Disorder and cluster formation during ion irradiation of Au nanoparticles in $\mathrm{Si}{\mathrm{O}}_2$

Au nanoparticles (NPs) have been formed by ion beam synthesis in $600\mathrm{nm}$ thin $\mathrm{Si}{\mathrm{O}}_2$. Subsequently the NPs were irradiated with $2.3\mathrm{MeV}$ Sn ions at liquid nitrogen temperature. Samples were analyzed using extended x-ray absorption fine structure (EXAFS) spectroscopy and small angle x-ray scattering (SAXS) as a function of Sn irradiation dose. Transmission electron microscopy shows that the NPs largely retain their spherical shape upon irradiation. However, we observe a reduction in average NP size and a concomitant significant narrowing of the size distribution with increasing irradiation dose as consistent with inverse Ostwald ripening. At lower irradiation doses, significant structural disorder is apparent with an effective bond length expansion as consistent with amorphous material. At higher irradiation doses, EXAFS measurements indicate dissolution of a significant fraction of Au from the NPs into the $\mathrm{Si}{\mathrm{O}}_2$ matrix (as monomers) and the formation of small Au clusters (dimers, trimers, etc.). We estimate the volume fraction of such monomers/clusters. Ion irradiation thus yields disordering then dissolution of Au NPs.

Figure 1

RBS spectra of the Au implanted $\mathrm{Si}{\mathrm{O}}_2$ layer. The solid dots show the profile prior to irradiation and the open dots after irradiation with $1\times {10}^{16}{\mathrm{cm}}^{-2}$ Sn ions. For clarity only the Au peak is shown.

Figure 2

$k^2$-weighted EXAFS spectra at $10\mathrm{K}$ of NP samples of series I. Irradiation doses are given in units of ${\mathrm{cm}}^{-2}$.

Figure 3

(a) Recorded SAXS pattern of the unirradiated NPs of sample series I and the integrated spectra of the unirradiated NPs of both sample series. SAXS intensities for the NP samples of (b) sample series I and (c) sample series II. Irradiation doses are given in units of ${\mathrm{cm}}^{-2}$. The solid lines show the corresponding fitted intensities using a maximum entropy method.

Figure 4

Size distributions of the NP samples extracted from SAXS for (a) sample series I and (b) sample series II. Irradiation doses are given in units of ${\mathrm{cm}}^{-2}$.

Figure 5

Cross-sectional TEM images of Au NPs of sample series I (a) unirradiated, (b) after irradiation with $3\times {10}^{15}{\mathrm{cm}}^{-2}$ Sn ions, and (c) after irradiation with $1\times {10}^{16}{\mathrm{cm}}^{-2}$ Sn ions. The insets in (a) show a high-resolution image and the corresponding electron diffraction pattern.

Figure 6

Fourier-transformed, $k^2$-weighted EXAFS spectra for (a) the unirradiated Au film and NP samples of both series and (b) for all NP samples of series I. The solid lines in (b) show the corresponding fits of the first coordination shell. The inset shows selected spectra of unirradiated and irradiated Au films for comparison.

Figure 7

Refined EXAFS parameters (a) CN, (b) Au-Au bond length, (c) D-W factor extracted from the first coordination shell for Au NP samples and Au films as function of ion irradiation dose given in units of ${\mathrm{cm}}^{-2}$. The dotted lines are plotted to indicate general trends. The dashed line in (b) shows the value for the unirradiated NPs of sample series I to highlight the bond length increase at low irradiation doses.

Figure 8

(a) Au-Au bond length and (b) CN as a function of NP diameter as extracted from SAXS. The solid line in (a) shows the bond length contraction expected from a simple liquid-drop model using a surface tension value of $3.4\pm 0.5\mathrm{J}∕{\mathrm{m}}^2$ (Ref. 14) (the dashed lines represent the associated error). The dotted line in (b) shows the expected CN for a spherical (fcc) NP. For a given sample series, the NP size decreases as the irradiation dose increases.

Figure 9

Estimated volume fraction of Au dispersed in the $\mathrm{Si}{\mathrm{O}}_2$ (as monomers and small clusters) as a function of the average CN of small clusters $(\text{monomers}=0)$ shown for selected irradiation conditions for sample series I.