Variable contact gap single-molecule conductance determination for a series of conjugatedmolecular bridges

Abstract

It is now becoming clear that the characteristics of the whole junction are importantin determining the conductance of single molecules bound between two metalcontacts. This paper shows through measurements on a series of seven conjugatedmolecular bridges that contact separation is an important factor in determining theelectrical response of the molecular junction. These data are obtained using theI(t) method developed by Haiss et al since the scanning tunnelling microscopetip to substrate separation can be controlled through choice of the set-point(I0) current and calibrated with current–distance curves and knowledge of the terminal toterminal length of the molecular wire. The contact gap separation dependence isinterpreted as arising from tilting of these molecules in the junction and this model isunderpinned by ab initio transport computations. In this respect we make the generalobservation that conductance increases rather dramatically at higher tilt angle away fromthe normal for conformationally rigid molecular wires and that this increase in conductancearises from increased electronic coupling between the molecular bridge and the goldcontacts.