Burrowing of Pt nanoparticles into SiO ₂ during ion-beam irradiation

Atomic force microscopy and cross-sectional transmission electron microscopy are used to characterize the evolution of nanoparticle/substrate interfaces during heavy-ion bombardment. Pt nanoparticles, prepared by annealing 3 Å Pt films on SiO ₂, embed into the substrates following 800 keV Kr ⁺ irradiation. For Pt particles with diameters 5-20 nm, the depth of the embedding increases with an ion dose until the particles are fully submerged at a dose of ∼10 ¹⁶cm ^-2. The results are explained by capillary driving forces and an ion-induced viscous flow of amorphous SiO ₂. The irradiation-induced viscosity of SiO ₂ needed to explain our results is ∼0.9×10 ²³Paioncm ^-2, consistent with previous measurements using stress relaxation. Similar results are obtained for 10 keV He ⁺ irradiation, suggesting that ion-induced viscosity arises from localized defects rather than from the creation of large melt zones. The embedding of Pt particles is inhibited, however, for energetically unfavorable substrates such as alumina.