engleski

How good conductors are pi-conjugated molecules ?

In the field of molecular nanoelectronics the transport properties of single molecule are of crucial impotance. Molecular junction with pi-conjugated molecules have often been studied primarily for their promising molecular wire behaviour. However, the discrepances between theoretical predictions and experimental data have blured out the ability of pi-conjugated systems. The precise knowledge of the contact geometry of the molecular junction, as the adsorption site and bonding between the molecule and the electrode has been difficult to achieve experimentally and the dependence of the conductivity on the length of the molecule has not been precisely known. In order to get a reliable description of the molecular conducting properties the electronic structure of the electrodes, the electrode surfaces, the molecule, and the molecule-induced gap states, should be fully considered. Moreover, as the junction is subject to the nonequilibrium conditions the transmission should be obtained self-consistently for each bias voltage applied across the junction. We used a full self-consistent ab initio method based on the density functional theory and nonequilibrium Green's function technique. We performed series of calculations of the conductance of certain classes of p-conjugated molecules: phenyl ethylene oligomers (Tour wires), phenylene vinylene oligomers (OPV) and polyynes. The considered geometry consisted of the molecule between the gold electrodes. In the low bias regime we found that polyynes show the highest conductance with the value of 100 microS at zero bias. In Tour wires and OPV molecules, the conductance shows the decrease proportional to the number of benzene rings in the molecule. We showed that the conductance of polyynes is stable with respect to the applied bias and the length of the molecule while in Tour wires and OPVn it exibits the nonlinear behaviour as a consequnce of different electronic structure around the Fermi level of the system.

Molecular junctions; Conductance

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