Charge transport through single-molecule junctions is dominated by image charges. This is demonstrated and explained by a combined experimental and theoretical effort from Delft (Quantum Nanoscience and Chemical Engineering departments) and the Leiden Institute of Physics in a recent paper in Nature Nanotechnology.
In a break-through gated break-junction experiment, the gate voltage and the molecule-electrode separation can be changed independently. As the zinc porphyrin molecule gets closer to the gold electrodes, both unoccupied and occupied orbital levels shift a few hundred meV, approaching the Fermi level. This confirms the strong effect of image charges in electronic transport through single-molecule junctions.
With their locally implemented non-equilibrium Green’s function (NEGF) method in our periodic DFT code BAND, Chris Verzijl and Jos Thijssen quantitatively explained the experiments. The fully self-consistent transport calculations with a voltage bias revealed the existence of states near the molecule-electrode interface acting as unoccupied transport orbitals. The experimentally observed level shifts were also be reproduced by electrostatic calculations.
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BAND, NEGF, charge transport, orbital levels
M. L. Perrin, C. J. O. Verzijl, C. A. Martin, A. J. Shaikh, R. Eelkema, J. H. van Esch, J. M. van Ruitenbeek, J. M. Thijssen, H. S. J. van der Zant, and D. Dulic Large tunable image-charge effects in single-molecule junctions. Nature Nanotech. 8, 282-287 (2013)Key conceptsBAND bonding analysis Charge Transport materials science molecular electronics NEGF periodic DFT