On the interaction of slow highly charged ions with 2D materials

Abstract

Slow ions in elevated charge states undergo a complex cascade of charge exchange and electronic deexcitation when brought in contact with a solid surface [1]. Electron transfer from the surface leads to dynamic charging of a nanometric material volume. The dynamics of these electronic processes is typically limited to a few 10 femtoseconds and is associated with keV of energy deposition in that timeframe. As a result, electronic sputtering can occur on the nanoscale with ultimate surface sensitivity, as shown for bulk surfaces of ionic crystals and a freestanding van-der-Waals (vdW) layer stack of 2D materials [2,3]. To understand this monolayer-sensitive lithography method, we investigate the charge exchange of slow highly charged ions (HCI) with freestanding 2D material membranes where the interaction time of the ion with the solid’s electron reservoir is naturally limited to a few femtoseconds. Suspended membranes also give also access to the ion after the transmission which we exploit to link charge exchange, kinetic energy loss, and secondary particle emission to the interaction with a surface-only material. In this contribution we present experimental results on HCI charge exchange and stopping during transmission through freestanding mono- and multilayers of graphene, MoS2 and hBN. The experimental results are accompanied by atomically resolved microscopy data for monolayers of these materials and vdW heterostructures thereof. The experimental results are supported by simulation methods which illuminate the underlying complex electronic processes in a solid surface [4]. We will put our recent findings into perspective to established charge exchange and secondary electron emission data from bulk materials under various incidence angles [1] and will discuss the importance of interatomic Auger-type energy release channels [5] for the incoming ion to comprehend the ultrafast HCI neutralization observed in ion-surface scattering over the past decades.