Physical Properties of Polyaniline Films: Assembled by the Layer-by-Layer Technique Manoj K. Ram,* ,† Marco Salerno, † Manuela Adami, † Paolo Faraci, † and Claudio Nicolini ‡ Polo Nazionale Bioelettronica, Via Roma 28, 57030 Marciana (LI), Italy, and Institute of Biophysics, University of Genoa, Corso Europa 30, 16132 Genoa, Italy Received June 19, 1998. In Final Form: November 9, 1998 Sequential addition of a polyanion, poly(styrene sulfonate), and a polycation, polyaniline, lead to the formation of layer-by-layer films at different solid surfaces. The prime variables which determine the films formation of poly(styrene sulfonate) (PSS)/polyaniline (PANI) were the polymer charge and ionic strength. The films were deposited by selecting organic/inorganic acid media at pH 2.8. The building up of such multilayer films was characterized by the increment of the adsorbed amount through UV-visible spectroscopy. A linear increase in the absorption magnitude was measured from 1 to 25 bilayers. The uniformity of the PSS/PANI layer-by-layer (LBL) films could be well-maintained, undoping the films in NaOH for obtaining an emeraldine base form of polyaniline. The built-up multilayers were investigated by atomic force microscopy, scanning tunneling microscopy, and cyclic voltammetric and electrical conductivity measurements. The interesting feature of the nearly equal grain size was noticed between 4 and 15 bilayer films of PSS/PANI. The surface roughness was distinguished beyond 15 bilayers of LBL films. The cyclic voltammogram showed the change in the peaks potential value going from 1 to 20 bilayers. The inhomogeneity incorporated inside the films slowed down the electrochemical kinetics in the PSS/ PANI bilayers while going from 1 to 25 bilayer films. The diffusion coefficient (D0) of PSS/PANI 10 bilayers was estimated to be 2 × 10 -8 cm 2 s -1 . Such multilayer films exhibit conductivity in the area of 0.1 S/cm. Introduction It is most interesting and challenging to construct ultrathin films with a supramolecular architecture in which the individual organic molecules are macroscopi- cally oriented and where the molecules with different functionality can be incorporated into individual layers. 1,2 Recently, layer-by-layer (LBL) assembly processes based on electrostatic or other molecular forces are a unique technique that presents a new approach to the formation of supramolecular architectures by adsorption of consecutively alternating polyelectrolytes. 3-5 The self- assembly of charged polyelectrolytes (i.e., proteins, conducting polymers, zirconium phosphate, optical dyes, metal nanoparticles, aluminosilicates, and clay) by LBL self-assembly can be considered as an alternative to the Langmuir-Blodgett, spin-coating, and chemical vapor deposition techniques. 6-13 The most substantial advan- tages of the LBL self-assembly is the quite accurately controlled average thickness of the polyelectrolyte layers, where the macroscopic properties of the molecular film can be controlled by the microscopic structure. 14 Besides the technological interest for the use of LBL bilayers in biosensors and microelectronics (i.e., LED and displays), a number of issues more exciting for fundamental science are the investigation of surface morphology, the kinetics of deposition, and the ionic strength on the deposition of polyelectrolyte LBL layers. 15-18 The stability of the film, the stoichiometry of multilayers, and the better under- standing of the film formation of the various polyelec- trolytes are also under investigation. 19 This technique was originally developed by Decher for polyelectrolytes and later extended to doped conjugated polymers by Rubner. 2,20,21 * To whom correspondence should be addressed. † Polo Nazionale Bioelettronica. ‡ University of Genoa. 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