Line shapes in inelastic electron tunneling spectroscopy of single-molecule junctions

Abstract

Spectroscopic line-shape analyses for single-${\mathrm{C}}_{60}$ vibrational modes are presented for two kinds of scanning tunneling microscope experiments. Inelastic electron tunneling spectroscopy is performed for ${\mathrm{C}}_{60}$ molecules with different adsorption geometries on Pb(111). Depending on the ${\mathrm{C}}_{60}$ adsorption site and rotational orientation, the lowest unoccupied molecular orbital exhibits varying degrees of overlap with ${\mathrm{C}}_{60}$ vibrational energies. Concomitantly, the line shapes of vibrational modes are affected according to expectations for on-resonance inelastic electron tunneling. Inelastic electron transport is further studied for decreasing tip-${\mathrm{C}}_{60}$ distances covering tunneling and contact ranges. Line-shape changes signaling a conductance increase rather than the expected decrease upon exciting vibrational quanta are observed. A phenomenological approach is suggested to understand the dissimilar behavior of the junction conductance in the different electron transport ranges.