Characterization of metal/organic molecule and metal/polymer interfaces by NEXAFS spectroscopy

Abstract

NEXAFS spectroscopy is a technique well suited to probing metal/organic molecule or metal/polymer interfaces. Two systems, related to the Pt/organic conducting polymer and Ni/polyacrylonitrile interfaces, are discussed in detail.For poly-3-alkylthiophene (and selenophene) thin films electrochemically deposited onto Pt, the C K-edge characteristics reveal that (i) the polymeric chain unit is composed of the same architecture as the monomer and (ii) the doping proceeds via a narrowing of the band gap with the appearance of metallic-like behaviour. The same tendencies are observed by varying the structure of the monomer, the nature of the dopant and the film thickness, which evidences a similar conduction mechanism for these five-membered polyheterocycles. The NEXAFS spectra reveal that the polymeric chains are well ordered on the metallic surface: in its undoped state, the layers lie flat on Pt with strong interactions between the electron states of the metal and the antibonding π* band of the polymer; when doped, the NEXAFS characteristics exhibit changes as a function of thickness. The first layer lies flat on the Pt surface while the other ones are randomly oriented, due to the intercalation of anions during the oxidation process. Finally, the orientation of the polymeric chain switches from a ‘lying-down’ to an ‘on-edge’ configuration when a long alkyl chain is added on the ring, this alkyl chain is oriented perpendicular to the Pt surface.Polyacrylonitrile (PAN) thin films electrochemically deposited on Ni have been studied by NEXAFS as a function of the film thickness and annealing treatment. For 20 A thick films, the polymer chains are oriented perpendicular to the surface with the CN groups parallel to it. Below a few A, no polymerization occurs, but molecules are adsorbed perpendicular to the surface. Annealing at 300 °C results in the loss of the majority of the N content of the film in contrast to the admitted mechanism for bulk PAN.