2.1. GAF Fabrication and Characterisation
To achieve graphene-based laminates with a high electrical conductivity, we introduce extremely large flake size graphene oxides (LGO) as the film’s building precursor to reduce the contribution of contact resistance. Fig. 1a is a schematic diagram of the fabrication process. The detailed fabrication procedures are described in Methods. With commercially standard equipment, GAF can be manufactured in a viable and scale manner, Supplementary Fig. 1-236. Fig. 1b is a transmission electron microscope (TEM) image presents a typical LGO sheet with a lateral size of 108 μm. In the corresponding size distributions curve from optical microscope measurement, Fig. 1c, the average LGO lateral size is around 110 μm, which is significantly larger than any previously reported study37,38. To compare, films made from typical size GO flakes, Supplementary Fig. 3, have also been fabricated as a control experiment and been tested accordingly. GAF was manufactured by an optimised compression and secondary high temperature (2850˚C) graphitization process. The XRD (Fig. 1d) and Raman spectroscopy (Fig. 1d and Supplementary Fig. 4) confirm GAF’s highly graphitized and defect free structure. Based on Raman measurement, a large crystallite size La = 1967.95 nm in the GAF is also confirmed according to Equation 139.
Small-angle x-ray scattering (SAXS) patterns of GAF, inserted in Fig. 1e, show a prismatic scattering pattern indicating GAF with highly aligned layered structure. In contrast, for typical flake size GO assembled film (TGF), Supplementary Fig. 5, no such pattern is identified40. Supplementary Fig.6 is the top view GAF SEM image, microfolds are uniformly distributed on the GAF surface. As shown in Supplementary Fig. 7, the cross-section SEM images of the GAF characterise a thin film with thickness of ~19 ± 0.5 μm. This thickness is similar to commercial copper foil applied in consumer electronic devices .
The electrical conductivity of all samples is measured by a four-probe method (Supplementary Fig. 8 and Supplementary Video 1). In Supplementary Fig. 9, the electrical conductivity of GAF and TGF fabricated under different conditions are characterized. The conductivity of GAF is 2.58 ± 0.06 × 106 S/m, which is 5 times higher than that of graphite film (5×105 S/m) and about 2 times higher than HOPG (1×106 S/m) and TGF (9×105 S/m), as shown in Fig. 1e. In Supplementary Fig.10, the electron transfer pathway between GAF and TGF is proposed as the mechanism for this ultrahigh electrical conductivity41. 5G WCE are powered by radio frequency (RF) signal, for which skin depth effect cannot be negligible. To better integrate in 5G WCE electronics, the thickness of conducting material in the device always demands a thickness of around 20 μm or below. The skin depth ẟ can be calculated by the following Equation 2.
where f is the frequency, μ is the permeability and σ is the conductivity of conductive material. For non-magnetic conducting materials, μ is taken as 4π×10-7 N/A2. Taking the Radio Frequency Identification (RFID) communication frequency band (860 MHz) specified in EPC Gen2 standard as an example, to reduce the resistance loss, the thickness of conductive material should be greater than skin depth. Regarding this, in Fig. 1f, a selective area which requires materials having electrical conductivity higher than 0.75 × 106 S/m is identified. For non-metal materials, it can be found that among all novel developed materials, only graphene films made by high temperature (over 2500 ℃) annealing can meet these requirements (Supplementary Fig. 11 and Supplementary Table 1-2,). Although metal materials, like gold (Au), silver (Ag), copper (Cu), aluminium (Al) and iron (Fe), has higher electrical conductivity, the apparent density of GAF (1.92 g/cm3) is significantly lower, shown in Supplementary Fig. 12-13. In Fig. 1g and Supplementary Video 2, we demonstrate that GAF can retain its ultrahigh conductivity even after 100,000 bending cycles test with a bending radius of 1.5 mm. The SEM images show the durability of GAF that can endure a zigzag folding without any breakage (Supplementary Fig. 14-15). A similar test was performed on a commercial copper foil with the thickness of 20 μm; the copper foil broke after only 12 folds (Supplementary Fig. 16).
To verify the excellent flexibility of GAF electronics, a flexible coplanar waveguide transmission line (FCPW TL) and a λ/4 short-circuited resonator based on silica gel film (Supplementary Fig. 17) are first designed and fabricated. As shown in Fig. 1h-i, FCPW TL’s transmission characteristics of the being bent and twisted at different conditions are investigated between the 10 MHz - 12 GHz frequency band., It is found that bending and twisting has negligible effects on the transmission coefficient (|S21|) of GAF transmission line. Fig. 1j-k illustrate the |S11| (between 10 MHz to 12 GHz band) of GAF λ/4 short-circuit resonator in the original state and different twist states. Like GAF flexible transmission line, the twist does not change the resonator’s performance.
2.2. GAF as a Dipole Antenna and its Highly Anticorrosion Properties
A GAF dipole antenna is used in this study as an example to compare with metal antennas in terms of device performance and corrosion resistance, which is a continuing concern in next-generation WCE working at higher frequencies and operating powers. Fig. 2a is the digital photo of GAF dipole antenna and copper antenna in the same pattern (design details in Methods and Supplementary Fig. 18). The measured gain of antennas in the 0.7 GHz - 1.0 GHz frequency band and at 865 MHz are shown in Fig. 2b-c, respectively. The GAF antenna has a gain of 1.89 dBi, which is comparable to the copper antenna of 1.94 dBi. This gain is much higher than that of graphite antenna (1.05 dBi), HOPG antenna (1.39 dBi), and TGF antenna (1.35 dBi). Fig. 2d-e present the 3D and 2D radiation patterns of GAF antenna at 865 MHz, also indicating a matching performance with copper antenna. The measurement environments for the antennas are exhibited in Fig. 2f-g. In Fig. 2h, the GAF dipole antenna exhibits the highest gain of any graphene-based dipole antenna reported in the literature42-46. In supplementary Fig. 19, the E-field distribution simulation results of the graphene dipole antenna in different phases indicates the current on the dipole arm is sinusoidally distributed.
The salt spray treatment is applied to the GAF and copper antennas to investigate their corrosion resistance, Supplementary Fig. 20. Fig. 2i-j are the digital photos of GAF antenna and copper anetnna after one week’s and two weeks’ salt spray: the copper antenna is rusted and then broken after the test, but the exterior of GAF antenna is unaffected. Fig. 2k illustrates the simulated and measured reflection coefficient |S11| at resonant frequencies of 865 MHz of the initial GAF antenna and after the salt spray treatments. The -10 dB bandwidth (BW) of the GAF antenna is from 790 MHz to 980 MHz, which covers the RFID communication frequency band. After salt spray treatment, the GAF’s |S11| does not change. However, the |S11| of copper antenna becomes only -6.25 dB at 865 MHz after two weeks of salt spray, Fig. 2l-m presents the measured gain at 865 MHz of GAF and copper antennas during the test, the gain of the GAF antenna is unchanged, but the copper antenna has a significantly lower gain of -2.74 dBi after two weeks of salt spray. This test confirms that, unlike metal based materials, because graphene is naturally more anticorrosive, no additional anti-corrosion coating is required in the GAF WCE to protect it from corrosion, which simplifies manufacturing and reduces weight.
2.3. Wearable Communication System and 5G Millimeter Wave Antenna with GAF
Fig. 3a illustrates the division of wireless communication spectrum with specified applications. Sub-6 GHz is currently the most mainstream mobile communication frequency band, such as 2.45 GHz in the Industrial Scientific Medical (ISM) frequency band and 3.5 GHz in the 5G communication frequency band47. In this work, we confirm that GAF antennas can be used to directly replace metal-based antennas in commercial sub-6 GHz electronic devices, like mobile phone and drone, without losing functionality, as shown in Fig. 3b-g, Supplementary Fig. 21-22, and Supplementary Video 3-4. To demonstrate GAF are capable of fabricating flexible Sub-6GHz WCE, we build an integrated terminal and base mobile communication system with all GAF based WCE, including flexible GAF wearable antenna, 2×2 GAF antenna array and GAF filter, as shown in Supplementary Fig. 23. The radiation patterns of GAF antenna and antenna array recorded for every 10° of rotation in microwave anechoic chamber, Fig. 3h. In Supplementary Fig. 24-25, a communication system integrated with all above GAF electronics is demonstrated. The gain and transmission coefficient of the flexible 5G GAF antenna and antenna array are measured, as shown in Fig. 3i. The gain of the two antennas reaches the maximum at the resonant frequency of 2.45 GHz, which is consistent with the measured |S21| (as in Fig. 3i, blue line). The measured results show that the GAF 5G WCE communication system can function reliably, and that the flexible 5G GAF antennas and transmission line are capable to fit and work on the human body to transmit signals.
Millimeter wave WCE, with the advantages of high directivity, small in size, high resolution, rich spectrum resources, and high information security, is an essential technology for the 5G mobile communication48. To investigate GAF’s abilities in millimeter wave 5G WCE, GAF-based 5G ultra-wideband (UWB) antenna and 5G antenna array are studied. Fig. 3j-k illustrate the geometry and present a digital photograph of the proposed 5G UWB antenna. The measured -10 dB bandwidth covers the frequency range of 1.6 GHz to 40 GHz with the bandwidth ratio of 25:1, Fig. 3l. Although the test frequency is up to 40 GHz, the actual impedance bandwidth of the 5G UWB antenna is much wider. To gain insight into the radiation characteristics, we simulated the surface current distribution of the UWB antenna at 6 GHz, 10 GHz, 20 GHz, and 40 GHz. As shown in Supplementary Fig. 26, current distribution demonstrates that the UWB antenna is working in its fundamental resonance mode at low frequency band and in higher resonance mode at high frequency band. A GAF 5G millimeter wave antenna array with the geometrical dimensions of 135 mm × 95 mm × 0.54 mm is fabricated, shown in Fig. 3m-n and Supplementary Fig. 27. The GAF 5G antenna array has 140 antenna elements (14 × 10), which accords with the Chebyshev current distribution. Fig. 3o illustrates the simulated and measured |S11| and gain from 25 GHz to 27 GHz. The GAF 5G antenna array operates at 26 GHz with a |S11| of -22.54 dB. The measured highest gain is 24.23 dBi, which matches with the simulated results. Fig. 3p-q and Supplementary Fig. 28 depict the measured E-plane and H-plane radiation patterns. Due to the anisotropy of GAF, the antenna array has a sidelobe performance below -20 dB in E-plane and H-plane: the beam width and sidelobe level of the E-plane and H-plane, respectively, are 8.5°, - 21.76 dB and 5.9°, - 24.94 dB. The simulated 3D radiation patterns of the GAF 5G antenna array at 26 GHz are shown in Fig. 3r, which also confirms the excellent sidelobe performance.
2.4. GAF for EMI Shielding
The EMI shielding performance of GAF in the frequency range of microwave band, millimeter wave band and teraherz (THz) band is explored. Firstly, the EMI shiedling effectiveness (SE) at the frequency range of 2.6 GHz - 40 GHz is simulated and tested by the rectangular waveguide method, Supplementary Fig. 29-30. Fig. 4a-c show the EMI SE of the GAF and commerical copper foils. The thickness of EMI SE produced from commercial copper foils (we tested thicknesses of 10 μm and 50 μm) has little effect on their EMI performance and presents an EMI SE of around 100 dB in the 2.6 GHz - 40 GHz frequency band. This is due to the fact that copper fully reflects electromagnetic waves and the absorption effect is negligible. Due to the high conductivity, GAF exhibits ultra-high EMI SE. Addtionlly, because of its natural laminate structure, which causes absorption and multiple internal reflections9, increasing the thickness of GAF can improve the EMI SE. The proposed GAF’s EMI shielding mechanism is in Supplementary Fig. 31. GAF's EMI SE with a thickness of 15 μm is greater than 80 dB and can achieve 90-100 dB above 6 GHz. When the thickness increase to 50 μm, the EMI SE of GAF rises to around 110 - 120 dB, especially at 13.5 GHz, where it reaches up to 127.3 dB. When the frequency exceeds 26 GHz, the GAF with a thickness of 15 μm exhibits the same EMI SE as copper foil with a thickness of 50 μm. Furthermore, We use the free space method (Supplementary Fig. 32) to evaluate GAF's EMI shielding performance over the frequency ranges of 40 GHz-67 GHz, 75 GHz-110 GHz, and 0.22 THz-0.325 THz. As shown in Fig. 4d-f, GAF outperforms copper foil in terms of EMI shielding as frequency increases. The EMI SE of the GAF with a thickness of 15 μm and 50 μm is around 60 - 80 dB and 80 - 100 dB, respectively, which is higher than the 40 - 60 dB of copper foil. We compared the recently reported EMI shielding performance of graphene-based and other materials, as shown in Fig. 4g. GAF is the electromagnetic shielding material that is closest to the ideal area, with the highest SE in the same thickness and the thinnest thickness in the same SE. Due to the importance of thickness to EMI shielding materials, SMI SE per unit thickness (SE/t) is used to characterize the EMI performance. Among other graphene structures, carbon nanotubes, carbon fibres, and MXene, GAF has the highest SE/t (6966 dB/mm), as shown and compared in Fig. 4h and Supplementary Table 3.
2.5. GAF Applied in FSS at Sub-6 GHz and Millimeter Wave Frequencies
In addition to complete electromagnetic shielding, selective shielding of the electromagnetic field is also important in many cases to ensure the normal transmission of other frequency bands. As a member of metamaterials, frequency selective surface (FSS) is formed by periodic arrangement of structural units, which can selectively absorb, reflect and transmit electromagnetic waves49, and thus is an effective way to realize frequency selection. To further explore the electromagnetic protection performance of GAF, we developed two transparent, flexible FSS that work in Sub-6 GHz and millimeter wave bands, respectively. Firstly, a miniaturized flexible FSS based on GAF that works at 3.5 GHz is designed, Supplementary Fig. 33. The GAF FSS is with 12×12 periodic elements, fabricating by a high precision laser engraving. The structure dimensions are 192 mm×192 mm×0.075 mm, as shown in Fig. 5a, with the optimised parameter values shown in Supplementary Fig. 34 and Table 4. GAF FSS is translucent and has a very low areal density of 0.0087 g/cm2. GAF FSS is very flexible and can conform to curved surfaces. Because of the presence of the bending line, the edge of the periodic element structure has a strong parasitic capacitance, as shown in Supplementary Fig. 35, which can miniaturise the FSS. A high parasitic capacitance can also assisst FSS maintain its angular stability. The equivalent circuit diagram of the GAF FSS element is shown in Supplementary Fig. 36. The transmission coefficient results of the periodic element are shown in Supplementary Fig. 37. The FSS has a resonance frequency of 3.5 GHz, agreeing with the simulation results. In Fig. 5b, the E-field distribution can be used to investigate the working mechanism of FSS. It is clear that the FSS resonates at 3.5 GHz and generates a large induced current, preventing electromagnetic waves from passing through the FSS. The measured transmission coefficient and shielding efficiency of GAF FSS under normal incidence of electromagnetic wave is shown in Supplementary Fig. 38-39. Supplementary Fig. 40 is the measuring environment. In the 3.38 - 3.91 GHz frequency band, the transmission coefficient of GAF FSS is less than -10 dB, which means that FSS can shield more than 90% of electromagnetic waves. In particular, GAF FSS can shield 99.4% of electromagnetic waves at 3.5 GHz. Outside of this frequency band, electromagnetic waves can pass GAF FSS in a large proportion, proving that GAF FSS has good frequency selection characteristics. Fig. 5c are the spectrum curve of GAF FSS at different incident angles of electromagnetic waves, the transmission coefficient and working bandwidth of GAF FSS remain essentially constant in the 0-25° range, indicating that GAF FSS has high angular stability.
We also designed a low profile and ultra-wideband flexible millimeter wave FSS with a thickness of 0.138λ. The element structure of millimeter wave FSS is illustrated in Supplementary Fig. 41. The digital photos shown in Fig. 5d and Supplementary Fig. 42 demostrate the FSS’s flexibility. The FSS has the physical dimensions of 125 mm × 125 mm × 0.065 mm implanting with 1600 elements. Supplementary Fig. 43 illustrates the simulated transmission coefficient. GAF millimeter wave FSS covers a wide -10 dB bandwidth of 8.16 GHz between 55.76 GHz and 63.92 GHz. Supplementary Fig. 44 is the surface current distribution of the GAF millimeter wave FSS at 60 GHz. It can be seen that the FSS resonates at 60 GHz and generates a strong induced current, hindering the passage of electromagnetic waves at 60 GHz. Supplementary Fig. 45 shows the measured transmission coefficient and shielding efficiency of GAF millimeter wave FSS under normal incidence of electromagnetic wave. The GAF FSS can shield 99.9% of electromagnetic waves at 60 GHz. Different bending states of GAF FSS and incident angles of electromagnetic waves can be achieved by varying the distance between two brackets and rotating the disc. As shown in Fig. 5e, the frequency selection performance of GAF FSS remains stable within a bending angle changing between 0° to 40°. Fig. 5f illustrates that GAF FSS maintains good frequency selection performance in the range of electromagnetic waves ± 30° incident angle.