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Session A6 - Chemical Sensitivity in Scanning Probe Microscopy.
MIXED session, Monday morning, March 18
Room 260, America's Center
Chemical modification of the force microscope probes and sample surfaces with self-assembled monolayers provides a reproducible system for quantifying adhesive and frictional forces between distinct functional groups in liquid media (This work has been done in collaboration with A. Noy and C. M. Lieber). Using this approach, termed Chemical Force Microscopy (CFM), we have studied a variety of intermolecular forces, including hydrogen bonding, van der Waals, hydrophobic and electrostatic interactions, between COOH, NH2, OH and CH3 functional groups. In organic solvent medium, forces detected between functionalized tips and substrates were found to be consistent with the expected chemical nature of the interactions. A direct correlation between normal (adhesive) and lateral (friction) forces was also observed for these structurally similar monolayer systems. The adhesion and friction data were quantitatively interpreted using the JKR model of contact mechanics. We demonstrated that the dependence of frictional forces on the tip terminal functionality constitutes a basis for the chemically sensitive lateral force imaging. In addition, CFM was used to study intermolecular forces in aqueous solutions, in particular hydrophobic and long-range electrostatic interactions. Measurements of the adhesion force versus pH provide a unique way to determine the local pKs of ionizable surface groups. The pH dependent behavior of the frictional forces was utilized to map the distribution of charged functional groups on surfaces. From these studies, CFM emerges as a versatile and rationally tunable technique for studying intermolecular interactions and for chemically specific imaging of surfaces in both organic and aqueous media.