Frictional force microscopic detection of anisotropy at NaCl (1 0 0), (1 1 0) and (1 1 1) surfaces

Frictional coefficients were measured at atom-flat NaCl (1 0 0), (1 1 0) and (1 1 1) surfaces with frictional force microscopy. At the electrically neutral (1 0 0) and (1 1 0) surfaces in 15% relative humidity, lower friction was detected along 1 1 0 direction where the same kind of ions are aligned, than along 1 0 0 direction where different ions alternate. Low and isotropic friction was observed at the polar (1 1 1) surface. The results were explained by the gradients of electrostatic interaction potentials between a dipole at the tip and the ions at the surface. Toward higher relative humidity up to 60%, frictions at the (1 0 0) surface decreased indicating lubrication effect by water. The changes observed for the (1 1 0) surface suggested presence of adsorbed water in stripe pattern along the 1 1 0 direction. In the higher humidity range, frictional force increased at all the surfaces, probably due to larger normal force caused by the meniscus force.     

STM study of the structure of the sulphur (1×2) overlayer on molybdenum (001) in air: ordered domains,phase boundaries and defects

A (001) surface of molybdenum, covered by one monolayer of sulphur was prepared in UHV and characterized by LEED, Auger and XPS. This surface was found to be stable in air for periods of several days. STM images of the surface, obtained in air in the topographic and local barrier height modes, revealed the atomic arrangement of sulphur atoms in domains with 1×2 and 2×1 periodicities. Boundaries between domains, adsorbate and substrate defect structures and crystallites formed during the initial oxidation of the Mo substrate were observed.