Detection mechanism of S-O tilt directions on CaSO4(100) by frictional force microscopy

Tilted S-O bonds on a CaSO4(100) surface exhibit asymmetry under frictional force microscopic observation. The detection mechanism of the directional bonds was explained as being an interaction between the probe surface and a n array of tilted mass-spring systems at the sample surface. When the scan speed of the probe was changed in the range of 5-91 mu m/s under 40% relati ve humidity, the asymmetric component of friction increased linearly with t he speed, while the total friction signal remained constant at speeds highe r than 25 mu m/s. A higher scan speed is more suitable fur the detection of the tilted bonds. At lower scan speeds, the total friction signal decrease d in intensity and the asymmetry signal became unstable. This is due to the condensation of water beneath the tip, A similar experiment under 60-65% r elative humidity yielded much smaller signals for both the total friction a nd the asymmetry component.