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(a) Continuous roll-to-roll CVD system using selective Joule heating to heat a copper foil suspended between two current-feeding electrode rollers to ∼1000 °C to grow graphene (Gra.). (b) Reverse gravure coating of a photocurable epoxy resin onto a PET film and bonding to the graphene/copper foil, followed by curing of the epoxy resin. (c) Spray etching of the copper foil with a CuCl2 solution. (d) Structure of the fabricated graphene/epoxy/PET film.
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(a) Optical microscopy image of the graphene film (J = 82 A/mm2) grown on copper foil after baking at 180 °C. The image is captured 20 mm from the edge of the copper foil. The light red areas in the image represent the copper surface covered with graphene and the darker areas represent the oxidized copper surface. (b) SEM image of the graphene film (J = 82 A/mm2) on an SiO2/Si substrate transferred from near the center of the copper foil. (c) Coverages of graphene at different positions from the edge of the copper foil having a (001) copper surface. (d) Raman spectrum of graphene on an SiO2/Si substrate measured at an excitation wavelength of 437 nm and a spot size of 1 μm. Raman G- and 2D-band peaks can be fitted with a single Lorentzian curve, indicating that the film consists of predominantly single-layer graphene. (e) Photograph of the graphene/epoxy/PET roll before doping. The widths of the graphene/epoxy and base PET film are 210 mm and 230 mm, respectively. (f) Optical transmittance of the base PET film (blue), AuCl3/graphene/expoxy/PET (red), and PET/adhesive/AuCl3/graphene/epoxy/PET (purple). The transmission loss includes the reflection at the film surface.
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(a) Decay of carrier density measured by the Hall effect in the van der Pauw geometry at a magnetic field of 0.7 T and electrode separations of 10 mm. Each data point corresponds to an average of 8 measurements taken at different points on the surface. The error bars correspond to the standard deviation. The purple curve represents the curve of best fit with a double-exponential function. The horizontal broken line represents the stable carrier density obtained by the fit. (b) Spatial distribution of sheet resistance R s taken at a position of 30 m along the graphene transparent conductive film, and measured 1 day after doping with AuCl3. Histograms of R s at positions of 30 m [(c)] and 92 m [(d)]. Measurements were carried out within the 210 mm × 300 mm area at a 15-mm interval (n = 280) before (blue) and 1 day after (red) doping with AuCl3. The R s value stated above each peak shows the mean ± standard deviation of each histogram obtained by a Gaussian fit (solid lines). (e) R s of the graphene/epoxy/PET film along the longitudinal direction measured every 1 m (red line). Each point on the red line is an average of 5 measurements taken at different positions in the transverse direction. The 5-m averages (total of 25 measurements) are also shown by the blue circle symbols. The horizontal broken lines represent the averages of all of the measured data in the 82 A/mm2 (0–52 m) and 83 A/mm2 (52–100 m) regions.
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(a),(b) Optical microscopy images of graphene films grown on a copper foil after baking at 180 °C. The darker red lines are microcracks observed only on plastically deformed areas. In (a), the displayed area is entirely deformed, while the non-deformed area remains in (b). Plastic deformation strongly depends on the temperature (current density) and heating time (line velocity). (c),(d) Electron backscatter diffraction maps of graphene films grown on copper foils. (c) Before plastic deformation, the copper foil is predominantly oriented to (001). (d) After plastic deformation, (101) or (111) surfaces and twin boundaries appear. The graphene films were grown at J = 83 A/mm2 [(a)] and J = 82 A/mm2 [(b)–(d)]. The line velocities were 0.1 m/min [(a) and (c)] and 0.02 m/min [(b) and (d)].
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A high-quality graphene transparent conductive film was fabricated by roll-to-roll chemical vapor deposition (CVD) synthesis on a suspended copper foil and subsequent transfer. While the high temperature required for the CVD synthesis of high-quality graphene has prevented efficient roll-to-roll production thus far, we used selective Joule heating of the copper foil to achieve this. Low pressure thermal CVD synthesis and a direct roll-to-roll transfer process using photocurable epoxy resin allowed us to fabricate a 100-m-long graphene transparent conductive film with a sheet resistance as low as 150 Ω/sq, which is comparable to that of state-of-the-art CVD-grown graphene films.
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