J. Vennik

Electron spectroscopy of transition metal oxide surfaces

Abstract A review of XPS and AES studies performed on clean transition metal oxide (TMO) surfaces is presented. A discussion of the main aspects of these spectra and the difficulties encountered in their interpretation is given. Recent developments in the calculation of binding energies and Auger kinetic energies are described and used to discuss interpretations of chemical shifts in XPS and AES. A possible correlation between extra-atomic relaxation and the ionicity of the compound, for the zinc chalcogenides and zinc halides, is put into perspective. Multiplet splitting is briefly discussed. The influence of the number of unpaired spins and of the nature of the chemical bond, is emphasized. Spectra of the oxides MnO, CoO and ZnO are shown and discussed together with the data of the metals Co, Ni, Cu and compounds of Cr, Mn, Fe. Multiple excitations are shown to be linked to the paramagnetic character of the compound. This is illustrated by the CuO, Cu 2 O, NiO, CoO and MnO spectra. Multiple effects in the corresponding Auger spectra are also discussed. It is shown that in some compounds the doubly excited state is sufficiently long lived to give rise to shake-up satellites in Auger spectra (NiO), but not in others (CuO and CoO). Valence band studies using XPS and AES are described. XPS-valence band spectra have been interpreted on the basis of band structure calculations (ZnO and ReO 3 ) or crystal field theory (NiO and Cr 2 O 3 ). For ReO 3 cross-section modulation effects are emphasized. The valence band structures of V 2 O 5 and of lower oxides (V 4 O 9 and V 6 O 13 ), present on V 2 O 5 after different reducing treatments, are discussed. A discussion of the influence of valence band structure in Auger spectra, based on the comparison of the O (KLL) spectra of different oxides, is presented. Finally, broadening effects, mainly lifetime broadening, of both photolines and Auger transitions are discussed. For the Zn chalcogenides the width of certain Zn lines in different chemical environments, is apparently linked to other properties, such as the ionicity of the bond.

Thermal and low energy electron bombardment induced oxygen loss of V2O5 single crystals: Transition into V6O13

Abstract Thermal treatment in UHV of clean V2O5 single crystals results in homogeneous oxygen loss, involving a rate-limiting surface reaction. Depending upon the pretreatment, aircleaved samples transform topotactically into V6O13, or into what we call a phase Q of probable composition V4O9 or V6O13.5. Low energy electron bombardment of clean UHV-cleaved V2O5(010) surfaces produces the transition V2O5 → V6O13 at room temperature. This effect is attributed to electron beam stimulated reactions. The influence on the transition of carbon-containing impurities is discussed. The nucleation of V6O13 on V2O5 is explained by a model based on a surface reaction, the rate of which is enhanced by the interaction with contaminating molecules and low energy electron bombardment. The presence of shear planes at the boundary between V2O5 and the V6O13 nuclei locally enhances the oxygen loss rate and allows the V6O13 nuclei to grow into the bulk. The enhanced mobility of the oxygen at these boundaries is thought to influence favorably the oxidation-regeneration rate of the V2O5-catalyst.

Auger kinetic energies and electronic relaxation phenomena in atoms and solids

Abstract A semi-empirical theory has been developed to calculate the kinetic energy of Auger electrons resulting from radiationless transitions in both free atoms and metals. Experimental electron binding energies and calculated two-electron interaction and relaxation energies are used. Relaxation energies are determined by means of hyper-Hartree—Fock hole-state calculations. To account for extra-atomic relaxation phenomena in metals, it is assumed that conductionband electrons occupy free-atom-like screening orbitais. The relationship of the present theory to recent work of Shirley et al., Larkins, Kim et al. and Watson et al. is discussed. The dependence of the Auger cross-relaxation energy on the ionicity of compounds is briefly discussed.

Electronic relaxation processes in the KLL′ auger spectra of the free magnesium atom, solid magnesium and MgO

Abstract The K -Auger spectrum of the free magnesium atom and the magnesium metal is calculated. The Friedel model is used to account for extra-atomic relaxation effects. Theoretical Auger electron energies are compared with the corresponding experimental values. In general the agreement between theory and experiment is good. The evolution of the extra-atomic relaxation energy in going from the metal to the oxide is presented.

Observation of surface phonons on the (001) and (100) surfaces of anatase minerals

Abstract The surface properties of TiO2-anatase are of great importance in chemical technology because this material is frequently used as a support for oxide catalysts. In the present work, surface phonons were observed on the (001) and (100) surfaces of anatase minerals by means of HREELS. After comparison with theoretical values and infrared data a preliminary assignment of the observed modes was achieved.

Signature and capture cross section of copper-related hole traps in p-type high-purity germanium

The hole capture and emission characteristics of deep levels in p-type detector-grade high-purity germanium have been investigated using double lock-in DLTS. The long voltage pulses applied to the high-resistivity semiconductor and the double lock-in mode necessitate corrections in order to derive the signature and the capture cross section accurately. The dominant deep levels that are probably all copper related have sigma p values in the range 10-12-10-13 cm2; they are regarded as deep acceptors which may cause important hole trapping and resolution loss in nuclear detectors.

DLTS of gold impurities in germanium

The band-gap levels introduced by gold in germanium have been investigated using DLTS. The samples were taken from well characterised n- and p-type germanium crystals and were doped with gold by diffusion at 700 degrees C followed by a quench to room temperature. Three acceptor levels at Ec-0.056 eV, Ec-0.215 eV and Ev+0.135 eV and one donor level at Ev+0.044 eV are observed, in agreement with published data obtained from Hall effect studies. All four levels are attributed to one amphoteric defect, i.e. the gold substitutional.

A HREELS study of the V2O5 (001) surface phonon spectrum

Abstract In this work, HREELS is used to investigate the surface vibrational properties of V 2 O 5 , a multi-phonon-mode, anisotropic, layered semiconductor. An adequate description of the complex loss spectrum is provided by the dielectric theory, extended to anisotropic materials, which together with infrared data allows to generate theoretical spectra. These simulations accurately reproduce the peak positions. The relative loss intensities are also in fair agreement with the experimental data. On comparing the HREELS spectra with infrared reflection data, the assignment of the single loss frequencies is straightforward.

Electronic structure of bulk and surface vanadyl oxygen vacancies in the layer compound V2O5

Abstract The electronic structure of the vanadyl oxygen vacancy in V2O5 is studied by means of tight binding methods. The localized states and changes in density of states in the conduction band are calculated by means of a Green function method based on the band structure of a single layer. Atomic relaxation effects are studied by means of cluster models. An inward shift of vanadium into the oxygen layer is found to be energetically favourable. The effect of the relaxation on the localized states is small. Due to the layer structure of V2O5, bulk and surface vacancies are similar.

Chemical information obtained from Auger depth profiles by means of advanced factor analysis (MLCFA)

Abstract The advanced multivariate statistical technique “maximum likelihood common factor analysis (MLCFA)” is shown to be superior to “principal component analysis (PCA)” for decomposing overlapping peaks into their individual component spectra of which neither the number of components nor the peak shape of the component spectra is known. An examination of the maximum resolving power of both techniques, MLCFA and PCA, by means of artificially created series of multicomponent spectra confirms this finding unambiguously. Substantial progress in the use of AES as a chemical-analysis technique is accomplished through the implementation of MLCFA. Chemical information from Auger depth profiles is extracted by investigating the variation of the line shape of the Auger signal as a function of the changing chemical state of the element. In particular, MLCFA combined with Auger depth profiling has been applied to problems related to steelcord-rubber tyre adhesion. MLCFA allows one to elucidate the precise nature of the interfacial layer of reaction products between natural rubber vulcanized on a thin brass layer. This study reveals many interesting chemical aspects of the oxi-sulfidation of brass undetectable with classical AES.