Hitoshi Shindo, Kozo Shitagami, Takashi Sugai and Sei-ichi Kondo
Frictional force microscopic observation of directional CaSO4(100), SrSO4(001) and BaSO4(001) surfaces showed their asymmetric nature due to the orientations of the sulfate ions. Friction levels change at monolayer steps where S–O tilt directions are reversed. The absolute tilt directions were determined by the shapes of monolayer etch pits. As intuitively expected, scanning the probe against the ‘nap’ of the surface gives higher friction, which can be explained by the larger force constant of the S–O stretching vibrations than bending modes.