Nanoﬁber Organic Semiconductors: The Effects of Nanosize on the Electrical Charge Transport and Optical Properties of Bulk Polyanilines F. Yakuphanoglu, 1,2 R. Mehrotra, 3 A. Gupta, 3 M. Mun ˜ oz 2 1 Department of Physics, Faculty of Arts and Sciences, Fırat University, 23119 Elazig, Turkey 2 Instituto de Fı ´sica Aplicada, Consejo Superior de Investigaciones Cientı ´ﬁcas (CSIC), Serrano 144, 28006, Madrid, Espan ˜a 3 Optical Radiation Standards, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India Received 6 December 2007; accepted 1 April 2008 DOI 10.1002/app.28535 Published online 15 June 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The electrical transport, optical, and micro- structural properties of bulk polyaniline (PANI) and nano-PANIs were investigated. A ﬁeld emission scanning electron microscopy (SEM) image of bulk PANI showed macroscopic and aggregated granular particles. A SEM image of the nanostructured PANI showed the formation of one-dimensional nano/microstructures. The formation of nanoﬁbers was observed from the transmission electron microscopy image. The electrical conductivities of the bulk and nanostructured PANIs increased with increasing temperature, which indicated semiconductor behavior. The electrical conductivities of the bulk and nanostruc- tured PANIs at room temperature were found to be 2.12  10 5 and 1.80  10 2 S/cm, respectively. The electrical conductivity of the nanostructured PANI was about 850 times higher than that of the bulk PANI. The obtained band gaps of the bulk and nanostructured PANIs were determined from diffuse reﬂectance measurements and were found to be 3.27 and 2.41 eV, respectively. The re- fractive index of the PANI samples changed from 1.3 to 1.61. The obtained results indicate that the electrical and optical properties of the PANI were inherently dependent on the nanostructure. V V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 794–799, 2009 Key words: activation energy; charge transport INTRODUCTION Semiconductor polyanilines (PANIs) have attracted considerable attention in electronic devices because of their electrical and optical properties. Polymeric molecules usually have dimensions ranging from 5 to 10 nm and are expected to exhibit size-dependent properties. 1 The behavior of organic polymers in nanoscopic systems has been found to be signiﬁ- cantly different from their behavior in bulk samples, not only because of their reduced dimension but also because of their changed morphology. 2 Spectral characteristics are evidence for their peculiar behav- iors at nanodimensional levels. 3 The strong depend- ence of UV spectra on nanoparticle size has been frequently observed. 1,4 The electronic 5 and optical properties 5,6 of these polymers in nanodimensions have been found to be different from those of bulk polymers. A fundamental understanding of how size and morphological changes inﬂuence the physico- chemical properties of PANI nanoparticles is of great technological interest. 7 PANIs seem to be suitable candidates for nanoelectronic applications because of their optical and electrical properties, such as refrac- tive index, optical band gap, and dielectric constant. The main goals of this study included the analysis of the electrical, optical, and microstructural proper- ties of PANI and a fundamental understanding of how size changes inﬂuenced the electrical and opti- cal properties of PANI. The effects of nanosize on the electronic and optical properties were analyzed by measurements of optical and electrical conductivity. EXPERIMENTAL The preparation of bulk and nanostructured PANIs was described elsewhere. 7 To study the electrical conductivity, bulk PANI and nano-PANI samples were pressed at 10 t/cm 2 to form a circular disc. Electrical conductivity was measured as a function of temperature with an alternating polarity method to eliminate electrical polarization and triboelectric and piezoelectric effects with a Keithley 6517A Journal of Applied Polymer Science, Vol. 114, 794–799 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: F. Yakuphanoglu (fyhanoglu@ﬁrat. edu.tr or fyhan@hotmail.com). Contract grant sponsor: Turkish Scientiﬁc and Technological Research Council of Turkey (TU ¨ BITAK); contract grant number: 105T137.