Abstract
Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.
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Acknowledgements
This research was carried out with financial support from the Dutch Foundation for Fundamental Research on Matter (FOM) and the European Union Seventh Framework Programme (FP7/2007-2013, under grant agreement no 270369, ‘ELFOS’). The authors would like to thank R. van Egmond for expert technical support and J. S. Seldenthuis for fruitful discussions.
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D.D. and H.v.d.Z. designed the project. C.M., H.v.d.Z. and J.v.R. designed the set-up and the devices. M.P. and C.M. fabricated the devices. A.S., R.E. and J.v.E provided the molecules. M.P and D.D. performed the experiments. C.V., M.P. and J.T. performed the calculations. M.P., C.V., D.D., J.T. and H.v.d.Z. wrote the manuscript. All authors discussed the results and commented on the manuscript.
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Perrin, M., Verzijl, C., Martin, C. et al. Large tunable image-charge effects in single-molecule junctions. Nature Nanotech 8, 282–287 (2013). https://doi.org/10.1038/nnano.2013.26
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DOI: https://doi.org/10.1038/nnano.2013.26
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