach law two Gaussian bands were generally included in the fitting procedure: one centered at 7.54 eV with FWHM of 0.6 eV (E band) and the other one centered at 7.9 eV with a FWHM of 0.4 eV. In some materials two minor bands 150
-1
m2 ¼ 2228:64 þ
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
Absorption coefficient (cm )
560
100
50
0 6
7
8
Energy (eV) Fig. 1. (d) VUV absorption spectrum of a sample of QPA. The full line plots the Urbach edge as determined by the fitting procedure. The Urbach energy is (72 ± 5) meV.
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
Materials
I
I301 QPA
62 72
1503 1468
II
HM H1
77 75
1438 1416
III
S311 S1 CNG5F
73 116 155
1251 1295 1244
F300 S300 SG
91 96 229
1481 1339 1222
IV Sol–gel
Urbach energy (meV)
Fictive temperature (K)
2
Urbach energy (meV)
150
100
50 1400
100
10
Type I Type II Type III Type IV Sol-gel
1300
QPA
120
60
250
1200
F300
80
at 6.25 eV and at 6.85 eV were required to get satisfying fitting. The measured values of Eu and of Tf of the not-irradiated materials are listed in Table 1 [11]. In Fig. 2, the Urbach energy of the different not irradiated materials is plotted versus their fictive temperature. With the relevant exception of the S311 material, as a general trend the fictive temperature is lower in the as-grown materials characterized by higher values of the Urbach energy. Different samples of each material were then subjected to different irradiation doses. Figs. 3(a) and (b) show typical results of the Urbach energy of the irradiated materials versus the irradiation dose. In Fig. 3(a) we report the results obtained in dry materials (QPA natural and F300 synthetic). The Urbach energies of the two as-grown materials are significantly different (Eu = 72 meV in QPA, Eu = 91 meV in F300). The Eu values grow with the irradiation dose in both materials, but the induced variation at the highest explored dose is different (DEu = 55 meV in QPA, DEu = 84 meV in F300). The results reported in Fig. 3(b) regard wet materials (HM natural and S1 syn-
200
180 160 140 120 100 80
1500
1600
Fictive temperature (K) Fig. 2. Urbach energy versus fictive temperature of (d) type I, (j) type II, (h) type III, (s) type IV and (m) sol–gel materials, respectively. The values of Eu and Tf of each material are listed in Table 1.
3
4
10
5
10 10 Irradiation dose (kGy)
6
10
6
10
10
7
b 250 Urbach energy (meV)
Type
a Urbach energy (meV)
Table 1 Urbach energies and fictive temperatures of the as-grown materials determined in this study
561
200
150
HM S1 SG
100
50 2 10
3
10
4
10
5
10
10
7
Irradiation dose (kGy) Fig. 3. Urbach energy versus the irradiation dose of (a) two dry materials (QPA, and F300) and of (b) two wet materials (HM and S1) and a sol–gel material (SG). The dashed lines indicate the value of the Urbach energy measured in each as-grown material.
thetic). The Urbach energies of the as-grown samples of the two materials are different (Eu = 77 meV in HM, Eu = 116 meV in S1), while in this particular case the amounts of the variation of Eu after irradiation are similar (DEu = 33 meV in HM, DEu = 32 meV in S1). Similar trends of the Urbach energy as a function of the irradiation dose were observed for the other industrial materials (wet or dry) examined. In Fig. 3(b) we also report the Urbach energy as a function of the irradiation dose in SG. The value of Eu in the sol–gel as-grown samples (Eu = 228 meV) is higher than that measured in any of the materials of industrial manufacture. Moreover, within the experimental error the Urbach energy does not change significantly after irradiation. We have also measured the fictive temperature of each sample examined. Fig. 4 shows the change of Tf with the irradiation dose for a wet material (S1), a dry one (F300) and the sol–gel one (SG). As far as the two materials of industrial manufacture are concerned, the value of Tf increases with the irradiation dose with a similar growth, while the amount of its variation depend on the material
562
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
F300 S1 SG
Fictive temperature (K)
2200 2000 1800 1600 1400 1200 2
10
3
10
4
10
5
10
6
10
7
10
Irradiation dose (kGy) Fig. 4. Fictive temperature versus irradiation dose for a dry material (F300), a wet material (S1) and the sol–gel one (SG). The dashed lines indicate the value of Tf in each material before irradiation.
considered (DTf = 570 K in F300; DTf = 360 K in S1). Similar trends were observed for the other industrial materials examined. The fictive temperature of the sol–gel material, instead, is almost not affected by irradiation within the experimental uncertainties. 4. Discussion The experimental values reported in Table 1 indicate that the Urbach energy and the fictive temperature show a high variability in different types of silica. In particular Eu and Tf varies respectively from 60 meV to 160 meV and from 1250 K to 1500 K in traditionally prepared materials, while in the sol–gel material Eu reaches the value of 230 meV and Tf is 1220 K. The fictive temperature of the sol–gel materials is lower than that of any of the industrial materials examined: this is the consequence of the fact that the highest temperature reached during the process of synthesis of these materials (1200 K) is lower than the others one (1600 K). Our results suggest, also, that the values of the Urbach energy and of the fictive temperature in glassy silica are not determined by being natural or synthetic nor by the SiOH content (wet or dry). Furthermore, one of the most interesting evidences is that, excepting the S311 material, in as-grown materials low values of the Urbach energy go with high values of the fictive temperature and vice versa. This result is somewhat unexpected since a higher disorder degree should correspond to higher values both of Tf and of Eu, an expectation not confirmed by our results. As far as the effect of irradiation is concerned, our results indicate a clear difference between the traditionally prepared materials and the sol–gel ones. In the first ones the Urbach energy increases monotonically with the irradiation dose in the range of doses from 104 to 5 · 106 kGy and in particular in the samples irradiated at a dose of 5 · 106 kGy Eu can reach a value even twice than that
of the pristine material. The effect of b-irradiation on the Urbach energy is similar to that previously observed in the literature as a consequence of UV laser irradiation [12], even if generally stronger. In the sol–gel material, instead, Eu does not change after irradiation within the experimental error. This result, together with the fact that the not irradiated sol–gel silica is characterized by a high value of Eu, suggests that in these materials, in spite of the low temperature reached during synthesis, the structural disorder is so marked to be almost not affected by irradiation. Apart from the sol–gel materials, the trends of growth of Eu and of Tf are similar to each other in all the examined materials. However, a remarkable difference in their growth is the dose range. In fact Eu starts increasing after a dose of 104 kGy, while Tf remain constant up to a dose of 105 kGy. A tentative explanation of this different behavior of Eu and Tf may be the following. The Urbach energy is influenced not only from the structural disorder peculiar to the amorphous structure, but also by the presence of point defects, which yield a relevant contribution to the density of the near-edge electronic localized states [13,14]. On the other hand, the fictive temperature, being related to the properties of the whole vitreous matrix, can vary only if a damage extended to the network as a whole is produced and this may occur only at higher irradiation doses. Finally it is worth noting that we found two different behaviors of the Urbach energy versus the fictive temperature in the as-grown materials and in the irradiated ones. Before irradiation the fictive temperature is higher for materials with a lower Urbach energy, while after irradiation both Eu and Tf increase. In order to understand this result it is important to consider that even if the Urbach energy and the fictive temperature both characterize the structural disorder of the system, at the same time they may be correlated to different structural aspects. From this point of view the fact that the correlation between Tf and Eu has different features in the as-grown materials and in the irradiated ones is an evidence of the complexity of the problem. In our opinion the study of the effect of irradiation on other structural parameters can be useful in order to get a deeper comprehension of this issue. One of these structural parameter is, for example, the distribution of n-membered rings, and in particular of the three-membered ones [15]. The investigation of the relationship of the variations of the rings distribution with Tf and Eu could bring useful information on this question. Work is in progress in this direction. Acknowledgements The authors thank S. Grandi (University of Pavia, Italy) for preparing sol–gel materials and G. Napoli and G. Tricomi for technical assistance. S. Esnouf and S. Guillous are acknowledged for taking care of b ray irradiation. We wish to thank M. Leone, S. Agnello and G. Buscarino for helpful discussions and suggestions.
E. Vella et al. / Journal of Non-Crystalline Solids 353 (2007) 559–563
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