Non-Contact Atomic Force Microscopy (nc-AFM)
In Non-Contact Atomic Force Microscopy (nc-AFM), the tip at the end of the AFM cantilever does not have any physical contact to the probed surface. Although there is no direct bending of the tip, as in contact AFM, one can still obtain information about the topology in such a configuration. This is due to the fact that the forces between probe and sample are still present even without direct contact, following a force gradient along the distance between tip and sample. Due to the relatively large distance, a direct effect, such as bending of the cantilever, cannot be observed. However, if the cantilever is excited to oscillate perpendicular to the surface, even weak normal forces (e.g. van-der-Waals forces) can have a profound impact on the oscillation characteristics such as amplitude and resonance frequency. The change of these characteristics as a function of the lateral x-y position of the oscillating tip can be used to construct an image of the sample topography.