Surface-induced vacancy loops and damage dispersion in irradiated Fe thin films

Transmission electron microscopy (TEM) in situ ion implantation is a convenient way to study radiation damage, but it is biased by the proximity of the free surfaces of the electron transparent thin sample. In this work this bias was investigated by performing irradiation of Fe in thin foil and bulk form with ions of energies between 50 keV and 100 keV using molecular dynamics simulations. The damage resulting from the subsequent displacement cascades differs significantly between the two sample geometries. The most remarkable difference is in the resulting 〈1 0 0〉 vacancy loops. Both their size and frequency are much greater in thin films, with loops reaching 4 nm in size. This is due to an imbalance between the number of vacancies and self-interstitials produced, since the faster self-interstitials can escape to the surfaces and remain there as ad-atoms. In addition, the self-interstitial clusters are smaller for thin foils and there is a larger dispersion of the induced damage in terms of defect number, defect clustering and defect morphology. The study discusses the impact of these results on the study of radiation effects during in situ experiments.