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The deduction of structure-property relationships is the key challenge for advanced electron microscopy methods. In the following, you will find as examples a number of challenging applications performed in the framework of larger projects.

Strain Measurements at GaAs Mesa Structure with a buried AlOx Current Aperture

Left: TEM cross-section of a AlOx-stressor structure. Right: experimental and simulated in-plane component of the exhibiting strain field.

An interesting derivative of off-axis electron holography is dark-field electron holography for measuring strain fields as developed by Hytch [1]. Here, a diffracted beam of an unstrained crystalline reference area is brought to interference with the corresponding reflection stemming from a strained area. The resulting phase gradient is a measure for local variations of geometrical phase. By repetition of the experiment with a second noncollinear reflection, the full 2D strain field can easily be evaluated by simple matrix algebra. This method is used at an example of a GaAs mesa structure with a buried AlOx current aperture where the in-plane tensile strain promotes the aimed nucleation of InAs quantum dots in the middle of the aperture [2]. Quantitative comparison with corresponding calculations (Andrei Schliwa) based on linear elasticity theory clearly shows that the strain relaxation in a TEM lamella even at a thickness of a few 100 nm has to be considered in the simulation. [3]

[1] M. Hytch et al., Nature 453 (2008) 1086.
[2] F. Kießling et al., Phys Rev. B 91 (2015) 075306.
[3] The financial support through DFG CRC 787 project A4 is gratefully acknowledged.

Virtual GaN Substrates

Aberration-corrected HRTEM micrograph of buffer structure.

In close collaboration with the IHP in Frankfurt/Oder (workgroup of Prof. Schroeder), we investigate the atomic structure of virtual GaN substrates on Si(111) via Sc2O3/Y2O3 buffers. Of special interest are extended defects in the buffer and in the GaN, the interface between Sc2O3 and the GaN, and the resulting polarity of the GaN film [1,2].

[1] Tarnawska et al., J. Appl. Phys. 113 (2013) 213507.
[2] Niermann et al., J. Appl. Phys. 113 (2013) 223501.

Heterogeneous Catalysis

Transmission electron microscopy (TEM) allows ex-situ and in-situ characterization of nanostructured materials with (sub-)nanometer resolution. Through members of the central facility for electron microscopy (ZELMI), as a support we provide structural and elemental characterization of novel catalytic materials as developed within the framework of the Center of Excellence "Unifying Concepts in Catalysis" (UniCat) [4]. Out of the high number of electron microscopy investigations, the following topics of UniCat collaborations are highlighted:

In the group of Prof. Schomäcker, a new method for the synthesis of catalytically active and stable Pt nanoparticles by thermodestabilization of microemulsions has been developed. In order to produce more active sites, the shape of the Pt particles was changed to an anisotropic structure. By the combined efforts of FHI and us, these investigations were strongly supported by various TEM methods showing the shape of the nanoparticles is a key factor for catalytic activity. [1]

In the group of Prof. Strasser, the morphology, crystallinity, and chemical state of well-defined Ir oxide nanoscale thin-film catalysts on Ti substrates and their activity in the electrocatalytic oxygen evolution reaction (OER) are investigated by an intense using of electron microscopy methods. As one of the results it is found that low-temperature amorphous Ir oxy-hydroxides are highly active and efficient catalysts for the OER. [2]

In the group of Prof. Lerch, a new crystal structure of Rutile-type ScTa2O5N was synthesized as a promising material for photocatalytic water splitting. Imaging by SEM and TEM allowed revealing the nature of the nanoscaled structure showing pores in the single-crystalline particles. Furthermore, high-resolution imaging and corresponding evaluation of Fourier spectra exhibit a good agreement with the proposed crystallographic structure. [3]

[1] Parapat, R., Wijaya, M., Schwarze, M., Selve, S., Willinger, M., Schomäcker, R., Particle shape optimization by changing from an isotropic to an anisotropic nanostructure: preparation of highly active and stable supported Pt catalysts in microemulsions. Nanoscale, 2013, 5, 796-805; DOI: 10.1039/c2nr32122j
[2] Reier, T., Teschner, D., Lunkenbein, T., Bergmann, A., Selve, S., Krähnert, R., Schlögl, R., Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. J. Electrochem. Soc., 2014, 161, F876-F882. DOI: 10.1149/2.0411409jes
[3] Cosgun, S., Rohloff, M., Göbel, C., Gernert, U., Fischer, A. and Lerch, M., Synthesis and Crystal Structure of Rutile-type ScTa2O5N. Z. anorg. allg. Chem., 2014, 640, 2771–2775. DOI: 10.1002/zaac.201400325
[4] The financial support by the DFG Center of Excellence UniCat is gratefully acknowledged.



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