F. Pump

Controlled Stability of Molecular Junctions

Using single molecules as the building blocks of electronic devices is the ultimate goal of molecular electronics. However, measuring, understanding and manipulating the transport of charge through molecules attached to nanosize electrodes remains a difficult task. It is thus important to fabricate molecular junctions exhibiting reproducible and stable electrical response, and evaluate their performance. [1,2] While chemists are able to synthesize molecules with amazing electronic, optical, magnetic, thermoelectric or electromechanical properties and with the potential to provide electronic devices with novel functionality, the integration into single molecular devices remains the main problem of the field “molecular electronics”. It has been demonstrated that the electronic transport through a molecule depends not only on its intrinsic properties but also on specif ic details of the contacts and the local environment. [3] Here we report a new strategy to monitor the conductance of such molecular junctions enabling to gain further insight into the detailed nature of the conductance of single molecules. Due to the stability and reproducibility of our junctions, our method provides a new perspective on studies of electronic transport on the nanometer scale.

Electrical transport through a mechanically gated molecular wire

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.

Quantum transport through STM-lifted single PTCDA molecules

Using a scanning tunneling microscope we have measured the quantum conductance through a PTCDA molecule for different configurations of the tip–molecule–surface junction. A peculiar conductance resonance arises at the Fermi level for certain tip to surface distances. We have relaxed the molecular junction coordinates and calculated transport by means of the Landauer/Keldysh approach. The zero bias transmission calculated for fixed tip positions in lateral dimensions but different tip–substrate distances show a clear shift and sharpening of the molecular chemisorption level on increasing the STM–surface distance, in agreement with experiment.