Mechanisms involved in the formation of onionlike carbon nanostructures synthesized by ion implantation at high temperature

The origin of the formation of multishell fullerenes (carbon onions) produced by carbon ion-implantations performed at high-temperature into silver is discussed on the basis of high-resolution transmission electron microscopy observations. For low carbon fluences, one observes the formation inside the silver matrix of poorly organized carbon nanostructures the shape of which is roughly spherical. They progressively evolve towards perfect onionlike structures when the implanted carbon fluence increases. The catalytic effect of silver, which lowers the graphitization temperature of the carbon, the irradiation-induced displacements of the carbon atoms, and the effect of the temperature are proposed to be the key mechanisms that control the formation of the onionlike structures observed in such experiments.