Peter J. Feibelman

Two-dimensional Ir cluster lattice on a graphene Moiré on Ir(111)

Lattices of Ir clusters have been grown by vapor phase deposition on graphene moirés on Ir(111). The clusters are highly ordered, and spatially and thermally stable below 500 K. Their narrow size distribution is tunable from 4 to about 130 atoms. A model for cluster binding to the graphene is presented based on scanning tunneling microscopy and density functional theory. The proposed binding mechanism suggests that similar cluster lattices might be grown of materials other than Ir.

Surface electromagnetic fields

Abstract Recent progress in the theory of surface electromagnetic fields is reviewed. Difficulties are noted in applying the textbook theory of refraction to fields withinA˚of a surface. These difficulties have been overcome with the development of a nonlocal description of surface optics, that has largely been applied to free-electron metal surfaces. Various model versions of non-local surface optics are described and quantitatively compared. Opportunities for, and actual contact between the theory of surface electromagnetic fields and experiment are described. Directions for future research in the area are discussed.

Stability of ionically bonded surfaces in ionizing environments

Abstract We present criteria for the stability of ionic materials in ionizing environments, confining ourselves to cases where the core hole Auger decay mechanism of Knotek and Feibelman is applicable. The main result is that Auger induced decomposition will not occur unless the cation species in the solid is ionized down to a relatively deep filled shell. This shell must be sufficiently deep that an Auger decay starting from it will release the energy necessary for decomposition. The degree to which covalency in bonding affects stability is discussed. We show how these concepts can be applied by examination of the periodic table and a table of electron binding energies.

Factors influencing graphene growth on metal surfaces

Graphene forms from a relatively dense, tightly bound C-adatom gas when elemental C is deposited on or segregates to the Ru(0001) surface. Nonlinearity of the graphene growth rate with C-adatom density suggests that growth proceeds by addition of C atom clusters to the graphene edge. The generality of this picture has now been studied by use of low-energy electron microscopy (LEEM) to observe graphene formation when Ru(0001) and Ir(111) surfaces are exposed to ethylene. The finding that graphene growth velocities and nucleation rates on Ru have precisely the same dependence on adatom concentration as for elemental C deposition implies that hydrocarbon decomposition only affects graphene growth through the rate of adatom formation. For ethylene, that rate decreases with increasing adatom concentration and graphene coverage. Initially, graphene growth on Ir(111) is like that on Ru: the growth velocity is the same nonlinear function of adatom concentration (albeit with much smaller equilibrium adatom concentrations, as we explain with DFT calculations of adatom formation energies). In the later stages of growth, graphene crystals that are rotated relative to the initial nuclei nucleate and grow. The rotated nuclei grow much faster. This difference suggests firstly, that the edge-orientation of the graphene sheets relative to the substrate plays an important role in the growth mechanism, and secondly, that attachment of the clusters to the graphene is the slowest step in cluster addition, rather than formation of clusters on the terraces.

Modification of transition metal electronic structure by P, S, Cl, and Li adatoms

We have evaluated the electronic perturbations induced on a thin Rh(001) film by the adsorption of 1/4 monolayer coverages of P, S, Cl or Li atoms. The self-consistent Surface Linearized Augmented Plane Wave (SLAPW) calculations indicate that the P, S, and Cl adlayers cause only very slight work function changes, and generally give rise to perturbations in the valence charge that are small beyond the nearest neighbor Rh atoms. We attribute the latter result to the effectiveness with which the charge density is screened. In contrast, reductions induced in the Fermi level Local Density of States ( E f -LDOS), whose magnitude reflects the ability of the surface to respond to the presence of reactants, are substantial even above next nearest neighbor Rh atoms, which are 8 a.u. away. The effect is strongest for Cl and weakest for P. When Li is adsorbed, the work function is reduced by almost 2 eV. This decrease is associated with an increase in electronic charge density over most of the surface, and at the same time, an increase in the E f -LDOS. Experimental observations of CO adsorption and dissociation on transition metal surfaces indicate a “poisoning” effect by coadsorbed P, S, and Cl atoms and a “promotion” effect by coadsorbed alkali atoms. The SLAPW results for the E f -LDOS parallel these observations.

Theory of H bonding and vibration on Pt(111)

Abstract Linearized augmented plane wave (LAPW) calculations of the structural energies of a H monolayer absorbed on a Pt(111) film predict a fcc site, a H-Pt bond length of 1.86 A, a symmetric stretch vibration frequency of 166 meV, and an asymmetric stretch frequency of 114 meV. The symmetric stretch is predicted to be weakly dipole active, with an effective charge of 0.054 e per unit cell. The results are found to be inconsistent with a nearest-neighbor spring model of the adsorbate dynamics. The site and surface charge density are compared with He diffraction results, and the vibrational frequencies are compared with electron energy loss and inelastic neutron spectra.

LAPW calculations of Rh(001) surface relaxation

Abstract Linearized Augmented Plane Wave total energy calculations for a Rh(001) surface predict a 5.1% contraction of the separation between the outer two Rh planes, relative to bulk, and virtually no change in the separation of the 2nd and 3rd layers. They also show that an adsorbed monolayer of H reduces the contraction of the outer Rh layer separation to about 1.4%. These results make it plausible that low energy electron diffraction experiments which imply that Rh(001) is unrelaxed were performed in the presence of undetected, adsorbed H.

Graphene Growth by Metal Etching on Ru (0001)

Low-energy electron microscopy (LEEM) reveals a new mode of graphene growth on Ru(0001) in which Ru atoms from a step edge are injected under a growing graphene sheet. The injected atoms can form under-graphene islands, or incorporate into the topmost Ru layer, thereby increasing its density and forming dislocation networks. Density functional calculations imply that Ru islands nucleated between the graphene layer and the substrate are energetically stable; scanning tunneling microscopy (STM) reveals that dislocation networks exist near step edges.

Lattice match in density functional calculations: ice Ih vs. beta-AgI.

Density functional optimizations of the crystal parameters of ice Ih and beta-AgI imply lattice mismatches of 4.2 to 7.9%, in a survey of eight common, approximate (non-hybrid) functionals, too large to allow a meaningful contribution from Density Functional Theory to the discussion of the significance of lattice match in ice nucleation.

Relaxation of the clean, Cu- and H-covered Ru(0001) surface

Abstract We have obtained and analyzed LEED- IV data for a cleaned Ru(001) surface and Ru(0001) covered with one monolayer of Cu. The analysis implies a 2.1% contraction of the first interlayer spacing of the cleaned surface. For Cu/Ru(0001), counter-intuitively, the first Ru interlayer spacing is found to contract by an additional 0.7% . Linearized augmented plane wave (LAPW) calculations suggest that this surprising result can be attributed to H contamination of the reference “cleaned” surface. For (1 × 1) clean, Cu- and Hcovered Ru(0001) slabs, the LAPW predicts 4.0%, 3.5% and 1.5% contractions, respectively, of the outermost Ru-layer separation. The agreement of these predictions with the LEED results is reasonably close if the experimental “cleaned” surface is identified with the H-covered slab of the calculations.