Ion-beam modification of fullerene

The response of thin films of fullerene (${\mathrm{C}}_{60}$) to energetic ion impact is investigated. The diagnostics employed include Fourier-transform infrared and Raman spectroscopies, cross-sectional transmission electron microscopy, and atomic force microscopy. By combining the information obtained from these diagnostics with that from the dependence of the conductivity on ion dose, it is concluded that each ${\mathrm{C}}_{60}$ molecule completely disintegrates when hit by an energetic ion. The cross section for the destruction is about 6×${10}^{\mathrm{-}13}$ ${\mathrm{cm}}^2$ for irradiation with 620-keV Xe ions. The disintegration occurs when C atoms are knocked out of the molecule either directly by the impinging ion or by an energetic knock-on C atom within the damage cascade. This process is quite different from the Coulomb-explosion mechanism previously proposed in the literature. For very low ion doses (${10}^{11}$ Xe/${\mathrm{cm}}^2$) most of the ${\mathrm{C}}_{60}$ molecules remain intact; however this dose is sufficient to completely disrupt the ordering of the ${\mathrm{C}}_{60}$ molecules in the van der Waals bonded ${\mathrm{C}}_{60}$ solid. Disruption of the lattice ordering at such low doses is considered to be attributable to the weakness of the van der Waals forces which bind the ${\mathrm{C}}_{60}$ clusters together into the molecular solid.