Electrochemical interference

Single molecule electronics has become one of the most active research fields in nano-electronics, an area which aims at maintaining a continuous rise in performance of digital devices even once the lower threshold for miniaturisation faced by the semiconductor industry has been reached. For realising the potential of this field, two essential aims need to be met. First, it is necessary for device design to understand and to be able to model physical effects on the nanoscale such as quantum interference, where it has been proven theoretically and experimentally that Kirchhoffs laws, which determine the conductance of two classical wires connected in parallel, do not apply anymore if the wires are branches of molecules. Secondly, in order to be useful for any practical application, a device must operate at room temperature, where it has been recently achieved to demonstrate diode and transistor features in the electron transport through organic single molecules with redox active metal centers in an electrochemical environment. Branched molecules containing such a redox active center in each of their two branches have never been considered before, although they might open up intriguing new possibilities which depend in their details on the electron transport regime in which the current flow occurs. If it is phase coherent electron tunnelling, wave like interference effects might be induced due to an asymmetry brought about by the use of different metals in both branches such as Osmium and Ruthenium and this might provide more flexibility in the related device design. In the hopping regime on the other hand a local gating effect might be achieved, because the oxidation state of the metal in one branch is likely to have an influence on the electron transport through the other, thereby offering a route towards chemical sensors. Within this project both possibilities will be investigated with theoretical simulations on the basis of density functional theory on a small range of target molecules. These molecules have been already synthesized and will be characterized with an electrochemical scanning tunnelling microscope by the projects experimental partners at Imperial College London, where the interplay of theory and experiment is essential for arriving at an atomistic understanding of the electron transfer processes involved. For the theoretical work in Vienna the semiclassical Marcus theory will be employed for describing electron hopping and a nonequilibrium Greens function approach for the description of phase coherent tunnelling. This project follows up the research carried out in two other FWF funded projects, namely Interference effects in molecular electronics P20267 and Electrochemical charge transport P22548, where the main idea is to build a bridge between the two essential aims in single molecule electronics mentioned at the top of this page.

Single molecule electronics has become one of the most active research fields in nano-electronics, an area which aims at maintaining a continuous rise in performance of digital devices even once the lower threshold for miniaturisation faced by the semiconductor industry has been reached. For realising the potential of this field, two essential aims need to be met. First, it is necessary for device design to understand and to be able to model physical effects on the nanoscale such as quantum interference, where it has been proven theoretically and experimentally that Kirchhoffs laws, which determine the conductance of two classical wires connected in parallel, do not apply anymore if the wires are branches of molecules. Secondly, in order to be useful for any practical application, a device must operate at room temperature, where it has been recently achieved to demonstrate diode and transistor features in the electron transport through organic single molecules with redox active metal centers in an electrochemical environment. Branched molecules containing such a redox active center in each of their two branches have never been considered before, although they might open up intriguing new possibilities which depend in their details on the electron transport regime in which the current flow occurs. If the current flow is based on phase coherent electron tunnelling, wave like interference effects might be induced due to an asymmetry brought about by the use of different metals in both branches such as Osmium and Ruthenium and this might provide more flexibility in the related device design. Within this project this possibility has been investigated with theoretical simulations on the basis of density functional theory in combination with a nonequilibrium Greens function approach on a small range of target molecules. In addition the applicability of atomic orbital and molecular orbital based schemes for the prediction of such interference effects has been critically assessed and their relationship to concepts from quantum chemistry such as the Coulson Rushbrook pairing theorem has been clarified. Another focus of this work was the competition between coherent tunneling and hopping as electron transport mechanisms. We found that while in molecular wires there is a crossover length for the two transport regimes, they can co-exist in some redox-active compounds which enables a mechanism for redox switches. The latter work was a close collaboration with experimentalists from IBM Zurich. In another attempt to guide the interpretation of measured current voltage curves, we studied the influence of different adsorption configurations in a scanning tunneling microscope setup, which was aimed at mimicking the measurements of our experimental partners at Imperial College London.