Scanning tunneling spectroscopy of small Ce-doped endohedral fullerenes on HOPG

We report about scanning tunneling spectroscopy (STS) on deposited cerium-doped endohedral fullerenes with ⩽60 carbon atoms, i.e. Ce@Cn (n=36, 44, 50, 60). The clusters have been produced in a laser vaporization source and mass-selected prior to soft-landing onto a highly oriented pyrolytic graphite (HOPG) substrate. The STS spectra of Ce@C36 and Ce@C44 reveal a metal-like density of states while the one of Ce@C50 and Ce@C60 on HOPG is semiconductor-like. The band gap observed for the two larger fullerenes is interpreted in terms of an on-site Coulomb repulsion within the framework of a Mott–Hubbard picture.