Resolving the Adsorption Geometries of Single m-Terphenyl Isocyanide Ligands on Cu(100) with Vibrational Microscopy and Spectroscopy*

Topographic images obtained using a scanning tunneling microscope (STM) frequently present a blend of electronic states and molecular structure, potentially causing confusion when determining the binding configuration of a molecule on surfaces. In contrast, vibrational microscopy offers distinctive fingerprinting characteristics that can shed light on the intricacies of molecule-surface interactions. Here, we utilize STM-inelastic electron tunneling spectroscopy (IETS) to determine the chemical structure of single m-terphenyl isocyanide ligands on a Cu(100) surface. This molecule exhibits both intrinsic stretching modes and rich surface-induced vibrational features concerning the motions of different functional groups. IETS mapping provides sub-molecular scale insights to the spatial distribution of these vibrational modes and can locate the functional group(s) contributing the most to these modes within a molecule. By locating the vibrational features concerning the motion of the ortho-methyl groups, we can now unambiguously distinguish the different adsorption configurations of the molecule. Furthermore, two almost identical adsorption geometries that cannot be distinguished by vibrational microscopy because of similar spatial distribution of ortho-methyl groups are differentiated by their different isocyanide stretching energies with vibrational spectroscopy.