Chemical Force Microscopy: Exploiting Chemically-Modified Tips To Quantify Adhesion, Friction, and Functional Group Distributions in Molecular Assemblies

Chemical force microscopy (CFM) has been used to measure adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers (SAMs) that terminate in distinct functional groups. Robe tips have been modified with SAMs using a procedure that involves coating commercial Si3N4 cantilevedtip assemblies with a thin layer of polycrystalline Au followed by immersion in a solution of a functionalized thiol. This methodology provides a reproducible means for endowing the probe with different chemical functional groups. The spring constants and radii of the chemically modified cantilevedtip assemblies have been characterized to allow for quantitative friction and adhesion measurements. Au-coated Si and Si substrates have been treated with functionalized thiols and silanes, respectively, to produce SAM coated substrates terminating with different functional groups. A force microscope has been used to characterize the adhesive interactions between probe tips and substrates that have been modified with SAMs which terminate with COOH, CH3, and N H 2 functional groups in EtOH and HzO solvents. Force vs distance curves recorded under EtOH show that the interaction between functional groups decreases as follows: COOWCOOH > CHdCH3 > COOWCH3. The measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact and thus show that the observed adhesion forces correlate with the surface free energy of the molecular groups in EtOH. Electrostatic contributions to adhesive forces have also been studied using a COO-/NH3+ tip/surface in aqueous solution. Force vs distance curves recorded as a function of ionic strength show that the observed adhesive interaction decreases with increasing ionic strength. These results have been interpreted in terms of contact and noncontact contributions to the experimentally measured adhesive force. The friction forces between tips and samples modified with COOH and CH3 groups have also been measured as a function of applied load. The magnitude of the friction force was found to decrease in the following manner with different tip/sample functionalities: COOWCOOH > CH3/CH3 > COOWCH3. Friction forces between different chemical functional groups thus correlate directly with the adhesion forces between these same groups. Specifically, high friction is observed between groups that adhere strongly, while low friction is observed between weakly interacting functional groups. The dependence of friction forces on the tip and sample functionality is shown to be the basis for chemical force microscopy in which lateral force images are interpreted in terms of the strength of both adhesive groups.