Critical evaluation of the synergy model for elongation of embedded nanoparticles by swift heavy ion irradiation

Elongation of metal nanoparticles (NPs) embedded in silica, induced by swift heavy ion irradiation, has received much attention since the discovery. While the mechanism is still under debate, one of the most likely mechanisms is the synergy effect of transient melting of NPs by thermal spike and in-plane stress from silica matrix by the ion hammering. This paper aims at a critical evaluation of this mechanism. As reported by Benyagoub et al., the ion hammering effect is strongly affected by the radiation-induced compaction of the silica matrix at low fluences where the track coverage ratio CR = πR²ϕ < 1, where R and ϕ denote the track radius and the ion fluence, respectively. A silica film of ∼ 10 μ m thick showed shrinkage at the low fluences (CR < 1) and then turned to in-plane dilation with increasing the fluence (CR > 1)