Kelvin probe force microscopy on surfaces of UHV cleaved ionic crystals.

Force spectroscopy and Kelvin probe force microscopy (KPFM) measurements taken on (001) surfaces of UHV cleaved NaCl, KCl and MgO are presented for the first time. With the help of force spectroscopy we show first that the charging of (001) surfaces of alkali halide crystals, which generally occurs after UHV cleavage, vanishes after a couple of days due to their sufficiently high ionic conductivity at room temperature. KPFM images of these (001) surfaces show that the surface potential is not uniform but exhibits variations of up to 1 V at a nanometre length scale. Variations on terraces as well as a strong contrast at step edges can be observed, of which the latter is probably due to trapped charges. On MgO(001), we observe strong changes in the surface potential, especially at previously reported adstructures. These changes explain why imaging MgO(001) is difficult.