NOVEL SURFACE CHEMISTRY OF SINGLE MOLECULES AND SELF-ASSEMBLED STRUCTURES BY SCANNING TUNNELING MICROSCOPY

by Maksymovych, Petro

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 singlemolecule 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 atomicallylocalized 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.