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Single molecular STM transport in well controlled experimental geometries

Olga Neucheva, Ruslan Temirov, Stefan Tautz

One of the foremost challenges in the field of molecular electronics is the insufficient structural and electronic contact definition in many of the single-molecule transport experiments reported up to date, demanding the performance and/or statistical analysis of hundreds of experiments to extract the relevant data. It has long been recognized that the scanning tunnelling microscope (STM) has the potential to make a difference in this respect. However, STM transport experiments often lack the tunability which, for example, can be achieved when an electromigrated molecular junction is combined with a gate electrode. Tunability of the transport junction and structural control are thus two important objectives which ideally should be realized in one and the same experiment, because this combination provides an optimal interface to ab-initio transport calculations. Recently we have reported a mechanically gated single-molecule transport experiment in which tunability and contact control both reach a very high level. In our experiment, the molecular wire was established by a chemically bonded point contact between the epitaxially adsorbed PTCDA molecule and the tip (see Fig1). For this purpose, the molecule should possess a functional group which is free to react with the tip. Because of its two interaction channels towards metals, PTCDA is well-suited for this experiment, since it is in fact possible to transfer the bond of one of the four carboxylic oxygen atoms from the substrate to the tip and thus establish a molecular wire with well defined contacts.

Transport Figure 1Figure 1 - A, Structural formula of PTCDA and schematic side view of PTCDA bonding to Ag(111), with primary π interaction and secondary interactions via the carboxylic oxygen atoms (vertical distortion is shown schematically). Yellow arrows: Charge transfer from metal to LUMO due to primary bond. B, Schematic side view of the junction with the PTCDA molecule incorporated between the STM tip and the surface.

After tip-molecule contact formation, the molecular wire junction can be stretched by retracting the tip, leading to a cleavage of the molecule-substrate bond and a simultaneous gating of the molecular wire. In the present example of the tip-PTCDA-Ag(111) wire, the transport spectra thus recorded not only reveal minute details about the metal-molecule bond breaking process, but also about the many-body transport physics in the molecular wire.

Publications:

R. Temirov, A.C. Lassise, F. Anders, F.S. Tautz, Kondo effect by controlled cleavage of a single molecule contact, Nanotechnology 19, 065401 (2008)
F. Pump, R. Temirov, O. Neucheva, S. Soubatch, F.S. Tautz, M. Rohlfing and G. Cuniberti, Quantum transport through STM-lifted single PTCDA molecules, Appl. Phys. A 93, 335, (2008)

Collaborations:

M. Rohlfing, Univ. Osnabrück,
J. Cuniberti, TU Dresden,
F. Anders, Univ. Bremen.


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