Copper Nanowires and Their Applications for Flexible, Transparent Conducting Films: A Review

Abstract

Cu nanowires (NWs) are attracting considerable attention as alternatives to Ag NWs for next-generation transparent conductors, replacing indium tin oxide (ITO) and micro metal grids. Cu NWs hold great promise for low-cost fabrication via a solution-processed route and show preponderant optical, electrical, and mechanical properties. In this study, we report a summary of recent advances in research on Cu NWs, covering the optoelectronic properties, synthesis routes, deposition methods to fabricate flexible transparent conducting films, and their potential applications. This review also examines the approaches on protecting Cu NWs from oxidation in air environments.

Keywords: ITO replacement; copper nanowires; flexible; optoelectronic properties; protection; transparent conducting films.

Figure 1

(a) Plot of the resistivity (ρ) of Ag nanowires (NWs) at 295 K and 4.2 K and the residual resistivity as a function of the NW diameter. The line is the fit to the data using Equation (1). The arrow is the bulk value of the resistivity of Ag, 1.629 × 10⁻⁸ Ωm. (Reproduced with permission from [36]. Copyright American Physical Society, 2006); (b) The dots represent measured sheet resistances (R_s) as a function of NW width for network pitches of 500 (blue), 700 (green), and 1000 nm (orange). The solid lines represent fitted plots for each pitch. (Reproduced with permission from [38]. Copyright American Chemical Society, 2012); (c) Experimental R_s values for silver NW thin films (points) as a function of the area fraction (AF) for the L/D values indicated. R_s from quasi-2D simulations of rods with specified L and D values use an effective contact resistance (R_c) to fit the simulations to the experimental data; the best fits correspond to R_c = 1.5 kΩ (dashed lines) and 2.5 kΩ (solid lines) (Reproduced with permission from [42]. Copyright American Chemical Society, 2013).

Figure 2

(a) The relationship between AF and transmittance (T). Different L/D in AgNW1 and AgNW2 lead to different trends in both T and R_s. (Reproduced with permission from [43]. Copyright Springer Science + Business Media on behalf of the American Coatings Association and the Oil and Colour Chemist Association, 2015); (b) Transmittance (550 nm) plotted as a function of sheet resistance (R_s) for thin films prepared from four nanostructured materials, graphene, single-walled carbon-based nanomaterials, Ag nanowires (NWs) and Ag flakes. The dashed lines represent fitted plots of the bulk regime using Equation (5), while the solid lines corresponds to the percolative regime using Equation (6); (c) The date in Figure 2a are re-plotted on the axes of (T^−1/2 − 1) and R_s on log-log scale. Note that (T^−1/2 − 1) is proportional to film thickness. The dashed lines represent fitted plot of the bulk regime using Equation (5), while the solid lines corresponds to the percolative regime using Equation (6). (Reproduced with permission from [37]. Copyright American Chemical Society, 2010); (d) Haze factor (HF) vs. AF of Ag NWs with diameters of 56, 100, and 153 nm; (e) Slope of HF(%) vs. AF, HF(%)/AF, as a function of Ag NW diameter. (Reproduced with permission from [45]. Copyright AIP Publishing, 2013).

Figure 3

(a) Scanning electron microscopy (SEM) image of general views of Cu NWs; (b) Photographic image of as-prepared Cu NWs in mother liquor; (c) SEM image of detailed views of Cu NWs; (d) Transmission electron microscopy (TEM) image of Cu NWs (Reproduced with permission from [55]. Copyright American Chemical Society, 2005); (e) X-ray diffraction pattern of Cu NWs. Inset, SEM (left) and TEM (right) images of as-grown Cu NWs; (f) Wavelength-dependent transmittance, sheet resistance, and corresponding SEM images of transparent conductors. Substrate contribution is excluded. (Reproduced with permission from [29]. Copyright American Chemical Society, 2015).

Figure 4

Scheme of Cu NW network electrode fabrication: (1) Electron-beam deposition of Cu film on the transparent substrate; (2) Electrospun polyacrylonitrile (PAN) nanofibers on the Cu-covered substrate; (3) Solvent vapor annealing. The insets show schematics of the fiber cross-sections before and after solvent vapor annealing; (4) Metal etching; (5) Removal of PAN fibers by dissolution. Cu NW network on transparent substrate after PAN is removed (Reproduced with permission from [73]. Copyright American Chemical Society, 2014).

Figure 5

Solution-printed highly aligned Ag nanowire (NW) arrays: (a) Schematic of the capillary printing process using a nanopatterned polydimethylsiloxane (PDMS) stamp to produce highly aligned NW arrays; (b) Schematic showing the alignment process during capillary printing of unidirectional NW arrays. The solvent-evaporation-induced capillary force produces highly aligned networks by dragging confined NWs at the solid-liquid-vapor contact line. (Reproduced with permission from [68]. Copyright American Chemical Society, 2015).

Figure 6

(a) Transmittance of electrospun Cu nanofibers before and after atomic layer deposition (ALD); inset is a cartoon (not to scale) of the Cu@AZO/Al₂O₃ nanofiber (b) Normalized sheet resistance (R_s) of bare and protected Cu NWs vs. baking time at 80, 120, and 160 °C. Solid circles refer to bare Cu nanofibers and hollow circles represent AZO-Cu nanofibers. In the case of bare Cu nanofibers at 120 and 160 °C, the resistance exceeded the multimeter limit (20 MΩ) in the end. (Reproduced with permission from [77]. Copyright American Chemical Society, 2012); (c) A plot of specular transmittance vs. number density of NWs, which shows the effect of increasing wire diameter on the film transmittance; inset is an energy-dispersive X-ray spectroscopy (EDS) mapping image of a cupronickel NW coated with 54 mol % nickel; (d) Plot of R_s vs. time for films of Ag NWs, Cu NWs, and cupronickel NWs stored at 85 °C (Reproduced with permission from [78]. Copyright American Chemical Society, 2012); (e) Transmittance as a function of R_s for various NW electrodes; (f) R_s vs. time for various NW electrodes during exposure in the natural environment. (Reproduced with permission from [81]. Copyright American Chemical Society, 2014).

Figure 7

(a) Bending test results of a GFRHybrimer film (sheet resistance, R_s = 35 Ω/sq) and a reference indium tin oxide (ITO)/polyethylene naphthalate (PEN) film (R_s = 15 Ω/sq). Top and right axes are for the ITO/PEN (the inset represents the experimental setup for the bending test; bending radius is 5 mm); (b) Thermal and oxidation stability tests of bare Cu NW film and CuNW-GFRHybrimer film at 80 °C; (c) A schematic example of flexible smartphone using Cu NWs in the future. The inset is a photograph of a Cu NW-GFRHybrimer film (scale bar is 3 cm). (Reproduced with permission from [88]. Copyright American Chemical Society, 2014).