  The extreme sensitivity of 2D materials to defects and nanostructure requires
precise imaging techniques to verify presence of desirable and absence of
undesirable features in the atomic geometry. Helium-ion beams have emerged as a
promising materials imaging tool, achieving up to 20 times higher resolution
and 10 times larger depth-of-field than conventional or environmental scanning
electron microscopes. Here, we offer first-principles theoretical insights to
advance ion-beam imaging of atomically thin materials by performing real-time
time-dependent density functional theory simulations of single impacts of
10-200 keV light ions in free-standing graphene. We predict that detecting
electrons emitted from the back of the material (the side from which the ion
exits) would result in up to 3 times higher signal and up to 5 times higher
contrast images, making 2D materials especially compelling targets for ion-beam
microscopy. We also find that the charge induced in the graphene equilibrates
on a sub-fs time scale, leading to only slight disturbances in the carbon
lattice that are unlikely to damage the atomic structure for any of the beam
parameters investigated here.
