Correct Height Measurement in Noncontact Atomic Force Microscopy

We demonstrate that topography measurements by noncontact atomic force microscopy are subject to residual electrostatic forces. On highly oriented pyrolitic graphite (HOPG) with a submonolayer coverage of ${\mathrm{C}}_{60}$, we monitor the step height from ${\mathrm{C}}_{60}$ to HOPG as a function of dc bias between tip and sample. Because of the different contact potential of ${\mathrm{C}}_{60}$ and HOPG ($\sim 50\mathrm{m}\mathrm{V}$), the step height is strongly dependent on the dc bias. The presented results and additional simulations demonstrate clearly that for correct height measurements it is mandatory to use a Kelvin probe force microscopy method with active compensation of electrostatic forces.