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Bouwe Kuiper

Size effects in epitaxial oxide thin films

Promotion date: January 30.

Promotor: Guus Rijnders

Assistant Promotor: Gertjan Koster

Materials from the perovskite type oxides comprises a vast range of interesting physical properties, like high temperature superconductivity and room temperature ferromagnetism. Due to their similar crystal structures and their common oxygen backbone, these complex oxides can be prepared in heterostructures. In such oxide heterostructures several types of material can be combined in order to search for new functionalities or enhance material properties. These properties can be tuned by applying epitaxial strain.

Upon reducing the size of such strained complex oxide materials down to a few atomic layers, properties are often found to be altered with respect to their bulk counterparts. This thesis work was focused on reducing the size of complex oxide heterostructures and the effect on the material properties was studied. A novel method for creating self-organized epitaxial SrRuO3 nanowires was developed, grown by pulsed laser deposition. Thin film nanopatterns were created using self-organization and the growth behavior was modeled.

The effects of size reduction on material properties and structure were studied in two material systems: SrCuO2 and SrRuO3. In both cases a structural transition was expected to occur upon reducing the film thickness to below ~2 nm. This transition was confirmed experimentally for SrCuO2 by using X-ray photoelectron diffraction. In this work the transition temperature in SrRuO3thin films was enhanced to nearly bulk values by means of adding a SrTiO3 capping layer. This novel mechanism is expected to be valid for other material systems as well.

Was your research application driven?

The research is to be placed on the forefront of a broader set of research topics originating from industry. Size reduction and size effects are very relevant for the type of materials I investigated: complex oxide thin films. The general question is what happens if we act on the brink of building smaller and smaller structures and patterns, in order to gain even more functionality per square nanometer.

Our strategy started from well-defined templates, then growing structures by using self organization during pulsed laser deposition. We explored these mechanisms and were able to fabricate desired patterns and structures. Also we developed simulation models on an atomic level that helped explain some crucial features. So, my research was a nice combination of experimental work, simulations and theory building activities.

Were there some special moments you recall, occurring during your PhD period?

The fabrication method, using self-organization in producing nanowires, was never practiced before. We successfully made use of the intrinsic layering present in complex oxide perovskites. We were able to control the surface structure at the nanometer scale, giving us an excellent platform to start growing patterns really bottom-up, without using etching techniques to prepare the thin layers. This led to my first publication, a memorable moment indeed.

Also I cherish the last months of my PhD period. Here I was able to link the experimental work, on various topics, with fundamental insights. It put together loose pieces of work in a more unifying perspective, leading to a logical story. This was a grateful experience.

In what way did you develop personally, as a researcher and scientist?

In this period I gained experience on many levels involving fabrication techniques, like pulsed laser deposition, and various experimental methods. For example, I used X-ray diffraction as well as X-ray photoemission and X-ray photoelectron diffraction techniques. Some challenging collaborations were part of the project, for example with the University in Nijmegen and the ESRF in Grenoble where I was lucky to use the synchrotron. In collaboration with the University of California, Irvine, I was able to perform high resolution magnetic measurements.

I gained confidence fulfilling complex projects, as the PhD work is a very diverse and complex piece of work indeed. Sometimes it is hard to have a clear overview and to keep setting clear goals along the way. Supervising the work of master and bachelor students was a very rewarding aspect of the PhD work as well. Noticing how they gain experience and knowledge along the way was very rewarding. In the end they came up with new ideas themselves, helping me greatly on some topics.

An instructive process was the migration of the COMAT vacuum system of our lab to the new building, here at the Nanolab. A lot of consultation, planning and technical work was involved in this complex and altogether practical project. We were able to rebuild our system, leading to good new results even.

What are your future plans?

I would like to work in industry, for example in research or development. Using the knowledge and experience gained during my study and PhD, I would like to address problems and challenges which are much closer to applications. I guess good chances are there to find this at the R&D and product design at a spin-off company that originated from the University of Twente.