KCS

A new memory device made of a carboxylic-acid-derivative of polyaniline, poly(o-anthranilic acid) (PARA), on gold electrodes shows a very high on/off ratio (~10⁵) of switching at low bias voltages (< 2 V) and also a strong pH-dependence of I-V characteristics^1-2.The switching seems to be controlled by chemical reactions such as oxidation and protonation, but the mechanism is not well-understood. As a step toward a better understanding of the switching mechanism of this PARA-based device, we calculated the electronic structure of a PARA oligomer in various oxidation and protonation states in aqueous solution, using quantum mechanics (density functional theory; B3LYP/6-31G**) combined with the Poisson-Boltzmann continuum-solvation model. Based on the calculated acid dissociation constants (pK_a) and reduction potentials (E⁰), we were able to identify the chemical states of PARA in various experimental conditions (pH and bias voltages). Changes in these chemical states, which can be induced by changing pH or by applying bias voltages, were shown to have a significant influence on the HOMO-LUMO gap and in turn the electron transport property of PARA. As a next step, we will build model devices of PARA sandwiched between gold electrodes in various chemical states and morphologies and then calculate their I-V characteristics using the non-equilibrium Green’s function formalism.----1S. Baek, D. Lee, J. Kim, S. H. Hong, O. Kim, M. Ree, Novel digital nonvolatile memory devices based on semiconducting polymer thin films, Adv. Funct. Mater., 17, 2637-2644 (2007). 2J. Kim, S. Cho, S. Choi, S. Baek, D. Lee, O. Kim, S.-M. Park, M. Ree, Novel electrical properties of nanoscale thin films of a semiconducting polymer: Quantitative current-sensing AFM analysis. Langmuir, 23, 9024-9030 (2007).