Mechanisms of formation of phase contrast in tapping mode atomic force microscopy

Abstract

Phase imaging for hard materials with atomic force microscope in tapping mode has been analyzed. It was shown by the example of imaging diamond-like carbon coating that the cantilever probes should have spring constant higher than 100 N/m to obtain reliable data. Experiments showed that the oscillating probe can operate in different regimes depending on the cantilever stiffness and set-point amplitude. The regimes are characterized by the interplay between the attractive and repulsive interactions between tip and sample. Repulsive condition of intermittent contact oscillation is effective for obtaining structural information of hard materials. The regime corresponding to the mixed attractive-repulsive tip-sample interaction provides the highest phase contrast. However, the operation in this regime produces significant artifacts in height imaging. A complex parameter is suggested to estimate the effect of contact adhesion in the tip-sample interaction. The parameter takes into account the cantilever characteristics, its oscillation condition and surface energy of the sample material. It can be used to define the regime of a cantilever oscillation during scanning in tapping mode.