An ab initio study of electron transport through nitrobenzene:: the influence of leads and contacts

We present density functional theory (DFT) based calculations for the conductance through a nitrobenzene molecule, which is anchored by thiol-groups to An electrodes. The emphasis is put on a comparison with the unsubstituted benzene molecule and the influence of segments of molecular poly-acetylene (PA) wires of different lengths inserted between the molecule and the Au surfaces. Our method uses partly occupied Wannier functions (WFs) as a maximally localized basis set for the transport calculations, which offers two distinct advantages over other similar schemes: (i) the WFs are expanded in terms of the eigenfunctions of DFT plane wave calculations, where the high accuracy of these calculations for the description of the electronic structure of the junction is preserved for the derivation of the transport characteristics; (ii) WFs are a natural tool for analysing the contributions to the transmission function coming from individual molecular orbitals (MOs), thereby facilitating a comparison between the structures investigated in this article. We illustrate within such an MO picture an earlier found mechanism for destructive interference, which explains the main differences in the transmission functions of benzene and nitrobenzene, and conclude with a qualitative discussion of their dependence on the PA wire length, where for small PA segments somewhat counter-intuitively an increase in the conductance is found.