Significant negative differential resistance predicted in scanning tunneling spectroscopy for aC60monolayer on a metal surface

Abstract

We theoretically predict the occurrence of negative differential resistance (NDR) in scanning tunneling spectroscopy for a pure ${\text{C}}_{60}$ monolayer deposited on a metal surface using metal tips, namely, on a Cu(111) surface and using various W tips. It is proposed that the likely reason why NDR has not been observed under such conditions is that NDR can be reduced if an oxidized or Cu-terminated tip is used. A detailed decomposition of the total tunneling current into its contributions from individual molecular orbitals reveals that only some of the orbitals on the tip and on the ${\text{C}}_{60}$ can be ``matched up'' to give a contribution to the current and that the NDR is a consequence of the mismatch between these specific orbitals within particular ranges of bias voltage. Moreover, the NDR characteristics, including the peak positions and the peak-to-valley ratios, are found to depend on the tip material, tip geometry, and tip-to-molecule position.