Prof. Dr. Marika Schleberger

Experimental Physics
University of Duisburg-Essen

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  • Charge Regulation at a Nanoporous Two-Dimensional Interface
    Ghosh, M. and Junker, M.A. and Van Lent, R.T.M. and Madauß, L. and Schleberger, M. and Lebius, H. and Benyagoub, A. and Wood, J.A. and Lammertink, R.G.H.
    ACS Omega 6 (2021)
    In this work, we have studied the pH-dependent surface charge nature of nanoporous graphene. This has been investigated by membrane potential and by streaming current measurements, both with varying pH. We observed a lowering of the membrane potential with decreasing pH for a fixed concentration gradient of potassium chloride (KCl) in the Donnan dominated regime. Interestingly, the potential reverses its sign close to pH 4. The fitted value of effective fixed ion concentration (C¯ R) in the membrane also follows the same trend. The streaming current measurements show a similar trend with sign reversal around pH 4.2. The zeta potential data from the streaming current measurement is further analyzed using a 1-pK model. The model is used to determine a representative pK (acid-base equilibrium constant) of 4.2 for the surface of these perforated graphene membranes. In addition, we have also theoretically investigated the effect of the PET support in our membrane potential measurement using numerical simulations. Our results indicate that the concentration drop inside the PET support can be a major contributor (up to 85%) for a significant deviation of the membrane potential from the ideal Nernst potential. © 2021 The Authors. Published by American Chemical Society.
    view abstract10.1021/acsomega.0c03958
  • Gate-Controlled Field Emission Current from MoS2 Nanosheets
    Pelella, A. and Grillo, A. and Urban, F. and Giubileo, F. and Passacantando, M. and Pollmann, E. and Sleziona, S. and Schleberger, M. and Di Bartolomeo, A.
    Advanced Electronic Materials 7 (2021)
    Monolayer molybdenum disulfide (MoS2) nanosheets, obtained via chemical vapor deposition onto SiO2/Si substrates, are exploited to fabricate field-effect transistors with n-type conduction, high on/off ratio, steep subthreshold slope, and good mobility. The transistor channel conductance increases with the reducing air pressure due to oxygen and water desorption. Local field emission measurements from the edges of the MoS2 nanosheets are performed in high vacuum using a tip-shaped anode. It is demonstrated that the voltage applied to the Si substrate back-gate modulates the field emission current. Such a finding, that it is attributed to gate-bias lowering of the MoS2 electron affinity, enables a new field-effect transistor based on field emission. © 2020 Wiley-VCH GmbH
    view abstract10.1002/aelm.202000838
  • Laser- and Ion-Induced Defect Engineering in WS2 Monolayers
    Asaithambi, A. and Kozubek, R. and Prinz, G.M. and Reale, F. and Pollmann, E. and Ney, M. and Mattevi, C. and Schleberger, M. and Lorke, A.
    Physica Status Solidi - Rapid Research Letters 15 (2021)
    Tungsten disulfide is one of the prominent transition metal dichalcogenide materials, which shows a transition from an indirect to a direct bandgap as the layer thickness is reduced down to a monolayer. To use (Formula presented.) monolayers in devices, detailed knowledge about the luminescence properties regarding not only the excitonic but also the defect-induced contributions is needed. Herein, (Formula presented.) monolayers are irradiated with (Formula presented.) ions with different fluences to create different defect densities. Apart from the excitonic contributions, two additional emission bands are observed at low temperatures. These bands can be reduced or even suppressed, if the flakes are exposed to laser light with powers up to 1.5 mW. Increasing the temperature up to room temperature leads to recovery of this emission, so that the luminescence properties can be modified using laser excitation and temperature. The defect bands emerging after ion irradiation are attributed to vacancy defects together with physisorbed adsorbates at different defect sites. © 2020 The Authors. Physica Status Solidi (RRL) – Rapid Research Letters published by Wiley-VCH GmbH
    view abstract10.1002/pssr.202000466
  • Nanopatterning surfaces by grazing incidence swift heavy ion irradiation
    Karlušić, M. and Mičetić, M. and Kresić, M. and Jakšić, M. and Šantić, B. and Bogdanović-Radović, I. and Bernstorff, S. and Lebius, H. and Ban-d'Etat, B. and Žužek Rožman, K. and O'Connell, J.H. and Hagemann, U. and Schleberger, M.
    Applied Surface Science 541 (2021)
    Nanopatterned surfaces play a key role for many applications exploiting unique features such as an enhanced surface area, long- and short-ranged morphology modulations or a spatial variation of electronic and chemical properties. Ion beam irradiation has been frequently used for nanostructuring bulk materials because it is efficient, fast, and cost-effective. In this paper we show that ion irradiation under extremely grazing incidence in conjunction with other scalable processing methods such as wet etching and thermal annealing, is a perfect tool for nanopatterning of dielectric surfaces. We demonstrate that by tuning ion energy and fluence, one can select different surface nanopattern morphologies like individual chains of nanohillocks, nanostripes, or nanoscaled ripples. Furthermore, chemical etching of the irradiated surface can be used to create a negative replica of the nanopattern as only the material making up the surface track is susceptible to the etching process and is thus removed. Also, a removal of the surface track can be achieved by thermal annealing in vacuum. All these presented strategies open up new ways for achieving control over nanoscale surface modifications using swift heavy ion beams. © 2020 Elsevier B.V.
    view abstract10.1016/j.apsusc.2020.148467
  • Time-of-flight mass spectrometry of particle emission during irradiation with slow, highly charged ions
    Skopinski, L. and Ernst, P. and Herder, M. and Kozubek, R. and Madauß, L. and Sleziona, S. and Maas, A. and Königstein, N. and Lebius, H. and Wucher, A. and Schleberger, M.
    Review of Scientific Instruments 92 (2021)
    We describe a setup for the analysis of secondary ions and neutrals emitted from solid surfaces and two-dimensional materials during irradiation with highly charged ions. The ultrahigh vacuum setup consists of an electron beam ion source to produce bunches of ions with various charge states q (e.g., Xe1+-Xe46+) and thus potential energies, a deceleration/acceleration section to tune the kinetic energy of the ions in the range of 5 keV to 20 × q keV, a sample stage for laser-cleaning and positioning of freestanding as well as supported samples, a pulsed excimer laser for post-ionization of sputtered neutrals, and a reflectron type time-of-flight mass spectrometer, enabling us to analyze mass and velocity distributions of the emitted particles. With our setup, contributions from potential and kinetic energy deposition can be studied independently of each other. Charge dependent experiments conducted at a constant kinetic energy show a clear threshold for the emission of secondary ions from SrTiO3. Data taken with the same projectile charge state, but at a different kinetic energy, reveal a difference in the ratio of emitted particles from MoS2. In addition, first results are presented, demonstrating how velocity distributions can be measured with the new setup. © 2021 Author(s).
    view abstract10.1063/5.0025812
  • A swift technique to hydrophobize graphene and increase its mechanical stability and charge carrier density
    Madauß, L. and Pollmann, E. and Foller, T. and Schumacher, J. and Hagemann, U. and Heckhoff, T. and Herder, M. and Skopinski, L. and Breuer, L. and Hierzenberger, A. and Wittmar, A. and Lebius, H. and Benyagoub, A. and Ulbricht, M. and Joshi, R. and Schleberger, M.
    npj 2D Materials and Applications 4 (2020)
    Despite the improvement of the quality of CVD grown single-layer graphene on copper substrates, transferring the two-dimensional layer without introducing any unintentional defects still poses a challenge. While many approaches focus on optimizing the transfer itself or on necessary post-transfer cleaning steps, we have focused on developing a pre-treatment of the monolayer graphene on copper to improve the quality and reproducibility of the transfer process. By pressing an ethylene-vinyl acetate copolymer foil onto the monolayer graphene on copper using a commercially available vacuum bag sealer graphene is stabilized by the attachment of functional carbon groups. As a result, we are able to transfer graphene without the need of any supporting layer in an all-H2O wet-chemical transfer step. Despite the general belief that the crumbling of graphene without a support layer in a H2O environment is caused due to differences in surface energy, we will show that this assumption is false and that this behavior is caused rather by the polar interactions between graphene and water. Suppressing these interactions protects graphene from ripping and results in extremely clean, highly crystalline graphene with a coverage close to 100%. © 2020, The Author(s).
    view abstract10.1038/s41699-020-0148-9
  • Apparent differences between single layer molybdenum disulphide fabricated via chemical vapour deposition and exfoliation
    Pollmann, E. and Madauß, L. and Schumacher, S. and Kumar, U. and Heuvel, F. and vom Ende, C. and Yilmaz, S. and Güngörmüs, S. and Schleberger, M.
    Nanotechnology 31 (2020)
    Innovative applications based on two-dimensional solids require cost-effective fabrication processes resulting in large areas of high quality materials. Chemical vapour deposition is among the most promising methods to fulfill these requirements. However, for 2D materials prepared in this way it is generally assumed that they are of inferior quality in comparison to the exfoliated 2D materials commonly used in basic research. In this work we challenge this assumption and aim to quantify the differences in quality for the prototypical transition metal dichalcogenide MoS2. To this end single layers of MoS2 prepared by different techniques (exfoliation, grown by different chemical vapour deposition methods, transfer techniques and as vertical heterostructure with graphene) are studied by Raman and photoluminescence spectroscopy, complemented by atomic force microscopy. We demonstrate that as-prepared MoS2, directly grown on SiO2, differs from exfoliated MoS2 in terms of higher photoluminescence, lower electron concentration and increased strain. As soon as a water film is intercalated (e.g. by transfer) underneath the grown MoS2, in particular the (opto)electronic properties become practically identical to those of exfoliated MoS2. A comparison of the two most common precursors shows that the growth with MoO3 causes greater strain and/or defect density deviations than growth with ammonium heptamolybdate. As part of a heterostructure directly grown MoS2 interacts much stronger with the substrate and in this case an intercalated water film does not lead to the complete decoupling, which is typical for exfoliation or transfer. Our work shows that the supposedly poorer quality of grown 2D transition metal dichalcogenides is indeed a misconception. © 2020 The Author(s).
    view abstract10.1088/1361-6528/abb5d2
  • Electron Irradiation of Metal Contacts in Monolayer MoS2Field-Effect Transistors
    Pelella, A. and Kharsah, O. and Grillo, A. and Urban, F. and Passacantando, M. and Giubileo, F. and Iemmo, L. and Sleziona, S. and Pollmann, E. and Madauß, L. and Schleberger, M. and Di Bartolomeo, A.
    ACS Applied Materials and Interfaces 12 (2020)
    Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsami.0c11933
  • Energy deposition of highly charged ions transmitted through single layer MoS2
    Creutzburg, S. and Schwestka, J. and Inani, H. and Tripathi, M.K. and Grande, P.L. and Heller, R. and Klingner, N. and Niggas, A. and Kozubek, R. and Madauß, L. and Facsko, S. and Kotakoski, J. and Schleberger, M. and Aumayr, F. and Wilhelm, R.A.
    Journal of Physics: Conference Series 1412 (2020)
    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. © 2020 Institute of Physics Publishing. All rights reserved.
    view abstract10.1088/1742-6596/1412/16/162018
  • Erratum: Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer (Journal of Physical Chemistry C (2020) 124: 4 (2689−2697) DOI: 10.1021/acs.jpcc.9b10120)
    Pollmann, E. and Morbec, J.M. and Madauß, L. and Bröckers, L. and Kratzer, P. and Schleberger, M.
    Journal of Physical Chemistry C 124 (2020)
    The scale bar in the abstract graphic was unintentionally inserted incorrectly. The corrected abstract graphic is provided here. We kindly apologize for any possible irritation while reading the publication. (Figure Presented). © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c01297
  • Freestanding and Supported MoS2Monolayers under Cluster Irradiation: Insights from Molecular Dynamics Simulations
    Ghaderzadeh, S. and Ladygin, V. and Ghorbani-Asl, M. and Hlawacek, G. and Schleberger, M. and Krasheninnikov, A.V.
    ACS Applied Materials and Interfaces 12 (2020)
    Two-dimensional (2D) materials with nanometer-size holes are promising systems for DNA sequencing, water purification, and molecule selection/separation. However, controllable creation of holes with uniform sizes and shapes is still a challenge, especially when the 2D material consists of several atomic layers as, e.g., MoS2, the archetypical transition metal dichalcogenide. We use analytical potential molecular dynamics simulations to study the response of 2D MoS2 to cluster irradiation. We model both freestanding and supported sheets and assess the amount of damage created in MoS2 by the impacts of noble gas clusters in a wide range of cluster energies and incident angles. We show that cluster irradiation can be used to produce uniform holes in 2D MoS2 with the diameter being dependent on cluster size and energy. Energetic clusters can also be used to displace sulfur atoms preferentially from either top or bottom layers of S atoms in MoS2 and also clean the surface of MoS2 sheets from adsorbents. Our results for MoS2, which should be relevant to other 2D transition metal dichalcogenides, suggest new routes toward cluster beam engineering of devices based on 2D inorganic materials. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsami.0c09255
  • High-frequency gas effusion through nanopores in suspended graphene
    Rosłoń, I.E. and Dolleman, R.J. and Licona, H. and Lee, M. and Šiškins, M. and Lebius, H. and Madauß, L. and Schleberger, M. and Alijani, F. and van der Zant, H.S.J. and Steeneken, P.G.
    Nature Communications 11 (2020)
    Porous, atomically thin graphene membranes have interesting properties for filtration and sieving applications. Here, graphene membranes are used to pump gases through nanopores using optothermal forces, enabling the study of gas flow through nanopores at frequencies above 100 kHz. At these frequencies, the motion of graphene is closely linked to the dynamic gas flow through the nanopore and can thus be used to study gas permeation at the nanoscale. By monitoring the time delay between the actuation force and the membrane mechanical motion, the permeation time-constants of various gases through pores with diameters from 10–400 nm are shown to be significantly different. Thus, a method is presented for differentiating gases based on their molecular mass and for studying gas flow mechanisms. The presented microscopic effusion-based gas sensing methodology provides a nanomechanical alternative for large-scale mass-spectrometry and optical spectrometry based gas characterisation methods. © 2020, The Author(s).
    view abstract10.1038/s41467-020-19893-5
  • Highly charged ion impact on graphene leading to the emission of low energy electrons
    Schwestka, J. and Niggas, A. and Creutzburg, S. and Kozubek, R. and Madauß, L. and Heller, R. and Schleberger, M. and Facsko, S. and Wilhelm, R.A. and Aumayr, F.
    Journal of Physics: Conference Series 1412 (2020)
    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. © 2019 Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/1412/20/202012
  • Ionization probability of sputtered indium atoms under impact of slow highly charged ions
    Herder, M. and Ernst, P. and Skopinski, L. and Weidtmann, B. and Schleberger, M. and Wucher, A.
    Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 38 (2020)
    In order to investigate the different role of kinetic and potential projectile energy for secondary ion formation, the authors have measured the ionization probability of indium atoms sputtered from a clean indium surface under irradiation with rare gas (Xeq+) ions of different charge states q at the same kinetic impact energy of 20 keV. In this energy range, the kinetic energy of the projectile is predominantly deposited via nuclear stopping, leading to a collision-dominated sputtering process. The authors find that the ionization probability increases significantly if a highly charged ion is used as a projectile, where the ionization energy becomes comparable to or even exceeds the kinetic energy, indicating that a higher level of electronic substrate excitation induced by the potential energy stored in the projectile can boost the secondary ion formation process. This experimental result is discussed in terms of microscopic model calculations describing the secondary ion formation process. At the same time, the authors observe a significant change of the emission velocity distribution of the sputtered particles, leading to a pronounced low-energy contribution at higher projectile charge states. It is shown that this "potential sputtering"contribution strongly depends on surface chemistry even under conditions where the surface is dynamically cleaned by interleaved 5 keV Ar+ ion bombardment. © 2020 Author(s).
    view abstract10.1116/6.0000171
  • Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer
    Pollmann, E. and Morbec, J.M. and Madauß, L. and Bröckers, L. and Kratzer, P. and Schleberger, M.
    Journal of Physical Chemistry C 124 (2020)
    Two-dimensional molybdenum disulfide on graphene grown by chemical vapor deposition is a promising van der Waals system for applications in optoelectronics and catalysis. To extend the fundamental understanding of growth and intrinsic properties of molybdenum disulfide on graphene, molybdenum disulfide on highly oriented pyrolytic graphite is a suitable model system. Here, we show experimentally and by density functional theory calculations that molybdenum disulfide flakes grow in two orientations. One of the orientations is energetically preferred, the other one is rotated by 30°, but both orientations are found to be stable at room temperature. Combined Kelvin probe microscopy and Raman spectroscopy measurements show that the flakes with a typical size of a few hundred nanometers are electron doped in the order of 1012/cm2, while the doping of a molybdenum disulfide single layer exfoliated on silicon dioxide is on the order of 1013/cm2. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b10120
  • Particle emission from two-dimensional MoS2 induced by highly charged ion impact
    Skopinski, L. and Ernst, P. and Herder, M. and Schleberger, M.
    Journal of Physics: Conference Series 1412 (2020)
    For many attractive applications of single layer MoS2 such as in optoelectronics e.g., the sample is supported by a substrate. Its importance for the modification through ion irradiation is here experimentally investigated by the analysis of sputtered particle of MoS2 on SiO2 and Au substrates under highly charged ion irradiation. The velocity distribution of the sputtered atoms is less affected by the substrate using highly charged projectiles than using slightly charged ones. Furthermore, we can show that potential sputtering causes additional emission of particles with lower kinetic energy. © 2019 Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/1412/20/202007
  • Selective proton transport for hydrogen production using graphene oxide membranes
    Madauß, L. and Foller, T. and Plaß, J. and Kumar, P.V. and Musso, T. and Dunkhorst, K. and Joshi, R. and Schleberger, M.
    Journal of Physical Chemistry Letters (2020)
    Graphene oxide has shown exceptional properties in terms of water permeability and filtration characteristics. Here the suitability of graphene oxide membranes for the spatial separation of hydronium and hydroxide ions after photocatalytic water splitting is demonstrated. Instead of relying on classical size exclusion by adjusting the membrane laminates' interlayer spacings, nonmodified graphene oxide is used to exploit the presence of its natural functional groups and surface charges for filtration. Despite a significantly larger interlayer spacing inside the membrane compared with the size of the hydrated radii of the ions, highly asymmetric transport behavior and a 6 times higher mobility for hydronium than for hydroxide are observed. DFT simulations reveal that hydroxide ions are more prone to interact and stick to the functional groups of graphene oxide, while diffusion of hydronium ions through the membrane is less impeded and aligns well with the concept of the Grotthuss mechanism. © 2020 American Chemical Society. All rights reserved.
    view abstract10.1021/acs.jpclett.0c02481
  • The effect of elevated temperatures on excitonic emission and degradation processes of WS2monolayers
    Kaupmees, R. and Walke, P. and Madauß, L. and Maas, A. and Pollmann, E. and Schleberger, M. and Grossberg, M. and Krustok, J.
    Physical Chemistry Chemical Physics 22 (2020)
    Controlled heating experiments in an inert environment have been performed on WS2 monolayers, in order to clarify the conflicting reports on the high-temperature photoluminescent response of 2D TMDs. We find that in contrast to some previous results on both WS2 and MoS2, the photoluminescent intensity shows a consistent reduction above room temperature. This is accompanied by an almost linear redshift of the peak maximum, and a nearly linear increase in the peak width, which is attributed to an enhanced interaction with optical phonons. Moreover, by fitting the photoluminescence integral intensity with an Arrhenius type dependence, we demonstrate that the center of the WS2 monolayer flake starts to undergo irreversible degradation at a temperature of 573 K in an inert environment. Regions close to flake edges in contrast, with a more intense room temperature PL response, remain stable. The macroscopic PL signal is largely recovered in these regions following subsequent cooling to room temperature. This journal is © the Owner Societies.
    view abstract10.1039/d0cp03248d
  • Understanding Mono- And Bivalent Ion Selectivities of Nanoporous Graphene Using Ionic and Bi-ionic Potentials
    Ghosh, M. and Madauß, L. and Schleberger, M. and Lebius, H. and Benyagoub, A. and Wood, J.A. and Lammertink, R.G.H.
    Langmuir 36 (2020)
    Nanoporous graphene displays salt-dependent ion permeation. In this work, we investigate the differences in Donnan potentials arising between reservoirs, separated by a perforated graphene membrane, containing different cations. We compare the case of monovalent cations interacting with nanoporous graphene with the case of bivalent cations. This is accomplished through both measurements of membrane potential arising between two salt reservoirs at different concentrations involving a single cation (ionic potential) and between two reservoirs containing different cations at the same concentration (bi-ionic potential). In our present study, Donnan dialysis experiments involve bivalent MgCl2, CaCl2, and CuCl2 as well as monovalent KCl and NH4Cl salts. For all salts, except CuCl2, clear Donnan and diffusion potential plateaus were observed at low and high salt concentrations, respectively. Our observations show that the membrane potential scaled to the Nernst potential for bivalent cations has a lower value (≈50%) than for monovalent cations (≈72%) in the Donnan exclusion regime. This is likely due to the adsorption of these bivalent cations on monolayer graphene. For bivalent cations, the diffusion regime is reached at a lower ionic strength compared to the monovalent cations. For Mg2+ and Ca2+, the membrane potential does not seem to depend upon the type of ions in the entire ionic strength range. A similar behavior is observed for the KCl and NH4Cl membrane potential curves. For CuCl2, the membrane potential curve is shifted toward lower ionic strength compared to the other two bivalent salts and the Donnan plateau is not observed at the lowest ionic strength. Bi-ionic potential measurements give further insight into the strength of specific interactions, allowing for the estimation of the relative ionic selectivities of different cations based on comparing their bi-ionic potentials. This effect of possible ion adsorption on graphene can be removed through ion exchange with monovalent salts. © 2020 American Chemical Society.
    view abstract10.1021/acs.langmuir.0c00924
  • Vanishing influence of the band gap on the charge exchange of slow highly charged ions in freestanding single-layer MoS2
    Creutzburg, S. and Schwestka, J. and Niggas, A. and Inani, H. and Tripathi, M. and George, A. and Heller, R. and Kozubek, R. and Madauß, L. and McEvoy, N. and Facsko, S. and Kotakoski, J. and Schleberger, M. and Turchanin, A. and Grande, P.L. and Aumayr, F. and Wilhelm, R.A.
    Physical Review B 102 (2020)
    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 high and low charge states, are observed. They are accompanied by smaller and larger kinetic energy losses, as well as scattering angles, 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. Independent 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. Considering the similarities of the zero band-gap material graphene and the 1.9 eV band-gap material MoS2, we suggest that electron transport on the femtosecond timescale is dominated by the strong influence of the ion's Coulomb potential in contrast to the dispersion defined by the material's band structure. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.045408
  • A concept to generate ultrashort ion pulses for pump-probe experiments in the keV energy range
    Breuers, A. and Herder, M. and Kucharczyk, P. and Schleberger, M. and Sokolowski-Tinten, K. and Wucher, A.
    New Journal of Physics 21 (2019)
    The impact of an energetic particle onto a solid surface generates a strongly perturbed and extremely localized non-equilibrium state, which relaxes on extremely fast time scales. In order to facilitate a time-resolved observation of the relaxation dynamics using established ultrafast pump-probe techniques, it is necessary to pinpoint the projectile impact in time with sufficient accuracy. In this paper, we propose a concept to generate ultrashort ion pulses via femtosecond photoionization of rare gas atoms entrained in a supersonic jet, combined with ion optical bunching of the resulting ion package. We calculate the photoion cloud generated by an intense focused laser pulse and show that Arq+ ions with q = 1-5 can be generated with a standard table-top laser system, which are then accelerated to energies in the keV range over a very short distance and bunched to impinge onto the target surface in a time-focused manner. Detailed ion trajectory simulations show that single ion pulses of sub-picosecond duration can be generated this way. The influence of space charge broadening is included in the simulations, which reveal that flight time broadening is insignificant for pulses containing up to 10-20 ions and starts to increase the pulse width above ∼50 ions/pulse. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab1775
  • Charge-exchange-driven low-energy electron splash induced by heavy ion impact on condensed matter
    Schwestka, J. and Niggas, A. and Creutzburg, S. and Kozubek, R. and Heller, R. and Schleberger, M. and Wilhelm, R.A. and Aumayr, F.
    Journal of Physical Chemistry Letters 10 (2019)
    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. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.jpclett.9b01774
  • Gas dependent hysteresis in MoS2 field effect transistors
    Urban, F. and Giubileo, F. and Grillo, A. and Iemmo, L. and Luongo, G. and Passacantando, M. and Foller, T. and Madauß, L. and Pollmann, E. and Geller, M.P. and Oing, D. and Schleberger, M. and Di Bartolomeo, A.
    2D Materials 6 (2019)
    We study the effect of electric stress, gas pressure and gas type on the hysteresis in the transfer characteristics of monolayer molybdenum disulfide (MoS2) field effect transistors. The presence of defects and point vacancies in the MoS2 crystal structure facilitates the adsorption of oxygen, nitrogen, hydrogen or methane, which strongly affect the transistor electrical characteristics. Although the gas adsorption does not modify the conduction type, we demonstrate a correlation between hysteresis width and adsorption energy onto the MoS2 surface. We show that hysteresis is controllable by pressure and/or gas type. Hysteresis features two well-separated current levels, especially when gases are stably adsorbed on the channel, which can be exploited in memory devices. © 2019 IOP Publishing Ltd.
    view abstract10.1088/2053-1583/ab4020
  • Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions
    Kozubek, R. and Tripathi, M. and Ghorbani-Asl, M. and Kretschmer, S. and Madauß, L. and Pollmann, E. and O'Brien, M. and McEvoy, N. and Ludacka, U. and Susi, T. and Duesberg, G.S. and Wilhelm, R.A. and Krasheninnikov, A.V. and Kotakoski, J. and Schleberger, M.
    Journal of Physical Chemistry Letters 10 (2019)
    Porous single-layer molybdenum disulfide (MoS 2 ) 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 MoS 2 . 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. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpclett.8b03666
  • Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: With zero to relativistic speeds
    Aumayr, F. and Ueda, K. and Sokell, E. and Schippers, S. and Sadeghpour, H. and Merkt, F. and Gallagher, T.F. and Dunning, F.B. and Scheier, P. and Echt, O. and Kirchner, T. and Fritzsche, S. and Surzhykov, A. and Ma, X. and Rivarola, R. and Fojon, O. and Tribedi, L. and Lamour, E. and Crespo López-Urrutia, J.R. and Litvinov, Y.A. and Shabaev, V. and Cederquist, H. and Zettergren, H. and Schleberger, M. and Wilhelm, R.A. and Azuma, T. and Boduch, P. and Schmidt, H.T. and Stöhlker, T.
    Journal of Physics B: Atomic, Molecular and Optical Physics 52 (2019)
    We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6455/ab26ea
  • 2D material science: Defect engineering by particle irradiation
    Schleberger, M. and Kotakoski, J.
    Materials 11 (2018)
    Two-dimensional (2D) materials are at the heart of many novel devices due to their unique and often superior properties. For simplicity, 2D materials are often assumed to exist in their text-book form, i.e., as an ideal solid with no imperfections. However, defects are ubiquitous in macroscopic samples and play an important - if not imperative - role for the performance of any device. Thus, many independent studies have targeted the artificial introduction of defects into 2D materials by particle irradiation. In our view it would be beneficial to develop general defect engineering strategies for 2D materials based on a thorough understanding of the defect creation mechanisms, which may significantly vary from the ones relevant for 3D materials. This paper reviews the state-of-the-art in defect engineering of 2D materials by electron and ion irradiation with a clear focus on defect creation on the atomic scale and by individual impacts. Whenever possible we compile reported experimental data alongside corresponding theoretical studies. We show that, on the one hand, defect engineering by particle irradiation covers a wide range of defect types that can be fabricated with great precision in the most commonly investigated 2D materials. On the other hand, gaining a complete understanding still remains a challenge, that can be met by combining advanced theoretical methods and improved experimental set-ups, both of which only now begin to emerge. In conjunction with novel 2D materials, this challenge promises attractive future opportunities for researchers in this field. © 2018 by the authors.
    view abstract10.3390/ma11101885
  • High resolution AFM studies of irradiated mica - Following the traces of swift heavy ions under grazing incidence
    Gruber, E. and Bergen, L. and Salou, P. and Lattouf, E. and Grygiel, C. and Wang, Y. and Benyagoub, A. and Levavasseur, D. and Rangama, J. and Lebius, H. and Ban-D'Etat, B. and Schleberger, M. and Aumayr, F.
    Journal of Physics Condensed Matter 30 (2018)
    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. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aac7f7
  • Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation
    Madauß, L. and Zegkinoglou, I. and Vázquez Muiños, H. and Choi, Y.-W. and Kunze, S. and Zhao, M.-Q. and Naylor, C.H. and Ernst, P. and Pollmann, E. and Ochedowski, O. and Lebius, H. and Benyagoub, A. and Ban-D'Etat, B. and Johnson, A.T.C. and Djurabekova, F. and Roldan Cuenya, B. and Schleberger, M.
    Nanoscale 10 (2018)
    Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8nr04696d
  • Hysteresis in the transfer characteristics of MoS2 transistors
    Di Bartolomeo, A. and Genovese, L. and Giubileo, F. and Iemmo, L. and Luongo, G. and Foller, T. and Schleberger, M.
    2D Materials 5 (2018)
    We investigate the origin of the hysteresis observed in the transfer characteristics of back-gated field-effect transistors with an exfoliated MoS2 channel. We find that the hysteresis is strongly enhanced by increasing either gate voltage, pressure, temperature or light intensity. Our measurements reveal a step-like behavior of the hysteresis around room temperature, which we explain as water-facilitated charge trapping at the MoS2/SiO2 interface. We conclude that intrinsic defects in MoS2, such as S vacancies, which result in effective positive charge trapping, play an important role, besides H2O and O2 adsorbates on the unpassivated device surface. We show that the bistability associated to the hysteresis can be exploited in memory devices. © 2017 IOP Publishing Ltd.
    view abstract10.1088/2053-1583/aa91a7
  • Ion-mediated growth of ultra thin molybdenum disulfide layers on highly oriented pyrolytic graphite
    Pollmann, E. and Ernst, P. and Madauß, L. and Schleberger, M.
    Surface and Coatings Technology 349 (2018)
    Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials are at the center of many novel devices. To prepare vdW heterostructures which are of the highest quality and suitable for applications, chemical vapour deposition (CVD) can be used to grow the 2D materials directly on top of each other and thus build the vdW heterostructure in a bottom-up fashion. However, obtaining layers of uniform quality by precisely controlling their growth poses a severe challenge. The aim of our work is to understand the growth mechanisms and we have chosen MoS2 layers on highly oriented pyrolytic graphite (HOPG) as a model system for the MoS2-graphene interface. In our model system we observe, that MoS2 layers do not grow on the HOPG terraces but are more likely to grow at HOPG edges, one-dimensional defects, which obviously acts as growth seeds. In graphene however, step edges are absent and the ever-improving quality of commercially available CVD graphene yields less and less defects per unit area. While this is clearly an advantage for most devices, in the light of our findings it constitutes a major disadvantage for the bottom-up preparation of vdW heterostructures. To overcome this obstacle we artificially introduce defects into the HOPG surface by highly charged ion irradiation. In this way we induce an easily controllable number of quasi zero-dimensional defects before the chemical vapour deposition of MoS2 takes place. We show that this treatment results in MoS2 island growth on top HOPG terraces. © 2018 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2018.05.031
  • Surface structure modification of single crystal graphite after slow, highly charged ion irradiation
    Alzaher, I. and Akcöltekin, S. and Ban-d'Etat, B. and Manil, B. and Dey, K.R. and Been, T. and Boduch, P. and Rothard, H. and Schleberger, M. and Lebius, H.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 420 (2018)
    Single crystal graphite was irradiated by slow, highly charged ions. The modification of the surface structure was studied by means of Low-Energy Electron Diffraction. The observed damage cross section increases with the potential energy, i.e. the charge state of the incident ion, at a constant kinetic energy. The potential energy is more efficient for the damage production than the kinetic energy by more than a factor of twenty. Comparison with earlier results hints to a strong link between early electron creation and later target atom rearrangement. With increasing ion fluence, the initially large-scale single crystal is first transformed into μm-sized crystals, before complete amorphisation takes place. © 2018 Elsevier B.V.
    view abstract10.1016/j.nimb.2018.01.024
  • The role of radiative de-excitation in the neutralization process of highly charged ions interacting with a single layer of graphene
    Schwestka, J. and Wilhelm, R.A. and Gruber, E. and Heller, R. and Kozubek, R. and Schleberger, M. and Facsko, S. and Aumayr, F.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 422 (2018)
    X-ray emission of slow (<1 a.u.) highly charged Argon and Xenon ions is measured for transmission through a freestanding single layer of graphene. To discriminate against X-ray emission originating from the graphene's support grid a coincidence technique is used. X-ray emission of 75 keV Ar17+ and Ar18+ ions with either one or two K-shell vacancies is recorded. Using a windowless Bruker XFlash detector allows us to measure additionally Ar KLL and KLM Auger electrons and determine the branching ratio of radiative vs. non-radiative decay of Ar K-shell holes. Furthermore, X-ray spectra for 100 keV Xe22+-Xe35+ ions are compared, showing a broad M-line peak for all cases, where M-shell vacancies are present. All these peaks are accompanied by emission lines at still higher energies indicating the presence of a hollow atom during X-ray decay. We report a linear shift of the main M-line peak to higher energies for increasing incident charge state, i.e. increasing number of M-shell holes. © 2018 Elsevier B.V.
    view abstract10.1016/j.nimb.2018.02.022
  • Tunable quantum interference in bilayer graphene in double-resonant Raman scattering
    Herziger, F. and Tyborski, C. and Ochedowski, O. and Schleberger, M. and Maultzsch, J.
    Carbon 133 (2018)
    The line shape of the double-resonant 2D Raman mode in bilayer graphene is often considered to be characteristic for a certain laser excitation energy. Here, in a joint experimental and theoretical study, we analyze the dependence of the double-resonant Raman scattering processes in bilayer graphene on the electronic broadening parameter γ. We demonstrate that the ratio between symmetric and anti-symmetric scattering processes sensitively depends on the lifetime of the electronic states, explaining the experimentally observed variation of the complex 2D-mode line shape. © 2018 Elsevier Ltd
    view abstract10.1016/j.carbon.2018.03.026
  • Charge equilibration times for slow highly charged ions in single layer graphene
    Gruber, E. and Wilhelm, R.A. and Schwestka, J. and Smejkal, V. and Kozubek, R. and Krasheninnikov, A.V. and Schleberger, M. and Facsko, S. and Aumayr, F.
    Journal of Physics: Conference Series 875 (2017)
    We report on charge exchange and energy loss measurements for slow highly charged Xeq+ (q ≤ 35) ions after transmission through a single layer of freestanding graphene. Surprisingly short charge equilibration times of only a few femtoseconds are found, which cannot be explained within currently available models.
    view abstract10.1088/1742-6596/875/12/112001
  • Corrigendum to “Creating nanoporous graphene with swift heavy ions” [Carbon 114 (April 2017) 511–518] (S0008622316310843) (10.1016/j.carbon.2016.12.015))
    Vázquez, H. and Åhlgren, E.H. and Ochedowski, O. and Leino, A.A. and Mirzayev, R. and Kozubek, R. and Lebius, H. and Karlušic, M. and Jakšic, M. and Krasheninnikov, A.V. and Kotakoski, J. and Schleberger, M. and Nordlund, K. and Djurabekova, F.
    Carbon 119 (2017)
    The authors regret that the address of authors H. Vázquez, A. A. Leino and F. Djurabekova was incorrectly given. The correct address is: Helsinki Institute of Physics and Department of Physics, P. O. Box 43, FIN-00014 University of Helsinki, Finland The authors would like to apologise for any inconvenience caused. © 2017 Elsevier Ltd
    view abstract10.1016/j.carbon.2017.03.065
  • Creating nanoporous graphene with swift heavy ions
    Vázquez, H. and Åhlgren, E.H. and Ochedowski, O. and Leino, A.A. and Mirzayev, R. and Kozubek, R. and Lebius, H. and Karlušic, M. and Jakšic, M. and Krasheninnikov, A.V. and Kotakoski, J. and Schleberger, M. and Nordlund, K. and Djurabekova, F.
    Carbon 114 (2017)
    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. © 2016 Elsevier Ltd
    view abstract10.1016/j.carbon.2016.12.015
  • Defect engineering of single- and few-layer MoS2 by swift heavy ion irradiation
    Madauss, L. and Ochedowski, O. and Lebius, H. and Ban-d'Etat, B. and Naylor, C. H. and Johnson, A. T. C. and Kotakoski, J. and Schleberger, M.
    2d Materials 4 (2017)
    We have investigated the possibility to use swift heavy ion irradiation for nano-structuring supported and freestanding ultra-thin MoS2 samples. Our comprehensive study of the ion-induced morphological changes in various MoS2 samples shows that depending on the irradiation parameters a multitude of extended defects can be fabricated. These range from chains of nano-hillocks in bulk-like MoS2, and foldings in single and bilayer MoS2, to unique nano-incisions in supported and freestanding single layers of MoS2. Our data reveals that the primary mechanism responsible for the incisions in the ultrathin supported samples is the indirect heating by the SiO2 substrate. We thus conclude that an energy of less than 2 keV pernmtrack length is sufficient to fabricate nano-incisions in MoS2 which is compatible with the use of the smallest accelerators.
    view abstract10.1088/2053-1583/4/1/015034
  • Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors
    Di Bartolomeo, A. and Genovese, L. and Foller, T. and Giubileo, F. and Luongo, G. and Croin, L. and Liang, S.-J. and Ang, L.K. and Schleberger, M.
    Nanotechnology 28 (2017)
    We study electrical transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 104 s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of ON current as high as 105 ON-OFF ratio, mobility of ∼1 cm2 V-1 s-1 and photoresponsivity. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa6d98
  • Fabrication of nanoporous graphene/polymer composite membranes
    Madauß, L. and Schumacher, J. and Ghosh, M. and Ochedowski, O. and Meyer, J. and Lebius, H. and Ban-D'Etat, B. and Toimil-Molares, M.E. and Trautmann, C. and Lammertink, R.G.H. and Ulbricht, M. and Schleberger, M.
    Nanoscale 9 (2017)
    Graphene is currently investigated as a promising membrane material in which selective pores can be created depending on the requirements of the application. However, to handle large-area nanoporous graphene a stable support material is needed. Here, we report on composite membranes consisting of large-area single layer nanoporous graphene supported by a porous polymer. The fabrication is based on ion-track nanotechnology with swift heavy ions directly creating atomic pores in the graphene lattice and damaged tracks in the polymer support. Subsequent chemical etching converts the latent ion tracks in the supporting polymer foil, here polyethylene terephthalate (PET), into open microchannels while the perfectly aligned pores in the graphene top layer remain unaffected. To avoid unintentional damage creation and delamination of the graphene layer from the substrate, the graphene is encapsulated by a protecting poly(methyl methacrylate) (PMMA) layer. By this procedure a stable composite membrane is obtained consisting of nanoporous graphene (coverage close to 100%) suspended across selfaligned track-etched microchannels in a polymer support film. Our method presents a facile way to create high quality suspended graphene of tunable pore size supported on a flexible porous polymeric support, thus enabling the development of membranes for fast and selective ultrafiltration separation processes. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c7nr02755a
  • Interatomic Coulombic Decay: The Mechanism for Rapid Deexcitation of Hollow Atoms
    Wilhelm, R.A. and Gruber, E. and Schwestka, J. and Kozubek, R. and Madeira, T.I. and Marques, J.P. and Kobus, J. and Krasheninnikov, A.V. and Schleberger, M. and Aumayr, F.
    Physical Review Letters 119 (2017)
    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. © 2017 American Physical Society.
    view abstract10.1103/PhysRevLett.119.103401
  • Monitoring ion track formation using in situ RBS/c, ToF-ERDA, and HR-PIXE
    Karlušić, M. and Fazinić, S. and Siketić, Z. and Tadić, T. and Cosic, D.D. and Božičević-Mihalić, I. and Zamboni, I. and Jakšić, M. and Schleberger, M.
    Materials 10 (2017)
    The aim of this work is to investigate the feasibility of ion beam analysis techniques for monitoring swift heavy ion track formation. First, the use of the in situ Rutherford backscattering spectrometry in channeling mode to observe damage build-up in quartz SiO2 after MeV heavy ion irradiation is demonstrated. Second, new results of the in situ grazing incidence time-of-flight elastic recoil detection analysis used for monitoring the surface elemental composition during ion tracks formation in various materials are presented. Ion tracks were found on SrTiO3, quartz SiO2, a-SiO2, and muscovite mica surfaces by atomic force microscopy, but in contrast to our previous studies on GaN and TiO2, surface stoichiometry remained unchanged. Third, the usability of high resolution particle induced X-ray spectroscopy for observation of electronic dynamics during early stages of ion track formation is shown. © 2017 by the authors.
    view abstract10.3390/ma10091041
  • On the threshold for ion track formation in CaF2
    Karlušić, M. and Ghica, C. and Negrea, R.F. and Siketić, Z. and Jakšić, M. and Schleberger, M. and Fazinić, S.
    New Journal of Physics 19 (2017)
    There is an ongoing debate regarding the mechanism of swift heavy ion (SHI) track formation in CaF2. The objective of this study is to shed light on this important topic using a range of complementary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV nm-1 is provided by both transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy in the channelling mode, which has direct consequences for the validity of models describing the response of CaF2 to SHI irradiation. Furthermore, information about the elemental composition within the ion tracks is obtained using scanning TEM, electron energy loss spectroscopy, and with respect to the stoichiometry of the materials surface by in situ time of flight elastic recoil detection analysis. Advances in the analyses of the experimental data presented here pave the way for a better understanding of the ion track formation. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/aa5914
  • SAXS investigation of un-etched and etched ion tracks in polycarbonate
    Hossain, U.H. and Rodriguez, M.D. and Schauries, D. and Hadley, A. and Schleberger, M. and Trautmann, C. and Mudie, S. and Kluth, P.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 409 (2017)
    Investigation of the ion track morphologies and track etching behaviour in polycarbonate (PC) films was carried out using synchrotron based small-angle X-ray scattering (SAXS) measurements. The tracks were induced by Au ions with kinetic energies of 1.7 and 2.2 GeV with applied fluences between 1 × 1010 and 1 × 1012 ions/cm2. The average radii of the un-etched tracks were studied as a function of the irradiation fluence, indicating a general ion induced degradation of the polymer, with a simultaneous increase in ion track radius from 2.6 ± 0.002 nm to 3.4 ± 0.03 nm. Chemical etching of the ion tracks in PC leads to the formation of cylindrical pores. The pore radius increases linearly with etching time. In 3 M NaOH at 55 °C, a radial etching rate of 9.2 nm/min is observed. © 2017 Elsevier B.V.
    view abstract10.1016/j.nimb.2017.04.023
  • Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions
    Karlusic, M. and Jaksic, M. and Lebius, H. and Ban-d'Etat, B. and Wilhelm, R. A. and Heller, R. and Schleberger, M.
    Journal of Physics D-applied Physics 50 (2017)
    Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm(-1). Obtained results are compared with predictions of the analytical thermal spike model.
    view abstract10.1088/1361-6463/aa678c
  • Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions
    Karlušić, M. and Jakšić, M. and Lebius, H. and Ban-D'Etat, B. and Wilhelm, R.A. and Heller, R. and Schleberger, M.
    Journal of Physics D: Applied Physics 50 (2017)
    Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm-1. Obtained results are compared with predictions of the analytical thermal spike model. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa678c
  • A new setup for the investigation of swift heavy ion induced particle emission and surface modifications
    Meinerzhagen, F. and Breuer, L. and Bukowska, H. and Bender, M. and Severin, D. and Herder, M. and Lebius, H. and Schleberger, M. and Wucher, A.
    Review of Scientific Instruments 87 (2016)
    The irradiation with fast ions with kinetic energies of >10 MeV leads to the deposition of a high amount of energy along their trajectory (up to several ten keV/nm). The energy is mainly transferred to the electronic subsystem and induces different secondary processes of excitations, which result in significant material modifications. A new setup to study these ion induced effects on surfaces will be described in this paper. The setup combines a variable irradiation chamber with different techniques of surface characterizations like scanning probe microscopy, time-of-flight secondary ion, and neutral mass spectrometry, as well as low energy electron diffraction under ultra high vacuum conditions, and is mounted at a beamline of the universal linear accelerator (UNILAC) of the GSI facility in Darmstadt, Germany. Here, samples can be irradiated with high-energy ions with a total kinetic energy up to several GeVs under different angles of incidence. Our setup enables the preparation and in situ analysis of different types of sample systems ranging from metals to insulators. Time-of-flight secondary ion mass spectrometry enables us to study the chemical composition of the surface, while scanning probe microscopy allows a detailed view into the local electrical and morphological conditions of the sample surface down to atomic scales. With the new setup, particle emission during irradiation as well as persistent modifications of the surface after irradiation can thus be studied. We present first data obtained with the new setup, including a novel measuring protocol for time-of-flight mass spectrometry with the GSI UNILAC accelerator. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4939899
  • Formation of swift heavy ion tracks on a rutile TiO2 (001) surface
    Karlušic, M. and Bernstorff, S. and Siketic, Z. and Šantic, B. and Bogdanovic-Radovic, I. and Jakšic, M. and Schleberger, M. and Buljan, M.
    Journal of Applied Crystallography 49 (2016)
    Nanostructuring of surfaces and two-dimensional materials using swift heavy ions offers some unique possibilities owing to the deposition of a large amount of energy localized within a nanoscale volume surrounding the ion trajectory. To fully exploit this feature, the morphology of nanostructures formed after ion impact has to be known in detail. In the present work the response of a rutile TiO2 (001) surface to grazing-incidence swift heavy ion irradiation is investigated. Surface ion tracks with the well known intermittent inner structure were successfully produced using 23MeV I ions. Samples irradiated with different ion fluences were investigated using atomic force microscopy and grazing-incidence small-angle X-ray scattering. With these two complementary approaches, a detailed description of the swift heavy ion impact sites, i.e. the ion tracks on the surface, can be obtained even for the case of multiple ion track overlap. In addition to the structural investigation of surface ion tracks, the change in stoichiometry of the rutile TiO2 (001) surface during swift heavy ion irradiation was monitored using in situ time-of-flight elastic recoil detection analysis, and a preferential loss of oxygen was found.Formation of ion tracks on a rutile TiO2 (001) surface after exposure to swift heavy ions under grazing incidence is studied using atomic force microscopy, grazing-incidence small-angle X-ray scattering and in situ time-of-flight elastic recoil detection analysis and a preferential loss of oxygen was found. © Marko Karlušic et al. 2016.
    view abstract10.1107/S1600576716013704
  • Irradiation of graphene field effect transistors with highly charged ions
    Ernst, P. and Kozubek, R. and Madauß, L. and Sonntag, J. and Lorke, A. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 382 (2016)
    In this work, graphene field-effect transistors are used to detect defects due to irradiation with slow, highly charged ions. In order to avoid contamination effects, a dedicated ultra-high vacuum set up has been designed and installed for the in situ cleaning and electrical characterization of graphene field-effect transistors during irradiation. To investigate the electrical and structural modifications of irradiated graphene field-effect transistors, their transfer characteristics as well as the corresponding Raman spectra are analyzed as a function of ion fluence for two different charge states. The irradiation experiments show a decreasing mobility with increasing fluences. The mobility reduction scales with the potential energy of the ions. In comparison to Raman spectroscopy, the transport properties of graphene show an extremely high sensitivity with respect to ion irradiation: a significant drop of the mobility is observed already at fluences below 15 ions/μm2, which is more than one order of magnitude lower than what is required for Raman spectroscopy. © 2016 Elsevier B.V.
    view abstract10.1016/j.nimb.2016.03.043
  • Swift heavy ion irradiation of CaF2 - From grooves to hillocks in a single ion track
    Gruber, E. and Salou, P. and Bergen, L. and El Kharrazi, M. and Lattouf, E. and Grygiel, C. and Wang, Y. and Benyagoub, A. and Levavasseur, D. and Rangama, J. and Lebius, H. and Ban-D'Etat, B. and Schleberger, M. and Aumayr, F.
    Journal of Physics Condensed Matter 28 (2016)
    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. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/28/40/405001
  • Ultrafast electronic response of graphene to a strong and localized electric field
    Gruber, E. and Wilhelm, R.A. and Pétuya, R. and Smejkal, V. and Kozubek, R. and Hierzenberger, A. and Bayer, B.C. and Aldazabal, I. and Kazansky, A.K. and Libisch, F. and Krasheninnikov, A.V. and Schleberger, M. and Facsko, S. and Borisov, A.G. and Arnau, A. and Aumayr, F.
    Nature Communications 7 (2016)
    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 10 12 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 Author(s) 2016.
    view abstract10.1038/ncomms13948
  • Nanostructuring graphene by dense electronic excitation
    Ochedowski, O. and Lehtinen, O. and Kaiser, U. and Turchanin, A. and Ban-D'Etat, B. and Lebius, H. and Karlušić, M. and Jakšić, M. and Schleberger, M.
    Nanotechnology 26 (2015)
    The ability to manufacture tailored graphene nanostructures is a key factor to fully exploit its enormous technological potential. We have investigated nanostructures created in graphene by swift heavy ion induced folding. For our experiments, single layers of graphene exfoliated on various substrates and freestanding graphene have been irradiated and analyzed by atomic force and high resolution transmission electron microscopy as well as Raman spectroscopy. We show that the dense electronic excitation in the wake of the traversing ion yields characteristic nanostructures each of which may be fabricated by choosing the proper irradiation conditions. These nanostructures include unique morphologies such as closed bilayer edges with a given chirality or nanopores within supported as well as freestanding graphene. The length and orientation of the nanopore, and thus of the associated closed bilayer edge, may be simply controlled by the direction of the incoming ion beam. In freestanding graphene, swift heavy ion irradiation induces extremely small openings, offering the possibility to perforate graphene membranes in a controlled way. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/26/46/465302
  • Response of GaN to energetic ion irradiation: Conditions for ion track formation
    Karlušić, M. and Kozubek, R. and Lebius, H. and Ban-d'Etat, B. and Wilhelm, R.A. and Buljan, M. and Siketić, Z. and Scholz, F. and Meisch, T. and Jakšić, M. and Bernstorff, S. and Schleberger, M. and Šantić, B.
    Journal of Physics D: Applied Physics 48 (2015)
    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. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/32/325304
  • Splitting of monolayer out-of-plane A1′ Raman mode in few-layer WS2
    Staiger, M. and Gillen, R. and Scheuschner, N. and Ochedowski, O. and Kampmann, F. and Schleberger, M. and Thomsen, C. and Maultzsch, J.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    We present Raman measurements of mono- and few-layer WS2. We study the monolayer A1′ mode around 420cm-1 and its evolution with the number of layers. We show that with increasing layer number there are an increasing number of possible vibrational patterns for the out-of-plane Raman mode: in N-layer WS2 there are N Γ-point phonons evolving from the A1′ monolayer mode. For an excitation energy close to resonance with the A excitonic transition energy, we were able to observe all of these N components, irrespective of their Raman activity. Density functional theory calculations support the experimental findings and make it possible to attribute the modes to their respective symmetries. The findings described here are of general importance for all other phonon modes in WS2 and other layered transition-metal dichalcogenide systems in the few-layer regime. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.195419
  • Studies of surface nanostructure formation due to swift heavy ion irradiation under grazing incidence
    Gruber, E. and Lattouf, E. and Bergen, L. and Salou, P. and Grygief, C. and Wang, Y.Y. and Ochedowski, O. and Benyagoub, A. and Levavasseur, D. and Rangama, J. and Lebius, H. and Ban D'Etat, B. and Schleberger, M. and Aumayr, F.
    Journal of Physics: Conference Series 635 (2015)
    In this contribution we present new experimental results for swift heavy ion irradiation under grazing incidence. This particular collision geometry forces the track formation to a region close to the surface, sometimes visible as a chain of individual nanodots, whose length can be controlled by the angle of incidence. For irradiation of mica surfaces a new track form is observed. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/635/3/032001
  • Swift Heavy Ion Induced Optical and Electronic Modifications of Graphene-TiO2 Nanocomposites
    Mishra, M. and Meinerzhagen, F. and Schleberger, M. and Kanjilal, D. and Mohanty, T.
    Journal of Physical Chemistry C 119 (2015)
    The effect of swift heavy ions irradiation on optical and electronic properties of chemically synthesized graphene-TiO<inf>2</inf> nanocomposites is presented. Modification of surface properties of these nanocomposites by irradiation with three different ions and with varying fluence was analyzed by Raman spectroscopy, transmission electron microscopy, and scanning Kelvin probe microscopy techniques. Raman spectra of irradiated samples exhibit systematic changes in the characteristic peaks of both graphene and TiO<inf>2</inf>. The nanocrystallite dimension calculated from Raman peak intensity decreases with fluence, indicating the occurrence of peripheral fragmentation. Furthermore, measurement of the surface contact potential difference using scanning Kelvin probe reveals that the work function of graphene-titanium dioxide nanocomposites can be effectively increased by more than 1 eV. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b07297
  • Application of HOPG and CVD graphene as ion beam detectors
    Kozubek, R. and Ochedowski, O. and Zagoranskiy, I. and Karlušić, M. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 340 (2014)
    Highly ordered pyrolytic graphite and graphene created via chemical vapor deposition have been irradiated with high energetic I6+ ions. By Raman mapping an increase of the ID/IG ratio could be identified which arises from the ion induced defects. This ratio grows with increasing fluence. Using this as a tool, HOPG and graphene can be utilized to determine the ion beam spot size and its homogeneity. Both systems seem to be suitable for size determination of the spot. But due to the much higher sensitivity of graphene to ion irradiation, more detailed information regarding the homogeneity of the beam can only be derived using this 2D system. By comparison of both systems we conclude, that CVD graphene is more suitable as an ion beam detector, while HOPG is sufficient for a rough spot size analysis. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.nimb.2014.07.034
  • Damage in graphene due to electronic excitation induced by highly charged ions
    Hopster, J. and Kozubek, R. and Ban-D'Etat, B. and Guillous, S. and Lebius, H. and Schleberger, M.
    2D Materials 1 (2014)
    Graphene is expected to be rather insensitive to ion irradiation. We demonstrate that single layers of exfoliated graphene sustain significant damage from irradiation with slow highly charged ions. We have investigated the ion induced changes of graphene after irradiation with highly charged ions of different charge states (q = 28-42) and kinetic energies (E<inf>kin</inf> = 150-450 keV). Atomic force microscopy images reveal that the ion induced defects are not topographic in nature but are related to a significant change in friction. To create these defects, a minimum charge state is needed. In addition to this threshold behaviour, the required minimum charge state as well as the defect diameter show a strong dependency on the kinetic energy of the projectiles. From the linear dependency of the defect diameter on the projectile velocity we infer that electronic excitations triggered by the incoming ion in the above-surface phase play a dominant role for this unexpected defect creation in graphene. © 2014 IOP Publishing Ltd.
    view abstract10.1088/2053-1583/1/1/011011
  • Double-resonant la phonon scattering in defective graphene and carbon nanotubes
    Herziger, F. and Tyborski, C. and Ochedowski, O. and Schleberger, M. and Maultzsch, J.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    We present measurements of the D′′ Raman mode in graphene and carbon nanotubes at different laser excitation energies. The Raman mode around 1050-1150 cm-1 originates from a double-resonant scattering process of longitudinal acoustic (LA) phonons with defects. We investigate its dependence on laser excitation energy, on the number of graphene layers, and on the carbon nanotube diameter. We assign this Raman mode to so-called inner processes with resonant phonons mainly from the Γ-K high-symmetry direction. The asymmetry of the D′′ mode is explained by additional contributions from phonons next to the Γ-K line. Our results demonstrate the importance of inner contributions in the double-resonance scattering process and add a fast method to investigate acoustic phonons in graphene and carbon nanotubes by optical spectroscopy. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.245431
  • Effect of contaminations and surface preparation on the work function of single layer MoS2
    Ochedowski, O. and Marinov, K. and Scheuschner, N. and Poloczek, A. and Bussmann, B.K. and Maultzsch, J. and Schleberger, M.
    Beilstein Journal of Nanotechnology 5 (2014)
    Thinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance. © 2014 Ochedowski et al.
    view abstract10.3762/bjnano.5.32
  • Field-regulated switching of the magnetization of Co-porphyrin on graphene
    Klar, D. and Bhandary, S. and Candini, A. and Joly, L. and Ohresser, P. and Klyatskaya, S. and Schleberger, M. and Ruben, M. and Affronte, M. and Eriksson, O. and Sanyal, B. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    Different magnetic coupling mechanisms have been identified for a few monolayers of Co-porphyrin molecules deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker intermolecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic field strength, the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and field dependence was probed. The nature of the magnetic couplings is identified by means of density functional theory and orbital-dependent susceptibilities. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.144411
  • Folding two dimensional crystals by swift heavy ion irradiation
    Ochedowski, O. and Bukowska, H. and Freire Soler, V.M. and Brökers, L. and Ban-D'Etat, B. and Lebius, H. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 340 (2014)
    Ion irradiation of graphene, the showcase model of two dimensional crystals, has been successfully applied to induce various modifications in the graphene crystal. One of these modifications is the formation of origami like foldings in graphene which are created by swift heavy ion irradiation under glancing incidence angle. These foldings can be applied to locally alter the physical properties of graphene like mechanical strength or chemical reactivity. In this work we show that the formation of foldings in two dimensional crystals is not restricted to graphene but can be applied for other materials like MoS2 and hexagonal BN as well. Further we show that chemical vapour deposited graphene forms foldings after swift heavy ion irradiation while chemical vapour deposited MoS2 does not. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.nimb.2014.07.037
  • Graphene on mica - Intercalated water trapped for life
    Ochedowski, O. and Bussmann, B.K. and Schleberger, M.
    Scientific Reports 4 (2014)
    In this work we study the effect of thermal processing of exfoliated graphene on mica with respect to changes in graphene morphology and surface potential. Mild annealing to temperatures of about 200°C leads to the removal of small amounts of intercalated water at graphene edges. By heating to 600°C the areas without intercalated water are substantially increased enabling a quantification of the charge transfer properties of the water layer by locally resolved Kelvin probe force microscopy data. A complete removal on a global scale cannot be achieved because mica begins to decompose at temperatures above 600°C. By correlating Kelvin probe force microscopy and Raman spectroscopy maps we find a transition from p-type to n-type doping of graphene during thermal processing which is driven by the dehydration of the mica substrate and an accumulation of defects in the graphene sheet.
    view abstract10.1038/srep06003
  • Graphitic nanostripes in silicon carbide surfaces created by swift heavy ion irradiation
    Ochedowski, O. and Osmani, O. and Schade, M. and Bussmann, B.K. and Ban-Detat, B. and Lebius, H. and Schleberger, M.
    Nature Communications 5 (2014)
    The controlled creation of defects in silicon carbide represents a major challenge. A well-known and efficient tool for defect creation in dielectric materials is the irradiation with swift (E kin ‰ 500 ‰keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.
    view abstract10.1038/ncomms4913
  • Hydration layers trapped between graphene and a hydrophilic substrate
    Temmen, M. and Ochedowski, O. and Schleberger, M. and Reichling, M. and Bollmann, T.R.J.
    New Journal of Physics 16 (2014)
    Graphene is mechanically exfoliated on CaF2(111) under ambient conditions. We demonstrate the formation of a several monolayer thick hydration layer on the hydrophilic substrate and its response to annealing at temperatures up to 750 K in an ultra-high vacuum environment. Upon heating, water is released, however, it is impossible to remove the first layer. The initially homogeneous film separates into water-containing and water-free domains by two-dimensional Ostwald ripening. Upon severe heating, thick graphene multilayers undergo rupture, while nanoblisters confining sealed water appear on thinner sheets, capable of the storage and release of material. From modeling the dimensions of the nanoblisters, we estimate the graphene/CaF2(111) interfacial adhesion energy to be 0.33 ± 0.13 J m-2, thereby viable for polymer-assisted transfer printing. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/16/5/053039
  • Photoluminescence of freestanding single- and few-layer MoS 2
    Scheuschner, N. and Ochedowski, O. and Kaulitz, A.-M. and Gillen, R. and Schleberger, M. and Maultzsch, J.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 instead originates from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate decreases the indirect band gap energy by up to ≈80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.125406
  • Detecting swift heavy ion irradiation effects with graphene
    Ochedowski, O. and Akcöltekin, S. and Ban-D'Etat, B. and Lebius, H. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 314 (2013)
    In this paper we show how single layer graphene can be utilized to study swift heavy ion (SHI) modifications on various substrates. The samples were prepared by mechanical exfoliation of bulk graphite onto SrTiO3, NaCl and Si(1 1 1), respectively. SHI irradiations were performed under glancing angles of incidence and the samples were analysed by means of atomic force microscopy in ambient conditions. We show that graphene can be used to check whether the irradiation was successful or not, to determine the nominal ion fluence and to locally mark SHI impacts. In case of samples prepared in situ, graphene is shown to be able to catch material which would otherwise escape from the surface. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.nimb.2013.03.063
  • Manipulation of the graphene surface potential by ion irradiation
    Ochedowski, O. and Kleine Bussmann, B. and Ban Detat, B. and Lebius, H. and Schleberger, M.
    Applied Physics Letters 102 (2013)
    We show that the work function of exfoliated single layer graphene can be modified by irradiation with swift (E k i n 92 MeV) heavy ions under glancing angles of incidence. Upon ion impact individual surface tracks are created in graphene on silicon carbide. Due to the very localized energy deposition characteristic for ions in this energy range, the surface area which is structurally altered is limited to ≈ 0.01 μ m 2 per track. Kelvin probe force microscopy reveals that those surface tracks consist of electronically modified material and that a few tracks suffice to shift the surface potential of the whole single layer flake by ≈ 400 meV. Thus, the irradiation turns the initially n-doped graphene into p-doped graphene with a hole density of 8.5 × 10 12 holes / cm 2. This doping effect persists even after heating the irradiated samples to 500 °C. Therefore, this charge transfer is not due to adsorbates but must instead be attributed to implanted atoms. The method presented here opens up a way to efficiently manipulate the charge carrier concentration of graphene. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4801973
  • Radiation hardness of graphene and MoS2 field effect devices against swift heavy ion irradiation
    Ochedowski, O. and Marinov, K. and Wilbs, G. and Keller, G. and Scheuschner, N. and Severin, D. and Bender, M. and Maultzsch, J. and Tegude, F.J. and Schleberger, M.
    Journal of Applied Physics 113 (2013)
    We have investigated the deterioration of field effect transistors based on two-dimensional materials due to irradiation with swift heavy ions. Devices were prepared with exfoliated single layers of MoS2 and graphene, respectively. They were characterized before and after irradiation with 1.14 GeV U28+ ions using three different fluences. By electrical characterization, atomic force microscopy, and Raman spectroscopy, we show that the irradiation leads to significant changes of structural and electrical properties. At the highest fluence of 4 × 1011 ions/cm 2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4808460
  • Routes to rupture and folding of graphene on rough 6H-SiC(0001) and their identification
    Temmen, M. and Ochedowski, O. and Kleine Bussmann, B. and Schleberger, M. and Reichling, M. and Bollmann, T.R.J.
    Beilstein Journal of Nanotechnology 4 (2013)
    Twisted few layer graphene (FLG) is highly attractive from an application point of view, due to its extraordinary electronic properties. In order to study its properties, we demonstrate and discuss three different routes to in situ create and identify (twisted) FLG. Single layer graphene (SLG) sheets mechanically exfoliated under ambient conditions on 6H-SiC(0001) are modified by (i) swift heavy ion (SHI) irradiation, (ii) by a force microscope tip and (iii) by severe heating. The resulting surface topography and the surface potential are investigated with non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). SHI irradiation results in rupture of the SLG sheets, thereby creating foldings and bilayer graphene (BLG). Applying the other modification methods creates enlarged (twisted) graphene foldings that show rupture along preferential edges of zigzag and armchair type. Peeling at a folding over an edge different from a low index crystallographic direction can result in twisted BLG, showing a similar height as Bernal (or AA-stacked) BLG in NC-AFM images. The rotational stacking can be identified by a significant contrast in the local contact potential difference (LCPD) measured by KPFM. © 2013 Komkova et al; licensee Beilstein-Institut.
    view abstract10.3762/bjnano.4.69
  • Tracks and voids in amorphous Ge induced by swift heavy-ion irradiation
    Ridgway, M.C. and Bierschenk, T. and Giulian, R. and Afra, B. and Rodriguez, M.D. and Araujo, L.L. and Byrne, A.P. and Kirby, N. and Pakarinen, O.H. and Djurabekova, F. and Nordlund, K. and Schleberger, M. and Osmani, O. and Medvedev, N. and Rethfeld, B. and Kluth, P.
    Physical Review Letters 110 (2013)
    Ion tracks formed in amorphous Ge by swift heavy-ion irradiation have been identified with experiment and modeling to yield unambiguous evidence of tracks in an amorphous semiconductor. Their underdense core and overdense shell result from quenched-in radially outward material flow. Following a solid-to-liquid phase transformation, the volume contraction necessary to accommodate the high-density molten phase produces voids, potentially the precursors to porosity, along the ion direction. Their bow-tie shape, reproduced by simulation, results from radially inward resolidification. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.110.245502
  • Transient metal-like electrical conductivity in swift heavy ion irradiated insulators
    Osmani, O. and Medvedev, N. and Juaristi, J.I. and Schleberger, M. and Rethfeld, B.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 317 (2013)
    The irradiation of insulators with swift heavy ions leads to a transient excitation of valence band electrons into the targets conduction band. The population of the conduction band gives rise to an increased electrical conductivity of the intrinsically non-conductive insulator. Due to the transient nature of the number density of excited electrons in the conduction band, the electrical conductivity is also strongly time dependent. In this work it is demonstrated how the transient electrical conductivity can be evaluated on the basis of a combination of the Monte Carlo method and a two temperature model. As a model system, the irradiation of SiO2 with 11.4 MeV/u Ca ions, corresponding to a stopping power of dE/dx=2.7 keV/nm, is chosen for the calculations. It is found, that the enhancement of the electrical conductivity is quite large and comparable to that of metals like gold. The striking magnitude of this enhancement may allow for an experimental measurement of the electrical conductivity of insulators during irradiation with swift heavy ions. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.nimb.2012.12.092
  • Ultra-thin MoS2 irradiated with highly charged ions
    Hopster, J. and Kozubek, R. and Krämer, J. and Sokolovsky, V. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 317 (2013)
    Single MoS2 layers exfoliated on KBr have been irradiated with highly charged Xe ions, i.e. with Xe35+ and Xe40+. By atomic force microscopy (AFM) we identified pits and hillocks induced by single ion impacts. The latter ones appear on single layer and bulk-like MoS 2 after both irradiations, whereas their diameter and height apparently depend on the charge state q and layer number. By comparison of contact mode and tapping mode AFM measurements we deduce that these ion induced defects are topographical hillocks accompanied by an enhanced friction. In contrast to this, pit-like structures were only observed on single layer MoS2 irradiated with q = 40. Taking into account the well known ion induced pit formation on KBr due to defect mediated sputtering, we deduce that pit formation takes place in the substrate and not in the MoS2 layer. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.nimb.2013.02.038
  • Comparison of ion beam and electron beam induced transport of hot charge carriers in metal-insulator-metal junctions
    Hopster, J. and Diesing, D. and Wucher, A. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1354 (2012)
    The generation of hot charge carriers within a solid bombarded by charged particles is investigated using biased thin film metal-insulator-metal (MIM) devices. For slow, highly charged ions approaching a metal surface the main dissipation process is electronic excitation of the substrate, leading to electron emission into the vacuum and internal electron emission across the MIM junction. In order to gain a deeper understanding of the distribution and transport of the excited charge carriers leading to the measured device current, we compare ion induced and electron induced excitation processes in terms of absolute internal emission yields as well as their dependence on the applied bias voltage. © 2011 Materials Research Society.
    view abstract10.1557/opl.2011.1083
  • Damage in crystalline silicon by swift heavy ion irradiation
    Osmani, O. and Alzaher, I. and Peters, T. and Ban D'Etat, B. and Cassimi, A. and Lebius, H. and Monnet, I. and Medvedev, N. and Rethfeld, B. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 282 (2012)
    We have studied damage of crystalline Si surfaces induced by electronic energy loss of swift heavy ions with an electronic stopping power of up to S e = 12 keV/nm. Scanning tunneling microscope images of the surface after irradiation under perpendicular as well as glancing angles of incidence showed no surface damage. We have performed theoretical calculations for the damage threshold within the two temperature model, resulting in Seth=8 keV/nm as the minimum stopping power to create a molten zone. We investigate the respective influence of the electron-phonon coupling, of the criterion at which the damage occurs and a possible effect of ballistic electrons. We show that the latter has the strongest effect on the calculated damage threshold. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.nimb.2011.08.036
  • Electronic characterization of single-layer MoS2 sheets exfoliated on SrTiO3
    Bußmann, B.K. and Marinov, K. and Ochedowski, O. and Scheuschner, N. and Maultzsch, J. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1474 (2012)
    Single layer regions of MoS2 on SiO2 and SrTiO 3 were identified by Raman spectroscopy and μ-photoluminescence before Kelvin probe force microscopy was performed. For the already known system MoS2/SiO2 we find 1.839 eV for the direct bandgap, in good agreement with earlier results. On MoS2/SrTiO3 the direct bandgap was determined to be 1.829 eV. From our Kelvin probe data we infer that the SrTiO3 substrate leads to a dipole layer at the interface of the MoS2 single layer. The corresponding μ-PL measurements however show no significant decrease of the bandgap. This shows, that in the case of MoS2 the carrier type as well as concentration is not significantly influenced by the choice of SrTiO3 as the substrate compared to SiO2. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.1463
  • Folding graphene with swift heavy ions
    Akcöltekin, S. and Bukowska, H. and Akcol̈tekin, E. and Lebius, H. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1354 (2012)
    Swift heavy ion induced modifications on graphene were investigated by means of atomic force microscopy and Raman spectroscopy. For the experiment graphene was exfoliated onto different substrates (SrTiO3 (100), TiO2(100), Al2O3(1102) and 90 nm SiO 2/Si) by the standard technique. After irradiation with heavy ions of 93 MeV kinetic energy and under glancing angles of incidence, characteristic folding structures are observed. The folding patterns on crystalline substrates are generally larger and are created with a higher efficiency than on the amorphous SiO2. This difference is attributed to the relatively large distance between graphene and SiO2 of d ≈ 1 nm. © 2011 Materials Research Society.
    view abstract10.1557/opl.2011.1278
  • Graphene on Si(111)7×7
    Ochedowski, O. and Begall, G. and Scheuschner, N. and El Kharrazi, M. and Maultzsch, J. and Schleberger, M.
    Nanotechnology 23 (2012)
    We demonstrate that it is possible to mechanically exfoliate graphene under ultrahigh vacuum conditions on the atomically well defined surface of single crystalline silicon. The flakes are several hundred nanometers in lateral size and their optical contrast is very faint, in agreement with calculated data. Single-layer graphene is investigated by Raman mapping. The graphene and 2D peaks are shifted and narrowed compared to undoped graphene. With spatially resolved Kelvin probe measurements we show that this is due to p-type doping with hole densities of n h6×10 12cm 2. The in vacuo preparation technique presented here should open up new possibilities to influence the properties of graphene by introducing adsorbates in a controlled way. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/23/40/405708
  • Laser cleaning of exfoliated graphene
    Ochedowski, O. and Bußmann, B.K. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1455 (2012)
    We have employed atomic force and Kelvin-Probe force microscopy to study graphene sheets exfoliated on TiO2 under the influence of local heating achieved by laser irradiation. Exfoliation and irradiation took place under ambient conditions, the measurements were performed in ultra high vacuum. We show that after irradiation times of 6 min, an increase of the surface potential is observed which indicates a decrease of p-type carrier concentration. We attribute this effect to the removal of adsorbates like water and oxygen. After irradiation times of 12 min our topography images reveal severe structural modifications of graphene. These resemble the nanocrystallite network which form on graphene/SiO2 but after much longer irradiation times. From our results we propose that short laser heating at moderate powers might offer a way to clean graphene without inducing unwanted structural modifications. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.1196
  • Resonant Raman profiles and μ-photoluminescence of atomically thin layers of molybdenum disulfide
    Scheuschner, N. and Ochedowski, O. and Schleberger, M. and Maultzsch, J.
    Physica Status Solidi (B) Basic Research 249 (2012)
    We present photoluminescence and resonant Raman measurements of atomically thin layers of molybdenum disulfide (MoS2) in the energy range of the A and B optical transitions. We found a Stokes shift of the transition energies in the resonant Raman measurements compared to the photoluminescence. The Stokes shift increases for the bilayer compared to the single layer. Furthermore, in resonance with the A transition, the Raman intensity of the A1g mode of the bilayer is six times larger than that of the single layer. Schematic crystal structure of molybdenum disulfide. (a) Top view (slightly tilted). (b) Side view. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201200389
  • Surface damage of silicon after swift heavy ion irradiation
    Peters, T. and Alzaher, I. and Ban D'Etat, B. and Cassimi, A. and Monnet, I. and Lebius, H. and Schleberger, M.
    Journal of Physics: Conference Series 388 (2012)
    In order to answer the long-standing question, if silicon surfaces can be damaged by swift heavy ions, a set-up to study ion-irradiation damage of reactive surfaces is presented. This set-up allows for the first time to avoid oxidization of the silicone surface during the experimental study. Scanning tunneling microscopy as well as low-energy electron diffraction was used to study the surfaces before and after irradiation. Silicon surfaces were prepared by flash-heating before irradiation with swift heavy ions (Xenon at 0.9 MeV/u). The targets stayed in ultra-high vacuum during preparation, irradiation and surface imaging. No surface damage was detected, at normal as well as at grazing incidence angle. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/388/13/132035
  • Doping of graphene exfoliated on SrTiO3
    Bußmann, B.K. and Ochedowski, O. and Schleberger, M.
    Nanotechnology 22 (2011)
    We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO3 substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be ΦSLG = 4.409 0.039eV and ΦBLG = 4.516 0.035eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/26/265703
  • Energy dissipation in dielectrics after swift heavy-ion impact: A hybrid model
    Osmani, O. and Medvedev, N. and Schleberger, M. and Rethfeld, B.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The energy dissipation after irradiation of dielectrics with swift heavy ions is studied applying a combination of the Monte Carlo (MC) method and the two-temperature model (TTM). Within the MC calculation the transient dynamics of the electrons in the excited dielectric is described: the primary excitation and relaxation of the target electrons as well as the creation of secondary electrons. From the MC data, it was observed that the electron system can be considered as thermalized after a time of t∼100 fs after the ion impact. Then the TTM is applied to calculate the spatial and temporal evolution of the electron and lattice temperature via the electron-phonon coupling using the MC data as initial conditions. Additionally, this MC-TTM combination allows to compute material parameters of strongly excited matter. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.214105
  • Raman spectra of graphene exfoliated on insulating crystalline substrates
    Bukowska, H. and Meinerzhagen, F. and Akcöltekin, S. and Ochedowski, O. and Neubert, M. and Buck, V. and Schleberger, M.
    New Journal of Physics 13 (2011)
    We have investigated single layer, bilayer and few-layer graphene exfoliated on SiO 2 and on single crystal surfaces of SrTiO 3, Al 2O 3 and TiO 2 using Raman spectroscopy. The typical 'fingerprint' 2D peak turns out to be indicative of the number of graphene layers independent of the substrate material. The morphological quality of the graphene is as good as on SiO 2 substrates for all the materials. We find evidence for substrate-induced changes due to doping. With most substrates, hole doping is observed, but with SrTiO 3 we have identified a dielectric substrate with which electron accumulation in graphene can be achieved. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/13/6/063018
  • Single ion induced surface nanostructures: A comparison between slow highly charged and swift heavy ions
    Aumayr, F. and Facsko, S. and El-Said, A.S. and Trautmann, C. and Schleberger, M.
    Journal of Physics Condensed Matter 23 (2011)
    This topical review focuses on recent advances in the understanding of the formation of surface nanostructures, an intriguing phenomenon in ionsurface 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. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/23/39/393001
  • Unzipping and folding of graphene by swift heavy ions
    Akcöltekin, S. and Bukowska, H. and Peters, T. and Osmani, O. and Monnet, I. and Alzaher, I. and D'Etat, B.B. and Lebius, H. and Schleberger, M.
    Applied Physics Letters 98 (2011)
    We show that graphene on a dielectric substrate sustains major modifications if irradiated with swift heavy ions under oblique angles. Due to a combination of defect creation in the graphene layer and hillock creation in the substrate, graphene is split and folded along the ion track yielding double layer nanoribbons. The folded parts are up to several 100 nm in length. Our results indicate that the radiation hardness of graphene devices is questionable but also open up a new way of introducing extended low-dimensional defects in a controlled way. © 2011 American Institute of Physics.
    view abstract10.1063/1.3559619
  • Energy dissipation in insulators induced by swift heavy ions: A parameter study
    Osmani, O. and Lebius, H. and Rethfeld, B. and Schleberger, M.
    Laser and Particle Beams 28 (2010)
    The irradiation of solids with high energy laser or particle beams has led to a deeper understanding of the relaxation processes inside the target material. However, a lot of open questions remain. In the present paper, we will examine the irradiation of the model system Xe23+ @ 93 MeV → SrTiO3 within the framework of the two-temperature-model and study the electron-phonon-coupling g and the electron diffusivity De as well as the lattice diffusivity Dp. These are crucial parameters for which no experimental data is available. Experimentally, g is very difficult to measure and therefore theoretical predictions are of great importance. With the approach presented here it is possible to determine the coupling-constant by one order of magnitude. © 2010 Cambridge University Press.
    view abstract10.1017/S0263034609990632
  • Energy threshold for the creation of nanodots on SrTiO3 by swift heavy ions
    Karlušić, M. and Akcöltekin, S. and Osmani, O. and Monnet, I. and Lebius, H. and Jakšić, M. and Schleberger, M.
    New Journal of Physics 12 (2010)
    We present experimental and theoretical data on the threshold behaviour of nanodot creation with swift heavy ions. A model calculation based on a two-temperature model that takes into account the spatially resolved electron density gives a threshold of 12keVnm-1 below which the energy density at the end of the track is no longer high enough to melt the material. In the corresponding experiments, we irradiated SrTiO3 surfaces under grazing incidence with swift heavy ions. The resulting chains of nanodots were analysed by atomic force microscopy (AFM). In addition, some samples of SrTiO3 irradiated under normal incidence were analysed by transmission electron microscopy (TEM). Both experiments showed two thresholds, which were connected with the appearance of tracks and the creation of fully developed tracks. The threshold values were similar for surface and bulk tracks, suggesting that the same processes occur at both glancing and normal incidence. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/12/4/043009
  • Excitation and relaxation of swift heavy ion irradiated dielectrics
    Osmani, O. and Medvedev, N. and Schleberger, M. and Rethfeld, B.
    e-Journal of Surface Science and Nanotechnology 8 (2010)
    The structural modifications of insulators after swift heavy ion irradiation are studied by applying a combination of the Monte-Carlo method (MC), used to describe the excitation and relaxation induced by the penetrating ion in the electronic subsystem, with the two temperature model (TTM) describing the temporal evolution of the lattice temperature heated by the hot electronic system. This MC-TTM combination demonstrates that secondary ionizations play a very important role for the track formation process and is capable to compute important material parameters which are often unaccessible by experiments. It is found that the secondary electron generation leads to an additional energy source in the heat diffusion equation related to energy storage in holes. © 2010 The Surface Science Society of Japan.
    view abstract10.1380/ejssnt.2010.278
  • Track creation after swift heavy ion irradiation of insulators
    Medvedev, N. and Osmani, O. and Rethfeld, B. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 268 (2010)
    The dynamics of structural modifications of insulators irradiated with swift heavy ions were investigated theoretically applying a combination of Monte-Carlo method (MC), used to describe SHI penetration and following excitation and relaxation of the electronic subsystem, with Two Temperature Model (TTM) describing the heating of the lattice. This MC-TTM combination demonstrates that secondary ionizations play a very important role for the track formation process. They lead to an additional term in the heat diffusion equation related to energy stored in the hole subsystem. This storage of energy causes a significant delay of heating and prolongs the timescales up to tens of picoseconds. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.nimb.2010.05.078
  • 2D materials

  • atomic force microscopy

  • dynamic force microscopy

  • electronic conductivity

  • graphene

  • graphs

  • ion bombardment

  • nanostructures

  • surfaces

  • swift heavy ions

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