Nanocrystalline and Polyicosahedral Structure of a Nanospike Generated on Metal Surface Irradiated by a Single Femtosecond Laser Pulse

Short pulse laser irradiation of metal targets can trigger a cascade of highly nonequilibrium processes leading to the formation of unique surface structures of interest to various practical applications. In this paper, we report the results of a large-scale atomistic simulation predicting the generation of a similar to 200 nm long frozen nanospike on the surface of a Ag target irradiated by a femtosecond laser pulse. The simulation provides, detailed information on the mechanisms responsible for the formation of the nanospike and the processes that define its complex nanostructure. The competing contributions of the epitaxial regrowth of the solid part of the target and the homogeneous nucleation of new crystallites triggered by the strong undercooling of the liquid regions are found to produce a remarkable variability of the structural motifs coexisting in different regions of the frozen nanospike. The homogeneous solidification, in particular, proceeds along two distinct paths selected at the nucleation stage and produces markedly different nanostructures in different parts of the nanospike, namely, nanograins with mixed fcc/hcp structure and a continuous network of pentagonal twinned structural elements arranged into a polyicosahedral structure.