New measurement technique brings molecular chips one step closer
Leiden and Delft researchers have determined what makes electron transport in a single molecule so difficult. They made this discovery thanks to a new measuring technique. Their findings bring us one step closer to electronic chips. The findings are published in Nature Nanotechnology.
A better insight into the factors that hinder electrical conductivity at molecular level brings us one step closer to applying molecular electronics. Prof. Jan van Ruitenbeek, who led the study together with Delft researcher Prof. Herre van der Zant, comments: ‘I expect the first applications to be used for large surfaces, for instance for displays, where there is a great advantage to having a single layer of molecules that can be put together simply and cheaply.’ The real challenge now is to design a molecule that can behave as a switch or a diode, with a sufficiently large difference in conductivity between the ‘on’ and ‘off’ positions.
The study offers insight into the fundamental physical behaviour of individual molecules. Being only a few nanometres in size, a molecule can act as a very sensitive sensor or extremely small transistor between two electrodes. But the problem in developing this kind of molecular electronics is that it is difficult to make electrical contact with a single molecule.
The Leiden researchers and their colleagues from the FOM Foundation and Delft University of Technology came up with the idea of using image charges to creat electronic molecular components. Image charges occur in a metal as a result of the proximity of a charge, such as that on a single molecule. These image charges in the metal in turn impact the energy levels of the molecule. It has been known for some time that image charges therefore play an important role in transporting charges through molecules. The image charges can strongly shift the position of the molecular energy levels with respect to the energy levels in the metal. In this way they lead to better or worse conductivity. The researchers first mapped this systematically for a single molecule.
By combining their respective expertise, the researchers from Delft and Leiden were able to develop a new technique for measuring conductivity in a molecule. The method is based on the mechanically driven break junction technique developed by Van Ruitenbeek. A small freely suspended bridge in a metal conductor is subjected to mechanical pressure, as a result of which it bends and breaks. The molecule subsequently attaches itself to the two clean break surfaces. The Delft researchers extended the technique by making it into a transistor. This makes it possible to vary the distance between the electrodes (and therefore the proximity of the molecule), and in this way to impact the image charge. As a result the researchers gained a unique degree of mechanical and electronic control over the energy levels of the molecule. In this way they were able to experimentally determine the role of image charges and express this in numerical terms.
(19 March 2013)
Mickael L. Perrin, Christopher J. O. Verzijl, Christian A. Martin, Ahson J. Shaikh, Rienk Eelkema, Jan H. van Esch, Jan M. van Ruitenbeek, Joseph M. Thijssen, Herre S. J. van der Zant and Diana Duliæ, Large tunable image-charge effects in single-molecule junctions, Nature Nanotechnology (2013).
Fundamentals of Science is one of the six themes for research at Leiden University.