Production and Characterization of Metal-Encapsulated Fullerenes
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PRODUCTION AND CHARACTERIZATION OF METAL-ENCAPSULATED FULLERENES
Robert D. Johnson, Costantino S. Yannoni, Ma.rk Hoinkis, Mattanjah de Vries, Jesse R. Salem, Mark S. Crowdert and Donald S. Bethune IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 tIBM Adstar, 5600 Cottle Rd., San Jose, CA 95193
ABSTRACT We report here the arc-production and spectroscopic characterization of fullerene-encapsulated metal atoms and metal-atom clusters. In particular, both solution and solid-state electron paramagnetic resonance (EPR) spectra of LaC 82, YC82, SCC 82 and Sc3 C82 have been obtained. Additional species containing rareearth atoms and clusters have been produced. The results suggest, for example, that the three scandium atoms in Sc 3 C82 form a molecule In the shape of an equilateral triangle - as was previously suggested for Sc 3 molecules Isolated In a cryogenic rare-gas matrix. The spectra of the MC82 species (M=La, Y, Sc) exhibit small hyperfine couplings and g-values close to 2, suggesting that they can be described as a +3 metal cations within -- 3 fullerene radical anion cages. Sc 2C 2n species - the most abundant metailofuilerenes In the scandlum-fullerene mass spectrum - are EPR-silent, even though Sc 2 is EPR-actlve In a rare-gas matrix at 4.2K. A broader Implication of this work Is that production of macroscopic quantities of metal-containing fullerenes may make possible the fabrication of exotic new structures with regular arrays of metal atoms or clusters Isolated in fullerene molecules, resulting in new types of host/guest nanostructured materials. The bulk production of fullerenes with entrapped atoms or clusters has a been a vigorously pursued goal since the advent of the Kraitschmer-Huffman technique for producing hollow fullerenes.1 , 2 Elegant carbon cluster-beam experiments using metal-impregnated graphite, 3-6 which began very soon after the fullerene structures for large carbon clusters were first proposed, 7 had already provided convincing evidence that endohedral metallofullerenes can be produced - at least as transients. More recently, the chemical stability reported for LaC 82 ,8 , 9 La 2C80 ,10 YC82, Y2C82'11, 12 SCC 8 2 , Sc 2 C 2 n' Sc C8213, 14 and FeC6015 in solution and in the solid state, has given new impetus to efforts to produce purified bulk quantitities of fullerenes with metal atoms and clusters entrapped within the cages. These novel fullerene variants could be the basis of a new class of materials with arrays of metal atoms or clusters isolated, stabilized, and held in position by a lattice of surrounding carbon shells. Fullerene encapsulation also provides a novel method for isolating and stabilizing reactive species under ambient conditions, opening up the possibility of examining them spectroscopically to learn about their detailed structure.
Mat. Res. Soc. Symp. Proc. Vol. 270. 91992 Materials Research Society
262
Following the success reported by Chai et al. 8 in producing lanthanumcontaining fullerenes, both by laser vaporization of metal-containing gra
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