The potential energy an ion is carrying, initially stored during its production process, can add up to several 10 keV for ions in very high charge states (e.g. Xe ions in charge states >30) and even exceed its kinetic energy. When using very slow ions (<100 keV), this high amount of potential energy is released already within the first few layers of a material upon target impact. We perform ion transmission studies of highly charged projectiles through ultimately thin (one up to only a few atomic layers) films, where the energy deposition within the first layer becomes crucial. In the case of these two-dimensional targets, the neutralization process of the ion is not fully completed and the projectile remains charged after the transmission. This allows us to study fundamental charge exchange processes of highly charged ions with different target materials. We measure the exit charge state of the ion after the transmission, the energy loss of the ion within the target as well as the released energy by emission of low energy electrons. In this way, we probe the electronic response of two-dimensional materials to ultrafast external perturbations, which is nowadays of great interest for the development of novel devices (e.g. in the field of optoelectronics).
Recently, we found an ultra-fast electronic response (only a few fs) of the prototype candidate of a 2D material, a monolayer of graphene [1]. We extend our studies on other monolayer materials with different electronic properties, e.g. semi-conducting MoS2. While the semi-metal graphene stays intact after ion bombardment, the high potential energy of the projectiles leads to defects and even nm-sized pores (increasing with increasing charge state) in a freestanding MoS2 layer. Thus, we cannot only probe the response of the investigated 2D material, we can further modify these thin films by slow highly charged ion bombardment.
[1] E. Gruber et al., Nat. Commun. 7 (2016) 13948.