Size-dependent ion-beam-induced anisotropic plastic deformation at the nanoscale by nonhydrostatic capillary stresses

We develop a phenomenological model for size-dependent anisotropic plastic deformation of colloidal nanoparticles under ion irradiation. We show that, at the nanoscale, nonhydrostatic capillary stresses drive radiation-induced Newtonian viscous flow, counteracting the stress state that initiates the anisotropic viscous strains in the high-temperature thermal spike region around the ion track. We present experimental data using colloidal silica nanoparticles in the 10-100 nm size range that show that the deformation is indeed strongly size dependent, in excellent agreement with the model. This work allows for the prediction of the ion-beam-induced shape modification of a whole range of nanostructures.