Analysis of catalyst surface wetting: the early stage of epitaxial germanium nanowire growth

In several nanotechnological applications the dewetting process is crucial. Although not all phenomena of dewetting are fully understood yet, especially with regard to metallic fluids, it is clear that the formation of nanoparticles, -droplets, and -clusters and their movement is strongly linked to their wetting behaviour. For this reason, the thermodynamic stability of thin metal layers (0.1 – 100 nm) with respect to its free energy is examined here. The decisive factor for the theoretical consideration is the interfacial energy. In order to achieve a better understanding of the interface interactions, three different models for the estimation of this energy are presented: i. fully theoretical, ii. empirical and iii. semi-empirical. The formation of nanometre-sized gold particles on silicon and silicon oxide is investigated in detail, elucidating the strengths and weaknesses of the three models, comparing the different substrates, and verifying the possibility of further processing of the gained particles as nanocatalysts. The importance of a persistent thin communication wetting layer between the particles and its effects on their size and number also becomes clear. In particular, the intrinsic reduction of the Laplace pressure of the system by material re-evaporation and Ostwald ripening is considered to describe the theoretically predicted and experimentally found effects. Thus dewetting phenomena of thin metal layers can be well-directed used for the manufacturing of nanostructured devices. From this viewpoint, the behaviour of gold droplets as catalysts to grow germanium nanowires on different substrates is described.