Preben J. Møller

Epitaxial and electronic structures of ultra-thin copper films on MgO crystal surfaces

The deposition of Cu on MgO(100) and MgO(111) surfaces by electron beam evaporation technique at room temperature in UHV was studied by LEED, AES and EELS. An epitaxial growth was seen, already from the initial deposition stage, of Cu(100) orientation both on freshly cleaved and on Ar+ bombarded MgO(100) surfaces and of Cu(111) orientation on freshly cleaved MgO(111) surfaces. The changes in the electron energy loss spectra with copper coverage are discussed and a model for copper epitaxial growth on MgO crystal surfaces is proposed.

Studies of the electronic structure of ultrathin Cu films on a TiO2(110) surface

Abstract The electronic structures of ultrathin Cu films deposited at room temperature upon a TiO 2 (110) surface have been studied by ultraviolet photoelectron spectroscopy (UPS) and electron energy-loss spectroscopy (EELS). The results clearly indicate that interactions between Cu3d levels and the substrate are very limited. The bonding between Cu adatoms and the rutile substrate is mainly contributed by Cu4s-p electrons, resulting in a charge transfer. A relation between the Cu3d binding energy and the degree of the charge transfer is revealed by means of a charge potential model. The influence of substrate defects on electronic structures of Cu overlayers has been examined during the initial formation of the interface. The defect sites seem to have capability to trap electrons donated from Cu atoms, thus mollifying the interface properties.

A photoemission study of the coadsorption of CO2 and Na on TiO2(110)-(1 × 1) and -(1 × 2) surfaces: adsorption geometry and reactivity

The coadsorption of CO2 and Na on TiO2(110)-(1 × 1) and -(1 × 2) surfaces have been investigated by synchrotron-radiation based core-level and valence band photoemission. We find that the initially adsorbed Na exhibits a core-level shift of 1.15 eV when the two surfaces are compared. From a simple adsorption model this binding energy shift is understood in terms of a difference in initial Na adsorption site on these surfaces. While the (1 × 1) surface seems to favor Na adsorption in a hollow site “between” bridging surface oxygen atoms, it is found that the (1 × 2) surface facilitates a chemically more advantageous Na adsorption “adjacent to” the bridging oxygen atoms. Valence band measurements support this model since Na adsorption on the (1 × 2) surface leads to emission characteristic of alkali-oxygen-like compounds while this is not the case for the NaTiO2(110)-(1 × 1) system. Finally, the relatively high resolution of the core-level emission allows in a direct way the various features contributing to the Na 2p core-level emission to be determined. With respect to adsorption of CO2 we find for the (1 × 2) surface that CO2 uptake saturates around 0.5 ML Na coverage compared to 1 ML for the (1 × 1) surface, indicating that the Na coverage required for saturation of CO2 uptake is proportional to the density of protruded oxygen rows present at the surface. The CO2 uptake, however, increases as the density of the oxygen rows decreases. Valence band photoemission data obtained from both interfaces show that a surface carbonate species is formed. At lower coverages/exposures there are, however, indications of the presence of a CO−2 species rather than carbonate, thereby suggesting that the carbonate species is formed through the surface reaction: 2CO−2→CO2−3 + CO.

Electron beam induced charging of Cu/MgO surfaces

Abstract The Auger beam surface charge build-up during deposition of thin layers of Cu upon MgO(100) and MgO(111) crystal surfaces was investigated. A three step model is proposed: 1) copper induced electron trapping, 2) formation of local conduction band in a thin metal film, and 3) formation of continuous isolated metal.

Resonant photoemission from TiO2(110) surfaces: implications on surface bonding and hybridization

Valence band photoemission obtained near the Ti 3p absorption edge has been used to investigate the surface electronic structure of TiO2(110)-(1 × 1), −(1 × 2) and Na/TiO2(110). The variation of the energy distribution curves with photon energy suggests that the observed resonant emission should be divided into two parts. A low-energy part is attributed to Ti 3p → 3d initial exitations, while a high-energy part is assigned to Ti 3p → 4s exitations. We find that both the intensity and the shape of the resonance profile as well as the spectral shape of the resonant emission vary substantially with surface conditions, the largest changes being associated with Ti 4s initial excitations. By applying a molecular orbital bonding scheme developed for rutile compounds and by considering the surface-induced perturbations to this, we are able to relate our experimental findings to recent structural models for the (1 × 1) and (1 × 2) surfaces. Particularly for the (1 × 1) surface the existence of bonding Ti 4s-O 2p hybridization observed at EB = 5.6 and EB = 6.7 eV is recognized for the first time. The observed restriction of this resonant emission to the clean (1 × 1) surface is readily associated with the contraction of Ti(5-fold)O bond lengths expected for this surface. For the (1 × 2) surface we observe a strong damping of the 4s-related bonding resonances, a decrease in 3d-bonding emission and an increase of the 3d-“non-bonding” emission. These observations are in accordance with a recent structural model for this surface. Finally, the resonant-emission data demonstrate that the surface Ti present at the (1 × 2) surface should be considered as Ti4+ states.

Annealing-induced microfaceting of the CoO(100) surface investigated by LEED and STM

Abstract During a LEED examination of the CoO(100) surface a number of extra spots were observed in the LEED pattern. These spots indicated the presence of a faceting of the surface, and appeared upon annealing to 1100 K. Further analysis of the extra spots indicates that the faces of the structures formed are the CoO 110 planes. An STM study of the reconstruction revealed it to consist of pyramidal surface depressions of sizes up to 500 A by 500 A.

On the defect center electron energy loss structures from MgO surfaces

Abstract Electron energy loss spectroscopy (EELS) was applied to surfaces of (1) clean MgO(100), (2) ultrathin (0.5 A average thickness) Cu layers deposited on MgO(100) by an electron beam evaporation technique, and (3) a carbon-contaminated MgO(100). The surface-defect-related energy loss peak was ascribed to the presence of surface states arising from the V s − centers rather than from the F s + centers. The copper deposit is supposed to be trapped by the magnesium ion vacancies and bonded to the oxygen ligands as ions. The new electronic structures caused by the Cu deposit aie explained in terms of Cu impurity levels.

X-ray photoelectron spectroscopy study of the vapor deposition of copper onto a MgO(100) surface

Abstract The formation of a vapor-deposited Cu overlayer on a MgO(100) crystal surface has been investigated using X-ray photoelectron spectroscopy (XPS). The results, in terms of electron energies, Auger parameters, intensity ratios, and Cu peak width as functions of the time of deposition, are consistent with the Stranskii-Krastanov growth model. The copper, deposited at room temperature, is at low coverages in a non-metallic state. During the deposition of Cu, a mixed monolayer of non-metallic and metallic Cu is formed, and finally the deposition is producing on top of it a continous layer of metallic Cu.

On the thermal stability of copper deposits on a (0001) sapphire surface

Abstract The thermal stability of copper deposited on a sapphire, α-Al 2 O 3 (0001)-1 × 1 substrate in UHV has been studied by Auger electron spectroscopy and electron energy-loss spectroscopy. The results indicate formation of a bond between the initially deposited copper and oxygen sites, and a Cu(I) state. The ultrathin Cu films, deposited upon room-temperature substrates, are stable on the substrate at temperatures below 430°C. For comparison, an ultraviolet photoelectron spectroscopy study was made on Cu deposited on Al 2 O 3 /A1(111).

Ultrathin films of Cu on ZnO(112̄0): growth and electronic structure

Abstract The growth of Cu on ZnO(1120) and the related electronic-structure changes were investigated for deposits until 4 nm thickness. The growth at 300 K follows the monolayer-simultaneous-multilayer mode with a premonolayer break at 0.6 ML coverage. A surface-related plasmon excitation was observed by EELS at 6.7 eV. Core-level synchrotron-based photo-emission demonstrates a monotonic decrease in binding energy with increasing deposition thickness, approaching the bulk Cu value. Initial- and final-state considerations suggest that the shift is due to final-state effects, i.e. that Cu is present in the metallic state. By annealing 0.6 nm-deposited surfaces to 875 K, however, an observed shift in the initial 5.7 eV plasmon-energy with d Cu corresponding to a 0.28 e charge transfer, and a missing dangling bond of O, suggest bonding of the small Cu islands to substrate O, leading to fully coordinated O atoms and Cu(I).