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We present a first qualitative description of the atomic dynamics in two-dimensional (2D) materials induced by the impact of slow, highly charged ions. We employ a classical molecular dynamics simulation for the motion of the target atoms combined with a Monte Carlo model for the diffusive charge transport within the layer. Depending on the velocity of charge transfer (hopping time or hole mobility) and the number of extracted electrons which, in turn, depends on the charge state of the impinging ions, we find regions of stability of the 2D structure as well as parameter combinations for which nanopore formation due to Coulomb repulsion is predicted.

Experimental results on the charge-state-dependent sputtering of metallic gold nanoislands are presented. Irradiations with slow highly charged ions of metallic targets were previously considered to show no charge state dependent effects on ion-induced material modification, since these materials possess enough free electrons to dissipate the deposited potential energy before electron-phonon coupling can set in. By reducing the size of the target material down to the nanometer regime and thus enabling a geometric energy confinement, a possibility is demonstrated to erode metallic surfaces by charge state related effects in contrast to regular kinetic sputtering.

The growing family of 2D materials led not long ago to combining different 2D layers and building artificial systems in the form of van der Waals heterostructures. Tailoring of heterostructure properties postgrowth would greatly benefit from a modification technique with a monolayer precision. However, appropriate techniques for material modification with this precision are still missing. To achieve such control, slow highly charged ions appear ideal as they carry high amounts of potential energy, which is released rapidly upon ion neutralization at the position of the ion. The resulting potential energy deposition is thus limited to just a few atomic layers (in contrast to the kinetic energy deposition). Here, we irradiated a freestanding van der Waals MoS2/graphene heterostructure with 1.3 keV/amu xenon ions in high charge states of 38, which led to nanometer-sized pores that appear only in the MoS2 facing the ion beam, but not in graphene beneath the hole. Reversing the stacking order leaves both layers undamaged, which we attribute to the high conductivity and carrier mobility in graphene acting as a shield for the MoS2 underneath. Our main focus is here on monolayer MoS2, but we also analyzed areas with few-layer structures and observed that the perforation is limited to the two topmost MoS2 layers, whereas deeper layers remain intact. Our results demonstrate that in addition to already being a valuable tool for materials processing, the usability of ion irradiation can be extended to mono- (or bi)layer manipulation of van der Waals heterostructures when the localized potential energy deposition of highly charged ions is also added to the toolbox.

The scattering of ions in solids is accompanied with momentum transfer and electronic excitations resulting in the slowing down of the ion. The amount of energy transferred in a single scattering event depends on the particular trajectory which can be traced back through the scattering angle. Performing scattering angle dependent measurements of slow highly charged Xe ions transmitted through freestanding single-, bi-, and trilayer graphene allows us to determine the charge exchange and energy loss for different minimal interatomic distances. Interestingly, the charge exchange shows an increase with scattering angle by a factor of less than 2, while the energy loss increases by more than a factor of 10 for ${3}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}$ compared to forward direction. Our results can be compared to a time-dependent potential model and show that determination of the stopping cross section is not straightforward even with angle-dependent data at hand.

The formation of nanopores by highly charged ion impacts on freestanding fluorine-functionalized graphene is demonstrated. The process is driven by potential sputtering, which becomes active by changing the semimetallic property of graphene into a strongly insulating state by fluorination. The interaction of fluorographene with highly charged ions is also studied in terms of charge exchange and kinetic energy loss. A higher number of captured electrons and a larger kinetic energy loss than in pristine graphene are observed, which can be well explained by an increase in the ion neutralization length and in the atomic areal density of the target, respectively. Using a computer code based on a time-dependent scattering potential model, a connection between the efficiency of charge exchange and the fluorine coverage is revealed. Our results suggest a competition of two distinct nanostructure formation processes, leading either to pore formation or fluorine desorption.

Over seventy years ago, Niels Bohr described how the charge state of an atomic ion moving through a solid changes dynamically as a result of electron capture and loss processes, eventually resulting in an equilibrium charge state. Although obvious, this process has so far eluded direct experimental observation. By peeling a solid, such as graphite, layer by layer, and studying the transmission of highly charged ions through single-, bi- and trilayer graphene, we can now observe dynamical changes in ion charge states with monolayer precision. In addition we present a first-principles approach based on the virtual photon model for interparticle energy transfer to corroborate our findings. Our model that uses a Gaussian shaped dynamic polarisability rather than a spatial delta function is a major step in providing a self-consistent description for interparticle de-excitation processes at the limit of small separations. Ion-solid interactions are governed by a range of complex processes the direct experimental observation of which pose their own set of challenges. Here, the authors present a joint experimental and first-principles approach to study and describe the underlying mechanism of electron capture for an ion travelling through layers of graphene with monolayer precision.

The formation of nanopores by highly charged ion impacts on freestanding fluorine-functionalized graphene is demonstrated. The process is driven by potential sputtering, which becomes active by changing the semi-metallic properties of graphene into a strongly insulating state by fluorination. The interaction of fluorographene with highly charged ions is also studied in terms of charge exchange and kinetic energy loss. A higher number of captured electrons and a larger kinetic energy loss than in pristine graphene are observed, which can be well explained by an increase in the ion's neutralization length and in the atomic areal density of the target, respectively. Using a computer code based on a time-dependent scattering potential model, a connection between the efficiency of charge exchange and the fluorine coverage is revealed. Our results suggest a competition of two distinct nanostructure formation processes leading either to pore formation or fluorine desorption.

As surface-only materials, freestanding 2D materials are known to have a high level of contamination—mostly in the form of hydrocarbons, water, and residuals from production and exfoliation. For well-designed experiments, it is of particular importance to develop effective clean- ing procedures, especially since standard surface science techniques are typically not applicable. We perform ion spectroscopy with highly charged ions transmitted through freestanding atomically thin materials and present two techniques to achieve clean samples, both based on thermal treatment. Ion charge exchange and energy loss are used to analyze the degree of sample contamination. We find that even after cleaning, heavily contaminated spots remain on single layer graphene. The contamination coverage, however, clusters in strand-like structures leaving large clean areas. We present a way to discriminate clean from contaminated areas with our ion beam spectroscopy if the heterogeneity of the surface is increased sufficiently enough. We expect a similar discrimination to be necessary in most other experimental techniques.

In the absence of a protecting atmosphere, the surfaces of rocky bodies in the solar system are affected by significant space weathering due to the exposure to the solar wind [1]. Fundamental knowledge of space weathering effects, such as optical changes of surfaces as well as the formation of an exosphere is essential for gaining insights into the history of planetary bodies in the solar system [2]. Primarily the exospheres of Mercury and Moon are presently of great interest and the interpretation of their formation processes relies on the understanding of all space weathering effects on mineral surfaces.Sputtering of refractory elements by solar wind ions is one of the most important release processes. We investigate solar wind sputtering by measuring and modelling the sputtering of pyroxene samples as analogues for the surfaces of Mercury and Moon [3, 4]. These measurements with thin film samples on Quartz Crystal Microbalance (QCM) substrates allow recording of sputtering yields in-situ and in real time [5]. For the simulation of kinetic sputtering from the ion-induced collision cascade we use the software SDTrimSP with adapted input parameters that consistently reproduce measured kinetic sputtering yields [4, 6].This study focuses on investigating the potential sputtering of insulating samples by multiply charged ions [7]. Changes of these sputtering yields with fluence are compared to calculations with a model based on inputs from SDTrimSP simulations. This leads to a very good agreement with steady-state sputtering yields under the assumption that only O atoms are sputtered by the potential energy of the ions. The observed decreasing sputtering yields can be explained by a partial O depletion on the surface [4]. Based on these findings expected surface composition changes and sputtering yields under realistic solar wind conditions can be calculated. Our results are in line with previous investigations (see e.g. [8, 9]), creating a consistent view on solar wind sputtering effects from experiments to established modelling efforts. References:[1] B. Hapke, J. Geophys. Res.: Planets, 106, 10039 (2001).[2] P. Wurz, et al., Icarus, 191, 486 (2007).[3] P.S. Szabo, et al., Icarus, 314, 98 (2018).[4] P.S. Szabo, et al., submitted to Astrophys. J. (2020).[5] G. Hayderer, et al., Rev. Sci. Instrum., 70, 3696 (1999).[6] A. Mutzke, et al., “SDTrimSP Version 6.00“, IPP Report, (2019).[7] F. Aumayr, H. Winter, Philos. Trans. R. Soc. A, 362, 77 (2004).[8] H. Hijazi, et al., J. Geophys. Res.: Planets, 122, 1597 (2017).[9] S.T. Alnussirat, et al., Nucl. Instrum. Methods Phys. Res. B, 420, 33 (2018).

Bodies in space without a dense atmosphere are affected by several erosive space weathering processes [1, 2]. These processes are responsible for modifications of surface properties as well as for the formation of an exosphere. During the BepiColombo mission, an on-board mass spectrometer will probe this exosphere on Mercury. Knowledge the different processes causing its formation is crucial for the interpretation of the obtained mass spectrometry data [3, 4]. Sputtering by solar wind ions is expected to be one of the key drivers of the particle release that leads to the formation of the exosphere. In addition, these ions can modify the regolith, become implanted and are released into the exosphere [5, 6].We emulate solar wind precipitating onto the surface of Mercury by irradiating analogue material with mass over charge selective ion sources. For this study in particular, a magnesium rich augite (Ca,Fe)(Mg,Fe)[Si2O6] sample was used as analogue. The material is deposited onto a Quartz Crystal Microbalance (QCM) as thin film. Chemical composition and thickness of these films was investigated by means of ion-beam analysis [7]. The QCM-technique allows for real time measurements of mass changes during the experiments. This approach enables us to determine sputter yields due to ion impact, as well as projectile implantation and rerelease. Furthermore, the target is heatable and desorbed atoms can be analyzed with a quadrupole mass spectrometer. The setup thus allows for Thermal Desorption Spectroscopy (TDS) measurements. Together with the QCM, temperature dependence of the projectile rerelease and the total mass change during heating cycles can be determined.Irradiation of the targets with He+ at solar wind energies of 4keV were performed and simulated using the program SDTrimSP [8, 9]. A significant amount of helium is implanted upon irradiation, leading to a fluence-dependent mass change rate. When an equilibrium of implantation and rerelease is reached the mass loss due to sputtering of target material is observed. This saturation happens after a fluence of about 1021 ions per m2, which corresponds to an irradiation by the solar wind of several hundred years on the surface of Mercury [3]. The study shows that helium is mobile during ion irradiation, and released thermally from the sample at about 400K. Combining TDS results and measurements of the mass change, the helium volume abundance after saturation was estimated to about 10-15%. The results of this study therefore provide a more detailed understanding of the interaction between helium from solar wind and Mercury analogues.References:[1] Hapke B.: J. Geophys. Res. Planet., 106, 10039, 2001.[2] Noble S.K., et al.: Sol. Syst. Res. 37, 31, 2003.[3] Wurz P., et al.: Planet. Space Sci., 58, 1599, 2010.[4] Benkhoff J., et al.: Planet. Space Sci. 58, 2, 2010.[5] Sasaki S., et al.: Adv. Space Res., 33, 2152, 2004.[6] Hartle R., et al.: J. Geophys. Res., 80, 3689, 1975.[7] Moro M.V., et al.: Thin Solid Films, 686, 137416, 2019.[8] Mutzke A., et al.: IPP Report, 2019.[9] Szabo P.S., et al.: Icarus, 314, 98, 2018.

We present a method to measure the kinetic energy of electrons emitted upon ion impact via their time-of-flight. Pulsed beams of H$^{+}$, D$_2^{+}$, He$^{+}$ and Ne$^{+}$ ions with velocities between 0.4 and 3.5 a.u. are transmitted through thin, self-supporting carbon and gold foils. Transmitted ions and secondary electrons are detected with a position-sensitive detector behind the sample and their respective energies are determined via their flight times. A coincidence criterium can be applied in the acquisition software. Measured electron energies range between 10 and 400 eV. Above ion velocities of 1 a.u. the most probable electron energy scales with ion velocity pointing towards a kinetic emission mechanism. At lower ion velocities, the electron energy stays constant and lies above the maximum energy transfer possible in a classical binary collision between ion and electron. Potential applications and technical challenges of measuring electron energies and yields with a time-of-flight approach are discussed., Comment: submitted to the VSI: 23rd International Workshop on Inelastic Ion-Surface Collisions proceedings

Synopsis Recent experiments found that the neutralisation of highly charged ions interacting with a freestanding single layer of graphene proceeds on a femtosecond time scale. This ultra-fast deexcitation was attributed to Interatomic Coulombic Decay (ICD), a process in which core holes in the projectile are filled by previously captured outer electrons and the energy is transferred to electrons of the surrounding carbon atoms. ICD therefore predicts the emission of many low energy electrons. We now present experimental evidence that e.g. Xe40+ indeed emits up to 85 electrons with energies below 20 eV.

Charge exchange and kinetic energy loss of slow highly charged xenon ions transmitted through freestanding monolayer MoS₂ are studied. Two distinct exit charge state distributions, characterized by low and high charge exchange, are observed. They are accompanied by smaller and larger kinetic energy losses, respectively. High charge exchange is attributed to two-center neutralization processes, which take place in close impact collisions with the target atoms. Experimental findings are compared to graphene as a target material and simulations based on a time-dependent scattering potential model. Independently of the target material, experimentally observed charge exchange can be modeled by the same electron capture and de-excitation rates for MoS₂ and graphene. A common dependence of the kinetic energy loss on the charge exchange for MoS₂ as well as graphene is also observed, which additionally underlines the common nature of the two-center Auger neutralization process. Considering the similarities of the zero band gap material graphene and the 1.9 eV direct band gap material MoS₂, we suggest that electron transport on the femtosecond time scale is dominated by the strong influence of the ion’s Coulomb potential in contrast to the dispersion defined by the material’s band structure.

Synopsis Highly charged ions (HCIs) are an efficient tool for the perforation of suspended 2D materials. Only a fraction of their potential energy is transferred to the atomically thin target during the very short interaction time and is available for pore formation. Charge exchange spectra were measured for highly charged xenon ions transmitted through suspended, single layer MoS2 in order to determine the deposited potential energy available for pore formation. Additionally, charge exchange dependent ion stopping responsible for kinetic sputtering was measured simultaneously.

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).

The neutralisation of ions due to their interaction with matter deals with fundamental aspects of ion-solid interaction, e.g.: How does the kinetic energy loss depend on the charge exchange? In order to investigate the neutralisation behaviour, classical ion beam foil experiments were performed using ultimately thin 2D materials as target. Because of their low thickness, an ion with a sufficiently high incident charge state does not reach charge equilibrium, which enables the measurement of the non-equilibrium exit charge state distribution. The influence of target material properties on the charge exchange is investigated for 2D materials consisting of graphene, MoS₂ and hBN, which show different band gap energies between 0 and 6 eV, conductance properties (metallic, semi-conducting and insulating) and layer structures.

Highly charged ions (HCIs) are an efficient tool for the perforation of suspended 2D materials. Only a fraction of their potential energy is transferred to the atomically thin target during the very short interaction time and is available for pore formation. Charge exchange spectra were measured for highly charged xenon ions transmitted through suspended, single layer MoS₂ in order to determine the deposited potential energy available for pore formation. Additionally, charge exchange dependent ion stopping responsible for kinetic sputtering was measured simultaneously. CA extern

Kontaminationen auf zweidimensionalen Materialien k��nnen elektronische, optische oder mechanische Eigenschaften des Materials beeinflussen oder sogar dominieren. Wir nutzen hochgeladene Xenonionen als Projektile f��r Ionenspektroskopie, die durch freistehende 2D-Materialien transmittiert werden. Dabei studieren wir fundamentale Ladungsaustauschprozesse zwischen dem Ion und der Probe. Wir verwenden eine Elektronenstrahlionenquelle (electron beam ion source, EBIS), um Ionen mit Ladungszust��nden im Bereich Xe1+- Xe44+ zu produzieren, die mit einstellbarer kinetischer Energie (1-400 keV) aus der Ionenquelle extrahiert werden. Die Projektile werden dann durch freistehende 2D-Materialien transmittiert und fangen sowie stabilisieren bei der Wechselwirkung Elektronen. Wir messen den Austrittsladungszustand der Prim��rionen nach Transmission durch die Probe in Koinzidenz mit Elektronen, die bei der Wechselwirkung vom Material emittiert werden. Zus��tzlich nehmen wir die Flugzeit der Xenonionen von der Position der Probe bis zu einem Detektor auf, um Informationen ��ber deren Energieverlust im Material zu erlangen. Sowohl der Austrittsladungszustand als auch der Energieverlust h��ngen kritisch von der Materialdicke ab und werden von Kontaminationen auf beiden Seiten des 2D-Materials beeinflusst. Um sicherzugehen, dass unsere zweidimensionalen Proben sauber sind und dass der Ausgangsladungszustand des Projektils sowie die detektierte Elektronenemissionsausbeute vom 2D-Material dominiert sind und nicht von Kontaminationen (wie z.B. Wasser, Kohlenwasser- stoffe, PMMA), versuche ich mit verschiedenen M��glichkeiten unsere Proben zu reinigen: die Verwendung von einem Heizdraht und von einem Laser (445 nm, 6 W) verlagert den Ausgangsladungszustand zu h��heren und die Flugzeit zu kleineren Werten und erzeugt somit saubere(re) Proben. Ich zeige, dass ich gut gereinigte Probenzust��nde erreiche, die zumindest mithilfe unserer M��glichkeiten nicht mehr verbessert werden k��nnen. Messungen mit gereinigten Proben von Mono-, Bi- und Trilagengraphen werden am Ende dieser Arbeit in Bezug auf Streuwinkel, Ladungsaustausch und Neutralisationszeiten von hochgeladenen Xenonionen jeweils in Abh��ngigkeit der Graphenlagen diskutiert., Contaminations on two dimensional materials can influence electronic, optical and mechanical material properties or even dominate them. We perform ion beam spectroscopy with highly charged ions as projectiles transmitted through free-standing 2D materials to study fundamental charge exchange processes between the ion and the sample. We employ an electron beam ion source to produce ions with charge states in the range of Xe1+ to Xe44+ which can be extracted from the source with tunable kinetic energies (1-400 keV). The projectiles are transmitted through the free-standing 2D materials and capture as well as stabilise electrons due to the interaction. We measure the exit charge state of the primary ions after transmission in coincidence with emitted electrons from the sample due to the ion impact. Additionally, we record the time of flight of Xe ions from the target position to the detector to gain information on their energy loss in the sample. Both, exit charge state and energy loss critically depend on the sample thickness and will be influenced by contaminations on either side of the two dimensional materials. To aim at reaching a clean 2D sample so that the projectile exit charge state and the measured electron emission yields are dominated by the 2D layer rather than by contaminations (e.g. water molecules, hydrocarbons, PMMA) I try different ways to clean our samples: employing a heating wire as well as a laser (445 nm, 6 W) creates clean(er) samples and shifts the exit charge state to higher and the time of flight to lower values. I show that I achieve a cleaned target state which I at least cannot further improve with the methods at hand. Measurements using cleaned single, bi- and trilayer graphene samples are discussed in the end of this thesis with respect to charge exchange in dependence of the number of graphene layers, scattering angles and neutralisation times of highly charged Xe ions transmitted through these samples.