Shaping of nanospheres into disks and rods with swift heavy ions

The swift heavy ion induced deformation of SiO2 and metal nanoparticles were studied intensively in the last years. Recently, we reported on ion beam shaping of Ge nanospheres. Surprisingly, Ge nanospheres in silica form oblate ellipsoids, what is opposite to prolate ellipsoids found for metal spheres.
This presentation reports on the experimental features of the shaping of Ge nanospheres and our theoretical search for the shaping mechanism(s). A stack of alternating Ge and SiO2 layers was sputtered on an oxidized Si wafer. The Ge layer thickness varied from 2.5 to 7.5 nm with 100 nm SiO2 in-between. Heating this layer stack to 950 °C for 300 s transformed each Ge layer into a layer of Ge nanospheres. With growing Ge layer thickness the mean diameter increases from 10 to 40 nm. After irradiation with (1-10)x1014 I7+cm-2 at 38 MeV, cross-section TEM images of as-prepared and irradiated samples showed that the largest Ge nanospheres kept spherical, whereas medium-size Ge spheres became oblates, and smaller ones shaped diamond-like. The different shaping of metal and semiconducting spheres is explained by their differences in thermodynamic properties.