- Email: [email protected]
A Raman spectroscopic study on the microscopic origin of the photoinduced fluidity effect
Physica B 296 (2001) 216}221
A Raman spectroscopic study on the microscopic origin of the photoinduced #uidity e!ect D.Th. Kastrissios , S.N. Yannopoulos *, G.N. Papatheodorou Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes, P.O. Box 1414, GR-26500 Patras, Greece Department of Chemical Engineering, University of Patras, GR-26500, Patras, Greece
Abstract We report on a sub-band gap light scattering (Raman) investigation for a-As S "bers subjected to external elongation stress in order to elucidate the photoinduced #uidity e!ect on a microscopic scale. Changes in the short-range order caused by the presence of the illuminating light have been detected. On the other hand, subtle modi"cations are revealed in the intermediate range order, manifested in the increase of the magnitude and in the modi"cation of the frequency dependence for the depolarization ratio } in the low-frequency part of the spectrum } as a function of the applied stress. The `buckling modela proposed to account for the optical absorption tails in chalcogenide glasses seems quite relevant for explaining certain aspects of the experimental data. 2001 Elsevier Science B.V. All rights reserved. Keywords: Chalcogenide glasses; Photoinduced #uidity; Raman scattering; Boson peak
1. Introduction When amorphous semiconducting or insulating materials are illuminated with light having energy close with that of the band gap of the material, electron}hole pairs are created. These pairs can separate enhancing the electrical response or recombine in a radiative or a non-radiative way. The last case is the most interesting since alterations may occur in structural, mechanical and optical properties of the material. The randomness of the amorphous state compared to the well-de"ned lattice periodicity in crystalline solids renders
* Corresponding author. Tel.: #30-61-965-252; fax: #3061-965-223. E-mail address: [email protected] (S.N. Yannopoulos).
it possible for such changes to occur in a number of di!erent ways. Therefore, more e!orts have been undertaken for elucidating the photoinduced phenomena in glasses and amorphous solids; see Refs. [1}3] for reviews. Furthermore, what is more striking is the fact that the changes in various properties of the materials can be reversible, i.e. they can be eliminated after annealing to the glass transition temperature ¹ or irreversible, i.e. they stay perma nently after exposition to illumination. Reversible photoinduced changes in chalcogenide glasses are up until now the most thoroughly studied. Outstanding among them are the reversible photodarkening (redshift in the optical absorption edge), the photoinduced anisotropy [4,5] (including photoinduced birefringence and photoinduced dichroism), the giant photoexpansion [6] (volume changes up to 5% have been measured
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 8 0 3 - 6
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
after exposition to light). Mechanical or rheological changes have also been observed (decrease of ¹ ) and softening of the elastic constants under illumination. Recently, Hisakuni and Tanaka demonstrated that the #uid state of a glassy semiconductor could be obtained by a process that does not require heat supply [7]. A local increase of the network #uidity emerges as a result of light illumination and can be revealed through the application of external stress. Speci"cally, the e!ect of the incident light (6328 As ) was to facilitate viscous #ow reducing the viscosity by more than two orders of magnitude. In other words, the isoviscous (+2;10 P) temperatures ¹ (without illumination) and ¹ (under illumination) present a di!erence ¹!¹+200 K. The e!ect becomes also macroscopically evident as an increasing elongation of a "ber's length under light illumination, manifested as a `neckinga e!ect on the illuminated point. Quite interesting is the fact that the viscosity of the photo-excited sample unexpectedly increases at higher temperatures. This fact indicates that the photoinduced #uidity is not the e!ect of temperature rise because then the opposite behavior should be observed, namely decreasing viscosity with increasing temperature. The microscopic origin of the photoinduced #uidity e!ect is still highly speculative where changes in both intramolecular (covalent) bonds and intermolecular (interlayer) bonds are invoked for its rationalization [7]. Further, Fritzsche [8] based on the concept of the formation of transient excitons presented a model where the change of the #uidity is caused by the cumulative e!ect of recombination-induced atomic motions and bond changes during illumination. In this work we have undertaken a Raman spectroscopic investigation of a-As S "bers subjected to external elongation stress. Analyzing the complete Raman spectrum, including both the internal vibrational modes and the low-energy excitations, and polarization geometries as well, we were able to inspect the role of both intra- and inter-molecular degrees of freedom involved in this e!ect. 2. Experiments Amorphous bulk quantities of As S , kindly provided by Prof. H. Fritzsche, were used as the
217
Fig. 1. Schematic representation of the stretching device. A: incident beam, B: scattered light, F: a-As S "ber, S: spring that provides the applied stress, D: calibrated screw, M: moving support, P: fastened end of the "ber.
starting material. Proper bulk amount of the material (&5 g) was placed in cylindrical fused silica tubes (o.d."13 mm, i.d. 10 mm) and "lled with Ar gas after evacuation. Then the material was heated for homogenization at a temperature (&6003C) far above the melting point for 1}2 h. Homogeneous thin amorphous "bers with variable diameters were produced after immersing and extracting with high speed a needle-like silica rod in the melt. The diameters of the "bers were measured through an optical microscope. Typical diameter magnitudes were found within 80}250 m. The micro-stretching device was constructed in our machine lab and is schematically illustrated in Fig. 1. One of the "ber's endpoints is fastened on a stable block P while the other one was attached on a moving support M driven by the spring S. The elongation of the later and hence the applied force was determined by the proper rotation of a calibrated screw D. The force constant of the spring has been predetermined. During the "ber stretching the spring was loaded only within the elastic regime where the Hookean behavior is ful"lled. Right angle Raman spectra were recorded by a 0.85 m double monochromator (Spex 1403). The excitation source was a Kr> laser (Spectra Physics, model 2017) operating at the 647.1 nm line with a power incident on the "ber of about 10 mW. The
218
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
Fig. 2. Stokes}side Raman spectra of a-As S "ber, d"225 m under elongated tress. The dotted line corresponds to the isotropic part of the spectrum that has included in the "gure to show better the rapid increase of the depolarized scattering with increasing external stress. Numbers besides the spectra denote the externally applied stress in units of 10 dyn cm\.
instrumental resolution was "xed at 2.5 cm\ for the whole set of measurements that were performed at ambient temperature. Both polarization scattering geometries, VV and HV, were employed. The signal after its detection from a water-cooled photomultiplier and its ampli"cation from standard electronic equipment was transferred to a computer. A more detailed description of light scattering apparatus and the conditions for spectra recording can be found in Ref. [9].
3. Results and discussion 3.1. Intramolecular modes Representative polarized and depolarized Stokes} side Raman spectra recorded for di!erent magnitudes of the applied stress are shown in Fig. 2 for
the a-As S "ber with diameter d"225 m. The dashed line represents the isotropic part of the spectrum calculated through the relation I'1-" I44!(4/3)I&4, which is also plotted to emphasize the changes that the spectra reveal. Indeed, drastic modi"cations are observed in the region 260}430 cm\ where according to the molecular model analysis [10] three vibrational modes can be identi"ed, speci"cally the and of the C pyr amidal AsS pyramidal unit at 342 and 310 cm\, respectively, and the of the C water}like unit As}S}As at 392 cm\. The increase of the applied external stress causes a monotonic increase of the depolarization ratio in the whole range of the spectrum, which is most discernible in the mentioned region 260}430 cm\. The increase of the depolarized scattering becomes more profound for larger values of the applied stress, as has been checked from studies when thinner "bers have been employed, leading eventually to an inversion of the depolarization ratio, i.e. I&4'I44, of the spectrum at the highest attainable applied stress. To distinguish between the changes brought about from the incident photon energy and the application of the stress itself we have undertaken a time-dependent study of the Raman spectra of a "ber that was not subjected to elongation stress. The results have shown that no detectable changes are discernible from the measured spectra and the depolarization ratio remains constant independent of the exposure time, which was several hours. As a consequence the aforementioned modi"cations of the depolarization ratio are due to the combined e!ect of illumination and the application of the external stress, where the latter facilitates reorganization of the structure. In order to quantify the dependence of the depolarization ration on the applied stress we have followed a description procedure for the 260}430 cm\ region using a three Gaussian sum approximation to "t the experimental spectra, I():
A ! G G I()" exp !2 (/2 G G G
#bl, (1)
where A is the area, the width, the position G G G and bl a baseline accounting for the background of
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
Fig. 3. Stress dependence of the depolarization ratio for the d"225 m "ber; squares: results from "tting with Eq. (1); circles: results from integration; solid triangle: depolarization ratio for the bulk glass. The continuous lines are guides to the eye.
the spectrum. The subscript i"1, 2, 3 denotes the peaks at 311, 339 and 384 cm\ respectively. The positions and the widths of the three peaks were found to be essentially stress}independent corroborating the perception that the applied stress has no e!ect on the bond lengths and the angles bridging the structural units. Instead, the mutual slipping between layer-like clusters of the amorphous medium relieves the elongation stress and hence micro-ordering e!ects have to be invoked to rationalize the depolarization ratio dependence on the stress. This argument is also supported by the outcome of the unstressed "ber experiment mentioned above. The stress dependence of the depolarization ratio (area ratio) for the individual vibrational modes corresponding to the peaks is depicted in Fig. 3. 3.2. Intermolecular modes It is a well-established fact that amorphous and glassy materials present universally in the lowfrequency Raman spectrum broad low-energy modes (i.e. below &100 cm\) which are manifestations of the non-conserved nature of the scattering wave vector [11]. This low-frequency part of the spectrum consists of the quasi-elastic scattering and the so-called Boson peak. The latter is frequently assigned to the presence of superstructural
219
units at the intermediate range order [12]. Although the microscopic origin of the universally present Boson peak is not yet undisputedly rationalized there are cases where its spectral form and its temperature and polarization dependence can assist to assign it a certain structural origin [9]. In particular, we have argued on the possibility that the Boson peak in layered glasses is a manifestation of the interlayer vibrations, which in the parent crystal are located precisely under its pro"le. In that sense, the position and the spectral form of the Boson peak of the stressed "ber could be employed as a rather sensitive indicator of subtle structural changes that take place } on an intermediate length scale } in the amorphous medium during the photoinduced #uidity e!ect. The moderate signal-to-noise ratio in our case does not facilitate the determination of the apparent Boson peak maximum directly from the raw HV spectra and hence we are forced to employ a spectral "tting analysis. A formal basis to carry out such a "tting procedure, that is relatively simple and does not involve a large number of parameters and approximations, is the log-normal model [13]. The mentioned model relies on the existence of clusters that de"ne micro-regions in the amorphous medium on an intermediate length scale. Under the reasonable assumption that the size of these clusters follows a log-normal distribution it can be shown that the vibrational density of states can also be modeled by the same log-normal form. The "nal "tting expression employed is a composite model consisting of the sum of a Lorenztian line that accounts for the quasi-elastic line centered at zero frequency and the log-normal function where the last reads as:
I (log !log . )
, I0()" exp ! 2 (2
(2)
where I0 is the reduced Raman intensity, I is an arbitrary intensity pre-factor, is the half-width of the log-normal distribution, and . is the posi tion of the Boson peak maximum. The reduction scheme followed here is that originally introduced by Shuker and Gammon [14] i.e. I0()" I/[n(, ¹)#1],C?@()g()/, where I is the intensity of the Stokes Raman spectrum, C?@()
220
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
the polarization ( )-dependent photon}phonon coupling coe$cient, g() the vibrational density of states, and n(, ¹)"[exp( /k ¹)!1]\ is the Bose population factor. The analysis showed that . is almost constant
irrespective of the stress applied to the "ber and further that is equal to the position of the Boson peak in the bulk glass. On the other hand, the depolarization ratio of the Boson peak increases when intensifying the external stress with a rate similar to that of the internal vibrational modes. Further, while a clear frequency dependence can be de"nitely evidenced in the depolarization ratio,
(), of the bulk glass and the unstressed (or slightly stressed) "ber this dependence is gradually smoothed out leading "nally to a constant () for the highest stress values. This signi"cant feature revealed by the depolarization ratio indicates a change in the scattering mechanisms that contribute in the frequency range 20}100 cm\ while the fairly insensitive position of the Boson peak maximum implies no change in some `correlation lengtha associated with structural disorder. It is interesting also here to note that the photon}phonon coupling coe$cient C?@() is the only one quantity contributing to the Raman spectrum that carries polarization information. Then by adopting the idea that the vibrational density of states does not experience substantial modi"cations during stretching we are led to the conclusion that the observed () behavior is re#ected on changes of the coupling coe$cient and subsequently on the atomic polarizability modulation. The buckling model [15] may help to understand at least qualitatively the observed spectral features. It has been proposed that the layer-like remnants (clusters) existing in amorphous chalcogenides could be buckled at some particular atoms. Thus, if the temperature is su$ciently low } namely lower than the thermal energy required for surmounting the barrier of the potential well in which electrons (or holes) are localized } an additional interlayer bond might be formed. The electrons can become delocalized either by thermal activation (temperature rise) or by photo-excitation. The removal of buckling may lead to a release of the excess strain and "nally to an e!ective slip-
ping between neighboring clusters, thus resulting in the observed increasing #uidity and the macroscopic elongation of the "ber. The employment of the idea that the removal of the constraint brought about by the `buckling atomsa facilitates the athermal #ow process seems also compatible with the results obtained by the examination of the low}frequency Raman spectra. Indeed, the established insensitivity of the log-normal distribution width as well as the . on the applied stress
forces us to envisage that breaking or disruption of the clusters does not take place. The applied external stress can be relieved after annulling the buckling constraints and the undulated layerlike cluster may "nally acquire a more `#ata geometry. On the other hand, at high temperatures according to the `buckling modela the interlayer buckling is thermally removed. Then photo-induced #uidity would be facilitated and expected to be more appreciable, which is not the case [7].
4. Conclusions In this contribution we have performed a Raman spectroscopic study for a-As S "bers subjected to external elongation stress in order to elucidate the role of the intra- and inter- molecular modes during the photoinduced #uidity e!ect. The analysis revealed that drastic changes are observed in the depolarization ratio of both modes as a function of the applied stress. On the other hand, the Boson peak position that is a probe of the intermediate range order (intermolecular interactions) seems quite insensitive to the external stress. This fact can lead us to the conclusion that weakening of the intermolecular bonds are not likely to take place during the photoinduced #uidity e!ect, within the accuracy of our experiment and the validity of the assumptions we have used.
Acknowledgements Financial support from the General Secretariat of Research and Technology in the framework of the PENED/99E44 grant is acknowledged.
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
References [1] S.R. Elliott, J. Non-Cryst. Solids 81 (1986) 71. [2] Ke Tanaka, Rev. Solid State Sci. 4 (1990) 641. [3] K. Shimakawa, A.V. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475. [4] A.V. Kolobov et al., Phys. Rev. B 55 (1997) 8788. [5] H. Fritzsche, Phys. Rev. B 52 (1995) 15854. [6] H. Hisakuni, K. Tanaka, Appl. Phys. Lett. 65 (1994) 2925. [7] H. Hisakuni, K. Tanaka, Science 270 (1995) 974. [8] H. Fritzsche, Solid State Commun. 99 (1996) 153.
221
[9] S.N. Yannopoulos, G.N. Papatheodorou, G. Fytas, J. Chem. Phys. 107 (1997) 1341. [10] G. Lucovsky, R.M. Martin, J. Non}Cryst. Solids 8}10 (1972) 185. [11] J. JaK ckle, in: W.A. Phillips (Ed.), Amorphous Solids: Low}Temperature Properties, Springer, Berlin, 1981, p. 151. [12] S.R. Elliott, Physics of Amorphous Materials, 2nd Edition, Longman, Essex, 1990. [13] T. Pang, Phys. Rev. B 45 (1992) 2490. [14] R. Shuker, R. Gammon, Phys. Rev. Lett. 25 (1970) 222. [15] J. Ihm, J. Phys. C 18 (1985) 4741.
A Raman spectroscopic study on the microscopic origin of the photoinduced #uidity e!ect D.Th. Kastrissios , S.N. Yannopoulos *, G.N. Papatheodorou Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes, P.O. Box 1414, GR-26500 Patras, Greece Department of Chemical Engineering, University of Patras, GR-26500, Patras, Greece
Abstract We report on a sub-band gap light scattering (Raman) investigation for a-As S "bers subjected to external elongation stress in order to elucidate the photoinduced #uidity e!ect on a microscopic scale. Changes in the short-range order caused by the presence of the illuminating light have been detected. On the other hand, subtle modi"cations are revealed in the intermediate range order, manifested in the increase of the magnitude and in the modi"cation of the frequency dependence for the depolarization ratio } in the low-frequency part of the spectrum } as a function of the applied stress. The `buckling modela proposed to account for the optical absorption tails in chalcogenide glasses seems quite relevant for explaining certain aspects of the experimental data. 2001 Elsevier Science B.V. All rights reserved. Keywords: Chalcogenide glasses; Photoinduced #uidity; Raman scattering; Boson peak
1. Introduction When amorphous semiconducting or insulating materials are illuminated with light having energy close with that of the band gap of the material, electron}hole pairs are created. These pairs can separate enhancing the electrical response or recombine in a radiative or a non-radiative way. The last case is the most interesting since alterations may occur in structural, mechanical and optical properties of the material. The randomness of the amorphous state compared to the well-de"ned lattice periodicity in crystalline solids renders
* Corresponding author. Tel.: #30-61-965-252; fax: #3061-965-223. E-mail address: [email protected] (S.N. Yannopoulos).
it possible for such changes to occur in a number of di!erent ways. Therefore, more e!orts have been undertaken for elucidating the photoinduced phenomena in glasses and amorphous solids; see Refs. [1}3] for reviews. Furthermore, what is more striking is the fact that the changes in various properties of the materials can be reversible, i.e. they can be eliminated after annealing to the glass transition temperature ¹ or irreversible, i.e. they stay perma nently after exposition to illumination. Reversible photoinduced changes in chalcogenide glasses are up until now the most thoroughly studied. Outstanding among them are the reversible photodarkening (redshift in the optical absorption edge), the photoinduced anisotropy [4,5] (including photoinduced birefringence and photoinduced dichroism), the giant photoexpansion [6] (volume changes up to 5% have been measured
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 8 0 3 - 6
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
after exposition to light). Mechanical or rheological changes have also been observed (decrease of ¹ ) and softening of the elastic constants under illumination. Recently, Hisakuni and Tanaka demonstrated that the #uid state of a glassy semiconductor could be obtained by a process that does not require heat supply [7]. A local increase of the network #uidity emerges as a result of light illumination and can be revealed through the application of external stress. Speci"cally, the e!ect of the incident light (6328 As ) was to facilitate viscous #ow reducing the viscosity by more than two orders of magnitude. In other words, the isoviscous (+2;10 P) temperatures ¹ (without illumination) and ¹ (under illumination) present a di!erence ¹!¹+200 K. The e!ect becomes also macroscopically evident as an increasing elongation of a "ber's length under light illumination, manifested as a `neckinga e!ect on the illuminated point. Quite interesting is the fact that the viscosity of the photo-excited sample unexpectedly increases at higher temperatures. This fact indicates that the photoinduced #uidity is not the e!ect of temperature rise because then the opposite behavior should be observed, namely decreasing viscosity with increasing temperature. The microscopic origin of the photoinduced #uidity e!ect is still highly speculative where changes in both intramolecular (covalent) bonds and intermolecular (interlayer) bonds are invoked for its rationalization [7]. Further, Fritzsche [8] based on the concept of the formation of transient excitons presented a model where the change of the #uidity is caused by the cumulative e!ect of recombination-induced atomic motions and bond changes during illumination. In this work we have undertaken a Raman spectroscopic investigation of a-As S "bers subjected to external elongation stress. Analyzing the complete Raman spectrum, including both the internal vibrational modes and the low-energy excitations, and polarization geometries as well, we were able to inspect the role of both intra- and inter-molecular degrees of freedom involved in this e!ect. 2. Experiments Amorphous bulk quantities of As S , kindly provided by Prof. H. Fritzsche, were used as the
217
Fig. 1. Schematic representation of the stretching device. A: incident beam, B: scattered light, F: a-As S "ber, S: spring that provides the applied stress, D: calibrated screw, M: moving support, P: fastened end of the "ber.
starting material. Proper bulk amount of the material (&5 g) was placed in cylindrical fused silica tubes (o.d."13 mm, i.d. 10 mm) and "lled with Ar gas after evacuation. Then the material was heated for homogenization at a temperature (&6003C) far above the melting point for 1}2 h. Homogeneous thin amorphous "bers with variable diameters were produced after immersing and extracting with high speed a needle-like silica rod in the melt. The diameters of the "bers were measured through an optical microscope. Typical diameter magnitudes were found within 80}250 m. The micro-stretching device was constructed in our machine lab and is schematically illustrated in Fig. 1. One of the "ber's endpoints is fastened on a stable block P while the other one was attached on a moving support M driven by the spring S. The elongation of the later and hence the applied force was determined by the proper rotation of a calibrated screw D. The force constant of the spring has been predetermined. During the "ber stretching the spring was loaded only within the elastic regime where the Hookean behavior is ful"lled. Right angle Raman spectra were recorded by a 0.85 m double monochromator (Spex 1403). The excitation source was a Kr> laser (Spectra Physics, model 2017) operating at the 647.1 nm line with a power incident on the "ber of about 10 mW. The
218
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
Fig. 2. Stokes}side Raman spectra of a-As S "ber, d"225 m under elongated tress. The dotted line corresponds to the isotropic part of the spectrum that has included in the "gure to show better the rapid increase of the depolarized scattering with increasing external stress. Numbers besides the spectra denote the externally applied stress in units of 10 dyn cm\.
instrumental resolution was "xed at 2.5 cm\ for the whole set of measurements that were performed at ambient temperature. Both polarization scattering geometries, VV and HV, were employed. The signal after its detection from a water-cooled photomultiplier and its ampli"cation from standard electronic equipment was transferred to a computer. A more detailed description of light scattering apparatus and the conditions for spectra recording can be found in Ref. [9].
3. Results and discussion 3.1. Intramolecular modes Representative polarized and depolarized Stokes} side Raman spectra recorded for di!erent magnitudes of the applied stress are shown in Fig. 2 for
the a-As S "ber with diameter d"225 m. The dashed line represents the isotropic part of the spectrum calculated through the relation I'1-" I44!(4/3)I&4, which is also plotted to emphasize the changes that the spectra reveal. Indeed, drastic modi"cations are observed in the region 260}430 cm\ where according to the molecular model analysis [10] three vibrational modes can be identi"ed, speci"cally the and of the C pyr amidal AsS pyramidal unit at 342 and 310 cm\, respectively, and the of the C water}like unit As}S}As at 392 cm\. The increase of the applied external stress causes a monotonic increase of the depolarization ratio in the whole range of the spectrum, which is most discernible in the mentioned region 260}430 cm\. The increase of the depolarized scattering becomes more profound for larger values of the applied stress, as has been checked from studies when thinner "bers have been employed, leading eventually to an inversion of the depolarization ratio, i.e. I&4'I44, of the spectrum at the highest attainable applied stress. To distinguish between the changes brought about from the incident photon energy and the application of the stress itself we have undertaken a time-dependent study of the Raman spectra of a "ber that was not subjected to elongation stress. The results have shown that no detectable changes are discernible from the measured spectra and the depolarization ratio remains constant independent of the exposure time, which was several hours. As a consequence the aforementioned modi"cations of the depolarization ratio are due to the combined e!ect of illumination and the application of the external stress, where the latter facilitates reorganization of the structure. In order to quantify the dependence of the depolarization ration on the applied stress we have followed a description procedure for the 260}430 cm\ region using a three Gaussian sum approximation to "t the experimental spectra, I():
A ! G G I()" exp !2 (/2 G G G
#bl, (1)
where A is the area, the width, the position G G G and bl a baseline accounting for the background of
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
Fig. 3. Stress dependence of the depolarization ratio for the d"225 m "ber; squares: results from "tting with Eq. (1); circles: results from integration; solid triangle: depolarization ratio for the bulk glass. The continuous lines are guides to the eye.
the spectrum. The subscript i"1, 2, 3 denotes the peaks at 311, 339 and 384 cm\ respectively. The positions and the widths of the three peaks were found to be essentially stress}independent corroborating the perception that the applied stress has no e!ect on the bond lengths and the angles bridging the structural units. Instead, the mutual slipping between layer-like clusters of the amorphous medium relieves the elongation stress and hence micro-ordering e!ects have to be invoked to rationalize the depolarization ratio dependence on the stress. This argument is also supported by the outcome of the unstressed "ber experiment mentioned above. The stress dependence of the depolarization ratio (area ratio) for the individual vibrational modes corresponding to the peaks is depicted in Fig. 3. 3.2. Intermolecular modes It is a well-established fact that amorphous and glassy materials present universally in the lowfrequency Raman spectrum broad low-energy modes (i.e. below &100 cm\) which are manifestations of the non-conserved nature of the scattering wave vector [11]. This low-frequency part of the spectrum consists of the quasi-elastic scattering and the so-called Boson peak. The latter is frequently assigned to the presence of superstructural
219
units at the intermediate range order [12]. Although the microscopic origin of the universally present Boson peak is not yet undisputedly rationalized there are cases where its spectral form and its temperature and polarization dependence can assist to assign it a certain structural origin [9]. In particular, we have argued on the possibility that the Boson peak in layered glasses is a manifestation of the interlayer vibrations, which in the parent crystal are located precisely under its pro"le. In that sense, the position and the spectral form of the Boson peak of the stressed "ber could be employed as a rather sensitive indicator of subtle structural changes that take place } on an intermediate length scale } in the amorphous medium during the photoinduced #uidity e!ect. The moderate signal-to-noise ratio in our case does not facilitate the determination of the apparent Boson peak maximum directly from the raw HV spectra and hence we are forced to employ a spectral "tting analysis. A formal basis to carry out such a "tting procedure, that is relatively simple and does not involve a large number of parameters and approximations, is the log-normal model [13]. The mentioned model relies on the existence of clusters that de"ne micro-regions in the amorphous medium on an intermediate length scale. Under the reasonable assumption that the size of these clusters follows a log-normal distribution it can be shown that the vibrational density of states can also be modeled by the same log-normal form. The "nal "tting expression employed is a composite model consisting of the sum of a Lorenztian line that accounts for the quasi-elastic line centered at zero frequency and the log-normal function where the last reads as:
I (log !log . )
, I0()" exp ! 2 (2
(2)
where I0 is the reduced Raman intensity, I is an arbitrary intensity pre-factor, is the half-width of the log-normal distribution, and . is the posi tion of the Boson peak maximum. The reduction scheme followed here is that originally introduced by Shuker and Gammon [14] i.e. I0()" I/[n(, ¹)#1],C?@()g()/, where I is the intensity of the Stokes Raman spectrum, C?@()
220
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
the polarization ( )-dependent photon}phonon coupling coe$cient, g() the vibrational density of states, and n(, ¹)"[exp( /k ¹)!1]\ is the Bose population factor. The analysis showed that . is almost constant
irrespective of the stress applied to the "ber and further that is equal to the position of the Boson peak in the bulk glass. On the other hand, the depolarization ratio of the Boson peak increases when intensifying the external stress with a rate similar to that of the internal vibrational modes. Further, while a clear frequency dependence can be de"nitely evidenced in the depolarization ratio,
(), of the bulk glass and the unstressed (or slightly stressed) "ber this dependence is gradually smoothed out leading "nally to a constant () for the highest stress values. This signi"cant feature revealed by the depolarization ratio indicates a change in the scattering mechanisms that contribute in the frequency range 20}100 cm\ while the fairly insensitive position of the Boson peak maximum implies no change in some `correlation lengtha associated with structural disorder. It is interesting also here to note that the photon}phonon coupling coe$cient C?@() is the only one quantity contributing to the Raman spectrum that carries polarization information. Then by adopting the idea that the vibrational density of states does not experience substantial modi"cations during stretching we are led to the conclusion that the observed () behavior is re#ected on changes of the coupling coe$cient and subsequently on the atomic polarizability modulation. The buckling model [15] may help to understand at least qualitatively the observed spectral features. It has been proposed that the layer-like remnants (clusters) existing in amorphous chalcogenides could be buckled at some particular atoms. Thus, if the temperature is su$ciently low } namely lower than the thermal energy required for surmounting the barrier of the potential well in which electrons (or holes) are localized } an additional interlayer bond might be formed. The electrons can become delocalized either by thermal activation (temperature rise) or by photo-excitation. The removal of buckling may lead to a release of the excess strain and "nally to an e!ective slip-
ping between neighboring clusters, thus resulting in the observed increasing #uidity and the macroscopic elongation of the "ber. The employment of the idea that the removal of the constraint brought about by the `buckling atomsa facilitates the athermal #ow process seems also compatible with the results obtained by the examination of the low}frequency Raman spectra. Indeed, the established insensitivity of the log-normal distribution width as well as the . on the applied stress
forces us to envisage that breaking or disruption of the clusters does not take place. The applied external stress can be relieved after annulling the buckling constraints and the undulated layerlike cluster may "nally acquire a more `#ata geometry. On the other hand, at high temperatures according to the `buckling modela the interlayer buckling is thermally removed. Then photo-induced #uidity would be facilitated and expected to be more appreciable, which is not the case [7].
4. Conclusions In this contribution we have performed a Raman spectroscopic study for a-As S "bers subjected to external elongation stress in order to elucidate the role of the intra- and inter- molecular modes during the photoinduced #uidity e!ect. The analysis revealed that drastic changes are observed in the depolarization ratio of both modes as a function of the applied stress. On the other hand, the Boson peak position that is a probe of the intermediate range order (intermolecular interactions) seems quite insensitive to the external stress. This fact can lead us to the conclusion that weakening of the intermolecular bonds are not likely to take place during the photoinduced #uidity e!ect, within the accuracy of our experiment and the validity of the assumptions we have used.
Acknowledgements Financial support from the General Secretariat of Research and Technology in the framework of the PENED/99E44 grant is acknowledged.
D.Th. Kastrissios et al. / Physica B 296 (2001) 216}221
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