Graphene films grown by chemical vapour deposition have unusual physical and chemical properties that offer promise for applications such as flexible electronics and high-frequency transistors1,2,3,4,5,6,7,8,9,10. However, wrinkles invariably form during growth because of the strong coupling to the substrate, and these limit the large-scale homogeneity of the film1,2,3,4,11,12. Here we develop a proton-assisted method of chemical vapour deposition to grow ultra-flat graphene films that are wrinkle-free. Our method of proton penetration13,14,15,16,17 and recombination to form hydrogen can also reduce the wrinkles formed during traditional chemical vapour deposition of graphene. Some of the wrinkles disappear entirely, owing to the decoupling of van der Waals interactions and possibly an increase in distance from the growth surface. The electronic band structure of the as-grown graphene films shows a V-shaped Dirac cone and a linear dispersion relation within the atomic plane or across an atomic step, confirming the decoupling from the substrate. The ultra-flat nature of the graphene films ensures that their surfaces are easy to clean after a wet transfer process. A robust quantum Hall effect appears even at room temperature in a device with a linewidth of 100 micrometres. Graphene films grown by proton-assisted chemical vapour deposition should largely retain their intrinsic performance, and our method should be easily generalizable to other nanomaterials for strain and doping engineering.
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The data that support the findings of this study are available from the corresponding author on reasonable request.
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We thank R. Liu for help with electron beam lithography. G.Y., X.H., H.Z., D.W., J.X. and L.G. acknowledge support from the National Key R&D Program of China (grant no. 2018YFA0305800), the NNSF of China (nos. 11674154, 51972163 and 11761131010), the Natural Science Foundation of Jiangsu Province (grant no. BK20190010) and the Fundamental Research Funds for the Central Universities (nos. 020414380094 and 020414380128). D.L. and X. Xi acknowledge support from the National Key R&D Program of China (grant nos. 2017YFA0303201 and 2018YFA0307000) and the NNSF of China (no. 11774151). Y.W. and J.S. acknowledge support from the National Key R&D Program of China (grant no. 2016YFA0300404), the NNSF of China (nos. 11574133, 11974162 and 11834006), the High Performance Computing Center of Collaborative Innovation Center of Advanced Microstructures, the High Performance Computing Center of Nanjing University and “Tianhe-2” in the NSCC-Guangzhou. W.C., X. Xie, J.Z. and Y.Z. acknowledge support from the National Key R&D Program of China (grant no. 2018YFA0306800), the NNSF of China (nos. 11774154 and 11790311) and the Fundamental Research Funds for the Central Universities (no. 020414380110). Q.-Q.Y. and S.-C.L. acknowledge support from the National Key R&D Program of China (grant no. 2014CB921103) and the NNSF of China (nos. 11774149 and 11790311).
The authors declare no competing interests.
Peer review information Nature thanks Marcelo Lozada-Hidalgo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, Top and profile view of the supercell of graphene, hydrogens and Cu(111). b, Theoretical simulation of bonding status for recombined hydrogen with H:C = 1:4 and 1:2 at 650 °C. Some of the hydrogen atoms form into H2 and increase their surface distance. c, AFM image of graphene grown on Cu(111) substrate (region 30 μm × 30 μm). Wrinkles are not apparent. d, Enlarged AFM image (5 μm × 5 μm). Wrinkles can be recognized by adjusting the height scale. e, Height profiles extracted from d. The wrinkles are usually lower than 1 nm. The root-mean-square (RMS) roughness is about 0.756 nm for c and 0.538 nm for d. Scale bars in c, d are 10 μm and 1 μm, respectively.
a, AFM images of graphene films grown on Cu(111) at different growth temperatures from 1,050 °C to 850 °C. b, Typical AFM images of graphene grown on Cu(111) at 1,000 °C, adjusting the ratio of H2:CH4 from 10:1 to 1,000:1. c, Enlarged AFM images of b with zoom-in height scale. Scale bars in a–c are 3 μm, 10 μm and 3 μm, respectively.
a, Raman spectra of monolayer graphene after ICP post-treatment at different temperatures T. b, Intrinsic graphene after post-treatment at different conditions. The Raman spectra are all collected at the same position. c, Encapsulated graphene after different treatments. Scale bar, 10 μm. d, Defective graphene sheets after different treatments.
a, AFM image of weakly defective graphene films after UHV annealing, subsequent H2 annealing and H2 ICP with light (see Methods) treatment. Inset, the related AFM phase images. The bubbles apparently occur only after ICP treatment. b, Raman spectra of a collected at similar position. The Raman frequencies show an obvious shift after H2 ICP treatment. c, AFM image of films with a medium defect level, after UHV annealing, subsequent H2 annealing, H2 ICP @ dark treatment and H2 ICP with light treatment. d, Raman spectra of c collected at the similar position, showing almost no shift. e, AFM image of the heavily defective films after UHV annealing, subsequent H2 annealing, H2 ICP with dark treatment and H2 ICP with light treatment. f, Raman spectra of e collected at similar position, showing no obvious shift. g, AFM image of the as-grown wrinkled graphene films on Cu(111), after He ICP treatment at 400 °C and 650 °C. h, Raman spectra collected from the similar position of g. i, Raman shifts collected from five different positions, showing random shift and no defects formed (no increase in the intensity at ~1,350 cm−1 of the D band). All the above results show that the atomic hydrogen, molecular hydrogen, He and He+ are not helpful in decoupling the defect-free graphene or weakly defective graphene, so only protons (and electrons) in an H2 plasma will permeate CVD-grown graphene to fully decouple it from the substrates. Scale bars in a, c, e and g are all 1 μm.
Schematic illustrations of evolution in wrinkles by ICP treatment. a, Wrinkles formed to release the high fstrain, along with the strong fcoupling. b, Partly released wrinkles when heated to 650 °C, with weak fstrain and strong fcoupling. c, Retained wrinkles after ICP treatment at 650 °C, with weak fstrain and weak fcoupling. d, Partly released wrinkles when cooling down, with stronger fstrain and weak fcoupling.
Extended Data Fig. 6 Inhomogeneous strain in ICP post-treated graphene, comparison of graphene on different Cu substrates and D2-assisted flattening or growth.
a, Typical Raman spectra of wrinkled graphene film after ICP treatment at 650 °C. b, Inhomogeneous distribution of ω2D collected from 2 mm × 2 mm region, showing that the strains are inhomogeneous on a large scale. c, Typical AFM image of Cu foil fully covered by graphene films. Scale bar, 1 μm. d, Typical Raman spectra of graphene films grown on different Cu substrates or suspended on a micro-sized hole. e, Raman spectra of wrinkled graphene films on Cu(111) after different treatments and of ultra-flat graphene films grown by ICP–CVD. All the processes here use D2. f, Distribution of Raman frequency collected from multiple points after different treatments.
a, Large-scale STM images of as-grown graphene films and after UHV-annealing at 250 °C, grown by ICP–CVD, and their related typical STS spectra. Insets are zoomed-in STM images, showing no periodic moiré pattern and no energy gap. b, Large-scale STM images of wrinkled graphene films grown by CVD under different bias voltages, and their related typical STS spectra. Inset is a zoomed-in STM image, showing moiré patterns with period of 5 nm and energy gap of 200–350 meV. c, Large-scale STM images of wrinkled graphene films after ICP post-treatment under different bias voltage, and their related typical STS spectra. Inset is zoomed-in STM image, showing no periodic moiré pattern and no energy gap. d, ARPES of ICP-CVD graphene films after UHV annealing at 500 °C, indicating the metastable state. e, Typical ARPES images extracted from different locations on the ICP post-treated graphene films on Cu(111). The doping is not homogeneous. f, The doping level changes gradually after UHV annealing, and the Dirac point will change from −120 meV (without annealing in e) to −170 meV (annealing at 200 °C) and finally reaches −320 meV (annealing at 400 °C). Scale bars in a–c are 4 nm, 6 nm and 4 nm, respectively. All the scale bars in the insets are 1 nm.
Extended Data Fig. 8 Variable-temperature Raman measurements for grown and transferred graphene on Cu, and transferred graphene on SiO2/Si.
a, Raman spectra with measurement temperature cycled from 10 K to 300 K and from 300 K to 10 K. The Raman frequency of the G band (ωG) and 2D band (ω2D) is plotted against the variable temperature, demonstrating that the coupling interactions are recovered and this variable-temperature measurement is non-destructive. b, Raman frequency shift for graphene films grown on Cu(111) by traditional CVD, before and after ICP treatment, and related FWHM of G and 2D bands and IG/I2D for the above spectra, showing that FWHM is slightly changed but the I2D/IG seems almost unchanged; Raman frequency shifts are also shown for as-grown graphene films on Cu(111) by ICP–CVD and on Cu foils by traditional CVD. c, Ex situ AFM height profiles for as-transferred and UHV-annealed graphene films. The thickness is decreased by about 5 Å after this UHV annealing. Insets are related AFM height images. In situ variable-temperature Raman measurements are also shown for the transferred and UHV-annealed films on SiO2/Si wafers, together with related ΔωG and Δω2D. Scale bars, 2 μm. d, Raman spectra of the transferred graphene on Cu(111) under different UHV annealing conditions, and in situ variable-temperature Raman spectra of as-transferred graphene films before (25 °C) and after UHV annealing at 180 °C and 430 °C. The variable temperatures are all from 10 K to 300 K.
a–c, AFM images of residual nanoparticles on transferred graphene films after UHV annealing. The films are grown by traditional CVD on Cu(111) (a), traditional CVD on Cu foil (b) and ICP–CVD on Cu(111) (c, 150 μm × 90 μm). Transferred ICP–CVD graphene films appear wrinkle-free and nanoparticle-free during this large-scale measurement. Scale bars in a–c are 1 μm, 1 μm and 30 μm, respectively.
Extended Data Fig. 10 Calculation of carrier mobility in the Hall bar devices and the robust QHE in the ultra-flat graphene films.
a, Typical transport characteristics of graphene FET with device linewidth of 100 μm measured at 300 K and 1.5 K (left panel); carrier mobilities extracted by field effect μFE ≈ 4,900 cm2 V−1 s−1 and ~4,700 cm2 V−1 s−1 for electrons and holes at 300 K, and ~5,600 cm2 V−1 s−1 and ~5,300 cm2 V−1 s−1 for electrons and holes at 1.5 K (centre panel); and carrier mobilities extracted by Hall effect when applying magnetic field from 0 to 7.5 T at 300 K, showing μHall is ~9,800 cm2 V−1 s−1 at 300 K with n ≈ 2.9 × 1011 cm−2 (right panel). b, Magnetotransport properties of Hall device with linewidth of 20 μm. c, Magnetotransport properties of Hall device with linewidth of 100 μm, showing that the first σxy plateaus appear at 2e2/h for both electrons and holes at RT under 7.5 T. d, Magnetotransport properties of Hall device with linewidth of 500 μm, showing that QHE still appears. Insets are optical images of the Hall devices. The device with linewidth 500 μm was fabricated by simple manual operation. Scale bars in b–d are 20 μm, 100 μm and 500 μm, respectively.
| Theoretical simulation of bonding status for recombined hydrogen between graphene and Cu. The video was generated by first-principles molecular dynamics simulation at at 298 K and 923 K for graphene/H/Cu(111) systems. C, H and Cu atoms are denoted as black, grey and yellow spheres, respectively. The supercell used for calculations contains monolayer graphene with 24 carbon atoms and 5-layers Cu(111) with 60 copper atoms, together with 6 or 12 hydrogen atoms (H:C = 1:4 or 1:2) inside. At 923 K, the supercell with H:C=1:4 has 6 hydrogen atoms, and the equilibrium of this system will reduce the surface distance. When increasing hydrogen density to H:C=1:2, some of the hydrogen atoms will rebond into hydrogen molecules, and the movement of H2 can eliminate their interlayer interaction and increase the average surface distance.
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Yuan, G., Lin, D., Wang, Y. et al. Proton-assisted growth of ultra-flat graphene films. Nature 577, 204–208 (2020). https://doi.org/10.1038/s41586-019-1870-3
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