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Vacuum breakdown (also known as arc or discharge) occurs when a sufficiently high electric field is applied between two electrodes in vacuum. The discharge is driven by the formation of an intensively glowing plasma at the cathode, which is followed by the ignition of an anode flare that gradually expands and fills the gap. Although it has been shown that the anode electrode does not play a significant role in the breakdown initiation, the nature of the anodic glow is of paramount importance for understanding the breakdown evolution. In this work, we use time- and space-resolved spectroscopy to study the anode flare. By using different anode and cathode materials, we find that excitations from both anode and cathode ions and neutrals contribute to the anodic glow. This implies that the cathodic plasma expands towards the anode without emitting any detectable light and starts glowing only when it reaches and interacts with the anode electrode. This interaction causes the introduction of anodic species in the plasma. The latter starts producing an expanding glow which contains spectra from both the cathode and anode materials and gradually fills the gap as the plasma equilibrates. Finally, we observe that after a breakdown, cathode material deposits on the anode electrode, gradually coating it. After hundreds of breakdowns, this coating covers the anode, resulting in the decay and possible elimination of the anode material signal in the spectra.

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.

Two-dimensional molybdenum-disulfide (MoS2) catlysts 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 controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface in this respect using swift heavy ion irradiation. The creation of nanometer-scale structures by the 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 enhance of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.

When a swift heavy ion (SHI) penetrates amorphous $\mathrm{Si}{\mathrm{O}}_{2}$, a core/shell (C/S) ion track is formed, which consists of a lower-density core and a higher-density shell. According to the conventional inelastic thermal spike (iTS) model represented by a pair of coupled heat equations, the C/S tracks are believed to form via ``vaporization'' and melting of the $\mathrm{Si}{\mathrm{O}}_{2}$ induced by SHI (V-M model). However, the model does not describe what the vaporization in confined ion-track geometry with a condensed matter density is. Here we reexamine this hypothesis. While the total and core radii of the C/S tracks determined by small angle x-ray scattering are in good agreement with the vaporization and melting radii calculated from the conventional iTS model under high electronic stopping power $({S}_{e})$ irradiations (g10 keV/nm), the deviations between them are evident at low-${S}_{e}$ irradiation (3--5 keV/nm). Even though the iTS calculations exclude the vaporization of $\mathrm{Si}{\mathrm{O}}_{2}$ at the low ${S}_{e}$, both the formation of the C/S tracks and the ion shaping of nanoparticles (NPs) are experimentally confirmed, indicating the inconsistency with the V-M model. Molecular dynamics (MD) simulations based on the two-temperature model, which is an atomic-level modeling extension of the conventional iTS, clarified that the ``vaporlike'' phase exists at ${S}_{e}\ensuremath{\sim}5$ keV/nm or higher as a nonequilibrium phase where atoms have higher kinetic energies than the vaporization energy, but are confined at a nearly condensed matter density. Simultaneously, the simulations indicate that the vaporization is not induced under 50-MeV Si irradiation $({S}_{e}\ensuremath{\sim}3$ keV/nm), but the C/S tracks and the ion shaping of nanoparticles are nevertheless induced. Even though the final density variations in the C/S tracks are very small at the low stopping power values (both in the simulations and experiments), the MD simulations show that the ion shaping can be explained by flow of liquid metal from the NP into the transient low-density phase of the track core during the first \ensuremath{\sim}10 ps after the ion impact. The ion shaping correlates with the recovery process of the silica matrix after emitting a pressure wave. Thus, the vaporization is not a prerequisite for the C/S tracks and the ion shaping.

We propose a method for efficiently coupling the finite element method with atomistic simulations, while using molecular dynamics or kinetic Monte Carlo techniques. Our method can dynamically build an optimized unstructured mesh that follows the geometry defined by atomistic data. On this mesh, different multiphysics problems can be solved to obtain distributions of physical quantities of interest, which can be fed back to the atomistic system. The simulation flow is optimized to maximize computational efficiency while maintaining good accuracy. This is achieved by providing the modules for a) optimization of the density of the generated mesh according to requirements of a specific geometry and b) efficient extension of the finite element domain without a need to extend the atomistic one. Our method is organized as an open-source C++ code. In the current implementation, an efficient Laplace equation solver for calculation of electric field distribution near rough atomistic surface demonstrates the capability of the suggested approach., 25 pages, 14 figures

The effect of low energy irradiation, where the sputtering is imperceptible, has not been deeply studied in the pattern formation. In this work, we want to address this question by analyzing the nanoscale topography formation on a Si surface, which is irradiated at room temperature by Arthorn ions near the displacement threshold energy, for incidence angles ranging from 0 degrees to 85 degrees. The transition from the smooth to ripple patterned surface, i.e., the stability/instability bifurcation angle is observed at 55 degrees, whereas the ripples with their wave-vector is parallel to the ion beam projection in the angular window of 60 degrees-70 degrees, and with 90 degrees rotation with respect to the ion beam projection at the grazing angles of incidence. A similar irradiation setup has been simulated by means of molecular dynamics, which made it possible, first, to quantify the effect of the irradiation in terms of erosion and redistribution using sequential irradiation and, second, to evaluate the ripple wavelength using the crater function formalism. The ripple formation results can be solely attributed to the mass redistribution based mechanism, as erosion due to ion sputtering near or above the threshold energy is practically negligible. Published by AIP Publishing.

Atomistic rigid lattice Kinetic Monte Carlo (KMC) is an efficient method for simulating nano-objects and surfaces at timescales much longer than those accessible by molecular dynamics. A laborious and non-trivial part of constructing any KMC model is, however, to calculate all migration barriers that are needed to give the probabilities for any atom jump event to occur in the simulations. We have calculated three data sets of migration barriers for Cu self-diffusion with two different methods. The data sets were specifically calculated for rigid lattice KMC simulations of copper self-diffusion on arbitrarily rough surfaces, but can be used for KMC simulations of bulk diffusion as well.

Stable C-C bonds existing in several sp hybridizations place carbon thin films of different structural compositions among the materials most tolerant to radiation damage, for applications in extreme environments. One of such applications, solid state electron stripper foils for heavy-ion accelerators, requires the understanding of the structural changes induced by high-energy ion irradiation. Tolerance of carbon structure to radiation damage, thermal effects and stress waves due to swift heavy ion impacts defines the lifetime and operational efficiency of the foils. In this work, we analyze the consequences of a single swift heavy ion impact on two different amorphous carbon structures by means of molecular dynamic simulations. The structures are constructed by using two different recipes to exclude the correlation of the evolution of sp2-to-sp3 hybridization with the initial condition. Both initial structures contain approximately 60% of sp2-bonded carbon atoms, however, with different degree of clustering of atoms with sp3 hybridization. We simulate the swift heavy ion impact employing an instantaneous inelastic thermal spike model. The analysis of changes in density, bonding content and the number and size of carbon primitive rings reveals graphitization of the material within the ion track, with higher degree of disorder in the core and more order in the outer shell. Simulated track dimensions are comparable to those observed in small angle x-ray scattering measurements of evaporation-deposited amorphous carbon stripper foils irradiated by 1.14 GeV U ions.

The cylindrical nanoscale density variations resulting from the interaction of 185 MeV and 2.2 GeV Au ions with 1.0 mu m thick amorphous SiNx:H and SiOx:H layers are determined using small angle x-ray scattering measurements. The resulting density profiles resembles an under-dense core surrounded by an over-dense shell with a smooth transition between the two regions, consistent with molecular-dynamics simulations. For amorphous SiNx:H, the density variations show a radius of 4.2 nm with a relative density change three times larger than the value determined for amorphous SiOx:H, with a radius of 5.5 nm. Complementary infrared spectroscopy measurements exhibit a damage cross-section comparable to the core dimensions. The morphology of the density variations results from freezing in the local viscous flow arising from the non-uniform temperature profile in the radial direction of the ion path. The concomitant drop in viscosity mediated by the thermal conductivity appears to be the main driving force rather than the presence of a density anomaly.

Ion beams are frequently used in industry for composition control of semiconducting materials as well as for surface processing and thin films deposition. Under certain conditions, low- and medium energy ions at high fluences can produce nanoripples and quantum dots on the irradiated surfaces. In the present work, we focus our attention on the study of irradiation of amorphous silicon (a-Si) target with 250 eV and 1 keV Ar+ ions under different angles, taking into special consideration angles close to the grazing incidence. We use the molecular dynamics (MD) method to investigate how much the cumulative displacement of atoms due to the simulated ion bombardment contribute to the patterning effect. The MD results are subsequently analysed using a numerical module Pycraters that allows the prediction of the rippling effect. Ripple wavelengths estimated with Pycraters are then compared with the experimental observations, as well as with the results obtained by using the binary collisions approximation (BCA) method. The wavelength estimation based on the MD results demonstrates a better agreement with the experimental values. In the framework of the utilized analytical model, it can be mainly attributed to the fact that the BCA ignores low energy atomic interactions, which, however, provide an important contribution to the displacement of atoms following an ion impact.

cited By 11; International audience; Surface segregation designates the phenomenon of variation in chemical composition between the surface and the bulk of an alloy, which can have a beneficial or detrimental effect on its physical and chemical properties. This is even more pronounced in nanoalloys, i.e., alloy systems comprised of nanoparticles, with significant surface-to-volume ratios. In this case study we demonstrate the element-specific Cr segregation in Ni-rich NiCr alloy nanoparticles and nanogranular films grown by gas-phase synthesis methods. In situ annealing measurements (300-800 K), performed under vacuum using aberration-corrected environmental transmission electron microscopy (E-TEM), and vibrating sample magnetometry (VSM) revealed progressive Cr segregation with annealing temperature and subsequent complete transformation into core-satellite structures at 700 K. Simultaneously, atomistic computer simulations (molecular dynamics (MD) and Metropolis Monte Carlo (MMC)) elucidated the resultant structures, explaining the driving force behind segregation energetically. Most importantly, we emphasize the significant effects of Cr segregation on magnetic properties, namely, (i) the highly nonsaturated M-H loops (below the Néel temperature of antiferromagnetic Cr) with reduced coercivities and (ii) the uncompensated high Curie temperatures, TC, compared to the NiCr bulk, which approach bulk Ni values upon annealing. Both are clear evidence that the distribution of Cr in the nearest-neighbor shells of Ni atoms differs from that of the bulk NiCr alloy, reconfirming our structural findings.

Understanding plasma initiation in vacuum arc discharges can help to bridge the gap between nano-scale triggering phenomena and the macroscopic surface damage caused by vacuum arcs. We present a new twodimensional particle-in-cell tool to simulate plasma initiation in direct-current (DC) copper vacuum arc discharges starting from a single, strong field emitter at the cathode. Our simulations describe in detail how a sub-micron field emission site can evolve to a macroscopic vacuum arc discharge, and provide a possible explanation for why and how cathode spots can spread on the cathode surface. Furthermore, the model provides us with a prediction for the current and voltage characteristics, as well as for properties of the plasma like densities, fluxes and electric potentials in a simple DC discharge case, which are in agreement with the known experimental values. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A selection of ion–solid interactions in the swift heavy-ion irradiation regime is reviewed. We consider the effects of electronic energy loss at tens of keV/nm on both bulk material and nanostructures embedded in a matrix. Specific examples include ion track formation at low ion fluences in bulk Si and Ge and porous layer formation at high ion fluences in bulk Ge. In addition, the intriguing shape and phase transformations observable at high ion fluences in Ge and metallic nanoparticles embedded in bulk SiO 2 are examined and compared. Experiment, modelling and simulation are combined synergistically as we seek fundamental atomistic insight into these unique yet poorly understood processes operative only at the extremes of electronic energy loss.

In this study a direct comparison between lattice spacings predicted by molecular dynamics (MD) simulations and values measured from sputtered Cu–Ta films by X-ray diffraction (XRD) is reported. The study spans the entire composition range between pure Cu and pure Ta and takes into account all the phases documented for the Cu–Ta system in the literature (i.e. α-Cu, α-Ta, β-Ta as well as X-ray amorphous structures). For both the experiments and the MD simulations the results are compared to the literature and discrepancies are critically discussed. A direct comparison between simulation and experiments shows that the MD simulations reproduce the interatomic distances observed in the experiments accurately over most of the composition range, with the exception of alloys with 15–30 at.% Ta content where the MD simulations show spontaneous amorphization whereas the experiments suggest that Ta-rich particles are formed in face-centered cubic solid-solution phase.

The variation in the composition, structure, and properties of CdTe films when bombarded with Ar+ ions in combination with annealing is investigated. It is established that the technological treatment makes CdTe film surfaces much smoother and the films become nearly perfect in their stoichiometric composition and properties.

An investigation of mechanisms of enhancement of irradiation-induced damage formation in GaN under molecular in comparison to monatomic ion bombardment is presented. Ion-implantation-induced effects in wurtzite GaN bombarded with 0.6 keV amu(-1) F, P, PF2, PF4, and Ag ions at room temperature are studied experimentally and by cumulative MD simulation in the correct irradiation conditions. In the low dose regime, damage formation is correlated with a reduction in photoluminescence decay time, whereas in the high dose regime, it is associated with the thickness of the amorphous/disordered layer formed at the sample surface. In all the cases studied, a shift to molecular ion irradiation from bombardment by its monatomic constituents enhances the damage accumulation rate. Implantation of a heavy Ag ion, having approximately the same mass as the PF4 molecule, is less effective in surface damage formation, but leads to noticeably higher damage accumulation in the bulk. The cumulative MD simulations do not reveal any significant difference in the total amount of both point defects and small defect clusters produced by light monatomic and molecular ions. On the other hand, increased production of large defect clusters by molecular PF4 ions is clearly seen in the vicinity of the surface. Ag ions produce almost the same number of small, but more large defect clusters compared to the others. These findings show that the higher probability of formation of large defect clusters is important mechanism of the enhancement of stable damage formation in GaN under molecular, as well as under heavy monatomic ion irradiation.

In this paper, we study the mechanisms of growth of Ag nanoclusters in a solid Ar matrix and the emission of these nanoclusters from the matrix by a combination of experimental and theoretical methods. The molecular dynamics simulations show that the cluster growth mechanism can be described as "thermal spike-enhanced clustering" in multiple sequential ion impact events. We further show that experimentally observed large sputtered metal clusters cannot be formed by direct sputtering of Ag mixed in the Ar. Instead, we describe the mechanism of emission of the metal nanocluster that, at first, is formed in the cryogenic matrix due to multiple ion impacts, and then is emitted as a result of the simultaneous effects of interface boiling and spring force. We also develop an analytical model describing this size-dependent cluster emission. The model bridges the atomistic simulations and experimental time and length scales, and allows increasing the controllability of fast generation of nanoclusters in experiments with a high production rate.

Metallic nanowires are known to break into shorter fragments due to the Rayleigh instability mechanism. This process is strongly accelerated at elevated temperatures and can completely hinder the functioning of nanowire-based devices like e.g. transparent conductive and flexible coatings. At the same time, arranged gold nanodots have important applications in electrochemical sensors. In this paper we perform a series of annealing experiments of gold and silver nanowires and nanowire junctions at fixed temperatures 473, 673, 873 and 973 K (200 degrees C, 400 degrees C, 600 degrees C and 700 degrees C) during a time period of 10 min. We show that nanowires are especially prone to fragmentation around junctions and crossing points even at comparatively low temperatures. The fragmentation process is highly temperature dependent and the junction region breaks up at a lower temperature than a single nanowire. We develop a gold parametrization for kinetic Monte Carlo simulations and demonstrate the surface diffusion origin of the nanowire junction fragmentation. We show that nanowire fragmentation starts at the junctions with high reliability and propose that aligning nanowires in a regular grid could be used as a technique for fabricating arrays of nanodots.

Performance enhancement of structural materials in extreme radiation environments has been actively investigated for many decades. Traditional alloys, such as steel, brass and aluminum alloys, normally contain one or two principal element(s) with a low concentration of other elements. While these exist in either a mixture of metallic phases (multiple phases) or in a solid solution (single phase), limited or localized chemical disorder is a common characteristic of the main matrix. Fundamentally different from traditional alloys, recently developed single-phase concentrated solid-solution alloys (CSAs) contain multiple elemental species in equiatomic or high concentrations with different elements randomly arranged on a crystalline lattice. Due to the lack of ordered elemental arrangement in these CSAs, they exhibit significant chemical disorder and unique site-to-site lattice distortion. While it is well recognized in traditional alloys that minor additions lead to enhanced radiation resistance, it remains unclear in CSAs how atomic-level heterogeneity affects defect formation, damage accumulation, and microstructural evolution. These knowledge gaps have acted as roadblocks to the development of future generation energy technology. CSAs with a simple crystal structure, but complex chemical disorder, are unique systems that allow us, through replacing principal alloying elements and modifying concentrations, to study how compositional complexity influences defect dynamics, and to bridge the knowledge gaps through understanding intricate electronic- and atomic-level interactions, mass and energy transfer processes, and radiation resistance performance. Recent advances in defect dynamics and irradiation performance of CSAs are reviewed, intrinsic chemical effects on radiation performance are discussed, and direction for future studies is suggested. (C) 2017 Elsevier Ltd. All rights reserved.

Self-sputtering of copper under high electric fields is considered to contribute to plasma buildup during a vacuum breakdown event frequently observed near metal surfaces, even in ultra high vacuum condition in different electric devices. In this study, by means of molecular dynamics simulations, we analyze the effect of surface temperature and morphology on the yield of self-sputtering of copper with ion energies of 0.1-5 keV. We analyze all three low-index surfaces of Cu, {100}, {110} and {111}, held at different temperatures, 300 K, 500 K and 1200 K. The surface roughness relief is studied by either varying the angle of incidence on flat surfaces, or by using arbitrary roughened surfaces, which result in a more natural distribution of surface relief variations. Our simulations provide detailed characterization of copper self-sputtering with respect to different material temperatures, crystallographic orientations, surface roughness, energies, and angles of ion incidence., The published version of this paper can be found here: https://doi.org/10.1016/j.nimb.2017.11.001

Atomistic rigid lattice Kinetic Monte Carlo is an efficient method for simulating nano-objects and surfaces at timescales much longer than those accessible by molecular dynamics. A laborious part of constructing any Kinetic Monte Carlo model is, however, to calculate all migration barriers that are needed to give the probabilities for any atom jump event to occur in the simulations. One of the common methods of barrier calculations is Nudged Elastic Band. The number of barriers needed to fully describe simulated systems is typically between hundreds of thousands and millions. Calculations of such a large number of barriers of various processes is far from trivial. In this paper, we will discuss the challenges arising during barriers calculations on a surface and present a systematic and reliable tethering force approach to construct a rigid lattice barrier parameterization of face-centred and body-centred cubic metal lattices. We have produced several different barrier sets for Cu and for Fe that can be used for KMC simulations of processes on arbitrarily rough surfaces. The sets are published as Data in Brief articles and available for the use.

Vacuum arcing (also known as breakdown), is a major limiting factor in various applications such as particle accelerators, fusion reactors etc. Although it is well-established that vacuum arcs appear after intense Field electron Emission (FE), the physical mechanism that leads from FE to the ignition of plasma is not yet understood. A common hypothesis is that intense FE leads to excessive heating of the cathode, which causes its deformation, and eventually material evaporation and plasma formation. However, this process has never been observed experimentally or fully understood theoretically. Here we present atomistic simulations of this process that give an insight to it and provide possible mechanisms that can explain the initiation of plasma. Our simulations take into account various physical processes, namely field-induced stresses, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that under certain conditions the cathode apex melts and the field-induced stress deforms it to become longer and sharper. This initiates a positive feedback process that leads to extremely high local temperatures, and causes evaporation of material. This mechanism might give a plausible explanation to the initiation of plasma.

We report in situ and ex situ fabrication approaches to construct p-type (CuO) and n-type (SnO2) metal oxide nanowire devices for operation inside an environmental transmission electron microscope (TEM). By taking advantage of their chemoresistive properties, the nanowire devices were employed as sensitive probes for detecting reactive species induced by the interactions of high-energy electrons with surrounding gas molecules, in particular, for the case of O-2 gas pressures up to 20 mbar. In order to rationalize our experimental findings, a computational model based on the particle-in-cell method was implemented to calculate the spatial distributions of scattered electrons and ionized oxygen species in the environmental TEM. Our approach enables the a priori identification and qualitative measurement of undesirable beam effects, paving the way for future developments related to their mitigation. Published by AIP Publishing.

Volume: 393 A new class of single phase metal alloys of equal atomic concentrations has shown very promising mechanical properties and good corrosion resistance. Moreover, a significant reduction in damage accumulation during prolonged irradiation has also been observed in these equiatomic multicomponent alloys. A comparison of elemental Ni with the two component NiFe- and the three component NiCoCr-alloy showed a substantial reduction in damage in both alloys, and an even larger difference was seen if only larger clusters were considered. One of the factors limiting the damage build-up in the alloys compared to the elemental material was seen to be dislocation mobility (Granberg et al., 2016). In this Article, we focus on a more thorough investigation of the mobility of edge dislocations in different cases of the Ni-, NiFe- and NiCoCr-samples. We find that even though the saturated amount of defects in the alloys is lower than in elemental Ni, the defect buildup in the early stages is faster in the alloys. We also find that the dislocation mobility in NiFe is lower than in Ni, at low stresses, and that the onset stress in NiFe is higher than in Ni. The same phenomenon was seen in comparison between NiFe and NiCoCr, since the three component alloy had lower dislocation mobility and higher onset stress. The dislocation velocity in elemental Ni plateaued out just under the forbidden velocity, whereas the alloys showed a more complex behaviour. (C) 2016 Published by Elsevier B.V.

In an increasing number of high-gradient linear accelerator applications, accelerating structures must operate with both high surface electric fields and low breakdown rates. Understanding the statistical properties of breakdown occurrence in such a regime is of practical importance for optimizing accelerator conditioning and operation algorithms, as well as of interest for efforts to understand the physical processes which underlie the breakdown phenomenon. Experimental data of breakdown has been collected in two distinct high-gradient experimental set-ups: A prototype linear accelerating structure operated in the Compact Linear Collider Xbox 12GHz test stands, and a parallel plate electrode system operated with pulsed DC in the kV range. Collected data is presented, analyzed and compared. The two systems show similar, distinctive, two-part distributions of number of pulses between breakdowns, with each part corresponding to a specific, constant event rate. The correlation between distance and number of pulses between breakdown indicates that the two parts of the distribution, and their corresponding event rates, represent independent primary and induced follow-up breakdowns. The similarity of results from pulsed DCto 12GHz rf indicates a similar vacuum arc triggering mechanism over the range of conditions covered by the experiments.

Ion beam processing of surfaces is well known to lead to sputtering, which conventionally is associated only with erosion of atoms from the material. We show here, by combination of experiments and a newly developed Monte Carlo algorithm, that in the case of nanoparticles in a regular two-dimensional array on surfaces, the redeposition of sputtered atoms may play a significant role on the system development. The simulations are directly compared to in situ experiments obtained using a dual focused Ga+ ion beam system and high resolution scanning electron microscopy, and explain the size evolution by a combination of sputtering and redeposition of sputtered material on neighboring particles. The effect is found to be dependent on the size of the nanoparticles: if the nanoparticle size is comparable to the ion range, the reposition is negligible. For larger nanoparticles the redeposition becomes significant and is able to compensate up to 20% of the sputtered material, effectively reducing the process of sputtering. The redeposition may even lead to significant growth: this was seen for the nanoparticles with the sizes much smaller than the ion range. Furthermore, the algorithm shows that significant redeposition is possible when the large size neighboring nanoparticles are present.

We propose a method using solid state detectors with directional sensitivity to dark matter interactions to detect low-mass Weakly Interacting Massive Particles (WIMPs) originating from galactic sources. In spite of a large body of literature for high-mass WIMP detectors with directional sensitivity, no available technique exists to cover WIMPs in the mass range, Most recent version

The slowing down of energetic ions in materials is determined by the nuclear and electronic stopping powers. Both of these have been studied extensively for ordinary-matter ions. For antiprotons, however, there are numerous studies of the electronic stopping power, but none of the nuclear stopping power. Here, we use quantum-chemical methods to calculate interparticle potentials between antiprotons and different atoms, and derive from these the nuclear stopping power of antiprotons in solids. The results show that the antiproton nuclear stopping powers are much stronger than those of protons, and can also be stronger than the electronic stopping power at the lowest energies. The interparticle potentials are also implemented in a molecular dynamics ion range calculation code, which allows us to simulate antiproton transmission through degrader foil materials. Foil transmission simulations carried out at experimentally relevant conditions show that the choice of antiproton-atom interaction model has a large effect on the predicted yield of antiprotons slowed down to low (a few keV) energies.

Onset of vacuum arcing near a metal surface is often associated with nanoscale asperities, which may dynamically appear due to different processes ongoing in the surface and subsurface layers in the presence of high electric fields. Thermally activated processes, as well as plastic deformation caused by tensile stress due to an applied electric field, are usually not accessible by atomistic simulations because of the long time needed for these processes to occur. On the other hand, finite element methods, able to describe the process of plastic deformations in materials at realistic stresses, do not include surface properties. The latter are particularly important for the problems where the surface plays crucial role in the studied process, as for instance, in the case of plastic deformations at a nanovoid. In the current study by means of molecular dynamics (MD) and finite element simulations we analyse the stress distribution in single crystal copper containing a nanovoid buried deep under the surface. We have developed a methodology to incorporate the surface effects into the solid mechanics framework by utilizing elastic properties of crystals, pre-calculated using MD simulations. The method leads to computationally efficient stress calculations and can be easily implemented in commercially available finite element software, making it an attractive analysis tool.

When an electron emitting tip is subjected to very high electric fields, plasma forms even under ultra high vacuum conditions. This phenomenon, known as vacuum arc, causes catastrophic surface modifications and constitutes a major limiting factor not only for modern electron sources, but also for many large-scale applications such as particle accelerators, fusion reactors etc. Although vacuum arcs have been studied thoroughly, the physical mechanisms that lead from intense electron emission to plasma ignition are still unclear. In this article, we give insights to the atomic scale processes taking place in metal nanotips under intense field emission conditions. We use multi-scale atomistic simulations that concurrently include field-induced forces, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that when a sufficiently high electric field is applied to the tip, the emission-generated heat partially melts it and the field-induced force elongates and sharpens it. This initiates a positive feedback thermal runaway process, which eventually causes evaporation of large fractions of the tip. The reported mechanism can explain the origin of neutral atoms necessary to initiate plasma, a missing key process required to explain the ignition of a vacuum arc. Our simulations provide a quantitative description of in the conditions leading to runaway, which shall be valuable for both field emission applications and vacuum arc studies.

By means of molecular dynamics simulations, we study the low energy ion irradiation effect on the short-range order (SRO) of metallic glass structures. Breaking the topological SRO in a controlled manner may lead to desirable modifications of deformation behavior of glass structures. These changes in SRO were also found to be stable during the thermal annealing at temperatures below the glass transition temperature ( T g ). The tensile deformation simulations performed for the Cu–Zr glass nanorods show that irradiated nanorods become softer compared to the pristine ones, changing the yielding mechanism – from a slip via formation of a single shear band to formation of a broad necking region.

We obtained the two- and three-component nanostructures by means of ion implantation of low-energy Co and Ba ions in the surface layers of Si and GaAs in combination with the post-implantation annealing. We show that flat shaped nanocrystals (nanoislands) of two- and three-component composition, Co-Si and Ga-Ba-As, start forming at ion fluences of Φ = (6¸8)×1014 cm-2. After the laser annealing and short time electron heating we observed the formation of CoSi2 and Ga0.4Ba0.6As nanocrystals. Our results show that the size effects are clearly seen in electronic properties (opening of the band gap) when the lateral size of nanocrystals is less than 15-30 nm, or in case of solid nanofilms, similar effect is observed at the thickness less than 3-4 nm. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metallic nanocrystals can exhibit intriguing properties due to large surfaces specific for low-dimensional objects. Recent experimental observation showed an interesting shape transformation of Au nanoparticle during simultaneous mechanical and electrical load. By applying finite element methods extended to include nanosize effects, we study the mechanisms of such transformations. We utilize fully coupled electro-thermal calculations with the nanoscale correction to the electric and thermal conductivities. The mechanical response of the material is simulated using the elastoplastic material model while the nanoscale mechanical interaction between gold and tungsten particles is simulated using adhesive contact modelling. We observe that Joule heating due to high electric currents increases the temperature of the nanoparticle to the value close to the melting point. In combination with the mechanical stress, this causes significant plastic deformations within the nanoparticle, which can explain the observed shape modification of the latter.

The presence of high electric fields may affect significantly the process of sputtering of metal surfaces by energetic ions, especially in the vicinity of rough surface features. The effect can be significant if the energy of ions is fairly low. Moreover, the nanosized rough surface features – invisible to a naked eye, both intrinsic ones due to technological processing of surfaces and those forming because of sputtering – may affect the topology of surface erosion under ion bombardment. In this work we study by means of concurrent electrodynamics–molecular dynamics the sputtering yield of Cu + ions hitting a flat Cu surface or a nanosized Cu protrusion as a function of both ion energy and electric field strength. The results show that the sputtering yield is significantly enhanced in the presence of an electric field in both cases.

The mechanisms of sputtering of nanoparticles embedded in the solid matrix under ion bombardment are presented. The mechanism of sputtering by multicharged ions is based on the increase of the potential energy associated with the Coulomb interaction of charges. It is shown that the increase of the lifetimes of holes formed under ionization of the material in the track of the nanoparticles enlarges the sputtering yield due to the confinement of charges. The modification of the thermal spike mechanism has been studied, when the modifying factor is the interface between the nanoparticle and the matrix, which is characterized in the phonon and electron subsystems by the corresponding coefficients of the wave reflection. The process of heat injection into the lattice and following averaging of the probability of atoms' evaporation is described in framework of the two-temperature phenomenology. The expressions for the sputtering yield Y are obtained, which are dependent in all cases on the size of the nanoparticles.

Heterogeneous gas-phase condensation is a promising method of producing hybrid multifunctional nanoparticles with tailored composition and microstructure but also intrinsically introduces greater complexity to the nucleation process and growth kinetics. Herein, we report on the synthesis and growth modeling of silicon–silver (Si–Ag) hybrid nanoparticles using gas-aggregated cosputtering from elemental Si and Ag source targets. The final Si–Ag ensemble size was manipulated in the range 5–15 nm by appropriate tuning of the deposition parameters, while variations in the Si–Ag sputtering power ratio, from 1.8 to 2.25, allowed distinctive Janus and core–satellite structures, respectively, to be produced. Molecular dynamics simulations indicate that the individual species first form independent clusters of Si and Ag without significant intermixing. Collisions between unlike species are unstable in the early stages of growth (

Swift heavy ions (SHI), of specific kinetic energies in the excess of 1 MeV/u, can create cylindrical regions of structural transformation in SiO 2 targets, also known as SHI tracks. Recent measurements of the track cross-sections in α -quartz show significant and consistent discrepancies across different experimental techniques used. In particular, the track radii obtained from channelling experiments based on the Rutherford Backscattering Spectrometry (RBS-c) method increase monotonically with the electronic stopping power, whereas the track radii obtained from the Small Angle X-ray scattering (SAXS) saturate past a certain stopping power threshold. We perform a systematic study of the structure of the α -quartz tracks obtained from the molecular dynamics (MD) simulations incorporating a time-dependent energy deposition based on the inelastic thermal spike model, which allows us to discuss the possible origins of these experimental discrepancies.

The influence of the bombardment by the Ar + , Ba + and Na + ions and the subsequent annealing on composition and electron structure of a surface of the CaF 2 /Si (1 1 1) films is studied. The energy band diagram of the epitaxial nanofilm systems of the Ca–CaF 2 –Si type is constructed. Optimum regimes of ion implantation and annealing for the production of three-componental nanodimensional structures of the Ca 1− X Ме X F 2 type are determined and parameters of their energy bands are estimated.

Ion and electron irradiation can be used to modify not only conventional materials such as silicon, but also nanostructures. This opens up exciting possibilities for basic science studies of how materials behave under an external force driving them off equilibrium, and creating new kinds of nanostructures of potential application interest. Radiation effects are almost without exception a multiscale phenomenon, and hence modelling them theoretically requires the use of multiple different levels of simulation tools. In this Article we discuss the multiscale modelling framework relevant for modelling nanoscale phenomena, review briefly the most widely used modelling tools relevant for them, and present some recent examples of their use.

Swift heavy ion (SHI) irradiation of amorphous SiO2 that contains metal nanocrystals can be used to transform the shape of the particles into peculiar asymmetric ones not easily achievable by other means. Using a molecular dynamics simulation framework augmented to include the electronic excitations of the SHIs, we predict that the reshaping of spherical particles into nanorods occurs continuously during consecutive ion impacts by a dynamic crystal–liquid–crystal phase transition of metal particle with the flow of liquid phase into an underdense track core in silica. The simulated nanocrystals are shown to have a saturation width that agrees with experiments.

Ion tracks were created in natural quartz and fluorapatite from Durango, Mexico, by irradiation with 2.2 GeV Au ions at elevated temperatures of up to 913 K. The track radii were analysed using small-angle X-ray scattering, revealing an increase in the ion track radius of approximately 0.1 nm per 100 K increase in irradiation temperature. Molecular dynamics simulations and thermal spike calculations are in good agreement with these values and indicate that the increase in track radii at elevated irradiation temperatures is due to a lower energy required to reach melting of the material. The post-irradiation annealing behaviour studied for apatite remained unchanged.

We used molecular dynamics simulations to study 2.3 MeV Au ion irradiation of silica. In this energy regime, the energy loss of the ion is divided almost equally between electronic and nuclear energy loss. The inelastic thermal spike model was used to model the electron–phonon interactions due to the high electronic energy loss. Binary collision approximation calculations provided input for the recoil energies due to MeV ions. We performed simulations of the damage due to the separate damage mechanisms as well as together, and found that the inelastic thermal spike is needed to accurately simulate the irradiation damage from MeV ions.

The amount of studies performed in the field of microscopic models of radiation nanophysics is rather limited and, with the exception of carbon nanostructures, they are non-systematic. The purpose of the present work is to initiate a systematic approach to build the theoretical models of radiation nanophysics with the account for fundamental properties of nano-objects. We choose at this stage to consider two basic phenomena (Frenkel pair formation and amorphization) modified by only one nanoproperty, i.e. the presence of an interface. We develop a mathematical framework to describe the modification on the nanoscale. From the general expression for the modified radiation phenomena, we find that any radiation effect can be modified at least by 48 ways. This predicts a very rich future of radiation nanophysics both in the fundamental and applied aspects. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular dynamics (MD) and binary collision approximation (BCA) computer simulations are employed to study surface damage by single ion impacts. The predictions of BCA and MD simulations of displacement cascades in amorphous and crystalline silicon and BCC tungsten by 1 keV Ar + ion bombardment are compared. Single ion impacts are studied at angles of 50 , 60 and 80 from normal incidence. Four parameters for BCA simulations have been optimized to obtain the best agreement of the results with MD. For the conditions reported here, BCA agrees with MD simulation results at displacements larger than 5 A for amorphous Si, whereas at small displacements a dierence between BCA and MD arises due to a material ow

This article has an erratum: DOI 10.1103/PhysRevB.95.099901 It is well established that when energetic ions are moving in crystals, they may penetrate much deeper if they happen to be directed in some specific crystal directions. This ‘channeling’ effect is utilized for instance in certain ion beam analysis methods and has been described by analytical theories and atomistic computer simulations. However, there have been very few systematic studies of channeling in directions other than the principal low-index ones. We present here a molecular dynamics-based approach to calculate ion channeling systematically over all crystal directions, providing ion ‘channeling maps’ that easily show in which directions channeling is expected. The results show that channeling effects can be quite significant even at energies below 1 keV, and that in many cases, significant planar channeling occurs also in a wide range of crystal directions between the low-index principal ones. In all of the cases studied, a large fraction (∼20–60%) of all crystal directions show channeling. A practical implication of this is that modern experiments on randomly oriented nanostructures will have a large probability of channeling. It also means that when ion irradiations are carried out on polycrystalline samples, channeling effects on the results cannot a priori be assumed to be negligible. The maps allow for easy selection of good ‘nonchanneling’ directions in experiments or alternatively finding wide channels for beneficial uses of channeling. We implement channeling theory to also give the fraction of channeling directions in amanner directly comparable to the simulations. The comparison shows good qualitative agreement. In particular, channeling theory is very good at predicting which channels are active at a given energy. This is true down to sub-keV energies, provided the penetration depth is not too small.