R.G. Egdell

Oxygen deficient SnO2 (110) and TiO2 (110): A comparative study by photoemission

Abstract Oxygen deficient SnO 2 (110) and TiO 2 (110) surfaces prepared by argon ion bombardment of stoichiometric crystals have been studied by He(I) photoemission spectroscopy. On TiO 2 (110), 3 d 1 polaronic states localised at crystallographic shear planes give rise to a band peaking 1 eV below the Fermi level. By contrast for SnO 2 (110) there is strong 5 s −5 p hybridisation that pushed the Sn 2+ lone-pair-like defect state down toward the valence band edge.

Electronic excitations at oxygen deficient TiO2(110) surfaces: A study by EELS

Abstract TiO 2 (110) surfaces with controlled oxygen deficiency introduced by 160 eV electron bombardment have been studied by XPS and EELS. Stoichiometry was monitored by the growth of core peaks due to Ti 3+ states in XPS. Oxygen desorption is characterised by an initial cross section of 3 × 10 −21 cm 2 that decreases with increasing oxygen loss, tending toward a limiting composition Ti 4 O 7 . The oxygen deficient surfaces display sub-bandgap excitations in electronic EELS, whilst in the vibrational region there is a selective downward shift and attenuation of the highest energy phonon loss. This is attributed to modification of the effective background dielectric constant by the defect excitations. Quantitative consideration of the changes in HREELS leads to an estimate of 0.1 for the oscillator strength of the defect electronic excitations. The high value supports the idea that electrons at oxygen deficient TiO 2 surfaces occupy states that are pulled down below the conduction band by polaronic self trapping.

The adsorption of water on SrTiO3 and TiO2: a reappraisal

Abstract The adsorption of water on both SrTiO 3 (100) and TiO 2 (ll0) has been studied by He(ll) photoelectron spectroscopy and high resolution EELS. Water does not adsorb on the defect free surfaces at room temperature, although non-dissociative adsorption is observed on cooling SrTiOa to approximately 150 K. Dissociative adsorption of H 20 occurs at room temperature on TiO2 surfaces with defects introduced either by electron or argon-ion bombardment. By contrast, on argon-ion bombarded SrTiO3 dissociated and non-dissociated species coexist, although mild annealing under H20 leads to complete dissociation.

HREELS studies of adsorbates on polar solids : Water on SrTiO3(100)

Abstract The adsorption of water on SrTiO 3 (100) has been studied by HREELS. Exposure of the defect free surface to water at 100K leads initially to non-dissociative adsorption on surface cation sites with development of losses due to OH stretching and bending modes at frequencies similar to those of free H 2 O. Further exposure results in progressive attenuation of the surface phonon losses of the substrate with development of a loss spectrum characteristic of an ice multilayer. Binding of water in oxygen vacancy sites generated by argon ion etching is much stronger than to the defect free surface.

Solid-state and surface chemistry of Sn-doped In2O3 ceramics

Abstract Tin-doped indium oxide (ITO) ceramics prepared by a high-temperature solid-state synthetic procedure have been studied over the composition range 1–6 at.% Sn by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron energy loss spectroscopy (HREELS). Surface tin enrichment is evident from XPS with a heat of segregation of around −40 kJ mole −1 . However, the surface free-carrier concentration probed via the conduction-to-valence band intensity ratio in UPS or the surface plasmon frequency in HREELS is lower than the nominal tin concentration. It is concluded that electrons associated with segregated Sn ions in the topmost surface plane occupy a lone-pair-like sp hybrid surface state, while the region immediately below the surface is depleted in free carriers as a result of donor trapping effects and evaporation of tin during preparation.

APPLICATION OF FOURIER-TRANSFORM TECHNIQUES TO DECONVOLUTION OF HREEL SPECTRA

Abstract The interaction of electrons with the surfaces of polar materials via long-range dipole coupling is so strong that in high-resolution electron energy-loss spectroscopy (HREELS) electrons may excite two or more quanta of surface excitations in separate loss events. We have developed a method based on Fourier transformation of the experimental spectrum for the determination of surface loss functions free this from overtone and combination structure.

Application of high-resolution electron-energy-loss spectroscopy to MBE grown GaAs(100)

Plasmon and phonon modes of silicon-doped MBE grown (100) oriented GaAs ( n ≈4×10 18 /cm 3 ) have been observed by high-resolution electron-energy-loss spectroscopy (HREELS). A protective arsenic overlayer allowed transfer of samples from the MBE growth chamber to the HREELS system without contamination of the underlying GaAs. Criteria are presented which allow appraisal of the success or failure of the arsenic desorption procedure used to prepare clean, ordered sample surfaces. The variation of intensity and energy of the loss features with exciting beam energy are analysed in terms of a model which incorporates a carrier free surface depletion layer on top of the undepleted bulk.

A SPECTROSCOPIC STUDY OF ELECTRON AND ION-BEAM REDUCTION OF SNO2(110)

The reduction of SnO2(110) by 160 eV electron bombardment has been studied by XPS. Oxygen desorption is characterised by an initial cross section of 2.8 × 10−20 cm2. Growth of core peaks due to Sn(0) accompany oxygen loss; this contrasts with reduction under Ar ion bombardment, where only Sn(II) is formed. Plasmon loss satellites 14 eV below the new core peaks identify the Sn(0) as metallic tin. Careful analysis of peak intensities shows that electron beam reduction does produce some Sn(II) in addition to the Sn(0) species.

Surface segregation of Ba in MgO

Abstract Surface segregation of Ba in doped polycrystalline and single crystal MgO has been studied by XPS. There is little variation in surface coverage of Ba over a range of bulk doping levels from 7 to 7000 ppm in ceramic pellets equilibrated at 1630 K. This remarkable behaviour is shown to be consistent with surface coverage calculated from heats of segregation derived from ionic model simulations of the surface if due account is taken of the strong variation of the segregation energy with coverage. The coverage of Ba on a MgO(001) single crystal surface doped by indiffusion of Ba deposited onto the crystal from a getter source can be made similar to that of the polycrystalline material. The Ba-segregated MgO(001) surface exhibits a complex LEED pattern.

The high-resolution electron-energy-loss spectrum of TiO2(110)

Abstract High-resolution electron-energy-loss spectra of TiO 2 (110) are presented and compared with model calculations of dielectric loss functions. The influence of crystal anisotropy on loss spectra is rather small, producing only minor changes in the relative intensities of loss peaks. By contrast, electronic excitations associated with surface oxygen vacancy detects exert a major influence on the vibrational loss spectra and lead to pronounced changes in peak intensities and positions.