IONIZATION OF C60 MOLECULE ON GRAPHENE

Abstract

Fundamental understandings of electron charging behavior of single atoms or molecules are important for the applications of nanoscale electronic devices. C60 is one of the great candidates for building blocks of molecular electronics, which can be tuned into metallic, insulating, and superconducting phases by changing the charge state. In this study, we investigated the ionization of individual fullerene molecules on a single-layer graphene by means of low-temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS). Single-layer graphene were grown on a Cu(111) single crystal through argon-assisted ultra-high vacuum chemical vapor deposition (UHV-CVD) method. Fullerene molecules were deposited onto graphene by thermal evaporation. When we applied high electric field to a fullerene molecule on graphene, the topography and electronic structures of fullerene was changed as a result of the anion formation. The change of charged states was confirmed by the observation of new gap states inside the HOMO-LUMO (Highest Occupied Molecular Orbital/Lowest Unoccupied Molecular Orbital) gap of C60 in STS data. In addition, the tip induced band bending (TIBB) was observed around the C60 anion due to the negatively charged fullerene. The threshold voltage for the formation of C60 anion was about 4.2 V, which corresponds to the first image-potential state (IPS) of C60 on graphene. Interestingly, the charging of C60 has not been observed for the fullerenes on the Cu(111) surface, indicating the important role of graphene as a support. It is believed that the ionization was induced by the resonance at IPS of C60/SLG/Cu(111) surface. The charged states of C60 were stabilized by the Jahn-Teller effect due to the high degeneracy of C60, and the electronic decoupling effect of graphene. Our results demonstrate a new method of tuning molecular electronic configuration by the Jahn-Teller effect, which can be utilized for individual ionization of a single molecule.