Manipulating Connectivity and Electrical Conductivity in Metallic Nanowire Networks Peter N. Nirmalraj, †,§,∥ Allen T. Bellew, †,§ Alan P. Bell, †,§ Jessamyn A. Fairfield, †,§ Eoin K. McCarthy, †,§ Curtis O’Kelly, †,§ Luiz F. C. Pereira, ‡,§ Sophie Sorel, ‡,§ Diana Morosan, ‡,§ Jonathan N. Coleman, ‡,§ Mauro S. Ferreira, ‡,§ and John J. Boland* ,†,§ † School of Chemistry, Trinity College Dublin, Dublin 2, Ireland ‡ School of Physics, Trinity College Dublin, Dublin 2, Ireland § Center for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin, Dublin 2, Ireland * S Supporting Information ABSTRACT: Connectivity in metallic nanowire networks with resistive junctions is manipulated by applying an electric field to create materials with tunable electrical conductivity. In situ electron microscope and electrical measurements visualize the activation and evolution of connectivity within these networks. Modeling nanowire networks, having a distribution of junction breakdown voltages, reveals universal scaling behavior applicable to all network materials. We demonstrate how local connectivity within these networks can be programmed and discuss material and device applications. KEYWORDS: Nanowire, network, conductivity, tunable W hile nanoscale materials have found applications in areas of devices, 1 sensors, 2 displays, 3 and medical technolo- gies, 4 early efforts to exploit the potential of individual wires have met with limited success due to property variations and challenges associated with placement. 5 Consequently, there has been a growing interest in the use of random nanowire networks (NWNs), 6,7 where placement is not important and differences in properties are averaged out. These advantages, in combination with superior mechanical performance 8 and the ability to spray-deposit networks over large areas, 9 have extended potential applications to include transparent, flexible conductors 8,10 or even artificial skin. 11−13 The global properties of any NWN are controlled by the connectivity between individual wires, which in turn depends on properties of the interwire junctions such as surface coating, work function, and contact geometry. Connectivity determines how information or charge is carried across the NWN from an array of electrodes that contact and interrogate it. Early studies addressed the onset of conduction and the formation of a percolation channel across ultrasparse wire networks or composites. 14,15 Here, we investigate NWNs comprised of Ag wires (individually coated with a nanoscale polymer passivation layer) and Ni wires with a passivating oxide (see S1, Supporting Information). While the former is qualitatively similar to earlier studies of nanoparticle and nanowire/polymer composites, 15,16 the network densities employed here are well beyond the percolation threshold. The latter is prototypical of non-noble (Cu, Ni, Co) NW systems. Several strategies have been reported to increase the conductivity of such networks. For example, the conductivity in Ag NWNs has been improved by using light−plasmon interactions to weld junctions, 17 but this method is applicable only to noble metals (Ag, Au, etc). The conductivity and stability of metal NWNs has also been improved by creating core−shell NW structures, such as Ni on Cu, that lead to oxidation resistant networks. 18 Here we report for the first time observations of tunable connectivity and conductivity within the same material system, in contrast with previous studies where conductivity was controlled by changing the volume fraction of conducting nanomaterials. 19,20 Under the action of an applied electric field, the resistive junctions in our NWNs undergo breakdown that activates electrical conduction within the network. We find that connectivity evolves under electrical stressing to create materials with controllable conductivity. The voltage threshold for network activation exhibits a unique scaling behavior, which theory and simulation reveal to be consistent with a random distribution of junction properties. Our theory predicts the same scaling for all resistive junction networks, independent of the nanowire material or surface coating. We confirm this prediction by comparing the behaviors and performance of Ag and Ni NWNs with different surface passivations. Network films were formed by spray deposition of nanowires onto Si substrates coated with 300 nm of thermal oxide. The nanowires were poly(vinylpyrrolidone) (PVP) surface-coated Ag NWs (Seashell Technology) and surface-oxide-passivated Ni NWs (Nanomaterials.it), with an average NW length Received: September 13, 2012 Revised: October 11, 2012 Published: October 12, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 5966 dx.doi.org/10.1021/nl303416h | Nano Lett. 2012, 12, 5966−5971