Preparation and characterization of ordered Poly(3,4-Ethylenedioxythiophene) monolayers on Au(111) surfaces

A novel method is developed to prepare orderly arranged mono-molecular layers of poly(3,4-ethylenedioxythiophene) (PEDOT) by surface polymerization of adsorbed 3,4-ethylenedioxythiophene (EDOT) monomers on Au(111) surfaces. To decrease the EDOT-substrate adhesion force, obtaining a highly ordered EDOT monolayer, the adsorption is performed in a phosphate buffer solution (PBS), rather than the acid solutions commonly utilized in the literature. Furthermore, potentials are applied on the electrode to regulate the adsorption rate of EDOT, and to simultaneously control the adsorption/polymerization mechanism of EDOT molecules. According to the observation of an in-situ electrochemical scanning tunneling microscopy (EC-STM), a highly ordered EDOT monolayer can be prepared by performing the adsorption in a PBS solution, as well as by slowly increasing the electrode potential, attributed to the slow adsorption of EDOT. In the following polymerization of the EDOT monolayer, if the reaction is performed at a constant potential (0.5 V vs. Ag/AgCl reference) or in an acid solution, the EDOT in the solution will take part in the reaction on the surface, leading to a disordered and multilayered structure of PEDOT film. Alternatively, by applying cyclic potentials between 0.0 and 0.5 V, as well as the utilization of a PBS solution, the polymerization could perform only on the pre-adsorbed EDOT monolayer, and an ordered PEDOT monolayer can be prepared. Impedance spectroscopy analysis indicates that the ordered PEDOT monolayer has a charge transfer resistance not only much lower than that of an EDOT monolayer, but also lower than that of the disordered PEDOT multilayer.