  Graphene is expected to be rather insensitive to ionizing particle radiation.
We demonstrate that single layers of exfoliated graphene sustain significant
damage from irradiation with slow highly charged ions. We have investigated the
ion induced changes of graphene after irradiation with highly charged ions of
different charge states (q = 28-42) and kinetic energies E_kin = 150-450 keV.
Atomic force microscopy images reveal that the ion induced defects are not
topographic in nature but are related to a significant change in friction. To
create these defects, a minimum charge state is needed. In addition to this
threshold behaviour, the required minimum charge state as well as the defect
diameter show a strong dependency on the kinetic energy of the projectiles.
From the linear dependency of the defect diameter on the projectile velocity we
infer that electronic excitations triggered by the incoming ion in the
above-surface phase play a dominant role for this unexpected defect creation in
graphene.
