Cobalt oxide films were prepared by oxidation of different amounts of cobalt deposited on Ir(100)-(1 × 1), where oxygen rich conditions were applied during deposition. The resulting oxide films with thicknesses of up to about 40 Å were investigated as regards their crystallographic structure and morphology, applying quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). It can be unequivocally shown that the spinel-type Co(3)O(4) phase develops, for which an excellent fit between measured and calculated LEED intensity spectra is achieved (Pendry R-factor R = 0.124). In spite of the quadratic unit cell of the substrate the oxide films are in the polar (111) orientation. Also, the native lattice parameter of the material is assumed, i.e. there is no pseudomorphic relation to the substrate. However, by means of orientational epitaxy, one of the unit-mesh vectors of the oxide and one of those of the substrate layer are aligned, leading to two mutually orthogonal domains in the oxide. The oxide is terminated by a sublayer of cobalt ions which in the bulk were tetrahedrally coordinated Co(2+) ions. There are drastic relaxations of layer spacings at and near the surface. As a consequence, the bond length between the surface terminating cobalt ions and oxygen ions below is considerably reduced, indicative of a substantial change of the ionicity of the cobalt and/or oxygen ions. This is interpreted as accounting for polarity compensation of the film, as surface reconstruction, oxygen vacancies and species adsorbed can be ruled out.

Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.

Cobalt oxides on the unreconstructed Ir(100) surface were prepared by reactive deposition of Co established by simultaneous oxygen flux at about 50 °C and subsequent annealing. The films were investigated by low-energy electron diffraction (LEED), scanning tunnelling microscopy (STM) and thermal desorption spectroscopy (TDS). We show that in spite of the quadratic unit mesh of the substrate, oxide films of (111) orientation develop. As long as oxygen-rich conditions are maintained they are of spinel-type Co(3)O(4)(111). They are non-pseudomorphic and transform to rocksalt-type CoO(111) when oxygen loss is induced by annealing at elevated temperatures. Thin films of CoO(111) are commensurate, and so, in order to realize that, they exhibit a slightly distorted unit cell when below a thickness equivalent to about seven cobalt monolayers. With increasing film thickness the uniaxial strain accompanied by the commensurability is gradually relieved by the insertion of dislocations so that eventually the film assumes ideal hexagonality. All CoO(111)-type surfaces are reconstructed at low sample temperatures equivalent to a [Formula: see text] superstructure. They reversibly transform into a (1 × 1) phase at about 50 °C.

The surface structures of CoO͑111͒ films epitaxially grown on Ir͑100͒ − ͑1 ϫ 1͒ are investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. A ͑ ͱ 3 ϫ ͱ 3͒R30°superstruc-ture is revealed for the films' ground states. It appears for film thicknesses Ն10 Å both for strained and unstrained films and so most likely applies also to the ͑111͒ surface of a bulk CoO crystal. The superstructure is interpreted as a stress-relieving reaction to the switch from rocksalt-type to wurtzite-type stacking below the surface which has been detected earlier.As is well known, transition-metal oxides play an important role in catalysis and modern material science. Concerning the latter the importance holds in particular for magnetoelectronic applications as, for example, spin valves and giant magnetoresistance devices as well as for beating the superparamagnetic limit of small clusters of a ferromagnetic material. Thereby, the phenomenon of exchange bias 1,2 is exploited, i.e., the shift of the magnetic hysteresis loop along the field axis and enhancement of the coercive field. This is observed in coupled ferromagnetic and antiferromagnetic thin films and is an interfacial phenomenon, i.e., interface properties have an essential influence on the device properties. These properties include the crystallographic structure of the interface as well as the interface roughness, that is, deviations from an atomically smooth interface. 3 It is common knowledge that the structure and roughness of catalyst surfaces are also essential for their catalytic activity.In this Rapid Communication we address the surface of CoO͑111͒ films. CoO plays an important role in the application and research of magnetoelectronic devices 3-8 and catalysts ͑e.g., Ref. 9͒. Recently we have shown that films grown on an unreconstructed Ir͑100͒ surface are terminated by a wurtzite-type stacking of layers in contrast to the rocksalttype stacking in native CoO͑111͒ and leading to a metallic surface. 10 Here we show that this termination in its low-temperature ground state ͑below 50°C͒ exhibits a ͑ ͱ 3 ϫ ͱ 3͒R30°superstructure with respect to the ͑111͒ bulk layer unit cell of CoO. We regard this structure to be of importance as it should influence both catalytic and interfacial properties. Both the wurtzite-type termination and the ͑ ͱ 3 ϫ ͱ 3͒R30°superstructure prevail in the thickness regime investigated, i.e., from 10 Å to values as large as 150 Å. Consequently, this special surface structure must be regarded as typical for CoO͑111͒ thin films and most likely also for the ͑111͒ surface of a CoO crystal. It possibly represents a new kind of stoichiometry-saving polarity compensation so far not known from the literature ͑for a review, see Ref. 11͒.Quantitative low-energy electron diffraction ͑LEED͒ and scanning tunneling microscopy ͑STM͒ were used to investigate the films within a two-stage ultrahigh vacuum apparatus as described earlier. 10 STM images were taken for the sample at room temperature. LEED intensity vs energy s...

The quasi one-dimensional Si(553)-Au surface is investigated with time-resolved two-photon photoemission and laser-based photoemission. Several occupied and unoccupied states in and outside the bulk band gap of silicon were found near the center of the surface Brillouin zone. A non-dispersing unoccupied state 0.62 eV above the Fermi level with a lifetime of 125 fs matches the spin-split silicon step-edge state predicted by density-functional-theory calculations. Two occupied bands can be associated with the bands calculated for non-polarized step-edge atoms.