Thermal stability of embedded metal nanoparticles elongated by swift heavy ion irradiation: Zn nanoparticles in a molten state but preserving elongated shapes.

Solid Zn and V nanoparticles (NPs) embedded in silica were elongated by swift heavy ion (SHI) irradiation with 200 MeV Xe(14+) ions to a fluence of 5.0 × 10(13) ions cm(-2). Isochronal annealing was carried out in a vacuum from 200 to 1000 °C in steps of 100 °C for 10 min each. The degree of shape elongation was evaluated at room temperature (RT) by two different optical methods: linear dichroism spectroscopy and birefringence spectroscopy. In the as-irradiated state, the samples showed an absorption band at 5 eV due to radiation-induced defects in the silica in addition to the anisotropic absorption due to the elongated metal NPs. After annealing at 400 °C the defect band had completely disappeared, while the degree of shape elongation was almost unchanged or rather slightly increased in both the Zn and V NPs. The elongation of the Zn NPs slightly decreased but maintained a certain value after annealing at 500 °C, which is much higher than the melting point (MP) of Zn NPs (~420 °C). This observation indicates that shape elongation is mostly maintained even if the Zn NPs are in the molten state to some extent during annealing. The elongation of the Zn NPs was almost eliminated after annealing at 600 °C. In the case of the V NPs, elongation was maintained up to 800 °C but mostly eliminated at 900 °C. Since the recovery temperature of 900 °C from the elongated to the spherical shape is much lower than the MP of bulk V (1890 °C), we consider that the elongation is eliminated without melting of V NPs, i.e. via solid state mass transportation. The melting of NPs is not the key factor for the recovery to the spherical shape.