Anisotropic deformation of colloidal particles under MeV ion irradiation

Spherical silica colloids with a diameter of 1.0 mum, made by wet chemical synthesis, were irradiated with 2-16 MeV Au ions at fluences ranging from 2 x 10(14) to 11 x 10(14) cm(-2). The irradiation induces an anisotropic pla stic deformation turning the spherical colloids into ellipsoidal oblates. A fter 16 MeV Au irradiation to a fluence of 11 x 10(14) cm(-2) a size-aspect ratio of 4.7 is achieved. The size polydispersity (similar to3%) remains u naffected by the irradiation. The transverse diameter increases with the el ectronic energy loss above a threshold value of similar to0.6 keV/nm. Non-e llipsoidal colloids are observed in the case that the projected ion range i s smaller than the colloid diameter. The deformation effect is also observe d for micro-crystalline ZnS and amorphous TiO2 colloids, as well as ZnS/SiO 2 core/shell particles. No deformation is observed for crystalline Al2O3 an d Ag particles. The data provide strong support for the thermal spike model of anisotropic deformation. (C) 2001 Elsevier Science B.V. All rights rese rved.