Effect of tip sharpness on the relative contributions of attractive and repulsive forces in the phase imaging of tapping mode atomic force microscopy

The way in which the sharpness of a tip affects phase imaging in tapping mo de atomic force microscopy was examined by performing experiments on polyst yrene and Si surfaces with tips of different sharpness. The dominant tip-sa mple interaction controlling the cantilever vibration can be made repulsive with a sharp tip, and attractive with a dull tip. When the dominant intera ction is either attractive or repulsive, the magnitude of the phase shift ( Delta phi) increases with decreasing set-point ratio r(5p)=A(sp)/A(0), and the Delta phi-vs.-r(sp) plots are nearly independent of the drive amplitude , A,. These observations indicate that the cantilever vibration is affected mainly by the amount of time the tip spends in going through the lower tur ning point of each oscillation and by the forces whose force derivatives ch ange sharply during this period. (C) 1999 Elsevier Science B.V. All rights reserved.