Tip-induced bond weakening, tilting, and hopping of a single CO molecule on Cu(100)

The interaction between a probing tip and an adsorbed molecule can significantly impact the molecular chemical structure and even induce its motion on the surface. In this study, the tip-induced bond weakening, tilting, and hopping processes of a single molecule were investigated by sub-nanometre resolved tip-enhanced Raman spectroscopy (TERS). We used single carbon monoxide (CO) molecules adsorbed on the Cu (100) surface as a model system for the investigation. The vibrational frequency of the C-O stretching mode is always redshifted as the tip approaches, revealing the weakening of the C-O bond owing to tip-molecule interactions. Further analyses of both the vibrational Stark effect and TERS imaging patterns suggest a delicate tilting phenomenon of the adsorbed CO molecule on Cu(100), which eventually leads to lateral hopping of the molecule. While a tilting orientation is found toward the hollow site along the [110] direction of the Cu(100) surface, the hopping event is more likely to proceed via the bridge site to the nearest Cu neighbour along the [100] or [010] direction. Our results provide deep insights into the microscopic mechanisms of tip-molecule interactions and tip-induced molecular motions on surfaces at the single-bond level.