Štěpán Pick

On the electronic structure of surface Pt–Sn alloys

The electronic structures of Pt3Sn(111),Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surfaces are studied using the linear-muffin-tin-orbital tight-binding method in the atomic-sphere approximation. Both ideal and rumpled surface terminations are considered. The hybridization between Pt d- and Sn p-electrons, respectively, leads to a lowering of the local density of electronic states at the Fermi level and to a downward shift of the Pt local d-band, which accounts for the lower reactivity of the Pt–Sn surfaces. The effect is more pronounced for rumpled surfaces. Generally, the situation is similar to that of bimetallic transition-metal surfaces. The initial-state approximation is used to predict Pt(4f) core-level shifts. We find moderate shifts of either sign, but the d-band centre of gravity moves to higher binding energies, as compared to Pt(111), in most cases. The correlation between the surface reactivity and core-level shifts, respectively, seems to be less favourable than at bimetallic transition-metal surfaces

Structure and magnetic properties of Co chains on a stepped Cu surface

Magnetic moments and the magnetic anisotropy energy (MAE) are calculated for Co chains on a stepped Cu(111) surface in a fully relaxed geometry. The Korringa–Kohn–Rostoker Green’s function method is used to determine parameters of N-body interatomic potentials and to fit the semi-empirical tightbinding electronic Hamiltonian. The strain relief at steps and in the Co chains is demonstrated to have a profound effect on the morphology of the substrate. Atomic relaxations are shown to decrease the magnetic moments and the MAE. The MAE and orbital moments are found to exhibit an oscillatory behavior with increasing size of the chains. (Some figures in this article are in colour only in the electronic version)

Hydrogen molecule dissociation on PtNi(111) systems

Abstract In the present work we consider with a molecular orbital model the H2 dissociation mechanism on a Pt layer grown on Ni(111), and comparatively, on the pure metal components. The metallic substrate was represented by a four layers cluster. In the bimetallic case, the overall adiabatic energy profile is less stable in comparison with Pt(111). All these results are in good agreement with experimental evidence and can be explained using electronic structure arguments.

TIGHT-BINDING MODEL OF CO CHEMISORPTION ON PD/W (110) AND PT/W (110) SURFACES

Abstract We study the CO chemisorption on epitaxial Pd and Pt monolayers grown on W (110), by using a simple tight-binding self-consistent recursion method model. The σ donation, π back-donation and the adsorption energy are reduced by the substrate presence. For the atop site above Pd/W (110), however, the situation does not differ much from the chemisorption at the atop position above Pd (111), and this site can be preferred on Pd/W (110) or Pd/Ta (110). Core-level shifts induced on metal atoms by the chemisorption are also discussed.

Tight-binding study of the electronic structure of Pd and Pt overlayers on W (011). The role of s electrons

Abstract The electronic structures of Pd and Pt epitaxial monolayers on a W (011) surface are studied within a self-consistent tight-binding scheme. The effective Pd (Pt) d-band filling found experimentally as well as in theoretical studies for similar systems is reproduced without the necessity of introducing any essential charge transfer. The phenomenon is related to the low-density tail in the local density of states at the Fermi level. The latter effect cannot be described satisfactorily without the inclusion of s electrons into the model. For the Nb and Ta substrates similar conclusions are reached. The attempt to predict the core-level shifts from the corresponding d-electron potential changes yields results that vary from good to unsatisfactory.

On the calculation of the heats of formation for the MTi and M3Ti (M=Ni, Pd, Pt) alloys

Abstract Heats of formation for transition metal alloys MTi and M 3 Ti, respectively (M = Ni, Pd, Pt) are calculated by a self-consistent tight-binding method. To avoid the problem with the construction of two-body repulsive potentials, a theorem of Pettifor is used. According to this theorem, contributions from the repulsive terms approximately cancel out if the calculation is performed at constant atomic volumes. To this goal, several choices of interatomic distance are tested. Comparison with experiments shows that the problem is more complicated for components with essentially differing atomic radii, and also if one goes from f.c.c. metals to b.c.c.-like alloys.

Density-functional study of the methoxy intermediates at Cu(111), Cu(110) and Cu(001) surfaces

Although the geometry of the methoxy intermediates on copper surfaces has been investigated in a number of experimental and also in several theoretical papers, the situation remains controversial for the (110) and (001) surface orientations. In the present study, we perform density-functional calculations for the Cu(111), (110) and (001) surfaces. The stress is laid upon the models and ideas proposed in the literature. At the (111) face, the fcc three-fold adsorption site is found to be slightly more favourable than the hcp one. At the (110) surface, we predict the methoxy to adsorb close to the short-bridge site in a tilted geometry. Metastable long-bridge positions are less stable by more than 0.3 eV. Since for coverages up to θ = 0.5 the interaction between the methoxy groups is weak, we see no reason for the presence of two different adsorbed methoxy forms unless the copper surface is reconstructed. For the (001) surface we obtain almost the same adsorption energy for the upright adsorbed methoxy in the hollow site, and a tilted methoxy form in a position between the bridge and the hollow site. The calculations agree semiquantitatively with the available experimental data, and admit the presence of two distinct methoxy surface forms.

Electronic structure of small Pt clusters on Ni(111)

Abstract In the present work, the electronic structure and the equilibrium spatial distribution of Pt atoms adsorbed on a Ni(11) surface have been studied. The mean adsorption energies for linear and triangular 3-atom aggregates, both in epitaxy with the substrate, are calculated and analyzed. Local density of states curves show important differences in comparison with those of the 7-atom aggregates. Experimental shifts of the Pt 4f core level seem to be in agreement with the predicted chemical shifts based on the calculated electronic structure.

Remarks on the hydrogen chemisorption on Pd(111) and Pt(111)

Abstract Chemisorption of the H(1×1) overlayer and isolated hydrogen atoms, respectively, on Pd and Pt(111) surfaces is studied within a simple self-consistent tight-binding model. The aim of the study is to calculate the local density of electronic states and help thus to resolve some puzzles found in photoemission experiments for these systems. The results partly correlate with measurements for H on Pd(111) at room temperature; the explanation of some differences between the data for Pd and Pt(111) faces is problematic. Initial-state contribution to core-level shifts induced by the chemisorption is also shortly discussed.

Tight-binding study of the electronic structure of Pd and Pt epitaxial overlayers on hcp transition metal surfaces

Abstract The electronic structures of Pd and Pt epitaxial monolayers on Re and Ru (0001) surfaces are studied within a self-consistent semi-empirical scheme. To this purpose an s—d-electron tight-binding Hamiltonian is treated by the recursion method. The over-layer d bands are shifted to higher binding energies with respect to the corresponding elemental metal position. No charge transfer between atoms is supposed in our model. The experimental Pd core-level shifts are not far from the calculated chemical shifts although the agreement is not perfect.