Scanning force microscopy with chemical specificity: An extensive study of chemically specific tip-surface interactions and the chemical imaging of surface functional groups

An extensive and systematic scanning force microscopy (SFM) study is presented. The observations are based on hydrogen bonding, van der Waals, and Coulombic interactions between the scanning probe (tip) and the substrate and provide the basis for scanning force microscopy with chemical specificity (SFMC).(1) The self-assembly (SA) of omega-functional n-alkanethiol compounds was used to chemically modify standard SFM probes and substrates with a variety of functional groups (CH3, OH, NH2, COOH, and CONH2). The Johnson-Kendall-Roberts (JKR) theory of adhesion mechanics provided the theoretical background for our measured data and enabled the calculation of surface free energies, the number of interacting molecules, and single-bond forces. Furthermore, a recently developed statistical analysis of the force distribution was applied to evaluate single-bond strengths and the number of molecules interacting between scanning probe and substrate. Good agreement was found between this analysis and the JKR theory. Furthermore, from the lateral force measurements the friction coefficients were determined and a direct relationship between friction and the measured adhesion forces was demonstrated. This relationship was used as the basis for chemically specific imaging of functional groups with predictable frictional force contrast. Imaging of up to three different functional groups on the same surface was demonstrated on chemically patterned surfaces. Experimentally, a chemical resolution of 100 nm in the lateral dimensions was achieved. pH-dependent adhesion force measurements were performed to study acid-base properties of surface-bound functional groups and were found to be a unique way to determine surface pK values. Finally, a new method was developed to differentiate and image the surface-bound functional groups on the basis of their acid-base properties.