A calibration error revealed via local tip position detection in atomic force microscopy

Atomic Force Microscopy (AFM) is a versatile tool in nanoscience. In conventional AFM, knowledge of the local 3D tip position is not accessible and tip trajectories are extrapolated from the cantilever deflection ($\Delta Z)$ which provides data of reduced dimensionality. The sensitivity (nm/V) of $\Delta Z$ is calibrated by taking slope of $\Delta Z $curve when the tip makes contact to a surface. Using a focused laser beam directly focused on the apex of the AFM tip, we have measured 3D positions of the tip as it interacts with a sample surface in fluid. We have observed a significant difference between the slope of ($\Delta Z)$ and that of the $Z$-tip position. This implies an erroneous calibration of sensitivity of $\Delta Z$ detection which we can now correct. Also, we have observed significant lateral slipping of tip as it touches the surface. These observations provide a comparison between tip and cantilever dynamics.