K. Nordlund

Ion and electron irradiation-induced effects in nanostructured 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...

Molecular dynamics simulation of ion ranges in the 1–100 keV energy range

Abstract Binary collision approximation methods have been conventionally used to describe the slowing down of recoiling ions. In order to better understand the slowing-down process, molecular dynamics methods are more and more used in the literature. However, the computer capacity limits the usefulness of the methods in most practical cases where ion implantation in the 1–100 keV energy range is used. We present an efficient molecular dynamics method for calculating ion ranges and deposited energies in the recoil energy region 100 eV to 100 keV. By taking into account only the interactions that are involved in the slowing-down process, range and deposited energy distributions at higher energies can be simulated. The method is demonstrated by range calculations of 40 keV H atoms in Si, 40 keV He atoms in Ta and 100 eV to 10 keV Si atoms in Si.

Molecular dynamics investigations of surface damage produced by kiloelectronvolt self-bombardment of solids

Abstract Molecular dynamics computer simulations were employed to study damage production mechanisms at solid surfaces during bombardment with kiloelectronvolt ions. Three separate mechanisms are identified: ballistic damage, viscous flow and microexplosions. Ballistic damage is created by the direct knock-on of atoms onto the surface as described within the binary collision approximation. Viscous flow refers to local melting and the forced flow of liquid onto the surface, and microexplosions occur when the high pressures in cascades lead to rupturing of the nearby surface. The relative importance of each mechanism depends on several parameters: atomic mass, melting temperature, atomic density, structure and atomic bonding of the target, and the mass and energy of the projectile. The simulations were performed for Pt. Au, Cu, Ni and Ge self-atom bombardment. Cascades in the interior of the targets were also examined to provide a comparison for the surface events. In addition several events of 4.5keV Ne an...

Analytical interatomic potential for modeling nonequilibrium processes in the W-C-H system

A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W–C–H. The model combines Brenner’s hydrocarbon potential with parameter sets for W–W, W–C, and W–H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W–H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides.

Modelling of compound semiconductors: analytical bond-order potential for gallium, nitrogen and gallium nitride

An analytical bond-order potential for GaN is presented that describes a wide range of structural properties of GaN as well as bonding and structure of the pure constituents. For the systematic fit of the potential parameters reference data are taken from total-energy calculations within the density functional theory if not available from experiments. Although long-range interactions are not explicitly included in the potential, the present model provides a good fit to different structural geometries including defects and high-pressure phases of GaN.

Comparison of TOF-ERDA and nuclear resonance reaction techniques for range profile measurements of keV energy implants

A comparative study on the range measurements of keV energy implants by the Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) and conventionally used nuclear resonance reaction methods has been performed for 20–100 keV 15N+ ions implanted into crystalline silicon. Range profiles of 15N atoms were chosen because they can be measured accurately using a very strong and narrow resonance at Ep = 429.6 keV in the reaction 15N(p,αγ)12C which provides a challenging test for other methods. The measured range profiles were simulated by molecular dynamics calculations where the interatomic NSi pair potential is deduced from first principles calculations. The electronic stopping power for 20–100 keV nitrogen ions in silicon is deduced from the comparison of the measured and simulated range profiles. The results are discussed in the framework of the applicability of the TOF-ERDA technique for keV energy ion range measurements.

Coherent displacement of atoms during ion irradiation

Ion irradiation is a common technique of materials processing, as well as being relevant to the radiation damage incurred in nuclear reactors. Early models of the effects of ion irradiation typically assumed that particles undergo two-body elastic collisions, like billiard balls colliding in three dimensions. Later descriptions invoked such phenomena as localization of kinetic energy, thermalization and localized melting. In all these descriptions, the displacement of atoms is chaotic in that slight variations in the ions trajectory produce completely different, unpredictable sets of atomic displacements. Here we report molecular-dynamics simulations of high-energy self-bombardment of copper and nickel, in which we see collective displacements of atoms. The high pressures developed in collision cascades centred well below the surface can cause a coherent displacement of thousands of atoms, over tens of atomic planes, in a shear-induced slip motion towards the surface. The mechanism leads to a significant increase in damage production near the surface, characterized by well-ordered islands of adsorbed atoms. Our findings suggest an explanation for some features of radiation damage, as well as for differences between ion and neutron irradiation.

Difference in formation of hydrogen and helium clusters in tungsten

The experimentally observed large difference in the depths of hydrogen and helium clusters formed in tungsten still lacks a fundamental explanation. Using density functional theory calculations, molecular dynamics simulations, and kinetic Monte Carlo calculations, we show that the fundamental mechanism behind the different clustering depths is significantly different behaviors of interstitial H and He atoms in W: H–H states are unstable for small interatomic distances whereas He–He states are strongly bound.

REPULSIVE INTERATOMIC POTENTIALS CALCULATED USING HARTREE-FOCK AND DENSITY-FUNCTIONAL THEORY METHODS

The repulsive part of the interatomic potential affects the outcome of computer simulations of many irradiation processes of practical interest, like sputtering and ion irradiation range distributions. The accuracy of repulsive potentials is studied by comparing potentials calculated using commonly available density-functional theory (DFT) and Hartreee Fock (HF) methods to highly accurate fully numerical HF and Hartree-Fock-Slater (HFS) calculations. We find that DFT calculations utilizing numerical basis sets and HF calculations using deconrrncted standard basis sets provide repulsive potentials which are significantly improved compared to the standard universal ZBL potential. The accuracy of the calculated potentials is almost totally governed by the quality of the one-particle basis set. The use of reliable repulsive potentials open up new avenues for analysis of ion irradiation experiments. 0 1997 Elsevier Science B.V.

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.