Kinetics of fullerene formation by electric-arc vaporization of graphite

Abstract

01 Since the discovery of the new exotic form of molecular carbon—fullerenes ~C60 , C70 , etc.! —obtained by laser vaporization of graphite, they have become an object of intense experimental and theoretical study. The introduction of a new method of synthesis of fullerenes by arc vaporization of graphite in a helium or argon atmosphere has made it possible to obtain these compounds in macroscopic quantities and has substantially motivated experimental studies of the physical-chemical properties of these extraordinary carbon formations. This method has been used by many research groups with C60 and C70 production yields typically at the 5–15% level, while Parkev et al. measured the total fullerene yield—from C60 to C240—and report a value of 44% of the material deposited on the cold reactor walls, where this material consisted of a mixture of soot and fullerenes. The method of laser vaporization of graphite, followed by vacuum expansion of the helium–carbon cluster mixture, proved to be most suitable for studies of mass-spectra and the isomeric composition of the carbon clusters. Quite a few papers in the pre-fullerene period examined the distribution of small carbon clusters Cn for n,25 ~both neutrals and positive and negative ions!. In particular, ‘‘magic’’ numbers were discovered for clusters of carbon and also of other elements ~metals and noble gases!, that is to say, concentrations of some carbon clusters in the observed massspectra significantly exceeded the mean level. Clusters with n526–32 were hardly ever observed; lines in the massspectra corresponding to odd mass numbers were either not observed or were significantly weaker than the even lines for n.32. In this wide spectral range significantly higher intensities are observed for ‘‘magic’’ clusters with n560, 70, and a number of others. The results of Ref. 8 show that for carbon clusters ~positive ions! linear structures ~chains! exist up to C10 1 . Some families of isomers have the structure of planar rings: monocyclic rings start at C 7 1 and extend to C40 1 ; bicyclic rings are first observed at C21 1 and extend to C40 1 , followed by tricyclic rings starting at C30 1 , and an extremely limited quantity of tetracyclic rings starting at C40 1 . Fullerenes, i.e., spherical hollow particles are first observed at C30 1 and become dominant above C50 1 . According to present thinking, the path of carbon cluster growth can be represented in the following way: carbon atoms → linear chains → monocyclic rings → polycyclic rings → fullerenes. A structural analysis of clusters in addi-