275 Mater. Res. Soc. Symp. Proc. Vol. 1 2 © 2011 Materials Research Society DOI: 10.1557/opl.2011.113 Self-Assembled Nano-Needles of Polyaniline, Efficient Structures in Controlling Electrical Conductivity Michael I. Ibrahim 1,2 , Maria J. Bassil 1 , Umit B. Demirci 2 , Georges El Haj Moussa 1 , Mario R. El Tahchi 1 and Philippe Miele 2 1 LPA-GBMI, Department of Physics, Lebanese University - Faculty of Sciences II, PO Box 90656 Jdeidet, Lebanon, email: gbmi@ul.edu.lb, Tel: +961 3 209688, Fax: +961 1 681553. 2 Université Lyon 1, CNRS, UMR 5615, Laboratoire des Multimatériaux et Interfaces, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France, email: umit.demirci@univ-lyon1.fr. ABSTRACT Polyaniline (PANI) is one of the most interesting conducting polymers with a wide and controllable conductivity range, synthesized easily via chemical or electrical route, stable chemically and environmentally, having high absorption in the visible range and high mobility of charge carriers. Under different conditions, PANI morphology can be controlled yielding to the creation of nano-tubes, belts, rods, fibers and particles. In this study, the chemical oxidative polymerization which consists of mixing aniline hydrochloride (A-HCl) with ammonium peroxydisulfate (APS) was used to synthesize HCl doped PANI. Fixing the weight ratio A-HCl/APS defined by the IUPAC while varying their quantities leads to the formation of PANI nanoparticles with variable diameters. In addition, PANI nano-needles of 60 nm average diameter at the center are also obtained. Different methods are used to investigate of 1-D morphologies. The electrical conductivity of bulk PANI pellets is measured using the four-point probe technique. The absorption in the visible range of PANI particles and nano-needles is determined by UV-Vis spectroscopy. XRD analysis was performed to study the effect of PANI particle size and morphology on the crystallinity of the powder. Such structures could be used in hybrid solar cells for higher conversion efficiencies. INTRODUCTION Since the discovery of conductive polymers, new applications such as plastic flexible circuits, hybrid photovoltaics and many others were achieved. Polyaniline (PANI) is one of the most interesting conducting polymers being widely used for its easiness of synthesis, chemical and environmental stability, high absorption in the visible range and high mobility of charge carriers [1]. For these reasons PANI has entered the wide door for many applications from which we cite organic electrodes for supercapacitors [2], gas sensors [3], hybrid solar cells [4] and corrosion protection [5]. Highly crystalline PANI films have proved their suitability in humidity sensing [6]. Varying the polymerization process, PANI morphology could be controlled resulting in the formation of nanofibers, nanoparticles, nanowires, needle-like shapes, and so on. Wang et al have proved that the rapid mixing polymerization of low aniline and APS concentrations leads to the formation of PANI nanofibers with diameter of 30-50 nm [7]. Due to PANI nanofibers high surface area, their response to gas vapors is faster and more intense than that of bulk PANI [8]. The morphology of PANI can be varied from spherical particles to micro and nanofibers using 31