Noncontact Atomic Force Microscopy

The atomic force microscope (AFM), invented by Binnig et al. in 1986 [1], has been used for developing a novel technique for obtaining high-resolution surface images of both conductors and insulators. For several layered and nonlayered materials [2–5], atomic resolution has been achieved in the contact mode. However, the question has been raised as to whether the AFM operating in the contact mode is really a microscope like the scanning tunneling microscope with a “true” atomic resolution. That is, most of the reported data obtained with the AFM has shown either perfectly ordered periodic atomic structures or defects only on a large lateral scale [4], but no atomic-scale defects routinely observed by a scanning tunneling microscope (STM). Moreover, the usual contact mode imposes a repulsive force of 0.5–5 nN [4,6] or higher [5,6], which is greater than the ~0.1 nN acceptable for a single atom on the tip apex of an AFM cantilever and for a single atom on the sample surface. Furthermore, the AFM in the contact mode mainly measures so-called atomic-scale friction with a lattice periodicity [6] rather than the topography. The contact-mode AFM thus seems to have a large contact area between the tip and the sample [6], although in a few experiments measured under restricted conditions such as in solutions [7,9] or at low temperature [8], monoatomic step lines [7–9] and atomic-scale point defects [9] could be observed even in the contact mode. Research into the true atomic resolution of the AFM has therefore focused on resolving two problems. These are (1) to observe atomic-scale defects and (2) to observe a Si(111)7×7 reconstructed surface which is the standard sample of the ultra-high vacuum STM (UHV-STM).