Swift-heavy-ion-induced shaping of spherical Ge nanoparticles into disks and rods

The work reports on experimental features and theoretical studies of swift-heavy-ion-induced shaping of Ge nanospheres into disks. A stack of alternating Ge and SiO2 layers was sputtered on an oxidized Si wafer. The Ge layer thicknesses varied from 2.5 to 7.5 nm. Thermal treatment above the melting temperature of Ge transformed each Ge layer into a layer of Ge nanospheres. With growing Ge layer thickness the mean diameter increases from 8 to 37 nm. Irradiation with low fluences (~1014cm-2) of 38 MeV I7+ shaped medium-sized Ge nanospheres into disks, whereas smaller ones became rod-like and larger ones remained unchanged. At higher fluences, the larger Ge nanospheres shrink due to Ge loss and shape into disks too. A new model is presented and atomistic Monte-Carlo simulations are shown which describe the shaping evolution and the size thresholds for shaping quantitatively. The volume change of Ge upon melting has been identified as driving force.