Subsurface-AFM: sensitivity to the heterodyne signal

Applying heterodyne force microscopy (HFM), it has been impressively demonstrated that it is possible to obtain subsurface information: 20 nm large gold nanoparticles that were buried 500 nm deep have been imaged. It is the heterodyne signal that contains the subsurface information. We elucidate, both theoretically and experimentally, the sensitivity to the heterodyne signal as a function of the tip–sample distance. This is crucial information for experiments as the distance, and therefore the sensitivity, is tunable. We show that the amplitude of the heterodyne signal has a local maximum in the attractive part of the tip–sample interaction, before it surprisingly reaches an even higher plateau, when the tip–sample interaction is repulsive. This can only be explained by a non-decreasing amplitude of the ultrasonic motion of the tip, although it is in full contact with the surface. We confirm this counterintuitive tip behavior experimentally even on a hard surface like silicon. (paper)