Atom-resolved surface chemistry studied by scanning tunneling microscopy and spectroscopy

We have used scanning tunneling microscopy and spectroscopy to study the reaction of Si(111)-(7×7) with ${\mathrm{NH}}_3$. We have found that by use of topographs obtained at different energies, as well as atom-resolved spectra, reacted and unreacted surface sites can be imaged selectively. Thus we have been able to probe the spatial distribution of the surface reaction on an atom-by-atom basis. We find that there are significant differences in reactivity between the various dangling-bond sites on the Si(111)-(7×7) surface. Specifically, rest-atom sites are more reactive than adatom sites and, moreover, center-adatom sites are more reactive than corner-adatom sites. We ascribe the reduced reactivity at adatom sites to the delocalized nature of their dangling-bond state. We suggest that a bonding interaction between adatoms and the Si atoms directly below them is responsible for this behavior—a suggestion supported by electronic-structure calculations. Thus, while reaction at a rest-atom site can be considered a dangling-bond saturation process, reaction at an adatom site involves the formation of a hypervalent (fivefold-coordinated) adatom. We tentatively ascribe the reactivity differences between center and corner adatoms to differences in the strain they induce upon reaction on the dimer bonds. Atom-resolved spectroscopy allows us to probe interactions and charge transfer between surface sites, and for the first time, we can directly observe how chemisorption affects the substrate electronic structure at neighboring unreacted sites.