Aqueous Synthesis, Degradation, and Encapsulation of Copper Nanowires for Transparent Electrodes

Abstract

Copper nanowires (CuNWs) have increasingly become subjected to academic and industrial research, which is attributed to their good performance as a transparent electrode (TE) material that competes with the one of indium tin oxide (ITO). Recently, an environmentally friendly and aqueous synthesis of CuNWs was demonstrated, without the use of hydrazine that is known for its unfavorable properties. In this work, we extend the current knowledge for the aqueous synthesis of CuNWs by studying their up-scaling potential. This potential is an important aspect for the commercialization and further development of CuNW-based devices. Due to the scalability and homogeneity of the deposition process, spray coating was selected to produce films with a low sheet resistance of 7.6 Ω/sq. and an optical transmittance of 77%, at a wavelength of 550 nm. Further, we present a comprehensive investigation of the degradation of CuNWs when subjected to different environmental stresses such as the exposure to ambient air, elevated temperatures, high electrical currents, moisture or ultraviolet (UV) light. For the oxidation process, a model is derived to describe the dependence of the breakdown time with the temperature and the initial resistance. Finally, polymer coatings made of polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA), as well as oxide coatings composed of electron beam evaporated silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) are tested to hinder the oxidation of the CuNW films under current flow.

Keywords: CuNWs; PDMS; PMMA; copper nanowires; degradation; encapsulation; solution-based; transparent electrode.

Figure A1

High-resolution FESEM-images of CuNWs for the precursor-to-solvent series with different mass ratios (precursor:solvent) of (a) 1:300, (b) 1:100, (c) 1:50 and (d) 1:33. The scale bar in (a) applies to all images.

Figure A2

Diameter histograms for the precursor-to-solvent series with different mass ratios (precursor:solvent) of (a) 1:300, (b) 1:100, (c) 1:50 and (d) 1:33.

Figure A3

Photo for the four-probe measurement across a CuNW film after electrical breakdown.

Figure A4

SEM-images for a CuNW film subjected to a relative humidity and temperature of 90%RH and 60 °C, for a duration of 24 h, under (a) a low and (b) a high magnification.

Figure A5

Normalized increase in resistance R/R₀ over time for CuNW films with a different initial resistance that were subjected to different electrical input powers. The x-axes for all films have been shifted to allow comparing the breakdown regions of each device.

Figure 1

(a) Schematic for the growth of a single copper nanowire (CuNW) along the (111)-plane. The blue and orange spheres indicate oleylamine (OM) head groups and copper atoms, respectively. (b) High-resolution field-emission scanning electron microscope (FESEM)-image for a single copper nanowire with the characteristic pentagonal shaped cross-section.

Figure 2

FESEM-images of CuNWs for the precursor-to-solvent series with different mass ratios (precursor:solvent) of (a) 1:300, (b) 1:100, (c) 1:50 and (d) 1:33.

Figure 3

Mean diameters extracted using DiameterJ from the FESEM-images illustrated in Figure A1 for the precursor-to-solvent series with different mass ratios. The dashed line serves as a guide to the eye.

Figure 4

(a) Normalized increase in resistance R/R₀ of two CuNW films with different transmittances exposed to ambient conditions as a function of the time; (b) FESEM-image of CuNWs exposed to ambient conditions for more than 25 days.

Figure 5

(a) Normalized increase in resistance R/R₀ over time for different temperatures. The CuNW films were placed on a hot plate in ambient air; (b) Breakdown time t_BD as a function of the annealing temperature.

Figure 6

(a) Photo of a CuNW film spray-deposited to a glass substrate and electrically contacted on each side by copper tape and conductive silver ink; (b) Normalized resistance R/R₀ as a function of time for CuNW heaters that show resistances in a range of 3.0 Ω to 5.5 Ω and were subjected to an electrical input power ranging from 2 W to 9 W; (c) R₀ as a function of the time-to-failure for CuNW heaters that are subjected to powers of 5 W, 6 W and 7 W, where the ellipses indicate the regions of time-to-failure for same power values.

Figure 7

Light microscope images for CuNW films with (a) a low and (b) a high wire density. (c) Transmittance evaluated at a wavelength of 550 nm and wire density as a function of the room temperature resistance. The wire densities corresponding to the images (a,b) are indicated by red hollow symbols.

Figure 8

(a) Photo for a CuNW heater with a sheet resistance of around 3.2 Ω/sq. after electrical breakdown. The heater was subjected to an electrical input power of 6 W for around 17 h; (b) Microscope images for the CuNW film after an electrical breakdown at a position with >MΩ sheet resistance; (c–e) SEM-images for the CuNW heater shown in (a,b) at a position with >MΩ sheet resistance; (f) R_S across the CuNW heater as a function of the measurement position, as indicated in (a).

Figure 9

R/R₀ plotted as a function of time for CuNW heaters with different R₀ that were subjected to different moisture levels of (a) 90%RH and (b) 20%RH, at a temperature of 60 °C and for a duration of 24 h. (c) R/R₀ as a function of R₀ for the two moisture levels. The inset shows a photo for a CuNW film.

Figure 10

(a) Normalized increase in resistance R/R₀ over time for CuNW films with a transmittance of 70% that were (i) subjected to ultraviolet-visible (UV-vis) light, (ii) shielded from UV-vis light and (iii) exposed to the ambient air. Each symbol represents a mean resistance that was determined by averaging over four different samples; (b) R/R₀ over time for four different CuNW films that are subjected to prolonged UV-vis exposure. The inset depicts the four CuNW samples that were subjected to UV-vis exposure along with a labeling that is in accordance with normalized resistance curves.

Figure 11

X-ray photoelectron spectroscopy (XPS) spectra for CuNW film that were subjected to different environmental stresses, under ambient air, i.e., (1) as-deposited, (2) exposure to ambient air for 3 months, (3) subjected to a temperature of 200 °C for a duration of 30 min, (4) heated at a power of 6 W until breakdown, (5) subjected to a relative humidity of 90%RH, at a temperature of 60 °C, and (6) UV-light exposure.

Figure 12

Photos for (a) an as-deposited CuNW film and (b) a CuNW film subjected to a temperature of 175 °C for a duration of 1 h. (c) Schematic of an oxidized CuNW along with the parameters that are used in the text to describe the oxidation mechanism.

Figure 13

Temperature-breakdown time dependence for the data from Figure 5b. The solid line represents a linear fit to the experimental data, in agreement with Equation (10).

Figure 14

Normalized increase in resistance R/R₀ as a function of the time for CuNW films coated with (a) PDMS, (b) PMMA and (c) SiO₂ and Al₂O₃. The CuNW films were heated over an effective area of 3.5 × 5 cm² by applying an electrical input power of 6 W.