Theory of the alkali-metal chemisorption on metal surfaces. II

Coverage dependent adsorption sites in the K/Cu(100) system: A crystal truncation rod analysis

The analysis of the intensity along the diffraction rods normal to the crystal surface (crystal truncation rods) has been used to analyse the adsorption sites of potassium atoms adsorbed at T = 330 K on atomically clean Cu(100) under ultra high vacuum conditions. At potassium coverages below about Θ = 0.18 ML (1 ML = 1.53 × 1015 atoms cm−2), where no ordered adsorbate superstructure forms, the potassium atoms are adsorbed in the fourfold hollow sites of the Cu(100) substrate at d 1 = 2.1(4) Å above the first Cu layer corresponding to an effective potassium radius of r eff = 1.5(3) Å which is close to the ionic radius of 1.33 Å. Within about 1 –2% the substrate interlayer spacings, d 12 and d 23 are not affected.

At high coverage (Θ ≈ 0.30 ML) where an ordered incommensurate overlayer forms, the adsorption site is partially on-top. These results are in correspondence with the model of alkalimetal adsorption on metals supposing extensive charge transfer from potassium to the substrate at low coverage.

Rational design of two-dimensional nanoscale networks by electrostatic interactions at surfaces

The self-assembly of aromatic carboxylic acids and cesium adatoms on a Cu(100) surface at room temperature has been investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy. The highly ordered molecular nanostructures are comprised of a central ionic coupling motif between the anionic carboxylate moieties and Cs cations that generate distinctive chiral arrangements of the network structures. The primary electrostatic interaction results in highly flexible bond lengths and geometries. The adsorbate-substrate coupling is found to be important for the determination of the structures. With the use of rod-like carboxylic linker molecules, the dimension of the porous networks can be tuned through the variation of the aromatic backbone length.

First-principles study of K and Cs adsorbed on Pd(111)

The adsorptions of K and Cs on Pd(111) were studied by the density functional calculations within the generalized gradient approximation. The site preference, bonding character, work function, and electron structure of the system were analyzed. For K and Cs adsorption, the hcp hollow site was found to be preferred for all the coverages investigated. The calculated adsorption geometries for (2 x 2) and (square root 3 x square root 3)R30 degrees phases are both in reasonable agreement with the observed results. The decrease of the work function upon the adsorption of K and Cs can be attributed to a dipole moment associated with the polarized adsorbate atom, which is characterized by depletion of the electron charge in the alkali metal layer and a charge accumulation in the interface region. Our results indicate that the bonding of alkali metal with the Pd(111) surface has a mixed ionic and metallic bond character at low coverage and a metallic bond of covalent character at high coverage.