AFM observation of monatomic step movements on NaCl(001) with the help of adsorbed water

AFM observation of cleaved NaCl(001) surface in air at room temperature revealed spontaneous motion of monatomic steps, depending on relative humidity, to minimize one-dimensional surface free energy. While step motion was recognized only at the parts having large positive curvature at 52% humidity, even straight steps moved at 57%, The motion was accelerated toward higher humidity. It is suggested that adsorbed water having 2-dimensional nature transports ions at the surface. By holding the AFM tip at one place, salt solution was collected beneath the tip by capillary action. Upon removal of the tip, the solution droplet turned into a bell-shaped hillock of salt as high as 78 nm. Although the hillocks gradually flattened spontaneously, the method has potential application in pattern formation.