Imaging Surface Electronic Structure with STM
Because the trajectory executed by an STM tip is determined by the overlap of the electronic-state density of the tip with the local electronic-state density of the surface, the STM can be used to extract chemical information associated with a particular surface structure. On the Si(l00) surface, for example, dimerization results in a concentration of valence electron density between dimerized atoms (bonding orbitals) and a depletion of valence electron density in "anti-bonding" orbitals, of which there is one for each surface atom. In the model below, which presents the bonding geometry of a pair of dimers on the surface of Si (100), the bonding orbitals are represented in red, and the antibonding orbitals in gold.
If the STM is operated with a positive tip-to-sample junction, so that electrons must tunnel from the surface to the tip, the concentration of surface valence electron density within dimers causes the STM image to display rows of dimers, as in image (a) below. The individual dimers, displayed in red in image (a) correspond to the dimer bonds shown in red in the model. If the polarity is reversed, and electrons tunnel from the tip to the antibonding orbitals of the surface, then the resulting image (b) will enhance individual atoms. Each gold spot in image (b) then corresponds to one antibonding orbital, which in turn is associated with a single surface atom, as shown in the model.