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Angelo Bongiorno1 Elisa Riedo2 Erio Tosatti3 Claire Berger4 Walt de Heer4 Tengfei Cao1 Yang Gao2

1, College of Staten Island - CUNY, Staten Island, New York, United States
2, The City University of New York, New York, New York, United States
3, SISSA, Trieste, , Italy
4, Georgia Institute of Technology, Atlanta, Georgia, United States

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,
a two-layer graphene film on SiC(0001) exhibits a transverse
stiffness and hardness comparable to those of diamond,
is resistant to perforation with a diamond indenter and
shows a reversible drop in electrical conductivity upon
indentation [1]. 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, density functional theory
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 [1]. Atomistic indentation simulations
show also that a SiC(0001) substrate coated by a stiff diamond-like
film yields a force versus indentation depth curve steeper than
that of the bare SiC substrate, whereas a five-layer graphene
film (which does not undergo any phase change) on SiC leads to
a significant softening of the transverse mechanical response.
Overall, experiments and calculations show that the hardening
effect exhibited by two-layer graphene on SiC(0001) arises
from a pressure-induced phase transformation to a diamond-like
film [1]. Our study opens up new ways to investigate
graphite-to-diamond phase transitions at room temperature in
low-dimensional systems, and it identifies supported two-layer
graphene as an interesting candidate for pressure-activated adaptive
ultrahard and ultrathin coatings and for force-controlled dissipation
switches.

[1] Yang Gao, Tengfei Cao, Filippo Cellini, Claire Berger,
Walter A. de Heer, Erio Tosatti, Elisa Riedo, and Angelo Bongiorno
Nature Nanotechnology (2017) doi:10.1038/s41565-017-0023-9

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