Vibrational Characterization of Spatial Inhomogeneity of Single Surface-Adsorbed DMP Isocyanides

We studied the site-dependent adsorption geometries and vibrational fingerprints of the 2,6-dimesitylphenyl (DMP) isocyanides on Au(111) with scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS) at 5 K and 77K. The STM topographic images reveal that the DMP isocyanides preferably bind to the surface sites with a relatively large curvature, such as the step edges and the herringbone elbow sites. IETS measurement shows a series of molecular vibrational features which demonstrate obvious spatial inhomogeneity among molecules adsorbed at different sites. It was found that certain vibrational modes exhibit energy shifts upon altering the electric field between the tip and molecules, which was tentatively assigned to first-order perturbation to the Hamiltonian by coupling to the molecular dipole or the Stark effect. Furthermore, the varying energy shifts of the same vibrational modes at different sites could indicate a spatial variation of the molecular dipole moments due to different adsorption geometries.