Abstract
Structuring of 2D materials and their heterostructures with ion beams is a challenging task, because typically low ion energies are needed to avoid damage to a substrate. In addition, at the very first monolayers of a material, ions are not yet in charge equilibrium, i.e. they may either charge up or neutralize depending on their velocity. The change in electronic structure of the ion during scattering affects the energy, which can be transferred to the recoil and therefore the energy available for defect formation. In order to make reliable use of ion beams for defect engineering of 2D materials, we present here a model for charge state and charge exchange dependent kinetic energy transfer. Our model can be applied to all ion species, ion charge states, and energies. It is especially powerful for predicting charge state dependent stopping of slow highly charged ions.
Highlights
Structuring of 2D materials and their heterostructures with ion beams is a challenging task, because typically low ion energies are needed to avoid damage to a substrate
Recent experimental work showed that heavy ions, especially in high initial charge states, undergo ultrafast neutralization and de-excitation within less than 10 fs when transmitted through a freestanding monolayer of graphene[23] or 1 nm thick polymeric carbon nanomembranes[24]
The neutralization is accompanied by a strong enhancement of the ion stopping[25], which depends on both, the ion incident charge state Qin and the amount of charge exchange ΔQ = Qin − Qout
Summary
The results for the total and nuclear energy losses are displayed in Fig. 3a for 40 keV Xe40+ ions impinging on a single layer of graphene as a function of the final ion charge state Qout They are calculated from the velocities of the Xe and C atoms after a distance of 50 atomic units from the graphene membrane. Since all parameters but one (α or ξ, respectively) used in the calculations are either taken from literature (γ(R)) or show only little influence on the results within a reasonable range of variation (λ, Rc, φ, μ, β), we see this as evidence for (1) a strong excitation of the graphene layer due to charge extraction such that target excitation always need to be considered, (2) charge extraction appears even when the projectile is still above the surface, and before the actual impact, and (3) ultrafast interatomic electron excitation energy release from the projectile without recharging, i.e. ICD during the collision. TDPot further enables a comprehensive understanding of ion beam spectroscopy measurements with wide applicability in techniques like Low and Medium Energy Ion Scattering, High-Resolution Rutherford Backscattering Spectroscopy and Elastic Recoil Detection where quantification of charge fractionization is important
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