We discuss existing and new computational analysis techniques to classify local atomic arrangements in large-scale atomistic computer simulations of crystalline solids. This article includes a performance comparison of typical analysis algorithms such as Common Neighbor Analysis, Centrosymmetry Analysis, Bond Angle Analysis, Bond Order Analysis, and Voronoi Analysis. In addition we propose a simple extension to the Common Neighbor Analysis method that makes it suitable for multi-phase systems. Finally, we introduce a new structure identification algorithm, the Neighbor Distance Analysis, that is designed to identify atomic structure units in grain boundaries.

Structure identification methods for atomistic simulations of crystalline materials

A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t...

Ion and electron irradiation-induced effects in nanostructured materials

When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.

Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering

Defect-interface interactions

Recently a new class of metal alloys, of single-phase multicomponent composition at roughly equal atomic concentrations ("equiatomic"), have been shown to exhibit promising mechanical, magnetic, and corrosion resistance properties, in particular, at high temperatures. These features make them potential candidates for components of next-generation nuclear reactors and other high-radiation environments that will involve high temperatures combined with corrosive environments and extreme radiation exposure. In spite of a wide range of recent studies of many important properties of these alloys, their radiation tolerance at high doses remains unexplored. In this work, a combination of experimental and modeling efforts reveals a substantial reduction of damage accumulation under prolonged irradiation in single-phase NiFe and NiCoCr alloys compared to elemental Ni. This effect is explained by reduced dislocation mobility, which leads to slower growth of large dislocation structures. Moreover, there is no observable phase separation, ordering, or amorphization, pointing to a high phase stability of this class of alloys.

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Mechanism of Radiation Damage Reduction in Equiatomic Multicomponent Single Phase Alloys

A comparative molecular dynamics simulation study of collision cascades in two elemental semiconductors and five fcc metals is performed to elucidate how different material characteristics affect primary defect production during ion irradiation. By using simulations of full 400 eV-10 keV collision cascades and contrasting the results on different materials with each other, we probe the effect of the mass, melting temperature, material strength, and crystal structure on the modification of the material due to the cascade. The results show that the crystal structure has a strong effect on many aspects of damage production, while other material characteristics are of lesser overall importance. In all materials studied, isolated point defects produced by the cascade are predominantly interstitials. In semiconductors, amorphous clusters are produced in the cascade core, whereas in metals most of the crystal regenerates, leaving only small vacancy-rich clusters. Large interstitial clusters found in a few events in the heavy metals were observed to form by the isolation of a high-density liquid zone during the recrystallization phase of a cascade.

/pdf/defect-production-in-collision-cascades-in-elemental-bad186v52a.pdf

Defect production in collision cascades in elemental semiconductors and fcc metals

The erosion of carbon by intensive hydrogen bombardment has been recently shown to decrease sharply at very high fluxes ( ;10 ions/cm s). This effect cannot be explained by standard sputtering or erosion models, yet understanding it is central for selection of fusion reactor divertor materials, and formulation of sputtering models for high-flux conditions. Using molecular dynamics computer simulations we now show that the effect is due to the buildup of a high hydrogen content at the surface, leading to a shielding of carbon atoms by the hydrogen. @S0163-1829 ~99!50344-5#

Suppression of carbon erosion by hydrogen shielding during high-flux hydrogen bombardment

It has recently become clear that electron irradiation can recrystallize amorphous zones in semiconductors even at very low temperatures and even when the electron beam energy is so low that it cannot induce atomic displacements by ballistic collisions. We study the mechanism of this effect using classical molecular dynamics augmented with models describing the breaking of covalent bonds induced by electronic excitations. We show that the bond breaking allows a geometric rearrangement at the crystal-amorphous interface which can induce recrystallization in silicon without any thermal activation.

/pdf/mechanism-of-electron-irradiation-induced-recrystallization-24l51vyn92.pdf

Mechanism of electron-irradiation-induced recrystallization in Si

Variable-temperature scanning tunneling microscopy is used to characterize surface defects created by 4.5 keV He ion bombardment of Si(001) at 80--294 K; surface defects are created directly by ion bombardment and by diffusion of bulk defects to the surface. The heights and areal densities of adatoms, dimers, and adatom clusters at 80 and 130 K are approximately independent of temperature and in reasonable agreement with molecular dynamics calculations of adatom production. At 180 K, the areal density of these surface features is enhanced by a factor of $\ensuremath{\sim}3$. This experimental result is explained by the migration and surface trapping of bulk interstitials formed within $\ensuremath{\sim}2\mathrm{nm}$ of the surface.

/pdf/surface-defects-and-bulk-defect-migration-produced-by-ion-3dxccwz2yd.pdf

Surface Defects and Bulk Defect Migration Produced by Ion Bombardment of Si(001)

We have studied defect formation and defect distributions in silicon under low-energy (25\char21{}800 eV) self-bombardment of the $2\ifmmode\times\else\texttimes\fi{}1$ terminated Si(001) surface We applied the classical molecular dynamics technique and collected statistically significant averages to be able to detect defect production trends in the energy dependence The number of defects created in implantations was found to be a superlinear function of energy at low energies $(l400\mathrm{eV})$ and larger than the defect production in the bulk up to about 1 keV We have also examined the depth dependence of close-to-surface damage and explored the energy and time dependence of the defect creation mechanisms and the sensitivity of the results to the choice of the model potential

J. Tarus

Papers

Defect production in collision cascades in elemental semiconductors and fcc metals

Suppression of carbon erosion by hydrogen shielding during high-flux hydrogen bombardment

Mechanism of electron-irradiation-induced recrystallization in Si

Surface Defects and Bulk Defect Migration Produced by Ion Bombardment of Si(001)

Effect of surface on defect creation by self-ion bombardment of Si(001)