Nanocluster Evolution under Ion Irradiation

An ensemble of nanoclusters embedded in a substrate evolves during thermal annealing by coarsening or Ostwald ripening, and, if the concentration of cluster material is sufficiently high, coalescence.
In this contribution it will be shown that by ion irradiation the evolution of nanoclusters can be modified or even dramatically changed. (i) Ion irradiation increases due to collisional detachments of atoms from clusters the steady-state solubility. Thus, cluster evolution is accelerated or happens at lower temperatures. (ii) Collisional detachment of atoms from nanoclusters is more efficient for large clusters than for small ones. This dependence on cluster size is opposite to that predicted by the Gibbs-Thomson law for thermally activated detachment. The "inverse" behaviour results in "Inverse Ostwald Ripening", i.e. a broad size distribution becomes narrower during annealing under ion irradiation. (iii) Under intense ion irradiation large clusters can evaporate and new nanoclusters will nucleate. A steady-state mean size, which is dependent on ion flux and temperature, can be found.
The new results listed above were predicted by an analytical theory and have been proven by kinetic lattice Monte-Carlo simulations and preliminary experiments.