Sliding friction between the tip of a friction force microscope and NaCl(100) was studied to deduce the velocity dependence of friction forces on the atomic scale. A logarithmic dependence of the mean friction force is revealed at low velocities. The experimental data are interpreted in terms of a modified Tomlinson model which is based on reaction rate theory.

Atomically resolved dynamic force microscopy ͑DFM͒ images of step and kink sites of NaCl films grown on the Cu͑111͒ surface are presented. Combining experimental results with an atomistic modeling of DFM imaging, we study the mechanism of contrast formation and extract more information about the tip and NaCl film structure. The experimental results and theoretical modeling systematically demonstrate the enhanced interaction of step and kink sites of one kind with the tip. This is explained by the enhanced gradient of the electrostatic potential at low-coordinated surface sites, and considerable displacements of the step edge and kink atoms from their sites due to the interaction with the tip upon approach. The theoretical analysis predicts that the silicon tip is effectively an insulator, and that the NaCl island cannot be thicker than two monolayers. We discuss the shape and chemical structure of the tip and the mechanism of damping during DFM imaging.

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Using a noncontact atomic-force and scanning-tunneling microscope in ultrahigh vacuum, we have measured the switching energy of a single molecule switch based on the rotation of a di-butyl-phenyl leg in a Cu-tetra-3,5 di-tertiary-butyl-phenyl porphyrin. The mechanics and intramolecular conformation of the switched leg is controlled by the tip apex of the noncontact atomic-force microscope. The comparison between experimental and calculated force curves shows that the rotation of the leg requires an energy less than 100 x 10(-21) J, which is 4 orders of magnitude lower than state-of-the-art transistors.

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