Authors: J. Heurich, J.C. Cuevas, W. Wenzel and G. Schön
Affilation: Forschungszentrum Karlsruhe, Germany
Pages: 90 - 93
Keywords: molecular electronics, conduction channels, IV-characterisitic
Present trends in the miniaturization of electronic devices suggest that ultimately single molecules may be used as electronically active elements in a variety of applications. Recent advances in the manipulation of single molecules now permit to contact an individual molecule between two electrodes (see Fig.1) and measure its electronic transport properties. In contrast to single-electron transistors based on metallic islands molecular devices have a more complicated, but in principle tunable, electronic structure. Interesting and novel effects, such as negative differential conductance, were observed in some of these experiments, which still, by-and-large, beg theoretical explanation. In addition to generic principles of nanoscale physics, e.g. Coulomb blockade the chemistry and geometry of the molecular junction emerge as the fundamental tunable characteristics of molecular junctions. Here we present an atomistic theory that bridges traditional concepts of mesoscopic and molecular physics to describetra sport through single organic molecules in qualitative agreement with recent break-junction experiments. We combine ab initio quantum chemistry calculations with non-equilibrium Green functions techniques to illustrate the emergence of conduction channels in a single-molecule junction from the molecular orbitals (MO). We further show how the specific properties of individual Mos are reflected in their contribution to the current. Using this data we provide insight into the microscopic origin of the nonlinear I-V characteristics (see Fig 2) observed experimentally and correlate their features to the specific properties of the molecule (see Fig 3). Our approach naturally accounts for the experimental observations and indicates that the current in these molecular junctions is mainly controlled by the electronic structure of the molecules and their local environment. We demonstrate that many molecular orbitals participate in a single conduction channel and provide examples where, surprisingly, the current is not dominated by the contribution from the energetically closest MOs. The theory provides a quantitative criterion to judge the importance of individual MOs to the current and thus paves the way for the design of molecular transport properties. Our results suggest that in thiol-bridged aromatic molecules, such as those investigated here, even a single conduction channel is comprised of the tails of many electronic levels. We discuss the implications of this finding on the suitability of such systems for the construction of molecular electronic devices.
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