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 SAM may be the reason for strong changes of the conductance observed by Schoen et al. in the ``slot'' geometry.