6. Artificial structures

Near-field microscopy techniques involve scanning a tip that is virtually in contact with the surface. The close proximity of the tip to the surface can lead to local, atomic-scale modifications of the surface structure. Such modifications are occasional and generally to be avoided when recording images of surface structure. On the other hand, the very strong interaction between the tip and the surface can be used to intentionally modify a local structure. It's easy to understand how much a tunneling microscope tip can disrupt a surface, if you consider that the electric fields between tip and surface are typically 10 ⁷ to 10 ⁸ V · cm ^–1 and that, although the tunnel currents are only a few nanoamperes, the associated current density is of the order of...