S.A. Komolov

CO2− intermediates in the CO/ZnO(0001) interface

Abstract Electron energy-loss and total current spectroscopy and photodesorption methods are employed in studies of CO adsorption on a ZnO(0001) surface. Predominantly CO2 molecules are registered in the light-induced desorption flow. In addition to the desorption stimulated by thermal action of light, a photodesorption component of CO2 is observed. The spectroscopy data provides evidence for CO oxidation on the ZnO(0001) surface and for formation of a (CO2−) chemisorption complex which is considered as an initial state for CO2 photodesorption.

A total current spectroscopy study of metal oxide surfaces: I. Unoccupied electronic states of ZnO and MgO

Unoccupied density of states (DOS) for the polar (0001) Zn, (000) O and non-polar (100) ZnO surfaces and for a MgO(001)-c(2 × 2) thin film were studied using total current spectroscopy (TCS). TC spectra of the ZnO surfaces reveal the Zn 4s-O 2p band (6.2 eV) and two DOS maxima in the Zn 4p-O 2p band which exhibited different energy positions on the Zn-terminated (8.1 eV) and O-terminated (11.0 eV) surfaces. In the TC spectra of the MgO thin film, the energy location of Mg-derived (8.0, 14.8 and 21.0 eV) and O-derived (11.6, 14.2 and 18.5 eV) maxima of DOS were determined. Previously reported data on DOS of these materials is discussed, and it is shown that additional detailed information on the energy structure of unoccupied DOS in ZnO and MgO in the 5-25 eV range above EF has been found.

Selective growth of a MgO(100)-c(2×2) superstructure on a SrTiO3(100)-(2×2) substrate

Abstract Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and electron energy loss spectroscopy were used to study a process of electron beam evaporated deposition of magnesium oxide film on a strontium titanate substrate. An annealed substrate exhibited a SrTiO 3 (100)-(2×2) surface reconstruction and consisted of nearly equal areas of TiO 2 and SrO terminated planes (slightly rich TiO 2 ). A selective deposition of magnesium oxide onto TiO 2 terminated planes of strontium titanate substrate was carried out, and a formation of a MgO(100)-c(2×2) superstructure discovered. A plausible nature of the superstructure is discussed in terms of a non-stoichiometric MgO(100) plane with a 1/4 density of oxygen vacancies.

A total current spectroscopy study of metal oxide surfaces: II. Unoccupied electronic states on TiO2(110) and SrTiO3(100) surfaces

Unoccupied densities of states (DOS) for the ordered and atomically clean TiO2(110) and SrTiO3(100) surfaces were studied using the total current spectroscopy (TCS) technique. Obtained TC spectra reveal the Ti-derived (Ti 3d(eg) - 5.2 eV, Ti 4s - 23.5 eV) and Sr-derived (Sr 3d - triplet: 7.3, 9.0 and 10.5 eV; Sr 5p/O 2p - 15 eV) maxima of unoccupied DOS. Main oxygen-derived maxima are located at 11.2 and 15.0 eV for TiO2 and at 11.8, 18.0 and 19.6 eV for SrTiO3. The results are discussed in comparison with the previously reported data on DOS of these materials and they provide additional detailed information on the energy structure of unoccupied DOS on TiO2(110) and SrTiO3(100) surfaces in the 4-25 eV range above the Fermi level.

Oxidation of ultrathin copper layers on zinc oxide polar surfaces: unoccupied electronic states

Total current spectroscopy (TCS) was used to study an evolution of unoccupied states through the process of thin-layer copper deposition and its oxidation on polar ZnO(0001) and surfaces. Zn-derived and O-derived unoccupied bands were identified in the TC spectrum at 7.7 and 9.8 eV, respectively, from the clean and well ordered polar ZnO faces, while the spectra from a thin CuO layer reveal an intensive fine structure in the 6 - 25 eV energy range above the Fermi level, which is discussed in comparison with previously reported results of experimental and theoretical investigations. The obtained data provide additional information on the energy location of the edges of unoccupied bands in CuO in the 6 - 25 eV range relative to .

VLEED from a ZnO(0001) substructure

Abstract The appearance of symmetrical sharp subsextets around each of the main hexagonally-ordered, very-low-energy diffraction spots from ZnO(0001) were analyzed and related to diffraction from thermally etched surface pits. The pit step-height and step-width were estimated from the diffraction pattern. The energy dependence of the variation in the dimensions of the subsextets was analyzed by total (target) current spectroscopy, which further showed a satisfactory agreement to the critical points of the ZnO band structure.

Laser-induced O2 desorption from TiO2 surfaces

Abstract Laser-induced oxygen molecule desorption from crystalline (110) and compressed-powder rutile TiO 2 surfaces is investigated by time-of-flight distributions and desorption signal dependencies on laser fluence. A double-component distribution of desorbed O 2 molecules (a fast one and a slow one) justifies validity of both photoelectronic and thermo-activation mechanisms. The defected surfaces are characterized by the more intensive oxygen escape supporting the desorption flow formation via the surface defect sites.

Influence of atomic Cu-layer epitaxy on Co2 and CO phtoinduced desorption from ZnO(0001)

Abstract CO adsorption is performed on an atomically clean ZnO(0001) surface, on a ZnO(0001) surface covered by 1/3 Cu monolayer and on a ZnO(0001) surface approximately fully covered by an average of 6 atomic Cu layers. Geometric and electronic properties of the initial and CO-saturated surfaces are characterized by LEED, EELS and TCS. Electron spectroscopy data give evidence for CO2- chemisorption-complex formation on the ZnO surface, and for Cu oxidation during CO exposure. CO2 molecules predominate in the photoinduced (Xe-lamp illumination) desorption flow from the clean ZnO(0001) surface, and it is possible to distinguish photodesorption and thermodesorption components. The covering of the ZnO surface with 1/3 layer of Cu induces an increase in CO2 desorption which indicates an increase of adsorption capacity and of oxidation activity due to Cu-particle action. At the 6 Cu-layer, (111)-oriented-patch-covered surface, catalytic CO oxidation is eliminated, however; a photodesorption mechanism is not valid here, but a predominant CO desorption according to a thermo-mechanism is observed.

Identification of Fe 3d empty states from the total current spectra of an (0001) surface

An intensive triplet line is revealed in the total current spectrum (TCS) of a reconstructed surface. The nature of the observed structure in the TCS is discussed in comparison with previously reported data from photoemission, inverse photoemission and theoretical investigations. It is found to be a consequence of the electron transitions into an Fe 3d band of empty states. Energy locations of Fe 3d-band extrema ( and above the bottom of the conduction band) and its splitting are estimated from the proposed schemes of electron transitions.

Oxygen effect on the conductivity of the CuxO/ZnO(0001) and (0001̄) systems

Abstract Thin copper oxide film formation on the polar ZnO(0001) and ZnO(0001) surfaces under ultra-high vacuum conditions was monitored by LEED, Auger, TCS and work-function measurements. The 2.5 nm thick discontinuous oxide films contained a mixture of CuO, Cu 2 O and ZnO. The effect of oxygen gas on the conductivity of the thin films has been measured, and a more pronounced increase of a relative resistance of the film is observed in the temperature range 400–500 K.