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Building Break Junctions for Molecular Electronics
Within the molecular electronics project, an important achievement during the first phase
of the NCCR Nanoscale Science has been the successful fabrication of mechanically controllable
break junctions (MCBJ) and their operation in liquid environment. Break junctions are an
essential tool to reliably prepare nanometer-size gaps between two metallic electrodes into
which single molecules can be trapped and electrically characterized. We start by coating a
flexible substrate (phosphor-bronze) with an insulating layer (polyimide). A metallic bridge,
with a constriction in its center, is then prepared by electron-beam lithography followed by
an etching step to form a free-standing bridge (Figure 1). The substrate is then mounted in a
three-point bending mechanism where the metallic bridge will be elongated and, ultimately,
broken. The gap between the two halves of the bridge can be controlled to sub-Angström
resolution thanks to the large mechanical reduction ratio of this simple mechanical set up.
The junction represents "ultimate nano-tweezers" to trap a molecule within the gap for
electrical characterization.
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Figure 1: Left: To form a break junction, a phosphor-bronze
substrate supporting a Au bridge prepared on polyimide is mounted in a three point
bending mechanism. By displacing vertically a push-rod, the Au wire can be broken in
a controlled manner and nanometer-size gaps formed between the two broken Au electrodes.
Right: Microfabricated Au bridge on top of the insulating polyimide layer shown in
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As illustrated in Figure 2, the conductance of the gold bridge decreases and exhibits steps
during the breaking process. This is due to the decreasing number of gold atoms within the
bridge, forming an atomic size contact surface. For gold, when the conductance reaches one
conductance quantum G0=2e2/h, this means that a single Au atom forms
the bridge. By further moving the push-rod upwards, the bridge breaks and a gap forms.
Thanks to collaborations with chemists within the NCCR (F. Diederich, ETHZ; A. Pfaltz,
M. Mayor, Basel) and beyond (in the framework of the Eurocores SONS program), we can now
investigate the electronic properties of various conjugated molecules trapped within break
junctions.
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Figure 2:
Conductance of a Au bridge as a function of the
vertical displacement of the push-rod measured in two distinct organic
solvents.
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Electrical Conductance of Atomic Contacts in Liquid Environments
Lucia Grüter, M. Teresa González, Dr., Roman Huber, Michel Calame, Dr., Christian Schönenberger, Prof. Dr.
Small, Volume 1, Issue 11, 1067 - 1070
Resonant tunnelling through a C60 molecular junction in a liquid environment
Lucia Grüter, Fuyong Cheng, Tero T Heikkilä, M Teresa González, François Diederich, Christian Schönenberger and Michel Calame
Nanotechnology 16, 2143
Contact:
Michel Calame |
Christian Schönenberger |
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Institute of Physics
University of Basel
Switzerland
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