Stripping voltammetry of Cu overlayers deposited on self-assembled monolayers: Field emission of electrons through a phenylene ethynylene oligomer

Copper overlayers were formed on electrodes coated with self-assembled mono layers (SAMs) of a molecular wire candidate, 1-thio-4-[4'-[(4'-thio)phenyle thynyl]-1'-ethynyl]-2',5'-(diethyl)phenylbenzene (TTEB), by electrochemical reduction of copper ions and by physical vapor deposition. Anodic strippin g voltammetry of copper was employed to study the electrochemical and elect ron transport properties of SAMs of these molecules. Electrochemical copper deposition revealed that SAMs of TTEB passivate the electrode to electroch emistry in a manner similar to alkanethiols. Migration of copper ions trapp ed within the TTEB SAM after copper oxidation was also observed. Reduction of the solvent prevented the application of a sufficient potential to depos it copper by conduction through the TTEB SAM, so evaporation of copper meta l was employed to coat the entire electrode. Anodic stripping voltammetry f irst removed the metal from defect sites, leaving behind copper islands con nected to the gold by TTEB molecules. At higher potentials the copper islan ds were oxidized via electron transport through the TTEB SAM. A barrier hei ght of 1.17 eV to charge injection was calculated from the tunneling curren t and overpotential by a Fowler-Nordheim-type analysis. Tunneling currents in TTEB molecules with metal contacts were found to be dominated by hole tr ansport.