Swift heavy ion irradiation of Pt nanocrystals embedded in SiO₂

Pt nanocrystals (NCs) formed by ion implantation in SiO₂ were irradiated with 185 MeV Au ions in the fluence range from 2 × 10¹² to 6 × 10¹⁴ cm^-2. Transmission electron microscopy (TEM) shows that spherical Pt NCs with diameter smaller than a certain threshold (∼70 Å) do not change in shape, but decrease in size with increasing irradiation fluence until dissolved in the matrix. On the other hand, NCs larger than ∼70 Å change from spheres (unirradiated) to prolate spheroids (2 × 10¹³ cm^-2), finally achieving a rod-like shape with the major axis aligned parallel to the beam direction for fluences up to ∼10¹⁴ cm^-2. The NC minor dimension decreases with increasing irradiation fluence, resulting in a nearly monodispersed distribution with mean value around 70 Å, while at the same time the major dimension increases significantly resulting in the formation of rod-like NCs with aspect ratio as great as 10.