Comparative investigation of the nucleation and growth of fcc-metal particles (Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) on amorphous carbon and SiO₂ substrates during vapor deposition at elevated temperatures

The condensation of various fcc metals (Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) from the vapor phase on thin film substrates of amorphous carbon and SiO₂ was systematically investigated by TEM, partly in situ, as well as by EDX. At temperatures up to 723 K, condensation of the metals was generally complete, except for Au and Ag on SiO₂. On SiO₂ all metals condensed as finely dispersed particles, and their number densities saturated by coalescence in the 10¹²/cm² range. The nucleation and growth kinetics could be well fitted by the "classical" atomistic model, with stable dimers, and surface diffusion of only monomers, with activation energies of around 1 eV. On a-C, Ir, Pt, Rh and Au assumed even higher particle number densities. In contrast, for Pd, Ag, Ni and Cu on a-C, the particle number densities were lower by more than one order of magnitude, and approached saturation long before coalescence. Moreover, in situ TEM experiments revealed a delay in nucleation, which was only slight for Au, but considerable for particles of Pd, Ag and Ni, which exhibited high growth speeds subsequently. Also, growth transients were observed upon beam interruptions. These effects could not be explained by the classical model. Rather, it seems that some material is trapped at special sites in the substrate, such that it does not contribute to nucleation and cluster growth. This was supported by Monte Carlo simulations. In addition, Pd, Ag and Ni atoms are relatively weakly bound to those sites such that they may be re-emitted in an activated process for later incorporation into nucleated clusters.