Ion Beam Spectroscopy with 2D Materials

The spectroscopic analysis of ions transmitted or backscat- tered through/from a solid is a standard procedure in ion beam analysis and reveals material composition, crystallog- raphy, surface roughness and other properties. The infor- mation on material properties is typically gained through the determination of the ion energy (or energy of emitted secondary particles) and thus from the ion stopping. Addi- tionally, the ion charge state may change upon interaction with the solid material, which is typically considered as a ’complication’ in experiment, e.g. in the context of charge fractionization when using charge state selective detectors (magnetic spectrometers or electrostatic analyzers). How- ever, charge exchange is also determined by material prop- erties and can yield additional information in conjunction with kinetic energy loss analysis. In this contribution I will show that charge exchange and stopping of highly charged ions (and ions in general) are closely coupled, which implies that nuclear and electronic energy loss are coupled as well. Our results in combina- tion with a quantitative model for impact parameter depen- dent charge exchange and stopping can serve as an addi- tional tool for material structure determination on the sub- nm scale for 2D materials, especially in cases where elec- tron microscopy may not yield atomically resolved data. In our experiments we use slow (v < v0) highly charged Ar and Xe ions with charge states of 1 − 18 and 1 − 40, respec- tively. We transmit the ions through freestanding layers of 2D materials, i.e. single-, bi-, and tri-layer graphene, single- layer hBN, carbon nanomembranes, single-layer MoS2, and other transition metal dichalcogenides [1]. Secondary elec- trons emitted from the interaction process are recorded in a high voltage biased surface barrier detector serving as a start signal for time-of-flight measurements and also allow- ing us to determine the absolute amount of emitted elec- trons. Ions are detected by a position sensitive MCP detec- tor using a delay line anode. Utilizing a set of slits and parallel deflection plates we relate the impact position at the MCP to the scattering angle and charge state of the ions. The timing signal from the impact serves as the stop trigger for the ion’s time-of-flight. Thus, we obtain angle- resolved, energy-resolved charge exchange spectra in coin- cidence with the number of emitted electrons for each individual ion.
Comparing our highly differential data with an atomistic model for time-dependent charge redistribution between the ion and the target atoms allows us to link structural proper- ties and defects to the observed charge state pattern. Our model is based on the statistical description of atoms and an interatomic distance dependent ion de-excitation rate adapted from the Interatomic Coulombic Decay process [2, 3].
Fig. 1 shows a sketch of the ion transmission process in- volving the emission of secondary particles on the time scale of only femtoseconds. Thus, ion transmission studies with atomically thin materials not only are useful for materials science, but also to study inelastic ion-surface interaction on the femtosecond time scale simply by varying the ion velocity or tilting the sample.