Unambiguous Interpretation of Atomically Resolved Force Microscopy Images of an Insulator

The (111) surface of $\mathrm{CaF}{}_2$ was imaged with dynamic mode scanning force microscopy and modeled using atomistic simulation. Both experiment and theory showed a clear triangular contrast pattern in images, and theory demonstrated that the contrast pattern is due to the interaction of a positive electrostatic potential tip with fluorine ions in the two topmost surface layers. We find a good agreement of position and relative height of scan line features between theory and experiment and thus establish for the first time an unambiguous identification of sublattices of an insulator imaged by force microscopy.