Characterization of atomic step structures on CaF₂(111) by their electric potential

The structure and polarity of step edges on cleaved CaF₂(111) are investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy. Ledges produced by cleaving the crystal appear with two distinctly different polarities denoted as type I and type II arising from the sectioning of ledges with steps having different polarities. With respect to the stoichiometric terrace, the surface potential is slightly reduced at ledges predominately composed of type I steps, while the potential of ledges predominantly composed of type II steps is significantly higher (typically 100 mV). We propose that the positive potential of type II steps stems from low coordinated Ca²⁺ ions inducing a dipole at step edges and confirm this by atomically resolved NC-AFM images revealing the Ca²⁺ ion sub-lattice with repulsive-mode imaging contrast.