Ion beam shaping of embedded metal nanoparticles by Si⁺ ion irradiation

Fine Co and Pt nanoparticles are nucleated when a silica sample is implanted with 400 keV Co⁺ and 1370 keV Pt⁺ ions. At the implanted range, Co and Pt react to form small Co_xPt_(1-x) nanoparticles during Si⁺ ion irradiation at 300 °С. Thermal annealing of the pre-implanted silica substrate at 1000 °С results in the formation of spherical nanoparticles of various sizes. When irradiated with Si⁺ ions at 300 °С, particles in the size range of 5 - 17 nm undergo rod-like shape transformation with an elongation in the direction of the incident ion beam, while those particles in the size range of 17 - 26 nm turn into elliptical shape. Moreover, it is suspected that very big nanoparticles (size > 26 nm) decrease in size, while small nanoparticles (size < 5 nm) do not undergo any transformation. During Si⁺ ion irradiation, the crystalline nature of the nanoparticles is preserved. The results are discussed in the light of the thermal spike model.