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

Monolayers of C-60 molecules epitaxially grown on GeS(001) in ultrahig h vacuum were investigated by friction force microscopy in air. The fr ictional force F-f on the GeS substrate was found to be proportional t o the normal force F-n in the first approximation, whereas the frictio n measured on the first C-60 monolayer fits excellently to a F-f propo rtional to F-n(2/3) dependence. This different frictional behavior cau ses a flip in contrast in spatially resolved friction force maps. Addi tionally, the second C-60 monolayer exhibited a lower friction than th e first layer at normal forces below the critical force where the cont rast flips. The measured data are analyzed using a generalized Hertzia n theory, which considers capillary condensation.