Vibrational Inelastic Electron Tunneling Spectroscopy of Surface Adsorbed Single Molecules at Sub-Kelvin Temperature

With a 600mk homebuilt UHV STM system, we studied molecular vibration at the solid surface with inelastic electron tunneling spectroscopy (IETS) of Acetylene single molecules adsorbed on Cu(100) surface and revealed five new vibrational modes that were previously inaccessible to STM-IETS at 8K temperature. The identification of vibrational IETS features with normalized conductance change (??/?) as low as 0.24% was demonstrated. Facilitated by the high energy resolution, we also revealed the anisotropic vibrational energy of carbon--monoxide (CO) molecule induced by substrate surface symmetry. The discrepancy in vibrational energy as small as 0.8meV can be resolved by STM-IETS. Our results also showed that the change in vibrational behavior of CO can be used to understand its environment. CO can be vertically transferred from substrate surface to STM tip and creates a scanning probe which has the characteristic vibrational signal of CO contained in the inelastic component of the tunneling current; i.e. the inelastic tunneling probe (itProbe). The itProbe senses the local potential energy landscape by imaging the spatial variations of CO hindered-translational mode and resolves the skeletal structure and bonding details of a surface adsorbed Cobalt-Phthalocyanine molecule (CoPc). The image contrast of itProbe also reveals the interaction between one CoPc and the substrate surface as well as the interaction paths between neighboring CoPcs.