XPS study of layer-by-layer deposited polypyrrole thin films

Layer-by-layer deposition of thin films (a few nm) of polypyrrole (PPY) was carried out on various substrates such as Ag, Pt, electrochemically oxidized Al and pretreated glass. Scanning electron microscopy showed that the deposited layers nucleate by an island-type mechanism and show first disjointed hillocks on hydrated alumina and KOH-pretreated (hydrophilic) glass before forming a continuous film, whereas continuous thin films are obtained on chromic-acid-pretreated (hydrophobic) glass and sputtered Ag or Pt on glass after only three or four deposition cycles, suggesting the nucleation of a continuous monomolecular film from adsorbed pyrrole. The mean deposition rate evaluated by X-ray photoelectron spectroscopy (XPS) for the first deposition cycles on Ag and Pt is 3 and 4 nm/cycle, respectively, in agreement with previous gravimetric determinations on thicker films (100–600 nm), proving the constancy of the deposition rate from one cycle to another for tens and hundreds of cycles. No chemical shifts related to PPY-substrate interactions were detected. However, the XPS study of the very thin films obtained by a few deposition cycles shows that the first PPY layers are dedoped by hydroxidic (basic) substrate surfaces, as seen by the presence of an imine (deprotonated) nitrogen component in the decomposition of the N(1s) core-level spectrum.