Adatom adsorption energy dependence

Adatom Adsorption Energy Dependence on Coordinative Unsaturation of Surface Atoms... 

In Section 10.3.2, we study the dependence of adatom adsorption energy on the position of the metal in the periodic system. 

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

Different computational approaches were also considered in order to describe UPD as the potential dependent adsorption. For example, the Monte Carlo (MC) simulations of Pb UPD on Ag(lll), preformed by Obretenov et al., showed that the experimental isotherms can be completely recovered from the MC simulations if the proper distribution of the adsorption energy states on substrate surface and the interaction energies among the adatoms in the UPD ML are taken into account. This work has also highlighted the importance of surface defects and strain in the UPD ML on specific voltammetry features observed experimentally. 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

Summarizing, it is clear that the indirect interaction between adatoms has a significant effect on the chemisorption properties of the system. Most noticeably, the chemisorption energy has a damped, oscillatory dependence on the adatom separation, as first quantified in (8.1) by Grimley. Thus, multi-atom adsorption occurs preferentially with the atoms at certain sites relative to one another. 

Generally, the bonding of adatoms other than hydrogen to a metal surface is highly coordination-dependent, whereas molecular adsorption tends to be much less discriminative. For the different metals the bond strength of an adatom also tends to vary much more than the chemisorption energy of a molecule. Atoms bind more strongly to surfaces than molecules do. Here we will discuss the quantum chemical basis of chemisorption to the transition metal surfaces. We will illustrate molecular chemisorption by an analysis of the chemisorption bond of CO [3] in comparison with the atomic chemisorption of a C atom. 

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