Conductive probe AFM study of Pt-thiol and Au-thiol contacts in metal-molecule-metal systems.

The charge transport mechanism between 1,8-octanedithiol (ODT, C(8)H(16)S(2)H(2)) and platinum and gold electrodes is studied by breaking bonds between single ODT molecules and atomic metal junctions using conductive probe atomic force microscopy. Histograms of conductance values show peaks that are obscured by background processes that differ from the metal-molecule-metal conduction path of interest. We introduce a new method to reduce greatly such backgrounds by dividing by a 1-octanethiol (OMT, C(8)H(17)SH) reference histogram, without data selection. The method reveals three series of conductance values for both platinum and gold contacts, which we associate with geometrically different configurations between thiol and metal atoms. The ordering of conductance values, Pt-ODT-Pt > Pt-ODT-Au> Au-ODT-Au, is consistent with a relative dependence on both the number of electron channels and the density of states.