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ResearchZernike (ZIAM)Nanostructures of Functional OxidesNanostructures of Functional Oxides


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The focus of our research is on ferroic materials (ferroelectrics, ferromagnets and multiferroics), as well as nanostructures based on ferroic patterns or other type of self-assembled patterns, such as nanodomains. Our aim is to gain access to the materials nanoscale by manipulating the growth of thin film oxides at atomic level. By growing structures atomic-layer by atomic-layer and tuning the strain, one can create novel materials with compositions and symmetries deliberately tailored for specific applications. This control also allows us to investigate the properties of domain walls and their distinct functionality (conductivity, polarity, magnetism).
Although our research is fundamental in nature, we are inspired by two main application areas that we believe will enable the next technological revolution: Piezoelectric Energy Harvesting for low power electronics and Novel materials for Neuromorphic Computing.

Fig. 1
Fig. 1
- “A rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films” by Y. Wei et al.

Nature Materials (22 October 2018) DOI: 10.1038/s41563-018-0196-0

Reducing the size of ferroelectric materials has been a research topic for more than 20 years. For ferroelectric materials to have any tangible application in microelectronics, ferroelectricity should be stabilized at nanoscopic sizes. Such nanosized-ferroelectrics became a reality with HfO2 based materials. This is also unconventional ferroelectricity because it becomes robust with decreasing size. In this work, we investigated the origins of this new-type of ferroelecticity. In the process we discovered a new polar rhombohedral phase in hafnia-based systems, that can be stabilized by large compressive strain from the substrates and the particle pressure due to nanoscopic sizes . These results also point towards arriving at guidelines to engineer this new-type of ferroelectricity in other types of simple oxides, potentially generating a vastly unexplored class of nanoscale ferroelectrics.

Fig. 2
Fig. 2
- “Ferroelectric BaTiO3 thin films under low strain” by A.S. Everhardt et al.

Advanced Electronic Materials (18 November 2015) DOI: 10.1002/aelm.201500214

In this paper, low-strain ferroelectric BaTiO3 films are experimentally realized for the first time, showing a fantastic agreement with the theoretical predictions of Khoukhar, Pertsev and Waser (2000).

Fig. 3
Fig. 3
- Novel phases at domain walls” by S . Farokhipoor et al.

Nature  (20 November 2014) DOI: 10.1038/nature13918

Due to large local stresses, domain walls can promote the formation of new phases and function in some sense as nanoscale chemical reactors. We have synthesized a novel 2D ferromagnetic phase at the domain walls of the orthorhombic perovskite TbMnO3, which was grown in thin layers under epitaxial strain on SrTiO3 substrates. This phase has never been observed before and cannot be made by other chemical routes. The density of the domain walls can be tuned by changing the epitaxial strain. In this way, the distance between ferromagnetic sheets can be made as small as 5 nanometres in ultrathin films, such that the new phase at domain walls represents up to 25% of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases is applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.

Commented by Philippe Ghosez and Jean-Marc Triscone in News & Views (Nature): Reactive walls

Explained for a general reader by René Fransen: Science Linx- RUG
Press releases: University of Groningen

Fig. 4
Fig. 4
- By Sylvia Matzen, Oleksiy Nesterov et al.

Nature Communications (14 July 2014) DOI: 10.1038/ncomms5415

With shrinking device sizes, controlling domain formation in nano ferroelectrics becomes crucial. Periodic nano domains that self-organize into ‘superdomains’ have been recently observed, mainly at crystal edges or in laterally confined nano objects. Here we show that in extended, strain-engineered thin films, superdomains with purely in-plane polarization form to mimic the single-domain ground state, a new insight that allows a priori design of these hierarchical domain architectures. Importantly, superdomains behave like strain- neutral entities whose resultant polarization can be reversibly switched by 90°, offering promising perspectives for novel device geometries

Last modified:14 March 2019 09.19 a.m.