Shape transformation of Sn nanocrystals induced by swift heavy-ion irradiation and the necessity of a molten ion track

We report on the spherical to rodlike shape transformation of Sn nanocrystals (NCs) embedded in amorphous SiO2 following irradiation with 185 MeV Au ions. Consistent with previous reports for other metals, transmission electron microscopy demonstrates that under irradiation, Sn NCs larger than a critical size (11 nm) elongate parallel to the incident ion direction, while smaller particles remain spherical. Irradiation-induced NC dissolution is significant, as evident from the formation of smaller NCs in place of their original larger counterparts. Using formation conditions that yield Sn NCs at the amorphous-SiO2/crystalline-Si interface, we show that the irradiation-induced shape change occurs only within the SiO2 layer, in the direction opposite to that of the incident ions. We suggest this demonstrates the necessity of a molten ion track and provides further evidence for an elongation process involving NC melting and flow.