Direct measurement of forces during scanning tunneling microscope imaging of graphite

The normal force acting on a scanning tunneling microscope tip while imaging a graphite surface in air has been measured directly. Forces in the range of 10-7 to 10-6 N are required to achieve tunneling. Further, the force needed to maintain a constant current varies considerably as the tip scans from one part of the graphite unit cell to another. Our results are consistent with a model, originally suggested by Mamin et al., in which the force between the tip and the surface is mediated by a contamination layer, and tunneling occurs at the end of an asperity which pierces this layer. However, we cannot rule out a model where a graphite flake is dragged across the graphite surface to generate an STM image. © 1989.