First-Principles Insights for Light-Ion Microscopy of Graphene

The properties of 2D materials are notoriously sensitive to defects and nanostructure, requiring precise characterization methods to verify desirable features. Ion-beam techniques including helium ion microscopy are a promising tool for this purpose. However, optimizing ion beam parameters for 2D materials requires improved understanding of their highly pre-equilibrium response to ion irradiation, which can fundamentally differ from their bulk counterparts. To this end, we simulate single impacts of 0.25 – 200 keV protons and helium ions in free-standing monolayer graphene using real-time time-dependent density functional theory. We find a channel-dependent anomalous effect in the energy transferred by ~6 keV protons that we associate with electron capture from σ and π bands. Furthermore, our first-principles results confirm analytic estimates of a threshold proton energy near 1 keV below which electron emission vanishes. Most importantly, we predict that anisotropic electron emission mechanisms result in up to 3 times stronger signal and 5 times higher contrast for exit-side (forward) emission than the typically detected entrance-side (backward) emission. At the same time, the ion-induced electronic excitations within the graphene delocalize on a sub-fs time scale, rendering their contribution to damage processes negligible. These findings will advance high-resolution, nondestructive imaging techniques for 2D materials.<br/><br/>This material is based upon work supported by the National Science Foundation under Grant No. OAC-1740219. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.