The electroluminescence and scanning tunneling microscopy of single molecules

The scanning tunneling microscopy (STM) of single molecules has become a prominent experimental method in the field of molecular electronics. It has been found that in STM experiments, when an electric current flows through a single molecule, the molecule may luminesce. This electroluminescence, in conjunction with traditional STM data, provides a potentially important additional degree of freedom for understanding nanoscale systems. This thesis describes exploratory theoretical work on the newly emerging phenomenon of molecular electroluminescence, and its relationship to the scanning tunneling microscopy of single molecules. A local electrode framework is progressively developed, in order to explain single-molecule electroluminescence data as well as simulating STM current-voltage characteristics and topographic maps for molecules on complex substrates. The molecule Zn(II)-etioporphyrin I is chosen for detailed analysis and comparison with experiment. Electron transport is studied using Landauer theory that relates electric current to the transmission probability for an electron to scatter through the molecule. The theoretical approach utilizes tight binding and extended Huckel approaches for the electrodes and molecule, a charge-conserving scheme to self-consistently model the influence of electric fields and electric currents on the molecular energy level structure, and Fermi's golden rule in calculating electroluminescence. A single coherent framework is ultimately achieved that explains for the first time both the electric current data and molecular electroluminescence in a molecular system and elucidates the physics underlying a rich and previously puzzling array of interlinked optical and transport phenomena.