Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate electron tunneling from a C60-terminated tip into a Cu(111) surface. Tunneling between a C60 orbital and the Shockley surface states of copper is shown to prod
uce negative differential conductance (NDC) contrary to conventional expectations. NDC can be tuned through barrier thickness or C60 orientation up to complete extinction. The orientation dependence of NDC is a result of a symmetry matching between the molecular tip and the surface states.
Recent low-temperature scanning-tunneling microscopy experiments [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)] observed the vibrationally induced flip motion of a hydroxyl dimer (OD)2 on Cu(110). We propose a model to describe two-level fluctua
tions and current-voltage characteristics of nanoscale systems which undergo vibrationally induced switching. The parameters of the model are based on comprehensive density-functional calculations of the systems vibrational properties. For the dimer (OD)2 the calculated population of the high and low conductance states, the I-V, dI/dV, and d2I/dV2 curves are in good agreement with the experimental results and underlines the different roles played by the free and shared OD stretch modes of the dimer.
The role of the tip in inelastic electron tunneling spectroscopy (IETS) performed with scanning tunneling microscopes (STM) is theoretically addressed via first-principles simulations of vibrational spectra of single carbon monoxide (CO) molecules ad
sorbed on Cu(111). We show how chemically functionalized STM tips modify the IETS intensity corresponding to adsorbate modes on the sample side. The underlying propensity rules are explained using symmetry considerations for both the vibrational modes and the molecular orbitals of the tip and sample. This suggests that single-molecule IETS can be optimized by selecting the appropriate tip orbital symmetry.
We study pentanedithiol molecular junctions formed by means of the break-junction technique with a scanning tunneling microscope at low temperatures. Using inelastic electron tunneling spectroscopy and first-principles calculations, the response of t
he junction to elastic deformation is examined. We show that this procedure makes a detailed characterization of the molecular junction possible. In particular, our results indicate that tunneling takes place through just a single molecule.
The charge flow from a single C60 molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contac
t conductance of a single molecule between two Cu electrodes can vary up to a factor of three depending on electrode geometry, the conductance of the C60-C60 contact is consistently lower by two orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C60-C60 distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.
Scanning tunneling spectra on single C60 molecules that are sufficiently decoupled from the substrate exhibit a characteristic fine structure, which is explained as due to the dynamic Jahn-Teller effect. Using electron-phonon couplings extracted from
density functional theory we calculate the tunneling spectrum through the C60- anionic state and find excellent agreement with measured data.
A dynamical method for inelastic transport simulations in nanostructures is compared with a steady-state method based on non-equilibrium Greens functions. A simplified form of the dynamical method produces, in the steady state in the weak-coupling li
mit, effective self-energies analogous to those in the Born Approximation due to electron-phonon coupling. The two methods are then compared numerically on a resonant system consisting of a linear trimer weakly embedded between metal electrodes. This system exhibits enhanced heating at high biases and long phonon equilibration times. Despite the differences in their formulation, the static and dynamical methods capture local current-induced heating and inelastic corrections to the current with good agreement over a wide range of conditions, except in the limit of very high vibrational excitations, where differences begin to emerge.
