The role of adhesion forces in nanoscale measurements of the conductive properties of organic surfaces using conductive probe AFM

Electronic and mechanical studies of metal-molecules-metal junction have been accomplished to evaluate how biasinduced adhesion forces influence the charge transport efficiency of these junctions. The conducting probe atomic force microscope (CP-AFM) measures the current through an organic film sandwiched between two metal electrodes as a function of the tip-sample separation simultaneously with detection of the force between the probe and the surface. By applying a voltage between the sample and the tip, an attractive electrostatic capacitance force is added to the adhesion force. This paper describes probe-sample capacitance forces in conducting probe (CP) microscopy of polythiophene and alkanethiol monolayers, both theoretically and experimentally. The importance of taking into account an offset in the interaction force determined by the bias-induced adhesion force in the electronic measurements is demonstrated using current-voltage (I-V) characteristics of the polythiophene monolayer and the dependence of the adhesion force as a function of applied tip bias.