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Artikelaktionen

Galerie der Bilder von 2018

Bild des Monats Januar 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

Investigating the lattice dynamics in ionic conductorsBild des Monats Januar

Chemical intuition has always led to the believe that a softer, more polarizable anion sub-lattice of cation conductors leads to lower activation barriers for diffusion and subsequently higher ionic conductivities. This intuitive approach has been supported by the high conductivity of cations in iodine compounds compared to sulfides and oxides. However, until today, the chemical concept of increasing polarizability and softness of the lattice has never been experimentally proven within one materials class.

The group investigates this influence of a soft, dynamic lattice on the ionic transport in order to tailor the ionic conductivities of solid ion conductors for solid state batteries. We show that the paradigm “the softer, the better” is not entirely correct. While indeed a softer lattice changes the ion dynamics across activation barrier (see figure) the entropy of migration is also decreased. Our work shows that it is necessary to reinvestigate the theoretical descriptors for ionic conduction in the solid state.

Publications:

[1] Kraft M.A. et al. “Influence of lattice polarizability on the ionic conductivity in the lithium superionic argyrodites Li6PS5X (X = Cl, Br, I)” J. Am. Chem. Soc. 2017, 139, 10909-10918. doi: https://doi.org/10.1021/jacs.7b06327

[2] Krauskopf T. et al. “Influence of lattice dynamics on Na+ transport in the solid electrolyte Na3PS4-xSexChem. Mater. 2017, 29, 8859-8869. doi: https://doi.org/10.1021/acs.chemmater.7b03474

Dieses Bild wurde eingereicht von Dr. Wolfgang Zeier.

Bild des Monats Februar 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

Wide bandgap III-V nanowires for photoelectrochemical applicationsBild des Monats Februar

GaN based nanowire structures are widely discussed for applications such as photocatalytic water splitting or pH-sensing. Our project was to investigate such structures by electrical bias dependent photoluminescence, photocurrent and electron paramagnetic resonance spin trapping experiments. For InGaN nanowires the results are in agreement to an established band bending model while for GaN nanowires oxidation and etching processes have to be taken into account. For GaNP the bias dependence of the carrier transfer across the semiconductor interface to the electrolyte is not reflected in the photoluminescence response, thus this material system suitable for water splitting purposes. InGaN and GaN nanowires are in addition good candidates for sensing applications.

Reference: J.M. Philipps, J. E. Stehr, D. M.Hofmann, I. A. Buyanova, and M. Eickhoff, Applied Physics Letters 110, 222101 (2017), doi: 10.1063/1.4984277

Dieses Bild wurde eingereicht von Prof. Dr. Detlev Hofmann.

Bild des Monats März 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

Shape-Controlled CeO2 Nanoparticles: Stability and Activity in the Catalyzed HCl Oxidation ReactionBild des Monats März

CeO2 is a promising catalyst for the HCl oxidation (Deacon process) in order to recover Cl2. Employing shape-controlled CeO2 nanoparticles (cubes, octahedrons, rods) exposing facets with preferential orientations ((100), (111), (110)), we studied the activity and stability under two reaction conditions (harsh: Ar:HCl:O2= 6:2:2 and mild: Ar:HCl:O2=7:1:2). It turns out that both activity and stability are structure-sensitive and are not reconciled with corresponding surface energies and the formation energies of O-vacancies. The apparent activation energies are about 50 kJ/mol for cubes and rods, while the octahedrons reveal an apparent activation energy of 65 kJ/mol. The reaction order in O2 is positive (0.26-0.32). Under mild reaction conditions all three morphologies are stable, consistent with corresponding studies of CeO2 powders and CeO2 nanofibers. Under harsh reaction conditions, however, cubes and octahedrons are both instable, forming CeCl3, while rods are still stable. The present stability and activity experiments in the catalytic HCl oxidation reaction over shape-controlled CeO2 nanoparticles may serve as benchmarks for future ab-initio studies of the catalyzed HCl oxidation reaction over well-defined CeO2 surfaces.

Recent Publication on this Topic:
C. Li, Y. Sun, I. Djerdj, P. Vöpel, C. Sack, T. Weller, R. Ellinghaus, J. Sann, Y. Guo, B.M. Smarsly, H. Over, Shape-controlled CeO2 Nanoparticles: Stability and Activity in the Catalyzed HCl Oxidation Reaction, ACS Catal. 7 (2017) 6453-6463

Dieses Bild wurde eingereicht von Prof. Dr. Herbert Over.

Bild des Monats April 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

Ab initio characterisation of skyrmion induced effects in MTJsBild des Monats April

Magnetic skyrmions are swirl like spin structures in an otherwise purely collinear environment.
When it comes to technological applications of skyrmions, the main functionalities one has to provide in order to build practical devices include the creation and deletion, transport, and reading of skyrmions.
The main focus of our research is the reading of skyrmions via magnetic tunnel junctions (MTJs) in which two conducting leads are divided by a thin insulating layer.
We calculate the transmission function which is influenced by several effects such as standard TMR but also non-collinear effects which arise from the change of the magnetization direction from atom to atom for various configurations.
As our tool of choice we use the Keldysh-formalism operating with the non-equilibrium Green‘s function (NEGF) implemented in our KKR code in order to get an accurate description of the energy dependent transmission through the tunneling barrier.

Dieses Bild wurde eingereicht von Jonas F. Schäfer, AG Prof. Dr. Christian Heiliger.

Bild des Monats Mai 2018

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Stabilizing the electrochemical deposition and dissolution for zinc anodesBild des Monats Mai

Zinc-based secondary batteries, especially zinc-air batteries, are promising candidates as energy storage device for electronic applications. They appeal due to their high specific energy density, environment friendliness and safe operation. However, the zinc anode suffers from limited cycling stability arising from the high solubility of oxidized zinc species in the alkaline electrolyte. This results in the so called shape change of the electrode and in loss of active material during cycling.

In the BMBF-project Zisabi (in cooperation with Prof. Abe, Kyoto University, Japan) we investigate the influence of a homogeneous coating with an anion-exchange ionomer on top of the zinc anode to confine the oxidized zinc species. In a recent publication we demonstrated that our concept (see schematic drawing) helps to confine oxidized zinc species as zinc oxide interlayer, evidenced by electrochemical analysis, morphology analysis via scanning electron microscopy and X-ray photoelectron spectroscopy depth-profiling. The coating applied keeps the active material as close as possible to its place of origin, reduces the electrode shape change and stabilizes the electrochemical deposition and dissolution processes at the zinc anode.

In a collaboration with the group of Prof. Dr. Derck Schlettwein, we are now working on the transfer of this concept to an applied zinc anode using a multitude of electronic conductive host structures that are covered with active material (shown in the scanning electron micrograph).

Publication: Stock D. et al., “Homogeneous Coating with an Anion-Exchange Ionomer Improves the Cycling Stability of Secondary Batteries with Zinc Anodes”, ACS Appl. Mater. Interfaces. 10 (2018) 8640–8648. doi: https://doi.org/10.1021/acsami.7b18623

Dieses Bild wurde eingereicht von Daniel Stock, AG Dr. Daniel Schröder.

Bild des Monats Juni 2018

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Friction force microscopy as a new tool to analyze phase transitionsBild des Monats Juni

In recent years, a number of papers from the Institute of Applied Physics of the Justus-Liebig University Giessen have shown that friction force microscopy comprises a highly sensitive tool to explore internal relaxation modes for materials like polymers or self-assembled monolayers. Now, this experimental approach could be transferred to the general analysis of phase transitions. More specifically, phase transitions, which are usually accompanied by mechanical anomalies, can be pinpointed by detecting the resulting frictional changes. On contrary to previous approaches, experiments are not performed as a series of fixed temperature measurements, but instead the lateral forces are constantly monitored during a continuous change of the sample temperature (see Fig.).

As a model system, the layered charge density wave material 1T-TaS2 was analyzed and it was found, that e.g. the first order phase transition between the NC-CDW and the C-CDW phase was accompanied by distinct friction peaks. This behavior can be explained by a theoretical model, where small mechanical perturbations by the AFM tip are assumed to preemptively trigger the spinodal transformation, if the free energy barrier preventing this transformation becomes small enough. This new experimental technique thereby opens up fascinating perspectives to probe phase transitions, while at the same time mechanically interacting with the materials.

Publication: Panizon et al., New Journal of Physics 20, 023033 (2018), doi: https://doi.org/10.1088/1367-2630/aaac00

Dieses Bild wurde eingereicht von Dr. Dirk Dietzel, AG Prof. Schirmeisen.

Bild des Monats Juli 2018

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Photosynthesis of novel organic π-conjugated materials extending the common benzene theme

Bild des Monats Juli

The reaction is based on the methodology developed in the group of H. A. Wegner using bidentate Lewis acids to catalyse inverse electron-demand Diels-Alder reactions.

The picture was taken by Sebastian Ahles.
Julia Ruhl is the co-worker on the project.

Dieses Bild wurde eingereicht von Prof. Dr. Hermann A. Wegner.

Bild des Monats August 2018

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Light-induced interaction between a gold nanoparticle and graphene

Bild des Monats August

At first sight, we conducted a rather simple experiment. We put a gold nanoparticle on a graphene sheet and probed the optical response of this hybrid system by scanning a laser beam across it.

However, the Raman scattered light from the graphene probed by the laser yields a surprising amount of information about the microscopic interaction between the gold nanoparticle and its graphene environment in terms of temperature, strain and hot carriers excited by the laser within the gold nanoparticle by plasmonic absorption and transferred into the graphene. By evaluating the frequency of the two most prominent features of graphene’s Raman spectrum, the G- and 2D-Mode, in each spectrum of the 2D-scan and plotting one frequency against the other, we are able to distinguish the three contributions due to their characteristic influence on the frequency of the particular Raman mode.

Such graphene-gold hybrids may be employed in light harvesting and (opto-) electronic devices. The size and shape dependence of the plasmonic properties offers additional degrees of freedom to tune the optical characteristics of such devices.

Dieses Bild wurde eingereicht von Prof. Dr. Peter J. Klar.

Bild des Monats September 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

In-plane stress development in mesoporous thin films

Bild des Monats September

The research area of mesoporous thin films became more popular in the last decade as these films enhance the material properties for several applications in (photo‑)catalysis, energy storage, photo-/electrochromics, thermal insulators or sensors. As there is still a lack of fundamental understanding the relationship between the pore structure and the mechanical properties of these mesoporous thin films, ordered mesoporous thin films of TiO2 and CexZr1-xO2 (x = 0, 0.5, 1) were prepared via an evaporation-induced self-assembly process (EISA) and subsequently investigated in terms of the developing intrinsic and residual in-plane stress. Within the cooperation with Kansai University (Osaka, Japan), these mechanical properties were determined by the curvature method which is based on the determination of the deflection of light due to concave or convex bending of the films on a substrate. This bending derives mainly from the difference of the thermal expansion coefficient α of the substrate and the film. Dense and mesoporous films were investigated in regards to the intrinsic stress during the heat treatment up to 500 °C yielding lower stress values for the PIB50-b-PEO45 templated film. A comparable behavior was observed for the residual stress at room temperature for several annealing temperatures indicating the distributed polymer and the corresponding mesopores act as relaxing agents for the system which was further verified by mesoporous CexZr1-xO2 (x = 0, 0.5, 1) thin films. Our work reveals the increase in the residual in-plane stress during the pore collapse which lays the foundation for further understanding the stress-related mechanical properties of mesoporous thin films.

Recent publication:
P. Cop, S. Kitano, K. Niinuma, B. Smarsly, H. Kozuka, Nanoscale, 2018, 15, 7002–7015.
doi: 10.1039/C8NR00793D

Dieses Bild wurde eingereicht von Pascal Cop, Arbeitsgruppe Prof. Dr. Bernd Smarsly.

Bild des Monats Oktober 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

Microstructuring of VO2 for improved transmittance properties

Bild des Monats Oktober

Vanadiumdioxide (VO2) is a material which changes its transmittance properties due to a temperature induced phase metal-insulator transition (MIT). The transmittance for infrared light in the monoclinic low temperature phase is much higher then in the tetragonal high temperature phase, thus VO2 is predestined for use as a smart window coating for an improved climate control in buildings.
To gain a better optical impression and a higher transmittance in the visible range, one attempts to reduce the VO2 covered area (AC) by microstructuring but on the other hand to maintain the switching effect.
Firstly, the VO2 thin films were deposited by radio frequency sputtering (RF sputtering), then photolithography was carried out in the MiNaLab. As last step ion beam etching (IBE) was used for structuring the samples to obtain different patterns in the VO2 thin film.
Producing different types of structures and using various etching parameters will be the next steps to gain the best relation between enhancing the transmittance in visible light and still a good switching efficiency for the MIT in the infrared range.

Dieses Bild wurde eingereicht von Hannes Giese, Arbeitsgruppe PD Dr. Angelika Polity.

Bild des Monats November 2018

Hier finden Sie wechselnde Einblicke in die Arbeiten der am LaMa beteiligten Arbeitsgruppen. Eine Sammlung aller bisher erschienenen Bilder finden sie in der Galerie der Bilder des Monats.

Sodium storage behavior in graphite

Bild des Monats November

The development of energy storage systems has been a main focus of research in recent decades. Sodium-ion batteries (SIBs) have attracted great attention by offering more cost-efficient energy stores, which can be by achieved by replacing lithium by earth abundant and low cost sodium. One of the main drawbacks of SIBs remains the absence of thermodynamically stable sodium-rich graphite intercalation compounds (GICs). In the same time, rich binary GICs exist for other alkali metals, e.g. for lithium (LiC6), potassium (KC8), rubidium (RbC8) and cesium (CsC8). The contrast of Na intercalation behavior in graphite compared to other alkali metals raises a question “What is the origin of the instability of sodium-graphite intercalation compounds?” For this purpose, we examined the formation of Na-GICs by density functional theory (DFT) calculations and distinguish between the binding contributions and the structural deformation contributions.

Recently, a new approach of utilizing graphite as a negative electrode in SIBs was discovered. In fact, graphite is capable to intercalate solvated sodium ions, e.g. from ether-based electrolytes. Moreover, despite the large volume expansion of graphite, the co-intercalation reaction shows the excellent reversibility. With the group of Prof. Dr. Philipp Adelhelm from the Friedrich-Schiller-University in Jena we investigate the intercalation of solvated sodium ions into graphite. The main goal of our research is to gain a better understanding how the Na ions and solvent molecules are stored during the Na+-solvent co-intercalation into graphite, and to analyze the structure and thermodynamic stability of the resulting ternary GICs.

Dieses Bild wurde eingereicht von Dr. Olena Lenchuk, Arbeitsgruppe Prof. Dr. Doreen Mollenhauer.

Bild des Monats Dezember 2018

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Electrochemical Deposition of Hierarchical ZnO Nanostructures

Bild des Monats DezemberWe study the electrochemical deposition of ZnO thin films in various structures onto conductive substrates [1] and they can be used as electrodes in photoelectrochemical devices or batteries [2, 3]. A well-functioning electrode requires a fast charge transport, good accessibility of its surface for e.g. liquid solutions and a high surface area [2, 3]. While the first two requirements are met for ZnO nano rods, the last requirement is not. Thus, we were looking for an electrode structure that maintains a fast charge transport and good surface accessibility but also has a large surface: “nano corn-dogs”. Highly porous ZnO is electrodeposited onto the surface of nano rods pre-deposited in a first independent step. By setting deposition parameters one can control the structural properties, e.g. a deposition of highly porous ZnO only at the bottom part (close to the substrate) of the nano rod or at the top part (solution side) of the nano rod, only . This technique allows the specific design of nanostructured ZnO electrodes depending on the specific demands.

[1] C. Lupó, D. Schlettwein. J. Electrochem. Soc. 166(1), D3182-D3189. 2019. DOI: 10.1149/2.0231901jes

[2] R. Ruess, S. Haas, A. Ringleb, D. Schlettwein. Electrochim. Acta 258, 591-598. 2017. DOI: 10.1016/j.electacta.2017.11.102

[3] M. Stumpp, D. Damtew, D. Stock, K. Hess, D. Schröder, D. Schlettwein. J. Electrochem. Soc. 165(10), D461-D466. 2018. DOI: 10.1149/2.0941810jes

We still hold the copyright of our SEM image of the nanostructures, the “real” corndogs stem from [Jonathunder, GFDL 1.2, https://commons.wikimedia.org/wiki/File:CornDog.jpg]

Dieses Bild wurde eingereicht von Prof. Dr. Derck Schlettwein.