Electronic structures of fullerenes and metallofullerenes studied by surface potential analysis

We investigated electronic structures of fullerenes and metallofullerenes with the aid of the surface potential analysis method. These fullerene-related molecules had a work function of 4.6–4.7 eV in the form of thin films, and that is slightly smaller than that of air-exposed Au. From results of the work function, ionization potential, and temperature dependence of the resistance, the band diagram of these molecular thin films was estimated and it was elucidated that the Fermi level is located above the midgap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), namely, they are $n$-type semiconductors. So far, the Fermi level was thought to be located in the middle of the HOMO and LUMO; however, it was concluded that this description was not valid from this work.