Anisotropy driven ultrafast nanocluster burrowing.

We explore the occurrence of low-energy and low-temperature transient cluster burrowing leading to intact cluster inclusions. In particular, the anomalously fast (ballistic) Pt nanocluster implantation into Al and Ti substrates has been found by molecular dynamics simulations using a tight-binding many-body potential with a 1-10 eVatom low impact energy. A similar behavior has also been found for many other cluster/substrate couples such as CuAl, NiTi, and CoTi. At this few eV/atom low impact energy regime, instead of the expected stopping at the surface we find the propagation of the cluster through a thin Al slab as thick as approximately 50 A with a nearly constant speed ( proportional, variant1 eVatom). Hence the cluster moves far beyond the range of the impact energy, which suggests that the mechanism of cluster burrowing cannot be explained simply by collisional cascade effects. In the couples with reversed succession (e.g., TiPt, AlPt) no burrowing has been found; the clusters remain on the surface (the asymmetry of burrowing). We argue that cluster penetration occurs at a few eV/atom impact energy when the cluster/substrate interaction is size mismatched and mass anisotropic atomically.