Electron microscopy study of carbon onions synthesized by ion implantation

The structure of concentric-shell carbon onions, which were synthesized by carbon-ion implantation into copper and silver substrates, is investigated by carrying out transmission electron microscopy (TEM) characterizations. TEM observations at high specimen temperatures reveal that carbon onions with coherent shells and a highly symmetric structure can be obtained by ion implantation. The onions show a decreasing intershell spacing towards their centre. Contrast features in bright- and dark-field images of carbon onions are compared with high-resolution lattice images. A structure model is suggested on the basis of local faceting of the onions and stacking order between the shells. The relationship between carbon onions and fullerenes with defects such as pentagonal or heptagonal rings in the shells is discussed.