- Email: [email protected]
Electroabsorption studies of structurally modified fullerene thin films
ARTICLE IN PRESS
Journal of Luminescence 112 (2005) 291–294 www.elsevier.com/locate/jlumin
Electroabsorption studies of structurally modified fullerene thin films G.F. Farrella,, G. Chambersb, H.J. Byrnea a FOCAS, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland Physics Department, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland
b
Available online 18 October 2004
Abstract The excited-state properties of C60 have been extensively studied in recent years through a number of processes and a good understanding of the active processes has begun to emerge. However, a number of key questions remain about the nature of the inter- and intra- molecular excited states of C60. The application of Electroabsorption (EA) spectroscopy to the study of the charge transfer characteristics of C60 has been shown to be an extremely valuable probe of these excited states. However, the effect of controlling the relative contributions from inter- and intra-molecular interactions has not been, thoroughly investigated. This work hopes to extend the knowledge base by monitoring the changes in the electroabsorption spectrum of fullerene thin films which are subjected to thermal annealing, a processes known to modify the electronic properties of solid state C60. It has been shown by comparison that while there is a large pronounced change in the absorption spectrum of annealed fullerene films, there is little or no change in the electroabsorption spectrum. r 2004 Published by Elsevier B.V. PACS: 71.20.Tx; 78.40.Ri; 61.48.+c; 78.66.Tr Keywords: Fullerenes; Electroabsorption; Lattice structure; Annealing
It is generally accepted that the EA spectrum of C60 is dominated by charge transfer states [1]. It has also been observed that these features become enhanced at temperatures below the well-known orientation phase transition temperature of solid state C60 at 249 K suggesting that changes in the Corresponding author. Fax: +353 1 402 7901.
E-mail address: [email protected] (G.F. Farrell). 0022-2313/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jlumin.2004.09.106
molecular packing due to the phase transition can modify or control the relative contribution of inter- and intra- molecular interactions. Indeed several methods in the past have shown structural and electronic changes incurred due to the phase transition can be mimicked at higher temperature. In particular, the formation of clathrates in which C60 crystals are exposed to various neutral solvents, which intercalate themselves into the
ARTICLE IN PRESS 292
G.F. Farrell et al. / Journal of Luminescence 112 (2005) 291–294
FCC lattice arrangement of C60, thereby reducing the symmetry of the crystals to an orthorhombic phase [2]. The overall net effect of this reduction in symmetry is to shift the absorption edge to higher energies and the orientational phase transition at 249 K to a higher temperature dependent on the degree of intercalation and the intercalating molecule itself. In addition to solvent inclusion it has been shown that the lattice structure of C60 can be affected by thermally annealing films. By annealing C60 thin films a partial phase transition to a HCP crystalline phase occurs. This transition results in a decrease in the optical oscillator strengths, which is also coupled by a red shift of up to 50 nm in the absorption spectrum. A number of studies on annealed films have been reported [3,4] where it was seen that UV/Vis and IR spectra show no indication of any chemical alteration of the sample but rather an apparent structural change is observed [3], similar to that reported for C60 clathrates. [5] This work will investigate the effects of the structural modification of C60 thin films primarily by changing the crystallinity of fullerene thin films via a systematic annealing procedure. Characterization of these films will primarily involve using UV/Vis and electroabsorption (EA) measurements. In both cases the aim is to correlate changes in charge transfer states to the well-known structural changes in the C60 lattice. Thin films of C60, (99.98% pure as purchased from SES Research) approximately 500 nm thick, were produced by vacuum deposition onto indium–tin oxide covered glass substrates. A transparent Al top electrode (7 nm thick) was then deposited through a shadow mask, onto the surface of the C60 film. Reference films were also produced by sublimation of C60 onto quartz substrates. A selection the films where annealed by postdeposition heating in a Varian vacuum oven at 2001C at a pressure of 105 mbar for 24 h. The temperature was somewhat arbitrarily chosen to be sufficiently below the sublimation temperature. Absorption spectra of annealed and unannealed films were taken using a Perkin–Elmer Lambda 900 UV/Vis/NIR. Annealed films were prepared
by heating films to 200 1C under vacuum. The electroabsorption (EA) measurements were preformed using a homemade system, similar to that described by Boxer et Al. [5]. Fig. 1(a) shows the absorption spectrum of a thin film of C60. Evident from this spectrum are a number of features, which are molecular in origin and some, which are the result of the solid state environment. In particular, the lowest energy transition in the solid state, which is maximum at 630 nm, possesses a vibrational structure similar to that of the isolated molecule. Furthermore, all of the higher order, allowed molecular transitions also appear in the solid-state spectrum, although slightly red shifted. On this basis, the solid state of C60, appears to behave like a classical molecular solid in which the molecular wave functions are minimally perturbed, the principal influence of the crystalline environment being in symmetry breaking. However, there is a broad, relatively strong absorption centred around 450 nm which is absent in the solution and gas phase EELS spectra [8,9] hence suggesting that it is not molecular in origin but rather the result of the solid-state environment. Further, this photoconductivity and electroabsorption similarly indicate a photoexcited species which is not molecular in origin [7,10]. Kazaoui et al. [7], have demonstrated, via electro-absorption that the feature at 460 nm derives from a series of charge transfer states, whereby the transition occurs between the HOMO of one
Fig. 1. (a) Shows the absorption spectrum of a pristine C60 film. (b) Shows the affect of annealing a C60 film for 24 h (dotted line).
ARTICLE IN PRESS G.F. Farrell et al. / Journal of Luminescence 112 (2005) 291–294
molecule to a higher unoccupied state whose wave function derives from a mixture of the molecular state with those of its next nearest neighbours. It is thus now widely accepted [11] that this feature consists of two distinct absorptions, one at 511 nm and the other at 460 nm.These are due to the solid states influence on the electronic properties of the molecule, hence it is not molecular in character but instead is the result of an intermolecular charge transfer excited state. As previously mentioned it has been shown as early as 1994, that the absorption spectrum of C60 can be altered due to external structural modification of the films [5]. Initially such work focused upon the thermal annealing of C60 thin films in which it was reported that a marked decrease in the oscillation strengths of transitions was observed due to the annealing process. [6] Fig. 1(b) showes the absorption spectrum obtained from a film which was annealed for 24 h and indeed there is an overall decrease in the oscillation strengths of the features between 200 and 400 nm. More noticeable however is the decrease in intensity of the features at 460 and 510 nm, the region of interest for this study. This decrease suggests a possible reduction in the charge transfer processes associated with these transitions through photoconductivity and electroabsorption measurements. There is also a continuous red shift of these solid-state features (although not entirely evident from Fig. 1). This apparent shifting accompanied by the loss in intensity can be attributed to a closer packing of the molecules in the solid-state phase. A theoretical analysis of the effects of crystal packing on both the spectral positioning and oscillator strengths of the p–p* transitions of C60 in the solid state has been performed by Wu and Ulloa [12]. The analysis indicated that the oscillator strength and spectral positioning of transitions can have considerable anisotropy due to different Coulomb interactions between molecules in different packing arrangements. This predicted sensitivity of the optical absorption spectrum to crystal packing is consistent with the variations observed in these thermally annealed films The electroabsorption spectra were obtained by measuring the change in transmission (DT) under
293
the influence of an electric field, (2w; w ¼ 190 Hz) which was normalised to the transmission (T) of the same sample. The following relationship DT=T ¼ dDa was used, where d is the film thickness and a is the absorption coefficient. As can be seen from Fig. 2 the spectrum of pristine C60 consists of a number of modulations with second-order contributions at 510, 460 and 423 nm. For the purposes of the EA measurements we will concentrate only on the modes located at 460 and 510 nm. The positioning of these modes and relative intensities are consistent with those reported in literature for the EA of C60. The strongest feature evident in Fig. 2 is the mode positioned at approximately 510 nm. Previously this mode has been attributed to Frenkel state due to its resemblance to a first derivative line shape [13]. However, subsequent work perform by Tsubo and others later attributed this feature to a CT state [9,14] with confirmatory data being provided by Pac et al. [15]. The next most prominent mode is located at 460 nm. This mode has been described by Pac. et al. [15] as arising from the splitting of the ð1=2; 1=2; 0Þ CT manifold by the off-diagonal charge transfer interactions. There are numerous other CT interactions associated with EA spectrum of C60, and detailed discussions can be found in the various cited works. However, in essence the electroabsorption spectrum of C60 can be thought of as a combination of various offdiagonal CT interactions coupled with nearby
Fig. 2. The main graph shows a full EA spectrum of a C60 film before and after annealing at 2001C for 24 h. (film 500 nm thick, field 100 kV).
ARTICLE IN PRESS 294
G.F. Farrell et al. / Journal of Luminescence 112 (2005) 291–294
inactive Frenkel states and intensity borrowing from high-energy allowed Frenkel excitions [15]. In order, to monitor any changes due to the thermal annealing, EA spectra were taken on the same films of C60 which were used to obtain the absorption spectra see figure. Upon annealing the general shape of the EA spectrum initially appears unaltered, however there are a number of subtle changes. In particular the mode position at 510 nm experiences an increase in intensity and a small red shift. Such a shift may be indicative of a change in the packing density as previously describe for the absorption spectrum by introducing changes to the charge transfer species. Indeed annealing processes have been reported to change the C60 lattice from a FCC type arrangement to a HCP type of arrangement, a finding confirmed by X-ray analysis of the films [4,16]. With respect to the electroabsorption it would be expected that any change to the lattice structure or the electronic properties of C60 due to the annealing processes could be readily detected. Since, a contraction of the lattice would suggest a small decrease in the distances between adjacent molecules and hence the electron–hole separation of the (1,0,0) lattice coordinate which is reported in literature to be 14.15 A˚ [9] for pristine C60. As a result of this decrease the Dm associated with this CT manifold would also be expected to experience a small decrease. However, upon examination of the feature at 460 nm there does not seem to be any noticeable difference between the two spectra. A possible explanation as to why there is no observable difference between the EA spectra of the annealed and unannealed films could be that the dipole moments and polarizabilities of the pristine films do not change drastically upon annealing and hence resulting changes in the charge separation are too small to be detected with our current set-up. In summary, it has been shown by various groups that the electronic transitions associated
with absorption spectroscopy can be altered by structural modification of the bulk lattice structure. Although, the aim of this work was to show any differences in the dipole moment due to the affect of thermal annealing, examination of the EA spectrum did not reveal any such differences. FOCAS is funded under the National Development Plan 2000-2006 with assistance from the European Regional Development Fund. References [1] K. Pichler, S. Graham, O.M. Gelsen R.H Friend, W.J. Romanow, J.P. Mc Cauley, N. Coustel, J.E. Fischer, A.B. Smith, J. Phys.: Cond. Matt 3 (1991) 9259. [2] K. Henderson, G. Chambers, H.J. Byrne, Synth. Met. 103 (1999) 2360. [3] M. Kaiser, W.K. Maser, H.J. Byrne, A. Mittelbach, S. Roth, Solid State Commun 87 (1993) 281. [4] L. Akselrod, H. J Byrne, T.E. Sutto, S. Roth, Chem. Phys. Lett. 233 (1995) 436. [5] J.R. Lawrence, G. Chambers, D. Fenton, K. Henderson, A.B. Dalton, H.J. Byrne, Synth. Met. 121 (2001) 1145. [6] G.U. Bublitz, S.G. Boxer, Annu. Rev. Phys. Chem. 48 (1997) 213. [7] S. Kazaoui, N. Minami, Y. Tanabe, H.J. Byrne, A. Eilmes, P. Petelenz, Phys. Rev. B 58 (12) (1998) 7689. [8] S. Leach, in: K. Prassides (Ed.), Physics and Chemistry of Fullerenes, NATO ASI Series, Kuluwer Academic Publishers, Vol. 117, 1994, p. 443. [9] C. Bulliard, M. Allan, S. Leach, Chem. Phys. Lett. 434 (1993) 209. [10] M. Kaiser, J. Reichenbach, H.J. Byrne, J. Anders, W. Maser, S. Roth, A. Zahab, P. Bernier, Solid State Commun 61 (1992) 81. [11] H.J. Byrne, in: H. Kuzmany, J. Fink, M. Mehring, S. Roth, (Eds.), Physics and Chemistry of Fullerenes and Derivatives, World Scientific Singapore, 1995, p. 183. [12] X. Wu, S.E. Ulloa, Phys. Rev. B 7825 (1994) 49. [13] S. Jeglinski, Z.V. Vardeny, D. Moses, V.I. Srdanov, F. Wudl, Synth. Met. 557 (1992) 49–50. [14] T. Tsubo, K. Nasu, Solid State Commun 91 (1994) 907. [15] B. Pac, P. Petelenz, M. Slawik, R.W. Munn, J. Chem. Phys. 109 (18) (1998) 7932. [16] A.H. Jayatissa, T. Gupta, A.D. Pandya, Carbon 42 (2004) 1143.
Journal of Luminescence 112 (2005) 291–294 www.elsevier.com/locate/jlumin
Electroabsorption studies of structurally modified fullerene thin films G.F. Farrella,, G. Chambersb, H.J. Byrnea a FOCAS, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland Physics Department, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland
b
Available online 18 October 2004
Abstract The excited-state properties of C60 have been extensively studied in recent years through a number of processes and a good understanding of the active processes has begun to emerge. However, a number of key questions remain about the nature of the inter- and intra- molecular excited states of C60. The application of Electroabsorption (EA) spectroscopy to the study of the charge transfer characteristics of C60 has been shown to be an extremely valuable probe of these excited states. However, the effect of controlling the relative contributions from inter- and intra-molecular interactions has not been, thoroughly investigated. This work hopes to extend the knowledge base by monitoring the changes in the electroabsorption spectrum of fullerene thin films which are subjected to thermal annealing, a processes known to modify the electronic properties of solid state C60. It has been shown by comparison that while there is a large pronounced change in the absorption spectrum of annealed fullerene films, there is little or no change in the electroabsorption spectrum. r 2004 Published by Elsevier B.V. PACS: 71.20.Tx; 78.40.Ri; 61.48.+c; 78.66.Tr Keywords: Fullerenes; Electroabsorption; Lattice structure; Annealing
It is generally accepted that the EA spectrum of C60 is dominated by charge transfer states [1]. It has also been observed that these features become enhanced at temperatures below the well-known orientation phase transition temperature of solid state C60 at 249 K suggesting that changes in the Corresponding author. Fax: +353 1 402 7901.
E-mail address: [email protected] (G.F. Farrell). 0022-2313/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jlumin.2004.09.106
molecular packing due to the phase transition can modify or control the relative contribution of inter- and intra- molecular interactions. Indeed several methods in the past have shown structural and electronic changes incurred due to the phase transition can be mimicked at higher temperature. In particular, the formation of clathrates in which C60 crystals are exposed to various neutral solvents, which intercalate themselves into the
ARTICLE IN PRESS 292
G.F. Farrell et al. / Journal of Luminescence 112 (2005) 291–294
FCC lattice arrangement of C60, thereby reducing the symmetry of the crystals to an orthorhombic phase [2]. The overall net effect of this reduction in symmetry is to shift the absorption edge to higher energies and the orientational phase transition at 249 K to a higher temperature dependent on the degree of intercalation and the intercalating molecule itself. In addition to solvent inclusion it has been shown that the lattice structure of C60 can be affected by thermally annealing films. By annealing C60 thin films a partial phase transition to a HCP crystalline phase occurs. This transition results in a decrease in the optical oscillator strengths, which is also coupled by a red shift of up to 50 nm in the absorption spectrum. A number of studies on annealed films have been reported [3,4] where it was seen that UV/Vis and IR spectra show no indication of any chemical alteration of the sample but rather an apparent structural change is observed [3], similar to that reported for C60 clathrates. [5] This work will investigate the effects of the structural modification of C60 thin films primarily by changing the crystallinity of fullerene thin films via a systematic annealing procedure. Characterization of these films will primarily involve using UV/Vis and electroabsorption (EA) measurements. In both cases the aim is to correlate changes in charge transfer states to the well-known structural changes in the C60 lattice. Thin films of C60, (99.98% pure as purchased from SES Research) approximately 500 nm thick, were produced by vacuum deposition onto indium–tin oxide covered glass substrates. A transparent Al top electrode (7 nm thick) was then deposited through a shadow mask, onto the surface of the C60 film. Reference films were also produced by sublimation of C60 onto quartz substrates. A selection the films where annealed by postdeposition heating in a Varian vacuum oven at 2001C at a pressure of 105 mbar for 24 h. The temperature was somewhat arbitrarily chosen to be sufficiently below the sublimation temperature. Absorption spectra of annealed and unannealed films were taken using a Perkin–Elmer Lambda 900 UV/Vis/NIR. Annealed films were prepared
by heating films to 200 1C under vacuum. The electroabsorption (EA) measurements were preformed using a homemade system, similar to that described by Boxer et Al. [5]. Fig. 1(a) shows the absorption spectrum of a thin film of C60. Evident from this spectrum are a number of features, which are molecular in origin and some, which are the result of the solid state environment. In particular, the lowest energy transition in the solid state, which is maximum at 630 nm, possesses a vibrational structure similar to that of the isolated molecule. Furthermore, all of the higher order, allowed molecular transitions also appear in the solid-state spectrum, although slightly red shifted. On this basis, the solid state of C60, appears to behave like a classical molecular solid in which the molecular wave functions are minimally perturbed, the principal influence of the crystalline environment being in symmetry breaking. However, there is a broad, relatively strong absorption centred around 450 nm which is absent in the solution and gas phase EELS spectra [8,9] hence suggesting that it is not molecular in origin but rather the result of the solid-state environment. Further, this photoconductivity and electroabsorption similarly indicate a photoexcited species which is not molecular in origin [7,10]. Kazaoui et al. [7], have demonstrated, via electro-absorption that the feature at 460 nm derives from a series of charge transfer states, whereby the transition occurs between the HOMO of one
Fig. 1. (a) Shows the absorption spectrum of a pristine C60 film. (b) Shows the affect of annealing a C60 film for 24 h (dotted line).
ARTICLE IN PRESS G.F. Farrell et al. / Journal of Luminescence 112 (2005) 291–294
molecule to a higher unoccupied state whose wave function derives from a mixture of the molecular state with those of its next nearest neighbours. It is thus now widely accepted [11] that this feature consists of two distinct absorptions, one at 511 nm and the other at 460 nm.These are due to the solid states influence on the electronic properties of the molecule, hence it is not molecular in character but instead is the result of an intermolecular charge transfer excited state. As previously mentioned it has been shown as early as 1994, that the absorption spectrum of C60 can be altered due to external structural modification of the films [5]. Initially such work focused upon the thermal annealing of C60 thin films in which it was reported that a marked decrease in the oscillation strengths of transitions was observed due to the annealing process. [6] Fig. 1(b) showes the absorption spectrum obtained from a film which was annealed for 24 h and indeed there is an overall decrease in the oscillation strengths of the features between 200 and 400 nm. More noticeable however is the decrease in intensity of the features at 460 and 510 nm, the region of interest for this study. This decrease suggests a possible reduction in the charge transfer processes associated with these transitions through photoconductivity and electroabsorption measurements. There is also a continuous red shift of these solid-state features (although not entirely evident from Fig. 1). This apparent shifting accompanied by the loss in intensity can be attributed to a closer packing of the molecules in the solid-state phase. A theoretical analysis of the effects of crystal packing on both the spectral positioning and oscillator strengths of the p–p* transitions of C60 in the solid state has been performed by Wu and Ulloa [12]. The analysis indicated that the oscillator strength and spectral positioning of transitions can have considerable anisotropy due to different Coulomb interactions between molecules in different packing arrangements. This predicted sensitivity of the optical absorption spectrum to crystal packing is consistent with the variations observed in these thermally annealed films The electroabsorption spectra were obtained by measuring the change in transmission (DT) under
293
the influence of an electric field, (2w; w ¼ 190 Hz) which was normalised to the transmission (T) of the same sample. The following relationship DT=T ¼ dDa was used, where d is the film thickness and a is the absorption coefficient. As can be seen from Fig. 2 the spectrum of pristine C60 consists of a number of modulations with second-order contributions at 510, 460 and 423 nm. For the purposes of the EA measurements we will concentrate only on the modes located at 460 and 510 nm. The positioning of these modes and relative intensities are consistent with those reported in literature for the EA of C60. The strongest feature evident in Fig. 2 is the mode positioned at approximately 510 nm. Previously this mode has been attributed to Frenkel state due to its resemblance to a first derivative line shape [13]. However, subsequent work perform by Tsubo and others later attributed this feature to a CT state [9,14] with confirmatory data being provided by Pac et al. [15]. The next most prominent mode is located at 460 nm. This mode has been described by Pac. et al. [15] as arising from the splitting of the ð1=2; 1=2; 0Þ CT manifold by the off-diagonal charge transfer interactions. There are numerous other CT interactions associated with EA spectrum of C60, and detailed discussions can be found in the various cited works. However, in essence the electroabsorption spectrum of C60 can be thought of as a combination of various offdiagonal CT interactions coupled with nearby
Fig. 2. The main graph shows a full EA spectrum of a C60 film before and after annealing at 2001C for 24 h. (film 500 nm thick, field 100 kV).
ARTICLE IN PRESS 294
G.F. Farrell et al. / Journal of Luminescence 112 (2005) 291–294
inactive Frenkel states and intensity borrowing from high-energy allowed Frenkel excitions [15]. In order, to monitor any changes due to the thermal annealing, EA spectra were taken on the same films of C60 which were used to obtain the absorption spectra see figure. Upon annealing the general shape of the EA spectrum initially appears unaltered, however there are a number of subtle changes. In particular the mode position at 510 nm experiences an increase in intensity and a small red shift. Such a shift may be indicative of a change in the packing density as previously describe for the absorption spectrum by introducing changes to the charge transfer species. Indeed annealing processes have been reported to change the C60 lattice from a FCC type arrangement to a HCP type of arrangement, a finding confirmed by X-ray analysis of the films [4,16]. With respect to the electroabsorption it would be expected that any change to the lattice structure or the electronic properties of C60 due to the annealing processes could be readily detected. Since, a contraction of the lattice would suggest a small decrease in the distances between adjacent molecules and hence the electron–hole separation of the (1,0,0) lattice coordinate which is reported in literature to be 14.15 A˚ [9] for pristine C60. As a result of this decrease the Dm associated with this CT manifold would also be expected to experience a small decrease. However, upon examination of the feature at 460 nm there does not seem to be any noticeable difference between the two spectra. A possible explanation as to why there is no observable difference between the EA spectra of the annealed and unannealed films could be that the dipole moments and polarizabilities of the pristine films do not change drastically upon annealing and hence resulting changes in the charge separation are too small to be detected with our current set-up. In summary, it has been shown by various groups that the electronic transitions associated
with absorption spectroscopy can be altered by structural modification of the bulk lattice structure. Although, the aim of this work was to show any differences in the dipole moment due to the affect of thermal annealing, examination of the EA spectrum did not reveal any such differences. FOCAS is funded under the National Development Plan 2000-2006 with assistance from the European Regional Development Fund. References [1] K. Pichler, S. Graham, O.M. Gelsen R.H Friend, W.J. Romanow, J.P. Mc Cauley, N. Coustel, J.E. Fischer, A.B. Smith, J. Phys.: Cond. Matt 3 (1991) 9259. [2] K. Henderson, G. Chambers, H.J. Byrne, Synth. Met. 103 (1999) 2360. [3] M. Kaiser, W.K. Maser, H.J. Byrne, A. Mittelbach, S. Roth, Solid State Commun 87 (1993) 281. [4] L. Akselrod, H. J Byrne, T.E. Sutto, S. Roth, Chem. Phys. Lett. 233 (1995) 436. [5] J.R. Lawrence, G. Chambers, D. Fenton, K. Henderson, A.B. Dalton, H.J. Byrne, Synth. Met. 121 (2001) 1145. [6] G.U. Bublitz, S.G. Boxer, Annu. Rev. Phys. Chem. 48 (1997) 213. [7] S. Kazaoui, N. Minami, Y. Tanabe, H.J. Byrne, A. Eilmes, P. Petelenz, Phys. Rev. B 58 (12) (1998) 7689. [8] S. Leach, in: K. Prassides (Ed.), Physics and Chemistry of Fullerenes, NATO ASI Series, Kuluwer Academic Publishers, Vol. 117, 1994, p. 443. [9] C. Bulliard, M. Allan, S. Leach, Chem. Phys. Lett. 434 (1993) 209. [10] M. Kaiser, J. Reichenbach, H.J. Byrne, J. Anders, W. Maser, S. Roth, A. Zahab, P. Bernier, Solid State Commun 61 (1992) 81. [11] H.J. Byrne, in: H. Kuzmany, J. Fink, M. Mehring, S. Roth, (Eds.), Physics and Chemistry of Fullerenes and Derivatives, World Scientific Singapore, 1995, p. 183. [12] X. Wu, S.E. Ulloa, Phys. Rev. B 7825 (1994) 49. [13] S. Jeglinski, Z.V. Vardeny, D. Moses, V.I. Srdanov, F. Wudl, Synth. Met. 557 (1992) 49–50. [14] T. Tsubo, K. Nasu, Solid State Commun 91 (1994) 907. [15] B. Pac, P. Petelenz, M. Slawik, R.W. Munn, J. Chem. Phys. 109 (18) (1998) 7932. [16] A.H. Jayatissa, T. Gupta, A.D. Pandya, Carbon 42 (2004) 1143.