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

Abstract

We report on the spherical to rodlike shape transformation of Sn nanocrystals (NCs) embedded in amorphous ${\text{SiO}}_{2}$ 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-${\text{SiO}}_{2}$/crystalline-Si interface, we show that the irradiation-induced shape change occurs only within the ${\text{SiO}}_{2}$ 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.