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.

• Received 6 August 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.266101