Transmission of highly charged xenon ions through a monolayer of molybdenum disulfide


Transmission of highly charged xenon ions through a monolayer of molybdenum disulfide

Creutzburg, S.; Schwestka, J.; Grande, P. L.; Inani, H.; Tripathi, M. K.; Heller, R.; Niggas, A.; Kozubek, R.; Madauß, L.; Facsko, S.; Kotakoski, J.; Schleberger, M.; Aumayr, F.; Wilhelm, R. A.

The modification of solids by ion irradiation is a longstanding research objective driven by the urge for controlled defect engineering. By incorporating defects in a host material, the electronic, optical and magnetic properties of a solid can be modified. Especially in low-dimensional materials, like 2D layers, the presence of defects may significantly change their performance in applications. Highly charged ions (HCIs) are an effective tool for nanostructure formation on surfaces [1,2]. For HCI impact on a surface, nanostructure formation is driven by the deposition of the potential energy in very shallow depths in the order of nanometers. Recently, pore formation in 2D materials, like carbon nanomembranes [1] or MoS₂ [2], was observed by HCI impact despite the fact that their atomic thickness is limiting the amount of energy, which can be deposited. By measuring the charge exchange of HCIs transmitted through a monolayer of MoS₂, we can provide an upper estimate for the energy transferred to the layer available for pore formation in MoS₂. Additionally, we can gain insights into non equilibrium charge state effects, i.e. the neutralization behavior of the projectile and the charge state dependent kinetic energy loss.
The exit charge state of the ions transmitted through a suspended monolayer of MoS₂ placed on a TEM grid is measured simultaneously with their time-of-flight (TOF) [3].
A two-dimensional charge state and scattering angle resolved spectrum is measured. Two distinct exit charge state distributions at high and low charge states are observed. The distribution at high charge states is accompanied by small scattering angles, which indicates collision events taking place at large impact parameters. On the contrary, the distribution at low charge states is characterized by larger scattering angles pointing to collisions occurring at small impact parameters. We can associate both exit charge state distributions with two well separated peaks in the TOF signal corresponding to slow and fast transmitted ions, i.e. high and low energy loss, respectively. The low exit charge state distribution was already observed for HCI interaction with graphene. Neutralization times of a few femtoseconds were determined previously [4], which could only be explained by ion de-excitation via an Interatomic Coulombic Decay process. This common neutralization behavior for graphene and MoS₂ implies a common de-excitation mechanism for HCI interaction for both target materials. The additional high exit charge state distribution for MoS₂ is interpreted as a feature related to the different crystalline structure of the material in contrast to graphene. Our experimental results are supported by computer simulations using the Monte-Carlo code TDPot [5], which also reveals two distinct interaction regimes of the incident ions within the unit cell of MoS₂.
[1] R. A. Wilhelm et al., 2D Mater. 2, 035009 (2015).
[2] R. Kozubek et al., J. Phys. Chem. Lett. 10, 904-910 (2019).
[3] J. Schwestka et al., Rev. Sci Instrum. 89, 085101 (2018).
[4] R. A. Wilhelm et al., Phys. Rev. Lett. 119, 103401 (2017).
[5] R. A. Wilhelm and P. L. Grande, Commun. Phys. 2, 89 (2019).

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