Amorphization of silicon induced by nanodroplet impact: A molecular dynamics study

Abstract

The hypervelocity impact of electrosprayed nanodroplets on crystalline silicon produces an amorphous layer with a thickness comparable to the droplet diameters. The phase transition is puzzling considering that amorphization has not been observed in macroscopic shock compression of silicon, the only apparent difference being the several orders of magnitude disparity between the sizes of the macroscopic and nanodroplet projectiles. This article investigates the physics of the amorphization by modeling the impact of a nanodrop on single-crystal silicon via molecular dynamics. The simulation shows that the amorphization results from the heating and subsequent melting of a thin layer of silicon surrounding the impact area, followed by an ultrafast quenching with cooling rates surpassing 1013 K/s. These conditions impede crystalline growth in the supercooled liquid phase, which finally undergoes a glass transition to render a disordered solid phase. The high temperature field near the impact interface is a localized effect. The significantly different temperatures and cooling rates near the surface and in the bulk explain why amorphization occurs in nanodroplet impact, while it is absent in macroscopic shock compression. Since these high temperatures and ultrafast quenching rates are likely to occur in other materials, nanodroplet impact may become a general amorphatization technique for treating the surfaces of most crystalline substrates.