Probing molecule-metal bonding in molecular junctions by inelastic electron tunneling spectroscopy.

We present first-principles calculations for the inelastic electron tunneling spectra (IETS) of three molecules, 1-undecane thiol (C11), alpha,omega-bis(thioacetyl)oligophenylenethynylene (OPE), and alpha,omega-bis(thioacetyl)oligophenylenevinylene (OPV), sandwiched between two gold electrodes. We have demonstrated that IETS is very sensitive to the bonding between the molecule and electrodes. In comparison with experiment of Kushmerick et al. (Nano Lett. 2004, 4, 639), it has been concluded that the C11 forms a strong chemical bond, while the bonding of the OPE and OPV systems are slightly weaker. All experimental spectral features have been correctly assigned.