Structural stability of Cu nanocrystals in SiO2 exposed to high-energy ion irradiation

Cu nanocrystals (NCs) were synthesized in SiO2 by ion implantation and thermal annealing. Annealing at two different temperatures of 950°C and 650°C yielded two different nanocrystal size distributions with an average diameter of 8.1 and 2.5nm, respectively. Subsequently the NCs were exposed to 5.0MeV Sn3+ ion irradiation simultaneously with a thin Cu film as a bulk reference. The short-range atomic structure and average NC diameter was measured by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. Consistent with the high regeneration rate of bulk elemental metals, no irradiation induced defects were observed for the reference, whereas the small NCs (2.5nm) were dissolved as Cu monomers in the matrix. The latter was attributed to irradiation-induced mixing of Cu, Si and O based on dynamic binary collision simulations. For the large NCs (8.1nm) only minor structural changes were observed upon irradiation, consistent with a more bulk-like pre-irradiation structure.