Scanning tunneling microscopy: A chemical perspective

In this review article, scanning tunneling microscopy (STM) is presented in a chemical perspective. The typical distance from the nucleus of the apex atom of the tip to the top-layer nuclei of the sample is 4-6 Å, where a strong attractive atomic force, i.e., a partial covalent bond, arises between the tip and the sample. The origin of the covalent bond is the back-and- forth transfer of electrons between two atoms, which Pauling has called resonance. While a bias voltage is applied between them, a net electron current in a specific direction arises. This tunneling current is a result of the overlap of the tip electronic state and the sample electronic state, same as the chemical bond. The imaging process of STM can be considered as a sequence of local bond forming and bond rupturing. A quantitative understanding of the STM imaging mechanism can be achieved in such a chemical perspective. A natural consequence of this perspective is that the tip, partially bonded with the sample, can play an active role in local chemical reactions. The tip can either involve directly in a chemical reaction with the atoms on the sample surface or induce local chemical reactions on the sample surface as a local catalyst.