Growth and structure of complex oxide thin films

Complex oxide materials form a technologically promising class of materials due to the wealth of physical properties they comprise. Using these materials in heteroepitaxial thin lm form, introduces challenging obstacles concerning the growth and results often in structures with unexpected physical properties. The structural discontinuity across the interface between the substrate and lm has a large eect on the nucleation, growth and nal atomic stacking. This thesis describes a study aiming at clearifying the atomic structure during and after growth of several complex oxide thin lm heteroepitaxial systems

DESY NanoLab

The DESY NanoLab is a facility providing access to nano-characterization, nano-structuring and nano-synthesis techniques which are complementary to the advanced X-ray techniques available at DESY’s light sources. It comprises state-of-the art scanning probe microscopy and focused ion beam manufacturing, as well as surface sensitive spectroscopy techniques for chemical analysis. Specialized laboratory x-ray diffraction setups are available for a successful sample pre-characterization before the precious synchrotron beamtimes. Future upgrades will include as well characterization of magnetic properties

Role of hydroxylation for the atomic structure of a non-polar vicinal zinc oxide

From the catalytic, semiconducting, and optical properties of zinc oxide (ZnO) numerous potential applications emerge. For the physical and chemical properties of the surface, under-coordinated atoms often play an important role, necessitating systematic studies of their influence. Here we study the vicinal ZnO(10 1 \uaf 4) surface, rich in under-coordinated sites, using a combination of several experimental techniques and density functional theory calculations. We determine the atomic-scale structure and find the surface to be a stable, long-range ordered, non-polar facet of ZnO, with a high step-density and uniform termination. Contrary to an earlier suggested nano-faceting model, a bulk termination fits much better to our experimental observations. The surface is further stabilized by dissociatively adsorbed H2O on adjacent under-coordinated O- and Zn-atoms. The stabilized surface remains highly active for water dissociation through the remaining under-coordinated Zn-sites. Such a vicinal oxide surface is a prerequisite for future adsorption studies with atomically controlled local step and terrace geometry

Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction

The imaging of active nanoparticles represents a milestone in decoding heterogeneous catalysts dynamics. We report the facet resolved, surface strain state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent x-ray diffraction imaging under catalytic reaction conditions. Density functional theory calculations allow us to correlate the facet surface strain state to its reaction environment dependent chemical composition. We find that the initially Pt terminated nanoparticle surface gets Rh enriched under CO oxidation reaction conditions. The local composition is facet orientation dependent and the Rh enrichment is non-reversible under subsequent CO reduction. Tracking facet resolved strain and composition under operando conditions is crucial for a rational design of more efficient heterogeneous catalysts with tailored activity, selectivity and lifetime.Comment: 15 pages, 4 figures, 32 reference

Pulsed laser deposition chamber for in-situ X-ray diffraction

A sample chamber has been constructed for studying the growth of thin films by pulsed laser deposition in situ with surface X-ray diffraction. The achievable temperature ranges from room temperature to 1073 K in a controlled oxygen environment. The partial pressure of the oxygen background gas covers the range from 0.1 to 105 Pa. The first results, showing intensity oscillations in the diffracted signal during homoepitaxial deposition of SrTiO3, are presented

Polar-discontinuity-retaining A-site intermixing and vacancies at SrTiO3/LaAlO3SrTiO_{3}/LaAlO_{3} interfaces

Our anomalous surface x-ray diffraction study of the interface between SrTiO3(001) substrates and thin polarfilms of LaAlO3shows La/Sr interdiffusion leading to neutrally charged Sr1−1.5xLaxO layers. From a thickness oftwo unit cells, the LaAlO3film’s interior retains its polar stacking sequence, which is compensated by a cationiccompositional change of the substrate’s terminating TiO2layer. While such valence-retainingA-site intermixingdoes not resolve the polar catastrophe, it may be a general feature at oxide interfaces influencing the electronicand magnetic properties