WHY ARE BUCKYONIONS ROUND

Multi-layered round carbon particles (onions) containing tens to hundr eds of thousands of atoms form during electron irradiation of graphite . However, theoretical models of large icosahedral fullerenes predict highly faceted shapes for molecules with more than a few hundred atoms . This discrepancy in shape may be explained by the presence of defect s during the formation of carbon onions. Here, we use the semi-empiric al tight-binding method for carbon to simulate the incorporation of pe ntagon-heptagon defects on to the surface of large icosahedral fullere nes. We show a simple mechanism that results in energetically competit ive derivative structures and a global change in molecular shape from faceted to round. Our results provide a plausible explanation of the a pparent discrepancy between experimental observations of round buckyon ions and theoretical predictions of faceted icosahedral fullerenes.