Mar 11, 2009
Single molecule electrical junctions with asymmetric contacts
In the last few years it has become experimentally possible to measure the electrical properties of molecules down to the single molecule level. This progress in experimental methodology, for example the development of STM or break-junction methods for forming single molecular electrical wires, has been complemented by advanced theoretical and computational methods for determining charge transport across single molecule junctions. This has provided a great deal of detail about charge transport through metal/single molecule/metal junctions.
To date, the focus of such single molecule studies has been on symmetric junctions, where both ends of the molecule are contacted to the respective metal terminals with chemically identical end groups. However, systems where the contacting groups at either end of the molecule are different (contact asymmetry) are also of potential interest in molecular electronics.
In such cases a "polar" orientation of mono-molecular films in devices can result, which has significance in molecular electronics because it has been previously shown that it can lead to electrically rectifying junctions with diode-like behaviour. This inspired our team to examine experimentally and theoretically how contact asymmetry influences the conductance of single molecules in an electrical junction, a study that has now appeared in Nanotechnology.
We measured and calculated the single molecule conductance of molecular wires with different combinations of end groups; either thiol groups at each end, carboxylic acids groups at each end, or mixed end group systems with a thiol group at one end and a carboxylic acid group at the other. We found that for molecular wires with mixed functional groups (X-bridge-Y) the single molecule conductance decreases with respect to the comparable symmetrical (X-bridge-X) molecules.
These differences are confirmed by theoretical computations based on a combination of density functional theory and the non-equilibrium Green's functions formalism and also by a heuristic tight-binding model, which captures the essential quantum mechanics. This study demonstrates that the apparent contact resistance, as well as being highly sensitive to the type of contacting group, is strongly influenced by contact asymmetry of the single molecular junction. This highlights that contact asymmetry is a significant factor to be considered when evaluating nano-electrical junctions incorporating single molecules.
About the author
The work was performed at the University of Liverpool and the University of Lancaster and is supported by the UK Engineering and Physical Sciences Research Council, the European Commission, Qinetiq, the British Department of Trade and Industry, Royal Society and Northwest Regional Development Agency. Dr Santiago Martin is a postdoctoral researcher at Liverpool University financially supported through a postdoctoral fellowship from the Ministerio de Educacion y Ciencia of Spain. David Zsolt Manrique is a PhD student in the Condensed Matter Physics group at Lancaster University. Dr Victor Garcia-Suárez is a post-doctoral researcher also from the Condensed Matter Physics group at Lancaster University. Dr Wolfgang Haiss is a post-doctoral researcher in the Chemistry Department at Liverpool University. Dr Simon Higgins is a reader in chemistry at Liverpool University. Prof. Colin Lambert is professor of theoretical condensed matter physics at Lancaster University and associate dean of research. Prof. Richard Nichols is a professor of physical chemistry at the University of Liverpool.