No abstract

Copper nanowire networks are considered a promising alternative to indium tin oxide as transparent conductors. The fast degradation of copper in ambient conditions, however, largely overshadows their practical applications. Here, we develop the synthesis of ultrathin Cu@Au core-shell nanowires using trioctylphosphine as a strong binding ligand to prevent galvanic replacement reactions. The epitaxial overgrowth of a gold shell with a few atomic layers on the surface of copper nanowires can greatly enhance their resistance to heat (80 °C), humidity (80%) and air for at least 700 h, while their optical and electrical performance remained similar to the original high-performance copper (e.g., sheet resistance 35 Ω sq at transmittance of ∼89% with a haze factor <3%). The precise engineering of core-shell nanostructures demonstrated in this study offers huge potential to further explore the applications of copper nanowires in flexible and stretchable electronic and optoelectronic devices.

Copper nanowire (Cu NW) based transparent conductors are promising candidates to replace ITO (indium-tin-oxide) owing to the high electrical conductivity and low-cost of copper. However, the relatively low performance and poor stability of Cu NWs under ambient conditions limit the practical application of these devices. Here, we report a solution-based approach to wrap graphene oxide (GO) nanosheets on the surface of ultrathin copper nanowires. By mild thermal annealing, GO can be reduced and high quality Cu r-GO core-shell NWs can be obtained. High performance transparent conducting films were fabricated with these ultrathin core-shell nanowires and excellent optical and electric performance was achieved. The core-shell NW structure enables the production of highly stable conducting films (over 200 days stored in air), which have comparable performance to ITO and silver NW thin films (sheet resistance ∼28 Ω/sq, haze ∼2% at transmittance of ∼90%).

We present experimental results for the transmission T, haze H, sheet resistance Rs, and its spatial fluctuations ΔRs for silver nanowire films. Mie light scattering theory of nanowires is developed to predict both T and H as a function of diameter D of wires and the surface fraction ϕs covered by the wires. Percolation theory is used to derive an equation for Rs in terms of D, the aspect ratio of wires D/L and ϕs. The critical exponent t for percolation of Rs is found to be 1.23 in close agreement with theoretical results for 2D random resistive networks (t = 1.3). These equations show the importance of both the distributions of diameter ⟨D⟩ and aspect ratio of wires ⟨D⟩⟨L⟩/⟨L2⟩ to predict the optical and electrical properties. Spatial fluctuations ΔRs/Rs can also be significant in these films and be greater than 10% as ϕs approaches the critical percolation concentration ϕc. We show that the calculated T versus Rs and H versus Rs curves are in good agreement with the experimental data. We propose figures of merit for percolating nanowire films in terms of high T, low H, and low Rs to order the quality of films for touch screen applications. The results show that D &lt; 50 nm and L &gt; 5 μm are needed to achieve low haze H &lt; 1%, high transmission T &gt; 90%, together with low Rs ∼ 100 Ω/sq for touch screen applications. Finally, we present experimental and theoretical results of the real and imaginary refractive indices of AgNW/polymer nanocomposites, and find that the Van De Hulst model is more accurate than the Maxwell Garnett models.