Conductance of Junctions with Acetyl-Functionalized Thiols: A First-Principles-Based Analysis

Thiol-based contacts are widely used in fabrication of molecular junctions but are associated with several drawbacks due to their chemical reactivity. In particular, their tendency to dimerize by forming sulfur–sulfur bonds is viewed as a barrier for large scale bridge fabrication. Instead, the use of functionalized sulfur end groups in the fabrication of the junctions is promoted. We analyze the effects of thiol functionalization by acetyl on the transport properties of porphyrin based bridges. In scanning tunneling microscopy (STM) experiments, where the conductance is measured as the tip is retracted, we observe molecular conductance steps due to junctions with acetyl protected thiols that are significantly lower than observed for junctions with deprotected thiols (by a factor of ≈5). Using a first-principles based computational approach, we explain the lower conductance of junctions with acetyl functionalized thiol and associate it to chemical changes of the sulfur, where essentially a tunneling barri...