Low-load friction behavior of epitaxialC60monolayers under Hertzian contact

Monolayers of ${\mathrm{C}}_{60}$ molecules epitaxially grown on GeS(001) in ultrahigh vacuum were investigated by friction force microscopy in air. The frictional force ${\mathit{F}}_{\mathit{f}}$ on the GeS substrate was found to be proportional to the normal force ${\mathit{F}}_{\mathit{n}}$ in the first approximation, whereas the friction measured on the first ${\mathrm{C}}_{60}$ monolayer fits excellently to a ${\mathit{F}}_{\mathit{f}}$∝${\mathit{F}}_{\mathit{n}}^{2/3}$ dependence. This different frictional behavior causes a flip in contrast in spatially resolved friction force maps. Additionally, the second ${\mathrm{C}}_{60}$ 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.