Transition from tunneling to hopping in single molecular junctions by measuring length and temperature dependence.

The charge transport characteristics of a family of long conjugated molecular wires have been studied using the scanning tunneling microscope break junction technique. The family consists of four wires ranging from 3.1 to 9.4 nm in length. The two shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer two wires show weakly length dependent and temperature variant behavior. This trend is consistent with a model whereby conduction occurs by two different mechanisms in the family of wires: by a coherent tunneling mechanism in the shorter two and by an incoherent charge hopping process in the longer wires. The temperature dependence of the two conduction mechanisms gives rise to a phenomenon whereby at elevated temperatures longer molecules that conduct via charge hopping can yield a higher conductance than shorter wires that conduct via tunneling. The evolution of molecular junctions as the tip retracts has been studied and explained in context of the characteristics of individual transient current decay curves.