Atomic force microscopy and friction force microscopy studies of ferroelastic crystal surfaces

Topography and frictional properties of freshly cleaved surfaces of ferroelastic crystals: K3Na(SeO4)2 (KNSe), and NH4LiH3(SO4)4 (ALHS), and Gd2(MoO4)3 (GMO) were investigated by combined scanning and friction force microscopy (FFM) under ambient and UHV conditions. Atomic force microscopy (AFM) images revealed stepped surfaces with step heights corresponding to the multiplication height of crystal lattice constant fractions e.g., c/2 or c/4. A frictional contrast was observed between terraces which are separated by steps and between different domains of ferroelastic crystals. We suggest that the frictional contrast is due to different molecular orientations at different terraces and/or at different domains of a ferroelastic crystal, in the case of GMO, surface bends at the domain boundary with the angle estimated to be about 2.35°. It was also shown that the ALHS crystal surface can be modified (e.g., by changing the step profile and creation of the artificial bumps) by an AFM tip imaging with normal force above 100 nN.