Imaging defects on CaF2(111) surface with frequency modulation atomic force microscopy

The cleaved (111) surfaces on CaF2 were imaged by frequency modulation atomic force microscopy (FM-AFM) in ultrahigh vacuum at room temperature. Hi-h-resolution imaging showed the perfect atomic periodicity of a F- sublattice in the outermost layer or a Ca2+ sublattice in the second layer. Gentle contact between a modified Si tip with a CaF2 nanocluster and the CaF2(111) surface induced formation of atomic size defects on the perfect atomic periodicity. We observed two types of atomic contrast on the point defect during successive imaging. In a constant-height FM-AFM image, one appeared as a dark spot corresponding to a region of weaker attraction between the tip and the surface, while the other appeared as a bright spot. The change in the atomic contrast on the point defect call be explained by the change in the sign of a tip-terminating ion.