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 monolayers (SAMs) of a molecular wire candidate, 1-thio-4-[4'-[(4'-thio)phenylethynyl]-1'-ethynyl]-2',5'-(diethyl)phenylbenzene (TTEB), by electrochemical reduction of copper ions and by physical vapor deposition. Anodic stripping voltammetry of copper was employed to study the electrochemical and electron transport properties of SAMs of these molecules. Electrochemical copper deposition revealed that SAMs of TTEB passivate the electrode to electrochemistry in a manner similar to alkanethiols. Migration of copper ions trapped within the TTEB SAM after copper oxidation was also observed. Reduction of the solvent prevented the application of a sufficient potential to deposit copper by conduction through the TTEB SAM, so evaporation of copper metal was employed to coat the entire electrode. Anodic stripping voltammetry first removed the metal from defect sites, leaving behind copper islands connected to the gold by TTEB molecules. At higher potentials the copper islands were oxidized via electron transport through the TTEB SAM. A barrier height of 1.17 eV to charge injection was calculated from the tunneling current and overpotential by a Fowler-Nordheim-type analysis. Tunneling currents in TTEB molecules with metal contacts were found to be dominated by hole transport.