  Molecular dynamics (MD) simulations were performed to study the formation
process of nanopores in a suspended graphene sheet irradiated by using
energetic ions though a mask. By controlling the ion parameters including mass,
energy and incident angle, different kinds of topography were observed in the
graphene sheet. Net-like defective strucutures with carbon atom chains can be
formed at low ion fluence, which provides the possibility to functionalize the
irradiated sample with subsequent chemical methods; finally a perfect nanopore
with smooth edge appears as the ion fluence is high enough. We found that the
dependence of ion damage efficiency on ion fluence, energy and incident angle
are different from that predicted by the semi-empirical model based on the
binary-collision approximation, which results from the special structure of
graphene. Our results demonstrate that it is feasible to fabricate controlled
nanopores/nanostructures in graphene via heavy ion irradiation.
