Imaging of soft structures: Dependence of contrast in atomic force microscopy images on the force applied by the tip

Forces acting on atomic force microscope tips during scanning of films of ionic surfactant molecules adsorbed from aqueous solutions onto hydrophilic substrates are measured. Near critical micellar concentration images of mica substrates show aggregate regions at the interface. Force versus distance curves indicate that aggregates are the thickest adsorbed structures on the substrate. However, topographic images registered at low scanning speed (15 μm/s) show that these aggregates appear as holes, consequently observed as inverted in contrast images. In atomic force microscope imaging of soft structures such as surfactants or biological material, inverted images may be observed when the tip penetrates the scanned layers. This penetration can be adjusted by changing the force applied by the tip, which results in different images. In order to obtain the conventional atomic force microscope contrast in scanned images the applied force set point is determined by the analysis of the force versus distance curves.