Atomic force microscopy under liquid: A comparative study of three different AC mode operations

Summary The image contrast mechanism of three newly proposed AC mode operations under liquid in three different frequency ranges is presented. They all rely on a strong repulsive force to damp the cantilever and the tip still ‘touches’ the sample surface. A direct comparison of the three different modes of operation with the conventional DC mode technique using the same gold sample under isopropanol was conducted. It was found that all the three AC modes exerted a much smaller lateral force than the DC mode although the normal loads were of the same order of magnitude. The suitability of such techniques in imaging physisorbed systems on hard substrates (such as soft biological samples) and the prospect of a true non‐contact repulsive mode operation are discussed.