Complete report - Donostia International Physics Center - Euskal ...

2011 DIPC Highlight
Atomic-scale engineering of
electrodes for single-molecule
contacts
G. Schull, T. Frederiksen, A. Arnau, D. Sanchez-Portal, and R. Berndt
Nature Nanotechnology 6, 23–27 (2011)
Contacts between a single molecule and a metal electrode can be good or bad depending on
the number of metal atoms that are in direct contact with the molecule.
One of the most challenging questions in novel molecular electronics is the fabrication of atomic-scale
contacts to single molecules. This requires a precise and detailed characterization of the current flow in
extremely small electrical circuits, so small that their components are individual atoms and molecules. It
is precisely the small dimension of the system, typically in the nanometer range, that makes the solution
difficult. In particular, for a junction formed by a single molecule the number of atoms making the contact,
as well as their positions, are expected to be crucial parameters for its ability to conduct electrical currents.
So far, there have not been any experiments where these variables have been controlled with sufficiently
high precision.
Thanks to a collaboration between scientists in San Sebastian and Kiel (Germany) it is now been proven
possible to determine and control the number of atoms making contact between a single C60 molecule
and a copper metal electrode while the current flow through the junction is registered, as shown in the
figure. In this way, these scientists have been able to quantify the changes in the electrical conductance
across the molecular junction (metal/molecule/metal) as a function of the contact area between the molecule
and the metallic electrodes. In brief, by changing the number of atoms in the contact one by one,
it was found that there is a change from low (bad contact) to high (good contact) conductance. In the
bad contact regime, the current flow is limited by the contact area, while in good contact regime the current
flow is limited by scattering at the molecule.
Artistic representation of a scanning tunneling microscope (STM) tip with a C60 molecule at its apex suspended
over a copper surface which has been manipulated at the atomic scale to accommodate clusters with different
number of copper atoms. The copper clusters are used to make different contacts to the molecule by gently approaching
the tip to each of them. The blue color surface in the lower part of the figure shows a real STM image
of the metal surface with different number of copper atoms on top of it as indicated by the orange spheres
[Image by G. Schull].
Molecular contacts at hand.
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