Surface Double Layer on (001) Surfaces of Alkali Halide Crystals: A Scanning Force Microscopy Study

In this Letter we consider the surface double layer on (001) surfaces of UHV cleaved and annealed alkali halide crystals (KCl, NaCl), which we studied with dynamic scanning force microscopy and Kelvin probe force microscopy. Kelvin images show bright and round patches at corner sites and steps. Images with atomic resolution always show kinks at the latter step sites. Our findings are in perfect agreement with the long-standing picture that, due to the presence of divalent impurity ions, the surface carries a net negative surface charge originating from negative cation vacancies at kinks.