Growth, Volmer-Weber

In practice, uniform films are obtained for only a limited number of film-substrate material combinations (17). The more common experience is that the deposited material forms 3D clusters. The clusters may form directly on the bare substrate, in the Volmer-Weber growth mode, or on top of a very thin but uniform film of the deposit, the Stranski-Krastanov growth mode. We now discuss a method for determining the equilibrium configuration of the deposited material. 

The first in situ STM study of metal deposition was for Ag on HOPG [430], which is representative of the Volmer-Weber growth mode. Since that time several other studies of metal deposition on HOPG have been reported Ag [393,431-433], Pb [129], Pt [434,435], Au [436], and Ni [437]. In these studies examining the small metal particles proved to be... 

Figure 3. Crystal growth modes (a) three-dimensional island (Volmer-Weber) growth, (b) layer-plus-island (Stranski-Kastanov) growth, and (c) layer-by-layer (Franck-van der Merwe) growth.

This is an intermediate combination of the two growth modes mentioned above. The substrate is first covered with one or a few monolayers grown in Frank-van der Merwe mode, then some three-dimensional islands are formed on the surface in the Volmer-Weber growth mode. This mode is a transition from two-dimensional growth to three-dimensional growth and has been observed in metal-metal and metal-semiconductor systems. 

In the case of weak vertical Me-S interaction, only 3D Me cluster formation takes place in the OPD range according to the Volmer-Weber growth mode (Fig. 3(a)), as foimd experimentally in the system highly oriented pyrolytic graphite HOPG(0001)/Ag [14]. 

In summary, a vacuum-based process must be capable of enhancing catalyst activity at the palladium surface, perhaps by atomic displacement, and of optimizing diffusion pathways, perhaps by formation of small grains and many grain boundaries. Finally, the film nucleation and growth process must favor production of a thin but pin-hole free film and avoidance of island formation known as Volmer-Weber growth. 

---