Tapping scanning force microscopy in air-theory and experiment

Ultrathin layers of micelles of a diblock copolymer with a polystyrene corona and a poly(2-vinylpyridine) core have been studied by tapping scanning force microscopy in air, probing the surface with varying forces depending on the setpoint of the probe and the tapping frequency. The compliance of the core of the micelles was varied by neutralization of the pyridine groups with HAuCl4 and incorporation of small particles. The apparent deformation of the globular micelles was compared with a simple model describing the probe as a forced oscillator which changes its effective spring constant depending on the direct contact with the surface. Consistent with the experiment, the model shows that the deformation and the shift in phase are minimized by tapping on the low-frequency side of the noncontact cantilever resonance.