Metal nanocrystal formation in magnesium aluminate spinel and silicon dioxide with high-flux Cu− ions

Intense Cu− ions of 60 keV spontaneously grow nanospheres embedded within a shallow depth in insulators, which exhibit optical nonlinearity. The in-beam growth of nanoparticles is preferred but subjected to phase instability. Spinel oxides may be a candidate substrate to realize the phase stability, because of good radiation resistance and sufficient transparency. Spinel of MgAl2O4 and amorphous(a-) SiO2 were irradiated with Cu− at dose rates up to 100 μA/cm2, at a total dose of 3.0 × 1016 ions/cm2. Nanocrystal morphology and optical absorption (hν=0.5 – 5 eV) varied depending on dose rate. At high dose rates, a-SiO2 showed a strong tendency of depth-dependent rearrangement and particle coarsening. The MgAl2O4 also showed spontaneous precipitation of nanoparticles but, in contrast, neither long-range rearrangement of implants nor particle coarsening, up to high dose rates. Therefore, the MgAl2O4 spinel is a promising substrate to realize fine and stable nanostructures.