- Fig. 1: Left: To form a break junction, a phosphor-bronze substrate supporting on Au bridge prepared on polyimide is mounted in a three point bending mechanism. By displacing a push-rod vertically, 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 green.
- Fig. 2: Conductance of an Au bridge as a function of the vertical displacement of the push-rod, measured in two distinct organic solvents.
Molecular Electronics. Molecular Electronics combines Nanoelectronics and Nanoelectrooptics with Synthetic Supramolecular Chemistry.
Scientists examine supramolecular structures and the way they conduct current in order to learn more about the way electrons flow through molecules and about single-molecule functions.
Tailored molecules with specific properties are examined. Their assembly on surfaces, their optical properties, electrical transport in single molecules and in assembled networks are of special interest.
Theoretical studies are going to address fundamental aspects of molecular charge transport.
Building Break Junctions for Molecular Electronics
Within the molecular electronics project, an important achievement 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 characterised.
We start by coating a flexible substrate (phosphorbronze) 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 (fig. 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 characterisation.
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.
In case of 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.
Grüter, L. et al., Small 1, 1067 (2005) Grüter, L. et al., Nanotechnology 16, 2143 (2005)