Preferential amorphisation of Ge nanocrystals in a silica matrix

Full text: Relative to bulk crystalline material, Ge nanocrystals in a silica matrix exhibit subtle structural perturbations including a non-Gaussian inter-atomic distance distribution. We now demonstrate such nanocrystals are extremely sensitive to ion irradiation. Using transmission electron microscopy, Raman spectroscopy and extended x-ray absorption fine structure spectroscopy, the crystalline-to-amorphous phase transformation in -8 nm diameter nanocrystals and bulk crystalline material has been compared Amorphisation of Ge nanocrytals in a silica matrix was achieved at an ion dose -100 times less than that required for bulk crystalline standards. This rapid amorphisation of Ge nanocrystals is attributed to the preferential nucleation of the amorphous phase at the nanocrystal/matrix interface, the pre irradiation higher-energy structural state of the nanocrystals themselves and an enhanced nanocrystal vacancy concentration due to the more effective trapping of irradiation induced interstitials at the nanocrystal/matrix interface and inhibited Frenkel pair recombination when Ge interstitials are recoiled into the matrix. To demonstrate the significance of the latter, we show ion irradiation of -2 nm diameter nanocrystals yields their dissolution when the range of recoiled Ge atoms exceeds the nanocrystal bounds. Copyright (2005) Australian Institute of Physics