Theory of Alkali-Metal Adsorption on Close-Packed Metal Surfaces

Abstract

Results of recent density functional theory calculations for alkali metal adsorbates on close-packed metal surfaces are discussed. Single adatoms on the (111) surface of Al and Cu are studied with the self-consistent surface Greenfunction method by which the pure adsorbate-substrate interaction may be analyzed. Higher coverage ordered adlayers of K on Al (111), Na on Al (111), and Na on Al (001) are treated using the ab-initio pseudopotential plane wave method which affords the prediction of coverage dependent stable and metastable adsorbate geometries and phase transitions of the adsorbate layers. Together, these studies give insight and understanding into current key issues in alkali metal adsorption, namely, the nature of the adsorbate-substrate bond at low coverage and the occurrence of hitherto unanticipated adsorbate geometries, and the associated electronic properties.

An orbital-overlap model for minimal work functions of cesiated metal surfaces

“…A dipole correction is applied to compensate for the symmetry-breaking with respect to the fixed and free surface of the slab, reducing the number of layers needed compared to a full relaxation of the entire slab 28 . Figure 1 shows the typical dependence of the work function on the cesium coverage as observed both in experiments 20,29 and DFT simulations 19,30 . Initially, the work function decreases approximately linearly with increasing coverage as the dipoles associated with individual adsorbate atoms practically do not interact with each other.…”

“…At sufficiently low coverage, a single alkali atom is predicted to be adsorbed as an ion 30,32,33 , with the corresponding dipole formed normal to the surface (e.g., Fig. 2a).…”

“…In the intermediate regime, however, the adsorbatesubstrate bond adopts a partially covalent character as purely covalent bonds start to form between the adsorbates themselves 30,31,33 (Fig. 3).…”

Analytic theory of hexagonal monolayer interacting with hexagonal substrate

“…However, in order to prove that adsorbate-substrate interactions play a decisive role in determining the rotational epitaxy of alkalis on Ag͑111͒, the exact values of V G coefficients should be calculated for ͑2 ϫ 2͒ and ͑ ͱ 7 ϫ ͱ 7͒R19.11°structures. This should be possible by employing density-functional theory ͑DFT͒ calculations, which have been frequently used for halogens 24,27,44,[46][47][48] and alkalis 43,49,50 adsorbed on metal surfaces. Doering 9 proposed a model based on symmetry considerations of the monolayer and identified a special trajectory in the ͑R , ␣͒ space for hexagonal monolayer on hexagonal substrate.…”

Density functional study of Ni bulk, surfaces and the adsorbate systems Ni(111)R30°–Cl, and Ni(111)(2×2)–K

“…An effective radius for Cl on Ni(111) could be computed by subtracting an effective nickel radius of 3 48 ).…”