Ranges and range fluctuations of Cu_N nanoclusters implanted in Cu(111): a molecular dynamics study

摘要

In recent years, the interaction of accelerated nanoclusters with surfaces has received increasing interest due to various existing or still anticipated practical applications of cluster beams. One of the fundamental questions is how the nonlinear spike effects (which normally happen during cluster implantation) can affect stopping power and, hence the range (penetration depth) of cluster atoms? In fact, there are two contradictory points of view. On one hand, some theoretical and experimental studies indicate significant enhancement of ranges of implanted cluster atoms with respect to implanted single-atomic ions at the same velocity (so-called "clearing-the-way effect" first described by P. Sigmund and V. Shulga). On the other hand, recent experiments of H.H. Andersen have shown that mean range of atoms of metallic clusters in copper with energy of 10 keV/atom is independent on cluster size, whereas significant enhancement of broadening of implantation profile takes place with increase of cluster size (so-called "within spike diffusion" effect). To extend these studies, in present work we carry out systematic computations of projected range distribution of atoms of Cu_N clusters (N=1, 6, 13, 55) implanted into Cu(111) at various energies per one incident atom - within (100...1000) eV/atom. Simulation was done by means of classical Molecular Dynamics (MD) method. It was demonstrated that both mean projected range and range straggling (broadening) of cluster atoms exceeds those of monomer at the same velocity. At the same time, the effect of range enhancement with increase of cluster size is found to be disappeared with growth of energy per one incident atom. On the other hand, the effect of strong enhancement of range straggling with increase of cluster size is not so much energy-dependent. The possible mechanisms responsible on these effects are discussed.