Distance Dependence of Electron Tunneling through Self-Assembled Monolayers Measured by Conducting Probe Atomic Force Microscopy: Unsaturated versus Saturated Molecular Junctions

Long-range electron transfer through monolayers and bilayers of alkanethiols in electrochemically controlled Hg[bond]neling junctions

The rates of electron tunneling through monolayers and bilayers of alkanethiols self-assembled in a potentiostatically controlled Hg-Hg junction are reported. An alkanethiolate monolayer is formed in situ on one or both Hg drops via oxidative adsorption at the controlled potential. Subsequently, the Hg drops are brought into contact using micromanipulators. The junction formation is instantly followed by the flow of a steady-state tunneling current between the two electrodes. A plot of the logarithm of the tunneling current density vs the total number of carbon atoms in each junction yields identical tunneling coefficients, beta = 1.06 +/- 0.04/-CH(2)- and beta = 1.02 +/- 0.07/-CH(2)-, for monolayers and bilayers of alkanethiols, respectively. Careful examination of the tunneling data indicates that the solvent and ions are ejected from the junction area. The tunneling current recorded for a bilayer of 1-octanethiol or 1-nonanethiol is ca. 2-fold larger than a corresponding tunneling current recorded for monolayers of 1-hexadecanethiol or 1-octadecanethiol, respectively. This result is explained in terms of weak electronic coupling across the noncovalent molecule/electrode interface.