Maureen L. Nietiadi

Influence of local curvature on sputtering

Sputtering depends on the local surface curvature; it is higher for convex and lower for concave surfaces than for a flat reference target. We analyze this dependence theoretically and show that the sputter yield primarily depends on a dimension-free curvature parameter, and second on the relative variance of the deposited-energy distribution in the direction along the ion beam. Using molecular dynamics, we study the effects in an exemplary way for 500 eV Ar impact on an a-Si target; here, we compare a wire and a valley structure (radii of curvature 2.5 nm) with a planar target. Our findings demonstrate that the sputter yield, the crater function, and the angular distribution of sputtered particles strongly depend on the curvature.

Collision‐Induced Melting in Collisions of Water Ice Nanograins: Strong Deformations and Prevention of Bouncing

Collisions between ice grains are ubiquitous in the outer solar system. The mechanics of such collisions is traditionally described by the elastic contact theory of adhesive spheres. Here we use molecular dynamics simulations to study collisions between nanometer-sized amorphous water-ice grains. We demonstrate that the collision-induced heating leads to grain melting in the interface of the colliding grain. The large lateral deformations and grain sticking induced considerably modify available macroscopic collision models. We report on systematic increases of the contact radius, strong grain deformations, and the prevention of grain bouncing.

Sputtering of silicon membranes with nanoscale thickness

A theoretical study of forward and backward sputtering produced by the impact of single 20 keV Ar ions on freestanding amorphous Si membranes is carried out. We use three techniques: Monte Carlo (MC) and molecular dynamics (MD) simulations, as well as analytical theory based on the Sigmund model of sputtering. We find that the analytical model provides a fair description of the simulation results if the film thickness d exceeds about 10%–30% of the mean depth of energy deposition a. In this regime, backward sputtering is nearly independent of the membrane thickness and forward sputtering shows a maximum for thicknesses d≈a. The dependence of forward sputtering on the ions incidence angle shows a qualitative change as a function of d: while for d≲a, the forward sputter yield has a maximum at oblique incidence angles, the maximum occurs at normal incidence for d≳a. As the membrane thickness is reduced below 0.1– 0.3a, the theorys predictions increasingly deviate from the MC results. For example, the predi...