Photoemission spectroscopy of clean and potassium-intercalated carbon onions

Hollow onionlike carbon (OLC), generated by annealing nanodiamond at 2140 K, has been studied by core-level and valence-band photoemission spectroscopy. Upon intercalation with potassium, core and valence states of the OLC show an almost rigid shift to higher binding energies, and the density of states at the Fermi level (E-F) is observed to increase. An asymmetric broadening of the C1s line from the OLC as intercalation proceeds indicates an increase in electron-hole pair excitations. Both core and valence-band spectra are consistent with charge transfer from the intercalated potassium to the OLC, and support the conclusion that the electronic structure of the carbon onions bears strong similarity to that of graphite, although differences do exist. In consequence the conclusion can be drawn that these species behave as graphite "nanocrystals" rather than as large fullerene molecules.