Electrical percolation in metal nanowire networks for bulk polymer nanocomposites and transparent conductors, and resistive switching in metal/polymer nano-gap devices

This dissertation describes the electrical properties of metal nanowire-polymer hybrid systems. The first part of the thesis discusses electrical percolation of metal nanowire networks in bulk polymer nanocomposites (3D) and nanowire films (quasi-2D). Specifically, we integrate simulations of rod networks and experiments of model metal nanowire systems to establish the dependence of their electrical properties on the nanowire aspect ratio (L/D) and network structure. For bulk polymer nanocomposites, we find that narrow Gaussian distributions in filler size do not impact the electrical percolation threshold, while mixing small fractions of high-L/D rods with modest-L/D rods significantly reduces the threshold. We also generalize the widely used excluded volume model of percolation, which was originally formulated for infinite-L/D, monodisperse rod networks, to account for both finite-L/D and arbitrary distribution in the rod dimensions. Next, we adapt our 3D simulation approach to model the electrical properties quasi-2D rod networks, which are relevant to nanowire films that are being pursued for flexible, transparent conductors. We present the first quantitative predictions of the dependence of the sheet resistance in nanowire films on the aspect ratio and areal density of monoand poly-disperse nanowires. Moreover, by combining our simulations of sheet resistance and an empirical diameter-dependent expression for the optical transmittance, we produced a fully calculated plot of optical transmittance versus sheet resistance, the primary performance criteria for transparent conductors. Further, by fitting simulation results to experimental data, we obtain an effective average contact resistance, Rc_effective, between two nanowires. Rc_effective extracted using our integrated approach enables direct comparisons between nanowires of different compositions or networks fabricated by distinct means. We also report the critical area fraction of rods required to form a percolated network in nanowire films and provide a semi-empirical analytical expression for the critical area fraction as a function of L/D for monoand polydisperse rods. Our simulations of electrical percolation in quasi-2D and 3D rod networks, coupled with our extensions of existing analytical models, provide critical guidance for engineering bulk conducting nanocomposites and nanowire films with well-defined properties that are optimized for specific applications. In the second part of this thesis, we demonstrate reversible resistive switching in silver/polystyrene/silver nano-gap devices comprised of Ag nano-strips separated by a nanoscale gap and encapsulated in polystyrene (PS). These devices show highly reversible switching behavior with high on-off ratios during cyclic switching tests over many cycles. We also observe evolution of the gap after extensive testing, which is consistent with metal filament formation as the switching mechanism in Ag/PS/Ag nano-gap devices. The reversible electrical bistability demonstrated here was accomplished with an electrically inactive polymer, thereby extending the range of polymers suitable for organic digital memory applications. Degree Type Dissertation Degree Name Doctor of Philosophy (PhD) This dissertation is available at ScholarlyCommons: http://repository.upenn. edu/edissertations/902 Graduate Group Materials Science & Engineering First Advisor Karen I. Winey