Exploring the Potential Energy Threshold: HCI Irradiation of 2D Materials and CaF₂ with Au⁷⁹⁺ at HITRAP

Abstract

Upon impact with a surface, slow highly charged ions (HCIs) deposit large amounts of potential energy within the top few atomic layers of the material. Depending on material properties, this can trigger relaxation processes that create nanostructures such as hillocks, craters, or pores in the material surface [1,2] - often in a similar manner as swift heavy ions (SHIs) [3]. For atomically thin graphene, so far no pore formation could be observed after irradiation with HCIs (up to charge states of typically 40+) [4], while SHIs have been shown to induce cracks [5]. This contrast suggests the existence of a material-specific potential energy threshold for nanostructuring, as it has previously been found for bulk samples [1], with CaF2 being a particularly well investigated case. Yet, irradiations with ions in very high charge states (>> 40+) remain scarce for both bulk and 2D materials. Within one of its first official beam times, HITRAP offered a perfect opportunity to explore this uncharted territory of (very) slow but highly charged ion impacts on surfaces: 6 keV/u Au79+ ions were supplied by HITRAP and used to irradiate a set of different samples: a CaF2 crystal, quasi- freestanding graphene (1-, 2- and 3-layer systems) as well as transition metal dichalcogenide monolayers on various substrates. Post-irradiation characterisation was performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). Despite the high charge state, we observed no defects that can be unambiguously attributed to individual HCI impacts. We attribute this to an ion fluence that was likely too low to produce defect densities resolvable by these microscopy techniques. In this contribution I will first introduce the topic of slow highly charged ion induced nanostructuring, followed by a discussion of our results from HITRAP-irradiated sample surfaces.