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

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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 ${R}_{0}$ $(1.3\text{ }\text{nm}\ensuremath{\le}{R}_{0}\ensuremath{\le}5.3\text{ }\text{nm})$ 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.