Chemical Force Microscopy - Frictional Force Mapping

Frictional Force Mapping

Chemical interactions can also be used to map prepatterned substrates with varying functionalities (see Figure 3). Scanning of a surface having varying hydrophobicity with a tip having no functional groups attached would produce an image with no contrast because the surface is morphologically featureless (simple AFM operation). Functionalizing a tip to be hydrophilic would cause the cantilever to bend when the tip scans across hydrophilic portions of the substrate due to strong tip-substrate interactions. This is detected by laser deflection in a position sensitive detector therefore producing a chemical profile image of the surface. Generally, a brighter area would correspond to a greater amplitude of deflection so stronger bonding corresponds to lighter areas of a CFM image map. When the cantilever functionalization is switched such that the tip is bent when encountering hydrophobic areas of the substrate instead, the complementary image is observed.

Frictional force response to the amount of perpendicular load applied by the tip on to the substrate is shown in Figure 4. Increasing tip-substrate interactions produce a steeper slope, as one would expect. Of experimental importance is the fact that contrast between different functionalities on the surface may be enhanced with an application of greater perpendicular force. Of course, this comes at the cost of potential damage to the substrate.