Surface-diffusion-mediated deposition

Bonifas AP, McCreery RL (2010) Soft Au, Pt and Cu contacts for molecular junctions through surface-diffusion-mediated deposition. Nat Nanotechnol 5 612-6172... 

Because of the widespread interest in growth of material systems by deposition, the subject of surface diffusion is one of enormous current interest. The example of surface diffusion being taken up here is of interest to our overall mission for several different reasons. First, as noted above, surfaces are one of the most important sites of communication between a given material and the rest of the world. Whether we interest ourselves in oxidation and corrosion, catalysis, the crystal surface is the seat of tremendous activity, most of which is mediated by diffusion. A second reason that we have deemed it important to consider the role of surface diffusion is that our analysis will reveal the dangers that attend the use of transition state theory. In particular, we will appeal to the existence of exchange mechanisms for diffusion that reveal that the diffusion pathways adopted on some crystal surfaces are quite different than those that might be suggested by intuition. 

The process of surfactant adsorption from a solution onto a solid surface is perhaps the most relevant in the field of tribology. It is the process by which all surfactant-mediated lubrication occurs. The physical model for this can be described by a series of steps. As with other systems, there will be diffusion from the bulk to a subsurface layer, adsorption of (initially) a monolayer of surfactant molecules, and the possibilities of subsequent rearrangement of molecules on the surface, deposition of a bilayer and subsequent further layers, and also desorption. The interactions that govern the adsorption and desorption processes can be hydrophilic, hydrophobic, and/or electrostatic, depending on the nature of the surfactant and surface concerned. Now the situation is very complex, and because of this, it is sensible to tackle the kinetics of nonionic and ionic surfactants separately. 

The first system that was based on the feedback mode aimed to deposit metals (Figure 15.10). Metal ions were incorporated in a polymer layer, for example, protonated polyvinyl pyridine, as a means of attaching them onto the surface. Hexaammineruthenium(III), Ru(NH3)e", was reduced at a UME, diffused to the substrate, and drove the reduction of AuCU and PdCl4 to their respective metals. The different factors that determined the size and pattern of deposited metal were examined. Although this initial system was expected to be simple and straightforward, it concealed complicated chemistry predominantly due to the diffusion of the mediator into the polymer. While gold ions were efficiently reduced by Ru(NH3)g" inside the polymer to form gold patterns, PdCl " ions formed different structures upon reduction. A detailed study of this system revealed that the... 

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