Effect of lateral tip motion on multifrequency atomic force microscopy

In atomic force microscopy (AFM), the angle relative to the vertical ($\theta_{i}$) that the tip apex of a cantilever moves is determined by the tilt of the probe holder and the geometries of the cantilever and actuated eigenmode $i$. Even though the effects of $\theta_{i}$ on static and single-frequency AFM are known (increased effective spring constant, sensitivity to sample anisotropy, etc), the higher eigenmodes used in multifrequency force microscopy lead to additional effects that have not been fully explored. Here we use Kelvin probe force microscopy (KPFM) to investigate how $\theta_{i}$ affects not only the signal amplitude and phase, but can also lead to behaviors such as destabilization of the KPFM voltage feedback loop. We find that longer cantilevers and modified sample orientations improve voltage feedback loop stability, even though variations to scanning parameters such as cantilever shake amplitude and lift height do not.