Electron spectroscopy studies of vacuum deposited chromium and cobalt layers
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Doctoral thesis (article-based)
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Dissertations / Laboratory of Physics, Helsinki University of Technology, 136
AbstractVacuum created model systems give the possibility for detailed study of the substrate–adsorbate interaction. In this work two systems have been created by vacuum deposition and these have been characterized with different electron spectroscopies. A chromium oxide model catalyst has been studied with X-ray photoelectron spectroscopy (XPS) and a magnetically interesting epitaxial Co/Cu system has been characterized with low energy electron diffraction (LEED). Alumina supported chromium oxides are widely used as dehydrogenation catalysts. The nature of the active sites and the reaction paths are still somewhat unclear. We have used the model catalyst approach in order to understand the properties of supported chromium oxide catalysts. The growth of chromium on a thin alumina film has been studied. The oxidation and reduction of this model catalyst and alumina supported chromium oxide catalysts have been compared. The model catalyst can be oxidized under vacuum to the active oxidation state Cr3+, but not further. In calcination and reduction the model catalyst was found to exhibit similar behavior as supported catalyst. This similarity suggests that the model catalyst can be used to study e.g. dehydrogenation reactions carried out on supported chromium oxides despite of the structural differences. The chemical state information obtained from XPS measurements by peak fitting is subject to many systematic and random errors. We have studied the effect of the background subtraction method on the derived chemical state proportions of chromium and iron oxides with two oxidation states. The effect of the background choice was found to be comparable to typical systematic errors related to uncertain peak fitting parameters. Epitaxial layers of Co and Cu show interesting magnetic properties, such as giant magnetoresistance. Undesired interface mixing has been observed when growing these layers. In this work Co was deposited onto a Cu(001) single crystal to model the interface structure. Co was found to form random surface alloys exhibiting substitutional copper sites. When the Co coverage increases to several monolayers, a transition to layer growth is observed. These findings can be used to model the magnetic properties of Co/Cu layers.
model catalyst, chromium oxide, cobalt, photoelectron spectroscopy, low energy electron diffraction
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