Atomistic modeling of the sputtering of silicon by electrosprayed nanodroplets

Abstract

The hypervelocity impact of electrosprayed nanodroplets on single-crystal silicon ejects a large number of atoms. Although sputtering by atomic, molecular, and gas cluster ions has been thoroughly studied, the significantly larger size of nanodroplets prevents a straightforward extrapolation of the physics governing the impact of these smaller projectiles. This motivates the present molecular dynamics simulations of nanodroplet impact on silicon, aimed at resolving the mechanisms and the effect of the projectile's size and velocity on sputtering. We find that both collision cascades and thermal sputtering contribute to the overall atom ejection, the former being active during the initial stages of the impact characterized by strong interactions between the molecules of the projectile and the atoms of the target, and the absence of partial thermodynamic equilibrium. In addition, for sufficiently large projectile diameters and impact velocities, conglomerates of atoms are ejected by hydrodynamic forces. The sputtering yield, defined as the average number of target atoms ejected per projectile's molecule, increases monotonically with the kinetic energy of the molecules and, at constant molecular kinetic energy, slightly decreases with projectile diameter as a result of enhanced backscattering of the ejected atoms by the projectile's molecules. For the ionic liquid considered in this study, sputtering is first observed at molecular energies near 12.7 eV and, at the highest energy simulated of 73 eV, the sputtering yield averages to 0.37.