Structure and morphology of the GWF

GWFs were grown by atmospheric CVD using copper meshes as substrates (see Methods section). Fig. 1a shows the three main steps used in the fabrication of GWFs: (i) CVD growth of graphene on copper mesh, (ii) removal of copper wires with FeCl 3 /HCl aqueous solution and (iii) collapse of graphene to form double layered GMRs. The CVD grown GWF retains the network configuration of the copper mesh. For the cases described here, the copper mesh consists of copper wires (~60 μm in diameter), arranged in a crisscross pattern. Fig. 1b shows the optical and plan-view scanning electron microscope (SEM) images of a copper mesh before and after graphene growth. In the next step, the GWFs can be collected from the liquid surface with desired target substrates. The details are described in the Methods section. Fig. 1c shows the as-obtained GWF films floating on water and deposited on glass and PET, clearly demonstrating the well-aligned arrays of GMR lines, with width and spacing of ~100 μm and ~150 μm, respectively. Every piece of the GWF is an integration of warp and weft GMRs through intersection. The enhanced contrast in Fig. 1c shows that the GMRs are clearly distinguished from the surrounding non-deposited areas, revealing the grid structure, thus demonstrating the precisely controlled dimension of the GWF pattern.

Figure 1 Fabrication of GWFs by CVD using copper wire meshes as substrates. (a) Schematic of steps for GWF preparation. (b) Macroscopic optical images (left), top-view SEM images (right) of copper meshes before (top) and after (bottom) graphene growth. Scale bars, 200 μm. (c) Optical images of GWF films floating on water and deposited on glass and PET. Scale bars, 5 mm. (d) TEM image of a GMR and selected area electron diffraction pattern from the region marked with a yellow box. Scale bars, 50 nm (left), 5 (1/nm) (right). Full size image

Fig. 1d shows the TEM image of a GMR and the corresponding electron diffraction pattern. TEM studies reveal that the GMR surface is clean and homogeneous. The electron diffraction pattern shows the typical six-fold symmetry expected for graphene and graphite. The diffraction pattern shows two sets of the hexagonal spots with 26° rotation between them. This occurs because the electron beam simultaneously probes the front and back layers of the GMR, revealing its overlapping structure (as shown in the inset). Diffraction spots are labeled using Miller-Bravais (hkil) indices. The analysis of the diffraction intensity ratio I {1–210} /I {0–110} (~0.7) reveals that the graphene is in fact few-layered21. Monolayer graphene has been grown on copper mesh; however, they are too fragile to form a monolithic structure upon copper etching. Raman spectra of the regions marked in Fig. S1 clearly show the evidences of bilayer and few-layer graphene, which are confirmed by the 2D-band (2700 cm−1) with a full width at half maximum of 40~60 cm−1 and by the moderate ratio (0.6~1.6) of the 2D, G peak intensities22, respectively. The weak peaks of D-band in the spectra reveal the presence of imperfections (e.g., surface defects, wrinkles, edges, grain boundaries) in the GWFs.

For comparison, nickel meshes were also used to grow GWFs by a non-equilibrium surface segregation process based on a carburization/decarburization mechanism. The multilayer nature of CVD graphene formed on nickel greatly delayed the etching process and left residual nickel-carbon compound cores inside GMRs. As shown in Fig. S2, the so-obtained GWFs possess higher structural integrity compared with GWFs grown on copper. To demonstrate its mechanical strength, the GWF was collected by a quartz O-ring and after drying it can be self-supported.

Fig. 2 provides a large-area optical image (Fig. 2a) and magnified top views (Fig. 2b) of regions of a representative sample deposited on a silicon wafer. It can be clearly seen that the so-obtained GWF has a planar structure produced by interlacing two sets of GMRs which pass each other essentially at right angles, forming a self-locked planar fibrous system. Less transparent areas in Fig. 2b can be attributed to the folding and overlap of a single layer or the overlap of multiple layers and the darkest areas result from crumpled regions. The mild etching process keeps the integrity and configuration of the textile structure of the copper mesh, as is evident by the cross-sectional view in Fig. 2c. The lower and upper parts of overlapped GMRs at the region of cross linking would adhere and stick together.

Figure 2 Large-area, CVD grown GWFs in supported and free-standing states. (a) Large-area optical image of a representative region of a GWF film. Scale bar, 100 μm. Inset shows the optical image of a nylon woven fabric for comparison. Scale bar, 200 μm. (b) Top-view optical image (Scale bar, 50 μm) and (c) High magnification SEM images of cross-sectional views of the interlacing points of GMRs. Scale bars, 5 μm (top), 1 μm (bottom). (d) Optical images of flexible GWF/PDMS composite films. Scale bar, 5 mm. The top inset shows the twisted GWF film by tweezers. The bottom inset shows the cross-section view SEM image of the composite film. Scale bar, 100 μm. Full size image

GWF/polymer composites

We then show the fabrication of GWF/polymer composite films (bonding with inert, transparent polymer, e.g., PDMS). To obtain the GWF/PDMS composite mesh, before etching away the copper skeleton by FeCl 3 /HCl solution, a thin layer of PDMS with controlled thickness was deposited on the surfaces of the graphene-coated copper wires, followed by cross-linking and curing the PDMS as a support to prevent graphene networks from collapsing during copper etching. As shown in Fig. 2d, real free-standing GWF/PDMS composite films can be obtained after copper etching. In spite of the thin layer of PDMS coating, this composite structure still has good optical and electrical properties. As revealed in the insets of Fig. 2d, the free-standing GWF/PDMS composite film can be twisted by tweezers without fracture. The use of the PDMS support layer was critical for preparing the free-standing GWF composite film. This method can keep the 3D structure of GWFs (see bottom inset of Fig. 2d). As further revealed in Fig. S3, the composite film is woven by interconnected graphene/PDMS micron-tubes, forming a multi-joint channel system. During CVD growth, two interconnected copper wires will melt and join together at the crossing point at 1000°C, resulting in a monolithic structure of the copper mesh. Carbon was introduced by decomposing methane and graphene was then grown on the surface of the interconnected copper wires. In the resulting composite (Fig. S4), graphene serves as a lining of the PDMS hollow structure and could be used as protective coating, reinforcement phase, electrical and thermal conducting channels in the composites.

To form fully filled GWF/PDMS composites (Fig. S3a and Fig. S4), a large amount of PDMS has been applied. The transparency of the composite film is high as the GWF is semi-transparent and the GWF mesh was planarized and stabilized by filling the spacing between GMRs with PDMS.

Tunable morphology and optoelectronic properties

The diameter of the copper wire and the mesh density directly determine the structure of GWFs. The three frames of Fig. 3a show optical images of three GWFs produced using copper meshes woven with 100 μm thick wires of different packing density. The corresponding representative small-area SEM views of these three samples are shown in Fig. 3b. Visual inspection of the samples and high-resolution images collected from them indicate excellent spatial uniformity and low defect density. The packing density of GMRs is also tunable by post manipulation (Fig. S5), for instance, controlling the liquid flow during film transfer.

Figure 3 Macroscale GWFs with tunable GMR packing density. (a) Macroscopic optical images of three different GWFs (Scale bars, 300 μm) and (b) corresponding representative small-area SEM views of these three samples. Scale bars, 200 μm. (c) Ultraviolet–visible-near infrared transmission spectra and (d) transparency (at 550 nm) versus sheet resistance plots. Full size image

Due to the unique configuration, the GWF becomes inherently strong (compared with polycrystalline graphene films) and conductive. The optoelectronic performance in terms of sheet resistance (R s ) and transparency of GWF strongly relies on the geometrical parameters, such as GMR width, spacing (related to graphene coverage density). As shown in Fig. 3c,d, the film transparency is varied owing to the cavity of the GWF. The R s of 500~2500 Ω/sq and 200~1200 Ω/sq after HNO 3 treatment at the corresponding transmittances of 50%~90% are obtained. The resistance values still cannot meet the criteria for transparent conducting applications that may replace the conventional indium tin oxide (ITO). The combined conductivity and transparency property of the GWF is also not advantageous due to its polycrystalline structure and the presence of surface wrinkles which will greatly reduce the conductivity and transparency of GMRs, in turn, GWFs. The conductivities of the GWFs are expected to be improved by synthesis process optimization23 and post acid doping24.

GWF/PDMS hybrid films

GWFs were transferred on PDMS substrates to make GWF/PDMS hybrid films and their mechanical properties (in tension) were studied. The interlacing (or crossing) points are the major locations where interactions between GMRs take place, through which the GWF forms an interlocked structure. We demonstrate that conductive GWF/PDMS hybrid films can act as electromechanical sensors under uniaxial tension. Tensile test accompanied by simultaneous electrical measurement shows a significant increase of resistance during the tensile cycle. The strain dependent resistance variations of GWF/PDMS films were investigated and summarized in Fig. 4a. As shown in the schematics (left panel), the X direction is defined as the direction of GMR lines and the XY direction is defined as the diagonal direction.

Figure 4 GWF/polymer hybrid films. (a) Resistance-strain curves for GWF/PDMS hybrids along different directions. Inset shows the schematics and corresponding optical images. Scale bars, 300 μm. (b) Electromechanical properties of the GWF/PDMS films. Resistance change relative to the original value (ΔR/R 0 ) recorded for a number of cycles at tensile strains of 2% and 5%. (c) Stretchable sensor fixed to a finger and relative changes in resistance for finger motion. Insets show corresponding photographs. Full size image

The electromechanical response differs along different directions and under different strains. The hybrid film shows certain flexibility and can be stretched within a relatively low strain range (<5%) and the resistance returns to the initial value upon unloading (Fig. 4a). Irreversible tensile strain and resistance changes occur at relatively high strain levels. The hybrid film shows a significant jump of resistance (by several orders of magnitude, up to 105 times) when it was stretched to a strain of 12% (Fig. 4a), due to the fracture occurring along the boundaries within the polycrystalline GMRs (Fig. S6). When the hybrid film is returned to 0% strain, the resistance increases to 3~5 times of its original value. In comparison, when stretched along the XY direction, the quadrilateral GMR network is self-adjusting. The stress applied on GMRs is smaller than that when the same strain is applied along X direction. If the PDMS substrate was pre-stretched before GWF deposition, the resistance variation becomes more gently. As shown in Fig. 4a, the resistance change is only 3 times when a pre-stretched sample was stretched to 10% along XY direction. Similar resistance change has been observed in several other graphene/PDMS25 and CNT/PDMS26 composites under stretching. Our GWF/PDMS hybrid film shows reversible resistance change (ΔR) relative to the original value (R 0 ) over many cycles and maintains a non-linear relationship versus strain in every cycle. Fig. 4b shows the results at maximum stains of 2% and 5% for a GWF/PDMS sample along X direction. The resistance shows an increase of 25 times under 2% strain and 230 times under 5% strain. Based on this result, the GWF can not only be used as a strain sensor for composites, if high resolution and large image fields of view could be obtained, the resistance variation and optical diffraction from optical images might provide information for the deformation of the composite. As a demonstration, the GWF/PDMS hybrid film was further fixed to an index finger to perform as a stretchable sensor. The resistance was detected at different finger motion. As shown in Fig. 4c, taking about 45° as a step, the resistance of the hybrid film increases with finger bending, which is similar to the results in the stretching experiments.

GWF/Si Schottky solar cells

Alternative approaches to achieve transparent electrodes have investigated the use of with conductive nanostructures27 such as graphene, CNTs and metallic nanowires, or conductive polymers such as poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS). Though the GWF is not a good window electrode material for transparent conductor applications, the periodic voids can be filled with PEDOT, or served as permeable membrane of electrolytes for photovoltaic applications. Our techniques have provided transparency on the order of 70%~80%, with conductivities in the 1000 Ω/sq range. GWF/Si solar cells were first fabricated from the GWF transparent electrodes and compared with similar devices fabricated using graphene film28. The solar cells with GWF electrodes outperformed devices having graphene transparent electrodes in efficiency (η) (>2% and up to 3.8%, see Fig. S7). While these parameters are sufficient for some flexible electronics applications, the resistive losses remain too high for many scaled applications. The discontinuity of the GWF films also leads to other issues impacting device performance. As shown in Fig. 5a, the GWF-Si solar cell was doped by HNO 3 fume. Acid infiltration of GWF networks boosts the cell efficiency by reducing the internal resistance to improve the fill factor (FF) and by forming photoelectrochemical units that enhance charge separation and transport29,30. In this way, the efficiency of solar cells could reach 6.1% (2-fold increase), with V oc of 0.53 V, short-circuit current density (J sc ) of 15.6 mA/cm2 and fill factor (FF) of 58.4 % at AM 1.5 (80 mW/cm2).

Figure 5 GWF-based solar cells. Device schematics, energy band structure diagrams and J-V curves of (a) GWF/Si solar cell, (b) PEDOT filled GWF/Si solar cell and (c), hybrid Schottky and PEC GWF/Si solar cell. Full size image