Mechanisms and energetics of site hopping and chemical reactions of O 2 molecules at Si(111)-7 × 7 surfaces

Abstract

With continuous-time imaging using a variable-temperature scanning tunneling microscope (STM), we observe the dynamic behavior of a bright species, which we believe to be single O 2 molecules on Si(111)-7 × 7 surfaces. We have found the hopping motion of this molecular species between neighboring adatom sites, which is mediated by two intermediate states. A model is proposed to explain the detailed mechanism of the site hopping. Using Arrhenius plots, we determine the activation energies of different hopping paths and the relative binding energies of different states. We also observe the chemical reaction process of two adsorbed O 2 molecules to form an atomic species. Our results reveal that the hopping motion and chemical reactions of oxygen molecules depend strongly on the configuration of dangling bonds at the surface and the oxygen bonding. This study provides a direct view of molecular dynamics at a solid surface.