Article

Ultrahard carbon film from epitaxial two-layer graphene

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Abstract

Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp2 to sp3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.

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Acknowledgements

The authors acknowledge support from the Office of Basic Energy Sciences of the US Department of Energy (grant no. DE-SC0016204). E.T. thanks the European ERC (320796 MODPHYSFRICT). The authors acknowledge support from the CUNY High Performance Computing Center and the Extreme Science and Engineering Discovery Environment (XSEDE). The authors thank T. Wang for support with TEM measurements, C. Dean for insights on the C-AFM measurements, and M. Moseler for discussions on indentation simulations.

Author information

Author notes

  1. Yang Gao and Tengfei Cao contributed equally to this work.

Affiliations

  1. Advanced Science Research Center, City University of New York, New York, NY, USA

    • Yang Gao
    • , Tengfei Cao
    • , Filippo Cellini
    •  & Elisa Riedo
  2. School of Physics, Georgia Institute of Technology, Atlanta, GA, USA

    • Yang Gao
    • , Claire Berger
    • , Walter A. de Heer
    •  & Elisa Riedo
  3. Department of Chemistry, College of Staten Island, City University of New York, Staten Island, NY, USA

    • Tengfei Cao
    •  & Angelo Bongiorno
  4. Institut Néel, CNRS- University Grenoble-Alpes, Grenoble, France

    • Claire Berger
  5. Abdus Salam ICTP, Trieste, Italy

    • Erio Tosatti
  6. SISSA, Trieste, Italy

    • Erio Tosatti
  7. Physics Department, City College of New York, City University of New York, New York, NY, USA

    • Elisa Riedo
  8. CUNY Graduate Center, Ph.D. Program in Physics, New York, NY, USA

    • Elisa Riedo
    •  & Angelo Bongiorno
  9. CUNY Graduate Center, Ph.D. Program in Chemistry, New York, NY, USA

    • Angelo Bongiorno

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Contributions

Y.G. and F.C. performed nanomechanics experiments and data analysis. T.C. carried out DFT calculations and indentation simulations. E.R. conceived and designed the experiments and analysed the data. A.B. and E.T. analysed the experimental data and delineated the modelling strategy. C.B. and W.A.d.H. synthesized the EG samples. All authors contributed to writing the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Elisa Riedo or Angelo Bongiorno.

Supplementary information

  1. Supplementary Information

    Supplementary Figures 1–17, Supplementary references.