Simon Schmitt

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Online Annual Report 2013

Scientific Highlights

Simple Production Method for Coveted Nanocrystals

The chemical element cerium belongs to the group of rare earth metals. Its oxide is widely used in industry in the nanocrystalline form, for example, for electrodes in fuel cells or for catalytic converters of motor vehicles where it converts toxic carbon monoxide into hydrocarbons. Last but not least, cerium oxide also serves as an abrasive or polish in the semiconductor industry. Dr. Christoph Hennig from the Helmholtz-Zentrum Dresden-Rossendorf and Dr. Atsushi Ikeda-Ohno from the Australian University of New South Wales were able to observe the growth mechanism for the first time ever.

In the past, it had been impossible to watch nanocrystalline cerium dioxide growing since the requisite analytical techniques were not available. When using electron microscopes or X‑ray diffractometers, for example, the nanocrystals had to be separated from the solution. Even though this permits the analysis of the particles, their creation remains hidden since this happens in the solution.

That is why the scientists combined diverse spectroscopic technologies such as dynamic light scattering, X‑ray absorption spectroscopy, and high energy X‑ray scattering. This permitted them to observe firsthand for the first time ever the creation of nanocrystalline cerium dioxide in an aqueous solution.

With these insights, it is possible to radically simplify the production process of the nanomaterial. The result: If the pH-value of tetravalent cerium is adjusted correctly in an aqueous solution, then this will create uniform particles of cerium dioxide. Any physical or chemical finishing treatment such as, for example, the addition of accelerator substances can be omitted.

The researchers also discovered that the cerium dioxide crystals, which are produced in such a simple manner, have a size of two to three nanometers; in fact, they are largely independent of the concrete environmental conditions. Thus, the nanoparticles are precisely in the range which is interesting for industrial products.

The scientists also consider it to be a key discovery that tetravalent cerium only forms cerium dioxide crystals in the nanometer range if it had existed in the solution either as a dimer or a primer.

  • Publication: Ikeda-Ohno, A.; et. al. (2013). Hydrolysis of Tetravalent Cerium for a Simple Route to Nanocrystalline Cerium Dioxide: An in-situ Spectroscopic Study of Nanocrystal Evolution. Chemistry – A European Journal, 19(23), 7348-7360 (DOI: 10.1002/chem.201204101)
  • Contact: Dr. Christoph Hennig, Institute of Resource Ecology

Radiotracer Allows the Detection of Tumour-associated Proteolytic Enzymes

A hallmark of malign solid tumours is their ability to form metastases in tissues distant from the original site. In the processes which enable tumour cells to invade the surrounding tissue a group of enzymes called cathepsins plays an important role. These enzymes are able to degrade proteins of the extracellular matrix by catalyzing the hydrolysis of their peptide bonds which paves the way for the tumour cells through the tissues. In addition, the cathepsins can also play a role in developing resistance to radiation and chemotherapy.

For the first time, researchers at the Helmholtz-Zentrum Dresden-Rossendorf were able to develop a radiotracer which allows the visualisation of these oncologically highly important proteases in vivo.

The research group headed by Dr. Reik Löser from the HZDR’s Institute of Radiopharmaceutical Cancer Research developed this imaging probe based on compounds which are capable of forming stable complexes with cysteine cathepsins by covalent attachment to their active sites. These compounds are referred to as azadipeptide nitriles, and the Rossendorf scientists functionalised one of these substance with the radionuclide fluorine‑18. With the help of positron emission tomography, it could thus become possible to assess the metastatic potential of tumours by molecular imaging of the cathepsins. In experiments with tumor‑bearing mice, Löser’s team was able to prove that the newly developed radiotracer is taken up by the neoplastic cells. As a result of this work, the precise diagnosis and characterisation of tumors which are indispensable for the successful fight against the disease have been taken another important step forward.

  • Publication: Löser, R.; et al. (2013). Synthesis and Radiopharmacological Characterisation of a Fluorine‑18‑Labelled Azadipeptide Nitrile as a Potential PET Tracer for in vivo Imaging of Cysteine Cathepsins. ChemMedChem, 8(8), 1330–1344 (DOI: 10.1002/cmdc.201300135)
  • Contact: Dr. Reik Löser, Institute of Radio­pharma­ceutical Cancer Research

New Insights into Particle-Loaded Bubble Columns

Ultraschnelle Röntgen-CT - ROFEX I
Ultrafast X‑ray tomography - ROFEX I
Photo: Frank Barthel, HZDR

With the help of an ultrafast X‑ray tomography, which had been developed at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), researchers headed by Dr. Markus Schubert from the Institute of Fluid Dynamics were able to explain the impact of catalyst particles on the flow patterns inside bubble column reactors. These devices, which resemble a large cylinder, are applied in the manufacturing process of many products, which are used in everyday life such as plastics or low sulfur fuels.

Gas is sparged into these liquid filled columns through many holes at the reactor bottom and then rises in the form of bubbles. This way, the gas is transported into the liquid and reaches the catalyst particles, which are often required for such chemical processes. The reaction between the gas and the liquid occurs at the surface of these particles. Thus, the gas‑liquid mass transfer determines the yield in the reactors and essentially depends on the actual flow.

So far, it has not yet been possible to completely explain, how precisely the catalyst particles affect the formation of gas bubbles and the flow patterns inside the columns. Researchers assumed that the particles primarily foster the formation of large bubbles through coalescence by bubbles collision, which would, though, not have been optimal for the processes. Because single large bubbles mean, in contrast to many small bubbles, that the bubble surface area for transporting the gas into the liquid is smaller.

In their investigations, however, the HZDR scientists were now able to demonstrate that the previous assumptions had only been partially correct. It is actually true that the catalyst particles have a decisive impact on the stability of the flow patterns inside the reactors – whether this has positive or negative consequences, though, depends primarily on their concentration. If the concentration is very low, then the particles attach to the small gas bubbles and stabilize them. Higher concentrations result in the formation of larger bubbles. If the concentrations increase even further, then the interactions between the particles and the bubbles, in turn, also increase, which causes the bubbles to break and, thus, intensifies the mass transfer inside the bubble columns.

These findings provide important starting points, which help to optimize manufacturing processes. And this could ultimately save not only money, but also a lot of energy.

  • Publication: Rabha, S.; Schubert, M.; & Hampel, U. (2013). Intrinsic Flow Behavior in a Slurry Bubble Column: A Study on the Effect of Particle Size. Chemical Engineering Science, 93 (DOI: 10.1016/j.ces.2013.02.034)
  • Contact: Dr. Markus Schubert, Institute of Fluid Dynamics

Novel High-Field Phase in Spinel Compound at High Magnetic Fields

Multiferroic materials, which exhibit concomitant magnetic and ferroelectric order, are of great interest both to fundamental and applied research. Such multiferroics could be used, for example, for spintronic applications since their dielectric and magnetic polarization can be influenced by external fields. This could be suitable for applications in new storage technologies.

But these materials also represent a great challenge for modern solid state physics. Among multiferroics, magnetic AB2X4 compounds with a spinel structure attracted considerable interest: They possess a significant spin-lattice coupling and magnetic frustration. Despite quenched orbital moments, these compounds reveal structural instabilities which are governed explicitly by the ordering of spins, e.g. giant magnetostriction, negative thermal expansion, and spin-Jahn-Teller instabilities.Messkurve Spinell-Verbindung

In 2013, combining ultrasound and magnetization measurements with high magnetic fields, researchers at the HZDR’s Dresden High Magnetic Field Laboratory (HLD) discovered some unusual and fascinating features in CoCr2O4. A novel magneto-structural high-field state has been revealed in this material above 42 Tesla unveiling a huge metastable region down to 0 Tesla.

While the magnetization evolves gradually with the field, step-like increments in the sound velocity (right figure) clearly signal a transition into a new high-field state (cf. the phase diagram in left figure). From the data analysis, the researchers ascribed the high-field phase to a high-symmetry magneto-structural state with longitudinal collinear spin arrangement while the transverse helical component remains disordered.

These investigations shed new light on the importance of magneto-elastic interactions in multiferroic materials and provide valuable information on the metastable character of some states. In addition to magnetic and electric fields, this could open new perspectives for mechanical stress to control different states in multiferroic materials.

  • Publication: Tsurkan, V.; et al. (2013). Unconventional Magnetostructural Transition in CoCr2O4 at High Magnetic Fields. Physical Review Letters, 110 (DOI: 10.1103/PhysRevLett.110.115502)
  • Contact: Dr. Sergei Zherlitsyn, Dresden High Magnetic Field Laboratory

Inverted Pyramids Induced by Ion Beam Irradiation of Semiconductor Surfaces

When the surfaces of solids are bombarded with low energy ions at room temperature, self-organizing structures are usually created in sizes that range below 100 nanometers. This results in the formation of wavelike or mound‑shaped patterns with symmetries that depend solely on the angle of incidence and the direction of the ion beam.

Researchers at the HZDR’s Ion Beam Center have observed recently a new, self-organized pattern formation mechanism. They bombarded the semiconductor germanium with argon ions at temperatures above the recrystallization temperature; that is, at more than 260 degrees Celsius. The scientists discovered that patterns with entirely new symmetries are now created on the crystalline surface: Patterns of inverted pyramids with a rectangular, triangular, or hexagonal base area are formed, depending on the surface orientation.

Continuous ion irradiation produces many vacancies in the surface by ion sputtering. These vacancies are trapped on the terraces where they are created. When they join, they form new pits. This creates a new surface instability which results in the formation of inverted pyramids.

Analogous to the formation of pyramids during epitaxial growth, this pattern formation is called “reverse epitaxy” as it removes atom by atom from the crystal.  Such a crystalline surface with non-equilibrium facets could be used, for example, to increase the efficiency of solar cells and of catalytic surfaces.

  • Publication: Ou, X.; et al. (2013). Reverse Epitaxy of Ge: Ordered and Faceted Surface Patterns. Physical Review Letters, 111 (DOI: 10.1103/PhysRevLett.111.016101)
  • Contact: Dr. Xin Ou, Institute of Ion Beam Physics and Materials Research