Vibrational Spectroscopy of a Single Molecule by Combining Scanning Tunneling Microscopy and Atomic Force Microscopy

Abstract

Inelastic electron tunneling spectroscopy (IETS) combined with scanning tunneling microscopy (STM) is a powerful tool for investigating the vibrational energies of a single molecule at the atomic scale resolution. However, it is known that the intensity of IETS varies strongly for various tips, and the vibrational energies acquired by IETS are significantly influenced by the distance between the tip and molecule. To overcome the aforementioned problems, we have incorporated atomic force microscopy into STM-IETS in collaboration with Franz J. Giessibl of Regensburg University. Consequently, we have found that a metallic tip whose apex consists of a single atom provides a stronger IETS signal to the CO molecule on a Cu (111) surface, whereas a metallic tip whose apex consists of multiple atoms provides a weaker IETS signal. Further, we have found that the vibrational energies of the CO molecule are highly influenced by a tip that exerts a stronger force. Subsequently, these findings have been rationalized by a classical mechanical model assuming that the motion of the CO molecule is similar to that of a double pendulum, where the perturbation force between the tip and molecule and the weakening of the chemical bond because of the tip are taken into account.