Research Topics

The electron microscopy group at Empa carries out independet research projects and is involved in various collaborative research projects. Some research topics are of methodogical character, aiming at advancing imaging and analysis methods in electron microscopy, others are of applied nature where advanced characterization methods are employed to advance the understanding of the structure-property relation of novel functional (nano-)materials.

 

Birth of Solids - Atomic-Scale Processes in Crystal Nucleation

For details, visit the CLUSTER project website.

This research topic is funded by an ERC consolidator grant.

Complex Functional Oxides

Complex functional oxides play a crucial role in many areas of advanced technologies. We are interested in investigating structural defects and their electronic impact on the functionality. In the case of photocatalytic water splitting, we have investigated the surface structure BiVO4 (BVO) and found that there is a distinct surface shell of reduced oxidation state which could impact the photocatalytica properties of BVO.  Another area where complex oxides find application is advanced electronic materials. We investigate novel lead-free multi-ferroic materials, which are grown epitaxially as thin layers on a well-chosen substrate, and analyze the impact of the epitaxial strain on the atomic and electronic structure of the material. We employ atomic resolution imaging, mostly in scanning transmission mode (STEM), combined with local electron energy-loss spectroscopy (EELS). The electronic structure information contained in the electron energy-loss spectrum is analyzed by comparing the experimental results with first-principle electronic structure calculations. This approach enables us to directly relate the atomic structure information, particularly at defects and interfaces, with electronic properties and thus to provide important information about the structure-property relation.

 

This research topic is funded by the Swiss National Science Foundation.

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Analysis of the electronic surface structure of BVO by STEM and EELS. The high-resolution STEM image shows the atomic structure of BVO. The vanadium L-edge spectrum is shifted at the surface revealing the presence of about 10-15% oxygen vacancies. Measuring the position of the vanadium L-peaks as a function of the distance towards the surface reveals a reduced surface shell of about 5 nm thickness. Density functional theory shows the impact of the reduced surface shell: a donor state (yellow) is present which impacts the photocatalytic properties of BVO. (Marta Rossell, Piyush Agrawal. For details: Rossell et al., Chem. Mater. 27 (2015) 3593.)
Novel semiconductor heterostructures

Novel technologies often require the implementation of novel materials or a new way of combining known materials. In this research topic, the electron microscopy group collaborates with other reseach groups on new semiconductor heterostructures. One sub-project concerns the development of an advanced X-ray detector based on Ge pillars grown on patterned silicon. Due to the large lattice mismatch between Si and Ge, the Ge is heavily strained due to the epitaxial growth. The strain is partially plastically relaxed by the formation of dislocations and the formation of other structural defects, like e.g. stacking faults. Characterization of these defects  is crucual for advancing the understanding of their occurance as a function of growth conditions and for developping the structure-property relation of the final device. 

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Dislocations in a strained Ge pillar which is grown on a patterned Si substrate. High resolution image of a small stacking fault in Ge which is formed by two partial dislocations marked by green arrows. (Yadira Arroyo, Marta Rossell. For details: Arroyo Rojas Dasilva et al., Appl. Phys. Lett. 107 (2015) 093501.)