Size-dependent effect of ion bombardment on Au nanoparticles on top of various substrates: Thermodynamically dominated capillary forces versus sputtering

Hexagonally ordered arrays of Au nanoparticles exhibiting narrow size distributions were prepared on top of Si wafers with either a thin native oxide or a thick thermally oxidized layer as well as on top of crystalline (0001)-oriented sapphire substrates. Subsequent irradiation of these nanoparticles by 200 keV ${\text{Ar}}^{+}$ and ${\text{Xe}}^{+}$, respectively, in combination with transmission electron microscopy analysis (TEM) corroborated the previously reported phenomenon of bombardment-induced burrowing of metallic nanoparticles into ${\text{SiO}}_x$ while conserving their spherical shape [X. Hu et al., J. Appl. Phys. 92, 3995 (2002)]. Performing the ion irradiations on particle ensembles of different radii $R_0$ $\left(1.3\text{nm}\le R_0\le 5.3\text{nm}\right)$ and determining the burrowing effect by atomic force microscopy combined with TEM provide sufficient statistics to allow a quantitative description of this effect. In addition to the thermodynamic driving forces necessary for the burrowing effect, sputtering of the nanoparticles due to the ion bombardment has to be included to arrive at an excellent theoretical description of the experimental data. The magnitude of sputtering can be quantified for the Au/sapphire system, where the burrowing effect is found to be completely suppressed. In that case, the theoretical description can even be improved by assuming a size-dependent sputtering coefficient for the Au nanoparticles. Combining this type of sputtering with the thermodynamically driven burrowing effect delivers a consistent model for all ion bombarded Au nanoparticles on top of ${\text{SiO}}_x$. Specifically, the residual heights of the ${\text{Ar}}^{+}$- or ${\text{Xe}}^{+}$-induced burrowing of Au nanoparticles can be scaled on top of each other if plotted versus the average displacements per target atom rather than versus the applied ion fluences.

Figure 1

Au nanoparticles (diameter 10.3 nm) prepared by a micellar approach on top of Si: (a) top view SEM image (30 kV, SE-detector) (b) bright-field TEM image (cross-sectional view).

Figure 2

(HR)TEM cross-section images of Au nanoparticles (diameter 10.3 nm) on top of Si and irradiated with 200 keV ${\text{Ar}}^{+}$ ions for various fluences: (a) $0.7\times {10}^{15}\text{ions}/{\text{cm}}^2$, $\left(\text{b}\right)/\left({\text{b}}^{\prime }\right)$ $1.7\times {10}^{15}\text{ions}/{\text{cm}}^2$, $\left(\text{c}\right)/\left({\text{c}}^{\prime }\right)$ $2.6\times {10}^{15}\text{ions}/{\text{cm}}^2$, and (d) $7.4\times {10}^{15}\text{ions}/{\text{cm}}^2$. Double panels such as (b) and $\left({\text{b}}^{\prime }\right)$ demonstrate the statistical scatter of particle behavior at identical ion fluences.

Figure 3

(Color online) (a) AFM image of Au nanoparticles (diameter 10.6 nm) on top of ${\text{SiO}}_2$. (b) Size distributions of as-prepared and ${\text{Ar}}^{+}$-irradiated Au nanoparticles (200 keV, fluences as given in the inset). Solid curves are fits to Gaussians.

Figure 4

(HR)TEM cross section for Au particles (diameter 10.6 nm) on top of ${\text{SiO}}_2$ after 200 keV ${\text{Ar}}^{+}$ irradiation (a) $1.7\times {10}^{15}\text{ions}/{\text{cm}}^2$, (b) $3.7\times {10}^{15}\text{ions}/{\text{cm}}^2$, and (c) $4.2\times {10}^{15}\text{ions}/{\text{cm}}^2$.

Figure 5

(a) Schematics of final equilibrium positions of solid spheres sinking into a viscous substrate depending on the wetting conditions as expressed by the parameter $a$ defined in Eq. (1). (b) Definition of the two lengths $z$ and $h$ used in the theoretical description.

Figure 6

(Color online) Decrease of the height of Au nanoparticles (starting diameter 8.8 nm) due to irradiation-induced burrowing by 200 keV ${\text{Ar}}^{+}$ ion. Closed squares are experimental data and dashed curve is a fit to Eq. (8) resulting in the parameters given in the inset.

Figure 7

(Color online) Scaled particle height $h/R_0$ versus scaled ion fluence $\varphi /R_0$ for various Au nanoparticle ensembles on top of ${\text{SiO}}_2$ with different starting radii $R_0$ as given in the inset.

Figure 8

(a) Bright-field TEM image of Au nanoparticles on top of a sapphire substrate after 200 keV ${\text{Ar}}^{+}$-ion irradiation $\left(6.5\times {10}^{15}\text{ions}/{\text{cm}}^2\right)$. The nanoparticles were embedded in a ${\text{SiO}}_x$ layer after irradiation for TEM preparational reasons. (b) (HR)TEM image of single nanoparticle on top of sapphire. Implantation defects appear as inhomogeneous Bragg contrast of the substrate.

Figure 9

Ion-induced erosion (sputtering) of Au nanoparticles (starting diameter 7.8 nm) by 200 keV ${\text{Ar}}^{+}$ ions as a function of the ion fluence. The experimental data as obtained by AFM height measurements (closed squares) are fitted by a straight line.

Figure 10

(Color online) Size-dependent sputter yields of spherical Au nanoparticles due to their bombardment with 200 keV ${\text{Ar}}^{+}$ ions. An overview is given in the inset, while the main panel magnifies the behavior for particles with radii below 25 nm. In this range, the calculated results (solid curve) can be fitted by a parabola $Y=k{R}^2$ (dashed curve).

Figure 11

(Color online) Ion-induced erosion (sputtering) of sapphire-supported Au nanoparticles (starting diameter 7.8 nm) by 200 keV ${\text{Ar}}^{+}$ ions as a function of the ion fluence. The experimental data as obtained by AFM height measurements (closed squares) are fitted to Eq. (12) with a parabolic sputter yield $Y\left(R\right)=k{R}^2$.

Figure 12

(Color online) Modeling of ion-induced (200 keV ${\text{Ar}}^{+}$) burrowing of Au nanoparticles (starting diameter 8.6 nm) on top of ${\text{SiO}}_2$ including sputtering as given in the inset.

Figure 13

Au nanoparticles on top of ${\text{SiO}}_2$ covered by Si prior to ion irradiation: bright-field TEM images (left) and (HR)TEM images of single particles (right) at two different ion fluences: (a) $4\times {10}^{15}\text{ions}/{\text{cm}}^2$ and (b) $6\times {10}^{15}\text{ions}/{\text{cm}}^2$. The higher fluence does not change the position of the particles but results in a more pronounced effect of sputtering.

Figure 14

(Color online) Ion-induced burrowing of Au nanoparticles (starting radius 3.8 nm) on top of ${\text{SiO}}_2$ induced by 200 keV ${\text{Xe}}^{+}$. Left panel: scaled particle height versus scaled ion fluence and fit according to Eq. (14). Right panel: scaled particle height versus displacements per atom as calculated from SRIM (Ref. 22) including results for ${\text{Ar}}^{+}$ (200 keV) irradiation of Au particles (starting radius 4.3 nm).