Depth sensing and dissipation in tapping mode atomic force microscopy

Tapping mode atomic force microscopy is frequently used to image the surface of soft materials; it is also a powerful technique for nanomechanical analysis of surfaces. We report here an investigation of the depth sensing of the method on soft polymers. The chosen approach is based on the analysis of phase images of a model filled elastomer material. It leads to the determination of the depths of the hard particles lying under the surface. We found that tapping mode can probe interfaces buried under up to 80 nm of polymer. Under given tapping conditions, the penetration depth of the tip into the polymer is observed to depend on the layer thickness. However we show that, for a given penetration depth, the dissipated energy is independent of the thickness of the polymer layer under the tip. This suggests that the phase signal does not originate in the bulk viscoelasticity of the elastomer. Our observations support the hypothesis that, in tapping mode experiments on elastomers, the phase signal has an adhesive origin. Then, on surfaces with uniform interfacial properties, the phase images may reflect the local elastic properties of the sample, since they modify the tip-surface adhesive interactions. (C) 2004 American Institute of Physics.