Chemically-Sensitive Imaging in Tapping Mode by Chemical Force Microscopy: Relationship between Phase Lag and Adhesion

Tapping mode atomic force microscopy has been used to record phase-lag images of patterned self-assembled monolayer (SAM) surfaces in alcohol−water solutions using probe tips functionalized with SAMs that terminate in distinct chemical groups. We find that phase contrast between chemically distinct monolayer regions and adhesion forces are directly correlated: increasing adhesion forces lead to increases in the phase lag. Data recorded in alcohol−water mixtures have been analyzed using a driven oscillator model and show that differences in phase shift between distinct regions of the patterned SAMs can be quantitatively related to differences in the work of adhesion, Wst. Because the adhesion forces are readily interpretable on the basis of surface chemical functionality, our results demonstrate that intermittent contact or tapping mode force microscopy can be used to image samples with chemical sensitivity. The implications and potential applications of this new form of chemical force microscopy are discussed.