A. Biaxially stretchable silver nanowire network

The fabrication procedure for creating a biaxially stretchable Ag nanowire film is shown in Fig. 1(a). One weight percent of Ag nanowires (Nanotron Corporation) are dispersed in water. The length and diameter of the Ag nanowires are 49 ± 18 µm and 100 ± 20 nm, respectively. The polydimethylsiloxane (PDMS) substrates are prepared using the same method as described elsewhere.^{Reference Ho, Tey, Liu, Cheng and Wei22} A PDMS substrate is clamped on all sides and equibiaxially prestrained by pushing a sphere of 5 cm diameter against the bottom surface of the PDMS. The elastic PDMS is strained equibiaxially as the sphere pushes the thin PDMS out of its original plane. A silver nanowire film, which forms on a Whatman Anodisc membrane filter via vacuum filtration, is transfer printed from the membrane filter to the equibiaxially stretched PDMS. Upon complete transfer, the mechanical prestrain in the PDMS substrate is released by moving the sphere back into its original position. As the PDMS relaxes, the Ag nanowire film buckles out of plane of the PDMS substrate. This is evident from the SEM image shown in Fig. 1(b). Small distinct areas of Ag nanowires appear brighter (circled in red). Ag nanowires in these areas buckle out of plane (additional emission of secondary electrons along the buckled portions of the nanowires is responsible for the brighter appearance during imaging). Buckling is absent when the Ag nanowire film is transfer printed to a PDMS substrate that was not prestrained, as shown in the SEM image in Fig. 1(c). Waviness of the buckled nanowire films results from the compressive stress which the nanowires accommodate by bending when the prestrain is released. The electrical and optical properties of the Ag nanowire film were measured. The sheet resistance was ∼50 Ω/sq and the transmittance at a wave length of 550 nm was ∼70%. Lower sheet resistance with higher transparency can be achieved if longer nanowires were used.^{Reference Lee, Lee, Lee, Yeo, Hong, Nam, Lee, Lee and Ko16}

FIG. 1.

(a) Schematic illustration of the fabrication of biaxially stretchable transparent conductor based on networks of silver nanowires (Ag NW) film. (b, c) SEM images of Ag nanowire films transfer printed onto a (b) prestrained and (c) non prestrained substrate, respectively. Selected areas (marked in red circles) of the Ag nanowire film in part (b) buckle out-of-plane.

The use of buckled nanowire networks has previously been reported to lend stretchability to the transparent conductor in a single axis^{Reference Lee, Lee, Lee, Yeo, Hong, Nam, Lee, Lee and Ko16} or two perpendicular axes.^{Reference Ho, Tey, Liu, Cheng and Wei22} This is the first time that 2D out-of-plane buckling of nanowire network is observed when isotropic prestrain of 25% is applied on the elastic substrate. The biaxially buckled network that results is expected to lend stretchability in all axes of the two-dimensional plane. Interestingly, this is found not to be the case as shown in Fig. 2(a). The sample examined in Fig. 2(a) has been prestrained biaxially. The diameter of the circular film of Ag nanowires missing from the membrane filter after transfer printing from the membrane filter to the prestrained PDMS was measured to be 25 mm. This is compared to the diameter of the circular film of Ag nanowires transfer printed onto the PDMS substrate upon release of the prestrain: 20 mm. Hence, a prestrain of 25% has been applied in each axis of the plane. Figure 2(a) shows the normalized resistance (R/R ₀) of the sample with applied uniaxial strain. R ₀ and R are the electrical resistances before and after applying the uniaxial strain, respectively. The electrical resistance before applying the uniaxial strain is 53 ± 16 Ω. The normalized resistance at an applied uniaxial strain of 1, 3, and 5% increases by 1.1, 1.2, and 1.3 times, respectively. The normalized resistance further increases by 2.0 times (±0.3) when a 10% uniaxial strain is applied. Even upon releasing the uniaxial strain, the electrical resistance does not recover to its original value, unlike a reversible stretching process that was demonstrated in Ref. Reference Wu, Zang, Rathmell, Zhao and Wiley33. The normalized resistance remains high at 1.8 times (±0.3) of the original value after complete relaxation. In a perfectly reversible stretching process, the nanowires return to be in good electrical contact again (and hence return to the original electrical resistance) after the strain is released.

FIG. 2.

Samples that have been equibiaxially prestrained but not undergone mechanical hot press: (a) Normalized resistance of the Ag nanowire films with applied uniaxial strain. The initial sheet resistance is 53 ± 16 Ω/sq. (b) SEM images of the Ag nanowire films upon stretching by 0 and 10%. The black arrow shows the direction in which the film was stretched. Scale bars in the left and right images are 10 and 2 µm, respectively. (c) Schematic illustration of stretching the equibiaxially prestrained Ag nanowire network in one axis. (d) SEM image of the Ag nanowire films. The nanowires are not fused at the junctions. (e) Normalized resistance of the Ag nanowire films with 20% applied strain. Samples that are strained uniaxially and biaxially are compared.

The change in network percolation is observed under a scanning electron microscope (SEM) as uniaxial strain is applied to understand the stretching mechanism in a 2D out-of-plane buckled silver nanowire network, as shown in Fig. 2(b). The images on the left and right of Fig. 2(b) show the silver nanowire network when unstretched and stretched by 10%, respectively. Before stretching, the nanowires were buckled out-of-plane. As strain is applied, the wavy nanowires start to “flatten out” along the axis of the uniaxial strain and buckle more out-of-plane in the axis perpendicular to the uniaxial strain, as shown in the right image of Fig. 2(b).

B. Mechanism for loss of contact between nanowires during stretching

The question that arises then is why there is an increase in electrical resistance for 2D out-of-plane buckled films but not for films that are buckled in-plane, even though the nanowires “straighten out” during subsequent stretching in all cases. A film that has been biaxially prestrained by 25% (in this study) buckles out-of-plane. However, out-of-plane buckling was not observed in the film that was biaxially prestrained by 10% in an earlier work.^{Reference Ho, Tey, Liu, Cheng and Wei22} Only in-plane wavy nanowires were present.^{Reference Ho, Tey, Liu, Cheng and Wei22} Out-of-plane buckling in this study occurs due to the accommodation of larger compressive stress built up when the 25% prestrain is released. When a uniaxial strain is applied to the out-of-plane buckled nanowires, the nanowires that run approximately parallel to the stretch axis are stretched, as indicated by the red arrows in Fig. 2(c). In the transverse axis, the nanowires are compressed due to the Poisson effect of the PDMS substrate, as indicated by the blue arrows. This results in the stretched nanowires “flattening out” and the compressed nanowires buckling out of plane even further. Hence, the nanowires pull vertically, i.e., in the out-of-plane direction, away from each other and the number of nanowire-to-nanowire contacts decreases rapidly, resulting in an increase in overall electrical resistance. Figure 2(d) shows a SEM image of the nanowires. The nanowires are not fused to one another at their intersections and can easily move out of contact from one another.

To verify this phenomenon, the normalized resistance of the out-of-plane buckled nanowire network is compared when the sample is stretched 20% biaxially or uniaxially. Normalized resistance of the sample under 20% biaxial strain is lower than the normalized resistance when it is under 20% uniaxial strain [Fig. 2(e)]. The normalized resistance at 20% biaxial strain and 20% uniaxial strain is 2.1 ± 0.5 and 5.7 ± 1.3, respectively. This is in agreement with the phenomenon proposed above because the contraction in the transverse axis is absent in biaxial stretching. Nonetheless, a small increase in electrical resistance is also observed when the sample is strained 20% biaxially. This can be attributed to the loss of good contact at some of the nanowire junctions.

C. Maintaining good contact between nanowires during stretching

Pressing the out-of-plane buckled nanowires onto the substrate surface and fusion of nanowires at the junctions are expected to circumvent this problem of nanowires pulling away from one another. Various methods such as annealing^{Reference Garnett, Cai, Cha, Mahmood, Connor, Christoforo, Cui, McGehee and Brongersma34–Reference Kang, Yun, Kim and Yang36} or mechanical pressing^{Reference Hu, Kim, Kim, Peumans and Cui21,Reference Tokuno, Nogi, Karakawa, Jiu, Nge, Aso and Suganuma37} have been explored to fuse nanowires at the junctions. By combining these two methods (applying a pressure of 4 MPa for 10 s at a temperature of 100 °C), we are able to (1) press the nanowires onto the plane of the substrate surface and (2) effectively fuse the nanowires together. Annealing alone is usually carried out at >200 °C to fuse the nanowires.^{Reference Lee, Lee, Kim and Lee35,Reference Kang, Yun, Kim and Yang36} High annealing temperature is usually used because the presence of insulating residual poly(vinylpyrrolidone) (PVP: a byproduct of Ag nanowire synthesis) on the surface of Ag nanowires impedes the fusion of the nanowires.^{Reference Garnett, Cai, Cha, Mahmood, Connor, Christoforo, Cui, McGehee and Brongersma34} As the polymeric substrate used in our devices is heat sensitive, we used a combination of mechanical pressing and gentle annealing at 100 °C (mechanical hot press). Mechanical pressing also presses the out-of-plane buckled nanowires onto the substrate surface. The mechanical pressure applied is relatively low at 4 MPa compared to 25 MPa^{Reference Tokuno, Nogi, Karakawa, Jiu, Nge, Aso and Suganuma37} and 81 GPa^{Reference Hu, Kim, Kim, Peumans and Cui21} used in other works. Much higher mechanical pressure can potentially deform the PDMS substrate. Performance of the prestrained sample that has undergone mechanical hot press is vastly improved. Figure 3(a) shows the normalized resistance of the sample with applied uniaxial strain. The initial sheet resistance is 16 ± 4 Ω/sq. As uniaxial strain is increased up to 10%, normalized resistance remains pretty similar to before stretching. In fact, the normalized resistance barely changes even after 100 cycles of stretching to 10% [Fig. 3(b)].

FIG. 3.

Samples that have been equibiaxially prestrained and undergone mechanical hot press: (a) Normalized resistance of the Ag nanowire films with applied uniaxial strain. The initial sheet resistance is 16 ± 4 Ω/sq. (b) Normalized resistance of the Ag nanowire film as a function of the number of stretch and relax cycles. The applied strain was 10%. (c) SEM images of the Ag nanowire films upon stretching by 0 and 10%. The black arrow indicates the direction in which the film was stretched. Scale bars in the images are 2 µm.

Figure 3(c) shows the SEM images of the prestrained nanowire network that has undergone mechanical hot press with applied uniaxial strain. Unlike the samples that have been prestrained but not undergone mechanical hot press, this network has been pressed onto the same plane. A flat transparent conductor film is also more desirable because it is essential in thin film devices such as thin film photovoltaic cells or light-emitting displays to prevent shorting of the devices. Although the nanowires in the network have been pressed onto the same plane, they remain wavy in-plane. As the strain increases from 0 to 10%, the nanowires in the network become less wavy in-plane but maintain electrical contact with one another [Fig. 3(a)].

As the nanowires are flexible, they are able to deform freely. This configuration is in principle similar to the serpentine microstructures used for stretchable electronics by Rogers and co-workers.^{Reference Rogers, Someya and Huang38–Reference Zhang, Xu, Fu, Lee, Su, Hwang, Rogers and Huang40} The group has carefully designed lithographically defined electrical interconnects with serpentine layouts which can bend and twist and improve stretchability. A random network of wavy nanowires can be perceived as a form of such interconnects but does not require expensive lithography because the nanowire dimensions are defined by their chemical synthesis process. However, the orientation of the nanowires with respect to one another is not well defined. Screen printing or spray deposition is expected to be able to define the dimensions of the nanowire network film as the nanowire network film is deposited from solution form. These methods enable large area printing at relatively low cost.

If only mechanical hot pressing without prestrain is performed on the nanowire network (“hot-press-only” sample), the normalized resistance also increases with applied uniaxial strain [Fig. 4(a)], despite the fusion between the nanowires in the network. A SEM image showing the fused nanowires in the hot-press-only sample is provided in Fig. 4(b). The normalized resistance at 3, 5, and 10% uniaxial strain is 1.3 ± 0.1, 1.7 ± 0.3, and 3.5 ± 0.8, respectively. The initial sheet resistance is 18 ± 8 Ω/sq. The increase in normalized resistance of the hot-press-only sample is higher than that of a prestrained-only sample. When the applied strain on the hot-press-only sample is released, the normalized resistance returns to 1.0, reflecting the reversibility of this process. In Fig. 4(c), one can observe the nanowires “stretching” in the direction of the strain axis as the hot-press-only sample is stretched. The source of the increase in electrical resistance of the hot-press-only sample with applied strain is examined next.

FIG. 4.

Samples that have undergone mechanical hot press only: (a) Normalized resistance of the Ag nanowire films with applied uniaxial strain. The initial sheet resistance is 18 ± 8 Ω/sq. (b) SEM image of the Ag nanowire film after mechanical hot press. Fused junction is circled in red. (c) SEM images of the Ag nanowire films upon stretching by 0 and 10%. The black horizontal arrow indicates the direction in which the film was stretched. Scale bars in the images are 2 µm.

During stretching, the applied tensile stress cannot be accommodated by the straight nanowires in the hot-press-only sample, so the nanowires are strained and their junctions are forced to degrade. During stretching of a prestrained sample, the tensile stress is relieved when the nanowires flatten back onto the plane of the substrate. In the case of a prestrained sample that has subsequently undergone mechanical hot pressing, the nanowires are very wavy (and they all lie on the same plane as the substrate). During stretching of such a sample, the wavy nanowires “straighten out”. Hence, both prestrain (for creating buckled nanowires) and mechanical hot press (for pressing the out-of-plane buckled nanowires onto the substrate surface to form an in-plane wavy nanowire network and improved contact at the nanowire junctions) are essential for durable biaxially stretchable transparent conductors.