Pt nanoparticles (NPs) formed by ion-beam synthesis in amorphous ${\text{SiO}}_2$ were irradiated with Au ions in the energy range of 27–185 MeV. Small-angle x-ray scattering (SAXS) and transmission electron microscopy were used to characterize an irradiation-induced shape transformation within the NPs. A simple yet effective way of analyzing the SAXS data to determine both NP dimensions is presented. A transformation from spherical to rodlike shape with increasing irradiation fluence was observed for NPs larger than an energy-dependent threshold diameter, which varied from 4.0 to 6.5 nm over 27–185 MeV. NPs smaller than this threshold diameter remained spherical upon irradiation but decreased in size as a result of dissolution. The latter was more pronounced for the smallest particles. The minor dimension of the transformed NPs saturated at an energy-dependent value comparable to the threshold diameter for elongation. The saturated minor dimension was less than the diameter of the irradiation-induced molten track within the matrix. We demonstrate that Pt NPs of diameter 13 nm reach saturation of the minor dimension beyond a total-energy deposition into the matrix of $20\text{keV}/{\text{nm}}^3$.

• Received 19 June 2008

DOI:https://doi.org/10.1103/PhysRevB.78.125413