Flyura Djurabekova

Molecular dynamics of single-particle impacts predicts phase diagrams for large scale pattern formation

Energetic particle irradiation can cause surface ultra-smoothening, self-organized nanoscale pattern formation or degradation of the structural integrity of nuclear reactor components. A fundamental understanding of the mechanisms governing the selection among these outcomes has been elusive. Here we predict the mechanism governing the transition from pattern formation to flatness using only parameter-free molecular dynamics simulations of single-ion impacts as input into a multiscale analysis, obtaining good agreement with experiment. Our results overturn the paradigm attributing these phenomena to the removal of target atoms via sputter erosion: the mechanism dominating both stability and instability is the impact-induced redistribution of target atoms that are not sputtered away, with erosive effects being essentially irrelevant. We discuss the potential implications for the formation of a mysterious nanoscale topography, leading to surface degradation, of tungsten plasma-facing fusion reactor walls. Consideration of impact-induced redistribution processes may lead to a new design criterion for stability under irradiation.

SAXS investigations of the morphology of swift heavy ion tracks in α-quartz

The morphology of swift heavy ion tracks in crystalline α-quartz was investigated using small angle x-ray scattering (SAXS), molecular dynamics (MD) simulations and transmission electron microscopy. Tracks were generated by irradiation with heavy ions with energies between 27 MeV and 2.2 GeV. The analysis of the SAXS data indicates a density change of the tracks of ~2 ± 1% compared to the surrounding quartz matrix for all irradiation conditions. The track radii only show a weak dependence on the electronic energy loss at values above 17 keV nm(-1), in contrast to values previously reported from Rutherford backscattering spectrometry measurements and expectations from the inelastic thermal spike model. The MD simulations are in good agreement at low energy losses, yet predict larger radii than SAXS at high ion energies. The observed discrepancies are discussed with respect to the formation of a defective halo around an amorphous track core, the existence of high stresses and/or the possible presence of a boiling phase in quartz predicted by the inelastic thermal spike model.

Formation Mechanism of Fe Nanocubes by Magnetron Sputtering Inert Gas Condensation

In this work, we study the formation mechanisms of iron nanoparticles (Fe NPs) grown by magnetron sputtering inert gas condensation and emphasize the decisive kinetics effects that give rise specifically to cubic morphologies. Our experimental results, as well as computer simulations carried out by two different methods, indicate that the cubic shape of Fe NPs is explained by basic differences in the kinetic growth modes of {100} and {110} surfaces rather than surface formation energetics. Both our experimental and theoretical investigations show that the final shape is defined by the combination of the condensation temperature and the rate of atomic deposition onto the growing nanocluster. We, thus, construct a comprehensive deposition rate-temperature diagram of Fe NP shapes and develop an analytical model that predicts the temporal evolution of these properties. Combining the shape diagram and the analytical model, morphological control of Fe NPs during formation is feasible; as such, our method proposes a roadmap for experimentalists to engineer NPs of desired shapes for targeted applications.

Cooperative effect of electronic and nuclear stopping on ion irradiation damage in silica

Radiation damage by ions is conventionally believed to be produced either by displacement cascades or electronic energy deposition acting separately. There is, however, a range of ion energies where both processes are significant and can contribute to irradiation damage. The combination of two computational methods, namely binary collision approximation and molecular dynamics, the latter with input from the inelastic thermal spike model, makes it possible to examine the simultaneous contribution of both energy deposition mechanisms on the structural damage in the irradiated structure. We study the effect in amorphous SiO2 irradiated by Au ions with energies ranging between 0.6 and 76.5 MeV. We find that in the intermediate energy regime, the local heating due to electronic excitations gives a significant contribution to the displacement cascade damage.

Inoculation of silicon nanoparticles with silver atoms

Silicon (Si) nanoparticles were coated inflight with silver (Ag) atoms using a novel method to prepare multicomponent heterostructured metal-semiconductor nanoparticles. Molecular dynamics (MD) computer simulations were employed, supported by high-resolution bright field (BF) transmission electron microscopy (HRTEM) and aberration-corrected scanning transmission electron microscopy (STEM) with a resolution ≤0.1 nm in high angle annular dark field (HAADF) mode. These studies revealed that the alloying behavior and phase dynamics during the coating process are more complex than when attaching hetero-atoms to preformed nanoparticles. According to the MD simulations, Ag atoms condense, nucleate and diffuse into the liquid Si nanoparticles in a process that we term “inoculation”, and a phase transition begins. Subsequent solidification involves an intermediate alloying stage that enabled us to control the microstructure and crystallinity of the solidified hybrid heterostructured nanoparticles.

Dislocation nucleation from near surface void under static tensile stress in Cu

We examine a possible mechanism for the formation of protrusions on a metallic surface held in a sufficiently high electric field in the presence of a near-surface void. By means of molecular dynamics simulations we show that the high tensile stress exerted on a Cu {110} surface with a near-surface void can promote the nucleation of dislocations on the void surface. These dislocations cause slip along {111} crystallographic planes leading to mass transport in the volume above the void. We find a linear correlation between the radius of the void and the maximum depth for the growth to occur.

Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2

We report on the observation of nanoscale density fluctuations in 2 μm thick amorphous SiO2 layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5 × 1012 ions/cm2, where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track “annihilation” process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.

Swift Heavy Ion Shape Transformation of Au Nanocrystals Mediated by Molten Material Flow and Recrystallization

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.

Temperature dependence of ion track formation in quartz and apatite

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.

Long-term stability of Cu surface nanotips

Sharp nanoscale tips on the metal surfaces of electrodes enhance locally applied electric fields. Strongly enhanced electric fields trigger electron field emission and atom evaporation from the apexes of nanotips. Together, these processes may explain electric discharges in the form of small local arcs observed near metal surfaces in the presence of electric fields, even in ultra-high vacuum conditions. In the present work, we investigate the stability of nanoscale tips by means of computer simulations of surface diffusion processes on copper, the main material used in high-voltage electronics. We study the stability and lifetime of thin copper (Cu) surface nanotips at different temperatures in terms of diffusion processes. For this purpose we have developed a surface kinetic Monte Carlo (KMC) model where the jump processes are described by tabulated precalculated energy barriers. We show that tall surface features with high aspect ratios can be fairly stable at room temperature. However, the stability was found to depend strongly on the temperature: 13 nm nanotips with the major axes in the [Formula: see text] crystallographic directions were found to flatten down to half of the original height in less than 100 ns at temperatures close to the melting point, whereas no significant change in the height of these nanotips was observed after 10 [Formula: see text] at room temperature. Moreover, the nanotips built up along the [Formula: see text] crystallographic directions were found to be significantly more stable than those oriented in the [Formula: see text] or [Formula: see text] crystallographic directions. The proposed KMC model has been found to be well-suited for simulating atomic surface processes and was validated against molecular dynamics simulation results via the comparison of the flattening times obtained by both methods. We also note that the KMC simulations were two orders of magnitude computationally faster than the corresponding molecular dynamics calculations.