Molecules of bisthiolterthiophene have been adsorbed on the two facing gold electrodes of a mechanically controllable break junction in order to form metal-molecule(s)-metal junctions. Current-voltage $(I-V)$ characteristics have been recorded at room temperature. Zero bias conductances were measured in the 10–100 nS range and different kinds of nonlinear $I-V$ curves with steplike features were reproducibly obtained. Switching between different kinds of $I-V$ curves could be induced by varying the distance between the two metallic electrodes. The experimental results are discussed within the framework of tunneling transport models explicitly taking into account the discrete nature of the electronic spectrum of the molecule.

• Received 17 November 1998

DOI:https://doi.org/10.1103/PhysRevB.59.12505