S.L. Cunningham

An analysis of the structure of the iridium (111) surface by low-energy electron diffraction

Abstract Elastic low-energy electron diffraction (LEED) intensity versus voltage ( I - V ) measurements for the clean Ir(111) surface have been obtained. Seven specular I - V spectra were measured from 15 to 975 eV at incident angles from 7° to 62.5° relative to the surface normal. The outermost atomic layer spacing of the unreconstructed Ir(111) surface was determined both by the convolution-transform method we have presented previously (including certain convenient modifications) and by dynamical calculations. Results from the analysis of the I - V spectra by the convolution-transform method indicate that the outermost Ir(111) layer spacing is either unrelaxed or contracts by 4% of its bulk value depending upon whether the θ=7° data or the θ = 25° data are used. In agreement with this, the dynamical calculations show that the outermost Ir(111) layer spacing contracts by 2.5± 5% and, in addition, that the registry of the first layer of the crystal surface is not shifted, maintaining the fcc structure.

Single atom chemisorption on a bcc metal

Abstract The effect of chemisorption of a single atom on the (001) surface of a bcc metal is investigated via the Greens function and the phase shift techniques using the LCAO method and the tight-binding approximation. In particular, we obtain the change in the electronic density of states Δn due to two different binding sites, the on-site and the centered four-fold-site. For each site, the adatom energy level Ea is placed both inside and outside the band, and several adatom-substrate interaction strengths σ are considered. By varying these parameters we obtain a qualitative understanding of the effects on Δn due to either the adsorption of different atoms or a readjustment of Ea arising from a flow of charge onto or away from the adsorbed atom. We compare our results to previous studies of chemisorption on the (001) surface of an s-band simple cubic crystal and find that, although some details distinctive of the bcc metal arise, the overall qualitative features are the same.

Surface relaxation of Ni(110), Al(110) and Ag(110) determined by the convolution-transform method☆

Experimental (LEED) intensity-voltage (I–V) beam profiles for clean Ni(110), Al(110) and Ag(110) have been analyzed by the convolution-transform method. Results indicate that the first layer spacing of Ni(110), Al(110) and Ag(110) is contracted by 5%, 4% and 7% of their bulk spacing, respectively. These results are in good agreement with the results obtained earlier using dynamical calculations.

Chemisorption of a monolayer of atoms on a bcc metal surface

Abstract The change in the density of states due to the adsorption of a monolayer of atoms on the (001) surface of a bcc metal is presented. The substrate is described by the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight-Binding (TB) approximation, and both the Greens function formalism and the phase shift technique are employed. Each adatom is represented by a single nondegenerate energy level. Two binding sites for the commensurate monolayer are considered: the on-site and the centered fourfold-site. By assuming that screening of the charges on the adatoms is complete within the surface layer of atoms, the selfconsistency condition of satisfying Friedels sum rule can be met by varying the orbital energies of the adatoms and the surface plane of atoms of the substrate. The changes in the density of states show strongly skewed bonding and antibonding resonances which occur at different energies for the two binding sites even though equal binding strengths are assumed. A comparison with previous single adatom results shows that the shape and position of the bonding resonance are dependent upon adatom coverage.

Existence of localized electronic states at interfaces

Abstract A simple self-consistent atomic model is used to show that both true localized interface states as well as interface resonances can occur at the junction between a metal and a non-metal. These results resolve a controversy of long standing over whether or not true localized states (Bardeen states) can exist when one of the electrodes is a metal. Two distinct physical situations are considered for the metal-non-metal interface showing that these interface states may or may not exist depending upon the band gap of the non-metal and the coupling strength across the interface. In addition, results are presented for the case when both electrodes are non-metals (heterojunctions).

Chemisorption on a model insulator

Abstract The adsorption both of a single atom and a monolayer of atoms on the (001) surface of a model two-band crystal with the CsCI structure is investigated using the Greens function formalism and the phase shift technique. The electronic structure of the surface is described within the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight Binding (TB) approximation. Each adatom is represented by a single non-degenerate energy level. The adatoms are placed on the surface in both the on-site and the centered fourfold-site configuration. The change in the density of electronic states upon chemisorption is found, and comparison is made with similar results on a metal surface. It is shown that many, but not all, of the qualitative features in chemisorption on metallic surfaces can be transferred to the case of an insulating surface. In addition, it is shown that there are systematic variations in the density of states with adatom coverage which depend upon the absorption site.

Surface reconstruction of a two-band crystal: II. Model results☆

Abstract The change in the total electronic energy which occurs upon reconstruction of the (001) face of a model two-band crystal with the CsCl structure is presented. The energy changes due to a (2 × 1) and a c(2 × 2) reconstruction are considered. The results are obtained by a Greens function perturbation technique developed in the previous paper. The Greens functions appropriate for this reconstructed surface are analytic. The question of whether or not reconstruction occurs on a surface is shown to be dependent upon the relative magnitudes of the electronic energy and the strain energy.

Surface reconstruction of a two-band crystal: I. Green's function formalism

Abstract A Greens function perturbation technique is presented which is appropriate for obtaining the change in the total electronic energy when the surface atoms of a crystal reconstruct in a periodic manner. The phase shift technique is used to determine the change in the density of states. The method is quite general and can be used to study arbitrary reconstructed surfaces. As an illustration, explicit equations appropriate to the (2 × 1) reconstruction of a square lattice are presented.