Anomalous length and voltage dependence of single molecule conductance

A systematic experimental study of the electrical conductance of single alkanedithiol molecules (HS–(CH₂)_N–SH) between gold contacts in air for N = 3–12 is presented. For all of these molecules, three different fundamental conductance groups (low, medium and high conductance) were observed. For long molecules (N > 7) the conductance decays exponentially with molecular length for all three conductance groups, as it has been reported previously. In contrast, for short molecules (N < 8), it is shown that the decay of conductance with molecular length gets less pronounced for decreasing length, approaching length independent conductance values for N < 5 where the voltage dependence of the tunnelling current exhibits an anomalous behaviour. Possible reasons for these findings, including the influence of the image potential on the effective mass of the tunnelling electron (hole), are discussed.