Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics (2022): Surface processes in ion-irradiated materials from first principles

First-principles modeling offers exciting prospects for computational design of not only materials, but also materials imaging and processing techniques. Focused ion beams are a particularly versatile tool: depending on beam parameters, energetic ions can allow nondestructive high-resolution imaging or intentional modification of atomic structure. While ion-irradiated bulk materials are fairly well studied, applications of ion irradiation often involve poorly understood surface processes like pre-equilibrium energy transfer and electron emission. The extreme case of 2D materials is especially compelling because of their departures from bulk properties and myriad applications relying on precise control of nanostructure. First-principles modeling can help elucidate important surface physics and predict optimal beam parameters for desired outcomes. Here, we first present methodological advances for simulating ion-irradiated surfaces and 2D materials using real-time time-dependent density functional theory. Then, we apply our extended framework to examine a variety of surface effects in ion-irradiated aluminum sheets and graphene. We uncover important differences in the energy deposited by energetic ions in these systems compared to bulk counterparts, including contributions from surface plasmon excitations and dissipation through electron emission. We also find interesting projectile charge state dynamics and that electron emission plummets below a threshold ion velocity. Finally, we offer insights into the early stages of ion-induced defect formation mechanisms. This work elevates our understanding of the basic physics of ion-irradiated surfaces and 2D materials and advances the ability to design ion beam techniques for specific applications.