- Email: [email protected]
Studies of aqueous colloidal solutions of fullerene C60 by electron microscopy
5 February 1999
Chemical Physics Letters 300 Ž1999. 392–396
Studies of aqueous colloidal solutions of fullerene C 60 by electron microscopy G.V. Andrievsky a,) , V.K. Klochkov a , E.L. Karyakina b, N.O. Mchedlov-Petrossyan c a
Institute for Therapy of the Academy of Medical Sciences of Ukraine, 2-a PostysheÕ str., 310039 KharkoÕ, Ukraine b Ukrainian State Research Institute of Refractories, 310024 KharkoÕ, Ukraine c Department of Physical Chemistry, KharkoÕ State UniÕersity, 310077 KharkoÕ, Ukraine Received 8 September 1998; in final form 4 December 1998
Abstract The systems of fullerene C 60 in water ŽC 60 FWS., i.e. aqueous colloidal solutions of buckminsterfullerenes, have been studied using transmission electron microscopy. The C 60 FWS are shown to be molecular–colloid systems, containing both single fullerene molecules and their fractal clusters in a hydrated state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Recently w1,2x we have shown the possibility of obtaining aqueous solutions of C 60 and C 70 with no stabilisers and chemical modification. The method is based on transferring fullerene from toluene into the water phase with the help of sonication. These solutions, stable over at least 18 months, we denote FWS. The solutions turned out to be colloidal systems with a finely dispersed fullerene phase w1–3x. The C 60 hydrosol with a concentration G 0.1 mg mly1 behaves as an ultramicroheterogeneous polydisperse hydrophobic colloidal system, with negatively charged particles w2,3x. C 60 dispersions, obtained by others w4x, are much more rough-sized
Žtypical suspensions. and dilute Ž0.001 mg mly1 .. Having modified and optimized the method of FWS preparation, we were able to obtain a C 60 FWS solution with a concentration up to 2.2 = 10y3 M Ž1.6 mg mly1 .. This extends the application field of fullerenes. Thus a more detailed study of FWS systems becomes important, in particular a more precise determination of the colloidal particle’s size. In the present Letter, we communicate the results of an investigation of C 60 FWS, mainly with a concentration of 1.85 = 10y4 M Ž0.13 mg mly1 ., using transmission electron microscopy ŽTEM..
2. Experimental
)
Corresponding author. Fax.: q380 572 726105; e-mail: [email protected]
Dispersions of C 60 were obtained from a sample of fullerene C 60 with a purity ) 99.5% ŽMER, Tuscon, AZ, USA..
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 3 9 3 - 1
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
For the TEM studies, the EMV 100 AK apparatus, operating at 75 kV, was used. The research was carried out by using the ‘suspension’ method; a drop of the colloidal solution was placed on a structureless nitrocellulose support. After air drying, the specimens were placed into the apparatus Žvacuum 3 = 10y5 Torr.. Samples obtained in a similar way from C 60 solutions in benzene were also studied for comparison. Care was taken to minimise exposure of the samples to the electron beam in order to avoid beam damage. Typical electron micrographs are presented in Figs. 1–3. 3. Results and discussion The plate-like morphology of the crystalline C 60 phase, formed after evaporation of benzene, completely agrees with literature data w5,6x. Contrary to it, the solid phase, obtained from C 60 FWS, consists of typical colloidal particles ŽFig. 1.. Sphere-shaped aggregates of 7–72 nm size, under close examination consisting of primary spherical particles, tend in their turn to further cluster formation. This proves the ultramicroheterogeneous and polydisperse nature of C 60 hydrosol, revealed earlier using other methods w2x. Electron diffraction patterns ŽFig. 1. show a crystal-like character of primary aggregates of C 60 .
393
Note that the size of colloidal particles of amino acids and dipeptide derivatives of C 60 in solutions is 1–10 mm w7x. In addition, the micrographs reveal less distinct, but graphic enough ‘ vague’ zones. In these regions, the solid phase is formed from conglomerates of particles with 1–4 nm size, the smallest of which are commensurate with the isolated C 60 molecule ŽFig. 2.. This micrograph also shows that the large C 60 aggregates are loose. Such aggregates are to be easily deformed during the specimen’s preparation Že.g., Fig. 2, bottom left. and consist of smaller particles of varying size. The results of ultracentrifugation w2x also show the presence of a ‘light’ fraction in the polydispersed colloidal C 60 solution which remains after 1 h of ultracentrifugation Ž145 = 10 3 g .. The coagulation of C 60 FWS by NaCl was studied as well ŽFig. 3.. The particles of the solid phase, obtained from coagulates, contain, along with preformed large C 60 spherical aggregates Žless contrast in the micrograph., a lot of even larger sphere-like ones Žmore contrast.. As we observed no ‘ vague’ zones ŽFig. 2, top right. in the micrographs of the coagulates ŽFig. 3., small particles seem to take part in the formation of the above aggregates having more contrast image.
Fig. 1. The electron micrograph of fullerene C 60 aggregates obtained from C 60 FWS Žon the right: diffraction pattern from C 60 aggregates..
394
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
Fig. 2. The electron micrograph with ‘ vague’ zones.
Thus, the majority of particles in the studied polydisperse systems are of 1–4 nm size. The numerical results obtained with other C 60 FWS samples are of similar character. Do the sizes of the sphere-shaped particles in C 60 FWS adhere to a certain regularity?
To clarify this problem, let us consider the C 60 aggregates in C 60 FWS from the point of view of the fractal cluster’s growth. Let us assume the model of the diffusion limited aggregation of clusters as a result of the attachment to it of separate particles. In this case, the relation between a number of particles
Fig. 3. The electron micrograph of fullerene C 60 coagulates.
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
in a cluster Ž n., its radius Ž R . and its fractal dimensionality Ž d f . has the form: ns
R
ž / r
df
,
Ž 1.
where r is the radius of a particle forming the cluster w8,9x. The smallest stable spherical aggregate of C 60 molecules with crystalline packing was predicted to be of 2.9–3.0 nm in size Žwith regard to van der Waals radii. and to consist of 13 C 60 molecules w10x. Earlier w11x, such an aggregate was detected mass spectrometrically, and was supposed to possess an icosahedral structure. On the other hand, scanning tunneling microscopy convinced us that the smallest spherical clusters in C 60 FWS have a mean size equal to 3.4 nm w2,12x. In accordance with formula Ž1. and with the value d f s 2.1, found for benzene solutions of C 60 w13x, these clusters also contain 13 C 60 molecules. As the diameter of the above clusters is 0.4–0.5 nm larger than that of the predicted ones we can presume the existence of strongly bound water molecules within the aggregates observed in aqueous systems. Probably, such fractal clusters were formed of hydrated fullerene molecules. By analogy to the existence of stable crystalline solvates of C 60 with molecules of organic solvents w14,15x, these fractal clusters may be considered to be crystalline hydrates. In this case, the character of crystalline packing ŽFig. 1. should differ from that of individual C 60 molecules w5x. By analyzing the sizes of spherical particles Že.g., Fig. 1., we revealed that their diameters Ž D s 2 R . regularly rise within the range from 7 to 36 nm and are equal to 7.1, 10.9, 14.5, 18.1, 21.8, 25.4, 28.8, 32.4 and 36.0 nm. Taking the smallest spherical C 60 cluster with diameter Ž d s 2 r . of 3.4 nm as the first member, we can see that each following particle is larger than the preceding one by 3.4–3.8 nm Žwhich makes 100–110% of d .. Note that the regularity found for the observed sphere-shaped particles will be valid only if the cluster of size 3.4 nm is taken as the primary one. Larger, sphere-like aggregates Žfrom 36 to 72 nm., sometimes appearing in the micrographs, seem to be composed of particles from the above row. The C 60 molecules are known to form unstable fractal clusters even in benzene w13,16x.
395
Judging by the TEM data, this trend is more markedly expressed in aqueous systems. Hence the fullerene colloidal particles in C 60 FWS are fractal clusters formed from primary spherical aggregates of 3.4 nm size. However the form of the large clusters, consisting of particles of 3.4 nm, can be close to a sphere only if they contain in addition isolated C 60 molecules as well. The latter, according to TEM, are present in C 60 FWS. The existence of clusters formed entirely of single hydrated C 60 molecules wC 60 ŽH 2 O. n x cannot be excluded. Such clusters, containing more water as compared with the above-mentioned ones, are to be looser and liable to decay to single C 60 molecules on drying of C 60 FWS. According to the few data available, the solubility of C 60 in water is negligible: 1.3 = 10y1 1 mg mly1 w17x. If one assumes this value as the equilibrium Žthermodynamic. solubility, then our solutions may be regarded as supersaturated. The hypothesis of a supersaturated molecular solution as a fraction of polydisperse C 60 sol seems quite probable, taking into account the present results of TEM. Note that the size of an isolated C 60 molecule corresponds to the lower border of the colloidal range of dispersity. Even in benzene and other solvents with low polarity Žtoluene, CS 2 , etc.. the tendency to form unstable clusters consisting of solvated C 60 molecules w13,15,16,18x allows us to ascribe some colloidal features to them. Probably in our aqueous solutions such properties of single C 60 molecules are more expressed. Some evidence in support of this may be found in the data on coagulation. On the whole, the TEM data allow us to consider the C 60 FWS to be molecular–colloid systems, containing both single fullerene molecules and their fractal clusters in a hydrated state. Such solutions may combine some properties of a typical colloidal solution with those of a true molecular solution. Their characteristics will be published in detail later.
References w1x G.V. Andrievsky, M.V. Kosevich, O.M. Vovk, V.S. Shelkovsky, L.A. Vashchenko, J. Chem. Soc., Chem. Commun. 12 Ž1995. 1281. w2x N.O. Mchedlov-Petrossyan, V.K. Klochkov, G.V. Andrievsky, J. Chem. Soc., Faraday Trans. 93 Ž1997. 4343.
396
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
w3x V.K. Klochkov, N.O. Mchedlov-Petrossyan, G.V. Andrievsky, Kharkov Univ. Bull., Chem. Ser. 1 Ž1997. 247. w4x W.A. Scrivens, J.M. Tour, K.F. Creek, L. Pirisi, J. Am. Chem. Soc. 116 Ž1994. 4517. w5x Y. Yoshida, Jpn. J. Appl. Phys. 31 Ž1992. L807. w6x P.J.F. Harris, R.E. Douthwaite, A.H.H. Stephens, J.F.C. Turner, Chem. Phys. Lett. 199 Ž1992. 631. w7x M.E. Vol’pin, E.M. Belavtseva, V.S. Romanova, A.I. Lapshin, L.I. Aref’eva, Z.N. Parnes, Mendeleev Commun. 4 Ž1995. 129. w8x R. Jullien, Fractal aggregates, Comments Condens. Matter Phys. 13 Ž1987. 177. w9x B.M. Smirnov, Physics of Fractal Aggregates, Nauka, Moscow, 1991, 136 pp. Žin Russian.. w10x Yu. Prilutsky, S. Durov, L. Bulavin, V. Pogorelov, V. Yaschuk, T. Ogul’chansky, E. Buzaneva, G.V. Andrievsky, P. Schaff, Mol. Cryst. Liq. Cryst. 324 Ž1998. 65. w11x T.P. Martin, U. Naher, H. Schaber, U. Zimmermann, Phys. ¨ Rev. Lett. 70 Ž1993. 3079.
w12x G.V. Andrievsky, V.K. Klochkov, A.D. Roslyakov, A.Yu. Platov, The 3rd International Workshop in Russia: Fullerenes and Atomic Clusters, St.-Petersburg, 1997, Abstracts of Invited Lectures and Contributed Papers, p. 262. w13x Q. Ying, J. Marecek, B. Chu, J. Chem. Phys. 101 Ž1994. 2665. w14x M.F. Meidine, P.B. Hitchcock, H.W. Kroto, R. Taylor, D.R.M. Walton, J. Chem. Soc., Chem. Commun., 1992, 1534. w15x A.L. Balch, J.W. Lee, B.C. Noll, M.M. Olmstead, J. Chem. Soc., Chem. Commun., 1993, 56. w16x A.V. Eletski, M.V. Okun, B.M. Smirnov, in: K.M. Kadish, R.S. Ruoff ŽEds.., Fullerenes, Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, Electrochem. Soc., Pennington, NJ, 1995, p. 1567. w17x D. Heymann, Fullerene Sci. Technol. 4 Ž1996. 509. w18x S.H. Gallagher, R.S. Armstrong, P.A. Lay, C.A. Reed, J. Phys. Chem. 99 Ž1995. 5817.
Chemical Physics Letters 300 Ž1999. 392–396
Studies of aqueous colloidal solutions of fullerene C 60 by electron microscopy G.V. Andrievsky a,) , V.K. Klochkov a , E.L. Karyakina b, N.O. Mchedlov-Petrossyan c a
Institute for Therapy of the Academy of Medical Sciences of Ukraine, 2-a PostysheÕ str., 310039 KharkoÕ, Ukraine b Ukrainian State Research Institute of Refractories, 310024 KharkoÕ, Ukraine c Department of Physical Chemistry, KharkoÕ State UniÕersity, 310077 KharkoÕ, Ukraine Received 8 September 1998; in final form 4 December 1998
Abstract The systems of fullerene C 60 in water ŽC 60 FWS., i.e. aqueous colloidal solutions of buckminsterfullerenes, have been studied using transmission electron microscopy. The C 60 FWS are shown to be molecular–colloid systems, containing both single fullerene molecules and their fractal clusters in a hydrated state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Recently w1,2x we have shown the possibility of obtaining aqueous solutions of C 60 and C 70 with no stabilisers and chemical modification. The method is based on transferring fullerene from toluene into the water phase with the help of sonication. These solutions, stable over at least 18 months, we denote FWS. The solutions turned out to be colloidal systems with a finely dispersed fullerene phase w1–3x. The C 60 hydrosol with a concentration G 0.1 mg mly1 behaves as an ultramicroheterogeneous polydisperse hydrophobic colloidal system, with negatively charged particles w2,3x. C 60 dispersions, obtained by others w4x, are much more rough-sized
Žtypical suspensions. and dilute Ž0.001 mg mly1 .. Having modified and optimized the method of FWS preparation, we were able to obtain a C 60 FWS solution with a concentration up to 2.2 = 10y3 M Ž1.6 mg mly1 .. This extends the application field of fullerenes. Thus a more detailed study of FWS systems becomes important, in particular a more precise determination of the colloidal particle’s size. In the present Letter, we communicate the results of an investigation of C 60 FWS, mainly with a concentration of 1.85 = 10y4 M Ž0.13 mg mly1 ., using transmission electron microscopy ŽTEM..
2. Experimental
)
Corresponding author. Fax.: q380 572 726105; e-mail: [email protected]
Dispersions of C 60 were obtained from a sample of fullerene C 60 with a purity ) 99.5% ŽMER, Tuscon, AZ, USA..
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 3 9 3 - 1
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
For the TEM studies, the EMV 100 AK apparatus, operating at 75 kV, was used. The research was carried out by using the ‘suspension’ method; a drop of the colloidal solution was placed on a structureless nitrocellulose support. After air drying, the specimens were placed into the apparatus Žvacuum 3 = 10y5 Torr.. Samples obtained in a similar way from C 60 solutions in benzene were also studied for comparison. Care was taken to minimise exposure of the samples to the electron beam in order to avoid beam damage. Typical electron micrographs are presented in Figs. 1–3. 3. Results and discussion The plate-like morphology of the crystalline C 60 phase, formed after evaporation of benzene, completely agrees with literature data w5,6x. Contrary to it, the solid phase, obtained from C 60 FWS, consists of typical colloidal particles ŽFig. 1.. Sphere-shaped aggregates of 7–72 nm size, under close examination consisting of primary spherical particles, tend in their turn to further cluster formation. This proves the ultramicroheterogeneous and polydisperse nature of C 60 hydrosol, revealed earlier using other methods w2x. Electron diffraction patterns ŽFig. 1. show a crystal-like character of primary aggregates of C 60 .
393
Note that the size of colloidal particles of amino acids and dipeptide derivatives of C 60 in solutions is 1–10 mm w7x. In addition, the micrographs reveal less distinct, but graphic enough ‘ vague’ zones. In these regions, the solid phase is formed from conglomerates of particles with 1–4 nm size, the smallest of which are commensurate with the isolated C 60 molecule ŽFig. 2.. This micrograph also shows that the large C 60 aggregates are loose. Such aggregates are to be easily deformed during the specimen’s preparation Že.g., Fig. 2, bottom left. and consist of smaller particles of varying size. The results of ultracentrifugation w2x also show the presence of a ‘light’ fraction in the polydispersed colloidal C 60 solution which remains after 1 h of ultracentrifugation Ž145 = 10 3 g .. The coagulation of C 60 FWS by NaCl was studied as well ŽFig. 3.. The particles of the solid phase, obtained from coagulates, contain, along with preformed large C 60 spherical aggregates Žless contrast in the micrograph., a lot of even larger sphere-like ones Žmore contrast.. As we observed no ‘ vague’ zones ŽFig. 2, top right. in the micrographs of the coagulates ŽFig. 3., small particles seem to take part in the formation of the above aggregates having more contrast image.
Fig. 1. The electron micrograph of fullerene C 60 aggregates obtained from C 60 FWS Žon the right: diffraction pattern from C 60 aggregates..
394
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
Fig. 2. The electron micrograph with ‘ vague’ zones.
Thus, the majority of particles in the studied polydisperse systems are of 1–4 nm size. The numerical results obtained with other C 60 FWS samples are of similar character. Do the sizes of the sphere-shaped particles in C 60 FWS adhere to a certain regularity?
To clarify this problem, let us consider the C 60 aggregates in C 60 FWS from the point of view of the fractal cluster’s growth. Let us assume the model of the diffusion limited aggregation of clusters as a result of the attachment to it of separate particles. In this case, the relation between a number of particles
Fig. 3. The electron micrograph of fullerene C 60 coagulates.
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
in a cluster Ž n., its radius Ž R . and its fractal dimensionality Ž d f . has the form: ns
R
ž / r
df
,
Ž 1.
where r is the radius of a particle forming the cluster w8,9x. The smallest stable spherical aggregate of C 60 molecules with crystalline packing was predicted to be of 2.9–3.0 nm in size Žwith regard to van der Waals radii. and to consist of 13 C 60 molecules w10x. Earlier w11x, such an aggregate was detected mass spectrometrically, and was supposed to possess an icosahedral structure. On the other hand, scanning tunneling microscopy convinced us that the smallest spherical clusters in C 60 FWS have a mean size equal to 3.4 nm w2,12x. In accordance with formula Ž1. and with the value d f s 2.1, found for benzene solutions of C 60 w13x, these clusters also contain 13 C 60 molecules. As the diameter of the above clusters is 0.4–0.5 nm larger than that of the predicted ones we can presume the existence of strongly bound water molecules within the aggregates observed in aqueous systems. Probably, such fractal clusters were formed of hydrated fullerene molecules. By analogy to the existence of stable crystalline solvates of C 60 with molecules of organic solvents w14,15x, these fractal clusters may be considered to be crystalline hydrates. In this case, the character of crystalline packing ŽFig. 1. should differ from that of individual C 60 molecules w5x. By analyzing the sizes of spherical particles Že.g., Fig. 1., we revealed that their diameters Ž D s 2 R . regularly rise within the range from 7 to 36 nm and are equal to 7.1, 10.9, 14.5, 18.1, 21.8, 25.4, 28.8, 32.4 and 36.0 nm. Taking the smallest spherical C 60 cluster with diameter Ž d s 2 r . of 3.4 nm as the first member, we can see that each following particle is larger than the preceding one by 3.4–3.8 nm Žwhich makes 100–110% of d .. Note that the regularity found for the observed sphere-shaped particles will be valid only if the cluster of size 3.4 nm is taken as the primary one. Larger, sphere-like aggregates Žfrom 36 to 72 nm., sometimes appearing in the micrographs, seem to be composed of particles from the above row. The C 60 molecules are known to form unstable fractal clusters even in benzene w13,16x.
395
Judging by the TEM data, this trend is more markedly expressed in aqueous systems. Hence the fullerene colloidal particles in C 60 FWS are fractal clusters formed from primary spherical aggregates of 3.4 nm size. However the form of the large clusters, consisting of particles of 3.4 nm, can be close to a sphere only if they contain in addition isolated C 60 molecules as well. The latter, according to TEM, are present in C 60 FWS. The existence of clusters formed entirely of single hydrated C 60 molecules wC 60 ŽH 2 O. n x cannot be excluded. Such clusters, containing more water as compared with the above-mentioned ones, are to be looser and liable to decay to single C 60 molecules on drying of C 60 FWS. According to the few data available, the solubility of C 60 in water is negligible: 1.3 = 10y1 1 mg mly1 w17x. If one assumes this value as the equilibrium Žthermodynamic. solubility, then our solutions may be regarded as supersaturated. The hypothesis of a supersaturated molecular solution as a fraction of polydisperse C 60 sol seems quite probable, taking into account the present results of TEM. Note that the size of an isolated C 60 molecule corresponds to the lower border of the colloidal range of dispersity. Even in benzene and other solvents with low polarity Žtoluene, CS 2 , etc.. the tendency to form unstable clusters consisting of solvated C 60 molecules w13,15,16,18x allows us to ascribe some colloidal features to them. Probably in our aqueous solutions such properties of single C 60 molecules are more expressed. Some evidence in support of this may be found in the data on coagulation. On the whole, the TEM data allow us to consider the C 60 FWS to be molecular–colloid systems, containing both single fullerene molecules and their fractal clusters in a hydrated state. Such solutions may combine some properties of a typical colloidal solution with those of a true molecular solution. Their characteristics will be published in detail later.
References w1x G.V. Andrievsky, M.V. Kosevich, O.M. Vovk, V.S. Shelkovsky, L.A. Vashchenko, J. Chem. Soc., Chem. Commun. 12 Ž1995. 1281. w2x N.O. Mchedlov-Petrossyan, V.K. Klochkov, G.V. Andrievsky, J. Chem. Soc., Faraday Trans. 93 Ž1997. 4343.
396
G.V. AndrieÕsky et al.r Chemical Physics Letters 300 (1999) 392–396
w3x V.K. Klochkov, N.O. Mchedlov-Petrossyan, G.V. Andrievsky, Kharkov Univ. Bull., Chem. Ser. 1 Ž1997. 247. w4x W.A. Scrivens, J.M. Tour, K.F. Creek, L. Pirisi, J. Am. Chem. Soc. 116 Ž1994. 4517. w5x Y. Yoshida, Jpn. J. Appl. Phys. 31 Ž1992. L807. w6x P.J.F. Harris, R.E. Douthwaite, A.H.H. Stephens, J.F.C. Turner, Chem. Phys. Lett. 199 Ž1992. 631. w7x M.E. Vol’pin, E.M. Belavtseva, V.S. Romanova, A.I. Lapshin, L.I. Aref’eva, Z.N. Parnes, Mendeleev Commun. 4 Ž1995. 129. w8x R. Jullien, Fractal aggregates, Comments Condens. Matter Phys. 13 Ž1987. 177. w9x B.M. Smirnov, Physics of Fractal Aggregates, Nauka, Moscow, 1991, 136 pp. Žin Russian.. w10x Yu. Prilutsky, S. Durov, L. Bulavin, V. Pogorelov, V. Yaschuk, T. Ogul’chansky, E. Buzaneva, G.V. Andrievsky, P. Schaff, Mol. Cryst. Liq. Cryst. 324 Ž1998. 65. w11x T.P. Martin, U. Naher, H. Schaber, U. Zimmermann, Phys. ¨ Rev. Lett. 70 Ž1993. 3079.
w12x G.V. Andrievsky, V.K. Klochkov, A.D. Roslyakov, A.Yu. Platov, The 3rd International Workshop in Russia: Fullerenes and Atomic Clusters, St.-Petersburg, 1997, Abstracts of Invited Lectures and Contributed Papers, p. 262. w13x Q. Ying, J. Marecek, B. Chu, J. Chem. Phys. 101 Ž1994. 2665. w14x M.F. Meidine, P.B. Hitchcock, H.W. Kroto, R. Taylor, D.R.M. Walton, J. Chem. Soc., Chem. Commun., 1992, 1534. w15x A.L. Balch, J.W. Lee, B.C. Noll, M.M. Olmstead, J. Chem. Soc., Chem. Commun., 1993, 56. w16x A.V. Eletski, M.V. Okun, B.M. Smirnov, in: K.M. Kadish, R.S. Ruoff ŽEds.., Fullerenes, Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, Electrochem. Soc., Pennington, NJ, 1995, p. 1567. w17x D. Heymann, Fullerene Sci. Technol. 4 Ž1996. 509. w18x S.H. Gallagher, R.S. Armstrong, P.A. Lay, C.A. Reed, J. Phys. Chem. 99 Ž1995. 5817.