Atomic force microscopic observation of step movements on NaCl(001) and NaF(001) with the help of adsorbed water

Cleaved (001) faces of NaCl, NaF and LiF single crystals have been observed with atomic force microscopy in air. Spontaneous movements of monatomic steps were observed, depending upon relative humidity, with the first two crystals. The step speed on NaCl showed a marked increase at about 60%, while only a gradual increase was observed with NaF. The step movements on NaCl were not disturbed very much by screw dislocations, but were substantially retarded by surface voids. The presence of adsorbed water in two-dimensionally and three-dimensionally condensed phases is a reasonable explanation for these effects. By fixing the AFM tip on flat terraces, crystalline blocks were formed on the NaF surface. They grew spontaneously, collecting salt from flat parts. The salt is pumped from the substrate to the top of the blocks. A mechanism to explain this is proposed here.