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We tested the reliability of the mechanical thinning procedure by repeating hydrofluoric acid pretreatment and deposition steps on Si samples that had been thinned and then subjected to TEM analysis. Au clusters are very similar in morphology (see Supporting Information File 1, Fig. S1) and obtained by depositing on bulk substrates. As a result, we deposited using standard wafers, and the samples were mechanically thinned and lightly ground for TEM observations.

The morphology of the samples was investigated by transmission electron microscopy (TEM). It has been reported in previous papers that sample preparation can alter the original and structural characteristics of Au particles. Thus, we used two different dilution procedures to prepare samples for TEM analysis. A sample was obtained after conventional TEM preparation techniques, including mechanical and energetic ion dilution (Ar + ions at 5 keV). Another alternative is to use low energy (0.1-1 keV) Ar + ions (see the details in the experimental section) prepared by the so-called "gentle grinding" procedure. The results are compared in Figure 1, which shows a plan view TEM of a silicon substrate after the same gold deposition process but respectively prepared according to standard and gentle milling procedures.

The morphology was significantly modified by the high energy Ar + ion thinning, where the average particle size increased by about 20% and the density increased by about 30% relative to the low energy Ar + beam thinning procedure. The particles in Figure 1a are more spherical in shape, with a light gray area of irregular shape, whereas in Figure 1b the clusters are elongated and we observe a square grayish area. Obviously, during the standard thinning process, the rearrangement of gold atoms occurs in the sample, which may be due to the higher Ar + ion energy resulting from sample heating. To avoid artifacts, all samples were subjected to TEM analysis using a gentle milling procedure.