Publications (9) View all
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Article: Electrical transport through a mechanically gated molecular wire
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ABSTRACT: A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment. Comment: 31 pages, 10 figures11/2010; -
Article: Simulating STM transport in alkanes from first principles
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ABSTRACT: Simulations of scanning tunneling microscopy measurements for molecules on surfaces are traditionally based on a perturbative approach, most typically employing the Tersoff-Hamann method. This assumes that the STM tip is far from the sample so that the two do not interact with each other. However, when the tip gets close to the molecule to perform measurements, the electrostatic interplay between the tip and substrate may generate non-trivial potential distribution, charge transfer and forces, all of which may alter the electronic and physical structure of the molecule. These effects are investigated with the ab initio quantum transport code SMEAGOL, combining non-equilibrium Green's functions formalism with density functional theory. In particular, we investigate alkanethiol molecules terminated with either CH3 or CF3 end-groups on gold surfaces, for which recent experimental data are available. We discuss the effects connected to the interaction between the STM tip and the molecule, as well as the asymmetric charge transfer between the molecule and the electrodes.02/2009; -
Article: Effects of self-interaction corrections on the transport properties of phenyl-based molecular junctions
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ABSTRACT: In transport calculations for molecular junctions based on density functional theory the choice of exchange and correlation functional may dramatically affect the results. In particular local and semi-local functionals tend to over-delocalize the molecular levels thus artificially increasing their broadening. In addition the same molecular levels are usually misplaced with respect to the Fermi level of the electrodes. These shortfalls are reminiscent of the inability of local functionals to describe Mott-Hubbard insulators, but they can be corrected with a simple and computationally undemanding self-interaction correction scheme. We apply such a scheme, as implemented in our transport code Smeagol, to a variety of phenyl-based molecular junctions attached to gold electrodes. In general the corrections reduce the current, since the resonant Kohn-Sham states of the molecule are shifted away from the contact Fermi level. In contrast, when the junction is already described as insulating by local exchange and correlation potentials, the corrections are minimal and the I-V is only weakly modified. Comment: 12 pages, 21 figures12/2007; -
Article: Molecules for organic electronics studied one by one.
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ABSTRACT: The electronic and geometrical structure of single difluoro-bora-1,3,5,7-tetraphenyl-aza-dipyrromethene (aza-BODIPY) molecules adsorbed on the Au(111) surface is investigated by low temperature scanning tunneling microscopy and spectroscopy in conjunction with ab initio density functional theory simulations of the density of states and of the interaction with the substrate. Our DFT calculations indicate that the aza-bodipy molecule forms a chemical bond with the Au(111) substrate, with distortion of the molecular geometry and significant charge transfer between the molecule and the substrate. Nevertheless, most likely due to the low corrugation of the Au(111) surface, diffusion of the molecule is observed for applied bias in excess of 1 V.Physical Chemistry Chemical Physics 08/2011; 13(32):14421-6. · 3.57 Impact Factor -
Article: Dynamical bi-stability of single-molecule junctions: A combined experimental/theoretical study of PTCDA on Ag(111)
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ABSTRACT: The dynamics of a molecular junction consisting of a PTCDA molecule between the tip of a scanning tunneling microscope and a Ag(111) surface have been investigated experimentally and theoretically. Repeated switching of a PTCDA molecule between two conductance states is studied by low-temperature scanning tunneling microscopy for the first time, and is found to be dependent on the tip-substrate distance and the applied bias. Using a minimal model Hamiltonian approach combined with density-functional calculations, the switching is shown to be related to the scattering of electrons tunneling through the junction, which progressively excite the relevant chemical bond. Depending on the direction in which the molecule switches, different molecular orbitals are shown to dominate the transport and thus the vibrational heating process. This in turn can dramatically affect the switching rate, leading to non-monotonic behavior with respect to bias under certain conditions. In this work, rather than simply assuming a constant density of states as in previous works, it was modeled by Lorentzians. This allows for the successful description of this non-monotonic behavior of the switching rate, thus demonstrating the importance of modeling the density of states realistically.09/2010;
