Challenges and Pitfalls in Molecular Tunnel Junctions

When: Wednesday, 12th March (2014), 12:00h
Place: Departamento de Física de la Materia Condensada, Facultad Ciencias, Module 3, Aula de Seminarios (5ª Planta).
Speaker: Christian A. Nijhuis, Chemistry Depart of the National University of Singapore.

During the talk I will discuss our recent progress in the development of molecular tunnel junctions based on self-assembled monolayers (SAMs) and how we apply them as electrical excitation sources of plasmons. We form SAM-based molecular junctions using top-electrodes of a non-invasive liquid-metal that forms stable features in microchannels. This liquid-metal alloy (a mixture of Ga and In stabilized by a conductive 0.7 nm thick layer of GaOx) forms good electrical contacts with SAMs. This platform makes it possible to perform detailed physical-organic studies of charge transport across SAMs and to address some long standing controversies in the field. The current that flows across a junction decays exponentially with the molecular length and is expressed in the so-called tunneling decay coefficient β. For aliphatic SAMs, the consensus value of β is 1.0 per CH2 moiety, but values as low as 0.4 have been reported. Even more problematic, for the same molecular architecture a great variation of around 8-9 orders of magnitude of the injection currents have been reported. For molecular electronics to continue to evolve, these discrepancies across test-beds have to be understood. We found that the topography of the electrode that supports the SAMs, and the supramolecular structure of the junctions, are of crucial importance. By minimizing the number of defects, e.g., induced by grain boundaries that easily exceed the molecular dimensions, we are able to explain low values of β or high values of the injection currents. This knowledge we used to improve, for instance, the performance of molecular diodes, or to turn around a diode at the molecular level.