Electrochemical charge transport

Molecular electronics has been identified as one of the most promising candidates in nano-electronics. In spite of the recent progress in experimental and theoretical research on single molecule conductivity in a ultra-high vacuum setup at cryogenic temperatures, the latter two conditions impose severe limits on any practical applications. Studies of electron transport in an electrochemical environment offer a new perspective, where individual transition metal complexes have already been demonstrated to display wire, diode and transistor features, notably at room temperatures. For a better understanding of electron transport in such an environment it is essential to arrive at a clear theoretical picture of its mechanism based on first principles calculations. The setup in charge transfer experiments with an electrochemical scanning tunneling microscop (STM) is very complex for a density functional theory (DFT) description. Not only are the transition metal complexes which need to be investigated rather large and potentially problematic in terms of an accurate description of the localization of charges, but also the influence of solvent and substrate further complicates the picture. A comprehensive approach within the framework of the semi-classical Marcus theory needs to be developed for electron transfer based on vibrationally induced hopping, where the simplest representative system is chosen for first test calculations (objective I). A starting point will be a Os-complex bonded to a small cluster of Au-atoms and the attempt to directly calculate the driving force G0 , the reorganization energy (at first only the inner part coming from a relaxation of the Os-complex in its two redox states of interest itself), and the electronic coupling matrix element Hab between molecule and substrate. The modelling of the solvent requires a conductor-like screening model. For a direct comparison of our DFT caclulations with experiments we need to calculate the electron transfer (ET) rate constant explicitly by using the Marcus equation. The main aim is to theoretically describe and explain the trends for the current and rate constant in dependence on the structure of the metal complex and the surrounding solvent. The second objective of this proposal is a close collaboration with the already commenced and ongoing experimental research efforts in Tim Albrecht`s group at Imperial College London on electrochemical STM measurements on Ru/Os complexes, where both coherent tunnelling and hopping transport are considered to be possible mechanisms. The research in Vienna will focus entirely on a theoretical description of such compounds but a vivid exchange of concepts and findings with the experimentalists in London is ensured in the proposal.

Molecular electronics has been identified as one of the most promising candidates in nano-electronics. In spite of the recent progress in experimental and theoretical research on single molecule conductivity in an ultra-high vacuum setup at cryogenic temperatures, the latter two conditions impose severe limits on any practical applications, where studies of electron transport in an electrochemical environment offer a new perspective. The aim of this project was to arrive at a clear theoretical picture of different electron transport mechanisms in such an environment based on first principles calculations. The setup in charge transfer experiments is very complex for a density functional theory (DFT) description. Not only are the transition metal complexes which needed to be investigated rather large and potentially problematic in terms of an accurate description of the localization of charges, but also the influence of solvent and substrate further complicated the picture. A comprehensive approach within the framework of the semi-classical Marcus theory needed to be developed for electron transfer based on vibrationally induced hopping (objective I) but also the theoretical description of the competing electron transport mechanism, namely coherent tunnelling required an adjustment of the oxidation state of the central redox system (objective II). Our work on objective I has been completed as anticipated in the original proposal: Within Marcus theory the calculation of the electronic coupling matrix element (or transfer integral) between molecule and substrate plays a crucial role. We developed several methods to calculate this quantity directly, which we tested extensively for two molecular systems, where reliable data existed in the literature for a direct comparison and also applied them to the Ru complex at the center of our project. For the computation of the driving force and the reorganization energy we used a method with DFT calculations only for the central molecule and its partial charge distribution combined with electrostatic continuum models for its interaction with the leads and for its solvation energies in different charging states which reached sufficient accuracy for our purpose. By applying the methods summarized above we were able to compare electron transfer rates for coherent tunnelling and electron hopping for a series of molecular junctions, which was the final aim of the project on objective I. Our work on objective II has also been completed as anticipated in the original proposal, where we described coherent electron tunnelling through a single molecule junction containing a redox-active Ru-complex with a special focus on the adjustment of the oxidation state of this complex. In addition we started a collaboration with IBM and the University of Zurich, because the interpretation of experimental data was one of the central aims of this project. This collaboration resulted in two publications about the influence of anchor groups on the conductance and on a description of its bias induced switching for a mono-nuclear Molybdenum complex combining both hopping (objective I) and tunnelling (objective II), where the latter work has been published in Nature Nanotechnology in November 2015.