  The effects of water wetting conditions on the transport properties of
molecular nano-junctions are investigated theoretically by using a combination
of classical molecular dynamics and first principles electronic transport
calculations. These are at the level of the non-equilibrium Green's function
method implemented for self-interaction corrected density functional theory. We
find that water effectively produces electrostatic gating to the molecular
junction, with a gating potential determined by the time-averaged water dipole
field. Such a field is large for the polar benzene-dithiol molecule, resulting
in a transmission spectrum shifted by about 0.6 eV with respect to that of the
dry junction. The situation is drastically different for carbon nanotubes
(CNTs). In fact, because of their hydro-phobic nature the gating is almost
negligible, so that the average transmission spectrum of wet Au/CNT/Au
junctions is essentially the same as that in dry conditions. This suggests that
CNTs can be used as molecular interconnects also in water-wet situations, for
instance as tips for scanning tunnel microscopy in solution or in biological
sensors.
