Browsing by Author "Azevedo, GDM"

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    Preferential amorphisation of Ge nanocrystals in a silica matrix
    (Australian Institute of Physics, 2005-01-31) Ridgway, MC; Azevedo, GDM; Elliman, RG; Wesch, W; Glover, CJ; Miller, R; Llewellyn, DJ; Foran, GJ; Hansen, JL; Nylandsted Larsen, A
    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.
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    Preferential amorphisation of Ge nanocrystals in a silica matrix
    (Elsevier, 2004-09-05) Ridgway, MC; Azevedo, GDM; Elliman, RG; Wesch, W; Glover, CJ; Miller, R; Llewellyn, DJ; Foran, GJ; Hansen, JL; Nylandsted Larsen, A
    Extended X-ray absorption fine structure and Raman spectroscopies have been used to compare the crystalline-to-amorphous phase transformation in nanocrystalline and polycrystalline Ge. We demonstrate Ge nanocrystals are extremely sensitive to ion irradiation and are rendered amorphous at an ion dose ∼40 times less than that required to amorphise bulk, crystalline standards. This rapid amorphisation is attributed to the higher-energy nanocrystalline structural state prior to irradiation, inhibited Frenkel pair recombination when Ge interstitials are recoiled into the matrix and preferential nucleation of the amorphous phase at the nanocrystal/matrix interface. © 2005 Elsevier B.V
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    Structural characterization of Ge nanocrystals in silica amorphised by ion irradiation
    (Elsevier, 2008-06) Araujo, LL; Giulian, R; Johannessen, B; Llewellyn, DJ; Kluth, P; Azevedo, GDM; Cookson, DJ; Ridgway, MC
    Ge nanocrystals (NCs) grown by ion implantation in amorphous silica matrices were irradiated with 5 MeV Si ions over a different fluence range (2 x 10(11)-2 x 10(13) cm(-2)) than previously reported. Size and depth distributions as well as structural disorder in the NCs were measured by RBS, TEM, SAXS and EXAFS. The EXAFS results show that the embedded Ge NCs are rendered amorphous at fluences similar to 40 times lower than bulk crystalline Ge (c-Ge). No significant changes in the size or depth distribution of the NCs are observed for all irradiation fluences. Compared to c-Ge, the higher-energy structural state of the NCs prior to irradiation and the presence of the nanocrystal/matrix interface are considered the main causes for the peculiar amorphisation behavior of embedded Ge NCs. © 2008, Elsevier Ltd.