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J. Phys. Chem. C 2010, 114, 22106–22112
Temperature-Dependent Resistive Switching in Bulk Silver Nanowire-Polystyrene Composites Sadie I. White,† Patrick M. Vora,‡ James M. Kikkawa,‡ John E. Fischer,† and Karen I. Winey*,† Department of Materials Science and Engineering, and Department of Physics and Astronomy, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, United States ReceiVed: August 28, 2010; ReVised Manuscript ReceiVed: October 29, 2010
We describe the temperature-dependent characterization of resistive switching behavior in bulk silver nanowire-polystyrene composites between 10 and 300 K. We propose that the resistive switching behavior is caused by the electroformation of silver filaments between adjacent nanowire clusters, resulting in an extension of the electrical percolation network in the on state. This process is reversible above 200 K, and irreversible below 100 K. The switching fields are shown to depend strongly on sample composition (i.e., proximity to the electrical percolation threshold), as well as measurement temperature. Introduction We recently reported the first observation of reversible resistive switching behaviors in bulk, glassy polymer nanocomposites under ambient conditions, specifically silver nanowire-polystyrene composites containing nanowires with average aspect ratios, 〈L/D〉, ∼8, ∼16, and ∼31.1 Prior to this study, reversible resistive switching in polymer nanocomposites had been exclusively reported in thin-film geometries.2-15 In our silver nanowire polymer nanocomposites, switching is only observed when the nanowire volume fractions, φ, are close to the electrical percolation thresholds, φc. To measure φc, the electrical conductivities of composites containing nanowires of each 〈L/D〉 were measured in a voltage range significantly below each composite’s switching voltage (i.e., the sample’s “off” state), and the φ-dependence of each conductivity was fit with a power law to obtain φc. When 〈L/D〉 ) 8.23 ( 0.57, 16.4 ( 1.5, or 31.0 ( 3.7, φc values are 0.083, 0.059, or 0.023, respectively. We observed resistive switching in these composites with compositions near φc, suggesting that the separation between the nanowires and their clusters is critical for electrical resistive switching. Given this dependence on nanowire density, we propose that the resistive switching is caused by the fieldinduced electroformation of silver filaments that form physical bridges between conductive silver nanowire clusters, Figure 1. These silver filaments extend the effective percolation network within the sample and lower the bulk resistivity. Additional key pieces of evidence supporting this switching mechanism are that polystyrene composites of carbon nanotubes and copper nanowires show no resistive switching under identical testing conditions. The oxide-free silver nanowire surfaces are susceptible to filament formation, unlike carbon nanotubes, which have covalently bonded surface atoms, and copper nanowires, which have surface oxide layers. Our measured switching fields are uncharacteristically low (
