Molecular electron transport changes upon structural phase transitions in alkanethiol molecular junctions.

We demonstrated structural phase dependency of conductance across thiolate self-assembled monolayers (SAMs) in different junctions. A structural phase transition from a hexagonal closed phase to a striped phase in 1-octanethiol (OT) and 1,8-octanedithiol (ODT) SAMs was revealed by high resolution scanning tunneling microscopy (STM) images. Electron tunneling characteristics were measured through STM-based individual molecular junctions and micropore-based large molecular junctions. The tunneling barrier height and the tunneling decay constant of the molecular junctions were used as measures of the intermolecular coupling for different structural phases of the thiolate SAMs. Electron transport through ODT SAMs was found to be more sensitive than that through OT SAMs, according to the structural phase transition. These results suggest that (1) the structural phase transition in the SAM induces a change in the electron tunneling distance through the pathway of through-bond tunneling and through-space tunneling, leading to a change in the tunneling barrier of molecular junctions, and (2) integrated intermolecular coupling in a large molecular junction leads to a significant change of the electron transport between two structural phases.