In situ mechanical characterization of silver nanowire/graphene hybrids films for flexible electronics

ABSTRACT Flexible transparent conductive films are indispensable for nowadays wearable electronic devices with various applications. However, existing solutions such as ITO and metal mesh were limited by their poor intrinsic stretching ability. In this work, we designed and fabricated silver nanowires (AgNWs) on graphene hybrid films for enhanced mechanical and electrical performance. In situ TEM characterizations show that, beside conductive paths, silver nanowire network, can also contribute to the toughening mechanisms of the hybrid films. Furthermore, bending and electrical tests were applied to examine the corresponding flexible electronics’ performance. Finally, we showed that the fabrication of our AgNW/graphene hybrid films could be scaled up for large film applications and extended to other 1D/2D hybrid systems. Graphical Abstract


Introduction
With the emerging demands for flexible electronics, highly transparent, conductive, and flexible films have attracted great research interests for their great potential in these applications [1][2][3]. The intrinsic brittleness nature of commercial indium tin oxide (ITO) has limited its applications in flexible electronics [3,4], novel films with highly transparent, electric, and flexible properties thusly need to be pursued. Metal nanowires and graphene have been selected to fabricate large scale flexible transparent conductive films due to their excellent optical, electrical, and mechanical performance [3][4][5][6][7]. Good electrical conductivity and large stretchability make silver nanowires (AgNWs) a good candidate for flexible transparent conductive films [8,9]. For instance, large-area transparent AgNWs films have been fabricated with sheet resistances less than 10 Ω/sq and transparency 80%-90% [6,[10][11][12]. Increased AgNWs density will increase conductivity, while also reduce transparency. Therefore, a trade-off in the Ag nanowire network density needs to be reached. Since flexible transparent films will also undergo many deformations in their device applications such as bending, stretching, and twisting [13,14], loose connections of the nanowires will result in the degradation of the mechanical and functional performance. To increase the connections of the Ag nanowires, techniques such as plasmonic welding, thermal annealing, room-temperature nanosoldering, roll compression, and joule heating have been developed with reduced contact resistance and increased mechanical stability [11,[15][16][17][18][19]. However, AgNWs show poor ambient and thermal stability. AgNWs can be easily oxidized in air and agglomerate into balls during thermal annealing due to their large surface-to-volume ratio, which has become a major challenge for the broad applications of AgNWs transparent films.
Due to the high electrical conductivity, high thermal conductivity, and high intrinsic strength of graphene, it also has been studied as flexible transparent conductive films [5]. Our previous work has demonstrated large-scale monolayer graphene with sample-wide elastic strain up to ~6%, and engineering tensile strength reaching ~50-60 GPa [20]. Multilayer graphene nanosheets show a decreasing trend in the ultimate fracture strength, and different fracture manner compared with the brittle fracture of the monolayer graphene [21]. However, the low toughness of graphene limits its applications in flexibles devices with the existence of defects [22]. Besides, large-area graphene grown on the substrate needs to be transferred to the desired substrate for application. Wrinkles and cracks formed during the transferring procedure will decrease the electrical and mechanical stability of graphene under real applications [23]. By integrating graphene with CNT, the hybrid film showed increased toughness, with the embedded CNTs deflecting and bridging the propagating cracks [24]. Theoretical simulations also indicate that the modulation of the interface interaction can effectively control the mechanical properties of graphene nanostructures [25][26][27]. Furthermore, through the integrating of metal nanowires and other nanomaterials, the electrochemical, mechanical stabilities, and other properties of the hybrids are also improved [28]. By combining graphene oxide film with AgNWs, the fabricated films show a transmittance of 86% and sheet resistance of 150 Ω/sq [29]. Through a filtration method, the fabricated AgNW/graphene hybrid paper shows excellent flexibility with only <5% electrical conductivity loss over 500 times mechanical bending [30]. Single-layer graphene and AgNWs networks and mesh are also utilized for the fabrication of flexible transparent conductive films for electrical, optical, and thermal applications with improved mechanical stability [31][32][33][34][35][36][37][38][39][40][41]. However, the microstructure evolution and mechanical deformation process of the AgNW/graphene hybrid film have not been investigated, which is fundamental for the understanding of the mechanism for electromechanical performance enhancement for real applications.
Here in this work, we used a spin coating method for the fabrication of the AgNW/ graphene hybrid film. We then demonstrate the excellent connection between the AgNWs and graphene, via step-by-step fracture manner, and the deflection of the crack propagation through observation during in situ TEM tensile testing. The hybrid film also shows improved electrical performance through cyclic bending and electrical tests. This work would give useful insights for the design and optimization of AgNW/graphenebased flexible transparent films for potential industrial applications in flexible and wearable devices.

Samples and experimentals
AgNW/graphene hybrid films were synthesized through a spin coating method as described in Methods. Heat treatment at 250°C for 10 min was further conducted to make the AgNWs welded together. The hybrid film was illustrated in Figure 1(a), AgNWs randomly distributed on the graphene film, with good contacts at the junctions, as shown with a red arrow in the zoom-in image (Figure 1(b)). The good connections of the AgNWs were confirmed in SEM images (Figure 1(c-d)), at the intersects of the AgNWs, the welded part expanded denoted with a red arrow. The mechanical tests at microscale were conducted as shown in Figure 1(e-f) using the Hysitron TM Pico-Indenter (PI) 95 TEM holder. 'Push-to-pull' (PTP) devices were mounted on the front part of the TEM holder, as denoted with red rectangle shape in Figure 1(e). The hybrid film was clamped between the two sides of the device, and then the device was mounted on the TEM holder for in situ mechanical tests.
The electromechanical tests at macroscale were carried out using a microtest machine (DEBEN) (Figure 1(g-h)). AgNW/graphene hybrid film on polyethylene terephthalate (PET) substrate was clamped on the microtest machine between the two grippers as denoted with red rectangle shape ( Figure 1(g)). The two bright bars in Figure 1(h) (indicated with red arrows) were Ag electrode sputtered on the PET substrate for electrical measurements. The hybrid film protected with a thin layer of PMMA covered on top of the Ag electrodes.

In situ micro-mechanical characterization of AgNW/graphene hybrid films
AgNW/graphene hybrid films synthesized were characterized using TEM ( Figure 2). The AgNWs distributed evenly on the graphene film ( Figure 2(a)). The AgNWs overlapped with each other and welded at the junctions. One of the junctions is shown in Figure 2 (b). The AgNWs underwent morphological changes at the crossing point. While away from the crossing point, the Ag nanowire kept its pristine morphology with a surface change controlled by the Rayleigh instability phenomenon [42,43]. The energydispersive spectroscopy mapping results also confirmed the Ag and C elements of the nanowire and the film beneath the nanowire, respectively (Figure 2(c-d)). HRTEM image of the edge of graphene (indicated with an arrow in Figure 2(b)) is shown in Figure 2(e), which suggests the graphene of multilayer nature and layer number of ~5. Furthermore, the HRTEM in Figure 2(g) shows a straight line at the junction between the two nanowires. The crystal orientation is quite different at the two sides of the line. Away from the line, the two nanowires kept a relatively uniform crystalline structure, making the line as a clear boundary between the two nanowires.
The fabricated AgNW/graphene hybrid film was transferred to the PTP device and attached on the PTP device with conductive silver glue. Then the film between the gap was cut into a rectangle shape for the tensile testing. A focused ion beam (FIB) cut sample for the in situ tensile testing is shown in Figure 3, and the hybrid film was cut and left with only a small part in the middle. The AgNWs are firmly attached to the graphene film. During the tensile testing process of the AgNW/graphene hybrid film, the gap will be opened, and the hybrid film will be under tension until fracture. The process is shown in Figure 3(a). The distance of the gap is increasing; thus, the hybrid film is under increasing tensile strain from top to bottom. The fracture showed a step-by-step fracture process phenomenon. With the increase of the displacement, the film fractured step-by-step, which is in accordance with three load drops in the load-displacement curve in Figure 3(b). The fracture strain of the hybrid film was calculated at ~4.2%. The load-displacement curve can be separated into two parts, first part with a higher slope of ~737 N/m, and after fracture the second part with a lower slope of ~162 N/m. The hybrid film sample and the PTP device both contribute to the load in the first part, while after fracture of the hybrid film, only the PTP device contributed to the load. Then the stiffness of the sample was calculated to be 575 N/m. The Young's modulus of the hybrid film was derived from the following equation: E ¼ k l t G nW [44], where E represents the Young's modulus of the sample; k is the stiffness of the sample. t G is the thickness of monolayer graphene (which is 0.335 nm), n is the layer number of the graphene film; l, W represent the length and the width of the hybrid film sample, therir values are 2.6 μm and 5.6 μm, respectively. The calculated E is ~160 GPa. Thus the fracture strength of the sample is ~6.7 GPa.

In situ macro-electromechanical characterization of AgNW/graphene hybrid films
The in situ macro-scale electromechanical properties of the AgNW/graphene film were further characterized. During the in situ bending tests, the displacement control was set at a speed of 1 mm/min for all the tested samples. As illustrated in Figure 4(a), the motor at the left part of the micro tester will acute the left gripper toward the right fixed gripper and resulting in the bending deformation of the sample in the middle. The electromechanical performance of AgNW/graphene hybrid film was compared with the pure AgNWs film (fabricated using the same volume and concentration of AgNWs solution with the hybrid film) and pure graphene film. The pristine current of the hybrid film (Figure 4(d)) is higher than the pure AgNWs (Figure 4(b)) and graphene films (Figure 4(c)). The three films all showed a decreasing trend of current with the increasing of the bending cycles. For the pure AgNWs film, the current decreased to nearly 0 A after six bending cycles (Figure 4(b)). The failure of the junctions may be contributed to the fast decreasing of the current during the bending tests. For the pure graphene film, the current reached a steady level after the initial current drop at the beginning of the bending test (Figure 4(c)), the relatively lower pristine current indicates the graphene has cracks which may be generated during the transfer process. The current quickly reached a steady level after the initial drop indicating the robustness of the graphene film (excellent mechanical properties of graphene) during the 14 cycles. For the AgNW/graphene hybrid film, the current showed a relatively slower decreasing trend during the 32 bending cycles (Figure 4(d))) compared with the pure AgNWs and pure graphene film. The response to the bending tests of the hybrid film is more sensitive as the peaks and valleys showed a current gap of ~5-10 mA. The good electrical conductivity of AgNWs and graphene, the mechanical robustness of graphene film, both contribute to the electrical performance under cyclic bending tests.

Discussion
The fracture morphology is further shown in Figure 3(c-e). The fracture path is not straight as shown with red dashed lines in Figure 3(c). The crack propagated and deflected at the AgNWs, which shows the AgNWs have an effect on the crack propagation in the hybrid film. The fracture surface of the AgNWs showed obvious necking and significant defects density at the fracture points, and this indicated the AgNWs underwent plastic deformation before fracture. The AgNWs and the graphene film both fracture at the same locations manifested that there was no sliding between the AgNWs and the graphene film, the interface interaction of the AgNWs and the graphene film is quite significant. Our hybrid film fabrication method can also be potentially extended to large-scale fabrication of other 1D/2D hybrid films through 'cold welding' and other methods [18,40,45].  The in situ tensile testing of the AgNW/graphene hybrid film results showed the hybrid film of Young's modulus of ~160 GPa and fracture strength of ~6.7 GPa at a strain of ~4.2%. The calculated Young's modulus of ~160 GPa is much smaller than the common values of ~900-1000 GPa for graphene. However, the interlayer shear stiffness and strength of bulk 2D materials is typically orders of magnitude lower than their in-plane Young's modulus and tensile strength. The interlayer shear effect of the multilayer graphene film may lead to the weakening of the stiffness [46,47]. Moreover, the van der Waals interface connection between the AgNWs and graphene film and the cross-link density of the nanowires also can contribute to the stiffness of the hybrid film [25][26][27]48,49]. Compared with a pure graphene film, the hybrid film showed a comparable fracture strain [20,21]. Besides, the length of the AgNWs may also have an effect on the mechniacl performance for large-sized hybrid films, as the longer nanowires would be easier formed into connected networks. The hybrid film showed higher modulus compared with AgNWs [9]. No sliding between the AgNWs and the graphene film showed an excellent interface connection. The above results indicate the AgNW/graphene film with better electromechanical performance under bending tests, makes it of great potential in the applications of transparent conductive films and flexible electronics.

Conclusion
In conclusion, the AgNW/graphene hybrid film was fabricated through a spin-coating method. The in situ TEM tensile testing of the hybrid film showed an excellent interface connection between the AgNWs and the graphene film and Young's modulus of ~160 GPa, and fracture strength of ~6.7 GPa at a strain of ~4.2% of the hybrid film. The AgNWs fractured with plastic deformation at the same point as the graphene film. Moreover, the crack would deflect at the AgNWs, which shows the AgNWs affect the crack propagation of the hybrid film. Furthermore, the macroscale electromechanical study of the hybrid film shows the hybrid with better cyclic performance under bending tests compared with pure AgNWs and pure graphene film, indicating the excellent combination of the AgNWs and graphene indeed enhanced both the mechanical and electrical performance of the hybrid films. We expect our works would contribute to develop high performance graphene-based metallic nanostructure network for advanced flexible and wearable electronics devices and systems.

Fabrication of the AgNW/graphene hybrid film
The AgNW/graphene hybrid film is fabricated through a spin coating method. First, AgNWs (with lengths ~10-30 μm, diameters ~20-50 nm)were dispersed into ethanol and ultrasonicated for 5 min. Then, AgNWs were spin-coated on the CVD synthesized graphene (1 cm *1 cm, a silicon substrate with a 300 nm layer of SiO 2 ) at a speed of 300-500 rpm. Then, the Ag NWs/graphene on the SiO 2 /Si substrate was put on a hot plate and heated to vaporize the ethanol to make the Ag NWs welded together and increase attachment between the Ag NWs and the graphene. After that, the Ag NWs/graphene hybrid film was covered with a Poly(methyl methacrylate) (PMMA) protective layer using a spin-coater (3000 rpm for 60 s), followed by heat treatment (180°C, 1 min) to cure the PMMA, and then dried in a vacuum chamber for 15 min to increase the attachment between different layers. Additionally, the PMMA coated Ag NWs/graphene hybrid film was removed from the SiO 2 /Si substrate through an etching method [50]: a 1 M KOH solution was prepared and heated up to 90° C; then, the PMMA coated Ag NWs/graphene hybrid film on the SiO 2 /Si substrate was put into the KOH aqueous solution to etch off the SiO 2 layer; finally, the suspended PMMA coated Ag NWs/graphene hybrid film was fished up using a substrate.

Transfer of the AgNW/graphene hybrid film for morphological characterization
For the morphological characterization of as-prepared Ag NWs/graphene hybrid film, a clean silicon wafer was used as the substrate to fish up the suspended hybrid film in the KOH aqueous solution. The silicon wafer was pretreated with acetone and followed by ethanol in an ultrasonic bath, each process lasting 10 min. Then the substrate was rinsed in deionized water and dried with nitrogen gas, and then oxygen plasma cleaned to make the surface hydrophilic. After fished up the hybrid film, the Si substrate was dried in a vacuum chamber for 30 min to make the hybrid film fully attached to the Si substrate. Finally, the PMMA layer was removed by immersing the sample into acetone. Furthermore, the morphology of the hybrid film was investigated inside SEM.

Electromechanical testing of the AgNW/graphene hybrid film
For the macro-scale electromechanical testing, the PMMA coated Ag NWs/graphene hybrid film was fished up with a PET substrate. The PET substrate was pretreated with acetone and followed by ethanol in an ultrasonic bath, each process lasting 10 min, and then the substrate was rinsed in deionized water and dried with nitrogen gas. Ag electrodes were deposited onto a rectangle shape PET substrate for electrical measurement through magnetron sputtering at room temperature. The Ag (99.99%) target was used to prepare the Ag thin film electrodes with a nominal total thickness of ~50 nm. The sputtering chamber was evacuated before deposition with pressure below 5 × 10 −4 Pa. The rotate speed of the plate was 10 r·min −1 with the 1.0 Pa working pressure during the deposition process to homogenize the alloy composition and film thickness. The following electromechanical tests were carried out with a microtest machine (DEBEN) at a speed of 1 mm/min for load-displacement control and with an electrochemical station (CHI 760E) for electrical measurement.

In situ TEM mechanical characterization of the AgNW/graphene hybrid film
For the micro-scale mechanical tensile testing, the PMMA coated Ag NWs/graphene hybrid film was fished up with a PTP device to study the mechanical properties of the hybrid film. Then the PTP device loaded with the hybrid film was left in the dry cabinet overnight. The in situ mechanical tensile testing was carried out inside TEM with the PI 95 TEM holder.