  Graphene is a sturdy and chemically inert material exhibiting an exposed
two-dimensional electron gas of high mobility. These combined properties enable
the design of graphene composites either based on covalent or non- covalent
coupling of adsorbates, or on multilayered structures. These systems have
exhibited novel tunable electronic properties such as bandgap- engineering,
reversible metal/insulating transition or supramolecular spintronics. Tunable
superconductivity is expected as well, but experimental realization is lacking.
Here, we show experiments based on me tal/graphene hybrid composites, enabling
the control of proximity coupling of an array of superconducting nanoparticles
onto a macroscopic graphene sheet. This material allows at low temperature to
tune the superconductivity down to a strongly insulating state. It results from
the combination of a proximity-induced superconductivity generated by the
metallic nanoparticles array with the two-dimensional and tunable metallicity
of graphene. The resulting hybrid material behaves, as a whole, like a granular
2D superconductor showing universal transition threshold and localization of
Cooper pairs in the insulating phase. This experiment sheds light on the
emergence of superconductivity in inhomogeneous superconduc- tors, and more
generally it demonstrates the potential of graphene as a versatile
building-block for the realization of novel superconducting materials.
