LOW-LOAD FRICTION BEHAVIOR OF EPITAXIAL C-60 MONOLAYERS UNDER HERTZIAN CONTACT

Monolayers of C-60 molecules epitaxially grown on GeS(001) in ultrahigh vacuum were investigated by friction force microscopy in air. The frictional force F-f on the GeS substrate was found to be proportional to the normal force F-n in the first approximation, whereas the friction measured on the first C-60 monolayer fits excellently to a F-f proportional to F-n(2/3) dependence. This different frictional behavior causes a flip in contrast in spatially resolved friction force maps. Additionally, the second C-60 monolayer exhibited a lower friction than the first layer at normal forces below the critical force where the contrast flips. The measured data are analyzed using a generalized Hertzian theory, which considers capillary condensation.