Adatom-Substrate Interaction

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

Since the presence of chemisorption (V and AT) states gives rise to the formation of localized covalent bonds between the adatom and substrate, we are interested in how the occurrence of localized states is governed by the values of the parameters za, zs and r/, which define the adatom-substrate interaction. Localized states exist, if one or both of (1.39) and (1.44) have real roots //,fc, which, since cosh /y,fc > 1 and e k > 1, exist for a given ri in regions of the zazs-plane depicted by the two hyperbolas... 

If chemisorbed species are polar, or dipole moments are induced by adatom-substrate interactions, there will be a dipole-dipole interaction between two adatoms. The interaction is given by... 

The driving force for ordering originates, just as with three-dimensional crystal formation, in the mutual atomic interactions. With adsorbates, an important distinction must be made between adatom-adatom and adatom-substrate interactions. In chemisorption the adatom-adatom forces are usually... 

As in (1.92), the chemisorption energy is the difference between the electronic energy of the adatom-substrate system before and after the interaction occurs, i.e.,... 

At low adsorbate coverages the surface structure of the deposited metal is determined by the substrate periodicity. Thus, under these conditions the adsorbate-substrate interaction is predominant. At higher coverages the adsorbate may continue to follow the substrate periodicity or form coincidence structures with new periodicities that are unrelated to the substrate periodicity. The ordering geometry of large-radius metallic adatoms (especially K, Rb and Cs) shows relatively little dependence on the substrate lattice they tend to form hexagonal close-packed layers on any metal... 

Fig. 4.33 Adatom-adatom pair interaction on the W (110) surface for (a) W-Ir and (b) Ir-Ir pairs. For the W-Ir pair, the maximum binding occurs at the nearest neighbor bond of the substrate, while for the Ir-Ir pair it occurs at twice the nearest neighbor bond distance. All the pair cohesive energies are less than 100...

The isotherm has a Langmuir basis and implies additivity of the various UPD adatom-substrate and adatom-adatom interactions. The AE q term represents the underpotential of the most positive stripping peak of the UPD adlayer where the UPD ML coverage approaches zero. The term is the Temkin parameter describing the UPD layer-substrate interactions such as the electrode work function change with the UPD adlayer coverage. The term is the Frumkin parameter representing the lateral adatom interactions within the UPD adlayer. 

---