Contrast formation in atomic resolution scanning force microscopy on CaF2(111): experiment and theory

We investigate mechanisms of contrast formation in atomic resolution imagin g of flat terraces on the CaF2(111) surface with scanning force microscopy operated in the dynamic mode. Experimental results are interpreted with a t heory based on atomistic modelling. Experiments reveal characteristic contr ast features in the form of triangles that can be explained by theory as be ing due to the interaction of a positively terminated tip with fluorine ion s from two different sublattices. Results for a tip with negative terminati on are found not to be compatible with experiments. We demonstrate that the ory correctly predicts the trend in contrast changes when varying the tip-s urface distance but is also limited in quantitative agreement due to the no n-ideal atomic structure of real tips, In a distance range where such pecul iarities do not play a major role, however, we find good quantitative agree ment between theoretical predictions and experimental results, The validity of the comparison between theory and experimental scan lines is discussed in detail using an extensive statistical image analysis.