Model for Nanopore Formation in Two-Dimensional Materials by Impact of Highly Charged Ions.

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.

Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and MoS_{2} Probed by the Emission of Electrons.

We compare the ion-induced electron emission from freestanding monolayers of graphene and MoS_{2} to find a sixfold higher number of emitted electrons for graphene even though both materials have similar work functions. An effective single-band Hubbard model explains this finding by a charge-up in MoS_{2} that prevents low energy electrons from escaping the surface within a period of a few femtoseconds after ion impact. We support these results by measuring the electron energy distribution for correlated pairs of electrons and transmitted ions. The majority of emitted primary electrons have an energy below 10 eV and are therefore subject to the dynamic charge-up effects at surfaces.

Nano-hillock formation on CaF2due to individual slow Au-cluster impacts.

We present a direct way to generate hillock-like nanostructures on CaF2(111) ionic crystals by kinetic energy deposition upon Au-cluster irradiation. In the past, the formation of similar nanostructures has been observed for both slow highly charged ions and swift heavy ions. However, in these cases, potential energy deposition of highly charged ions or the electronic energy loss of fast heavy ions, respectively, first leads to strong electronic excitation of the target material before the excitation energy is transferred to the lattice by efficient electron-phonon coupling. We now show that the kinetic energy deposited by slow single Au-clusters directly in the lattice of CaF2(111) leads to the production of nano-hillocks very similar to those found with slow highly charged and swift heavy ions, with heights between 1 and 2 nm. Our results are in good agreement with previous cluster irradiation studies regarding energy deposition and hence nano-structuring of surfaces, and we present Au-cluster irradiation as novel tool to fine-tune nanostructure formation.

A high temperature dual-mode quartz crystal microbalance technique for erosion and thermal desorption spectroscopy measurements.

An improved quartz crystal microbalance measurement method is described, which allows us to determine erosion, implantation, and release rates of thin films, during changing temperatures and up to 700 K. A quasi-simultaneous excitation of two eigenmodes of the quartz resonator is able to compensate for frequency drifts due to temperature changes. The necessary electronics, the controlling behavior, and the dual-mode temperature compensation are described. With this improved technique, quantitative in situ temperature-programmed desorption measurements are possible and the quartz crystal microbalance can be used for quantification of thermal desorption spectroscopy measurements with a quadrupole mass spectrometer. This is demonstrated by a study of the retention and release behavior of hydrogen isotopes in fusion-relevant materials. We find that more than 90% of the deuterium implanted into a thin film of beryllium is released during a subsequent temperature ramp up to 500 K.

Atomic-Scale Carving of Nanopores into a van der Waals Heterostructure with Slow Highly Charged Ions.

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.

Charge-Exchange-Driven Low-Energy Electron Splash Induced by Heavy Ion Impact on Condensed Matter.

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.

A versatile ion beam spectrometer for studies of ion interaction with 2D materials.

We present an ultrahigh vacuum setup for ion spectroscopy of freestanding two-dimensional solid targets. An ion beam of different ion species (e.g., Xe with charge states from 1 to 44 and Ar with charge states from 1 to 18) and kinetic energies ranging from a few 10 eV to 400 keV is produced in an electron beam ion source. Ions are detected after their transmission through the 2D target with a position sensitive microchannel plate detector allowing the determination of the ion's exit charge state as well as the scattering angle with a resolution of approximately 0.04°. Furthermore, the spectrometer is mounted on a swiveling frame covering a scattering angle of ±8° with respect to the incoming beam direction. By utilizing a beam chopper, we measure the time-of-flight of the projectiles and determine the energy loss when passing a 2D target with an energy uncertainty of about 2%. Additional detectors are mounted close to the target to observe emitted secondary particles and are read-out in coincidence with the position and time information of the ion detector. A signal in these detectors can also be used as a start trigger for time-of-flight measurements, which then yield an energy resolution of 1% and an approximately 1000-fold larger duty cycle. First results on the interaction of slow Xe30+ ions with a freestanding single layer of graphene obtained with the new setup are compared to recently published data where charge exchange and energy were measured by means of an electrostatic analyzer.

High resolution AFM studies of irradiated mica-following the traces of swift heavy ions under grazing incidence.

High resolution AFM imaging of swift heavy ion irradiated muscovite mica under grazing incidence provides detailed insight into the created nanostructure features. Swift heavy ions under grazing incidence form a complex track structure along the surface, which consists of a double track of nanohillocks at the impact site accompanied by a single, several 100 nm long protrusion. Detailed track studies by varying the irradiation parameters, i.e. the angle of incidence (0.2°-2°) and the kinetic energy of the impinging ions (23, 55, 75, 95 MeV) are presented. Moreover, the track formation in dependence of the sample temperature (between room temperature and 600 °C) and of the chemical composition (muscovite mica and fluorphlogopite mica) is studied.

Interatomic Coulombic Decay: The Mechanism for Rapid Deexcitation of Hollow Atoms.

The impact of a highly charged ion onto a solid gives rise to charge exchange between the ion and target atoms, so that a slow ion gets 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 deexcitation of the ions proceeds on a sub-10 fs time scale. We further demonstrate that a multiple Interatomic Coulombic Decay (ICD) model can describe the observed ultrafast deexcitation. Other deexcitation mechanisms involving nonradiative decay and quasimolecular orbital 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.

Ultrafast electronic response of graphene to a strong and localized electric field.

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.

Tuning the Fabrication of Nanostructures by Low-Energy Highly Charged Ions.

Slow highly charged ions have been utilized recently for the creation of monotype surface nanostructures (craters, calderas, or hillocks) in different materials. In the present study, we report on the ability of slow highly charged xenon ions (^{129}Xe^{Q+}) to form three different types of nanostructures on the LiF(100) surface. By increasing the charge state from Q=15 to Q=36, the shape of the impact induced nanostructures changes from craters to hillocks crossing an intermediate stage of caldera structures. A dimensional analysis of the nanostructures reveals an increase of the height up to 1.5 nm as a function of the potential energy of the incident ions. Based on the evolution of both the geometry and size of the created nanostructures, defect-mediated desorption and the development of a thermal spike are utilized as creation mechanisms of the nanostructures at low and high charge states, respectively.

Swift heavy ion irradiation of CaF2 - from grooves to hillocks in a single ion track.

A novel form of ion-tracks, namely nanogrooves and hillocks, are observed on CaF2 after irradiation with xenon and lead ions of about 100 MeV kinetic energy. The irradiation is performed under grazing incidence (0.3°-3°) which forces the track to a region in close vicinity to the surface. Atomic force microscopy imaging of the impact sites with high spatial resolution reveals that the surface track consists in fact of three distinct parts: each swift heavy ion impacting on the CaF2 surface first opens a several 100 nm long groove bordered by a series of nanohillocks on both sides. The end of the groove is marked by a huge single hillock and the further penetration of the swift projectile into deeper layers of the target is accompanied by a single protrusion of several 100 nm in length slowly fading until the track vanishes. By comparing experimental data for various impact angles with results of a simulation, based on a three-dimensional version of the two-temperature-model (TTM), we are able to link the crater and hillock formation to sublimation and melting processes of CaF2 due to the local energy deposition by swift heavy ions.

Charge exchange and energy loss of slow highly charged ions in 1 nm thick carbon nanomembranes.

Experimental charge exchange and energy loss data for the transmission of slow highly charged Xe ions through ultrathin polymeric carbon membranes are presented. Surprisingly, two distinct exit charge state distributions accompanied by charge exchange dependent energy losses are observed. The energy loss for ions exhibiting large charge loss shows a quadratic dependency on the incident charge state indicating that equilibrium stopping force values do not apply in this case. Additional angle resolved transmission measurements point on a significant contribution of elastic energy loss. The observations show that regimes of different impact parameters can be separated and thus a particle's energy deposition in an ultrathin solid target may not be described in terms of an averaged energy loss per unit length.

Single ion induced surface nanostructures: a comparison between slow highly charged and swift heavy ions.

This topical review focuses on recent advances in the understanding of the formation of surface nanostructures, an intriguing phenomenon in ion-surface interaction due to the impact of individual ions. In many solid targets, swift heavy ions produce narrow cylindrical tracks accompanied by the formation of a surface nanostructure. More recently, a similar nanometric surface effect has been revealed for the impact of individual, very slow but highly charged ions. While swift ions transfer their large kinetic energy to the target via ionization and electronic excitation processes (electronic stopping), slow highly charged ions produce surface structures due to potential energy deposited at the top surface layers. Despite the differences in primary excitation, the similarity between the nanostructures is striking and strongly points to a common mechanism related to the energy transfer from the electronic to the lattice system of the target. A comparison of surface structures induced by swift heavy ions and slow highly charged ions provides a valuable insight to better understand the formation mechanisms.

Potential sputtering.

The potential energy stored in multiply charged ions is liberated when the ions recombine during impact on a solid surface. For certain target species this can lead to a novel form of ion-induced sputtering, which, in analogy to the usual kinetic sputtering, has been termed 'potential sputtering'. This sputtering process is characterized by a strong dependence of the observed sputtering yields on the charge state of the impinging ion and can take place at ion-impact energies well below the kinetic sputtering threshold. We summarize a series of recent careful experiments in which potential sputtering has been investigated for hyperthermal highly charged ions' impact on various surfaces (e.g. Au, LiF, NaCl, SiO(2), Al(2)O(3) and MgO), present the different models proposed to explain the potential sputtering phenomenon and also discuss possible applications of potential sputtering for nanostructure fabrication.