Studying Surface Chemistry Atom-by-Atom Using the Scanning Tunneling Microscope

A solid surface provides a great variety of sites for a possible chemical reaction. A perfect surface may have several strucutrally inequivalent sites while a real life surface may have steps, a multitude of point and extended defect sites, adsorbed foreign atoms, etc. It is clear then that in order to even begin discussing the chemistry of a particular surface we have to have a good idea about its structure. This recognized need has led to the development of a great variety of surface structural techniques [16.1]. Most of these techniques provide information averaged over a macroscopic area of the sample defined by the size of the probe beam. Moreover, diffraction-based structural probes require long-range order which is not present in many cases of interest. Even when the structure of the surface is known, the understanding of the reactivities of the various sites requires that a correlation be made between the chemical behavior of a particular site and its local electronic structure. Thus, ideally one needs a surface technique or techniques which will allow the study of the topography and valence electronic structure of surfaces with atomic resolution.