Anisotropic deformation of colloidal particles under MeV ion irradation

Spherical silica colloids with a diameter of 1.0 µm, made by wet chemical synthesis, were irradiated with 2-16 MeV Au ions at fluences ranging from 2×10¹⁴ to11×10¹⁴ cm². The irradiation induces an anisotropic plastic deformation turning the spherical colloids into ellipsoidal oblates. After 16 MeV Au irradiation to a fluence of 11×10¹⁴cm² a size-aspect ratio of 4.7 is achieved. The size polydispersity (~3%) remains unaffected by the irradiation. The transverse diameter increases with the electronic energy loss above a threshold value of ~0.6 keV/nm. Non-ellipsoidal colloids are observed in the case that the projected ion range is smaller than the colloid diameter. The deformation effect is also observed for micro-crystalline ZnS and amorphous TiO₂ colloids, as well as ZnS/SiO₂ core/shell particles. No deformation is observed for crystalline Al₂O₃ and Ag particles. The data provide strong support for the thermal spike model of anisotropic deformation.