Supporting Information. A Tough and High-Performance Transparent Electrode from a. Scalable Transfer-Free Method

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1 Supporting Information A Tough and High-Performance Transparent Electrode from a Scalable Transfer-Free Method Tianda He, Aozhen Xie, Darrell H. Reneker and Yu Zhu * Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, 170 University Circle, Akron, Ohio , United States *Address correspondence to: Yu Zhu (yu.zhu@uakron.edu) Solvent Annealing (Wetting) Process The compressed air was passed through hot DMF to the funnel. The temperature of DMF was controlled by a water bath at 75 ± 5 C. The sample was put at the mouth of the funnel for one to three minutes.

2 Figure S1. A Photograph of the setup for the PAN nanofiber solvent annealing process. A Sample with Different Transmittance. This cm sample was prepared by adjusting the wetting process only. The sample with the PAN nanofibers was put on the edge of the funnel with vapor flow rate (1000 sccm). This experiment indicated that the transmittance of the transparent electrodes can be easily controlled by wetting process. Figure S2. A stepwise wetted sample that shows gradual transmittance changes from 60% (Zone

3 D) to 90% (Zone A). The bottom four figures a, b, c and d show the optical microscope images for Zone a, b, c and d, respectively. Etching Process. With the support of a sample holder, the sample was rinsed by M FeCl 3 aqueous solution. The volume of solution is determined by the thickness of the copper film. For the copper film with thickness of 100 nm, 1 liter of M FeCl 3 solution was required. To remove the Cu residues completely, an additional rinsing with another 1 liter of M FeCl 3 aqueous solution was necessary. Figure S3. A photograph of the setup for copper etching process.

4 SEM EDAX Mapping. In order to provide evidence that Cu nanowires were well protected by the PAN nanofibers after etching (Figure 2e and 2f), EDAX mapping was carried out on the sample shown in Figure 2e and 2f. The results are shown in Figure S3. Figure S4. SEM EDAX mapping of the PAN nanofibers covered copper wire on the glass substrate. a) magnification SEM image under 10 kv voltage. b,c) The positions of red and yellow spots indicate the signals from C K α and Cu L α, respectively.

5 AFM Images. The following AFM images in Figure S5 show that the thicknesses of deposited Cu on glass are 100 nm and 50 nm. Figure S5. AFM images of the Cu nanowires on glass substrates with original deposited copper thickness of a) 50 nm: The average step height of the nanowires is nm. b) 100 nm: The average step height of the nanowires is nm.

6 Percolation Theory. Percolation theory can be used to predict the non-linear relation between sheet resistance R s and transmittance T in nanowire networks. 1, 2 Figure S6 shows the fitting results of the performance of the copper nanowire electrodes in this work based on the following percolation equation: T = [1 + 1 Π (Z 0 R s ) 1 n+1 ] 2 (1) where T is the light transmittance, R s is the sheet resistance, Z 0 is the impedance of free space (377 Ω), 2 n is the percolation exponent and Π is the percolative figure-of-merit (FOM). Π follows the relation: σ Π = 2[ dc /σ 1 op (Z 0 t min σ op ) n] n+1 (2) where σ dc is the dc conductivity, σ op the optical conductivity, t min the thickness below which dc conductivity becomes thickness dependent. Previous report shows that large values of Π and low values of n are required for high performance percolative transparent electrode. 2 Based on the fitting results, the percolation exponent n and FOM Π are 0.78 and 89 for this work, respectively. The results are compared with literature sources in Table S1.

7 log(t ) log(z 0 /R s ) Figure S6. Copper nanowire networks fit percolation theory Table S1. The percolation parameters of this work and other literatures. n Π Cu nanowire (this work) Metal nanowire Silver nanowire Cu nanotrough

8 AFM images of the transparent electrode. The following 100 µm 100 µm AFM image indicates that the resulting transparent electrode is free of large protruding. Figure S7. a) A 100 μm 100 μm AFM image of copper nanowires on glass substrate. b) A 3D presentation of the same AFM image. The height of the copper nanowire is 100 nm.

9 Touch Screen Fabrication. The fabrication procedure of resistive touch screen is shown in Figure S8 a. In comparison to the Figure 1, scotch tapes were used at step 3 to protect the copper on the edge. At step 5, the scotch tapes were peeled off. The Cu stripes on the edges work as part of the circuit. Finally, the electrode was cut to the right size to fit the bottom ITO substrate (Figure S8 b) with a digital connection to the computer (purchased from TVI Electronics LLC). The assembled touch screen was well functionalized (see Figure S8 c and SI video 2). Figure S8. a) Fabrication process of a touch screen electrode using Cu nanowire network. b) A photograph of the touch screen with the top electrode made of the Cu nanowire on the PET substrate. c) A photograph of the Cu nanowire touch screen testing result.

10 Reference 1. Hu, L.; Hecht, D. S.; Gruner, G. Percolation in Transparent and Conducting Carbon Nanotube Networks. Nano Lett. 2004, 4, De, S.; Coleman, J. N. The Effects of Percolation in Nanostructured Transparent Conductors. MRS Bull. 2011, 36, Scardaci, V.; Coull, R.; Lyons, P. E.; Rickard, D.; Coleman, J. N. Spray Deposition of Highly Transparent, Low-Resistance Networks of Silver Nanowires over Large Areas. Small 2011, 7, Wu, H.; Kong, D.; Ruan, Z.; Hsu, P. C.; Wang, S.; Yu, Z.; Carney, T. J.; Hu, L.; Fan, S.; Cui, Y. A Transparent Electrode Based on a Metal Nanotrough Network. Nat. Nanotechnol. 2013, 8,