Orientation-dependent friction between a carbon nanotube and graphite

We performed experiments to measure the friction force against the relative orientation between the carbon nanotube probe and graphite substrate. As the sample stage revolved one full circle, six sharp peaks were registered and each separated by 60�. Furthermore, these six peaks formed into three pairs, exhibiting the threefold symmetry, which can be explained with contribution beyond the top layer of the graphite. Yet, due to the prominent layer structure of the graphite, the energy loss caused by friction was dissipated mostly through its top layer