Mechanical and charge transport properties of alkanethiol self-assembled monolayers on a Au(111) surface: The role of molecular tilt

The relationship between charge transport and mechanical properties of alkanethiol self-assembled monolayers (SAMs) on Au(111) films has been investigated using an atomic force microscope with a conductive tip. Molecular tilts induced by the pressure applied by the tip cause stepwise increases in film conductivity. A decay constant beta = 0.57 +/- 0.03 angstrom(-1) was found for the current passing through the film as a function of tip-substrate separation due to this molecular tilt. This is significantly smaller than the value of similar to 1 angstrom(-1) found when the,separation is varied by changing the length of the alkanethiol molecules. Calculations indicate that, for isolated dithiol molecules S-bonded to hollow sites, the junction conductance does not vary significantly as a function of molecular tilt. The impact of S-Au bonding on SAM conductance is discussed.