  Many of the proposed future applications of graphene require the controlled
introduction of defects into its perfect lattice. Energetic ions provide one
way of achieving this challenging goal. Single heavy ions with kinetic energies
in the 100 MeV range will produce nanometer-sized defects on dielectric but
generally not on crystalline metal surfaces. In a metal the ion-induced
electronic excitations are efficiently dissipated by the conduction electrons
before the transfer of energy to the lattice atoms sets in. Therefore, graphene
is not expected to be irradiation sensitive beyond the creation of point
defects. Here we show that graphene on a dielectric substrate sustains major
modifications if irradiated under oblique angles. Due to a combination of
defect creation in the graphene layer and hillock creation in the substrate,
graphene is split and folded along the ion track yielding double layer
nanoribbons. Our results indicate that the radiation hardness of graphene
devices is questionable but also open up a new way of introducing extended
low-dimensional defects in a controlled way.
