Velocity Dependence and Limitations of Friction Force Microscopy of Mica and Graphite

We investigate the behavior of a friction force microscope tip on a crystal at tip velocities above 1 µm/s. Cleaved mica and highly oriented pyrolytic graphite (HOPG) are observed by a friction force microscope in ambient operation. A regular saw-toothed signal corresponding to atomic-level stick-slip is observed in each case at tip velocities lower than 1.5 µm/s. However, in the case of observing mica at tip velocities above 1.5 µm/s, regularity of the signal becomes unstable. Sometimes no saw-toothed signal is observed and even if there were a saw-toothed signal, the pitch of the saw-toothed signal would be larger than that corresponding to the crystal structure. In the case of observing HOPG, clear regularity is detected at tip velocities above 20 µm/s. The difference is considered to be caused by the hydrophilic/hydrophobic property of each crystal. For HOPG, the mechanical property of the cantilever is the limitation. For a 200-µm-long and 20-µm-wide rectangular cantilever, 95 kHz and 417 kHz spectrums, corresponding to the natural frequency of the cantilever for deflection and torsion with the tip in contact, start to mask the stick-slip signal as the scanning velocity is increased.