The Si (100) substrate was cleaned according to the procedure described in the experimental section. Some of these were then immersed in DHF (diluted hydrofluoric acid, 6% hydrofluoric acid) for 10 seconds, some other 240 seconds. The electroless gold deposition was then carried out by dipping in an electroplating solution for 3 seconds. The TEM images of the respective samples are reported in Figures 2a, b, respectively. The morphology of gold changes with the time of DHF pretreatment. After 10 seconds in the DHF (see Fig. 2a), the Au atoms are uniformly assembled on the Si substrate and have a long and narrow structure of several tens of nanometers. After 4 minutes in DHF (Fig. 2b), they were arranged as spherical small particles with a radius of less than 10 nm. Although an image is taken in the bright field mode, it is clearly shown that the light gray, thin square area (labeled 1 in Figure 2b) and the dark, thick circular particle (marked 2 in Figure 2b) Of the huge quality contrast.

The two samples were also analyzed by Rutherford backscatter (RBS) measurements with 2 MeV He + ions to obtain the areal density of the deposited gold atoms, we have estimated the equivalent average thicknesses of 3 nm and 10 nm, respectively. In other words, in the first case, layer-by-layer growth is dominant, while DHF pre-processing for 240 seconds facilitates 3D alignment. It is well known that hydrofluoric acid strongly changes the silicon surface roughness and wetting properties.

After 10 seconds (etch rate 500 ?/ min), native SiO2 is etched completely, so nucleation takes place on the surface of the silicon hydride. The roughness of the Si surface after pretreatment in DHF for 4 minutes was measured by AFM and the obtained root mean square (RMS) was 3 nm, which was about one order of magnitude higher than that of a typical Si wafer. Thus, for a coarser surface, the surface free energy is increased and the formation of 3D clusters is facilitated by providing a preferred site for nucleation as a vertex or surface discontinuity.