Effects of gating and contact geometry on current through conjugated molecules covalently bonded to electrodes

We study the effects of gating and contact geometry on current through self-assembled monolayers of conjugated molecules strongly coupled to gold electrodes by sulfur "anchor groups." The current changes by more than an order of magnitude depending on the angle between the axis of the benzene-dithiolate molecules and the normal to the electrode on the less coordinated "top site" position. The effect of gating is also much stronger in this case compared to higher coordinated "hollow site" binding of the molecule on a Au(111) surface. The large hybridization of the molecular states with electrode states for the hollow site leads to practically ohmic current-voltage characteristics. Changes in molecule-electrode geometry accompanying the gating of the self-assembled monolayer may be the reason for strong changes of the conductance.