Morphology and electric potential of pristine and gold covered surfaces with the fluorite structure

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Title: Morphology and electric potential of pristine and gold covered surfaces with the fluorite structure
Authors: Pieper, Hans Hermann
Thesis advisor: Prof. Dr. Michael Reichling
Thesis referee: Prof. Dr. Joachim Wollschläger
Abstract: In this thesis, the morphology and electric potential of pristine and gold covered surfaces with the fluorite structure are investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). The pristine (111) surface of calcium difluride is prepared by cleaving. Cleavege ledges are composed of short alternating type I and type II steps having different atomic structures and polarities. With respect to the stoichiometric terrace, the surface potential is slightly reduced at ledges predominately composed of type I steps, while the potential of ledges predominantly composed of type II steps is significantly higher. Pristine (111) surfaces of ceria single crystals and ceria thick films are prepared by repeated cycles of sputtering and annealing. Annealing bulk samples at 1100 K results in small terraces with rounded ledges and steps with predominantly one O-Ce-O triple layer height while annealing at 1200 K produces well-ordered straight step edges in a hexagonal motif and step bunching. The morphology and topographic details of films are similar, however, films are destroyed upon heating above 1100 K. NC-AFM and KPFM images exhibit uniform and atomically perfect terraces on a single crystal surface while films exhibit significant inhomogeneities even for best possible preparation conditions. Applying X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS), a significant contamination of the bulk ceria sample with fluorine within the first 5 nm below the surface is found while a possible fluorine contamination of films is below the detection limits. After deposition of gold, a surprisingly strong interaction of gold adatoms to the (111) surface of bulk ceria is found. The pinning of individual adatoms to one specific surface site and the stability against sintering for annealing temperatures up to 520 K is discussed considering the fluorine contamination of bulk ceria samples. Additionally nanoscale gold clusters supported on ceria are fully characterized by employing elaborate NC-AFM and KPFM experiments gaining detailed information on their shape, crystalinity, epitaxy and binding to the substrate.
Subject Keywords: NC-AFM; KPFM; CaF2; CeO2; PrOx; gold; surface; atomic structure; fluorite; electric potential
Issue Date: 9-Nov-2012
Type of publication: Dissertation oder Habilitation [doctoralThesis]
Appears in Collections:FB04 - E-Dissertationen

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