Juha Samela

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.

Crater functions for compound materials: A route to parameter estimation in coupled-PDE models of ion bombardment

Abstract During the ion bombardment of targets containing multiple component species, highly-ordered arrays of nanostructures are sometimes observed. Models incorporating coupled partial differential equations, describing both morphological and chemical evolution, seem to offer the most promise of explaining these observations. However, these models contain many unknown parameters, which must satisfy specific conditions in order to explain observed behavior. The lack of knowledge of these parameters is therefore an important barrier to the comparison of theory with experiment. Here, by adapting the recent theory of “crater functions” to the case of binary materials, we develop a generic framework in which many of the parameters of such models can be estimated using the results of molecular dynamics simulations. As a demonstration, we apply our framework to the recent theory of Bradley and Shipman, for the case of Ar-irradiated GaSb, in which ordered patterns were first observed. In contrast to the requirements therein that sputtered atoms form the dominant component of the collision cascade, and that preferential redistribution play an important stabilizing role, we find instead that the redistributed atoms dominate the collision cascade, and that preferential redistribution appears negligible. Hence, the actual estimated parameters for this system do not seem to satisfy the requirements imposed by current theory, motivating the consideration of other potential pattern-forming mechanisms.

Atomistic simulations of fracture in silica glass through hypervelocity impact

Fracture of silica glass through hypervelocity impact was studied using large-scale classical molecular-dynamics simulations. The fracture is found to proceed through the coalescence of nanopores created upon straining of the glass by the pressure wave originating in the impact. Cratering of the glass substrate according to a two-phase compression and expansion mechanism accompanies the fracture.

Classical molecular dynamics simulations of hypervelocity nanoparticle impacts on amorphous silica

We have investigated the transition from the atomistic to the macroscopic impact mechanism by simulating large Argon cluster impacts on amorphous silica. The transition occurs at cluster sizes less than

Emergence of non-linear effects in nanocluster collision cascades in amorphous silicon

50\text{ }000

A quantitative and comparative study of sputtering yields in Au

atoms at hypervelocity regime (22 km/s). After that, the crater volume increases linearly with the cluster size opposite to the nonlinear scaling typical of small cluster impacts. The simulations demonstrate that the molecular dynamics method can be used to explore atomistic mechanisms that lead to damage formation in small particle impacts, for example, in impacts of micrometeorites on spacecraft.

Origin of complex impact craters on native oxide coated silicon surfaces

Cluster ion beams create considerably more damage in silicon and other substrates and eject more material than single ions that deposit at the same kinetic energy on the substrate. The mechanisms that causes the non-linear growth of damage and sputtering are interesting from the point of view of both basic materials research and industrial applications. Using classical molecular dynamics, we analyse the dynamics of collision cascades that are induced in amorphous silicon by small noble gas nanoclusters. We show that the sputtering and other non-linear effects emerge due to the high-energy density induced in a relatively small region in the substrate during the cluster stopping phase and because of the timing of consequent events that dissipate the energy over a larger volume of the substrate.