Effect of Surrounding Medium on Resistance of a Molecular Monolayer Junction

We report a comparative study of charge transport across a n-decanethiol (C10SH) self-assembled monolayer (SAM) on Au(111) using conducting atomic force microscopy (C-AFM). The experiments are performed in nonpolar liquids (hexadecane and OMCTS) and ambient air. AFM force measurements in liquid mediums reveal discrete solvation layering close to the SAM interface. The liquid layering between the AFM probe and the SAM surface does not affect the measured contact resistance value, provided all the confined liquid layers are squeezed out of the contact. The contact resistance, however, can vary by orders of magnitude when measured in different mediums. This is mainly governed by the mechanical behavior of the monolayer. Force vs current measurements and contact stiffness measurements using sample modulation AFM show how the mechanical response of the SAM junction varies in the different surrounding mediums and supports the observed variation in measured contact resistance values.