The interaction of highly charged ions with freestanding 2D materials; invited talk

Abstract

In the last decade, free-standing atomically thin materials have proven to be excellent candidates to study ion-solid interaction on a fundamental basis. Not only do they allow for direct observation of the projectiles after the interaction, they also offer the opportunity to study target characteristics while showing pure surface effects, i.e., no sub-surface effects need to be considered. Thereby, we were able to discuss the ultrafast response of suspended semi-metallic single-layer graphene sheets upon excitation via highly charged Xe impact: while the ions were found to neutralise within a few fs only and thus extracting a high number of electrons from the target at the same time, no pore formation could be determined via transmission electron microscopy proposing ultrahigh local current densities [1]. For free-standing semi-conducting molybdenum disulfide (MoS2), however, pore formation via highly charged ion impact under similar conditions was shown recently [2]. Still, the neutralisation of the projectile is comparable to the case of single-layer graphene when taking an increased interaction time into account, following from the three-layered nature of MoS2, where Mo atoms are sandwiched in- between two layers of S atoms (cf. figure 1) [3]. In this contribution we will focus on the electron emission from both materials induced by highly charged ion impact. For single-layer graphene, Schwestka et al. presented a high number of emitted low-energy electrons lately [4], being in accordance with the predictions of Wilhelm et al. proposing a two-center Auger process - the interatomic Coulombic decay - as dominant mechanism in the de-excitation of these highly charged projectiles [5]. As MoS2 and single-layer graphene are comparable in their hexagonal structure (cf. top view of graphene and MoS2 sheets in figure 1), any discrepancies in the electron emission may be retraced to their significantly different electronic properties, i.e., zero vs. non-zero bandgaps.