Novel surface chemistry of single molecules and self-assembled structures by scanning tunneling microscopy

This thesis demonstrates the richness of Scanning Tunneling Microscopy (STM) as a method to understand surface chemistry and physics and to explore the new frontiers in single-molecule surface reactions and molecular self-assembly. Organosulfur molecules on the Au(111) surface were studied to address unresolved and controversial issues about self-assembled monolayers of alkanethiol molecules on gold surfaces. The key new finding is that the thermal surface chemistry of alkanethiol molecules occurs in a dynamic chemical environment that involves reactive gold adatoms to which the alkanethiol molecules chemically bond. The problem of alkanethiol self-assembly is thus transformed from the realm of adsorption on a surface toward organometallic surface chemistry, which is anticipated to have broad implications for the field. Molecules containing a disulfide (S-S) bond were also found to be a spectacular model system for exploring electron-induced surface chemistry. In particular, the atomically-localized injection of electrons from the metal tip of the tunneling microscope is capable of producing highly delocalized chemical reactions by means of surface current of hot-electrons. Chemical reactions can therefore be a unique approach to the measurement of the local transport of hot-electrons on metal surfaces. Finally the concepts of self-assembly and electron-induced chemistry are combined through an observation of an unusual process that flips the chirality of molecules self-assembled on the surface by a radical-like chain reaction. This experiment demonstrates how self-assembly enables a new reaction coordinate by optimizing the steric factor of the chemical reaction.