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Galerie der Bilder von 2022

Bild des Monats Januar 2022

Plasma crystals in Einstein's elevator

Microparticles (red dots) inside the "PKE-Nefedov" plasma chamber pick up electrons from the plasma (purple glow) and interact electrostatically with each other forming a regular structure, a so-called plasma crystal (large image). Here we can study the behaviour of a crystalline structure using a simple video camera. For optimum conditions we do this in weightlessness (microgravity), e.g. onboard the international space station ISS or in a drop tower. In September 2021 our group visited the "Einstein Elevator" at the University of Hannover, a new facility that allows to drop the setup from 20m height (see scheme on the left side), thus creating 2 seconds of microgravity during the free fall (according to Einstein's Gedankenexperiment). The video data of more than 50 drops are now under investigation for a PhD thesis. Inset: The group members of the Einstein Elevator (left) and JLU (right) with the plasma chamber in front of the elevator's gondola.

Image credits: C. Lotz, HITec, Leibniz University Hannover; M. Kretschmer (JLU, Institute of Experimental Physics I). WWW: einstein-elevator.de

Dieses Bild wurde eingereicht von Dr. Michael Kretschmer (AG Prof. Dr. Markus Thoma).


Weitere Einblicke in die Arbeiten der am ZfM beteiligten Arbeitsgruppen finden sie in der Galerie der Bilder des Monats.

Bild des Monats Februar 2022

Li(Ni,Co,Mn)O2-Dünnfilmkathoden als Modellsysteme

Li(Ni,Co,Mn)O2-basierte Kathoden leiden meist unter parasitären Nebenreaktionen in Kontakt mit dem Elektrolyten, wodurch die Leistungsfähigkeit von Lithiumionenbatterien reduziert wird. Eine Möglichkeit, die Kathodenoberfläche vor dem Elektrolyten zu schützen, ist das Aufbringen einer wenige Nanometer dünnen Beschichtung. Um den Einfluss einer Schutzschicht auf die elektrochemischen Eigenschaften der Kathode besser zu verstehen, wurden Li(Ni,Co,Mn)O2-basierte Dünnfilme hergestellt. Elektrochemische Charakterisierungen zeigen den positiven Effekt der Beschichtung auf die Zyklisierbarkeit der Dünnschichtkathoden. Die Dünnschichten erleichtern hierbei die Untersuchung möglicher Reaktionsprodukte und struktureller Änderungen aufgrund des Zyklisierens mit Hilfe von oberflächensensitiven Methoden, wie z.B. der atomaren Rasterkraftmikroskopie (AFM) oder Röntgenphotoelektronenspektroskopie (XPS). Weiter Informationen finden sich unter: H. Hemmelmann, J. Dinter & M.T. Elm, Advanced Materials Interfaces 8, 2002074 (2021), https://doi.org/10.1002/admi.202002074

Dieses Bild wurde eingereicht von Dr. Matthias Elm.


Weitere Einblicke in die Arbeiten der am ZfM beteiligten Arbeitsgruppen finden sie in der Galerie der Bilder des Monats.

Bild des Monats März 2022

Resistivity calculation using MD-KKR approach

The ab initio calculation of the temperature dependent electrical resistivity of materials still represents a great challenge. Methods such as the lowest order variational approximation to the Boltzmann transport equation are widely used, while the Keldysch formalism has been employed in our workgroup. (https://doi.org/10.1103/PhysRevB.96.165121) However, both of these formalisms require a prior calculation of the electron-phonon interaction self-energy. To eliminate this need, the MD-KKR formalism has recently been devised. A MD simulation of the material in question (here: copper) is carried out to obtain screenshots of a material at a certain temperature. These screenshots are used as input structures for calculating the transmission with the KKR code. From the tranmission, temperature dependent resistivity and electron-phonon self-energies are easily obtainable.
Also $k_\parallel$ dependent transmissions are obtained in this way and are displayed for different temperatures. At 0 K this transmission is 1 if states exist at the Fermi energy for this $k_\parallel$ vector and 0 otherwise. With increasing temperature the transmission of these states decreases due to the electron-phonon scattering effects within the material.

Dieses Bild wurde eingereicht von Prof. Dr. Christian Heiliger.


Weitere Einblicke in die Arbeiten der am ZfM beteiligten Arbeitsgruppen finden sie in der Galerie der Bilder des Monats.

Bild des Monats April 2022

The liquid-phase exfoliation of molybdenum trioxide for the production of 2D nanosheets colloidal inks

The interest in layered 2D nanomaterials has witnessed an impressive growth in the last years, bringing to the discovery of many new species and methods for their preparation. The liquid-phase exfoliation (LPE) of crystalline bulk powders is certainly the most suitable method for scaled-up production, allowing also the convenient access to solution processing techniques for the direct utilization of the produced 2D material colloidal inks. Given the large number of reports on LPE processes for different 2D materials, today, it is necessary to specifically define the results of similar investigations, so as to provide the scientific community with clear guidelines for identifying design rules and applying standardized procedures.

In our work, we describe a systematic study on the LPE process for α-MoO3, a stable high band gap semiconductor, which in its 2D form has been employed for many purposes, ranging from catalysis to energy/optoelectronic devices and sensing. We investigate the effect of different low-toxicity solvents and instruments for its LPE and provide new insights into the structural and electronic properties of the resulting 2D nano-inks in a joint experimental–computational effort, which will represent a solid source of information for the future implementation of liquid-dispersed layered α-MoO3 nanosheets in different fields. In particular, we resorted to X-ray absorption spectroscopy (XAS), carried out at the Synchrotron Soleil facilities in France, to probe the atomic distances in the bulk and LPE materials, evidencing the occurrence of a strain in the latter which was never described before. For this work, we collaborated with Prof. Silvia Gross and Prof. Stefano Agnoli from the University of Padua (Italy) and with Dr. Silvio Osella from the University of Warsaw (Poland).

A related publication can be found at https://doi.org/10.1021/acs.jpcc.1c09221

Dieses Bild wurde eingereicht von Dr. Teresa Gatti.


Weitere Einblicke in die Arbeiten der am ZfM beteiligten Arbeitsgruppen finden sie in der Galerie der Bilder des Monats.

Bild des Monats Mai 2022

1,2-Carboboration of allenes: A new way to synthesize 1,4-dienes

The direct carboboration of a double or triple bond, simultaneously forming a new C−C and C−B bond, is an attractive method for the synthesis of organboranes, important intermediates for pharmaceuticals. However, direct carboboration reactions are usually limited to specific Lewis acidic boranes.  We reported now found a more general way for synthesizing Lewis acidic alkenylboranes, which readily undergo carboboration reactions (https://doi.org/10.1002/chem.202200470). These boranes transfer under mild conditions the alkenyl group in a regioselective 1,2-carboboration to arylallenes. The products of this reaction are well suited for the palladium-catalyzed synthesis of aryl-substituted 1,4-dienes. The picture shows crystals of one of the 1,4-dienes and the molecular structure that was derived from single-crystal X-ray diffraction.

Dieses Bild wurde eingereicht von Dr. Urs Gellrich.


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Bild des Monats Juni 2022

Conformable metal oxide platelets – A smart surface armor for green tribology

Laser induced surface modification is usually considered as a promising route to improve the tribological properties, i.e., friction and wear, of metallic surfaces. Recently, a new approach was explored as a collaboration between the Universities of Marburg and Giessen. Using different irradiation parameters, a range of samples was created by nanosecond laser pulses (a), where subsequent melting and cooling resulted in hierarchical surface structures of metal oxide platelets based on vertical nanotubes (b). Tribological tests using a homebuilt tribometer (c) then revealed how these surfaces layers can prevent friction and wear, with typical friction reduction by about 50-70%. In particular the unique resilience of the flexible surface layers against mechanical stress makes them attractive for potential technological applications.

Reinhardt et al., Tribology International 162, 137108 (2021). DOI: 10.1016/j.triboint.2021.107138.

Dieses Bild wurde eingereicht von Dr. Dirk Dietzel.


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Bild des Monats Juli 2022

Hierarchical magnetic materials: A novel strategy for tuning the characteristics of a material by arranging magnetic nanoparticles into mesocrystals

The arrangement of nanometer-sized ferromagnetic entities of well-defined size, shape, and orientation into highly ordered, periodic structures introduces additional degrees of freedom to tune the macroscopic magnetic response of the sample. In such so-called mesocrystals, the response can not only be manipulated on the macroscopic or the atomic scale, but also on the mesoscopic scale. For example, a manipulation of the mutual interaction between the nanoparticles can be achieved by altering their size, shape, and arrangement. Furthermore, the magnetic response depends on the shape of nanoparticle arrangements. This tunability is reflected by the occurring resonant magnetic excitations, which can be studied by ferromagnetic resonance spectroscopy and Brillouin light scattering microscopy.

We study 2D mesocrystals built of spherical magnetite nanoparticles of diameter dM regularly arranged with spacings dS. Depending on the external conditions, such as the external driving microwave field of frequency fext and the applied static magnetic field Bext, multiple resonant excitations occur in the spectra of these 2D mesocrystals of defined shape. Often, the main signal is accompanied by a satellite signal. Typically, the main signal originates from the center of the arrangement, whereas the active areas of the satellite signal are located near its rim.

The manipulation of the interparticle interaction within the mesocrystal, e.g. by varying the spacing dS between the nanoparticles, is reflected in the variation of the resonance positions and linewidths of the excitations. A precise control of the spacing can be achieved by varying the surfactant shell which surrounds the nanoparticles. As the occurrence of multiple signals is crucially affected by the interparticle coupling strength, the relative signal intensities and line widths can be tuned specifically.

Such magnetic mesocrystals are not only ideal model systems for studying the interplay of magnetic interactions, but are also of interest as building blocks of miniaturized devices in the field of magnetotronics.

Dieses Bild wurde eingereicht von Nils Neugebauer, AG Prof. Klar.


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Bild des Monats September 2022

Correlating the nonlinear optical activity with electronic and structural properties of molecular clusters

Bild des Monats September

Special molecular clusters based on the (hetero)adamantane structure were shown to emit white light upon irradiation with a continuous wave infrared laser (doi: 10.1002/cptc.202100130). This strongly nonlinear optical effect and its origin can be investigated by atomistic calculations, e.g., in the framework of the density functional theory. These demonstrate that the magnitude of the spatial overlap between occupied and unoccupied orbitals separated by a given energy correlates with the intensity of the optical activity at that energy. This allows to estimate the contribution of specific sub-structures to the optical response. The knowledge of the intertwinement between structural and optical properties can be then used to tailor materials optimized for specific applications.

Dieses Bild wurde eingereicht von Prof. Dr. Simone Sanna.


Weitere Einblicke in die Arbeiten der am ZfM beteiligten Arbeitsgruppen finden sie in der Galerie der Bilder des Monats.