  The nanofriction of Xe monolayers deposited on graphene was explored with a
quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene
was grown by chemical vapor deposition and transferred to the QCM electrodes
with a polymer stamp. At low temperatures, the Xe monolayers are fully pinned
to the graphene surface. Above 30 K, the Xe film slides and the depinning onset
coverage beyond which the film starts sliding decreases with temperature.
Similar measurements repeated on bare gold show an enhanced slippage of the Xe
films and a decrease of the depinning temperature below 25 K. Nanofriction
measurements of krypton and nitrogen confirm this scenario.This thermolubric
behavior is explained in terms of a recent theory of the size dependence of
static friction between adsorbed islands and crystalline substrates.
