Bifurcation Of Nanoscale Thermolubric Friction Behavior For Sliding On Mos2

We present atomic force microscopy experiments of wearless sliding between nanoscale tips and both bulk and monolayer MoS2 in ultrahigh vacuum across a wide range of temperatures (150-450 K) and scanning speeds (5 nm/s to 500 mu m/s). Atomic lattice stick-slip behavior is consistently resolved. However, a bifurcation of behavior is seen, with some measurements showing a strong decrease in friction with increasing temperature and others showing athermal and low friction under nominally identical conditions. The difference between thermal and athermal behavior is attributed to a change in the corrugation of the potential energy surface, possibly due to trace amounts of adsorbed contaminants. While the speed dependence at a given temperature is consistent with the thermal Prandtl-Tomlinson model for atomic-scale friction, that is not the case for the temperature dependence (when it is present), nor can the temperature dependence be described by other existing models. We discuss the limitations of these models considering the measured results.