In this work, we firstly showed that it is possible to grow thin<br />V2O3(0001) films on Au(111) and W(110). The preparation process<br />consists of an evaporation of metallic vanadium in an oxygen<br />atmosphere, followed by an annealing at 700 K in 5.10-8 mbar<br />of oxygen. The low energy electron diffraction (LEED) patterns<br />obtained for both substrates exhibit sharp spots, indicating a<br />well-defined surface structure. The stoichiometry of the film has<br />been characterized by X-ray photoelectron spectroscopy (XPS) and<br />near edge X-ray absorption fine structure spectroscopy (NEXAFS).<br />The XP spectrum in the binding energy range 500-540 eV shows three<br />features corresponding to the V 2p3/2, V 2p1/2 and O 1s<br />lines, respectively. Relevant parameters for the determination of<br />the stoichiometry of the oxide are the distance between the O 1s<br />and V 2p signals, the Full Width at Half Maximum (FWHM) and the<br />shape of the spectra. Our spectra show good agreement with those<br />found in the literature for V2O3 single crystals. V L-edge NEXAFS<br />spectra present noticeable chemical shifts characteristic of the<br />different vanadium valencies and their shape depends implicitly on<br />the local symmetry of the vanadium cation. Each vanadium oxide<br />type therefore displays a typical spectrum. A comparison of our<br />spectrum to reference spectra permits the identification of our<br />vanadium oxide thin film to V2O3.<br /><br />We proved with infrared absorption spectroscopy (IRAS) the<br />existence of two possible terminations of the V2O3 (0001) surface.<br />These two terminations differ only by the presence or not of<br />oxygen atoms on the top of the surface, forming vanadyl groups<br />with the surface vanadium atoms. The first termination, called<br />-V=O termination, is obtained after the preparation process. The<br />second termination - the -V termination - is obtained by heating<br />the -V=O surface up to 600 K with electron bombardment.<br /><br />We studied with UV photoelectron spectroscopies (UPS), XPS and<br />NEXAFS the electronic structure of our V2O3 (0001) thin films. The<br />UP spectra of the -V=O terminated surface clearly show a gap for<br />the -V=O terminated surface. These data therefore evidence a metal<br />to insulator transition induced by the formation of the vanadyl<br />groups on the surface. This result is confirmed by our NEXAFS O K<br />edge and XPS results. The NEXAF O K edge spectra consist of two<br />features. The first one is attributed to the tansition to the<br />unoccupied V 3d egΠ and a1g (t2g) states with O 2p<br />character and the second one to the unoccupied V 3d egΣ states.<br />For the -V=O termination, both features of the spectrum exhibit a<br />shift towards higher energy relative to the spectrum for the -V<br />termination. This shift can be explained by the changes in the<br />electronic structure due to the metal to insulator transition. The<br />XP spectra exhibit enhanced satellite features in the case of the<br />-V=O termination, which can be attributed to poorly screened final<br />states. We also observed a shift of the O 2p band towards lower<br />binding energies for the -V=O terminated surface relative to the<br />-V terminated surface. We tried to explain this phenomenon with a<br />band bending model. Finally, we proposed two models for the<br />surface geometry of the -V=O terminated surface. In the first one,<br />the oxygen atoms sit on top of the vanadium atoms. In the second<br />one, the oxygen atoms sit on quasi regular bulk positions.<br /><br />We performed high resolution electron energy loss spectroscopy<br />(HREELS) measurements and presented a phonon spectrum for each<br />termination. Differences in phonon intensities observed between<br />both surface terminations can be interpreted as a screening effect<br />of electronic carriers. We compared our spectra with a spectrum of<br />the isomorphic Cr2O3(0001) and found out that the<br />metal-oxygen bond is not so strong in V2O3 as in Cr2O3.<br /><br />We studied the water adsorption properties of both surface<br />terminations. The experiment consists of the adsorption of water<br />at 90 K, yielding the formation of ice on the sample surface. The<br />sample then is heated up to 190 K. The species present on the<br />surface at this temperature are analyzed with UPS, XPS and HREELS.<br />The adsorption path seems to depend on both the termination and<br />the exposure. We observed molecularly adsorbed water on both<br />surface terminations for low exposures. The adsorbed water shows<br />only weak interaction with the substrate. For large exposures,<br />water dissociates and OH- groups were detected. When the OH-<br />desorb of the primary -V=O terminated surface, the surface left is<br />-V terminated. In the case of the -V=O terminated surface, the<br />water molecule is assumed to adsorb on the surface vanadium atom<br />through its oxygen atom. The oxygen double bonded to the vanadium<br />can interact with the hydrogen of the water molecule to form a OH<br />radical, breaking its double bond to the vanadium. This<br />dissociation mechanism may imply charge redistribution, explaining<br />why the V 3d emission in UPS increases upon water adsorption. This<br />model explains why the vanadyl oxygen atoms desorb with the OH<br />groups. For the -V terminated surface, we observed a charge<br />transfer from the V 3d substrate to the adsorbate, producing<br />OH- groups. Therefore, we proposed a model in which the<br />vanadium a1g or egΠ orbital forms a Σ bond with oxygen<br />lone-pair orbitals of OH-.<br /><br />We performed CO2 adsorption experiments with UPS, XPS, HREELS and<br />IRAS. The UP results for the -V=O surface exhibit small features<br />which we assigned to physisorbed CO2. The CO2 adsorption on the<br />-V terminated surface is more complex. The analyze of the IRAS<br />results leads us to the conclusion that CO2 adsorbs in a bent<br />configuration. With UPS and XPS, we could evidence the formation<br />of carbonates upon heating up to 200 K.<br /><br />The CO adsorption properties follow a similar trend as for CO2 :<br />only small quantities adsorb on the -V=O surface while the -V<br />surface seems to be much more reactive. On the -V=O surface, CO<br />adsorbs molecularly and we concluded from the angle resolved UPS<br />data that the CO molecule is strongly tilted on the surface. With<br />NEXAFS and IRAS, we showed the formation of CO2 on the -V<br />surface.<br /><br />To our knowledge, we are the first to report a surface effect<br />resulting in a metal to insulator transition. This very complex<br />phenomenon is very exciting for the surface scientist. Further<br />work on V2O3 (0001) should therefore involve theorists in order to<br />explain properly why the formation of vanadyl groups on the<br />surface induces a metal to insulator transition. A simulation of<br />the angle resolved UPS data could determine which model for the<br />surface geometry is correct. Further experimental work could be<br />thermal desorption spectroscopy (TDS) and IRAS with isotopes in<br />order to identify the formation path of CO2 by CO adsorption on<br />the -V terminated surface.
| Identifer | oai:union.ndltd.org:CCSD/oai:tel.archives-ouvertes.fr:tel-00096698 |
| Date | 17 October 2002 |
| Creators | Dupuis, Anne-Claire |
| Source Sets | CCSD theses-EN-ligne, France |
| Language | English |
| Detected Language | English |
| Type | PhD thesis |
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