Polypyrrole Self-Assembled Monolayers and Electrostatically Assembled Multilayers on Gold and Platinum Electrodes for Molecular Junctions

Self-assembled monolayers (SAMs) and electrostatically assembled multilayers of some soluble polypyrroles (including a thiol-functionalized polypyrrole) were produced on gold and platinum electrodes. Gold and platinum surfaces were used either bare or primed with an anionic monolayer of 3-mercaptopropylsulfonate or Nafion. Poly(sodium-p-styrenesulfonate) and Nafion were used as polyanions for electrostatic self-assembly (ESA). The layers were investigated by cyclic voltammetry, infrared reflection absorption spectroscopy, quartz crystal microbalance, and atomic force microscopy (AFM). ESA proceeds with a growth rate that is linear with the number of bilayers and particularly high (stored charge 30−60 μC cm-2 bilayer-1) with Nafion, which appears to be the primer for all the investigated polypyrroles. Gold−polymer−gold junctions, formed by contact between a polypyrrole SAM on gold and a gold-coated AFM tip, rectify current in polypyrrole sulfosuccinate, whereas they are ohmic with Simmons characteristics in poly(N-hexylcyclopenta[c]pyrrole). In the former case alternation of rectifying and ohmic characteristics is observed with progressive ESA.