Formation of diamond in carbon onions under MeV ion irradiation

Spherical carbon onions are generated by irradiating graphitic carbon soot with ${\mathrm{Ne}}^{\mathrm{+}}$ ions of 3 MeV energy. Under continued irradiation, a transformation of their cores to cubic diamond crystals is observed. In comparison to earlier electron irradiation experiments, the yield of diamond is much higher. The output of the irradiation experiment is characterized by electron microscopy and electron energy loss spectroscopy. Knock-on displacements of carbon atoms by ${\mathrm{Ne}}^{\mathrm{+}}$ ions are assumed to be responsible for a self-compression of the onions, leading to the nucleation of diamond in their cores. The increased diamond yield is explained by the higher displacement cross-section, the higher energy transfer, and the higher total beam current on the specimen.