Interfacial chemical effect evidenced on SERS spectra of polyaniline thin films deposited on rough metallic supports

Surface-enhanced Raman scattering (SERS) has proved to be an effective tech nique for studying the structural properties of conducting polymer thin fil ms. The enhancement process has a twofold origin, electromagnetic and chemi cal, The electromagnetic enhancement, which is the dominant mechanism in SE RS generation, consists in the excitation of localized and delocalized surf ace plasmons (SPs) in the metallic support of the thin film, The Raman emis sion of the adsorbed molecules on the metal surface (the most efficient bei ng Ag, Au and Cu) is due to the intense evanescent electromagnetic field lo cated at the interface between the metal and the surrounding medium. The se cond enhancing mechanism for SERS is of chemical origin, involving the form ation of new chemical bonds between the molecules and the metal surface, wi th the polarizability thus becoming considerably higher than that of the fr ee molecules. This mechanism is as a rule accompanied by a metal-molecule o r molecule-metal charge transfer, which partly accounts for the success of the SERS studies on conducting polymers. Unfortunately, these studies have revealed that the chemical effects at the polymer-metal interface varied su bstantially depending on the various types of polymer and metallic support. In this context, polyaniline containing two different entities (a reduced and an oxidized state) in its repeating units exhibits specific alterations in its SERS spectra depending on the type of metallic support. This paper presents new results concerning the structure of emeraldine-base and emeral dine-salt polyaniline thin films deposited on rough Ag and Au supports. The effect on the SERS spectra of the polymer-metal and polymer-ambient interf ace chemical reactions is also described. The presence of an interface comp ound depending on the oxidizing properties of the metallic support has a st rong influence on the SERS spectra, no matter how the PAN films were deposi ted on the support (whether by solvent evaporation or by an electrochemical process, i.e. cyclic voltammetry). When an emeraldine base is doped with H SO4- ions, it turns into an emeraldine salt, which exhibits a disordered st ate in its macromolecular chain, leading to a modified profile of the simil ar to 1162 cm(-1) Raman line which is associated with the C-H bond of the q uinoid ring. The Lorentzian profile is altered by the addition of a Gaussia n profile component. For a rough Ag support, the SERS spectra show that the transformation of emeraldine base films into emeraldine salt films is a re versible process, Copyright (C) 1999 John Wiley & Sons, Ltd.