Evolution of surface to bulk tunneling spectrum by scanning tunneling microscopy

By decreasing tip-to-surface separation on the Si (111) -7 x 7 surface, we have observed symmetry change in topographic images and evolution from surfacelike to bulklike energy spectra. Because the tunneling is via Si adatoms interfacing to the bulk, the barrier heights at Si atoms of different dangling bonds are inhomogeneous. The observed phenomena can be numerically explained by a model of shifting chemical potential of the surface electrons due to the competing coupling strengths between the states of the tip, the surface, and the bulk. It leads to the conclusion that we have probed the interfacial coupling between electronic states of surface atoms and the bulk with atomic resolution.