2014

We use inelastic electron tunneling spectroscopy first-principles simulations to identify the different chemical bonds present at metal-molecule junctions. We unambiguously identify the nature of these bonds from two distinctive features in the calculated spectra: (i) the presence (or absence) of active vibrational modes and (ii) the dependence of vibrational frequencies on electrode separation. We use this method to present a study of the vibrational properties of alkanes bound to the electrodes via highly conducting Au-C links. In the experiment, these links were formed from molecules synthesized with trimethyl-tin (SnMe₃) terminations, where the SnMe₃ groups were removed in situ at the junction, in a process involving both breaking and formation of bonds [Cheng, Z.-L.; Skouta, R.; Vázquez, H.; Widawsky, J. R.; Schneebeli, S.; Chen, W.; Hybertsen, M. S.; Breslow, R.; Venkataraman, L. Nat. Nanotechnol. 2011, 6, 353-357]. We obtain the vibrational fingerprint of these direct Au-alkane links and extend this study to the other scenario considered in that paper (bonding via SnMe₂ groups), which may be relevant under other experimental conditions. We also explore the effect of deuteration on inelastic electron tunneling spectroscopy (IETS). Complete deuteration of the molecules diminishes the differences of the spectra corresponding to the two bonding geometries, making identification more difficult. IETS of an isolated SnMe₃ fragment provides an additional basis for comparison in the characterization of the molecular junction.

Carbon-based nanostructures are attracting tremendous interest as components in ultrafast electronics and optoelectronics. The electrical interfaces to these structures play a crucial role for the electron transport, but the lack of control at the atomic scale can hamper device functionality and integration into operating circuitry. Here we study a prototype carbon-based molecular junction consisting of a single C₆₀ molecule and probe how the electric current through the junction depends on the chemical nature of the foremost electrode atom in contact with the molecule. We find that the efficiency of charge injection to a C₆₀ molecule varies substantially for the considered metallic species, and demonstrate that the relative strength of the metal-C bond can be extracted from our transport measurements. Our study further suggests that a single-C₆₀ junction is a basic model to explore the properties of electrical contacts to meso- and macroscopic sp² carbon structures.

We extend the simple and efficient lowest order expansion (LOE) for inelastic electron tunneling spectroscopy (IETS) to include variations in the electronic structure on the scale of the vibration energies. This enables first-principles calculations of IETS line shapes for molecular junctions close to resonances and band edges. We demonstrate how this is relevant for the interpretation of experimental IETS using both a simple model and first-principles simulations.

A theory of action spectroscopy, i.e., a reaction rate or yield as a function of bias voltage, is presented for single-molecule reactions induced by the inelastic tunneling current with a scanning tunneling microscope. A formula for the reaction yield is derived using the adsorbate resonance model, which provides a versatile tool to analyze vibrationally mediated reactions of single adsorbates on conductive surfaces. This allows us to determine the energy quantum of the excited vibrational mode, the effective broadening of the vibrational density of states (as described by Gaussian or Lorentzian functions), and a prefactor characterizing the elementary process behind the reaction. The underlying approximations are critically discussed. We point out that observation of reaction yields at both bias voltage polarities can provide additional insight into the adsorbate density of states near the Fermi level. As an example, we apply the theory to the case of flip motion of a hydroxyl dimer (OD)₂ on Cu(110) which was experimentally observed by Kumagai et al. [Phys. Rev. B 79, 035423 (2009)]. In combination with density functional theory calculations for the vibrational modes, the vibrational damping due to electron-hole pair generation, and the potential energy landscape for the flip motion, a detailed microscopic picture for the switching process is established. This picture reveals that the predominant mechanism is excitation of the OD stretch modes which couple anharmonically to the low-energy frustrated rotation mode.