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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.

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.

2D materials offer extraordinary optical, electronic and mechanical properties, which make them interesting for future applications. Modification techniques, like ion irradiation, allow to alter their properties to specific applications. However, because of their atomic thickness applied techniques have to address mainly the surface region. Highly charged ions are a novel tool for nanostructure formation. They carry potential energies up to tens of keV, which trigger the process of nanostructure formation due to the energy deposition in shallow depths in close vicinity of the surface region. Recently, HCI impact induced nanoholes in carbon nanomembranes [1] and in suspended monolayer MoS₂ [2] have been observed despite their atomic thickness limiting the amount of potential energy to be transferred. Here, we investigate the charge exchange of highly charged xenon ions passing a monolayer of MoS₂, from which we can obtain an upper estimate of the deposited energy. The exit charge states of transmitted ions are measured simultaneously with their scattering angle as well as with their time-of-flights [3]. Two distinct distributions at high and low charge states are visible. The different scattering angles indicate the existence of two impact parameter regimes leading to two (exit charge state) distributions. Our experimental results are supported by computer simulations using the Monte-Carlo code TDPot [4]. [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 and P. L. Grande, Commun. Phys. 2, 89 (2019).

2D materials are promising candidates for electronic and photonic applications in future devices. Their properties can be tailored by surface sensitive modification techniques, like ion irradiation. Usually single charged ions are used, which deposit the main part of their kinetic energy in depths well below the surface. Only a tiny fraction is converted into the sputtering of target atoms. In contrast, highly charged ions (HCIs) carry additional potential energy in the keV regime, which may even exceed their kinetic energy. The deposition of the potential energy causes intense excitation and ionization of the electronic system of the target atoms in a shallow surface region. The high density of electronic excitations may lead to local temperatures above the sublimation point and finally to the creation of nanostructures. Even for freestanding 2D materials, like carbon nanomembranes, which consist of only a few atomic layers, pore formation induced by HCIs was observed. The small thickness of 2D materials enables spectroscopic measurements of the HCIs after transmission with respect to their charge state and kinetic energy. From our data we can estimate the amount of energy deposited in the material and therefore available for pore formation. In the present study, the influence of target properties on the charge exchange and on the neutralization dynamics is investigated. Spectroscopic measurements of the charge state of transmitted ions were performed using monolayers consisting of graphene, MoS₂ and hBN, which show different band gap energies between 0 and 6 eV, conductance properties (semi-metallic, semi-conducting and insulating) and layer structures.

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.

Low-energy electrons (LEEs) are of great relevance for ion-induced radiation damage in cells and genes. We show that charge exchange of ions leads to LEE emission upon impact on condensed matter. By using a graphene monolayer as a simple model system for condensed organic matter and utilizing slow highly charged ions (HCIs) as projectiles, we highlight the importance of charge exchange alone for LEE emission. We find a large number of ejected electrons resulting from individual ion impacts (up to 80 electrons/ion for Xe40+). More than 90% of emitted electrons have energies well below 15 eV. This “splash” of low-energy electrons is interpreted as the consequence of ion deexcitation via an interatomic Coulombic decay (ICD) process.

Porous single-layer molybdenum disulfide (MoS2) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS2. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process, and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.

2D materials are promising candidates for electronic and photonic applications in future devices. Their properties can be tailored by surface sensitive modification techniques, like ion irradiation. Usually single charged ions are used, which deposit the main part of their kinetic energy in depths well below the surface. Only a tiny fraction is converted into the sputtering of target atoms. In contrast, highly charged ions (HCIs) carry additional potential energy in the keV regime, which may even exceed their kinetic energy. The deposition of the potential energy causes intense excitation and ionization of the electronic system of the target atoms in a shallow surface region. The high density of electronic excitations may lead to local temperatures above the sublimation point and finally to the creation of nanostructures. Even for freestanding 2D materials, like carbon nanomembranes, which consist of only a few atomic layers, pore formation induced by HCIs was observed. The small thickness of 2D materials enables spectroscopic measurements of the HCIs after transmission with respect to their charge state and kinetic energy. From our data we can estimate the amount of energy deposited in the material and therefore available for pore formation. In the present study, the influence of target properties on the charge exchange and on the neutralization dynamics is investigated. Spectroscopic measurements of the charge state of transmitted ions were performed using monolayers consisting of graphene, MoS₂ and hBN, which show different band gap energies between 0 and 6 eV, conductance properties (semi-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.

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

The impact of highly charged ions onto a solid gives rise to charge exchange between the ions and target atoms, so that slow ions get neutralized in the vicinity of the surface. Using highly charged Ar and Xe ions and the surface-only material graphene as a target, we show that the neutralization and de-excitation of the ions proceeds on a sub-10 fs time scale. We further demonstrate that a multiple interatomic coulombic decay (ICD) model of highly charged ions can describe the observed ultrafast de-excitation. Other de-excitation mechanisms involving non-radiative decay and quasi- molecular orbitals formation during the impact are not important, as follows from the comparison of our experimental data with the results of first-principles calculations. Our method also enables the estimation of ICD rates directly.

Slow highly charged ions carry a large amount (several 10 keV) of potential energy, which gets released by target excitation and secondary particle emission upon impact on a solid surface. The energy release can trigger permanent material modifications on semi-conducting and insulating materials [1]. To understand the energy release mechanism and get information on it’s time scale, we use freestanding 2D materials, limiting the interaction time of the ions upon transmission to a few femtoseconds. We detect the ions after the interaction by means of charge state, energy, and angle resolved detection techniques. Further, we detect emitted secondary electrons in coincidence with a particular charge exchange. Using freestanding single layer graphene, our experimental findings revealed an ultrafast charge exchange and projectile de-excitation mechanism [2,3]. We also determined the in-plane current density in the material, which is transiently active to supply electrons to the ion, to be in the order of 1012A/cm2. Still, graphene is able to sustain these large current densities for a fs-time-scale without rupture. Here we go one step further and present results of ion transmission spectroscopy of single layer hBN and MoS2, which are insulating and semi-conducting, respectively.

We examine swift heavy ion-induced defect production in suspended single layer graphene using Raman spectroscopy and a two temperature molecular dynamics model that couples the ionic and electronic subsystems. We show that an increase in the electronic stopping power of the ion results in an increase in the size of the pore-type defects, with a defect formation threshold at 1.22–1.48 keV/layer. We also report calculations of the specific electronic heat capacity of graphene with different chemical potentials and discuss the electronic thermal conductivity of graphene at high electronic temperatures, suggesting a value in the range of 1 Wm−1 K−1. These results indicate that swift heavy ions can create nanopores in graphene, and that their size can be tuned between 1 and 4 nm diameter by choosing a suitable stopping power.

The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 1012 A cm 2, the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.

The impact of highly charged ions onto a solid gives rise to charge exchange between the ions and target atoms, so that slow ions get neutralized in the vicinity of the surface. Using highly charged Ar and Xe ions and the surface-only material graphene as a target, we show that the neutralization and de-excitation of the ions proceeds on a sub-10 fs time scale. We further demonstrate that a multiple interatomic coulombic decay (ICD) model of highly charged ions can describe the observed ultrafast de-excitation. Other de-excitation mechanisms involving non-radiative decay and quasi- molecular orbitals formation during the impact are not important, as follows from the comparison of our experimental data with the results of first-principles calculations. Our method also enables the estimation of ICD rates directly.

The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 1012 A cm 2, the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.

Studying ion-solid interaction has a long standing tradition both in fundamental research and for technological applications. The main parameter in this respect is the ion stopping force, i.e. the kinetic energy loss per unit length in a solid. The stopping force depends on the kinetic energy of the ions as well as on the degree of ionization [1]. The latter fact is usually disregarded, because after a few nanometers in a solid the ion accommodates an equilibrium charge state independent of it’s initial charge state. Stopping force in charge equilibrium is very well known. Here we use slow (v ≪ v0) highly charged ions (Q Z) to study stopping force far from charge equilibrium and the charge equilibration dynamics [2,3]. Using novel two-dimensional materials as target material allows us to limit the ion trajectory in the solid with monolayer precision and thus study non-equilibrium effects. The left side of fig. 1 shows schematically the experimental conditions with an ion beam transmitted under normal incidence through a freestanding single layer graphene sheet. The ion energy and charge state are measured with an electrostatic analyzer. A typical charge state distribution for Xe30+ ions at 40 keV (310 eV/amu) transmitted through graphene is also shown (right). Ions at this low velocity (0.1v0) are not fully neutralized. Hence, they still capture and stabilize about 20 electrons within the collision time of only 1-3 fs. Especially stabilization of the electrons is surprising, since the classical-over-barrier model for charge exchange [4] predicts the population of highly excited states with principal quantum numbers of n > 10 and a subsequent Auger electron cascade. Such a cascade would lead to the reemission of electrons and thus to a recharging of the ion. Present results show the need for a model beyond classical-over-barrier. This ultrafast charge exchange process is accompanied by a kinetic energy loss of up to ∆E /E ≈ 10 %, which is about 1 order of magnitude l

We investigated the response of wurzite GaN thin films to energetic ion irradiation. Both swift heavy ions (92 MeV Xe23+, 23 MeV I6+) and highly charged ions (100 keV Xe40+) were used. After irradiation, the samples were investigated using atomic force microscopy, grazing incidence small angle x-ray scattering, Rutherford backscattering spectroscopy in channelling orientation and time of flight elastic recoil detection analysis. Only grazing incidence swift heavy ion irradiation induced changes on the surface of the GaN, when the appearance of nanoholes is accompanied by a notable loss of nitrogen. The results are discussed in the framework of the thermal spike model.