Control of Relative Tunneling Rates in Single Molecule Bipolar Electron Transport

Abstract

The influence of relative electron tunneling rates on electron transport in a double-barrier single-molecule junction is studied. The junction is defined by positioning a scanning tunneling microscope tip above a copper phthalocyanine molecule adsorbed on a thin oxide film grown on the NiAl(110) surface. By tuning the tip-molecule separation, the ratio of tunneling rates through the two barriers, vacuum and oxide, is controlled. This results in dramatic changes in the relative intensities of individual conduction channels, associated with different vibronic states of the molecule.

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…The crossing bias eV c , at which the state moves below the Fermi level, hereby shifts to lower values when the tip approaches the surface (∆z becomes more negative). 84 In general, the dielectric response of an oxide film in a biased STM junction induces a shift of the experimental energy positions of oxide or adsorbate electronic states, whose magnitude is given by thickness and dielectric properties of the film. This effect needs to be considered whenever conductance spectroscopy is performed on insulating surfaces with a limited screening ability.…”

“…83,84 Due to the small electron density, oxides are often unable to screen the tip-electric field directly at their surface. As a consequence, a fraction of the applied junction bias drops inside the oxide film, leading to a bending of the oxide bands ( Fig.…”

Forces from periodic charging of adsorbed molecules

“…STM permits injecting charge carriers very locally in the weak coupling limit, namely, in the tunneling regime. [28][29][30][31][32][33][34][35] Atomic force microscopy (AFM) in combined STM/AFM setups complements STM experiments by providing information of the molecular geometry-independent from the conductance. 36,37 In recent years, this has been applied to gain insight into the geometry changes associated with toggling single-molecule switches.…”

Spin Splitting Unconstrained by Electron Pairing: The Spin-Vibronic States

“…The vibrational energies should be ~15% less than the peak spacing obtained from the slopes in Fig. 1(e), (f), and (g) [9,12,13]. The corrected vibrational energies are 11 meV and 3.4 meV.…”