Atom-substrate interface

Among the factors crucial for a SAM structure, the thiol-substrate bond has been a subject of particular controversy over more than 15 years. It is worth discussing the SAM/substrate interface in some detail since it is important not only for understanding SAM structures in general but also for electrochemical metal deposition when metal is intercalated at the SAM/Au interface (Section 5.4.3). As indicated above we limit the discussion to SAMs on Au(l 11) as this is the interface where theoretical and experimental work is sufficiently detailed to allow for a discussion at the atomic level. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

The atomic images at diamond/substrate interfaces is shown later. The presence or absence and the structures of the interface layers depend on the method of substrate pretreatment, the diamond CVD method and the growth conditions used. The investigation of the atomic interface structures will give us useful insight into the nucleation and heteroepitaxial growth of diamond. 

Several mechanisms of interaction between particles of solids are known [3]. Mechanical adhesion is achieved by flowing a metal into the support pores. The molecular mechanism of adhesion is based on the Van der Waals forces or hydrogen bonds, and the chemical mechanism on the chemical interaction of the metal particles with the support. The electric theory relates adhesion to the formation of an electric double layer (EDL) at the adhesive-substrate interface. Finally, the diffusion mechanism implies interpenetration of the molecules and atoms of the interacting phases, which results in the interface blurring. These insights into the nature of adhesion can be revealed in the papers about the interaction of transition metal... 

---