On a model of fullerene formation from polymers under MeV ion impact

Fullerenes - large even-numbered carbon clusters with a closed three-dimensional structure - can be formed in the gas phase by different methods, including laser ablation of graphite and other carbon containing materials. A method for production of macroscopic amounts of predominantly C60 by electric arc graphite vaporization in He atmosphere has lead to an explosive growth in fullerene research. Recently it has been demonstrated that fullerenes are also formed in the electronic sputtering of a specific polymer -poly(vinylidenedifluoride), PVDF - by swift MeV atomic ions. In a model for formation of fullerenes in MeV ion impact on polymer surfaces we assume that fullerenes result from carbon atom condensation in the dense axially expanding plasma in the MeV ion infratrack. Gas-flow concepts are employed to explain the ejection of the fullerenes. The temporal and radial dependence of the temperature in the track region is described on the basis of the linear thermoconductivity theory for thermal sources with cylindrical symmetry. The dependence of the fullerene yield on the MeV ion stopping power and on the number of atoms in the cluster is calculated by taking into account a requirement of minimal plasma temperature for fullerene formation. Calculations for the average cluster velocity are also performed and compared to experimental data. Finally we argue that both the model and the experimental findings bear implications for the mechanism of fullerene formation in general, providing estimates of the required time frame and spatial domain. (orig.)