Monolayer film, pseudomorphic

Frank-van der Merwe growth mode (film formation) Layer-by-layer growth where there is strong interaction between the depositing atoms and the substrate. Complete coverage of the substrate is attained with a few monolayer film thickness. See also Volmer-Weber (island) growth Stranski-Krastanov (pseudomorphic) growth. 

The effects of adsorbate coverage (film thickness) on the Pd 3d5/2 XPS peak positions of the Pd/W(l 1 0), Pd/ Re(0001), and Pd/Mo(l 10) systems were systematically investigated [63]. The peak positions reported for Pd coverage in excess of 1 ML represent a product of electrons emitted from surface and subsurface atoms. For the case of Pd(lOO), theoretical calculation suggest that the Pd 3d5/2 XPS BE of the surface atoms is 0.4 eV lower than that of bulk Pd. A similar difference has been observed experimentally for Ni and Pt surfaces. These shifts in BE are a consequence of variations in the coordination number of the surface atoms compared to bulk atom. If we reference the combined peak of bulk and surface atoms in 40 ML of Pd on W(1 1 0) to that of Pd(l 00) a difference of —0.8 eV is obtained between the Pd 3ds/2 BE of a pseudomorphic monolayer of Pd on W(110) and that of the surface atoms of Pd(l 00). The corresponding shifts... 

To all these intrinsic reasons, one would have to add the expected modifications in the electronic structure of the growing film as it thickens, due to the decreasing influence of the substrate. This can be better judged for a system that is not pseudomorphic, such as Ag/Cu(lll). The large (12%) mismatch between Ag and Cu would provoke such a tremendous compressive stress for a pseudomorphic layer that the Ag layers keep their own lattice parameter from the first monolayer on. For 1 ML of Ag/Cu(lll), the surface state has been found to be 120 meV lower in energy than for bulk Ag(lll) [79], and shifts with increasing Ag coverage to the bulk value. 

Fig. 1 Side view of slab models of various bimetallic structures often used in computational studies. In each case, the bottom layers of the material are defined using the structure of a specified bulk material. The number of surface and bulk layers varies in different studies, (a) In the sandwich structure the surface is one component, often the same component as the bulk material and the second layer is another component. This structure is often used to determine ligand effects, (b) The pseudomorphic monolayer structure combines strain and ligand effects in one structure by placing a second component on top of a bulk material, (c) The near surface alloy combines strain, ligand and ensemble effects in one structure by considering an alloy film defined by just a few atomic layers on top of an ordered bulk material.

Fig. 11 Valence photoemission spectra for the adsorption of CO on Ta(l 10)-supported Pd films (a) On a thick (> 3ML) Pd(l 1 l)-like film, (b) on a pseudomorphic Pd monolayer. Reprinted fi-om ref. [30].

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