Smallest conceivable switch developed
The wire produced by the scientists from Kiel and San Sebastian is barely two atomic bonds long and one atom wide. Thinner and much shorter is not possible, the researchers say. A special mechanism of action makes it possible to control the current through this wire.
But first, however, the scientists had to solve a seemingly trivial problem, which in practice seems to be quite tricky: How can such a nanowire switch be contacted? In order to measure the current flowing through the nanowire, its two ends must be connected to a metal electrode, as is the case with larger circuits. But there are no metal clips that would be small enough to make nanoscale electrical contacts. “Contacting individual molecules in an electrical circuit is a problem that has not yet been satisfactorily solved and is a matter of much discussion in research,” explains prinicpal author Jasper-Tönnies from the research group of Professor Richard Bernd in Kiel.
The scientists developed the new wire from one molecule in order to facilitate electrical contact. The special feature of this wire is that it can be placed vertically on a metal surface. Thus, one of the two necessary contacts is already built into the wire. To this end, the participants are using an approach that was also developed in Kiel, where former colleagues of Jasper-Tönnies had developed a molecular platform. They put the wire on such a platform. It has good conductivity and can be attached to a metal surface like a suction cup – the beginning of a circuit is made.
For the second contact required, the research team used a scanning tunneling microscope (STM). With a metal tip, it “palpates” a sample and creates an image of its surface down to the scale of a few nanometers. In this way, individual atoms are also visible. In their experiments, the Kiel researchers used a particularly fine metal tip for the STM, with only one atom sitting at the end. They were able to electrically contact the second end of the wire, close the circuit and measure the current. This approach exhibited a very good repeatability and a low variance.
During their measurements, the scientists also found that quantum mechanical forces act between the metal tip of the STM and the nanowire, which can be used to mechanically bend the wire. If the wire is only slightly bent, the current is reduced. However, the current increases with more severe bending. “By bending the wire, we were able to turn the current on or off. Although our wire is so simple, it behaves very complex – we were surprised by that,” says Jasper-Tönnies.
The scientists see the reason for the unusual current conduction of nanowire in its molecular structure. This is supported by the calculations of Dr. Aran Garcia-Lekue and Professor Thomas Frederiksen from San Sebastián. Due to the quantum mechanical forces, individual atoms of the wire form new chemical bonds with the atom of the STM tip. This changes the geometry of the molecule and its properties. In fact, small geometric differences can have a very big effect. This is why it is important to be able to adjust and measure the geometry of a molecule as accurately as possible – and we achieve this by precisely contacting the nanowire and using the STM images in atomic resolution,” said Jasper-Tönnies.
The results of the research teams were published in the journal Physical Review Letters. The first author was Torben Jasper-Tönnies.
Original publication: https://doi.org/10.1103/PhysRevLett.119.066801
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