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 funct ional groups. Probe tips have been modified with SAMs using a procedur e that involves coating commercial Si3N4 cantilever/tip assemblies wit h a thin layer of polycrystalline Au followed by immersion in a soluti on of a functionalized thiol. This methodology provides a reproducible means for endowing the probe with different chemical functional group s. The spring constants and radii of the chemically modified cantileve r/tip assemblies have been characterized to allow for quantitative fri ction and adhesion measurements. Au-coated Si and Si substrates have b een treated with functionalized thiols and silanes, respectively, to p roduce SAM coated substrates terminating with different functional gro ups. A force microscope has been used to characterize the adhesive int eractions between probe tips and substrates that have been modified wi th SAMs which terminate with COOH, CH3, and NH2 functional groups in E tOH and H2O solvents. Force vs distance curves recorded under EtOH sho w that the interaction between functional groups decreases as follows: COOH/COOH > CH3/CH3 > COOH/CH3. The measured adhesive forces were fou nd to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact and thus show that the observed adhe sion forces correlate with the surface free energy of the molecular gr oups 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 str ength. These results have been interpreted in terms of contact and non contact contributions to the experimentally measured adhesive force. T he friction forces between tips and samples modified with COOH and CH3 groups have also been measured as a function of applied load. The mag nitude of the friction force was found to decrease in the following ma nner with different tip/sample functionalities: COOH/COOH > CH3/CH3 > COOH/CH3. Friction forces between different chemical functional groups thus correlate directly with the adhesion forces between these same g roups. Specifically, high friction is observed between groups that adh ere strongly, while low friction is observed between weakly interactin g functional groups. The dependence of friction forces on the tip and sample functionality is shown to be the basis for chemical force micro scopy in which lateral force images are interpreted in terms of the st rength of both adhesive and frictional interactions between different functional groups.