Sonderforschungsbereich 546

"Struktur, Dynamik und Reaktivität von Übergangsmetalloxid-Aggregaten"

Teilprojekt C8

Allgemeine Angaben
Thema:  Structure determination of VOx surfaces, thin films and interfaces based on scanned-energy mode photoelectron diffraction

Fachgebiete und Arbeitsrichtung: Surface Physics, Surface and Interface Structure, Oxides, Thin Films



Prof. David Phillip Woodruff
Physics Department
University of Warwick
Coventry CV4 7AL, U.K.
Telefon: 00 44 24 76523378   
Telefax: 00 44 24 76692016   



This sub-project is concerned with gaining quantitative structural information on VOx surfaces, thin films and interfaces, primarily using the technique of scanned-energy mode photoelectron diffraction (PhD) pioneered by the Warwick (Woodruff)-FHI (Bradshaw) collaboration over some 15 years using the BESSY facility. Currently there is a remarakable dearth of quantitative structural information on oxide surfaces in general, and PhD is ideally suited to determining the local geometry of adsorbed molecules and ultra-thin films on well-characterised surface. A particular strength of the technique is its ability to provide local structural information which is element and chemical-state specific. Our overall objective is two-fold: (1) to understand the structure of ultra-thin VOx films on well-characterised substrates and (2) to determine the local geometry of molecular adsorbates on well-characterised VOx films. Initially we have focussed on the system VO2/TiO2(110), also performing some molecular adsorption studies on the TiO2(110) substrate as a model system which is already rather well characterised. While we have succesfully identified the geometry of formate and hydroxyl surface species on this surface following reaction with formic acid, the quality of the VO2 films grown on this surface appear poor, and in the earliest stages of growth V appears to cluster on the surface at low coverage; this means the question of the local adsorption geometry is ill-posed. Some recent complementary X-ray standing wave experiments promise to provide further characterisation of these films and we still hope to be able to study adsorbates on the VO2 surface. More recently, however, we have turned our attention to VOx on Pd(111); good quality thin films of V2O3 can be grown using this system with a single rotational domain, and it is this system we wish to focus on in the future. Initial data have already been collected on the ordered sub-monolayer VOx film and on the thicker V2O3 films. Preliminary results offer some hope that we may achieve some structural information on the surface vanadyl species identified on this surface. In addition, we aim to study model adsorbates on this surface including CO, water and atomic hydrogen as a potential route to forming surface hydroxyl spoecies in a controlled way. One complication we anticipate in the analysis of such data is the current absence of any quantitative structure determination of the clean surface. Structural models have been proposed, and indeed are widely accepted, yet they remain unproven experimentally. The adsorbate PhD data will provide some insight into this problem, but an independent determination of the clean surface structure would be of great value, and to this end we propose to perform complementary quantitative low energy electron diffraction measurements. We anticipate that these surface structural studies will bear particularly strongly on some of the theoretical work on minimum energy structures within Sfb 546, but also themselves build on prior experimental characterisation work performed by other experimental groups with in project.

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