Annealing atmosphere effects on Zn nanoparticles in SiO2 and transformation to ZnO nanoparticles

The effects of annealing atmosphere (oxygen or vacuum) on Zn nanoparticles (NPs) embedded in SiO2 fabricated by ion implantation are studied. The oxidation effect depends on the ratio of the diffusion length of O2 molecules in the substrates to the projectile range of implanted ions. Up to 400 °C, annealing for 1 h in oxygen and vacuum induces almost the same increase of the optical absorption band of Zn NPs at ∼ 5 eV. In this temperature range, the diffusion length of O2 molecules is less than ∼ 1 nm, i.e., much shorter than the projectile range. After annealing at 600 °C for 1 h, the vacuum atmosphere increases but the oxygen atmosphere decreases the absorption band of Zn NPs. At 600 °C, some fraction of Zn NPs transforms to ZnO NPs under the oxygen atmosphere since the diffusion length of O2 molecules increases to ∼ 10 nm. Annealing in oxygen at 700 °C for 1 h gives rise to a complete transformation of Zn NPs to ZnO NPs, since the diffusion length increases to ∼ 30 nm, which is comparable to the projectile range. Similar results are also observed for Ni NPs and Cu NPs in the same temperature range, supporting the diffusion controlled oxidation of the NPs in SiO2.