Monitoring surface reactions at an AFM tip: An approach to follow reactionkinetics in self-assembled monolayers on the nanometer scale

The kinetics of alkaline hydrolysis of ester groups in self-assembled monol ayers (SAMs) were monitored by a combination of atomic force microscopy (AF M) on the nanometer scale and FT-IR spectroscopy in the continuum limit. Th e main objective was to study surface reactions in situ with chemical speci ficity, from the nanometer perspective, using an atomic force microscope. T his could not be achieved by conventional AFM friction or force measurement s due to insufficient resolution, and instrumental or thermal drift, respec tively. These problems were circumvented by a novel approach, which we term ed "inverted" chemical force microscopy (ICFM). In ICFM, chemical reactions , which take place at the surface of the tip coated with reactants, are pro bed in situ by force-distance measurements on a scale of less than 100 mole cules. The pull-off forces of different reactive SAMs were shown to vary wi th the extent of the reaction. Reactivity differences for these monolayers observed in this manner by AFM on the nanometer scale agree well with macro scopic behavior observed by FT-IR and can be related to differences in the SAM structure. These results, together with additional force microscopy dat a, support the conclusion that, for closely packed ester groups, the reacti on spreads from defect sites, causing separation of the homogeneous surface s into domains of reacted and unreacted molecules.